magazine spread | editorial design

1. COVER STORY
Detecting failure modes
endless diversity in form and function of the devices. Each type of device
Figure 2: Wyko NT profiler image
has different complexity in the manufacturing process, a different geometry
of a MeMS fabricated microphone.
(Courtesy of of Texas Knowles Acoustics)
(from tens of microns to tens of millimeters in width), and vastly different
performance requirements or designed responses to external stimuli. Thus,
while curvature, surface roughness, and switching time may be critical for
in today’s MEMS
micromirror arrays, co-planarity of the capacitor parts and linewidths
may be the most crucial parameters for an accelerometer. Initial
or long-term failure of a device may also be driven
by an array of factors, such as geometric
errors in production, contamina-
tion, improper removal
by Noushin Dowlatshahi and Bob Chanapan, Veeco instruments inc.
M icroelectricalmechanical systems (MEMS)
are taking a whole new role in our day-to-
day life, and are much more widely used
than ever before, due to a wide range of benefits including their low
mass, fast mechanical response, low power consumption, and potential
for lowering end costs. Meanwhile, test and quality control of such
devices has become more critical to facilitate the insertion of this
technology into critical applications. Therefore, it is not surpris-
of sacrificial
ing that precision metrology has a huge role in this steady layers, stiction,
environmental
advancement of MEMS technology. attack, fatigue,
electrostatic
Unlike traditional semiconductor devices, today’s MEMS devices clamping, fusing,
require characterization in both their static state and under actuation. delamination, or
Parameters of interest include shape, dimensions, surface roughness, side- electrical damage.
wall angles, film thickness, residual stress, feature volumes, response times, During research
thermal properties, resonance frequencies, stiction, environmental compat- and development, there
ibility, and more. The greatest difficulty in MEMS metrology is the nearly is also a need to map the
behavior of devices and
materials placed under
external stresses (e.g., tem-
perature, pressure, or corro-
Figure 1: Wyko NT
sive agents) in order to fully
dynamic measurement
understand the time-course
reveals a hidden defect
in a micro-mirror.
evolution of these processes.
(Courtesy of of Texas Tech University)
Early detection of failure modes
not only enables improvement at
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2. Early detection
of failure modes
not only enables
improvement at the
next fabrication
cycle and increased
yields, but it
also identifies
the next fabrication cycle and increased yields, but it also These arrays typically consist of simple cantilever beams, has pros and cons, and thus the velocity of the moving part. Displacement can also
identifies whether the problem is isolated or systematic, torsion beams, tethered (piston-style) beams, circular mem- selecting the proper solu- be measured through integration of this signal. These sys-
whether the
directly impacting field failure quality control. branes, and oval membranes. One technique of fabricating tion depends on a number tems are very sensitive to motion, with sub-nanometer reso-
the micromirrors is to use low-temperature adhesive wafer of factors, from device type lution in the direction perpendicular to the test beam. More
problem is isolated
bonding to deposit a thin layer of monocrystalline silicon to stage of manufacturing. sophisticated systems enable the measurement point to be
RF MeMS
MEMS applications in the domain of radio-frequency (RF) device to a CMOS wafer. The digital micromirror device SEMs generally offer the scanned across the surface, to build up a complete picture of
or systematic,
circuits serve as a perfect example. For certain capaci- (DMD) developed by Texas Instruments, which incorporates highest lateral resolution the MEMS out-of-plane motion, though phase information
tive MEMS switches, it is crucial to examine the stiction more than 500,000 individually addressable micromirrors, has available for static MEMS between each of the monitoring points is lost due to the time
directly impacting
between the metal layer (top electrode) and the dielectric made great strides in both performance and reliability of such imaging. These systems lag between measurements. The lasers on these systems can
layer covering the bottom electrode. The charge build-up devices. Digital light processing technology based on DMD operate by focusing a beam also be coupled to other optical systems directly or via fiber
field failure
in the dielectric material can result in what would typi- has been used in such diverse products as projection displays of electrons on a small area options, such as stroboscopic bright-field microscopes, so one
cally be called a “failure mode.” The actuation voltage is with film-like projected images and photographic-quality and detecting the electrons can get the in-plane-motion through a separate series of mea-
quality control.
directly affecting this charge build-up. Researchers need printers. Reliability testing of the DMD has demonstrated that are scattered from the surements with that equip-
to understand how an increase or decrease in the actua- greater than 100,000 operating hours and more than 1 tril- surface. Measurement time ment. However, all systems are
Manufacturers
tion voltage can impact the lifetime of typical MEMS. This lion mirror cycles. is typically between 5–10s sensitive to the roughness of
actuation voltage is typically related to the device geometry, It is crucial for micromirror array manufacturers to exam- after the sample has been the surface, requiring enough
have relied on a
mechanical and material properties, and residual stresses ine the uniformity in how the micromirrors tilt in an array, loaded into the system. light to be scattered back into
in the devices. When voltage is applied to a typical capaci- the time it takes them to tilt, and how repeatable this behav- Lateral resolutions down to the nanometer level are possible, the detector for adequate sig-
range of methods
tive RF MEMS switch, the electrostatic charges cause a dis- ior is. This is in addition to the traditional parameters of mir- and depth-of-focus of such systems can be quite large. SEMs nal-to-noise. Therefore, very
tributed electrostatic force, leading to deformation of the ror curvature/deflection angle roughness and surface topog- provide an invaluable tool for evaluating sidewall angles and smooth or rough surfaces can
to identify
micro-structure. Later, due to the storage of elastic energy, raphy. Moreover, as the devices are packaged in hermetically roughness of very steep parts, and are often the only systems degrade accuracy.
the structure tends to return to its original state. Accurate sealed packages, the encapsulation process can further affect capable of these measurements in such applications as micro- Digital holography (DH)
structural failure
metrology can play a significant role in characterizing such a the operational performance. Environmental factors includ- fluidics and capacitive accelerometers. The primary limitations has been employed for many
switch by defining and evaluating the membrane’s deforma- ing temperature and humidity also impact on such devices, of the technique are that samples must be placed in vacuum, years to measure vibrating
modes in MEMS.
tion and analyzing thermal impact, as well as the material creating the need of a fourth-dimensional capable, non-con- and the penetration depth of electrons is very small, limiting devices. In this technique,
and mechanical properties of the metal. tact metrology solution. Inspecting the device in a meaning- effective analysis on packaged parts. Also, parts must often a digital camera is used to
ful manner can monitor and minimize fabrication errors and be cross-sectioned in order to achieve the proper orientation record a hologram produced
positively impact the micromirrors components within an for best measurement, which can be cost prohibitive. Lastly, by interfering a high-quality reference beam with a beam
Micromirrors
Another widely used type of MEMS device that exhibits array. edges may produce artifacts if they become charged, leading reflected from the sample under test. As the test object is later
particular metrology needs is the micromirror array, which to inaccurate data. deformed, the modified object beam is compared with the
is finding use in adaptive optics for both space and consumer Laser Doppler vibrometers are frequently used for rapid original digitally recorded hologram, and the deformation can
inspection strategies
electronics, as well as in projectors, televisions, and digital Manufacturers have relied on a range of methods to iden- characterization of out-of-plane MEMS motion. These sys- be quantified using standard phase-shifting or other interfero-
cameras. These mirror devices must be 100% reliable and tify structural failure modes in MEMS: optical microscopy, tems employ a beam of modulated laser light that is reflected metric techniques. These systems can therefore achieve nano-
accurate. Because of variations in device geometry that result stylus profilometrey, infrared (IR) thermal imaging, focused from the moving test piece. The returned light is Doppler- meter-level measurement of out-of-plane motion of devices.
from the micro-fabrication process, the performance from one ion beams (FIB), scanning electron microscopy (SEM), digital shifted from the original modulated beam, and the two beams Using high-speed cameras, deformations of up to several thou-
device to the next can vary, making it difficult to reliably holography (DH), atomic force microscopy (AFM), and trans- are then compared. A spectrum analyzer or other electronics sand hertz can be measured. The systems can also be com-
ensure accuracy and repeatability of the actuator positions. mission electron microscopy (TEM). Each of these techniques can be used to determine the magnitude of the shift, and bined with stroboscopic methods to measure motions up to
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3. It is now possible
to perform
non-destructive,
rapid, extremely
precise MEMS
characterization
through packaging
and under operation
several megahertz, to the device under test and the other part to a high-quality
similar to the strobed reference surface. The light recombines to form a pattern of
on a single
systems mentioned interference “fringes.” The reference surface is then trans-
previously. Like the lated relative to the test surface, and multiple frames of data
measuring tool.
other optical meth- are obtained during the translation. The resulting series of
ods, digital holo- fringe patterns are analyzed to calculate the surface profiler of
graphic systems can the device. Optical profilers provide rapid, 3-D, non-contact
measure parts through a transparent cover glass. How- sample surface characterization. Consequently, this method
ever, as the source employed is a laser with long coher- has proven quite useful for MEMS production processes. How-
ence length, they can suffer speckle problems from diffuse ever, this method has traditionally also been inadequate for
MEMS surfaces and may also have problems with transpar- determining a device’s dynamic ability to perform, such as
ent films on a surface. actuation, deformation, rotation, etc.
Atomic force microscopes (AFM) employ nanoscale tips CalTech overcame this primary limitation of optical pro-
attached to the end of a cantilever that is brought very close filing by combining it with stroboscopic illumination for
to the test surface. Measurement of the tip deflection allows dynamic measurement of MEMS devices in motion. Strobo-
full 3-D characterization of the surface, and van der Waals scopic illumination effectively “freezes” the motion of MEMS
forces between the tip and sample can be detected. AFMs structures, allowing nanometer-resolution measurement of
utilize a variety of measurement modes, including “contact their surface shape. By varying the amplitude, phase, and
mode” (where the tip lightly touches the sample) and “tap- frequency of the drive signal, multiple measurements can be
ping mode” (where high-frequency tip oscillations are used taken to describe the device’s full range of actuation/defor-
to minimize contact with the sample while maintaining the mation. Moving MEMS structures can then be analyzed for
high signal-to-noise ratio). AFMs have the highest lateral flatness variation, tilt, lateral translation, linearity of motion,
and vertical resolutions available of any three-dimensional stiction, and other key device parameters.
metrology instrumentation, with sub-nanometer features Today’s top-of-the-line optical profilers incorporate this
visible in all three dimensions. However, the field of view technology along with other advances to provide a best mea-
for a scan is typically only about 120µm2, and vertical limits surement solution for MEMS researchers and manufacturers.
are about 10µm in height. In addition, AFM measurements Veeco’s ninth-generation Wyko NT profilers incorporate LED
are relatively slow. Due to the scan speed and contact nature illumination for higher light levels and In-Motion capability for
of the scan, these systems are incapable of measuring MEMS measuring devices under actuation from 0–2.4mHz with ang-
while under actuation. strom-level resolution. Objective modules that can visualize
Optical profiling (white light interferometry) has long parts through packaging are also commercially available.
served as a standard technique for measuring surface topog- Thus, it is now possible to perform non-destructive, rapid,
raphy of MEMS and optical MEMS devices. In a white-light extremely precise MEMS characterization through packaging
optical microscope, the illumination source is traditionally a and under operation on a single measuring tool.
tungsten-halogen bulb, coupled into an optical system with
several interferometric microscope objectives. Light from the NoushiN Dowlatshahi is marketing product engineer at Veeco Instruments
Inc. Bob Chanapan is marketing product manager at Veeco Instruments Inc.
source is split in the objective, with part of the beam traveling
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