High Resistivity CZ Crystal for RF SOC Applications
The integration of RF circuits into CMOS devices favors a shift to a very high silicon substrate resistivity. RF circuitry requires linear, analog devices with low noise and precision passive components (e.g., resistors, capacitors and inductors). Very high substrate resistivity decreases capacitively-coupled cross-talk between digital, analog and RF components, improving noise isolation. High- res silicon also improves the quality factor of spiral inductors by decreasing eddy current losses.
Float Zone crystal has typically been the only silicon option for applications requiring a resistivity greater than 1,000 ohm.cm, but FZ crystal is not currently readily available at 200/300mm. SEH has now developed and patented several techniques to manufacture CZ crystal with a resitivity of greater than 1,000 ohm.cm, and these wafers are available in production quantities today. (U.S. patent 6,544,656B1)
The main issue in High Res CZ is the management of Oi. High levels of Oi lead to thermal donor generation which creates unstable resistivity in the temperature range 350°c to 500°c common in back-end processes.
Oi must be reduced by special prescription thermal cycles (ref. U.S. patent #6,544,656) or the Oi level must be kept low by MCZ pulling processes