Calcium fluoride single crystal ultra-precision machining method based on chemico-mechanical polishing and ion beam polishing combined process

Abstract

The invention disclose a calcium fluoride single crystal ultra-precision machining method based on a chemico-mechanical polishing and ion beam polishing combined process. The method comprises the following steps of: polishing a calcium fluoride single crystal by adopting a chemico-mechanical polishing, removing surface damages and sub-surface damages on a calcium fluoride surface, and exposing an ideal atom layer to obtain a super-smooth crystal surface; and improving the surface figure accuracy of the super-smooth crystal surface obtained in the previous step by adopting an ion beam polishing method to realize ultra-precision machining of a calcium fluoride single crystal with nano-grade or sub-nano-grade surface figure accuracy and a super-smooth surface. Due to the adoption of the machining method disclosed by the invention, respective technological advantages of chemico-mechanical polishing and ion beam polishing can be utilized fully; and moreover, the machining efficiency and the machining accuracy are high, and the product quality is good.

Description

technical field [0001] The invention relates to the technical field of ultra-precision processing of calcium fluoride crystals, in particular to an ultra-precision processing method for calcium fluoride single crystals by combining multiple polishing methods. Background technique [0002] Calcium fluoride crystals have excellent transmittance in the 0.125nm-10μm band, and have excellent achromatic and apochromatic capabilities, and are widely used in the manufacture of lenses, prisms and window optical components. In particular, with the transfer of lithography technology to short wavelengths, calcium fluoride has become a lithography system and high-energy Lens material of choice for UV laser systems. Excellent optical surfaces are the basis for high optical performance. The application of ultraviolet band not only requires the accuracy of calcium fluoride surface shape, but also requires extremely small surface roughness, and high anti-laser damage threshold and ultravio...

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Problems solved by technology

[0003] However, calcium fluoride crystals have low macroscopic hardness, are easily broken, are anisotropic, and have a relatively high thermal expansion coefficient, which poses a huge challenge to ultra-precision processing. One of the key factors in the development of optical systems

The traditional grinding and polishing processing methods and single-point diamond cutting are difficult to obtain the ideal surface shape accuracy. The processed calcium fluoride (111) crystal plane presents a fan-shaped structure with three high regions and three low regions (called "three-lobe effect"). ”), this is because the hardness of calcium fluoride is related to the surface crystal orientation, so that the material removal rate is anisotropic, and there are brittle areas and plastic areas during turning, and small broken points are easily generated in the brittle areas, resulting in brittle damage

In addition, the polishing powder in the traditional grinding and polishing liquid is easy to agglomerate, and damages such as scratches and impurity embedding will be formed on the soft calcium fluoride surface.

Magneto-rheological polishing technology can overcome the "three-lobe effect" by changing the residence time or material removal rate in the local area, but it is difficult to prevent the iron powder and polishing powder in the magnetorheological fluid from embedding into the polished optical surface

Float polishing and chemical mechanical polishing are ideal methods to obtain ultra-smooth surfaces without damage, but it is difficult to obtain surfaces with nanometer surface accuracy, especially spherical and aspheric surfaces, which are beyond the processing range of existing float polishing and chemical mechanical polishing

Ion beam polishing is the best way to obtain a surface with nanoscale or even sub-nanometer surface accuracy without causing surface and subsurface damage, but it is difficult for ion beam polishing to remove scratches, impurity embedding, brittle damage, etc. formed on the initial surface at the same time damage, which are difficult to remove with existing techniques

Therefore, it is difficult to meet the high-precision and ultra-smooth surface requirements of calcium fluoride in the ultraviolet band by using the above methods alone.

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