Chapter 13 MEMS slides 110407 - Elsevier · PDF fileEE141 3 System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 3 Topics Introduction MEMS Testing Considerations Test Methods

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System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 1

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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13.3.1 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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13.3.2 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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13.3.2 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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13.3.4 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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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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Motivation for MicrofluidicsMotivation for Microfluidics

Test tubes

Robotics

MicrofluidicsAutomation

Integration

Miniaturization

Automation

Integration

Miniaturization

Automation

Integration

Miniaturization

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Technology OverviewTechnology Overview

Digital microfluidic biochips

Manipulation of liquids as discrete droplets

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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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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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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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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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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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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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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


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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


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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.


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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 .


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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.


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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.


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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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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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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


Sensing the differential capacitance change, we know the displacement x, hence the acceleration a.

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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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Comb Accelerometer: Normal OperationComb Accelerometer: Normal Operation


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


Measure the voltage

level VMs in movable mass and compare with

expected good device

value.

)(0

0

tsqrVd

xVMs ω

=

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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.


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.


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.

Documents

Chapter 13 MEMS slides 110407 - Elsevier · PDF fileEE141 3 System-on-Chip Test Architectures Ch. 13 - MEMS Testing - P. 3 Topics Introduction MEMS Testing Considerations Test Methods