Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part application of U.S. patent application Ser No. 11/241049 filed Sep. 30, 2005, the entirety of which is hereby incorporated in this application. 

   FIELD OF THE INVENTION 
   This subject invention relates to pressure sensors. 
   BACKGROUND OF THE INVENTION 
   One version of a differential pressure sensor includes spaced diaphragms and a resonator structure in a cavity between the diaphragms. The diaphragms move together in response to a differential pressure causing the mass structure of the resonator to move resulting in a strain which can be measured and correlated with a force measurement such as pressure. 
   Traditional differential pressure sensors are designed to determine the differential pressure between the two sides of the sensor. By way of example, traditional differential pressure sensors detect the differential pressure between two regions of interest by evaluating the net effect of the pressure forces of the two regions on a component or components of the sensor. When employed in harsh industrial environments, traditional pressure sensors often require a more robust construction. For example, if a differential pressure sensor is exposed to relatively high-pressure and/or high-temperature environments, the exposed components of the pressure sensor benefit from a construction robust enough to accommodate these conditions. 
   The features and attributes that facilitate operation in such high pressure (i.e., harsh) environments, however, can negatively impact the resolution of the sensor. Some traditional differential pressure sensors that are robust enough to withstand high-pressure environments, for example, cannot detect the pressure differential between the two regions of interest in orders of magnitude less than the pressure difference in the environment. For example, a resonating differential pressure sensor robust enough to withstand pressures of 5000 pounds per square inch (psi), and beyond, generally does not have sufficient resolutional capabilities to detect a pressure differential of +/−10 psi, for instance. This is because traditional resonating pressure sensors contain a vacuum within the closed enclosure between the diaphragms of the pressure sensor and therefore, with high pressures acting on the each of the diaphragm, the diaphragms may tend to bulge inside. 
   Thus, there is a need for a pressure sensing system and method that can provide differential pressure sensing capabilities with high resolution, while withstanding high line-pressures. 
   In another example, high line pressure differential pressure sensors are used to measure a small differences between two high pressures. A typical application is to measure flow in an oil pipeline. A calibrated obstruction is placed in the pipeline and the pressure difference between the two sides of the obstruction is a function of the flow rate. Typically the differential pressure is less than 10 psi while the pressure in the pipeline is 3000 psi. 
   In certain fault conditions, the full line pressure is applied to one side of the diaphragm with ambient pressure applied to the other. Without overpressure protection, the sensor would be destroyed. Many sensors on the market have some sort of overpressure protection mechanism built in. 
   Some pressure sensors of this type use a silicon pressure sensing element which can be provided with stops. A known example has a boss on the diaphragm which is a monolithic part of the silicon. There is a small gap between the boss and a substrate. 
   This design, however, may not provide a sufficient degree of protection for a high line pressure sensing application because the flexible region of the diaphragm is unsupported. This region has to be sufficiently flexible to sense the differential pressure which necessarily means that it is too flexible to stand the line pressure. Also, the boss stop provides protection in only one direction. 
   The deflection of a silicon pressure diaphragm is typically sensed by diffused strain gauges. The amount of strain needed to obtain a satisfactory signal takes the strain in the material as far towards its breaking strain consistent with an adequate margin of safety. 
   A stop for a flat diaphragm would need to act over the whole area and conform to the shape of the surface of the diaphragm. The movement of the diaphragm is very small. Therefore, it would be difficult to fabricate a conformal stop with sufficient accuracy. 
   When a diaphragm is limited by a conformal stop at high pressure, the forces on the surface are high and there is a risk that some damage of deformation may result. Such damage on the flexible part of the diaphragm carries the risk that the elastic behavior is changed and, as a result, the calibration of the instrument is altered. This is highly undesirable. It is a normal practice to replace instruments that have been subjected to line pressure in order to guarantee a know calibration. 
   BRIEF SUMMARY OF THE INVENTION 
   The subject invention features a pressure sensor with a more robust construction. The pressure sensor is operable in high pressure environments and has better resolution. The inventive pressure sensor is also stable. 
   The subject invention results from the realization that a more robust pressure sensor operable in high pressure environments with better resolution is effected by the combination of a special diaphragm structure together with an overload stop. 
   In one embodiment, this subject invention features a diaphragm structure comprising a first substrate including a first surface with an annular groove therein and a second opposing surface with an annular groove on each side of annular groove in the first surface defining a first diaphragm. A second substrate also includes a first surface with an annular groove therein and a second opposing surface with an annular groove on each side of the annular groove in the first surface defining a second diaphragm. There is a diaphragm overload stop behind the first and second diaphragms. 
   The first substrate first surface may include channels across the first diaphragm and the second substrate first surface may also include channels across the second diaphragm. Typically, a resonator structure is suspended in the channels. Preferably, there are means for preventing sticking of the diaphragms to their respective overload stops. In one example, there is a rough surface on at least one of the diaphragms and the overload stops. In another example, there is a coating on at least one of the diaphragms and the overload stops. 
   The subject invention also features a diaphragm structure comprising a substrate including a first surface with an annular groove therein and a second opposing surface with an annular groove on each side of the annular groove in the first surface defining a first diaphragm and a diaphragm overload stop structure spaced from the diaphragm. 
   The subject invention, however, in other embodiments, need not achieve all these objectives and the claims hereof should not be limited to structures or methods capable of achieving these objectives. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which: 
       FIG. 1  is a schematic three-dimensional top view showing the primary portions of one half of an example of a pressure transducer in accordance with the subject invention; 
       FIG. 2  is a schematic three-dimensional partial cross-sectional view showing a portion of the pressure transducer shown in  FIG. 1 ; 
       FIG. 3  is a schematic three-dimensional partial cross-sectional view of a pressure transducer in accordance with subject invention now showing the other half or second wafer secured to the first wafer shown in  FIG. 1 ; 
       FIG. 4  is a highly schematic three-dimensional cross-sectional view of a portion of the pressure transducer shown in  FIG. 3 ; and 
       FIG. 5  is a highly schematic three-dimensional partial cross-sectional view showing another embodiment of a pressure transducer in accordance with the subject invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer. 
     FIG. 1  shows an example of a portion of a pressure transducer in accordance with the subject invention. Substrate  10   a  (typically silicon) includes surface  12   a  with annular groove  14   a  therein defining diaphragm  18   a . Typically, channels  16   a - 16   c  cross diaphragm  18   a . Channel  16   d  is shown intercepting channels  16   a - 16   c  and also shown are channels  16   e - 16   j  in frame portion  20   a . These channels house resonator structure  22  suspended in the channels as shown. One or more anchor structures as shown at  25  may be provided. 
     FIG. 2  shows a portion of substrate  10   a  without the resonator structure. In  FIG. 2 , it can be seen that the surface opposite surface  12   a  in the substrate includes annular grooves  30   a  and  32   a , one on each side of annular groove  14   a  in surface  12   a . Diaphragm overload stop  40   a  is also shown disposed adjacent diaphragm  18   a  as shown. There is a small gap between diaphragm  18   a  and stop  40   a.    
   Two such structures joined together are shown in  FIG. 3 . Substrate  10   b  also includes annular groove  14   b  and opposite side annular grooves  30   b  and  32   b  define diaphragm  18   b . Resonator structure  22  resides between diaphragm  18   a  and  18   b  and may be insulated therefrom via layers of oxide as shown at  150   a  and  150   b . Diaphragm overload stop structures  40   a  and  40   b  are also shown in  FIG. 3 . 
   Such a structure may made by bonding two silicon wafers together which are etched so that a sealed cavity is formed between them as discussed above. This cavity is then evacuated and contains a sensing resonator. The resonator structure may vary in design from that shown in  FIG. 1 . The preferred geometry is designed so that the evacuated cavity can withstand a full line pressure, in one particular example, 300 bar on both sides. Also, the resonator is coupled to the diaphragm so that a reasonable change of frequency, in one example 20%, results from a differential pressure of 1 bar. The diaphragms are able to withstand the full line pressure on one side with the provision of the stops at a spacing such as 10 microns from the diaphragm. The flexibility to measure differential pressure results from the system of grooves  14 ,  30 , and  32  with an overall racetrack shape, i.e. semicircular ends with a short straight section in between. The resonator can be a variation of the lever of the design shown in European Patent No. 1 273 896, incorporated herein by this reference. One resonator structure  22  typically includes two central tynes  50   a  and  50   b ,  FIG. 1  which form a basic double-ended-tuning-fork which is put into tension by the applied differential. The outer tynes  52   a  and  52   b  would have a comb drive, not shown, along most of their length. At the ends of the outer tynes there may be sensing levers and a link between the two halves that provides the coupling necessary to differentiate between the in-phase and out-of-phase modes. The remainder of the resonator layers are not shown in the drawings. 
     FIG. 4  shows how the “tongue” portion  61  is twisted to give a sideways displacement to the end of a resonator tyne.  FIG. 4  shows the deformation caused by the static pressure applied to one side only. Most of the stress is compressive for which silicon is very strong. The maximum tensile stress is about 3000 microstrain. The dimensions of the grooves  14 ,  30 , and  32  were optimized using a model. In one example, they were 200 microns wide and the wall thickness was 45 microns. The grooves can be formed by a deep reactive ion etching a slot 20 microns wide and then opening the slot by an isotropic etching process. An acid (HF—HNO 3 -Acetic) mixture could be used but the more precise gaseous XeF 2  etching technology is preferred. 
   The design of the diaphragm of this invention has the feature that the flexible part  61  is intrinsically strong enough to withstand the full line pressure because it has to support the contained vacuum. This means that stop  40   a  need act only on the non-flexing central area  60   a . Other sensing means can be used with a diaphragm of this design besides the resonator structure discussed above. 
   One resonator is essentially a stretched string of silicon. This structure could be used instead as a strain gauge by passing a current along it and measuring the change of resistance. 
   Also, as shown in  FIG. 5 , capacitive sensing could be achieved by replacing the resonator with flat plate  70  suspended around the edges so that it stays in the same position when the diaphragm moves. It extends through a cavity in the central solid region of the diaphragm. The gap between the surface of the plate and the bottom of the cavity is small and changes with diaphragm deflection thus changing the capacitance. The cavity may have an array of props between the two halves of the diaphragm extending through apertures in the plate to support the line pressure. 
   Note that the flexible regions  72   a  and  72   b  are vertical walls which are in overall compression. Silicon, like most brittle materials, is considerably stronger in compression. This geometry enables the designer to achieve the sufficient strength combined with the sufficient compliance. Note that the wall thickness could be defined by a boron etch stop. Only one overload stop is shown at  74  in  FIG. 5 . Also shown is variable capacitor gaps  80 , oxide insulation layer  82 , support pillar  84 , and aperture  86  in fixed capacitor plate  70 . 
   When an over pressure event occurs, the surface of a diaphragm may be pressed hard against the stop and may stick to the stop. Normally, silicon surfaces are optically flat and prone to forming bonds that are typified by bringing optically flat surfaces together. There may be weak chemical bonds forming in which case the diaphragm could stay stuck to the stop or the flow of the pressure medium into the very small gap is slowed by the viscosity effects. 
   Thus, it preferred in accordance with the subject invention that either the surface of the stops  40 ,  FIG. 3  and/or the surface of diaphragms  18  be made rough. There can be a system of grooves in either the stops or the diaphragms, for example, to conduct the pressure medium into the gap. In still another example, the diaphragms and/or the stops could be coated with layers that are not prone to sticking. Suitable coatings include, silicon nitride, titanium oxide, and diamond-like carbon which are known to be particularly good for chemical inertness and hardness. 
   The result in any embodiment is a more robust pressure sensor operable in high pressure environments with better resolution and enhanced stability. 
   Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. Other embodiments will occur to those skilled in the art and are within the following claims. 
   In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant can not be expected to describe certain insubstantial substitutes for any claim element amended.

Technology Category: 3