MEMS (Micro Electro-Mechanical Systems) Technology

In less than 20 years, MEMS (micro electro-mechanical systems)
technology has gone from an interesting academic exercise to
an integral part of many common products. But as with most
new technologies, the practical implementation of MEMS technology
has taken a while to happen. The design challenges involved in
designing a successful MEMS product (the ADXL2O2E) are
described in this article by Harvey Weinberg from Analog Devices.

In early MEMS systems a multi-chip approach with the sensing
element (MEMS structure) on one chip, and the signal conditioning
electronics on another chip was used. While this approach is
simpler from a process standpoint, it has many disadvantages:

* The overall silicon area is generally larger.

* Multi chip modules require additional assembly steps.

* Yield is generally lower for multi chip modules.

* Larger signals from the sensor are required to overcome the
stray capacitance of the chip to chip interconnections, and
stray fields necessitating a larger sensor structure.

* Larger packages are generally required to house the two-chip structure.

Of course, history teaches us that integration is the most cost
effective and high performance solution. So Analog Devices
pursued an integrated approach to MEMS where the sensor
and signal conditioning electronics are on one chip.

Figure 1

The latest generation ADXL2O2E is the result of almost a decades
worth of experience building integrated MEMS accelerometers.
It is the world's smallest mass-produced, low g, low cost,
integrated MEMS dual axis accelerometer.

The mechanical structure of the ADXL2O2E is shown in Figure 1
along with some key dimensions in Figure 2.

Figure 2

Polysilicon springs suspend the MEMS structure above the
substrate such that the body of the sensor (also known as the
proof mass) can move in the X and Y axes. Acceleration causes
deflection of the proof mass from its centre position. Around the
four sides of the square proof mass are 32 sets of radial fingers.

These fingers are positioned between plates that are fixed to the
substrate. Each finger and pair of fixed plates make up a differential
capacitor, and the deflection of the proof mass is determined by
measuring the differential capacitance.

This sensing method has the ability of sensing both dynamic
acceleration (i.e. shock or vibration) and static acceleration
(i.e. inclination or gravity).

The differential capacitance is measured using synchronous
modulation/demodulation techniques. After amplification, the X
and Y axis acceleration signals each go through a 32KOhm
resistor to an output pin (Cx and Cy) and a duty cycle modulator
(the overall architecture can be seen in the block diagram in
Figure 3). The user may limit the bandwidth, and thereby lower
the noise floor, by adding a capacitor at the Cx and Cy pin.

The output signals are voltage proportional to acceleration
and pulse-width-modulation (PWM) proportional to acceleration.
Using the PWM outputs, the user can interface the ADXL2O2
directly to the digital inputs of a microcontroller using a counter
to decode the PWM.

Figure 3

Challenges in MEMS Design

The mechanical design of microscopic mechanical systems,
even simple systems, first requires an understanding of the
mechanical behaviour of the various elements used. While the
basic rules of mechanical dynamics are still followed in the
miniaturised world, many of the materials used in these
structures are not well mechanically characterised. For
example, most MEMS systems use polysilicon to build
mechanical structures. Polysilicon is a familiar material in
the IC world, and is compatible with IC manufacturing
processes.

Until recently, little work has been done to fully understand
polysilicon's mechanical properties. In addition, many materials
mechanical properties change in the microscopic world. Again,
polysilicon is a good example. In the macro world it is rarely
used as a mechanical element. It is too brittle and fragile to
withstand all but small mechanical deflections. But in the
extremely small movements of MEMS structures (less than
a few pm), it turns out to be an almost ideal material.

The electronic design of MEMS sensors is very challenging.
Most MEMS sensors (the ADXL2O2E included) mechanical
systems are designed to realise a variable capacitor.
Electronics are used to convert the variable capacitance
to a variable voltage or current, amplify, linearise, and in
some cases, temperature compensate the signal. This is
a challenging task as the signals involved are very minute.
In the case of the ADXL2O2E for example, the smallest
resolvable signal is approximately 2OzF and this is on top
of a common mode signal several orders of magnitude
greater than that! Of course, for cost reasons the
electronics must be made as compact as possible at
the same time.

The integrated approach presented further challenges.
Many standard production steps that improve the mechanical
structure degrade the electronics and vice versa. For example,
the usual method for flattening out the Polysilicon mechanical
structure is annealing (where the structure is exposed to
controlled high temperatures). While the annealing process is
beneficial to the mechanical structure, it can degrade or destroy
the BiMOS transistors used in the signal conditioning electronics.
So compatible mechanical and electronic process methods had
to be devised.

Another roadblock for the MEMS designer has been the
unavailability of standard design software. Modern integrated
circuits are rarely designed by hand. Complex CAD and simulation
software is used to help design and optimise the designers
concepts.

MEMS design software is still in its infancy, and most MEMS
manufacturers develop part or all of their CAD and simulation
software to suit their particular needs.

The fabrication process design challenge is perhaps the
greatest one. Techniques for building three-dimensional MEMS
structures had to be devised. Chemical and trench etching
can be used to "cut out" structures from solid polysilicon, but
additional process steps must be used to remove the material
underneath the patterned polysilicon to allow it to move freely.

Standard plastic injection molded IC packaging cannot be used
because of the moving parts of the MEMS structure. A cavity of
some type must be maintained around the mobile MEMS structure.
So alternative low-cost cavity packaging was developed.

In addition, this package must also be mechanically stable as
external mechanical stress could result in output changes.

Even mundane tasks, such as cutting the wafer up into single
die, becomes complicated. In a standard IC the particle residue
created by the sawing process does not effect the IC. In a
moving MEMS structure these particles can ruin a device.

The Users Challenge
MEMS sensors, like almost all electronic devices, do not
exhibit ideal behaviour. While most designers have learned
how to handle the non-ideal behaviour of op-amps and
transistors, few have learned the design techniques used
to compensate for non-ideal MEMS behaviour. In most
cases, this type of information is not available in textbooks
or courses, as the technology is quite new. So generally
designers must get this type of information from the MEMS
manufacturer.

Analog Devices, for example, maintains a web site with
design tools, reference designs, and dozens of application
notes specific to its MEMS accelerometers to ease the
users work.


Raiza Pernia
CI. V.-17.528.555
CRF

http://www.sensorland.com/HowPage023.html