Capacitive Humidity Sensing Characeristics of Porous Silicon Oxide Via Electrodeposition Assisted Stripping and Metal Assisted Chemical Etching Process

Abstract

Sensor devices are widely used for increasing human convenience to detect various environmental parameters such as humidity, temperature, pressure and lots of hazardous chemicals. Also, specific sensor devices are utilized for private security using iris or finger print. Among them, humidity sensors are widely utilized in the fields of food science, meteorology, preservation of relics, so on [1-2]. Therefore, measuring the humidity is substantial interest in research fields. For detecting the humidity, many materials including silicon, silicon oxide, metal oxide, conducting polymers, and carbon group materials have been used. Among them, silicon oxide is considered because of its low process cost and simple fabrication process. Moreover, silicon oxide is hydrophilic materials that water molecules were easily adsorbed on its surface. In order to increasing sensing characteristics, lots of researchers put an enormous efforts such as increasing surface to volume ratio, improving the structure of sensor device, and doping the impurities to electrode materials and so on. Among them, increasing the effective surface area is the most common and simple way to enhancing the sensing characteristics. Electrodeposition assisted stripping (EAS) process is the one of the most effective kerf-free method for obtaining the thin silicon film [3]. The EAS process is using a spalling which is the unique mode of brittle fracture, caused by high tensile stress. High tensile stressed layer can induce fracture at a certain depth, parallel to substrate interface, which was deposited by electrochemical deposition of nickel layer on the silicon surface. In order to control the thickness of spalled silicon layer, current density is the critical parameter of electrodeposition process. Metal assisted chemical etching (MACE) process is utilized to fabricate the porous structure. The MACE process is using the anisotropic etching phenomenon through metal catalysts in wet etching solution, which is consisted of hydrogen peroxide and hydrofluoric acid. Compared to dry etching process, MACE can fabricate the high aspect ratio of porous structure much easier and cheaper than vacuum based process. In this study, we fabricated the porous silicon oxide film using EAS, MACE and oxidation process and it was characterized as humidity sensor device. Before the EAS process, the thickness of 20 and 200 nm of titanium and nickel layer was deposited by E-beam evaporator. The current density of 10 mA/cm2 was conducted to deposit the high tensile stressed nickel layer in watt bath which was consisted of nickel sulfate, nickel chloride and sodium citrate. The thickness of spalled silicon layer was around 90 ~ 100 um. After the spalling the silicon, MACE and oxidation process was conducted in consecutive order. Tin and silver electro-less deposition was conducted to make the metal catalyst and hydrogen peroxide and hydrofluoric acid were used for etching electrolyte. The oxidation process was conducted in ambient air condition at 1000 ℃ in 3-zone quartz tube furnace. Surface morphology and thickness of spalled silicon were observed with field-emission scanning electron microscope (FE-SEM). Electrical characteristics of porous Si humidity sensor were characterized by 2-point probe station. The humidity was controlled by changing the ratio of dry gas to wet gas. Variation of capacitance of humidity sensor was measured and recorded using an LCR meter. The sensitivity is represented by (CDry - CRH) / CRH, where CRH is the capacitance with the varied humidity and CDry is the capacitance with the lowest humidity. Moreover, response and the recovery time are defined as the time to reach 90% of the total variation. In order to verifying the humidity sensing mechanism, electrochemical impedance spectroscopy (EIS) analysis was conducted. From the EIS results, proper equivalent circuit of humidity sensor and sensing mechanism were suggested. REFERENCES [1] P. Hurjes, A. Kovacs, Cs. Ducso, M. Adam, B. Muller, U. Mescheder, “Porous silicon-based humidity sensor with interdigital electrodes and internal heaters”, Sensors and Actuators B: Chemical 2003, 95, 140-144. [2] Z.M. Rittersma, A. Splinter, A. Bodecker, W. Benecke, “A novel surface-micromachined capacitive porous silicon humidity sensor” Sensors and Actuators B: Chemical 2000, 68, 210-217. [3] Youngim Kwon, Changyeol Yang, Sang-Hwa Yoon, Han-Don Um, Jung-Ho Lee and Bongyoung Yoo, “Spalling of a Thin Si Layer by Electrodeposit-Assisted Stripping”, Applied Physics Express, 2013, 6 (11), 116502.