Abstract:
A pressure sensing device for producing an output proportional to an applied pressure irrespective of vibration and acceleration of the device, the device including: a first deflecting diaphragm formed in a first wafer and including a first plurality of piezoresistors mounted thereon, the first diaphragm being responsive to the applied pressure and vibration of the device; and, a second deflecting diaphragm formed in the first wafer and including a second plurality of piezoresistors mounted thereon, the second diaphragm being responsive only to vibration of the device; wherein, the first and second pluralities of piezoresistors are electrically coupled together to provide a common output such that they cooperatively at least partially cancel a portion of the common output associated with the vibration of the device.

Description:
FIELD OF INVENTION 
     The present invention relates to pressure sensing devices and more particularly to a pressure sensing device adapted for use in environments which subject the device to harsh vibrations or accelerations. 
     BACKGROUND OF INVENTION 
     Semiconductor pressure transducers are frequently used in applications which require operation in environments that subject the transducers to substantial vibrations and/or accelerations. A typical transducer includes a piezoresistive sensor device formed on a diaphragm which deflects in response to an applied pressure. Errors can be caused by undesired vibrations and/or accelerations. These errors are due to undesirable deflection of the sensor diaphragm in response to these undesired vibrations and accelerations. A typical application is the measuring of pressures inside of a jet-engine, where a sensor is exposed to substantial vibrations, accelerations and decelerations. It is desirable to compensate the stress sensing network of piezoresistors used in such applications to eliminate vibration and/or acceleration induced errors. 
     Accordingly, it is an object of the invention to provide a pressure sensing device adapted to accurately measure pressure in an environment which subjects the device to substantial vibrations and accelerations. 
     SUMMARY OF INVENTION 
     A pressure sensing device for producing an output proportional to an applied pressure irrespective of vibration and acceleration of the device, the device including: a first deflecting diaphragm formed in a first wafer and including a first plurality of piezoresistors mounted thereon or otherwise formed therein, the first diaphragm being responsive to the applied pressure and also vibration of the device; and, a second deflecting diaphragm formed in the first wafer and including a second plurality of piezoresistors mounted thereon or otherwise formed therein, the second diaphragm being responsive only to the vibration of the device; wherein, the first and second pluralities of piezoresistors are electrically coupled together to provide a common output such that they cooperatively at least partially cancel a portion of the common output associated with the vibration of the device. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 illustrates a partial plan-view of a pressure sensing device according to the present invention; 
     FIG. 2 illustrates cross-section  2 — 2  of the device of FIG. 1; 
     FIG. 3 illustrates cross-section  3 — 3  of the device of FIG. 1; 
     FIG. 4 illustrates an electrical representation of the device according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Generally, the device according to the present invention includes a single “chip” or piece of silicon containing at least two stress deflecting diaphragms. There is a half Wheatstone bridge including two piezoresistors formed in series on each diaphragm. One piezoresistor of each pair increases its resistance with a positive stress normal to the plane of the diaphragm and the other one decreases its resistance with a positive stress normal to the plane of the diaphragm. The two diaphragms are both exposed to the vibrational and acceleration stresses, however only one diaphragm is exposed to a pressure to be measured. The half-bridges from each diaphragm are electrically coupled to form a full bridge such that for a positive stress applied substantially normal to the diaphragm, the bridge output of one half-bridge will subtract from the other. Thus, the signal output is responsive to the pressure as applied to one diaphragm while the signal response to vibration applied to both diaphragms is canceled. 
     More particularly, if the two deflecting diaphragms have a substantially same size and thickness, and the piezoresistive characteristics of each half bridge are substantially the same, the output voltages from the same positive normal stress applied to each diaphragm will sum to zero. If however, both diaphragms are subjected to the same stress by vibration or acceleration and only one of the two diaphragms is also subjected to a positive pressure normal to that diaphragm, a voltage output from the full Wheatstone bridge will result. The resulting voltage is proportional to the positive normal pressure applied to the one diaphragm. Thus, such a dual device can provide an output proportional to an applied pressure to be measured and independent of vibrations and/or accelerations as these signal components are canceled. 
     Referring now to FIGS. 1-4, like references identify like elements of the invention. FIG. 1 illustrates a partial plan-view of a pressure sensing device  10  according to the present invention. In the preferred embodiment, two separate deflectable diaphragms  20 ,  30  each respectively containing two piezoresistors  40 ,  50  and  60 ,  70  electrically coupled in series are formed in wafer  100 . Piezoresistors  40 ,  60  each decrease with positive normal stress and piezoresistors  50 ,  70  each increase with positive normal stress. The piezoresistors  40 ,  50 ,  60 ,  70  are electrically interconnected to form a full bridge such as is illustrated in FIG.  4 . 
     Referring now also to FIG. 2, therein is illustrated cross-section  2 — 2  of the device  10  of FIG.  1 . The two deflecting diaphragms  20 ,  30  are in turn covered by another member  200  which has therein an aperture  205  accessible to the first diaphragm  20  but not the second diaphragm  30 . Thus, application of a pressure  215  on the covering member  200  will cause the first diaphragm  20  to deflect, while application of various vibration or acceleration stresses to the device  10  will cause both diaphragms  20 ,  30  to deflect. If both diaphragms  20 ,  30  are formed in sufficient proximity to one another, the deflection caused in each of the diaphragms  20 ,  30  by vibration and/or acceleration will be by the same amount. Thus, the device  10  is responsive to normal pressure applied to the cover  200  but not to vibration of or acceleration of the device  10 . 
     Commonly assigned U.S. Pat. No. 5,955,771, entitled “SENSORS FOR USE IN HIGH VIBRATIONAL APPLICATIONS AND METHODS FOR FABRICATING SAME”, issued Sep. 21, 1999, the entire disclosure of which is hereby incorporated by reference as if being set forth in its entirety, teaches a hermetically sealed sensor which can be advantageously used with the present invention. It is understood that other structures can be used as well though. The resulting structure will be an ultra-thin sensor without external leads being suitable for direct mounting to a structure such as a fan blade in a jet engine, for example. Furthermore, such a sensor is suitable for high temperature operation. For instance, such a device could be mounted on an appropriate pre-glazed kovar “shim” and then welded directly to a fan blade of a jet engine for example or to the blade of a helicopter. 
     The wafer  100  is preferably fabricated using the method disclosed in commonly assigned U.S. Pat. No. 3,930,823, entitled, “HIGH TEMPERATURE TRANSDUCERS AND HOUSING INCLUDING FABRICATION METHODS”, the entire disclosure of which is hereby incorporated by reference as if being set forth herein in its entirety. Alternatively, any conventional wafer processing technique which enable dielectically isolated piezoresistive sensor elements  40 ,  50 ,  60 ,  70  to be formed on semiconductor material using dielectric films of SiO 2  or the like could be used. The elements  40 ,  50 ,  60 ,  70  are preferably formed of highly doped (P+) silicon. It is understood that a number of such sensors can be made at the same time on a large substrate. The circuit nodes of the Wheatstone bridge include four oversized P+ diffused silicon electrical contact areas or fingers  72 ,  74 ,  76 ,  78  which are mainly located in non-active areas of the wafer  100 . It should be understood the active portions of the wafer  100  can be defined as those portions defined by the diaphragms  20  and  30 , as these portions deflect. The remaining portions are referred to as the non-active regions. The term “finger” is used to indicate those areas  72 ,  74 ,  76 ,  78  which project from the piezoresistors  40 ,  50 ,  60 ,  70  to the metal contacts  80 ,  82 ,  84 ,  86 . The metal contacts  80 ,  82 ,  84 ,  86  within the contact area are preferably rectangular in shape, although other shapes could of course be utilized. 
     Referring now also to FIG. 3, therein is illustrated a cross-section  3 — 3  of the device  10  of FIG. 1. A glass wafer  300  is preferably bonded to and can be used to electrically contact the fingers  72 ,  74 ,  76 ,  78 . Wafer  300  preferably includes four apertures  310 ,  320 ,  330 ,  340  disposed in areas which will overlie the metalized portions  80 ,  82 ,  84 ,  86  of the contacts  72 ,  74 ,  76 ,  78  when wafer  300  is secured or bonded to wafer  100 . Preferably, apertures  310 ,  320 ,  330 ,  340  on the side of the glass that contact the silicon wafer  100  are just slightly larger than the metalized regions  80 ,  82 ,  84 ,  86  but smaller than the width of the fingers  72 ,  74 ,  76 ,  78  to insure proper sealing with the wafer  100 . However, the apertures  310 ,  320 ,  330  and  340  are preferably tapered in dimension to accommodate and secure small metallic spheres  350 . In this case, the glass wafer  300  is preferably electrostatically bonded to the silicon wafer  100 . The apertures  310 ,  320 ,  330  and  340  are preferably filled with an unfired metal glass frit  360  and small spheres or balls of metal  350  are inserted into the unfired frit so as to leave a small portion of each sphere  350  protruding past an outermost edge of the glass wafer  350 . Of course, other shapes of small metal could be used instead of spheres  350 . The structure can then be fired to make an electrical contact between the spheres  350  and the metalized regions  80 ,  82 ,  84 ,  86  of the fingers  72 ,  74 ,  76 ,  78 . 
     On the other hand, successively filling and firing the glass-metal frits insures that after firing the glass-metal frit extend beyond the apertures so spheres  350  are not needed. 
     Referring now also to FIG. 4, therein is illustrated an electrical representation of the device according to the present invention. Basically, as set forth, it includes piezoresistive elements  40 ,  50  from diaphragm  20  being coupled in a full bridge configuration with piezoresistive elements  60 ,  70  of diaphragm  30 . Accordingly, they cooperatively cancel any output of the full bridge resulting from common exposure of both diaphragm  20  and  30 , i.e. output attributable to vibration and/or acceleration of the device  10  for example. 
     Although the invention has been described and pictured in a preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example, and that numerous changes in the details of construction and combination and arrangement of parts may be made without departing from the spirit and scope of the invention as hereinafter claimed. It is intended that the patent shall cover by suitable expression in the appended claim, whatever features of patentable novelty exist in the invention disclosed.