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Process for Maintaining H(3)PO(4) Concentration

IP.com Disclosure Number: IPCOM000081106D
Original Publication Date: 1974-Apr-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Pridans, JV: AUTHOR [+2]

Abstract

Silicon nitride layers used in the formation of certain semiconductor devices are etched using phosphoric acid with a silicon dioxide mask on a silicon wafer. To maintain the concentration of the phosphoric acid for a given boiling temperature, a steam bath is provided over the acid solution during the etching process.

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Process for Maintaining H(3)PO(4) Concentration

Silicon nitride layers used in the formation of certain semiconductor devices are etched using phosphoric acid with a silicon dioxide mask on a silicon wafer. To maintain the concentration of the phosphoric acid for a given boiling temperature, a steam bath is provided over the acid solution during the etching process.

In the present process, the boiling rate of the solution is controlled. A temperature controller is used to maintain the set temperature of the phosphoric acid. The steam input is at a rate to compensate for evaporation losses, and a moderate amount of constant cooling is provided to the tank cover which has partial openings to the atmosphere, e.g., 150 watts cooling for 600 watts heater input for approximately two liters of acid. The amount of exhausted steam is adjusted to equal the steam input rate, minus the amount of steam lost to the atmosphere through the tank cover openings.

A certain heat input is necessary to maintain the acid solution at a given boiling temperature, e.g., 180 degrees C. Beyond this value, excess heat is necessary to evaporate the water condensing out of the steam on the inside of the tank cover and refluxing into the solution, which is in excess of the amount of water necessary to maintain the solution concentration. The greater the excess refluxing, the greater is the boiling rate of the solution and, hence, by judiciously selecting the variable parameters, the boiling rate and, hence, concentration of the solution can be controlled. For examp...