Mems process and device

Abstract

A method of fabricating a micro-electrical-mechanical system (MEMS) transducer comprises the steps of forming a membrane on a substrate, and forming a back-volume in the substrate. The step of forming a back-volume in the substrate comprises the steps of forming a first back-volume portion and a second back-volume portion, the first back-volume portion being separated from the second back-volume portion by a step in a sidewall of the back-volume. The cross-sectional area of the second back-volume portion can be made greater than the cross-sectional area of the membrane, thereby enabling the back-volume to be increased without being constrained by the cross-sectional area of the membrane. The back-volume may comprise a third back-volume portion. The third back-volume portion enables the effective diameter of the membrane to be formed more accurately.

What is claimed is: 
     
         1 . A micro-electrical-mechanical system (MEMS) transducer, comprising:
 a substrate;   a membrane formed on a first side of the substrate; and   a cavity which extends between a first opening in the first side of the substrate and a second opening in the second side of the substrate;   wherein the cross sectional area or diameter of the second opening is greater than the cross-sectional area or diameter of the first opening respectively.   
     
     
         2 . A MEMS transducer as claimed in  claim 1 , wherein the cavity comprises a first cavity portion and a second cavity portion, the first cavity portion being separated from the second cavity portion by a step in a sidewall of the cavity. 
     
     
         3 . A MEMS transducer as claimed in  claim 2 , wherein the step comprises substantially a right angle. 
     
     
         4 . A MEMS transducer as claimed in  claim 2 , wherein the step comprises a discontinuity in the cross-sectional area of the back volume in a plane parallel to the substrate. 
     
     
         5 . A MEMS transducer as claimed in  claim 2 , wherein the step is curved. 
     
     
         6 . A MEMS transducer as claimed in  claim 2 , wherein the step comprises a change in the gradient of the sidewall. 
     
     
         7 . A MEMS transducer as claimed in  2 , wherein the first cavity portion has a cross-sectional area that is smaller than the cross-sectional area of the membrane in a plane where the first cavity portion and the membrane meet. 
     
     
         8 . A MEMS transducer as claimed in  claim 2 , wherein the cross-sectional area of the second cavity portion is greater than the cross-sectional area of the membrane. 
     
     
         9 . A MEMS transducer as claimed in  claim 2 , wherein the first cavity portion has substantially vertical walls. 
     
     
         10 . A MEMS transducer as claimed in  claim 2 , wherein the first cavity portion has sloped walls. 
     
     
         11 . A MEMS transducer as claimed in  claim 2 , wherein the second cavity portion has substantially vertical walls. 
     
     
         12 . A MEMS transducer as claimed in  claim 2 , wherein the second cavity portion has sloped walls. 
     
     
         13 . A MEMS transducer as claimed in  claim 1 , wherein the shape of one or both of the first opening and the second opening may be circular, square or rectangular. 
     
     
         14 . A MEMS transducer as claimed in  claim 1 , wherein the membrane overlies the back volume. 
     
     
         15 . A communications device comprising a micro-electrical-mechanical system (MEMS) transducer as claimed in  claim 1 . 
     
     
         16 . A portable telephone device comprising a micro-electrical-mechanical system (MEMS) transducer as claimed in  claim 1 . 
     
     
         17 . A package comprising a MEMS transducer as claimed in  claim 1 . 
     
     
         18 . A package as claimed in  claim 17 , wherein the package comprises a substrate having a recess, and wherein the MEMS transducer is situated in said recess. 
     
     
         19 . A package as claimed in  claim 17 , wherein the package comprises a lid. 
     
     
         20 . A package as claimed in  claim 17 , wherein the package forms an RF cage.