JPS6179228A - Projective optical device - Google Patents

Abstract

PURPOSE:To enable the correction of variation of the optical characteristics due to variation of temperature and atmospheric pressure by forming a hermetic space in the middle of an optical path from a mask to a projective substrate and supplying a gas which can change as refractive index into the space. CONSTITUTION:The selected gas is supplied from supply sources 13 and 23 into gas reservoirs 14 and 24. At this time, intervals 3c and 3f are filled with the gas and the air which has filled those intervals 3c and 3f is substituted for the gas. When the substituting operation is carried out sufficiently, the intervals 3c and 3f are insulated from the outside. At this time, capacities of the gas reservoirs 14 and 24 enhance so as to equal the internal pressures to the atmospheric pressure. As a result, pressure of the gas inside the intervals 3c and 3f also becomes equal to the atmospheric pressure. After that, the gas pressure inside the gas reservoirs 14 and 24 are modulated so as to be equal to the atmospheric pressure by expansion or contraction of the reservoirs and the variation of magnification or focus is corrected by the dependence on pressure of a refractive index of the gas by itself.

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention relates to an apparatus that uses a projection optical system to expose a pattern of a mask onto a photoreceptor (wafer), and in particular, relates to an apparatus that uses a projection optical system to expose a pattern of a mask onto a photoreceptor (wafer), and in particular, to The present invention relates to a projection optical device that is stabilized in the following state.

(Background of the Invention) In recent years, reduction projection exposure apparatuses have been widely introduced to VLSI production sites and have brought great results, and one of their important performances is overlay matching accuracy. Among the factors that affect this matching accuracy, an important one is the magnification error of the projection optical system. The size of patterns used in VLSIs is becoming increasingly smaller year by year, and the need for improved matching accuracy is also becoming stronger. Therefore, the need to maintain the projection magnification at a predetermined value has become extremely high. Currently, the magnification of the projection optical system is adjusted at the time of installation of the apparatus, so that the magnification error can be ignored. However, there is an increasing demand to prevent magnification errors from occurring even when environmental conditions change, such as slight temperature changes during operation of the device or slight pressure fluctuations in a clean room.

Furthermore, changes in environmental conditions cause not only a change in magnification but also a so-called focus change in which the position of the imaging plane of the projection optical system changes in the direction of the optical axis. Therefore, if this focus fluctuation is left as it is, the projected pattern image of the mask will become defective on the wafer, which is the photoreceptor, and this will lead to defects in the VLSI.

Such defects are also caused by exposure light passing through the projection optical system. This occurs because a portion of the exposure light energy is absorbed by optical elements within the projection optical system, causing a temperature change. Therefore, even if the environmental conditions are stable, variations in magnification and focus will occur while the device is in operation. The focus fluctuation can be easily corrected, for example, by adjusting the distance between the projection optical system and the projection target substrate. However, regarding magnification changes, the projection optical system is a so-called zoom lens (
This could not be easily corrected unless a variable magnification optical system was used.

(Objective of the Invention) In order to solve the above-mentioned problems, the present invention aims to provide a projection optical device that can easily correct magnification errors and focus fluctuations caused by changes in temperature and atmospheric pressure. ) The present invention relates to a device for projecting a pattern on a mask onto a projection target substrate via a projection optical system, and the present invention provides means for forming an airtight space cut off from outside air in an optical path from the mask to the projection target substrate, and the airtight space. The technical point is that a gas supply means for supplying a gas whose refractive index can be changed is provided in the interior, and the projection optical characteristics from the mask to the projection target substrate are adjusted by changing the refractive index of the supplied gas. It is said that

(Embodiment) FIG. 1 is a diagram showing a schematic configuration of a reduction projection type exposure apparatus according to a first embodiment of the present invention. A condenser lens 1 that condenses exposure light illuminates a reticle or a mask (hereinafter collectively referred to as a mask) 2 with light from a light source in a uniform illuminance distribution. The light image of the pattern drawn on the mask 2 is reduced by 115 or 1/10 by the projection lens 3, and is focused on the wafer 4 coated with a photosensitizer. The wafer 4 is placed on a step-and-repeat type moving stage 5.

The projection lens 3 is composed of a plurality of lens elements,
In this embodiment, an air gap 3C between specific lens elements 3a and 3b and an air gap 3f between lens elements 3d and 3e are sealed with a gas having different pressure characteristics in terms of refractive index. , so as to self-correct magnification fluctuations and focus fluctuations due to changes in atmospheric pressure. Air spacing 3c.

3F has an air storage structure that is isolated from the outside air, and the air gap is 3C.
A hole provided in the lens barrel communicates with a bellows-shaped gas reservoir 14 via a pipe 10 and a solenoid valve 11. 'This gas reservoir 14 further communicates with a first gas supply source (hereinafter referred to simply as the first supply source) 13 via a solenoid valve 12, and also communicates with an exhaust system via a solenoid valve 15. The first supply source 13 includes argon, carbon dioxide, nitrogen, helium,
A gas having a single composition such as chlorofluorocarbon or benzene, or a mixed gas obtained by mixing some of these gases at a predetermined ratio is supplied to the gas reservoir 14. On the other hand, the air gap 3f also communicates with a bellows-shaped gas reservoir 24 from a hole provided in the lens barrel via a pipe 20 and a solenoid valve 21. This gas reservoir 24 further communicates with a second gas supply source (hereinafter simply referred to as the second supply source) 23 via a solenoid valve 22, and also communicates with an exhaust system via a solenoid valve 25. The second supply source 23 also supplies a single composition gas or a mixed gas to the gas reservoir 24. Also, gas reservoir 14゜2
4 have a structure in which the internal volume can be changed so that the internal gas pressure follows changes in atmospheric pressure, that is, the gas pressure and atmospheric pressure are always equal.

Now, in this embodiment, the two air gaps 3c in the projection lens 3
, 3f are used to self-correct fluctuations in optical characteristics due to changes in atmospheric pressure. Here, for example, the projection magnification of the projection lens 3 can be adjusted by changing the refractive index of the gas within the air gap 3C. It is assumed that the focus fluctuation can be adjusted by changing the refractive index of the gas within the interval 3f. In other words, when a plurality of air intervals in the projection lens 3 are changed by the same constant pressure, among these air intervals, there are two air intervals that have a significant effect on changes in projection magnification and an air gap that has a significant effect on focus changes. and are selected from optical calculation simulations. The gas supplied within the wide air gaps 3c, 3f not only has a different refractive index with respect to the atmosphere, but also has different pressure-dependent properties. In other words, both the atmospheric and gas pressures are J
When P changes, the amount of change in the refractive index of the atmosphere, ΔN, and the change in the refractive index of the gas, ik, 1. Ng will have different values.

Further, the values of i and Ng themselves also vary depending on the composition of the gas. It is well known that the refractive index of gas increases as the pressure increases, so when the air gaps 3c and 3f are used as so-called gas lenses, the gaps are selected such that the gas lenses originally have negative power. (Even if the gas lens is convex in shape, it will have negative power because both sides of the lens are made of a medium with a fairly high refractive index, such as glass.) In general, the projection lens 3 is a convergent system. As a whole, it can be thought of as a single lens with positive power. Therefore, when the atmospheric pressure increases by i, P, the convergence of the projection lens 3 becomes weaker, and the original image forming plane position shifts (focus variation) by Δ, Z in the direction away from the projection lens 3, resulting in the original focus 1. One projected image at the dust surface position is also magnified by iM (variation in magnification).

Therefore, in order to always correct the focus variation amount ΔZ1 and the magnification variation amount ΔM to zero regardless of the atmospheric pressure variation ΔP, it is necessary to self-adjust the projection lens 3 to strengthen its convergence as the atmospheric pressure increases. .

For this purpose, an air gap 3c in the projection lens 3 is required.

It is sufficient to weaken the negative power of the gas lens due to 3f.

In order to weaken the negative power (reduce the difference in refractive index between glass and gas), it is sufficient to increase the pressure of the gas constituting the gas lens. Therefore, when the atmospheric pressure increases, if the gas pressure within the air gaps 3c and 3f is increased accordingly, it becomes possible to self-correct the focus fluctuations and magnification fluctuations. In this embodiment, the pressure of the gas within the air gaps 3c and 3f is set to always be equal to the atmospheric pressure, so
In order to perform self-correction, it is necessary to adjust the pressure dependence of the refractive index of the gas in advance. FIG. 2 is a diagram schematically showing how to find such a gas. In FIG. 2, the horizontal axis represents the atmospheric pressure fluctuation amount ΔP, and the vertical axis represents the projection lens 3 and the distance 3c.

It represents the amount of change in convergence (positive power) and divergence (negative power) when 3f etc. are considered as a single lens element. The amount of change in the positive direction with respect to the reference value (zero) on the same vertical axis indicates that the divergence becomes lower, and the amount of change in the negative direction indicates that the convergence becomes lower. In order to simplify the explanation, Fig. 2 shows convergence and divergence characteristics A due to atmospheric pressure fluctuations only due to the air gap 3C, and convergence and divergence characteristics B due to other optical elements (lenses and other air gaps). The convergence/divergence characteristic T and three atmospheric pressure characteristics of the entire projection lens 3 are shown. The property T is expressed by the algebraic sum of the properties A and B. It is clear from these characteristics! Like J1, projection lens 3
In order to keep the magnification fluctuation and focus fluctuation almost zero, the characteristic T
All you have to do is eliminate the slope. For that purpose, the slope θa of characteristic A
There are two methods: to change the characteristic to a larger value, or to change the slope θb of the characteristic B to a characteristic B1, which has a smaller slope.

The air gap 3c in this embodiment was selected so that it originally had negative power (divergence) as shown in characteristic A in the uncorrected state filled with air, so the gas was changed to change characteristic A to A. stipulate. In order to set characteristic A to Ae, let iNo be the amount of change in the refractive index with respect to a unit pressure change in air (atmosphere), and let ΔNg be the amount of change in refractive index with respect to a unit pressure change in the air pressure to be sealed, then ΔNg> It is sufficient to select a gas that satisfies the iNo relationship. Also, contrary to this embodiment, the originally positive power (
If we choose an air interval that has convergence), then ΔI'Jg<Δ,N, so that characteristic B changes to characteristic B=.

All you have to do is choose a gas that satisfies the relationship. helmet, ΔNg
The difference between , i, and No depends on the configuration of the projection lens 3 (lens type and arrangement of lens elements). All you have to do is find the most suitable gas (single composition or mixture) while measuring with the above methods.

Now, the operation of the apparatus shown in FIG. 1 will be explained.

When the gas selected as described above is sent to the first supply source area, magnification fluctuations and focus fluctuations due to changes in atmospheric pressure are simultaneously corrected.

Now, when exposure using the mask 2 is started in the above configuration, magnification variation and focus variation occur due to the incidence of exposure light. Since the amounts are sequentially input to the control device 60, the two gases G1. The mixing ratio of G is determined. It is assumed that the relationship between the mixture ratio and the refractive index of the gas mixture is known in advance through experiments, etc., and also the relationship between changes in the refractive index of the gas mixture (changes in the mixture ratio), magnification, and changes in the focal position. It is assumed that this is determined experimentally in advance. The control device 60 stores an arithmetic expression expressing these relationships, and determines the mixture ratio based on the expression.

The mixing ratio is, for example, 40.41 in the mixing chamber per unit time.
It is measured by detecting the ratio of the amount (volume) of the gas Gl flowing into the gas G1 and the amount (volume) of the gas G3 flowing into the air using a flowmeter or the like.

As described above, in this embodiment, at least two
Since the power of the gas lens is changed by changing the mixing ratio of the two gases, there is an advantage that it is possible to actively compensate for fluctuations in optical characteristics caused by incidence of exposure light in addition to fluctuations in atmospheric pressure.

As described above, in the two embodiments of the present invention, two specific air gaps 3c and 3f in the projection lens 3 are selected to seal a gas different from the atmosphere (air) whose refractive index can change. However, there can be any number of air intervals,
It is also possible to create one air spacing, or a combination of several air spacings, such that both magnification variation and focus variation can be corrected simultaneously. Further, a self-correcting gas as described in FIG. 1 or a mixed gas as described in FIG. 3 may be supplied into the space between the mask 2 and the projection lens 3. In this case, the object side of the projection lens 3 (
If the mask 2 side) is a non-telecentric optical system, the projection magnification can be adjusted.

(Effects of the Invention) According to the present invention, an airtight space is provided in the optical path from the mask to the projection target substrate, and a gas having a refractive index different from that of the atmosphere (air) or a gas having a change in refractive index is sealed in the space. As a result, the optical characteristics can be kept constant without mechanically adjusting the relative positions of the mask, the projection optical system, and the substrate to be projected in the optical axis direction. can. Therefore, the reliability and long-term stability of the device can be improved.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing a schematic configuration of a projection type exposure apparatus according to a first embodiment of the present invention, and FIG.
FIG. 3 is a diagram illustrating the change characteristics due to convergent pressure fluctuations, and is a diagram illustrating a schematic configuration of a projection exposure apparatus according to a second embodiment of the present invention. [Explanation of symbols of main parts] 2... Mask, 3... Projection lens, 3c, 3f... Air spacing, 4... Wafer, 13.30th. 1 gas supply source, 23.31...Second gas supply, 14.15...Gas reservoir, 40.41...Mixing chamber, 60...Control device applicant Nippon Kogaku Kogyo Agent Takashi Watanabe Co., Ltd.

Claims (2)

[Claims] (1) In an apparatus for projecting a pattern on a mask onto a projection target substrate via a projection optical system, means for forming an airtight space shielded from outside air in an optical path from the mask to the projection target substrate; in the airtight space; and a gas supply means for supplying a gas whose refractive index can be changed, and the projection optical characteristic is adjusted from the mask to the projection target substrate by changing the refractive index of the gas. . (2) The gas supply means includes a mixer for mixing a plurality of gases having different refractive indexes at an arbitrary ratio and supplying the mixture to the airtight space. The device described.