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Polysilicon Mask for Porous Anodic Etching

IP.com Disclosure Number: IPCOM000084252D
Original Publication Date: 1975-Oct-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 1 page(s) / 11K

Publishing Venue

IBM

Related People

Aboaf, JA: AUTHOR [+3]

Abstract

In some electronic applications, it is important to make some areas of a silicon wafer porous. This can be done easily on semiconductor wafers where pockets are diffused with n+ (p+) dopant. The n+ (p+) areas will be made porous while the unmasked N (P) [N (P) type wafers used] areas will not be made porous. Porous areas can be made by anodization of silicon using a solution of hydrofluoric acid as an electrolyte; a current density of 5ma/cm/2/ or higher can be used.

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Polysilicon Mask for Porous Anodic Etching

In some electronic applications, it is important to make some areas of a silicon wafer porous. This can be done easily on semiconductor wafers where pockets are diffused with n+ (p+) dopant. The n+ (p+) areas will be made porous while the unmasked N (P) [N (P) type wafers used] areas will not be made porous. Porous areas can be made by anodization of silicon using a solution of hydrofluoric acid as an electrolyte; a current density of 5ma/cm/2/ or higher can be used.

This is however limited by the usual depths obtained in p+ diffusion, i.e., 2.3 microns. For very deep pockets (p+ diffusion would take a very long time), it is necessary to have a good mask on the parts of the silicon wafer that should not be attacked. Polysilicon films of very high resistivity perform well as a mask against porous anodic etching.

The following process can be used for masking very deep pockets, up to the thickness of an entire wafer. 1. Polysilicon deposition by chemical vapor deposition at about 750 degrees C to a thickness of 3000 angstroms. 2. Conventional etching of polysilicon film using standard photolithographic techniques. 3. Porous formation of pockets delineated by the polysilicon mask.

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