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Multi-level exposures and 3-D X-ray patterning for high-aspect ratio microstructures
MICROELECTRONIC ENGINEERING ELSEVIER
Microelectronic Engineering 41/42 (1998) 493-496
Multi-Level Exposures and 3-D X-ray Patterning for High-Aspect Ratio Microstructures C. Khan-Malek" , R. W o o d b, Z. Ling', B. Dudleyb, and S. StadleP ": Center for Advanced Microstructures and Devices, Louisiana State University, 3990 West Lakeshore Drive, Baton Rouge, LA 70803. b: MCNC, 3031 Cornwallis Rd., RTP, NC 27709-288. An alignment system has been developed to address the special needs for deep X-ray lithography (DXRL). The system is based on optical registration method of mask and wafer prior to insertion into the Xray beam. A simple fixture allows for translation of the mask and rotation of the wafer. The fabrication of masks with optically transparent windows specially developed for that purpose at MCNC, a description of the fixture, and the alignment tests at CAMD will be reported. Results demonstrated +/- 5 micron alignment tolerances. The fixture also allows exposure of mask/wafer systems tilted with respect to the X-ray beam for fabrication of multi-level devices with tapered walls. First tests of dynamic X-ray exposures have been performed using rotation of a resist sample in the X-ray beam during exposure using a modified microlathe.
1.
INTRODUCTION
Requirements for today's MEMS applications include extended X-ray lithographic capabilities to produce more complex 3D geometries with inclined walls as well as multi-level systems where two or more levels must be aligned with respect to each other. Typical alignment tolerances for LIGA applications are currently on the order of 5 p.m. Though this is not particularly aggressive in modern IC terms, the relatively large resist thickness involved in LIGA prevent a straightforward implementation of alignment strategies designed for IC production. Conventional pattern transfer methods with mask and wafer positions fixed with respect to each other during exposure are limited to printing straight walls. Using non-standard concepts (dynamic exposure instead of static exposure) (1,2), more complex shapes can be achieved. A simple and modular fixture has been developed to expand the lithographic.capability of X-ray micromachining to 3D including alignment and variable shaping in the third dimension with oblique irradiation. The alignment is based on optical registration, necessitating the development of X-ray masks with optically transparent windows. First tests of dynamic X-ray exposures have also been performed rotating a resist sample in the X-ray beam. 0167-9317/98/$19.00 © Elsevier Science B.V All fights reserved. Plh S0167-9317(98)00115-4
2. MULTI-LEVELEXPOSURE 2.1 Motivation The HI-MEMS Alliance is an integrated US effort funded by (D)ARPA for 2 years (94-96). It was chartered to establish a base and methods for cost-effective, low-to-mid volume batch fabrication of primary parts for high-aspect-ratio MEMS (Micro-Electro- Mechanical Systems) manufacturing with LIGA (3,4). The Alliance has established infrastructure and networking to support prototyping and distributed manufacturing. CAMD functions as the "X-ray print-shop" and test-bed for DXRL for the Alliance, providing routine operation for exposure of thick resist. MCNC is the front-end and mask supplier for the Alliance. Among the prototypes which have been fabricated in the Alliance, one can list: • a magnetic planar mini-motor for a disk drive by IBM (5). This project, the most complex of the Alliance, included 5 lithographic levels with a mix-and match of optical and X-ray lithography (3 levels patterned by X-rays). It has necessitated successive planarization and alignment from one level to the next. • a micro-injection system for a drug delivery system (6), which includes a micro-valve formed by coupling a silicon diaphragm with a valve seat and plunger made by LIGA. The innovation in
494
C. Khan-Malek et aL / Microelectronic Engineering 41/42 (1998) 493-496
this approach consisted in using the capability of forming walls with any angle using tilted exposures. A combination of rotation and tilt was used to produce tapered walls with an angle close to but not matching the angle imposed by the crystalline orientation of silicon crystal.
2.2 Alignment strategy The system described here relies on external alignment of mask and wafer prior to insertion into the X-ray beam. It is based on optical registration of targets on an X-ray mask with optically transparent windows with respect to alignment marks on the wafer. A simple alignment fixture allowing rotation and translation of the mask with respect to the wafer has been developed. During the first irradiation step, the first level with the alignment marks is transferred into the resist. In most cases, the resist is developed and the open areas are filled in an electroplating step. The final sample surface consisting of remaining resist and metal often requires planarization before resist application for the second level to correct for uneven height distribution and roughness caused by the electroplating process. After planarization and application of the second resist layer, the mask is positioned on top of the wafer, at a given proximity distance fixed by spacers. The fixture (Fig. 1) which consists of a flex-frame on top of a fixed plate provides f'me positioning (translation X and Y, rotation 0) of the mask with respect to the wafer while viewing transparent alignment windows on the X- ray mask under a microscope with a long focal length. Adjustment screws are used to deform the fixture frame and convey the mask to the desired location. The frame also secures the mask vertically against the proximity shims in contact with the wafer. Once aligned, the mask and wafer are secured in their position and inserted into the Xray scanner. DXRL presents several challenges to patterning of aligned layers. The first is the fact that silicon membranes suitable for fabrication of X-ray masking patterns are only translucent to visible light, preventing direct through-the-mask optical alignment. This limitation was dealt with through the use of smaller "outrigger" windows which contain the alignment fiducials on optically transparent membranes which are much smaller
(a)
(b)
Fig. 1: Modular fixture for multi-exposure capability: a): alignmenffrotation fixture, b): tilting module (side view). than the active pattern area. Secondly, the PMMA layer acting as the X-ray resist is typically 50 to several hundred microns thick, and is conventionally machined (fly-cut) to achieve an accurate final height. This can create surface structure which prevents direct observation of any alignment targets beneath the PMMA layer. These targets must therefore be placed outside the region to be patterned by the second and succeeding Xray layers. Finally, DXRL is a proximity printing technique requiring a gap between the mask and the resist. This is dictated by the fragile nature of the X-ray mask which must be protected from contacting the resist surface. Proximity gaps of one to several hundred microns are typical. This gap, plus the height of the resist above the alignment marks can amount to hundreds of microns. Obtaining sufficient depth of field to view both alignment targets on the wafer and alignment fiducials on the mask requires trade-off of optical resolution, hence alignment accuracy. In our applications, the largest useful objective numerical aperture is about 0.06 when PMMA height of 300 ~tm and proximity gap of 200 Ixm are used. When performing manual alignments through a microscope, one must consider the vernier acuity of the human eye, being on the order of 1 to 10 seconds of arc (7). Assuming a visual acuity of 10 seconds of arc, a virtual viewing distance of 25 cm, and numerical aperture of 0.06, one can compute a resolution limit of about 0.6 ~tm. Working at ten times this resolution limit would correspond to a resolution accuracy of about 6 txm. This value is indeed compatible with many DXRL applications requiring multiple levels of exposure.
C. Khan-Malek et al./Microelectronic Engineering 41/42 (1998) 493-496
2.3 Alignment Results The alignment accuracy of this approach was evaluated in the fabrication of the IBM integrated mini-motor (5), where a stator/via hole pattern (L2) overlays a conductor stripe pattern (L1) (Fig. 2). Fig. 3 shows a picture of the mask with the stator pole which is positioned on top of the first level of winding layer. The mask has 27 mm x 15 mm transparent outrigger membranes for alignment. The patterns on the mask and resist are observed simultaneously on the videoscope screen of a Nikon measuroscope and alignment targets on the mask and the corresponding targets on the wafer are overlaid.
Fig. 2: SEM of two aligned Fig. 3: Mask with X-ray levels for a mini-motor, stator pole piece of The vias holes of the PMMA the mini-motor. stator template are aligned to the conductor level underneath. Wafers were analyzed for L2-to-L1 pattern misalignment after patterning of the L2 PMMA, by measuring offsets between the top of the L2 via with respect to its corresponding L1 conductor, at the four comers of the 4 cm by 4 cm exposure field. This represents one data point for each of the four motors in the mask layout. Measurement was performed on an Axiomat optical microscope with a calibrated reticule, and replicate measurements indicate a one-sigma measurement error no greater than 0.4 ~tm. Systematic offsets due to lack of microscope confocality were assumed to be negligible. Analysis of data was done so as to extract offsets with respect to center line misalignment, making the final offsets independent of image absolute size. Thus, mask and lithography bias do not contribute to the offsets. Alignment within +/- 5 ~tm is routinely achieved.
495
2.4 X-ray masks with optically transparent windows MCNC provides masks for the HI-MEMS Alliance. The standard and well characterized Xray mask fabrication process is based on the formation of 2-3 ~tm thick boron diffused silicon membranes (mono or multi-membrane type). LIGA designs requiring alignment have necessitated the fabrication of X-ray masks with optically transparent windows, i.e. allowing more than 50% transmission at 632 nm. Key to the process is the placement of two additional membranes or "outriggers" on the mask, positioned outside of the central X-ray exposure membrane. The fabrication sequence involved the photolithographic delineation of the two alignment outriggers, accomplished by a simple modification to the existing backside membrane chrome mask. After RIE of the backside silicon nitride layer, the wafers received the standard frontside fabrication sequence. This included the formation of the lithographically defined plating stencil, the electrochemical formation of the Au absorber structures, followed by the removal of both the photoresist plating stencil and the Cr/Au plating base. After the frontside processing was completed, the silicon was etched to a thickness of 2-3 ktm in heated aqueous KOH, utilizing the deep boron diffusion as the etch stop. Although optically opaque, this thickness of silicon produces structurally rigid and stable membranes. The Xray masks were then modified to incorporate the optically transparent alignment windows by placing the completed X-ray mask horizontally in a holder with the backside facing up. An isotropic silicon etch solution was placed in and confined to the alignment outriggers. After a few minutes, the silicon thickness was reduced, thereby rendering these regions optically transparent. 3.
COMPLEX PATTERN TRANSFER
3.1 Tilted exposure capability development Normally the surface of the sample is perpendicular to the direction of synchrotron radiation. The fixture developed here includes a rotation stage (Fig. l-a) for rotation of the mask/wafer assembly around its normal axis and can also be attached to a tilting module (Fig. l-b) which allows exposure under oblique angles with
496
C. Khan-Malek et al. / Microelectronic Engineering 41/42 (1998) 493-496
respect to the incident X-ray beam (allowing fabrication of devices with tapered walls). The rotation stage can be set at any angle between 0 and 360 deg. The combination of rotation and tilt is opening further varieties of elements such as mechanical dove-tail structures (Fig 4). The formation of oblique structures with several tapered sidewalls requires multiple exposures, adequate simulation of the dose deposition mechanism within the resist and proper control of the gap between mask and resist layer. In particular multiple exposures of the same areas can result in irradiation-induced damage in the resist as well as swelling detrimental to the mask.
early stages of development. Alignment and complex exposure strategies are under development and not yet standardized as in optical and X-ray lithographies. The requirements for LIGA products however become increasingly demanding for multilevel exposures and nonstandard pattern transfer exploiting the full capabilities of 3D "micro-structurization". In this phase, the technical solutions need to be flexible, rapidly available and also inexpensive. The system described here is an example for such a response driven by specific applications in the HIMEMS Alliance. Free 3D-patterning can be obtained by more elaborate exposure schemes involving movements of mask and resist samples in the X-ray beam, allowing complex 3-dimensional objects to be machined with sub-micron precision. However, this technology is still far from routine applications. 5. ACKNOWLEDGMENTS
Fig. 4: Structures with inclined walls (double exposure with tilt at 450 and rotation of 180°).
Fig. 5: PMMA rod rotated in a microlathe behind a grid during X-ray exposure.
3.2. Dynamic exposures Conventional pattern transfer methods allow for a wide variety of geometries with sloped walls but offer only limited capability for complete freedom of pattern formation such as samples with curved surfaces. Using non-standard concepts (dynamic exposure instead of static exposure) derived from conventional micromachining (2), complex shapes can be achieved using rotation/ translation of a resist sample in the X-ray beam or moving the mask during exposure. A micro-lathe for conventional micromachining work was modified and aligned with respect to the X-ray beam and a PMMA rod was rotated behind a grid mask (Fig. 5).
The authors would like to thank all coworkers and collaborators involved in the realization and success of this work. This work was supported by the State of Louisiana, and the Advanced Research Project Agency Program under Technology Reinvestment Project Development Agreement MDA972-94-3- 0043, REFERENCES I.
2.
3. 4.
http ://mems. mcnc. org/Hiclick, html
5.
6.
4. CONCLUSION LIGA-based micromachining is still in the
D.P. Siddons, E.D. Johnson, H. Guckel, Synchrotron Radiation News, Vol 7, No 2, 1994, 16. D. Feinerman, R. Lajos, V. White, and D. Denton, J. Microelectromechanical Systems, 5(4), 250 (1996). H. Guckel, Rev. Sci. Instrum. 67(9), 1 (1996). For the Alliance members, see
7.
E. O'Sullivan, E. Cooper, L. T. Romankiw, S. Krongelb, et al., HARMST '97, Madison, WI, June 20-21, 1997. T.W. Tsuei, R. Wood, C. Khan Malek, M. M. Donnelly, and R. 13. Fair, HARMST '97, Madison, WI, June 20-21, 1997. W. Smith, "Modem Optical Engineering", McGraw-Hill, 1966.
Microelectronic Engineering 41/42 (1998) 493-496
Multi-Level Exposures and 3-D X-ray Patterning for High-Aspect Ratio Microstructures C. Khan-Malek" , R. W o o d b, Z. Ling', B. Dudleyb, and S. StadleP ": Center for Advanced Microstructures and Devices, Louisiana State University, 3990 West Lakeshore Drive, Baton Rouge, LA 70803. b: MCNC, 3031 Cornwallis Rd., RTP, NC 27709-288. An alignment system has been developed to address the special needs for deep X-ray lithography (DXRL). The system is based on optical registration method of mask and wafer prior to insertion into the Xray beam. A simple fixture allows for translation of the mask and rotation of the wafer. The fabrication of masks with optically transparent windows specially developed for that purpose at MCNC, a description of the fixture, and the alignment tests at CAMD will be reported. Results demonstrated +/- 5 micron alignment tolerances. The fixture also allows exposure of mask/wafer systems tilted with respect to the X-ray beam for fabrication of multi-level devices with tapered walls. First tests of dynamic X-ray exposures have been performed using rotation of a resist sample in the X-ray beam during exposure using a modified microlathe.
1.
INTRODUCTION
Requirements for today's MEMS applications include extended X-ray lithographic capabilities to produce more complex 3D geometries with inclined walls as well as multi-level systems where two or more levels must be aligned with respect to each other. Typical alignment tolerances for LIGA applications are currently on the order of 5 p.m. Though this is not particularly aggressive in modern IC terms, the relatively large resist thickness involved in LIGA prevent a straightforward implementation of alignment strategies designed for IC production. Conventional pattern transfer methods with mask and wafer positions fixed with respect to each other during exposure are limited to printing straight walls. Using non-standard concepts (dynamic exposure instead of static exposure) (1,2), more complex shapes can be achieved. A simple and modular fixture has been developed to expand the lithographic.capability of X-ray micromachining to 3D including alignment and variable shaping in the third dimension with oblique irradiation. The alignment is based on optical registration, necessitating the development of X-ray masks with optically transparent windows. First tests of dynamic X-ray exposures have also been performed rotating a resist sample in the X-ray beam. 0167-9317/98/$19.00 © Elsevier Science B.V All fights reserved. Plh S0167-9317(98)00115-4
2. MULTI-LEVELEXPOSURE 2.1 Motivation The HI-MEMS Alliance is an integrated US effort funded by (D)ARPA for 2 years (94-96). It was chartered to establish a base and methods for cost-effective, low-to-mid volume batch fabrication of primary parts for high-aspect-ratio MEMS (Micro-Electro- Mechanical Systems) manufacturing with LIGA (3,4). The Alliance has established infrastructure and networking to support prototyping and distributed manufacturing. CAMD functions as the "X-ray print-shop" and test-bed for DXRL for the Alliance, providing routine operation for exposure of thick resist. MCNC is the front-end and mask supplier for the Alliance. Among the prototypes which have been fabricated in the Alliance, one can list: • a magnetic planar mini-motor for a disk drive by IBM (5). This project, the most complex of the Alliance, included 5 lithographic levels with a mix-and match of optical and X-ray lithography (3 levels patterned by X-rays). It has necessitated successive planarization and alignment from one level to the next. • a micro-injection system for a drug delivery system (6), which includes a micro-valve formed by coupling a silicon diaphragm with a valve seat and plunger made by LIGA. The innovation in
494
C. Khan-Malek et aL / Microelectronic Engineering 41/42 (1998) 493-496
this approach consisted in using the capability of forming walls with any angle using tilted exposures. A combination of rotation and tilt was used to produce tapered walls with an angle close to but not matching the angle imposed by the crystalline orientation of silicon crystal.
2.2 Alignment strategy The system described here relies on external alignment of mask and wafer prior to insertion into the X-ray beam. It is based on optical registration of targets on an X-ray mask with optically transparent windows with respect to alignment marks on the wafer. A simple alignment fixture allowing rotation and translation of the mask with respect to the wafer has been developed. During the first irradiation step, the first level with the alignment marks is transferred into the resist. In most cases, the resist is developed and the open areas are filled in an electroplating step. The final sample surface consisting of remaining resist and metal often requires planarization before resist application for the second level to correct for uneven height distribution and roughness caused by the electroplating process. After planarization and application of the second resist layer, the mask is positioned on top of the wafer, at a given proximity distance fixed by spacers. The fixture (Fig. 1) which consists of a flex-frame on top of a fixed plate provides f'me positioning (translation X and Y, rotation 0) of the mask with respect to the wafer while viewing transparent alignment windows on the X- ray mask under a microscope with a long focal length. Adjustment screws are used to deform the fixture frame and convey the mask to the desired location. The frame also secures the mask vertically against the proximity shims in contact with the wafer. Once aligned, the mask and wafer are secured in their position and inserted into the Xray scanner. DXRL presents several challenges to patterning of aligned layers. The first is the fact that silicon membranes suitable for fabrication of X-ray masking patterns are only translucent to visible light, preventing direct through-the-mask optical alignment. This limitation was dealt with through the use of smaller "outrigger" windows which contain the alignment fiducials on optically transparent membranes which are much smaller
(a)
(b)
Fig. 1: Modular fixture for multi-exposure capability: a): alignmenffrotation fixture, b): tilting module (side view). than the active pattern area. Secondly, the PMMA layer acting as the X-ray resist is typically 50 to several hundred microns thick, and is conventionally machined (fly-cut) to achieve an accurate final height. This can create surface structure which prevents direct observation of any alignment targets beneath the PMMA layer. These targets must therefore be placed outside the region to be patterned by the second and succeeding Xray layers. Finally, DXRL is a proximity printing technique requiring a gap between the mask and the resist. This is dictated by the fragile nature of the X-ray mask which must be protected from contacting the resist surface. Proximity gaps of one to several hundred microns are typical. This gap, plus the height of the resist above the alignment marks can amount to hundreds of microns. Obtaining sufficient depth of field to view both alignment targets on the wafer and alignment fiducials on the mask requires trade-off of optical resolution, hence alignment accuracy. In our applications, the largest useful objective numerical aperture is about 0.06 when PMMA height of 300 ~tm and proximity gap of 200 Ixm are used. When performing manual alignments through a microscope, one must consider the vernier acuity of the human eye, being on the order of 1 to 10 seconds of arc (7). Assuming a visual acuity of 10 seconds of arc, a virtual viewing distance of 25 cm, and numerical aperture of 0.06, one can compute a resolution limit of about 0.6 ~tm. Working at ten times this resolution limit would correspond to a resolution accuracy of about 6 txm. This value is indeed compatible with many DXRL applications requiring multiple levels of exposure.
C. Khan-Malek et al./Microelectronic Engineering 41/42 (1998) 493-496
2.3 Alignment Results The alignment accuracy of this approach was evaluated in the fabrication of the IBM integrated mini-motor (5), where a stator/via hole pattern (L2) overlays a conductor stripe pattern (L1) (Fig. 2). Fig. 3 shows a picture of the mask with the stator pole which is positioned on top of the first level of winding layer. The mask has 27 mm x 15 mm transparent outrigger membranes for alignment. The patterns on the mask and resist are observed simultaneously on the videoscope screen of a Nikon measuroscope and alignment targets on the mask and the corresponding targets on the wafer are overlaid.
Fig. 2: SEM of two aligned Fig. 3: Mask with X-ray levels for a mini-motor, stator pole piece of The vias holes of the PMMA the mini-motor. stator template are aligned to the conductor level underneath. Wafers were analyzed for L2-to-L1 pattern misalignment after patterning of the L2 PMMA, by measuring offsets between the top of the L2 via with respect to its corresponding L1 conductor, at the four comers of the 4 cm by 4 cm exposure field. This represents one data point for each of the four motors in the mask layout. Measurement was performed on an Axiomat optical microscope with a calibrated reticule, and replicate measurements indicate a one-sigma measurement error no greater than 0.4 ~tm. Systematic offsets due to lack of microscope confocality were assumed to be negligible. Analysis of data was done so as to extract offsets with respect to center line misalignment, making the final offsets independent of image absolute size. Thus, mask and lithography bias do not contribute to the offsets. Alignment within +/- 5 ~tm is routinely achieved.
495
2.4 X-ray masks with optically transparent windows MCNC provides masks for the HI-MEMS Alliance. The standard and well characterized Xray mask fabrication process is based on the formation of 2-3 ~tm thick boron diffused silicon membranes (mono or multi-membrane type). LIGA designs requiring alignment have necessitated the fabrication of X-ray masks with optically transparent windows, i.e. allowing more than 50% transmission at 632 nm. Key to the process is the placement of two additional membranes or "outriggers" on the mask, positioned outside of the central X-ray exposure membrane. The fabrication sequence involved the photolithographic delineation of the two alignment outriggers, accomplished by a simple modification to the existing backside membrane chrome mask. After RIE of the backside silicon nitride layer, the wafers received the standard frontside fabrication sequence. This included the formation of the lithographically defined plating stencil, the electrochemical formation of the Au absorber structures, followed by the removal of both the photoresist plating stencil and the Cr/Au plating base. After the frontside processing was completed, the silicon was etched to a thickness of 2-3 ktm in heated aqueous KOH, utilizing the deep boron diffusion as the etch stop. Although optically opaque, this thickness of silicon produces structurally rigid and stable membranes. The Xray masks were then modified to incorporate the optically transparent alignment windows by placing the completed X-ray mask horizontally in a holder with the backside facing up. An isotropic silicon etch solution was placed in and confined to the alignment outriggers. After a few minutes, the silicon thickness was reduced, thereby rendering these regions optically transparent. 3.
COMPLEX PATTERN TRANSFER
3.1 Tilted exposure capability development Normally the surface of the sample is perpendicular to the direction of synchrotron radiation. The fixture developed here includes a rotation stage (Fig. l-a) for rotation of the mask/wafer assembly around its normal axis and can also be attached to a tilting module (Fig. l-b) which allows exposure under oblique angles with
496
C. Khan-Malek et al. / Microelectronic Engineering 41/42 (1998) 493-496
respect to the incident X-ray beam (allowing fabrication of devices with tapered walls). The rotation stage can be set at any angle between 0 and 360 deg. The combination of rotation and tilt is opening further varieties of elements such as mechanical dove-tail structures (Fig 4). The formation of oblique structures with several tapered sidewalls requires multiple exposures, adequate simulation of the dose deposition mechanism within the resist and proper control of the gap between mask and resist layer. In particular multiple exposures of the same areas can result in irradiation-induced damage in the resist as well as swelling detrimental to the mask.
early stages of development. Alignment and complex exposure strategies are under development and not yet standardized as in optical and X-ray lithographies. The requirements for LIGA products however become increasingly demanding for multilevel exposures and nonstandard pattern transfer exploiting the full capabilities of 3D "micro-structurization". In this phase, the technical solutions need to be flexible, rapidly available and also inexpensive. The system described here is an example for such a response driven by specific applications in the HIMEMS Alliance. Free 3D-patterning can be obtained by more elaborate exposure schemes involving movements of mask and resist samples in the X-ray beam, allowing complex 3-dimensional objects to be machined with sub-micron precision. However, this technology is still far from routine applications. 5. ACKNOWLEDGMENTS
Fig. 4: Structures with inclined walls (double exposure with tilt at 450 and rotation of 180°).
Fig. 5: PMMA rod rotated in a microlathe behind a grid during X-ray exposure.
3.2. Dynamic exposures Conventional pattern transfer methods allow for a wide variety of geometries with sloped walls but offer only limited capability for complete freedom of pattern formation such as samples with curved surfaces. Using non-standard concepts (dynamic exposure instead of static exposure) derived from conventional micromachining (2), complex shapes can be achieved using rotation/ translation of a resist sample in the X-ray beam or moving the mask during exposure. A micro-lathe for conventional micromachining work was modified and aligned with respect to the X-ray beam and a PMMA rod was rotated behind a grid mask (Fig. 5).
The authors would like to thank all coworkers and collaborators involved in the realization and success of this work. This work was supported by the State of Louisiana, and the Advanced Research Project Agency Program under Technology Reinvestment Project Development Agreement MDA972-94-3- 0043, REFERENCES I.
2.
3. 4.
http ://mems. mcnc. org/Hiclick, html
5.
6.
4. CONCLUSION LIGA-based micromachining is still in the
D.P. Siddons, E.D. Johnson, H. Guckel, Synchrotron Radiation News, Vol 7, No 2, 1994, 16. D. Feinerman, R. Lajos, V. White, and D. Denton, J. Microelectromechanical Systems, 5(4), 250 (1996). H. Guckel, Rev. Sci. Instrum. 67(9), 1 (1996). For the Alliance members, see
7.
E. O'Sullivan, E. Cooper, L. T. Romankiw, S. Krongelb, et al., HARMST '97, Madison, WI, June 20-21, 1997. T.W. Tsuei, R. Wood, C. Khan Malek, M. M. Donnelly, and R. 13. Fair, HARMST '97, Madison, WI, June 20-21, 1997. W. Smith, "Modem Optical Engineering", McGraw-Hill, 1966.