Rate of Dissolution in Aqueous HF

Bergman (232) conducted extensive measurements on the rates of solution of quartz and tridymite in 0.1 and 1.0 A/ HF solution at 25 C. Reasonably reproducible rates expressed as milligrams per square meter per hour were obtained using particles in the micron size range. Tridymite dissolved in 0.1 M HF about seven times faster than quartz and vitreous silica about 45 times faster, but in I.O A/ HF there was little difference. 

I M HF, cristobalite and tridymite dissolved about 10 times faster than quartz and vitreous silica 100 times faster, based on unit surface area (233). 

There have been several studies of the etching rate of HF on silica in relation to processes in the electronics industry. A patented mixture of HF and NH F gives a desirable etching rate for thin films (234). 

Judge (235) concludes that the rate of attack of HF on SiOj films is due to HF and HF, the former acting four to five times faster than the latter. Pliskin and Lehman (236) uses a solution of 2.26% HF and 2.15% HNO, to etch Si02 films on silicon. The film formed by oxidation at 1000°C, which is probably nonporous. is etched at the rate of 2 A sec , which is 1600 mg m hr . Using vitreous silica spheres. Leko measured the rates of dissolution in HF solutions up to 20% concentration and at 20-80 C. Conditions for removing a certain thickness of silica can be calculated from the data (237). 

A fundamental study of the attack of HF on vitreous silica, preceded by a literature review, was published by Blumberg and Stavrinou (238). They developed equations for the reaction rates and determined the velocity constants. At 32°C, the typical value was = 5 x I0 g SiOj see cm Mhf. 

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