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Home > Implementing MEMS Technology in Today's Medical Electronics
Implementing MEMS Technology in
Today's Medical Electronics
Robert R. Swafford, Harold Joseph, and Vladimir Vaganov
Created 1997-10-01 03:00
[1]
Implementing MEMS Technology in Today's Medical
Electronics [2]
October 01, 1997
By: Robert R. Swafford, Harold Joseph, and Vladimir Vaganov
[3]
Find more content on: Components [4] Feature [5] Technology [6]
A Medical Electronics Manufacturing Fall 1997 Feature
MEMS
Robert R. Swafford, Harold Joseph, and Vladimir Vaganov
Improvements in the design, manufacture, and packaging of microelectromechanical
systems (MEMS) are lowering the costs and increasing the capabilities of these tiny
devices.
MEMS have been an important and growing part of the medical industry since the 1980s.
The microstructures are well suited for medical applications that require very small and
reliable sensors. The emphasis on developing portable devices for care in settings outside
the hospital has made MEMS sensors ever more attractive.
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Page 1 of 9Implementing MEMS Technology in Today's Medical Electronics
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2. Probably the most important development spurring the growth of the MEMS sensor
industry has been the decreasing cost and increasing computing power of
microprocessors. In medical sensing, most sensing needs are already known but may not
be adequately met because of limitations in the computing technology required to make
necessary calculations. Typically, microprocessors are first improved to address more
sophisticated sensing needs, and then new sensors are developed to supply input to the
microprocessors.
Silicon microvalves regulate pressure on vessels
containing fluids.
Manufacturers are meeting the increasing demand with
better and less costly sensors all the time. Improvements in
integrated circuit and semiconductor manufacturing
equipment, which made the combination of electronic
elements and mechanical structures in silicon and hybrid
materials possible in the first place, have led to significant
reductions in MEMS manufacturing costs. Manufacturers
have invested billions of dollars in developing silicon-
processing technology to produce low-cost integrated
circuits. Applying the resulting technological improvements
to sensors has allowed the industry to decrease silicon
sensor-element costs in much the same way that it has
decreased IC prices. Sensing elements are now typically produced in a wafer fabrication
facility and processed in batches of 25015,000 sensors, depending on size and
complexity.
MEMS sensors can also be produced for medical products with higher accuracy and
reliability than ever before. Currently available micromachining equipment allows
dimensional control for precise structures and the integration of mechanics and electronics
in ever smaller sizes with cost and power savings.
One of the most important and ongoing areas of improvement in MEMS manufacturing
technology is in sensor packaging design. A sensor's packaging can actually represent
more than 90% of the material cost of the product, and testing the packaging can be as
much as 20% of the overall cost of the product.
MEMS applications range from blood pressure monitoring to DNA testing, requiring a wide
variety of packaging designs. But, although the packaging types differ, they are all
undergoing improvements in design that are making more powerful and less expensive
MEMS possible.
Pressure Sensors
A pressure sensor comprising a Wheatstone bridge piezoresistive silicon microstructure is
one of the most common MEMS devices used in the medical industry. This sensor is
made to withstand pressures ranging from less than 0.1 to more than 10,000 psi.
The sensor's design combines piezoresistive elements and a diaphragm structure etched
to a thickness that is appropriate for the intended pressure range. Ion-implanted resistors
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3. are strategically placed to show changes in value for a given deflection of the diaphragm,
which is only a few millionths of an inch, thus creating a virtually motionless sensor.
Both dual-in-line and TO-8 packages are used in medical
applications.
In a Wheatstone bridge sensor, four resistors are
implanted and electrically connected so that opposite sides
of the bridge change by similar magnitudes and in the
same direction when the diaphragm is deflected. The
resulting imbalance in the bridge will cause a change in the
sensor's output voltage when excited by a constant current or constant voltage power
supply.
Measuring blood pressure is the foremost medical application for these types of MEMS.
The pressure sensor is an integral part of a system that consists of a saline solution bag
and tubing, to which the sensor is attached. The components are then connected to the
patient to provide signals to a monitor. Fluid passes through the tubing into the patient,
and when the heart beats a pressure wave moves up the fluid path and is detected by the
sensor.
In the early 1980s, external blood pressure sensors became widely available. Early
systems consisted of sensors on metal base plates and wire bonds contained in a plastic
housing. The saline solution passed through this housing. The housing contained a
flexible diaphragm that covered the pressure sensor and separated it from the fluid.
However, sterilizing and calibrating these reusable sensors actually cost several times as
much as buying replacement disposable units would. Disposables were, therefore, soon
developed.
In disposable systems, the sensing element is mounted on a plastic or ceramic base with
a plastic cap over it, designed to fit into the manufacturer's housing. A gel is used to
separate the saline solution from the sensing element. The disposable sensor is
manufactured and tested on an automated production line.
Figure 1. A cutaway diagram of a
pressure sensor.
Several design improvements have
helped bring down the cost of these
sensors. The basic specifications for
the sensors have been standardized
across the industry so that
performance requirements will not
change from one manufacturer to
another. The resistance of the
sensing bridge has also been
changed so that impedance-
matching electronics are no longer
needed to interface with medical monitors.
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4. The cap and ceramic base can be modified to interface with a variety of manufacturers'
assemblies. Sensors are tested on an automated assembly line, either on a tape-and-reel
system or on a multi-up ceramic substrate (see Figure 2). The multi-up ceramic sheet can
accommodate 120 sensors and can be fed directly into an automated assembly line or
divided and packaged as individual sensors. A typical manufacturer can produce from 1 to
4 million sensing systems per year. Design changes have reduced the cost of pressure
sensors to a fifth of their original value and have also allowed manufacturers to reduce
assembly costs.
PCB-Mounted Packages
A wide range of medical applications require sensors to be mounted on PCBs as part of
dual in-line or single in-line packages (DIPs and SIPs), TO-8 packages, or surface-mount
packages.
One of the early sensor packages was a TO-8 transistor can with a tube attached that
allowed the manufacturer to connect to the pressure source that was to be measured.
This package was widely available in the early 1980s and the pin-out was standardized so
manufacturers could easily lay out their PC boards. There were two classes of these
products, with several performance grades within each class.
One of the classes included a sensing die mounted in a package. It provided good
linearity, but the manufacturer needed to provide extensive correction for offset and
temperature errors. The second class, which was more expensive, provided correction of
the offset and temperature errors through the use of hard-wired resistors.
Several companies offered this second class of packages as their standard products,
giving OEMs several suppliers to choose from. Competition then encouraged the
development of thick-film ceramics with trimmable resistors, which obviated the need to
install the hard-wired resistors. A version of this package that has lower installation costs
is still used today.
DIP packages offered a number of advantages over the TO-8 design, and are now the
most common replacement for this earlier design. Ceramic is the base of the DIP
package. Gain trimming has been added, and improvements in die processing have
allowed for the consolidation of performance ranges. The leads are fixed in the package,
which is designed to be used with automatic placement machines.
The DIP package can be tested completely before leads and tubes are attached. This
means that the sensor can be stocked by range, and the proper tube and lead orientation
can be added once the order is received. This provides a faster response for the
manufacturer and a reduction in inventory. The package is also designed so that the
material being measured is ported to the back of the sensor for gage pressure
applications. The back of the sensor provides greater resistance than the front to moisture
or impurities in the measurement media.
DIP packages offer reduced costs compared to TO-8 designs. Part of the savings comes
from a reduction in inventory, but most is a result of the way these sensors are tested. The
DIP sensors are manufactured and tested in a batch containing 849 sensors. Therefore,
there is no need for individual handling, as there is in the TO-8 design. The DIP design
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5. also allows for a reduction in the mass of the test heads used in temperature testing of the
sensors, allowing a reduction of test time of more than 60%. The result of these testing
improvements is a cost reduction of more than 20% compared with TO-8 sensors.
Sensors on DIPs can be used for a wide range of medical purposes. These packages can
be used in portable vital sign and blood pressure monitors. They can be used to measure
breathing for ventilators. They can provide barometric pressure correction for blood-gas
analyzers. Drug-delivery inhalers can incorporate pressure sensors on DIPs to help
control the release of medication.
The least expensive way to package sensors is as surface-mount components. Most
sensor manufacturers now offer surface-mount pressure sensors. Such sensors are not
made with the performance of TO-8 or DIP packages, which allows a reduction in the size
of the sensing die and simplified testing. Manufacturing and testing are done on
automated production lines.
PCB-mounted pressure sensors are also available with amplified outputs. They are more
expensive than unamplified sensors, but they simplify PCB design, which can shorten the
product development cycle. Application-specific integrated circuits (ASICs) are now
available specifically for sensors, and some incorporate memory to allow automatic
temperature correction of the sensor.
Systems that use PCB-mounted sensors include eye surgery equipment that measures
vacuum and barometric pressure. These sensors can also be used to regulate the hospital
beds of burn victims. In this application, the sensors monitor the inflation levels of a series
of chambers within it to allow sections of it to be relieved of pressure, augmenting the
healing process and alleviating pain. The sensors can also be used in sleep disorder
monitors, which can be used, for example, in sleep apnea detection.
Flush Diaphragm Pressure Sensors
Flush diaphragm sensors measure fluid pressure with a plastic or metal diaphragm that
isolates the fluid from the MEMS sensor. A silica gel or silicone oil inside the sensor
package transmits the pressure on the diaphragm to the sensing element. Sometimes a
disposable plastic set is designed with a plastic membrane that mates with the pressure
diaphragm.
Figure 2. A cutaway diagram of an
accelerometer.
One method of providing additional
protection for a silicon sensor is to
place the sensing element in an oil-
filled, stainless-steel assembly. The
sensor is protected from the media to
be measured by a high-grade
stainless-steel diaphragm. Many
sensor and transducer companies
offer this type of sensor package.
Because stainless-steel diaphragm
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6. sensors are compatible with most media, these packages are the standard for a wide
range of industries. In fact, there are so many applications for these sensor packages that
there are now a multitude of fittings, electrical outputs, and housing and connection
standards for these products.
One medical application for this type of sensor package is to measure inlet and outlet
pressures in kidney dialysis machines. Blood flows through catheters into the dialysis
machine, where it is cleaned and then put back into the patient. The blood flows across a
thin permeable membrane where a special solution removes waste products by osmosis.
A stainless-steel pressure sensor with a flush isolating diaphragm, rather than a threaded
fitting, provides inlet and outlet pressure data used to regulate the fluid and blood
pressures during the operation of the dialysis system.
Accelerometers
A piezoresistive design can also be used to make silicon accelerometers. Accelerometers
are used for a variety of measurements from those related to high-volume manufacturing
to high-performance testing. An accelerometer can measure frequency and amplitude of
vibrations or can be used in steady-state conditions to determine a tilt angle or the
amplitude of a single shock or pulse.
An accelerometer operates with a sensing mass, supported by beams, that moves when
the structure is vibrated or when its orientation is rotated. As with pressure dies, ion-
implanted resistor elements are strategically positioned to reflect changing resistance with
the compression or relaxation of the beams. The sensor gives a constant voltage directly
related to the position of the mass. Mechanical stops prevent damage in the case of an
over-range condition.
Unlike pressure sensors, accelerometers do not have to interface directly with the media
being measured. Therefore, package design is driven only by environmental and
mechanical mounting concerns. How these packages are handled in assembly and test
can have a major effect on the product cost. A sensing element and ASIC can form a
complete amplified and calibrated device. The sensor is placed in a hermetically sealed
package, which is designed to mount either flat or on its side. The package can come in
plastic mounting tubes or as tape-and-reel systems. Both methods are designed for use
with automatic assembly equipment.
Accelerometers can measure frequency and amplitude of
vibrations, or they can be used in steady-state conditions.
Accelerometers are often used in sleep and motion studies
as components of patient-activity monitors. Sensors can be
mounted in three orthogonal axes and are sensitive
enough to detect small changes in patient position.
Accelerometers are also used in pacemakers to monitor
the activity level of a patient. When that activity level increases, the patient's heart must
beat faster. Newer pacemaker designs increase the output signal rate with the information
the device's microprocessor obtains from the accelerometer. The pacemaker is an internal
device, so reliability, size, and power are key concerns for accelerometers used in them.
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7. Specialized MEMS
Several important new MEMS packaging designs have been developed for very
specialized applications. Some OEMs with unusual device needs purchase individual
sensing die and then mount the sensors themselves in their own custom packages.
Catheter tip sensors for applications such as measuring intercranial pressure, intravenous
blood pressure, and intestinal pressure have required the development of die as small as
0.71.0 mm.
A remote temperature sensor has been made by combining an etched silicon diaphragm
with a thinly deposited layer of nitride-oxide to form a thermocouple. The diaphragm is
used to isolate the sensor from the immediate environment. The sensor, which is known
as a thermopile, is being evaluated for use as a remote temperature sensor in the inner
ear.
Flow restrictors provide the precise low-flow-rate control
needed for drug infusion.
Infusion pumps and drug-delivery systems meter
medication through a disposable set or tube inserted into
the patient. The sensing element is used to indicate an
alarm condition. It measures a pressure spike, which could
occur when a blockage exists. A custom MEMS device has
recently been developed to act as a force sensor. The
expandable plastic tube's sidewall is directly attached to the microstructure. The sensor
can be a pressure device, strain gage, or custom microstructure. The pressure range for
such applications is usually about 10 psi. In some cases, the set contains a diaphragm
that interfaces with a pressure sensor.
Another custom microstructure for drug infusion is a flow restrictor. Precise low-flow-rate
control is required for drug infusion and can be easily monitored with a silicon
microstructure. The manufacturing process is low in cost, repeatable, and precise, making
it an attractive alternative to conventional glass-capillary tubing.
Life-science research is spurring the development of custom silicon microstructures for
applications including DNA synthesizers, protein sequencers, and polymerase chain
reaction (PCR) instrumentation. In many of these instruments, complicated fluid-handling
systems are controlled pneumatically. Silicon microvalves have been designed to regulate
the pressures on vessels containing reagents and other fluids. A multichannel pressure-
management system provides precise proportional control to move very small amounts of
liquid while avoiding direct contact with the substances. In a silicon microvalve, a
diaphragm valve is electrically actuated. The valve uses a heated bimetallic structure to
provide the operating force. These valves provide fully proportional control of flows in the
range of 0300 cm3
/min at input pressures from near 0 to more than 100 psig.
PCR, which is used in the analysis of DNA, requires a carrier with a series of very small
wells containing a reagent. An 11 x 11-mm array can provide up to 48 wells. Rapid,
reproducible DNA amplification in silicon-based materials can be accomplished in a carrier
with very accurate temperature-controlled chambers, precise fluid manipulation, and fast
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8. test times. The precision of the fabrication process allows the use of small reaction
volumes, dramatically reducing analysis costs.
Future Trends
MEMS packaging design continues to improve. Companies are currently redesigning
many monitoring products that incorporate MEMS to simplify them and reduce their cost.
As hospitals and insurance companies work together to reduce the high costs of medical
care, putting greater emphasis on outpatient and in-home care, the demand grows for low-
cost MEMS products. Portable monitors, handheld devices, field life-science
instrumentation, and expanding overseas medical markets are all contributing to the
increasing need for new and improved MEMS devices.
Bibliography
Joseph H, and Vaganov V, "Matching Customer Application Requirements with Cost-
Effective Sensor Packaging," Philadelphia, Sensors Expo, October 22, 1996.
Joseph H, Swafford R, and Terry S, "Medical Applications for MEMS Devices,"
Philadelphia, Sensors Expo, October 23, 1996.
Robert R. Swafford is the product marketing manager, Harold Joseph is the director of
sales and marketing, and Vladimir Vaganov is the principal scientist at EG&G IC Sensors
(Milpitas, CA).
Copyright ©1997 Medical Electronics Manufacturing
Author:
Robert R. Swafford, Harold Joseph, and Vladimir Vaganov
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