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Physical characterisation of selective stress coupling for resonant pressure sensors
Sensors and Actuators A 115 (2004) 230–234
Physical characterisation of selective stress coupling for resonant pressure sensors P.K. Kinnell a,∗ , M.C.L. Ward a , R. Craddock b a
Research Centre for MicroEngineering and Nanotechnology, School of Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK b GE Druck Limited, Fir Tree Lane, Groby, Leicester LE6 0FH, UK
Received 22 September 2003; received in revised form 15 January 2004; accepted 21 January 2004 Available online 2 April 2004
Abstract This paper presents a selective stress coupling structure for resonant sensor applications. The structure is designed to selectively couple a resonant strain gauge into strain in one degree of freedom only. It constrains the displacement of the resonator such that it always stays aligned. A resonant pressure sensor is proposed that incorporates the stress isolation chip to allow a resonant strain gauge to be mounted to a large pressure sensitive silicon diaphragm. The chip behavior has been modeled using finite element analysis. Physical testing using a Wyko non-contact surface profiler has been carried out to investigate the selective stress coupling structure performance. © 2004 Elsevier B.V. All rights reserved. Keywords: Stress isolation; Strain gauge; Resonant; Packaging; Pressure
1. Introduction Resonant MEMs devices are now widely exploited in many sensor applications. While it is relatively easy to produce stable high Q resonators, packaging these devices in such a way that they are sensitive to the measurand but not affected by packaging stress presents a challenging problem [1,2]. In the case of resonant pressure sensors a micro-resonant strain gauge is typically mounted on to a micro-pressure sensitive diaphragm. This imposes limitations on the pressure ranges that can be measured. For low pressure measurement, a very flexible diaphragm is required. However, as the diaphragm stiffness decreases, the resonator becomes the controlling stiffness in the system, making it more difficult to measure pressure induced strain. An alternative solution would be to increase the size of the diaphragm. A larger diaphragm produces more mechanical work, so should be able to impart a measurable strain on the resonator for a lower pressure, while still retaining the dominant stiffness. The problem with using a larger diaphragm is that as the resonator must be coupled into points ∗ Corresponding author. Tel.: +44-121-414-4217; fax: +44-121-414-3958. E-mail address: [email protected] (P.K. Kinnell).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.01.061
on the diaphragm that are further apart, increasing the effects of residual packaging stress. To overcome these problems, a novel selective stress coupling chip is proposed. The aim of the selective stress coupling chip is to allow a micro-engineered resonant strain gauge to be coupled to a silicon diaphragm greater than 5000 m in diameter. The chip works by constraining the resonator to movement in one degree of freedom only. It is designed to selectively transmit strain in this degree of freedom to the resonator. Most importantly, it keeps the alignment of the resonator with its drive and pick-off electrodes, which is essential for optimum performance of the resonator.
2. The resonant strain gauge The principle of using a resonant strain gauge mounted on a pressure sensitive diaphragm has been well documented [1–3]. For the purposes of this work, a double-ended tuning fork (DETF) resonator is being used as the strain gauge. It is excited electro-statically using a parallel plate actuator. The change in capacitance of a second parallel plate electrode will be used to pick-off the frequency of the device (see Fig. 1). A DETF structure was chosen because it may be driven in a dynamically balanced mode, allowing a high mechanical Q to be developed [1].
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Fig. 1. Schematic of a double-ended tuning fork resonator with drive and pick-off electrodes. Fig. 3. Schematic of proposed resonant pressure sensor assembly, the cut-out shows an enlarged view of the double-ended tuning fork resonator.
3. Selective stress coupling structure Previous work has shown that it is possible to develop on-chip stress reduction zones, which shield sensitive structures from the unwanted affects of packaging or thermally induced stress [4,5]. In the case of a resonant strain gauge the stress isolation must be selective. The affects of packaging stress must be decoupled from the resonator; however, the resonator must still remain sensitive to the strain it was designed to measure. In this work, the aim is to mount a resonator across a span that may be more than five times its length. The simplest solution would be to incorporate stiff fixing brackets that take up the difference in length, as shown in Fig. 2. Stiff mounting brackets make sure that any longitudinal displacements of the diaphragm mounting points are transferred directly to the resonator causing a measurable strain. While this results in an optimum transferral of strain from the diaphragm to the resonator it also means that any residual parasitic packaging stress may also be directly transferred to the resonator. So, while achieving a tolerance in the mounting positions of ±1 or 2 m across the mounting points may be deemed acceptable, if this was applied directly to the ends of the resonator it would have many negative effects. The resulting misalignment between the resonator and its drive electrodes would cause a non-uniform drive force. This would cause an unpredictable resonator performance that may result in the excitation of suboptimal modes. Taken to the extreme if the resonator was to touch the drive electrode this would cause a mechanical and electrical failure of the device.
Fig. 2. Schematic illustrating the use of stiff fixing brackets to mount a micro-resonator to a large diaphragm.
To overcome this problem, the supports must be given a degree of flexibility to take up the unwanted effects of packaging. If the supports were more flexible than the resonator, less misalignment would happen. This is done at the cost of making the resonator less sensitive to the strain it was designed to measure. The solution should therefore offer a compromise between achieving sensitivity to strain caused by pressure, whilst also reducing the unwanted effects of packaging induced strain, and keeping the resonator aligned. To achieve this, a novel selective stress coupling structure is proposed. The aim of this structure is to act as a coupling between the resonant strain gauge at the heart of the pressure sensor and the pressure sensitive diaphragm. Fig. 3 shows a schematic of the proposed resonant pressure sensor. The stress coupling structure can be seen mounted onto a silicon diaphragm. In turn, mounted onto the stress coupling structure is the strain sensitive resonator. (Note: Drive and pick-off electrodes have been omitted from the schematic.)
Fig. 4. Selective stress coupling chip, showing the effect various displacements at the chip mountings have on the chip central area.
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The stress coupling structure consists of three main areas, the mounting points, flexible supports, and the central chip area, see Fig. 4. The mounting points are where the chip is fixed to the diaphragm. The flexible supports are designed to give a degree of compromise between the two extremes mentioned above. They are designed to be significantly stiffer than the combined central chip area and resonator in tension, however, less stiff in all other degrees of freedom. The central chip area consists of two independent areas constrained to linear motion relative to the longitudinal axis of the resonator. The dimensions of the central chip area mean that it is much stiffer than the flexible supports in all degrees of freedom except for along the longitudinal axis of the resonator. In this way, the resonator may be kept aligned regardless of the displacements applied to the chip mounting points. The flexible supports also mean that the chip selectively couples the resonator in to longitudinal strain.
4. Finite element analysis The finite element analysis (FEA) software Abaqus CAE was used to design and model the behavior of the stress coupling structure. Analysis shows the structure couples longitudinal displacement to the DETF strain gauge well, while lateral in and out of plane displacements generate much less strain in the DETF. Fig. 5. shows the strain developed in the DETF as a result of displacements made to the stress isolation chip supports. Displacements were made in the longitudinal direction of the resonator, laterally (in plane with the stress isolation chip), and vertically (out of the chip plane). From the chart, it can clearly be seen that longitudinal displacements of the stress isolation chip develop approximately 50–100 times more strain in the resonator than displacements in the other degrees of freedom. As well as reducing strain developed in the DETF resonator as a result of non-longitudinal stress, the chip also ensures that the DETF always stays correctly aligned and
Fig. 5. Schematic showing the strain developed in the resonator as a result of displacements made to the selective stress coupling chip.
Fig. 6. Schematic showing the effect of non-longitudinal loading. The central area of the chip with the resonator, stays aligned.
is only subjected to plain longitudinal strain; this is crucial to maintain optimum performance of the resonator. Fig. 6 shows the exaggerated deformation of the stress isolation chip after being subjected to non-longitudinal displacements. From Fig. 6, it can clearly be seen that the central area of the chip stays flat and aligned.
5. Fabrication As a first step towards the final device, a test stress isolation chip etched in bulk silicon was fabricated using an surface technology systems (STS) inductively coupled plasma (ICP) etching machine, the processing route is shown in Fig. 7. A device layer wafer was bonded to a handle wafer using photo-resist. This allows through etching of the device wafer. The device wafer was patterned then etched using the STS-ICP process. The individual stress isolation chips were then removed from the handle wafer using nitric acid. Rather than mounting a resonator on the chip, for the purpose of initial testing a pseudo resonator was built in to the stress isolation chip, see Fig. 8. This allowed a prototype stress isolation chip to be fabricated so mechanical testing could be done to verify the performance of the chip. Holes were added to the chip mounting points to allow locating and fixing of the device during testing. Fig. 9 shows the stress isolation chip sitting on the wafer it was etched from.
Fig. 7. Processing route for pseudo stress coupling chip.
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Fig. 8. Pseudo stress coupling chip 500 m thick, with 10 m × 1000 m beam added to the center of the chip as a pseudo resonator.
Fig. 10. Wyko measurement of the resonator mounting points showing approximately 20 nm difference in height resulting from a 2 m vertical displacement at the ends of the stress coupling chip.
Fig. 9. Manufactured selective stress coupling chip, sitting on the wafer it was etched from.
An advantage of this processing route is that dicing of the wafer is not required as the stress isolation chips are etched free from the wafer.
After applying out of plane displacements to the stress isolation structure the surface profiler could be used to develop an accurate deformation profile of the structure. An example of deformation caused at the chip flexible supports using the test rig is shown in Fig. 10; this was the result of a 2 m vertical displacement at one end of the chip. As the aim of the chip is to create mounting points for a resonator that are constrained to movement along the longitudinal axis of the resonator (Fig. 4), these mounting points were examined. The relative change in height of the mounting points as a result of a 2 m displacement applied to the stress isolation chip is approximately 20 nm, as shown in Fig. 11.
6. Testing A Wyco non-contact surface profiler was used to determine the chip performance. The chip was mounted to a test rig capable of applying fine displacements at the chip mounting points. The chips were located on to the test rig by two pins and fixed with a hard Loctite adhesive to the test rig.
7. Discussion The stress coupling structure allows for easier packaging of resonant devices by making the resonator its self less sensitive to packaging induced stress. It has two main func-
Fig. 11. Comparison of stress coupling chip with another theoretical alternative.
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tions: to couple the resonator to strain in one degree of freedom only, and ensure package induced stress does not cause misalignment of the resonator. The finite element analysis has demonstrated that the structure selectively couples the resonator into longitudinal strain over other degrees of freedom by a factor of more than 50 times. This selectivity means the job of packaging is made easier as packaging induced strain in only one degree of freedom significantly affects the resonator. The second function of the structure is to ensure correct alignment of the resonator despite the presence of packaging induced stress. A comparison of the stress isolation chip with the alternative stiff support bracket solution is given in Fig. 11. For each case, the ratio of out of plane displacement at the mounting points compared to the displacement at the resonator ends is given. The measurements of the stress coupling structure were taken using the Wyco surface profiler. They show that the selective stress coupling structure reduces unwanted out of plane displacement by 100 times. While accurate measurements for in plane lateral displacements of the structure are not possible using the surface profiler FEA suggests similar performance. This demonstrates that the structure effectively gives well constrained mounting mounts for a resonator to be fixed to. The resonator is allowed to remain sensitive to strain while being constrained in other degrees of freedom to ensure it stays aligned with drive and pick-off parallel plate electrodes. As maintaining alignment is important for parallel plate actuators or comb drives that may be used to drive resonant devices this structure allows for a more robust device.
8. Conclusion A novel selective stress coupling structure was presented, with a view towards mounting a micro-resonator on to a macro-scale pressure sensitive diaphragm. As the increased diaphragm size means packaging stress will be increased the structure is designed to make the resonator less sensitive to the adverse affects of packaging. Design and analysis work using FEA shows the potential to selectively couple in to strain in one degree of freedom with a selectivity of 50–100 times. Prototype chips were manufactured using deep reactive ion etching. Initial characterization of the chip using a Wyco
non-contact surface profiler give results that show the chip also reduces out of plane misalignment of the resonator by a factor of 100. Future work will be to fabricate the coupling structure with resonator mounted to it.
References [1] S.P. Beeby, G. Ensell, N.M. White, Silicon resonant strain gauges fabricated using SOI wafers. Micromachining technologies for industry (Ref. No. 2000/032), Presented at the IEE Seminar, 2000, pp. 2/1–2/4. [2] S.P. Beeby, G. Ensell, N.M. White, Microengineered silicon doubleended tuning fork resonators, Eng. Sci. Educ. J. 9 (6) (2000) 265–271. [3] J.C. Greenwood, D.W. Satchell, Miniature silicon resonant pressure sensor, IEE Proc.-Control Theory Appl. 135 (5) (1988) 369–372. [4] H.L. Offereins, H. Sandmaier, B. Folkmer, U. Steger, W. Lang, Stress free assembly technique for a silicon based pressure sensor, in: Digest of Technical Papers, TRANSDUCERS’91, 24–27 June 1991, pp. 986–989. [5] V.L. Spiering, S. Bouwstra, R.M.E.J. Spiering, M. Elwenspoek, Onchip decoupling zone for package-stress reduction, in: Digest of Technical Papers, TRANSDUCERS’91, 24–27 June 1991, pp. 982–985.
Biographies P.K. Kinnell studied mechanical engineering at the University of Birmingham and received his MEng in 2001. He is currently a PhD student at the University of Birmingham, Research Centre for MicroEngineering and Nanotechnology doing research on resonant MEMS devices. His research interest is in resonant pressure sensors, and is currently looking at novel packaging techniques for MEMS pressure sensors. M.C.L. Ward obtained his first degree in physics from Imperial College, London in 1981 and his PhD from Warwick University in 1985 for the study of the passive films on stainless steels using EXAFS. He then joined RSRE Malvern where he worked on silicon micro-electronics and micro-sensors. He is now senior lecturer in micro-systems engineering at the University of Birmingham. His current research interests include micro-systems and distributed sensor systems. R. Craddock graduated with a BSc in chemistry with electronics from Southampton University and a PostGraduate diploma in physics and electronics, and MSc in semiconductor devices from Lancaster University. Initial research work in the design, processing, and compensation of pressure and acceleration sensors at the Lucas Research Centre lead to a transfer to Lucas Nova Sensor in the US to work on automotive accelerometers. He returned to the UK in 1991 to develop pressure sensor structures, processes and compensation methods for GE Druck, where he has responsibility for all silicon engineering and production.
Physical characterisation of selective stress coupling for resonant pressure sensors P.K. Kinnell a,∗ , M.C.L. Ward a , R. Craddock b a
Research Centre for MicroEngineering and Nanotechnology, School of Engineering, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK b GE Druck Limited, Fir Tree Lane, Groby, Leicester LE6 0FH, UK
Received 22 September 2003; received in revised form 15 January 2004; accepted 21 January 2004 Available online 2 April 2004
Abstract This paper presents a selective stress coupling structure for resonant sensor applications. The structure is designed to selectively couple a resonant strain gauge into strain in one degree of freedom only. It constrains the displacement of the resonator such that it always stays aligned. A resonant pressure sensor is proposed that incorporates the stress isolation chip to allow a resonant strain gauge to be mounted to a large pressure sensitive silicon diaphragm. The chip behavior has been modeled using finite element analysis. Physical testing using a Wyko non-contact surface profiler has been carried out to investigate the selective stress coupling structure performance. © 2004 Elsevier B.V. All rights reserved. Keywords: Stress isolation; Strain gauge; Resonant; Packaging; Pressure
1. Introduction Resonant MEMs devices are now widely exploited in many sensor applications. While it is relatively easy to produce stable high Q resonators, packaging these devices in such a way that they are sensitive to the measurand but not affected by packaging stress presents a challenging problem [1,2]. In the case of resonant pressure sensors a micro-resonant strain gauge is typically mounted on to a micro-pressure sensitive diaphragm. This imposes limitations on the pressure ranges that can be measured. For low pressure measurement, a very flexible diaphragm is required. However, as the diaphragm stiffness decreases, the resonator becomes the controlling stiffness in the system, making it more difficult to measure pressure induced strain. An alternative solution would be to increase the size of the diaphragm. A larger diaphragm produces more mechanical work, so should be able to impart a measurable strain on the resonator for a lower pressure, while still retaining the dominant stiffness. The problem with using a larger diaphragm is that as the resonator must be coupled into points ∗ Corresponding author. Tel.: +44-121-414-4217; fax: +44-121-414-3958. E-mail address: [email protected] (P.K. Kinnell).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.01.061
on the diaphragm that are further apart, increasing the effects of residual packaging stress. To overcome these problems, a novel selective stress coupling chip is proposed. The aim of the selective stress coupling chip is to allow a micro-engineered resonant strain gauge to be coupled to a silicon diaphragm greater than 5000 m in diameter. The chip works by constraining the resonator to movement in one degree of freedom only. It is designed to selectively transmit strain in this degree of freedom to the resonator. Most importantly, it keeps the alignment of the resonator with its drive and pick-off electrodes, which is essential for optimum performance of the resonator.
2. The resonant strain gauge The principle of using a resonant strain gauge mounted on a pressure sensitive diaphragm has been well documented [1–3]. For the purposes of this work, a double-ended tuning fork (DETF) resonator is being used as the strain gauge. It is excited electro-statically using a parallel plate actuator. The change in capacitance of a second parallel plate electrode will be used to pick-off the frequency of the device (see Fig. 1). A DETF structure was chosen because it may be driven in a dynamically balanced mode, allowing a high mechanical Q to be developed [1].
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Fig. 1. Schematic of a double-ended tuning fork resonator with drive and pick-off electrodes. Fig. 3. Schematic of proposed resonant pressure sensor assembly, the cut-out shows an enlarged view of the double-ended tuning fork resonator.
3. Selective stress coupling structure Previous work has shown that it is possible to develop on-chip stress reduction zones, which shield sensitive structures from the unwanted affects of packaging or thermally induced stress [4,5]. In the case of a resonant strain gauge the stress isolation must be selective. The affects of packaging stress must be decoupled from the resonator; however, the resonator must still remain sensitive to the strain it was designed to measure. In this work, the aim is to mount a resonator across a span that may be more than five times its length. The simplest solution would be to incorporate stiff fixing brackets that take up the difference in length, as shown in Fig. 2. Stiff mounting brackets make sure that any longitudinal displacements of the diaphragm mounting points are transferred directly to the resonator causing a measurable strain. While this results in an optimum transferral of strain from the diaphragm to the resonator it also means that any residual parasitic packaging stress may also be directly transferred to the resonator. So, while achieving a tolerance in the mounting positions of ±1 or 2 m across the mounting points may be deemed acceptable, if this was applied directly to the ends of the resonator it would have many negative effects. The resulting misalignment between the resonator and its drive electrodes would cause a non-uniform drive force. This would cause an unpredictable resonator performance that may result in the excitation of suboptimal modes. Taken to the extreme if the resonator was to touch the drive electrode this would cause a mechanical and electrical failure of the device.
Fig. 2. Schematic illustrating the use of stiff fixing brackets to mount a micro-resonator to a large diaphragm.
To overcome this problem, the supports must be given a degree of flexibility to take up the unwanted effects of packaging. If the supports were more flexible than the resonator, less misalignment would happen. This is done at the cost of making the resonator less sensitive to the strain it was designed to measure. The solution should therefore offer a compromise between achieving sensitivity to strain caused by pressure, whilst also reducing the unwanted effects of packaging induced strain, and keeping the resonator aligned. To achieve this, a novel selective stress coupling structure is proposed. The aim of this structure is to act as a coupling between the resonant strain gauge at the heart of the pressure sensor and the pressure sensitive diaphragm. Fig. 3 shows a schematic of the proposed resonant pressure sensor. The stress coupling structure can be seen mounted onto a silicon diaphragm. In turn, mounted onto the stress coupling structure is the strain sensitive resonator. (Note: Drive and pick-off electrodes have been omitted from the schematic.)
Fig. 4. Selective stress coupling chip, showing the effect various displacements at the chip mountings have on the chip central area.
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The stress coupling structure consists of three main areas, the mounting points, flexible supports, and the central chip area, see Fig. 4. The mounting points are where the chip is fixed to the diaphragm. The flexible supports are designed to give a degree of compromise between the two extremes mentioned above. They are designed to be significantly stiffer than the combined central chip area and resonator in tension, however, less stiff in all other degrees of freedom. The central chip area consists of two independent areas constrained to linear motion relative to the longitudinal axis of the resonator. The dimensions of the central chip area mean that it is much stiffer than the flexible supports in all degrees of freedom except for along the longitudinal axis of the resonator. In this way, the resonator may be kept aligned regardless of the displacements applied to the chip mounting points. The flexible supports also mean that the chip selectively couples the resonator in to longitudinal strain.
4. Finite element analysis The finite element analysis (FEA) software Abaqus CAE was used to design and model the behavior of the stress coupling structure. Analysis shows the structure couples longitudinal displacement to the DETF strain gauge well, while lateral in and out of plane displacements generate much less strain in the DETF. Fig. 5. shows the strain developed in the DETF as a result of displacements made to the stress isolation chip supports. Displacements were made in the longitudinal direction of the resonator, laterally (in plane with the stress isolation chip), and vertically (out of the chip plane). From the chart, it can clearly be seen that longitudinal displacements of the stress isolation chip develop approximately 50–100 times more strain in the resonator than displacements in the other degrees of freedom. As well as reducing strain developed in the DETF resonator as a result of non-longitudinal stress, the chip also ensures that the DETF always stays correctly aligned and
Fig. 5. Schematic showing the strain developed in the resonator as a result of displacements made to the selective stress coupling chip.
Fig. 6. Schematic showing the effect of non-longitudinal loading. The central area of the chip with the resonator, stays aligned.
is only subjected to plain longitudinal strain; this is crucial to maintain optimum performance of the resonator. Fig. 6 shows the exaggerated deformation of the stress isolation chip after being subjected to non-longitudinal displacements. From Fig. 6, it can clearly be seen that the central area of the chip stays flat and aligned.
5. Fabrication As a first step towards the final device, a test stress isolation chip etched in bulk silicon was fabricated using an surface technology systems (STS) inductively coupled plasma (ICP) etching machine, the processing route is shown in Fig. 7. A device layer wafer was bonded to a handle wafer using photo-resist. This allows through etching of the device wafer. The device wafer was patterned then etched using the STS-ICP process. The individual stress isolation chips were then removed from the handle wafer using nitric acid. Rather than mounting a resonator on the chip, for the purpose of initial testing a pseudo resonator was built in to the stress isolation chip, see Fig. 8. This allowed a prototype stress isolation chip to be fabricated so mechanical testing could be done to verify the performance of the chip. Holes were added to the chip mounting points to allow locating and fixing of the device during testing. Fig. 9 shows the stress isolation chip sitting on the wafer it was etched from.
Fig. 7. Processing route for pseudo stress coupling chip.
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Fig. 8. Pseudo stress coupling chip 500 m thick, with 10 m × 1000 m beam added to the center of the chip as a pseudo resonator.
Fig. 10. Wyko measurement of the resonator mounting points showing approximately 20 nm difference in height resulting from a 2 m vertical displacement at the ends of the stress coupling chip.
Fig. 9. Manufactured selective stress coupling chip, sitting on the wafer it was etched from.
An advantage of this processing route is that dicing of the wafer is not required as the stress isolation chips are etched free from the wafer.
After applying out of plane displacements to the stress isolation structure the surface profiler could be used to develop an accurate deformation profile of the structure. An example of deformation caused at the chip flexible supports using the test rig is shown in Fig. 10; this was the result of a 2 m vertical displacement at one end of the chip. As the aim of the chip is to create mounting points for a resonator that are constrained to movement along the longitudinal axis of the resonator (Fig. 4), these mounting points were examined. The relative change in height of the mounting points as a result of a 2 m displacement applied to the stress isolation chip is approximately 20 nm, as shown in Fig. 11.
6. Testing A Wyco non-contact surface profiler was used to determine the chip performance. The chip was mounted to a test rig capable of applying fine displacements at the chip mounting points. The chips were located on to the test rig by two pins and fixed with a hard Loctite adhesive to the test rig.
7. Discussion The stress coupling structure allows for easier packaging of resonant devices by making the resonator its self less sensitive to packaging induced stress. It has two main func-
Fig. 11. Comparison of stress coupling chip with another theoretical alternative.
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P.K. Kinnell et al. / Sensors and Actuators A 115 (2004) 230–234
tions: to couple the resonator to strain in one degree of freedom only, and ensure package induced stress does not cause misalignment of the resonator. The finite element analysis has demonstrated that the structure selectively couples the resonator into longitudinal strain over other degrees of freedom by a factor of more than 50 times. This selectivity means the job of packaging is made easier as packaging induced strain in only one degree of freedom significantly affects the resonator. The second function of the structure is to ensure correct alignment of the resonator despite the presence of packaging induced stress. A comparison of the stress isolation chip with the alternative stiff support bracket solution is given in Fig. 11. For each case, the ratio of out of plane displacement at the mounting points compared to the displacement at the resonator ends is given. The measurements of the stress coupling structure were taken using the Wyco surface profiler. They show that the selective stress coupling structure reduces unwanted out of plane displacement by 100 times. While accurate measurements for in plane lateral displacements of the structure are not possible using the surface profiler FEA suggests similar performance. This demonstrates that the structure effectively gives well constrained mounting mounts for a resonator to be fixed to. The resonator is allowed to remain sensitive to strain while being constrained in other degrees of freedom to ensure it stays aligned with drive and pick-off parallel plate electrodes. As maintaining alignment is important for parallel plate actuators or comb drives that may be used to drive resonant devices this structure allows for a more robust device.
8. Conclusion A novel selective stress coupling structure was presented, with a view towards mounting a micro-resonator on to a macro-scale pressure sensitive diaphragm. As the increased diaphragm size means packaging stress will be increased the structure is designed to make the resonator less sensitive to the adverse affects of packaging. Design and analysis work using FEA shows the potential to selectively couple in to strain in one degree of freedom with a selectivity of 50–100 times. Prototype chips were manufactured using deep reactive ion etching. Initial characterization of the chip using a Wyco
non-contact surface profiler give results that show the chip also reduces out of plane misalignment of the resonator by a factor of 100. Future work will be to fabricate the coupling structure with resonator mounted to it.
References [1] S.P. Beeby, G. Ensell, N.M. White, Silicon resonant strain gauges fabricated using SOI wafers. Micromachining technologies for industry (Ref. No. 2000/032), Presented at the IEE Seminar, 2000, pp. 2/1–2/4. [2] S.P. Beeby, G. Ensell, N.M. White, Microengineered silicon doubleended tuning fork resonators, Eng. Sci. Educ. J. 9 (6) (2000) 265–271. [3] J.C. Greenwood, D.W. Satchell, Miniature silicon resonant pressure sensor, IEE Proc.-Control Theory Appl. 135 (5) (1988) 369–372. [4] H.L. Offereins, H. Sandmaier, B. Folkmer, U. Steger, W. Lang, Stress free assembly technique for a silicon based pressure sensor, in: Digest of Technical Papers, TRANSDUCERS’91, 24–27 June 1991, pp. 986–989. [5] V.L. Spiering, S. Bouwstra, R.M.E.J. Spiering, M. Elwenspoek, Onchip decoupling zone for package-stress reduction, in: Digest of Technical Papers, TRANSDUCERS’91, 24–27 June 1991, pp. 982–985.
Biographies P.K. Kinnell studied mechanical engineering at the University of Birmingham and received his MEng in 2001. He is currently a PhD student at the University of Birmingham, Research Centre for MicroEngineering and Nanotechnology doing research on resonant MEMS devices. His research interest is in resonant pressure sensors, and is currently looking at novel packaging techniques for MEMS pressure sensors. M.C.L. Ward obtained his first degree in physics from Imperial College, London in 1981 and his PhD from Warwick University in 1985 for the study of the passive films on stainless steels using EXAFS. He then joined RSRE Malvern where he worked on silicon micro-electronics and micro-sensors. He is now senior lecturer in micro-systems engineering at the University of Birmingham. His current research interests include micro-systems and distributed sensor systems. R. Craddock graduated with a BSc in chemistry with electronics from Southampton University and a PostGraduate diploma in physics and electronics, and MSc in semiconductor devices from Lancaster University. Initial research work in the design, processing, and compensation of pressure and acceleration sensors at the Lucas Research Centre lead to a transfer to Lucas Nova Sensor in the US to work on automotive accelerometers. He returned to the UK in 1991 to develop pressure sensor structures, processes and compensation methods for GE Druck, where he has responsibility for all silicon engineering and production.