窒化ケイ素膜を用いた生体用誘導電極

抄録

Metallic electrodes are employed as biomedical sensing electrodes. However, biomedical signals are distorted in case of metallic electrodes due to the noise generated by the electrochemical reaction between metal and electrolyte. The present study concerns introduction of a dielectric silicon nitride film between electrolyte and electrode in order to minimize the effects of noise and facilitate biomedical signal detection.
The silicon nitride (Si₃N₄) film was deposited on a silicon substrate by the process of vapor phase growth of SiH₄ and NH₃ at 950°C. The silicon substrate was of 0. 001 ohm-cm N-type silicon, 0..25, mm thick, (111) oriented, and chemically polished to a mirror finish. The relative flow rates were as follows SiH₄ 5 ml/min, NH₃ 1 ml/min, H₂ 3. 51/min and H₂ was used as a carrier gas.
The typical current-voltage characteristics in positive and negative directions of a Au-Si₃N₄, , . (1 000Å thick, 50 mm² area) -Si structure at room temperature and at a low field are virtually identical and ohmic. The impedance Z_o of this film can be easily transformed into parallel equivalent circuit of the resistance Z_R and the reactance Z_c. Z_c is much larger than Z_R from ultra low frequency to 10 Hz. Z_R is much larger than Z_c from 10 Hz and above. Therefore, when the frequency is lower than about 10 Hz, Z_o is resistive. When the input impedance of the amplifier is high, DC signal can be also detected. Noise level between this electrode (80 mm² area) and calomel reference electrode in 0.9% saline solution is much lower than the case in which calomel reference and other electrodes (thermal silicon oxide film, etc.) are combined. This electrode is stable and has the same noise level as the Ag-AgCl electrode (disk type). It was designed for EEG but it could be used for ECG and EMG as well.