STRESSES AND STRAINS IN LATTICE-MISMATCHED STRIPES, QUANTUM WIRES, QUANTUM DOTS, AND SUBSTRATES IN SI TECHNOLOGY

We discuss recent advances made in the theory and measurements of stre sses and strains in Si-based heterostructures containing submicron- an d micron-size features. Several reports on theoretical as well as expe rimental studies of stresses in the substrates with local oxidation of silicon structures on the surface have been published recently. With the advent of GexSi1-x strained layers and stripes extensive studies o f both the stripe and the substrate stresses have also been made. Unli ke the previous calculations and analytical models, recent finite elem ent (FE) calculations take into account the coupling between the film- substrate stresses without making the approximation that the interface is rigid or that there is no variation of stresses in the stripes in a direction perpendicular to the interface. The results of these calcu lations have been compared with the analytical models and limitations of the analytical models have been pointed out. Micro-Raman measuremen ts of the stresses in the stripes, quantum wires, quantum dots, and su bstrates have been made. The measured values of stresses in GeSi strip es and quantum structures agree well with the calculated values by the FE method. The micro-Raman measurements showed that as the ratio R=2l /h (2l is the width and h is the thickness of the stripe) decreases, t he shape of the measured normal stresses in the substrate under the st ripe (plotted in a direction parallel to the interface) changes dramat ically, from concave upward to convex upward. Generation of dislocatio ns in laterally small layers is also discussed briefly. FE calculation s of trench-induced stresses which include the effect of the anisotrop y of Si have also been made recently. In these calculations realistic experimental conditions were simulated to determine the oxide shape, o xide-interface stresses, and intrinsic and thermal stresses of the pol ysilicon fill. These values were then used as inputs for the FE calcul ations. Calculations of stresses induced by oxide-filled trenches were also made assuming that Si is isotropic and that the oxide fill has t he same elastic constants as Si. These calculations and results of an earlier analytical model implemented under the same assumptions gave i dentical results; however, the calculated stress values were in error of 20%-30%. The maximum resolved shear stress for the 60 degrees dislo cation induced by a trench is 30% more if it is aligned in [110] direc tion rather than in the [100] direction. This explains the common obse rvation that the [100]-oriented trenches cause fewer dislocations than the [110] trenches. The characteristics of trench isolated as well as junction isolated bipolar transistors have been studied. The trench i solated transistors had 20% higher gain; however, the collector-base c apacitance was higher by up to 50% in the trenched transistors. The in crease in capacitance was caused by the anomalous diffusion of the ant imony dopant from the buried collector layer induced by the stress fie ld of the trenches. The effect could be eliminated by increasing the d epth of the trench. The trenched devices also had higher emitter-colle ctor leakage current caused by the dislocations generated by the trenc h induced stress field. (C) 1996 American Institute of Physics.