Surface treatment method of calcium fluoride diaphragm, calcium fluoride diaphragm and deep ultraviolet laser

Abstract

The invention provides a surface treatment method of a calcium fluoride diaphragm (planar). The surface treatment method comprises the steps that an MgF2 thin film layer is deposited on the surface of the calcium fluoride diaphragm through electron beam thermal evaporation deposition; according to the surface treatment method of the calcium fluoride diaphragm, a MgF2-CaF2 diaphragm-MgF2-SiO2 film system structure is formed, and the two sides of the calcium fluoride diaphragm are plated with MgF2 antireflection films, so that the transmissivity of the calcium fluoride substrate in the deep ultraviolet band can be effectively improved, the transmittance of the calcium fluoride substrate in the deep ultraviolet band can be effectively improved, and the transmittance of the calcium fluoride diaphragm in the deep ultraviolet band can be effectively improved. According to the technology for depositing the SiO2 protective layer through the magnetron sputtering technology, a smooth, uniform and compact film layer with the average surface roughness smaller than 0.4 nm can be obtained through the technology, the calcium fluoride substrate and the magnesium fluoride anti-reflection layer can be effectively prevented from being damaged by volatile organic compounds, and therefore the stability of an optical system is greatly improved, and the service life of the optical system is greatly prolonged. In addition, the invention further provides a calcium fluoride diaphragm subjected to surface treatment and a deep ultraviolet laser.

Description

Technical Field

[0001] The present application relates to the field of optical films, and in particular to a surface treatment method of a calcium fluoride window, a calcium fluoride window and a deep ultraviolet laser. Background Art

[0002] Due to its excellent performance, deep ultraviolet (DUV) lasers are gradually being used in more and more fields such as semiconductor manufacturing, material processing, and biomedicine.

[0003] Calcium fluoride (CaF2) substrates have become the most widely used deep ultraviolet laser windows due to their high transmittance in the deep ultraviolet band, good physical and chemical stability, and high hardness. However, deep ultraviolet lasers usually have high power, and calcium fluoride windows may be damaged by thermal effects in deep ultraviolet lasers, thus affecting their service life. When a high-energy deep ultraviolet laser beam passes through a calcium fluoride window, certain areas of the window absorb the laser energy and convert it into heat energy. If the thermal conductivity of the window is not sufficient to quickly disperse this heat, the temperature around the absorption point will rise sharply. Since the thermal conductivity of calcium fluoride is relatively low, this causes the local temperature to rise, while other parts of the material remain at a lower temperature. This temperature gradient will generate thermal stress inside the material. In addition, the thermal expansion coefficient of calcium fluoride will also affect the stability of the window. When the material expands due to heat, different parts expand to different degrees, which will lead to the accumulation of internal stress. If the thermal stress exceeds the strength limit of the material, the window will break.

[0004] The reasons why thermal effects cause damage to calcium fluoride windows mainly include the following aspects:

[0005] (1) High-energy laser beam: Deep ultraviolet lasers usually produce laser beams with high energy density. When these laser beams pass through the window, part of the energy will be absorbed by the material and converted into heat energy;

[0006] (2) Thermal conductivity: Calcium fluoride has a relatively low thermal conductivity, which means that heat cannot diffuse away quickly from the absorption point. This local heating can lead to the formation of temperature gradients, which in turn cause thermal stress;

[0007] (3) Thermal expansion coefficient: Temperature differences at different locations can cause different parts of the material to expand or contract at different rates, which can also increase thermal stress, especially at the edges or center of the window.

[0008] (4) Optical damage: When the local temperature increases, the material may undergo structural changes, such as phase transition or melting, which will change its optical properties, such as changes in refractive index, and ultimately lead to a decrease in the quality of the laser beam;

[0009] (5) Mechanical damage: When thermal stress accumulates to a certain extent, it may cause cracks or breakage in the window.

[0010] In addition, since volatile organic compounds are ubiquitous in deep ultraviolet optical systems, the photodecomposition of these organic compounds under 193nm laser irradiation is one of the important sources of optical surface contamination. If these pollutants continue to accumulate over a period of time, they will cause a serious drop in transmittance and lead to degradation of the optical system. If the anti-reflection layer is directly exposed to the outermost part, cleaning the contaminants may cause the anti-reflection layer to be destroyed, thereby affecting the overall transmittance of the optical system. Summary of the invention

[0011] In view of this, it is necessary to provide a surface treatment method for a calcium fluoride window, a calcium fluoride window and a deep ultraviolet laser to improve the thermal damage threshold and service life of the calcium fluoride window in order to address the current technical problem of calcium fluoride window damage caused by thermal effects.

[0012] To solve the above problems, this application adopts the following technical solutions:

[0013] One of the purposes of the present application is to provide a surface treatment method for a calcium fluoride window, comprising the following steps:

[0014] Depositing a MgF2 thin film layer on the surface of the calcium fluoride window by electron beam thermal evaporation deposition;

[0015] A SiO2 protective layer is deposited on the MgF2 thin film layer by using magnetron sputtering technology.

[0016] In some embodiments, the step of depositing a MgF2 thin film layer on the surface of a calcium fluoride window by electron beam thermal evaporation deposition specifically includes the following steps:

[0017] Select high-purity MgF2 film material and add it into the evaporation crucible of the electron beam evaporation coating machine, and place the calcium fluoride window piece in the coating umbrella of the electron beam evaporation coating machine;

[0018] The vacuum chamber of the electron beam evaporation coating machine was evacuated to less than 8× using a mechanical pump and an oil diffusion pump.

[0019] 10 -4 Pa, using a heating wire to raise the temperature of the vacuum chamber to 270° C.;

[0020] 55 sccm of oxygen is introduced into the radio frequency ion source body of the electron beam evaporation coating machine, and 8 sccm of argon is introduced into the ion source neutralizer to generate plasma for substrate cleaning;

[0021] After the substrate is cleaned, a MgF2 thin film layer is deposited on a surface of the calcium fluoride window piece by electron beam thermal evaporation;

[0022] After the MgF2 thin film layer is deposited on the upper surface of the calcium fluoride window, the calcium fluoride window is taken out after cooling in the vacuum chamber for 60 minutes;

[0023] The calcium fluoride window sample was turned over 180 degrees, and the above steps were repeated to deposit a MgF2 thin film layer on the other surface of the calcium fluoride window.

[0024] In some of the embodiments, in the step of introducing 55 sccm of oxygen into the RF ion source body of the electron beam evaporation coating machine and 8 sccm of argon into the ion source neutralizer to generate plasma for substrate cleaning, the electron beam evaporation coating machine plasma substrate cleaning time is 90 seconds, the BEAM voltage is 700V, and the current is 700mA.

[0025] In some of the embodiments, after the substrate is cleaned, in the step of depositing a MgF2 thin film layer on a surface of the calcium fluoride window by electron beam thermal evaporation, the MgF2 thin film deposition rate is 0.3 nm / s, the electron gun generating the electron beam is a 270-degree deflection electron gun, the high voltage is set to 6 KV, and the current is controlled according to the real-time deposition rate.

[0026] In some embodiments, after the substrate is cleaned, in the step of depositing a MgF2 thin film layer on a surface of the calcium fluoride window by electron beam thermal evaporation, the deposition temperature of the electron beam evaporation coating machine is 270°C.

[0027] In some of the embodiments, in the step of depositing the SiO2 protective layer on the MgF2 thin film layer using magnetron sputtering technology, the following steps are specifically included: transferring the calcium fluoride window plate on which the MgF2 thin film layer is deposited to the fixture plate of the sputtering coating machine, and selecting high-purity elemental silicon as the sputtering target;

[0028] Use mechanical pump and molecular pump to evacuate the vacuum chamber of the sputtering coating machine to less than 8×10 -4 Pa, use the heating wire to raise the fixture plate temperature to 150°C;

[0029] High-purity elemental silicon sputters Si ions in the sputtering gas Ar, and the sputtering coating machine ion source is filled with high-purity O2 and Ar to generate a plasma beam. The target sputtering power supply and the sputtering coating machine ion source work together to obtain SiO2 thin film material, which is deposited on the MgF2 thin film layer.

[0030] In some of the embodiments, in the step of sputtering Si ions from high-purity silicon in the sputtering gas Ar, filling the ion source of the sputtering coater with high-purity O2 and Ar to produce a plasma beam, the target material sputtering power supply and the ion source of the sputtering coater work together to obtain the SiO2 thin film material, and depositing it on the MgF2 thin film layer, the gas flow rate of the sputtering gas Ar is 35sccm, the sputtering power of the sputtering coater is 2000W, the charging power of the ion source of the sputtering coater is 2500W, the gas flow rate of the gas Ar is 40sccm, and the gas flow rate of the gas O2 is 50sccm.

[0031] The second purpose of the present application is to provide a calcium fluoride window obtained by the surface treatment method of the calcium fluoride window, comprising: a calcium fluoride window, a MgF2 thin film layer deposited on the surface of the calcium fluoride window, and a SiO2 protective layer deposited on the surface of the MgF2 thin film layer.

[0032] The third object of the present application is to provide a deep ultraviolet laser, comprising the calcium fluoride window.

[0033] This application adopts the above technical solution, and its beneficial effects are as follows:

[0034] The surface treatment method of the calcium fluoride window provided by the application comprises depositing a MgF2 thin film layer on the surface of the calcium fluoride window by electron beam thermal evaporation deposition; depositing a SiO2 protective layer on the MgF2 thin film layer by magnetron sputtering technology. The surface treatment method of the calcium fluoride window provided by the application forms a film system structure of MgF2-CaF2 window-MgF2-SiO2. The process of plating MgF2 anti-reflection film on both sides of the calcium fluoride window can effectively improve the transmittance of the calcium fluoride substrate in the deep ultraviolet band. The process of depositing the SiO2 protective layer by magnetron sputtering technology can obtain a smooth, uniform and dense film layer with an average surface roughness of less than 0.4nm, which can effectively prevent the calcium fluoride substrate and the magnesium fluoride anti-reflection layer from being damaged by volatility, thereby greatly improving the stability and service life of the optical system. It is worth noting that this film system design only evaporates the SiO2 protective layer on one side of the calcium fluoride window. Because it is in direct contact with the air, it is more susceptible to contamination by volatile organic compounds. This design improves the overall coating efficiency and reduces film material consumption while protecting the magnesium fluoride anti-reflection layer and the calcium fluoride substrate, and is the most economical solution. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments of the present application or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0036] Figure 1 A flow chart of the steps of the surface treatment method of the calcium fluoride window provided in an embodiment of the present application.

[0037] Figure 2 A flow chart of the steps of depositing a MgF2 thin film layer on the surface of a calcium fluoride window using electron beam thermal evaporation deposition provided in an embodiment of the present application.

[0038] Figure 3 This is a theoretical spectrum of the transmittance of a calcium fluoride window in which a MgF2 thin film layer is deposited on the surface of a calcium fluoride window by electron beam thermal evaporation deposition in this embodiment.

[0039] Figure 4 A flow chart of the steps of depositing a SiO2 protective layer on the MgF2 thin film layer using magnetron sputtering technology provided in an embodiment of the present application.

[0040] Figure 5 An atomic force microscope image of depositing a SiO2 protective layer on the MgF2 thin film layer using magnetron sputtering technology provided in an embodiment of the present application.

[0041] Figure 6 A schematic diagram of the structure of a surface-treated calcium fluoride window provided in an embodiment of the present application. DETAILED DESCRIPTION

[0042] Embodiments of the present application are described in detail below, and examples of the embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are intended to be used to explain the present application, and should not be construed as limiting the present application.

[0043] In the description of the present application, it should be understood that the terms "upper", "lower", "horizontal", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present application.

[0044] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, the meaning of "plurality" is two or more, unless otherwise clearly and specifically defined.

[0045] In order to make the objectives, technical solutions and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0046] See also Figure 1 The step flow chart of the surface treatment method of the calcium fluoride window provided in the embodiment of the present application includes the following steps S110 to S120, and the implementation method of each step is described in detail below. The specific implementation method is described in detail below.

[0047] Step S110: depositing a MgF2 thin film layer on the surface of the calcium fluoride window piece by electron beam thermal evaporation deposition.

[0048] See also Figure 2 , which is a flow chart of the steps of depositing a MgF2 thin film layer on the surface of a calcium fluoride window piece by electron beam thermal evaporation deposition, specifically includes the following steps:

[0049] Step S111: Select high-purity MgF2 film material and add it into the evaporation crucible of the electron beam evaporation coating machine, and place the calcium fluoride window piece in the coating umbrella of the electron beam evaporation coating machine.

[0050] Step S112: Use a mechanical pump and an oil diffusion pump to evacuate the vacuum chamber of the electron beam evaporation coating machine to a vacuum level less than 8×10 -4 Pa, the temperature of the vacuum chamber was raised to 270°C using a heating wire.

[0051] Step S113: 55 sccm of oxygen is introduced into the radio frequency ion source body of the electron beam evaporation coating machine, and 8 sccm of argon is introduced into the ion source neutralizer to generate plasma for substrate cleaning.

[0052] Specifically, the electron beam evaporation coating machine plasma substrate cleaning time is 90 seconds, the BEAM voltage is 700V, and the current is 700mA.

[0053] Step S114: after the substrate is cleaned, a MgF2 thin film layer is deposited on a surface of the calcium fluoride window piece by electron beam thermal evaporation.

[0054] Specifically, the deposition rate of the MgF2 film is 0.3 nm / s, the electron gun generating the electron beam is a 270-degree deflection electron gun, the high voltage is set to 6 KV, and the current is controlled according to the real-time deposition rate.

[0055] Specifically, the deposition temperature of the electron beam evaporation coating machine is 270°C.

[0056] Step S115: After the MgF2 thin film layer is deposited on the upper surface of the calcium fluoride window, the calcium fluoride window is taken out after cooling in the vacuum chamber for 60 minutes.

[0057] Step S116: Flip the calcium fluoride window sample 180 degrees, and repeat the above steps to deposit a MgF2 thin film layer on the other surface of the calcium fluoride window.

[0058] See also Figure 3 , is the theoretical spectrum of transmittance of the calcium fluoride window piece in which the MgF2 thin film layer is deposited on the surface of the calcium fluoride window piece by electron beam thermal evaporation deposition in this embodiment, from Figure 3 It can be seen that the present application utilizes electron beam thermal evaporation deposition to deposit a MgF2 thin film layer on the surface of the calcium fluoride window, which can effectively improve the transmittance of the calcium fluoride window in the ultraviolet band.

[0059] Step S120: depositing a SiO 2 protective layer on the MgF 2 thin film layer by using magnetron sputtering technology.

[0060] See also Figure 4 , which is a flow chart of the steps of depositing a SiO2 protective layer on the MgF2 thin film layer using magnetron sputtering technology provided in this embodiment, includes the following steps:

[0061] Step S121: transfer the calcium fluoride window plate on which the MgF2 thin film layer is deposited to the fixture plate of the sputtering coating machine, and select high-purity silicon as the sputtering target.

[0062] Step S122: Use a mechanical pump and a molecular pump to pump the vacuum chamber of the sputtering coating machine to a vacuum of less than 8×10 -4 Pa, and the fixture plate temperature is raised to 150°C using a heating wire.

[0063] Specifically, the gas flow rate of the sputtering gas Ar is 35 sccm, the sputtering power of the sputtering coater is 2000 W, the ion source charging power of the sputtering coater is 2500 W, the gas flow rate of the charging gas Ar is 40 sccm, and the gas flow rate of the gas O2 is 50 sccm.

[0064] Step S123: High-purity elemental silicon sputters Si ions in the sputtering gas Ar, the sputtering coating machine ion source is filled with high-purity O2 and Ar to generate a plasma beam, the target sputtering power supply and the sputtering coating machine ion source work together to obtain SiO2 thin film material, and deposit it on the MgF2 thin film layer.

[0065] After the SiO2 film deposition is completed, it is cooled in the vacuum chamber for 60 minutes, and the sample is taken out to obtain a surface-treated calcium fluoride window.

[0066] See also Figure 5 , which is an atomic force microscope image and data of depositing a SiO2 protective layer on the MgF2 thin film layer using magnetron sputtering technology provided in this embodiment, by Figure 5It can be seen that in this embodiment, a smooth SiO2 protective film layer with a surface roughness Ra<0.4 nm can be obtained by ion sputtering deposition, which is beneficial to increasing the thermal damage threshold and service life of the calcium fluoride window.

[0067] It can be understood that the existing technology has higher requirements for the protective layer film material. The preparation of F-SiO2 raw materials is relatively difficult and needs to be customized. The protective layer film material provided in the present application is ordinary SiO2, which is low in cost and easy to purchase; at the same time, in the prior art, in order to enhance the density of the protective layer, it is necessary to additionally use high-temperature deposition (>300°C), reverse mask deposition technology, in-situ or post-deposition plasma ion treatment, and the process flow is complex and demanding. The present application adopts ion sputtering deposition to form a dense and uniform SiO2 protective layer by optimizing the deposition process, thereby greatly improving production efficiency; in addition, in the prior art, the surface roughness of the protective layer is not controlled, and the size of the surface roughness will affect the flux and service life of the optical system.

[0068] See also Figure 6 , the schematic diagram of the structure of the surface-treated calcium fluoride window provided in the above embodiment of the present application, including a calcium fluoride window (CaF2 substrate), a MgF2 film layer deposited on the surface of the calcium fluoride window, and a SiO2 protective layer deposited on the surface of the MgF2 film layer. Please refer to the table below for the specific number of layers and thickness of the film system design provided in one of the embodiments.

[0069] Number of layers (starting from the laser cavity) Material thickness 1 <![CDATA[MgF2]]> 33.6nm 2 <![CDATA[CaF2]]> 50mm 3 <![CDATA[MgF2]]> 33.6nm 4 <![CDATA[SiO2]]> 5nm

[0070] The surface-treated calcium fluoride window provided in the above-mentioned embodiments of the present application can be used to prepare a deep ultraviolet laser.

[0071] The surface treatment method of the calcium fluoride window provided in the above embodiment of the present application forms a film system structure of MgF2-CaF2 window-MgF2-SiO2, and the process of plating MgF2 anti-reflection film on both sides of the calcium fluoride window can effectively improve the transmittance of the calcium fluoride substrate in the deep ultraviolet band. The process of depositing the SiO2 protective layer using magnetron sputtering technology can obtain a smooth, uniform and dense film layer with an average surface roughness of less than 0.4nm, which can effectively prevent the calcium fluoride substrate and the magnesium fluoride anti-reflection layer from being damaged by volatility, thereby greatly improving the stability and service life of the optical system.

[0072] It can be understood that the technical features of the above-described embodiments can be arbitrarily combined. In order to make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0073] The above are only preferred embodiments of the present application, and only specifically describe the technical principles of the present application. These descriptions are only for explaining the principles of the present application and cannot be interpreted as limiting the scope of protection of the present application in any way. Based on the explanation here, any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application, and other specific implementation methods of the present application that can be associated with the technicians in this field without creative work, should be included in the scope of protection of the present application.

Claims

1. A surface treatment method for a calcium fluoride window, characterized in that: The steps include: Depositing a MgF2 thin film layer on the surface of the calcium fluoride window by electron beam thermal evaporation deposition; A SiO2 protective layer is deposited on the MgF2 thin film layer by using magnetron sputtering technology.

2. The surface treatment method of calcium fluoride window according to claim 1, characterized in that: The step of depositing a MgF2 thin film layer on the surface of a calcium fluoride window piece by electron beam thermal evaporation deposition specifically includes the following steps: Select high-purity MgF2 film material and add it into the evaporation crucible of the electron beam evaporation coating machine, and place the calcium fluoride window piece in the coating umbrella of the electron beam evaporation coating machine; The vacuum chamber of the electron beam evaporation coating machine was evacuated to a vacuum less than 8×10 -4 Pa, using a heating wire to raise the temperature of the vacuum chamber to 270° C.; 55 sccm of oxygen is introduced into the radio frequency ion source body of the electron beam evaporation coating machine, and 8 sccm of argon is introduced into the ion source neutralizer to generate plasma for substrate cleaning; After the substrate is cleaned, a MgF2 thin film layer is deposited on a surface of the calcium fluoride window piece by electron beam thermal evaporation; After the MgF2 thin film layer is deposited on the upper surface of the calcium fluoride window, the calcium fluoride window is taken out after cooling in the vacuum chamber for 60 minutes; The calcium fluoride window sample was turned over 180 degrees, and the above steps were repeated to deposit a MgF2 thin film layer on the other surface of the calcium fluoride window.

3. The surface treatment method of calcium fluoride window according to claim 2, characterized in that: In the step of introducing 55 sccm oxygen into the RF ion source body of the electron beam evaporation coating machine and 8 sccm argon into the ion source neutralizer to generate plasma for substrate cleaning, the electron beam evaporation coating machine plasma substrate cleaning time is 90 seconds, the BEAM voltage is 700V, and the current is 700mA.

4. The surface treatment method of calcium fluoride window according to claim 2, characterized in that: After the substrate is cleaned, in the step of depositing a MgF2 thin film layer on one surface of the calcium fluoride window piece by electron beam thermal evaporation, the MgF2 thin film deposition rate is 0.3nm / s, the electron gun generating the electron beam is a 270-degree deflection electron gun, the high voltage is set to 6KV, and the current is controlled according to the real-time deposition rate.

5. The surface treatment method of calcium fluoride window according to claim 2, characterized in that: After the substrate is cleaned, in the step of depositing a MgF2 thin film layer on a surface of the calcium fluoride window plate by electron beam thermal evaporation, the deposition temperature of the electron beam evaporation coating machine is 270°C.

6. The surface treatment method of calcium fluoride window according to claim 1, characterized in that: In the step of depositing the SiO2 protective layer on the MgF2 thin film layer by using magnetron sputtering technology, the following steps are specifically included: transferring the calcium fluoride window plate on which the MgF2 thin film layer is deposited to the fixture plate of the sputtering coating machine, and selecting high-purity elemental silicon as the sputtering target; Use mechanical pump and molecular pump to evacuate the vacuum chamber of the sputtering coating machine to less than 8×10 -4 Pa, use the heating wire to raise the fixture plate temperature to 150°C; High-purity elemental silicon sputters Si ions in the sputtering gas Ar, and the sputtering coating machine ion source is filled with high-purity O2 and Ar to generate a plasma beam. The target sputtering power supply and the sputtering coating machine ion source work together to obtain SiO2 thin film material, which is deposited on the MgF2 thin film layer.

7. The surface treatment method of calcium fluoride window according to claim 6, characterized in that: In the step in which high-purity elemental silicon is sputtered to produce Si ions in the sputtering gas Ar, the ion source of the sputtering coater is filled with high-purity O2 and Ar to produce a plasma beam, the target material sputtering power supply and the ion source of the sputtering coater work together to obtain the SiO2 thin film material, and deposit it on the MgF2 thin film layer, the gas flow rate of the sputtering gas Ar is 35sccm, the sputtering power of the sputtering coater is 2000W, the charging power of the ion source of the sputtering coater is 2500W, the gas flow rate of the gas Ar is 40sccm, and the gas flow rate of the gas O2 is 50sccm.

8. A calcium fluoride window obtained by the surface treatment method of a calcium fluoride window according to any one of claims 1 to 6, characterized in that: include: A calcium fluoride window piece, a MgF2 film layer deposited on the surface of the calcium fluoride window piece, and a SiO2 protective layer deposited on the surface of the MgF2 film layer.

9. A deep ultraviolet laser, characterized in that: The invention comprises the calcium fluoride window piece as claimed in claim 8.

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