Impact of germanium doping on the mechanical strength of low oxygen concentration Czochralski silicon wafers

ABSTRACT

The lack of interstitial oxygen for locking and pinning of dislocations has made the ultra-high resistivity silicon wafers for radio frequency applications prone to slip and warpage during electronic device manufacturing. In this work, we investigate the role of germanium doping on the dislocation mobility in very low oxygen concentration, Czochralski-grown silicon wafers. Mechanical bending tests are used to study the average dislocation velocity and unlocking stress for germanium-doped, nitrogen-doped, and undoped silicon wafers with similar oxygen levels. In contrast to the previously reported effect of germanium on retarding dislocation motion in high oxygen concentration silicon wafers, we find that germanium by itself at a high concentration (7–9 × 10¹⁹ atoms/cm³) does not lock dislocations by solid solution strengthening, nor does it appreciably change the oxygen dislocation locking behaviour in silicon. The lack of dislocation pinning and locking effects by germanium in silicon is explained from the small lattice mismatch (4%) and low chemical interactions of substitutional Ge atoms with silicon comparing to interstitial impurities, such as oxygen and nitrogen.

KEYWORDS:

• Silicon
• germanium
• doping
• dislocation mobility
• dislocation locking
• wafer strength
• mechanical bending

Acknowledgements

We would like to thank MEMC LLC, a GlobalWafers Company, for providing the silicon wafers and the bending fixture used in this work. The authors also acknowledge financial support from Washington University in St. Louis and the Institute of Materials Science and Engineering for the use of the SEM and staff assistance.

Disclosure statement

No potential conflict of interest was reported by the authors.