1. Field of the Invention
The present invention relates to methods for manufacturing optical elements, and more particularly, relates to a method for manufacturing an optical element formed of a CaF2 material having resistance against plasma damage.
2. Description of the Related Art
CaF2 is a necessary and essential material for forming optical elements used in a vacuum ultraviolet region. The optical elements formed of a CaF2 material have been used in various optical devices such as a semiconductor exposure apparatus and a broadcasting camera.
Optical elements are manufactured by polishing, cleaning, and film-forming steps, as shown in FIG. 10. After a raw material primarily composed of CaF2 for an optical element is polished to form a desired shape in the polishing step, fragments of an abrasive powder and organic materials adhered to the CaF2 are then removed in the cleaning step, and in the film-forming step, an optical thin film such as an antireflection film is formed on a surface of the CaF2 substrate.
As a cleaning method performed in the cleaning step, for example, there may be mentioned ultraviolet cleaning using ultraviolet rays or plasma cleaning using plasma. In particular, in the case of the plasma cleaning, compared to the ultraviolet cleaning, the cleaning time can be significantly decreased.
As a film-forming method used in the film-forming step, vacuum deposition, sputtering, or the like may be mentioned. In the case of sputtering using plasma, compared to the case of vacuum deposition, the film thus formed has a high film density, and an optical thin film having superior laser and environment resistance can be formed.
However, as is the cleaning step or the film-forming step using plasma described above, when treatment generating plasma is carried out in a chamber in which an optical element formed of CaF2 is placed, plasma damage is generated in the optical element. In this specification, the plasma damage means the cause of optical loss which is generated by bombardment of charged particles onto a CaF2 substrate when treatment (hereinafter referred to as xe2x80x9cplasma treatmentxe2x80x9d) generating plasma, such as plasma cleaning or sputtering, is performed in a chamber in which the CaF2 substrate is placed. This plasma damage results in decrease in transmittance of an optical element.
That is, in a method for manufacturing an optical element, when plasma treatment is performed in a cleaning step or a film-forming step, the cleaning time can be significantly decreased, or an optical thin film having superior laser or environmental resistance can be formed. However, on the other hand, a problem may arise in that the transmittance of the element thus formed is decreased by plasma damage.
FIG. 11 shows the difference in transmittance of a plasma-damaged CaF2 substrate from that of a CaF2 substrate free from plasma damage as optical loss (ratio of decrease in transmittance). In FIG. 11, the horizontal axis indicates wavelengths, and the vertical axis indicates values obtained by deducting the transmittance of a CaF2 substrate which is processed with plasma treatment from that of a CaF2 substrate which is not processed therewith. That is, this value of the optical loss represents the decrease in transmittance of the CaF2 substrate caused by plasma treatment. The optical loss caused by plasma damage depends on output power of plasma or time exposed thereto; however, in a visible region of a wavelength from 350 to 800 nm and a vacuum ultraviolet region of a wavelength of 250 nm or less, the optical loss becomes serious.
An object of the present invention is to provide a method for manufacturing an optical element using CaF2 as a raw material, in which even when plasma treatment is performed, degradation of optical properties of the optical element caused by plasma damage can be avoided or suppressed.
A method for manufacturing an optical element, according to one embodiment of the present invention, comprises the steps of polishing a CaF2 substrate so that a process-altered layer formed by the polishing has a thickness of 30 nm or less, and performing plasma treatment in a chamber in which the polished CaF2 substrate is placed.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.