Slide 16 of 64
This slide shows an early test wafer run which was used to confirm whether CAD tools were predicting circuit speed accurately (they were not). A key die is the RPI register file and adder ring oscillator test chip. Eventually adjustments were required to operate the GaAs HBTs at currents which would avoid dopant redistribution current density limits in the emitter. Anisotropy in the polyimide dielectric also caused some disappointments. These problems were designed around eventually, but this redesign effort took an additional 3 years. Modeling accuracy is the key to success.