Back when silicon prices were north of $400/kg, finding ways to use less material was seen as a keen opportunity, from different formfactors to thinner wafers. One group of efforts centered on "upgraded metallurgical-grade" (UMG) silicon, which was essentially a grade of silicon slightly less than the ultrapurity required by the electronics industry. Those efforts broadly lost their appeal, though, once silicon prices plunged to $20/kg and below, though prices have started showing signs of rising slightly.