{HEAD ~ The first step} of {TAIL ~ the contact etch} is accomplished using a boron trichloride BCL3 etch gas which selectively etches BPSG without etching the underlying silicon nitride 
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{ HEAD ~ The first step } of { TAIL ~ the contact etch } is accomplished using a boron trichloride BCL3 etch gas which selectively etches BPSG without etching the underlying silicon nitride
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