Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2002-04-25
2004-06-01
Le, N. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S661000, C073S514180, C073S862626, C073S862610
Reexamination Certificate
active
06744264
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to MEMS circuits and, more specifically, to a method and circuit for testing the characteristics of the individual capacitances of a MEMS sensor that has been sealed or packaged that may be integrated with another circuit or packaged with another circuit.
BACKGROUND OF THE INVENTION
One prior art model of a MEMS sensor integrated with a circuit is shown in FIG.
1
. The MEMS sensor
100
is represented by a first capacitor
110
and a second capacitor
120
sharing a common node
130
a
that represents a moving element. When an excitation voltage is applied to the plates
110
a
and
120
a
of a MEMS sensor and fluctuations occur on the moving element
130
a
due to an input stimulus like acceleration or pressure, the moving element changes its position according to the input stimulus. When this occurs, capacitances between
110
and
120
change. The output
130
of the moving element of the MEMS sensor
100
is fed into a first amplifier input
141
of a capacitor-voltage (C-V) converter
143
. The other input to the amplifier is connected to a reference voltage
142
. During a reset, the reference voltage is also applied to the two plates
181
and
182
of the MEMS sensor
100
. The reference voltage
142
can be hardwired to the sensor and amplifier directly through the pins of the package or can be controlled by an on-chip control
150
, such as an ASIC or other control logic. In either case, after the packaged MEMS sensor and IC have been reset by switch
192
, the voltage applied to the plates of the sensor is excited by changing the voltage directly applied to the pins of the package or by programming the control logic to switch between various voltages supplied to the package. The excitation voltage applied to plates
181
and
182
start at the voltage reference after reset and then are excited to an excitation voltage that is equal in magnitude and opposite in polarity. For instance, the voltage applied to the first plate
181
would step from the voltage reference to an excitation voltage (Vexcite) at the same time that the voltage applied to the second plate
182
steps from the reference voltage Vref to a negative excitation voltage (−Vexcite). Each step function would then alternate to its original reference voltage state and back again so that any fluctuations on the moving element
130
a
would cause corresponding fluctuations on the capacitors
110
and
120
.
The amplifier
140
produces a C-V output voltage
155
reflecting the difference between the first and second capacitances
110
and
120
experienced by fluctuations in the moving member caused by the input stimulus. The C-V output voltage
155
is typically modified by a feedback capacitance Cref represented by feedback path
145
(and reset by switch
192
) such as to produce an output voltage Vout=−[(C
1
−C
2
)/Cref]*(Vexcite−Vref). The C-V output voltage
155
is then signal conditioned as needed by other integrated circuitry
160
, such as filters, gain and offset trim and the like. The final output voltage
170
of the integrated device represents the physical activity of the MEMS sensor and is used in various applications such as accelerometers, pressure sensors, gyroscopes.
To reduce failure rates, the MEMS sensors are tested before being packaged. However, before and during packaging with the circuit, additional processing problems cause some MEMS sensors to malfunction or become damaged. Some problems may occur due to moisture ingress into the capacitor, for example. Common MEMS problems involve stiction where the moving element
130
a
or proof mass comes into contact with the fixed plates
181
,
182
. Additionally, breakages or holes may occur in the moving element. Because the MEMS sensor's moving element is extremely delicate, often resulting in capacitance changes in the few femto-farad range, direct connections to sensor elements such as the moving element are problematic as any probing would make the measurements inaccurate. Additionally, once the circuit and sensor are packaged, the only measurement available always reflects the difference between both sensor capacitances and does not assist in identifying problems with the individual capacitances.
Accordingly, what is needed is a packaged sensor device that allows for more accurate testing of the MEMS sensor after it has been packaged with an IC.
REFERENCES:
patent: 5325065 (1994-06-01), Bennett et al.
patent: 5345824 (1994-09-01), Sherman et al.
patent: 5465604 (1995-11-01), Sherman
patent: 5503016 (1996-04-01), Koen
patent: 5540095 (1996-07-01), Sherman et al.
patent: 5612494 (1997-03-01), Shibano
patent: 5616863 (1997-04-01), Koen
S. J. Sherman et al., “A Low Cost Monolithic Accelerometer; Product/Technology Update”, Analog Devices, Inc. Wilmington, MA, IEEE 1992, 0-7803-0817-4/92, pp. 19.1.1-19.1.4.
Henry V. Allen et al., “Self-Testable Accelerometer Systems”, IC Sensors, Milpitas, CA, 1989 IEEE, TH0249-3/89/0000/0024, pp. 113-115.
Farzad Purahmadi et al., “Silicon Accelerometer With New Thermal Self-Test Mechanism”, Lucas NovaSensor, Fremont, CA 94539, 1992 IEEE, 0-7803-0456-X/92, pp. 122-125, no month available.
Leland Chip Spangler et al., “Isaac—Integrated Silicon Automotive Accelerometer”, Ford Microelectronics, Inc., Colorado Springs, CO, The 8thInternational Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, Jun. 25-29, 1995, pp. 585-588.
Thomas Tschan et al., “A High-Performance, Low Cost Capacitive Accelerometer Trimmed Using Nonvolatile Potentiometers (EEPOT's)”, Silicon Microstructures Inc. Fremont, CA, The 8thInternational Conference on Solid-State Sensors and Actuators, and Eurosensors IX, Stockholm, Sweden, Jun. 25-29, 1995, pp. 601-603.
Gogoi Bishnu P.
Jo Sung Jin
Le N.
Motorola Inc.
Nguyen Vincent Q.
LandOfFree
Testing circuit and method for MEMS sensor packaged with an... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Testing circuit and method for MEMS sensor packaged with an..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Testing circuit and method for MEMS sensor packaged with an... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3365690