Fabrication of Thin Film of Porous Silicon Using Electrodeposition Assisted Stripping (EAS) Process and Its Humidity Sensing Properties
Authors: Park Soobin , Kwak Byungkwan , Yoo Bongyoung 
Published: February 2020

Humidity is one of the most important factors in human life to live comfortably. Especially, detecting and measuring humidity has been attracted in various fields such as food science, meteorology, agriculture, and so on. At present, most of the humidity sensors, fabricated to detect humidity through variation of the electrical signals using metal oxide which was zinc oxide1, copper oxide2, titanium oxide3, conducting polymer4, and silicon based materials5-6. Especially, among the silicon based materials, porous silicon was currently one of the promising materials because of its easy fabrication process and low cost.

Silicon is used as a key material for the electrical devices. The silicon occupies about 40 percent of the device production cost. Therefore, there were lots of study using spalling process for reducing the thickness of the silicon wafer. One of the most effective and inexpensive techniques is the Electrodeposition Assisted Stripping (EAS) process7. Electrodeposition assisted stripping (EAS) process was used to obtain a thin Si substrate by electroplating high tensile stress layer, which induced the lift-off behavior of thin Si layer. Ni buffer layer was deposited using electrodeposition process with the Watts bath. The buffer layer protects the Si substrate from the high stress layer which can cause damage to the substrate. Also, buffer layer thickness can control the spalled Si thickness.

In this study, we fabricated thin porous Si using EAS process and thin porous Si was characterized as humidity sensor. The porous silicon was fabricated by dry etching process. Before the EAS process, titanium and nickel layer was deposited by E-beam evaporator. The thickness of titanium and nickel layer was 20 nm and 200 nm, respectively. The thickness of thin porous Si layer was controlled by controlling current density of Ni electrodeposition process. The current density was controlled 10 mA/cm2 to 50 mA/cm2. The thickness of spalled porous silicon layer was controlled 100 um to 60 um. Surface morphology, and thickness of porous Si layer were observed with a field emission scanning electron microscope (FE-SEM). Electrical characteristics of porous Si humidity sensor were measured by 2-point probe station. The humidity was controlled by changing the ratio of dry gas to wet gas. The resistance of humidity sensor was recorded using an electrical source meter. The sensitivity is represented by RDry/RRH, where RRH is the resistance with the varied humidity and RDry is the resistance with the lowest humidity. Moreover, response and the recovery time are defined as the time to reach 90% of the total variation.

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