Chapter 13 MEMS slides 110407 - Elsevier · PDF fileEE141 3 System-on-Chip Test Architectures...
Transcript of Chapter 13 MEMS slides 110407 - Elsevier · PDF fileEE141 3 System-on-Chip Test Architectures...
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Chapter 13Chapter 13
MEMS TestingMEMS Testing
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What is this chapter about?What is this chapter about?
Microeletromechanical Systems (MEMS) Have emerged as a successful technology by utilizing the
existing infrastructure of the integrated circuit (IC) industry. MEMS along with IC has created new opportunities in
physical, chemical and biological sensor and actuator applications.
Focus on Testing considerations for MEMS, test methods and
instrumentation for MEMS. Overview of testing approaches for RF MEMS, Optical MEMS,
Fluidic MEMS, Accelerometers, Gyroscopes, and Microphones.
Testing Digital Microfluidic Biochips, DFT and BIST for MEMS.
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TopicsTopics
Introduction MEMS Testing Considerations Test Methods and Instrumentation for MEMS
Electrical, Optical, and Mechanical Test Methods Material Property Measurements, Failure Mode and
Analysis, and Environmental Testing
Test Methods for RF MEMS, Optical MEMS, Fluidic MEMS, Accelerometers,
Gyroscopes, and Microphones Digital Microfluidic devices.
DFT and BIST for MEMS Overview of DFT and BIST techniques, and MEMS BIST
examples
Concluding Remarks
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13.1 Introduction13.1 Introduction MEMS devices are miniature electromechanical sensors and actuators
fabricated using VLSI processing techniques. Typical sizes for MEMS devices range from nanometers to millimeters (100 nm to 1000 µm).
MEMS enhances realization of system-on-chip (SOC) by integration of mixed domain technologies such as electrical, optical, mechanical, thermal, and fluidics.
Typical examples for commercial MEMS devices include Analog Devices’ ADXL series accelerometers, FreeScale Semiconductor’s pressure sensors and accelerometers, Texas Instruments’ digital light processing (DLP) displays, and Knowles Electronics’ SiSonic MEMS microphone.
Microfluidics-based biochips, also referred to as lab-on-a-chip, are replacing cumbersome and expensive laboratory equipment for applications such as high-throughput sequencing, parallel immunoassays, protein crystallization, blood chemistry for clinical diagnostics, and environmental toxicity monitoring.
To ensure the testability and reliability of these MEMS-based SOCs, MEMS devices need to be thoroughly tested, particularly when used for safety-critical applications such as in the automotive and healthcare industry. Therefore, there is a pressing need for design for testability(DFT) and built-in self-test (BIST) of MEMS
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13.2 MEMS Testing Considerations 13.2 MEMS Testing Considerations
MEMS devices necessitate special considerations during fabrication processes such as handling, dicing, testing, and packaging.
The micromechanical parts need to be protected from shock and vibration during transport and packaging.
Extreme care must be taken to avoid particle contamination at various processing steps involved in MEMS fabrication.
As a common practice in MEMS industry, the backside of a fully processed wafer is attached to an adhesive plastic film and then mounted in a rigid frame for dicing at the wafer-processing facility.
Handle with Care!
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13.2 MEMS Testing Considerations (Continued)13.2 MEMS Testing Considerations (Continued)
MEMS devices often require packaging before dicing—that is, 0-level packaging at the wafer level by either wafer-to-wafer bonding or local bonding of minature caps (e.g., Si or glass) over the MEMS structure using a hermetic sealing ring.
MEMS test methods and instrumentation vary depending on whether the testing is performed at the wafer level (i.e., unpackaged die) or on packaged devices.
Wafer-level testing is carried out using precision-controlled wafer probers that step from die to die on the wafer, making electrical contact using needle probes.
Fully packaged MEMS devices can be tested with the electrical and non-electrical inputs required for the sensor to function. A variety of environmental test methods commonly used for testing packaged ICs can be directly employed for testing packaged MEMS devices.
Many standard tests are common to both ICs and MEMS, such as thermal cycling, high temperature storage, thermal shock, and high humidity. However, many MEMS packages need to fulfill additionalspecifications.
Testing die vs. Packaged device
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13.3 Test Methods and Instrumentation for MEMS13.3 Test Methods and Instrumentation for MEMS
Test Instrumentation for testing MEMS MEMS encompass a wide variety of applications such as inertial sensors
(accelerometers and gyroscopes), RF MEMS, optical MEMS, and bio or fluidic MEMS.
Test instrumentation depends on the specific type of MEMS device and the desired performance characteristics. For example, inertial MEMS sensors require different test instrumentation than RF MEMS.
Functionality Testing vs. Reliability/Failure Testing MEMS testing can be categorized as (1) functionality and performance
testing and (2) reliability/failure testing.
In functional testing, the characteristic performance parameters are measured and compared against benchmark specifications to verify the intended operation of the MEMS device.
In reliability/failure testing, the performance degradation over sustained operation or shelf life and eventual failure of the device are investigated. Quite often the borderline between functional testing and reliability testing is not always clear. Have emerged as a successful technology by utilizing the existing infrastructure of the integrated circuit (IC) industry.
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13.3 Test Methods and Instrumentation for MEMS13.3 Test Methods and Instrumentation for MEMS
13.3.1 Electrical Test Method Electrical tests are one of the most important methods employed to
characterize MEMS. A typical electrical test setup consists of a probe station interfaced with the required test instrumentation.
A wide range of electrical test equipment used for VLSI testing is commonly used to perform electrical characterization of MEMS devices.
Typical electrical test instrumentation includes current, voltage, and resistance measurement systems, capacitance-voltage measurement systems, impedance analyzers for low-frequency characterization, network analyzers for high-frequency characterization, and signal analyzers.
Probe lengths and wire types (shielded and unshielded) must also be carefully considered. For instance, resistance measurements mustinclude a means for reducing contact errors. Capacitance measurements need to take into account the stray capacitance in test lines.
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Test Methods and Instrumentation for MEMSTest Methods and Instrumentation for MEMS
13.3.1 Electrical Test A typical experimental setup used for testing an electrostatically actuated MEMS
relay is shown below. The setup shown consists of an Agilent 33220A function generator, Krohnit 7600 Wideband Amplifier, HP 54501A Oscilloscope, MM8060 Micromanipulator Probe Station, and HP3468A 4-wire Multimeter.
The basic test setup described here can be used to test a variety of actuators including electrostatic, thermal, and piezoelectric.
Source: L. Almeida, R. Ramadoss, R. Jackson, K. Ishikawa, and Q. Yu, Study of Electrical Contact Resistance of Multi-Contact MEMS relay fabricated using MetalMUMPs process, J. Micromechanics and Microengineering, 16(6), pp. 1189–1194, July 2006.
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13.3 Test Methods and Instrumentation for MEMS13.3 Test Methods and Instrumentation for MEMS
13.3.2 Optical Test Methods MEMS actuators typically include mechanical motion associated with
the electrical signals. Optical profilometers, such as an optical microscope, confocal
microscope, optical interferometers, and laser Doppler vibrometer, are useful for making static and dynamic measurements of MEMS devices.
An optical microscope equipped with high-resolution objectives and accurate graticule can be used to measure MEMS features in a two-dimensional plane view.
Modern confocal microscopes employ low-cost lasers and computers to scan a thin slice through the specimen.
The optical interferometers make use of white light (e.g., a sodium lamp) or of coherent monochromatic light (a laser light). Optical interferometers are useful for measuring noncontact three-dimensional profiles of MEMS devices.
Examples of optical interferometers include Wyko series manufactured by Veeco Instruments, NewView 6000 series manufactured by Zygo, PhotoMap 3-D profilometers by Fogale nanotech, and the Xi-100 developed by Ambios Technology.
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13.3 Test Methods and Instrumentation for MEMS13.3 Test Methods and Instrumentation for MEMS
13.3.2 Optical Test Setup – Laser Doppler Vibrometer (LDV) Laser Doppler vibrometry (LDV) is based on the modulation of laser interference fringes
caused by motion of the device under test (DUT). The fringe pattern in a Doppler vibrometeris moving at a rate proportional to the device motion.
By measuring the time rate of change in distance between successive fringes, a vibrometercan measure displacement as well as velocity. The direction of motion can be determined by observing the Doppler effect on the modulation frequency. LDV is useful for measuring transient and steady-state responses of MEMS devices.
A wide variety of LDVs for MEMS applications are available such as Polytec’s MSA-400 Microsystem Analyzer, which uses white light interferometry for static surface topography, laser Doppler vibrometry for measuring out-of-plane vibrations, and stroboscopic video microscopy for measuring in-plane motion.
Static surface topography Out-of-plane vibrations
Amplitude
Phase
Stroboscopic video microscopy
Source: MEMS Geometry and vibrations, Laser Measurement Systems Application Note VIB-M-05, PolytecGmbH, www.polytec.com, 2007.
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13.3 Test Methods and Instrumentation for MEMS13.3 Test Methods and Instrumentation for MEMS13.3.3 Material Property Measurements
Material properties and processing parameters influence the functionality and reliability of MEMS. Relevant material properties include elastic modulus, Poisson's ratio, fracture toughness and mechanisms, electrical properties (resistivity, migration), interfacial strength, and coefficient of thermal expansion.
MEMS-based test structures such as cantilever beams, clamped-clamped beams, and Guckel rings are often co-fabricated on the wafer for making stress and strain measurements. Optical profile measurements of these test structures can be used for estimation of the strain gradient, residual strain, and material properties.
For example, the curvature of cantilever beams can be used to obtain the stress gradient present in the film. Buckling behavior of fixed-fixed test structures can be used to obtain compressive stresses in the film. A Guckel ring can be used to obtain tensile stress information.
Guckel Ring
Cantilever Beam Clamped-Clamped Beam
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13.3 Test Methods and Instrumentation for MEMS13.3 Test Methods and Instrumentation for MEMS
13.3.4 Failure Modes and Analysis MEMS have specific failure modes, such as fatigue, wear, fracture, and stiction. Several kinds of test structures are commonly used to study materials related reliability issues
such as fatigue. Typically, samples with a preformed notch are used, such that the growth of a crack during functioning can be studied, either by direct optical observation or by a study of the influence on the Eigen-frequency of a beam or similar structure, for example
Surface roughness can affect issues such as stiction, wear, contact degradation, and contact resistance. Contact profilometers such as Dektak stylus profilers can be used to measure the surface roughness and the thickness of thin films.
Atomic force microscopy (AFM) is a useful tool for measuring surface roughness. It should be pointed out that the roughness of the top surface of a moving MEMS part is not necessarily the same as the roughness of the bottom surface.
To measure the bottom side roughness, the moving part can simply be removed destructively or in some cases even cut with a focused ion beam (FIB) and examined. An AFM can also be used to obtain information on mechanical parameters, contact resistance as a function of force, or even tribological information such as adhesion forces.
Also nanoindentor systems are frequently used to study MEMS: they can provide information on the Young's modulus of materials by physically indenting them, and they can also be used to obtain force-displacement curves of moving parts.
Several failure analysis (FA) techniques that are conventionally used for chips and packages can also be used for MEMS. Especially useful is the scanning electron microscope (SEM) for inspection and the focused ion beam (FIB) to make local cross sections.
Additional techniques include transmission electron microscopy (TEM), photon emission microscopy (PEM), scanning acoustic microscopy (SAM), infrared inicroscopy (In), x-ray, and Raman spectroscopy.
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13.3.4 Overview of Mechanical MEMS Devices13.3.4 Overview of Mechanical MEMS Devices
They can be modeled as second order systems consisting of:
proof mass spring damping System dynamics modeled by: where x1(t) is the input; x2(t) is the output
x2(t)
Spring, k
Damper, c
Mass, m
x1(t)
Mass, m
Frame
Spring, k
Air damping
Electrode
0)()( 12122 =−+−+ xxkxxcxm &&&&
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TransmissibilityTransmissibility
T(s) is defined as:
Natural frequency:
Quality factor:
Transmissibility:
22
2
1
2
)(
)()(
nn
nn
sQ
s
sQ
sX
sXsT
ωω
ωω
++
+
==
mk
n =ω
c
kmQ =
2222
42
)()(
)(
)(
Q
QjT
nn
nn
ωωωω
ωωω
ω
+−
+
=
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Transmissibility PlotTransmissibility Plot
For Q ≥ 5, the resonant
peak of |T(jω)| occurs at approximately ω = ωn
For Q ≥ 5, the
magnitude of |T(jω)| at ω = ωn is approximately equal to Q
Transmissibility Vs. Normailzed Frequency
-50
-30
-10
10
30
50
70
0 2 4 6 8 10
Normalized Frequency, Hz
Mag
nit
ud
e,
dB Q=1000
Q=100
Q=10
Q=1
Q=0.1
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Electromechanical ShakerElectromechanical Shaker
Can subject an attached device to sinusoidal motion
Adjustable amplitude
Adjustable bandwidth
Useful in measuring
|T(jω)| of a MEMS device
Photograph of an LDS model V408 electromechanical shaker with an
attached accelerometer (courtesy Auburn University).
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Rate TableRate Table
A machine used to rotate attached devices
Provides electrical feedthroughs for functional testing
Useful for angular rate testing
Useful for variable acceleration testing using centripetal force:
r
ω
2ωrac =
An illustration of a rate table
Rotating Plate
Slip Ring Assembly
Motor
A photograph of a simple rate table (courtesy Auburn University)
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13.3.6 Thermal Testing13.3.6 Thermal Testing
Evaluation of a device as a function of temperature
Static thermal evaluation
Thermal cycling
High temperature or low temperature storage
Thermal shock
A photograph of a box oven (courtesy Auburn University
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Humidity TestingHumidity Testing
Evaluation of a device as a function of humidity
Easiest type of chemical testing to perform
Usually performed in a controlled humidity chamber
Controlled humidity level
Controlled temperature
Humidity and temperature cycling is possible
Functional testing of MEMS devices during humidity testing is possible
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Pressure TestingPressure Testing
Evaluation of a device as a function of pressure
Pressures above or below ambient pressure may be of interest
For example: the testing of MEMS pressure sensors
A bell jar system is useful for pressures below ambient
The price for a suitable pump increases as the pressure decreases
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13.4 RF MEMS Devices 13.4 RF MEMS Devices
MEMS employed in radio-frequency (RF) applications are called RF MEMS. These represent a new class of devices and components that exhibit low insertion loss, high isolation, high Q, small size and low power consumption; and enable new system capabilities.
The application of MEMS in RF technology can be broadly classified into two categories: active (moving) devices, which involve mechanical motion (e.g., RF MEMS switch, RF MEMS capacitors, RF MEMS resonators, etc.) and static (non-moving) components (e.g., micromachined transmission lines, resonators, etc.).
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13.4 RF MEMS Devices13.4 RF MEMS Devices
RF MEMS Switches MEMS relays are more preferable than other conventional semiconductor based
switching devices such as field effect transistors, due to low-loss, low power consumption, absence of intermodulation distortion and broad-band operation from DC to the microwave frequency range.
An ohmic contact switch uses a metal-to-metal contact between the two electrodes for signal transmission.
Ohmic Contact Switches The operating voltage required to obtain electrical continuity can be obtained from
measuring the R-V characteristics using the experimental setup discussed in Electrical Testing.
RF characteristics of RF MEMS switches are obtained by measuring the S-parameters in both the ON and OFF states of the switch. S-parameters are most commonly used for electrical characterization of devices, components and networks operating at RF and microwave frequencies.
Capacitive Contact Switches In a capacitive contact switch, a thin dielectric layer is present between the two
electrodes. Capacitive contact RF MEMS switches can be characterized by measuring the
capacitance-voltage (C-V) characteristics. A C-V meter or an impedance analyzer equipped with a bias-T can be used in conjunction with a probe station to obtain C-V characteristics. The pull-down voltage can be determined from the C-V characteristics.
RF characteristics of RF MEMS switches are obtained by measuring the S-parameters using a network analyzer.
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13.4 RF MEMS Devices13.4 RF MEMS Devices Reliability of RF MEMS
The reliability of MEMS switches has been a major concern that limits the use of MEMS in real world applications. Ohmic contact MEMS switch reliability issues, such as failure due to stiction and contact degradation, have been observed to be the key failure modes.
In capacitive contact type MEMS switches, reliability issues such as stiction due to charge accumulation in the dielectric layer and capacitance degradation with actuation are commonly encountered failure modes.
A low frequency electrical test setup for reliability testing of RF MEMS switches is shown in the figure below. The setup consists of two signal generators, a filter, and a demodulator. The RF MEMS switches are driven by an actuation signal from Generator 1. A low frequency RF signal from Generator 2 is superimposed on the actuation signal.
The modulated signal is detected using a demodulator to obtain switch characteristics such as pull-in voltage, rise-time, fall-time, and capacitance change for capacitive switches or contact resistance change for ohmicswitches. Reliability of switches can be quantified by measuring the drift in any of these parameters.
Source: W. M. van Spengen, R. Puers, R. Mertens, and I. De Wolf, A low frequency electrical test set-up for the reliability assessment of capacitive RF MEMS switches, J. Micromechanics and Microengineering, 13(5), pp. 604–612, May 2003.
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13.4 RF MEMS Devices13.4 RF MEMS Devices
Resonators A mechanical filter is composed of multiple coupled lumped
mechanical resonators. Mechanical filters transform electrical signals into mechanical energy, perform a filtering function, and then transform the remaining output mechanical energy back into an electrical signal.
MEMS technology has been applied to the miniaturization of mechanical resonators and filters.
MEMS resonators and filters are characterized by measuring the frequency response characteristics. The performance parameters such as the resonant frequency, Q-factor and bandwidth are obtained from the measured frequency response characteristics. The equivalent circuit parameters can be extracted from the measured frequency response characteristics.
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13.4 RF MEMS Devices13.4 RF MEMS Devices Resonators – Disk Resonator Example
A MEMS disk resonator in a one-port configuration is shown in Figure (a). The contour-mode disk resonator consists of a resonating circular disk, two input electrodes, and a bottom output/bias electrode.
A typical test setup for testing a one-port contour-mode disk RF MEMS resonator is shown in Figure (a). The required test instrumentation includes a network analyzer, a DC voltage source, a bias-T and a vacuum chamber.
The measured transmission spectrum obtained from a one-port measurement of a 156 MHz disk resonator is shown in Figure (b). From the measured results, the equivalent circuit model (shown in Figure (c)) parameters have been extracted to be Rx = 22.287 kΩ, Lx = 70.15 mH, Cx=14.793 aF, and Co = 57.78 fF.
b) Measured transmission
spectrum
c) Equivalent
circuit modela) Test setup for a disk resonator
Source: J. R. Clark, W.-T. Hsu, and C. T.-C. Nguyen, Measurement techniques for capacitively-transduced VHF-to-UHF micromechanical resonators, in Proc. Int. Conf. on Solid-State Sensors & Actuators, pp. 1118–1121, June 2001.
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13.5 Optical MEMS13.5 Optical MEMS The integration of micro-optics and MEMS has created a new class of devices,
termed optical MEMS or Micro-Opto-Electro-Mechanical-Systems (MOEMS). The advantages of optical MEMS devices include high functionality, high performance, and low-cost.
Piston Micromirror A typical piston micromirror consists of a mirror segment supported by four springs and
is capable of movement in the direction normal (i.e., vertical) to the mirror surface. Arrays of piston micromirrors are employed in adaptive optics to compensate for variable optical aberrations.
The two important characteristics of interest are: 1) static characteristics (i.e., vertical displacement versus applied voltage characteristics), and 2) dynamic characteristics (i.e., transient response).
The deflection versus applied voltage characteristics can be obtained by measuring the optical profile of the micromirror for various applied voltages. Dynamic characteristics of piston mirrors can be measured using laser Doppler vibrometers.
a) Piston Micromirrors b) Optical profile measured using
Zygo interferometer
Source: A. Tuantranont, L.-A. Liew, V. M. Bright, J. Zhang, W. Zhang, and Y. C. Lee, Bulk-etched surface micromachined and flip-chip integrated micromirror array for infrared applications in Proc. IEEE/LEOS Int. Conf. on Optical MEMS, pp. 71–72, August 2000.
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13.5 Optical MEMS13.5 Optical MEMS Tilt Micromirror
A typical tilt micromirror consists of a flat mirror segment supported by two torsionalsprings. Tilt micromirrors change the angle of reflection of incident light by angular or torsional rotation of micromirror structures.
The two important characteristics of interest are: 1) static characteristics (i.e., vertical displacement versus applied voltage characteristics), and 2) dynamic characteristics (i.e., transient response).
To measure the tilt angle versus applied voltage characteristics, a laser beam is directed on the mirror surface while the reflection of the laser beam off the mirror surface is projected onto a screen mounted vertically and parallel to the scanner’s chip surface.
The dynamic characteristics of tilt mirrors can be measured using laser Doppler vibrometers. As an example, dynamic characteristics of an Applied MEMS DurascanTM two-axis tilt mirror measured using Polytec’s Laser Doppler vibrometerare shown below. Dynamic parameters such as switching time and settling times can be obtained from these results.
a) Applied MEMS Durascan tilt mirrorb) Optical profile measured using
Zygo interferometer
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13.6 Testing of 13.6 Testing of MicrofluidicMicrofluidic Devices Devices
MEMS technology can be used to realize miniature plumbing systems for fluid based applications
Testing may be limited to leak testing and/or functional testing
Reusable microfluidic devices may be easier to test than one-time-only devices
FlowFETS functionally behave as MOSFETS except that they control fluid flow instead of electrical current
Potentially useful for implementing functional testing algorithms in microfluidic devices
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13.6.1 MEMS13.6.1 MEMS Pressure Sensor Pressure Sensor
Pressure sensors are one of the most successful MEMS devices with a wide-range of applications in automotive systems, industrial process control, environmental monitoring, medical diagnostics and monitoring.
A MEMS pressure sensor consists of a mechanical membrane present at the interface between a sealed cavity and the external environment. The pressure difference between the sealed cavity and the surrounding environment produces a deflection of the diaphragm.
Pressure sensors are characterized by measuring the output response for various applied pressures.
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13.6.1 MEMS Pressure Sensor13.6.1 MEMS Pressure Sensor
The measurement setup for testing capacitive pressure sensors is shown in the Figure below.
The setup consists of two components: 1) a custom made pressure chamber which can withstand large pressures, and 2) the signal conditioning circuitry. The pressure chamber is made of Teflon with dimensions of 9.5″×8.5″×3″.
A pressure gauge is used to monitor the pressure inside the chamber. The pressure sensor is placed inside the chamber.
When the pressure inside the chamber exceeds the atmospheric pressure, the movable diaphragm starts deflecting downwards, thereby increasing the capacitance between the top and bottom electrodes. The signal conditioning board (MS3110BDPC from Microsensors Inc.) outputs a voltage corresponding to a change in the sensor capacitance.
Source: J. N. Palasagaram and R. Ramadoss, MEMS capacitive pressure sensor fabricated using printed circuit processing techniques, IEEE Sensors J., 6(6), pp. 1374–1375, December 2006.
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13.6.2 MEMS Humidity Sensor13.6.2 MEMS Humidity Sensor
The Hygrometrix HMX2000 is an example MEMS humidity sensor
Four cantilevered beams coated with a moisture absorbing polymer
Wheatstone bridge configured piezoresistivesensing
The sensor is small enough for use in evaluating the hermeticity of sealed cavities
Front and backside photographs
of a HMX2000 MEMS humidity sensor [Dean 2005]
Characterize sensor performance prior to cavity evaluation
Also evaluate as a function of temperature
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13.7 13.7 DyanmicDyanmic MEMS DevicesMEMS Devices
Dynamic MEMS devices are micromachinesthat possess one or more members that respond to an applied force by acceleration, resulting in mechanical motion.
The applied force could be internally generated, such as the force resulting from a microactuator, or externally generated, such as the force resulting from interaction with the environment.
A number of MEMS sensors can be accurately described as dynamic MEMS devices, including microphones, accelerometers, and gyroscopes.
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13.7.1 MEMS Microphone13.7.1 MEMS Microphone
MEMS microphones have been successfully commercialized for use in cell phones, cameras, PDA’s, and other high volume consumer electronics.
The microphones are characterized by measuring sensitivity, frequency response, and noise. The sensitivity (mV/Pa) is obtained by exciting the microphone at a chosen sinusoidal sound pressure level (SPL) and measuring the output voltage of the microphone for various DC bias voltages.
The frequency response is obtained by exciting the microphone with a periodic noise over the desired operating frequency rangeand measuring the sensitivity of the microphone. The relative gain and resonance frequency can be obtained from the frequency response characteristics.
The noise measurements are performed by measuring the frequency response of the microphone in an anechoic chamber.
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13.7.1 MEMS Microphone13.7.1 MEMS Microphone A typical test setup for acoustical test and
characterization of the integrated microphone is shown in Figure (a).
The instrumentation includes a signal analyzer and amplifier. The reference microphone, MEMS microphone, and test speaker are located inside the anechoic chamber.
The dimensions of the chamber are chosen such that standing waves are avoided in the frequency range of interest. The inside of the chamber is covered with sound absorbing material to minimize the influence of reflections as well as external noise. This results in an approximate free sound-pressure field.
The loudspeaker is driven by a dynamic signal analyzer, which uses a reference microphone in a feedback loop to maintain the output of the loudspeaker at a specified level in the frequency range of interest. The amplifier is used to boost the signal output from the reference microphone.
An example frequency response of the Knowles SiSonic MEMS microphone is shown in Figure (b).
a) Experimental Test Setup
b) Knowles SiSonicTM MEMS microphone
Source: M. Pedersen, W. Olthuis, and P. Bergveld, High-performance condenser microphone with fully integrated CMOS amplifier and DC-DC voltage converter, J. Microelectromechanical Systems, 7(4), pp. 387–394, December 1998.
Source: P. V. Loeppert and S. B. Lee, SiSonic: the first commercialized MEMS microphone, in Digest of Papers, Solid-State Sensors, Actuators, and Microsystems Workshop, pp. 27–30, June 2006.
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13.7.2 MEMS Accelerometer 13.7.2 MEMS Accelerometer
A very widely used type of MEMS device
Measures translational or linear acceleration
Can be tested using a rate table
The applied acceleration input can be varied by adjusting the angular rate
Some MEMS accelerometers have BIST features where an externally applied input signal emulates the effect of a specific applied acceleration
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13.7.3 MEMS Gyroscope13.7.3 MEMS Gyroscope
A gyroscope detects the presence of rotational motion about a predefined axis
A gyroscope can be tested using a rate table by varying the angular rate and measuring the sensor’s output signal
MEMS gyroscopes are often sensitive to high frequency mechanical vibrations present in the operating environment
An electromechanical shaker is useful for evaluating this sensitivity
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13.8 Testing Digital 13.8 Testing Digital
MicrofluidicsMicrofluidics BiochipsBiochips
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Testing of Digital Microfluidic BiochipsTesting of Digital Microfluidic Biochips
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Motivation for Microfluidic Motivation for Microfluidic
BiochipsBiochips
Conventional Biochemical Analyzer
Shrink
Lab-on-a-chip
Goal
Carry out biochemical laboratory
experiments on a microchip
Advantages
Higher throughput
Minimal human intervention
Smaller sample/reagent consumption
Higher sensitivity
Increased productivityMicrofluidic
Biochip
20nl sample
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 41
ApplicationsApplications of Biochipsof Biochips
Clinical diagnostics, e.g., healthcare for premature infants, point-of-care diagnosis of diseases
“Bio-smoke alarm”: environmental monitoring
Massive parallel DNA analysis,
automated drug discovery, protein crystallization
Robust test techniques needed
Outcome of biochemical results must be reliable
Testing must be low-cost: disposable devices ($1/chip)
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 42
Motivation for MicrofluidicsMotivation for Microfluidics
Test tubes
Robotics
MicrofluidicsAutomation
Integration
Miniaturization
Automation
Integration
Miniaturization
Automation
Integration
Miniaturization
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 43
Technology OverviewTechnology Overview
Digital microfluidic biochips
Manipulation of liquids as discrete droplets
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 44
What is Digital Microfluidics?What is Digital Microfluidics?
Droplet actuation is achieved through electrowetting
Electrical modulation of the solid-liquid interfacial tension
No Potential Applied Potential
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 45
MicrofluidicsMicrofluidics Continuous-flow biochips: Permanently etched
microchannels, micropumps and microvalves
Digital microfluidic biochips: Manipulation of liquids as discrete droplets
(University of Michigan) 1998
(Duke University) 2002
Control electronics (shown) are suitable for handheld or
benchtopapplications
Printed circuit board lab-on-a-chip –
inexpensive and readily manufacturable
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 46
AdvantagesAdvantages No bulky liquid pumps are required
Electrowetting uses microwatts of power
Can be easily battery powered Standard low-cost
fabrication methods can be used
Continuous-flow systems use expensive lithographic techniques to create channels
Digital microfluidic chips are possible using solely PCB processes
Droplet Transport on PCB (Isometric View)
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 47
CapabilitiesCapabilities
Digital microfluidics-based biochips
MIXERSMIXERSTRANSPORTTRANSPORT DISPENSINGDISPENSING REACTORSREACTORS
INTEGRATE
Digital Microfluidic
Biochip
DETECTIONDETECTION
Basic microfluidic functions
(transport, splitting, merging,
and mixing) have already been demonstrated on a 2-D array
Digital microfluidics-based
biochip is a highly
reconfigurable system
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More on ApplicationsMore on Applications
Droplet-based microfluidic biochip
Drug discovery
and biotechnologyEnvironmental and
other applications
Proteomics
High-throughput
screening
Genomics
Countering
bioterrorism
Micro-optics
Air/water/agro
food monitoring
Clinical
chemistry
Nucleic
acid tests
Immunoassays
Medical diagnostics and
therapeutics
Burns, Science 2002
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Glass Chip Platform DevelopmentGlass Chip Platform Development
Top Plate (Optional) (i.e. glass or plastic)
Gasket Layer (100 to 600 µm) (proprietary)
Hydrophobic Layer (50 nm) (i.e. Teflon dip coated)
Insulator Layer (1 to 25 µm) (i.e. parylene)
Patterned Metal on Substrate(i.e. chrome on glass via lift-off process)
Top plate is either glued or fixed in place by pressure
Contacts are made either through the top or bottom
Droplets are either dispensed by hand or formed from on-chip reservoirs
Chip Assembly
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 50
PCB Chip Platform DevelopmentPCB Chip Platform DevelopmentFabrication Process
Flash Plating(Copper)
PCB
PCB Material – Mitsui BN300 – 64 mil
Top Metal Layer (Electrodes) – Cu – 15µm
Bottom Metal Layer (Contacts) – Cu – 15µm
Dielectric – LPI Soldermask – 25 µm
Via Hole Filling – Non-conductive Epoxy
Hydrophobic Layer – Teflon AF – 0.05 to 1.0 µm
Gasket (spacer) – Dry Film Soldermask (Vacrel 8140) – 4 mils (~95µm after processing)
Gasket Layer(Dry Soldermask)
Hydrophobic Layer(Teflon AF)
Dielectric(LPI Soldermask)
Top Metal Layer(Copper)
Bottom Metal Layer(Copper)
Via Hole Filling(Non Conductive Epoxy)
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 51
Biochip for Multiple Assays Biochip for Multiple Assays (Circa 2002)(Circa 2002)
2-layer metal process
Pitch = 500µm
Gap = 85 µm
4-phase outer bus
3-phase inner bus
8 reservoirs for sample, reagents, waste, calibrantsetc
Each reservoir with a loading port
Dedicated mixing region
One detection siteWasteSample
Glucose Calibrants
ControlsLactate
Urea Buffer
G
G
L U
s
M
M M
s
L
U B
C
S
C
4 phase bus
3 phase bus
mixing
detection
G
s
L
U
B
G
L
U
M
C
Sample
Glucose
Lactate
Urea
Mixed product
Buffer
Control/ Calibrant
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 52
Fault ClassificationFault Classification Catastrophic faults
Causes: dielectric breakdown, degradation of the insulator, etc.
Parametric faults
Causes: geometrical parameter deviation, change in viscosity of droplet and filler medium, etc.
Single-electrode faults
Electrode open
Two-electrodes faults
Short between the adjacent electrodes
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Catastrophic Defects for BiochipsCatastrophic Defects for Biochips
Unintentional droplet
operations or stuck
droplets
Electrode-stuck-
on (the electrode
remains
constantly
activated)
1Irreversible
charge
concentration
on an
electrode
Electrode
actuation for
excessive
duration
Droplet undergoes
electrolysis, which
prevents its further
Transportation
Droplet-electrode
short (a short
between the
droplet and the
electrode)
1Dielectric
breakdown
Excessive
actuation
voltage
applied to an
electrode
Observable
error
Fault
model
No. of
cells
Defect
type
Cause of
defect
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Catastrophic Defects (ContCatastrophic Defects (Cont’’d)d)
Fragmentation of
droplets and their
motion is prevented
Dielectric islands
(islands of
Teflon coating)
1Non-uniform
dielectric
layer
Coating
failure
Droplet transportation
without activation
voltage
Pressure gradient
(net static
pressure in some
direction)
1Misalignment
of parallel
plates
(electrodes
and ground
plane)
Excessive
mechanical
force
applied to
the chip
Observable
error
Fault
model
No. of
cells
Defect
type
Cause of
defect
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Catastrophic Defects (ContCatastrophic Defects (Cont’’d)d)Observable
error
Fault
model
No. of
cells
Defect
type
Cause of
defect
A droplet resides in the
middle of the two shorted
electrodes, and its transport
along one or more directions
cannot be achieved
Electrode
short (short
between
electrodes)
2Metal
connection
between two
adjacent
electrodes
Failure to activate the
electrode for droplet
Transportation
Electrode
open
(electrode
actuation is
not possible)
1Broken
wire to control
source
Failure of droplet
transportation
Floating
droplets
(droplet are
not anchored
)
1Grounding
Failure
Abnormal
metal
layer
deposition
and etch
variation
during
fabrication
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Catastrophic Defects (ContCatastrophic Defects (Cont’’d)d)Observable
error
Fault
model
No. of
cells
Defect
type
Cause of
defect
Assay results are
outside the range of
possible outcomes
Contamination
Droplet transportation
is impeded.
Resistive open
at electrode
1Sample
residue on
electrode
surface
Protein
absorption
during a
bioassay
Electrode short2A particle
that
connect
two
adjacent
electrodes
Particle
contamination
or liquid
residue
A droplet resides in
the middle of the two
shorted electrodes, and
its transport along one
or more directions
cannot be achieved
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Electrical Detection MechanismElectrical Detection Mechanism Minimally invasive
Easy to implement (alleviate the need for external devices)
Fault effect should be unambiguous
Capacitive changes reflected in electrical signals (Fluidic domain to electrical domain)
• If there is a droplet, output=1; otherwise, output=0
• Fault-free : there is a droplet between sink electrodes
Faulty: there is no droplet.
Electrically control
and track test stimuli droplets
Droplet
150 pF
74C14
5 K
1N914
1N52315.1V
1N914
Gnd
+ 5 V
10 K
Output
Periodic
square
waveform
Chip-
under-test
Capacitivesensing circuit
Microscope & CCD camera
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Examples of DefectsExamples of Defects
Degradation of electrode
Short between electrodes
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DefectDefect--Oriented Experiment Oriented Experiment Understand the impact of certain defects on
droplet flow, e.g., for short-circuit between two electrodes
Experimental Setup To evaluate the effect of an electrode short on
microfluidic behavior
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DefectDefect--Oriented Experiment (ContOriented Experiment (Cont’’d)d) Results and Analysis
Experimental results and analysis for the first step.
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DefectDefect--Oriented Experiment (ContOriented Experiment (Cont’’d)d) Experimental results and analysis for the second step.
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Testing and Diagnosis: SummaryTesting and Diagnosis: Summary
“Edge-dependent” nature of some defects Testing based on the Hamiltonian path is not sufficient
Formulate the test planning problem in terms of the Euler circuit and Euler path problems
Key idea: Model array as an undirected graph; use Euler Theorem to find an efficient test flow path
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Test Planning MethodsTest Planning Methods Euler-path based testing
Manipulate single test droplet to transverse the
microfluidic array
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Experiments with Fabricated ChipExperiments with Fabricated Chip
PCB microfluidic platform for DNA
sequencing
Known a priori to contain one defect
Reservoirs
Reserved cells
Defect sites
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Experiments with Fabricated ChipExperiments with Fabricated Chip
Euler-path based testing
Testing: 57 seconds; Diagnosis: 173 seconds
Parallel scan-like testing
Testing: 44 seconds; Diagnosis: 44 seconds
Source
SinkPseudo
sinks
Pseudo
sources
Test
Droplets
Source
SinkPseudo
sinks
Pseudo
sourcesSource Source
SinkPseudo
sinks
Pseudo
sources
Test
Droplets
Test
Droplets
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13.9 DFT and BIST for MEMS13.9 DFT and BIST for MEMS
13.9.1 Overview of DFT and BIST Approaches13.9.1 Overview of DFT and BIST Approaches
13.9.2 MEMS BIST Examples13.9.2 MEMS BIST Examples
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Test Stimuli for MEMS BISTTest Stimuli for MEMS BIST Diversity of stimuli for MEMS devices acceleration pressure heat chemical concentration, etc. In most cases it is not convenient to generate real input
test stimuli for MEMS devices. Alternative test stimuli (such as electrical voltage) which
are somewhat equivalent, but easier to generate, will be used for MEMS BIST.
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Overview of DFT and BIST for MEMSOverview of DFT and BIST for MEMS
BIST of a comb accelerometer using electrostatic force [Mir 2006]
MEMS BIST: using voltage-induced electrostatic force Example: in-field BIST of ADXL comb accelerometers Electrostatic force induced by self-test voltage is used to
mimic the effect of input acceleration. Calibration needed, not good for manufacturing test.
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Pneumatic actuation of a pressure sensor for self-test [Puers 2001]
MEMS BIST: using electrically induced pneumatic actuation Example: self-testing of a piezoresistive pressure sensor Pulse voltage applied to resistor heater embedded in cavity The air inside the cavity is heated: air pressure increased Output response sensed by piezoresistive gauge in the
membrane and compared with good device response
Overview of DFT and BIST for MEMSOverview of DFT and BIST for MEMS
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Thermal actuation of an infrared imager pixel for BIST [Mir 2006]
MEMS BIST: using electrically induced resistor heating to mimic thermal radiation input of infrared imager
Electrical voltage applied to heating resistor on suspended membrane of each pixel
Membrane is heated up as by incident infrared radiation in normal operation
Overview of DFT and BIST for MEMSOverview of DFT and BIST for MEMS
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MEMS BIST: Oscillation-based Test Methodology (OTM) measuring indirect parameters was demonstrated for a MEMS magnetometer
Direct parameters (e.g., sensitivity) are effective to verify the device function, but not always easy to measure.
Electrically induced Lorentz force in magnetic field is used as test stimuli.
The DUT is reconfigured into an oscillating device with a feedback circuit.
Some indirect parameters (e.g., oscillation frequency and amplitude) which are easier to observe, are measured for testing the MEMS device.
Overview of DFT and BIST for MEMSOverview of DFT and BIST for MEMS
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MEMS BIST: Symmetry testing based on device structure symmetry.
Most MEMS devices have certain degree of structure symmetry (e.g., left-right, top-bottom or rotation symmetry).
Symmetry BIST is effective in detecting local defects which change the device symmetry.
No calibration needed, can be used for manufacturing test. Example: symmetry BIST for a pressure sensor with
internal redundancy [Rosing 2000a] The movable shuttle is activated twice: first toward left and
then toward right. The output responses from both activations are compared. Any difference indicates the existence of a local defect
leading to a structure asymmetry of the device .
Overview of DFT and BIST for MEMSOverview of DFT and BIST for MEMS
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 73
Symmetry BIST for CMOS MEMS accelerometers [Deb 2002]
Movable shuttle of the accelerometer is divided into two conductors which are physically connected by an insulator layer while electrically insulated from each other.
By comparing the voltage outputs from both conductors of the movable shuttle, structure asymmetry caused by local, hard-to-detect defects is detected.
Symmetry BIST that divides fixed instead of movable parts of symmetrical capacitive MEMS devices [Xiong2005a].
Good for MEMS devices (e.g., ADXL accelerometers, comb resonator) in which the movable parts are not divided.
Overview of DFT and BIST for MEMSOverview of DFT and BIST for MEMS
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 74
Pseudo-random BIST of MEMS cantilevers [Mir 2006]
MEMS BIST: Pseudo-random MEMS BIST [Mir 2006] Voltage pulses applied to a heating resistor on the cantilever. The cantilever deflects due to the induced heat. Deflection measured by piezoresistor Wheatstone bridge in anchor. Pseudo-random maximum-length binary sequences are generated by
linear-feedback-shift-registers (LFSRs). The output bridge voltage is converted to digital values and analyzed for
input-output cross-correlation function (CCF). Test signature compared with expected values for Go/No-Go decision.
Overview of DFT and BIST for MEMSOverview of DFT and BIST for MEMS
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MEMS BIST ExampleMEMS BIST Example Accelerometer is used to explain basic idea of MEMS
BIST, because it is widely used in industry.
ADXL series, such as ADXL190, ADXL 330 of Analog Devices, all implemented BIST.
A voltage Vs activates self-test pin, an electrostatic force is generated and results in about 20% of full-scale acceleration. A voltage change can be observed from output pin.
This BIST technique can be used for in-field testing where external test equipment are unavailable.
BIST for accelerometers is used to discuss basic working principles of MEMS BIST.
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MEMS BIST ExampleMEMS BIST Example
MEMS BIST: how to generate test stimulus? how to analyze output response?
Most BIST methods for accelerometers generate test stimuli using electrostatic input, thermal input, or real acceleration input, or pseudo-random input. Test response w.r.t. the actuation is measured using a sensing circuit and compared with expected response.
This discussion is mainly focused on surface-micromachined comb accelerometers due to its popularity in industry.
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MEMS BIST ExampleMEMS BIST Example
A typical surface-micromachined
MEMS comb accelerometer.
0
0
21
)(
d
hLnCC
ff∆−
==ε
In static mode,
where C1(C2): left (right) differential
capacitance, nf: total number of differential
capacitance groups, ε0: dielectric constant of air, Lf: length of movable finger, ∆: non-overlapped length at the
root of each movable finger, h: device thickness.
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 78
The schematic diagram of
differential capacitance (one
finger group)
ak
aM
k
Fx sa
∝⋅−
==
),1()(
)(
)(
00
0
0
0
1d
x
d
hLn
xd
hLnC
ffff−
∆−≈
+
∆−=
εε
).1()(
)(
)(
00
0
0
0
2d
x
d
hLn
xd
hLnC
ffff+
∆−≈
−
∆−=
εε
If there is acceleration a, the inertial force Fa=-Msa
results in deflection x of the beams and movablefingers
Differential capacitance changed to
MEMS BIST ExampleMEMS BIST Example
Sensing the differential capacitance change, we know the displacement x, hence the acceleration a.
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 79
MEMS Fault Modeling and SimulationMEMS Fault Modeling and Simulation
Carnegie Mellon University (USA) CARAMEL (contamination and reliability analysis of
micro-electromechanical layout) [Kolpekwar 1998a] Lancaster University (United Kingdom) FMEA (failure modes and effect analysis) approach
[Rosing 2000a] Inductive fault analysis [Shen 1985] TIMA (France) Failure mechanisms and fault classes for CMOS MEMS
[Castillejo 1998]
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Carnegie Mellon ApproachCarnegie Mellon Approach Fault analysis method was developed as a tool called
CARAMEL (contamination and reliability analysis of microelectromechanical layout).
In CARAMEL, a defective MEMS structure is represented by a 3-D representation, which is then extracted to mesh netlist for mechanical simulation.
Faults considered include: stiction for ADXL75, particulate contamination for microresonator, vertical stiction, foreign particles, etch variation for resonators and accelerometers.
Effects of these faults to resonant frequency was also identified.
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Lancaster University ApproachLancaster University Approach This technique integrates qualitative analysis and
quantitative fault simulation to generate realistic faults for MEMS transducers.
Industrial failure modes of sensor/actuator are analyzed and simulated by inductive fault analysis (IFA) and finite element simulation.
Analog and mixed-signals are also simulated using inductive fault analysis (defect-related faults) and process variation analysis (parametric faults).
Faults are then described by a behavioral model for test purpose.
Major faults considered: local defects, global and local parameters out of tolerance, wear, environmental hazards, problems from imperfection in design process.
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TIMA ApproachTIMA Approach Instead of using IFA, fabrication process of MEMS is
analyzed to determine realistic defects or failure mechanisms.
Failure mechanisms are divided into those occurred in CMOS process, and those occurred in micromachining process.
Defects are classified into gauge (e.g., sending circuit) faults and microstructure faults. Each class is further divided into catastrophic faults and parametric faults.
Gauge faults: shorts, opens, or changes in width, length and metal resistivity.
Microstructure faults: break-around-gauge, stiction, nonreleased microstructure, asymmetrical microstructure, or change of Young’s modulus.
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BIST Structure of Comb AccelerometerBIST Structure of Comb Accelerometer
BIST structural diagram of a comb accelerometer [Deb 2002]
Simplified comb accelerometer structure for BIST functions M1-M8: movable fingers Ms: central mass D1-D8: fixed driving fingers S1-S8: fixed sensing fingers
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Test Stimulus GenerationTest Stimulus Generation
MEMS comb accelerometer [Deb 2002]
Test stimulus generation: use electrostatic force Fd to mimic the effect of inertial force.
Voltage Vd applied to fixed driving fingers D1, D3, D5, D7 Nominal voltage Vnom applied to Ms and D2, D4, D6, D8 Induced electrostatic force Fd on movable mass Ms:
2
2
0
2
)(
d
VVSF nomd
d
−=
ε
Displacement of massx=Fd /k
Measure the resulted differential capacitance change and compare with expected good device value
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 85
Comb Accelerometer: Normal OperationComb Accelerometer: Normal Operation
MEMS comb accelerometer [Deb 2002]
Normal operation mode Modulation voltage Vmp=V0sqrt(ωt) applied to S1, S3, S5, S7 Modulation voltage Vmn=-V0sqrt(ωt) applied to S2, S4, S6, S8
Input acceleration a results in displacement of movable massx=-Ms·a/k
The voltage in the movable mass
)(0
0
tsqrVd
xVMs ω
=
Measuring the voltage
level VMs in the
movable mass, we know the value of displacement x, hence
the acceleration a.
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Comb Accelerometer: Sensitivity BISTComb Accelerometer: Sensitivity BIST Comb Accelerometer: Sensitivity BIST [Analog 2007]
Test driving voltage Vd is applied to D1, D3, D5, D7
Nominal voltage Vnom applied to D2, D4, D6, D8,M1, M5, M4, M8
Movable mass is activated upward by electrostatic force
Modulation voltage Vmp=V0sqrt(ωt) applied to S1, S3, S5, S7 Modulation voltage Vmn=-V0sqrt(ωt) applied to S2, S4, S6, S8
Displacement of massx=-Fd /k
Voltage in movable mass
MEMS comb accelerometer [Deb 2002]
Measure the voltage
level VMs in movable mass and compare with
expected good device
value.
)(0
0
tsqrVd
xVMs ω
=
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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 87
Comb Accelerometer: Symmetry BISTComb Accelerometer: Symmetry BIST Comb Accelerometer: Symmetry BIST [Deb 2002] Movable mass is divided into two (left and right) equal conductors
connected by an insulator layer.
Movable mass activated by electrostatic force as in sensitivity BIST
Modulation voltage Vmp=V0sqrt(ωt) applied to S1, S3, S5, S7
Modulation voltage Vmn=-V0sqrt(ωt) applied to S2, S4, S6, S8 The difference between voltage Vs1 from left movable fingers M2,
M3 and voltage Vs2 from right movable fingers M6, M7 is sensed
by a differential amplifier.
MEMS comb accelerometer [Deb 2002]
Any local defect changing
device left-right symmetry
results in difference
between Vs1 and Vs2 and will be detected.
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Comb Accelerometer: Symmetry BISTComb Accelerometer: Symmetry BIST Comb Accelerometer: Symmetry BIST [Xiong 2005a] For comb accelerometers in which the movable mass is not divided (e.g.,
ADXL accelerometers), symmetry BIST needs to be implemented in adifferent way [Xiong 2005a]
Movable mass activated by electrostatic force as in sensitivity BIST Modulation voltage Vmp=V0sqrt(ωt) applied to S1, S5 Modulation voltage Vmn=-V0sqrt(ωt) applied to S3, S7 Due to device symmetry, capacitance C1 between S1, S5 and M2, M3
should always equal to capacitance C2 between S3, S7 and M6, M7. Hence, for good device,
VMs=0.
MEMS comb accelerometer [Deb 2002]
Any local defect changing device left-right symmetry results in non-zero VMs
and will be detected. It divides fixed instead of movable
capacitance plates.
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Concluding RemarksConcluding Remarks A majority of microelectromechanical systems (MEMS) devices are
inherently mechanical in nature and therefore require some special considerations during various manufacturing stages and testing. This chapter discussed some of the important handling considerations during dicing, packaging, and testing.
A wide variety of test methods, such as electrical, optical, mechanical, and environmental, for characterization of various MEMS devices. This chapter reviewed the instrumentation, typical setup, and important characteristics for testing a wide variety of MEMS devices, including accelerometers, gyroscopes, humidity sensors, RF MEMS, optical MEMS, pressure sensors, and microphones
Microfluidics-based biochips have a great potential for replacing cumbersome and expensive laboratory equipment. Test techniques for digital microfluidic chips have been discussed.
MEMS DFT/BIST techniques and examples have been discussed. It should be noted that the diversity of MEMS devices and their principles remain a challenge in developing universal DFT and BIST solutions.