Wolfgang Theiss

Formation of porous silicon on patterned substrates

Abstract Application of porous silicon in device structures requires the formation of micron-size porous areas. Therefore, selective area anodization on photolithographically patterned p-doped substrates was investigated. As shown in this work, porosity and layer thickness vary from the edge to the middle of the structures. This inhomogeneity strongly depends on the doping level of the substrate and the lateral size of the structure. When organic photoresists are used, an anisotropic undercutting of up to several 10 μm occurs at the edge of the structures. This can largely be reduced by using thermally treated Si 3 N 4 deposited by plasma-enhanced chemical vapour deposition as a masking layer. In this case an isotropic undercutting of the masking layer is observed permitting fabrication of porous silicon structures in the μm range by photolithography.

Color-Sensitive Si-Photodiode Using Porous Silicon Interference Filters

A new method for the fabrication of color-sensitive Si-photodiodes is presented. Color sensitivity was achieved by using porous silicon multilayer stacks which act as interference filters if the formation parameters are controlled carefully. These filters were integrated in the upper, p+-type part of a p+n-junction. As expected, the spectral response of the photodiodes was determined by the transmission spectra of the filters, while the porous silicon had no significant influence on the electrical characteristics. The great advantage of this method over conventional ones is that it makes very cheap, fast filter fabrication requiring no expensive deposition process possible.

IR spectroscopy of porous silica gels

IR spectroscopy is a well established tool in chemical and semiconductor industry giving information about the composition and geometry (film thickness) of the samples. The application of this non-destructive method to characterize porous silica gels obviously faces the problem of dealing with inhomogeneities. The samples are basically two-component systems of SiO 2 and air (or vacuum). A rather simple theoretical model is presented being able to describe the effects of the inhomogeneities and to fit experimental spectra quite well. It gives information about the volume fraction and the strength of the SiO 2 network.

Determination of the effective dielectric function of silver powders produced by inert gas evaporation

Silver powder samples were prepared by evaporation of silver in an inert gas atmosphere. They were investigated by dc-conductivity-, reflectance- and photoacoustic (PAS) measurements. Model spectra based on the Bergman ansatz for the effective dielectric function εeff, the effective thermal and electrical conductivities λeff, and σeff, were fitted to the experiments. Good agreement has been found assuming a spongily packed, only weakly percolating topology and an increased damping of the free carriers in the small particles (classical size effect).

Optical Interference Filters Made of Porous Silicon

Porous silicon (PS) layers can easily be formed by an electrochemical etch process using a mixture of hydrofluoric acid (HF) and ethanol. The microstructure and porosity of the layers depend on the HF concentration, the doping level of the substrate and the current density applied during the etch process. Changing the current density during the etch process will result in a well defined layer structure consisting of layers with different porosities. Each single layer can be treated as an effective medium exhibiting a refractive index depending mainly on the porosity of the layer. Using reflectance measurements we have investigated the dependence of the refractive index of PS layers on the formation current density for different substrates. In addition the etch rate was determined by thickness measurements with an electron microscope. Based on these results various kinds of optical interference filters were studied. We have formed samples consisting of discrete single layers with different porosities (e.g. Bragg reflectors) as well as samples with continuous variation of the refractive index (rugate filters). Combining these PS filters with standard photolithography steps, microoptical devices such as spectral sensitive photodiodes can be realized.

Fabrication and properties of multilayer porous silicon filters

Porous silicon multilayer systems formed by different techniques were investigated. Type I layer systems are fabricated by changing the current density during the anodic etch process. Type II samples are formed with a constant current density but using a substrate with alternating doping levels. The superlattice structure is clearly visible in transmission electron microscope pictures. The quality of the interfaces depends on the formation technique. Porous silicon multilayer systems exhibit sharp peaks in the reflectance spectrum and can be used as filters. The line narrowing of the broad photoluminescence band of porous silicon by a Fabry- Perot filter structure is demonstrated.

Surface-enhanced Infrared Absorption Spectroscopy (SEIRA) Using Multireflection ATR-elements

This work shows the enhancement of spectral absorptions by noble metals (Ag, Au) deposited in nm-thick layers on the surface of ATR-crystals. By the surface-enhanced infrared absorption (SEIRA) technique, the absorption bands of p-nitrobenzoic acid can be enhanced by a factor of up to 40. This enables the detection of substance concentrations in the mM range. The structure of the metal deposit has been investigated by atomic force microscopy and correlated with the enhancement factor.

Seeing more, looking at less: applications of IR microscopy

Applications of IR microscopy in solid state physics are shown, namely, the investigations of layer thickness variations in semiconductor systems and of the scattering characteristics of small particles, both making use of the ability to take spectra from small sample spots.

IR-spectroscopical investigations on the glass structure of porous and sintered compacts of colloidal silica gels

The forming and sintering of fumed silica powders is an interesting route for the preparation of large, very pure or doped silica glasses with a precise geometry. The processing from the shaping of a porous compact to the sintering of transparent silica glass can be successfully investigated with optical spectroscopy. As only the dielectric function DF (a dielectric function is the square root of the complex refractive index) characterizes the material, the vibrational bands were calculated from reflectance measurements. In compacts of fine particles, the topology cannot be neglected. Therefore, the models describing topological effects are briefly reviewed. With these model calculations it could be proven that new bands in the compacts and the significant shifts in the reflectance spectra during sintering are mainly caused by topological effects and that changes in the glass structure play only a secondary role.

Advanced FTIR techniques for photoresist process characterization

Several applications of Fourier transform IR spectroscopy (FTIR) for the characterization of photoresist thin films are demonstrated. The applications are accurate resist thickness measurements, monitoring of solvent loss during the post-apply-bake, determination of the glass transition temperature, and deprotection reaction kinetics. Model based, spectral analysis is applied for the determination of photoresist thickness from mid-FTIR spectra and is shown to have linear correlation to measurements with UV-visible spectroscopic ellipsometry. Using this capability in conjunction with an external reflection accessory and rapid data acquisition hardware and software, measurements are performed on Shipley SPR-510L photoresist during the post apply bake step, deriving thickness and solvent loss information. The use of this approach is also explored for making glass transition measurements of an environmentally stable chemical amplification positive resist photoresist. Finally, in-situ PEB studies are illustrated for APEX-E photoresist. For off-line analysis, an in-sample compartment mapping accessories is applied to the characterization of multiple open frame exposure matrices on 200 mm double-side polished wafers.