Problems of chemical-dynamic polishing in the technology of silicon p-i-n photodiodes

Abstract

During the preparation of silicon substrates for the manufacture of silicon p-i-n photodiodes, the effect of the presence of chemical-dynamic polishing and the depth of etching on the electrical parameters of the FD was observed. The quality of the polishing operation also affected the optical and photoelectric parameters of the samples. These effects required detailed study. The article investigates defect formation during the operation of chemical-dynamic polishing in the technology of silicon p-i-n FDs and determines the optimal modes of polishing the front and back sides of the substrates. The composition of the etching mixture was HNO₃:HF:CH₃COOH = 9:2:4.

CDP has a significant effect on dark currents. This operation should be performed before heat treatments and before diffusion in the reverse direction to obtain minimum values of the dark current. The optimal polishing depth of the front side of the substrate is 20–25 microns (if we take into account the etching of the reverse side – 35–40 microns in total), the reverse side before boron diffusion – 10–15 microns.

The main factors that affect the appearance of defects during polishing have been elucidated. Thus, when the concentration of the components of the polisher medicine for the СDP changes significantly, its properties change significantly – the polisher can become selective, since its composition is similar to the selective polisher of Dash. In order to avoid this, it is necessary to carry out incoming control of component acids. If there are inclusions of another phase in the volume of ingots and, accordingly, on the surface of the substrates, clusters of point microdefects of different sizes, or defects acquired in the process of mechanical processing, there is unevenness of etching and violation of planar parallelism. At a high speed of rotation of the tank with the polisher, the depleted layer of the solution may not have time to form near the surface of the substrates or be “broken off” from the surface by the flow of liquid, which leads to selective etching of the plates. This effect can be manifested in the so-called texturing of substrates, which increases the level of dark current and reduces sensitivity. In the case of the reverse side of the СDP, before boron diffusion, the front side is protected with a chemically resistant varnish. If the thickness of the varnish is insufficient, the formation of punctures in the protective layer is possible. In this case, during the polishing operation, the herbaceous material flows under the varnish layer and the brightening or masking oxide is etched. To prevent the described phenomena, we recommend applying two layers of varnish with intermediate drying. Before the end of the polishing process, it is inadmissible to remove the plates from the container with the herbal agent, because drops of the herbal agent remain on the surface of the substrates. In places where there are drops, a violent reaction begins with the release of nitric acid vapors and strong heating. As a result of this reaction, the plates are strongly etched and spots, depressions-craters and black coating of silicon oxide are formed on their surface. In the event of the appearance of the described damaged surface and the unsuitability of the substrates for further technological operations, it is necessary to remove the surface layers by mechanical methods and again to perform the СDP.