Integration of Functional Oxide Films on Semiconducting Substrates

Abstract

System-on-chip (SoC) applications have attracted the attention of the modern community due to their ability to facilitate various functionalities within a single device which can meet the requirements of day-to-day life. To augment current devices with new functionalities, materials with either new properties or enhanced capabilities need to be integrated within existing electronic platforms. Functional oxides exhibit a wide range of properties, many of which are crystal orientation dependent and hence require deposition of epitaxial or oriented thin films. Si-based microelectronics technology is well established. Therefore, developing a common platform through which these oxides can be epitaxially integrated with silicon would help enable SoC applications significantly. One of the most successful ways of realizing epitaxial films on Si thus far is by molecular beam epitaxy (MBE). The work done as part of this thesis takes off from the preceding one on this topic in which it was demonstrated that (100) and (111) oriented yttria stabilized zirconia (YSZ) films could be deposited on Si by sputtering, which is a more technology friendly deposition method than MBE. Even though the oxide films in the previous work were oriented, the degree of orientation was very poor. The full width at half maximum (FWHM) of x-ray rocking curves were determined to be 26° and 16° for (100) and (111) YSZ films respectively. Good quality oriented and epitaxial films have values that are <1°. In the work done as part of this thesis stress driven grain growth was first evaluated as a mechanism to improve the degree of orientation in the deposited films. A significant improvement in texture was observed on annealing. The FWHM of the symmetric rocking curves for the (100) and (111) YSZ was reduced from the previously quoted values to 14° and 7° respectively. It was observed that a higher initial compressive stress (-1.4 GPa) resulted in significant (~150%) grain growth and reduction (~56%) in FWHM. The effect of intrinsic film stresses on grain growth and texture evolution during annealing of an oxide film has been analyzed and used to explain these results. It can be noted that these values of FWHM are still not low enough. The main reason is the presence of an amorphous native oxide of Si on the surface which hinders crystalline orientation in the deposited film. The oxide of Si is particularly stable, whereas that of its neighboring element Ge is not. Thermodynamic calculations showed that an oxide free Ge surface would be obtained above 521°C at 2.5 mTorr oxygen partial pressure. As YSZ has a low (~25) dielectric constant and hence would reduce the effective dielectric constant, barium strontium titanate (BST) films were directly grown on Ge at a temperature of 550°C. This experiment yielded a textured sputtered BST films with a (00h) rocking curve FWHM of <4° which is the lowest reported till date. Degradation in crystallinity for BST films in presence of the amorphous native oxide was observed. This improvement in crystalline quality, which was obtained by tuning sputtering parameters as discussed in this thesis, yielded a 5x increment in the dielectric constant from 19 to 82. While Ge substrates yield oxide free surfaces at temperatures greater than 521°C, fact remains that the melting point of Ge is 940C. Higher temperatures in crystal growth are essential for better crystalline quality. Hence, similar calculations were performed for Si and it was found that oxide free surface can be obtained at temperatures above 932°C at 2.5 mTorr oxygen partial pressure. To reach these temperatures inside a vacuum chamber, a strip heater which can raise substrate temperature up to 1400°C was hence developed. Due to lattice matching and stability, as a case study SrTiO3 films were deposited on Si within the thermodynamic window. Highly textured sputtered (h00) SrTiO3 films, with the lowest FWHM<2.5° ever reported, on Si was obtained. Again the improvement in texture by tuning sputtering parameters resulted in an increase in dielectric constant from 53 to 101. The fabrication of the strip heater resulted in a serendipitous discovery. Silver from the paint used for the electrical contacts was found to evaporate and deposit on the Si substrate in an epitaxial manner with an orientation relationship (001) Ag // (001) Si and <001> Ag // <001> Si. The epitaxial nature of the Ag film was confirmed by on and off axis x-ray diffraction (rocking curve and phi) scans. The on-axis rocking curve FWHM was found to be <0.4°. This process was developed to yield a Si substrate with a continuous uniform epitaxial Ag film. Oxide films deposited on this epitaxial Ag/Si platform has FWHM ~1.5°. Thus, in summary, the research done as part of this thesis has helped in the development of highly oriented oxide films and an epitaxial Ag platform on Si substrates which are expected to pave the way for further integration of oxides on to Si by methods such as sputtering and PLD that are simpler than MBE