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Formation of Porous Silicon in Dilute HF Solutions

IP.com Disclosure Number: IPCOM000085648D
Original Publication Date: 1976-May-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Burkhardt, PJ: AUTHOR

Abstract

Present porous silicon technology teaches the use of aqueous HF solutions containing 10% or greater HF by weight. Although 10% is not a critical number, films grown below 10% are expected to be grown at very low-current densities and to have higher porosities than those grown at equivalent current densities, but at higher HF concentrations. Further, as the current density is reduced the growth rate decreases.

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Formation of Porous Silicon in Dilute HF Solutions

Present porous silicon technology teaches the use of aqueous HF solutions containing 10% or greater HF by weight. Although 10% is not a critical number, films grown below 10% are expected to be grown at very low-current densities and to have higher porosities than those grown at equivalent current densities, but at higher HF concentrations. Further, as the current density is reduced the growth rate decreases.

It has been found that when dilute HF solutions are acidified by the addition of hydrochloric acid, suitable porous films can be produced at HF concentrations far below the 10% level. The following table shows the behavior of several types of HF-HCl electrolytes on P+ silicon at a current density of 10 ma./cm/2/. The pH of the solution is estimated from the concentrations and the HF equilibrium constants. Ionic Growth Rate

Composition pH Strength %HF Porosity(%) (Mu/min) HF-H(2)O -0.05 0.9 26.2 18 1.5 HF-H(2)O 1.2 0.17 1.9 etch --- HF-NaCl-H(2)O 1.2 5.8 1.9 etch --- HF-HCl-H(2)O -0.77 5.8 1.7 52 0.54 HF-HCl-H(2)O -1.1 11.7 1.6 41 0.6O HF-HCl-H(2)O -1.1 --- 0.74 57 0.43 HF-HCl-H(2)O -1.1 --- 0.49 85 0.29.

There is the obvious safety advantage of using more dilute HF solutions. Also, the durability of masking materials which may be used is greatly enhanced. For example, a silicon nitride mask etches more than ten times as fast in 25% HF than it does in 3% HF. This means that not only can a thinner mask be used...