MEMS devices require a variety of driving
waveforms. An individual device (e.g., Microengine) may require up
to 4 channels to drive plus ground. Custom waveforms may be
required for optimized performance. In general, clipped square and
sine functions are typical (0 to +100V). To generate these
signals, Sandia typically utilizes either a National Instruments AT-A0-6 High Performance Analog Output Board (installed in computer) or a
stand alone (external) function generator such as the Agilent 33120A/33220A or Keithley Instruments 2400.
The Analog Output Card card requires specialty software such as Super µDriverTM (Tegam
Inc.), MEMScriptTM or LabViewTM.(National
Most function generator outputs are in the range of 0V to 10V (peak
to peak). Many of the MEMS devices utilize electrostatic actuation
which require voltages on the order of 100V, hence, amplification is
required. If the devices being studied are low power and frequency
(bandwidth), the an amplifier such as TEGAM's 2375 will suffice. However if high frequency and or
high power is required, Ktech's SNLKP24W or Micro
Tech Instruments' MTI24KP (both Sandia licensed designs) would be
appropriate. Higher power (200mA, ??V) is required for thermal
actuators when compared to a TRA (Torsional Ratchet Actuator).
It is good practice to use an oscilloscope (Agilent
Technologies 54624A) during MEMS characterization. This allows the
tester to verify the signals being sent to the device and is useful for
Dual In Line Package
In order to test packaged parts, a Dual In Line
Package (DIP) is convenient means to make electrical connections to the
components. At Sandia National Laboratories Light Laboratory, the
HP 16058-6007, HP 16147-6002 and HP 16058-60006 (Hewlet Packard 18, 24
and 28 pin DIPs, respectively) socket modules are utilized. Agilent also provides a source for socket modules.
In order to test devices still on the wafer or individual die, a probe
station is required. For the purposes of this document, we define
a probe station to consist of a microscope, probes, and platform
(stage). One may decide to assemble a probe station from separate
components or purchase a complete setup. The Alessi REL-6100 (Cascade
Microtech) and the Signatone CAP-463 (Signatone)
are two systems currently used at Sandia.
Micromanipulators and Probes
Sandia surface micromachined MEMS devices require two to five
micromanipulators for actuation. Micromanipulators
also referred to as micropositioners have three degrees of
freedom (x,y and z) and are made to allow fine movement of the probe
tips. These are held to the platform either by vacuum (requires
house vacuum or an additional pump) or magnetic force. Surface
micromachined devices fabricated at Sandia typically require two to five
probes two actuate; one can employ Alessi MH2 or the Signatone S-926 micropositioning
The microscope should include objectives between 10X and 100X - a
three objective turret is adequate. Objectives should have a long
working distance (~2-4cm) as to provide ample space for probes. In addition to
the eyepieces for standard viewing, a camera mount is a requirement.
The ability to simultaneously view and record images is critical for
efficient image analysis. For MEMS inspection, illumination must
be top down (many "standard" microscopes used in other applications
illuminate from the bottom through the sample). Dark field and
brightfield illumination allow for enhanced visual inspection. The
ability to switch between more than one light source is desired but not
a strict requirement (CW or continuous wave illumination and a strobe
source is very useful). If interference microscopy is desired in
order to include optical height and critical dimension measurements of
MEMS physical structures, there are additional requirements.
Generally, such measurements are done on a separate system specifically
designed for such an application.
The ability for microscope head (consisting of the objectives,
eyepieces and associated optics) to move independently of the stage is
important. This allows one to adjust the field of view while the
electrical probes are making contact with the running device.
A standard white light source comes with most microscope systems (Halogen source from EKE from Micro-Tech).
A strobe light source with frequency and phase adjustment (relative to
actuation waveform) is vital to observe high frequency actuation. Sandia utilizes the Olympus ALS-12000S (Carsen
Video camera mounted to the microscope provides a source to
the image capture / video card in the computer as well as external
video/DVD recording devices and viewing monitor.
When using the strobe in conjunction with the
video/image capture tools, one can acquire detailed device position vs.
time information. This is done by adjusting the phase of the strobe relative to the drive signal.
While characterizing devices, we often measure the
distance an actuator or shuttle travels. We use systems that provide
an overlay on a video monitor (Boeckler
Instruments VIA-100) and systems that provide image capture with
measurement capabilities (Visual Inspector Imaging Software from Cimarron Computer
For extremely fast events, such as the sudden
uncoiling of a spring, a high speed camera may also be utilized. To see an example of
what one can do with a high speed camera (3500 frames/sec), select
either the 270 or 360 degree micro
coiled spring video clip.
Vibration isolation will improve image quality, avoid damage from
probe tip vibration. There are several types of vibration isolation systems in use at Sandia. Additional
sources include Kinetic Systems and Technical Manufacturing
Clean Benches - Vertical Laminar Flow
A clean area is required when probing non-packaged parts.
Vertical Laminar flow clean benches are adequate for probe stations -
enough space should be afforded to be able to handle parts in the clean
environment. Electronic support equipment such as the computer,
oscilloscope, amplifiers etc, do not have to be in the clean
environment. There are several levels of clean
environments utilized at Sandia, including clean benches from Envirco. Other sources include the Baker Company who offers a short Introduction to Clean Benches. Terra Universal is another
source for vertical laminar flow stations. Tech Rite also is a Vertical Laminar Flow station as well as Portable Clean Room Kits.
Characterization Technologists should wear clean room gloves and
smocks when testing devices. Hairnets and face masks ("beard
bags") will also reduce the risk of particulate contamination of open
parts. Parts should always be handled within the laminar clean
environment. In addition, the proper grounding of equipment, probe
station components and technologists will reduce the possibility of
electrostatic discharge (ESD) damage to the micro devices.