Reduction of intrinsic stresses during the chemical vapor deposition of diamond
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Reduction of intrinsic stresses during the chemical vapor deposition of diamond Sumit Nijhawan, Susan M. Jankovsky, and Brian W. Sheldon Division of Engineering, Brown University, Providence, Rhode Island 02912
Barbara L. Walden Physics Department, Trinity College, Hartford, Connecticut 06106 (Received 18 July 1997; accepted 8 July 1998)
Intrinsic stresses which arise during the chemical vapor deposition (CVD) of diamond were controlled by multistep processing. Film stresses (thermal and intrinsic) were measured with the bending plate method. The thermal stresses are compressive and arise due to the mismatch in thermal expansion coefficient between the film and substrate. The dominant intrinsic stresses are tensile and evolve during the deposition process. These stresses increase with deposition time. An intermediate step consisting of annealing the film when the diamond crystallites are only partially coalesced reduces the intrinsic stress by more than 50%. Annealing at longer growth times (i.e., after complete coalescence) does not produce large reductions in intrinsic stress. Our results are consistent with stress generation due to the formation of nonequilibrium grain boundary structures. The intermediate annealing step does not produce a large, direct stress reduction; instead, it alters the film microstructure in some subtle way which reduces stress generation during subsequent growth.
I. INTRODUCTION
The potential applications of diamond films include optical windows, electronic components, heat sinks for electronic devices, and protective coatings. A serious impediment to these applications is the large internal stresses that develop during growth. These stresses are detrimental because they can cause debonding between film and substrate or induce an unacceptable amount of surface curvature. The internal stresses that arise during processing consist of thermal and intrinsic components. The thermal stress is generated during cooling by the difference in the thermal expansion coefficient of the film and substrate. On Si substrates these stresses are compressive and can readily be calculated using T
sth
Zd Ta
sDad
E dT , s1 2 nd
(1)
where Eys1 2 nd is the biaxial modulus of the film, Da is the difference in the thermal expansion coefficient of the film and substrate, and Td and Ta are the deposition and ambient temperatures, respectively. Calculated values of thermal stress on Si substrates are shown in Table I.1–4 The intrinsic component of stress is associated with the growth mechanism and is independent of thermal expansion effects. Although these stresses have been observed by several researchers,1–7 there is considerable 1046
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J. Mater. Res., Vol. 14, No. 3, Mar 1999
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debate about what causes them. As noted above, the thermally induced stresses are readily predicted. However, it is generally more difficult to analyze intrinsic stresses in polycrystalline films since a variety of different mecha
