Ordinarily, you use (fuming) nitric acid. This dissolves the epoxy packaging, but leaves silicon and metal intact. You need the HF to etch the top layers of m

Ordinarily, you use (fuming) nitric acid. This dissolves the epoxy packaging, but leaves silicon and metal intact. You need the HF to etch the top layers of metal/silicon from the die. These layers are above the actual circuitry, and help increase security so that an attacker can't steal IP by using nitric acid and microprobes or UV light to modify the operation of the IC. Flylogic has a special technique by which they selectively etch areas of the metal layers, but not the circuitry below.