SOI diaphragm sensor

The present invention relates to the fabrication of diaphragm pressure sensors utilizing silicon-on-insulator technology where recrystallized silicon forms a diaphragm which incorporates electronic devices used in monitoring pressure. The diaphragm is alternatively comprised of a silicon nitride having the necessary mechanical properties with a recrystallized silicon layer positioned thereon to provide sensor electronics.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of pressure sensors, and more 
particularly to the field of diaphragm sensors using silicon-on-insulator 
(SOI) technology. 
The use of pressure sensors or transducers has found numerous applications 
in a variety of fields in which it is desirable to monitor fluid flow, 
liquid level or pressure. The electronics industry has attempted to 
accommodate the need for low-cost and dependable sensors by utilizing 
integrated circuit (IC) fabrication techniques in the design and 
manufacture of microfabricated diaphragm transducers on IC chips. The 
basic types of silicon pressure transducers include (1) piezo-junction 
devices, (2) piezoresistive sensors, and (3) capacitive pressure 
transducers. 
Piezoresistive sensors have been formed in polycrystalline silicon, laser 
recrystallized silicon and bulk single crystal silicon. The resistors have 
been fabricated by dopant implantation followed by annealing and 
metallization. The diaphragm of the sensor has been separately formed by 
patterning a silicon wafer with an oxide insulator, depositing 
polycrystalline silicon over the insulator/wafer surface and removing the 
oxide from between the diaphragm and the wafer. 
A microfabricated capacitive pressure transducer is formed, for example, by 
diffusing a dopant into a region in a silicon wafer that serves as a lower 
electrode, and forming a compliant diaphragm of polysilicon as a second 
electrode that is separated from the diffused region by an oxide spacer. 
The oxide spacer can be removed by etching through an opening in the 
backside of the wafer. See R. S. Hijab and R. S. Muller, "Micromechanical 
Thin-Film Cavity Structures For Low Pressure and Acoustic Transducer 
Applications," IEEE, CH 2127, 178, September, 1985. 
SUMMARY OF THE INVENTION 
A method of forming a microfabricated pressure sensor is in which a 
compliant membrane is formed that encloses a sacrificial insulating 
material. The insulating material is subsequently removed through an 
opening in the membrane to form a pressure sensitive diaphragm. Electrical 
elements are positioned in a single crystal silicon layer formed on or in 
the diaphragm to detect movements thereof and produce electrical signals 
proportional to the diaphragm displacement. The single crystal layer is 
formed by depositing a polycrystalline silicon layer and then zone-melt 
recrystallizing the film to form a high quality single crystal silicon 
suitable for CMOS circuitry. 
The method can be used to form both silicon and silicon nitride diaphragm 
pressure sensors. An oxide is formed on a semiconductive wafer such as 
silicon using a local oxidation of silicon (LOCOS) planar process. A 
silicon nitride or polysilicon layer is formed over a region of the oxide. 
Electrical elements such as piezo-resistors are formed over the nitride or 
silicon layers respectively and one or more holes are etched through the 
layer to permit removal of the oxide spacer with a suitable etchant. 
Removal of the oxide insulator forms a cavity between the nitride or 
silicon layer such that the layer forms a diaphragm that is highly 
sensitive to pressure variations to which the diaphragm is exposed. The 
holes or openings in the diaphragm are then sealed. The displacement of 
the diaphragm and the resulting electrical signals generated by 
piezo-resistors is directly correlated with changes in pressure. Any 
suitable transducer can be formed in the diaphragm structure, or 
alternatively, a capacitive type of transducer can be formed using 
electrodes formed in the diaphragm and the silicon wafer. Implanted 
piezo-resistors can be formed in a standard Wheatstone bridge arrangement 
used in microsensor systems. Other integrated circuits, driving elements 
or signal processing circuits can be fabricated in the recrystallized 
material to produce so-called "smart" microsensors. 
A number of additional features can be integrated with "smart" microsensor 
systems such as selfcalibration, system diagnostics, and redundant 
systems. 
The removal of the LOCOS oxide to produce a silicon diaphragm is 
accomplished by performing an anisotropic etch, for instance, preferably 
in the center of the diaphragm, to expose a small surface area of the 
oxide. An oxide etch is applied through the opening in the diaphragm which 
removes the underlying oxide. Where the diaphragm is comprised of a 
silicon nitride, one or more holes can be formed along the periphery of 
the diaphragm to expose a portion of the underlying oxide. 
The above, and other features of the invention including various novel 
details of construction and combination of parts, will now be more 
particularly described with reference to the accompanying drawings and 
pointed out in the claims. It will be understood that the particular 
method of fabricating silicon-on-insulator pressure sensors embodying the 
invention is shown by way of illustration only and not as a limitation of 
the invention. The principal features of this invention may be employed in 
various embodiments without departing from the scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
The silicon diaphragm pressure sensor of the present invention is the 
simplest type to fabricate in SOI technology. The ISE (isolated silicon 
epitaxy) process described below produces a film that is ideal for sensor 
applications, particularly because it can produce a diaphragm under very 
slight tensile stress. ISE technology is based upon existing processes 
known as lateral epitaxy by seeded solidification (LESS) and zone-melt 
recrystallization (ZMR). U.S. Pat. No. 4,371,421 entitled "Lateral 
Epitaxial Growth by Seeded Solidification" describes these processes in 
greater detail. The ISE process has produced wafers of semiconductor 
material having low defect density, low warpage and no measureable 
impurities. Since the ISE material is extremely high quality, 
piezoresistors fabricated in this material to produce SOI microsensors 
have properties as good as, or exceeding those obtainable with bulk 
materials. 
FIG. 1 is a process flow diagram illustrating the fabrication of a silicon 
diaphragm pressure sensor. Silicon nitride grown using a high temperature 
LPCVD process is deposited on a silicon wafer 10 and patterned into an 
array of rings 12 which will ultimately represent the perimeter of the 
diaphragms. One such ring 12 is shown in the cross-sectional view of FIG. 
1A (with background sections removed for clarity) Each ring 12 need not 
necessarily be circular in shape, but could be square or some other shape. 
Note, however, that the preferred embodiment produces a circular 
diaphragm. 
A layer 14 of oxide is then grown on the exposed silicon by a local 
oxidation of silicon planar process. In this process, the nitride ring 12 
acts as a mask and prevents the growth of oxide on the silicon surface 
beneath the nitride. This process is often referred to as LOCOS, for local 
oxidation of silicon (FIG. 1B). After LOCOS, the nitride ring 12 is 
stripped down to the silicon to form a support surface or ring 15 and a 
high purity polysilicon layer 18 is then deposited by LPCVD (liquid phase 
chemical vapor deposition) over the entire wafer (FIG. 1C). 
The polysilicon layer 18 is now capped with another silicon nitride film 
(not shown) and subjected to zone-melt recrystallization (ZMR). This 
process, which is described below in greater detail, converts the 
polysilicon 18 to a thin film layer of high quality single crystalline 
silicon. This layer 18 is suitable for the fabrication of high density 
CMOS circuits and, in particular, for the fabrication of piezoresistive 
elements 16. These devices 16 and other additional circuits are fabricated 
in the ISE layer 18 at this point, using standard SOI circuit fabrication 
technology. 
A preferred embodiment of the present method utilizes zone-melt 
recrystallization, however, other types of crystallization procedures such 
as laser recrystallization, can also be used. One type of system for 
zone-melt recrystallization uses a scanning line heating source that is 
translated across the material to be recrystallized and a second heating 
system to raise the temperature of the entire sample close to the melting 
point of the material. As the line heater is activated, it induces melting 
of a zone in the sample directly beneath the heater. The line heater is 
then translated across the surface of the sample causing the melted zone 
to move in unison with the heater. As the melted zone moves past a portion 
of the sample it solidifies and results in lateral epitaxial growth of a 
single crystal structure. The diaphragm that results may be a simple layer 
or a multi-layered structure which includes the recrystallized film. The 
diaphragm is secured along its periphery, either directly to the substrate 
material or to intermediate layers which partially enclose the cavity in 
which the diaphragm moves. 
After circuit processing, the wafer is coated with a second silicon nitride 
layer 20 for passivation and to protect the circuitry during subsequent 
processing. As shown in FIG. 1D, the nitride is patterned and the silicon 
diaphragm 18 is anisotropically etched to form a pit or opening 22 which 
exposes a small portion of the underlying oxide. This step is critical as 
the opening 22 in the diaphragm needs to be kept as small as possible, 
perhaps only a fraction of a micron, so that it can easily be sealed later 
on. Preferably more than one opening can be used to simplify oxide 
removal, however, only one is illustrated here. An anisotropic silicon 
etch is employed to form the opening 22 in order to ease the lithographic 
stress. This etchant forms etch pit sidewalls 28 and 29 which slope at 
about 55.degree., gradually narrowing with depth. Therefore, the pit will 
be wider at the top than at the bottom. This structure results in a more 
stable seal in which the sidewall geometry prevents movement of the 
sealing material into the sensor cavity. Note that while the opening 28 is 
located in the center of the diaphragm, other locations in the diaphragm 
can be used. 
At this point, the wafer will be exposed to a concentrated HF etchant which 
attacks the silicon oxide 26, even through very small holes. Once the 
oxide is consumed, the HF exits the cavity 27 due to surface energy 
considerations. Only the oxide under the diaphragm is etched since it is 
completely isolated by the ring 15 from the surrounding oxide layer (FIG. 
1D). 
Finally, the hole in the diaphragm is sealed with a layer 24 of material 
using an LPCVD process (FIG. 1E). Any number of materials could be 
deposited including: TEOS (Tetraethyloxysilicate), a low temperature 
oxide, a silicon nitride, polycrystalline silicon, etc. Alternatively, 
thermal oxidation could be used to seal the hole. This layer 24 may be 
patterned to limit its extent and therefore its effect on the properties 
of the diaphragm. Existing silicon diaphragm sensors have tended to remove 
the oxide spacer either through an opening in the substrate, or laterally 
from the side of the diaphragm. The present device opens the moving 
portion of diaphragm to remove the oxide and then seals the opening. This 
structure operates to shift the stress experienced by the diaphragm in a 
manner that can enhance sensor performance depending upon the specific 
operating conditions. The sealing material 24 can thus function as a 
"stress riser" by locating the resistors 16 at locations of higher stress 
in the diaphragm by using specific patterns of the material 24. 
For example, FIG. 4 shows a sensor having a substrate 70, an oxide 
insulator 78, a single crystal diaphragm 80 located in the recrystallized 
layer 76 having two or more piezoresistive elements 82, a nitride cap 74, 
and a patterned sealant 72. The sealant 72 has a stiffness greater than 
the silicon of the diaphragm 80 so that as the diaphragm 80 is displaced 
under pressure, the silicon portions of the diaphragm 80 will undergo 
greater strain. The piezoresistive elements 82 will thus experience 
greater stress than would normally occur. 
FIG. 4 also illustrates the introduction of additional electronic 
components 84 and 86 into the recrystallized layer 76 adjacent the 
diaphragm 80. The components 84 and 86 could be used to introduce higher 
levels of circuit functionality into the sensor to provide, for example, 
self-calibration of the sensor, diagnostic or computational capabilities, 
compensation for non-linear characteristics or communication with other 
systems. Also due to the small size of each sensor, a number of sensors 
could be used to provide redundancy, where the failure of any one sensor 
could be detected and its output electronically rejected. 
A silicon nitride diaphragm pressure sensor could be fabricated using a 
process flow similar to the one described above. As illustrated in FIGS. 
2A-2F, the process sequence starts with a patterned silicon nitride 32 
formed on a silicon wafer 30, and a LOCOS oxide 34, which in this case 
defines the region which will eventually be under the diaphragm (FIGS. 2A 
and 2B). In FIG. 2C, the nitride 32 has been repatterned and etched back 
away from the oxide area 34 to create a greater perimeter around the 
diaphragm. The wafer is again oxidized but this time to a much lesser 
extent, perhaps as little as 2000 A, to form a thin peripheral layer 36 
through which the LOCOS oxide is later removed. The silicon nitride, which 
forms the diaphragm material, is now deposited as layer 38. This must be a 
high quality silicon nitride layer and is likely to be deposited using an 
LPCVD process. This nitride diaphragm must have the appropriate mechanical 
properties including a controlled thickness so that a well-defined 
relationship exists between applied pressure and the resulting 
displacement or strain of the diaphragm. 
In FIG. 2D, the nitride is capped with a thin oxide layer 40. This oxide 
layer 40 can be deposited using any number of techniques or may be 
thermally grown on the nitride. This layer 40 serves to protect the 
polysilicon from the nitride during the recrystallization step. Next, 
polysilicon is deposited, capped and zone-melt recrystallized to form a 
single crystal silicon layer 42. The capping layer must be removed (not 
shown) prior to fabrication of device elements. In FIG. 2E, the resistors 
44 are patterned and any additional electronics are fabricated in the 
recrystallized silicon 46 that remains around the periphery of the 
diaphragm. 
Holes 48 are then etched down to the thin oxide layer 36 as shown in FIG. 
2F. These holes provide a path for the subsequent HF etch, which will be 
used to remove the oxide 34 from beneath the diaphragm to form cavity 50. 
The sensor is completed when a passivation layer is deposited and the 
etchout holes sealed. 
If the sensor is to be a capacitive type, the resistors would be replaced 
with a counter electrode and a substrate contact is used to provide the 
capacitive transducer. FIG. 3 illustrates a capacitive type transducer in 
which a dopant is implanted in the wafer 64 in layer 65 prior to diaphragm 
fabrication to form an electrode 62 in the substrate underneath the cavity 
60. The holes have been sealed using a deposited silicon nitride 68 or any 
of the processes referenced above in connection with FIG. 1E. The 
electrodes 66 serve as counterelectrodes in the capacitive circuit. 
Calibration of a sensor is essential to its proper performance. The usual 
approach is to connect the sensor to a calibration standard and to adjust 
the zero (or offset) and full-scale (or gain) values by first applying a 
minimum pressure level and adjusting the zero, and then applying a maximum 
pressure level and adjusting the span. In traditional sensors, this 
involves either adjusting potentiometers or trimming resistors which are 
components in the amplification circuit. A preferred embodiment of the 
present sensor uses a programmable memory to store information about the 
zero and span. With this type of sensor one can readjust the values after 
the sensor has been in use for a period of time and may have drifted out 
of calibration. This readjustment can be performed without removing the 
sensor from its location. This provides greater serviceability for the 
system. 
A further embodiment employs a sensor having circuitry that can perform 
self-diagnostic functions. This capability provides information such as 
whether the device is either shorted or open. In this event, any error can 
be compensated for in a sensor system having redundancy. The sensors can 
be extremely small with diameters in the tens of microns so that many 
sensing elements can be associated with one sensor chip. If one sensor 
fails, the error is reported and a new sensing element is brought on-line. 
With sufficient computational power, the sensor can take environmental 
effects into consideration. For example, piezoresistive pressure sensors 
are well known to have significant temperature coefficients. Normally, the 
current through the sensing bridge is used to compensate for the 
temperature dependence of the pressure reading using an analog circuit. 
With the present sensors, environmental effects are compensated for during 
calibration. Even a very non-linear environmental effect is corrected by 
providing the sensor with a programmed process and tabulated data which 
describes the effect. This method can also be applied to the intrinsic 
non-linearity of the sensing element itself. In both the capacitive and 
piezoelectric type sensors, the output is not a linear function of the 
applied pressure which can be corrected using this procedure. 
Communications in a noisy environment can be a problem for sensor 
applications. Incorporating digital communication components, particularly 
microprocessor based circuits, can provide the best combination of noise 
immunity and error detection possible. 
In a preferred method of fabrication, the materials and processes found in 
a typical integrated circuit fabrication can be used to make microsensors. 
These sensors are therefore "compatible" with the integrated circuit 
fabrication process. 
Polysilicon has been employed as the diaphragm material in one embodiment. 
Polysilicon is normally used in conjunction with gates in CMOS and NMOS 
circuits. A significant problem to be addressed with the use of this 
material was the internal stress in the polysilicon. To solve this 
problem, careful analysis of the deposition conditions was conducted and 
the appropriate conditions determined for the deposition of polysilicon 
with a slight tensile stress. Diaphragms with a tensile stress tend to 
stretch themselves taut while diaphragms with compressive stress will 
buckle. Both piezoresistive and capacitive pressure sensors have been 
fabricated having diaphragms under tensile stress. 
As indicated previously, the devices of the present invention can be 
fabricated in SOI using conventional CMOS fabrication techniques. These 
devices have demonstrated substantially better high temperature 
characteristics than equivalent bulk devices. SOI wafers have excellent 
characteristics for use as a high temperature material due to its good 
crystal quality and high purity. Measurements on large (gate width to 
length ratio W/L=400/10) enhancement mode MOS transistors have indicated 
much lower leakage current for devices fabricated in SOI wafers versus 
equivalent devices fabricated in bulk silicon. FIG. 5 shows a plot of the 
leakage current versus inverse temperature for both a bulk device and an 
SOI device. The significant improvements in the leakage current obtained 
for SOI device provide important advantages for sensors fabricated with 
SOI material including high temperature operation of sensors and 
associated circuitry, CMOS compatible processing, simple sensor 
fabrication, much smaller size compared to conventional sensor 
technologies, redundancy of sensor elements, capability of multiscale 
sensor arrays, and ease of integration with circuits. 
Capacitive sensors offer better temperature performance and better pressure 
sensitivity than piezoresistive devices. In order to take advantage of 
capacitive devices, it is important to place signal conditioning 
electronics near the devices themselves. This is because the capacitance 
of a microfabricated pressure sensor is very small. If the electronics are 
remote from the sensor, stray capacitance associated with the leads and 
noise induced by the environment degrade the signal from the sensor. With 
piezoresistive type pressure sensors, the measurement has better immunity 
to noise, especially for small devices, but piezoresistive devices are 
very temperature sensitive (in fact the temperature coefficient can exceed 
the piezoresistive coefficient by an order of magnitude). 
The simplest and most straight forward application of SOI material is to 
employ the underlying oxide as an etch stop. FIG. 6 shows the application 
of an etch stop 96 to a anisotropically etched piezoresistive pressure 
sensor 88. In order to fabricate this type of sensor, a typical process 
might start with the deposition of epitaxial silicon 90 onto a 
crystallized layer 92. The deposition increases the thickness of the layer 
to the desired value and than permits electronics and piezoresistor 94 
fabrication. In order to obtain the high temperature performance possible 
with SOI materials, the transistors and electrical devices that are used 
in the amplification circuit need to be isolated from one another. After 
the electronic components have been fabricated and metallization defined, 
the wafer is coated with a silicon nitride layer. Silicon nitride is 
extremely resistant to KOH etching and can act as an encapsulant for the 
final product. Other anisotropic etchants could be used such as EDP or 
Hydrazine. The back side of the wafer is now patterned and the nitride 
removed in the areas to be etched. Special alignment equipment is required 
to make sure that the front and back surfaces are registered properly. 
After anisotropic etching, the oxide is removed from the back of the 
diaphragm. This will improve the performance of the diaphragm, since 
silicon oxide layers are under significant stress. The wafer is completed 
by opening wirebond windows in the silicon nitride. 
FIG. 7 shows a modification of the process to provide fully isolated 
piezoresistors 100. In this case, a silicon wafer 70 with an epitaxial 
layer 102 is used as the starting material. An SOI wafer is then formed 
from this substrate having silicon oxide layer 106 and crystallized layer 
104. The epitaxial layer, typically formed of single crystal silicon, is 
chosen to be the thickness of the desired diaphragm. It is also chosen 
such that it's dopant type will be different from that of the wafer. In 
this way, several known etch stopping techniques can be employed such as 
stopping on a p+ layer or an n-layer using an electrochemical etchant. The 
crystallized silicon 104 is now used to form the resistors 100 and other 
electronic devices required. Full isolation of the resistors and the 
electronics is the result. 
FIG. 8 shows one embodiment in which SOI wafers can be used to create high 
temperature pressure sensors. In order to fabricate such a device, first 
etch through both the epitaxial layer 114 and the lower oxide 112 in order 
to define the area which will become the diaphragm. Next, the epitaxial 
region above this area is removed and the wafer is oxidized and patterned. 
Next the CVD diaphragm 118 is deposited and defined. If the sensor is to 
be a piezoresistive type, then a second polysilicon layer must be 
deposited and defined over silicon oxide layer 124. Electronics 122 and 
metallization follows and the final step is to etch out the cavity 116 by 
cutting holes in the CVD layer 118 outside the perimeter of the diaphragm 
and to etch the underlying oxide layer. The holes are then sealed using an 
additional thermal oxidation process or by depositing a CVD sealing layer 
120 or both. In this approach, a capacitive pressure sensor would involve 
fewer processing steps. 
FIG. 9 shows another similar approach in which the epitaxial layer 114 is 
used as the silicon diaphragm 128 for a capacitive pressure sensor. In 
this approach, a CVD deposited nitride layer 126 is used to support the 
128 diaphragm from above rather than below. All other processing steps are 
the same as outlined above. 
It should be noted that if electrical isolation of the diaphragm is 
required, additional insulating layers can be employed, or the CVD layers 
must be electrically insulating. In addition, etched cavities can be 
accessed from both the back and front surfaces if the silicon wafer 70 
were etched from behind as is the case for etch cavity type pressure 
sensors. 
A further embodiment employing the concepts described above, FIG. 10 shows 
a differential pressure sensor with both a lower and upper overpressure 
stop. The upper cavity 132 can be created with a deposited oxide 136 or 
other sacrificial layer. As in the previous example, the silicon diaphragm 
130 is formed from the epitaxial layer. Access to the diaphragm is 
achieved from both sides of the resulting die. Thus, a lower cavity 134 
can be accessed through a rear opening 138 in substrate 70. 
For applications other than pressure sensors, wafers allow simple 
micromachining of mechanical structures. As shown in FIG. 11 the 
underlying oxide can be used as a sacrificial layer for such structures as 
single crystal silicon bridges 144 formed from epitaxial layer 142 and 
double tuning forks, and for cantilever structures such as accelerometers. 
One of the advantages of the technology described herein is the ability to 
use alternate underlying insulating layers or multiple underlying layers. 
These insulating layers can be chosen to have properties that are suitable 
as mechanical materials or can be chosen to become sacrificial. 
A simple example of the use of alternate layers is shown in FIG. 12. A 
silicon nitride layer 150 is used in place of the lower oxide. Silicon 
nitride is known to have good mechanical properties for microsensors. In 
this case, the nitride 150 is used as the material from which a 
cantilevered accelerometer 156 is formed. A piezoresistive element, a 
diode, or an active device can be used as a stress detector 158 at the 
base of the cantilever 156 and can be fabricated in a mesa created in the 
epitaxial layer. High temperature electronics 160 are fabricated in this 
layer and in close proximity to the sensing element. To free the 
cantilever from the substrate, an anisotropic etchant is used. This 
etchant undercuts the cantilever structure at a rapid rate while stopping 
at the edges of the defined area. A mass 162 can be secured to cantilever 
156 to define its mechanical characteristics. 
Typical micromachined accelerometers are etched from Si wafers and can 
typically be about 0.35-0.5 cm on a side and 200-300 microns thick. For 
reliability this mass is supported, in one embodiment with 4 springs that 
are integral to the sensor. These springs have diffused resistors that 
sense the motion of the mass. 
By using materials with higher densities than silicon, a microaccelerometer 
can be manufactured without the lengthy micromachining currently used. 
Such a material is tungsten (W) with a density of 19.4 (which is roughly 
10 times that of silicon), the thickness being reduced by a factor of 10 
and still having the same mass. The area would increase to 0.5 cm on a 
side. A 20 micron layer of W can be selectively deposited using chemical 
vapor deposition. 
The frequency of the accelerometer can be expressed by the following 
equation: 
##EQU1## 
The frequency, f.sub.o, is inversely proportional to the mass but follows 
the spring constant K. The stiffness of the supporting members determines 
K. For a typical accelerometer, the springs are 20 microns thick and 
80-100 microns long. A ISE accelerometer has springs in the range of 1-2 
microns thick. When this thickness can support the high stresses seen by 
the springs, the accelerometer has a sensitivity of: 
##EQU2## 
Note that the spring constant K has an inverse relationship to the 
sensitivity. The thinner springs will have a lower K value. B is the 
transduction efficiency and is a function of the resistors. Such thin 
springs yield vary high B values and very low K values and thus enhance 
the overall sensitivity. 
Multiple springs can be employed to give the device survivability. This can 
increase the resonant frequency and thus decrease sensitivity. 
The use of more than one lower insulating layer provides even greater 
advantages. Another accelerometer is shown in FIG. 13. In this case, the 
lower insulator consists of a layer of oxide 152 upon which a layer of 
silicon nitride 150 has been deposited. The epitaxial layer is again 
patterned into mesas for the sensing elements 158 and electronics 160. 
Afterwards, the nitride is patterned and the oxide etched to undercut the 
nitride and free the cantilever from the substrate. If undercutting of the 
boundary layer is a problem in a given application, one could resort to a 
double masking. This involves depositing a masking material. In this case, 
the masking material is a deposited oxide. This layer is patterned to the 
desired final nitride profile. A second photoresist step provides a mask 
whose openings are smaller than those of the final nitride profile. Both 
the nitride 150 and the oxide 152 are etched 166 using a plasma technique. 
Prior to removing the photoresist, the underlying oxide 152 is etched from 
beneath the cantilever. This will remove the perimeter of the cavity as 
well. The photoresist is now removed and the silicon nitride 150 is again 
etched using the upper oxide as a mask. If the mask set is properly 
designed, the nitride 150 can be cut back such that the undercutting from 
the original oxide is completely compensated. 
FIGS. 14A-14E illustrate a process sequence for a preferred method of 
making a silicon diaphragm sensor. First, as seen in FIG. 14A, a substrate 
170 is oxidized to form layer 172 and patterned to form base support 
regions 174. In FIG. 14B, a polycrystalline silicon layer 176 is followed 
by a capping layer 178. Layer 176 is then zone-melt recrystallized using a 
scanning energy source 180 as shown in FIG. 14C to provide a single 
crystal silicon layer 182. In FIG. 14D, the structure is patterned to 
reveal the diaphragm structure 184. Finally, the oxide layer 172 is etched 
through the side openings to suspend the diaphragm 184. 
FIGS. 15A and 15B show more detailed side cross-sectional and top views of 
a structure fabricated using the process shown in FIGS. 14A-E. Base 
support regions 190 are used to support diaphragm 184. First 186 and 
second 188 oxide regions are used to facilitate processing. Regions 192 
and 194 are undercut to suspend the diaphragm structure. 
FIGS. 16 and 17 illustrate cantilever devices that have been fabricated 
using the methods outlined above to form accelerometers. FIG. 16 shows a 
beam 204 of single crystal silicon mounted at base 202 onto substrate 200. 
Diffused or implanted piezoresistors 208 are formed in the beam which has 
a mass 206 mounted on the free end. Due to the use of a single crystal 
beam 204 the piezoresistors are about three times more sensitive than 
polysilicon structures. 
FIG. 17 shows a capacitive type acceleromator having a generally circular 
free end 212 for beam 210 with mass 214 mounted thereon. A diffused 
silicon electrode 218 can be formed in the substrate 200 that is connected 
through channel 220 with sensor circuit 216 that is formed in an epitaxial 
layer overlying the substrate 200.