ESD protection of ISFET sensors

• US 5,407,854 A
• Filed: 10/13/1994
• Issued: 04/18/1995
• Est. Priority Date: 01/19/1994
• Status: Expired due to Term

• Current Assignee: General Signal Corporation (SPX Corporation)
• Original Assignee: General Signal Corporation (SPX Corporation)
• Inventors: Fogel, John D., Baxter, Ronald D., Silverthorne, Spencer V., Connery, James G.
• Primary Examiner(s): Chaudhuri, Olik
• Assistant Examiner(s): DUTTON, BRIAN KEITH
• Application Number:
  US08/322,226
• Time in Patent Office: 187 Days
• Field of Search: 437/54, 437/60, 437/47, 437/919, 204/406, 204/416, 204/418, 257/253, 257/322, 257/355, 257/414
• US Class Current: 438/49
• CPC Class Codes: G01N 27/4148 Integrated circuits therefor, e.g. fabricated by CMOS processing CMOS processing per se H01L21/82
• CPC Class Codes: H01L 27/0248 for electrical or thermal protection, e.g. electrostatic discharge [ESD] protection
• CPC Class Codes: H01L 27/0251 for MOS devices
• CPC Class Codes: H01L 2924/00 Indexing scheme for arrangements or methods for connecting or disconnecting semiconductor or solid-state bodies as covered by H01L24/00
• CPC Class Codes: H01L 2924/0002 Not covered by any one of groups H01L24/00, H01L24/00 and H01L2224/00

Abstract

Methods, apparatus and chip fabrication techniques are described which provide electrostatic discharge (ESD) protection to ion-sensitive field effect transistor (ISFET) based devices used to selectively measure ions in a liquid. According to one aspect of the invention, an ESD protection circuit, made up of conventional protective elements, is integrated onto the same silicon chip on which the ISFET is formed, along with an interface that is in contact with the liquid being measured and which does not open up paths for D.C. leakage currents between the ISFET and the liquid. According to a preferred embodiment of the invention, a capacitor structure is used as the interface between the protection circuit and the liquid sample. Further aspects of the invention are directed to methods per se for providing ESD protection for ISFET sensors utilizing the interface means (e.g, capacitor structure) referred to hereinabove, and processes for fabricating the novel interface on a silicon wafer.

Claims

1. A method for providing electrostatic discharge (ESD) protection to ion sensitive field effect transistor (ISFET) based ion selective electrodes, comprising the steps of:

(a) forming an ISFET circuit on a silicon chip;

(b) integrating a protection circuit onto said chip on which said ISFET is formed; and

(c) integrating, onto said chip, an interface between said protection circuit and said liquid, wherein said interface provides a contact with said liquid without opening up paths for D.C. leakage currents between the ISFET and said liquid.

2. A method as set forth in claim 1 wherein the interface integrated onto said chip is a capacitor structure.

3. A process for fabricating a capacitor to serve as an interface between a liquid being measured and a protection circuit included on an ion sensitive field effect transistor (ISFET) chip, wherein said ISFET chip is used to measure ions in said liquid and further wherein said chip includes a silicon substrate, a field oxide layer and at least one chemical barrier layer, and a diffused P+ region for connecting said capacitor to said protection circuit, comprising the steps of:

(a) opening a via in said field oxide and said at least one chemical barrier layer;

(b) sputter depositing a metal film onto said chip to create a lower electrode for said capacitor;

(c) connecting said film to said diffused P+ region through said via to place said film in electrical communication with said protection circuit by way of said diffused P+ region; and

(c) forming an oxide of said metal film to serve as the dielectric for said capacitor whereby, when said liquid being measured comes in contact with said oxide, said capacitor serves as an interface between the liquid being measured and said protection circuit.

4. A process as set forth in claim 3 wherein said step of forming an oxide is performed by chemically oxidizing said metal film.

5. A process as set forth in claim 3 further wherein said step of forming an oxide is performed by anodizing said metal film.

6. A method for providing electrostatic discharge (ESD) protection to an ion sensitive field effect transistor (ISFET) based sensor chip used to measure ion activity in a liquid test sample, comprising the steps of:

(a) building up charge in said test sample during an ESD event; and

(b) transferring charge built up in said liquid test sample, as a result of said ESD event, simultaneously to the source, drain and substrate said ISFET.