Thick single crystal silicon MEMS with high aspect ... - Semantic Scholar
Thick single crystal silicon MEMS with high aspect ratio vertical air-gaps Pejman Monajemi and Farrokh Ayazi School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332
ABSTRACT This paper presents recent advances in the HARPSS micromachining technology, which enables implementation of movable Single Crystal Silicon (SCS) structures with high aspect ratio vertical air gaps on low-resistivity silicon substrate. This is suitable for applications of micro-gravity accele rometers, low voltage tunable capacitors, and highresolution gyroscopes that require aspect ratios as large as 100:1 to achieve high sensitivity and wide tuning range. The single-sided HARPSS process eliminates the need for double-sided processing, and wafer bonding to subsequently package the device. The device thickness and gap spacing can be varied in a wide range, 30-150µm and 0.2-2µm respectively, to select the performance range. The movable MEMS elements are made of bulk silicon substrate, resulting in higher mass, higher quality factor (Q), and better shock resistance, compared to using polysilicon (poly) as the movable structure. This process provides a mechanism for creating corrugation in SCS electrodes to reduce the Brownian noise of sensors. This technique realxes the need to reduce the noise by using the maximum available mass and through-wafer etch. The corrugations are created by a DRIE technique for etching poly surrounded by oxide inside the isolation trenches. Also, uniform capacitive gap spacings are created by growing sacrificial oxide inside the trenches. Keywords: Single Crystal Silicon (SCS), Polysilicon (poly), HARPSS, Accelerometer, Tunable capacitor, Tuning fork gyroscope, Low Pressure Chemical Vapor Deposition (LPCVD), Deep Reactive Ion Etching (DRIE).
1. INTRODUCTION Due to their high sensitivity and good thermal properties, capacitive sensors and actuators play the main role in the precision MEMS and MOEMS transducers. Implementation of all kinds of electrostatic transduction mechanisms, including in-plane, out of plane and torsional movement with enough force requires high aspect ratio bulk micromachining. Moreover, bulk micromachined sensors are free of thin-film stress and have large mass to reduce the mechanical damping.1 Silicon on Insulator (SOI) and wafer bonding techniques have been widely used to fabricate MEMS and MOEMS sensors and actuators.1-4 Single wafer technologies have shown to be good candidates for low cost production of capacitive transducers.5,6 High resolution capacitive sensors need large mass, high quality factor (Q), and small transduction gap, or equivalently high aspect ratio gap spacing between the fixed and the movable components. With silicon as the major substrate material, many micro and nanostructures use trench-refilled poly as their structural material due to its stress-free nature, high strength, and good thermal matching to the substrate.7,8 Examples of poly resonant devices have been shown earlier to achieve high-resolution gyroscopes.9 Poly springs have been used to suspend a large silicon accelerometer.5,6 The major deposition method for poly is LPCVD, which limits the thickness of the poly film to a few microns. The main motivation behind using SCS as the structural material, as opposed to using poly are: (1) SCS movable structures are void and stress-free, which is required for high-Q devices.10 (2) Creating very wide (>20µm) beams to increase the mass is only possible with exploiting silicon from the substrate. (3) The SCS suspended mass has higher shock resistance and fracture strength, because it is directly connected to the rest of the substrate, as compared to poly suspension of mass, which requires anchoring of poly to the substrate. HARPSS technology has been introduced earlier as a process to achieve high aspect ratio microstructures.7 In the new method, the moving parts (main body and supporting springs) are made of SCS, and the stationary parts (electrodes and shock absorbers) are made of trench-refilled poly. This work demonstrates the development of a 4-mask HARPSS process for realization of different types of SCS sensors and actuators; a micro-gravity lateral accelerometer, a lowvoltage wide-range gap-tunable capacitor, and a low-noise low-voltage lateral tuning fork gyroscope.
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2. SCS HARPSS PROCESS DEVELOPMENT Figure 1 demonstrates the diagram and the close-up view of the electrodes for an inertial grade lateral (X-axis) accelerometer or a tunable capacitor, which can be fabricated using our technology. The accelerometer has a perforated SCS proof mass suspended by four tethers attached to the bulk. The stationary sense and drive electrodes are made of poly and are anchored on silicon nitride. In order to adjust the air damping and reduce the mechanical noise floor of the sensors , the electrodes are corrugated in the vertical direction by DRIE etching of silicon in certain places inside the SCS electrodes. If the SCS electrode is wide enough, this will have little effect on the stiffness of the SCS electrode, but since the poly electrode is a few microns wide, doing the same for the trench-refilled poly can reduce the stiffness and strength of the poly electrode. Ladder-shaped stiffeners are used in the fixed electrodes to increase their robustness in the lateral direction. The gap-tunable capacitor has the same diagram, except it does not have any proof mass or corrugated electrodes. Figure 2 shows the fabrication steps for the electrode area. First, LPCVD silicon nitride is deposited on a lowresistivity (1µm) low stress nitride films. By adjusting the ratio of Si2 H2 Cl2 :NH3, the film stress can be minimized above –300Mpa. The process consists of six furnace runs, as listed in Table 1. After trench refill, poly is etched inside the isolation trenches that define the borders of SCS fingers, using sacrificial oxide as the mask. The etching of poly inside deep trenches surrounded by oxide sidewalls requires a sequence of high frequency (13.56MHz) and low frequency (380kHz) plasma sources with gradual power ramping, the details of which are given in Table.2. The low frequency source is required to allow the long lifetime electrons to recombine with fluorine ions to neutralize the positive charge build up on the oxide sidewalls.13
Z
Y X X SCS proof mass
Poly electrode
Stiffener
Corrugation SCS finger
A SCS spring
Nitride anchor
A’
Figure 1: Left: Diagram of the lateral HARPSS silicon accelerometer/tunable capacitor; Right: Close-up of the electrodes/anchors.
139
It is necessary to minimize the poly etch duration, to create poly electrodes with smooth surfaces. After etching poly inside the isolation trenches, the 3-4µm wide trenches will be bridged over by spin coating a thick photoresist, such as AZ4620. The moving structure is then released by a combination of anisotropic and isotropic dry etch to undercut the SCS device at the bottom. An excessive isotropic etching may completely remove the SCS tethers and electrodes, so it is important to minimize the upward isotropic etching of the SCS part cycle, as highlighted in Fig. 2(e). This can be achieved by using two methods: (1) The release holes should be placed as close as possible relative to each other and their spacing should be equal all over the device, so that the whole SCS movable structure will be released together without any need to over-etch any non-released part. (2) After anisotropic SCS etch step, a thin layer of oxide is grown and directionally etched at the surface and bottom of trench, followed by isotropic etch of SCS. Using this method makes it possible to make the SCS electrodes/spring/mass even thicker than the poly electrodes. To conclude the process, the sacrificial oxide is removed in an HF:H2 O solution. The lateral etch rate of HF is not constant and is dependent on the oxide thickness. Etching a 1µm oxide between 25µm wide SCS/poly fingers takes half an hour. The boron-doped poly survives the long HF release, and its surface roughness is less than 70nm, as measured by an atomic force microscope. Due to the low overall stiffness and large electrode area, which are required to achieve high resolution, stiction of SCS and poly electrodes during rinse/evaporation of DI water, can be a major processing issue for such large devices. Using the super critical dryer will reduce the possibility of stiction. Also rinsing the device inside an ultrasonic bath will help removing any extra residues.
(a)
(d)
(b)
(e) Upward SCS etch
SCS spring Poly electrode SCS electrode
(c)
(f) Nitride
gap
Oxide
Poly
SCS
Figure 2: Process flow for the electrode/anchor area (cross section A-A’ in Fig. 1): (a) Pad isolation, trench etch; (b) Oxide growth, poly trench refill; (c) Anisotropic poly etch; (e) Anisotropic SCS etch; (e) Isotropic SCS etch; (f) Oxide etch in HF.
Table 1. Furnace run parameters Process
Pressure
Temperature
Low-stress LPCVD Nitride
300mT
800°C
Wet Oxidation Atmosphere
950°C
LPCVD Poly
250mT
588°C
Boron doping
Atmosphere
1050°C
Annealing
Atmosphere
1050°C
Gas Flow
Deposition Rate
Stress
NH3: 30sccm 50°A/min -300MPa SiH 2Cl2: 150sccm O2: 1000sccm 7hrs to grow 1 H2: 185sccm µm SiH 4: 100sccm 100MPa 60°A/min O2: 200sccm N2: 5000sccm N2: 5000sccm -
140
Table 2. DRIE parameters for etching poly inside high aspect ratio trench covered by oxide sidewalls Step Pressure Platen RF Power/Frequency Coil RF Power Etch/Passivation Initial DRIE 20mT 10W/ 13.56MHz 600W 10sec/8sec Isotropic etch of poly at sidewalls Remove poly at bottom of trench
30mT 20mT
10W, 0.1W/min/ 380kHz 12W, 0.1W/min/ 380kHz
600W 600W
12sec/8sec 10sec/8sec
3. FABRICATION AND CHARACTERIZATION RESULTS 3. 1. Fabrication and Testing of Lateral HARPSS Accelerometers In order to verify the scalability of the process, prototypes of 30-100µm thick lateral accelerometers with 1-2µm gap spacing have been fabricated and tested. Figure 3 shows the SEM view of a 60µm thick inertial-grade lateral accelerometer, fabricated through this process. The sensor has a 3mm×1mm perforated SCS proof mass with 2500 release holes and is suspended by four SCS tethers. The mass is about 1mg and the total area is around 0.25cm2 .
Shock absorber
Figure 3: SEM micrograph of the 60µm thick HARPSS lateral accelerometer. Figure 4(a) shows a close-up of the electrodes, SCS spring, nitride anchor, corrugated SCS fingers, and perforated mass.
Anchor
Proof mass
Poly electrode
1µm sense gap
SCS electrode
Z-axis shock stop 50µm cavity gap
SCS spring SCS finger
(a)
(b) Figure 4: (a) Enlarged view of electrode/tether area; (b) SCS finger and local off-plane shock absorbers.
141
The release holes in the proof mass are shifted relative to each other to increase the stiffness. A distributed array of Zaxis shock stops is formed by local overhang of poly on top of SCS fingers, as shown in Fig. 4(b). The SCS fingers are 25µm wide, separated by 30µm corrugations. The poly electrodes have been broken to verify the shape and surface roughness of the oxidized SCS fingers, as shown in Fig. 5(a). The enlarged view of the in-plane shock stop is shown in Fig. 5(b). Both types of local X-axis and distributed Z-axis shock stops resist excitations up to 500g. Since the stiffness in the Y-direction is huge, no shock absorber is placed in that direction.
1µm gap
Vertical undercut (a)
(b) Figure 5: (a) Broken corrugated SCS fingers; (b) Enlarged view of the X-axis shock stop.
Figure 6(a) and (b) show the profile of thermal oxide, grown over a 100µm deep trench to verify the 1:1 uniformity. Figure 6(c) shows the poly and SCS electrodes right after the vertical poly etch step. The hollow trench is about 3.5µm.
Poly-etched trench
Oxide (a) (b) (c) Figure 6: Profile of the oxide grown on a 100µm deep trench: a) top of trench; b) bottom of trench; c) electrodes after poly etch step.
Due to deflection of the SCS electrodes, the air molecules will be squeezed in the sense gap and will escape laterally, but since the velocity of molecules is very small, this will not create any turbulence, and so the squeeze film damping of the two near fingers will be independent. Assuming there is no damping correlation between the fingers, and the cavity gap is much larger than the sense gap, the total damping, D, and the mechanical Noise Equivalent Acceleration (NEA) for a set of N-corrugated fingers can be modeled as:14 N
D = µ Eff H
∑ (d
Li
i =1 N
D = µ Eff Li
∑ (d i =1
NEA( Brownian) =
)3
(Ns/m)
; LiH
(1)
0
4k BTD M
BW =
4k BTω 0 BW MQ
(2)
Here µEff is the air viscosity, d0 is the sense gap, H and Li are the height and length of the electrodes, BW is the measurement bandwidth, Q is the quality factor (Q1µm) and enough mass (M>0.05mg) is below 1°/hr/√Hz.
146
4. CONCLUSION Reported in this paper are the recent results on fabrication and characterization of an advanced mixed-mode (surface+bulk) micromachining technique for creation of high aspect ratio capacitive silicon MEMS devices on lowresistivity silicon substrate. The single sided/single wafer HARPSS process creates thick (30-150µm) Single Crystal Silicon (SCS) movable structures and fixed stress-free poly electrodes with narrow (0.2-2µm) capacitive gaps to achieve high aspect ratio vertical air gaps. This is suitable for fabrication of inertial-grade accelerometers and gyroscopes, and low-voltage tunable capacitors, that require aspect ratio of better than 100:1 for high sensitivity and wide tuning range. The new process is suitable for creating corrugations in SCS electrodes with different widths (>3µm) to reduce the Brownian noise floor of the capacitive sensors. A new technique for etching poly surrounded by oxide sidewalls is developed to enable corrugation of SCS electrodes. By growing sacrificial oxide, an aspect ratio of 90:1 has been achieved for the HARPSS MEMS devices. The fabrication technique is inherently stress-free, and can be used to create high-Q SCS MEMS structures. The fabricated devices include a micro-gravity lateral accelerometer with measured static sensitivity of 4.5pF/g and estimated total noise floor of 1µg/√Hz, a low-voltage tunable capacitor with a tuning ratio of 2:1, and a lateral tuning fork gyroscope with a Q of 30,000 and sub-°/hr/√Hz mechanical noise floor.
ACKNOWLEDGMENTS The Authors wish to thank Babak Vakili, Tanya L. Wright, and the CMOS group at the Georgia Institute of Technology Microelectronics Research Center for their assistance.
REFERENCES 1. Yazdi, N., Ayazi, F., and Najafi, K., “Micromachined Inertial Sensors,” Invited Paper, Special Issue of IEEE Proceedings, 8, pp. 1640-1659, 1998. 2. Milanovic, V., Kwon, S and Lee L.P., “High Aspect Ratio Micromirros with Large Static Rotation and Piston Actuation,” IEEE Photonics Technology Letters. 16, pp. 1891-1893, 2004. 3. Vakili, B., Pourkamali, S., Ayazi, F., “A High Resolution, Stictionless, CMOS Compatible SOI Accelerometer with a Low Noise, Low Power, 0.25µm CMOS Interface,” in Proc. IEEE MEMS, pp. 572-575, 2004. 4. Zaman, M. F., Sharma, A., Vakili, B., Ayazi, F., “Towards Inertial Grade Vibratory Microgyros: A High-Q In-Plane Silicon on Insulator Device,” The International Conference on Solid-State Sensors and Actuators, pp.384-385, 2004. 5. Chae, J. and Najafi, K.,”An in-plane high sensitivity micro-g accelerometer,” in proc.IEEE MEMS, pp.466– 469, 2003. 6. Yazdi, N. and Najafi, K.,” A high-sensitivity silicon accelerometer with a folded-electrode structure” J. Microelectromechanical. Syst, 12, pp.479–486, 2003. 7. Ayazi, F. and Najafi, K., “High Apect-Ratio Combined Polysilicon and Single-Crystal Silicon (HARPSS) MEMS Technology,” J. Microelectromechanical. Syst, 9, No.3, pp.288-294, 2000. 8. Ballarini, R., et. al., “The Fracture Toughness of Polysilicon Microdevices” Materials Research Society Symposium N:Microelectromechanical Structures for Materials Research, San Francisco, 1999. 9. Ayazi, F., Najafi, K., “A HARPSS Polysilicon Vibrating Ring Gyroscope,” J. Microelectromechanical. Syst., 10, pp.169-179, 2001. 10. Pourkamali, S., et. al., “High-Q single crystal silicon HARPSS capacitive beam resonators with self-aligned sub100nm transduction gaps,” J. Microelectromechanical Syst., 12, No.4, pp. 487-496, 2003. 11. Pyzyna, A. M, et. al., “Thermal Oxidation-Induced Strain In Silicon Nano beams,” Proc. MEMS, pp.189-192, 2004. 12. Abdolvand, R., Ho, G. K., and Ayazi, F., “Poly -Wire -Coupled Single Crystal Silicon HARPSS Micromechanical Filters Using Oxide Islands,” in Tech. Dig.Hilton Head, SC, pp. 242-245.13, 2004. 13. McAuley, S.A., et. al., “Silicon Micromachining Using a High-Density Plasma Source,” J. Phys. D: Appl. Phys., 34, pp. 2769-2774, 2001. 14. Veijola, T., “Model for Gas film damping in a Silicon Accelerometer,” Proc. Transducers, June 1997, pp.1097-1100. 15. Vakili, B., et. al., “A 2.5V 14-bit Sigma-Delta CMOS-SOI Capacitive Accelerometer,” in Tech. Dig. IEEE International Solid-State Circuits Conference (ISSCC 2004), San Francisco, CA, pp. 314-315, 2004. 16. Borwick, R. L., et. al, “A high Q, large tuning range MEMS capacitor for RF filter systems,” Sensors and Actuators A, 103, pp. 33-41, 2003. 17. Zou, J., et. al., “Development of a wide tuning range MEMS tunable capacitor for wireless communication systems ,” in Proc. IEDM 2000, pp. 17.2.1-17.2.4, 2000.
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ABSTRACT This paper presents recent advances in the HARPSS micromachining technology, which enables implementation of movable Single Crystal Silicon (SCS) structures with high aspect ratio vertical air gaps on low-resistivity silicon substrate. This is suitable for applications of micro-gravity accele rometers, low voltage tunable capacitors, and highresolution gyroscopes that require aspect ratios as large as 100:1 to achieve high sensitivity and wide tuning range. The single-sided HARPSS process eliminates the need for double-sided processing, and wafer bonding to subsequently package the device. The device thickness and gap spacing can be varied in a wide range, 30-150µm and 0.2-2µm respectively, to select the performance range. The movable MEMS elements are made of bulk silicon substrate, resulting in higher mass, higher quality factor (Q), and better shock resistance, compared to using polysilicon (poly) as the movable structure. This process provides a mechanism for creating corrugation in SCS electrodes to reduce the Brownian noise of sensors. This technique realxes the need to reduce the noise by using the maximum available mass and through-wafer etch. The corrugations are created by a DRIE technique for etching poly surrounded by oxide inside the isolation trenches. Also, uniform capacitive gap spacings are created by growing sacrificial oxide inside the trenches. Keywords: Single Crystal Silicon (SCS), Polysilicon (poly), HARPSS, Accelerometer, Tunable capacitor, Tuning fork gyroscope, Low Pressure Chemical Vapor Deposition (LPCVD), Deep Reactive Ion Etching (DRIE).
1. INTRODUCTION Due to their high sensitivity and good thermal properties, capacitive sensors and actuators play the main role in the precision MEMS and MOEMS transducers. Implementation of all kinds of electrostatic transduction mechanisms, including in-plane, out of plane and torsional movement with enough force requires high aspect ratio bulk micromachining. Moreover, bulk micromachined sensors are free of thin-film stress and have large mass to reduce the mechanical damping.1 Silicon on Insulator (SOI) and wafer bonding techniques have been widely used to fabricate MEMS and MOEMS sensors and actuators.1-4 Single wafer technologies have shown to be good candidates for low cost production of capacitive transducers.5,6 High resolution capacitive sensors need large mass, high quality factor (Q), and small transduction gap, or equivalently high aspect ratio gap spacing between the fixed and the movable components. With silicon as the major substrate material, many micro and nanostructures use trench-refilled poly as their structural material due to its stress-free nature, high strength, and good thermal matching to the substrate.7,8 Examples of poly resonant devices have been shown earlier to achieve high-resolution gyroscopes.9 Poly springs have been used to suspend a large silicon accelerometer.5,6 The major deposition method for poly is LPCVD, which limits the thickness of the poly film to a few microns. The main motivation behind using SCS as the structural material, as opposed to using poly are: (1) SCS movable structures are void and stress-free, which is required for high-Q devices.10 (2) Creating very wide (>20µm) beams to increase the mass is only possible with exploiting silicon from the substrate. (3) The SCS suspended mass has higher shock resistance and fracture strength, because it is directly connected to the rest of the substrate, as compared to poly suspension of mass, which requires anchoring of poly to the substrate. HARPSS technology has been introduced earlier as a process to achieve high aspect ratio microstructures.7 In the new method, the moving parts (main body and supporting springs) are made of SCS, and the stationary parts (electrodes and shock absorbers) are made of trench-refilled poly. This work demonstrates the development of a 4-mask HARPSS process for realization of different types of SCS sensors and actuators; a micro-gravity lateral accelerometer, a lowvoltage wide-range gap-tunable capacitor, and a low-noise low-voltage lateral tuning fork gyroscope.
138
2. SCS HARPSS PROCESS DEVELOPMENT Figure 1 demonstrates the diagram and the close-up view of the electrodes for an inertial grade lateral (X-axis) accelerometer or a tunable capacitor, which can be fabricated using our technology. The accelerometer has a perforated SCS proof mass suspended by four tethers attached to the bulk. The stationary sense and drive electrodes are made of poly and are anchored on silicon nitride. In order to adjust the air damping and reduce the mechanical noise floor of the sensors , the electrodes are corrugated in the vertical direction by DRIE etching of silicon in certain places inside the SCS electrodes. If the SCS electrode is wide enough, this will have little effect on the stiffness of the SCS electrode, but since the poly electrode is a few microns wide, doing the same for the trench-refilled poly can reduce the stiffness and strength of the poly electrode. Ladder-shaped stiffeners are used in the fixed electrodes to increase their robustness in the lateral direction. The gap-tunable capacitor has the same diagram, except it does not have any proof mass or corrugated electrodes. Figure 2 shows the fabrication steps for the electrode area. First, LPCVD silicon nitride is deposited on a lowresistivity (1µm) low stress nitride films. By adjusting the ratio of Si2 H2 Cl2 :NH3, the film stress can be minimized above –300Mpa. The process consists of six furnace runs, as listed in Table 1. After trench refill, poly is etched inside the isolation trenches that define the borders of SCS fingers, using sacrificial oxide as the mask. The etching of poly inside deep trenches surrounded by oxide sidewalls requires a sequence of high frequency (13.56MHz) and low frequency (380kHz) plasma sources with gradual power ramping, the details of which are given in Table.2. The low frequency source is required to allow the long lifetime electrons to recombine with fluorine ions to neutralize the positive charge build up on the oxide sidewalls.13
Z
Y X X SCS proof mass
Poly electrode
Stiffener
Corrugation SCS finger
A SCS spring
Nitride anchor
A’
Figure 1: Left: Diagram of the lateral HARPSS silicon accelerometer/tunable capacitor; Right: Close-up of the electrodes/anchors.
139
It is necessary to minimize the poly etch duration, to create poly electrodes with smooth surfaces. After etching poly inside the isolation trenches, the 3-4µm wide trenches will be bridged over by spin coating a thick photoresist, such as AZ4620. The moving structure is then released by a combination of anisotropic and isotropic dry etch to undercut the SCS device at the bottom. An excessive isotropic etching may completely remove the SCS tethers and electrodes, so it is important to minimize the upward isotropic etching of the SCS part cycle, as highlighted in Fig. 2(e). This can be achieved by using two methods: (1) The release holes should be placed as close as possible relative to each other and their spacing should be equal all over the device, so that the whole SCS movable structure will be released together without any need to over-etch any non-released part. (2) After anisotropic SCS etch step, a thin layer of oxide is grown and directionally etched at the surface and bottom of trench, followed by isotropic etch of SCS. Using this method makes it possible to make the SCS electrodes/spring/mass even thicker than the poly electrodes. To conclude the process, the sacrificial oxide is removed in an HF:H2 O solution. The lateral etch rate of HF is not constant and is dependent on the oxide thickness. Etching a 1µm oxide between 25µm wide SCS/poly fingers takes half an hour. The boron-doped poly survives the long HF release, and its surface roughness is less than 70nm, as measured by an atomic force microscope. Due to the low overall stiffness and large electrode area, which are required to achieve high resolution, stiction of SCS and poly electrodes during rinse/evaporation of DI water, can be a major processing issue for such large devices. Using the super critical dryer will reduce the possibility of stiction. Also rinsing the device inside an ultrasonic bath will help removing any extra residues.
(a)
(d)
(b)
(e) Upward SCS etch
SCS spring Poly electrode SCS electrode
(c)
(f) Nitride
gap
Oxide
Poly
SCS
Figure 2: Process flow for the electrode/anchor area (cross section A-A’ in Fig. 1): (a) Pad isolation, trench etch; (b) Oxide growth, poly trench refill; (c) Anisotropic poly etch; (e) Anisotropic SCS etch; (e) Isotropic SCS etch; (f) Oxide etch in HF.
Table 1. Furnace run parameters Process
Pressure
Temperature
Low-stress LPCVD Nitride
300mT
800°C
Wet Oxidation Atmosphere
950°C
LPCVD Poly
250mT
588°C
Boron doping
Atmosphere
1050°C
Annealing
Atmosphere
1050°C
Gas Flow
Deposition Rate
Stress
NH3: 30sccm 50°A/min -300MPa SiH 2Cl2: 150sccm O2: 1000sccm 7hrs to grow 1 H2: 185sccm µm SiH 4: 100sccm 100MPa 60°A/min O2: 200sccm N2: 5000sccm N2: 5000sccm -
140
Table 2. DRIE parameters for etching poly inside high aspect ratio trench covered by oxide sidewalls Step Pressure Platen RF Power/Frequency Coil RF Power Etch/Passivation Initial DRIE 20mT 10W/ 13.56MHz 600W 10sec/8sec Isotropic etch of poly at sidewalls Remove poly at bottom of trench
30mT 20mT
10W, 0.1W/min/ 380kHz 12W, 0.1W/min/ 380kHz
600W 600W
12sec/8sec 10sec/8sec
3. FABRICATION AND CHARACTERIZATION RESULTS 3. 1. Fabrication and Testing of Lateral HARPSS Accelerometers In order to verify the scalability of the process, prototypes of 30-100µm thick lateral accelerometers with 1-2µm gap spacing have been fabricated and tested. Figure 3 shows the SEM view of a 60µm thick inertial-grade lateral accelerometer, fabricated through this process. The sensor has a 3mm×1mm perforated SCS proof mass with 2500 release holes and is suspended by four SCS tethers. The mass is about 1mg and the total area is around 0.25cm2 .
Shock absorber
Figure 3: SEM micrograph of the 60µm thick HARPSS lateral accelerometer. Figure 4(a) shows a close-up of the electrodes, SCS spring, nitride anchor, corrugated SCS fingers, and perforated mass.
Anchor
Proof mass
Poly electrode
1µm sense gap
SCS electrode
Z-axis shock stop 50µm cavity gap
SCS spring SCS finger
(a)
(b) Figure 4: (a) Enlarged view of electrode/tether area; (b) SCS finger and local off-plane shock absorbers.
141
The release holes in the proof mass are shifted relative to each other to increase the stiffness. A distributed array of Zaxis shock stops is formed by local overhang of poly on top of SCS fingers, as shown in Fig. 4(b). The SCS fingers are 25µm wide, separated by 30µm corrugations. The poly electrodes have been broken to verify the shape and surface roughness of the oxidized SCS fingers, as shown in Fig. 5(a). The enlarged view of the in-plane shock stop is shown in Fig. 5(b). Both types of local X-axis and distributed Z-axis shock stops resist excitations up to 500g. Since the stiffness in the Y-direction is huge, no shock absorber is placed in that direction.
1µm gap
Vertical undercut (a)
(b) Figure 5: (a) Broken corrugated SCS fingers; (b) Enlarged view of the X-axis shock stop.
Figure 6(a) and (b) show the profile of thermal oxide, grown over a 100µm deep trench to verify the 1:1 uniformity. Figure 6(c) shows the poly and SCS electrodes right after the vertical poly etch step. The hollow trench is about 3.5µm.
Poly-etched trench
Oxide (a) (b) (c) Figure 6: Profile of the oxide grown on a 100µm deep trench: a) top of trench; b) bottom of trench; c) electrodes after poly etch step.
Due to deflection of the SCS electrodes, the air molecules will be squeezed in the sense gap and will escape laterally, but since the velocity of molecules is very small, this will not create any turbulence, and so the squeeze film damping of the two near fingers will be independent. Assuming there is no damping correlation between the fingers, and the cavity gap is much larger than the sense gap, the total damping, D, and the mechanical Noise Equivalent Acceleration (NEA) for a set of N-corrugated fingers can be modeled as:14 N
D = µ Eff H
∑ (d
Li
i =1 N
D = µ Eff Li
∑ (d i =1
NEA( Brownian) =
)3
(Ns/m)
; LiH
(1)
0
4k BTD M
BW =
4k BTω 0 BW MQ
(2)
Here µEff is the air viscosity, d0 is the sense gap, H and Li are the height and length of the electrodes, BW is the measurement bandwidth, Q is the quality factor (Q1µm) and enough mass (M>0.05mg) is below 1°/hr/√Hz.
146
4. CONCLUSION Reported in this paper are the recent results on fabrication and characterization of an advanced mixed-mode (surface+bulk) micromachining technique for creation of high aspect ratio capacitive silicon MEMS devices on lowresistivity silicon substrate. The single sided/single wafer HARPSS process creates thick (30-150µm) Single Crystal Silicon (SCS) movable structures and fixed stress-free poly electrodes with narrow (0.2-2µm) capacitive gaps to achieve high aspect ratio vertical air gaps. This is suitable for fabrication of inertial-grade accelerometers and gyroscopes, and low-voltage tunable capacitors, that require aspect ratio of better than 100:1 for high sensitivity and wide tuning range. The new process is suitable for creating corrugations in SCS electrodes with different widths (>3µm) to reduce the Brownian noise floor of the capacitive sensors. A new technique for etching poly surrounded by oxide sidewalls is developed to enable corrugation of SCS electrodes. By growing sacrificial oxide, an aspect ratio of 90:1 has been achieved for the HARPSS MEMS devices. The fabrication technique is inherently stress-free, and can be used to create high-Q SCS MEMS structures. The fabricated devices include a micro-gravity lateral accelerometer with measured static sensitivity of 4.5pF/g and estimated total noise floor of 1µg/√Hz, a low-voltage tunable capacitor with a tuning ratio of 2:1, and a lateral tuning fork gyroscope with a Q of 30,000 and sub-°/hr/√Hz mechanical noise floor.
ACKNOWLEDGMENTS The Authors wish to thank Babak Vakili, Tanya L. Wright, and the CMOS group at the Georgia Institute of Technology Microelectronics Research Center for their assistance.
REFERENCES 1. Yazdi, N., Ayazi, F., and Najafi, K., “Micromachined Inertial Sensors,” Invited Paper, Special Issue of IEEE Proceedings, 8, pp. 1640-1659, 1998. 2. Milanovic, V., Kwon, S and Lee L.P., “High Aspect Ratio Micromirros with Large Static Rotation and Piston Actuation,” IEEE Photonics Technology Letters. 16, pp. 1891-1893, 2004. 3. Vakili, B., Pourkamali, S., Ayazi, F., “A High Resolution, Stictionless, CMOS Compatible SOI Accelerometer with a Low Noise, Low Power, 0.25µm CMOS Interface,” in Proc. IEEE MEMS, pp. 572-575, 2004. 4. Zaman, M. F., Sharma, A., Vakili, B., Ayazi, F., “Towards Inertial Grade Vibratory Microgyros: A High-Q In-Plane Silicon on Insulator Device,” The International Conference on Solid-State Sensors and Actuators, pp.384-385, 2004. 5. Chae, J. and Najafi, K.,”An in-plane high sensitivity micro-g accelerometer,” in proc.IEEE MEMS, pp.466– 469, 2003. 6. Yazdi, N. and Najafi, K.,” A high-sensitivity silicon accelerometer with a folded-electrode structure” J. Microelectromechanical. Syst, 12, pp.479–486, 2003. 7. Ayazi, F. and Najafi, K., “High Apect-Ratio Combined Polysilicon and Single-Crystal Silicon (HARPSS) MEMS Technology,” J. Microelectromechanical. Syst, 9, No.3, pp.288-294, 2000. 8. Ballarini, R., et. al., “The Fracture Toughness of Polysilicon Microdevices” Materials Research Society Symposium N:Microelectromechanical Structures for Materials Research, San Francisco, 1999. 9. Ayazi, F., Najafi, K., “A HARPSS Polysilicon Vibrating Ring Gyroscope,” J. Microelectromechanical. Syst., 10, pp.169-179, 2001. 10. Pourkamali, S., et. al., “High-Q single crystal silicon HARPSS capacitive beam resonators with self-aligned sub100nm transduction gaps,” J. Microelectromechanical Syst., 12, No.4, pp. 487-496, 2003. 11. Pyzyna, A. M, et. al., “Thermal Oxidation-Induced Strain In Silicon Nano beams,” Proc. MEMS, pp.189-192, 2004. 12. Abdolvand, R., Ho, G. K., and Ayazi, F., “Poly -Wire -Coupled Single Crystal Silicon HARPSS Micromechanical Filters Using Oxide Islands,” in Tech. Dig.Hilton Head, SC, pp. 242-245.13, 2004. 13. McAuley, S.A., et. al., “Silicon Micromachining Using a High-Density Plasma Source,” J. Phys. D: Appl. Phys., 34, pp. 2769-2774, 2001. 14. Veijola, T., “Model for Gas film damping in a Silicon Accelerometer,” Proc. Transducers, June 1997, pp.1097-1100. 15. Vakili, B., et. al., “A 2.5V 14-bit Sigma-Delta CMOS-SOI Capacitive Accelerometer,” in Tech. Dig. IEEE International Solid-State Circuits Conference (ISSCC 2004), San Francisco, CA, pp. 314-315, 2004. 16. Borwick, R. L., et. al, “A high Q, large tuning range MEMS capacitor for RF filter systems,” Sensors and Actuators A, 103, pp. 33-41, 2003. 17. Zou, J., et. al., “Development of a wide tuning range MEMS tunable capacitor for wireless communication systems ,” in Proc. IEDM 2000, pp. 17.2.1-17.2.4, 2000.
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