Abstract:
A carrier structure for semiconductor transducers is disclosed. The carrier structure mounts as a single unit to a force-impacted base substrate and includes multiple piezoresistive elements integrally formed with the carrier structure, such that the elements are located on a predetermined elevation level as the carrier structure while maintaining their electrical contacts and their precise positions by using metal traces on a silicon substrate among the piezoresistive elements and bonding wires between the structure the elements.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to semiconductor transducers and, more particularly, to such transducer assemblies that employ a single block semiconductor carrier structure for supporting semiconductor piezoresistive elements (e.g., strain gages). 
     Semiconductive transducers are widely used in automotive, biomedical and a variety of other applications because of their relatively small dimensions, reliability and high signal output relative to other devices. In performing the transducer function, one or more piezoresistive semiconductor elements are utilized, in which the resistance thereof varies according to the intensity or magnitude of an applied force upon an associated diaphragm to which the elements are mechanically coupled (bonded). The diaphragm to which the force is applied is typically a metal or semiconductor membrane-like substrate upon which the piezoresistive elements are dielectrically mounted. A force applied to the diaphragm deflects the diaphragm and hence causes the associated piezoresistive element to vary resistance in accordance with the deflection. The force being measured is transferred to the piezoresistive element, which is strain responsive, causing the element to expand or compress, thereby producing a change in the electrical resistance of the element. The piezoresistive elements are typically arranged in a Wheatstone bridge circuit with one to four of the bridge legs being active. Other circuit configurations are also possible. 
     A desire is to continue improving the characteristics of transducers of the foregoing type so that devices may be manufactured that are relatively small, sensitive and which produce a relatively large resistance in a relatively small area. The devices should also exhibit a linear operation and small deviations in sensitivity and null offset over a wide range of temperatures, as well as having a relatively low manufacturing cost. 
     A problem in achieving these improved characteristics and cost efficiencies, however, relates to the manner in which prior art transducers are manufactured and assembled. Prior art transducer assemblies require multiple piezoelectric elements, i.e., “gages,” to be manufactured as discrete elements. Manufacturing process and material variations contribute to a lack of uniformity of these elements. The gages must subsequently be electrically normalized based upon their mechanical deviation and their deviation from optimum bonding position on the diaphragm, and it is therefore difficult to achieve consistent results from one transducer assembly to the next. 
     SUMMARY OF THE INVENTION 
     The present invention, accordingly, describes a cost effective transducer assembly having a semiconductor carrier structure that integrally supports multiple piezoelectric elements for mounting as a single unit to a diaphragm or other force-impacted substrate. 
     The integrally formed piezoelectric elements, i.e., gages, are separated from the carrier structure in a space defined by the structure while maintaining their electrical contact and their precise position relative to the structure by using electrically conductive, metallic bonding wires and thin film metalized traces attached to both the structure and the elements. The carrier structure maintains the precise, relative position of the elements throughout the manufacturing process. The transducer assembly is thus well suited for precise registration and mounting to a substrate by an automated process. 
     In one embodiment, a carrier structure for semiconductor transducers mounts as a single unit to a force-impacted substrate and includes multiple piezoelectric elements integrally formed with the carrier structure, such that the elements are located on the same elevation level as the carrier structure while maintaining their electrical contacts and their precise positions relative to the structure by using electrically conductive wires attached to both the structure and the elements. 
     In another embodiment, a carrier structure for semiconductor transducers mounts as a single unit to a force-impacted substrate and includes multiple piezoelectric elements integrally formed with the carrier structure, such that the elements are located on the same elevation level as the carrier structure, and the elements are connected into a bridge circuit form by metalized traces, and connected to the carrier structure through bonding wires. Therefore, in the assembly process, it is not necessary to handle and mount individual elements separately, since they are already in the bridge circuit form. Accordingly, automated handling and assembly machines can be used for better quality control and reduced manufacturing cost. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of an embodiment of a transducer assembly employing a carrier structure. 
     FIG. 2 is a partial cross-section view of an embodiment of the assembly taken through line  2 — 2  of FIG.  1 . 
     FIG. 3 is a top plan view of four gages connected by metal traces on a silicon wafer according to the present invention. 
     FIG. 4 is a top plan view of another embodiment of a transducer wherein the gages are connected and supported by metal traces on the same silicon substrate and whereas the connections between the carrier structure and conductive solder or bonding outputs are made by bonding wires. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An improved transducer assembly is described in FIG.  1  and FIG. 2 wherein all components are situated at the same elevation level. 
     As shown in FIGS. 1 and 2, the transducer assembly  10  includes a semiconductor carrier structure  12  having integrally-formed piezoelectric elements  14 , (e.g., strain gages), a diaphragm  16  serving as a substrate for the carrier structure, and an electronics board  18 . Carrier  12  and the piezoresistive elements  14  are preferably bonded to diaphragm housing  16  by glass bond  20 . Diaphragm housing  16  includes an active region  22  and a fluid chamber  28 . 
     While not shown, it is understood that a fluid, such as air, oil, water or the like, entering or leaving the bottom portion of the chamber  28 , as shown in FIG. 2, imparts a force to be measured to an active region of the diaphragm  16 . This causes the diaphragm  16  to be deflected, causing the piezoresistive elements  14  mounted on the diaphragm  16  to expand or compress, thereby producing a change in the resistance of the piezoresistive elements  14  which is then measured by conventional circuitry (not shown), some or all of which may be located on either or both of the carrier structure  12  or the electronics board  18 . 
     The carrier structure  12  is comprised of a semiconductor material, such as a monolithic or built up laminate consisting of silicon and dielectric material such as oxides, nitrides, polymers and the like, intercalated to create a block of bulk material. The block may be fabricated using a single crystal of silicon, gallium, germanium or other semiconductor, for example. 
     The carrier structure  12  and the piezoresistive elements  14  are formed using standard photolithographic and other etching techniques. The etching is performed on a block of material forming the carrier structure  12  to define the piezoresistive elements  14  such that the piezoresistive elements are carved out and separated from the carrier structure  12 , but have interconnecting electrical paths connecting the piezoresistive elements  14  to the carrier structure  12 . Techniques utilized include, for example, wet etching of the block where isotropic or anisotropic etchants such as KOH or TMAH are used, and dry etching where reactive ion etching is used. The carrier structure  12  is fabricated as shown in FIG. 1 to define an opening, preferably a window  12   a,  in which at least one piezoresistive element  14  resides. 
     Conducting lands  30  are deposited and patterned on the carrier  12  (using shadow masking techniques, for example) for providing electrically conductive paths for the piezoresistive elements  14 . The lands  30  may be constructed of aluminum or any other conductive metal and further may be insulated by depositing insulating material thereon. 
     Electrical bonding between the carrier structure  12  and the piezoresistive elements  14  can be made by bonding wires  31   a , as shown in FIG. 2, which also physically support the piezoresistive elements  14  within the window  12   a . Once the manufacturing process is completed and the piezoresistive elements  14  are bonded (as described below) to the diaphragm  16 , the bonding wires  30   a  are the only connections between the piezoresistive elements  14  and the carrier structure  12 . Since the piezoresistive elements  14  are mechanically decoupled from the rest of the carrier structure  12 , except maintaining the electrical connections to the carrier structure  12 , the piezoresistive elements  14  are able to more accurately and sensitively measure localized forces exerted on the diaphragm  16  at the individual locations of the piezoresistive elements  14 . Moreover, the bonding wires  30   a  can be made of certain special metals having some predetermined characteristics so that, although the wires  30   a  are conductively connected to the isolated piezoresistive elements  14 , they still will not affect the accuracy and sensitivity of the piezoresistive elements  14 . 
     In the normal practice of semiconductor manufacturing, wire bonds are regularly made between a plurality of bond pads on a chip to pads in a package housing assembly after each chip is sawed off from a wafer. It is a feature of this invention to selectively bond the carrier structure  12  and the piezoresistive elements  14  on the same chip. Thus, the piezoresistive elements  14  and the carrier structure  12  can be made on the same substrate, i.e, a wafer, and processed together, thereby reducing the cost. Further, a simple wire bonding process can be done between the piezoresistive elements  14  and the carrier structure  12  on the wafer before it is further divided into multiple transducers. 
     Output pads  32 ,  34 ,  36  are provided at the terminus (or other location) of the lands  30  for electrically connecting the carrier structure  12  to external electronics. The pads  32 - 36  may employ solder bump technology to alter the height of the pads in order to provide space and relief between the carrier structure  12  and external electronics such as the electronics board  18 . As shown in FIG. 2, an electrically conductive solder or bonding  40  connects the lands  30  to the electronics (not shown) of the electronics board  18 . While not shown, it is understood that the electronics of the electronics board  18  may include any type of electronic elements for use with the assembly  10 , including normalizing resistors and interconnecting circuits, peripheral feature circuits and the like. 
     A series-resistance chain made up of resistors  42 ,  44 ,  46  (FIG. 1) is also included as part of the electrically conductive lands  30 . In forming the resistors  42 - 46 , it is understood that resistive trimming utilizing a laser or other tool may be performed to change the resistance values thereof. Similar techniques may be utilized on the lands  30 , as well. Those similar techniques may additionally be used on the piezoresistive elements  14  to change the dimensions, or modify the resistance, of the piezoresistive elements. 
     Other circuits  48  and  50  are included in the carrier structure  12  for integrating additional active signal conditioning and electronics processing with the carrier structure  12 . The other circuits  48  and  50  may be made integral to the carrier structure  12  by etching, masking and other processes, or, alternatively, may be deposited on the carrier structure  12 . 
     Once the carrier structure  12  is in place, the electronics board  18  is bonded to the carrier structure  12  by aligning the conductive contact points of the board  18  with those of the carrier structure  12 . In this manner the remainder of the electronics contained in the board  18  and associated with the assembly  10  are quickly and accurately connected to the carrier structure  12 . It is thus apparent that both the mounting of the carrier structure  12  to the diaphragm  16  and the mounting of the electronics block  18  to the carrier structure  12  are readily capable of being performed by automated equipment, thereby reducing labor costs and improving the quality of the end product, i.e., the assembly  10 . 
     FIG. 3, a layout of four piezoresistive elements  14  with metal traces connected among them, illustrates another embodiment of the present invention. Instead of completely decoupling the piezoresistive elements  14  from the carrier structure  12  on the same silicon substrate, the piezoresistive elements  14  are connected by depositing metal traces  30   b  between them to make them into a bridge circuit form. The metal traces  30   b  can be metalized aluminum or other similar metals. This configuration gives at least two advantages. 
     The first advantage is that the metal traces  30   b  and the silicon substrate (not shown) provide a strong foundation for all the piezoresistive elements  14 . Therefore, the entire unit is more easily handled during assembly and other applications because of the strength of the metal traces. Further, since the piezoresistive elements are connected in a recognizable bridge circuit form, various vision equipment can be used to identify the piezoresistive elements  14  swiftly, thereby allowing the assembly process to be automated. 
     A second advantage is that the number of bonding wires  31   a  is reduced by the on-silicon connections made by the deposited metal traces  31   b . At least three wires  31   a  can be saved by connecting four piezoresistive elements  14  in this configuration. This configuration also maintains the close wafer identity of all the piezoresistive elements  14  until at least the assembly stage, whereas, in some situations, one or two piezoresistive elements  14  may be broken apart when mounting on larger diaphragms. By maintaining the close wafer identity, the piezoresistive elements  14  will have uniform silicon characteristics and, thus, electrical characteristics. 
     FIG. 4 illustrates a layout of the transducer design with four piezoresistive elements  14  connected to each other by metal traces  31   b  and further connected to solder bumps  40  by bonding wires  31   a . First, the layout of the piezoresistive elements  14  and the metal pads for the solder bumps  40  for connecting to other circuits can be efficiently planned since they are all now on the same wafer. In an optimal design, the size of the entire bridge circuit is relatively compact. For example, as it is shown in FIG. 4, four piezoresistive elements are arranged in a “L” shape in order to save space on the silicon substrate (not shown). 
     In addition, as mentioned above, the metal traces  31   b  on the same silicon substrate give all the piezoresistive elements  14  a solid foundation. Further, since the bridge circuit form can easily be recognized by automated assembly machines, the wire bonding process can be simplified. 
     Hence, several efficiencies and improvements over other transducer arrangements are achieved with various embodiments of the present invention. First, the designs and locations of the carrier structure  12  and the piezoelectric elements  14  relative to the diaphragm  16  are determined in the design stage. The advantage of being able to arrange the piezoresistive elements  14  at the design stage in a bridge circuit form is that the piezoresistive elements  14  can be assured to have comparatively uniform characteristics even if any one of them may be severed from the group later in the assembly process. 
     Additionally, even though the piezoresistive elements  14  are on the same wafer as the carrier structure  12 , the piezoelectric elements  14  are not adversely affected by heat or electrical interference from the additional circuitry, since only the thin bonding wires  31   a  are the connections to the rest of the circuitry. 
     Further, using the metal traces to connect and arrange the piezoelectric elements  14  in a bridge circuit form not only increases the mechanical support to each piezoelectric element, but also reduces the number of bonding wires needed, which in turn, further reduces the cost of mass manufacturing the transducers. 
     As thus described, the invention enables improved quality control and reduced manufacturing costs for transducer assemblies. Improved accuracy is also ensured during the assembly and placement of the piezoresistive elements  14  on the diaphragm substrate  16 . 
     The above disclosure provides many different embodiments, or examples, for implementing different features of the invention. Techniques and requirements that are only specific to certain embodiments should not be imported into other embodiments. Also, specific examples of signals, components, and processes are described above to help clarify the invention. These are, of course, merely examples and are not intended to limit the invention from that described in the claims. 
     Although illustrative embodiments of the present invention have been shown and described, a latitude of modification, change and substitution is intended in the foregoing disclosure, and in certain instances, some features of the invention will be employed without a corresponding use of other features. For example, electrical device or circuit elements other than piezoresistive elements may be employed and the application may involve electrical circuit or electromechanical assemblies other than transducer assemblies. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.