Browse Prior Art Database

METHOD OF PROCESSING HIGH PRECISION PRESSURE SENSORS

IP.com Disclosure Number: IPCOM000007318D
Original Publication Date: 1994-Oct-01
Included in the Prior Art Database: 2002-Mar-14
Document File: 2 page(s) / 113K

Publishing Venue

Motorola

Related People

Uko Udofia: AUTHOR [+3]

Abstract

In piezoresistive pressure sensors, control ofpres- sure sensitivity has posed a tough technical chal- lenge since it is inversely proportional to the square of the diaphragm thickness. This makes the control of diaphragm thickness very critical to achieving high precision pressure sensors.

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MOTOROLA Technical Developments Volume 23 October 1994

METHOD OF PROCESSING HIGH PRECISION PRE$SURE SENSORS

by Uko Udofia, Karen Bartholomew and Bruce Parmelee

  In piezoresistive pressure sensors, control ofpres- sure sensitivity has posed a tough technical chal- lenge since it is inversely proportional to the square of the diaphragm thickness. This makes the control of diaphragm thickness very critical to achieving high precision pressure sensors.

  To minimize pressure sensitivity variation caused by across wafer variation of the diaphragm thick- ness, many solutions have been advanced. Some have gone to the silicon on insulator (SOI) structure with the hope of obtaining a thin and flat diaphragm along with thickness uniformity Some have gone to a more sophisticated approach like the electrochemical etch stop. This is a technique that provides a rapid etch region in a substrate material formed by a deep impu- rity difision with a high surface concentration. When differentially etched, that rapid etch region rapidly etches away leaving the other portions of the sub- strate intact.

  Evaluation ofall available methods to processing pressure sensors with higher sensitivity calls for thin- ner diaphragms. With current techniques, as the dia- phragm thickness becomes thinner, we accentuate the problems with diaphragm strength, diaphragm rupture and control, manufacturability issues and wafer breakage.

Based upon the problems encountered so far with

the various methods of diaphragm control, it was necessary to develop a simple cost effective method ofprocessing high precision pressure sensors.

  A controllable silicon wet etching process that is capable ofetching silicon films without the neces- sity for protective measures on the opposite surface was used to etch cavities in the silicon substrates. One of the surfaces of the wafer is protected by a curtain of nitrogen while the opposite side is being etched. An outward nitrogen flow keeps chemicals

from curling around the wafer to damage the non etched surface. This eliminates the need for protec-

tion using photoresist tape (or other current protec- tive methods) and all the~:associated processing steps for deposition and removal ofthese protective films.

  The etch profile, roughness of the surface, and etch rate can be judiciously selected by varying the composition of the etch~ mixture. The etch rate of the silicon etch is determined by the composition of the solution. Most silicon etches utilize a HF/HN03 based solution. For this high p...