阅读说明：本技术 以倾斜入射角于样本表面上的检验光束成形 (Inspection beam shaping on sample surface at oblique incidence angle ) 是由 许志伟 黄春圣 李晴 于 2019-02-11 设计创作，主要内容包括：本发明揭示一种用于光学检验工具的光束成形器,其包含：聚焦透镜,其用以使光束以一倾斜入射角聚焦到目标上；及相位调制器,其用以在所述光束以所述倾斜入射角聚焦到所述目标上时使所述光束的顶部在所述目标的平面中基本上变平。(A beam shaper for an optical inspection tool is disclosed, including: a focusing lens for focusing the light beam onto the target at an oblique incident angle; and a phase modulator to substantially flatten the top of the beam in the plane of the target when the beam is focused onto the target at the oblique angle of incidence.)

1. A beam shaper for an optical inspection tool, comprising:

a focusing lens for focusing the light beam onto the target at an oblique incident angle; and

a phase modulator to substantially flatten a top of the light beam in a plane of the target when the light beam is focused onto the target at the oblique angle of incidence.

2. The beam shaper of claim 1, further comprising a chuck on which the target is to be mounted, wherein the focusing lens is located in an optical path of the beam between the phase modulator and the chuck.

3. The beam shaper of claim 2, wherein the beam incident on the phase modulator is substantially gaussian.

4. The beam shaper of claim 2, wherein there is no optical element between the phase modulator and the focusing lens.

5. The beam shaper of claim 1, wherein the optical modulator is configured to flatten the top of the beam such that an intensity of the beam across a width corresponding to an inspection track does not change by more than 10% in the plane of the target when the beam is focused onto the target at the oblique angle of incidence.

6. The beam shaper of claim 1, wherein the phase modulator comprises a Diffractive Optical Element (DOE).

7. The beam shaper of claim 6, wherein the DOE illustrates a convolution of a symmetric phase term with an asymmetric phase term to produce a substantially symmetric beam profile in the plane of the target when the beam is focused onto the target at the oblique angle of incidence.

8. The beam shaper of claim 7, wherein a cross-section of the DOE perpendicular to a radial axis of the beam is substantially wedge-shaped.

9. The beam shaper of claim 7, wherein the DOEs are shaped to produce respective beam profiles having distinct phases for distinct segments of the DOE, the distinct segments being perpendicular to a plane of incidence of the beam, wherein a combination of the distinct phases smoothes a spike in the top of the beam.

10. The beam shaper of claim 6, wherein the DOE is symmetric and positioned off-center with respect to an optical axis.

11. The beam shaper of claim 1, wherein the phase modulator comprises an aspheric lens.

12. The beam shaper of claim 11, wherein the aspheric lens is positioned off-center with respect to the optical axis.

13. The beam shaper of claim 1, wherein the phase modulator comprises a lens selected from the group consisting of a rod lens and a powell lens.

14. The beam shaper of claim 13, wherein the lens is positioned off-center with respect to the optical axis.

15. The beam shaper of claim 1, wherein:

the target is a semiconductor wafer; and is

The phase modulator is configured to substantially flatten the top of the beam in a plane corresponding to a surface of the semiconductor wafer on which the beam is focused.

16. A beam shaper for an optical inspection tool, comprising:

a focusing lens for focusing the light beam onto the target at an oblique incident angle;

a DOE to substantially flatten a top of the beam in a plane of the target when the beam is focused onto the target at the oblique angle of incidence; and

a chuck on which the target is to be mounted, wherein:

the focusing lens is located in the optical path of the beam between the DOE and the chuck;

the beam incident on the DOE is substantially gaussian; and is

A cross-section of the DOE perpendicular to a radial axis of the beam is substantially wedge-shaped.

17. A method of beam shaping, comprising:

a phase modulated light beam; and

focusing the light beam onto a target at an oblique angle of incidence;

wherein the light beam phase modulated and focused onto the target at the oblique angle of incidence has a substantially flattened top in the plane of the target.

18. The method of claim 17, wherein:

performing the phase modulation using a phase modulator prior to the focusing; and is

The method further includes providing the optical beam to the phase modulator, wherein the optical beam provided to the phase modulator is substantially gaussian.

19. The method of claim 17, wherein the phase modulation is performed using a DOE that produces a substantially symmetric beam profile in the plane of the target by instantiating a convolution of a symmetric phase term with an asymmetric phase term.

20. The method of claim 19, wherein:

shaping the DOE to produce respective beam profiles having distinct phases for distinct segments of the DOE, the distinct segments being perpendicular to a plane of incidence of the beams; and is

The distinct phases combine to smooth a spike in the top of the beam.

Technical Field

The present invention relates to optical inspection tools (e.g., for semiconductor inspection), and more particularly, to beam shaping in optical inspection tools.

Background

Optical inspection tools are used to inspect an object (e.g., a semiconductor wafer) by providing a beam of light (also referred to as an illumination beam) that is scattered off of the object. The presence of defects (e.g. particles) on the surface of the target within the trajectory illuminated by the beam (i.e. within the "inspection trajectory") will affect the way in which the beam is scattered, allowing the detection of defects. However, the shape of the light beam limits the sensitivity of the optical inspection tool. For example, if the beam is approximately Gaussian (Gaussian) such that the intensity drops towards the edges of the inspection track, this drop in intensity limits the sensitivity threshold for detecting defects within the track. Alternatively, the beam intensity for the optical tool may have to be increased to achieve the desired sensitivity. Further, the light beam may have an oblique incident angle with respect to the target such that the incident angle is not a right angle (i.e., the light beam is obliquely incident on the target). When the angle of incidence is an oblique angle, the shape of the beam in the plane of the target will be different from the shape of the beam in the radial plane.

Disclosure of Invention

Accordingly, in view of the oblique angle of incidence of the beam with respect to the target, there is a need for methods and systems that shape the beam to flatten the top of the beam in the plane of the target.

In some embodiments, a beam shaper for an optical inspection tool includes: a focusing lens for focusing the light beam onto the target at an oblique incident angle; and a phase modulator to substantially flatten the top of the beam in the plane of the target when the beam is focused onto the target at the oblique angle of incidence.

In some embodiments, a method of beam shaping includes phase modulating a light beam and focusing the light beam onto a target at an oblique angle of incidence. The light beam phase modulated and focused onto the target at the oblique angle of incidence has a substantially flattened top in the plane of the target.

Drawings

For a better understanding of the various described embodiments, reference should be made to the following detailed description, taken in conjunction with the following drawings, which are not to scale.

Fig. 1 is a schematic illustration of a beam shaper that shapes a beam to substantially flatten the top of the beam in the plane of a target, according to some embodiments.

Fig. 2 shows simulated beam profiles produced in the plane of a target using simulations of different examples of the beam shaper of fig. 1, according to some embodiments.

Fig. 3 shows a simulated beam profile in the plane of a target derived from convolution of a symmetric phase term and an asymmetric phase term in a Diffractive Optical Element (DOE) phase modulator in the beam shaper of fig. 1, in accordance with some embodiments.

Fig. 4 shows a DOE in which the surface in the path of the beam of fig. 1 is lithographically shaped and thus has steps, according to some embodiments.

Fig. 5 shows a cross-section of the DOE of fig. 4, in accordance with some embodiments.

Fig. 6 shows a simulated beam profile produced in the plane of a target using simulations involving DOEs according to some embodiments.

Fig. 7 is a flow diagram of a beam shaping method according to some embodiments.

Like reference numerals refer to corresponding parts throughout the drawings and the description.

Detailed Description

Reference will now be made in detail to the various embodiments, examples of which are illustrated in the accompanying drawings. In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of various described embodiments. However, it will be apparent to one of ordinary skill in the art that the various described embodiments may be practiced without these specific details. In other instances, well-known methods, procedures, components, circuits, and networks have not been shown in detail to avoid unnecessarily obscuring aspects of the embodiments.

Fig. 1 is a schematic illustration of a beam shaper 100 that shapes a beam 102 to substantially flatten a top of the beam 102 in a plane of a target 108 (e.g., on a surface of the target 108) according to some embodiments. The beam shaper 100 is part of an illumination system of an optical inspection tool, such as a semiconductor optical inspection tool. Having a radius w having a beam waist₀Is provided by an illumination source (e.g., a laser) in the illumination system. The beam shaper 100 includes a phase modulator 104, such as a phase modulation plate located in the path of the light beam 102, and a focusing lens 106. The phase modulator 104 implements a phase modulation function Φ and thereby modulates the phase of the optical beam 102. The focusing lens 106 focuses the light beam 102 onto the target 108 at an oblique angle of incidence (as measured from the surface normal of the target 108). The target 108 may be mounted on a chuck 110.

The phase modulation function Φ of the phase modulator 104 phase modulates the optical beam 102 such that the top of the optical beam 102 is substantially flat (e.g., uniform to within 10%) in the plane of the target 108 (e.g., on the surface of the target 108). According to some embodiments, the beam shaper 100 does not include an intensity modulator and thus does not use intensity modulation to shape the beam. Because no intensity modulation is used, the beam shaper 100 does not degrade the intensity of the beam 102, which is desirable for sensitive, photon-hungry applications. This optical loss reduction of the illumination system provides high enclosed energy to the light beam 102.

In some embodiments, the phase modulator 104 is positioned before the focusing lens 106 such that the focusing lens 106 is positioned in the optical path of the beam 102 between the phase modulator 104 and the chuck 110. In some embodiments, there is no optical element between the phase modulator 104 and the focusing lens 106. The illumination system may include additional optical elements (e.g., apertures, magnifiers, polarizers, lenses, mirrors, etc.) prior to the phase modulator 104. The illumination system may also include additional optical elements (e.g., one or more mirrors to direct the beam 102 to the target 108) behind the focusing lens 106 (i.e., between the focusing lens 106 and the chuck 110).

In some embodiments, the shape of the beam 102 is substantially gaussian. For example, the beam 102 is dominated by a substantially Gaussian pattern. Thus, the cross-sectional profile of the beam intensity (i.e., in a radial direction perpendicular to the optical axis) will resemble a gaussian (e.g., such that each point in the cross-sectional profile has an intensity that differs from the intensity expected by a gaussian by no more than 10% or no more than 5%).

The propagation of a gaussian beam in an illumination system can be described by the following equation:

F＝FFT(G) (3)

where P is the electric field, w is the beam spot size (i.e., field spot size), w₀Is the beam waist (i.e., waist point size), λ is the beam wavelength (e.g., laser wavelength), k is the wavenumber, Φ is the phase modulation function of the phase modulator 104, F is the focal length of the focusing lens 106, d is the distance from the focusing lens 106 to the target 108, y is the in-plane position on the wafer_rIs the radial position (i.e., in a plane perpendicular to the optical axis), the FFT is a fast Fourier transform, and I_tIs the beam intensity at the target 108. The phase modulation function Φ is selected such that the optical beam 102 has a substantial component in the plane of the target 108An upper flat top.

For a beam normally incident on the target (as opposed to incident at an oblique angle), the following solution of the phase modulation function Φ yields a substantially flat top beam (assuming sufficient Numerical Aperture (NA)):

wherein w_FTIs the width of the flat top, erf is the error function, and all other variables are as defined above for equations 1-5. The phase modulator may be implemented according to equation 8. For example, a rod lens or Powell lens may provide a low-order approximation of the solution of equation 8, an aspheric lens may be designed to provide a higher-order approximation of the solution of equation 8, and a Diffractive Optical Element (DOE) may be designed to provide a discrete implementation of the solution of equation 8. The phase modulation of equation 8 involves optical axis symmetry.

However, for a beam 102 having an oblique angle of incidence on the target 108, symmetric phase modulation results in an asymmetric beam profile in the plane of the target 108. This asymmetry occurs because the optical paths 112 and 114 for both ends of the top of the beam profile are asymmetric, as shown in fig. 1. A phase modulator optimized for normal incidence will therefore produce an asymmetric beam profile for oblique incidence in the plane of the target 108.

In some embodiments, the asymmetry compensation is provided by positioning the phase modulator 104 (e.g., a rod lens, a powell lens, an aspheric lens, or a symmetric DOE) off-center with respect to the optical axis. The asymmetry compensation may only be partially effective so that some asymmetry remains present in the beam profile. For example, fig. 2 shows simulated beam profiles 200 and 202 for respective examples of the beam 102 in the plane of the target 108 generated using simulations in which the phase modulator 104 is a respective symmetric DOE with off-center alignment (i.e., off-center positioning). (the beam width may vary for different implementations). The beam profile 200 is generated by simulation of the beam shaper 100 in which the phase modulator 104 is a symmetric positive phase DOE with off-center alignment. The beam profile 202 is produced by simulation of the beam shaper 100 in which the phase modulator 104 is a symmetric negative-phase DOE with off-center alignment. The tops of the beam profiles 200 and 202 are substantially flat in the wafer plane (or the plane of another target 108) and nearly but not exactly symmetrical in that plane.

In other embodiments, an asymmetric DOE is used to compensate (in whole or in part) for the asymmetry between optical path 112 and optical path 114 (fig. 1). The simulated beam profile 204 (fig. 2) is an example of a beam profile for a corresponding example of the beam 102 generated using a simulation in which the phase modulator 104 is an asymmetric DOE. To achieve the beam profile 204, the DOE is designed such that its phase modulation function Φ includes an asymmetric phase term:

wherein c is₁、c₂And c₃Is a coefficient that can be adjusted numerically to improve (e.g., optimize) the flatness of the top of the beam, other variables and functions are as defined above, and c₃The items are asymmetrical. The asymmetric phase term may take the form of the correlation term in equation 9, a polynomial approximation of equation 9, or another valid equivalent.

The beam profile of an asymmetric DOE in the plane of the target 108 may thus result from the convolution of the symmetric phase term with the asymmetric phase term:

wherein G is_symDOECorresponding to symmetric phase terms and G_asymDOECorresponding to an asymmetric phase term. FIG. 3 illustrates aThe simulation results of this convolution according to some embodiments are illustrated: the simulated beam profile 304 in the wafer plane or plane of another target 108 results from the convolution of the symmetric phase term 300 with the asymmetric phase term 304 (where the symmetry and asymmetry of the respective phase terms 300 and 302 are with respect to the optical axis). As fig. 3 shows, the beam profile 304 in the plane of the target 108 is substantially symmetric (in practice almost exactly symmetric) in the plane of the target 108. Thus, the DOE may illustrate a convolution of the symmetric phase term and the asymmetric phase term to produce a substantially symmetric beam profile in the plane of the target 108 when the beam 102 is focused onto the target 108 at an oblique angle of incidence.

A DOE (e.g., an asymmetric DOE) used as the phase modulator 104 can be made using a lithographic process such that there is a step in the phase modulation profile. Fig. 4 shows a device in which a surface 402 in the path of the beam 102 (fig. 1) is lithographically shaped and thus has a stepped DOE 400, according to some embodiments. An optical axis 406 extends through the DOE 400, intersecting both the surface 402 and the opposing surface 404. In the example of fig. 4, optical axis 406 passes through a minimum point on surface 402 (i.e., a minimum point on the step curve of surface 402).

The step size of the DOE 400 is programmable. The quality of the DOE 400 and the resulting flatness of the top of the beam 102 varies depending on the step size. A small step increases the quality of the DOE and thus the flatness of the top of the beam 102, but increases manufacturing costs. However, too coarse a step size introduces a phase error that degrades the flatness of the top of the beam. FIG. 6 shows a simulated beam profile 600 of the beam 102 in the plane of the target 108 when the step size is π/4. This coarse step results in the top of the beam 102 in the plane of the target 108 having a spike rather than being substantially flat, as shown in fig. 6.

In some embodiments, to mitigate uneven beam profiles resulting from coarse steps of surface 402, surface 404 intersects optical axis 406 at an oblique angle, as shown in fig. 5 in accordance with some embodiments. FIG. 5 shows directly out of the page in FIG. 4, parallel to the optical axis 406 (or coincident with the optical axis 406), and perpendicular to the radial axis y_rA cross section of the DOE 400 in the plane of (a). In some embodiments, each such cross-section of the DOE 400 has the shape shown in fig. 5. DOE400 may thus be in the tangential direction (i.e., the direction of the optical axis 400, which is perpendicular to the radial axis y)_r) Having a wedge angle and may have a substantially wedge-shaped cross-section (in this example, the cross-section is not precisely wedge-shaped because it does not taper). Respective segments of the DOE in the tangential direction and in respective planes parallel to the page of fig. 4 (or in the page of fig. 4) (and thus perpendicular to the plane of incidence of the beam) produce respective beam profiles that combine to smooth the spikes in the beam profile 600. (FIG. 4 shows an example of such a segment.) the inclination of surface 404 means that the distance 408 between surface 402 and surface 404 is different for different segments. The resulting simulated beam profile 602 for the beam 102 with a substantially flat top in the plane of the target 108 is shown in fig. 6. The geometry of fig. 5 thus allows the use of a DOE with a relatively coarse step (e.g., a step size of pi/4) as the phase modulator 104 to achieve a flat top beam in the plane of the target 108.

Fig. 7 is a flow diagram of a beam shaping method 700 according to some embodiments. Method 700 may be performed using beam shaper 100 (fig. 1).

In some embodiments of method 700, optical beam 102 is provided 702 to phase modulator 104. The optical beam 102 provided to the phase modulator 104 may be substantially gaussian.

The optical beam is phase modulated 704 (e.g., using phase modulator 104). In some embodiments, the phase modulation is performed using (706) a DOE, an aspheric lens, a rod lens, or a powell lens. For example, a DOE is used (708), which produces a substantially symmetric beam profile 304 (fig. 3) in the object plane by illustrating the convolution of the symmetric phase term 300 with the asymmetric phase term 302. The DOE may be shaped (710) to produce respective beam profiles having distinct phases for distinct segments of the DOE that are perpendicular to a plane of incidence of the beam (e.g., according to fig. 5). The distinct phases combine to smooth the spike in the top of the beam.

In another example, phase modulation is performed using (712) a symmetric DOE positioned off-center with respect to the optical axis. In yet other examples, the phase modulation is performed using (714) an aspheric lens, a rod lens, or a powell lens that is off-center aligned with respect to the optical axis.

The beam 102 is focused 716 at an oblique angle of incidence onto a target 108, such as a semiconductor wafer. The light beam 102, which is phase modulated and focused onto the target 108 at the oblique angle of incidence, has a substantially flattened top in the plane of the target 108 (e.g., on the surface of the target 108).

The order independent steps in method 700 may be reordered.

The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the scope of the claims to the precise forms disclosed. Many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles underlying the claims and their practical application to thereby enable others skilled in the art to best utilize the embodiments with various modifications as are suited to the particular use contemplated.