High overpressure low range pressure sensor

A low range pressure sensor includes a base plate of brittle material, and a diaphragm plate mounted on the base plate and sealed around a periphery to the base plate. Pressures are introduced to cause the diaphragm to deflect toward the base plate, and the deflection of the diaphragm is sensed through strain gauges to provide an indication of the pressure. The diaphragm is provided with a plurality of individual support posts on a side facing the base plate, so that when the diaphragm is deflected toward the base plate under high overpressures the support posts will support the diaphragm against movement to avoid failure or breakage of the diaphragm. The number of support posts can be varied as desired. In a preferred form of the invention, the base plate is provided with a very thin layer of silicon dioxide, which is a good electrical insulator and which also serves to help accommodate slight movement of the support posts as they touch the surface of the base plate to reduce stress buildup in the diaphragm as the overpressure increases.

BACKGROUND OF THE INVENTION 
The present invention relates to a deflecting diaphragm utilizing a brittle 
material diaphragm that deflects under pressure relative to a base plate 
and which is supported against damage when under high overpressures 
relative to the normal sensor range. 
Semiconductor pressure sensors, which utilize a backing plate and a 
diaphragm that deflects under pressure, are known in the prior art. The 
deflection is measured in such prior art sensors in a number of ways, 
including capacitive sensing, and using strain gauges that are mounted on 
the diaphragm. The use of brittle material such as silicon, glass and 
quartz for diaphragms has also been known. 
The need for protection of brittle material diaphragms when subjected to 
overpressure has been recognized. In general, such support has been 
obtained by resting the diaphragm surface across a facing surface of the 
base plate, which may be configured to match the deflected diaphragm 
configuration, so that when the diaphragm slightly exceeds its maximum 
design pressure sensing range it will be supported on the base. Preferably 
a substantial portion of the diaphragm will rest on a facing surface of 
the base. Recesses have been used in base plates and on diaphragms to 
attempt to ensure a full, continuous overpressure support across the 
deflecting portion of the diaphragm. 
SUMMARY OF THE INVENTION 
The present invention relates to a pressure sensor that utilizes a base 
plate and a diaphragm which is deflected under pressure relative to the 
base plate. The diaphragm is made of brittle material and has suitable 
sensing means thereon to provide an output indicative of the pressure 
applied to the diaphragm. 
Overpressure protection is provided by a plurality of short posts or bumps 
formed on a diaphragm, or if desired on the base plate, which will support 
the deflecting diaphragm under overpressure at a plurality of locations on 
the diaphragm to provide support and protection against overpressure. By 
using a plurality of short posts, the diaphragm is not stiffened and thus 
it behaves as though the posts were not there until excess pressure is 
applied, and it also eases the manufacturing process, therefore widening 
the overpressure stop gaps because the less stiff diaphragm can travel 
farther than one with a large stiff center. 
In the form shown, a silicon wafer is etched to provide a thin portion that 
forms a deflecting web of a diaphragm, and is masked to leave a plurality 
of posts that have a height equal to the original thickness of the wafer 
from which the diaphragm is being formed. A rim surrounding the plurality 
posts is left for attachment to the base plate. 
During the etching process the support posts become substantially pyramid 
shaped. 
It has also been discovered that having a layer of silicon dioxide on the 
surface of the base plate, which is not as brittle as pure silicon, 
permits the posts to shift slightly in the thin layer of silicon dioxide 
after the posts first contact the support layer and as the overpressure 
increases. The curve or bow of the deflecting diaphragm causes the posts 
to be at a slight angle as they first contact the base plate. As pressure 
increases, there is a slight shifting of the outer ends of the posts 
relative to the support layer. The silicon dioxide layer forms electrical 
isolation for the base plate from the surface it is to be mounted upon. 
The silicon dioxide layer also enhances the overpressure capability of the 
sensor by allowing the support posts to adjust position to avoid raising 
higher stress levels in the diaphragm. The hard silicon surface of the 
base layer, without the silicon dioxide layer, reduces the ability of the 
posts to shift as the overpressure increases.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a pressure sensor made according to the present 
invention is illustrated at 10. The pressure sensor is a low range 
pressure sensor, for example, in the range of 30 psi gauge, either gauge 
or absolute, but having the ability to handle overpressures that are a 
substantial multiple of such range. The gauge pressure sensor would have 
an opening through the base plate, open to the diaphragm area. The 
pressure sensor of the present invention could be a capacitive sensing 
unit. Comparable prior art pressure sensors generally will withstand three 
to eight times the rated pressure without breakage. The present pressure 
sensor will withstand 5 to 10 times more overpressure than a comparable 
diaphragm without posts. 
The pressure sensor 10 comprises a base plate or layer of a brittle 
material indicated at 12. Such material may be a semiconductor such as 
silicon, which is the preferred material, but could also be other brittle 
material such as glass, sapphire, quartz or the like. The base plate 
supports a diaphragm plate assembly 14, which includes a peripheral rim 
member 16 which bounds a deflecting diaphragm or web section 18. The 
deflection of the diaphragm web section 18 versus pressure is determined 
by the diaphragm web thickness in a bounding region adjacent to the rim 
16, which region is shown at 20 on the sides of the diaphragm web in FIGS. 
1 and 2 and also by the thickness between the posts. The amount of 
deflection of the diaphragm web is very small across the operative range, 
and if excessive deflection occurs the diaphragm will break. 
The rim 16 surface is suitably bonded to the base plate 12. For example, a 
glass frit in the interface region indicated at 24 can be used for bonding 
the diaphragm to the base plate upper surface 28, or the base and 
diaphragm can be joined at the rim using anodic bonding or fusion 
techniques. The bonding of the diaphragm rim to the base plate leaves a 
chamber indicated at 26 beneath the diaphragm web section 18 and above the 
upper surface 28 of the base plate 12. A vacuum is created when the die is 
sealed to the base plate 28 with the glass frit 24. The pressure or force 
to be measured is applied as indicated by the arrow 22 against an upper 
surface 30 of the diaphragm web 18. This will cause the diaphragm web to 
bow toward the base plate 12, and the amount of deflection of the 
diaphragm web can be measured with suitable strain gauges indicated at 32 
on the upper surface 30 of the diaphragm web to provide an indication of 
the amount of force or pressure applied, as a function of the deflection. 
In the preferred embodiment, in order to provide a support for the 
diaphragm web 18 as it bows downwardly, and to prevent the diaphragm from 
being fractured, damaged or otherwise broken, a plurality of support posts 
indicated at 36 are integrally formed on the diaphragm web 18 and on a 
side of the diaphragm web opposite from the application of force or 
pressure. These posts 36 face toward the upper surface 28 of the base 
plate or layer 12. As shown, in FIG. 2, there are 16 such posts, but a 
different number can be used. For example, four, eight or nine posts for 
supporting the diaphragm web 18 can be used for overpressure protection, 
depending in part on the size of the diaphragm. 
The posts 36 are formed by etching a wafer of silicon or other material 
from which the diaphragm is made, as the diaphragm web 18 is etched to 
proper thickness, in a batch process, utilizing a suitable mask that forms 
the posts 36 into substantially pyramid-like sections that, as shown, have 
a flat tip 38 that is the same height from the surface 30 as the surface 
of the rim 16 that contacts the base plate 12. The posts can be of 
different heights, if desired, to achieve substantially uniform contact 
with a base plate with a flat surface across the diaphragm taking into 
account the bowed shape of the diaphragm under pressure. 
As shown in FIG. 1A, the upper surface 28 of base plate 12 is recessed in 
selected areas from the plane of the edge portions where the diaphragm rim 
is bonded. For a 16 post configuration, sections 28A are along two sides 
and between adjacent corner sections 28C. Corner sections 28C are less in 
depth than the other parts. Sections 28A are recessed more than sections 
28C, but are closer to the posts than the center section indicated at 28B 
(most recess depth). The difference in recess depth is to accommodate the 
differences in deflection, because the diaphragm web 18 will deflect more 
in the center than it will adjacent to the rim 16, and the steps in the 
surface 28 of base plate 12 provide substantially simultaneous contact 
between the corners and outer rows of posts 36 and the surface portion 28C 
and 28A and between the posts located near the center of diaphragm web 18 
and the surface portion 28B. Again, as stated, the same effect can be 
obtained with a flat surface on base plate 12 and post 36 of different 
heights, with the shortest posts 36 near the center of the diaphragm web 
18. 
As the diaphragm web 18 deflects, the tips 38 of the posts 36 will contact 
their respective surface portions 28A, 28B and 28C, and be held against 
further deflection under increasing pressure. The overpressure applied 
along the line 22 can exceed the normal operating pressure by many times 
without damaging the diaphragm web 18. 
It has also been noted that when silicon is used as a base plate, it is 
desirable to have an electrically insulating layer on the surface, and as 
shown in FIGS. 1 and 3, a layer 40 (about three microns thick) represented 
by dotted lines is silicon dioxide that has been grown on the surface of 
the silicon base plate. However, it has been found that as the diaphragm 
web 18 is bowing toward the base plate, the posts 36 will be slighted 
canted, so that a corner or edge indicated at 42 of the flat top 38 may 
first contact the upper surface 28 of the base plate 12, rather than 
having the flat surface 38 of posts 36 contact the layer 40 across the 
full width of the flat surface 38. As the diaphragm web 18 deflects 
further under greater pressure, it has been found that these posts 36 will 
tend to rotate generally as shown by the arrow 44 as the diaphragm web 
becomes less bowed to fully seat the end surfaces 38. If the diaphragm web 
18 moves slightly after initial contact, the post corner will dig into the 
silicon dioxide layer and will tend to form a little channel or groove 
whereas, if the untreated silicon surface is engaged, there will be less 
sliding, and the diaphragm may fail in tension. 
It should also be noted in FIGS. 1, 2 and 4 that at the bases of each of 
the posts 36, due to interaction of the etching materials and the post 
masks, complex surface configurations 48 are formed. The etching leaves 
tapered surface 48 extending from the corners, and small spurs 48A extend 
laterally from the tapered surfaces or ribs at locations spaced from the 
posts at a shallow angle, so that the tapered base surfaces of the posts 
intersect in regions shown at 50. A dotted line representation at 52 in 
FIG. 1 is the true designed form of the base post with the thickness of 
the remaining diaphragm web between posts equal to that shown at 20. These 
tapered surfaces are depicted in FIG. 2, only very schematically and as 
can be seen, the tapers are not squared with the major sides of the posts. 
Thus, FIG. 1 is an illustrated cross section in the tapered surface 
region. The configuration of the surface between the posts can be 
different without substantially affecting the performance. 
The silicon dioxide layer 40 permits the posts to shift slightly and reduce 
diaphragm breakage. The overpressure protection posts 36 are quite easily 
manufactured, as are the other components of the sensors, so costs are not 
a substantial factor. 
The posts 36 are shown as being formed integrally as part of the diaphragm, 
but the posts also could be formed to extend up from the base plate to 
provide overpressure protection. 
The recessed regions of the base plate surface will change in shape, size 
and location as the number of posts change to provide the benefits of the 
posts all contacting the surface at substantially the same time. For 
example, a diaphragm with four posts would be used with one planar 
surface, recessed from the rim. Different numbers of posts require 
different surface depths to digitally approximate the bowed diaphragm 
shape when under pressure for adequate support. 
A suitable housing or pressure containment plate will be used over the 
diaphragm for containing the pressure acting on the diaphragm. Such 
housing is not shown because they are well known. 
The same conceptual construction can be used with capacitive sensing for 
sensing diaphragm deflections, by using a conductive film on a plate 
spaced from the diaphragm to form a capacitive plate insulated from the 
diaphragm, with suitable leads attached to the conductive film and the 
diaphragm. 
Although the present invention has been described with reference to 
preferred embodiments, workers skilled in the art will recognize that 
changes may be made in form and detail without departing from the spirit 
and scope of the invention.