WO1987005747A1

WO1987005747A1 - Ambient sensing transcucer devices with isolation

Info

Publication number: WO1987005747A1
Application number: PCT/US1987/000625
Authority: WO
Inventors: Imants R. Lauks
Original assignee: Integrated Ionics Inc
Current assignee: Integrated Ionics Inc
Priority date: 1986-03-21
Filing date: 1987-03-19
Publication date: 1987-09-24
Anticipated expiration: 1988-09-21
Also published as: JPS63503402A; EP0261208A4; EP0261208A1

Abstract

A transducer structure which may be used to transduce all manner of chemical and physical phenomena of a chemical environment. In this structure, the transducer means (146) and the active electronics (148) (140) are formed on discrete regions of the same surface of an inert substrate (130) and are interconnected by a connetor means (150). A wall (182) separates the active electronics (148) (140) from the transducer means (146) so as to protect the active electronics (148) (140) from the environment to which the transducer (146) is exposed. Advantageously, the active electronics (148) (140) are in a semiconductor material which is formed epitaxially on the substrate (130) to provide a monolithic structure; and the transducer (146), the active electronics (148) (140) and the connector means (150) are formed by conventional photolithographic techniques.

Description

AMBIENT SENSING TRANSDUCER DEVICES WITH ISOLATION

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of application

Serial No. 693,907 filed January 23, 1985 for "Ambient Sensing Device with Isolation", which application is incorporated herein by reference.

Related applications are "Ambient Sensing Extended Gate Transistor, "Serial No. 572,182; "Ambient Sensing

Devices," Serial No. 572,199; "Integrated Ambient Sensing Devices and Methods of Manufacture," Serial No. 572,185; "Ambient Sensing Devices Using Polyimide," Serial No. 572,213, filed January 19, 1984; and "Amorphous Metal Oxide Electrodes," Serial No. 441,902, filed November 15, 1982, which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

This relates to ambient sensing devices such as solid state physical and electrochemical transducers and to- methods of manufacturing such devices and in particular to devices that have a high degree of protection from the ambient to which they are exposed. It frequently is desirable to monitor some physical or chemical quantity (temperature, displacement, velocity, chemical composition, pH, acceleration, force, pressure, flow, etc.) of a chemical environment to regulate chemical or biochemical processes, to determine air or water quality, or to measure parameters of interest in biomedical, agri methodology, digital operation, some degree of signal preconditioning or intelligence, small size, high chemical sensitivity with selectivity, multiple species information with specificity, choice of reversible or integrating response to chemical species, temperature insensitivity or compensation and low power operation. In addition the measurement apparatus should have good long term electrochemical stability, good physical resiliency ^"and strength and good resistance to corrosion and chemical attack. In the case of electrical measurement devices, the outputs from the devices should also have low electrical impedance to provide good signal to noise ratios. With chemically sensitive devices, the devices should also have a Nernstian response to the chemical phenomena being measured. One method for the detection, measurement and monitoring of the chemical properties of a substance involves the measurement of an electric potential where the potential is dependent upon the chemical activity being measured. Bergveld has proposed that hydrogen and sodium ion activities in an aqueous solution be measured by a metal oxide semiconductor field-effect transistor (MOSFET) modified by removal of the gate metal. P. Bergveld, "Development, Operation, and Application of the Ion-Sensitive Field-Effect Transistor as a Tool for Electrophysiology" IEEE Transactions of Biomedical Engineering, Vol. BME-19, pages 342-351 (September, 1972) . In particular, if a MOSFET with no gate metal were placed in an aqueous solution, Bergveld suggested that the silicon dioxide insulation layer would become hydrated and then, because of impurities in the hydrated layer, ion selective. After hydration of the insulation layer of the MOSFET, Bergveld believed the device could be used for ion activity measurement by immersing the device in the solution in question and then recording conductivity changes of the device. Thus, the Bergveld device is commonly referred to as an ion-sensitive field effect transistor (ISFET) .

Bergveld*'s work led to other developments in the field of ion sensitive electrodes such as the chemical sensi- tive field effect transistor (CHEMFET) device described in U.S. Patent 4,020,830 which is incorporated herein by reference. As described in the '830 patent, the CHEMFET is a MOSFET is which the gate metal has been replaced by a chemically sensitive system that is adapted to interact with certain substances to which the system is exposed. Thus as shown in Figs. 1 and 2 of the '830 patent, the CHEMFET is identical in structure to a MOSFET except for a layer or membrane 38 that is deposited in place of a metal gate layer on the oxide insulator above the channel region of the transistor and, optionally, an impervious layer 44 that covers all other parts of the CHEMFET that might be exposed to the solution. Numerous variations on CHEMFET structures are disclosed, for example, in U.S. Patents 4,180,771, 4,218,298,. 4,232,326, 4,238,757, 4,305,802, 4,332,658, 4,354,308, 4,485,274 and 4,397,714. Further improvements in these structures are disclosed in Application Serial No. 441,902.

Physical transducers are also known in the art for transducing physical quantities. See, for example, the physical transducers described in Electronics Engineers

Handbook, D. J. Fink, pages 26-52 through 26-57 (McGraw-Hill 1982) ; Measurement Systems Application and Design, E. 0. Doebelin (3rd Ed., McGraw-Hill 1983); Transducers Theory & Applications, J.A. Allocca and A. Stuart (Reston 1984) . With the development of solid-state electronic techniques, physical transducers can be miniaturized with accurate dimensions in order to remedy some of the above deficiencies and at the same time to retain the same or better performance characteristics as those of regular size. Despite this intense development of new designs, there is still considerable work to be done to each solid state transducer to achieve some of the desirable transducer properties described above. One continuing problem has been the susceptibility of these devices to corrosion and chemical attack from the solutions to which they are exposed.

In the art of measuring chemical or ionic species through chemical means, several techniques have been proposed to overcome this difficulty. For example, in U.S. Patent 4,180,771, a structure is described for reducing this problem by locating the chemically sensitive system apart from the gate layer and connecting it thereto by a wire. However, this device has not proven to be very satisfactory in operation and in any event is relatively complicated to manufacture. The extended gate field effect transistors (EGFET) are one of a family of electronic devices that are useful in measuring chemical phenomena that are remote from the main body of the electronic device. For a general discussion of these devices see Application Serial No. 572,182. In general such devices are available to measure potential or current. Advantageously, as disclosed in the 182 application, such devices include four components: a chemical sensing means on a first portion of a semiconductor substrate, some form of active electronics on a second portion of the substrate remote from and separable from the first portion, a means for connecting the sensing portion to the active electronics and a guard for the conductor between the chemical sensing means and the active electronics. However, all these devices are susceptible to degradation in performance as a result of interaction with the solution which they monitor. Ions of

the solution migrate through to the silicon in sufficient quantities to affect measurements and, ultimately, to destroy the effectiveness of the device as a chemical monitor. In Application Serial No. 693,907, a structure is described to avoid these problems by locating the chemical sensing means and the active electronics on discrete regions of the same surface of an inert substrate and interconnecting them by a guarded conductor. Specifically, the active electronics are in a semiconductor material which is formed epitaxially on the substrate to provide a monolithic structure; and the sensor, the active electronics and the guarded conductor are formed by conventional photolithographic techniques. The chemical sensing means is separated from the active electronics by a wall through which the substrate and guarded conductor pass. As a result the active electronics are protected from the chemical environment to which the chemical sensing means is exposed.

SUMMARY OF THE INVENTION

We have devised a transducer structure which may be used to transduce all manner of chemical and physical phenomena of a chemical environment. In this structure, the transducer means and the active electronics are formed on discrete regions of the same surface of an inert substrate and are interconnected by a connector means. A wall separates the active electronics from the transducer means so as to protect the active electronics from the environment to which the transducer is exposed. Advantageously, the active electronics are in a semiconductor material which is formed epitaxially on the substrate to provide a monolithic structure; and the transducer, the active electronics and the connector means are formed by conventional photolithographic techniques. BRIEF DESCRIPTION OF DRAWINGS

These and other objects, features and advantages of our invention will be more readily apparent from the following detailed description of a preferred embodiment of invention in which:

Fig. 1 is perspective view of a chemical sensor with isolation;

Fig. 2 is a perspective view of a preferred embodiment of the invention; Fig. 3 is a series of flow diagrams, and plan views illustrating the formation of a portion of the device of Fig. l;

Fig. 4 is a prespective view of a preferred embodiment of a connector of the present invention; and Fig. 5 is a cross-sectional view along lines a-a of

Fig. 4.

BRIEF DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 is reproduced from Fig. 2 of Application

Serial No. 693,907. As shown in Fig. 1, an ambient sensing device having chemical sensing means and isolation comprises a substrate 30, active electronics 40, first and second chemically-sensitive layers 50, 55, guarded conductors 60, 65, respectively, connecting layers 50, 55 to the active electronics, conductive leads 70, 72, 74 and hermetic seal 80.

Illustratively, the substrate is sapphire (A1_0_) . The hermetic seal likewise can be made of the same material formed integrally with the substrate. Alternatively any other hermetic sealant can be applied to the device photolithographically at the wafer level of fabrication. Examples of other hermetic sealants are CVD or sputtered ceramics, parylene or polyimide. The chemically-sensitive layers can be made of any material suitable for monitoring the chemical or ionic species to be studied. For example sputtered iridium oxide is suitable for measuring pH. Numerous other examples of such materials are given in the above-referenced applications and patents. These layers are formed on substrate 30 so as to be integral therewith.

The guarded conductors comprise a signal line 62, 67 such as doped polycrystalline silicon and a guard 64, 69 that likewise may be formed of doped polycrystalline silicon. As described, for example, in Application Serial No. 572,182, the active electronics typically is a monolithic integrated circuit field effect transistor amplifier, or several such amplifiers; and the output of the amplifier is advantageously bootstrapped to the coaxial guard 64, 69 of lines 60, 65. The three conductive lines 70, 72, 74 illustratively provide a ground, an output and a voltage supply, respectively.

Preferably, the device is mounted to a wall 82, through which the chemically-sensitive layers 50, 55 protrude. In that configuration the chemically-sensitive layers might be used to monitor the chemical properties of a fluid flowing in a tube defined by said wall. As a result, only the chemically-sensitive layers contact the fluid while the active electronics 40 and conductive lines 70, 72, 74 remain outside the wall protected from the fluid.

While the ambient sensing device of Fig. 1 provides considerable improvement over the EGFET and CHEMFET in reliability and lifetime because it separates the chemically-sensitive layer from the electronics, it is limited for use in monitoring chemical species through chemical means, exclusively.

In accordance with the present invention, the same protection against the chemical environment is provided to all transducer devices by forming measuring means and active electronics on discrete regions of the same surface of an inert substrate and by interconnecting the measuring means and the active electronics by a connector that passes through a structure such as a wall that separates the active electronics from the chemical environment. The substrate is selected so that the active electronics can be formed in a semiconductor grown epitaxially on the substrate to provide a monolithic integrated circuit (i.e., one "whose elements are formed in situ upon or within a semiconductor substrate with at least one of the elements formed within the substrate" — IEEE Standard Dictionary of Electrical and Electronics Terms, (2d. Ed. 1977) ; and the measuring means, active electronics and connector means can all be formed by photolithographic processes. In the case where silicon is the semiconductor, the substrate preferably is sapphire.

An example of a preferred embodiment of a device formed in accordance with the invention is shown in Fig. 2. It comprises a substrate 130, active electronics 140, first and second thin film optical filters 142, 144, respectively, optical cell 146, optical detector 148, optical waveguide 150 connecting thin film filters 142, 144, optical cell 146 and optical detector 148 to a light source 160, conductive leads 170, 172, 174 and hermetic seal 180.

Illustratively, the substrate is sapphire (A1_0 ) . The hermetic seal likewise can be made of the same material formed integrally with the substrate. Alternatively any other hermetic sealant can be applied to the device photo- lithographically at the wafer level of fabrication. Examples of other hermetic sealants are CVD or sputtered ceramics, parylene or polyimide.

First and second thin film filters 142, 144 can be made of any material suitable for removing certain wavelengths from the transmittance spectra. These thin film filters are formed on substrate 130 using photolithographic techniques so as to be integral therewith. Optical cell 146 illustratively is a layer of chemically sensitive material that alters its optical properties and hence modulates a beam of light passing through the layer when it is placed in a chemically specific ambient. For pH measurement, for example, the layer can be a pH absorbing dye that changes its absorbence over a given wavelength range with concentration of pH.

Alternatively, the optical cell can be a chemically sensitive layer comprising materials having fluorescent chemistries. In this case, specific chemicals in the ambient trigger the optical cell to fluoresce typically at a wavelength different from the light signal travelling along the waveguide and through the optical cell. For optimum measurements, the waveguide sampling the fluorescent intensity is oriented at a right angle to the activating radiation.

Optical detector 148 can be any solid state device that efficiently detects optical signals applied to it and generates an electrical signal in response thereto. Some commonly used solid state optical detectors include phototransistors and photodiodes such as the PIN photodiode and the avalanche diode. The detector is formed on substrate 130 using photolithographic techniques so as to be integral therewith.

Optical waveguide 150 is a transmission medium for optical signals and can be made of ultrapure fused silica, glass or transparent plastic. It is formed on solid substrate 130 using sputtering or diffusion techniques so as to interconnect thin film filters 142, 144, optical cell 146 and optical detector 148 to a light source 160.

Active electronics 140 typically is a monolithic integrated circuit amplifier, or several such amplifiers as well as signal conditioning means. The three conductive lines 170, 172, 174 illustratively provide a ground, an output and a voltage supply, respectively. A light source 160 is applied to one end of waveguide 150. Typically, the light signal generated by the source is white light although it can be monochromatic. As shown, the light source 160 is located off the substrate although where appropriate it can be fabricated so as to be integral with the substrate. For example, an LED semiconductor device can be fabricated into the substrate to form a hybrid configuration with the substrate. Alternatively, an LED semiconductor device can be fabricated on a monolithic substrate having optical band gap properties that are compatible to those of the waveguide material and the monolithic substrate can then be fabricated into substrate 130. Advantageously, by fabricating a light source integral to the device the necessity of coupling a light signal from a discrete light source that is typically transmitted along an optic fiber is eliminated.

The light signal from light source 160 is transmitted along the waveguide through thin film filter 142 optical cell 146, thin film filter 144 and is applied to optical detector 148. The output signal from the optical detector typically is an electric potential that is proportional to the intensity of the light signal applied to the optical detector. This electrical signal appears across conductive lines 170 and 172.

In practicing the invention, a voltage reading is taken between conductive lines 170 and 172 to provide a reference voltage. Subsequently, the optical cell of the device is exposed to an environment containing specific chemical species to be measured. Advantageously, the specific chemical species interacts with the chemical layer of the optical cell and alters its optical properties so as to modulate the light signal passing through it and thus, the voltage across conductive lines 170 and 172. The difference between the modulated voltage and reference voltage taken across conductive lines 170 and 172 corresponds to the concentration in the ambient of the specific chemical species that is measured.

To illustrate the use of the present invention, a wall 182 is shown in Fig. 2 through which the optical cell 146 protrudes. In this configuration the optical cell might be used to monitor the chemical properties of a fluid flowing in a tube defined by said wall. As a result, only the optical cell contacts the fluid while the active electronics 140 and conductive lines 170, 172, 174 remain outside the wall protected from the fluid.

Again, we do not restrict our invention to an optical sensing means, although that is preferred, and we again emphasize that measuring means can be used with our device having sensitivity to different physical properties such as temperature, gravity, pressure and the like.

The practice of the invention in the photolitho¬ graphic fabrication of an optical transducer is shown in the flow chart and plan views of Fig. 3. The plan views are shown on the right hand side of Fig. 3 and depict one portion of the upper surface of an inert substrate 230 in which one such optical transducer is to be formed. As in conventional photolithography, numerous identical devices are formed simultaneously, each in a different area of the substrate. For convenience, however, we will describe the formation of only one such device.

Substrate 230 typically is a monolithic wafer of sapphire. An epitaxial layer of silicon is processed photolithographically to give an isolated area 240 containing active electronic devices including optical detector 248. A waveguide 250 is formed by deposition of a thin film optical conductor and photolithographic patterning. The ground, the output and the voltage supply conductor leads 270, 272, 274 are then formed by deposition of metal and photolithographic patterning. Ground lead 270 and voltage supply lead 274 are connected to appropriate portions of electronics 240. Output lead 272 is connected to output portion of electronics 240. The device is then passivated by formation of a layer of thin film dielectric 280. Contact windows to the optical waveguide line and metallizations 271, 273, 275 are formed photolithographically. Windows are then defined in the waveguide for placement of thin film filters 242, 244, and optical cell 246. These elements are deposited and formed photolithographically.

The apparatus of Figs. 1-3 permits the monitoring of physical and chemical properties of the environment in which the sensing device is located. Wall 82, 182 can be the wall of a forty-eight inch diameter pipe or that of a micro- pipette or anything in between. It could be the hull of a ship or the fuselage of an airplane. One situation of special interest is where the wall is part of a container such as a test tube, a pipette, a flask or beaker which one might want to make disposable.

It is only practical to make such containers disposable because they can be mass-produced with accompanying reductions in manufacturing cost. Because of similar economies inherent in the manufacture of semiconductor integrated circuits, the costs of manufacturing the integrated circuits and sensors of Figs. 1-3 also can be made to be quite low at high volumes of manufacture. Thus, it is possible to make the devices of Figs. 1-3 in the form of disposable equipment.

One difficulty in using such disposable equipment is to provide a reliable connection between the disposable elements and the apparatus which supplies power to the disposable elements and uses signals produced by them.

According to one prior art connector system, conductive wire leads are soldered directly to the leads of a microelectronic component of an ambient sensing device. However, this technique requires all wiring and soldering to be done by hand. Great care must be taken to prevent solder leakage paths on other wiring or on other surfaces of the substrate. It is also difficult to change circuitry, since this requires drilling out or reflowing the solder connections, with the result that the solder is either particulated and scattered, or is reduced to a molten state for flowing on other surfaces of the terminal board, causing contamination and electrical shorting of the unchanged ciruitry.

To overcome these problems other methods have been utilized. In one method a rigid female connector is connected at one end to a wire. The aperture of the block receives and holds an electrical contact on the monolithic chip. While this method permits easy attachment of the wire to the chip it is not economically feasible to design a different connector for changes in the design of a package such as changes in the number of electrical conductors on the chip to be monitored.

An illustrative embodiment of a connector system 310 for an ambient sensing device 320 is shown in perspective view in Fig. 4 and in cross-section in Fig. 5 along line a-a of Fig. 4. The ambient sensing device comprises a substrate 330 on which are formed sensors 350, 355, electronics 340 and electrical contact pads 371, 373, 375. The connector system 310 futher comprises a female connector 380 which includes connectors 391, 393, 395 positioned so as to establish ohmic contact with pads 371, 373, 375 respectively when connector 380 is mated with substrate 330.

Sensors 350, 355 are typically a plurality of chemical or ionic sensitive surfaces each of which develops a electrical potential upon contact with a specific chemical or ion. Purely for the sake of example, these sensors can be of the type generally disclosed in Application Serial No. 572,192. Electrical conductors 362, 367 transmit these potentials to active electronics 340 and illustratively are fabricated using well known semiconductor fabrication techniques.

Active electronics 340 typically is a monolithic integrated circuit comprising amplifiers 342, 344 and multiplexer 346. Each amplier 342, 344 receives the instantaneous signal from chemically sensitive surfaces 350, 355, respectively, and generates an amplified, filtered signal at its output proportional to said signal. Multiplexer 346 time multiplexes the analog information from the amplifiers 342, 344 onto electrical conductor 348. Advantageously, such multiplexing minimizes the number of electrical connections coming off the chip. .

Three conductive lines 370, 372, 374 provide a ground, an output from and a voltage supply to active electronics 340. They are fabricated using well known semiconductor fabrication techniques.

Electrical contact pads 371, 373, 375 are provided at the end of each conductive line 370, 372, 374, respectively, and are also fabricated using well known semiconductor fabrication techniques. The pads are fabricated preferably from a polysilicon material so as to be mechanically durable when connector 380 is engaged with the substrate. Additionally, these pads are fabricated sufficiently large so as to readily connect with contact fingers 391, 393, 395, respectively, of connector 380.

Connector 380 can be any conventional female connector and preferably is a standard female connector used with printed circuit boards. Contact fingers 391, 393, 395 are located along one side of aperture 381 in connector 380. A conductive lead 396 comprising three wire leads is electrically connected at one end to contact fingers 391, 392, 395. Typically, each wire lead is connected by solder to one of the contact pads. At the other end the conductive lead is connected to electrical equipment. Specifically, the wire associated with contact finger 393 is connected to monitoring equipment while the wires associated with contact finger 395, 391 are connected to positive and negative terminals, respectively, of a voltage supply.

To illustrate the use of the present invention, the ambient sensing device is inserted into the aperture of connector 380 so that electrical contact pads 371, 373, 375 of the ambient sensing device are in electrical contact with contact fingers 391, 393, 395 of the connector 380. The wire leads from the pads in the plug in connector are connected to a measuring device and power source. A chemical to be monitored is then inserted into the environment container defined by wall 382 so that the chemical sensing elements 350, 355 of the device interact with chemical species in the ambient and generate a signal at the measuring device in response thereto. This signal is transmitted through electric contact pad 373 and contact finger 393 to the electrical measuring equipment for measurement.

Numerous modifications of the invention will be apparent from the foregoing.

Claims

What is claimed is:

1. An ambient sensing solid state transducer device comprising: an inert substrate; a monolithic semi-conductive device formed on a first portion of a first surface of said substrate and integral therewith; means for sensing at least one property of the ambient formed on and integral with a second portion of the first surface of said substrate removed from said first portion; and means for connecting said sensing means to said semi-conductive device.

2. The device of claim 1 wherein the inert substrate is sapphire and the semi-conductive device is made of silicon.

3. The device of claim 1 wherein the semi- conductive device includes at least one transistor.

4. The device of claim 1 wherein the sensing means is an optical transducer.

5. The device of claim 1 wherein the sensing means is a thermal transducer.

6. The device of claim 1 wherein the sensing means is a gravimetric transducer.

7. The device of claim 1 further comprising means for sealing said semi-conductive device from the ambient to which said sensing means is exposed.

8. The device of claim 7 wherein said sealing means is integrally formed with the substrate.

9. The device of claim 1 wherein the substrate protrudes through a wall with the sensing means on one side of the wall and the semi-conductive device on the other.

10. A method of forming an ambient sensing solid state transducer device comprising the steps of: forming a_<semi-conductive device on a first portion of a first surface of an inert substrate by growing a semi- conductive layer epitaxially on said first surface and defining said device photolithographically in said semi- conductive layer; forming by photolithographic means on a second portion of the surface of said substrate removed from said first portion means for sensing at least one property of the ambient; connecting said sensing means to said semi- conductive device; and sealing said semi-conductive device from the ambient to which said sensing means are exposed.

11. The method of claim 10 wherein the inert substrate is sapphire and the semi-conductive layer is silicon.

12. The method of claim 10 wherein the step of sealing the semi-conductive device comprises the step of forming a seal around said semi-conductive device integral with the substrate.

13. A connector system for ambient sensing device comprising: an ambient sensing device having means for sensing at least one property of the ambient, electrical contact pads formed on a first portion of a first surface of said device and integral therewith; electronic circuitry integral with said device, means for interconnecting said sensing means to said electronic circuitry and said electronic circuitry to said electrical contact pads, and means for connecting said electrical contact pads to electrical equipment.

14. The system of claim 13 wherein the electronic circuitry includes a multiplexer.

15. The system of claim 13 wherein the connector means is further characterized by a female connector plug which has at one end an aperture and electrical contact pads formed on an inside surface of said aperture, wherein said aperture receives said electrical contact pads on said ambient sensing device and holds said contact pads firmly against said electrical contacts on said aperture surface.