HIROSHIMA, July 5 -- Micron Technology Inc. (NASDAQ: MU) has spent the better part of three years watching SK Hynix collect the richest margins in memory chips almost alone. On Saturday, in a ceremony at its existing Higashi-Hiroshima plant, Micron finally put a number on how much it costs to stop watching: 1.5 trillion yen, roughly $9.3 billion.

The money buys 28,000 square meters of new cleanroom space built specifically to make high-bandwidth memory, the stacked chips that sit next to processors like Nvidia's and feed them data fast enough to keep up with AI workloads. Tokyo is covering roughly a third of it. Japan's Ministry of Economy, Trade and Industry has committed up to 500 billion yen, about $3.2 billion, on top of subsidies it...