Miniature actuator

A ferroelectric motor comprises a single layer of ferroelectric material electrically excited by an array of electrical contacts and an electrical excitation source for supplying phased electrical signals to the contacts thereby creating a travelling wave of mechanical deformation in the ferroelectric layer and actuating an actuator. In alternative embodiments of the invention, the actuator may be linear or rotary. The motor may be fabricated on a single integrated circuit die, in which case the layer of ferroelectric material may be a thin film of PZT. In other embodiments a motor may comprise two dies which are sandwiched together by wafer to wafer bonding. Portions of a die may be removed to permit a linear actuator to project beyond the die.

BACKGROUND OF THE INVENTION 
This invention relates to silicon-based microfabricated motors, and 
particularly to motors for miniature robots. 
Several groups are working on integrated circuit motors or actuators. There 
have been some early successes, but hurdles still remain. Most of the 
existing microfabricated structures were developed to produce 
microsensors. Pressure sensors, chemical sensors and accelerometers are 
just a few of the wide variety of microsensors now commercially available 
which convert other variables to electrical signals in order to transduce 
some physical phenomenon into information. Due to the fact that these 
sensors can be both mass produced and fabricated with on-chip signal 
processing, they are rapidly finding homes in such places as the 
automobile and appliance industries. 
The basic technology which makes it possible to fabricate freely moving 
components on chips is termed silicon micro-machining. Micro-machining 
processes etch structural shapes in silicon for mechanical purposes. 
Mechanical structures such as cantilever beams and bridges have been 
etched in silicon in sizes on the order of a few tens of microns. 
Known microfabricated motors rely on variable capacitance forces to create 
motion. A typical microfabricated motor is the variable capacitance 
micromotor, which uses capacitive forces to cause a rotor to slide around 
a bearing. In this motor, friction between the bearing and rotor creates 
losses. 
Other, large scale motors have been designed using piezoelectric phenomena 
for mechanical force. For example, macroscopic travelling wave motors made 
out of bulk ceramic PZT have been designed with very high efficiency, and 
some even appear commercially (see Inaba et al., "Piezoelectric Ultrasonic 
Motor", Proceedings of the 1987 IEEE Ultrasonics Symposium, pp. 747-756, 
and Kumada, "A Piezoelectric Ultrasonic Motor", Japanese Journal of 
Applied Physics, Vol. 24 (1985) Supplement 24-2, pp. 739-741). 
Travelling wave motors also have inherent gear reduction due to the 
rectification of high frequency vibratory motion into continuous 
unidirectional motion. The resulting high power, low mass, low speed 
motors are ideal for robotic applications as complex gearing can be 
omitted. 
Of the thirty-two different crystal classes, twenty-one have lattice 
formations with an inherent asymmetry. Twenty of those twenty-one crystals 
exhibit piezoelectric properties, which means that application of a 
voltage across the material causes a mechanical deformation and, 
conversely, stressing the material produces an electrical signal. 
Of the twenty substances that have piezoelectric attributes, ten contain an 
electric dipole moment in the unstrained condition, which leads to 
pyroelectric characteristics. A pyroelectric material creates an 
electrical signal when the crystal is exposed to a change in temperature. 
Some of the ten pyroelectric materials also display ferroelectric traits. A 
ferroelectric material can have its polarization dipole reoriented in 
direction through the application of a strong electric field. After the 
electric field is removed, the crystal retains the polarization direction, 
effectively acting as a solid-state switch. 
Ferroelectrics, then, being a subset of pyroelectrics and piezoelectrics, 
contain attributes of all three. In addition, ferroelectric materials are 
characterized by having very high dielectric constants. 
Piezoelectric, pyroelectric, and ferroelectric phenomena have been used in 
a wide variety of applications. Materials that are predominantly 
piezoelectric are often used in items such as speakers, touch sensors or 
microphones and can be found in bulk ceramic or thin film form. Ceramics 
with large pyroelectric coefficients are used in applications which sense 
changes in infrared energy such as burglar alarms or night-vision scopes. 
Recently, some materials which exhibit ferroelectric properties have been 
produced in thin film form and incorporated into memory chips to create 
non-volatile random access memories. 
Known robots are designed to individually complete such tasks as window 
washing, automated manufacturing, or other physical duties which are too 
hazardous, difficult, or mundane for humans to perform. It has been found 
that the physical strength of robots can easily exceed that of human 
beings, but the programming of such robots to perform all but the most 
repetitive tasks has proven difficult. 
Known robot designs are quite expensive, owing mostly to the expense of 
suitable mechanical actuators and the attendant power systems. 
SUMMARY OF THE INVENTION 
In a first aspect, the invention features a ferroelectric motor constructed 
with a single layer of ferroelectric material and operated by stimulating 
the ferroelectric material with a spatially varying electrical signal, 
producing a travelling wave in the thin film. The travelling wave moves a 
linear or rotary actuator frictionally mounted on the stator. 
In a second aspect, the invention features a ferroelectric motor in which 
the ferroelectric layer is formed as a thin film of ferroelectric 
material. In preferred embodiments, the ferroelectric material is PZT 
deposited on a silicon substrate; the film is coupled to the actuator 
through electrical contacts which also serve as elastic bodies for 
mechanically amplifying the travelling wave; a wear layer is provided 
between the contacts and the moving actuator to reduce wear; an infrared 
sensor is also formed in said ferroelectric thin film. 
In a third aspect, the invention features a ferroelectric motor having two 
stators each formed of thin film ferroelectric material on a semiconductor 
substrate. In preferred embodiments, the stators are configured to produce 
linear motion along mutually transverse directions; an appendage is 
mechanically coupled to said actuators; one actuator moves the axis of 
rotation of the appendage and the other engages the appendage at a 
location spaced from the axis of rotation. 
In a fourth aspect, the invention features a robot provided with two linear 
motors and an appendage driven by the motors; the appendage is 
mechanically coupled to the two linear motors so that movement of one 
motor causes translation of the axis of rotation of the appendage and 
movement of the other causes rotation of the appendage about the axis. 
In a fifth aspect, the invention features a linear ferroelectric motor. In 
preferred embodiments, a strap is provided for retaining the linear 
actuator of the motor, two ends of the strap being connected to the 
substrate and the mid region of the strap extending over the linear 
actuator; capacitive regions are provided for attracting the actuator 
toward the stator to provide the necessary frictional engagement; the 
actuator can also be positioned between and frictionally engaged with two 
stators. 
In a sixth aspect, the invention features a thin-film ferroelectric motor 
and capacitive regions on the stator of the motor for attracting the 
actuator to achieve the required frictional engagement. 
In a seventh aspect, the invention features the manufacturing method of 
forming a linear actuator extending beyond the boundary of the substrate 
on which the actuator is formed by removing portions of the substrate 
(e.g., by etching or breaking off portions). 
Other advantages and features of the invention will apparent from the 
following description of preferred embodiments and from the claims.

All of the figures are somewhat diagrammatic and are not drawn to scale. 
FIG. 1 shows a gnat robot in accordance with the invention. The robot is 
approximately cubic in shape, and about two or three millimeters in size. 
It has a plastic body 10 to which a number of limbs 20 are affixed. Limbs 
20 may be used as legs for locomotion of the robot. At other times pairs 
of the limbs may operate in tandem as a manipulator or gripper. 
The structure of such a robot is hybrid. Individual silicon dies 22 are 
implanted into sockets in a laser sculpted plastic body 10. The dies are 
separately fabricated to contain control electronics, sensors, electronic 
motors, and actuators. The dies are mounted on the plastic body using 
microfabricated appendages and sockets, as illustrated in FIG. 3. 
Laser-deposited tungsten wires 24 connect the silicon dies together to 
form a complete gnat robot. 
Such a robot may be mass-produced in analogous manner to the production of 
hybrid integrated circuits. Using mass production, gnat robots will be 
relatively inexpensive to build and thus in some applications may be 
disposable. However, owing to dense integration of computing power, each 
gnat robot can possess levels of intelligence similar to that found in 
much larger previous robots. 
When properly programmed, a large number (or "swarm") of gnat robots can, 
in parallel, perform tasks currently assigned to single large robots, or 
other tasks not currently performed by robots. Gnat robot swarms can take 
advantage of their large numbers to accomplish highly distributed tasks, 
and, owing to their low cost, the gnat robots can be disposed of when the 
job is complete. 
Swarms of gnat robots can perform tasks such as, for example, operating as 
distributed sensors. By gathering information via audio, visible light or 
infrared radiation, each robot can be used to locate distributed 
information. For example, each robot may be programmed to eject a dye when 
sensing underwater noise. A swarm of such robots floating on the surface 
of the ocean forms a submarine or whale detector. In another example, a 
swarm of robots may sample the soil of an unexplored planet. Corner laser 
reflectors on the robot bodies may be used to communicate information to a 
centralized location (possibly an orbiting satellite) via a scanning laser 
at the central location. In another example, gnat robots are equipped with 
microaccelerometers and vibration sensors. A swarm of these robots can 
form a sophisticated distributed seismographic sensor. 
Gnat Robot Manufacture 
The robot body 10 is produced using three-dimensional free-form laser 
molding as illustrated in FIG. 2. In this process, a platform 30 sits just 
beneath the surface of a liquid bath of polymer 40. A steerable, 
ultraviolet laser 50 is used to draw a pattern on top of platform 30. The 
polymer solidifies at every place exposed by the laser, creating a very 
thin solid structure 60 sitting on top of platform 30. Platform 30 is 
slowly lowered into the polymer bath 40 and new layers of solid polymer 
are drawn on top of the existing layers. Some of the layers are drawn so 
that sockets 25 (FIGS. 1 and 5) are left in the surface of the body. 
Eventually, a partially hollow, three-dimensional structure 10 is built. 
This structure is approximately cubical and about two or three millimeters 
in diameter. 
The preferred polymer 40 for laser molding is methyl-acrylate. 
Methyl-acrylate is part of a broad range of organic photosensitive resists 
which are the cornerstone of the integrated circuit industry. The 
photoresist, polymethyl-methacrylate (PMMA) is often used for defining 
features down into the submicron and deep-submicron range (1.0-0.01 .mu.m) 
in state of the art silicon processes. Thus, the granularity of the 
methyl-acrylate polymer will not limit the size or resolution of the 
proposed robots. The above photo polymers are also inexpensive and are 
nontoxic when polymerized. 
Other embodiments of the invention may use lithographic methods for free 
form molding, such as those disclosed by Feely et al. in "3-D Latent Image 
in an Acid Hardening Resin", Proceedings of SPSE Photochemistry for 
Imaging Symposium, Jun. 26-29, 1988, ISBN 0-89208-140-6. 
The surface of body 10 has many sockets 25 spaced such that the barbed 
appendages 32 of the electrical elements 22 will interlock with one set of 
sockets 25 in many possible positions on the robot surface (FIGS. 3, 4). 
In this embodiment, exact placement of the electrical elements is not 
required. However, programming of the robot must include a mechanism for 
calculating and accounting for variation of possible positions of the 
elements on the robot body. 
Exact placement of the electrical elements can be performed by a recursive 
manufacturing process. The electrical elements 22 are manually attached to 
prototype gnat robot bodies using precision manipulators. The prototype 
robots are designed to perform attachment of electrical elements to 
further robots, thus limiting the use of precision manipulators to the 
initial manufacture of the prototypes. 
The appendages and sockets are shown in more detail in FIGS. 3 and 4. The 
appendages and sockets are made either by bulk micro-machining the silicon 
die edges into barbed appendages 32 which will fit into sockets in the 
3-dimensional body 10 or, conversely, by forming barbed appendages on the 
surface of the body 10 which snap into bulk micro-machined sockets in the 
dies. 
A more expensive alternative technique for attaching silicon dies is to 
form the appendages 32 and sockets 25 so that exact placement is required. 
More complex, and thus more retentive, shapes can be used for the 
appendages and sockets. But this connection technique is more time 
consuming, and therefore more expensive, owing to the small scale and high 
precision which must be exercised throughout. 
After silicon dies 22 are attached, appendages can be added to the 
electrical elements which contain motors (each appendage having been 
independently manufactured using free form laser molding techniques 
discussed above), or as illustrated in FIG. 15, appendages can be 
manufactured on the same silicon die as the motors in an integrated 
fashion. 
Electrical elements 22 are electrically connected after insertion. This is 
accomplished by selectively depositing "wires" 24 of conductive material 
(e.g. tungsten) on the surface of the body 10 between the elements using 
laser pantography, as disclosed by Herman et al, in their series 
"Wafer-Scale Laser Pantography: I-V", Lawrence Livermore National 
Laboratory, UCRL-88314, UCRL-88537, UCRL89399, UCRL-89350, UCRL-89639, 
available from NTIS. A laser 110 (FIG. 5) scans the desired wire path 
through an ambient vaporized metal gas, causing a chemical reaction. The 
reaction deposits conductive lines 24 on the body surface. 
Locomotion for the gnat robot is provided by a piezoelectric thin film 
ultrasonic motor. This motor is fabricated on a single chip. 
Lead zirconium titanate, otherwise known as PZT, is used for the 
piezoelectric thin film. Referring to FIGS. 6A and 6B, the crystal 
structure of PZT contains inherent asymmetries in its lattice (caused by 
the bistable location of Pb, Zr, and Ti ions). For illustrative purposes, 
two bistable states of the central Zr or Ti atom in the PZT lattice are 
depicted in FIG. 6A and FIG. 6B, respectively. 
FIG. 7 illustrates a sol-gel process for creating crack-free thin films of 
PZT from 6000 angstroms to 1.2 microns thick. In this process a sol of PZT 
is spun onto a wafer to a desired thickness, and then annealed at high 
temperature to form the crystalline lattice. After the anneal, the 
material is poled to induce the desired piezoelectric properties. Further 
details on the processing of PZT thin films can be found in Budd et al. 
"Sol-Gel Processing of PbTiO.sub.3, PbZrO.sub.3, PZT, and PLZT Thin 
Films", British Ceramics Proceedings, Vol. 36:1985, pages 107-121, 
incorporated by reference herein. 
FIG. 8 shows the mechanism employed in a rotary embodiment of such a motor. 
A travelling wave of mechanical deformation is induced in a piezoelectric 
thin film 120 and then mechanically amplified through the use of attached 
elastic bodies 130, which also serve as the electrodes for exciting the 
PZT thin film. The thin film 120 and elastic bodies 130 effectively act as 
a stator. An inert rotary actuator 140 rests on top of the stator and is 
caused to rotate by a tangential frictional force at every point which is 
in contact with the stator. A load can easily be attached to this rotor as 
there is no need to levitate the rotary actuator (as in variable 
capacitance motors). Indeed, friction is actually necessary here to cause 
actuation. 
In order to transduce electrical to mechanical energy, a ferroelectric 
material, such as PZT, must be "poled" (the process of permanently 
aligning the random polarized crystal orientations in the ceramic) through 
the application of a strong electric field. Once poled, a ferroelectric 
material will expand piezoelectrically when a voltage of one polarity is 
applied across it and contract when a voltage of the opposite polarity is 
applied. 
FIGS. 9A through 9E show the bending moments due to this piezoelectric 
effect. A phased, spatially varying AC voltage (supplied, e.g., by AC 
voltage sources V.sub.1, V.sub.2, V.sub.3, V.sub.4, ground plane 145, and 
elastic bodies/contacts 160) is applied to a PZT thin film 120. By 
maintaining a fixed phase relationship between the AC excitation voltages, 
a travelling wave of elastic deformation is produced in the PZT thin film 
120. For example, in FIGS. 9A through 9E, positive applied voltage creates 
a contraction of the PZT film, and negative applied voltage creates an 
expansion of the PZT film. 
In FIG. 9A, the voltage sources are turned off, and thus the PZT film 120 
is unstrained. In FIGS. 9B through 9E, the voltage sources are energized 
by phased, sinusoidal voltage waveforms, thus creating sinusoidal 
mechanical bending stresses in the PZT film. Each of FIGS. 9B through 9E 
is a "snapshot" of the bending strain in the film at a moment in time. The 
figures represent one full cycle in the oscillation of the PZT film in 
response to the excitation voltages. 
In FIG. 9B, V.sub.2 is at the most positive value in its sinusoidal 
oscillation. V.sub.1, which is delayed from V.sub.2 by a quarter cycle, is 
at a zero value in its sinusoidal oscillation. V.sub.4, which is delayed 
from V.sub.1 by a quarter cycle, is at the most negative value in its 
sinusoidal oscillation, and V.sub.3, which is again delayed by another 
quarter cycle, is at a zero value in its sinusoidal oscillation. The 
bending stresses in the PZT film contract the film at the contact at 
voltage V.sub.2, and expand the film at the contact at voltage V.sub.4, 
creating a sinusoidal deformation in the film as shown. 
FIG. 9C depicts a moment one quarter cycle after FIG. 9B. V.sub.2 has now 
reduced in value to 0 volts, V.sub.1 has increased in value to the most 
positive value in the oscillation, V.sub.4 has increased in value to 0 
volts, and V.sub.3 has decreased in value to the most negative value in 
the oscillation. The sinusoidal deformation of the film is now similar to 
that shown in FIG. 9B, but the points of maximum deformation have moved to 
the right, owing to the similar movements of the points of maximum 
voltage. 
FIG. 9D shows the deformation of the film one quarter cycle later. The 
points of maximum deformation now appear at the contacts at voltages 
V.sub.2 and V.sub.4, but in opposite phase to the deformation shown in 
FIG. 9B. 
Finally, one quarter cycle later, as shown in FIG. 9E, the points of 
maximum deformation appear at the contacts at voltages V.sub.1 and 
V.sub.3, but in opposite phase to the deformation shown in FIG. 9C. One 
quarter cycle later, a full cycle has elapsed, and the deformation of the 
film will again be as shown in FIG. 9B. 
It will be appreciated from the above illustration that the deformation 
pattern in the PZT film is a sinusoidal travelling wave, e.g. moving to 
the right in FIGS. 9B through 9E. The travelling wave is created by the 
discrete, phased voltages applied to the film, and its velocity is 
controlled by the frequency of the voltage sources and the spacing of the 
contacts. A greater variation of voltage phases provides more precise 
control of the film's deformation. Therefore, other embodiments of the 
invention may use more voltage phases to obtain more precise control. If 
less than four phases are used, the deformation of the film will become 
difficult to accurately control. 
FIGS. 10A through 10F show a travelling wave of deformation induced by 
applying phased, alternating voltages to the PZT film 120. As this 
travelling wave moves through the PZT film and the attached elastic body 
(the movement is to the right in FIGS. 10A through 10E), every point in 
the film and body moves such that it traces out a counterclockwise, 
elliptical trajectory over time. This motion is illustrated by the 
relative locations of points A, B, C and D, and is summarized in bottom of 
FIG. 10F. 
The ellipse trajectory amounts to a coupling between the motion that comes 
about from expanding and contracting with the energy of the traveling wave 
moving to the right. When the travelling wave is moving to the right, the 
film and body move to the left at point D. If a rotary or linear actuator 
is placed on top of the elastic body, then it will contact the stator only 
when the frictional surface of the stator is in position D, and thus will 
move to the left. 
An advantage to the ferroelectric motor is that friction is used as a 
feature. Gravity, or some normal force holding the actuator and stator in 
contact, is necessary in order to make use of the tangential force at each 
point D to generate continuous motion of the actuator. 
FIG. 11 shows a cross section of a rotary piezoelectric micromotor 
according to the invention. The motor is formed by laying down thin films 
of material onto a silicon substrate 175. The first layer is an insulating 
mechanical membrane 165, which allows mechanical deformations, and 
isolates substrate 175 from the PZT excitation Voltages. Membrane 165 is 
preferably of a nitride or silicon oxide grown onto the substrate. In the 
region underneath the micromotor, the silicon substrate 175 is etched 
away, leaving a cavity 177. This allows the area of membrane 165 under the 
motor to deform in response to excitation of the PZT. 
The next layer in the motor structure is a ground plane 155 for providing a 
reference potential for the excitation sources, preferably of platinum 
metallization. Platinum is preferred because it provides a diffusion 
barrier to prevent the PZT from reacting with the silicon to form stable, 
low K lead silicates. Other metallizations, such as Aluminum, will not 
perform this function. In addition, the metallized ground plane aids in 
the adhesion of following layers. 
Next, piezoelectric material such as PZT is spun onto the substrate, 
forming a micro-mechanical surface 150. Electrodes 160 are patterned out 
of gold or platinum on top of PZT surface 150 in a rotary configuration. 
(Aluminum may not be used for the contacts because it has a very stable 
oxide surface which makes contact to the PZT difficult.) Rotary actuator 
183 with integral axle 185 rests on top of electrodes 160 and is actuated 
through a travelling wave of mechanical deformation of PZT surface 150, 
ground plane 155, and membrane 165. The electrodes 160 are connected to 
external sources, through bonding wires 180, or through suitably isolated 
metallization. As seen in FIG. 11, rotary actuator 183 and axle 185 rest 
frictionally atop the electrodes 160 and are supported by an integrated 
support structure 190 and/or a bearing hole 195 through the substrate. A 
wear layer 182, e.g. diamond or silicon nitride, is deposited between the 
electrodes and the rotary actuator to increase the life of the motor. 
During fabrication, the rotary actuator 183 and support structure 190 are 
suspended by sacrificial oxide layers 161, 63. Once the rotary actuator 
and support structure are fabricated, a hole for the axle is created by a 
laser. In one embodiment, the hole is filled by polysilicon or similar 
material, forming axle 185. In other embodiments, the axle is 
prefabricated and manually inserted in the hole. The sacrificial oxide 
layers are then etched away, resulting in a free-floating rotary actuator 
and axle within the supports structure. 
Other embodiments of a rotary motor may not fabricate an axle and support 
structure. Rather, a central bearing and capacitive rings would hold the 
rotary actuator in place, and the torque would be coupled out of the motor 
by microfabricated gears. The fabrication of the central bearing and 
capacitive rings would be as disclosed in U.S. patent application No. 
07/52,725 filed on May 20, 1987 by Roger T. Howe et al for an 
"Electrostatic Micromotor", incorporated by reference herein. 
FIG. 12 shows the arrangement of electrodes 160 in the rotary motor. As can 
be seen, the electrodes are excited by sinusoidal voltages having fixed 
phase correlation. 
FIGS. 13A and 13B show a linearly actuated piezoelectric micromotor 
according to the invention. The motor is shown in perspective view in FIG. 
13C. The motor is formed in similar fashion to the rotary motor of FIG. 
11, by laying down thin films of material onto a silicon substrate 175. 
The mechanical membrane 165, cavity 177, ground plane 155, and 
piezoelectric surface 150 are formed as set forth in conjunction with FIG. 
11 above and perform the same functions as in FIG. 11. 
Electrodes 160 are patterned out of gold or platinum on top of PZT surface 
150 in a linear configuration, and are connected to external phased AC 
voltage sources. The linear actuator 183, preferably fabricated of 
polysilicon, rests on top of electrodes 160 and is actuated in the 
direction indicated by the arrow through a travelling wave of mechanical 
deformation of PZT surface 150, ground plane 155, and membrane 165. 
Actuator 183 is held in place by a strap 190 extending transversely across 
the actuator midway along its length. During fabrication the linear 
actuator and strap are suspended by sacrificial oxide layers 186, 187. 
These layers are later etched away, resulting in a free-floating linear 
actuator within the strap. 
As seen in FIG. 13B, additional electrodes 184 capacitively attract linear 
actuator 183 to the electrodes during operation. Electrodes 184 do not 
project as far above the substrate surface as electrodes 160, and thus do 
not touch linear actuator 183. By varying the bias voltage of capacitive 
electrodes 184, the friction between the linear actuator 183 and 
electrodes 160 can be optimally adjusted. When the power is off, the 
linear actuator is held in place by strap 190. To allow the linear 
actuator 183 to project beyond the substrate 175, all of the substrate 
beyond line 188 may be completely etched away. 
FIG. 14 shows an alternative embodiment of a linear motor structure. The 
strap 190 is eliminated by providing two sets of stators on two 
substrates, and bonding the substrates together using wafer-to-wafer 
bonding. The result is a "sandwich" structure with the linear actuator in 
the center. The wafer bonding is aligned using sacrificial oxide layers 
186, 187, which are then etched from the sandwich, freeing the linear 
actuator 183 to slide across the electrodes 160. To allow the linear 
actuator 183 to project beyond the wafer sandwich, all of the sandwich 
structure beyond line 188 may be completely etched away. 
FIG. 15 shows an integrated appendage/motor structure according to the 
invention. A electrical element 22 is fabricated with two linear PZT 
motors 142, 144, having motion in the vertical and horizontal direction. 
(The actuator in motor 144 has two straps is held to the element 22 by two 
straps.) An appendage 20 is also fabricated, along with bearings 152, 154 
for controlling appendage position. During fabrication, appendage 20 is 
supported by a portion of the substrate 162 which is later etched away 
(preferably during the steps used to form cavities 177 of FIGS. 11-14). In 
this way, appendage 20 may be fabricated by conventional means, but may 
extend beyond the boundary of the surface of the resulting electrical 
element. Independent control of the two linear motors 142, 144 provides 
two-dimensional positioning of the appendage 20. Motor 142 translates the 
axis of rotation of appendage 20. Both motors are capable of rotating the 
appendage about the axis of rotation. 
Robots and other electrical and electro-mechanical devices need sensors in 
order to interact with their environment. Infrared sensors have many 
advantages for image recognition. Some conventional infrared sensors use a 
cooled CCD. However, cooling conventional CCD infrared sensors is not 
practical in low power applications, such as gnat robots. 
According to the invention, to avoid cooling, dielectric films are used as 
static, room temperature infrared sensors. The sensors are produced by a 
VLSI process, and are combined with conventional VLSI circuitry for 
sensing and processing the signals from the sensor sites. 
To use dielectric material in conjunction with standard silicon processes, 
potential contamination and thermal problems are dealt with. The three 
major effects of impurities which can ruin MOS transistors are: high 
electron-hole recombination rates due to deep-level donor impurities, 
voltage threshold shifts due to mobile ion impurities in gate oxides, and 
large interface charges due to surface contamination. Some materials that 
cannot be present during MOS transistor fabrication are, for example, 
gold, copper, iron, and zinc, which are deep level donors, and sodium, 
potassium, and lithium, which are mobile ions in oxides. If present during 
deposition or thermal growth of thin films, these materials and others 
will also cause excessive interface charges and in extreme cases, 
fermi-level pinning (which will destroy MOS transistor action). 
Many processing steps can be also be ruined by later steps that use high 
temperatures. For instance, it is desirable to dope the source and drain 
regions of small MOSFETs to only very shallow levels, but subsequent 
processing steps may require high temperatures which will inadvertently 
cause the doped junctions to diffuse to deeper levels. To prevent these 
junctions from diffusing inappropriately, the wafer cannot be exposed to 
temperatures above 800.degree. C. for any length of time. In addition, 
high temperature cycling of wafers tends to both reduce mobilities and 
overstress wafers, affecting transistor yield and performance. 
Unlike cross-contamination issues, thermal management problems do not 
prevent mechanical and device wafers from being processed in the same lab 
using the same equipment. This allows for sharing of resources which is 
very helpful in reducing expenses. Wafers containing films that could 
possibly contaminate the normal fabrication procedures can be protected or 
passivated with special layers. Finally, special layers can be applied on 
top of fabricated transistors and the wafers removed to a separate 
fabrication laboratory with a much reduced subset of equipment for final 
processing. 
The sensing circuitry for the dielectric sensor makes use of switched 
analog circuit techniques, measuring the very small, heat-related 
capacitance changes of the dielectric film. The dielectric constant 
.epsilon. of the films, and thus the measured capacitance, is a strong 
function of temperature. To enhance the sensitivity of .epsilon. to 
temperature, the sensors may use a ferroelectric film which has a 
composition close to, but just above, its Curie temperature T.sub.C at 
ambient temperature. In ferroelectrics close to T.sub.C, 
.delta..epsilon./.delta.T reaches very high values. In one embodiment of 
the invention, the ferroelectric material is PZT. 
Accuracy is maintained by automatic calibration and noise cancellation. By 
measuring the actual capacitance of the dielectric film, as opposed to the 
currents produced from the sensors, a static infrared picture is generated 
by the sensor, eliminating the need for a chopper device. 
Because the sensor sites can be manufactured directly over the sensor 
circuitry, the "fill factor" (i.e. the percentage of the chip area 
containing sensing sites) is improved over known CCD sensors. In silicon 
CCD sensors, part of the chip surface area must be devoted to silicon 
sensor sites, because the sensors are diffusion areas in the substrate. 
This means that a CCD sensor has a fill factor of much less than one, 
leading to below optimal sensor resolution. 
The sensing requirements of a gnat robot may not be completely fulfilled by 
a infrared sensor. Therefore, the gnat robot is also provided with a 
visible light sensor, and simple algorithms, for example, optical flow 
algorithms, are used to control the robot to avoid objects. 
Ideally, an infrared system would be coupled to a visible-light system, the 
strengths of one system being used to complement the other. If CMOS 
compatibility is essential, the visible light sensors may use photo 
transistors, implemented by the parasitic bipolar capacitors which are 
formed naturally in a CMOS process. Examples of sensors of this type can 
be found in "A Two-Dimensional Visual Tracking Array" by Stephen P. 
DeWeerth and Carver A. Mead, proceedings MIT Conference on VLSI, 1988, 
available from the MIT Press and incorporated by reference herein in its 
entirety. 
In preferred embodiments, the visible light sensor sites take up 
approximately half of the chip area, and the processing circuitry 
transistors for both the visible and infra-red sensors take up the other 
half. A thin dielectric film is then deposited directly over the 
processing circuitry transistors, thus utilizing the second half of the 
chip for infra-red sensor sites. In these embodiments, the fill factor of 
the chip approaches 100%; 50% of the chip senses visible light and 50% 
senses infra-red. Note that, in these embodiments, no more silicon area is 
used than that required by a conventional visible-light sensor with no 
sensor sites over the transistors. 
FIG. 16 shows one embodiment of a combination infrared and optical sensor. 
The sensor is formed on a silicon substrate 200, in accordance with 
standard MOS processing. MOS transistor sources 202, drains 204, and gates 
206 are grown and deposited on the substrate, along with field oxide 210. 
At the same time, diffusion area 215 is introduced, creating a CCD 
visible-light sensor. The MOS transistor 202, 204, 206 then insulated by a 
thermally insulating glass 220 (such as Aergel), which prevents the 
thermal behavior of the MOSFET from influencing the signal from the sensor 
230 which overlays it. The MOSFET is connected to the sensor 230 by a 
small strip of metallization 222, which connects to the sensor 230 through 
a small opening in the insulator 220. The small size of the opening limits 
the thermal interaction of the MOSFET and the sensor 230. A reference 
sensor 240 is formed in direct contact with the substrate. 
In operation, sensors 230, 240 and diffusion area 215 are exposed to light. 
The capacitance of the sensor 230 is compared to the reference capacitance 
of thermally grounded (i.e. in direct contact to the substrate) sensor 
240, and the difference signal is processed by switched capacitor 
amplifiers. 
FIG. 17 shows a suitable switched capacitor amplifier circuit. The 
capacitances C.sub.PYRO 230 and C.sub.REF 240 are compared by an op-amp 
280 (implemented by MOS transistors) and switched capacitors. The circuit 
comprises an input voltage source V.sub.IN, switches 250, parasitic 
capacitances C.sub.P 260 (formed naturally from the substrate capacitances 
of the various diffusion regions tied to the `+` and `-` nodes of the 
op-amp), and matched input capacitors C.sub.M 270. The input capacitors 
C.sub.M and the parasitic capacitors C.sub.P are exactly matched to 
enhance the accuracy of the circuit, as discussed below. 
To auto-zero the offset of op-amp 280, switches D and E are closed, and 
switches A, B, and C are grounded. This zeros the voltage on the input 
capacitors C.sub.M 270 and the signal capacitors C.sub.PYRO 230 and 
C.sub.REF 240, and also drives the offset voltage of op-amp 280 onto the 
parasitic capacitors C.sub.P 260. 
After auto-zeroing the circuit, to compare C.sub.PYRO and C.sub.REF, 
switches D and E are opened, switches B and C are connected to the `+` and 
`-` inputs of op-amp 280, and switch A is connected to the input voltage 
V.sub.IN (typically 1 to 5 volts). Input capacitors C.sub.M 270 then 
charge to a voltage near V.sub.IN by drawing charge off of parasitic 
capacitors C.sub.P 260 and signal capacitors C.sub.PYRO 230 and C.sub.REF 
240. The majority of the charge drawn onto input capacitors C.sub.M is 
drawn off of capacitors C.sub.PYRO 230 and C.sub.REF 240 because they have 
capacitance values much larger than those of the parasitic capacitors 
C.sub.P. In addition, because the capacitance values of the input 
capacitors C.sub.M and the parasitic capacitors C.sub.P are very closely 
matched, the charge drawn off of capacitors C.sub.PYRO and C.sub.REF will 
be substantially equal. If there are differences in the capacitance of 
C.sub.PYRO and C.sub.REF (caused by infrared radiation received by the 
sensor site), however, when equal amounts of charge are drawn off of 
C.sub.PYRO and C.sub.REF, the resulting voltages on the two capacitors 
will not be equal. Therefore, the output voltage from op-amp 280 will be 
directly proportional to the difference in capacitance of C.sub.PYRO and 
C.sub.REF, and will be indicative of the infrared radiation received by 
the associated sensor site. 
The gain of the circuit is a function of the ratio of the capacitances 
C.sub.M and C.sub.REF. This gain will ordinarily be less than one, and 
thus the output of the circuit will need to be followed by additional 
amplification stages. However, because the signals have low-level voltage 
swings (smaller than 1 volt), very accurate MOS op-amp designs may be 
implemented to amplify the difference signal. The accuracy of the circuit 
will also be affected by the matching of the channel "charge pumping" of 
the MOS switches 270 in use. This factor can be controlled by careful 
matching of the geometry of the switches. 
Further discussion of the design and implementation of high-precision 
switched capacitor circuits can be found in "Charge Circuits for Analog 
LSI" by Robert H. McCharles and David A. Hodges, IEEE Transactions on 
Circuits and Systems, Vol. CAS-25, July 1978, pg. 490-497, incorporated by 
reference herein. 
OTHER EMBODIMENTS 
Other embodiments are within the scope of the following claims. For 
example, the motor fabrication techniques described above may be used for 
other purposes than gnat robot propulsion.