CN110551979B - Silicon carbide surface modification method - Google Patents

Abstract

The invention relates to a silicon carbide surface modification method, which adopts PVD (physical vapor deposition) modification, and comprises the steps of cleaning a substrate, detecting the roughness before modification, putting the substrate into a clean coating machine, and putting coating materials of nickel (Ni) and silicon (Si) at corresponding positions of the coating machine; and (3) improving the vacuum degree in the vacuum chamber of the film coating machine, plating film material nickel (Ni) and film material silicon (Si) after the vacuum degree reaches the modification standard, and detecting the roughness after the modification is finished. Compared with the traditional PVD (physical vapor deposition) modification method, the method can directly reduce the surface roughness of the substrate, save the optical polishing process after modification, save the manpower, material resources and financial resources consumed in the optical polishing process, and avoid corresponding risks.

Description

Silicon carbide surface modification method

Technical Field

The invention relates to a SiC surface modification method, belongs to the technical field of optical equipment, and can be applied to the technical field of film optical preparation.

Background

SiC materials have gained widespread use in recent years in the aerospace field due to their good physical, mechanical and thermal properties. Due to many factors such as production process, material characteristics, component composition and the like, many micropores exist on the surface of SiC, and the surface compactness degree is far less than that of a glass material.

And (3) carrying out surface polishing on the SiC mirror blank with the defects of residual micropores and the like, wherein the limit range of the surface roughness after polishing is 3-4 nm, and at the moment, the optical polishing cannot further improve the surface precision and cannot meet the technical requirement of the surface roughness required by the reflector of the high-precision space camera. The surface scattering and the surface roughness are in positive correlation, if the SiC reflector is directly applied, a large amount of scattering can cause light energy loss of the space optical system, and meanwhile, the generated stray light can influence the imaging quality of the space optical system. In addition, since SiC has high hardness and chemical stability, it is difficult to directly machine it into a high-precision optical surface (mirror surface having a small RMS value).

In order to further improve the surface precision of the SiC reflector and obtain a high-precision SiC mirror blank, the SiC mirror blank needs to be subjected to surface modification treatment, and when the SiC mirror blank is modified by adopting a Physical Vapor Deposition (PVD) method, a dense modification layer Si with a certain thickness is essentially plated on the surface of the SiC mirror blank, and the thickness of the modification layer Si is generally more than 10 mu m, so that the surface defect is improved and the surface precision is improved. After the coating process of the modified layer is finished, the surface roughness of the SiC mirror blank is still high, even higher than the roughness before modification, and the modified layer on the surface of the SiC mirror blank needs to be subjected to optical cold processing, namely the surface of the mirror blank is subjected to optical precision grinding and polishing, so that a smooth surface with the roughness of about 1nm is finally obtained, and the requirement of a corresponding surface shape is met.

The traditional modification method has the following risks from the step of plating a modified layer to the step of polishing the modified layer:

sputtering points are easy to appear in the PVD modified coating process, and the sputtering points can be easy to become modified layer falling points in the polishing process, so that modification failure is caused;

the modified layer is thick, so that the time for plating the modified layer is long, the equipment and personnel are tested, and certain risks exist;

there is a risk of over-processing the modified layer during the optical polishing process and wearing through the modified layer. After the modified layer is worn, the modified layer needs to be plated again, and optical polishing is carried out again, so that the time and the labor are consumed, and the processing period is prolonged.

Disclosure of Invention

The technical problem solved by the invention is as follows: the defect that the surface roughness is unchanged or increased after the traditional silicon carbide surface modification is overcome, and the silicon carbide surface modification method is provided, so that the problems that after the traditional modification method is finished, the roughness needs to be reduced by continuous polishing, and time and labor are consumed are solved.

The technical scheme of the invention is as follows: a silicon carbide surface modification method comprises the following steps:

1) selecting coating materials of nickel and silicon, and respectively placing the coating materials in a coating machine evaporation-resistant boat and a crucible;

2) vacuumizing, and starting heating when the vacuum degree reaches high vacuum;

3) when the temperature T is reached, nickel is plated;

4) keeping the temperature T, and continuously plating silicon;

5) after the two coatings are plated, the coating is bombarded by an ion source.

The high vacuum degree is less than or equal to 3x10^-3Pa。

The T is 290-310 ℃.

The T is 300 DEG C

The thickness of the nickel plated in the step 3) is 25nm-30 nm.

The thickness of the plated silicon in the step 4) is 700nm-900 nm.

The bombardment time is 4-6 minutes.

The ion source bombardment time was 5 minutes.

The ion source working parameters are 120V and 150 mA.

The distance d between each evaporation source of the film plating machine and the plated piece is less than 1 m.

Compared with the prior art, the invention has the advantages that:

(1) according to the invention, two coating materials are selected, and proper coating process parameters are applied, so that the roughness of the surface of the silicon carbide is reduced, the surface scattering is inhibited, the surface precision is improved, the optical grinding and polishing processing of the substrate is not required, and the time and the labor are saved;

(2) the traditional modification technology needs to plate a thicker modified layer on a substrate to cover the surface defects. In the existing method, two modification layers are selected, so that the surface roughness of SiC meets the precision requirement of directly plating a reflective film, and meanwhile, the thickness of a plated film material is much thinner than that required by conventional modification, so that some risks of conventional modification, such as abrasion of elements of a film plating machine, occurrence of splash points and the like, are avoided, and the success rate of modification is greatly increased;

(3) the traditional method for modifying the silicon carbide surface relates to a subsequent optical polishing process, and the method skips the optical polishing process after SiC modification, thereby saving a large amount of related resources such as manpower, material resources, financial resources and the like to a great extent and avoiding related risks of the optical polishing.

Drawings

FIG. 1 is a schematic view of a film structure;

FIG. 2 is a schematic view of a coating process;

FIG. 3 is a schematic diagram of roughness before coating;

FIG. 4 is a schematic diagram of roughness after plating.

Detailed Description

The invention avoids the risk in the process of plating the modified layer, skips the subsequent polishing optical processing process, and provides a novel method for plating the modified silicon carbide surface by utilizing a film material growth and refinement interaction mechanism, thereby finally obtaining the high-precision optical mirror surface of the SiC mirror blank. Nickel (Ni) and silicon (Si) are deposited on the surface of the unmodified silicon carbide by using a thermal evaporation process, and the structural design of the film layer is shown in fig. 1. The essence is that the growth of the silicon carbide surface in the holes is completed by utilizing the growth characteristics of the material, so that the purpose of filling the holes is achieved, meanwhile, the interaction of the film materials has a refining mechanism, so that the surface roughness is reduced, the surface scattering after film coating is inhibited, the surface modification of the SiC mirror blank is completed, and the surface precision of the SiC reflector is improved.

The invention utilizes a film material growth mechanism, selects film materials of nickel (Ni) and silicon (Si), designs a related film system, adjusts film coating parameters including film thickness, ion source parameters and other data, and a film coating process schematic diagram of each working element placing machine in a film coating machine is shown in figure 2, wherein the specific design method comprises the following steps:

(1) cleaning the substrate, and testing the substrate before roughness coating, wherein the test result is shown in figure 3;

(2) selecting film material nickel (Ni) and film material silicon (Si);

(3) placing film material nickel (Ni) on a steaming-resistant boat;

(4) placing film material silicon (Si) in an electron gun 1;

(5) vacuumizing, and when the vacuum degree reaches 3x10 according to the actual state of the film coating machine^-3Heating is started when the pressure is Pa;

(6) when the temperature reaches 300 ℃, firstly plating nickel (Ni) with the thickness of 30 nm;

(7) keeping the temperature at 300 ℃, and continuously plating silicon (Si) with the thickness of 800 nm;

(8) after the two coatings are plated, bombarding for 5 minutes by an ion source with working parameters of 120V and 150 mA.

(9) And after the coating is finished, carrying out roughness detection on the substrate, wherein the test result is shown in figure 4.

The basic principle of the vacuum degree required by PVD coating is that the mean free path of gas molecules is larger than the distance between an evaporation source and a piece to be coated. According to the gas molecule motion theory, the gas molecule mean free path:

where σ is the gas molecule diameter, n is the density of the gas molecules, p is the gas pressure corresponding to n, and if the distance d from the evaporation source to the article to be plated is 1m, the degree of vacuum required for determining PVD should be such that

The distance d between each evaporation source of the film plating machine and the plated piece is less than 1m, and the vacuum degree is 3x10^-3And (3) coating a film layer when the pressure is Pa, so that the theoretical design requirement is met.

The high temperature is as follows:

1) removing residual gas adsorbed on the surface of the substrate to increase the binding force between the substrate and deposited molecules;

2) the physical adsorption of the film layer is promoted to be converted into chemical adsorption, the action among molecules is enhanced, the film layer is compact, the adhesive force is increased, and the mechanical capacity is improved;

3) the difference between the crystallization temperature of vapor molecules and the temperature of the substrate is reduced, the film layer concentration is improved, the film layer hardness is increased, and the internal stress is eliminated.

The film structure change or the film material decomposition can be caused by overhigh temperature, the good interaction between the film materials is reduced, and the final temperature is selected to be 300 ℃ through experimental verification.

The SiC substrate adopting the new modification method is subjected to roughness test at the same position of the substrate before and after modification, and the test result shows that the distance between the highest point and the lowest point of the surface of the substrate before modification is 62.69+ 89.45-152.14 nm, the distance between the highest point and the lowest point of the surface after modification is 47.30+ 45.39-92.69 nm, the PV value of the surface of the substrate is reduced, which shows that the surface of the substrate tends to be smoother and the roughness is reduced. Test results show that the new modification method is adopted, namely after the surface of the SiC substrate is plated with film materials of nickel (Ni) and silicon (Si), the surface roughness of the SiC substrate is reduced, and the roughness reaches the precision requirement of plating a reflector; the scattering is suppressed; various resources required by the optical polishing process are saved.

The method overcomes the defect that the surface roughness is unchanged or increased after the traditional silicon carbide surface is modified, and solves the problems that the traditional modification method needs to continuously grind and polish to reduce the roughness and is time-consuming and labor-consuming.

The invention is not described in detail and is within the knowledge of a person skilled in the art.

Claims (1)

1. A silicon carbide surface modification method is characterized by comprising the following steps:

1) selecting film materials of nickel and silicon, and respectively placing the film materials in a film plating machine evaporation-resistant boat and an electronic crucible;

2) vacuumizing, and starting heating when the vacuum degree reaches high vacuum;

3) when the temperature T is reached, nickel is plated;

4) keeping the temperature T, and continuously plating silicon;

5) after the two kinds of film materials are plated, bombarding by an ion source;

the high vacuum degree is less than or equal to 3x10^-3Pa；

The T is 290-310 ℃;

the thickness of the nickel plated in the step 3) is 25nm-30 nm;

the thickness of the plated silicon in the step 4) is 700nm-900 nm;

the bombardment time of the ion source is 4-6 minutes;

the working parameters of the ion source are 120V and 150 mA;

the distance d between each evaporation source of the film plating machine and the plated piece is less than 1 m.

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