Robotic pressure imagers

A fine-scale array of pressure transducers which mimic biological nerve endings and are particularly useful in robotic architecture are provided.

CROSS-REFERENCE 
The inventions disclosed and claimed in co-assigned U.S. patent 
applications Ser. No. 267,235, filed May 26, 1981, and Ser. No. 282,218, 
filed July 10, 1981, which may be material to the examination of this 
application, are incorporated herein by reference. 
BACKGROUND OF THE INVENTION 
Robots and the science of robotics are receiving increasing interest and 
attention both in terms of use in industrial environments and research to 
improve existing robots and develop far more sophisticated robots. 
Robots of the current generation may be equipped with elementary artificial 
vision and tactile sensors, e.g., television cameras and pressure 
sensitive switches or load cells, respectively. The ability to accurately 
sense small variations in pressure, ideally on the scale exhibited by 
biological fingers, and utilize that information in an interactive or 
feedback system, is predicted to become an increasingly important area of 
robotic technology in the future. 
SUMMARY OF THE INVENTION 
The pressure imager of the invention consists of a plurality of sensing 
cells or regions arranged in an array or pattern in a body of 
semiconductor material, a layer of the oxide of the semiconductor material 
situated on and contiguous with a substantial portion of the top major 
surface of the body, and an adherent layer of piezo-electric material over 
the oxide layer. Preferably, the piezo-electric material is a flexible, 
compliant, and tough polymer such as polyvinylidene fluoride. The small 
size of the individual sensing regions yields a highly sensitive imager 
ideally suited for use in robotic architecture. 
The individual sensing cells are of the piezo-electric gate controlled 
diode (PZGCD) type or the piezo-electric field effect transistor (PZFET) 
type selected primarily with reference to the operating environment. The 
PZGCDs are characterized by a small diameter substantially cylindrical 
hole which extends completely or substantially through the thickness of 
the body between the major top and bottom surfaces of the body and a 
substantially cylindrical semiconductor region of generally uniform 
cross-section which is substantially concentric with the hole and extends 
between the major surfaces. The conductivity type of the semiconductor 
region is made opposite to that of the body of semiconductor material, 
thus a substantially cylindrical P-N type junction extending between the 
major top and bottom surfaces of the body is formed with the body. The 
individual cells are separated by a gridwork of excavations in the top 
surface of the body or, preferably, by a gridwork of heavily doped regions 
extending a short distance into the interior of the body from the top 
major surface. 
Cells having piezo-electric field effect transistors (PZFETs) are similar 
to cells having PZGCDs, except there are two spaced holes with their 
associated substantially concentric cylindrical semiconductor regions per 
cell.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to FIG. 1, there is shown in schematic cross-section a 
portion of a robotic pressure imager 10 of the present invention. 
Illustratively, an object, i.e., bolt 5 whose size, shape, and weight are 
to be sensed, is shown situated on imager 10. 
The principal elements of imager 10 are a plurality of sensing regions 20 
situated in a pattern or array in a body 15 of semiconductor material, a 
layer 16 of the oxide of the semiconductor material covering at least a 
substantial portion of top surface 22 of body 15, and a layer 17 of 
piezo-electric material situated over and adherent to oxide layer 16. 
Sensing regions 20 are considerably smaller than the features of the 
object to be sensed, thus the imager of this invention has high 
sensitivity. 
The semiconductor material of body 15 may be selected from those materials 
which are known by the practitioners of the art of the construction of 
semiconductor or microelectronic devices. Suitable materials include 
silicon, germanium, compounds of a group III element of the periodic table 
and a group V element (e.g., gallium arsenide) and compounds of a group II 
and a group VI element (e.g., cadmium telluride). Silicon is presently 
preferred due to its availability and the ease with which a passivating 
film, e.g., layer 16, may be formed. Body 15, and imager 10, are in the 
form of thin, i.e., on the order of about 6 to about 100 mils in 
thickness, wafers or chips whose shape is determined in accordance with 
the dictates of the particular robotic architecture with which imager 10 
is to be used. The thickness dimension, measured as the perpendicular 
distance between top major surface 22 and bottom major surface 23, is, 
therefore, small in comparison to the lateral dimensions of top 22 and 
bottom 23 major surfaces of body 15. 
Conventional piezo-electric materials, e.g., quartz, Rochelle salt, and 
lithium sulphate monohydrate (Li.sub.2 SO.sub.4.H.sub.2 O), can be used 
for layer 17. However, since the aforementioned conventional materials are 
typically hard, stiff and brittle, a preferred alternate is a 
thermoplastic fluorocarbon polymer such as polyvinylidene fluoride 
(PVF.sub.2). These polymers are compliant, flexible, and tough and are 
available commercially as sheets as thin as 6 .mu.m in rolls 1000 meters 
long and over 1 meter wide. Moreover, in such quantities, they are 
relatively inexpensive compared to the conventional piezo-electric 
materials. In addition, PVF.sub.2, like Teflon.RTM., is chemically inert, 
electrically insulating, and has been used as a protective coating for 
metallic surfaces. These polymers can be easily applied as adherent layer 
17 by the use of suitable adhesives, such as rubber cement or epoxy, or by 
heating to about 200.degree. C., with pressure applied, for a time 
sufficient to render the polymer tacky and then cooling. 
In FIG. 2, there is shown schematically in more detail a single sensing 
region or cell 20. Region 20 of FIG. 2 is a piezo-electric gate controlled 
diode (PZGCD) made in accordance with the method disclosed and claimed in 
the above-referenced Ser. No. 267,235 application. For purposes of 
illustration, the material of body 15 is silicon with a substantially 
uniform distribution of atoms of an impurity element, i.e., a dopant, 
therein. The concentration of the dopant atoms is typically measured in 
terms of the resistivity of body 15 and, if selected properly, will impart 
P-type or N-type conductivity to the silicon. As is known by the 
practitioners of the semiconductor arts, if the dopant atoms are Al, Ga or 
B, for example, the silicon will exhibit P-type conductivity and if the 
dopant atoms are As, Sb, or P, for example, the silicon will exhibit 
N-type conductivity. Illustratively, the silicon of body 15 of FIG. 2 is 
lightly doped, i.e., has a low concentration of impurity atoms and is of 
the N.sup.- type as indicated by the symbol N.sup.-. Body 15 is lightly 
doped so that its conductivity type may easily be inverted to the opposite 
type conductivity by the application of an electrostatic field as 
discussed further below. 
Hole 21 is a cylindrical cavity extending substantially perpendicularly 
between major top 22 and bottom 23 surfaces of body 15 through the 
thickness dimension of body 15. Hole axis 24 is substantially parallel to 
substantially cylindrical inner surface 25 of hole 21. 
Holes 21 are best produced by the laser drilling process disclosed in the 
cross-referenced Ser. No. 267,235 application. Briefly described, a laser 
such as ESI, Inc. Model 25 Laser Scribing System modified with a 10 watt 
(maximum) optoacoustic Q-switched Nd:YAG head manufactured by U.S. Laser 
Corp. is used. The laser is operated in a repetitively Q-switched mode 
with a focused beam size of about 20 microns, a depth of focus of about 
250 microns, an individual pulse duration of about 200 nanoseconds and a 
repetition rate of about 3 KHz. At a power level of about 2 watts, 
measured independently in a continuously pulsed mode, ten pulse trains of 
5 msec duration separated by a 10 msec delay drill approximately 5 holes 
per second. Using the above parameters, holes 21 as small as about 3/4 mil 
in diameter (D) with axis 24-to-axis 24 spacings as close as about 1.5D 
can be drilled through 12-mil thick silicon wafers by the laser beam means 
without spalling, cracking, or introducing stresses or strains, i.e., 
damage, into the material of semiconductor body 15 adjacent to holes 21. 
Region 26 shown in FIG. 2 is a semiconductor region of generally uniform 
cross-section substantially concentric with hole 21 and extending between 
surfaces 22 and 23. Region 26 is made by diffusing impurity atoms radially 
a distance t into body 15 from surface 25 by gas diffusion or from an 
adherent solid state source in accordance with the method described in 
more detail in the cross-referenced Ser. No. 267,235 application. In the 
PZGCDs of this application, region 26 will have at least a different type 
conductivity from that of body 15. 
Interface 27 formed between region 26 and the semiconductor material of 
body 15, is substantially concentric with hole 21, extends between 
surfaces 22 and 23, and is situated away from inner surface 25 by the 
distance, t, to which the impurity atoms diffuse into body 15 from surface 
25. Since, as illustratively shown on FIG. 2, the material of body 15 is 
of N-type conductivity and region 26 is of P-type conductivity, interface 
27 will be a P-N type junction. 
Longitudinally-extending regions 28 serve to isolate adjacent sensing 
regions 20. Regions 28 may be excavations below surface 22, but, 
preferably, regions 28 are semiconductors having the same conductivity 
type as body 15, but are more heavily doped as indicated by the symbol 
N.sup.+. Doped isolation regions 28 may be formed by diffusing the dopant 
into body 15 or by ion implantation techniques conventionaly known to 
those skilled in the art of semiconductor device manufacture. Isolation 
regions 28 should extend at least about 2 microns into body 15 from 
surface 22. As noted above, the diameter, D, of hole 21 is typically 1 
mil. The center line 24-to-center line 24 distance between adjacent cells 
20 should be about 2D, thus regions 28 will be about 1 mil from center 
line 24. This spacing represents a good trade-off between cell resolution 
which is a measure of the size of the object which can be sensed and cell 
sensitivity which is a measure of the cell's ability to detect small 
changes in pressure per cell surface contact area. 
After regions 28 are formed, layer 16 of the oxide of the material of body 
15 is formed in contact with surface 22. Since the area of holes 21, as 
viewed looking down on surface 22, is small in comparison to the surface 
area of cell 20, as delineated by regions 28, substantialy all of the 
surface of cell 20 will be covered by oxide layer 16. Thereafter, layer 17 
of the piezo-electric material, preferably PVF.sub.2, is affixed on top of 
layer 16 as discussed above. Layer 38, which is optional, is discussed in 
detail in a subsequent section below. 
The operation of sensing region 20 is shown schematically in FIG. 2A 
whereon certain details of FIG. 2 have been omitted for clarity and others 
added to aid the following description. Any pressure, P, on PVF.sub.2 film 
17 generates a polarization, P.sub.Q, in film 17 that induces a charge, 
Q.sub.s, on surface 29 of the film in accordance with equations (1) and 
(2) 
EQU P.sub.Q =.alpha.P (1) 
and 
EQU Q.sub.s =BP.sub.Q (2) 
where .alpha. and .beta. are material constants. As with any piezo-electric 
material, the application of stress generates an electrostatic charge 
within the material. The polarity of the charge, positive or negative, 
will be a function of the type of stress, e.g., tensile or compressive, 
and will either be unique to the material or, as with PVF.sub.2, may be 
imparted by manufacture. 
Surface charge Q.sub.s of the proper polarity first causes a space-charge 
or depletion zone to form in near-surface region 30 and, with increasing 
pressure, the conductivity type of region 30 changes to the type opposite 
to that of the material of body 15, as shown in FIG. 2A. With further 
increases in pressure, region 30 is extended farther away from surface 22. 
While the pressure-induced changes in region 30 are small, they are much 
larger and more readily measured than the electrostatic charges in layer 
17. Thus, the changes in region 30, including the formation of a depletion 
zone, may be accurately measured by means of suitable instrumentation, 
e.g., a capacitance bridge, connected between regions 26 and body 15. 
In FIG. 3, there is shown schematically a single sensing region or cell 20 
of the piezo-electric field effect transistor (PZFET) type. The 
nomenclature of FIG. 2 is carried over to FIG. 3 and is the same except as 
indicated in the following discussion. 
Blind holes or cylindrical cavities 21 and 21A extending from bottom 
surface 23 into and terminating in the thickness dimension of 
semiconductor body 15 can reproducibly be made by carefully controlling 
the number of pulses from the above-described laser operated with the 
parameters described above. Blind holes are an alternative embodiment of 
the through-thickness holes shown in FIGS. 2 and 2A, thus the PZGCDs of 
FIGS. 2 and 2A and the PZFET of FIG. 3 may be made either with 
through-thickness holes or blind holes. With blind holes 21 and 21A, the 
dopant is diffused from inner walls 25 as well as from bottoms 33 of the 
holes to form semiconductor regions 26. Regions 26 will be substantially 
in the form of right circular cylinders if the cavities are laser drilled 
substantially completely through body 15, i.e., to within about 10% of the 
thickness dimension of body 15. In FIG. 3, for illustrative purposes, the 
conductivity type of body 15 and semiconductor regions 26 have been 
selected opposite to those shown in FIGS. 2 and 2A, and therefore, the 
polarity of layer 17 is also selected opposite to that of FIGS. 2 and 2A. 
As in the case of the PZGCD, the diameter of cavities 21 and 21A is about 
1 mil. The center line 24-to-center line 24 distance, L, between holes 21 
and 21A of one cell 20 is about 2D, or 2 mils, and the distance between 
the nearest cavities of adjacent cells 20 is also about 2D, thus regions 
28 are approximately equidistant between center lines 24 of adjacent 
cells. 
Pressure, P, is shown applied across the entire sensing surface 29 of cell 
20, thus regions 30 extend out to regions 28 and a continous region 30 is 
formed between semiconductor regions 26 between holes 21 and 21A. 
Pressure, P, as sensed by PZFET cell 20 of FIG. 3 is best detected and 
measured in terms of resistance changes measured between semiconductor 
regions 26 surrounding holes 21 and 21A. Incremental increases in 
pressure, P, from that depicted in FIG. 3 drives regions 30 deeper into 
body 15 and produces a further incremental detectable change in 
resistance. For lighter pressures or pressures over a smaller area of 
surface 29 than is illustrated in FIG. 3, regions 30 will be less 
extensive and may not form a continuous region between semiconductor 
regions 26, however, a detectable change from the unstressed material will 
be produced. 
The robotic pressure imager 10 of FIG. 1 consists of a plurality of cells 
20 arranged in an array. By the term array it is meant that cells 20 are 
arranged in a periodic repeating geometric pattern. An example of an array 
is shown in FIG. 4 which is imager 10 of FIG. 1 when viewed by looking at 
a portion of bottom surface 23. Illustratively, individual cells 20 are of 
the piezo-electric gate controlled diode type shown in FIGS. 2 and 2A. 
Cells 20 are bounded by the gridwork formed by intersecting isolation 
regions 28. When viewed from top surface 22, isolation regions 28 are 
continuous between their points of intersection 38, but are shown as 
dotted lines in the bottom view of FIG. 4. The center lines of holes 21 
are located at the orthogonal intersections of a first set of parallel 
lines 34 separated from each other by the distance M and a second set of 
parallel lines 35 separated from each other by the distance N which, in 
FIG. 4, is equal to M. Concentric with holes 21 are semiconducting regions 
26 and interfaces 27. 
The array of FIG. 4 is illustrative and is not intended to be limiting as 
other arrays compatible with the robotic functions to be performed are 
within the contemplation of the invention. For example, it may be 
advantageous for the array to consist of a grouping of a small number of 
cells in an array with the groupings themselves arranged in a larger array 
configuration, i.e., a hierarchy of arrays. Similarly, isolation regions 
28 may be in a form other than the straight line segments of FIG. 4, e.g., 
a plurality of circles whose peripheries do or do not touch or intersect. 
Further shown on FIG. 4 are means for obtaining the information from cells 
20 of imager 10. A first series of parallel conductive strips 36 are 
placed in contact with or in the proximity of bottom surface 23, but in 
contact with semiconductor areas 26 using conventional semiconductor 
device manufacturing techniques. On upper surface 22, there is similarly 
provided in contact with the portions of semiconductor regions 26 
accessible from top surface 22 a second series of parallel conductive 
strips 37 which are orthogonal to the first set. Microelectronic 
solid-state devices, for example, (not shown) may be provided to permit 
each cell 20 of imager 10 to be addressed individually. 
The second set of semiconductor strips 37 may be provided on the same 
surface as first set 36 as is typical in the prior art. The uniqueness of 
through-thickness diodes 26, i.e., the combination of hole 21 plus 
semiconductor region 26, however, permits the conductive strips to be 
placed on opposite surfaces thus greatly reducing the potential for 
interference in the form of cross-talk which may arise in prior art 
devices. A protective covering layer (not shown) overlying bottom surface 
23 may optionally be provided. 
Use of the piezo-electric gate controlled diode (PZGCD) is generally 
preferred for cell 20 when imager 10 is used in the presence of floating 
potentials and in the presence of static electricity. In this type 
environment, it may be advantageous to place a grounded metal layer over 
piezo-electric layer 17 to shield imager 10 from stray charges. Such a 
grounded layer is shown schematically in FIG. 2 as layer 38. Layer 38 may 
be provided as a thin sheet applied over layer 17 or formed in place by 
such techniques as sputtering or evaporation of a metal such as aluminum. 
The thickness of layer 38 should be on the order of about 1 micron so as 
not to interfere with the ability of cells 20 to independently sense 
pressure changes. The piezo-electric field effect transistor (PZFET) is 
generally preferred for cell 20 when imager 10 is used in environments 
having alternating current (AC) type noise. Oxide layer 16 may also be 
considered optional with cells 20 and imager 10, however, use of layer 16 
is preferable since it prevents the buildup of stray charges from impurity 
ions, for example, at top surface 22. 
Those skilled in the art will readily recognize that other changes, 
omissions and additions from the form and detail of the preferred 
embodiments shown herein may be made without departing from the spirit and 
scope of the invention.