WO2019170036A1

WO2019170036A1 - Ultrafast Laser Fabrication Method and System

Info

Publication number: WO2019170036A1
Application number: PCT/CN2019/076682
Authority: WO
Inventors: Shih-Chi Chen; Qiang GENG; Dien Wang; Pengfei Chen; Dapeng Zhang
Original assignee: The Chinese University Of Hong Kong
Priority date: 2018-03-06
Filing date: 2019-03-01
Publication date: 2019-09-12
Also published as: CN111819500A; CN111819500B

Abstract

A laser fabrication method and a laser fabrication system, the laser fabrication system comprises: an ultrafast laser source (101), configured to output a laser beam; and a digital micromirror device (DMD) (106), configured to receive, shape, and scan the laser beam, wherein more than one binary holograms are synthesized to form a scanning hologram applied to the DMD (106). The shaped laser beams, containing one or multiple focal points, leaving the DMD (106) are focused to the sample for fast laser fabrication.

Description

Ultrafast Laser Fabrication Method and System

Cross Reference to Related Applications

This application claims priority of US provisional application No. 62,639/245 filed on March 6, 2018, the whole of which is incorporated herein by reference.

Technical field

The present application relates to an ultrafast laser fabrication system.

Background

Two-photon polymerization is an important additive manufacturing method that is typically performed via raster-scanning the focus of a femtosecond laser in photoresists, thereby inducing a nonlinear absorption process to polymerize submicron features as nanoscale building blocks. Although it enables nanoscale features to be additively created, the processing rate is limited by the point-by-point and layer-by-later serial scanning procedure. While parallel processing methods have been proposed, e.g., microlens array or spatial light modulator, they are often limited to fabricating periodic structures or have compromised resolution. It is desired to have a solution of substantially improved throughput (multi-point writing) and writing capability (random-access scanning) without sacrificing the fabrication resolution.

Summary

In the present application, a laser fabrication method and the associated optical system are proposed in which the binary hologram-based technique is used to perform multi-point laser point control with simultaneously beam shaping and wavefront correction using a single DMD.

According to an embodiment of the present application, the laser fabrication system comprises: an ultrafast laser source, configured to output a laser beam; a digital micromirror device (DMD) , configured to receive and shape the laser beam, wherein more than one binary holograms are synthesized (or superposed) to form a scanning hologram applied to the DMD; the shaped laser beam leaving the DMD is focused to photoresists for fabrication of single or multiple points in parallel. Since more than one binary holograms are synthesized to form the scanning hologram which is applied to the DMD, i.e., one or multiple focal points can be generated by a single hologram, multi-focal point scanning can be achieved by using a single DMD.

According to an embodiment, since the DMD may introduce negative angular dispersion to the laser beam, the system may further comprise a dispersion compensation unit, configured to transfer the laser beam from the laser source to the DMD with a positive angular dispersion for neutralizing the negative angular dispersion. For example, the dispersion compensation unit includes a blazed transmission grating and a mirror.

According to an embodiment, wavefront information for wavefront correction is further included in the scanning hologram so that wavefront correction is achieved simultaneously with the beam shaping for the multi-focal point scanning.

According to an embodiment, the laser beam from the laser source may be expanded to match an aperture of the DMD.

According to an embodiment, the system may further comprise an objective lens via a set of relay optics such as a 4-f system, consisting of two lenses, configured to focus the shaped laser beam.

According to an embodiment, a spatial filter may be placed at a back focal plane of the relay optics such as the 4-f system to spatially select a non-zero order beam such as the -1st or +1st order beam diffracted from the hologram.

According to an embodiment, the system may further comprise a microscopic imaging system, configured to monitor fabrication processes performed by the shaped laser beams.

According to an embodiment, the system may further comprise a position stage, configured to mount and maneuver samples to be laser processed.

According to an embodiment, a method for laser fabrication comprises outputting a laser beam from a laser source; shaping the laser beam by a digital micromirror device (DMD) with a scanning hologram synthesized from more than one binary holograms; and focusing the shaped laser beam to photoresists for parallel fabrication with a single or multiple shaped laser points.

According to an embodiment, the method may further comprise neutralizing a negative angular dispersion that is introduced by the DMD to the laser beam.

According to an embodiment, wavefront information for wavefront correction is further included in the scanning hologram.

According to an embodiment, the method may further comprise spatially selecting a non-zero order, e.g., the -1st or +1st order diffraction of the scanning hologram.

Brief Description of the Drawing

Fig. 1 presents the optical configuration of the DMD TPP fabrication system according to an embodiment of the present application.

Fig. 2 (a) shows an exemplary spiral path that can be achieved by the DMD scanner, where each point along the path corresponds to a different hologram; and Figs. 2 (b) and 2 (c) show the holograms of two different points on the scan path (B and C) , respectively.

Fig. 3 (a) shows planned trajectories for truss array according to an experiment of the present application; and Figs. 3 (b) -3 (d) show SEM results of truss arrays with different scale bars.

Figs. 4 (a) -4 (d) are planned trajectories for single-, two-, three-, and four-focus fabrication of woodpile structures respectively.

Figs. 5 (a) -5 (d) present the images of single-, two-, three-, and four-focus fabrication processes respectively, collected by the CCD camera; and Figs. 5 (e) and 5 (f) present the SEM image of the fabrication results after development.

Fig. 6 (a) shows a CAD model of the London Bridge; and Fig. 6 (b) is a TPP fabricated London Bridge.

Fig. 7 illustrates a flow chart of a laser fabrication method according to an embodiment of the present application.

Detailed Description

To realize high precision and high-throughput two-photon polymerization, a multi-focus DMD random-access scanner based on binary holography is proposed. Specifically, the DMD is used as a programmable binary mask, coded with holograms, to modulate the incident femtosecond laser wavefront. 3-D scanning can be achieved by using holographic patterns of spherical wavefronts with adjusted tilted phases. To enhance the scanning resolution, holograms for wavefront correction can be designed and synthesized to the scanning holograms, realizing simultaneous arbitrary beam shaping and 3-D laser scanning via a single DMD. By combining the individual focal points via superposition, multi-focus random-access scanning can be achieved. Accordingly, multi-focus scanning trajectory can be planned arbitrarily to fabricate structures with optimal mechanical properties. Parametric models and computer algorithms are also developed to deterministically link the system performance to DMD parameters. Fabrication experiments have been performed and proved the performance of the new multi-point laser writing system.

According to an embodiment of the present application, the laser fabrication system comprises an ultrafast laser source configured to output a laser beam; a digital micromirror device (DMD) configured to receive, shape, and scan the laser beam via synthesized binary holograms. The system may further include a position stage configured to mount the photoresists or sample. Since one or multiple focal points are generated by the hologram, fabrication with multiple focal points can be realized. Accordingly, the laser fabrication system can implement scanning of multiple focal points at the same time, significantly reducing the time for fabrication. Further, wavefront information for wavefront correction may also be included in the scanning hologram, so that wavefront correction is also achieved simultaneously with the beam shaping for the multi-focal point scanning.

The DMD will shape the laser beam with a negative angular dispersion. To neutralize the negative angular dispersion, a dispersion compensation unit may be provided between the DMD and the laser source so as to transfer the laser beam from the laser source to the DMD with a positive angular dispersion. For example, the dispersion compensation unit may include a blazed transmission grating and a mirror. The laser beam from the laser source may be expanded to match an aperture of the DMD.

In addition, the laser fabrication system may include an infinity-corrected objective lens via a 1: 1 telescope, consisting of an achromatic lens and a tube lens, configured to focus the shaped laser beam on the photoresists. An iris diaphragm may be placed at a back focal plane of the achromatic lens to spatially select a -1st order diffraction of the scanning hologram.

An illustrative embodiment of the laser fabrication system of the present application is shown in Fig. 1. As an example, the laser source 101 is a regenerative femtosecond Ti: sapphire laser amplifier (Spectra-Physics, Spitfire Pro) with a center wavelength of 800 nm. The laser is configured to have a repetition rate of 10 kHz with 100 fs pulse width and 4 W average power. An alternative choice of the laser source may be a Ti: sapphire laser (Coherent, Chameleon Ultra II, 3.5W at 800 nm; repetition rate: 80 MHz. ) . A conservative calculation indicates the laser amp can simultaneously process 100+ focal points with sufficient energy for TPP, while the oscillator can process 15 points.

First, the laser beam issued from the laser source 101 is appropriately expanded by two achromatic lenses (L1, L2) 103 and 104 to ensure the DMD aperture (DLP 4100 0.7” XGA, 1024 × 768 pixels, Texas Instrument) is fully filled. Lenses L1 and L2 together form a beam expander. As the DMD 106 functions both as a programmable binary hologram and a blazed grating, it introduces negative angular dispersion to the laser beam. To remove the angular dispersion, a blazed transmission grating 102 and a mirror (M1) 105 are included in the light path to generate positive angular dispersion. The blazed transmission grating 102 and the mirror (M1) 105 collectively function as a dispersion compensation unit. The mirror M1 105 may be a high-reflectivity mirror. After the DMD 106, the dispersion-free laser beam is guided to an infinity-corrected objective lens 111 via a 1: 1 telescope, consisting of an achromatic lens L3 107 and a tube lens L4 109. An iris diaphragm 108 is placed at the back focal plane of L3 to spatially select the -1st order diffraction of the binary hologram. The photoresists are mounted on a motorized precision XYZ stage 112 for positioning. To monitor the fabrication process in situ, a microscopic imaging system may be built in conjunction with the fabrication setup. As shown in Fig. 1, the microscope shares the objective with the fabrication system via a dichroic mirror 110. An epi-illumination light source 115 is coupled into the system for sample illumination. The image/video of the fabrication process is recorded by a CCD camera 114 after the 50: 50 beam splitter (BS) 113.

Hereinafter, feature of the laser fabrication system are described in details.

Dispersion compensation

Dispersion compensation is critical to the DMD scanner and the related parameters can be determined mathematically. The general form of grating equation is given by

d (sin θ _i+sin θ _m) =mλ, (1)

where d is the distance of groove centers; m is an integer that specifies the diffraction order; θ _i and θ _m are incident angle and diffraction angle at the m ^th diffraction order respectively. Differentiating Eq. 1 yields the angular dispersion

Hence, the ratio of cone angles Δθ _G and Δθ _D can be written as

where the subscripts G and D denote the related parameters of the grating and DMD respectively. L1 and L2 form a 4-f optical configuration to expand the beam size and adjust the dispersion angle after the grating. Accordingly, the angular dispersion introduced by DMD is entirely compensated. As Δθ _G and Δθ _D are small, they can be approximated as:

The important parameters in this setup include the central wavelength, λ=800nm; pixel size of the DMD, d _D=19.35μm ; pitch of the grating, d _G=0.83μm; the diffraction order of the grating and DMD, m _G=1 and m _D=10 respectively; and the corresponding diffraction angle, θ _mG=27°, θ _iD=17° respectively. Substituting these values into Eq. 2 and Eq. 3, there is a relationship f _L2 = 2.5×f _L1, thus f _L1 = 100 mm and f _L2 = 250 mm may be chose to fully compensate the angular dispersion introduced by DMD. Note that when employing different DMD units or models or gratings, a suitable 4-f system can always be found to compensate the angular dispersion because θ _iD can be adjusted continuously.

Arbitrary trajectory planning

In conventional TPP systems, the fabrication process is achieved by a pair of galvanometric scanners that scans the x-y plane and a linear stage that moves the sample axially. For example, the commercial system from Nanoscribe GmbH (https: //www. nanoscribe. de/) . Accordingly, structures can only be fabricated in a layer-by-layer fashion, which limits the printing speed of complex structures. In contrast, the DMD-based TPP system can scan any trajectories (continuous or discontinuous) with an equal speed (22.7 kHz or 5mm/s) . Importantly, gray scale control can be easily achieved by extending the dwell time of the laser focal point at any selected points within the work volume of the DMD scanner. Fig. 2 (a) shows an exemplary spiral path that can be achieved by the DMD scanner, where each point along the path corresponds to a different hologram. The holograms of two different points on the scan path (B and C) are shown in Figure 2 (b) and 2 (c) , respectively.

Axial scanning may be realized by applying binary holograms of spherical wavefronts of increasing or decreasing focal lengths to the DMD; and lateral scanning may be realized by changing the tilted phase term in Lee hologram, i.e., R (x, y) /T, where R (x, y) determines the bias and tilted angle of the fringe patterns and T determines the period of the fringes. Accordingly, simultaneous axial and lateral scanning (i.e., 3-D random-access scanning) is achieved via superposing and rapidly modulating the designed binary holograms on the DMD. When pairing with a 40x objective, the DMD scanner has a scanning range of 103, 206, 524 microns in the X, Y, Z axes; and a scanning resolution (i.e., minimum step size) of 270 nm and 130 nm in the axial and lateral directions respectively.

Multi-focus scanning

Multiple focal points may be simultaneously generated by superposing holograms of individual focus in the DMD work space. To mathematically realize this, let

be the target wavefront containing k focal points, where A (x, y) ∈ [0, 1] and

represent the amplitude and phase of the electric field; x and y are the coordinates in Cartesian coordinate system. Binary holograms with the desired intensity distribution among the k focal points may be synthesized via the equation below, which is derived based on Lee hologram:

where h (i, j) represents the binary value of the micromirrors on the DMD at (i, j) . B _k, R _k (x, y) , T _k and φ _k are the relative amplitude factor, tilted phase, grating period, and phase for the k ^th focal point respectively.

Importantly, via binary hologram, the intensity distribution among the focal points may be arbitrarily controlled, realizing single exposure grayscale control. Simultaneous multi-focus fabrication has been a long-sought goal in additive manufacturing as it substantially reduces the processing time. Comparing with the existing methods of multi-focus fabrication, which mainly combines spatial light modulators and mechanical scanners, the system according to the present application could achieve the goal with a single DMD.

Adaptive wavefront correction

Using the DMD-scanner, wavefront correction algorithms can be designed and added to the scanning holograms via superposition, enabling the point-specific wavefront optimization and minimizing the voxel sizes across the DMD work space. To perform wavefront correction, the system aberration is first measured by collecting a sequence of images at low exposure level in fluorescent solutions, and is then determined based on the modal wavefront sensing method, where the aberration is considered as a summation of orthogonal modes, e.g., Zernike polynomials (Note a EMCCD may be used for the calibration. ) . The quality metric may be set to be total intensity for the optimization. Basis modes are then chosen and converted to binary holograms and combined with the scanning holograms. Next, a sequential quadratic maximization process is applied to identify the optimal modal coefficients. Once the optimal modes are determined, the aberration measurement and correction are applied to the beam scanning and shaping holograms for high-speed TPP fabrication (22.7 kHz) , i.e., the point-specific wavefront correction is applied to every point the DMD work space.

Accordingly, multi-point beam scanning and wavefront correction can be simultaneously performed. Note that all focal points generated by the DMD can be individually controlled (both location and intensity) and wavefront optimized. Mathematically, they can be described as:

Here,

is target wavefront containing k focal points, where A (x, y) ∈ [0, 1] and

represent the amplitude and phase of the electric field; x and y are the coordinates in Cartesian coordinate system. h (i, j) represents the binary value of the micromirrors on the DMD at (i, j) . B _k, R _k (x, y) , T _k and φ _k are the relative amplitude factor, tilted phase, grating period, and phase for the k ^th focal point respectively. φ _w, k is the additional wavefront information to be included in the hologram for controlling the size and shape of the focal points.

Experiments

A prototype system is developed for precision 3-D TPP printing based on ultrashort pulse laser and beam shaping. Preliminary experimental data show that the results are repeatable and achieve much higher throughput than any existing systems.

To validate the fabrication resolution and speed, truss arrays are fabricated, their trajectories are shown in Fig. 3 (a) . Each truss array consists of about 60,000 points, corresponding to about 60000 binary patterns, laser power was set at 30 mw, DMD worked at its maximum pattern rate (22.7 kHz) . Under this condition, each truss was fabricated within 3 seconds. Fig. 3 (b) –3 (d) present the SEM images of the fabricated truss arrays at different magnification. The results show the system according to the present has achieved equal or better resolution than the state-of-the-art commercial system from Nanoscribe.

To demonstrate the multi-focus parallel fabrication capability, two, three, and four focal points are used respectively to fabricate the woodpiles. The programed the scanning trajectories and the fabrication results are presented in Fig. 4 and Fig. 5 respectively.

Figs. 4 (a) -4 (d) show planned trajectories for (a) single- (b) two- (c) three-, and (d) four-focus fabrication of woodpile structures, respectively.

Figs. 5 (a) -5 (d) present the images of single-, two-, three-, and four-focus fabrication processes respectively, collected by the CCD camera, where the multi focus capability can be clearly observed. Figs. 5 (e) and 5 (f) present the SEM image of the fabrication results after development. The zoom-in image in Fig. 5 (f) shows the details of the woodpile and confirm again the writing resolution (～500 nm, limited by diffraction) .

Lastly, the DMD TPP system is used to fabricate a microscale London Bridge, demonstrating the capability of arbitrary path planning. First, the CAD model of the London Bridge is decomposed into a point array; each point in space corresponds to a specific binary hologram. Trajectories can be easily planned by arbitrarily arrange the hologram sequence in the DMD memory. For multi-point processing, selected holograms of selected points can be synthesized to generate a new hologram. Accordingly, the total number of hologram (or fabrication time) is reduced.

Fig. 6 (a) and 6 (b) present the CAD model and fabrication results of the London Bridge respectively. The London Bridge consists of 160, 000 points (or binary patterns) . For single point scanning, the fabrication time is 7 seconds only. The pixel dwell time is 44 μs.

According to the present application, with enough laser power, up to 100 points can be simultaneously generated and individually controlled for precision nano-fabrication, e.g., two-photon polymerization, thereby realizing high-throughput, high precision nano-fabrication.

Comparing with existing ultrashort laser based 3-D printing technologies, the laser fabrication system of the present application has the following distinct advantages: (1) arbitrary scanning trajectory in space, (2) high throughput, (3) multi-focus scanning, (4) superior accuracy and repeatability, (5) modification of mechanical and optical properties of printed structures, and (6) low cost.

It is understood that, on the basis of the laser fabrication system as described above, a laser fabrication method is also proposed in the present application. The laser fabrication method according to the present application may be implemented by the laser fabrication system as described above.

Fig. 7 illustrates a flow chart of a laser fabrication method according to an embodiment of the present application. As shown, at step 710, a laser beam is output from a laser source. At step 730, the laser beam is shaped by a digital micromirror device (DMD) with a scanning hologram synthesized from more than one binary holograms. At step 750, the shaped laser beam is focused to photoresists for parallel fabrication with a single or multiple shaped laser points. According to an embodiment, a step of neutralizing a negative angular dispersion that is introduced by the DMD to the laser beam is further included. According to an embodiment, wavefront information for wavefront correction is further included in the scanning hologram. According to an embodiment, a step of spatially selecting a -1st order diffraction of the scanning hologram may be further included, It is noted that all technical features described above for the laser fabrication system are also applicable to the laser fabrication method.

Although the preferred examples of the present application have been described, those skilled in the art can make variations or modifications to these examples upon knowing the basic inventive concept. The appended claims are intended to be considered as comprising the preferred examples and all the variations or modifications fell into the scope of the present application.

Claims

A laser fabrication system, comprising:

a laser source, configured to output a laser beam; and

a digital micromirror device (DMD) , configured to receive and shape the laser beam, wherein more than one binary holograms are synthesized to form a scanning hologram applied to the DMD; the shaped laser beam leaving the DMD is focused to photoresists for parallel fabrication with a single or multiple shaped laser points.
The laser fabrication system according to claim 1, wherein the DMD introduces negative angular dispersion to the laser beam; and the system further comprises:

a dispersion compensation unit, configured to transfer the laser beam from the laser source to the DMD with a positive angular dispersion for neutralizing the negative angular dispersion.
The laser fabrication system according to claim 1, wherein wavefront information for wavefront correction is further included in the scanning hologram.
The laser fabrication system according to claim 1, wherein the laser beam from the laser source is expanded to match an aperture of the DMD.
The laser fabrication system according to claim 1, further comprising:

an objective lens via a set of relay optics configured to focus the shaped laser beam.
The laser fabrication system according to claim 5, wherein the set of relay optics is a 4-f system consisting of two lenses.
The laser fabrication system according to claim 5, wherein a spatial filter is placed at a back focal plane of the set of relay optics to spatially select a non-zero order beam diffracted from the hologram.
The laser fabrication system according to claim 7, wherein the non-zero order beam is -1st or +1st order beam.
The laser fabrication system according to claim 2, wherein the dispersion compensation unit includes a blazed transmission grating and a mirror.
The laser fabrication system according to claim 1, further comprising:

a microscopic imaging system, configured to monitor fabrication processes performed by the shaped laser beams.
The laser fabrication system according to claim 1, further comprising:

a position stage, configured to mount and maneuver samples to a location where the sample will be laser processed.
A method for laser fabrication, comprising:

outputting a laser beam from a laser source;

shaping the laser beam by a digital micromirror device (DMD) with a scanning hologram synthesized from more than one binary holograms; and

focusing the shaped laser beam to photo-sensitive resins for parallel fabrication with a single or multiple shaped laser points.
The method according to claim 12, further comprising:

neutralizing an angular dispersion introduced by the DMD to the laser beam.
The method according to claim 12, wherein the scanning hologram comprises wavefront modulating information for wavefront correction.
The method according to claim 12, further comprising:

spatially selecting a non-zero diffracted order of the scanning hologram.
The method according to claim 15, wherein the non-zero diffracted order is -1st or +1st order.