Interface studies by second-harmonic generation during silicon-based materials processing

Surface and interface properties increasingly govern device performance in microelectronics and photovoltaics due to a continuous trend of decreasing critical dimensions and increasing performance demands. This thesis is aimed at contributing to bridging the gap between the rapid developments in Si-based materials processing and limitations brought by poor understanding of surface and interface characteristics during processing. The ability to specifically measure surface and interface properties and their evolution during processing leads to a better understanding of the underlying mechanisms of Si-based materials processing and can lead to process and device improvements. Moreover, this approach also provides means for online process control and monitoring. To reveal the properties of surfaces and interfaces during Si-based materials processing, the nonlinear optical technique of second-harmonic generation (SHG) has been implemented both spectroscopically and in real time. SHG is the method of choice as it is noninvasive, highly surface and interface sensitive, and applicable during processing. Moreover, SHG has proven to be an ultrasensitive probe for surface and interface states such as dangling bonds and strained Si-Si bonds in crystalline Si (c -Si) surface science. The SHG experiments described in this thesis were carried out using amplified and unamplified femtosecond Ti:sapphire laser systems. To circumvent the complexity inherent to a (plasma) processing environment, the studies were performed under well-defined conditions in high vacuum setups using separate ion and radical beams. SHG has been applied in various areas of Si-based materials processing, predominantly in thin film geometries. These areas include: (1) c -Si etching, (2) Si thin film deposition, (3) the introduction of high-¿ dielectrics, such as Al2O3, in Si technology, and (4) novel Si-based device technologies employing nanocrystals. The ion-assisted etching of c -Si has been investigated using low-energy (70-1000 eV) Ar+ ions. These ion energies are representative for plasma processing and have been found to create a layer of amorphous Si (a -Si) with a thickness of several nanometers in the surface region of the c -Si. From spectroscopic SHG in combination with a critical point model it has been shown that the SHG signal from this a -Si/c -Si system is dominated by a contribution from the a- Si/c- Si interface, with an additional a -Si surface contribution. The results suggest that the a- Si/c- Si interface is relatively sharp and that the surface and interface properties are virtually ion energy independent. The growth of hydrogenated amorphous Si (a -Si:H) thin films on c -Si has been studied during hot-wire chemical vapor deposition. Also in this system the SHG signal displays a strong contribution originating from theSummary 165 a -Si:H/c- Si interface, with an additional contribution from the a- Si:H. In addition to SHG, two other noninvasive and all-optical diagnostics with realtime applicability have been used simultaneously. Spectroscopic ellipsometry (SE) has been used to deduce thicknesses and linear optical properties of Si films and substrates, while attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) has been applied to characterize the hydrogen content in Si thin films during deposition. For many applications, such as silicon heterojunction solar cells and passivation of c -Si surfaces, an extremely abrupt interface between the a -Si:H film and the c -Si substrate is required. It is demonstrated that real-time SHG provides a method to distinguish between direct heterointerface formation and undesired nanometer-level epitaxial growth at a very early stage of film growth with a sensitivity unprecedented by other real-time probes. The technique of SHG is known to be sensitive to internal electric fields within materials created for example by the presence of fixed charge. This feature has been employed to characterize the surface passivation of c -Si by thin films of Al2O3 deposited with plasma-assisted atomic layer deposition. Spectroscopic SHG combined with critical point modeling has revealed the presence of a high fixed charge density with negative polarity in the Al2O3. After calibration, negative fixed charge densities in the range of 1011-1013 cm-2 have been deduced from the SHG spectra. This negative charge is an important factor responsible for the excellent surface passivation properties of Al2O3. SHG has also been applied to investigate Si nanocrystals embedded in SiO2. This study has been carried out by spectroscopic cross-polarized two-beam SHG (XP2-SHG), which yields a more efficient generation of SHG radiation from nanocomposites than single beam SHG. XP2-SHG revealed spectral features that can possibly be attributed to the Si nanocrystals and their interfaces with the SiO2 matrix. This approach has great potential to lead to more insight into Si nanocrystal interface states and their influence on the emission of light. In conclusion, the work in this thesis shows that SHG is capable of bridging the gap between materials processing and surface science. It is demonstrated that SHG can enhance the insight into a broad range of surface and interface properties during and after materials processing, as required for next-generation advanced devices