Surface Kinetics and Uniformity Issues in Low-Temperature Epitaxy of Sige and Si:P Layers
Authors: Smirnov Andrey , Shustrov Yury , Zimina Daria , Galyukov Alex 
Published: December 2024

Fabrication of the modern Si/SiGe-based devices normally necessitates the application of reduced temperatures (in the range about 550-750 °C) during the growth, which imposes significant kinetic limitations. As a result, the growth rate, layer composition, and dopant incorporation show high sensitivity to the process parameters and even small temperature non-uniformities over the wafer may result in considerable variation of the SiGe layer thickness and composition.

In the growth of Si:P layers, the intrinsic Si growth kinetics is supplemented with the influence of the P concentration on the Si:P growth rate that may differ significantly as compared to the undoped Si despite P incorporation in concentrations of 0.1 % and even lower. In addition, the trend of the growth rate variation (increase or decrease) is different at the atmospheric and reduced operating pressures.

Under all these inputs, control of the thickness and composition uniformity over the 300 mm wafer becomes challenging and should involve the adjustment of both temperature distribution and process parameters, including zonal distribution of the gas flow rates.

To approach the solution of this problem, we have developed models of SiGe growth from SiH4/SiH2Cl2 (DCS) and GeH4 as well as Si:P growth from DCS and PH3 in the atmosphere of hydrogen with possible addition of HCl. The models account for high coverages of the surface with the adsorbed H2 and HCl, resulting in the growth limitation by low density of the free adsorption sites (kinetically limited growth mode). In case of SiGe, the model accounts also for the dependencies of surface kinetic parameters on the Ge concentration in the layer.

The models were carefully verified using numerous experimental data on the layer growth rate and composition versus temperature and precursor flow rates. Calculations with the developed models reproduce the effect and facilitate interpretation of the following experimentally observed trends: higher growth rate and lower Ge concentration at elevated temperatures and the opposite tendency at a higher HCl supply. This behavior is explained in terms of stronger kinetic limitations for the Si part of the alloy that weaken with the temperature increase and contribution of the added HCl to the surface site blocking.

The strong and non-monotonic dependence of Si:P growth rate on the PH3 supply is attributed to the competition between the adsorbed HCl and P-containing species. Addition of PH3 to the reactive gas mixture results, on the one hand, in additional coverage of the surface with these species but, on the other hand, in removal of the adsorbed HCl from the surface due to its interaction with these species, followed by the formation of volatile products.

Next, the developed approaches have been applied to the detailed modeling of the process in widely used industrial reactors to address the uniformity issue in dependence of the operating parameters and reactor settings. In particular, variation of the uniformity for different temperature distributions over the wafer and gas injection parameters will be discussed. Also, conclusions are made on the relative sensitivity of the thickness and composition to possible non-uniformities of the wafer temperature and details of species transport inside the reactor.

Figure 1