Diamond and Related Materials 6 ( 1997) 1759- I771 Simulation of morphological instabilities during diamond chemical vapor deposition P. Mahalingam, D.S. Dandy * Departnlent of Chemical Engineering, Colorado State University, Fort Collins, CO 80523, USA Received 7 January 1997; accepted I5 May 1997 Abstract The diamond chemical vapor deposition (CVD) process has been investigated theoretically and the morphological instabilities associated with the growth of diamond films have been examined with a model based on the continuum species conservation equation coupled to surface reaction kinetics. A linear stability analysis and numerical calculations have been carried out to determine critical parameters affecting the diamond deposition layer morphology. A two-dimensional model describes the evolution of the gas-solid interface. The dynamic behavior of the interface depends on the reactants’ diffusivity and surface kinetics. These factors depend upon the reactant material properties and film growth conditions such as the reactor temperature and pressure. From the analyses, it has been found that the ratio (2/k) of gas phase diffusivity (9) to the surface reaction rate constant (k) plays the critical role in promoting diamond morphological instabilities because the film morphology stabilizing processes of surface diffusion and re-evaporation are absent or negligible during diamond CVD. It is found that the film nonuniformity increases as the ratio (2/k) decreases. Increasing growth rates also result in increasing morphological instability. leading to rough surfaces. It is shown that increasing reactor pressure and decreasing gas-phase temperature and/or substrate temperature promote deposition layer nonuniformity. An approach to avoiding these instabilities is proposed. 0 1997 Elsevier Science S.A. Kew~ords: Diamond growth; Morphology evolurion; Modeling 1. Introduction The synthesis of diamond at low pressures is a current topic of intense worldwide interest due to the inherent physical properties of this material [I]. The feasibility of synthesizing diamond at low pressures enables the exploitation of these properties in a large range of applications such as semiconductor devices, cutting and grinding tools, and windows for visible and infrared transmission. Diamond films may be synthesized using a variety of low-pressure techniques whereby mixtures of hydro- carbons such as methane, and hydrogen are activated with energetic sources such as microwaves to generate plasmas, direct current (DC) glow discharges. hot fila- ments, combustion flames and arcjets. Broadly speaking, diamond synthesis processes may be classified by rates at which diamond can be synthesized. The low growth rate methods include the hot filament [2] and many types of microwave plasma [3], which typically have * Corresponding author 0925-9635/97/$17.00 0 1997 Elsevier Science S.4. All rights reserved. PII SO925-9635(97)00135-O growth rates less than 1 pm h ‘. whereas the high growth rate techniques, such as DC or radio frequency (RF) arc discharges [4] and atmospheric combus- tion synthesis devices [5]. have growth rates of 30-l 50 pm h - ‘. Ideally, it is desirable to obtain the highest deposition rate possible, but it has been found that the film uniformity often decreases with increasing deposition rate. A “fingerlike” morphology develops as a result of surface instabilities which accompany higher deposition rates of diamond [6] and these instabilities are magnified with time. The control of film quality and morphology is strongly coupled to the control of condi- tions in the deposition chamber, that is. the processing time. reactor throughput. and related economic consid- erations. The characterization of diamond film morphol- ogy and its control have become increasingly important because the thermal and electrical properties of a poly- crystalline diamond film is adversely affected by factors such as porosity, inhomogeneities and inclusions, and zones of differing morphology. In this work, a numerical model is developed ‘0 analyze the factors which control diamond film morphol-