Thick oxidized porous silicon layer as thermo-insulating material for high temperature operating thin and thick film gas sensors

Silicon Porosification: State of the Art

“…185 It has been shown that micro-and meso-PSi layers are excellent materials for local thermal isolation on bulk silicon 106,186 and for fabrication of micro-hotplates for low-power sensors. [187][188][189] Micro-and meso-PSi could also be used as isolation material for radio-frequency (rf) applications because of its high resistivity and compatibility with modern very-largescale-integration IC technology. 190 The list of possible applications is not yet exhausted; there are many more possible applications of porous semiconductors covered in the reviews, 4,12,27,30,32,38,153,157,171,[191][192][193][194][195][196][197] which we will not address here.…”

“…110,[476][477][478] Many oxidation methods exist, such as aging in ambient, 479 exposure to water vapor, [480][481][482] anodic oxidation in a non-fluoride electrolyte, 142,179,[483][484][485] 479 and thermal oxidation in wet and dry O 2 fluxes, 189,381,[488][489][490][491] including rapid thermal oxidation. 477 A data review regarding a wide range of oxidation treatments (e.g., thermal treatments, electrochemical and chemical oxidation, pthotochemical oxidation, etc.)…”

“…189,492 It was found that after a short-term annealing at a high oxidation temperature above 1000 • C in wet O 2 , thick Oxidized PSi (OPSi) films had the same properties as thermal SiO 2 of bulk silicon. 489 Commonly used modes of thermal oxidation of porous Si are presented in Table 10.…”

“…59 According to Yon et al, 495 a pre-oxidation of porous silicon layers at low temperatures in the range of 300 • C is sufficient for creation of an oxide layer all along the pore walls, which prevents restructuring of the sample during further heating at higher temperatures up to 800 to 900 • C. 495 Therefore, it was established that the procedure: preoxidation at 300 • C for structure stabilization; oxidation at 750 to 800 • C for complete porous silicon transformation into silicon dioxide; and final densification at 1050 to 1090 • C for obtaining a compact oxide equivalent to standard thermal silicon dioxide, is the most optimal for reproducible forming of oxide layers from porous silicon. 189,492,495 It was also found that the oxidation of PSi is being controlled by a surface reaction both at the surface of PSi and the wall of pores in PSi layers. Both Si 2 O 3 and SiO 2 are produced by low temperature oxidation below 400 • C, and physically absorbed water exists in an OPSi film formed by wet O 2 oxidation at temperatures below 700 • C. It was found that at a high oxidation temperature, Si is being increasingly transformed to SiO 2 , with consequential loss of crystalline structure, resulting in a partly amorphous form.…”

Multi-walled microchannels: free-standing porous silicon membranes for use in /spl mu/TAS

“…The large surface area of PS and its relative inertness make it an ideal material to increase the area of an analytical device [5]. Oxidized PS has also been used as an insulating material in gas sensors [6], whereas metallized PS can be used in microelectronics [7].…”