Noise in Different Micro-bolometer Configurations with Silicon-Germanium Thermo-sensing Layer

Abstract

A recent interest in thermal detectors is due to plasma deposited materials with large temperature coefficient of resistance (TCR) and micro-machining technology for thermal isolation. Both are compatible with dominating Si CMOS technology that makes it possible developing device-on-chip configurations. In our previous works we have reported on the study of fabrication and characterization of single cell micro-bolometers based on silicon-germanium thermo-sensing films deposited by low frequency plasma [1-3]. Noise measurements are very important part of device characterization, which have been poorly reported in literature. Models proposed by present time for noise description in non-crystalline samples (either in films or in device structures) are still debated and no one can be considered as the only accepted. The goal of this work is to study experimentally noise spectral density in several configurations of micro-bolometers with silicon-germanium (a-SixGey:H) as thermo-sensing films. We studied four configurations: planar structure with a) a-SixGey, y=0.5, b) a-SixGeyBz:H, x=0.5, y=0.45 and z=0.05 ; c) x=0.5, y=0.5, z=0 and d)sandwich structure with x=0.5, y=0.5. These samples were characterized by SIMS (composition), FTIR (H-bonding and H content), conductivity measurements (σ(T), activation energy, TCR), current-voltage characteristics in dark and under illumination (responsivity). Noise spectral density (NSD) versus frequency S(f) was studied in the range of frequency f=1 to f=103 Hz under IR illumination modulated with this frequency and constant bias. The measurements were performed in vacuum chamber with P=10 mTorr. Generally S(f) dependence demonstrated three regions separated by two corner frequencies: fc1 and fc2: 1) f ≤ fc1 S1 α f -γ γ =0.15 to 0.5, 2) fc1 ≤ f ≤ fc2, S2 α f-γ γ = 0.7 to 1.34 and 3) f ≥ fc2 S3 const (f). Different samples studied showed different values of fc1, fc2 γ and S3. These noise characteristics observed experimentally are analyzed in comparison with data reported in literature and possible mechanisms for noise in the frequency regions studied are discussed.