ON-AXIS LINEAR FOCUSED SPOT SCANNING MICROSTEREOLITHOGRAPHY SYSTEM: OPTOMECHATRONIC DESIGN, ANALYSIS AND DEVELOPMENT

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1 Journal of Advanced Manufacturing Systems Vol. 12, No. 1 (2013) c World Scientific Publishing Company DOI: /S ON-AXIS LINEAR FOCUSED SPOT SCANNING MICROSTEREOLITHOGRAPHY SYSTEM: OPTOMECHATRONIC DESIGN, ANALYSIS AND DEVELOPMENT PRASANNA GANDHI, SUHAS DESHMUKH, RAHUL RAMTEKKAR, KIRAN BHOLE and ALEM BARAKI Department of Mechanical Engineering Indian Institute of Technology Bombay Powai, Mumbai, Maharashtra, , India gandhi@iitb.ac.in Microstereolithography (MSL) is technology of fabrication of three-dimensional (3D) components by using layer-by-layer photopolymerization. Typical design goals of MSL system are: small features, high resolution, high speed of fabrication, and large overall size of component. This paper focuses on design and development of such a system to meet these optomechatronic requirements. We first analyze various optical scanning schemes used for MSL systems along with the proposed scheme via optical simulations and experiments. Next, selection criteria for various subsystems are laid down and appropriate design decisions for the proposed system are made. Further, mechanical design of the scanning mechanism is carried out to meet requirements of high speed and resolution. Finally, system integration and investigation in process parameters is carried out and fabrication of large microcomponent with high resolution is demonstrated. The proposed system would be useful for fabrication of multiple/large microcomponents with high production rate in various applications. Keywords: Microstereolithography; 3D microfabrication; large microstructures; scanning schemes. 1. Introduction As market for miniaturized products grows rapidly, there is an increasing need for high resolution complex microstructures. Various microcomponents such as accelerometers, pressure sensors, micromirrors, microcantilevers, microfluidic devices and biosensors have been developed for various applications ranging from automobile to biomedical. 1 3 These are fabricated using 2D or 2 1 / 2 D microfabrication technologies such as bulk micromachining, surface micromachining, LIGA, and so on. These technologies mostly utilize mask-based processes and have limited flexibility in fabricating complex shapes in third (depth) dimension. Microstereolithography (MSL) is a promising technology to achieve 3D microdevices by using layer-by-layer photopolymerization of liquid resin. 43

2 44 P. Gandhi et al. For a good MSL system the following are the major desired system design requirements: (1) High resolution of microcomponents, (2) high speed of fabrication (high productivity), (3) large overall size over which component can be fabricated. This requirement is for either fabrication of large size component having several micro features (for example, lab-on-chip device) or fabrications of several small size components in one go. Two major approaches found in the literature for MSL are; (1) Projection MSL (each layer of the 3D part is exposed in one irradiation step by projecting its image on the surface of photosensitive resin), 1,4 8 and (2) scanning MSL (every layer is produced by scanning a focused laser spot on the surface of photosensitive resin). 1,9 13 Projection MSL is very efficient in high speed production but has inherent limitation on resolution due to diffraction of light and larger range would require large expensive dynamic mask. Desired system design requirements get translated into the following performance parameters of the scanning system: (1) Spot characteristics (spot size and intensity profile) at various scan distances (see Appendix A for connection between intensity and solidified spot), (2) resolution of spot positioning, (3) speed of scanning, and (4) range of scanning. Various scanning MSL systems such as pre-objective, post-objective, and offaxis lens scanning have been developed and others use liquid resin tank scanning. Focused spot scanning methods mentioned previously have good speed of scanning however suffer from spot distortion during scanning and liquid tank scanning method suffers from fabrication of wavy microstructures (summary in Sec. 2 and see Refs. 14 and 15 for more detailed description of these systems). Recently published scanning method (developed using axiomatic approach 16,17 ) gives uniform spot characteristics during scanning but may have limited speed of fabrication and spot positioning resolution. This paper presents a systematic way to carry out relevant analysis towards a design of MSL system that overcomes the previous limitations to effectively realize the desired requirements of MSL system. This paper is organized as follows. Section 2 summarizes results of numerical optical analysis supported with experimental results of various optical scanning schemes along with proposed scanning scheme. Section 3 presents proposed high precision, high speed optomechanical scanning system based on XY planar flexural mechanism so as to achieve high speed fabrication with high resolution spot positioning. The section presents experimental

3 On-axis Linear Focused Scanning MSL System 45 identification of XY flexure mechanism, results of implementation of PID control toward this end. Section 4 explains design and selection criteria of various subsystems of scanning MSL system. Section 5 presents experimental investigation of process parameters of photopolymerization in scanning MSL system and fabrication of microstructures. Section 6 presents concluding remarks. 2. Optical Analysis As a first step in the design, appropriate scanning scheme need to be chosen. The schemes previously available in the literature are to be analyzed for their appropriateness toward the design requirements mentioned in Sec. 1. Next subsections first summarize various previously available scanning schemes used in scanning MSL systems, choose the proposed scanning scheme and then summarize results of optical analysis pertaining the same Scanning schemes Two distinctly different approaches used in MSL for focused laser spot scanning on the resin surface are: (1) Moving photoreactor tank (see Fig. 1), and (2) moving optics (see Fig. 2). In the first case, the photoreactor tank is scanned in horizontal direction and the beam focusing optics is held stationary. 9,18 As the optics (mirror and lens) is fixed (see Fig. 1), the focused spot characteristics are uniform during entire scan range. Fig. 1. Photoreactor tank scanning. 1

4 46 P. Gandhi et al. (a) Pre-objective scanning 18,19 (b) Post-objective scanning 10,18 (c) Off-axis lens scanning 11,12 (d) Axiomatic approach of scanning 16,20 Fig. 2. Optomechanical scanning systems for focused laser spot scanning in MSL.

5 On-axis Linear Focused Scanning MSL System 47 However, moving resin tank at higher speed generates waves on the low viscosity monomer resin surface which spoil the desired resolution. 10,18 In the second approach where tank is kept fixed and the optics is moved, the following schemes have been used: (1) Galvanomirror pre-objective scanning, 18,19 (2) galvanomirror post-objective scanning, 10,18 (3) off-axis scanning, 11,12 and (4) scanning system developed using axiomatic design approach. 16,20 Scanning methods (such as pre-objective, 18,19 post-objective, 10,18 and off-axis lens scanning 11,12 ) produce variation of the spot characteristics either due to defocusing or due to misalignment of optical axis of lens with respect to laser beam axis during scanning. Recently developed scanning MSL system based on axiomatic design approach 16,20 has uniform spot characteristics and large range but may have limited spot positioning resolution and scan speed. Proposed Optomechanical Scanning: Principle Based on the previous discussion, the optomechanics scanning scheme similar to that presented in Refs. 16 and 20 is chosen. Figure 3 shows basic principle of operation of the proposed optical scanning. One-dimensional scanning is achieved by moving mirror and lens in the direction of the laser beam as shown in Fig. 3. This motion maintains alignment of the optical axis of the lens with laser beam (reflected from mirror). Two-dimensional scanning is achieved by introducing one Fig. 3. Basic principle of on-axis scanning method.

6 48 P. Gandhi et al. more mirror and linear scanning stage into previous scheme (see Fig. 3). Method of optical scanning mentioned above maintains alignment of the optical axis of the lens and axis of the laser beam during entire range of the scan. Such scanning produces uniform spot characteristics over entire range of the scan. Section 2.2 summarizes optical analysis results originally presented in Refs. 14 and 15 to see the effect of the spot size and intensity profile on fabrication resolution and variation of spot characteristics in previously mentioned scanning schemes Optical simulations Resolution of microcomponent in scanning MSL system depends on two major factors: (a) the laser spot size, (b) intensity distribution within the spot which in turn governs the solidification process. Effect of intensity profile and spot size on microfabrication is presented in Appendix A. Optical analysis of scanning systems mentioned in Sec. 2.1 involves determination of the focused spot characteristics (size and intensity profile) over entire scan range as the mechanical motion is imparted to optical elements. Optomechanical analysis approach similar to that in Ref. 14 is used to determine intensity profile at image plane. Fundamental principle involves wave-optics approach along with geometrical ray tracing 21 to get the point spread function (also called impulse function) on the image plane. Various optical elements viz. lens, mirror along with their locations in various scanning schemes are modeled in simulator OSLO 22 for the same. Point spread function (PSF) is obtained numerically using OSLO with FFT technique to reduce computations. Intensity profile at image plane is determined by convolving 23 Gaussian intensity profile at object plane and point spread function of optical system. Intensity can be expressed mathematically by, I i (x y )= I 0 (x, y)h(x, y, x y )dxdy, (1) where I i (x ; y ) is intensity profile at image plane, I o (x; y) is intensity profile at object plane and h(x; y; x ; y ) is point spread function of optical system. Further PSF data is taken from OSLO and discrete convolution of object data with PSF data is carried out using MATLAB to find the irradiance or intensity distribution of the spot formed at the image plane. Simulation is carried out for four scanning systems: pre-objective (see Fig. 2(a)), post-objective (see Fig. 2(b)), off-axis lens (see Fig. 2(c)) and proposed on-axis scanning/axiomatic scanning (see Figs. 3 and 2(d)). Figures 4(a) and 4(b) show variation of the spot size and peak intensity at the focused plane along the scan distances. Simulation results clearly show zero variation of spot size and peak intensity in case of proposed on-axis scanning and scanning system developed using axiomatic design approach, whereas significant degradation in spot and peak intensity is observed in pre-objective, postobjective, and off-axis lens scanning.

7 On-axis Linear Focused Scanning MSL System 49 (a) Variation of RMS spot size (b) Variation of peak intensity Fig. 4. Comparison of optomechanical scanning systems based on variation of spot size and peak intensity (A-Off-axis lens scanning Y -direction, B-Post-objective galvanomirror scanning, C-Preobjective galvanomirror scanning, D-Proposed optomechanical scanning scheme/scanning scheme developed using axiomatic design approach.) Optical simulations presented in this section shows that axiomatically designed scanning scheme and proposed on-axis lens scanning scheme gives uniform spot characteristics over entire scan range. These simulation results are further supported with experimental results presented in Sec Experimental validation Figures 3 and 5 illustrate experimental setup for proposed optomechanical scanning and combined pre-objective and off-axis scanning system respectively. Combined pre-objective and off-axis scanning setup consists of Ar+ Ion laser system (λ = 0 : 351 µm, Power = 140 mw), 24 beam profilometer (BEAM STAR Fx ) and optomechanical scanning system assembled using the optomechanical components supplied by M/S Holmarc. 26 Optomechanical scanning system consists of rotational and translational stages, UV lens (NEWPORT SPX010), UV mirror from NEWPORT (10QM20EM.15) 27 and its mount. The lens is chosen to give smaller spot size of 6.31 µm, suitable for microfabrication in MSL as mentioned in Appendix A. Pre-objective scanning is achieved by rotation of the mirror by using rotational motion stage and off-axis scanning is achieved by translational motion stage. Figures 6(a), 6(c), 6(e) and 6(b), 6(d), 6(f) show comparison of simulation and experimental results for proposed scanning scheme, pre-objective scanning, and offaxis lens scanning scheme, respectively. It is observed that there is close matching of simulations with experimental results. In case of the proposed scanning scheme, no distortion of the spot size and intensity profile is observed (see Fig. 6(b)) even in worst case condition i.e. at position of (4,4). (Note that the pre-objective and off-axis lens scanning show significant distortion even at spot position (0,2).)

8 50 P. Gandhi et al. Fig. 5. Experimental setup: Combined pre-objective and off-axis scanning schemes. (a) Simulation: Proposed and axiomatically (b) Experimental: Proposed and axiomatically designed systems (X, Y )=(4, 4) designed systems (X, Y )=(4, 4) (c) Simulation: Pre-objective (X, Y ) =(0, 2) (d) Experimental: Pre-objective (X,Y ) =(0, 2) Fig. 6. Comparison of simulation and experimental results (spot intensity profiles).

9 On-axis Linear Focused Scanning MSL System 51 (e) Simulation: Off-axis lens (X,Y ) =(0, 2) (f) Experimental: Off-axis lens (X,Y ) =(0, 2) Fig. 6. (Continued) (a) Variation of 1/e 2 spot size (b) Variation of peak intensity Fig. 7. Variation of 1/e 2 spot size and peak intensity over range of the scan in X- and Y - direction.

10 52 P. Gandhi et al. Figure 7 shows a variation of spot size (see Fig. 7(a)) and peak intensity (see Fig. 7(b)) along the scan distance for proposed scanning scheme. It is observed here that, spot size vary within 8.2% (Note: these measurements are 1 mm away from theoretical focus) over a scan distance of 8mm (±4 mm) for proposed scanning system and variation of peak intensity is 3.7%. Optical simulations are carried out again to obtain similar variations in spot and intensity at distance 1 mm away from theoretical focus to match with experimental settings. The results show 5.7% and 3.72% variation respectively in spot and intensity which is close to that of experiments. Optical analysis presented previously shows a superior performance of axiomatically designed scanning system and proposed on-axis scanning system in spot characteristics in comparison with the other scanning systems for scanning a focused spot. Further, Sec. 3 presents development of optomechatronics system around proposed scanning scheme and implementation of PID control to achieve high speed scanning. 3. XY Optomechatronics System As per previous discussion, proposed scanning scheme and axiomatically designed scanning scheme are best suitable optical scanning scheme among other, as they both give uniform spot characteristics over entire scan range. Further, to achieve high positioning resolution at high speed of the scanning, XY flexural mechanism is designed and developed around proposed on-axis scanning scheme and discussed in further subsections XY scanning mechanism Flexure mechanism (no friction) is used here due to zero friction and backlash during motion. A flexural mechanism is interfaced with noncontact sensors and actuators (see Fig. 8). Sensors and actuators are co-located to have ease of control despite of nonlinear flexibility effects. The actuator specifications are designed to have desired high speed motion of 10 mm/s in scanning. To have a high speed implementation without compromising accuracy, high speed data acquisition system dspace DS1104 is employed in our mechatronic design. Figure 13(a) shows a photographic view of complete integrated scanning MSL system Modeling and identification Experimental identification of XY mechanism is carried out to determine transfer functions for both motion stages. Determined transfer function is further used to tune PID control parameters. Following form of transfer function is identified for Y -motion stage, 45s 2 +90s e05 G Y = s s s e04s e06. (2)

11 On-axis Linear Focused Scanning MSL System 53 Fig. 8. Photographic view of designed mechatronics system. Similarly, the identified transfer function for X-motion stage is given by, G X = s 4 +90s s s (3) Figures 9(a) and 9(b) show a closed matching of frequency response of identified model with experimental results for both motion stages. Using these identified transfer functions, PID controller parameters such as Kp proportional gain, Kd derivative gain, and Ki integral gain are tuned so that desired positioning accuracy is achieved. Tuned parameters are further implemented for real-time control of XY trajectory of focused spot on the resin surface to achieve high speed and high positioning resolution. Next section discusses the high speed scanning results High speed scanning High speed in scanning is achieved by using low inertia devices such as actuators and noncontact optical encoders. Further system integration of XY flexural mechanism with PC is carried out using dspace data acquisition system. A PID control algorithm is implemented for XY scanning of focused spot. Standard slicing and scan-path generation algorithm 3 is used to evaluate X- andy -motion trajectories of focused spot from 3D CAD model. Smooth acceleration and deceleration at end points are ensured by appropriate modification of trajectory paths. These trajectories are further used for real-time control of focused spot on the resin surface. Scan path data is generated for various scan speeds up to 5 mm/s, and related experiments are carried out. Figure 10 shows a high speed scanning results at 5 mm/s.

12 54 P. Gandhi et al. (a) Fourth-order transfer function of Y -motion stage (b) Second-order transfer function of X-motion stage Fig. 9. System identification of XY-motion flexural mechanism. It is observed from XY plot (see Fig. 10(a)) that at scan speed of 5 mm/s positioning resolution achieved is less than 0.35 µm. Positioning error in scanning (see Fig. 10(a)) achieved for constant speed part (most relevant for part fabrication with raster scan) is less than 0.35 µm.

13 On-axis Linear Focused Scanning MSL System 55 (a) XY Positioning (b) Error in µm Fig. 10. PID control results (vector-by-vector tracing). (a) XY plot results at high scanning speed (5 mm/s) and (b) Y -motion scanning. So far we have discussed two main important aspects to improve fabrication resolution of scanning MSL system. First aspect is spot characteristics and second one is positioning resolution at high speed of the scanning. First aspect is achieved by designing new optomechanical scanning scheme and second is achieved by developing a flexural mechanism around proposed scanning scheme. Moreover, fabrication resolution in scanning MSL system depends on other subsystems such as layer preparation stage, optical switch, laser system, DAQ (data acquisition system) and computer and resin system. Next section presents issues of selection of these subsystems for proposed scanning MSL system. 4. Design and Selection Criteria of Subsystems of MSL and System Integration Figure 11 shows a block diagram representation of proposed scanning MSL system. It shows various subsystems and flow of the information between these subsystems Selection criteria of subsystems for scanning MSL Scanning MSL system consists of following subsystems: (1) UV source (Laser System), (2) resin system, (3) optical shutter (AOM (Acousto-Optical Modulator)), (4) optical system, (5) mechanical XY scanning stage, (6) layer preparation system i.e. Z-stage, and (7) Computer and interfacing electronics.

14 56 P. Gandhi et al. Fig. 11. Schematic diagram of scanning MSL system. Selection criterion and design considerations for each subsystem of scanning MSL is given below, Ar+ laser is chosen as UV laser source as its viability has been practically tested for scanning MSL systems. 28,29 The Ar+ ion laser system has stable beam parameters such as beam intensity, pointing stability. 24 The choice of resin system depends on factors 1,28,29 such as; (1) The resin should be photosensitive to operating wavelength, (2) the resin should have low viscosity and also produce uniform and smooth layer on already polymerized part, (3) finished cured part should have good toughness and it should have low shrinkage during liquid to solid transformation, (4) also should have low penetration depth to achieve high resolution in Z-direction. For proposed system, HDDA (1-6-hexanediol diacrylate)-based resin is chosen with BEE (benzoil ethyl ether) as photoinitiator due to its low viscosity of 6 cps, high toughness after curing and low shrinkage during fabrication process. This resin system is already tested for MSL system in order to fabricate ceramic microstructures. 28,29 Optical shutter is usually used to control the laser beam (i.e. ON/OFF and intensity of beam) according to CAD model information. Major disadvantage of mechanical shutter is low response time (i.e. 1 ms) whereas acousto-optic modulators have a response time of few nanoseconds. AOM is preferred here due to its low rise time and perfect control over an intensity and power of laser beam as compared to mechanical shutters. Suitable AOM (Model No BR) is selected from NEOS tech. 31 Optical system and scanning scheme are key elements of scanning MSL system. Optical system decides focused spot size (here UV compatible lens (Model No.:

15 On-axis Linear Focused Scanning MSL System 57 SPX010) is selected from Newport 27 having Focal length = 12.7 mm corresponding to diffraction limited spot diameter of 6.31 µm)). Proposed optical scanning scheme is best suitable for scanning MSL (see optical analysis carried out in Sec. 2) which gives high positioning resolution at high speed of scanning using XY flexural mechanism (see design and analysis of XY mechanism in Sec. 3) Layer preparation stage At focal plane minimum spot size is obtained. Hence, it is necessary to have precise position of the build platform below XY scanner at focal plane. To ensure precise positioning of build platform at focal plane high stiffness flexural-based manual Z- stage is designed and developed (see Fig. 12). Precision manual positioning flexural Z-stage consists of double flexural manipulator (DFM), resin tank, build platform, positioning sensor (Renishaw Optical Encoder RGF25, having a positioning resolution of 20 nm), tank positioning micro-stage, precision screw (having a pitch of 0.3 mm), precision nut, and frame for DFM. Z-stage is positioned below XY mechanism so that focused laser spot lie on the build platform. Further, resin tank is mounted in such a manner to have build platform inside the tank as shown in Fig. 12. When precision screw is rotated in clockwise direction (which is mounted Fig. 12. Layer preparation system: Manual Z-stage for layer preparation.

16 58 P. Gandhi et al. in frame of DFM), tip of the screw applies a force on motion stage of DFM. As build platform is rigidly fixed to motion stage, build platform also moves in upward direction. Therecoatingprocessisasfollows: Initially Z-translational stage is positioned at focal plane of the lens then resin tankismovedinupwarddirectionbyusingz-stage so that first layer of liquid resin is spread uniformly on build platform. XY scanner is then used to solidify the first layer. Further, build platform (i.e. Z-translation stage) is moved down a distance more than a desired layer thickness after first layer solidification is completed. Platform needs to be deeply dipped in the tank to overcome surface tension effects thereby allowing liquid to flow in from the edges of the platform and/or on previously formed layer. After deep dipping build platform is again positioned manually at desired layer thickness, and then next layer solidification is carried out using XY scanner. Deep dipping and precise positioning of build platform, and XY scanning of laser spot for each layer is carried out until the complete microstructure is built System integration Complete proposed scanning MSL consists of four main subsystems: (1) CAD-Software system, (2) mechanical scanning system, (3) optical system, and (4) layer preparation or Z-stage. CAD software system: Stereolithography process often begins with the creation of a model using a CAD modeler (e.g. Pro-E, AutoCAD, SolidWorks, etc.). All CAD modeling software can easily generate STL (stereolithography file) file which converts CAD model into triangular facets. STL file is then sliced to generate data about successive layers which are then constructed one layer at a time. Sliced data is further used to create a scan path data for each layer. Scan path data include X and Y position of laser spot and laser ON-OFF conditions for each layer. Slicing and scan path generation algorithm is coded using MATLAB script program which generate XY position data and laser ON-OFF conditions for each layer. Generated data is then sent using serial communication scheme to the hardware through dspace DS1104 microcontroller. This information is used by the control program to expose photopolymer in commanded way to create each of the layers. Graphics User Interface (GUI) for the fabrication of the component is developed in Control Desk software provided along with the dspace DAQ. Real-time control of X, Y position, and laser ON-OFF is done using ControlDesk Software developed by dspace Gmbh. Table 1 and Fig. 13 show complete mechatronics interfacing. Figure 13(a) depicts a complete integrated scanning MSL system. Figure 13(b) shows a block diagram

17 On-axis Linear Focused Scanning MSL System 59 Table 1. System integration: Integration of subsystems of developed scanning MSL system with PC. Component Mechanical scanning system Y -Motion Stage Actuator BEI- KimcoLA A 32 X-Motion Stage Actuator H2W-tech NCC X 33 X Y -Motion stage sensor Renishaw RGH25F 34 Optical Shutter Acousto-optic modulator BR 31 Manual Z-stage Z-Motion stage sensor Renishaw RGH25F 34 Interface dspace1104 microcontroller and LCAM amplifier dspace1104 microcontroller and LCAM amplifier dspace1104 microcontroller and interpolator RGF2000 dspace1104 microcontroller and RF driver AM dspace1104 microcontroller and interpolator RGF2000 representation of interfacing of subsystems of MSL with PC through dspace DS1104 microcontroller. Details of interfacing of XY mechanism, AOM and layer preparation Z-stage is discussed further. XY mechanism: XY flexural mechanism consists of two voice coil motors (VCM) and two optical encoders. VCM needs a current signal whereas dspace board gives output as voltage 10 V with 5 ma current. Voltage to current amplifier (here Quanser current amplifier LCAM is used) is necessary to provide sufficient power to VCM. DAC channel of dspace board is connected to LCAM and output of LCAM is connected to VCM. Optical encoders are connected to encoder channels of dspace CLP board through interpolator RGF2000 provided by Renishaw. Encoders used in present application have resolution of 20 nm. Acousto-optic modulator (AOM): AOM is used to vary and control laser beam intensity. A Bragg configuration gives a single first-order output beam, whose intensity is directly linked to the power of RF control signal. Acoustic waves in acoustic crystal are generated using RF driver. AOM is mounted in the optical path of laser beam so that laser beam passes through center of the acousto-optic crystal. AOM mount has sufficient adjustments for Bragg angle, and horizontal position adjustments. RF power given to modulator is controlled by controlling DAC voltage signal. Figure 13(b) shows a connection of RF driver with DAC channel of dspace board. Maximum voltage given to RF driver is 1 V. Bragg angle is adjusted so that diffracted first-order beam is at highest intensity. Pinhole is mounted at sufficient distance from AOM to allow sufficient separation of diffracted beams. Pinhole is to be adjusted in vertical plane so that only first-order beam is passed through it and all other orders of beams are blocked. By varying DAC voltage between 0 and 1 V, power of first-order laser beam can be controlled. For 0 V input to RF driver first-order beam has a zero power and for 1 V input first-order beam has maximum

18 60 P. Gandhi et al. (a) Photographic view of developed scanning MSL setup (b) Schematic diagram of system integration Fig. 13. Newly developed scanning MSL system. power. Power of first-order beam depends on input voltage to RF driver; hence necessary calibration is carried out. Layer preparation/z-stage: Description of flexural-based manual Z-stage is presented in Sec Optical encoder is mounted on the Z-stage so that position of build platform is precisely measured. Optical encoder is interfaced with PC through RGF2000 interpolator provided by Renishaw. Figure 13(b) shows interfacing of

19 On-axis Linear Focused Scanning MSL System 61 Z-stage with PC using dspace data acquisition system. Optical encoder has resolution of 20 nm and manual positioning can be done within 300 nm easily. Figure 14 shows results of manual positioning resolution of build platform using Z-stage, and it is observed that positioning resolution achieved is up to 300 nm. Fig. 14. Layer preparation system: Manual positioning precision achieved in developed Z-stage. Fig. 15. Front panel for developed scanning MSL system.

20 62 P. Gandhi et al. Real-Time Workshop toolbox from MATLAB-SIMULINK is used to develop a program to measure a position of the build platform in resin tank. Developed program is build and loaded to SRAM of dspace DS1104 microcontroller. Real- Time position of build platform is measured using ControlDesk software provided by dspace Gmbh. Front panel of integrated MSL system is shown in Fig Experimental Investigation and Microfabrication To verify the proposed performance of the developed MSL setup, the fabrication results of various 3D microstructures are demonstrated. This section presents first the results of prefabrication experiments and characterization of fabricated microstructures. In later part, section demonstrates the fabrication of high resolution, high range and high aspect ratio microstructures Experimental investigation As per previous discussion, resolution of the microstructure obtained in scanning MSL system depends on spot characteristics and positioning accuracy. Size of fabricated microlobe depends on various parameters such as focused spot size, properties of photopolymer, laser power, scanning speed and positioning resolution of scanning system. In present system, focused spot size is fixed i.e µm. Properties of polymer depends on concentration of photo-initiator in monomer solution. For experimental investigation, fresh HDDA solution with the photo-initiator (BEE) concentration of 4 wt.% is used. Resolution obtained in line scanning experiment depends on scan speed and laser power. A typical line scanning pattern (see Fig. 10) is used for investigation of photopolymerization process. Scan speed and laser exposure energy are varied and cured line width is measured using high resolution optical microscope. Figure 16(a) shows typical cured lines during investigation experiments (a) Cured line width, Lw = 6 µm (b) Exposure energy versus cured line width Fig. 16. Experimental and theoretical investigations of photopolymerization process.

21 On-axis Linear Focused Scanning MSL System 63 and Fig. 16(b) shows a variation of line width with increasing exposure energy, which shows close matching of simulation and experiment results. Fabrication capability of the developed MSL system is finally demonstrated by fabrication of various 3D microstructures such as micro-cones, micro-pyramid, micro-channels and large range micro-tanks. (a) Micro-cones: Fabricated dimensions: (b) Micro-pyramid: Fabricated dimensions: Diameter of cone at base = µm, Base of micro-pyramid = µm, height of cone = µm top land of pyramid = µm (c) Micro-channel: Fabricated dimensions: (d) Large Range Micro-tanks: Fabricated Overall dimension = 3993 µm 3989 µm, dimensions: Outer dimension of large channel dimension = 146 µm, square tank = 3990 µm 3995µm, counter hole reservoir = diameter 740 µm innermost high aspect ratio square and 417 µm tank = 696 µm 697 µm Fig. 17. Successfully fabricated 3D multi-layer and high range micro-structures. Process parameters for micro-cones and micro-pyramid: scan speed = 0.8mm/s, laser power 60 µw. Process parameters for micro-channel and micro-tanks: scan speed = 0.8mm/s, laser power 100 µw and 80 µw, respectively.

22 64 P. Gandhi et al Microfabrication The high aspect ratio microstructures such as micro-cones, micro-pyramid, microtanks and high range microstructures such as micro-channel and micro-tanks were fabricated with multiple layers on silicon substrate. Designed details of each of the structure and their fabrication result (see Fig. 17) are discussed below: Micro-cones: Figure 17(a) shows a SEM image of four microcones having designed dimension of base diameter 450 µm and height 420 µm (12 layers with 35 µm thicknesses). Micro-pyramid: Figure 17(b) is SEM image of micro-pyramid. It has an overall size of 1 mm 1 mm with four square cavities one on each side of the base having a size of 275 µm 275 µm. Pyramid is fabricated with 12 numbers of layers with 50 µm layer thickness. SEM image of pyramid shows that features of CAD model are completely reflected in fabricated structure and dimensional accuracy achieved is less than 5 µm. Micro-channel: Figure 17(c) is SEM image of micro-channel. The CAD model of microstructure is prepared with an overall outer square of dimensions 4 mm 4mm with microchannels of 150 µm width. The counter shape reservoir has diameter 750 µm on top face and diameter 425 µm at the bottom face. Structure is fabricated with four layers of each 100 µm thickness. Large range Micro-tanks: Large range square tank of outer size 4 mm 4mm along with high aspect ratio square tank of innermost dimensions 700 µm 700 µm and height 640 µm is fabricated in single structure. Figure 17(d) shows SEM image of the fabricated structure. Micro-tank structure is fabricated with number of layers varying from 2 to 8 with a step of two layers from outer large range tank to innermost high aspect ratio tank. Uniform layer thickness of 80 µm is maintained during this fabrication. 6. Conclusion This paper discussed, design, analysis, and development of scanning MSL system. The proposed novel way of optical scanning, that gives uniform spot characteristics, involves linear scanning of mirror and lens so that optical axis of the lens and laser beam axis is aligned during entire scan range. Optical analysis shows a zero variation of the spot characteristics as against other scanning scheme. Linear XY scanning scheme of focused spot is realized by using friction-free, backlashfree flexural mechanism. A mechatronics system is designed and developed around proposed scanning scheme. Further, optomechatronics system is integrated with PC using high speed data acquisition system dspace DS1104. Most popular PID control is implemented to have a XY scanning at high speed with submicron positioning accuracy. Experimental investigation of process parameters is carried out to

23 On-axis Linear Focused Scanning MSL System 65 achieve high resolution microfabrication. High precision fabrication of microstructures such as micro-cones, micro-pyramid, micro-channels and large range microtanks are demonstrated using the investigated process parameters. The system can further be useful for fabrication of ceramic and metallic components by seeding photopolymer with metal or ceramic micro/nanoparticles. Acknowledgments This work is partially supported by generous funding from alumnus Mr. Raj Mashruwala and through two MHRD projects. Authors would like to acknowledge Mr. Amit Phatak for part of this work. Appendix A. Resolution of Microcomponent in MSL Resolution of microcomponent in MSL system depends on two major factors: (a) the laser spot size, (b) intensity distribution within the spot which in turn govern the solidification process. Spot size of a focused laser beam is given by Airy disc formula 35 : W 0 = 2.44λF NO, (A.1) where W 0 is the diameter of the focused spot, F is focal length of the lens, NO is diameter of aperture/diameter of the laser beam. Toward the goal of fabrication of high resolution microcomponent, spot size required should be as small as possible. Hence, we need focal length as small as possible to get a smaller focus spot. From practical implementation perspective and available UV compatible optics we have selected SPX010 from Newport 27 (Focal length = 12.7 mm corresponding to diffraction limited spot diameter of 6.31 µm) for this work. Our results of simulation and experiments are based on this single lens used in all three different scanning methods for reasonable comparison. Intensity within the laser spot is another important factor affecting the resolution. During photopolymerization, light energy absorbed at any point x; y along the Z-direction in the photopolymer is explained by Beer Lambert s Law, I (x,y,z) = I (x,y,0) e z/dp, (A.2) where Dp is the penetration depth (characteristic of a photopolymer) and I(x; y;0) is the intensity on the surface of the liquid vat. Thus intensity at any point x; y goes on decreasing as the depth z increases. According to exposure threshold model, 1,36 when laser spot is focused on a layer of liquid vat, polymerization within the laser spot takes place at all points where intensity exceeds a critical intensity value Ec. Shape of the solidified part during line scanning is characterized by cured width Yp and cured depth Zp. If the spot focused on liquid vat has Gaussian intensity

24 66 P. Gandhi et al. Table A.1. Line widths obtained at 0.8 mm/s scan speed. Line width (µm) Laser power (µw) Fig. A.1. Effect of spot intensity profile on polymerized shape and optical modeling of system. variation, the solidified shape (Ypand Zp) using Eq. (A.2) is given by, 1,36 2 P L E C = e 2Y 2 P W 0 e Z P D P, (A.3) π W 0 V S where P L, W 0,andV s are laser power, beam width, and scanning speed, respectively. Equations (A.2) and (A.3) clearly show the dependence of cured shape Yp and Zp on the spot size W 0 and intensity profile of the spot (which is Gaussian in this case). Table A.1 shows the variation in line width of cured photopolymer line against laser power at constant velocity of scan 0.8 mm/s. The cross section of the shape formed along with the corresponding Gaussian intensity profile during exposure of liquid resin to focused Gaussian laser beam is shown in Fig. A.1. Thus, we can see that if the intensity profile is distorted or spot sizes are changing over the scan range then the resolution of microcomponent in Z- and Y -directions would degrade. Hence, to obtain high resolution microcomponent it is necessary to maintain intensity profile and size of the beam as uniform as possible over the entire scan range. References 1. V. K. Varadan, X. Jiang and V. V. Varadan, Microstereolithography and other Fabrication Techniques for 3D MEMS (John Wiley Sons, New York, 2001). 2. N. Maluf, An Introduction to Micromechanical Systems Engineering (Artech House Inc., Norwood, 2004). 3. P. M. Pandey, N. V. Reddy and S. G. Dhande, Slicing procedures in layered manufacturing: A review, Rapid Prototyping Journal 9 (2003)

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