JP2005040736A - Washing method and apparatus of optical parts - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a means, in which various organic matter contamination adhered to an optical element, in particular, a surface of a fluorite substrate is removed by fluorine active species within a vacuum deposition chamber. <P>SOLUTION: According to the washing method of the optical element, the organic matter contamination adhered to the fluorite substrate can be removed by the use of a DC ion source and the irradiation of the fluorine active species produced from plasma at a low energy and at a high density without giving any damage to the substrate. Further, by washing with a fluorine gas, even if a deposition starts immediately after washing, the residual gas incorporated in a film does not cause the optical absorption in 157 nm laser wavelength range. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a cleaning method performed immediately before applying an antireflection film or an increased reflection film to an optical element used in a semiconductor exposure apparatus, and in particular, a fluoride for a fluoride lens used in a vacuum ultraviolet wavelength region having a wavelength of 200 nm or less. The present invention relates to a cleaning method immediately before forming an optical thin film.

  In recent years, the miniaturization of semiconductor elements has further progressed, and there has been a demand for higher resolution of a reduction projection system exposure apparatus used in the semiconductor element manufacturing process. In order to increase the resolution in a semiconductor exposure apparatus, it is necessary to shorten the wavelength of the light source. Currently, development of a semiconductor exposure apparatus using an F2 laser having a wavelength of 157 nm as a light source is in progress. A material that transmits light in this wavelength region is practically limited to fluoride, and a calcium fluoride crystal is used for the lens.

  In the reduction projection system exposure apparatus, several tens of optical elements coated with fluoride coating are used. Therefore, even if light absorption per optical element is very small, the transmittance is reduced to a degree that cannot be ignored when the entire optical element is combined. Therefore, organic contaminants and the like adhering to the optical element and having a large light absorption at a wavelength of 157 nm must be removed as much as possible.

As a method of removing contaminants such as organic substances adhering to the optical element, the effectiveness of UVO3 cleaning using ultraviolet light and ozone (see Patent Document 1) and plasma cleaning using active oxygen species has been confirmed. In addition, by installing these cleaning mechanisms in the film forming chamber and performing cleaning immediately before the film forming, a large effect can be obtained by removing contaminants that have re-adhered during the vacuum evacuation or heating process in the chamber.
Japanese Patent Laid-Open No. 2000-66003

  However, these dry cleaning methods are all cleaning methods using oxygen gas, and there is no problem when the substrate or film forming material is an oxide or in the visible light region, but oxygen molecules in the vacuum ultraviolet region. Has light absorption, and if oxygen remaining after cleaning is taken into the interface between the substrate and the film or during film formation, the transmittance is reduced. In addition, since the material used in this wavelength region is mainly fluoride, the deposition material is oxidized by the cleaning method in which a large amount of oxygen is poured in the film formation tank. Furthermore, in the case of plasma cleaning, generally higher pressure can be cleaned without damaging the substrate, but from the viewpoint of forming a film in the same chamber after cleaning, it is not possible to raise the pressure in the chamber extremely. It is difficult to avoid a situation where the substrate is easily damaged by plasma cleaning. In particular, in fluorite, color centers are likely to occur due to high-energy ion irradiation, which causes an increase in absorption rate.

  Therefore, an object of the present invention is to provide a method for cleaning a fluoride substrate with low energy plasma without using oxygen gas in the production of a fluoride optical thin film for the vacuum ultraviolet region.

  The optical element cleaning apparatus according to the present invention includes a film formation tank, a plasma generation mechanism that supplies an inert gas into the film formation tank to generate plasma, and a gas supply mechanism that supplies fluorine gas.

  In the substrate cleaning method according to the present invention, the inert gas is converted into plasma, the fluorine gas supplied from another supply mechanism is mixed, and the substrate is irradiated with fluorine excited by the plasma converted inert gas. It is characterized by efficiently removing adhered contaminants such as organic matter.

Here, the inert gas to be converted into plasma includes at least one kind of gas of Ar, Kr, Xe, Rn, He, and N ₂ , and the plasma generating means uses a DC power source. To do.

  Further, the optical element cleaning method according to the present invention excites fluorine gas with plasma-generated inert gas, and does not flow fluorine directly into the plasma generator, so that the plasma generator is resistant to corrosion by fluorine gas. It is characterized by not depending on.

  In addition, when fluorine can flow in the plasma generator, a mixed gas of an inert gas and a fluorine gas may be used.

  In general, it has been pointed out that, in a cleaning method using plasma, charged particles having kinetic energy collide with the surface of the optical element and cause damage causing optical absorption loss. In particular, the fluorite used in the ultraviolet wavelength region has a problem in that a color center is generated due to the chemical bond on the surface being cut, and the optical absorption in the ultraviolet region is increased, causing the transmittance deterioration of the optical component. .

  In order to solve the adverse effects associated with the plasma cleaning, it is necessary to suppress the ion energy causing damage to a low value. The excitation voltage required to excite the plasma depends on the ionization energy of the gas to be plasmatized. Conventional plasma cleaning uses oxygen plasma, but in the cleaning method according to the present invention, by selecting a xenon gas having a lower ionization energy than oxygen, the substrate is damaged as much as possible with an excitation voltage less than half that of the conventional method. It is characterized in that organic contamination on the surface can be removed without any problems.

  Until now, cleaning with oxygen plasma in the film formation tank can effectively remove organic contaminants, but when film is formed on an optical element used in the vacuum ultraviolet region, it has light absorption in this wavelength region. If oxygen remains in the chamber and is taken into the film during film formation, it causes a large decrease in transmittance. However, optical elements and film materials used in the vacuum ultraviolet wavelength region are currently mainly fluorides. When these materials are used, the gases used for cleaning in the present invention are inert gas and fluorine gas. Therefore, after the cleaning, it is possible to shift to the film formation without being influenced by the residual gas used in the cleaning.

  In more detail, the present invention can solve the above problems by the following configuration.

  (1) A method of cleaning an optical component in a film forming tank having a vacuum film forming tank including a vacuum sealed space, a mechanism for generating plasma in the vacuum film forming tank, and a gas supply mechanism for supplying gas.

  (2) The plasma generating apparatus according to (1) is a mechanism using a hollow cathode type DC ion source, and an ion gun for irradiating plasma is installed in a film forming tank. Method.

  (3) The method for cleaning an optical component according to (1) or (2), wherein the plasma gas supplied to the plasma generating means is an inert gas.

(4) the (3) the inert gas is Ar as described, Kr, Xe, Rn, Ne , He, method for cleaning an optical component, which comprises at least one gas of _{N 2.}

(5) The method for cleaning an optical component according to (1), wherein the gas supplied to the gas supply mechanism is F ₂ gas.

  (6) The optical component according to any one of (1) to (4) is an optical lens used in a vacuum ultraviolet wavelength region of 200 nm or less, the material of the optical lens is fluoride, and fluoride is added to the optical lens. The method for cleaning an optical component immediately before film formation according to any one of (1) to (4), wherein an antireflection film and an increased reflection film are formed with a material.

  (7) A cleaning apparatus for optical components in a film forming tank having a vacuum film forming tank including a vacuum sealed space, a mechanism for generating plasma in the vacuum film forming tank, and a gas supply mechanism for supplying gas.

  (8) The plasma generating apparatus according to (7) is a mechanism using a hollow cathode type DC ion source, and an ion gun for irradiating plasma is installed in a film forming tank. apparatus.

  (9) The apparatus for cleaning an optical component according to (7) or (8), wherein the plasma gas supplied to the plasma generating means is an inert gas.

(10) wherein (9) the inert gas is Ar as described, Kr, Xe, Rn, Ne , He, optical components of the cleaning device, characterized in that it comprises at least one gas of _{N 2.}

(11) The optical component cleaning apparatus according to (7), wherein the gas supplied to the gas supply mechanism is F ₂ gas.

  (12) The optical component according to any one of (7) to (10) is an optical lens used in a vacuum ultraviolet wavelength region of 200 nm or less, the material of the optical lens is fluoride, and fluoride is added to the optical lens. The apparatus for cleaning an optical component immediately before film formation according to any one of (7) to (10), wherein an antireflection film and an increased reflection film are formed with a material.

  As described above, according to the present invention, the plasma generation mechanism for generating active fluorine or the like is provided in the film formation tank, so that the substrate is not affected by the gas used for cleaning in the film formation of the fluoride material. Surface contaminants can be removed. Thereby, the fluoride optical thin film corresponding to the vacuum ultraviolet wavelength region having low absorption and excellent optical characteristics can be produced in a short time. Because of its low absorption and excellent durability, the frequency of maintenance such as replacement of optical element parts due to laser damage is reduced in the exposure apparatus for vacuum ultraviolet, and the time required for the manufacturing process can be shortened, so that the productivity of the exposure apparatus is improved. .

  Embodiments of a fluoride substrate cleaning method and a fluoride thin film manufacturing apparatus according to the present invention will be described below, but the present invention is not limited to this example.

  FIG. 1 is a schematic view showing an embodiment of an optical thin film device used in the present invention.

The fluoride thin film manufacturing apparatus of FIG. 1 is a load-lock type composed of three vacuum chambers, a film formation tank 1, a transfer tank 15, and a heating tank 16, and the fluorite substrate 4 introduced from the heating tank 16 is here. Then, the pressure is reduced from atmospheric pressure to 10 ⁻⁵ Pa, heated to the film formation temperature, and then carried into the film formation tank 1 via the transfer tank 15.

The film formation layer 1 is decompressed to 10 ⁻⁵ Pa by the exhaust pump 14, and is kept at a constant temperature by the heater 6. In the film forming tank 1, a vapor deposition boat 3 for containing the vapor deposition material 2 and a substrate holder 5 for fixing the fluorite substrate 4 are provided.

  In addition, a plasma generator comprising an ion gun (cathode) 7, a neutralizer (anode) 8, and a DC power supply is incorporated.

  In the plasma generating apparatus, an inert gas is introduced into the ion gun (cathode) 7 and the neutralizer (anode) 8 from the gas cylinder 9 via the mass flow controllers 11 and 12, respectively, and a DC voltage is applied for discharge. The fluorine-based gas is introduced into the film formation tank 1 from the fluorine gas cylinder 10 via the mass flow controller 13. The pressure of the fluorine gas to be introduced is adjusted by the mass flow controller 13. In addition, the pressure is controlled by the mass flow controllers 11 and 12 so that the plasma generator easily discharges.

  A cleaning method immediately before film formation of a fluoride substrate according to an embodiment of the present invention will be described.

First, the optical element fluorite substrate 4 is plasma-cleaned in the heating tank 16 and then fixed to the substrate holder 5. Then, the heating layer 16 is evacuated from atmospheric pressure to 10 ⁻⁵ Pa and heated to the film formation temperature.

  After heating, the fluorite substrate 4 is carried into the film formation tank 1 through the transfer tank 15. For optical elements used in the vacuum ultraviolet region, even if they are thoroughly washed before being put into the heating tank, reattachment of contaminants that cause light absorption in the vacuum ultraviolet region is inevitable during the exhaust, heating, and carry-in processes. . Therefore, it is effective to perform cleaning immediately after the film is loaded into the film formation layer.

  Meanwhile, 35 sccm of Xe gas is introduced into the ion gun (cathode) 7 via the mass flow controller 11. Further, 10 sccm of Xe gas is introduced into the neutralizer (anode) 8 via the mass flow controller 12. The neutralizer (anode) 8 is provided with a filament, and thermoelectrons are generated by passing an electric current through the filament. In this state, Xe plasma discharge occurs by applying a DC voltage of 30 V between the ion gun (cathode) 7 and the neutralizer (anode) 8.

  As a gas species used for the inert gas, Ar is generally used, but Xe having a relatively low ionization energy is effective for fluoride optical parts such as fluorite, which are feared to be damaged.

  Furthermore, when 3 sccm of fluorine gas is introduced from the fluorine gas supply pipe 10 into the film formation tank 1 in the Xe plasma discharge state via the mass flow controller 13, the fluorine gas is excited and fluorine active species such as fluorine radicals and fluorine ions are generated. appear.

  The fluorite substrate 4 fixed to the substrate holder 5 is cleaned by irradiating with active fluorine gas for 30 seconds. Thereafter, the discharge of the Xe plasma is stopped and each supply gas is also stopped.

  After cleaning, the inert gas and fluorine gas used for cleaning remain in the film formation tank, but unlike oxygen, which is often used in normal plasma cleaning, it is absorbed in the vacuum ultraviolet region even if taken into the film during deposition. Therefore, when cleaning was performed using oxygen plasma, it took about 1 hour to exhaust the residual gas, whereas when using the method of the present invention, exhausting for about 5 minutes was sufficient.

  Thereafter, the fluoride material 2 placed on the vapor deposition boat 3 is heated and evaporated to form a film on the substrate.

  Fluoride materials include magnesium fluoride, calcium fluoride, lithium fluoride, lanthanum fluoride, aluminum fluoride, neodymium fluoride, gadolinium fluoride, yttrium fluoride, dysprosium fluoride, barium fluoride, fluoride Sodium, bismuth fluoride, strontium fluoride, lead fluoride, selenium fluoride and the like.

  An example of an antireflection film actually formed by performing the above substrate cleaning method immediately before film formation will be described.

After the fluorite substrate is plasma-cleaned, it is placed in the heating tank 16, evacuated from atmospheric pressure to 10 ⁻⁵ Pa, heated to 200 ° C., and then transferred to the film formation tank 1 via the transfer tank 15. To do. Xe gas is supplied to the ion gun at 35 sccm, and the neutralizer is supplied at 10 sccm, and a voltage of 30 V is applied to the anode to discharge the Xe plasma. Further, by introducing fluorine gas into the film forming tank 1 in the Xe plasma discharge state, active fluorine gas is generated and the substrate is irradiated for 30 seconds. This anode voltage is less than half of the applied voltage of oxygen plasma discharge required for the conventional cleaning method using oxygen plasma, and it is considered that the substrate is hardly damaged.

After stopping the plasma discharge, the gas supply was also stopped, and the film formation tank 1 was evacuated until the pressure in the film formation tank 1 reached 10 ⁻⁵ Pa, and the deposition of the fluoride thin film was started. The film was formed by sequentially depositing two layers of gadolinium fluoride and magnesium fluoride on a fluorite substrate to form an antireflection film having a wavelength of 157 nm. The film formation rate is 0.2 nm / sec.

  The antireflection film formed after the pre-deposition cleaning showed a transmittance of about 99% at a wavelength of 157 nm, but the antireflection film had a transmittance of about 97 when the pre-deposition cleaning was not performed. %, The transmittance increased by about 2%. From this result, it is considered that the contaminants adhering to the substrate before film formation could be removed.

  FIG. 2 shows how much the gas used for cleaning affects the absorptance of the optical element after film formation. FIG. 2 is a graph showing the absorptance of the film alone, with the horizontal axis representing the residual gas exhaust time from cleaning to the start of deposition, and the vertical axis representing the absorptance of the substrate from the absorptance of the entire optical element after film formation. It is. The longer the waiting time from cleaning to vapor deposition start, the more the influence of residual gas used in cleaning can be eliminated. As conventional cleaning before film formation, oxygen plasma was used, so the case of using oxygen gas was compared with the case of the present invention. Since oxygen has light absorption at a wavelength of 157 nm, when the standby time is about 30 minutes, residual oxygen is taken into the film during the film formation, or oxygen adheres to the surface of the substrate, resulting in an absorption rate after the film formation. On the other hand, it can be seen that the cleaning method according to the present invention is hardly affected by the residual gas even when the standby time is 5 minutes.

It is a figure which shows the vacuum vapor deposition apparatus which installed the plasma generator and the gas line which supplies fluorine gas which make inert gas into plasma. It is a figure which shows the waiting time after washing | cleaning just before film-forming, and the film | membrane absorption after film-forming.

Explanation of symbols

1 Deposition tank 2 Vapor deposition material (fluoride material)
3 Deposition boat 4 Fluoride substrate (fluorite substrate)
5 Substrate holder 6 Heater 7 Ion gun (cathode)
8 Neutralizer (Anode)
9 Inert gas cylinder 10 Fluorine gas cylinder (fluorine gas supply pipe)
11 Mass Flow Controller 1
12 Mass Flow Controller 2
13 Mass Flow Controller 3
14 Exhaust pump 15 Transfer tank 16 Heating tank

Claims (12)

  A method of cleaning an optical component in a film forming tank having a vacuum film forming tank including a vacuum sealed space, a mechanism for generating plasma in the vacuum film forming tank, and a gas supply mechanism for supplying gas.   2. A method for cleaning an optical component, wherein the plasma generating apparatus according to claim 1 is a mechanism using a hollow cathode type DC ion source, and an ion gun for irradiating plasma is installed in a film forming tank.   3. The method of cleaning an optical component according to claim 1, wherein the gas to be plasma supplied to the plasma generating means is an inert gas. Inert gas Ar of claim 3, wherein, Kr, Xe, Rn, Ne , He, a method of cleaning an optical component, which comprises at least one gas of _{N 2. 2. The} optical component cleaning method according to claim 1, wherein the gas supplied to the gas supply mechanism is F2 gas.   The optical component according to any one of claims 1 to 4 is an optical lens used in a vacuum ultraviolet wavelength region of 200 nm or less, the material of the optical lens is fluoride, and the optical lens is antireflective with a fluoride material, and The method of cleaning an optical component immediately before film formation according to any one of claims 1 to 4, wherein an increased reflection film is formed.   A cleaning apparatus for optical components in a film forming tank having a vacuum film forming tank including a vacuum sealed space, a mechanism for generating plasma in the vacuum film forming tank, and a gas supply mechanism for supplying gas.   8. The optical component cleaning apparatus according to claim 7, wherein the plasma generating apparatus is a mechanism using a hollow cathode type DC ion source, and an ion gun for irradiating the plasma is installed in the film forming tank.   9. The plasma generating mechanism according to claim 7, wherein the gas to be plasma supplied to the plasma generating means is an inert gas. Inert gas Ar of claim 9, wherein, Kr, Xe, Rn, Ne , He, optical components of the cleaning device, characterized in that it comprises at least one gas of _{N 2.} In claim 7, the gas supplied to the gas supply mechanism, the cleaning device of the optical component, which is a F ₂ gas.   The optical component according to any one of claims 7 to 10 is an optical lens used in a vacuum ultraviolet wavelength region of 200 nm or less, the material of the optical lens is fluoride, and the optical lens is antireflective with a fluoride material, and The apparatus for cleaning an optical component immediately before film formation according to any one of claims 7 to 10, wherein an increased reflection film is formed.

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