JP2003057401A - Optical element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such problems that the distribution of the refractive index in conventional quartz glass is inhomogeneous, deposited substances dropping from the inner wall of a furnace are mixed in an ingot and the glass contains bubbles, foreign matters and striae. SOLUTION: In the method for manufacturing quartz glass by injecting a Si compound gas and a combustion gas from a burner to combust and by depositing quartz glass on a target to form an ingot, the ventilation amount of a ventilating means to discharge the quartz glass powder not depositing on the target is controlled so as to manufacture quartz glass having uniform distribution of the refractive index and no bubbles, foreign matter or striae.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a silica glass optical element, and particularly to a field requiring a synthetic silica glass optical element requiring high homogeneity, such as photolithography, high precision spectroscope, laser, etc. The present invention relates to an optical element made of high-homogeneous synthetic silica glass, which is useful for precision optical instruments.

[0002]

2. Description of the Related Art Conventionally, an exposure apparatus called a stepper has been used in an optical lithography technique for exposing and transferring a fine pattern of an integrated circuit on a wafer such as silicon.

The light source of this stepper has a g-line (436 nm) to an i-line (3
65 nm), and further KrF (248 nm) and ArF
(193 nm) Shorter wavelengths are being promoted to excimer lasers.

In general, the optical glass used as an illumination system of a stepper or a projection lens has a low light transmittance in the wavelength region shorter than the i-line, and therefore synthetic quartz glass or CaF ₂ (fluorite) is used instead of the conventional optical glass. It has been proposed to use a fluoride single crystal such as stone.

As described above, quartz glass used as an optical element for ultraviolet lithography is required to have high transmittance in the ultraviolet region and high homogeneity of the refractive index. In order to realize high transmittance in the ultraviolet region, it is necessary to suppress the impurity concentration in quartz glass. Therefore, a Si compound gas (a carrier gas for sending out the Si compound gas is used at the same time) as a raw material of the quartz glass and a combustion gas (O ₂ gas and H ₂ gas) for heating and reaction are discharged from the burner. ,
The flame hydrolysis synthesis method of depositing quartz glass in a flame is commonly used.

[0006]

However, in the production of quartz glass by the flame hydrolysis method as described above,
Quartz glass powder that was not deposited on the target adhered to the exhaust port and exhaust pipe and clogged, which was a variable factor that hindered steady exhaust.

Further, when these deposits are intermittently peeled off, the exhaust amount also intermittently greatly changes.

These fluctuations in the exhaust amount cause fluctuations in the atmosphere inside the furnace (the temperature inside the furnace, the temperature at the composite surface at the top of the ingot, etc.), causing inhomogeneity in the refractive index of the silica glass.

Further, since the quartz glass powder adheres to the exhaust port and the exhaust pipe, the amount of exhaust is reduced, and the quartz glass powder also adheres to the inner wall of the furnace. When the deposit and the ingot come into contact with each other and the deposit (quartz glass powder) is rolled up, the deposit is mixed into the ingot, and bubbles, striae, and inhomogeneity of the refractive index occur.

Therefore, in the conventional quartz glass, the refractive index distribution was non-uniform, and the deposits peeled off from the inner wall of the furnace were mixed in the ingot, and bubbles, foreign matters, and even striae were present.

[0011]

In order to solve the above problems, the present inventors have installed a control device for controlling the rotation speed of an exhaust fan in an exhaust device arranged in a quartz glass manufacturing apparatus. When the amount of exhaust gas was selected at arbitrary time intervals, the silica glass powder not deposited on the target was smoothly discharged from the exhaust port and the exhaust pipe without hindering steady exhaustion.

That is, the present invention provides a method for producing a quartz glass in which a Si compound gas and a combustion gas are jetted from a burner and burned to deposit quartz glass on the target to form an ingot. By adjusting the exhaust volume of the exhaust means for exhausting the silica glass powder that was not formed, it was possible to manufacture silica glass having a uniform refractive index distribution and no bubbles, foreign matters, or striae.

The optical element of the present invention thus obtained is
“The homogeneity of the refractive index in the lateral direction of the ingot is Δn ≦ 4 × 1
0 is ^-6, an optical element ", characterized in that and the gradient of the refractive index distribution has a quartz glass is 1 × 10 ^-6 or less.

When an optical element is manufactured using quartz glass, it is usually cut out perpendicularly to the growth direction (side surface direction) of the ingot. Then, it is known that variations in the refractive index in the plane perpendicular to the growth direction of the ingot affect the optical characteristics of the optical element, and measures have been taken to eliminate this.

According to the present invention, not the variation of the in-plane refractive index but also the variation of the refractive index in the growth direction of the ingot,
It was found that it affects the optical performance of the optical member, and in order to control this, the exhaust volume of the exhaust means of the quartz glass manufacturing equipment shall be adjusted, and the homogeneity of the refractive index in the lateral direction of the ingot and the refractive index distribution. The result is that the gradient of is suppressed.

The present invention is preferably characterized in that striae are not found in any of the three directions.

During the synthesis of the quartz glass, the exhaust amount for exhausting the silica glass powder not deposited on the target is adjusted so that the exhaust port and the exhaust pipe are not clogged and the atmosphere inside the furnace (the temperature inside the furnace and the upper part of the ingot) The temperature of the composite surface) is kept constant, and as a result, striae do not occur.

The present invention is also preferably "0.01 mm.
It is characterized in that it does not contain the above bubbles and foreign matters. By adjusting the exhaust amount of the exhaust means, the silica glass powder does not adhere to the inner wall of the furnace due to the decrease in the exhaust amount with time, and the deposits are not mixed in the ingot.

The adjustment of the exhaust gas amount is performed by measuring the change in the exhaust gas amount due to the fluctuation factors such as clogging of the exhaust pipe by an exhaust gas amount sensor or an exhaust pressure system installed in the exhaust pipe, and the measured value is measured by an exhaust gas control device. Can be achieved by giving feedback to.

It should be noted that it is conceivable to keep the exhaust amount large enough to prevent the exhaust pipe from being clogged at all times (for example, to the maximum exhaust amount of the exhaust device). The temperature is too low to vitrify.

Further, for the intermittent fluctuation of the exhaust amount, it is not enough to feed back and adjust the exhaust amount as described above. Therefore, in the present invention, preferably, the quartz glass powder that has been clogged can be removed while maintaining the temperature of the composite surface by changing the exhaust amount in a pulsed manner. If the change and interval of the pulsed exhaust gas are properly selected according to a program, the intermittent change of the exhaust gas amount can be eliminated.

The silica glass optical element of the present invention is mainly composed of a lens, a prism, a reflector and a base material thereof. The step of obtaining the optical element from the base material is arbitrary, but the outer peripheral portion of the base material is scraped off, cut and re-formed as necessary to be processed into an arbitrary shape. Then, after annealing (heat treatment) to eliminate the internal strain, polishing and coating steps are performed to obtain an optical element.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for producing quartz glass used in the present invention will be described below with reference to examples.

[0024]

EXAMPLE FIG. 1 is a conceptual diagram showing an example of a quartz glass manufacturing apparatus.

The burner 2 is installed from the top of the furnace 1 toward the target 4 with its tip end facing. An observation window (not shown) and an exhaust pipe are provided on the furnace wall.

Further, a target 4 for forming an ingot is installed in the lower part of the furnace. From the quartz tube at the tip of the burner 2, SiCl ₄ and carrier gas as raw material gases, and O ₂ gas and H ₂ gas as combustion gases respectively flow out.

Then, by the flame of the combustion gas, Si
Cl ₄ is flame-hydrolyzed into synthetic quartz, which is deposited on the target to form an ingot. The synthetic surface on top of the ingot is covered with a flame due to combustion gas.

FIG. 2 shows a conceptual diagram of a scrubber which is the exhaust means of this embodiment.

In this embodiment, a thermocouple or IR
The temperature inside the furnace and the upper part of the ingot (composite surface) are measured by a camera (not shown), and the measured values are fed back to the frequency of the exhaust fan for control. That is, when the synthesis surface and the temperature in the furnace change from the beginning of the synthesis, the frequency of the exhaust fan is controlled and the temperature is automatically controlled so as to be kept constant.

A sequencer is used to control the frequency, and the frequency of the inverter connected to the induction motor driving the exhaust fan is changed at arbitrary time intervals according to the program to intermittently increase the exhaust amount in a pulse form. It was The relationship between the frequency change, the exhaust gas amount, and the temperature inside the furnace is shown in FIG. The most effective way of increasing the pulsed exhaust amount is to maximize the capacity of the exhaust treatment device (scrubber) used, and the shorter the pulse width T1, the more effective the clogging removal.

Several kinds of program exhaust schedules were performed as shown in FIG. 5, but considering the stability of the atmosphere inside the furnace and the influence on the ingot (foam, homogeneity of refractive index, etc.), FIG.
The schedule like this (Pattern 1 in FIG. 5) was the most effective.

[0032]

As described above, by maintaining a constant displacement for a long period of time, there is no fluctuation in the atmosphere inside the furnace (temperature inside the furnace, temperature on the composite surface above the ingot, etc.).
It is possible to prevent deterioration of the homogeneity of the refractive index of quartz glass.

Further, since it is possible to prevent the silica glass powder from adhering to the inner wall of the furnace, the adhered matter does not mix with the ingot to generate bubbles or striae, and the homogeneity of the refractive index is not deteriorated. This has made it possible to provide quartz glass used in the optical element of the present invention.

[Brief description of drawings]

FIG. 1 is a conceptual diagram of a quartz glass manufacturing apparatus used in the present invention.

FIG. 2 is a conceptual diagram of a scrubber which is an example of an exhaust unit of a quartz glass manufacturing apparatus used in the present invention.

FIG. 3 is a graph showing changes in the exhaust amount and the temperature inside the furnace due to changes in the scrubber fan frequency during synthesis, depending on the presence or absence of program exhaust.

FIG. 4 shows an optimal schedule for program exhaust.

FIG. 5 shows a program exhaust schedule.

[Explanation of symbols]

1 furnace 2 burners 3 ingots 4 targets

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hiroki Jimbo             Marunouchi 3 2-3 No. 3 shares, Chiyoda-ku, Tokyo             Ceremony Company Nikon (72) Inventor Hiroyuki Hiraiwa             Marunouchi 3 2-3 No. 3 shares, Chiyoda-ku, Tokyo             Ceremony Company Nikon F-term (reference) 4G014 AH19

Claims (3)

[Claims] 1. The homogeneity of the refractive index in the lateral direction of the ingot is Δ.
n ≦ 4 × 10 ⁻⁶ and the gradient of the refractive index distribution is 1 × 1
An optical element characterized by using quartz glass having a size of 0 ^-6 or less. 2. The optical element according to claim 1, characterized in that quartz glass is used which has no striae in any of the three directions. 3. The optical element according to claim 1, wherein the optical element is 0.
An optical element characterized by using quartz glass that does not contain bubbles or foreign matter of 01 mm or more.