Effect of adatom surface diffusivity on microstructure and intrinsic stress evolutions during Ag film growth

Abstract

The effect of the adatom surface diffusivity on the evolution of the microstructure and the intrinsic stress of thin metal films was investigated for the case of growth of polycrystalline Ag films on amorphous SiO 2 (a-SiO 2) and amorphous Ge (a-Ge) substrates, with high and low Ag adatom surface diffusivity, respectively. The surface diffusivity of the deposited Ag adatoms on the aGe substrate is suppressed also after coalescence of Ag islands due to the continuous (re)segregation of Ge at the surface of the growing film as evidenced by in-situ XPS. An assessment could be made of the role of adatom surface diffusivity on the microstructural development and the intrinsic stress evolution during film growth. As demonstrated by ex-situ TEM and ex-situ XRD, the Ag films grown on the a-SiO 2 and aGe substrates possess strikingly different microstructures in terms of grain shape, grain size, and crystallographic texture. Nevertheless, the real-time in-situ stress measurements revealed a compressive ! tensile ! compressive stress evolution for the developing Ag films on both types of substrates, however on different time scales and with stress-component values of largely different magnitudes. It was concluded that (i) the microstructural development of metallic thin films is predominated by the surface diffusivity of the adatoms and (ii) the intrinsic stress evolution is largely controlled by the developing microstructure and the grain-boundary diffusivity. V

Atom insertion into grain boundaries and stress generation in physically vapor deposited films

“…[1][2][3] The origin and evolution of intrinsic stresses in physical vapor deposited polycrystalline films have been thoroughly investigated during the past decades, 1,3,4 highlighting the role of grain boundaries for stress generation. [5][6][7][8][9][10][11] Films deposited at homologous temperatures, T s /T m , well below 0.2 tend to be in a tensile stress state due to attraction between neighboring grains over underdense grain boundaries, 5,6 formed as a result of the limited mobility of film forming species (adatoms) at these conditions. 12 On the other hand, when deposition is performed at relatively high T s /T m (typically above 0.2), adatoms have sufficient mobility for dense films to form, 12 which is accompanied by generation of a compressive growth stress after formation of a continuous film.…”

“…12 On the other hand, when deposition is performed at relatively high T s /T m (typically above 0.2), adatoms have sufficient mobility for dense films to form, 12 which is accompanied by generation of a compressive growth stress after formation of a continuous film. 1,4,10 This phenomenon has been explained based on results from in situ monitoring of intrinsic stress evolution during and after deposition, growth simulations, and analytical models 8,9,[13][14][15][16][17] to be the effect of diffusion of adatoms into grain boundaries driven by a chemical potential difference between the latter and the film surface. 8,9 However, direct experimental evidence for insertion of film forming species into grain boundaries is not available in the literature, since this would require observation of atoms exiting grain boundaries after deposition is ceased.…”

Atomistic view on thin film nucleation and growth by using highly ionized and pulsed vapour fluxes

“…When deposition is performed at relatively high Ts/Tm (typically above 0.2) adatoms have sufficient mobility for formation of dense films to occur [3]. This is accompanied by generation of a compressive growth stress after formation of a continuous film which relaxes on deposition interruptions [67,80] as seen in the left panel in Fig.1. Using in situ monitoring of intrinsic stress evolution during and after deposition, growth simulations and analytical models, a number of research groups [77,78,[90][91][92][93][94][95] have attributed compressive stress generation to diffusion of adatoms into grain boundaries.…”

“…Compressive stresses are typically observed before film coalescence and it is widely agreed upon that they originate from Laplace pressure leading to a smaller than bulk lattice spacing frozen in during island growth [73,74]. During the coalescence stage neighbouring grains deform elastically to form grain boundaries as it is favourable to trade surface energy for strain and grain boundary energy [75][76][77][78][79][80][81][82][83][84]. This process causes tensile stresses ( Fig.1) [75][76][77][78][79][80][81][82][83][84].…”

“…During the coalescence stage neighbouring grains deform elastically to form grain boundaries as it is favourable to trade surface energy for strain and grain boundary energy [75][76][77][78][79][80][81][82][83][84]. This process causes tensile stresses ( Fig.1) [75][76][77][78][79][80][81][82][83][84]. This stress state prevails after the completion of the island coalescence in the continuous growth stage when films are deposited at relatively low homologous temperatures, Ts/Tm (typically well below 0.2), that favour the formation of columnar structures with underdense grain boundaries, i.e., porous grain boundaries with mass density significantly smaller to that of the grains (see right panel in Fig.1) [3].…”