# US 6,038,928 A · Miniature gauge pressure sensor using silicon fusion bonding and back etching > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 6,038,928 A - Document ID: US6038928A - Status: Granted - Kind: A - Application number: 08/944,733 - Filed: 1997-10-06 - Granted: 2000-03-21 - Earliest claimed: 1997-10-06 - Assignees: Lucas NovaSensor - Inventors: Nadim I. Maluf; John R. Logan; Gertjan van Sprakelaar - Web page: https://patentgrep.com/patent/US6038928A ## CPC - G01L 9/0042: {Constructional details associated with semiconductive diaphragm sensors, e.g. etching, or constructional details of non-semiconductive diaphragms (details about the integration or bonding of piezoresistor in or on the diaphragm G01L9/0052 and G01L9/0057 respectively)} - G01L: MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE (weighing G01G) ## Abstract A gauge or differential pressure sensor has a base portion having walls which define a cavity within the base portion and a diaphragm portion positioned over the cavity. The base portion comprises silicon; the diaphragm portion comprises silicon; the substrate has a passageway from a surface of the substrate into the chamber; the walls of the cavity form an angle with the diaphragm of no more than ninety degrees; and the chamber has a depth of at least about 5 microns. Preferably, the pressure sensor has a lip within the passageway which prevents an adhesive used to glue the sensor to a base from flowing to the diaphragm and fouling it. The pressure sensor is made by forming a cavity in a first wafer, fusion bonding a second wafer over the first wafer in an oxidizing environment, and using the thin oxide formed when fusion bonding the wafers as an etch stop when opening the cavity to the atmosphere. Etch conditions are selected to form the preferred lip in the passageway. The pressure sensor has improved accuracy and reliability as well as small size. ## Claims ### Claim 1 A method of making a structure which is useful to form a pressure sensor, said method comprising a) etching a first silicon substrate to form a cavity, wherein the cavity has walls and the cavity has an opening to at least one surface of the first silicon substrate; b) forming a composite structure by fusion bonding the first silicon substrate to a second silicon substrate so that a portion of the second silicon substrate covers said opening of the cavity of the first silicon substrate and performing said fusion bonding in an oxygenated environment, thereby forming a thin protective oxide film on the walls of the cavity and on said portion of the second silicon substrate; c) etching the composite structure to form a channel in the composite structure, which channel opens to a surface of the composite structure and which channel has, as a wall, the protective oxide film; and d) etching the protective oxide film which forms said wall of the channel to provide a passageway through the composite structure and into the cavity within the first silicon substrate. ### Claim 2 (depends on 1) The method of claim 1 wherein the oxygenated environment consists essentially of oxygen. ### Claim 3 (depends on 1) The method of claim 1 wherein the act of etching the first silicon substrate to form the cavity comprises anisotropically etching the first substrate. ### Claim 4 (depends on 1) The method of claim 1 wherein the channel is formed by etching the first silicon substrate. ### Claim 5 (depends on 4) The method of claim 4 further comprising fusion bonding a third silicon substrate to the first silicon substrate in an oxygenated environment and etching the third silicon substrate to form a second channel adjoining the first channel. ### Claim 6 (depends on 1) The method of claim 1 wherein the act of etching the composite structure to form the channel comprises anisotropically etching the composite structure. ### Claim 7 (depends on 1) The method of claim 1 wherein the act of etching the protective oxide film comprises wet etching the protective oxide film. ### Claim 8 (depends on 1) The method of claim 1 wherein the second silicon substrate comprises a silicon wafer and an epitaxial silicon layer, and wherein the epitaxial silicon layer of the second silicon substrate is fusion bonded to the first silicon substrate to form the composite structure. ### Claim 9 (depends on 1) The method of claim 1 further comprising removing a sufficient amount of said portion of the second silicon substrate covering the opening of the cavity to form a diaphragm. ### Claim 10 (depends on 1) The method of claim 1 further comprising removing a sufficient amount of said portion of the second silicon substrate covering the opening of the cavity to form a diaphragm by polishing the second silicon substrate. ### Claim 11 (depends on 1) The method of claim 1 further comprising removing a sufficient amount of said portion of the second silicon substrate covering the opening of the cavity to form a diaphragm by isotropically etching the second silicon substrate. ### Claim 12 (depends on 1) The method of claim 1 wherein the etching conditions are selected such that the walls of the cavity converge in a direction from the surface of the wafer to the interior of the wafer. ### Claim 13 (depends on 1) A substrate made by the method of claim 1. ### Claim 14 A method of making a structure which is useful to form a pressure sensor, said method comprising a) etching a first silicon substrate to form a cavity, wherein the cavity has walls and the cavity has an opening to at least one surface of the first silicon substrate; b) forming a composite structure by fusion bonding the first silicon substrate to a second silicon substrate so that a portion of the second silicon substrate covers said opening of the cavity of the first silicon substrate and performing said fusion bonding in an oxygenated environment, thereby forming a protective oxide film on the walls of the cavity and on said portion of the second silicon substrate; c) etching the composite structure with an etchant having a high selectivity for etching silicon instead of oxide to form a channel in the composite structure, which channel opens to a surface of the composite structure and which channel has, as a wall, the protective oxide film; and d) etching the protective oxide film which forms said wall of the channel to provide a passageway through the composite structure and into the cavity within the first silicon substrate. ### Claim 15 (depends on 14) The method of claim 14 wherein the etchant comprises aqueous tetramethylammonium hydroxide. ### Claim 16 (depends on 14) The method of claim 14 wherein the act of etching the first silicon substrate to form the cavity comprises anisotropically etching the first silicon substrate to form the cavity under conditions sufficient to produce cavity walls which converge in a direction from the surface of the substrate being etched toward the interior of the substrate. ### Claim 17 (depends on 16) The method of claim 16 wherein the channel is formed using an anisotropic etchant which produces channel walls which converge in a direction from the surface of the substrate being etched toward the interior of the substrate. ### Claim 18 (depends on 17) The method of claim 17 wherein the act of etching the protective oxide film comprises etching the protective oxide film using a wet etchant. ### Claim 19 (depends on 17) The method of claim 17 wherein the channel is etched under conditions sufficient to form a lip where the channel joins with the cavity. ### Claim 20 (depends on 14) A substrate made by the method of claim 14. ### Claim 21 A method of making a structure which is useful to form a pressure sensor, said method comprising a) etching a first silicon substrate to form a cavity, wherein the cavity has walls and the cavity has an opening to at least one surface of the first silicon substrate; b) forming a composite structure by fusion bonding the first silicon substrate to a second silicon substrate so that a portion of the second silicon substrate covers said opening of the cavity of the first silicon substrate and performing said fusion bonding in an oxygenated environment, thereby forming a protective oxide film on the walls of the cavity and on said portion of the second silicon substrate; c) etching the composite structure to form a channel in the composite structure, which channel opens to a surface of the composite structure and which channel has, as a wall, the protective oxide film, and wherein the channel is etched under conditions sufficient to form a lip where the channel joins with the cavity; and d) etching the protective oxide film which forms said wall of the channel to provide a passageway through the composite structure and into the cavity within the first silicon substrate. ### Claim 22 A substrate which is useful in forming a gauge pressure sensor or differential pressure sensor, said substrate comprising a base portion having walls which define a cavity, and a diaphragm portion positioned over the cavity to form a chamber which includes the cavity; and wherein the base portion comprises silicon; the diaphragm portion comprises silicon; the substrate has a passageway from a surface of the substrate into the chamber; the walls of the cavity form an angle with the diaphragm of no more than ninety degrees; and the chamber has a depth of at least about 5 microns; and wherein the substrate has a lip within the passageway into the chamber. ### Claim 23 A substrate which is useful in forming a gauge pressure sensor or differential pressure sensor, said substrate comprising a base portion having walls which define a cavity, and a diaphragm portion positioned over the cavity to form a chamber which includes the cavity; and wherein the base portion comprises silicon; the diaphragm portion comprises silicon; the substrate has a passageway from a surface of the substrate into the chamber; the walls of the cavity form an angle with the diaphragm of no more than ninety degrees; the chamber has a depth of at least about 5 microns; the base portion is formed from a first silicon wafer and the diaphragm portion is formed from a second silicon wafer; and the base portion is further formed from a third silicon wafer fusion bonded in an oxygenated environment to the first silicon wafer.