Process Metrology for Ultrathin Gate Dielectrics

One of the major challenges is to be able to grow and accurately measure such thicknesses of gate oxides. In addition, the gate oxide thickness may change during the polysilicon deposition, which is important to measure from the device point of view. The use of variable angle spectroscopic ellipsometer (VASE) has been shown to provide a technique for directly measuring the thickness of the oxide underneath polysilicon. The objective of this study Is to use VASE (energy range from 0.78 eV to 6 eV, which corresponds to wavelengths from 200 to 1600 nm) to measure ultra thin silicon dioxide layers with and without polysilicon. Thin oxides have been grown under different experimental conditions in a furnace. The first set of wafers had only native oxide, the second set of wafers followed a five minutes soak, and the third one followed a ten minutes soak at 750 C in dry oxygen. A 430 nm polysilicon layer has been deposited on these wafers by low pressure chemical vapor deposition (LPCVD). To extract useful information, a model is constructed that describes the sample structure. It has been possible to measure thin oxides without the polysilicon. With polysilicon, however, problems are encountered. The optical constants change significantly with polysilicon processing conditions and the model must be adjusted to take into account these changes. The effective media approximation (EMA) is often used that describes mixing of two phases — crystalline and amorphous. After experiment, we found that something changed in the oxide during the polysiicon deposition. It can either be the thickness of this oxide, or some contaminants can be incorporated between the oxide and the polysilicon, changing the refractive index