THE STABILITY OF SI1-XGEX STRAINED LAYERS ON SMALL-AREA TRENCH-ISOLATED SILICON

The combined effects of isolation stress, active area size, and SiGe m isfit strain on dislocation generation in an advanced SiGe heterojunct ion bipolar transistor (HBT) process were studied. Eight-inch wafers w ere patterned with polysilicon-filled deep, and oxide-filled shallow t rench isolation similar to that used in IBM's analog SiGe HBT technolo gy, Half of the wafers were subjected to an additional stress-producin g oxidation prior to SiGe growth. Si1-xGex films containing 0, 5.5, 9, and 13 at. % Ge were grown epitaxially by ultrahigh vacuum chemical v apor deposition (UHV CVD). The films were of constant thickness with a n intrinsic Si cap. Some samples received an additional relaxation ann eal following deposition. After the growth and anneal cycles, the disl ocation density was determined by transmission electron microscopy (TE M). On nonstressed samples, no dislocations were observed in the devic e areas, even at Ge concentrations which are not stable to misfit disl ocation generation in blanket form. This small area effect has been ob served on patterned substrates that do not have functional device isol ation. On the stressed-isolation wafers, the compressive stress from t he oxidation of the trench sidewalls was found to intensify stress in the SiGe films, and to lower the critical strain at which misfit dislo cations appeared. In large active areas on these wafers, two distinct dislocation regions were observed. Defects at the edge resembled those caused by isolation stress, while the defects in the center were more typical of the misfit dislocations associated with lattice-mismatch e pitaxial films. It is clear that isolation stress must be minimized wh en fabricating integrated circuits using SiGe epitaxial films. It is a lso evident that SiGe films grown on nonstressed isolation exhibit the same increase in critical thickness with decreasing lateral dimension that has been observed on much simpler patterned substrates.