Clean Metal Oxide Semiconductor Systems

Aspects of sodium contamination of metal-oxide-silicon (MOS) systems associated with silicon surface preparation, the oxidation plenum, and post-oxidation procedures, including contact application, are discussed. The validity of neutron activation analysis to determine sodium distributions in MOS samples is established. Refractory materials for furnace construction are examined and shown to contain as much as 1 - 5 percent sodium or potasium. Use of multiwall quartz furnace tubes as well as cold wall r-f heated systems is shown to give low sodium content oxides. Sodium levels as low as 4×1015 atoms/cc in the oxide, have been attained. Silicon crystals are shown to contain ¿ 1014 atoms-Na/cc. A typical chemically polished silicon surface generally retains an oxide contaminated at ¿ 1013 atoms-Na/cm2. This native oxide is sufficient to account for the shape of the sodium distribution, as well as the amount of sodium found in thermal oxides. Additional discussion is made of the impurity levels associated with contact application. The combination of oxidation and contact procedures is shown to yield MOS samples which suffer no change in C-V characteristics after stress at 300° C and 1 x 106 V/cm applied across the oxide.