14th Automotive Materials Conference: Ceramic Engineering and Science Proceedings, Volume 8, Issue 9/10

145 Pages·1987·2.253 MB·English

14th Automotive Materials Conference
Proceedings of the
14th Automotive Materials Conference
Bob R. Powell and Adolph L. Micheli
Conference Chairmen
A Collection of Papers Presented at the
14th Automotive Materials Conference
Sponsored by the
Department of Materials and
Metallurgical Engineering
and
Michigan Section
The American Ceramic Society, Inc.
November 19, 1986
University of Michigan
Ann Arbor, Michigan
ISSN 0196-6219
Published by
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CESPDK Vol. 8, NO. 9-10, pp. 997-1134, 1987
I
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Preface
This is the second time in the history of the Automotive Materials Confer-
ence Series that Sensors and Actuators has been chosen as a conference topic.
The technology of, and indeed, the possibile applications for sensws and ac-
tuators in the automotive environment have changed greatly since that first
conference in 1979. Microsensor technology has emerged with its promise of
being able to fabricate sensors using thin film technology. New concepts have
developed for air/fuel sensors based on oxygen ion transport which can ex-
tend the range of operation well into the lean region. Other sensors that will
contribute to improved vehicle performance, ride comfort, and safety are under
development. Finally, our understanding of sensor operation and the impor-
tance of processing has improved.
The opportunities are great. However, the new technology also contains
caveats. Greater attention should be given to the entire sensor system with
regard to packaging and interfacing. Packaging requires hard decisions about
partitioning and emerging modularity demands standardization.
The Michigan Section of the American Ceramic Society, and the Depart-
ment of Materials Science and Engineering at the University of Michigan spon-
sored the 14th Automotive Materials Conference to address these developments
in sensor and actuator technology.*
Bob R. Powell
Adolph L. Micheli
General Motors Research Laboratories
*The support of the following companies is gratefully acknowledged: AC Spark
Plug, Allied Automotive Autolite Division, Ford Motor Company, General
Motors Research Laboratories, The Harshaw/Filtrol Partnership, Hitachi
America, Ltd., and NGK-Locke, Inc.
iii
Each issue of Ceramic Engfneering and Science Proceedings includes a collection of
technical articles in a general area of interest. such as glass, engineering ceramics,
and refractories. These articles are of practical value for the ceramic industries. The
issues are based on the proceedings of a conference. Both The American Ceramic
sodety. Inc.. and non-Society conferences provide these technical articles. Each issue
is organized by an editor who selects and edits material from the conference. Some
issues may not be complete representations of the conference proceedings. There is
no other review prior to publication.
iv
Table of Contents
...............
Microsensor Packaging and System Partitioning 997
Stephen D. Senturia and Rosemary L. Smith
....
Integrated Solid-state Sensors for Automated Manufacturing .lo10
K. D. Wise
.............................
Silicon Resonant Microsensors .lo19
Martin A. Schmidt and Roger T. Howe
Effect of Liquid Phase on the PTCR Behavior of BaTiO,. ....... .lo35
K. R. Udayakumar, K. G. Brooks, J. A. T. Taylor, and
V. R. W. Amarakoon
Strain Sensing Transducer for On-Vehicle Load
......................................
Measuring Systems .lo44
William J. Fleming and John Hutchinson
...............
Air-to-Fuel Sensors Based on Oxygen Pumping .lo58
E. M. Logothetis
Air-Fuel Ratio Sensors for Automotive Use Utilizing
........................................
ZrO, Electrolytes .lo74
Takao Sasayama, Seiko Suzuki, Minoru Ohsuga, and
Sadayasu Ueno
Performance of Commercially Manufactured ZrO, Oxygen
Sensors at High Temperatures and Low PO, Atmospheres ...... .lo88
Michael J. Hanagan and Paul F. Johnson
Tin Oxide Gas Sensing Microsensors From Metallo-Organic
..............................
Deposited (MOD) Thin Films .lo95
Adolph L. Micheli, Shih-Chia Chang, and David B. Hicks
.................
Recent Sensors for Automotive Applications. .1106
Masataka Naito
Grain Boundary Engineering of Semiconducting Tin Oxide
....................................
Via Sol-Gel Coatings .1120
F. A. Selmi and V. R. W. Amarakoon
Sol-Gel Processes for Fibers and Films of
................................
Multicomponent Materials .1128
William C. LaCourse and Sunuk Kim
V
Ceramic Engineering and Science Proceedings
Bob R. Powell, Adolph L. Micheli
copyrightQThe American Ceramic Society, Inc., 1987
Ceram. Eng. Sci. Proc., 8 [9-101 pp. 997-1009 (1987)
Microsensor Packaging and System Partitioning
STEPHEN D. SENTURIA AND ROSEMARYL . SMITH
Microsystems Technology Labs, Dept. Electrical Eng.
Massachusetts Institute of Technology
Cambridge, MA 02139
While the problem of packaging of conuentional microelectronic components can
be neatly partitioned into two nearly separate disciplines, the design of packaging
for microsensors must be undertaken as an integral part of the ouerall sensor-
package-instrument design. Three issues figure prominently: (I ) sensors must in-
teract with their environment (by definition): (2)c onuentional packaging methods
do not necessarily prouide the correct combination of deuice isolation (forr eliabili-
ty) and deuice access (to the environment);a nd (3)t he package design can aflect-
even dominate-many aspects of sensor performance and reliability. This paper
outlines an approach to package design for microsensors, and illustrates how the
approach was used in the development of the flex-circuit ribbon-cable package for
the commercial rnicrodielectrometer sensor.
Introduction
The purpose of this paper is to address some general issues involving the use
of microfabrication technologies to create new types of measurement devices.
The tone is informal and descriptive under the assumption that regular technical
papers provide good examples of how various investigators choose to confront the
issues raised here.
We begin with some definitions. Transducers, as classically defined, are devices
that convert one form of energy to another. In the present context, it is more useful
to consider such devices as elements that convert a physical (or chemical) variable
into an electrical quantity, regardless of whether the energy is obtained from the
physical system or from energy sources associated with the transducer. Transducers
can be used for measurement, for actuation, and for display. This paper emphasizes
issues associated with measurement, although there are obvious parallels in the
other areas.
A microsensor is a measurement transducer made with techniques of
microfabrication. Some of these techniques are well-established in the integrated
circuits industry; others are specific to microsensors (microfabrication methods
are discussed further under Microfabrication). There is nothing particularly new
about microsensors. Examples are well known (see Fig. 1). The photodiode con-
verts incident optical energy into electrical energy, and is an example of a direct
energy-conversion transducer. The photoconductor works differently: incident light
energy changes the relative populations of electrons in various quantum states, thus
changing the conductivity of the element. The energy in the electric circuit, however,
is supplied from an external source. The phototransistor shares features of both
other devices. The base current in the transistor is supplied by electron-hole genera-
tion from incident light, but most of the energy in the collector circuit is supplied
by an external power source. These latter two transducers are examples of
parametric microsensors, in which the physical variable modifies a parameter of
the sensor element, which is then measured or detected through the element’s
behavior in an electrical circuit.
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System Issues
The title of this paper suggests that “Microsensor Packaging and System Par-
titioning” is a discipline that has well-understood principles, and that examples
will readily illustrate how these principles impact any given engineering design
problem. Unfortunately, such a suggestion would be imprecise, at best. While there
have been some excellent publications on specific techniques used for microsen-
sor packaging, most attempts to elucidate principles have produced more con-
troversy than agreement. The nub of the controversy is the so-called “smart sen-
sor”, which merges sophisticated electronic data processing with the microsen-
sor. The reader is hereby alerted that while some of the assertions presented here
are based on technical judgments that are quantitatively defendable, some are also,
to a certain extent, based on strongly felt opinions of the authors, and may not
be universally accepted. Caveat emptor.
Measurement systems have a great deal of modularity. This is illustrated
schematically in Fig. 2. Three modules are identified. The transducer’s function
has already been discussed. The schematic packaging boundary suggests that
transducer packaging presents special problems: some of the transducer requires
environmental access while the rest may require protection from the interface (the
packaging issue is discussed further in the section on Packaging). The interface
circuit supplies excitation to the transducer (if needed), accepts the response, and
performs additional functions such as amplification, linearization, or data conver-
sion. The data system provides overall control, and accepts the data for subse-
quent use. The various components communicate with one another over highly
standardized interconnections, such as the 4-20 mA or RS-232C interfaces between
data system and interface circuit. Furthermore, the design of transducers is often
made to provide for easy replacement; hence, the interface circuits are designed
to connect to standardized types of transducers. Among the benefits of this modulari-
ty is the fact that transducers, interfaces, and data systems can all be designed and
optimized separately. Microsensors which merge the transducer with other parts
of the measurement system do not have such modularity. The implications are
discussed under Microfabrication.
Measurement systems must be calibrated. Figure 3 illustrates a highly simplified
piezoresistive bridge that could be used to measure pressure. Two of the resistors
are presumed to be pressure dependent, the other two are not. (This can be achieved
by fabricating two of the resistors in a thinned diaphragm portion of the microsen-
sor which is allowed to deform under pressure, straining the resistors and chang-
ing their values.) The excitation voltage x(r) is supplied by the interface circuit,
and the output voltage y(t) is returned to the interface circuit. It is seen that the
relation between the inferred pressure p(t) and the output y(t) is nonlinear. The
calibration expresses the accuracy with which the inferred pressure reflects the
actual pressure applied to the diaphragm.
Where does the calibration reside? It resides in the precision and repeatibility
with which the resistors and the deformable diaphragm are manufactured, in the
stability of these components, in the extent to which the response y(r) can be made
independent of all other physical effects, notably temperature and package-induced
stresses, in the accuracy with which the excitation waveformx(r) is produced, and
in the accuracy of the amplification and data-conversion portions of the system.
In summary, the calibration resides everywhere in the system; it is a system issue.
Conceptually, modularity aids in calibration because the functional performance
of each module can be independently discovered, optimized, and compensated.
On the other hand, there are examples where cost advantages are achieved by allow-
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ing one portion of the system (such as the data system) to implement a compen-
sating correction for the calibration of individual transducers. Thus, in consider-
ing a microsensor design, there is a system-level decision to be made: whether
to trim individual devices to standard calibrations (the modular approach), or to
use the interface circuit or data system to compensate for device-to-device varia-
tions (the system approach). Both approaches are used.
Microfabrication
Microfabrication refers to the collection of techniques used by the electronics
industry for the manufacture of integrated circuits. The success of microfabrica-
tion is immense, combining the economies of batch fabrication with the dimen-
sional precision of photolithography. A variety of materials are compatible with
batch-fabrication techniques, and these materials can be deposited and patterned
in many ways. In addition, and of great significance for the field of microsensors,
the technologies of microfabrication are needed by, and are therefore supported
by, the electronics industry. Thus, it is not necessary for the sensor industry to
provide the capital development costs for most of the process technologies.
Standard integrated circuit processing techniques have been used to make a
variety of devices that function as sensors. Table I lists types of microsensor devices
that are already well established. In many cases, these are simply microfabricated
versions of existing “macro” transducer. In other cases, notably the charge-coupled
optical imaging devices, there is no corresponding macro-device. Careful examina-
tion of the Table, however, shows that the well-established devices are those for
which the packaging problem is most readily solved. Heat and magnetic fields readi-
ly penetrate standard encapsulation materials, and hermetic window technologies
for optical devices have a long history. Encapsulation technologies for pressure
microsensors are in commercial use, but in many cases, packaging artifacts limit
either accuracy or drift specifications. Chemical microsensors are not yet able to
take full advantage of the microfabrication technologies, in part, because of packag-
ing limitations.
In addition to new fabrication capabilities for existing types of sensors,
microfabrication offers the promise of new types of devices, based either on u-
nique properties of microelectronic devices (such as the charge-coupled device or
carrier-domain magnetometer devices), on new fabrication capabilities such as
microma~hiningo,~r ~on~ the promise of being able to merge signal conditioning
and signal processing with the primary sensing device (the smart sensor).
Micromachining refers to a set of special deposition and/or etch processes with
which mechanically complex structures can be fabricated either in or on planar
substrates. Diaphragms, cantilevers, moveable capacitor plates, and through-
substrate holes are among the types of structures that are readily created. The smart
sensor is an attractive idea, at least at first. One imagines a batch-fabricated device
that performs an entire measurement, and presents an output signal in a form readily
accepted by a microprocessor. This is the promise; but there are problems:
The principal difficulty with the smart-sensor concept is the loss of modulari-
ty. Because the design of microfabricated parts must be done in monolithic fashion,
every detail of the device must be designed at once. It is no longer possible to
have the interface expert work independently of the sensor expert. Furthermore,
because the production specification for a smart sensor consists of a mask set and
a process description, any change in either the sensor or the interface design re-
quires a completely new mask set. Fabrication errors in either the sensor or the
interface can ruin both when they are fabricated together. Of perhaps greater
999
significance is the fact that the optimization of process sequences for electronic
components may not be compatible with the optimum process with which to fabricate
the transducer. By trying to create a merged design, the quality of both types of
components may be compromised. Finally, the sensor package must be designed
along with the microsensor. Thus, the design overhead in a non-modular merged
smart sensor can be very large. There may still be good economic justifications
for building smart sensors, but in the many public discussions of this topic, only
the attractiveness has been emphasized; the hazards have often been ignored.
The capabilities of microfabrication, with their potential for new and improved
microsensors, create a set of partitioning decisions for the designer. The system
must be partitioned between the batch-fabricated microsensor and the rest of the
system. The process technology must be partitioned between standard process steps
that are readily available in the integrated circuit industry, and non-standard pro-
cess steps required for the specific microsensor design. Both packaging and the
need to optimize process technology impose constraints on these partitioning and
design decisions. The rest of the paper explores these issues, starting with packaging.
Packaging
As used here, packaging refers to first-level packaging, enough encapsulation
to permit device handling, performance evaluation, and actual use in at least some
applications. The packaging problem, as stated earlier, is that the details of the
package affect every level of microsensor design, including how the measurement
system is set up, how it is partitioned, and how the microsensor part of the system
is designed. Therefore, it is necessary to design the microsensor and the package
at the same time. This surprisingly simple suggestion often meets with great op-
position. One problem is that packaging people are usually not the same as sensor
people, and getting them to work together can be difficult. Further, because package
design can be expensive, there is a reluctance to commit effort without some
evidence that the microsensor will work. Nevertheless, it is the authors’ opinion
that without a package design-even a temporary, simple package design-effort
spent on microsensor development can be a mistake. The cost of fabrication of
a microsensor does not depend heavily on details of the layout. Therefore, one
might as well use a layout that can be successfully tested in a package. By putting
some effort into the packaging problem early in the design, unrea!istic designs that
cannot be packaged are avoided.
The approach involving simultaneous package and sensor design is described
with reference to Fig. 4. The design sequence is conceived at four levels: parti-
tioning, specification of interfaces, design specifications, and detailed design. Itera-
tion at all levels is assumed. Each of the levels is now discussed:
Partitioning
For a microsensor measurement system, the partitioning decision addresses
how much of the system is to be merged into the batch-fabricated microsensor part
(the chip), and how much is to be off-chip. The position taken here is very simple:
minimize the on-chip part of the system.
This approach is useful for several reasons. It forces the designer to think
through which of the many possible functional parts (such as extra resistors, tran-
sistors for amplification, switches for multiplexing) are actually essential for suc-
cessful performance of the microsensor’s task. It also forces the designer to ad-
dress questions of operating environment, control of parasitic responses, and overall
system architecture early in the design. Adding functionality may increase the pro-
1000
cess complexity (hence the cost), may reduce the yield (hence increase the cost),
and increase the size of packaging complexity (and hence the cost) of the device.
The only justification for such increased costs is a documentable performance
benefit, either at the sensor level or at the overall system level. There are correctly
partitioned examples where the microsensor consists of a simple set of electrodes
with no added functionality, and other correctly partitioned examples where the
microsensor contains a sensing element plus transistor amplification and trimmable
resistors for adjustment of temperature compensation and calibration. The idea is
to avoid seduction by complexity just because highly merged smart sensors are
possible-they should also be required by the application at hand before the smart
sensor-designer should build them.
The partitioning of the microdielectrometry system6,’ provides a useful exam-
ple. The microdielectrometer is intended for measurements of dielectric constant
and conductance in very insulating materials (see a system schematic in Fig. 5).
It is based on an interdigitated electrode pair, one of which is driven, and the other
of which collects charge through the medium under test. Because of the intended
high-impedance application, beyond the capabilities of a simple passive electrode
pair, the sense electrode must be physically close to the first stage of amplifica-
tion; otherwise the ieakage currents in wiring insulation would limit the measure-
ment. Hence, the device as conceived requires one transistor. Given that one is
required, a second is also required to allow an accurate differential measurement,
providing temperature and pressure compensation, and cancelling out process-
induced variations. There are many examples where this minimal device set is ap-
propriate: a property-dependent element and a matched pair of transistors (see Fig.
6). In the case of the microdielectrometer, an additional diode is added to the chip
to provide a temperature measurement capability. This adds no process complexi-
ty, no additional area, and greatly enhances the performance of the sensor. Hence
it is justified under the minimalist approach. However, the next stage of analog
amplification in Fig. 5, which could have been added with relatively modest cost,
is not included on chip because it is not necessary. A benefit of this decision was
the discovery (after the fact) that the basic sensor of Fig. 6 would operate suc-
cessfully at temperatures much higher than would have been possible had the analog
electronics been added to the chip.
Define System Interfaces
A microsensor has a variety of interfaces: an electronic interface to the measure-
ment system; a mechanical interface with the environment (with attendant chemical,
thermal, and pressure/ stress characteristics) and a cabling or interconnect require-
ment with the measurement system; materials requirements for chemical stability,
thermal stability, mechanical properties, and in some cases, biocompatibility . During
the process of defining these various interfaces, it is useful to list possible elec-
trical, chemical, mechanical, and thermal parasitic effects that can arise either in
the microsensor itself or in its package. It is often possible to make relatively modest
design changes to eliminate serious parasitics-the best time to find them is before
the first prototype is built.
Design Specifications
As the various system requirements and interfaces become clear, possible com-
binations of microsensor and package can be evaluated for their success in dealing
with each of the requirements and possible parasitics. Out of this process, it becomes
possible to be specific about how the microsensor should be built and how it should
1001