Stress Engineering of Dielectric Films on Semiconductor Substrates

International audienceDielectric thin films deposited by plasma enhanced chemical vapor deposition (PECVD) have beenextensively studied over the last decades due to their interesting optical and electrical properties besidestheir many applications in microelectronic and optoelectronic devices. Recently published studies haveshown the impact of the mechanical properties of amorphous dielectric films on semiconductor substrates[1]. In strain engineering, stressed films are used to control on demand the physical properties ofsemiconductors at the surface such as bandgap energy, dielectric constant, and refractive index. We aimto study in this work how to control the distribution of the strain field beneath a dielectric film and how thechanging of the residual stress affects the physical properties of the dielectric film itself. We depositedhydrogenated amorphous silicon nitride a-SiN:H films on Si, InP, and GaAs substrates using a capacitivelycoupled plasma reactor CCP-PECVD with a radiofrequency (RF) power at 13.56 MHz.The a-SiN:H filmswere deposited at 280 °C, with a thickness of approximatively 500 nm, using a SiH4/NH3/N2/Ar precursor mixture. The RF power injected into the plasma allows a tunable residual stress and a wide range of built-in stress, from tensile (+ 300 MPa) to compressive (– 400 MPa). To evaluate the residual stress in ourdeposited thin films, we used the standard method of wafer curvature measurements. The thickness andthe refractive index were characterized by variable angle spectroscopic ellipsometry (VASE). Thedetermination of Young’s modulus and hardness of the a-SiN:H films was performed by nanoindentation.We noticed that the adjustment of the residual stress leads to the modification of the film in terms ofoptical and mechanical properties. In order to investigate the deformation induced in the semiconductor,an understanding of the semiconductor mechanical behavior on a microscopic scale is required. Thus, weperformed a detailed investigation of the effect of strain on the degree of polarization (DOP) of thephotoluminescence signal on direct bandgap substrates [2]. After examining the DOP profiles beneath thefilm, it is interesting to note that the anisotropic deformation extends to significant depths (~ 8 μm), asillustrated in figure 1, while the horizontal distribution of the stress can propagate beyond the edge of thesample by a few microns (see figure 2). The confinement of light in some photonic devices such asphotoelastic planar waveguides can be achieved by a photo-elastic effect in semiconductor using stresseddielectric films [3]