Sacrificial polysilicon surface micromachining is emerging as a
technology that enables the mass production of complex
microelectromechanical systems by themselves, or integrated
with microelectronic systems. Early versions of these
micromachined systems-on-a-chip found application in the
commercial world as acceleration sensors for airbag
deployment—for example, ADI's ADXL50. There were two advances
in manufacturing techniques for micromachined systems-on-a-chip
that made possible great leaps ahead in the complexity of the
systems. The first was a three-layer polysilicon micromachining
process which included a fourth polysilicon electrical
interconnect layer. The other was a single-layer (+ second
electrical interconnect level) polysilicon surface
micromachining process integrated with 1.25 micron CMOS.
Samples of systems-on-a-chip built in these processes are
pop-up mirrors and multi-axis accelerometers.
Integrated Microelectronic/Micromechanical Systems-on-a-Chip
A great deal of interest developed in manufacturing processes
that make possible the monolithic integration of
MicroElectroMechanical Systems (MEMS) with driving,
controlling, and signal processing electronics. This
integration promises to improve the performance of
micromechanical devices, as well as reduce the cost of
manufacturing, packaging, and instrumentation for these
devices, by combining the micromechanical devices with an
electronic sub-system in the same manufacturing and packaging
process. For example, Analog Devices developed and marketed an
accelerometer which demonstrates the viability and
commercial potential of integration. They accomplished this by
interleaving, combining, and customizing their manufacturing
processes that produce the micromechanical devices with those
that produce the electronics. In another approach, researchers
at Berkeley developed a modular integrated approach in
which the aluminum metallization of CMOS is replaced with
tungsten to enable the CMOS to withstand subsequent
As summarized in a review paper by Howe, micromechanical structures require long,
high-temperature anneals to ensure that the stress in the
structural materials of the micromechanical structures is
completely removed. CMOS technology requires planarity of the
substrate to achieve high-resolution in the photolithographic
process. If the micromechanical processing is performed first,
the substrate planarity is sacrificed. If the CMOS is built
first, it (and its metallization) must withstand the
high-temperature anneals of the micromechanical
Figure 1: Micromachined resonators (left) next to their CMOS driving electronics (right) fabricated using the embedded micromechanics integration process.
A unique micromechanics-first approach, which overcomes the planarity issues of
building the MEMS before the CMOS, was developed at Sandia. In
this approach, micromechanical devices were fabricated in a
trench etched on the surface of the wafer. After these devices
were complete, the trench was refilled with oxide, planarized
using chemical-mechanical polishing, and sealed with a nitride
membrane. The wafer with the embedded micromechanical devices
was then processed using conventional CMOS processing.
Additional steps were added at the end of the CMOS process in
order to expose and release the embedded micromechanical
devices. Completed devices are shown in Figure 1. A
cross-section of this technology is shown in Figure 2.
This technology was named as one of the recipients of the 1996
R&D 100 Award.
Important developments resulted from a collaboration with
designers from the Berkeley Sensor and Actuator Center (BSAC).
BSAC designs for inertial measurement units (three-axis
acceleration and three-axis rotation rate) were built using Sandia's Integrated
MicroElectroMechanical Systems (IMEMS) Technology. Wafer lots
of devices were fabricated, and further information on the
accelerometers fabricated with this technology is reported in
the next section.
Figure 2: A schematic cross-section of the embedded micromechanics approach to CMOS/MEMS integration.
One of the principal commercial products fabricated using
surface-micromachining, is inertial sensors, examples of which
are Analog Devices' ADXL150 and Motorola's XMMAS40GWB. The primary application of these
accelerometers is as airbag-deployment sensors in automobiles,
but they are also being used as tilt or shock sensors. (In the
following discussion, please note the use of g for
gravitational acceleration and the word "gram" for mass). The
Motorola device is a 40g (full scale) single-axis
accelerometer with a noise floor of 400 mg (400 Hz bandwidth,
peak) and has an analog output. The Analog Devices accelerometer is
available as either a single (ADXL150) or dual-axis device
(ADXL250), has a 50g full scale output, an analog output, and
a noise floor of 10 mg (100 Hz bandwidth, rms). Please note that the use of peak vs. rms
noise specifications together with the difference in bandwidth
specifications for the devices makes a direct comparison
The application of these types of accelerometers as inertial
measurement units is limited by the need to manually align and
assemble them into three-axis systems, the resulting alignment
tolerances, their lack of on-chip A/D conversion circuitry, and
their lower limit of sensitivity. In order to overcome some of
these limitations, a three-axis, force-balanced accelerometer
was designed at U.C. Berkeley to be fabricated using the integrated
MEMS/CMOS technology described in the previous section. This
three-axis accelerometer system-on-a-chip is shown below in
Figure 3: Three-axis accelerometer micrograph with labeling of functional units as reported by Lemkin et al.
The performance of the device is summarized in Table
1 below. Approximately an order of magnitude increase in
sensitivity is seen over the commercial devices described
previously. The accelerometer chip also includes clock
generation circuitry, a digital output, and photolithographic
alignment of the sense axes. Thus, this system-on-a-chip is a
realization of a full three-axis inertial measurement unit that
does not require manual assembly and alignment of sense
Although the bias stability of this accelerometer system has
yet to be assessed, the noise numbers indicate that there are
potential commercial applications for this system such as:
- automotive control
- automotive diagnostics
- automotive navigation
- virtual reality environmental sensing.
A combined X/Y-axis rate gyro and a Z-axis rate gyro were
designed by researchers at U.C. Berkeley and fabricated in this
manufacturing process to yield a full six-axis inertial
measurement unit on a single chip. The estimated size for this
system was approximately 4 mm by 10 mm.
Table 1: Performance of the three-axis accelerometer
as reported by Lemkin, et al.
Multi-Level, Planarized Polysilicon Systems-on-a-Chip
Micromechanical actuators have not seen the wide-spread
industrial use that micromechanical sensors have achieved. Two
principal stumbling blocks to their widespread application have
been low torque and difficulty in coupling tools to engines.
Sandia National Laboratories developed devices that overcome
these difficulties. Our three-layer polysilicon micromachining
process made possible the fabrication of devices
with increased complexity that greatly enhanced the ability to
couple tools to engines.
The three-layer process includes three movable levels of
polysilicon in addition to a stationary layer for a total of
four layers of polysilicon. The polysilicon layers are
separated from one another by sacrificial oxide layers. A total
of eight mask layers are involved in this process. An
additional friction-reducing layer of silicon nitride is placed
between the layers that form bearing surfaces. Inset (lower
right) in Figure 4, is a cross-section view of a bearing
formed between two layers of mechanical polysilicon. The
balance of the Figure 4 illustration is a picture of two
sets of comb-drive actuators that drive a pair of linkages that
then drive a pair of rotary gears. The whole system is driven
by transmitting drive signals that are 90° out of phase with
each other to the comb drive actuators. The small gear has been
operated at speeds in excess of 300,000 revolutions per minute.
The operational lifetimes of these small devices can exceed
8x10 revolutions. The small gear is shown driving
a larger (1.6 mm diameter) gear in Figure 4. This large gear has been
driven at speeds as fast as 4800 rpm.
Figure 4: Two sets of linear comb-drive actuators drive the small gear (shown in cross-section in the inset). The smaller gear drives a 1.6 mm diameter shutter gear in the lower left of the photo. The inset (lower right) is a focused ion-beam (FIB) cross-section image of the small gear.
To increase the torque available from a rotary drive, a
multi-layer microtransmission was developed. This transmission, shown in Figure 5,
employs sets of small and large gears mounted on the same shaft
that mesh with other sets of gears to transfer power while
providing torque multiplication and speed reduction. The
structure in Figure 5 shows the output gear of a
microengine, similar to the one in Figure 4, meshed with
a linear rack to provide linear motion with a high degree of
Figure 5: An electrostatic microengine output gear meshed with a two-layer gear train that drives a linear rack. This gear train provides a speed-reduction/torque-multiplication ratio of 9.6 to 1.
The microengine in combination with the microtransmission can
be used to drive a pop-up mirror up, out of plane. With the
torque increase resulting from this combination, the mirror can
be elevated by the mechanism, alone—without manipulation from
the outside using probes.(Figure 6).
Figure 6: The microtransmission driving a linear rack to elevate a mirror out-of-plane.
A technology involving micromachined devices embedded below
the surface of a wafer, prior to fabrication of microelectronic
devices, was developed and applied to build complex sensor
systems on a single chip. A three-layer polysilicon process
made possible intricate coupling mechanisms that link linear
comb-drive actuators to multiple rotating gears. This
technology has been used to build devices such as microengines,
microtransmissions, and micromirrors. These devices were also
combined to yield intricate mechanical systems-on-a-chip.
Courtesy Sandia National Laboratories, SUMMiT Technologies,