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A novel high-accuracy microstereolithography method employing an adaptive electro-optic mask
Journal of Materials Processing Technology 107 (2000) 167±172
A novel high-accuracy microstereolithography method employing an adaptive electro-optic mask M. Farsari*, F. Claret-Tournier, S. Huang, C.R. Chatwin, D.M. Budgett, P.M. Birch, R.C.D. Young, J.D. Richardson University of Sussex, School of Engineering, Laser and Photonics Systems Group, Falmer, Brighton BN1 9QT, UK
Abstract A new stereo-photolithography technique to create three-dimensional micro-components using a planar, layer-by-layer, process of exposure has been developed. With this procedure, it is possible to build components with dimensions in the range 50 mm±50 mm, and feature sizes as small as 5 mm with a resolution of less than 1 mm. This newly developed system consists of eight elements: a Computer Aided Design environment supporting solid or surface modelling; a resin bath with an integrated high resolution translation stage and component built platform; an ultraviolet laser light source operating at 351.1 nm; an optical shutter; a diffractive optical element designed to re-distribute the irradiance of the laser beam from a Gaussian to a top-hat pro®le; a polysilicon thin ®lm twisted nematic SVGA resolution (800 600) spatial light modulator; a multi-element lithographic reduction lens system; and a comprehensive control system. In this paper, the experimental set-up is described and examples of the microcomponents fabricated by the system are shown. # 2000 Published by Elsevier Science B.V. Keywords: Microstereolithography; Rapid prototyping; Spatial light modulators; Diffractive optics
1. Introduction Commercially available stereolithography systems use a focused scanning laser beam to partially solidify components, layer-by-layer, into a honeycomb structure. Components are then fully cured in a post-curing process. In these systems the maximum component resolution is approximately 0.1 mm, which is a limitation imposed by using a focused laser beam. Efforts have been made to increase the resolution by either using a layer-by-layer process with a liquid crystal display as a mask [1], or by silicon microfabrication [2]. However, the former method only operates at visible wavelengths, where there are no stereolithographic materials commercially available; with silicon processing there are constraints on the height of components (third dimension). The technique described in this paper is a new rapid prototyping process capable of fabricating micro-components for micro-sensors, micro-actuators, micro-robot grippers, micro-robots, micro-motors, micro-drives, microconnectors, micro-optical components, packaging and
* Corresponding author. E-mail address: [email protected] (M. Farsari).
0924-0136/00/$ ± see front matter # 2000 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 6 7 2 - 5
inter-connection technologies; in the medical ®eld there are applications involving the manufacture of implants and prostheses. The system operates in the UV (351.1 nm) and utilises a spatial light modulator to create 3D components using a completely planar (layer-by-layer) process of exposure. This is a major advance over alternative methods because layers are now concurrently cured over the entire surface as opposed to incrementally building the layer itself. Furthermore, it takes advantage of widely available commercial UV-curable resins designed for conventional stereolithography. 2. Experimental set-up The prototype system is shown in Fig. 1. The principal physical components of the system consist of: an ultraviolet laser light source; an optical shutter; a spatial light modulator; a multi-element lithographic lens system; a highresolution translation stage. The spatial light modulator is the critical interface between electronic and optical systems. 2D images are generated by level slicing a 3D solid or surface model of the required component (DUCTTM surface modeller with TRIFIXTM and additional component slicing software sup-
168
M. Farsari et al. / Journal of Materials Processing Technology 107 (2000) 167±172
Fig. 1. Experimental set-up.
plied by DELCAM International plc are used in this case); the slices are made at uniform increments along the chosen axis. Each slice is then converted to a bitmap format and loaded onto the spatial light modulator which acts as a dynamic lithographic mask, modulating the top-hat UV laser beam with the level slice image. The multi-element lens reduces the image by 10 or 20 times and re-images it to the plane of the resin surface. Formation of the component takes place in the resin bath, the photopolymer resin and incident light being selected such that incident light produces cationic polymerisation and initiates the selective curing of regions of the resin. The shutter, placed in the beam path, controls the duration of the exposure. Once curing of a layer is complete, the component is lowered further into the bath such that a new layer of resin ¯ows over the surface that was last cured. It is then raised to the correct position just below the surface so that a thin liquid layer covers the component. The process then repeats over the same sequence of operations for the next layer until the component is completed. 3. Optical systems The experimental optical system is divided into two sections: beam conditioning and spatial light modulator. 3.1. Beam conditioning optics The light source for the miniature rapid prototyping experiment is an argon ion laser operating at 351.1 nm.
The nominally Gaussian irradiance distribution is reshaped to a rectangular uniform irradiance in order to obtain a uniform beam energy distribution within the subsequent optical components and maximise the contrast ratio of the spatial light modulator. The diffractive optical element used is a Digital Optics Corporation Gaussian to square beam converter. This is an eight-level device designed for a wavelength of 351.1 nm. In practice, the argon ion laser gives a poor approximation to a Gaussian which results in the diffractive optical element not giving a ¯at irradiance pattern. The method is, however, considerably more energy ef®cient than sampling the central section of a Gaussian beam with an aperture. For example, if the central section of a Gaussian beam is sampled such that the intensity variation was 5% across the aperture, only 5% of the beam power can used. The low quality of the beam produced by the diffractive optic can be improved by the incorporation of a rotating holographic diffuser. This also destroys spatial coherence and reduces speckle in the image plane of the reducing lens. The disadvantage of this arrangement is that the beam is no longer collimated and it becomes depolarised. The polarisation is linearised before the beam enters the spatial light modulator by the addition of a linear polariser sheet after the ground glass screen. 3.2. Spatial light modulator The device was supplied by CRL Smectic Technologies, with the normal input or output polarisers removed. It has Super VGA resolution (800 600) with a pixel size of
M. Farsari et al. / Journal of Materials Processing Technology 107 (2000) 167±172
26 mm 24 mm. The total active area of the device is 26:4 mm 19:8 mm with a pixel ®ll factor of 50%. The contrast ratio of the device is quoted as 200:1 with a grey scale modulation of at least four levels. A Coreco Ultra-II frame grabber board with dual SVGA video output is used to drive the SLM. The image to be displayed on the device is written to the frame buffer of a Coreco card with the rate of 60 frames per second; however, the liquid crystal material will limit the actual speed of the spatial light modulator. The rise time of the liquid crystal is 7 ms but the fall time is 25 ms (40 Hz). The dead space between the pixels in both devices is opaque, and although this produces light loss from diffraction into higher orders the optical system is designed such that only the zeroth order is sampled. The optical ¯atness of the spatial light modulator is not of importance since the input laser beam has its spatial coherence removed with the rotating diffuser before entering the spatial light modulator. A cubic UV polarising beam-splitter is used as the output analyser enabling the spatial light modulator to act as an intensity modulator. The CRL device is not damaged by wavelengths longer than 350 nm. However, shorter wavelengths will cause damage to the spatial light modulator's indium tin oxide electrodes and the liquid crystal material. 4. System requirements The principal components of the system and application sequencing are identi®ed above and in Fig. 1. The subsystems and their performance speci®cations are summarised in Table 1. The control system manipulates image data and control signals in real-time. The translation stage is a key component used for dipping the target part into the resin for re-coating, and returning the part to a pre-determined distance below the liquid resin surface. A PCI interface card is used to control the translation stage. The objective of the spatial light modulator interface is to send a speci®c sequence of level-slice data (represented by a raster bitmap), as de®ned by the CAD representation of the component, to the spatial light modulator; this displays the data as a mask which is illuminated and imaged onto the resin bath surface to build a layer of the micro-component. The spatial light modulator is driven by a standard SVGA
169
signal. The host computer has dual SVGA outputs to support concurrent access to a regular computer monitor and the spatial light modulator. Image acquisition is used to provide a path for visual closed loop control of the curing process. The mask supplied to the spatial light modulator may be dynamically altered to correct partial curing of the present layer before applying the mask for the next layer. A video frame grabber is used to capture images from a CCIR format CCD camera. Further IO control for accessories such as shutters was provided using a combination of RS-232 and digital TTL lines. The operating system was Windows NT and microstereolithography system software was developed in LabVIEW#. This provided high level support for the variety of I/O interfaces utilised. 5. Component built In order to decide which stereolithographic resin to use, holographic measurements were performed on a series of materials [3]. The stereolithographic resin chosen is the Ciba-Geigy CibatoolTM SL 5180, the main reason being its viscosity, which is lower than the other materials investigated. Figs. 2±6 show SEM photographs of several components that have been built. These components consist of 45±105 layers, each one clearly visible in the SEM photograph. The component in Fig. 2 is a micro-gear and it is made up of 50 layers, 50 mm thick each. It consists of a hollow cylindrical bearing, which is attached to the outer truncated cone by four spokes. The component in Fig. 3 is a free-standing double helix, and it consists of 105 layers, 50 mm thick each. Fig. 4 shows a micro-pyramid, which consists of a base of 20 layers, 50 mm thick each, and the pyramid structure, which consists of 31 layers, 35 mm thick each. Fig. 5 shows a fourpyramid structure, consisting of 45 layers including the base, each 50 mm thick. Finally, Fig. 6 shows a micro-wheel which is made of 5 mm thick layers. The laser irradiance in the material follows the Beer± Lambert's law [4], which states that in depth z the laser irradiance E(z) is E z E0 exp ÿz=Dp
(1)
Table 1 Interface and performance speci®cation Component
Interface
Requirements
Supplier
Opto-electronic mask (SLM) Translation stage
Binary/grey level video interface PCI card
Coreco Coherent-Ealing Electro Optics
Optical shutter
RS 232
Digital dial test indicator
Custom serial interface
600 800 (SVGA) 20 nm resolution, 5 mm step size, 0.1 mm repeatability 2 ms minute exposure time, delay time 1 ms, repetition rate 0±400 Hz 1 mm resolution
Uniblitz Mitutoyo
170
M. Farsari et al. / Journal of Materials Processing Technology 107 (2000) 167±172
Fig. 2. A micro-gear.
where E0 resin and The laser calibrated
is the laser irradiance on the surface of the Dp is the penetration depth of the material. irradiance used to polymerise each layer was so that it would cure only the liquid layer of
the resin over the already built component. That is to say, if the component built consists of 50 mm layers at a depth of 50 mm, the laser irradiance should equal the critical energy for the material Ec. In the case of Ciba-Geigy CibatoolTM SL
Fig. 3. A double helix.
M. Farsari et al. / Journal of Materials Processing Technology 107 (2000) 167±172
Fig. 4. A micro-pyramid.
Fig. 5. A four micro-pyramid structure.
171
172
M. Farsari et al. / Journal of Materials Processing Technology 107 (2000) 167±172
Fig. 6. A micro-wheel.
5180, Dp 0:132 nm and Ec 16:2 mJ=cm2 , so E0 is calculated as E0 23:66 mJ=cm2 . Since the manufacturer's values are approximate and vary according to the stereolithography method used, we measured the E z 50 mm experimentally. This was done by building several layers and incrementally lowering the laser irradiance until the top cured layer did not adhere to the previous layer.
Acknowledgements
6. Conclusion
References
In summary, we have developed a new method for microstereolithography which operates in the UV and uses a spatial light modulator as a dynamic lithographic mask to manufacture micro-parts which are complex in shape and could not be manufactured with either micro-component silicon processing methods or conventional stereolithography. The ®rst components built using this method show good accuracy and resolution. By using different laser irradiances, we expect to be able to build components with even higher resolution.
The authors wish to thank CRL Smectic Technologies Ltd. for supplying the SLM devices, and Mr. D.P. Randall for taking the SEM pictures. We gratefully acknowledge the EPSRC, Design and Integrated Production Program, for supporting this research.
[1] A. Bertsch, J.Y. JeÂzeÂquel, J.C. AndreÂ, Study of the spatial resolution of a new 3D micro-fabrication process: the microstereophotolithography using a dynamic mask-generator technique, J. Photochem. Photobiol. A 107 (1±3) (1997) 275±281. [2] M. Pottenger, B. Eyre, E. Kruglick, G. Lin, MEMS: the maturing of a new technology, Solid State Technol. 40 (9) (1997) 89±95. [3] M. Farsari, S. Huang, R.C.D. Young, M.I. Heywood, P.J.B. Morrell, C.R. Chatwin, Holographic characterisation of epoxy resins at 351.1 nm, Opt. Eng. 37 (10) (1998) 2754±2759. [4] P.F. Jacobs, Rapid Prototyping and Manufacturing: Fundamentals of Stereolithography, The Society of Manufacturing Engineers, Dearborn, MI, 1992 (ISBN 0-87263-425-6).
A novel high-accuracy microstereolithography method employing an adaptive electro-optic mask M. Farsari*, F. Claret-Tournier, S. Huang, C.R. Chatwin, D.M. Budgett, P.M. Birch, R.C.D. Young, J.D. Richardson University of Sussex, School of Engineering, Laser and Photonics Systems Group, Falmer, Brighton BN1 9QT, UK
Abstract A new stereo-photolithography technique to create three-dimensional micro-components using a planar, layer-by-layer, process of exposure has been developed. With this procedure, it is possible to build components with dimensions in the range 50 mm±50 mm, and feature sizes as small as 5 mm with a resolution of less than 1 mm. This newly developed system consists of eight elements: a Computer Aided Design environment supporting solid or surface modelling; a resin bath with an integrated high resolution translation stage and component built platform; an ultraviolet laser light source operating at 351.1 nm; an optical shutter; a diffractive optical element designed to re-distribute the irradiance of the laser beam from a Gaussian to a top-hat pro®le; a polysilicon thin ®lm twisted nematic SVGA resolution (800 600) spatial light modulator; a multi-element lithographic reduction lens system; and a comprehensive control system. In this paper, the experimental set-up is described and examples of the microcomponents fabricated by the system are shown. # 2000 Published by Elsevier Science B.V. Keywords: Microstereolithography; Rapid prototyping; Spatial light modulators; Diffractive optics
1. Introduction Commercially available stereolithography systems use a focused scanning laser beam to partially solidify components, layer-by-layer, into a honeycomb structure. Components are then fully cured in a post-curing process. In these systems the maximum component resolution is approximately 0.1 mm, which is a limitation imposed by using a focused laser beam. Efforts have been made to increase the resolution by either using a layer-by-layer process with a liquid crystal display as a mask [1], or by silicon microfabrication [2]. However, the former method only operates at visible wavelengths, where there are no stereolithographic materials commercially available; with silicon processing there are constraints on the height of components (third dimension). The technique described in this paper is a new rapid prototyping process capable of fabricating micro-components for micro-sensors, micro-actuators, micro-robot grippers, micro-robots, micro-motors, micro-drives, microconnectors, micro-optical components, packaging and
* Corresponding author. E-mail address: [email protected] (M. Farsari).
0924-0136/00/$ ± see front matter # 2000 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 0 ) 0 0 6 7 2 - 5
inter-connection technologies; in the medical ®eld there are applications involving the manufacture of implants and prostheses. The system operates in the UV (351.1 nm) and utilises a spatial light modulator to create 3D components using a completely planar (layer-by-layer) process of exposure. This is a major advance over alternative methods because layers are now concurrently cured over the entire surface as opposed to incrementally building the layer itself. Furthermore, it takes advantage of widely available commercial UV-curable resins designed for conventional stereolithography. 2. Experimental set-up The prototype system is shown in Fig. 1. The principal physical components of the system consist of: an ultraviolet laser light source; an optical shutter; a spatial light modulator; a multi-element lithographic lens system; a highresolution translation stage. The spatial light modulator is the critical interface between electronic and optical systems. 2D images are generated by level slicing a 3D solid or surface model of the required component (DUCTTM surface modeller with TRIFIXTM and additional component slicing software sup-
168
M. Farsari et al. / Journal of Materials Processing Technology 107 (2000) 167±172
Fig. 1. Experimental set-up.
plied by DELCAM International plc are used in this case); the slices are made at uniform increments along the chosen axis. Each slice is then converted to a bitmap format and loaded onto the spatial light modulator which acts as a dynamic lithographic mask, modulating the top-hat UV laser beam with the level slice image. The multi-element lens reduces the image by 10 or 20 times and re-images it to the plane of the resin surface. Formation of the component takes place in the resin bath, the photopolymer resin and incident light being selected such that incident light produces cationic polymerisation and initiates the selective curing of regions of the resin. The shutter, placed in the beam path, controls the duration of the exposure. Once curing of a layer is complete, the component is lowered further into the bath such that a new layer of resin ¯ows over the surface that was last cured. It is then raised to the correct position just below the surface so that a thin liquid layer covers the component. The process then repeats over the same sequence of operations for the next layer until the component is completed. 3. Optical systems The experimental optical system is divided into two sections: beam conditioning and spatial light modulator. 3.1. Beam conditioning optics The light source for the miniature rapid prototyping experiment is an argon ion laser operating at 351.1 nm.
The nominally Gaussian irradiance distribution is reshaped to a rectangular uniform irradiance in order to obtain a uniform beam energy distribution within the subsequent optical components and maximise the contrast ratio of the spatial light modulator. The diffractive optical element used is a Digital Optics Corporation Gaussian to square beam converter. This is an eight-level device designed for a wavelength of 351.1 nm. In practice, the argon ion laser gives a poor approximation to a Gaussian which results in the diffractive optical element not giving a ¯at irradiance pattern. The method is, however, considerably more energy ef®cient than sampling the central section of a Gaussian beam with an aperture. For example, if the central section of a Gaussian beam is sampled such that the intensity variation was 5% across the aperture, only 5% of the beam power can used. The low quality of the beam produced by the diffractive optic can be improved by the incorporation of a rotating holographic diffuser. This also destroys spatial coherence and reduces speckle in the image plane of the reducing lens. The disadvantage of this arrangement is that the beam is no longer collimated and it becomes depolarised. The polarisation is linearised before the beam enters the spatial light modulator by the addition of a linear polariser sheet after the ground glass screen. 3.2. Spatial light modulator The device was supplied by CRL Smectic Technologies, with the normal input or output polarisers removed. It has Super VGA resolution (800 600) with a pixel size of
M. Farsari et al. / Journal of Materials Processing Technology 107 (2000) 167±172
26 mm 24 mm. The total active area of the device is 26:4 mm 19:8 mm with a pixel ®ll factor of 50%. The contrast ratio of the device is quoted as 200:1 with a grey scale modulation of at least four levels. A Coreco Ultra-II frame grabber board with dual SVGA video output is used to drive the SLM. The image to be displayed on the device is written to the frame buffer of a Coreco card with the rate of 60 frames per second; however, the liquid crystal material will limit the actual speed of the spatial light modulator. The rise time of the liquid crystal is 7 ms but the fall time is 25 ms (40 Hz). The dead space between the pixels in both devices is opaque, and although this produces light loss from diffraction into higher orders the optical system is designed such that only the zeroth order is sampled. The optical ¯atness of the spatial light modulator is not of importance since the input laser beam has its spatial coherence removed with the rotating diffuser before entering the spatial light modulator. A cubic UV polarising beam-splitter is used as the output analyser enabling the spatial light modulator to act as an intensity modulator. The CRL device is not damaged by wavelengths longer than 350 nm. However, shorter wavelengths will cause damage to the spatial light modulator's indium tin oxide electrodes and the liquid crystal material. 4. System requirements The principal components of the system and application sequencing are identi®ed above and in Fig. 1. The subsystems and their performance speci®cations are summarised in Table 1. The control system manipulates image data and control signals in real-time. The translation stage is a key component used for dipping the target part into the resin for re-coating, and returning the part to a pre-determined distance below the liquid resin surface. A PCI interface card is used to control the translation stage. The objective of the spatial light modulator interface is to send a speci®c sequence of level-slice data (represented by a raster bitmap), as de®ned by the CAD representation of the component, to the spatial light modulator; this displays the data as a mask which is illuminated and imaged onto the resin bath surface to build a layer of the micro-component. The spatial light modulator is driven by a standard SVGA
169
signal. The host computer has dual SVGA outputs to support concurrent access to a regular computer monitor and the spatial light modulator. Image acquisition is used to provide a path for visual closed loop control of the curing process. The mask supplied to the spatial light modulator may be dynamically altered to correct partial curing of the present layer before applying the mask for the next layer. A video frame grabber is used to capture images from a CCIR format CCD camera. Further IO control for accessories such as shutters was provided using a combination of RS-232 and digital TTL lines. The operating system was Windows NT and microstereolithography system software was developed in LabVIEW#. This provided high level support for the variety of I/O interfaces utilised. 5. Component built In order to decide which stereolithographic resin to use, holographic measurements were performed on a series of materials [3]. The stereolithographic resin chosen is the Ciba-Geigy CibatoolTM SL 5180, the main reason being its viscosity, which is lower than the other materials investigated. Figs. 2±6 show SEM photographs of several components that have been built. These components consist of 45±105 layers, each one clearly visible in the SEM photograph. The component in Fig. 2 is a micro-gear and it is made up of 50 layers, 50 mm thick each. It consists of a hollow cylindrical bearing, which is attached to the outer truncated cone by four spokes. The component in Fig. 3 is a free-standing double helix, and it consists of 105 layers, 50 mm thick each. Fig. 4 shows a micro-pyramid, which consists of a base of 20 layers, 50 mm thick each, and the pyramid structure, which consists of 31 layers, 35 mm thick each. Fig. 5 shows a fourpyramid structure, consisting of 45 layers including the base, each 50 mm thick. Finally, Fig. 6 shows a micro-wheel which is made of 5 mm thick layers. The laser irradiance in the material follows the Beer± Lambert's law [4], which states that in depth z the laser irradiance E(z) is E z E0 exp ÿz=Dp
(1)
Table 1 Interface and performance speci®cation Component
Interface
Requirements
Supplier
Opto-electronic mask (SLM) Translation stage
Binary/grey level video interface PCI card
Coreco Coherent-Ealing Electro Optics
Optical shutter
RS 232
Digital dial test indicator
Custom serial interface
600 800 (SVGA) 20 nm resolution, 5 mm step size, 0.1 mm repeatability 2 ms minute exposure time, delay time 1 ms, repetition rate 0±400 Hz 1 mm resolution
Uniblitz Mitutoyo
170
M. Farsari et al. / Journal of Materials Processing Technology 107 (2000) 167±172
Fig. 2. A micro-gear.
where E0 resin and The laser calibrated
is the laser irradiance on the surface of the Dp is the penetration depth of the material. irradiance used to polymerise each layer was so that it would cure only the liquid layer of
the resin over the already built component. That is to say, if the component built consists of 50 mm layers at a depth of 50 mm, the laser irradiance should equal the critical energy for the material Ec. In the case of Ciba-Geigy CibatoolTM SL
Fig. 3. A double helix.
M. Farsari et al. / Journal of Materials Processing Technology 107 (2000) 167±172
Fig. 4. A micro-pyramid.
Fig. 5. A four micro-pyramid structure.
171
172
M. Farsari et al. / Journal of Materials Processing Technology 107 (2000) 167±172
Fig. 6. A micro-wheel.
5180, Dp 0:132 nm and Ec 16:2 mJ=cm2 , so E0 is calculated as E0 23:66 mJ=cm2 . Since the manufacturer's values are approximate and vary according to the stereolithography method used, we measured the E z 50 mm experimentally. This was done by building several layers and incrementally lowering the laser irradiance until the top cured layer did not adhere to the previous layer.
Acknowledgements
6. Conclusion
References
In summary, we have developed a new method for microstereolithography which operates in the UV and uses a spatial light modulator as a dynamic lithographic mask to manufacture micro-parts which are complex in shape and could not be manufactured with either micro-component silicon processing methods or conventional stereolithography. The ®rst components built using this method show good accuracy and resolution. By using different laser irradiances, we expect to be able to build components with even higher resolution.
The authors wish to thank CRL Smectic Technologies Ltd. for supplying the SLM devices, and Mr. D.P. Randall for taking the SEM pictures. We gratefully acknowledge the EPSRC, Design and Integrated Production Program, for supporting this research.
[1] A. Bertsch, J.Y. JeÂzeÂquel, J.C. AndreÂ, Study of the spatial resolution of a new 3D micro-fabrication process: the microstereophotolithography using a dynamic mask-generator technique, J. Photochem. Photobiol. A 107 (1±3) (1997) 275±281. [2] M. Pottenger, B. Eyre, E. Kruglick, G. Lin, MEMS: the maturing of a new technology, Solid State Technol. 40 (9) (1997) 89±95. [3] M. Farsari, S. Huang, R.C.D. Young, M.I. Heywood, P.J.B. Morrell, C.R. Chatwin, Holographic characterisation of epoxy resins at 351.1 nm, Opt. Eng. 37 (10) (1998) 2754±2759. [4] P.F. Jacobs, Rapid Prototyping and Manufacturing: Fundamentals of Stereolithography, The Society of Manufacturing Engineers, Dearborn, MI, 1992 (ISBN 0-87263-425-6).