Black silicon as secondary standard of the black·body

Abstract

One of the important problems in thermography of microsystems is scaling of the infrared radiation in very small areas. We describe an application of dry etched silicon surface, so called black silicon as the secondary standard of the black body in the Far Field Thermography (FFT). In our experiments the black silicon surface topography is studied with Scanning Electron Microscopy (SEM) and Atomic Force Microscope (AFM). Sample preparation is discussed also. Dependence of emission coefficient on temperature is studied in the wide temperature range and is compared with traditional black body model. 1. Introduction Black silicon and other materials like for instance diamond-like carbon films are the materials which due to simplicity of their fabrication (simple technological process) and possibility of location of appropriate pre-prepared items on chips containing microsystems, are suitable for applications as secondary standards of black body. However, such applications require optimisation of technological process from the point of view of emission coefficient 8, whose value should be close to 1. Materials with emission and thus absorption coefficients close to 1 are also important components of radiation thermometers. Such material used as absorber of infrared sensor ensures relatively high response of the sensor [1). In the work preliminary results of investigation on application of black silicon in thermographical studies of microsystems will be described. 2. Process of fabrication and properties of black silicon Phenomena of random micromasking is often observed in such processes as Reactive Ion Etching (RIE) in strong anisotropy conditions and selective silicon etching both in SF60 2 gas mixture with AI mask as well as in chlorine or chlorine containing gas. The structure of black silicon is formed when the whole silicon surface is randomly covered with particles, clusters or other residues coming from reactor chamber walls, surface of cathode or material of masking layer. These elements define random microsurfaces which mask successfully some areas of silicon and cause that during etching process, a great number of densely packed needles microstructures originate and form a layer similar to dense grass on the silicon surface. In Fig. 1 a scheme of above described process of microcolumn etching is shown [2). Physical properties of so formed surface of black silicon depend on the density of packaging of silicon needles (nanowires), their geometrical dimensions (diameter and height) and the depth of etching. Forming of black silicon has been considered as an interesting but side effect of the process of interaction of ions with silicon surface up to now, so the literature on this subject does not contain much information about its physical properties [2). It is possible that the structure of numerous silicon high needles (nanowires) is a trap for incident andlor emitted infrared radiation. It may be assumed that emission orland absorption coefficient of black silicon will rise with the increase of packaging density of nanowires, extension in etching depth and simultaneous decrease in the needles diameter. Due to the random character of the R!ocess, the final effect is determined by the average values of the above mentioned parameters.