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 a technique1 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 secret to manufacturing such high resistivity CZ crystal lies in an annealing process that precipitates most of the oxygen that is incorporated in the crystal during the growing process. CZ crystal that is grown without the addition of a dopant will have inherently high resistivity, but this resistivity drops quickly as the material is heated and the oxygen atoms generate thermal donors. SEH's patented Full Oxygen Precipitation (FOP) cycle effectively lowers the interstitial oxygen concentration to less than 5 ppma, eliminating it as a donor even after prolonged heating at 450° C. (See Fig. 1)

The high level of oxygen precipitation also provides significant gettering (IG) benefits. Additional benefits of the FOP anneal include greater strength and slip resistance because dislocations are trapped by the oxygen precipitates.

Fig. 1