EUROSENSORS XIV
`The 14th European Conference on Solid-State Transducers
`August 27-30, 2000, Copenhagen, Denmark
`
`T3W4
`Miscellaneous physical sensors
`
`A Wireless Batch Sealed Absolute Capacitive Pressure Sensor
`
`Orhan Akar1, Tayfun Akin1, Tim Harpster2, and Khalil Najafi2
`1Middle East Technical University, Dept. of Electrical and Electronics Eng., Ankara, Turkey
`2Center for Integrated Microsystems, The University of Michigan, Ann Arbor, MI 48109
`e-mail: tayfun-akin@metu.edu.tr http://www.eee.metu.edu.tr/~tayfuna
`
`Summary. This paper reports the development of an absolute wireless capacitive pressure sensor.
`The sensor has a simple structure and is obtained using a capacitive pressure sensor and a gold-
`electroplated planar coil that form an LC circuit. Applied pressure deflects the 6 m m-thin silicon
`diaphragm, changing the capacitance, and hence, the resonant frequency of the circuit. This
`change is sensed remotely with inductive coupling, eliminating the need for wire connection or
`implanted RF telemetry circuits to monitor the applied pressure. The sensor is fabricated using
`boron-etch stop technique. Fabricated devices measure 2.6x1.6 mm 2 in size and houses 24 turns
`of gold-electroplated coil that has a measured inductance of 1.2 m H. The sensor is designed to
`provide a resonant frequency change between 95-103MHz for a pressure change in the range 0-
`50mmHg w.r.t. the ambient pressure, providing a pressure responsivity and sensitivity of 160
`KHz/mmHg and 1553ppm/mmHg, respectively. The measured pressure responsivity and
`sensitivity of the fabricated device are 120KHz/mmHg and 1579ppm/mmHg, respectively.
`
`Keywords: wireless sensor, sealed sensor, and capacitive pressure sensor
`
`Introduction
`
`Sensor structure
`
`Absolute pressure sensors are required in many
`applications, including industrial process control,
`environmental monitoring, and biomedical systems.
`Capacitive pressure sensors provide very high
`pressure sensitivity, low noise, and low temperature
`sensitivity and are preferred in many emerging high-
`performance applications. However, to fabricate
`absolute pressure sensors with sealed cavities that also
`allow easy lead transfer from inside of the cavity to
`outside
`requires
`relatively complex
`fabrication
`technologies [1]. In addition to the need for batch-
`sealed absolute pressure sensors, many emerging
`applications require these sensors to operate via a
`wireless link. One such application is in continuous
`long-term monitoring of the pressure for various
`medical applications [2]. Some wireless telemetry
`systems use active
`transmitters, however,
`these
`devices are often big and require a power source in
`the form of an implanted battery, or an implanted
`active circuit that can receive power through inductive
`telemetry [3]. Both of these complicate the system
`design and operation.
`This paper presents a new structure for wireless
`absolute capacitive pressure sensors. The structure
`utilizes a parallel inductor-capacitor resonant circuit.
`The capacitor is formed by the pressure sensor and
`changes in response to changes in pressure. The on-
`chip inductor is fabricated inside the sealed cavity of
`the sensor and is connected to the pressure sensitive
`capacitor. Because the inductor is inside the sealed
`cavity of the pressure sensor, lead transfer outside of
`the cavity is not needed, which in turn allows one to
`batch seal pressure sensors at the wafer level.
`Furthermore, the sensor can be operated using a
`passive telemetry approach [4-7], as discussed later.
`
`Figure 1 shows the structure of the wireless pressure
`sensor. It consists of a boron-doped silicon diaphragm
`with a thickness of ~3-6m m, which is supported by
`anchors that fix it to a glass substrate. The diaphragm
`is suspended over the glass by a gap of about 2m m and
`can deflect when the pressure across it changes. This
`forms a variable-gap capacitor. To obtain high
`pressure sensitivity, the capacitive gap should be as
`small as possible, usually in the range of 1-2µm. The
`capacitor gap should also house the gold-electroplated
`coil. The planar coil is placed in a recessed glass area
`around the metal capacitor plate on the glass. By
`making the recess as deep as possible, it is possible to
`fabricate very thick on-chip coils with high Q. This is
`necessary to obtain high sensitivity for the sensor.
`Figure 2 shows the cross-section and electrical
`equivalent of the wireless pressure sensor. The
`variable capacitor and the electroplated coil inside it
`form an LC circuit, whose resonant frequency
`changes with the applied pressure. The change in the
`resonant frequency is sensed remotely with inductive
`coupling, eliminating the need for wire connection to
`monitor the applied pressure.
`
`Diaphragm
`
`p++ silicon
`
`Fixed Capacitor Plate
`
`Electroplated Coil
`
`Glass Substrate
`
`Fig. 1: Structure of the wireless pressure sensor.
`
`ISBN 87-89935-50-0
`
`585
`
`Abbott
`Exhibit 1026
`Page 001
`
`

`

`O. Akar et al., A Wireless Batch Sealed Absolute Capacitive Pressure Sensor, pp. 585-588
`
`T3W4
`Miscellaneous physical sensors
`
`Pressure Sensitive Capacitor
`
`Silicon
`
`L Cx
`
`Metal plate
`
`Glass
`
`Planar Inductor
`
`Glass Recess
`
`(a)
`(b)
`Fig. 2: Wireless capacitive pressure sensor: (a) cross-
`section, (b) electrical equivalent. The change in the
`resonance frequency due to capacitance change is
`sensed remotely with inductive coupling, eliminating
`the need for wire connection.
`
`Re
`
`~
`
`Le
`
`M
`
`R
`
`L
`
`C
`
`Zs
`
`Re
`~
`
`Le
`
`(w M)2/Zs(w )
`
`External coil
`Pressure sensor
`Reflected load
`Fig. 3: Equivalent circuit model of the telemetric
`readout approach.
`
`Inductive coupling link
`
`Figure 3 shows the equivalent circuit model of the
`inductive coupling link used in the telemetric readout.
`The sensor is modeled with an inductor, L, a series
`resistance of the inductor, R, and a variable capacitor,
`C. The resonance frequency of the sensor is given by
`
`=
`
`f
`
`0
`
`2
`
`1
`LC
`
`
`
`(Eq.1)
`
`The resonance frequency changes in response to
`pressure and it can be detected by inductive telemetry
`with an external coil antenna. Due to inductive
`coupling, the external coil stimulates the sensor
`circuit, and the load impedance is reflected back to the
`antenna. Reflected impedance, Xl, can be found as a
`function of the sensor impedance, Zs, and the mutual
`inductance, M, between the coil antenna and the
`integrated inductance in the sensor, as
`(
`)
`ww
`2
`M
`(
`Z
`
`=
`
`X
`
`l
`
`)
`
`s
`
`(Eq.2)
`
`(Eq.3)
`
`(Eq.4)
`
`(Eq.5)
`
`=
`LLkM
`e
`(cid:247)ł(cid:246)(cid:231)Ł(cid:230) -
`1
`+
`=
`www
`)
`(
`R
`L
`j
`C
`where w
` is the angular frequency, k is the coupling
`coefficient. The impedance seen at the external coil
`due to coupling is given by [8-9]
`(
`)
`wwww
`M
`)
`(
`Z
`where Re and Le are the series resistance and the
`inductance of the external coil. At the resonant
`frequency of the pressure sensor equivalent circuit,
`the impedance ZS becomes purely resistive and
`
`where,
`
`Z s
`
`Z
`
`(
`
`=
`
`)
`
`+
`
`R
`e
`
`+
`
`Lj
`e
`
`2
`
`s
`
`586
`
`reduces to only R, therefore, the impedance of the
`external antenna becomes
`(
`)
`
` www += +
`
`
`(
`)
`j
`R
`L
`e
`
`2
`
`(Eq.6)
`
`RM
`
`o
`
`Z
`
`o
`
`e
`
`o
`
`By monitoring overall impedance change of the
`external coil due to the reflected impedance, it is
`possible to detect the sensor resonance frequency.
`This change is more detectable if the phase of the Z is
`monitored.
` The approximate magnitude of the
`impedance phase dip is given by
`(cid:247)(cid:247)ł(cid:246)(cid:231)(cid:231)Ł(cid:230)@D -
`wj
`tan
`
`2
`
`M e
`
`1
`
`DIP
`
`o
`RL
`The impedance phase dip is maximized when the
`series resistance of the electroplated coil, i.e., R, is
`minimized and its inductance, L, is maximized for
`larger M. However, it is important to note that, when
`L is increased by increasing the number of turns in the
`coil, then the parasitic capacitance of the coil also
`increases, decreasing the self resonant frequency of
`the planar coil [8-9]; but, the coil self resonant
`frequency should be much higher then the operating
`frequency for proper device operation. Therefore, it is
`more important to decrease the series resistance of the
`electroplated coil for a sensitive detection of the
`resonant frequency of the sensor. The increase in the
`series resistance of
`the coil at high operation
`frequencies due to skin effects should also be
`considered during the design phase [9].
`
`(Eq.7)
`
`Fabrication Process
`
`The fabrication process of the sensor is based on the
`bulk silicon dissolved wafer process [10] with some
`additional steps needed to integrate the on-chip
`electroplated coil with the pressure sensor. A silicon
`wafer is selectively etched with KOH to create a 2m m
`recess to define the capacitive gap. Then, two high-
`temperature boron diffusion steps are applied to
`define the 12m m support anchors and the 3-6m m
`diaphragm of the pressure sensor. The diaphragm
`thickness can be reduced down to 2.5 m m if necessary.
`Meanwhile, a glass wafer is processed to define the
`fixed plate of the capacitor, which is formed by
`depositing and patterning a layer of Ti/Pt/Au. The
`glass wafer is selectively etched to have a 7m m recess,
`so that 6m m-thick electroplated coil does not touch to
`the diaphragm of the capacitive pressure sensor when
`front sides of glass and silicon wafers face each other
`and are attached using silicon-to-glass anodic
`bonding.
`This process has a number of advantages. It is a
`simple, batch, and high yield process, and
`it
`eliminates the problems associated with the discrete
`coil attachment. The coil is sealed inside the pressure
`sensor cavity, so that the harsh external environment
`does not affect it. It should be noted that for
`biomedical applications, all the materials used in this
`process are biocompatible, there is no need for
`additional protective coatings.
`
`Abbott
`Exhibit 1026
`Page 002
`
`p
`

`

`EUROSENSORS XIV
`The 14th European Conference on Solid-State Transducers
`August 27-30, 2000, Copenhagen, Denmark
`
`T3W4
`Miscellaneous physical sensors
`
`Fabrication and test results
`
`A number of different sensor structures have been
`designed and fabricated. Figure 4 shows a SEM
`photograph of the gold electroplated coil windings on
`the glass substrate. The width and the height of the
`gold lines are 7µm and 6µm, respectively, while the
`separation of the lines is 7µm. Figure 5 shows a
`fabricated sensor seen through the glass substrate.
`This device measures 2.6mmx1.6mm and houses 24
`turns of gold-electroplated coil. Table 1 shows
`performance parameters of the sensor. The sensor is
`optimized for a pressure range of 0-50mmHg.
`Figure 6 shows the SEM photograph of the fabricated
`sensor showing the cross-sectional view of the
`capacitive gap. Figure 7 shows another SEM
`photograph of the fabricated sensor, which is broken
`to show the electroplated coil windings inside the
`sealed cavity. Gold electroplated coils are measured
`to be 1.2m H. With the expected capacitance variation
`due to changes in pressure, the resonant frequency of
`the sensor is expected to change between 103 and 95
`MHz for a pressure range of 0-50mmHg w.r.t. the
`ambient pressure, providing a pressure sensitivity of
`1553ppm/mmHg and a pressure responsivity of
`160KHz/mmHg.
`
`Table 1: Summary of the device characteristics.
`
`Parameter
`Diaphragm thickness
`Capacitor plate separation
`Full scale deflection
`Dynamic range
`Total diaphragm area
`Device size
`Inductance
`Capacitance change for 0-50mmHg
`Frequency shift for 0-50mmHg
`Pressure sensitivity
`Pressure responsivity
`
`Value
`6m m
`2m m
`0.4m m
`0-50mmHg
`2x(680x680m m2)
`2.6mmx1.6mm
`1.2m H
`2.0-2.35pF
`103-95MHz
`1553ppm/mmHg
`160KHz/mmHg
`
`Fig. 5: Photograph of a fabricated sensor seen through
`the
`glass
`substrate. This
`device measures
`2.6mmx1.6mm and houses 24
`turns of gold-
`electroplated coil. The capacitive plate separated into
`two parts to increase the dynamic range.
`
`Electroplated Coil
`
`Thin Silicon Diaphragm
`
`Thin Silicon Diaphragm
`
`P++ Support
`
`Electroplated Coil
`
`Glass
`
`Capacitive Gap
`
`Fig. 6: SEM photograph of a fabricated sensor
`showing the cross-sectional view.
`
`P++ Thick Silicon Support
`
`Fixed capacitor plate
`
`P++ Thin Silicon
`Diaphragm
`
`Coil
`windings
`
`Glass recess
`
`Glass Substrate
`
` Electroplated Coil
`
`Fixed
`Capacitor
`Plate
`
`Fig. 4: SEM photograph of the gold electroplated coil
`windings.
`
`Fig. 7: Another SEM photograph of the fabricated
`sensor, which is broken to show the electroplated coil
`windings inside the sealed cavity.
`
`ISBN 87-89935-50-0
`
`587
`
`Abbott
`Exhibit 1026
`Page 003
`
`

`

`T3W4
`Miscellaneous physical sensors
`
`O. Akar et al., A Wireless Batch Sealed Absolute Capacitive Pressure Sensor, pp. 585-588
`
`Fig. 8: External coil phase shift due to the resonant
`frequency shift of the sensor with the applied pressure.
`
`80
`
`75
`
`70
`
`65
`
`60
`
`Resonance Frequency (MHz)
`
`A number of measurements were performed to
`verify the device operation and to characterize its
`performance. The measurements were completed
`using an external coil, a pressure chamber, and an
`impedance analyzer. The phase shift on the external
`coil due to inductive coupling is monitored using HP
`4395A Network/Impedance Analyzer.
` Figure 8
`shows the impedance phase changes on the external
`stimulating coil at different pressure values w.r.t. the
`ambient pressure. The monitored resonant frequency
`chance is between 76MHz and 70MHz in the
`0-50mmHg pressure range, resulting in a pressure
`responsivity of 120kHz/mmHg and a pressure
`sensitivity of 1580ppm/mmHg. Figure 9 shows
`remotely detected sensor resonance frequency change
`with respect to the applied pressure change from zero
`pressure to the over pressure value of 100mmHg.
`It should be noted here that the resonant frequency
`of the sensor seems lower than the design value. The
`reason is believed to be the difference between the
`actual pressure inside the cavity and the ambient
`pressure. The pressure in the cavity is not ambient
`pressure, but the applied pressure is monitored with
`respect to the ambient pressure. The pressure inside
`the cavity is lower than the ambient pressure, since
`during the sealing process with electrostatic glass to
`silicon anodic bonding, the structure is heated upto
`400(cid:176) C. When the device cools down, the air inside
`the sealed cavity contracts and pulls the diaphragm
`closer to the fixed metal capacitor plate on the glass,
`increasing the zero pressure capacitance, hence,
`decreasing the resonant frequency of the sensor.
`Although
`the measured
`resonant
`frequency
`is
`different than the designed resonant frequency, the
`measured pressure sensitivity of
`the device
`is
`1580ppm/mmHg, which very close to the design
`sensitivity of 1550ppm/mmHg. The device sensitivity
`can be increased by increasing the device dimensions.
`These measurements show that the device is
`functional and allows to measure the pressure of a
`sealed capacitive pressure sensor remotely, without
`requiring lead transfer from the sealed cavity.
`
`Conclusions
`
`An absolute wireless capacitive pressure sensor has
`been designed and fabricated. The sensor consists of
`a pressure sensitive capacitor and an integrated
`inductor forming a pressure sensitive resonant circuit.
`The resonance frequency of the sensor is measured by
`inductive
`telemetry. Fabricated devices measure
`2.6x1.6 mm2 in size and are optimized to provide a
`dynamic range of 0-50mmHg. The fabricated device
`is operational, and its resonant frequency is monitored
`to change between 76MHz and 70MHz when a
`pressure between 0 and 50mmHg is applied with
`respect to the ambient pressure. This corresponds to a
`pressure
`responsivity
`and
`sensitivity
`of
`120KHz/mmHg or 1580ppm/mmHg, respectively.
`
`588
`
`0
`
`10
`
`20
`
`30
`
`60
`50
`40
`Pressure (mmHg)
`Fig. 9: Resonant frequency change of the sensor with
`the applied pressure w.r.t. the ambient pressure.
`
`70
`
`80
`
`90
`
`100
`
`110
`
` Acknowledgements
`This work is supported by NSF-International Grant No:
`9602182, which is provided to Prof. Najafi and Prof. Akin
`for US-Turkey Cooperative Research. Authors would like to
`thank Dr. Babak Ziaie for his valuable discussions.
`
`References
`[1] A. Chavan and K. D. Wise. 9th Int. Conf. on Solid-State
`Sensors & Actuators, (TRANSDUCERS’97) , June 16-
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`[7] T. Harpster, S. Hauvespre, M. Dokmeci, and K. Najafi.
`IEEE Int. Conf. on Micro Electro Mechanical Systems,
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`[8] F. E. Terman. Radio Engineer’s Handbook, McGraw-
`Hill, New York, 1943.
`[9] E. G. Weber. Inductance; magnetic materials. Radio
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`Dev., 35-12 (1988) 2355-2362.
`
`Abbott
`Exhibit 1026
`Page 004
`
`