阅读说明：本技术 用于亚微米增材制造的系统和方法 (Systems and methods for sub-micron additive manufacturing ) 是由 苏拉布·萨哈 罗伯特·帕纳西 陈世祈 于 2018-12-06 设计创作，主要内容包括：公开了一种用于通过加工光敏聚合物抗蚀剂材料来执行增材制造操作以形成结构的设备。该设备可以包括：激光器,用于生成激光光束；以及可调掩模,用于接收该激光光束,该可调掩模具有光学色散元件。掩模将该激光光束分成多个出射光束,每个出射光束具有不同强度或相同强度的子集细光束,每个细光束从该掩模的照射区域的唯一子部分出射。准直仪对出射光束中的至少一个进行准直以形成准直光束。一个或更多个聚焦元件将准直光束聚焦为聚焦光束,该聚焦光束在抗蚀剂材料上或抗蚀剂材料内被投影为聚焦成像平面。所述聚焦光束同时照射抗蚀剂材料的层来以并行方式加工整个层。(An apparatus for performing an additive manufacturing operation to form a structure by processing a photosensitive polymeric resist material is disclosed. The apparatus may include: a laser for generating a laser beam; and a tunable mask for receiving the laser beam, the tunable mask having an optical dispersive element. A mask divides the laser beam into a plurality of exit beams, each exit beam having a subset beamlet of different or the same intensity, each beamlet exiting from a unique sub-portion of the illuminated area of the mask. The collimator collimates at least one of the outgoing beams to form a collimated beam. One or more focusing elements focus the collimated light beam into a focused light beam that is projected as a focused imaging plane on or within the resist material. The focused beam simultaneously irradiates a layer of resist material to process the entire layer in a parallel manner.)

1. An apparatus for performing an additive manufacturing operation by processing a photosensitive polymeric resist material to form a structure, the apparatus comprising:

a laser source for generating a laser beam;

a tunable mask for receiving the laser beam, wherein the tunable mask comprises an optical dispersive element;

the tunable mask is configured to split the laser beam into a plurality of exit beams, wherein each of the exit beams exiting the tunable mask comprises a subset of beamlets of different or the same intensity, and wherein each of the beamlets exits a unique sub-portion of an illumination area of the tunable mask;

a collimator for collecting and collimating at least one of the outgoing beams from the tunable mask to form a collimated beam;

one or more focusing elements for focusing the collimated light beam into a focused light beam projected as a focused imaging plane on or within the photopolymer resist material, wherein the tunable mask, the collimator and the focusing elements are oriented and positioned to produce the same optical path length between the tunable mask and the imaging plane for the laser light beam of all optical frequencies; and

wherein the focused beams simultaneously irradiate the layer of photosensitive polymeric resist material.

2. The apparatus of claim 1 wherein the entrance aperture of the collimator is large enough to collect all wavelengths contained in a single one of the outgoing beams exiting the tunable mask, but sufficiently small to block all wavelengths of all other outgoing beams in the outgoing beam.

3. The apparatus of claim 1, wherein the collimator comprises a convex lens or a concave mirror.

4. The apparatus of claim 1, further comprising a moving stage for supporting the resist material and moving the resist material relative to the focal imaging plane.

5. The apparatus of claim 4, further comprising a moving stage for supporting the one or more focusing elements and moving the focused imaging plane axially toward or away from the resist material.

6. The apparatus of claim 1, wherein at least one of the one or more focusing elements comprises focus-adjustable optics comprising an electrically-adjustable lens (ETL).

7. The apparatus of claim 1, further comprising a power monitoring system for monitoring the power of at least one of the outgoing beams exiting the tunable mask unfocused on the resist material.

8. The apparatus of claim 7, further comprising a power control unit comprising at least one of:

a rotating half-wave plate followed by a polarizing beam splitter for controlling the power of the beam received by the adjustable mask; or

A rotating neutral density filter wheel for controlling the power of the beam received by the tunable mask.

9. The apparatus of claim 1, wherein the tunable mask comprises a strain tunable pleat structure having a strained membrane supported on a flexible substrate.

10. The apparatus of claim 1, wherein the adjustable mask comprises a Digital Micromirror Device (DMD).

11. The apparatus of claim 1, further comprising a control unit for adjusting the tunable mask upon receiving a timing signal from an internal clock or an external clock.

12. The apparatus of claim 4, further comprising a control unit for adjusting the tunable mask upon receiving a synchronization signal from the moving stage supporting the resist material.

13. The apparatus of claim 5, further comprising a control unit for adjusting the adjustable mask upon receiving separate synchronization signals from the moving stage supporting the resist material and the moving stage supporting the focusing element.

14. The apparatus of claim 1, further comprising an imaging system using an incoherent optical source to monitor a focused beam of light illuminating the photosensitive polymeric resist material.

15. The apparatus of claim 1, wherein the tunable mask comprises a Spatial Light Modulator (SLM).

16. An apparatus for performing an additive manufacturing operation by processing a photosensitive polymeric resist material to form a structure, the apparatus comprising:

a laser source for generating a pulsed laser beam;

a tunable mask for receiving the pulsed laser beam, wherein the tunable mask comprises a Digital Micromirror Device (DMD) comprising a plurality of individually controllable pixels that can be turned on or off;

the tunable mask is configured to split the pulsed laser beam into a plurality of exit beams, wherein each of the exit beams exiting the tunable mask includes a subset of beamlets that are of different intensities or the same intensity, and wherein each of the beamlets exits a unique pixel;

a collimator for collecting and collimating all wavelengths of only selected ones of a plurality of outgoing beams from the tunable mask, and the collimator is configured to block all wavelengths of all other outgoing beams in the outgoing beam;

one or more focusing elements for focusing a collimated beam into a focused beam projected as a focused imaging plane on or within the photopolymer resist material, wherein the tunable mask, the collimator, and the focusing elements are oriented and positioned to produce the same optical path length between the tunable mask and the focused imaging plane for the pulsed laser beam of all optical wavelengths;

a moving stage for supporting the photopolymer resist material and moving the photopolymer resist material relative to the focal imaging plane; and

wherein the focused beams simultaneously irradiate the layer of photosensitive polymeric resist material.

17. The apparatus of claim 16, wherein the tunable mask is oriented at an angle to the received pulsed laser beam so as to generate a blazed grating condition for a center wavelength of the received pulsed laser beam.

18. The apparatus of claim 16, further comprising a power monitoring unit for collecting and measuring power of at least one of the plurality of outgoing beams not used to generate the focused imaging plane.

19. The apparatus of claim 18, further comprising a power control unit comprising one of:

a rotating half-wave plate followed by a polarizing beam splitter; or

A rotating neutral density filter wheel for controlling the power of the pulsed laser beam received by the tunable mask.

20. The apparatus of claim 16, further comprising a control unit for adjusting the DMD upon receiving a trigger signal from an internal clock or an external clock.

21. The apparatus of claim 16, further comprising a control unit for adjusting the DMD upon receiving a synchronization signal from the moving stage supporting the photopolymer resist material.

22. The apparatus of claim 16, wherein one of the focusing elements comprises an Electrically Tunable Lens (ETL) for optically translating an axial position of a focal plane toward or away from the photosensitive polymer resist material.

23. The apparatus of claim 22, further comprising a control unit for adjusting the DMD upon receiving a synchronization signal or a trigger signal from the moving stage supporting the photopolymer resist material, the ETL, and an internal or external clock.

24. A method for performing an additive manufacturing operation by processing a photosensitive polymeric resist material to form a structure, the method comprising:

generating a laser beam;

directing the laser beam toward a tunable mask, wherein the tunable mask comprises an optical dispersive element;

collecting at least one exit beam from a plurality of exit beams exiting the tunable mask and directing the at least one exit beam through collimating optics to generate a collimated beam, wherein each of the exit beams from the tunable mask comprises a plurality of beamlets of different or the same intensity, and wherein each of the beamlets exits a unique sub-portion or area of the tunable mask illuminated by the laser beam;

focusing the collimated light beam into a focused light beam by one or more focusing elements, the focused light beam projected as an imaging plane on or within the photopolymer resist material, wherein the tunable mask, the collimating optics and the focusing elements are oriented and positioned to produce the same optical path length between the tunable mask and the focused imaging plane for the laser light beam of all optical frequencies; and

maintaining a selected pattern of sub-portions on the tunable mask for a finite duration to cause simultaneous polymerization of selected portions of the photopolymer resist material corresponding to the selected pattern at which time a combined dose effect from the duration of laser irradiation and the intensity of light exceeds a threshold dose for polymerization of the photopolymer resist material.

25. The method of claim 24, further comprising monitoring the power of at least one emergent beam produced by the tunable mask but not used to cause polymerization of the photopolymer resist material.

26. The method of claim 25, further comprising adjusting the power of the laser beam directed toward the adjustable mask in proportion to the monitored power of the at least one exit beam.

27. The method of claim 24, further comprising optically translating an axial position of the focused imaging plane by using a focus-adjustable focusing element as one of the focusing elements.

28. The method of claim 24, further comprising supporting the photosensitive polymer resist material on a stage and moving the focusing element relative to the stage to write out the layer of photosensitive polymer resist material.

29. The method of claim 28, further comprising controllably moving the moveable stage while the focused laser beam irradiates the photosensitive polymer resist material.

30. The method of claim 29, further comprising moving both the movable stage and the focusing element to create a continuous structure.

31. A method for performing an additive manufacturing operation by processing a photosensitive polymeric resist material to form a structure, the method comprising:

generating a pulsed laser beam;

directing the pulsed laser beam toward a tunable mask, wherein the tunable mask is an optical dispersive element;

collecting at least one exit beam exiting the tunable mask and directing the at least one exit beam through collimating optics to generate a collimated beam, wherein each exit beam from the tunable mask comprises a plurality of beamlets of different or the same intensity, and wherein each of the beamlets exits a unique sub-portion of an illumination area of the tunable mask;

focusing the collimated light beam on an X-Y imaging plane on or within the photopolymer resist material by one or more focusing elements, wherein the tunable mask, the collimating optics, and the focusing elements are oriented and positioned to produce the same optical path length between the tunable mask and the imaging plane for the laser light beam of all optical frequencies; and

wherein the plurality of beamlets simultaneously irradiate the layer of photosensitive polymer resist material.

32. The method of claim 31, further comprising the steps of: changing a pattern of sub-portions on the tunable mask; and maintaining each pattern for a finite duration, wherein a combined dose effect from the duration of laser irradiation and the intensity of light exceeds a threshold dose for polymerization of the resist material only in a portion of the fully irradiated regions of the pattern corresponding to each sub-portion in the resist material.

33. The method of claim 31, further comprising the step of supporting the resist material on a movable stage, wherein the stage is movable along multiple axes simultaneously or sequentially.

34. The method of claim 33, further comprising the steps of: moving the resist material relative to the X-Y imaging plane, changing the pattern of sub-portions on the tunable mask, and maintaining the pattern for a finite duration to simultaneously irradiate the layer of photosensitive polymeric resist material, wherein a cumulative combined dose effect from the duration of laser irradiation and the intensity of light exceeds a threshold dose for polymerization of the resist material in only a portion of the layer of resist material that is each simultaneously irradiated.

Technical Field

The present disclosure relates generally to additive manufacturing systems and methods, commonly referred to as 3D printing (three-dimensional printing), and more particularly to methods and apparatus for high-speed additive manufacturing of structures with sub-micron features using multiphoton, nonlinear light absorption processes, wherein the systems and methods enable the manufacture of features smaller than diffraction-limited focused illumination spots.

Background

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Two-photon polymerization, sometimes also referred to as two-photon lithography, is a popular technique today for additive manufacturing of complex 3D structures with sub-micron building blocks. The technique uses a non-linear light absorption process to polymerize sub-micron features inside a photosensitive polymeric resist material. After irradiation of the desired structures inside the photoresist volume and subsequent development (washing away of the non-irradiated areas), the polymerized material retains the prescribed three-dimensional form. One example of a system that can be modified for two-photon polymerization is described in U.S. patent publication No. 2016/0199935a1, published 2016, 7, 14, the entire contents of which are incorporated herein by reference.

Two-photon polymerization is a direct-write technique that enables the fabrication of large-scale complex 3D structures with sub-micron features. In the most common implementation form of this technique, writing of complex structures is achieved by a serial writing technique, where high light intensity spots are sequentially scanned in 3D space to generate the entire structure. Due to the serial writing scheme, the writing rate is fundamentally limited to the inability to perform two-photon lithography on a large number of features. Although attempts have been made in the past to increase the rate via parallelization, such attempts have failed to achieve the same degree of pattern complexity that can be achieved with the dot scan serial technique. Specifically, past parallelization work has either generated an arrangement of the same features or been used to print 2D parts that do not have depth-resolvability.

Although two-photon lithography enables the passage of light therethroughHis additive manufacturing technique cannot achieve length scale to manufacture features, but the serial writing scheme used for two-photon lithography limits the method to about 0.1mm³Low treatment rate per hour. This prevents the full exploitation of sub-micron geometry control of two-photon lithography to fabricate features. Due to the slow point-to-point serial illumination technique of existing systems, technical and scientific challenges have arisen to address this low processing rate limitation. In the past, the problem of performing parallel two-photon lithography ("TPL") without adversely affecting the ability to fabricate arbitrarily complex 3D components has not been solved. There are two general approaches in the prior art that partially address the parallelization problem of TPL: (i) "splitting" a beam and focusing it at multiple points simultaneously to produce the same feature at multiple points (see Vizsnyiczai, G., Kelemen, L., and Ormos, P.,2014, "Hologranic multi-focus 3D two-photo-polymerization with real-time calculated holes", Opt. express,22(20), pp. 24217-24223); (ii) projecting an arbitrarily complex 2D image into the resist to produce a 2D structure with no depth resolvability. (See alsoMills,B.,Grant-Jacob,J.A.,Feinaeugle,M.,and Eason,R.W.,2013,"Single-pulse multiphotonpolymerization of complex structures using a digital multimirror device",Opt.Express,21(12),pp.14853-14858)。

The first method is not suitable for TPL magnification because in this method magnification is achieved by printing the structure simultaneously at a plurality of points in a periodic array. Since the same beam is split into multiple identical beams, each beam will produce the same features. Therefore, when any complex non-periodic structure is printed using this technique, no magnification can be achieved.

The second approach is not suitable for printing of complex 3D structures because of the loss of depth resolution in these projection techniques. Depth-resolvability refers to the ability to process a thin section of resist material without having to process anything below or above the processed section. For sub-micron additive manufacturing, depth resolution (i.e., the thickness of the processed cross-section of the resist) is desired in the range of less than 1 μm to several microns. However, in this second approach, when a 2D image is projected through the resist material, a single focal plane perpendicular to the 2D projection image cannot be uniquely registered. Instead, the same 2D image is "focused" at multiple planes, such that a thick 3D cured volume is generated in the form of a 2D image extrusion over the entire thickness of the resist layer. Thus, this approach cannot be used to print 3D structures with depth-resolved features such as those present in 3-D truss structures.

The fundamental reason for this failure is the key difference between the physical mechanisms of multiphoton lithography ("MPL") and the physical mechanisms of imaging or material removal^a× (time)^bWherein 'a' and 'b' are positive real numbers). This non-linear form for the exposure dose is due to a combination of the non-linear light absorption process and the underlying chemical reaction kinetics of the polymerization process. Therefore, the prior art technique of dose control by time-averaging the light intensity is not suitable for non-linear dose control in MPL. If this technique is used in MPL, either spots of over-exposed structures are generated or structures with under-exposed regions are obtained. Herein, tools and techniques are presented for proper dose control in parallelized MPL.

Accordingly, there remains a need for systems and methods that can significantly increase the rate of two-photon lithography without adversely affecting the ability to fabricate arbitrarily complex 3D structures.

Disclosure of Invention

In one aspect, the present disclosure is directed to an apparatus for performing an additive manufacturing operation by processing a photosensitive polymeric resist material to form a structure. The apparatus may include: a laser source for generating a laser beam; and a tunable mask for receiving the laser beam mode. The tunable mask may include an optical dispersive element. The tunable mask may be configured to split the laser beam into a plurality of exit beams, wherein each exit beam exiting the tunable mask comprises a subset of beamlets of different or the same intensity, and wherein each beamlet exits a unique sub-portion of the illumination area of the tunable mask. A collimator may be included for collecting and collimating at least one of the outgoing beams from the tunable mask to form a collimated beam. One or more focusing elements may be included for focusing the collimated light beam into a focused light beam that is projected as a focused imaging plane on or within the photopolymer resist material. The tunable mask, collimator, and focusing element are oriented and positioned to produce the same optical path length between the tunable mask and the imaging plane for laser beams of all optical frequencies. The focused beam simultaneously irradiates a layer of photosensitive polymeric resist material.

In another aspect, the present disclosure is directed to an apparatus for performing an additive manufacturing operation by processing a photosensitive polymeric resist material to form a structure. The apparatus may include: a laser source for generating a pulsed laser beam; and a tunable mask. The tunable mask may be for receiving a pulsed laser beam and includes a Digital Micromirror Device (DMD) having a plurality of individually controllable pixels that may be turned on or off. The tunable mask may be configured to split the pulsed laser beam into a plurality of exit beams, wherein each exit beam exiting the tunable mask comprises a subset of beamlets of different or the same intensity, and wherein each beamlet exits a unique pixel. The collimator may be used to collect and collimate all wavelengths of only selected ones of the plurality of outgoing beams from the tunable mask, and wherein the collimator is configured to block all wavelengths of all other outgoing beams in the outgoing beam. One or more focusing elements may be used to focus the collimated light beam into a focused light beam that is projected as a focused imaging plane on or within the photopolymer resist material. The tunable mask, collimator, and focusing element are oriented and positioned to produce the same optical path length between the tunable mask and the focused imaging plane for all optical wavelengths of the pulsed laser beam. A moving stage may be included for supporting the photopolymer resist material and moving the photopolymer resist material relative to the focal imaging plane. The focused beam simultaneously irradiates a layer of photosensitive polymeric resist material.

In yet another aspect, the present disclosure is directed to a method for performing an additive manufacturing operation by processing a photosensitive polymeric resist material to form a structure. The method can comprise the following steps: generating a laser beam; and directing the laser beam toward a tunable mask, wherein the tunable mask includes an optical dispersive element. The method may further include collecting at least one exit beam from the plurality of exit beams exiting the tunable mask and directing the at least one exit beam through collimating optics to generate a collimated beam. Each outgoing beam from the tunable mask comprises a plurality of beamlets of different or the same intensity, and each beamlet emerges from a unique sub-portion or area of the tunable mask that is illuminated by the laser beam. The method may further comprise focusing the collimated light beam into a focused light beam projected as an imaging plane on or within the photopolymer resist material by one or more focusing elements, wherein the tunable mask, the collimator and the focusing elements are oriented and positioned to produce the same optical path length between the tunable mask and the focused imaging plane for laser beams of all optical frequencies. The method may further include maintaining the selected pattern of sub-portions on the tunable mask for a finite duration to cause simultaneous polymerization of selected portions of the photopolymer resist material corresponding to the selected pattern at which a combined dose effect from the duration of the laser irradiation and the intensity of the light exceeds a threshold dose for polymerization of the photopolymer resist material.

In yet another aspect, the present disclosure is directed to a method for performing an additive manufacturing operation by processing a photosensitive polymeric resist material to form a structure. The method may include generating a pulsed laser beam and directing the pulsed laser beam toward a tunable mask, wherein the tunable mask is an optical dispersive element. The method may further include collecting at least one exit beam exiting the tunable mask and directing the at least one exit beam through collimating optics to generate a collimated beam. Each outgoing beam from the tunable mask includes multiple beamlets of different or the same intensity, and each beamlet exits a unique sub-portion of the illumination area of the tunable mask. The method may further include focusing the collimated light beam on an X-Y imaging plane on or within the photopolymer resist material by one or more focusing elements. The tunable mask, collimator, and focusing element are oriented and positioned to produce the same optical path length between the tunable mask and the imaging plane for laser beams of all optical frequencies. The plurality of beamlets may simultaneously irradiate a layer of photopolymer resist material.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

Drawings

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

Fig. 1 is a high-level block diagram of an apparatus for performing an additive manufacturing operation to produce a sample component having sub-micron features according to one embodiment of the present disclosure;

FIG. 2 is a high level block diagram of another embodiment of the present disclosure, which is somewhat similar to the apparatus of FIG. 1, but which also utilizes a power meter and a beam power control unit to control the power of the machining beam in real time;

FIG. 3 is a high-level block diagram of another embodiment of the present disclosure that encompasses critical functions into a subsystem;

FIG. 4 is a graph showing dose laws empirically derived in parallel two-photon lithography with different feature pitches for a particular polymer material being processed;

fig. 5a and 5b show exemplary patterns of DMD "on" states (i.e., bright regions) for gray scale control, where fig. 5a shows a pattern with both vertical and horizontal bright stripes and fig. 5b shows a pattern with only horizontal bright stripes. Fig. 5c shows the desired printed pattern in the photoresist material corresponding to the DMD pattern of fig. 5a and 5 b.

FIG. 5c is an isometric view of an actual part produced using the mask of FIGS. 5a and 5 b;

FIG. 5d shows an image obtained using a scanning electron microscope of a part fabricated using the teachings of the present disclosure in the foreground and showing the depth resolvability of various Z planes used to resolve the part in the present disclosure;

FIG. 6 is a high-level flow chart illustrating basic operations that may be performed using the method of the present disclosure; and

FIG. 7 is a high-level flow chart showing basic operations that may be performed using the method of the present disclosure, as well as steps to achieve grayscale printing and super-resolution printing.

Detailed Description

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

The present disclosure overcomes the fabrication rate limitations with two-photon lithography described above with systems and methods that implement parallel illumination techniques. The parallel illumination technique projects the entire plane of about one million points simultaneously, rather than the single point illumination technique of existing commercial systems, to increase the rate by at least a factor of 100.

The present disclosure is also distinguished from other popular 3D printing techniques (commonly referred to as projection micro-stereolithography) by the use of high peak power pulsed laser sources and their ability to generate features inside the material being processed ("resist material"). A high peak power pulse source is used in this technique to ensure that non-linear light absorption is observed in addition to single photon linear light absorption. The photopolymer resist material can be selected such that it exhibits a "threshold" behavior, i.e., it undergoes a phase change (typically from liquid to solid) upon exposure to light only when the nonlinear exposure dose exceeds a minimum threshold (which is referred to as a "threshold dose"). Resist materials in which the light exposed regions become more soluble in the solvent may also be used. Since the dose in multiphoton lithography is non-linearly proportional to the light intensity in the exposed material, a steeper dose gradient can be generated in the material during non-linear absorption compared to linear absorption. This steeper dose gradient results in machined features that are smaller than the diffraction limited illumination spot; this steeper gradient also enables the generation of individual point-like voxel features (or volume pixels) inside the resist material by focusing the laser spot at an inner point. Thus, the present apparatus and method differ from conventional micro-stereolithography in both form and function. The forms differ in the use of pulsed laser sources (with the present apparatus and method during multiphoton lithography) and the use of incoherent sources (in conventional micro-stereolithography); however, the functionality differs in the ability to fabricate sub-micron features in the interior of the resist (i.e., the present apparatus and method) versus the ability to fabricate diffraction limited features on the surface of the resist (e.g., using conventional microstereolithography).

Within the field of multiphoton lithography (MPL), the apparatus and method of the present disclosure differ from existing implementations in their ability to simultaneously focus a set of points inside a resist material (i.e., focus a "projected image") without providing significant light intensity above or below the depth of focus. Thus, the technique significantly increases the processing rate by parallelizing the generation of sub-micron features. It is important to note that with the apparatus and methods described herein, the dose at each individual focal point can be independently adjusted to generate arbitrarily complex patterns. Thus, the apparatus and method of the present disclosure is different from and significantly improves upon previous existing MPL implementations that split the same beam into multiple focal points with the same intensity profile. Additionally, by incorporating features for non-linear dose gradients, the apparatus and methods of the present disclosure differ from and significantly improve upon those previously existing multi-point MPL implementations that fail to maintain the steep dose gradients experienced during single-point focusing. With the present apparatus and method, a steep spatial gradient of the dose is achieved by exploiting the time dependence of the intensity in the beam generated by the pulsed laser source. In particular, a steep dose gradient is achieved by temporal focusing of a broadband femtosecond pulsed laser.

The apparatus and methods of the present disclosure utilize a technique known as "time focusing". Temporal focusing refers to a phenomenon in which the duration of a femtosecond pulse (nominally 100fs or less) is gradually shortened with spatial focusing of a light beam. Since the peak intensity during a pulse depends on both the spatial size of the beam and the duration of the pulse, the intensity can be independently adjusted by changing either of the spatial size of the beam and the duration of the pulse. In a serial spot scanning implementation, focusing of the beam to a single spot is achieved by adjusting only the spatial size of the beam without any adjustment to the duration of the pulse. In contrast, the apparatus and method of the present disclosure implement an optical projection scheme in which the pulse duration is gradually reduced in proportion to the size of the beam so that the beam is focused both spatially and temporally on the surface of the resist material or inside the resist material, and in which the positions of the spatial and temporal focal points overlap. This ensures that even when the projected image is large (due to multiple focal points), a steep dose gradient is achieved on the projected imaging plane. It is important to note that this projection scheme differs from conventional projection micro-stereolithography implementations in form and function because it relies on the temporal characteristics of a broadband femtosecond pulsed laser source to achieve the focusing described herein. An important factor in the temporal focusing technique is that the optical path length after the projection mask is designed to match only all optical frequencies in the beam on the focused imaging plane, but not all optical frequencies in the beam on all other planes. Thus, temporal focusing stretches the pulse by introducing a "chirp" (chirp), and selectively minimizes (and ideally eliminates) this chirp only at the focused imaging plane.

The fundamental reason for this failure is the key difference between the physical mechanisms of multiphoton lithography (MPL) and the physical mechanisms of imaging or material removal^a× (time)^bWherein 'a' and 'b' are positive real numbers). Therefore, the prior art technique of dose control by time-averaging the light intensity is not suitable for dose control in MPL. If this technique is used in MPL, either spots of over-exposed structures are generated or structures with under-exposed areas are obtained. Herein, tools and techniques are presented for proper dose control in parallelized MPL.

It is important to note that the non-linear dose characteristics also affect the correct choice of laser source. Since the threshold dose is determined by the peak intensity of the light (i.e., the maximum instantaneous intensity), the peak intensity for all focus points in the parallel scheme must be similar to the peak intensity in the serial scan technique. In serial scanning techniqueIn operation, the focusing intensity is 0.1TW/cm²To about 2TW/cm²Within the range of (1). This indicates that for a field of view of a few hundred microns (i.e., focused beam size), the peak beam power should be in the range of about 1 GW. High repetition rate (>10s MHz) femtosecond laser oscillator and low repetition rate (about 1kHz to 10kHz) femtosecond laser amplifier are among the potential choices of pulsed laser sources. Laser amplifiers with high peak power, despite similar average powers, are preferred sources for parallel sub-micron additive manufacturing due to their 4 to 5 orders of magnitude (i.e., 10,000 to 100,000 times) higher peak power.

Fig. 1 shows a device 10 according to one embodiment of the present disclosure. Apparatus 10 may include a pulsed laser source 12 in the form of a laser amplifier, an optical parametric amplifier ("OPA") 14, a beam expander 16, a first highly reflective ("HR") mirror 18, a beam homogenizer 20, a second HR mirror 22, a second beam expander 24, a tunable mask 26, an electronic digital mask control system 27 (which includes a processor, memory, and I/O), a concave mirror 30, a neutral density ("ND") filter 32, a short pass filter ("SF") 36, a charge-coupled display ("CCD") camera 40, an electrically tunable lens ("ETL") 42, an objective lens 44, a sample (including a "photosensitive polymer resist material" supported on an optically transparent slide) 48, a movable stage 50, and a lamp 52 that projects a beam 54 toward sample 48 for imaging of a processing area. The lamp may be an incoherent light source having a wavelength spectrum such that it does not affect the photopolymerization of the resist material.

In operation, the laser source 12 may be a pulsed laser source that provides laser light to drive the writing process. A key feature of the laser source 12 is that the laser source 12 generates pulses having a broad wavelength spectrum rather than a single wavelength. One example of a suitable laser source is a femtosecond titanium sapphire regenerative laser amplifier with a center wavelength of 800nm, a pulse duration of 35fs, and a bandwidth of 40 nm. As shown in fig. 1, the light from the laser source 12 has the following wavelengths: its wavelength is modified by OPA14, in one example from 800nm to 325nm or 500nm, before being further modified by the first beam expander 16, beam homogenizer 20 and second beam expander 24. The beam homogenizer modifies the shape of the beam from a non-uniform gaussian profile to a uniform flat-top beam profile. The beam expander 16 and the beam expander 24 control the diameter of the beam 25 that illuminates the digital mask 26. Multiple beam expanders may be required to match the size of the beam to the apertures of various components such as the beam homogenizer and tunable mask, respectively.

In one example, the adjustable mask 26 may be a digital micromirror device ("DMD"). The components are commercially available from various manufacturers, such as Texas instruments Inc. (Texas instruments Inc. of Dallas, Tex.). Alternatively, the tunable mask 26 may be formed by a Spatial Light Modulator (SLM). The tunable mask may also be a strain-driven tunable diffraction grating, such as those formed by corrugating a support film (e.g.,see alsoS.k. saha and m.l. currpepper, Biaxial Tensile Stage for fabiating and tufting Wrinkles, us patent 9,597,833B 2, March 2017, incorporated herein by reference) or may be a fixed uniform grating or a non-uniform grating mounted on a movable (rotating and/or translating) base. A key feature of the tunable mask 26 to achieve temporal focusing is that the tunable mask 26 is a dispersive optical element, i.e., the tunable mask 26 is capable of spatially separating different optical frequencies (or wavelengths) of an incident beam. The DMD, SLM and tunable pleated membrane may all act as a dispersive element due to their periodic structure that diffracts light. When a laser source is incident on such a dispersive element, it diffracts into a plurality of beams. The angular position of the diffracted beam is determined by the diffraction pattern. Each of these diffracted beams contains complete information about the illumination-patterned sub-portions of the tunable mask in the form of respective beamlets corresponding to these sub-portions. These patterned sub-portions may correspond to the respective peaks of the respective mirrors or corrugated gratings in the DMD, where the sub-portions themselves are tunable. If the laser source is a broadband source, the beamlets (and beams) that emerge from the mask may diverge rather than be in the form of individual beamlets or beams. This is because the angular position of the diffracted beamlets (and beams) depends on the particular wavelength. This spatial divergence (caused by the dispersive mask) lengthens the pulse duration due to the inverse relationship between spectral bandwidth and pulse duration, andthis spatial divergence is a key feature to ensure temporal focusing. The adjustability of the mask ensures that structures with various feature geometries can be printed. For the purposes of the following discussion, it will be assumed that tunable mask 26 is formed from a DMD.

In the case of a DMD used as the tunable mask 26, a key feature is that each micromirror of the DMD can be considered to form a pixel site, and each pixel site can be individually turned on or off. This is achieved by rotating the mirror by a small angular amount (typically + or-12 degrees) between two predetermined positions. In one predetermined position, the pixel (i.e., micromirror) forms an "on" state in which the intensity of light exiting the micromirror pixel via reflection and diffraction in a particular set of directions is high, while another predetermined position forms an "off state in which the intensity of exiting light in the same set of directions is zero or a low value. Here, the threshold values for the qualitative terms "high" and "low" are determined by the particular downstream application. Typically, commercially available DMD systems are designed such that the intensity ratio for the off-state to the on-state along a particular direction of propagation is almost zero for incoherent light. This illumination adjustment is sufficiently high for two-photon lithography to achieve two different exposure states (high exposure and zero exposure). For the high exposure state, exposure of the resist by a limited number of laser pulses is sufficient to affect polymerization. For the low exposure state, due to the threshold behavior of the polymer resist material, the intensity is too low to affect the polymerization even with an infinite number of pulses. The beamlets produced by each of the "on" micromirrors in the tunable mask 26 form diverging beams, indicated by reference numeral 28, and collectively form an image produced using the tunable mask 26. Thus, only beamlets generated from "on" pixels within tunable mask 26 are used to affect polymerization of material within sample 48. The remaining beamlets (i.e., all beamlets exiting the "off" pixels) may be redirected to one or more optical traps. It is important to note that several diffracted beams exit the mask, and each of these diffracted beams comprises an "on" beamlet. These beams differ in angular position (diffraction mode) and energy (mode efficiency). For polymerization, it is preferable to use only the light beam from the mode having the highest diffraction efficiency (i.e., the highest energy). Other modes (beams) may be used for diagnostics or redirection to the optical traps. The additional diffracted beams are not shown in fig. 1, but one additional diffracted beam is shown in fig. 2. The collimating optics (i.e., concave mirror 30) converts the diverging beam 28 into a collimated beam 34. Although the collimating optics are shown as concave mirrors 30 in this example, suitable lenses may be used to provide the required collimation of the diverging beam 28.

The collimated beam 34 then passes through the neutral density ND filter 32, through the short pass filter 36, through the ETL 42, the objective lens 44, and is focused onto the X-Y plane inside the sample 48, as indicated by the focused beam 46. As described above, the sample 48 may include a photosensitive polymeric resist material. The focus X-Y plane is the conjugate plane of the tunable mask 26. An example of an objective lens suitable for use as objective lens 44 is an oil immersed infinite objective lens (such as a40 x 1.4NA lens) of high numerical aperture but low to medium magnification.

At the focused imaging plane on or within sample 48, when the laser illumination is maintained for a finite duration, the exposure dose at each point in the resist corresponding to an "on" pixel in tunable mask 26 is above the threshold dose for the material comprising sample 48, and conversely, the exposure dose at each point in the resist corresponding to an "off pixel is below the threshold exposure dose⁶Or more orders of magnitude). Thus, the ability to form each layer of sample 48 using multiple beamlets written in parallel results in a significant reduction in the time required to make a finished part from the photopolymer resist material of sample 48.

Three-dimensional structures may be fabricated by moving the focused imaging plane relative to the sample 48 using a movable stage 50. In practice, movement of the moveable stage 50 in the X-Y-Z plane may be controlled by an electronic control system 56. Alternatively, the moveable stage 50 may be a fixedly supported stage (i.e., non-moveable) as the objective lens 44 is moved in the Z-plane as desired. In addition, it is possible that both the movable stage 50 and the objective lens 44 may be moved simultaneously. However, it is anticipated that for most applications it will be preferable to move only one or the other of the moveable stage 50 or objective 44. Further, the focal plane of the objective lens 44 may be optically scanned in the axial Z (i.e., depth into the sample) direction using an electrically adjustable lens (ETL) 42. ETL provides the ability to move the final time-focused imaging plane quickly without any mechanical movement of the objective lens or movable stage, resulting in a rate increase of up to 10 times. More complex part geometries can be created by replacing the moveable X-Y-Z stage with a 6-axis moveable stage capable of motion in all six degrees of freedom (i.e., capable of X, Y, Z translational and tip, tilt, and rotational displacements).

For apparatus 10 described above, an important feature is thus the adjustment of the laser light from laser source 12, adjustable mask 26, the axis of concave mirror 30 (i.e., the collimating optics), and the relative size and position of the collimating optics (summarized in fig. 3). The size and position of the collimating optics is related to the 3D printing throughput due to the divergence of the light beam exiting the tunable mask and the multi-beam characteristics. For the purpose of temporal focusing, all parts of at least one diverging beam (i.e. one diffraction mode) have to be collected by the collimating optics. If only a portion of the diffraction patterns are collected, the gradient of light intensity at the focal imaging plane will decrease. This reduced intensity gradient will result in a loss of depth resolution during printing. In addition, local portions of the beam from other diffraction modes should be blocked to minimize intensity variations of the projected image. In addition, in order to improve the optical transmission efficiency, three elements (laser, mask, collimating optics) are arranged such that a blazed grating condition is obtained for the tunable mask 26. The blazed grating condition requires that light be incident on the blazed tunable mask 26 at a specific angle determined by the pixel pitch on the tunable mask, the center wavelength of the laser beam, and the blaze angle of the grating (i.e., the angle at which the mirrors in the DMD are adjusted). A description of the blazed grating condition is provided in the Gu, c, Zhang, d, Wang, d, Yam, y, Li, c, and Chen, s-c, 2017, "parallel finite laser light shade micro-machining based on temporal focusing," Precision Engineering,50, pp.198-203, the entire contents of which are incorporated by reference into the present disclosure.

To ensure high quality printing, concave mirror 30 (i.e. the collimating optics) is arranged such that only the diffraction orders corresponding to the blaze condition are collected. Typically, this requires concave mirror 30 (i.e., the collimating optics) to be placed at a predetermined angle with respect to the face of tunable mask 26 and blocking other orders by introducing an aperture. This sets the condition for the maximum angular aperture. In addition, it must be ensured that concave mirror 30 (i.e., a collimating lens) collects all wavelengths (within the bandwidth of laser source 12). This condition sets the minimum angular aperture for the collimating optics, since different wavelengths exit at (slightly) different angles. It is therefore necessary that the angular aperture through which the beamlets of beam 28 must be collected onto concave mirror 30 (i.e., collimating optics) be located within a small frequency band. Outside this frequency band, the performance of the device 10 may drop significantly to such an extent that depth resolution for 3D printing is lost. While such additional aperture-based design features may appear to be obvious design goals in light of this disclosure, past attempts at parallelizing multiphoton lithography using similar optical configurations failed to demonstrate 3D depth resolvability: (See alsoMills et al, supra), indicating that it is not trivial to design and configure an optical system capable of depth-resolved parallel multi-photon lithography even where the system uses known components. It is important to note that these past attempts have been successful in melt-based machining operations, despite the failure to achieve polymerization-based depth-resolved multi-photon lithography. Thus, the success of the thermal drive machining process does not automatically guarantee that the underlying system can also successfully 3D print depth-resolved polymer structures.

The device 10 is alsoImplementations of a grayscale printing method that ensures that high quality parts can be manufactured are facilitated. The grayscale printing method includes a sequence of operations and a selection of writing conditions in those operations that cause non-uniform "doses" during printing within the same projection imaging plane. The term "dose" refers to the combined non-linear effect of light intensity and duration of exposure (in the form of dose: (intensity)^a× (time)^bWhere 'a' and 'b' are positive real numbers). Writing occurs at a point where the dose is above the threshold for a given photopolymer resist material. To write, the pixel must be continuously on for a duration longer than the threshold exposure time (represented by the vertical axis in fig. 4) at the incident light intensity (represented by the horizontal axis in fig. 4). Non-uniform dose can be achieved by selectively turning some pixels on or off or selectively increasing or decreasing the non-linear dose in the plane of the resist material. In practice, this may be achieved by sending a series of patterns (i.e., a map of pixel "on" and "off states) to the DMD and holding each of the patterns for a finite duration. These mode illumination durations will then be shorter than the maximum exposure time required for any point within the projection field. The net nonlinear dose at any point within the resist material is the cumulative combined dose from each projected image. Here, the projected field refers to the largest area on/in the resist material onto which any focused image can be projected. Therefore, it may be necessary to project a series of non-intuitive DMD patterns to print a desired structure on a focal plane by a process of sequentially projecting several DMD patterns and combining the effects of illumination intensity and duration of illumination non-linearly and cumulatively.

In determining the range of net doses that can be adjusted by this method, the minimum duration that a pixel can be on and the rate at which the power of the incident beam can be adjusted must be considered. The minimum duration that a pixel can be "on" is determined by the pulse repetition rate of the laser source 12. The gray scale control enables the total exposure time of each pixel to be adjusted between zero and the maximum required duration in steps of the inverse of the pulse repetition rate. For example, if the projection field requires a maximum of 20 pulses, the exposure time can be adjusted discretely in 1ms steps between 0ms and 20ms using a laser source with a pulse repetition rate of 1 kHz. This is accomplished by loading a new single bit image onto the DMD26 every 1 ms. Further adjustments to dose can be made if a high speed (i.e., faster than about 10ms response time) beam power control unit is incorporated into the system. Without this additional power control unit, the dose can be controlled discretely over several grey levels separated by the derivative of the pulse repetition rate. Beam power control enables finer dose control than without the additional power unit. It is important to note that the gray scale method for dose control disclosed herein is different from the intensity control scheme of commercial DMD masks (i.e., projectors). In commercial DMD projectors, the time-averaged intensity of the pixels can be controlled over several levels by varying the ratio of the rates at which the mirrors switch between on and off states as they cycle continuously between on and off states.

It has also been experimentally observed that, as shown in fig. 4, the threshold dose depends on the proximity of features within the sample 48. The figure shows the minimum threshold exposure time required to affect polymerization in the resist material when irradiated with a particular peak intensity and for different feature pitches. In portions of the sample containing closely spaced features and sparsely spaced features, providing a uniform dose results in over-or under-exposed based defects. The gray scale dose control of the above gray scale printing method enables non-uniform control of dose in the same focal plane. This is achieved by utilizing the pulse characteristics of the laser light from laser source 12 and the multi-pulse exposure threshold behavior of the material comprising sample 48. Typically, one image is kept projected at each focal plane within the specimen 48 until the desired uniform dose is achieved at a particular plane within the specimen. However, an important difference with the present method is that instead of projecting the same image, multiple images may be projected sequentially on the same imaging plane. The digital image being created is altered such that pixels of the tunable mask 26 having a local dose exceeding the non-uniform dose threshold are turned off in subsequent images. This adjustment of sequential images in the sample 48 on the same imaging plane can be performed either a priori by calibration of dose regularity (such as in fig. 4) or in real time by optically sensing changes in contrast of images captured by a real-time imaging system (such as the imaging subsystem 118' in fig. 3). The optical sensing system includes a separate lamp 52 but shares the same focusing elements 42 and 44 with the processing system to generate an image of the processing plane on the camera. This enables real-time imaging and recording of the printing process. To ensure that the optical sensing/visualization system does not interfere with the printing process, the illumination wavelength in the system is selected to be outside the absorption spectrum of the resist material.

Thus, implementing the gray-scale dose control technique as described herein by producing multiple successive images on the same imaging plane but with different doses for each pixel of the image enables non-uniform portions to be printed without creating defects due to over-exposure or under-exposure. In particular, it is now possible to print uneven parts having closely spaced but different characteristics. Fig. 5a and 5b show a representative gray-scale digital mask (a pattern of DMD pixels in the "on" state) required to print the structure of fig. 5 c. Fig. 5D is an obtained scanning electron microscope image of an actual part manufactured using the teachings of the present disclosure, as shown in the foreground image. The foreground image is flipped by rotating the plane around the Y-axis edge of the plane on the bottommost plane. The foreground images show the respective depth-resolved Z-planes of the printed pillar structures, while the background images are shown in an upright orientation "as printed".

It should also be noted that time-averaged intensity is generally not a reliable measure of exposure dose during multiphoton lithography. Therefore, commercial intensity control techniques that rely on time-averaging of intensities cannot be used for reliable dose control. In addition, the proposed gray scale technique, although used to adjust the total exposure time, cannot adjust the instantaneous or peak intensity. Although the intensity can be adjusted by controlling the net power of the incident beam, feedback for such adjustment is generally not available in real time. Since the diffraction efficiency of the DMD26 tends to vary with the spatial frequency of the image, one calibration of the emitted power for all images is not accurate. This problem has been solved with another embodiment of the present disclosure shown as device 100 in fig. 2. Apparatus 100 is somewhat similar to apparatus 10 in that apparatus 100 includes a pulsed laser source 102, a half-wave plate 104, a polarizing beam splitter 106, a mirror 108, a tunable mask 110, an electronic control system 112 for controlling tunable mask 110, a pair of collimating lenses 114 and 115 each for collimating a light beam received thereby, a beam monitoring power meter 116, a beam power control unit 117, the imaging device 118 (in this example the electronic control system 112 also controls the imaging device 118), a lens 120, a beam splitter 122, a lamp 124, a dichroic mirror 126, an objective lens 128, a movable stage 130 positioned elevationally adjacent to (e.g., below the objective lens 128) the objective lens for supporting a sample 132 (i.e., a photopolymer resist) thereon, and an electronic control system 134 for controlling at least one (or possibly both) of the movement of the movable stage 130 or the objective lens 128. Alternatively, a single electronic subsystem (e.g., system 112 or system 134) may be used to perform all control operations for device 100.

The main difference between apparatus 100 and apparatus 10 is the ability of apparatus 100 to continuously monitor one of the unprocessed diffracted beams (i.e., "mth diffraction order" beam 111' in FIG. 2) from tunable mask 110 and to detect changes in beam power using a beam monitor power meter 116. The beam to be processed is denoted by reference numeral 111, which may be referred to as the "processing beam" emitted from the adjustable mask 110. The beam power control unit 117 is coupled to the power meter 116 for controlling the real-time beam intensity of the beam incident on the adjustable mask. To control the power of the incident beam, the power control unit may control a rotating half-wave plate 104 followed by a polarizing beam splitter 106. Polarization-based power control schemes are effective because pulsed laser light sources typically emit linearly polarized light. When implementing polarization-based power control, the polarization-dependent anisotropy of the multiphoton polymerization process can be minimized by introducing a quarter-wave plate to convert linearly polarized light into circularly polarized light before the light enters the objective lens. Another power control technique may rotate one of several neutral density filters to a position in the path of the process beam 111 before entering the objective lens, or by introducing a filter into the path of the beam incident on the tunable mask 110.

Referring to fig. 3, a high-level diagram is presented showing the major subsystems of the device 100' in accordance with the present disclosure in a broader manner. The apparatus 100' utilizes a pulsed laser beam 102' received by a beam conditioning subsystem 104 '. The pulsed laser beam exits the beam conditioning subsystem 104' and is reflected from the mirror 108' to the adjustable mask 110 '. The tunable mask 110 'generates a beam that is directed to a collimator 115'. The collimated output beam from collimator 115' is reflected from mirror 116' toward focusing element 128 '. The imaging system 118' may be used to image a focal plane. The control unit 140' may be used to control the laser, the beam conditioning unit, the imaging system, the moving stage or the focusing element by receiving feedback signals or synchronization trigger signals from the respective units connected to the control unit.

Referring briefly to FIG. 6, there is illustrated a high level flow chart 200 of various operations that may be performed by the apparatus 10 or the apparatus 100' in performing the method of the present disclosure. At operation 202, a pulsed laser beam is generated. At operation 204, the adjustable mask (26 or 110) may be used to digitize the beam (i.e., the sub-portions of the discrete pattern have high and low intensities), and selectively turn on only certain ones of the pixels within the mask to produce "machining beams" (i.e., beams 28 or 111) for machining the layer of the part (i.e., sample 132). Alternatively, one of the beams not used for machining (i.e., the "mth diffraction pattern") may be selected and its power monitored, as indicated by operation 206. Further optionally, if operation 206 is performed, the power of the machining beam may be adjusted in real time based on the measured power of the mth beam at operation 208.

At operation 210, the machining beam may be collimated. At operation 212, the collimated processing beam may be used to begin/continue parallel processing of an entire layer within or on the sample (i.e., the photopolymer resist material). At operation 214, the movable stage (50 or 130) and/or focusing elements (e.g., objective lens 44 or 128 and/or Electrically Tunable Lens (ETL)) may be controlled as desired during the polymerization process. At operation 216, a check is made as to whether the current layer is machined, and if not, operations 206 through 216 may be repeated. If the check for processing completion of the current layer at operation 216 yields a "yes" answer, then a check is made at operation 218 as to whether the entire sample part (i.e., all processed/formed layers) is complete. If the check at operation 218 produces a "yes" answer, the process ends, but if the check at operation 218 produces a "no" answer, then the digital information for writing to the next layer of the component may be obtained, as indicated at operation 220, and operations 204 through 216 may be repeated to write out the next layer. In order to synchronize the various components of the system including the tunable mask, the control unit may wait for a synchronization signal from the moving stage (for the sample or objective) or from the camera of the imaging system, or for a trigger signal from an internal or external clock (such as the pulsed laser itself). Without synchronization, printing of undesired locations of the resist material may be observed.

The synchronization of the laser illumination, the adjustable mask and the moving stage also enables the resolution of the printing process to be increased by "super-resolution" printing techniques with sub-pixel resolution. In this technique, the moving stage is moved before the exposure dose at a certain point exceeds the threshold dose. By moving the stage only a small amount to overlap with the Point Spread Function (PSF) of the illumination at the previously illuminated point, the threshold dose can be exceeded in only a small portion of the overlap region. Thus, such super-resolution printing enables printed features to be smaller than available features without the need to move the stage during or between projections. Such super-resolution printing may be achieved separately or in combination with each other along all three axes (X, Y, Z) to print finer features. Super-resolution printing can be achieved without any stage movement by means of projecting a series of images through a tunable mask, where the images are offset from each other by at least one pixel. To obtain sub-pixel feature resolution, the exposure duration of these images should be lower than the exposure duration corresponding to the threshold exposure dose at the laser illumination intensity. When stage movement is combined with this pixel-shift projection technique, super-resolution printing can be obtained along multiple axes simultaneously.

While the primary focus of the present disclosure is on the printing of sub-micron features, the systems and methods disclosed herein may also be used to print larger features on the scale of a few microns. This can be achieved by simply replacing the high Numerical Aperture (NA) objective with a low NA objective. Since the feature size is determined by the spatiotemporal distribution of light intensity, low NA objectives produce larger features. The advantage of using a low NA lens is that one can benefit from the low magnification and wider field of view of such lenses, such as a low NA10 or 25 lens and a high NA40 or 100 lens. Furthermore, such a lens would significantly increase the area of light projection, thereby increasing the overall printing rate by one to two orders of magnitude. Thus, a trade-off balance between print rate and feature size resolution can be achieved by a properly selected combination of super-resolution printing and objective lens. Referring briefly to FIG. 7, there is illustrated a high level flow chart 300 of various operations that may be performed by apparatus 10 or apparatus 100' in performing the method for grayscale or super resolution printing of the present disclosure. Flowchart 300 differs from flowchart 200 in the ability to change the mask pattern or move the stage and/or focusing elements before writing of a particular layer is completed. This capability is crucial to achieving grayscale printing and super-resolution printing. More specifically, operations 302 through 320 in fig. 7 correspond to operations 202 through 220 of fig. 6, previously described. However, as described above, using the method illustrated in fig. 7, if the check at operation 316 indicates that the currently processed layer is not complete, then operations 304 and operations 310 through 316 (or alternative operations 304 through 316) are re-executed. Re-executing operation 304 enables control of the digital mask and/or the movable stage to achieve grayscale or super-resolution printing, if desired.

The various embodiments and methods of the present disclosure described herein propose a new parallel two-photon lithography technique that ensures a depth resolution on the order of a single micron and in-plane feature sizes of less than about 350 nm. Thus, arbitrarily complex structures can be generated by projecting a series of patterned "light sheets" that are dynamically adjusted through a tunable mask (26 or 110). While the methods described herein appear functionally similar to conventional DMD-based parallelization used in today's projection micro-stereolithography systems, the apparatus and methods of the present disclosure achieve a fundamentally different optical system that ensures that the light sheet (i.e., the projected image) is focused both spatially and temporally. By overcoming this obstacle to depth resolvability in femtosecond projection optics, the present disclosure successfully scales the rate up to 100 times while still maintaining the <350nm feature size resolution of high quality serial technologies. Thus, the apparatus and methods of the present disclosure eliminate the fundamental obstacles to scale up sub-micron additive manufacturing and transform two-photon lithography into a viable system for high volume additive manufacturing of functional components with nanoscale features.

The various embodiments and methods of the present disclosure are expected to have broad applicability, for example, in 3D printing applications in the microelectronics industry, in the manufacture of high energy laser targets; for printing photonic crystals (i.e., sensors) in 3D printing applications, in mechanical metamaterials (e.g., low density, high strength engineered metamaterials) and in microfluidics (e.g., for biomedical diagnostic strips), to name just a few examples of potential applications.

While various embodiments have been described, those skilled in the art will recognize that modifications or variations may be made without departing from the disclosure. These examples illustrate various embodiments and are not intended to limit the disclosure. Accordingly, the specification and claims should be interpreted liberally with only such limitations as are necessary in the pertinent art.

Example embodiments are provided so that this disclosure will be thorough and will fully convey the scope to those skilled in the art. Numerous specific details are set forth, such as examples of specific components, devices, and methods, in order to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to those skilled in the art that specific details need not be employed, that example embodiments may be embodied in many different forms and that neither the specific details nor the example embodiments should be construed to limit the scope of the disclosure. In some example embodiments, well-known processes, well-known device structures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting. As used herein, the singular forms "a", "an" and "the" may be intended to include the plural forms as well, unless the context clearly indicates otherwise. The terms "comprises," "comprising," "including," and "having" are inclusive and therefore specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It should also be understood that additional or alternative steps may be taken.

When an element or layer is referred to as being "on," "engaged to," "connected to," or "coupled to" another element or layer, it can be directly on, engaged, connected, or coupled to the other element or layer, or intervening elements or layers may be present. In contrast, when an element is referred to as being "directly on," "directly engaged to," "directly connected to" or "directly coupled to" another element or layer, there may be no intervening elements or layers present. Other words used to describe the relationship between elements should be interpreted in the same manner (e.g., "between," "directly between," "adjacent" and "directly adjacent," etc.). As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

Although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another region, layer or section. Terms such as "first," "second," and other numerical terms when used herein do not imply a sequence or order unless clearly indicated by the context. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the example embodiments.

Spatially relative terms, such as "inner," "outer," "below," "lower," "above," "over," and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the example term "below" can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.