Micromachined pressure sensors are being incorporated into many products today in which it was impractical to incorporate larger conventional pressure sensors. Micromachined pressure sensors are being incorporated into such diverse equipment as medical instruments, laboratory instruments, industrial equipment, and automotive circuitry. Smaller, more accurate pressure sensors are needed for new generations of equipment in the medical, analytical, and industrial fields, yet the cost of these pressure sensors must remain low in order to provide instruments and equipment at reasonable prices.
Micromachined pressure sensors which measure absolute pressure are somewhat easier to fabricate than micromachined pressure sensors which measure gauge pressure or a pressure differential. One typical micromachined absolute-pressure sensor is made by forming a cavity within a first silicon wafer and then attaching a second silicon wafer to the first wafer and thinning the second wafer above the cavity, thereby providing a diaphragm over a sealed chamber. The absolute-pressure sensor measures pressure by sensing how much the pressure acting on the front side of the diaphragm deflects the diaphragm into the sealed chamber. Because standard fabrication procedures provide a very clean environment when the second wafer is attached to the cavity-containing wafer, essentially no dirt or debris which can interfere with deflection of the diaphragm enters the chamber. Further, the chamber and diaphragm can be fabricated to fairly precise dimensions, since the chamber is sealed and isolated from further processing which could modify the dimensions of the cavity and/or diaphragm.
Micromachined pressure sensors which measure gauge or differential pressure are more difficult to fabricate than micromachined absolute-pressure sensors. The cavity in the first silicon wafer must remain open during processing or must be opened at some point during processing to provide the channel to the back-side of the diaphragm that is necessary to allow gauge or differential pressure to be measured. Dirt or debris from processing steps can enter the cavity and remain within the cavity, interfering with the diaphragm as it deflects and causing the pressure sensor to supply inaccurate read-out of the pressure measured by the pressure sensor.
Also, since the cavity is exposed to subsequent processing, the cavity and back-side of the diaphragm are exposed to further processing steps that can etch away some of the materials from which the cavity and diaphragm are constructed. When diaphragm thickness is diminished, the diaphragm deflects further under a given pressure than a diaphragm having the expected thickness. The thinned diaphragm consequently provides a pressure sensor having a less-accurate readout of pressure than expected. The thinner diaphragm can also crack more easily during use, leading to reliability issues. Further, the shape of the cavity can be changed during subsequent processing, and if portions of the cavity supporting the diaphragm are removed, the accuracy and reliability of the pressure sensor can be adversely affected. The shape of the cavity supporting the diaphragm determines the shape (length and width) of the diaphragm, and when the shape of the pressure sensor's diaphragm differs from its expected shape, the pressure sensor provides a less accurate readout of pressure. If too much of the supporting walls of the cavity are removed, the diaphragm can be too large to withstand the forces imposed on it, and the diaphragm will fail.
It is desirable for a pressure sensor to have a rectangular cross sectional shape as shown in FIG. 1(a) in order to minimize the overall dimensions of the sensor. The minimum size and geometrical dimensions are established by the minimum length l.sub.d and width of the diaphragm 101 (for a particular pressure) and the minimum length l.sub.f and width of the frame 102 required for attaching the sensor 100 to an appropriate package.
One type of micromachined gauge pressure sensor is manufactured using anisotropic back etch processes to etch a single wafer and expose a thin silicon diaphragm supported over a silicon frame, as illustrated in FIG. 1(b). The wafer is subsequently etched from the backside to form a thin diaphragm upon which the piezoresistors are formed using typical means such as implantation and diffusions.
However, in actuality, it is known that the shape of the back etched structure is limited by the crystallographic planes of silicon. When (100) silicon wafers are used, as is often the case, the cavity takes on the shape of a trapezoid which tapers inwardly from the outer surface of the substrate to the diaphragm, as shown in FIG. 1(b). As a result, the die size is substantially increased beyond the minimum desired geometrical dimensions (FIG. 1(a)) by an amount proportional to the thickness d of the wafer and given by the formula 0.708*d. For example, commonly used 4" and 6" wafers are typically 500-700 .mu.m thick resulting in an unnecessary increase in die size of 700 to 1000 .mu.m.
Alternative fabrication methods have shown that the adverse effect of the slope of crystallographic planes on the overall die size can be reduced or eliminated for absolute pressure sensors. Silicon fusion bonding, where two silicon wafers can be bonded to each other to form one structural element, has been used to bond a top wafer 201, which is later thinned down to form the diaphragm, over a previously etched cavity 202 in a bottom wafer 203. The details of silicon fusion bonding are well published in the literature. As shown in FIG. 2, a cavity 202 is etched in the front side of the bottom wafer 203, and a diaphragm is made by fusion bonding a wafer 201 over the cavity and thinning the upper wafer to form a diaphragm. The cavity walls are now sloped inwards, tapering toward each other the further the walls are from the diaphragm The sealed cavity provides a fixed and controlled pressure environment for the absolute pressure sensor 200.
There are problems with extending the above technique to the manufacture of gauge pressure sensors 300 by allowing the front side cavity to etch through the entire wafer, thus creating a pressure inlet 301 on the backside as illustrated in FIG. 3 before bonding the top wafer on and forming the diaphragm. While the proposal is theoretically possible, it is impractical because of the problems that arise due to lodging minute particulates in the cavity during processing. The particulate interferes with proper operation of the diaphragm, leading to faulty read-out of the pressure. To avoid this problem, it becomes necessary to form the back side pressure inlet after completion of all front-side processing thus guaranteeing cleanliness inside the cavity. The challenge in this final etch step is to ensure the integrity of the thin diaphragm and the cavity walls supporting the thin diaphragm. In other words, once the backside pressure inlet is fully etched, the etch should immediately stop and should not etch the thin diaphragm or chamber walls which are now exposed to the etching chemistry.
One way to perform the final etch yet insure integrity of the diaphragm and cavity walls is to grind the bottom wafer until the cavity is exposed, as disclosed by Petersen et al. in the article "Silicon Fusion Bonding for Pressure Sensors," Proceedings of IEEE Solid-State Sensor and Actuator Workshop, (Hilton Head, N.C.), 1988, p. 144. This method provides a pressure sensor as illustrated in FIG. 3. Others have deposited or thermally grown a layer of oxide over the cavity to protect it and the diaphragm during this back-side etch. See, e.g., U.S. Pat. Nos. 5,295,395 and 5,576,251. The pressure sensors disclosed in these patents can suffer from other problems. When etching is performed under conditions which produce a shallow cavity in the first wafer as shown in U.S. Pat. Nos. 5,295,395 and 5,576,251, often the thin diaphragm becomes stuck to the bottom floor of the cavity during processing. This stiction reduces the number of functional sensors that are produced from a wafer, which increases the cost of producing functional sensors. Removal of the thick oxide layer deposited in the chamber of the pressure sensor in the '395 patent also results in some of the silicon of the chamber and/or diaphragm being removed, since it takes a comparatively long period of time to remove all of the thick oxide within the chamber. Etching debris may also remain within the cavity and interfere with movement of the diaphragm when the oxide layer formed on the cavity of the first chamber prior to fusion bonding is etched using reactive ions.
As pressure sensors are fabricated to smaller dimensions, it becomes much more difficult to fabricate gauge-pressure sensors cleanly and precisely with little variance in diaphragm and cavity dimensions. Minor processing variations which can be tolerated for larger pressure sensors reduce the yield of acceptably-accurate and precise miniaturized gauge pressure sensors. Small variations in diaphragm thickness or length or shape result in large deviations in the pressure indicated by the pressure sensor. Consequently, a new method of making a gauge pressure sensor is needed to address the foregoing problems.