JPS6028613A - Projection optical device - Google Patents

Abstract

PURPOSE:To adjust finely magnifications easily with a high precision without generating an asymmetrical magnification distribution by varying the pressure of an air chamber shut out from outside air to adjust the projection magnification of a projection object lens. CONSTITUTION:A projection object lens 1 reduces and projects the pattern of a reticle R, which is illuminated by an illuminating device 2, onto a wafer W on a table 3. Two independent air chambers 10 and 20 in the lens 1 are connected to pressure controllers 12 and 22 on the outside of the object lens by tubes 11 and 21. Constant-pressure air is supplied to controllers 12 and 22 from a pressurized air supplier 4 through filters 13 and 23, and magnification variations DELTAX1 and DELTAX2 per unit pressure in individual chambers 10 and 20, image-forming surface variations DELTAZ1 and DELTAZ2, and a magnification variation DELTAX and an image-forming surface variation DELTAZ per unit pressure of an air pressure are stored preliminarily by pressure sensors 14 and 24 and an operator 5. The operator 5 detects an air pressure variation DELTAP by the signal of a measuring instrument 6 and calculates pressure variations DELTAP1 and DELTAP2 and outputs signals to controllers 12 and 22 to give pressure variations DELTAP1 and DELTAP2 to individual chambers 10 and 22.

Description

[Detailed description of the invention] The present invention relates to a projection optical device that can be corrected and a method thereof.

(Background of the Invention) In recent years, reduction projection exposure devices (hereinafter referred to as steppers) have become extremely
, has been widely introduced in SI production sites and has brought great results, and one of its important performances is overlay matching accuracy. An important factor among the factors that affect this matching accuracy 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, in order to correct magnification errors caused by changes in prefectural border conditions such as slight temperature changes during equipment operation or slight pressure fluctuations in the clean room, and to obtain even higher matching accuracy than the initial setting, the magnification There is a high demand for fine-tuning the system.

Conventionally, in projection optical systems other than steppers, methods have been used to change the projection magnification by mechanically changing the distance between the object (reticle) and the projection lens, or by moving the lens element in the projection lens in the optical axis direction. was. However, if the method of changing the optical member in the optical axis direction as described above is adopted for a device that requires extremely high-precision magnification setting such as a stepper, the optical axis It is difficult to apply displacement while maintaining the correct position. As a result, the optical system including the object is coaxially blurred, resulting in a disadvantage that a magnification distribution asymmetrical with respect to the optical axis occurs on the image plane. In addition, in order to accurately set the magnification so that an error of 0.05 μm or less occurs on the wafer, it is necessary to control the amount of change in the optical member to within a few μm or 1 μm, including eccentricity (shift, tilt). There are many difficulties involved in realizing this.

(Object of the Invention) An object of the present invention is to eliminate these drawbacks and provide a projection optical device and a method thereof that allow fine adjustment of magnification with high precision and ease without causing an asymmetric magnification distribution. do.

(Summary of the Invention) Technical points of the present invention include forming at least one space shielded from the outside air in the portion of the projection optical system through which the light beam passes, and changing the projection magnification by changing the air pressure in the space. It is said that In other words, we created a space in the part of the projection optical system that is shielded from the outside air through which the light rays pass, and by changing the air pressure in that space, we discovered that the projection magnification could be changed accurately. This enables accurate magnification error correction. However, the relationship between the change in atmospheric pressure in the space and the change in magnification varies depending on the projection magnification and lens type, and also varies depending on the space in which the pressure is changed for a given lens system. Changing not only changes the magnification but also generally changes the position of the imaging plane, but the degree of this effect also differs depending on the space.

Therefore, one of the lens intervals of the existing projection objective lens
When the pressure in this air chamber changes by a unit pressure from the initial magnification setting, the amount of change in magnification, that is, the change in magnification at a predetermined off-axis image point on the imaging plane. Assume that the amount of displacement is ΔX. Also, assume that the pressure in the air intervals other than this air chamber changes approximately equal to the atmospheric pressure, and for the entire interval excluding this one air chamber, the amount of magnification change ΔX with respect to the change in unit pressure of atmospheric pressure
Suppose that At this time, if the atmospheric pressure changes by △P, the pressure in the sealed air chamber changes by △P1, and (1) △p, ・△X, + △P - △X =.

By satisfying the relationship, the change in magnification can be corrected.

However, if the pressure changes in only one air chamber, even if magnification correction is possible, it is difficult to simultaneously correct changes in the imaging plane. For this reason, it is desirable to newly provide a second air chamber that is isolated from the outside air. In this case, the amount of change in the imaging plane due to the first air chamber with respect to a change in unit pressure is expressed as △z1
Assuming that the amount of change in magnification due to the second air chamber is △X7, and the amount of change in the imaging plane is △2, then if r and c, then the pressure in the first air chamber is increased by ∆PI so that the two conditions are satisfied at the same time. Adjust the pressure in the second air chamber.

By changing the respective values, it becomes possible to easily correct the fluctuations in both the magnification and the imaging plane that occur in the entire remaining air chamber.

In order to deal with fluctuations in the imaging plane, for example, if a configuration is adopted in which the automatic focus detection device is always activated to precisely move the stage that supports the wafer up and down, then the above-mentioned 1. The magnification correction may be performed by providing air chambers at only one location and controlling the pressure of these air chambers.

(Examples) Hereinafter, the present invention will be described based on Examples of the present invention. FIG. 1 is a lens arrangement diagram showing an example of a projection objective lens used in a stepper, and a predetermined pattern on a reticle (R) is reduced and projected onto a wafer (5) by this objective lens. In the figure, light rays representing the conjugate relationship between on-axis object points between the wafer and the reticle are shown. This objective lens consists of a total of 14 lenses, Ll, L21...Ll4, in order from the reticle (6) side, and the distance between each lens and the reticle (mother, wafer (5)), in order from the reticle side.
, b, c, ..., 0, a total of 15 air intervals are formed. Table 1 shows the specifications of this objective lens.

However, r is the radius of curvature of each lens surface, D is the center thickness and air gap of each lens, and N is the i-line of each lens (λ = 36s, o
In the table, the leftmost number represents the order from the reticle side. Further, Do represents the distance between the reticle (plane) and the front/first lens surface, and D31 represents the distance between the final lens surface and the wafer (E).

Now, in this objective lens, the air distance is a.

b,...If the atmospheric pressure at 0 is changed by +137.5 mmHg, the relative refractive index of each air gap is 1,
Table 2 shows the change in magnification and the change in the imaging plane, that is, the conjugate plane with the reticle (R). However, the magnification change ΔX is the amount of movement of the image point, which is formed at a position 5.66 degrees away from the optical axis when there is no change in air pressure on the imaging plane, after changes in air pressure in each air interval, in μm.
It is expressed in units, and a positive sign indicates a case where the image is projected larger (enlargement) on the image forming plane when there is no atmospheric pressure fluctuation, that is, on a predetermined wafer surface. Further, the change Δ2 in the imaging plane is shown as a change in the imaging point on the axis, and the case where it moves away from the objective lens is shown as a positive sign. Both values are in μm.

From Table 2 above, the change in the imaging plane due to the 8th space is the least, the 8th space is optimal for using as an air gap for magnification correction, and the change in magnification due to the 14th space n is the most It can be seen that it is ideal for correcting the imaging plane. Therefore, the eighth space (n) and the fourteenth space (n) are made into air chambers that are isolated from the outside air, and the magnification correction and the imaging plane correction are performed by controlling the pressure inside these air chambers. It is assumed that the spaces other than the 8th space and the 14th space n are not isolated from the atmosphere and change with the atmospheric pressure. As mentioned above (2)
Rewriting the equation, the conditions for correcting the magnification and imaging plane due to atmospheric pressure fluctuations in the above objective lens are as follows. Here, ΔP11 is the pressure change in the eighth space, Δx
h is the magnification change amount for the unit pressure change in the 8th chamber, △
zh is the image forming plane change for a unit pressure change in the 8th space, △Pn is the pressure change in the 14th space n, and △Xn is the 1st
The change in magnification with respect to a unit pressure change in the 4th space n, ΔZn, is the change in the imaging plane with respect to a unit pressure change in the 14th space n.

Also, △P is the atmospheric pressure change, △X is the magnification change with respect to the unit pressure change in all spaces other than space h-n, and △2 is the space h・
This is the image plane change for a unit pressure change in all spaces other than n. The unit of pressure change is mmHg, and the unit of magnification change and image plane change is μm/mm Hg.

Table 2 lists the magnification changes and imaging plane changes when the pressure change in each space is +137.5 Hg, so △xh, △Xn, △X, △zh, △Zn in equation (3), When Δz is determined from this, equation (3) can be rewritten in the following form.

If we consider △Ph and △Pn that satisfy this equation (4), we get △P
h=-8,2ΔP and ΔPn=-23,5ΔP are obtained. To give a more specific example, fluctuations in atmospheric pressure -I
When O+III+lHg drops, the 8th space is 82mH
g and pressurizes the 14th space by 235++ onHg, it is possible to correct both the change in magnification and the change in the imaging plane due to fluctuations in atmospheric pressure.

FIG. 2 is a schematic diagram of a projection optical device that can perform magnification correction and imaging plane correction by controlling the pressure of the air chamber as described above. Projection objective lens (Illumination device (2)
) The pattern on the reticle (R) uniformly illuminated by the wafer (R) is reduced and projected onto the wafer (2) placed on the stage (3). Two independent air chambers 00)clllJ are formed in the projection objective (1), corresponding to the 8th air spacing and the 14th air spacing n shown in Figure @1, and each air chamber (10 , 20) are pressure controllers Oa provided outside the objective lens by pipes (II, 21), respectively.
and Qa. and each pressure controller (12
, 22) are constantly supplied with air at one foot pressure from the pressurized air supply device (4) in line with the filter Q3 and the filter Q3. On the other hand, a pressure sensor Q4 (24) is provided on the side surface of each air chamber to detect its internal pressure, and this output signal is sent to a computing unit (5).
A measured value signal of atmospheric pressure is also input from the measuring device (6). As mentioned above, the computing unit (5) includes each air chamber (to
, 2+) ) magnification change per unit pressure △
X7. ΔXt and the amount of change in the imaging plane △right, Δz2 and the amount of change in magnification per unit pressure of atmospheric pressure △X and the amount of change in the imaging plane ΔX
is stored in advance. Then, the calculator (5) detects the amount of change in atmospheric pressure △P based on the signal from the measuring device (6), and sets the pressure required in each air chamber to satisfy both conditions of equation (2) mentioned above. It calculates the changes △"l I △P, and issues signals to each pressure controller (12, 22) to make these pressure changes. In each pressure controller (12, 22), a disable device (5 ) Based on the signal from △PI l △
Give a pressure change of P. Note that the atmospheric pressure measuring device can also be used as a barometer for a light wave interferometer (not shown) equipped in the stepper.

In this way, a constant projection magnification is always maintained despite atmospheric pressure fluctuations, and high-precision matching as a stepper is stably achieved. In the above embodiment, the signal from the pressure sensor installed in each air chamber is fed to the pressure controller via the computing unit, and the pressure controller is constantly activated. It can also be configured to allow a human to read the measurements, calculate the required pressure changes for each air chamber, and manually activate each pressure controller as needed.

In addition, a temperature sensor (7) is provided near the lens barrel of the projection objective lens (1) or inside the lens barrel, and the output signal is sent to a calculator (
5), and the arithmetic unit can generate f'f4 so as to correct not only atmospheric pressure changes but also fluctuations due to temperature changes around the projection objective lens and temperature changes of the projection objective lens itself. .

As described above, if there is at least a space in the optical path of the projection optical system in which the atmospheric pressure can be independently controlled, changes in both the projection magnification and the convergence plane position 1d can be controlled. At this time, the lens element in the projection lens is moved in the optical axis direction,
If a method of changing the distance between the reticle and the projection lens is used, two or more spaces for controlling the air pressure are not necessarily required.

Furthermore, if the stepper is equipped with a function of detecting and following changes in the position of the imaging plane, the air pressure control of the space can be performed for only one space by focusing only on the change in magnification. If you want to limit the air pressure control to one space using a stepper that does not have the ability to detect the image plane position, it is better to choose a space where the image plane changes less, such as the 8th space shown in Figure 1. Seem. For spaces where the atmospheric pressure is not controlled, it may be better to make a hole in the lens barrel so that the same fluctuations as the atmospheric pressure in the outside air occur. It is possible to provide

As in the embodiment shown in FIG. 2 above, fine adjustment of magnification is achieved by forming an air chamber at a specific distance between lenses within the projection objective lens, which is isolated from the outside air, and controlling the pressure of this air chamber. However, there are various methods of operating such a magnification fine adjustment means.

First, as in the example shown in Fig. 2, the factors that affect the magnification change of the stepper and the extent of their influence are investigated in advance, and the amount of variation in each element (for example, the environment This is a method in which the magnification fine adjustment means is operated by measuring the amount of change in magnification that has occurred by measuring the amount of change in the magnification (m improvement in quality or the amount of change in atmospheric pressure). In this case, it is desirable to configure a servo system that measures the valley effect accumulation in real time and immediately adjusts the magnification automatically, as shown in Figure 2.
It is also possible to manually adjust the magnification based on the measurement (+K).

In addition, the projection lens of Sugetsuba generally absorbs part of the light energy If, and its temperature rises. For this reason, if the projection lens is continuously irradiated with exposure light for a long time, or if the exposure operation is performed continuously for a long time, the magnification may change slightly.
There is a potential. Therefore, it is desirable to feed the energy accumulated in the projection lens directly to the needle side and feed it to the magnification fine adjustment means.

In addition, the energy stored in the pair and proboscis lens is directly i++++
Rather than setting a fixed value, the relationship between sunlight time, continuous operation time, and magnification change should be investigated in advance through experiments and calculations.
Information on the exposure time and continuous operation time may be fed back to the magnification fine adjustment means.

Furthermore, it is also possible to provide the stepper with a projection magnification measurement function and feed the measurement results to the magnification fine adjustment means. If the magnification can be measured in real time, it is also possible to use a servo system that immediately adjusts the magnification. If the measurement takes time, the measured value may be displayed once and the magnification may be finely adjusted manually based on that value. It is also easy to create a sequence that adjusts the magnification based on the measured value and then rechecks the magnification. Note that since the projection magnification can be known by actually exposing a wafer with a stepper and measuring the wafer, it is also possible to feed this information back to the magnification adjustment means.

By the way, until now N21 contained in air as atmospheric pressure
We have treated only the total pressure of each gas, such as o2+ co, lN20, etc., without considering the partial pressure. However, since the important thing in the present invention is to control the refractive index of air, it is usually possible to use only N2 instead of air, or to change the refractive index of air by controlling the partial pressure of each gas while keeping the total pressure constant. are also naturally included in the present invention.

The present invention provides a method that allows fine adjustment of the magnification.1 It is clear that this invention is useful not only for keeping the magnification constant, but also for intentionally varying the magnification. .

(Effects of the Invention) As described above, according to the present invention, the projection magnification of the stepper can be adjusted easily and with high precision, so it is easy to respond to changes in the environmental conditions of the machine, and a high matching tIv degree can be maintained. It is possible to provide a stepper that greatly contributes to improving the productivity of VLSI.

[Brief explanation of drawings]

FIG. 1 is a lens diagram of an objective lens of an investment trust for a stepper according to an embodiment of the present invention, and FIG. (Explanation of symbols of main parts) 1... Projection objective lens, 10, 20... Air chamber,
12, 22...pressure controller, R...reticle,
W...Wafer applicant Takao Watanabe, agent of Nippon Kogaku Co., Ltd. Figure 1! Z diagram

Claims (1)

[Claims] 1. A projection optical device having a projection objective lens for projecting and exposing a pattern on a reticle onto a wafer,
An air chamber isolated from outside air is provided in at least one space between the lenses in the projection objective, and a pressure controller is provided to control the pressure in the air chamber, and the pressure controller controls the pressure in the projection objective. 1. A projection optical device characterized in that the projection magnification of the projection objective lens can be adjusted by changing the pressure of the air chamber. 2. In a projection method for projecting a pattern on a reticle onto a wafer at a predetermined magnification using a projection objective lens,
A change in the element that changes the predetermined magnification is measured, a pressure in at least one air gap in the projection objective is controlled corresponding to the amount of change in the element, and a change in magnification due to the element is controlled in accordance with the amount of change in the element. A projection method characterized by correcting to a predetermined magnification. 3. In a projection method for projecting a notch on a reticle onto a Ueno at a predetermined magnification using a projection objective lens, the magnification of the pattern image on the reticle projected onto the Ueno is measured; calculating the difference between the magnification of the pattern image projected onto the wafer and the predetermined magnification, and controlling the pressure in at least one air gap in the projection objective in response to the difference in magnification; A projection method characterized by correcting pronuclear magnification differences.