Self-assembly and non-destructive inspection of the silicon-on-nothing and germanium-on-nothing structures

Abstract

In this dissertation, the fabrication and non-destructive inspection of double-stacked membrane-cavity structure are reported. During high-temperature annealing of semiconductor materials, the surface of the material transforms to the shape with minimum surface energy due to surface diffusion. Using the surface diffusion of hole patterned substrate, self-assembled membrane-cavity structure can be obtained. Until recently, the research related to self-assembly has been mainly focused on silicon to fabricate silicon-on-nothing (SON) structures for various applications. However, in the previous SON research, only single SON structure consisting of a membrane-cavity-substrate in the vertical direction was employed. The theoretically reported multi-stacked SON structures can maximize the advantage of self-assembly in that facile method to fabricate membrane-cavity structures. For example, in case of the conventional bonding or selective etching methods multiple fabrication process should be applied in series to fabricate multi-stacked SON structures while one fabrication can result in multi-stacked SON structures from self-assembly. To fabricate double-stacked SON structures, a high-temperature vacuum furnace is employed with the novel cover method for preventing chemical desorption and surface defects during high-temperature vacuum annealing. Using the proposed annealing method single and double-stacked SON structures with a width of ~ 30 µm are successfully fabricated. It is also observed that the taper of initial hole patterns much affect the cavity height than membrane thickness. On the other hand, only reported GON structure has thin membrane (~ 0.3 µm) for easy detachment of membrane from substrate. However, the nanometer scale hole pattern for thin membrane has limitation in the obtainable membrane thickness (~ 0.1 µm), although it is more advantageous for annealing condition than micrometer scale hole patterns. To fabricate GON structures with the micrometer scale membrane, the annealing duration near melting temperature is thoroughly investigated for the micrometer scale hole patterns, and the model for annealing duration according to annealing temperature is proposed. Moreover, effect of the sidewall profile of the initial hole pattern on the post-annealing shape that can be single or double layer is analyzed. Non-destructive inspection method for fabricated membrane-cavity structures is also investigated. In previous studies, the destructive method in sequence of cleaving and imaging was used to examine the fabricated SON structure. However, it is practically difficult to have the high reliability or reproducibility for destructive inspection due to small width of SON structures. Moreover, The destructive inspection has a fatal disadvantage that the cleaved samples cannot be used for post fabrication processes and thus restricts device applications of self-assembled structures. To overcome the limitation of current destructive inspection, NIR interferometry setup is established. The each component of the setup is precisely employed considering the thickness (~ 2 µm) of the fabricated SON structure as well as coherence length of interferogram. To capture the interferogram that becomes comparable with noise due to multiple reflection and transmission at each interface, the noise is minimized as 1% of signal by multiple scanning. The second membrane of double-stacked SON structure is also measured with deviation of 0.01 µm from SEM image while the measured membrane thickness and cavity height show deviation less than 0.13 µm from SEM images for single SON structures. The membrane thickness of GON structures is also measured with the established NIR interferometry setup. It is observed that the surface structure that is originated from initial hole patterns much attenuates the subsurface interferogram signal and the membrane thickness can not be measured, especially for germanium that absorbs incident light. On the other hand, the interferogram signal from bottom side of smooth membrane is captured and the membrane thickness is obtained with deviation less than 0.05 µm from SEM image. As a result, through the measurements of SON and GON structures, it is found that when the ratio of optical depth difference of two adjacent interfaces are higher than 0.9, the membrane thickness and cavity height could be measured with a difference of 6% compared to SEM. It is expected that extension of the number of stacked layers and the thickness of membrane-cavity structures as well as non-destructive inspection method will serve as basic technology for various applications such as complementary metal–oxide–semiconductor(CMOS) compatible single and double-stacked cavity wafers, perfect reflecting substrates, and multi-usable epitaxial growth templates. In addition, a photo detector with optimum sensitivity and a CMOS substrate with wireless communication can also be supplied based on the current results. Furthermore, the established NIR interferometry microscope will facilitate post-annealing shape analysis according to the initial hole pattern by time-effective dense-parametric-study.