阅读说明：本技术 紧凑型对准传感器布置 (Compact alignment sensor arrangement ) 是由 T·M·T·A·M·埃拉扎里 J·L·克勒泽 F·G·C·比基恩 K·肖梅 于 2019-08-22 设计创作，主要内容包括：一种用于确定衬底的对准的设备和系统,其中,利用空间相干辐射照射周期性对准标记,然后将所述空间相干辐射照射提供给紧凑集成光学器件以创建对准标记的自身图像,该图像可以被操纵(例如进行镜像、偏振)和合并以获取与标记位置和标记内变形有关的信息。还公开了一种用于确定衬底的对准的系统,其中,利用空间相干辐射照射周期性对准标记,然后将所述空间相干辐射照射提供给光纤布置以获得诸如标记的位置和标记内的变形之类的信息。(An apparatus and system for determining alignment of a substrate in which periodic alignment marks are illuminated with spatially coherent radiation, which is then provided to compact integrating optics to create a self-image of the alignment marks that can be manipulated (e.g., mirrored, polarized) and combined to obtain information about the mark positions and distortions within the marks. A system for determining the alignment of a substrate is also disclosed, wherein periodic alignment marks are illuminated with spatially coherent radiation, which is then provided to an optical fiber arrangement to obtain information such as the position of the marks and deformations within the marks.)

1. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam of coherent radiation illuminating the portion of the grating from a second angle, the first and second beams of coherent radiation being diffracted by the portion of the grating; and

a multimode interference device comprising:

a first input port arranged to receive a positive diffraction order of the first coherent radiation beam and a positive diffraction order of the second coherent radiation beam diffracted by the grating portion;

a second input port arranged to receive a negative diffraction order of the first coherent radiation beam and a negative diffraction order of the second coherent radiation beam diffracted by the grating portion;

a first output port arranged to output a first spatially superposed image comprising an image of the grating portion illuminated by the first coherent radiation beam and a mirror image of the grating portion illuminated by the second coherent radiation beam; and

a second output port arranged to output a second spatially superposed image comprising an image of the grating portion illuminated by the second beam of coherent radiation and a mirror image of the grating portion illuminated by the first beam of coherent radiation.

2. The apparatus of claim 1, further comprising:

a first detector arranged to receive the first spatially superimposed image and to generate a first signal indicative of an intensity of the first spatially superimposed image; and

a second detector arranged to receive the second spatially superposed image and to generate a second signal indicative of an intensity of the second spatially superposed image.

3. The device of claim 2, further comprising a processor arranged to receive the first and second signals and configured to determine a characteristic of the portion of the grating based at least in part on the first and second signals.

4. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam of coherent radiation illuminating the portion of the grating from a second angle, the first and second beams of coherent radiation being diffracted by the portion of the grating; and

a plurality of multimode interference devices, each of the multimode interference devices being disposed adjacent to a corresponding segment of the grating portion and comprising

A first input port arranged to receive a positive diffraction order of the first coherent radiation beam and a positive diffraction order of the second coherent radiation beam diffracted by the corresponding segment of the grating portion;

a second input port arranged to receive a negative diffraction order of the first coherent radiation beam and a negative diffraction order of the second coherent radiation beam diffracted by the corresponding segment of the grating portion;

a first output port arranged to output a first spatially superposed image comprising an image of a segment of the grating portion illuminated by the first coherent radiation beam and a mirror image of a segment of the grating portion illuminated by the second coherent radiation beam; and

a second output port arranged to output a second spatially superposed image comprising an image of the segments of the grating portions illuminated by the second coherent radiation beam and a mirror image of the segments of the grating portions illuminated by the first coherent radiation beam.

5. The apparatus of claim 4, wherein the plurality of multimode interference devices are arranged as a linear array parallel to the portion of the grating.

6. The apparatus of claim 4, further comprising:

a plurality of first detectors each arranged to receive the first spatially superposed image and to generate a first signal indicative of an intensity of the first spatially superposed image; and

a plurality of second detectors each arranged to receive the second spatially superposed image and to generate a second signal indicative of an intensity of the second spatially superposed image.

7. The device of claim 6, further comprising a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

8. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam of coherent radiation illuminating the portion of the grating from a second angle, the first and second beams of coherent radiation being diffracted by the portion of the grating; and

an optical fiber-based device comprising

A first input port arranged to receive a positive diffraction order of the first coherent radiation beam and a positive diffraction order of the second coherent radiation beam diffracted by the grating portion;

a second input port arranged to receive a negative diffraction order of the first coherent radiation beam and a negative diffraction order of the second coherent radiation beam diffracted by the grating portion;

a first output port arranged to output a first spatially stacked image comprising a p-polarized image from the negative diffraction order and an s-polarized image from the positive diffraction order; and

a second output port arranged to output a second spatially superposed image comprising an s-polarized image from the negative diffraction order and a p-polarized image from the positive diffraction order.

9. The apparatus of claim 8, further comprising:

a first detector arranged to receive the first spatially superimposed image and to generate a first signal indicative of an intensity of the first spatially superimposed image; and

a second detector arranged to receive the second spatially superposed image and to generate a second signal indicative of an intensity of the second spatially superposed image.

10. The apparatus of claim 9, further comprising a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

11. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam of coherent radiation illuminating the portion of the grating from a second angle, the first and second beams of coherent radiation being diffracted by the portion of the grating; and

a plurality of optical fiber-based devices, each optical fiber-based device disposed adjacent to a corresponding segment of the grating portion and comprising

A first input port arranged to receive a positive diffraction order of the first coherent radiation beam and a positive diffraction order of the second coherent radiation beam diffracted by the grating portion;

a second input port arranged to receive a negative diffraction order of the first coherent radiation beam and a negative diffraction order of the second beam diffracted by the grating portion;

a first output port arranged to output a first spatially stacked image comprising a p-polarized image from the negative diffraction order and an s-polarized image from the positive diffraction order; and

a second output port arranged to output a second spatially superposed image comprising an s-polarized image from the negative diffraction order and a p-polarized image from the positive diffraction order.

12. The apparatus of claim 11, wherein the plurality of optical fiber-based devices are arranged in a linear array parallel to the portion of the grating.

13. The apparatus of claim 12, further comprising:

a plurality of first detectors each arranged to receive the first spatially superposed image and to generate a first signal indicative of an intensity of the first spatially superposed image; and

a plurality of second detectors respectively arranged to receive the second spatially superposed images and to generate second signals indicative of intensities of the second spatially superposed images.

14. The apparatus of claim 13, further comprising a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

15. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating a first illumination beam and a second illumination beam that illuminate at least a portion of the grating;

a first input optical element arranged to receive a positive diffraction order of the first illumination beam and a positive diffraction order of the second illumination beam diffracted by the grating;

a second input optical element arranged to receive the negative diffraction orders of the first and second illumination beams diffracted by the grating;

a first imaging optical element optically coupled to the first input optical element for generating a first image based on the positive diffraction order;

a second imaging optical element optically coupled to the second input optical element for generating a second image based on the negative diffraction order;

a first transformation optical element for transforming the first image to generate a first transformed image;

a second transformation optical element for transforming the second image to generate a second transformed image;

a first combined optical element for spatially superimposing the first image and the second transformed image; and

a second combined optical element for spatially superimposing the second image and the first transformed image.

16. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating at least one beam of coherent radiation that illuminates a portion of the grating, the at least one beam of coherent radiation being diffracted by the portion of the grating to produce a diffracted beam; and

multimode interference device comprising

A first input port arranged to receive a positive diffraction order of the diffracted beam;

a second input port arranged to receive a negative diffraction order of the diffracted beam;

a first output port arranged to output a first spatially superimposed image; and

a second output port for outputting a second spatially superimposed image.

17. The apparatus of claim 16, wherein the at least one light source coaxially illuminates the portion of the grating.

18. The apparatus of claim 16, wherein the at least one light source off-axis illuminates the portion of the grating.

19. The apparatus of claim 16, wherein the first input port comprises a first single mode waveguide and the second input port comprises a second single mode waveguide.

20. The apparatus of claim 16, wherein the first input port comprises a first sub-wavelength structure and the second input port comprises a second sub-wavelength structure.

Technical Field

The present disclosure relates to fabricating devices using lithographic techniques. In particular, the present disclosure relates to apparatus for sensing and analyzing alignment marks on reticles and wafers to characterize and control semiconductor photolithography processes.

Background

Lithographic apparatus can be used, for example, to manufacture Integrated Circuits (ICs). For this application, a patterning device (alternatively referred to as a "mask" or a "reticle") may be used to generate a circuit pattern to be formed on an individual layer of the IC. Such a pattern may be transferred onto a target portion (e.g., comprising part of, one die, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically performed by imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are successively patterned.

The known lithographic apparatus comprises: so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time; and so-called scanners, in which each target portion is irradiated by synchronously scanning the substrate parallel or anti-parallel to a given direction (the "scanning" direction) while the radiation beam scans the pattern in that direction. The pattern may also be transferred from the patterning device to the substrate by imprinting the pattern onto the substrate.

ICs are built layer by layer, and modern ICs can have 30 or more layers. On Product Overlay (OPO) is a measure of the ability of the system to print these layers exactly On top of each other. Successive layers or multiple processes on the same layer must be precisely aligned with the previous layer. Otherwise, the electrical contact between the structures will be poor and the resulting device will not meet specification requirements. Good overlay improves device yield and enables printing of smaller product patterns. Overlay errors between successive layers formed in or on a patterned substrate are controlled by portions of an exposure apparatus of a lithographic apparatus.

Process induced wafer errors are a significant obstacle to OPO performance. Process-induced errors result from the complexity of the printed pattern and the increase in the number of printed layers. This error has a relatively high spatial variation: within a given wafer, this variation varies from wafer to wafer.

In order to control a lithographic process to accurately place device features on a substrate, one or more alignment marks are typically provided on, for example, the substrate, and the lithographic apparatus includes one or more alignment sensors with which the positions of the marks can be accurately measured. The alignment sensor may effectively act as a position measuring device. Different types of markers and different types of alignment sensors are known from different times and different manufacturers. Measuring the relative position of several alignment marks within the field can correct process-induced wafer errors. Alignment error variations within a field can be used to fit a model for correcting OPO within the field.

Lithographic apparatus are known which use multiple alignment systems to align a substrate relative to the lithographic apparatus. For example, the data may be acquired using any type of Alignment Sensor or technique, such as a smash (smart Alignment Sensor hybrid) Sensor, as described in U.S. patent No. 6,961,116 entitled "fatty cosmetic Apparatus, Device Manufacturing Method, and Device Manufactured theory," published on 11/1/2005, the entire contents of which are incorporated herein by reference, which employs a self-referencing interferometer having a single detector and four different wavelengths, and extracts the Alignment signals in software; alternatively, ATHENA (advanced Technology using High order ENhancement of Alignment), as described in U.S. Pat. No. 6,297,876 entitled "Lithographic Projection Apparatus with an Alignment System for Alignment Substrate on Mask", published on 10/2 2001, which is incorporated herein by reference in its entirety, directs each of the seven diffraction orders to a dedicated detector, respectively.

Reference is made in particular to european application No. EP 1372040 a1 entitled "lubricating Apparatus and Device Manufacturing Method", granted on 5.3.2008, which is incorporated herein by reference in its entirety. EP 1372040 a1 describes an alignment system using a self-referencing interferometer that produces two superimposed images of an alignment marker. The two images are rotated 180 deg. with respect to each other. EP 1372040 a1 also describes detecting intensity variations of the interfering fourier transforms of the two images in the pupil plane. These intensity variations correspond to the phase difference between the different diffraction orders of the two images and position information is derived from this phase difference, which is required for the alignment process. Reference is also made to U.S. patent No. 8,610,898, "Self-Referencing Interferometer, Alignment System, and lithographical Apparatus," published 2013, 12, month 17, which is incorporated herein by reference in its entirety.

Existing alignment systems and techniques suffer from certain drawbacks and limitations. For example, they are generally not capable of measuring distortion in the alignment mark field, i.e., in-field distortion. These systems are also typically relatively bulky. They also do not support finer alignment grating pitches, e.g., grating pitches less than about 1 um. Therefore, there remains a need for an alignment sensor that can be aligned with nanometer precision to alignment grating marks printed on a wafer.

In addition, it is desirable to be able to use a larger number of alignment marks, since using a larger number of alignment marks provides the possibility of higher alignment accuracy. However, current alignment sensors typically only measure one position of one alignment mark at a time. Therefore, attempting to measure the position of many marks using current alignment sensor technology will result in a significant time and throughput loss. It is therefore desirable to have a sensor that can be used in arrangements that measure multiple alignment marks simultaneously.

Therefore, there is a need for a compact alignment sensor capable of measuring in-field deformations that can support finer alignment grating pitches and simultaneously measure multiple marks.

Disclosure of Invention

The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is intended to neither identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments. Its sole purpose is to present some concepts of one or more embodiments in a simplified form as a prelude to the more detailed description that is presented later.

According to one aspect of an embodiment, a system for determining alignment of a substrate is disclosed in which alignment marks are illuminated with spatially coherent radiation to create self-images of the alignment marks that can be manipulated (e.g., mirrored, polarized) and combined together to provide information about the position of the marks and any in-mark distortions within the marks.

According to an aspect of an embodiment, there is disclosed an apparatus for sensing an alignment pattern including a grating on a substrate, the apparatus comprising at least one light source for generating a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam of coherent radiation illuminating the portion of the grating from a second angle, and a multimode interference device, the multimode interference device comprising: a first input port arranged to receive the positive diffraction orders of the first and second beams diffracted by the grating portion; a second input port arranged to receive the negative diffraction orders of the first and second beams diffracted by the grating portion; a first output port arranged to output a first spatially superimposed image comprising an image of the grating portion illuminated by the first beam and a mirror image of the grating portion illuminated by the second beam; and a second output port arranged to output a second spatially superposed image comprising an image of the grating portion illuminated by the second beam and a mirror image of the grating portion illuminated by the first beam. The apparatus may further include: a first detector arranged to receive the first spatially superimposed image and to generate a first signal indicative of an intensity of the first spatially superimposed image; and a second detector arranged to receive the second spatially superposed image and to generate a second signal indicative of an intensity of the second spatially superposed image. The apparatus may further comprise a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

According to an aspect of the embodiments, there is also disclosed an apparatus for sensing an alignment pattern including a grating on a substrate, the apparatus comprising at least one light source for generating a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam of coherent radiation illuminating the portion of the grating from a second angle, and a plurality of multimode interference devices, each of the multimode interference devices being arranged adjacent to a corresponding segment of a grating portion and comprising: a first input port arranged to receive a positive diffraction order of the first beam and a positive diffraction order of the second beam diffracted by the corresponding segment of the grating portion; a second input port arranged to receive the negative diffraction orders of the first and second beams diffracted by the corresponding segments of the grating portion; a first output port arranged to output a first spatially superimposed image comprising an image of a segment of the grating portion illuminated by the first beam and a mirror image of a segment of the grating portion illuminated by the second beam; and a second output port arranged to output a second spatially superposed image comprising an image of the segment of the grating portion illuminated by the second beam and a mirror image of the segment of the grating portion illuminated by the first beam. The plurality of multimode interference devices may be arranged as a linear array parallel to the portion of the grating. The apparatus may further include: a plurality of first detectors each arranged to receive the first spatially superposed images and to generate first signals indicative of the intensity of the first spatially superposed images; and a plurality of second detectors each arranged to receive the second spatially superposed images and to generate second signals indicative of the intensity of the second spatially superposed images. The apparatus may further comprise a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

According to an aspect of an embodiment, there is also disclosed an apparatus comprising at least one light source for generating a first beam of coherent radiation and a second beam of coherent radiation, the first beam illuminating a portion of the grating from a first angle and the second beam illuminating the portion of the grating from a second angle, the first and second beams being diffracted by the portion of the grating, and an optical fiber-based device comprising: a first input port arranged to receive the positive diffraction order of the first beam and the positive diffraction order of the second beam diffracted by the grating portion; a second input port arranged to receive the negative diffraction order of the first beam and the negative diffraction order of the second beam diffracted by the grating portion; a first output port arranged to output a first spatially stacked image comprising a p-polarized image from the negative diffraction order and an s-polarized image from the positive diffraction order; and a second output port arranged to output a second spatially superposed image comprising an s-polarized image from the negative diffraction order and a p-polarized image from the positive diffraction order. The apparatus may further include: a first detector arranged to receive the first spatially superimposed image and to generate a first signal indicative of an intensity of the first spatially superimposed image; and a second detector arranged to receive the second spatially superposed image and to generate a second signal indicative of an intensity of the second spatially superposed image. The apparatus may further comprise a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

According to an aspect of an embodiment, there is also disclosed an apparatus comprising at least one light source for producing a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam illuminating the portion of the grating from a second angle, the first and second beams being diffracted by the portion of the grating, and a plurality of fiber-based devices each disposed adjacent to a corresponding segment of a grating portion and comprising: a first input port arranged to receive the positive diffraction order of the first beam and the positive diffraction order of the second beam diffracted by the grating portion; a second input port arranged to receive the negative diffraction order of the first beam and the negative diffraction order of the second beam diffracted by the grating portion; a first output port arranged to output a first spatially stacked image comprising a p-polarized image from the negative diffraction order and an s-polarized image from the positive diffraction order; and a second output port arranged to output a second spatially superposed image comprising an s-polarized image from the negative diffraction order and a p-polarized image from the positive diffraction order. The plurality of optical fiber-based devices may be arranged as a linear array parallel to the portion of the grating. The apparatus may further include: a plurality of first detectors each arranged to receive the first spatially superposed images and to generate first signals indicative of the intensity of the first spatially superposed images; and a plurality of second detectors each arranged to receive the second spatially superposed images and to generate second signals indicative of the intensity of the second spatially superposed images. The apparatus may further comprise a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

According to an aspect of the embodiments, there is also disclosed an apparatus for sensing an alignment pattern including a grating on a substrate, the apparatus comprising: at least one light source for generating a first illumination beam and a second illumination beam that illuminate at least a portion of the grating; a first input optical element arranged to receive the positive diffraction order of the first beam and the positive diffraction order of the second beam diffracted by the grating; a second input optical element arranged to receive the negative diffraction order of the first beam and the negative diffraction order of the second beam diffracted by the grating; a first imaging optical element optically coupled to the first input optical element for producing a first image based on the positive diffraction order; a second imaging optical element optically coupled to the second input optical element for producing a second image based on the negative diffraction order; a first transformation optical element for transforming the first image to generate a first transformed image; a second transformation optical element for transforming the second image to generate a second transformed image; a first combined optical element for spatially superimposing the first image and the second transformed image; and a second combined optical element for spatially superimposing the second image and the first transformed image.

According to an aspect of the embodiments, there is also disclosed an apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising at least one light source for generating at least one coherent radiation beam, said at least one beam illuminating a portion of said grating, and a multimode interference device, said at least one beam being diffracted by said portion of said grating to produce a diffracted beam. The multimode interference device comprises: a first input port arranged to receive a positive diffraction order of the diffracted beam; a second input port arranged to receive a negative diffraction order of the diffracted beam; a first output port arranged to output a first spatially superimposed image; and a second output port for outputting a second spatially superimposed image. The at least one light source may illuminate the portion of the grating coaxially or off-axis. The first input port includes a first single mode waveguide and the second input port includes a second single mode waveguide. The first input port includes a first sub-wavelength structure and the second input port includes a second sub-wavelength structure.

Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments, are described in detail below with reference to the accompanying drawings.

Drawings

The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate by way of example, and not of limitation, methods and systems of embodiments of the present invention. Together with the detailed description, the drawings serve to explain the principles and to enable a person skilled in the pertinent art to make and use the methods and systems presented herein. In the drawings, like reference numbers can indicate identical or functionally similar elements.

FIG. 1 depicts selected portions of a lithography system, such as may be used in accordance with aspects of embodiments disclosed herein.

Fig. 2 depicts selected portions of a known alignment system for explaining the operational principles thereof.

Fig. 3A and 3B depict a multi-mode interference device based alignment sensor in accordance with an aspect of embodiments disclosed herein.

FIG. 4 depicts a linear array of alignment sensors in accordance with an aspect of embodiments disclosed herein.

Fig. 5A and 5B depict arrangements of MMI illumination devices and MMI detectors according to an aspect of embodiments disclosed herein.

FIG. 6 depicts another arrangement of MMI illumination devices and MMI detectors according to an aspect of embodiments disclosed herein.

Fig. 7 depicts another arrangement of an MMI illumination device and an MMI detector in accordance with an aspect of embodiments disclosed herein.

Fig. 8 depicts an arrangement of sensors in accordance with an aspect of embodiments disclosed herein.

Fig. 9 depicts an arrangement of sensors on a sensor wafer in accordance with an aspect of an embodiment disclosed herein.

Fig. 10 depicts an optical fiber-based alignment sensor in accordance with an aspect of an embodiment disclosed herein.

Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Other embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

Detailed Description

Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. However, it may be apparent in some or all cases that any of the embodiments described below may be practiced without employing the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments. The following presents a simplified summary of one or more embodiments in order to provide a basic understanding of the embodiments. This summary is not an extensive overview of all contemplated embodiments, and is intended to neither identify key or critical elements of all embodiments nor delineate the scope of any or all embodiments.

Embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include solid-state memory, read-only memory (ROM), random-access memory (RAM), magnetic disk storage media, optical storage media, flash-memory devices; electrical, optical, acoustical or other form of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, procedures, and instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc.

FIG. 1 schematically depicts a lithographic apparatus. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or other suitable radiation); a support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM configured to accurately position the patterning device in accordance with certain parameters; a substrate table (e.g. a wafer table) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters; and a projection system (e.g. a refractive projection lens system) PL configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure can hold the patterning device using mechanical, vacuum, electrostatic or other clamping techniques. The support structure may be, for example, a frame or a table, which may be fixed or movable as required. The support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device".

The term "patterning device" used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples of patterning devices include displays, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include binary, alternating phase-shift, and attenuated phase-shift types, as well as various hybrid types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

The term "projection system" used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered as synonymous with the more general term "projection system".

As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such "multiple stage" machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being exposed.

The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term "immersion" as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

Referring again to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The light source and the lithographic apparatus may be separate entities, for example when the light source is an excimer laser. In such cases, the source is not considered to form part of the laser apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing and/or beam expanding mirrors. In other cases, the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ -outer and σ -inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. IN addition, the illuminator IL may comprise various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the support structure (e.g., mask table MT), and is patterned by the patterning device. After passing through the mask MA, the radiation beam B passes through the projection system PL, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometer, linear encoder, two-dimensional encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in fig. 1) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as shown occupy dedicated target portions, they may be located in spaces between target portions (these alignment marks are referred to as scribe-lane alignment marks). Similarly, in situations in which multiple dies are provided on multiple masks MA, the mask alignment marks may be located between two dies. The wafer may also include additional markings, such as markings that are sensitive to changes in the Chemical Mechanical Planarization (CMP) process used in the wafer preparation step.

Targets P1 and/or P2 on substrate W may be, for example, (a) resist layer gratings printed to form bars from solid resist lines after development; or (b) a product layer grating; or (c) a composite grating stack in an overlay target structure comprising resist gratings overlaid or interleaved on the product layer gratings. The grid may alternatively be etched into the substrate.

Fig. 2 shows a schematic view of a known alignment system 10. The light source 11 emits a beam of spatially coherent radiation which illuminates an alignment mark WM on a substrate (e.g. a wafer) which reflects the radiation as a positive diffraction order + n and a negative diffraction order-n. These diffracted orders are collimated by the objective lens 12 and enter the self-referencing interferometer 13. The self-referencing interferometer outputs two images of the input that are rotated 180 ° relative to each other, which overlap and thus can interfere. In the pupil plane 14, the superimposed fourier transform of these images (where the different diffraction orders are separated) can be seen and can interfere. A detector 15 in the pupil plane detects the interfered diffraction orders to provide position information. Based on this position information, the substrate can be accurately aligned with respect to the lithographic apparatus. The right part of fig. 2 shows the formation of two superimposed images in the pupil plane 14; for one image, + n 'and-n' are rotated +90 with respect to the input diffraction orders + n and-n; for the other image, + n "and-n" are rotated-90 deg. relative to the input diffraction orders + n and-n. In the pupil plane, the (+ n ' and-n ') orders and the (+ n "and-n ') orders interfere, respectively.

A disadvantage of the known alignment systems is that they are relatively expensive, since their optical design may require the use of specially manufactured optical components. Another disadvantage of the known alignment systems is that the known self-referencing interferometers are often very bulky.

Fig. 3A illustrates an alignment sensor in accordance with an aspect of an embodiment. Fig. 3A shows a first light source 100 and a second light source 110, the first light source 100 and the second light source 110 being arranged to illuminate one or more gratings in an alignment mark 120, such as a TIS or PARIS plate. The first light source 100 produces a first beam of coherent light 130 and the second light source 110 produces a second beam of coherent light 140. Although the first and second light sources 100 and 110 are shown as separate light sources, one of ordinary skill in the art will appreciate that the first and second beams 130 and 140 may be formed by splitting a single beam. Coherence is provided between the first beam 130 and the second beam 140.

The first reflector 150 reflects the first beam 130 along a first path onto the alignment mark 120, wherein a grating in the alignment mark 120 diffracts the first beam 130. The second reflector 160 reflects the second beam 140 along a second path onto the alignment mark 120, wherein the second beam 140 is diffracted by the grating in the alignment mark 120. Light 170 from one positive order of the first beam diffraction and one positive order of the second beam diffraction travels along a first path to a first single mode channel 180 of a multi-mode interference device (MMI) 200. Light 190 from one negative order of the first beam diffraction and the other negative order of the second beam diffraction travels along a second path to a second single mode channel 210 of the MMI 200. In this respect, see also fig. 3B. Although the description is divided into two figures, so that the beam paths can be seen separately, it should be understood that the reception of the illumination and diffraction orders of the MMI200 occurs substantially together continuously rather than sequentially.

Thus, the illumination light is in the form of two illumination beams incident on the alignment grating marks from two different directions, which are opposite to the light direction for detection. It is necessary to maintain a stable phase difference between the two irradiation beams 130 and 140. For example, in the case where beams 130 and 140 are generated by a laser having a resonant cavity, this can be accomplished by using an actuator (e.g., a piezoelectric device) to apply a force to the resonant cavity and control the length of the resonant cavity.

Thus, the light source emits a beam of spatially coherent radiation that illuminates alignment marks 120 on the substrate (e.g., wafer) that reflect the radiation as diffraction orders. These diffraction orders are collimated by the lens or curved surface 220 and enter the MMI 200.

When illuminated with a coherent light source, a periodic structure image, such as the alignment grating 120, is reproduced at some regular distance from the grating. These Principles are described, for example, in "Optical Multi-Mode Interference Devices Based on Self Imaging" by Lucas B.Soldano et al, Principles and Applications ", Journal of Lightwave Technology, Vol.13, No.4 (4. 1995), the entire contents of which are incorporated herein by reference. This phenomenon is used in multimode interference devices. Interference between modes within the MMI will reproduce the own image of the incident field. In other words, self-imaging is a property of multimode waveguides; with this property, the input field profile is reproduced at periodic intervals along the propagation direction of the guide in a single or multiple images.

In the arrangement shown, the length of the MMI200 is adjusted so that a first self-image 230 appears in the position shown in fig. 3B, and a second self-image 240 appears in the position shown in fig. 3B. These images are then mirrored and merged about the mirror axis 250 such that the first overlaid image 260 of the self-image 230 and the 180 ° mirror image of the self-image 240 appear in the position shown, and the second overlaid image 270 of the self-image 240 and the 180 ° mirror image of the self-image 230 appear in the position shown. Thus, mirroring and superposition result in spatially superposed diffraction orders, similar to those occurring in self-referencing interferometers.

The first and second overlaid images 260, 270 are coupled out of the MMI200 by a first and second single mode waveguide 280, 290, respectively. The first detector 300 receives the first overlay image 260 and the second detector 310 receives the second overlay image 270. The raw scan signals generated by the first detector 300 and the second detector 310 are received by the processor 320, either directly or indirectly. The raw scan signal may be processed to extract information about the surface being scanned. For example, the relative phase between spatially superposed pairs of diffraction orders may be varied as they are scanned across the mark, thereby generating a modulation signal from which an alignment position error may be determined.

The MMI 220 collects the positive and negative diffraction orders reflected from the grating as it is translated relative to the sensor. The MMI length may be set to create a dual image at the output ports 280 and 290. Output ports 280 and 290 serve as coherent summing and coherent differencing channels, respectively, that are 180 degrees out of phase.

One advantage of the described arrangement is that it can be made very compact compared to existing alignment sensors, and thus multiple sensors can be aligned in parallel to cover any field on the wafer. As shown in fig. 4. The linear array of N alignment sensor MMI 300-300N extends across the width of the alignment grating 120 in the direction of the arrow to capture diffracted radiation by exciting the single mode waveguides 180-180N and 210-210N of the respective MMI waveguides. As described above, the waveguide fundamental mode can be matched using a lens or using only curved surfaces. The parallel sensors so arranged can collectively cover a wider field of view, i.e., align multiple markers simultaneously to correct for intra-field distortions. It also allows the use of an alignment grating pitch of less than 1 um.

The arrangement shown in the figure produces a coherent summation channel. By adjusting the MMI200, for example by adding a pi phase delay line, a coherent sum channel and a coherent difference channel can be generated. Also, it should be understood that other or additional diffraction orders may be collected by locating the single mode channels of the MMI 200.

The novel alignment sensor described above can be prepared using photolithographic preparation techniques and is therefore easier to prepare than alignment sensors using conventional optical elements (e.g., lenses and mirrors) that require precision grinding and polishing. Thus, the novel alignment sensor is also less expensive to manufacture.

In addition to the illustrated illumination scheme, illumination may be achieved using, for example, a lens, an optical fiber, another MMI, or the like. In general, the illumination arrangement may provide illumination that is on-axis (substantially at right angles to the grating) and off-axis (at oblique angles to the grating). Off-axis illumination allows alignment information to be obtained from smaller (finer pitch) diffraction gratings.

Fig. 5A shows an arrangement in which an illuminating MMI 330 is used to provide coaxial illumination to the alignment mark 120 through a single mode output coupler 340. The diffracted orders of light diffracted by the alignment marks 120 are coupled into the collecting MMI200 through single-mode input ports 180 and 190. The collecting MMI200 and the illuminating MMI 330 are angled at a relatively small angle theta from each other.

Fig. 5B shows an arrangement in which the illuminating MMI 350 is used to provide off-axis illumination to the alignment mark 120 through single-mode output ports 360 and 370. The diffracted orders of light diffracted by the alignment marks 120 are coupled into the collecting MMI200 through single-mode input ports 180 and 190. The collecting MMI200 and the illuminating MMI 350 are angled at a relatively small angle theta from each other.

Operation over an extended wavelength range may be achieved by using sub-wavelength structures or gratings (instead of single mode waveguide input and output ports) designed to be optically coupled into or out of the MMI. Multimode optical fibers can be used to collect light from the output port to a photodetector that is positioned remotely from the optics. To accommodate different grating pitches, additional sensors can be fabricated by appropriately designing the multimode waveguide length and setting the appropriate spacing between the input and output ports. The wafer alignment grating may be illuminated using a one-by-one (1 x 1) illumination MMI, where the illumination beam is several degrees off normal incidence.

Such an arrangement is shown in fig. 6. The illumination MMI 330 is used to illuminate the alignment marks 120. The illumination beam from the output coupler 340 is only a few degrees off normal incidence. The positive diffraction order is incident on coupler 500, and coupler 500 is configured to couple light to a sub-wavelength structure or grating in the detection MMI 200. The negative diffraction orders are incident on a coupler 510, which coupler 510 is configured as a sub-wavelength structure or grating. The diffraction order pairs are present at the couplers 520, 530, the couplers 520, 530 also being configured to couple light out of the sub-wavelength structure or grating that detects the MMI 200. The light is incident on the detector 540, 550, and the detector 540, 550 may be arranged to receive the light directly or through an optical fiber.

Figure 7 shows an arrangement in which the illuminating MMI 350 has two outputs to provide off-axis illumination of the alignment mark 120. The illuminating MMI 350 has two output ports 600, 610. The positive diffraction order is incident on coupler 500, and coupler 500 is configured to couple light to a sub-wavelength structure or grating that detects MMI 350. The negative diffraction orders are incident on the coupler 510, and the coupler 510 is also configured as a sub-wavelength structure or grating. The diffraction order pairs are present at couplers 520, 530, and couplers 520, 530 are also configured to couple light out of the sub-wavelength structure or grating that detects MMI 350. The light is incident on the detector 540, 550, and the detector 540, 550 may be arranged to receive the light directly or through an optical fiber.

As mentioned, one advantage of the described arrangement compared to existing alignment sensors is that it can be made very compact and thus multiple sensors can be arranged side by side to cover any field on the wafer, as shown in fig. 4. Several sensors can be placed in series above the wafer and their positions can be adjusted individually, for example using screws or as shown in fig. 8, by means of a scissor spreader which can control the spacing between the sensors to be changed synchronously so that they match the position of the field of a specific layer layout on the wafer (for example the X-direction). As shown, a row of integrated optical sensors 700 is positioned in a row above the wafer on a scissor expander 710. Sensor 700 is coupled to scissor expander 710 at branch intersection 720. The lateral position of the sensor can be adjusted by moving one or both of the outer movable branches 730 of the scissor spreader 700, wherein the branches 740 are fixed. When alignment on the wafer needs to be measured at different pitches of the alignment marks, scissor expanders 710 reposition sensors 700 relative to each other in the X-direction. All mark positions within a line of fields (in the X direction) can be addressed by a small movement of the wafer under this line of sensor-fields. All rows can be addressed by moving the wafer under the row sensor-field by a field distance in the Y-direction.

Because the layout of the alignment marks is fixed per field and per wafer, an integrated optical sensor wafer 750 may be used for each layer layout, as shown in FIG. 9. Fig. 9 shows a wafer 760 having field arrays 770, each field array 770 having an alignment mark pattern 780. Sensor wafer 750 is disposed over wafer 760. Sensor wafer 750 has field arrays 790, each field array 790 having a sensor array 800 corresponding to the location of an alignment mark on wafer 760. Each sensor field 790 is connected to a connector 810 by a respective fiber optic cable 820, only some of which are shown for clarity. Connector 810 and optical cable 820 connect illumination source 830 and detector 840 to sensor wafer 750. This arrangement is possible because there is no need to have a light source or detector on the sensor wafer 750. When changing a customer product layout, sensor wafer 750 may be replaced with another sensor wafer that is dedicated to the new product layout. Within the scanner, a storage compartment (e.g., for reticles and wafers) may be provided for the sensor wafer 750.

Systems providing similar benefits may also be implemented using optical fibers. Such an arrangement is shown in figure 10. The optical fiber can be used to collect the positive/negative diffraction orders. The diffraction order pairs collected by a single fiber may be combined using a fiber combiner to generate an alignment signal. More specifically, referring to FIG. 10, positive and negative diffraction orders 170 and 190 are coupled to a first fiber-based polarizing beam splitter 400 and a second fiber-based polarizing beam splitter 410, respectively. The first fiber-based polarizing beam splitter 400 splits the positive diffracted order into an s-polarized beam + _ s and a p-polarized beam + _ p. Similarly, the second fiber-based polarizing beam splitter 410 splits the negative diffraction order into s-polarized beam _ s and p-polarized beam _ p. The beams + _ p and-s are combined in a first fiber-based beam combiner 420 to produce combined (+ _ p and-s) beams. The beams p and + _ s are combined in a second fiber-based beam combiner 430 to produce combined (- _ p and + _ s) beams. Each combined beam is relayed to a respective fiber/free space based polarization splitter/combiner 440, 450 that separates the beams. Detectors 460, 470 represent the coherent sum and coherent difference channels, respectively. As described above, the scan signal may be processed to extract information about the surface being scanned. For example, the relative phase between spatially superposed pairs of diffraction orders may be varied as they are scanned across the mark, thereby generating a modulation signal from which an alignment position error may be determined. As described above, the fiber-based sensors may be arranged to cover all or some of the field of the alignment grating 120.

Embodiments may be further described using the following aspects:

1. an apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam of coherent radiation illuminating the portion of the grating from a second angle, the first and second beams of coherent radiation being diffracted by the portion of the grating; and

multimode interference device comprising

A first input port arranged to receive a positive diffraction order of the first coherent radiation beam and a positive diffraction order of the second coherent radiation beam diffracted by the grating portion;

a second input port arranged to receive a negative diffraction order of the first coherent radiation beam and a negative diffraction order of the second coherent radiation beam diffracted by the grating portion;

a first output port arranged to output a first spatially superposed image comprising an image of the grating portion illuminated by the first coherent radiation beam and a mirror image of the grating portion illuminated by the second coherent radiation beam; and

a second output port arranged to output a second spatially superposed image comprising an image of the grating portion illuminated by the second beam of coherent radiation and a mirror image of the grating portion illuminated by the first beam of coherent radiation.

2. The apparatus of aspect 1, further comprising:

a first detector arranged to receive the first spatially superimposed image and to generate a first signal indicative of an intensity of the first spatially superimposed image; and

a second detector arranged to receive the second spatially superposed image and to generate a second signal indicative of an intensity of the second spatially superposed image.

3. The apparatus of aspect 2, further comprising a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

4. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam of coherent radiation illuminating the portion of the grating from a second angle, the first and second beams of coherent radiation being diffracted by the portion of the grating; and

a plurality of multimode interference devices, each of the multimode interference devices being arranged adjacent to a corresponding segment of the grating portion and comprising

A first input port arranged to receive a positive diffraction order of the first coherent radiation beam and a positive diffraction order of the second coherent radiation beam diffracted by the corresponding segment of the grating portion;

a second input port arranged to receive a negative diffraction order of the first coherent radiation beam and a negative diffraction order of the second coherent radiation beam diffracted by the corresponding segment of the grating portion;

a first output port arranged to output a first spatially superposed image comprising an image of a segment of the grating portion illuminated by the first coherent radiation beam and a mirror image of a segment of the grating portion illuminated by the second coherent radiation beam; and

a second output port arranged to output a second spatially superposed image comprising an image of the segments of the grating portions illuminated by the second coherent radiation beam and a mirror image of the segments of the grating portions illuminated by the first coherent radiation beam.

5. The apparatus of aspect 4, wherein the plurality of multimode interference devices are arranged as a linear array parallel to the portion of the grating.

6. The apparatus of aspect 4, further comprising:

a plurality of first detectors each arranged to receive the first spatially superposed image and to generate a first signal indicative of an intensity of the first spatially superposed image; and

a plurality of second detectors each arranged to receive the second spatially superposed image and to generate a second signal indicative of an intensity of the second spatially superposed image.

7. The apparatus of aspect 6, further comprising a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

8. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam of coherent radiation illuminating the portion of the grating from a second angle, the first and second beams of coherent radiation being diffracted by the portion of the grating; and

an optical fiber-based device comprising

A first input port arranged to receive a positive diffraction order of the first coherent radiation beam and a positive diffraction order of the second beam diffracted by the grating portion;

a second input port arranged to receive a negative diffraction order of the first coherent radiation beam and a negative diffraction order of the second coherent radiation beam diffracted by the grating portion;

a first output port arranged to output a first spatially stacked image comprising a p-polarized image from the negative diffraction order and an s-polarized image from the positive diffraction order; and

a second output port arranged to output a second spatially superposed image comprising an s-polarized image from the negative diffraction order and a p-polarized image from the positive diffraction order.

9. The apparatus of aspect 8, further comprising:

a first detector arranged to receive the first spatially superimposed image and to generate a first signal indicative of an intensity of the first spatially superimposed image; and

a second detector arranged to receive the second spatially superposed image and to generate a second signal indicative of an intensity of the second spatially superposed image.

10. The apparatus of aspect 9, further comprising a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

11. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating a first beam of coherent radiation illuminating a portion of the grating from a first angle and a second beam of coherent radiation illuminating the portion of the grating from a second angle, the first and second beams of coherent radiation being diffracted by the portion of the grating; and

a plurality of optical fiber-based devices each disposed adjacent to a corresponding segment of the grating portion and comprising

A first input port arranged to receive a positive diffraction order of the first coherent radiation beam and a positive diffraction order of the second coherent radiation beam diffracted by the grating portion;

a second input port arranged to receive a negative diffraction order of the first coherent radiation beam and a negative diffraction order of the second coherent radiation beam diffracted by the grating portion;

a first output port arranged to output a first spatially stacked image comprising a p-polarized image from the negative diffraction order and an s-polarized image from the positive diffraction order; and

a second output port arranged to output a second spatially superposed image comprising an s-polarized image from the negative diffraction order and a p-polarized image from the positive diffraction order.

12. The apparatus of aspect 11, wherein the plurality of fiber-based devices are arranged in a linear array parallel to the portion of the grating.

13. The apparatus of aspect 12, further comprising:

a plurality of first detectors each arranged to receive the first spatially superposed image and to generate a first signal indicative of an intensity of the first spatially superposed image; and

a plurality of second detectors each arranged to receive the second spatially superposed image and to generate a second signal indicative of an intensity of the second spatially superposed image.

14. The apparatus of aspect 13, further comprising a processor arranged to receive the first signal and the second signal and configured to determine a characteristic of the portion of the grating based at least in part on the first signal and the second signal.

15. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating a first illumination beam and a second illumination beam that illuminate at least a portion of the grating;

a first input optical element arranged to receive a positive diffraction order of the first illumination beam and a positive diffraction order of the second illumination beam diffracted by the grating;

a second input optical element arranged to receive the negative diffraction orders of the first and second illumination beams diffracted by the grating;

a first imaging optical element optically coupled to the first input optical element for producing a first image based on the positive diffraction order;

a second imaging optical element optically coupled to the second input optical element for producing a second image based on the negative diffraction order;

a first transformation optical element for transforming the first image to generate a first transformed image;

a second transformation optical element for transforming the second image to generate a second transformed image;

a first combined optical element for spatially superimposing the first image and the second transformed image; and

a second combined optical element for spatially superimposing the second image and the first transformed image.

16. An apparatus for sensing an alignment pattern comprising a grating on a substrate, the apparatus comprising:

at least one light source for generating at least one beam of coherent radiation that illuminates a portion of the grating, the at least one beam of coherent radiation being diffracted by the portion of the grating to produce a diffracted beam; and

multimode interference device comprising

A first input port arranged to receive a positive diffraction order of the diffracted beam;

a second input port arranged to receive a negative diffraction order of the diffracted beam;

a first output port arranged to output a first spatially superimposed image; and

a second output port for outputting a second spatially superimposed image.

17. The apparatus of aspect 16, wherein the at least one light source coaxially illuminates the portion of the grating.

18. The apparatus of aspect 16, wherein the at least one light source off-axis illuminates the portion of the grating.

19. The apparatus of aspect 16, wherein the first input port comprises a first single mode waveguide and the second input port comprises a second single mode waveguide.

20. The apparatus of aspect 16, wherein the first input port comprises a first sub-wavelength structure and the second input port comprises a second sub-wavelength structure.

Systems as described above can generally be used not only to improve overlay alignment, but also to diagnose, monitor, and/or adjust scanner performance. The correlation of the scan signal to a step/scanner key performance indicator can be analyzed. The analysis results may be displayed and/or stored in any of a number of ways, such as for immediate display on a monitoring device or for compilation in a report.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, Liquid Crystal Displays (LCDs), thin-film magnetic heads, etc. Those skilled in the art will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" is considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track or a coating and developing system (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and is not limited to optical lithography, where the context allows. In imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist provided to the substrate, the resist being cured by applying electromagnetic radiation, heat, pressure or a combination thereof. After the resist is cured, the patterning device is moved out of the resist, leaving a pattern therein.

The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including Ultraviolet (UV) radiation (e.g. having a wavelength of or about 365nm, 355nm, 248nm, 193nm, 157nm or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, electromagnetic and electrostatic optical components.

The invention has been described above with the aid of functional building blocks illustrating the implementation of specific functions and relationships thereof. Boundaries of these functional components have been arbitrarily defined herein for convenience of description. Other boundaries may be defined so long as the specified functions and relationships thereof are appropriately performed.

The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such as specific embodiments without undue experimentation and without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance.

The spirit and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.