- Email: [email protected]
Planar very high aspect ratio microstructures for large loading forces
MICROELECTRONIC ENGINEERING
ELSEVIER
Microelectronic Engineering 30 (1996) 527-530
P l a n a r V e r y H i g h A s p e c t Ratio M i c r o s t r u c t u r e s for L a r g e L o a d i n g F o r c e s A. Jazairy and N.C. MacDonald School of Electrical Engineering and the National Nanofabrication Facility Cornell University, Ithaca NY 14853-5401 USA We report on a novel technology called Scream for High Aspect Ratio Proportions (SHARP) which adds a new dimension to the Single Crystal Reactive Etching And Metallization (SCREAM) process. The fabrication technology increases the height and capacitive area of mechanical beams to improve the planarity and further extend the loading capabilities of very deep actuator structures. The process is based on a repeated sequence of thermal oxidations and reactive ion etches (RIE). Part of the technique consists of oxidizing through single crystal silicon (SCS) structural beams of a device by thermally growing a thick silicon dioxide (SIO2) layer (H2/O2 precursors) after the first trench RIE etch, thus yielding an ultra-thick (>30 gin) SiO2 etch mask for further deep vertical etching (>100 gm) to form high aspect ratio structures. 1. Introduction Recent trends in mesoscopic physics and dynamics are leading to the development of high aspect ratio microelectromechanical systems (MEMS) and microanalytical instruments capable of generating milli Newton-range forces quasistatically and several 10 ~tm displacements. A new micro loading device that can apply compressive or tensile forces of 1.5 mN at 50 V has been fabricated and tested [1]. For these large scale micromechanical structures, buckling due to intrinsic stresses becomes an important issue. Typical designs for actuators include parallel and perpendicular "comb drive" capacitive structures. Parallel capacitive structures are usually preferred for the generation of large forces and displacements. Figure 1 shows a typical parallel capacitive comb drive structure with its translational motion being parallel to the finger orientation. On the SEM, the released fingers (light-colored) are interdigitated with the fixed fingers (dark-colored) and their vertical dimension is about 10 ~tm. Large MEMS are usually composed of numerous actuators since their capacitive force increases linearly with the number of fingers that form the individual capacitor plates of the comb drives. Therefore, high structural planarity in order to maximize the capacitance area is one of the key parameters in the successful performance of these large MEMS. In this work, we report on a novel fabrication technology which increases the structural height of actuator devices to improve their planarity and further extend their loading capabilities. The total capacitive force generated
between the comb drive plates is proportional to Nh/d, where N is the total number of fingers, h is the height and d is the separation between the fingers. In the standard Single Crystal Reactive Etching And Metallization (SCREAM) process [2,3], sputtered aluminum is used as the main electrical conduction medium for bulkmicromachined structures with vertical heights of 10-20 ~tm. However, conformal and uniform coating for very deep (>50 gm) structures with large h/d ratios is difficult. A typical isolation scheme that can be used for the fabrication of very deep actuator structures is to design oxide isolating segments to separate the biased comb drives from the grounded parts. Thus, the single crystal silicon (SCS) core (heavily doped or with low resistivity -0.001 ~ - c m ) serves as the main electrical conduction medium instead of the metal.
Figure 1. SEM micrograph of capacitive comb drive structure with fingers parallel to the motion.
0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. All fights reserved. SSD I 0167-9317(95)00301-0
A. Jazairy, N.C. MacDonald / Microelectronic Engineeling 30 (1996) 527-530
528
SiO2 mask (i) m n ! Si Substrate Photolithographyand pattemtransferinto SiO2
H !
n
(ii)
M
~
n
~
SiO2mask
Silicon
_9~i
n
SiliconC12RIE #1
The si°2
!
ThermalSiO2 i
!
Thermaloxidation#2 for oxidizedthinbeams
Thesio2
!
Thermaloxidation#1 for totallyoxidizedbeams (iv)
~
Thermal SiO2
~
n
Silicon (vii)
n
Float SiO2 removalfor t hickmask formation
I
!
Floor SiO2 removal
Silicon Silicon (viii) !
!
Deep SiliconC12RIE #2
SiliconSF6releaseetch
!
Figure 2. One version of SHARP for the fabrication of high aspect ratio released actuators. The outer cross sections represent released comb fingers and frame structures, and the inner cross section depicts an oxide isolating segment between the two sets of suspended comb drives. The outer cross sections in step (viii) show typical top, sidewall and overhang thermal oxide film thicknesses on the SCS beams. 2. F a b r i c a t i o n
We have developed a novel technology called Scream for High Aspect Ratio Proportions (SHARP) [4,5] which adds a new dimension to the SCREAM process. The technique uses a repeated sequence of thermal oxidations and reactive ion etchings (RIE). It is a single-mask, self-aligned process which allows the formation of releasable three-dimensional (3D) objects of large width with vertical dimensions exceeding 100 ~tm. In Figure 2, steps (i)-(viii) illustrate, in diagrammatic form, the fabrication of very high aspect ratio released actuators and frame structures ~r large force actuation. The starting substrate is a boron doped, p-type (100) SCS substrate which is coated with a 2 to 5 Bm-thick oxide mask. Such a thick initial masking layer is used for deep reactive ion etching (RIE). Thereafter, a first etch mask can be formed by lithographically patterning a vertical and smooth resist layer. The photoresist pattern is then
transferred to the underlying SiO2 masking layer by reactive ion etching in CHF 3 plasma to yield vertical sidewalls with large wafer uniformity [Fig. 2(i)]. After stripping the resist, the pattern in the SiO2 is transferred to the silicon substrate via the first trench PIE using a C12/BC13 plasma [6] resulting in an etch depth of 20-30 I.tm [Fig. 2(ii)]. Thereafter, a thick thermal silicon dioxide layer (H2£) 2 precursors) is grown to oxidize through all the structural beams of the device [Fig. 2(iii)]. The floor silicon dioxide is removed using another CHF3 RIE process, thus forming a vertical 20-30 I.tm-thick second etch mask for trench RIE #2 [Fig. 2(iv)]. This oxide thickness will form the initial top oxide for our device beams. It would be practically impossible to deposit the same amount on a wafer. A second trench RIE step forms sidewalls exceeding 100 gm in depth in accordance with the desired vertical dimensions [Fig. 2(v)]. Thereafter, another thermal SiO2 is grown to completely oxidize through thin oxide
529
A. Jazairy, N.C. MacDonald / Microelectronic Engineering 30 (1996) 527-530
isolating segments [Fig. 2(vi)]. This thin layer of thermal oxide film is also used for sidewall passivation during the subsequent release etch. Finally, the floor silicon dioxide is removed with a RIE using CHF 3 plasma [Fig. 2(vii)I, and the entire structure can be released off the wafer by using an isotropic SF 6 RIE to undercut the high aspect ratio beams [Fig. 2(viii)]. Figure 3a shows a cross section of a smooth structure with vertical depth h-100 [tm and anisotropy (or verticality) A~0.99. Figure 3b is a cross section of the same device, comparing the etch depth with the entire wafer thickness.
(a)
(b)
Figure 3. a) SEM micrograph of a 100 ktm-deep vertical and smooth frame structure, b) SEM micrograph of the same structure showing the entire wafer thickness (-375 ktm). Such deep micromachined structures have been proposed for micro-optics applications [7]. High energy synchrotron sources can also be used to generate very deep structures as in the LIGA technique [8-10]. Other methods include cryogenic dry etching at very low temperatures [11] and electroplating of moderate aspect ratio resist profiles [12]. SHARP is a simple, low-cost technique for the generation of very deep structures. It comprises a new technology for: (i) forming ultra-thick (>30 ktm in depth) etch masks using thermal oxidation; (ii) fabricating very high aspect ratio structures with, to our knowledge, record etch depths (up to 160 ].tm) using c o n v e n t i o n a l R/E systems; (iii) obtaining very smooth sidewalls and structural surfaces by repeated thermal oxidation steps;
(iv) controlling lateral dimensions during the fabrication steps for the formation of threedimensional objects of large width; (v) achieving high out-of-plane stiffness for large force actuation and integration of micromechanical with on-chip circuitry; (vi) allowing lift and displacement of entire devices off the wafer. 3. Structural Performance
A systematic theoretical study has determined the effect of intrinsic stress that develops in mechanical beams due to top, sidewall and overhang thin film coatings [13,14]. A careful control of the effect of stresses has generated structures from high planarity to large vertical deflections [15,16]. For these large scale micromechanical structures, buckling due to intrinsic stresses becomes an important issue. As the structural single crystal silicon (SCS) core height is increased, the overall curvature of the device decreases. Structures become planar for heights larger than 50 btm. Figure 4 shows a side view of a highly planar released 85 gm-deep cantilever suspended on a frame structure. The sidewall and overhang oxide thicknesses on the SCS beams for this device are 0.9 Ixm and 15 Ixm, respectively.
~
--
i
~s~:~
,
IIII
Figure 4. SEM micrograph of a highly planar released 85 ktm-deep cantilever on-a-frame. The large vertical depth provides a high out-ofplane stiffness for the structure [17]. Similarly, comb drive structures with planar and large surface area cantilevered (finger) beams can be designed to generate large actuation forces.
530
A. Jazairy, N.C. MacDonaM / Microelectronic Engineering 30 (1996) 527-530
Experimentally, we verify that a released frame structure has a large positive curvature for a height of 15 btm as shown in Figure 5a, while it becomes planar with a height of 70 btm as shown in Figure 5b. In general, sidewall and overhang oxide films tend to generate positive curvature (with upward bending) while top oxide films tend to generate negative curvature (with downward bending). For shallow structural beams (<50 ~tm), the competing effects of thin oxide films can cause large deflections in either directions. For deeper structural beams, the effects of top, sidewall and overhang oxides are still relevant but with low influence on bending. For example, if the curvature is 1/pl for a beam with 10 btm deep SCS, and l/p2 for a similar beam with 50 ~tm deep SCS, then l/p2 can be an order of magnitude smaller than 1/p 1.
(a)
(b)
Figure 5. a) SEM micrograph of a 15 ~tm-deep released bowed frame structure (scale bar: 50/.tm), b) SEM micrograph of a 70 ~tm-deep released planar frame structure (scale bar: 100 ~tm).
4. Summary A novel low-cost bulk micr0machiningbased technology called SHARP has been developed and used for the fabrication of very high aspect ratio SCS micromechanical actuators and frame structures with vertical dimensions exceeding 100 btm. SHARP has generated, to our knowledge, :record depths (up to 160 ktm) using conventional RIE systems. Experimental results have indicated that a large structural depth improves both the planarity and the loading force of micromechanical devices.
Acknowledgments The fabrication process was developed at the National Nanofabrication Facility, which is supported by the National Science Foundation under Grant No. ECS-9319005. The authors
would like to acknowledge funding from the Advanced Research Projects Agency in a project monitored by the Optoelectronics Technology Center under Contract No. OTC-II-MDA972-941-0002. One of the authors (A.J.) would like to thank Dr. A. Faria for fruitful discussions.
References [1]M. T. A. Saif and N. C. MacDonald, IEEE Technical Digest, 8th International Conference on Solid-State Sensors and Actuators (Transducers '95), vol. 2, pp. 60-63, Stockholm, June 25-29, 1995. [2]Z. L. Zhang and N. C. MacDonald, J. Micromech. Microeng. 2, 1, 31, (1992). [3]Z. L. Zhang and N. C. MacDonald, J. Vac. Sci. Technol. BU, 2538, (1993). [4]N. C. MacDonald and A. Jazairy, SPIE 2383, 125, (1995). [5]A. Jazairy and N. C. MacDonald, J. Vac. Sci. Technol. (submitted), (1996). [6]J. J. Yao, S. C. Arney and N. C. MacDonald, J. of Microelectromech. Sys. 1, 14, (1992). [7]A. Jazairy and N. C. MacDonald, paper WH4, presented at the Optical Society of America AnnuaJ Meeting, Portland, OR, Sept. 10-15, 1995. [8]H. Guckel, K. J. Skrobis, J. Klein, T. R. Christenson and T. Wiegele, SPIE 2045, 290, (1994). [9]W. Ehrfeld, P. Bley, F. G6tz, J. Mohr, D. Miinchmeyer, W. Schelb, H. J. Baving and D. Beets, J. Vac. Sci. Technol. B6 (1), 178, (1988). [10]W. Ehrfeld and D. Miinchmeyer, Nucl. Instrum. Methods Phys. Res. A303, 523, (1991). [ll]M. Esashi, Microsystem Technologies 1, 2, (1994). [12]G. Engelmann, O. Ehrmann, R. Leutenbauer, H. Schmitz and H. Reichl, SPIE 2045, 306, (1994). [13]M. T. A. Saif and N. C. MacDonald, SPIE 2441, 329, (1995). [14]M. T. A. Saif and N. C. MacDonald, SPIE 2448, 93, (1995). [15]W. Fang and J. A. Wickert, J. Micromech. Microeng. 4, 116, (1994). [16]B. P. van Driernhuizen, J. F. L. Goosen, P. J. French and R. F. Wolffenbuttel, Sensors and Actuators A37-38,756, (1993). [17]A. Jazairy and N. C. MacDonald, SPIE 2640 (to appear), (1995).
ELSEVIER
Microelectronic Engineering 30 (1996) 527-530
P l a n a r V e r y H i g h A s p e c t Ratio M i c r o s t r u c t u r e s for L a r g e L o a d i n g F o r c e s A. Jazairy and N.C. MacDonald School of Electrical Engineering and the National Nanofabrication Facility Cornell University, Ithaca NY 14853-5401 USA We report on a novel technology called Scream for High Aspect Ratio Proportions (SHARP) which adds a new dimension to the Single Crystal Reactive Etching And Metallization (SCREAM) process. The fabrication technology increases the height and capacitive area of mechanical beams to improve the planarity and further extend the loading capabilities of very deep actuator structures. The process is based on a repeated sequence of thermal oxidations and reactive ion etches (RIE). Part of the technique consists of oxidizing through single crystal silicon (SCS) structural beams of a device by thermally growing a thick silicon dioxide (SIO2) layer (H2/O2 precursors) after the first trench RIE etch, thus yielding an ultra-thick (>30 gin) SiO2 etch mask for further deep vertical etching (>100 gm) to form high aspect ratio structures. 1. Introduction Recent trends in mesoscopic physics and dynamics are leading to the development of high aspect ratio microelectromechanical systems (MEMS) and microanalytical instruments capable of generating milli Newton-range forces quasistatically and several 10 ~tm displacements. A new micro loading device that can apply compressive or tensile forces of 1.5 mN at 50 V has been fabricated and tested [1]. For these large scale micromechanical structures, buckling due to intrinsic stresses becomes an important issue. Typical designs for actuators include parallel and perpendicular "comb drive" capacitive structures. Parallel capacitive structures are usually preferred for the generation of large forces and displacements. Figure 1 shows a typical parallel capacitive comb drive structure with its translational motion being parallel to the finger orientation. On the SEM, the released fingers (light-colored) are interdigitated with the fixed fingers (dark-colored) and their vertical dimension is about 10 ~tm. Large MEMS are usually composed of numerous actuators since their capacitive force increases linearly with the number of fingers that form the individual capacitor plates of the comb drives. Therefore, high structural planarity in order to maximize the capacitance area is one of the key parameters in the successful performance of these large MEMS. In this work, we report on a novel fabrication technology which increases the structural height of actuator devices to improve their planarity and further extend their loading capabilities. The total capacitive force generated
between the comb drive plates is proportional to Nh/d, where N is the total number of fingers, h is the height and d is the separation between the fingers. In the standard Single Crystal Reactive Etching And Metallization (SCREAM) process [2,3], sputtered aluminum is used as the main electrical conduction medium for bulkmicromachined structures with vertical heights of 10-20 ~tm. However, conformal and uniform coating for very deep (>50 gm) structures with large h/d ratios is difficult. A typical isolation scheme that can be used for the fabrication of very deep actuator structures is to design oxide isolating segments to separate the biased comb drives from the grounded parts. Thus, the single crystal silicon (SCS) core (heavily doped or with low resistivity -0.001 ~ - c m ) serves as the main electrical conduction medium instead of the metal.
Figure 1. SEM micrograph of capacitive comb drive structure with fingers parallel to the motion.
0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. All fights reserved. SSD I 0167-9317(95)00301-0
A. Jazairy, N.C. MacDonald / Microelectronic Engineeling 30 (1996) 527-530
528
SiO2 mask (i) m n ! Si Substrate Photolithographyand pattemtransferinto SiO2
H !
n
(ii)
M
~
n
~
SiO2mask
Silicon
_9~i
n
SiliconC12RIE #1
The si°2
!
ThermalSiO2 i
!
Thermaloxidation#2 for oxidizedthinbeams
Thesio2
!
Thermaloxidation#1 for totallyoxidizedbeams (iv)
~
Thermal SiO2
~
n
Silicon (vii)
n
Float SiO2 removalfor t hickmask formation
I
!
Floor SiO2 removal
Silicon Silicon (viii) !
!
Deep SiliconC12RIE #2
SiliconSF6releaseetch
!
Figure 2. One version of SHARP for the fabrication of high aspect ratio released actuators. The outer cross sections represent released comb fingers and frame structures, and the inner cross section depicts an oxide isolating segment between the two sets of suspended comb drives. The outer cross sections in step (viii) show typical top, sidewall and overhang thermal oxide film thicknesses on the SCS beams. 2. F a b r i c a t i o n
We have developed a novel technology called Scream for High Aspect Ratio Proportions (SHARP) [4,5] which adds a new dimension to the SCREAM process. The technique uses a repeated sequence of thermal oxidations and reactive ion etchings (RIE). It is a single-mask, self-aligned process which allows the formation of releasable three-dimensional (3D) objects of large width with vertical dimensions exceeding 100 ~tm. In Figure 2, steps (i)-(viii) illustrate, in diagrammatic form, the fabrication of very high aspect ratio released actuators and frame structures ~r large force actuation. The starting substrate is a boron doped, p-type (100) SCS substrate which is coated with a 2 to 5 Bm-thick oxide mask. Such a thick initial masking layer is used for deep reactive ion etching (RIE). Thereafter, a first etch mask can be formed by lithographically patterning a vertical and smooth resist layer. The photoresist pattern is then
transferred to the underlying SiO2 masking layer by reactive ion etching in CHF 3 plasma to yield vertical sidewalls with large wafer uniformity [Fig. 2(i)]. After stripping the resist, the pattern in the SiO2 is transferred to the silicon substrate via the first trench PIE using a C12/BC13 plasma [6] resulting in an etch depth of 20-30 I.tm [Fig. 2(ii)]. Thereafter, a thick thermal silicon dioxide layer (H2£) 2 precursors) is grown to oxidize through all the structural beams of the device [Fig. 2(iii)]. The floor silicon dioxide is removed using another CHF3 RIE process, thus forming a vertical 20-30 I.tm-thick second etch mask for trench RIE #2 [Fig. 2(iv)]. This oxide thickness will form the initial top oxide for our device beams. It would be practically impossible to deposit the same amount on a wafer. A second trench RIE step forms sidewalls exceeding 100 gm in depth in accordance with the desired vertical dimensions [Fig. 2(v)]. Thereafter, another thermal SiO2 is grown to completely oxidize through thin oxide
529
A. Jazairy, N.C. MacDonald / Microelectronic Engineering 30 (1996) 527-530
isolating segments [Fig. 2(vi)]. This thin layer of thermal oxide film is also used for sidewall passivation during the subsequent release etch. Finally, the floor silicon dioxide is removed with a RIE using CHF 3 plasma [Fig. 2(vii)I, and the entire structure can be released off the wafer by using an isotropic SF 6 RIE to undercut the high aspect ratio beams [Fig. 2(viii)]. Figure 3a shows a cross section of a smooth structure with vertical depth h-100 [tm and anisotropy (or verticality) A~0.99. Figure 3b is a cross section of the same device, comparing the etch depth with the entire wafer thickness.
(a)
(b)
Figure 3. a) SEM micrograph of a 100 ktm-deep vertical and smooth frame structure, b) SEM micrograph of the same structure showing the entire wafer thickness (-375 ktm). Such deep micromachined structures have been proposed for micro-optics applications [7]. High energy synchrotron sources can also be used to generate very deep structures as in the LIGA technique [8-10]. Other methods include cryogenic dry etching at very low temperatures [11] and electroplating of moderate aspect ratio resist profiles [12]. SHARP is a simple, low-cost technique for the generation of very deep structures. It comprises a new technology for: (i) forming ultra-thick (>30 ktm in depth) etch masks using thermal oxidation; (ii) fabricating very high aspect ratio structures with, to our knowledge, record etch depths (up to 160 ].tm) using c o n v e n t i o n a l R/E systems; (iii) obtaining very smooth sidewalls and structural surfaces by repeated thermal oxidation steps;
(iv) controlling lateral dimensions during the fabrication steps for the formation of threedimensional objects of large width; (v) achieving high out-of-plane stiffness for large force actuation and integration of micromechanical with on-chip circuitry; (vi) allowing lift and displacement of entire devices off the wafer. 3. Structural Performance
A systematic theoretical study has determined the effect of intrinsic stress that develops in mechanical beams due to top, sidewall and overhang thin film coatings [13,14]. A careful control of the effect of stresses has generated structures from high planarity to large vertical deflections [15,16]. For these large scale micromechanical structures, buckling due to intrinsic stresses becomes an important issue. As the structural single crystal silicon (SCS) core height is increased, the overall curvature of the device decreases. Structures become planar for heights larger than 50 btm. Figure 4 shows a side view of a highly planar released 85 gm-deep cantilever suspended on a frame structure. The sidewall and overhang oxide thicknesses on the SCS beams for this device are 0.9 Ixm and 15 Ixm, respectively.
~
--
i
~s~:~
,
IIII
Figure 4. SEM micrograph of a highly planar released 85 ktm-deep cantilever on-a-frame. The large vertical depth provides a high out-ofplane stiffness for the structure [17]. Similarly, comb drive structures with planar and large surface area cantilevered (finger) beams can be designed to generate large actuation forces.
530
A. Jazairy, N.C. MacDonaM / Microelectronic Engineering 30 (1996) 527-530
Experimentally, we verify that a released frame structure has a large positive curvature for a height of 15 btm as shown in Figure 5a, while it becomes planar with a height of 70 btm as shown in Figure 5b. In general, sidewall and overhang oxide films tend to generate positive curvature (with upward bending) while top oxide films tend to generate negative curvature (with downward bending). For shallow structural beams (<50 ~tm), the competing effects of thin oxide films can cause large deflections in either directions. For deeper structural beams, the effects of top, sidewall and overhang oxides are still relevant but with low influence on bending. For example, if the curvature is 1/pl for a beam with 10 btm deep SCS, and l/p2 for a similar beam with 50 ~tm deep SCS, then l/p2 can be an order of magnitude smaller than 1/p 1.
(a)
(b)
Figure 5. a) SEM micrograph of a 15 ~tm-deep released bowed frame structure (scale bar: 50/.tm), b) SEM micrograph of a 70 ~tm-deep released planar frame structure (scale bar: 100 ~tm).
4. Summary A novel low-cost bulk micr0machiningbased technology called SHARP has been developed and used for the fabrication of very high aspect ratio SCS micromechanical actuators and frame structures with vertical dimensions exceeding 100 btm. SHARP has generated, to our knowledge, :record depths (up to 160 ktm) using conventional RIE systems. Experimental results have indicated that a large structural depth improves both the planarity and the loading force of micromechanical devices.
Acknowledgments The fabrication process was developed at the National Nanofabrication Facility, which is supported by the National Science Foundation under Grant No. ECS-9319005. The authors
would like to acknowledge funding from the Advanced Research Projects Agency in a project monitored by the Optoelectronics Technology Center under Contract No. OTC-II-MDA972-941-0002. One of the authors (A.J.) would like to thank Dr. A. Faria for fruitful discussions.
References [1]M. T. A. Saif and N. C. MacDonald, IEEE Technical Digest, 8th International Conference on Solid-State Sensors and Actuators (Transducers '95), vol. 2, pp. 60-63, Stockholm, June 25-29, 1995. [2]Z. L. Zhang and N. C. MacDonald, J. Micromech. Microeng. 2, 1, 31, (1992). [3]Z. L. Zhang and N. C. MacDonald, J. Vac. Sci. Technol. BU, 2538, (1993). [4]N. C. MacDonald and A. Jazairy, SPIE 2383, 125, (1995). [5]A. Jazairy and N. C. MacDonald, J. Vac. Sci. Technol. (submitted), (1996). [6]J. J. Yao, S. C. Arney and N. C. MacDonald, J. of Microelectromech. Sys. 1, 14, (1992). [7]A. Jazairy and N. C. MacDonald, paper WH4, presented at the Optical Society of America AnnuaJ Meeting, Portland, OR, Sept. 10-15, 1995. [8]H. Guckel, K. J. Skrobis, J. Klein, T. R. Christenson and T. Wiegele, SPIE 2045, 290, (1994). [9]W. Ehrfeld, P. Bley, F. G6tz, J. Mohr, D. Miinchmeyer, W. Schelb, H. J. Baving and D. Beets, J. Vac. Sci. Technol. B6 (1), 178, (1988). [10]W. Ehrfeld and D. Miinchmeyer, Nucl. Instrum. Methods Phys. Res. A303, 523, (1991). [ll]M. Esashi, Microsystem Technologies 1, 2, (1994). [12]G. Engelmann, O. Ehrmann, R. Leutenbauer, H. Schmitz and H. Reichl, SPIE 2045, 306, (1994). [13]M. T. A. Saif and N. C. MacDonald, SPIE 2441, 329, (1995). [14]M. T. A. Saif and N. C. MacDonald, SPIE 2448, 93, (1995). [15]W. Fang and J. A. Wickert, J. Micromech. Microeng. 4, 116, (1994). [16]B. P. van Driernhuizen, J. F. L. Goosen, P. J. French and R. F. Wolffenbuttel, Sensors and Actuators A37-38,756, (1993). [17]A. Jazairy and N. C. MacDonald, SPIE 2640 (to appear), (1995).