A METHOD OF ESTABLISHING ELECTRICAL CONNECTIONS AT A SEMICONDUCTOR DEVICE

Abstract: This invention relates to semiconductor sensors for use as chemical sensitive field effect transistor devices (CSFET) and more particularly to an improved CSFET device and method of making such devices to provide reliable, commer­ cial chemical sensitive field effect transistor devices.

Description: 3ACKCROUND OF THE INVENTION Reference may be made to the following United States patents of interest: 3,831,432; 4,020,830; 4,198,851; 4,218,298; and 4,269,682. As can be seen by reference to the above-listed United States patents, as well as to various publications cited in these patents, it has been proposed to utilize semiconductor sensors, specifically field effect transistors (FET) containing separate source and drain electrodes with a gate region therebetween, to detect and/or measure gases, enzymes, ion activity, and a variety of substances in for instance chemical, biochemical and electrochemical applicaticns. Such proposed devices have been known as chemical sensitive field effect transistors, and may be referred to hereinafter as a CSFET. The CSFET is formed on a semiconductor chip containing a field effect transistor. Such CSFET devices incorporate a chemical sensitive membrane or film in the gate region exposed to a particular substance to be sensed, detected, or measured, with the remainder of the device being encapsulated so as to hermetically seal the device, except for the gate region. A typical FET measures about 0.030 inch by 0.030 inch, with the gate region generally being about 0.015 inch by 0.020 inch and the FET electrodes being about 0.005 inch wide. The conven tional method of connecting the FET electrodes on a semiconduc tor chip to an outer electrical circuit is to bond wire between the FET electrodes and the electrical circuit by a thermal compression bonding technique or ultrasonic means. Since the diameter of the bonding wire varies from about 0.001 inch to 0.003 inch, the wire is extremely brittle and must be protected from mechanical stress and strain, vibrations, thermal expansion and compression. Normally, the bonded wire is protected and reinforced with an adhesive or epoxy material and, of course, must be encapsulated to seal the wires from the particular substance being sensed. In In view of the above-described requirements, and particularly in view of the FET, gate region, and wire size limitations, constructing such CSFET devices have involved expensive, time consuming and very tedious operations, and thus have been restricted to merely making one of a kind laboratory devices. It is-thererore desired to provide a method of manufacturing a CSFET device in large quantities and with a reliability commensurate with other commercial semiconductor devices. It is specifically desired to eliminate the require ment for wire bonding of the electrodes on the semiconductor chip to an outer electrical circuit, and yet provide an electrical connection therewith of increased reliability. In accordance > nL'tE one aspect of the present invention, electrical connectinns are established with a semiconeuctor device, such as with contact points or electrodes on a semiconductor chip, by mounting the chip on a base or substrate. Conductive elements then are printed, such as by screen printing with a conductive adhesive material, on the substrate surface and onto the respective chip contact points or electrodes. In accordance with another aspect of the present invention, there is provided a chemical sensitive field effect transistor (CSFET) device in which the semiconductor chip containing a field effect transistor (FET) is mounted on a base or substrate and conductive elements are screen printed on the substrate surface and directly onto the FET electrodes. This eliminates any requirement for wire bonding, soldering or other previously required means to establish electrical connections to the FET electrodes. In accordance with another aspect of the present invention, there is provided a sandwich configuration in which the semiconductor chip containing the FET is mounted within a suitably sized cavity provided within the base or substrate so that the top of the chip is flush with the substrate surface. A very thin dielectric adhesive pad is then screen printed on the substrate surface and onto a portion of the adjacent chip to maintain the chip in position. Alternatively, or in addition, the substrate cavity walls may be lined with epoxy so that when heat is applied, the FET chip will be bonded in the cavity. Conductors are then screen printed onto the substrate surface and extended onto respective FET electrodes so as to simultaneously result in an electrical connection therewith, thereby avoiding the previously required wire bonding. A thin film of dielectric adhesive material may then be placed over the substrate surface, excluding the FET gate region, so as to insulate the substrate, the screen printed conductor pattern, the FET and its edges (excluding the FET gate region). This hermetically seals the FET, except for the gate region, from the environment. A chemical sensitive membrane material, selected for detecting and/or measuring particular gases, enzymes, ion activity, or other substances, can then be placed in the gate region. If desired, an insulating cover can be placed on top of the assembled components to increase the handling properties of the CSFET. Thus, in accordance with the above-described preferred embodiment of the invention, wire bonding is eliminated along with the time consuming and expensive requirement for making the wire bonding. Instead, utilizing the preferred form of the invention allows the FET chip to be readily mounted within the substrate cavity and the required electrical connections to the FET electrodes to be made simultaneously during screen printing of the conductors on the substrate. In addition, except for the gate region, the remainder of the device is readily and reliably hermetically sealed in a few steps and utilizing only a few components as compared to the timely, costly and painstaking procedure of the prior art which resulted in a much less reliable CSFET device. In practicing this preferred form of the invention, it has been found that of 50 CSFET devices so constructed, 48 devices (g68) operated satisfactory and were of commercial grade quality. In contrast, prior art CSFET devices are typically onlY useful as laboratory items, and even then do not even approach the construction reliability percentage achieved with th. resent invention In another embodiment of the invention, following mounting of the FET chip in the substrate cavity and screen printing conductors on the substrate and onto the FET electrodes, the sub-assembly can then be subjected to injection molding. The area above the gate region is provided with a spring loaded plunger in the mold to enable the gate region to be left open. While this embodiment of the invention provides the advantages of eliminating undesired wire bonding, the heat developed during the molding process provides less reliabilty in the resulting CSFET device than the preferred embodiment. In a further embodiment of the invention, the FET chip is mounted to the substrate surface and the FET electrodes are then wire bonded to the substrate conductors. This sub-assembly may then be injection molded as previously described. While this embodiment provides advantages in ease of construction when compared to presently known CSFET devices, it has the disadvantages of requiring wire bonding and injection molding. BRIEF DESCRIPTION OF THE DRAWINGS The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings in which like reference numerals identify like elements in the several figures and in which: Figure 1 is a perspective view of a CSFET device in one embodiment of the invention in a sandwich configuration; Figure 2 is an exploded perspective view illustrat ing the several components of the embodiment shown in Figure I; Figure 3 is a sectional perspective view taken generally along the section lines shown in Figure 2, and illustrating screen printed conductors on the substrate overlying and in electrical contact with the FET electrodes; Figure-4 is a plan view partly cutaway of a CSFET device in the preferred embodiment of the invention in a sandwich configuration; ; Figure 5 is a sectional view taken along the section line 5-5 of Figure 4; Figure 6 is a sectional perspective view illustrating another embodiment of the invention with the FET mounted on a substrate surface; Figure 7 is a sectional elevational view illustrating the sub-assembly of Figure 6 in a mold for encapsulating the sub-assembly by injection molding; and Figure 8 is a perspective view of a CSFET device constructed in accordance with this embodiment of the present invention. In one embodiment of the invention illustrated in Figures 1-3, a CSFET device 10 is in a sandwich configura tion including a base or substrate 12, intermediate thin film 14 of dielectric adhesive material, and a cover 16 which may be bonded to the other components with epoxy, ultrasonic means, or other well-known techniques. A semiconductor chip 18 contains a field effect transistor (FET). An adhesive frame 20 which may be pre-formed of epoxy, is located within a suitably sized cavity 22 within top surface 24 of base or substrate 12. As can be seen most clearly in Figure 3, FET 18 and adhesive frame 20, when fitted into cavity 22 and with heat applied, results in a bonding of the FET chip within substrate 12 such that FET chip top surface 26 is substantially level or flush with substrate surface 24. Thus, FET electrodes 28 located on surface 26 are substantially flush with substrate surface 24. "Substrate" as used herein refers to the base on which a semiconductor chip is mounted and does not refer to an enzymatic substance. Conductive elements 30 are screen printed onto substrate surface 24 utilizing well-known techniques. Referring to Figure 3, in accordance with the principles of the present invention, conductors 30 are extended so as to overlyingly contact FET electrodes 28 and thereby provide electrical connection thereto, without requiring wire bonding as in prior devices. Dielectric film 14 and top cover 16 include respective openings 32 and 34 located so as to be in registered alignment with the gate region on semiconductor FET chip 18. A chemical sensitive membrane 36 is then inserted into openings 34 and 32 and onto the FET gate region. The membrane may be formed of a predetermined chemical sensitive material selected for detecting and/or measuring particular gases, enzymes, ion activity, or other substances in order that the CSFET device may properly sense desired biological or chemical characteristics. Reference may be made to the aforementioned U.S. patents which describe such chemical sensitive membrane materials. In constructing CSFET 10, base or substrate 12 is formed of a polymer material such as polyvinylchloride, ABS, or similar polymers. Cavity 22 is then formed in the substrate to accommodate a heat activated, self-levelling, pre-formed adhesive, such as epoxy, frame 20 which is placed into and lines the cavity area. The FET chip is then positioned into the epoxy-lined cavity area. Heat is applied, resulting in curing and bonding of the FET chip flush with the substrate surface. The conductive pads are then screen printed onto the substrate and extended over the FET chip and onto respective FET electrodes so as to make electrical connection therewith. As is well-known in the art, screen printing can be accomplished with a conductive paste material utilizing air drying adhesives and a silver filled polymer. Heat curable adhesives or ultraviolet light curable adhesives may also be utilized. The conductive paste is applied using standard screen printing techniques. Dielectric adhesive film 14 is then applied over the substrate and FET sub-assembly, excluding the FET gate area, so as to insulate the substrate, screen printed conductor pattern, the FET and the semiconductor chip edges to thereby hermetically seal the unit from the outside environment. Cover 16 is then placed on top of the assembled substrate, FET and adhesive film and bonded thereto with epoxy or other suitable adhesives to improve the handling, and cosmetic appearance of the CSFT unit. Cover 16 can be formed of a polymer material similar to that of substrate 12. A preferred embodiment of the invention is shown in Figures 4 and 5, wherein semiconductor chip 18 is placed in cavity 22 of base or substrate 12 with an interference fit. Dielectric adhesive pad 23 formed of a polymer material is screen printed on substrate surface 24 and onto a portion of chip 18 to securely maintain the chip in cavity 22. The thin dielectric pad may be 0.125 inch long by 0.020 inch wide by less than 0.005 inch thick. If desired, the walls of cavity 22 may be lined with an epoxy adhesive prior to insertion of chip 18. Conductors 30 are then screen printed on the substrate surface and extended over dielectric pad 23 so as to make electric contact with respective FET electrodes 28. A thin dielectric adhesive coating 25 of polymer material is applied, such as by screen printing, over the substrate surface to cover conductors 30, FET electrodes 28, and chip 18, excluding FET gate region 27. Coating 25 may be about 0.005 inch thick. Chemical sensitive membrane 36 may then be applied at the FET gate region as previously described with respect to Figures 1-3. As an alternative embodiment, the pre-assembled substrate 12 and FET chip mounted into cavity 22 can be subjected to injection molding to form a CSFET device. While this alternative embodiment also employs screen printing directly onto the FET electrodes to eliminate wire bonding, the additional heat generated during the injection molding process increases the possibility of damaging the CSFET components. Therefore, the embodiments of Figures 4-5 and Figures 1-3 are preferred. A further alternative embodiment is illustrated in Figures 6-8, which embodiment utilizes an injection molding process to form a CSFET device 40. In this embodiment, FET semiconductor chip 18 is bonded with an epoxy material 44 formed as a bead around the semiconductor chip edges and extending to surface 46 of substrate 48. Conductors 50 are screen printed onto substrate surface 46. Respective bond wires 52 are connected through standard thermal compression wire bonding techniques so as to interconnect substrate conductors 50 and respective FET electrodes 28. After wire bonding is established, the thin, rather fragile wires 52 are protected with a covering of a room temperature vulcanized elastomer so as to protect the wires against thermal shock, mechanical stress and vibration. After a suitable curing time, the sub-assembly is positioned into an injection mold such as top mold cavity 54 and bottom mold cavity 56. Suitable locating pins in the top and lower mold hold the substrate and FET chip subassembly in position. A spring loaded plunger 58 located in a suitable cavity in top mold 54 rests on top of the FET gate region. When the mold is closed, plunger 58 is mechanically locked with a set screw 60. The plunger has a movement allowance of 0.003 inch or less to avoid possible flow of the encapsulating material into the gate region. After the mold is closed, encapsulating material is then injected into the mold cavity and the molten plastic fills the cavity forming an encapsulation 60 while leaving the FET gate region open. The complete CSFET device 40 is shown in Figure 6. A chemical sensitive membrane material 36 is then placed into the CSFET so as to cover the gate area. This embodiment of the invention, while enabling a more reliable and considerably less painstaking construction than prior CSFET devices, still utilizes wire bonding and injection molding. Thus, the embodiments and construction technique shown and described in connection with Figures 1-3 and 4-5 are the preferred embodiments of t invention. The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

Claims: CLAIMS 1. A method of establishing electrical connections to a semiconductor device comprising: providing a semiconductor chip with a plurality of contact points; providing a base to hold and position the chip; mounting the chip to the base; and printing a plurality of conductive elements on the base and onto the contact points of the chip to provide electrical connections thereto. 2. The method of claim 1, including the steps of providing a cavity in said base adapted to accommodate said semiconductor chip, adhesively mounting said chip in said cavity, and screen printing said conductive elements on the base and onto the contact points of the chip. 3. A method of manufacturing a chemical sensitive field effect transistor (CSFET) device comprising the steps of: providing a semiconductor chip with a field effect transistor (FET) having a gate region and a plurality of respective conductive electrodes; providing a substrate for said field effect transistor; mounting said field effect transistor to said substrate; printing a plurality of conductive elements on said substrate and onto corresponding respective FET conductive electrodes to form respective electrical connections therewith; and hermetically sealing said FET and substrate from the environment except for said FET gate region. 4. The method of claim 3, including the steps of providing a cavity in said substrate adapted to accommodate said semiconductor chip, adhesively mounting said semiconductor chip in said substrate cavity, and wherein said printing step is provided by screen printing. 5. The method of claim 4, including the steps of providing an insulating cover having an opening adapted to said FET gate region, and mounting said insulating cover on said substrate with said opening in alignment with said FET gate region. 6. The method of claim 3, including the steps of injection molding said FET and substrate to encapsulate same except for said FET gate region. 7. A method of manufacturing a chemical sensitive field effect transistor (CSFET) device comprising the steps of: providing a semiconductor chip with a field effect transistor (FET) having a gate region and a plurality of respective conductive electrodes; providing a substrate having a surface and a cavity therein adapted to receive said semiconductor chip; insertably mounting said semiconductor chip in said cavity so that said conductive electrodes are substantially flush with the substrate surface; printing a dielectric adhesive pad on said substrate surface to overlie a portion of said chip and thereby adhesively secure said chip in said cavity; printing a plurality of conductive elements on said substrate surface and-onto corresponding respective FET conductive electrodes to form respective electrical connections therewith; ; printing a dielectric adhesive on said substrate surface, conductive elements, and said chip to hermetically seal said surface, elements and chip except for said FET gate region. 8. The method of claim 7, including the step of applying a chemical sensitive material to said FET gate region. 9. A method of manufacturing a chemical sensitive field effect transistor (CSFET) device comprising the steps of: providing a semiconductor chip with a field effect transistor (FET) having a gate region and a plurality of respective conductive electrodes; providing a substrate having a surface and a cavity therein adapted to receive said semiconductor chip; insertably mounting said semiconductor chip in said cavity so that said FET conductive electrodes are substantially flush with the substrate surface; printing a plurality of conductive elements on said substrate surface and onto corresponding respective FET conductive electrodes to form respective electrical connections therewith; applying a dielectric film to hermetically seal said substrate, plurality of conductive elements, and semiconductor chip except for said FET gate region; ; providing an insulating cover having an opening; and bonding said insulating cover to said dielectric film with said opening in registration with said FET gate region. 10. The method of claim 9, including the steps of providing a pre-formed epoxy mounting frame, inserting said epoxy mounting frame into said substrate cavity, inserting the semiconductor chip into the epoxy mounting frame, and bonding the semiconductor chip to the substrate with the FET electrodes substantialy flush with the substrate surface. -11. The method of claim 9, including the steps of applying a chemical sensitive material to the FET gate region. 12. A method of manufacturing a chemical sensitive field effect transistor (CSFET) device comprising the steps of: providing a semiconductor chip with a field effect transistor (FET) having a gate region and a plurality of respective conductive electrodes; providing a substrate for said field effect transistor; printing a plurality of conductive elements on a surface of said substrate; mounting said semiconductor chip to said substrate surface; bonding wire to form respective electrical connections between said plurality of conductive elements and said FET conductive electrodes; applying a room temperature vulcanizable material to said wire; and encapsulating by injection molding said substrate and semiconductor chip except for said FET gate region. 13. A semiconductor device comprising: a base; a semiconductor chip mounted on said base, said chip having a plurality of contact points; and a plurality of printed conductive elements on the base and extending onto the contact points of the chip to provide electrical connections thereto. 14. A semiconductor device according to claim 13 wherein said base includes a cavity and further including means for adhesively mounting said chip in said cavity. 15. A chemical sensitive field effect transistor (CSFET) device comprising: a semiconductor chip including a field effect transistor (FET) having a gate region and a plurality of conductive electrodes; a substrate including means for mounting said semiconductor chip to said substrate; a plurality of elongated conductive elements located on said substrate and extending onto said semiconductor chip in direct electrical connection with respective conductive electrodes; and means for hermetically sealing said semiconductor chip except for said FET gate region. 16. A CSFET device according to claim 15, wherein said substrate includes a surface having a cavity adapted to receive said semiconductor chip and enable said FET conductive electrodes to lie substantially flush with said substrate surface. 17. A CSFET device according to claim 16, including a dielectric adhesive pad adapted to adhesively mount said chip in said cavity. 18. A CSFET device according to claim 16, including a pre-formed adhesive mounting frame adapted to be located within said substrate cavity and to receive said semiconductor chip. 19. A CSFET device according to claim 16, wherein said hermetically sealing means includes a dielectric film bonded to said substrate, said dielectric film including an opening adapted to register with said FET gate region. 20. A CSFET device according to claim 191 including an insulating cover bonded to said dielectric film, said insulating cover having an opening in registered alignment with said dielectric film opening and said FET gate region. 21. A chemical sensitive field effect transitor (CSFET) device comprising: a semiconductor chip including a field effect transistor (FET) having a gate region and a plurality of conductive electrodes; a substrate having a surface and a cavity within said surface, said cavity adapted to receive said chip; a dielectric adhesive pad printed on said substrate surface to overlie a portion of said chip and thereby adhesively secure said chip in said cavity; a plurality of elongated conductive elements printed on said substrate surface and extending onto corresponding respective FET conductive electrodes to form electrical connections therewith; a dielectric adhesive film printed on said substrate surface, conductive elements, and said chip to hermetically seal said surface, elements and chip except for said FET gate region. 22. A CSFET device according to claim 21, including a chemical sensitive material applied to said FET gate region. 23. A chemical sensitive field effect transistor (CSFET) device comprising: a semiconductor chip including a field effect transistor (FET) having a gate region and a plurality of conductive electrodes; a substrate having a surface and a cavity within said surface, said cavity adapted to receive said semiconductor chip; means for securing said semiconductor chip within said cavity, with said FET conductive electrodes substantially flush with said substrate surface; a plurality of elongated conductive elements printed on said substrate surface and extending over said FET enabling respective conductive elements to be in direct electrical contacting engagement with respective FET conductive electrodes; ; a dielectric film member mounted to said substrate, said dielectric film having an opening adapted to said FET gate region, so that the semiconductor chip is hermetically sealed except for said FET gate region. 24. A CSFET device according to claim 23, including a pre-formed adhesive mounting frame adapted to be inserted within said substrate cavity and to receive said semiconductor chip for mounting said chip to said substrate. 25. A CISFET device according to claim 24, including an insulating cover having an opening, said insulating cover mounted to said substrate with said cover opening in registered alignment with said FET gate region. 26. A chemical sensitive field effect transistor (CSFET) device comprising: a semiconductor chip including a field effect transistor (FET) having a gate region and a plurality of conductive electrodes; a substrate having a surface for mounting said semiconductor chip; a plurality of elongated conductive elements printed on said substrate surface; means for mounting said semiconductor chip on said substrate surface; a plurality of bonding wires respectively electrically connected intermediate said respective plurality of conductive elements and said respective plurality of FET conductive electrodes; and means for encapsulating said semiconductor chip except for said FET gate region by injection molding.

Inventors: BOSE, ASHESH ERIC
Applicants: ABBOTT LABORATORIES
Application year: 1983
Main classification: H01L21
IPC codes: G01N27, H01L29, H01L23, H01L21