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Liquid etching of silicon at manufacturing of hologram optical elements

Keywords:

S.B. Odinokov - Dr.Sc. (Eng.), Associate Professor, Bauman Moscow State Technical University
E-mail: odinokov@ bmstu.ru
H.R. Sagatelyan - Dr.Sc. (Eng.), Associate Professor, Bauman Moscow State Technical University
E-mail: h_sagatelyan@mail.ru
Ye.A. Drozdova - Junior Research Scientist, Bauman Moscow State Technical University
E-mail: drozdova.e.a@yandex.ru
A.Yu. Betin - Engineer, Bauman Moscow State Technical University


Hologram optical elements (HOE) are optical parts containing the image forming calculated relief of the functional surface, due to which they are able to convert any input wave front of optical radiation into any output wave front. HOE, which is a sign-symbolic information indicator, considers. This HOE provides an image of the sighting mark in the holographic riflescope.
A kind of HOE, working on reflection, is discusses. It is offered to make it of monocrystalline silicon on the polished surface of wafers - substrates of integrated circuits. The image-restoring pattern of the working surface of the HOE was obtained by exposure of the photoresist using the photomask manufactured by the method of electron-beam lithography. This article examines the technological possibilities of the formation of the functional topography of this HOE by polishing liquid etching of silicon through a mask obtained by photolithography.
The mixture of hydrofluoric acid, nitric acid and acetic acid was considered as an etcher during the experimental studies in accordance with the existing recommendations. To determine the influence of each component of the solution on the performance characteristics of the HOE, the 23 plan of experiment was implemented. An option of etching through a mask made of photoresist was applied. The output parameter of each trial was the quality of image restoration with a sample containing the resulting relief. The quality of image restoration was estimated by the method of expert evaluations by the value Q on a one hundred-point scale.
Based on the results of this experiment, a mathematical model is obtained that establishes the dependence of the quality of the re-covered image Q on the studied factors and the interactions of these factors. The highest quality of the restored image is provided by a etcher containing components at the following quantities of parts by mass: HF-1 part.; HNO3 – 10 parts.; CH3COOH – 3 parts. Further experimental studies were carried out using this ratio of components.
The experimental research of the quality of image reconstruction by varying the time of etching through a mask of photoresist in the range of 14 – 23 seconds was conducted. Upper limit of this interval corresponds to the complete dissolution of the mask. Along with estimating the quality of the recovered image, the depth of the microrelief, formed because of etching, was measured by the profilo-graph-profilometer. It was found that the image recovery quality is the highest (Q = 98%) when the depth of the relief approaches a quarter of the wavelength. On the depths of relief of about the half of wavelength the quality of the recovered image is somewhat reduced (Q = 85 – 90%). At the depths of about three-quarters of the wavelength the image quality increases again (Q = 95%).
Etching of silicon using a chrome mask showed that when the depth of relief is less than 1 µm the application of chromium does not increase the restoration quality of the image relative to the etching through a mask of photoresist. With the increase in the duration of the process of liquid etching of silicon through the chrome mask, the depth of the relief linearly increases to depths H = 5 – 6 m. This indicates the absence of passivation of the etched surface.
There is no reason to improve the quality of the restored image by increasing the depth of the relief using the method of liquid etching of silicon. This is due to the isotropic nature of liquid etching under the studied conditions. The increase in the depth of the relief is accompanied by an increase in distortions of the restoring pattern of the surface. With increasing the etching depth by liquid etching of silicon the quality of image restoration as a whole decreases, and when using a mask made of photoresist this declining is significantly more intensive than when using a chrome mask.
The highest quality of image restoration is observed at relatively small etching depths, and the use of a photoresist mask is more pre-ferable than the use of a chrome mask. In the latter case, additional pattern distortions occur when performing two operations - liquid chromium etching through a photoresist mask and liquid silicon etching through a chromium mask, and when using a photoresist mask – only in the course of one operation – liquid silicon etching through a photoresist mask.

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May 29, 2020

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