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Fabrication and Electrical Characterization of Amorphous Black Silicon on Metal Electrodes
Keywords: Amorphous black silicon, flexible substrates, electrical break down
In this paper we report the fabrication of amorphous black silicon on metal electrodes. Applications for amorphous black silicon (ab-Si) include packaging of Micro Electrical Mechanical Systems (MEMS) and microfluidics. When two ab-Si surfaces are brought into contact, the interlocking needles can provide a seal to the outside environment. A key advantage of ab-Si is that it can be made on a wide range of substrate types and configurations, which include plastics and flexible substrates. ab-Si also has applications in photovoltaics, terahertz emission, micro-chemical, and micro-biological reactors. We investigated two distinct methods to create ab-Si on metal electrodes. One procedure involves using the established Bosch process to create highly dense amorphous silicon needles on an underlying layer of Titanium (Ti) and Nickel (Ni)/ Chrome (Cr) electrodes. By varying the process parameters, such as R.F. power, gas flow, bias voltage, and etch/ passivation time, we can obtain diverse etch profiles on silicon. Depending on the process parameters established this can lead to nanostructures of amorphous silicon that appears black and reflects minimal visible light. Our cross-section consists of an R.F. sputtered amorphous silicon layer on top of a Ti/silicon dioxide /p-type wafer. The Ti film was electron-beam deposited on a thermally grown oxide that serves as an isolation layer. The Ni/Cr film was similarly deposited. This process can be applied to any substrate/metal that can adhere to silicon. Our process is performed at low temperatures, which is important for low-thermal budget processes. We also studied a wet chemical etch method to produce amorphous black-silicon on a flexible substrate. This method involves using a mixture of hydrofluoric acid, hydrogen peroxide and water to nanostructure the surface. We can vary the etch time, which leads to different needle lengths. Prior to etching the surface, the cross-section consists of a silver layer on top of silicon with an underlying gold (Au) layer that is adhered to a flexible substrate. Post-etch, the silver is washed away and the remaining surface is ab-Si with an under layer of Au on a flexible substrate. In our initial study, we demonstrate selective amorphous black silicon formation on top of a metal contact for applications in gas sensing and exible interconnects. We developed a breakdown voltage gas sensor with a reduced breakdown voltage (< 200 V). Currently our device is packaged using an epoxy resin to hold the device together. In order to better understand ab-Si properties for potential applications in packaging, we investigated the electrical and surface properties of the material on top of metal electrodes. We analyzed the ab-Si samples under Scanning Electron Microscopy, Atomic Force Microscopy, and conventional current-voltage measurements. Future work involves bonding different substrates together using amorphous black silicon in photonics, photovoltaics, and sensors. In this presentation we will describe the fabrication process of amorphous black silicon on Ti and Ni/Cr based on the Bosch process. We will also discuss a wet chemical etching method that can be used on flexible substrates. In addition, we plan to present preliminary research data on the electrical breakdown properties of ab-Si with its respective surface roughness.
George A. Hernandez,
Auburn University
Auburn, AL

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