JPH11150053A - Exposure method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately correct the variation in the image formation property by the absorption of energy of illumination light in a projective optical system, even in the case of performing double exposure while exchanging the reticle pattern for each wafer. SOLUTION: An operation of exposing the pattern of a first reticle under exposure condition A in the period TA, and exposing the pattern of a second reticle under exposure condition B in the period TB is repeated alternately The coefficient of expressing the model of the ripple of image formation property in the period TA and the coefficient of expressing the model of the ripple of the image formation property in the period TB are determined severally, according to the ratio of the quantity of application energy under exposure condition A to that under exposure condition B, and based on these coefficients, the quantity ΔP of change of image formation property is forecast in each period TA and TB, and the image formation property is compensated so as to offset this quantity ΔP of change.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for transferring a mask pattern onto a photosensitive substrate when manufacturing a semiconductor device, a liquid crystal display device, a thin film magnetic head, or the like by using a photolithography technique. Exposure method,
And the exposure apparatus is particularly suitable for use when performing multiple exposure.

[0002]

2. Description of the Related Art In a projection exposure apparatus such as a stepper used for manufacturing a semiconductor device or the like, imaging characteristics (distortion, best focus position, etc.) fluctuate due to absorption of illumination light for exposure by a projection optical system. And various correction methods have been proposed. One example of the correction method is to measure the incident energy with respect to the projection optical system, predict the amount of change in the imaging characteristics according to a previously obtained model of the change characteristics of the imaging characteristics, and determine a predetermined amount based on the prediction result. This is a method of driving some optical elements of the projection optical system via a correction mechanism.

In this regard, in recent years, in order to further improve the resolving power, a modified illumination technique or a phase shifter for illuminating a reticle as a mask with illumination light from a secondary light source having an annular shape or a plurality of apertures has been provided. A technique using a reticle (phase shift reticle) has also been proposed. When these techniques are used, the intensity distribution of the illumination light in the projection optical system greatly changes, and the above-mentioned fluctuation characteristics of the imaging characteristics change. Therefore, as disclosed in Japanese Patent Application Laid-Open No. 62-229838. A technique has been proposed in which a model of the variation characteristic is changed according to exposure conditions and used.

However, in an actual exposure process, for example, after exposing one lot of wafers under a certain exposure condition, the next lot of wafers may be switched to another exposure condition and exposed. In this case, the exposure under the new exposure condition is started in a state where the influence of the previous exposure condition remains in the projection optical system, and the imaging characteristic changes discontinuously. It is difficult to make a highly accurate prediction calculation using a model of the fluctuation characteristics of the imaging characteristics under the above exposure conditions. Further, even if a model of the fluctuation characteristic under these two exposure conditions is obtained in advance and a method of switching the model of the fluctuation characteristic when the exposure condition is switched is used, the imaging characteristic is not good when the exposure condition is switched. Since it changes continuously, there is a possibility that a correction error of the imaging characteristic remains.

For this reason, for example, Japanese Patent Application Laid-Open No. 6-45217 proposes a method of suspending exposure under a new exposure condition until the influence of the previous exposure condition becomes sufficiently small. Alternatively, as disclosed in JP-A-7-94393, the discontinuity (offset) of the imaging characteristic at the time of change is measured in advance, and when the imaging characteristic is switched, the imaging characteristic after the switching is changed. There is also proposed a method of adding the offset to the data.

[0006]

As described above, the conventional method for correcting the image forming characteristics uses a model of the fluctuation characteristics of the image forming characteristics determined in advance in accordance with the exposure conditions, and uses an offset when switching the exposure conditions. In this method, the amount of change in the imaging characteristic is predicted by performing the correction for the minute, and the imaging characteristic is corrected so as to cancel the predicted amount of the fluctuation. However,
In the actual exposure process, for a while after switching the exposure conditions, the imaging characteristics under the two exposure conditions are in a state of being mixed, and even if the offset of the imaging characteristics is taken into account. However, if a single model of the fluctuation characteristic is used, there is a possibility that a correction error of the imaging characteristic remains.

Further, in recent years, a double exposure method has attracted attention as an exposure method for further improving the imaging performance. In this method, two or more (or two or more) different reticle patterns are overlapped and exposed on the same layer on a wafer. Specifically, for example, when it is desired to expose a pattern in which a dense pattern and an isolated pattern coexist on the same layer, since the dense pattern and the isolated pattern have significantly different states of generation of diffracted light, optimal exposure conditions (for example, The numerical aperture (NA), illumination conditions, exposure amount, and best focus position are different. Therefore, the dense pattern and the isolated pattern are drawn on the first and second reticles different from each other, and the patterns of these two reticles are double-exposed under the respective optimal exposure conditions. Both patterns can be transferred with a higher resolution than a method of exposing a pattern in which both are mixed under both intermediate exposure conditions.

When the double exposure method is applied, when a photosensitive material (photoresist) which has a small change in characteristics even if the storage time from exposure to development is long as in the prior art is used, for example, one lot of wafers After exposing the pattern of the first reticle to the wafer, the pattern of the second reticle can be exposed to the wafer of that one lot. However, recently, in order to enhance the resolution, excimer laser light such as shorter wavelength KrF (wavelength 248 nm) or ArF (wavelength 193 nm) has been used as illumination light. The chemically amplified resist, which is a photoresist suitable for excimer laser light, is suitable for forming fine patterns.However, the chemical stability after exposure is not very good, so that the time between exposure and development is reduced. In order to shorten the time, it is necessary to perform the developing process within a certain time after the exposure. For this reason, when performing double exposure using a chemically amplified resist, after exposing the pattern of the first reticle for each wafer in one lot, the reticle is replaced to perform the second reticle. And the exposed wafers must be sequentially transferred to a development process.

As described above, when two reticle patterns are double-exposed on one wafer under optimum exposure conditions, the fluctuation of the imaging characteristics of the projection optical system due to the absorption of the heat energy of the illumination light. Is corrected, the imaging characteristics fluctuate in such a tendency that the imaging characteristics under two exposure conditions are mixed. For this reason, it is difficult to accurately correct the fluctuation of the imaging characteristic simply by considering the offset of the imaging characteristic at the time of reticle replacement as in the related art. Further, in the method of waiting until the influence of the previous exposure condition almost disappears after the exposure of the pattern of the first reticle for each wafer, the throughput (productivity) of the exposure process deteriorates and is not realistic. However, it is difficult to apply the method to a photosensitive material such as a chemically amplified resist which requires a short resting time.

In view of the above, the present invention provides an exposure method capable of suppressing a change in imaging characteristics and accurately maintaining a desired imaging state when performing exposure while alternately switching a plurality of exposure conditions. The primary purpose is to provide. Further, the present invention provides an exposure apparatus capable of accurately correcting fluctuations in imaging characteristics due to absorption of illumination light energy in a projection optical system, even when performing multiple exposures while exchanging a reticle pattern for each wafer. It is a second object to provide a method.

Another object of the present invention is to provide an exposure apparatus which can use such an exposure method.

[0012]

According to an exposure method of the present invention, an image of a mask pattern (R) is projected on a substrate (W) through a projection optical system (PL) under a predetermined exposure energy beam.
In an exposure method for exposing upward, when performing exposure while sequentially switching a plurality of different exposure conditions on the substrate, the image forming device corrects an imaging characteristic of a projection optical system (PL) in accordance with an operation of switching the exposure conditions. It is.

According to the present invention, when exposure is performed while alternately switching a plurality of exposure conditions, such as in the case of performing multiple exposure such as double or triple exposure, the image forming characteristics are substantially different under a plurality of exposure conditions. Attention is paid to the fact that the characteristics can be regarded as being mixed, and that the ratio of the mixing can be regarded as substantially constant according to the switching operation, that is, for example, the ratio of the irradiation energy amount under each exposure condition. Then, in the long term, the imaging characteristics are considered to fluctuate based on the average fluctuation characteristics determined by the ratio according to the switching operation, and the fluctuation amount of the imaging characteristics is determined according to the average fluctuation characteristics. The prediction is performed, and the imaging characteristics are corrected based on the result. In other words, when the exposure condition is switched at a cycle sufficiently shorter than the time constant of the fluctuation of the imaging characteristic, correction is performed on the assumption that the imaging characteristic fluctuates according to the average fluctuation characteristic. The imaging characteristics can be accurately maintained in a desired state only by performing a complicated calculation.

In other words, for example, in the case of using the double exposure method, it is possible to correct the fluctuation of the imaging characteristic due to the irradiation of the exposure energy beam by considering that a plurality of fluctuation characteristics are substantially mixed. Was. In this case, it is desirable to correct the imaging characteristics of the projection optical system according to the ratio of the respective exposure times of the plurality of exposure conditions. Since the mixture ratio of the imaging characteristics is determined according to the exposure time ratio, the ratio of the exposure time is used to accurately predict the average fluctuation amount of the imaging characteristics with a small amount of calculation. it can.

A plurality of mask patterns may be prepared, and the images of the plurality of mask patterns may be exposed to the same sensitive layer on the substrate under different exposure conditions. This means that a predetermined pattern is exposed by performing multiple exposure on one layer on the substrate while switching exposure conditions. At this time, especially when performing multiple exposure while exchanging a mask pattern for each substrate, the imaging characteristics are considered to fluctuate based on the average fluctuation characteristics of a plurality of imaging characteristics corresponding to each mask pattern. By applying the present invention, the desired imaging characteristics can be accurately maintained with simple control.

Examples of the exposure conditions include the illumination condition of the exposure energy beam, the numerical aperture of the projection optical system,
And the type of the pattern of the mask. For example, since the transmittance or the like changes depending on the type of the mask pattern, the amount of incident energy to the projection optical system changes. Further, when exposing under the plurality of different exposure conditions, the image forming characteristic of the projection optical system (PL) is corrected according to the ratio of the amount of the exposure energy beam incident on the projection optical system (PL) under the respective exposure conditions. You may make it. Since the ratio of the mixture of the imaging characteristics under a plurality of exposure conditions can be almost accurately estimated by the ratio of the amount of incident energy, the amount of change in the imaging characteristics can be almost accurately predicted based on the ratio. .

Next, an exposure apparatus according to the present invention is an exposure apparatus for exposing an image of a mask pattern (R) onto a substrate (W) via a projection optical system (PL) under a predetermined exposure energy beam. An exposure control unit (1) for performing exposure while sequentially switching a plurality of different exposure conditions on the substrate.
4) and an image forming characteristic correction unit (8, 1) for correcting the image forming characteristic of the projection optical system according to the switching operation of the exposure condition.
5, 16, 17, 19, 21).

According to the exposure apparatus of the present invention, the exposure method of the present invention can be used. Also, an exposure control unit (14)
Is stored in advance for each of a plurality of exposure conditions a model of the fluctuation characteristics of the imaging characteristics due to the absorption of the exposure energy beam of the projection optical system, for example, when performing exposure by the double exposure method, as an example It is desirable to correct the fluctuation of the imaging characteristic due to the exposure energy beam irradiation based on a model obtained by averaging the fluctuation characteristic model according to the ratio of the irradiation energy amount under each exposure condition.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a projection exposure apparatus according to the present embodiment. In FIG. 1, KrF (wavelength 248 n
m), ArF (wavelength 193 nm), or F ₂ (wavelength 157 nm)
Illumination light IL as an exposure energy beam emitted from an exposure light source 1 comprising an excimer laser light source such as an excimer laser light source.
Is incident on the shaping optical system for shaping the cross-sectional shape of the illumination light, and the illuminance uniforming optical system 2 including a fly-eye lens and the like for uniforming the illuminance distribution. Note that a mercury lamp or the like can be used as the exposure light source 1.

The exit surface of the illumination uniforming optical system 2 corresponds to an optical Fourier transform surface (pupil surface) with respect to the pattern surface of the reticle (reticle R1 in FIG. 1) to be transferred. A turret plate 3 on which various aperture stops for switching the illumination conditions for the turret are formed. That is, around the rotation axis of the turret plate 3, a circular aperture stop for performing normal illumination, a small circular aperture stop 3a for setting a small σ value as a coherence factor, and a ring aperture for performing illumination. A ring-shaped aperture stop 3b, a deformed stop composed of four apertures arranged around the optical axis, and the like are arranged, and a main control system 14 that supervises and controls the operation of the entire apparatus is driven by a drive motor 3g. By rotating the turret plate 3, a desired aperture stop can be set on the exit surface.

Illumination light IL emitted from the illuminance uniforming optical system 2 and passing through a predetermined aperture stop in the turret plate 3
Illuminates the illumination area on the pattern surface (lower surface) of the reticle R1 via an optical system 4 including a relay lens and a field stop, a mirror 5 for bending the optical path, and a condenser lens 6. In the optical system 4, an integrator sensor for branching a part of the illumination light IL and detecting the light amount,
Also, a reflectance monitor or the like for detecting the amount of reflected light from the reticle side is provided. The image of the pattern in the illumination area of the reticle R1 is projected at a projection magnification β via the projection optical system PL.
(Β is ４ ，, ５, etc.) and is transferred to an exposure area on the surface of a semiconductor wafer (hereinafter simply referred to as “wafer”) W.

A variable aperture stop (not shown) is provided on an optical Fourier transform plane (pupil plane) of the reticle R1 in the projection optical system PL with respect to the pattern plane. A so-called light-blocking pupil filter that blocks illumination light, a phase filter in which a phase member is formed in a predetermined area, and the like can be installed as necessary. In addition, a chemically amplified resist is applied to the surface of the wafer W in this example, and the surface is exposed to the projection optical system P during exposure.
L is held so as to match the image plane of L. Hereinafter, the Z axis is taken parallel to the optical axis AX of the projection optical system PL, the X axis is taken parallel to the plane of FIG. 1 in a plane perpendicular to the Z axis, and the Y axis is taken perpendicular to the plane of FIG. explain.

First, the reticle R1 is sucked and held on the reticle holder 7, and the reticle holder 7 has three driving elements 8 (2 in FIG.
Only the driving elements 8 appear. The reticle stage RST is mounted on the reticle base 9 movably in the X direction, the Y direction, and the rotation direction by, for example, a linear motor system. The amount of expansion and contraction of the drive element 8 is set by the imaging characteristic control system 16 under the control of the main control system 14. A moving mirror on reticle holder 7 and an external laser interferometer 10 measure the X coordinate, Y coordinate, and rotation angle of reticle stage RST (reticle R), and the measured values are used as reticle stage drive system 11 and main control system. The reticle stage drive system 11 controls the operation of the reticle stage RST based on the measured values, control commands from the main control system 14, and the like.

A reticle exchanging device 12 is provided near the reticle base 9, and a second reticle R 2 is mounted on a first slider 13 of the reticle exchanging device 12. In response to a control command from the main control system 14, a second slider (not shown) and a first slider
The reticle R1 on the reticle holder 7 is replaced with a second reticle R2 via the slider 13 of FIG. Thereafter, reticle R1 and reticle R2 are alternately set on reticle holder 7 by reticle exchange device 12. On the reticles R1 and R2 of this example, a pattern for double exposure is formed (details will be described later).

On the other hand, the wafer W is held by suction on a sample table 23 via a wafer holder (not shown), and the sample table 23 is placed on a wafer stage WST for positioning the wafer W in the X and Y directions by, for example, a linear motor system. Fixed.
The sample stage 23 is also provided with a Z leveling drive mechanism for controlling the position (focus position) of the wafer W in the Z direction and the tilt angle within a predetermined range. A moving mirror on the sample stage 23,
Then, the X coordinate, the Y coordinate, and the rotation angle of the sample table 23 (wafer W) are measured by the external laser interferometer 25, and the measured values are supplied to the wafer stage drive system 26 and the main control system 14, and the wafer stage is driven. System 26 controls the operation of wafer stage WST based on the measured values, control commands from main control system 14, and the like.

At the time of exposure, while the reticle R1 (or reticle R2) and one shot area on the wafer W are kept stationary in a predetermined positional relationship, an image of the pattern of the reticle R1 is projected through the projection optical system PL. After exposure to the shot area, the wafer stage WST is step-moved, the next shot area on the wafer W is moved to the exposure area, and the pattern image of the reticle R1 is exposed by a step-and-repeat method. The exposure is repeatedly performed on each shot area on the wafer W. As described above, the projection exposure apparatus of the present embodiment is of a batch exposure type (stepper type).
The present invention can be applied to a scanning exposure type projection exposure apparatus such as a step-and-scan method. In the case of the scanning exposure type, the reticle stage RST is also provided with, for example, a continuous movement function in the Y direction. At the time of scanning exposure, the reticle R1 and the wafer W are moved in the Y direction via the reticle stage RST and the wafer stage WST. Scanning is performed synchronously with the projection magnification β as the speed ratio.

In order to hold the surface of the wafer W on the image plane of the projection optical system PL by the autofocus method at the time of the above exposure, the wafer W near the exposure area is provided on the side surface of the projection optical system PL.
A projection optical system 27 for projecting a slit image obliquely to a measurement point on the surface of the wafer W, and by receiving reflected light from the surface of the wafer W and re-imaging the slit image, the image of the measurement point from the image plane is obtained. An oblique incidence type focus position detection system (hereinafter, referred to as “AF sensors 27 and 28”) including a light receiving optical system 28 that outputs a focus signal corresponding to the defocus amount is disposed. The focus signals of the AF sensors 27 and 28 are transmitted to the main control system 14 and the wafer stage drive system 2.
The wafer stage drive system 26 controls the operation of the Z leveling drive mechanism in the sample stage 23 so that the focus signal becomes a target value (initial value is 0) set by the main control system 14. .

Next, it is assumed that the projection exposure apparatus of the present embodiment exposes a certain layer on the wafer W to a predetermined circuit pattern image by the double exposure method. The circuit pattern image is, for example, a pattern image in which a dense pattern image 32 and an isolated pattern image 33 are mixed as shown in FIG. 3A, and each pattern image corresponds to a light shielding pattern. In this case, the dense pattern and the isolated pattern are mutually optimal exposure conditions (the aperture stop of the illumination system, the numerical aperture of the projection optical system PL,
3A, the reticle pattern corresponding to FIG. 3A is different from the pattern of the first reticle R1 shown in FIG. 3B and the pattern of the second reticle R2 shown in FIG. And has been broken down. In this case, the pattern of the first reticle R1 is formed of a dense pattern 32A and a light-shielding pattern 33A for covering an isolated pattern, and the pattern of the second reticle R2 is a light-shielding pattern 32B for covering the dense pattern. It is formed from an isolated pattern 33B. Note that, for example, a phase shifter may be provided in the dense pattern 32A, and the reticle R1 may be a phase shift reticle.

Then, the first reticle R1 is placed on the wafer W.
3C is used as the aperture stop of the illumination system when exposing the pattern image of FIG. 3C, and when exposing the pattern image of the second reticle R2, the aperture is used. An aperture stop 3a for a small σ value shown in FIG. 3E is used as the stop. In addition, since the first reticle R1 and the second reticle R2 have different pattern abundances and, consequently, transmittances for the illumination light IL, the incident energy to the projection optical system PL changes depending on which reticle is used. .

As described above, in this embodiment, the exposure is performed by the double exposure method. In this case, too, the projection optical system is controlled by environmental conditions such as the temperature around the projection optical system PL and the amount of energy incident on the projection optical system PL. The imaging characteristics of the system PL gradually change. Therefore, the projection exposure apparatus of the present example is provided with a sensor for measuring environmental conditions and a mechanism for measuring the amount of energy incident on the projection optical system PL. That is, an atmospheric pressure sensor,
Detection signals from an environment sensor 30 including a temperature sensor and a humidity sensor are transmitted to the main control system 14 via a signal processing device 29.
The main control system 14 recognizes the atmospheric pressure, temperature, and humidity around the projection optical system PL from the detection signal.

An irradiation monitor 24 composed of a photoelectric detector is installed near the wafer W (wafer holder) on the sample table 23, and a detection signal of the irradiation monitor 24 is transmitted to the main control system 14.
Is supplied to By moving the light receiving surface of the irradiation amount monitor 24 to the exposure area of the projection optical system PL and detecting the amount of received light, the actual incident energy amount with respect to the projection optical system PL and the transmittance (pattern) of the reticle R1 (or R2) Abundance rate) can be measured. Further, even during the exposure, the exposure amount (incident energy to the projection optical system PL) on the wafer W can always be indirectly monitored via the integrator sensor in the optical system 4, and the reflectance monitor in the optical system 4 can be monitored. Reflectivity of the wafer W, and thus the wafer W
It is configured such that the amount of illumination light reflected by and returned to the projection optical system PL can also be obtained.

Next, a description will be given of a mechanism for correcting the imaging characteristics of the projection exposure apparatus of the present embodiment. First, when the projection optical system PL is telecentric on the reticle side, the drive element 8 for driving the reticle R in the optical axis AX direction is used for correcting a symmetric distortion component. In the case of being non-telecentric, it is used for correcting the projection magnification. Further, in FIG. 1, the projection optical system P
A 3-element, such as a piezo element, that can expand and contract in the Z direction
The lens element L2 is held in the lens frame 20 via the three drive elements 19, and the lens element L1 is held in the lens frame 22 via three drive elements 21 which can be extended and contracted in the Z direction on the lens frame 20. Have been. Drive element 19,
The amount of expansion / contraction of 21 is controlled by the imaging characteristic control system 16. Of the lens elements constituting the projection optical system PL in this manner, the lens element L close to the reticle R1
2 and L1 are respectively moved in the direction of the optical axis AX and tilted within a predetermined range, thereby making it possible to correct, for example, a projection magnification, a field curvature, a symmetric distortion component, and the like.

Further, there is provided a pressure variable section 15 such as a bellows pump for controlling the pressure inside the sealed space 17 between predetermined lens elements inside the projection optical system PL. By controlling the pressure in the closed space 17 via the variable section 15, for example, magnification, coma, field curvature and the like can be corrected. Further, the projection optical system PL
If the amount of change in the image plane (best focus position) of the wafer stage can be predicted, the main control system 14
To the target value of the focus signal detected by the AF sensors 27 and 28 so as to add an offset corresponding to the fluctuation amount, thereby causing the surface of the wafer W to follow the fluctuation of the image plane. It is possible. It is necessary to prepare these correction mechanisms in a number corresponding to the items (degrees of freedom) requiring correction. When the above-mentioned aberrations cannot be completely independently corrected, a combination of the driving amount of each correction mechanism and the corresponding amount of variation of the aberration is represented by a simultaneous equation. The driving amount of each correction mechanism for obtaining the amount (correction amount) may be obtained.
As the correction amount of each aberration (imaging characteristic) is set by the main control system 14, the imaging characteristic control system 16 obtains the driving amount of the corresponding correction mechanism and drives each correction mechanism.

Next, a description will be given of a calculation method for predicting the amount of change in the imaging characteristics of the projection optical system PL due to absorption of the illumination light IL. Main control system 14 includes a wafer stage WST
The energy amount of the illumination light IL incident on the projection optical system PL and the transmittance of the reticle R are measured by the upper irradiation amount monitor 24. Further, when monitoring the energy amount of the light beam passing through the projection optical system PL with higher accuracy, the main control system 1
Numeral 4 calculates an energy amount of a light beam reflected by the wafer W and returned to the projection optical system PL using a detection signal from a reflectance monitor (not shown) provided in the optical system 4. Further, the main control system 14 determines at which timing the illumination light IL is incident on the projection optical system PL based on, for example, a detection signal of an integrator sensor (not shown) in the optical system 4, and
Can be recognized for the irradiation period of the illumination light IL. The internal temperature of the projection optical system PL changes due to the balance between the absorption of the illumination light IL and the heat radiation, and the imaging characteristics change accordingly.

FIG. 2 shows how the imaging characteristics of the projection optical system PL change in such a manner. In FIG. 2, the horizontal axis indicates the elapsed time t, and the vertical axis indicates the amount of change ΔP in the imaging characteristics. I have. Further, solid lines 31A and 31B represent changes in the imaging characteristics when the reticles R1 and R2 are used, for example. In the following, the first reticle R1,
Exposure conditions (illumination conditions, type of reticle pattern, numerical aperture of projection optical system PL, exposure amount, etc.) for exposing the pattern of second reticle R2 are referred to as exposure condition A and exposure condition B, respectively. As shown by the curves 31A and 31B in FIG. 2, after the irradiation of the projection optical system PL with the illumination light IL is started at the time point t0, the imaging characteristics gradually change, and when the irradiation is further continued, the irradiation characteristics gradually change. The above-mentioned absorption and radiation are balanced, and the imaging characteristic saturates to a certain value. Also,
When the irradiation is stopped at time t1, the imaging characteristics gradually return to the original state. In this case, since the illumination conditions are different between the exposure conditions A and B, the intensity distribution of the illumination light in the projection optical system PL is different even with the same amount of incident energy as a whole, so that as shown by the curves 31A and 31B. , Change characteristics are different. This change characteristic is obtained in advance by an experiment or the like, and is stored in the storage unit in the main control system 14 as a model of the change characteristic of the imaging characteristic.

The change amount ΔP of the imaging characteristic is, for example, a change amount (defocus amount) of the best focus position, an error of the projection magnification β, or an amount of distortion.
And B are assumed to be ΔPA and ΔPB, respectively. Then, the time constants under the exposure conditions A and B are respectively τ
Assuming that the saturation values of A and τB and the change amounts ΔP under the exposure conditions A and B are PA and PB, respectively, in an example of a model of the fluctuation characteristics, the change amounts ΔPA and ΔPB are respectively functions of time t as follows. expressed. In the following model, the irradiation start time t0 in FIG.
≤ t ≤ t1.

ΔPA = PA {1−exp (−t / τA)} (1A) ΔPB = PB {1−exp (−t / τB)} (1B) Also, in FIG. 2, the fluctuation during t1 <t The characteristic model is obtained by calculating the change amounts ΔPA and ΔPB at t = t1 by PA
Assuming that 1, PB1, it can be expressed as follows as an example. In addition, as a model of the fluctuation characteristic, the elapsed time t
May be used such as a table for.

ΔPA = PA1 · exp {− (t−t1) / τA｝ (2A) ΔPB = PB1 · exp {− (t−t1) / τB｝ (2B) For these models, By successively substituting the elapsed time t, the main control system 14 can obtain the change amounts ΔPA and ΔPB of the imaging characteristics at the elapsed time t. Note that the integrated incident energy amount during t0 ≦ t ≦ t1 is proportional to the elapsed time t.
Can be made to correspond to the integrated incident energy amount.

As described above, different models of the fluctuation characteristics correspond to different ones for one exposure condition. In these cases, the saturation values PA and PB are coefficients (usually the amount of energy and the amount of change are proportional) indicating how much variation (at a saturation level) occurs for a given energy. The constants τA and τB are coefficients indicating the speed of change (how long it takes to reach saturation). Also,
Since the values of the saturation values PA and PB and the time constants τA and τB vary depending on the amount of incident energy (illuminance) per unit time, the saturation values PA and PB and the time constants τA and τ
The value of B is stored, for example, as a function of the illuminance of the illumination light.

Further, since the imaging characteristics of the projection optical system PL fluctuate depending on the atmospheric pressure, temperature, humidity, and the like detected through the environment sensor 30, the storage section of the main control system 14 stores these environmental conditions. A variation characteristic model for obtaining the amount of change in the imaging characteristic with respect to the variation of the image is also stored. And
The main control system 14 supplies the change amount of the imaging characteristic according to the change of the environmental condition and the sum of the change amount of the imaging characteristic according to the amount of incident energy to the imaging characteristic control system 16 to form the image. The characteristic control system 16 corrects the imaging characteristics via the driving elements 8, 19, 21 and the like so as to cancel the sum of the supplied changes in the imaging characteristics. Further, regarding the defocus correction, the main control system 14 changes the offset of the focus signals of the AF sensors 27 and 28 with respect to the target value with respect to the wafer stage drive system 26.

Next, an example of the operation of correcting the image forming characteristic in the case of performing the exposure by the double exposure method in the projection exposure apparatus of this embodiment will be described. In this case, in this example, in order to shorten the time for depositing the chemically amplified resist on the wafer, an operation of exposing the pattern image of the first reticle R1 to each wafer in one lot under the exposure condition A is performed. The operation of exposing the pattern image of the second reticle R2 under the exposure condition B is alternately repeated. In order to reduce the frequency of reticle exchange and increase the throughput of the exposure process, after exposing the first wafer, the reticle is not replaced and the second wafer is first replaced with the second wafer. It is desirable that the pattern image of the reticle R2 be exposed, and then the reticle exchange be performed to expose the pattern image of the first reticle R1, and that the reticle exchange be performed at an intermediate point of the exposure of each wafer.

When each wafer is sequentially exposed under the exposure conditions A and B in this manner, the image forming characteristics are characteristics in which the fluctuation characteristics of the image forming characteristics under the two exposure conditions A and B are mixed at a constant rate. Can be considered as changing. However, in a state where the imaging characteristics of the exposure conditions A and B are mixed, the projection optical system P
Even when L is irradiated, the passing position of the illumination light IL is different between the exposure conditions A and B, and the change characteristic for the exposure condition A and
Since the change characteristics are different for the exposure condition B, different coefficients (saturation value and time constant) are used for the exposure conditions A and B.
It is necessary to calculate the amount of change in the imaging characteristics.

FIG. 4 shows a method of determining the coefficients of a model representing the amount of change in the imaging characteristics under the exposure conditions A and B when the imaging characteristics of the exposure conditions A and B can be regarded as being mixed. In FIG. 4, the horizontal axis represents the ratio ε of the irradiation energy amount under the exposure condition B to the total irradiation energy amount under the two exposure conditions A and B. The irradiation energy amount includes the transmittance of the reticle, the illuminance on the reticle determined by the exposure condition of the resist (incident energy per unit time with respect to the projection optical system PL monitored by the integrator sensor in the optical system 4), and the irradiation. When the irradiation energy amounts under the exposure conditions A and B are ΣEA and ΣEB, respectively, the ratio ε is as follows.

Ε = ΣEB / (ΣEA + ΣEB) (3) This ratio ε can be calculated from the output of the dose monitor 24 in FIG. 1 and parameters (for example, the dose and the number of shots) that determine the exposure sequence. Alternatively, the dose monitor 24 is set in the exposure area instead of the wafer W, and the above-described double exposure operation is actually performed, that is, by executing a dummy exposure sequence, the output of the dose monitor 24 is integrated. It can also be determined from the actual measured value.

The vertical axis on the left side of FIG. 4 represents the coefficient kA under the exposure condition A, and the vertical axis on the right side represents the coefficient kB under the exposure condition B, and the solid lines 34A and 34B represent the coefficients kA and kB, respectively. ing. The coefficients kA and kB are, for example, those shown in FIG.
Saturation values PA and PB (formula (1A) described with reference to
(See equation (1B)) from the illuminance of the illumination light. In this case, the exposure condition A when the ratio ε is 0
The value kA ₀ of the coefficient kA in the above is a coefficient representing the fluctuation characteristic of the imaging characteristic in the case of continuously exposing one lot of wafers only under the exposure condition A. When the ratio ε is 1, The value kB ₀ of the coefficient kB under the exposure condition B is a coefficient representing the fluctuation characteristic of the imaging characteristic when the exposure of one lot of wafers is continuously performed only under the exposure condition B.

In the range of 0 <ε <1 where the double exposure is performed for each wafer, the effects of the exposure conditions A and B are mixed, and in this example, the exposure energy of the exposure condition B is, for example, the same as that of the irradiation energy B. However, since the rate of change is large, the ratio ε is 0
Becomes larger, the coefficient kA under the exposure condition A gradually increases as shown by a curve 34A. Conversely, as the ratio ε decreases from 1, the coefficient kB under the exposure condition B gradually decreases as indicated by the curve 34B. In this case, for example, the value kA ₁ of the coefficient kA when the ratio ε is close to ₁ and the value kB ₁ of the coefficient kB when the ratio ε is close to 0 may be determined by simulation or the like. For example, the value k
A ₁ is a coefficient indicating a change amount under the exposure condition A when greatly influenced by the exposure condition B. Coefficients kA and kB in a range of 0 <ε <1 are two exposure conditions. It can be regarded as an average coefficient in which the effects of A and B are mixed.

The curve 34A is approximately expressed as ε =
1, the value of the coefficient kA becomes (kA₁-KB
₀), Ie, kA for each ε₀To
(KA₁-KA₀) · Calculates the value obtained by adding ε
You may. Similarly, curve 34B approximates ε = 0
Where the value of the coefficient kB is (kA₀-K
B ₁), Ie, kB for each ε₀
From (kB₀-KB₁) · (1-ε) is subtracted from
You may ask for it. Thus, the values of the coefficients kA and kB
Is considered to be the average of the coefficients.
Eggplant

Alternatively, in order to more accurately determine the characteristics of the curves 34A and 34B, an exposure experiment is performed while changing the ratio ε from 0 to 1 in predetermined steps, and the values of the coefficients kA and kB are confirmed. The characteristics of the curves 34A and 34B may be obtained as a function related to the ratio ε (for example, a quadratic function) or a table for each predetermined step of the ratio ε. In this case, the function or the table may be stored in the storage unit in the main control system 14, and the values of the coefficients kA and kB may be obtained from the ratio ε during the double exposure. Further, the coefficients kA, k in FIG.
B is a coefficient for obtaining the saturation values PA and PB, and the time constants τA and τB may be obtained by obtaining the same characteristics as the curves 34A and 34B in FIG.

Next, with reference to FIGS. 5 and 6, a description will be given of the operation of correcting the imaging characteristics at the time of double exposure. FIG. 5 shows a case where double exposure is performed while the exposure conditions A and B are alternately repeated from the time point tS when the projection optical system PL in FIG. 1 is in a sufficiently cooled state. In FIG. 5, the horizontal axis represents the elapsed time t. The vertical axis indicates the absolute value of the correction amount C of the imaging characteristic. in this case,
The correction amount C is a value having the same absolute value and the opposite sign to the change amount ΔP of the imaging characteristic. In addition, one wafer
Since the double exposure method is performed while changing the reticle every time, the exposure period TA under the exposure condition A and the exposure period TB under the exposure condition B are alternately repeated at a substantially constant cycle. In this case, it is assumed that the ratio ε of the irradiation energy amount between the exposure condition A and the exposure condition B is εx. At this time, the main control system 14 stores the stored curves 34A and 34B in FIG.
From the characteristics of the above, the coefficient k under the exposure conditions A and B at the ratio εx
The values kAx and kBx of A and kB are obtained. Similarly, the time constant coefficient is obtained from the ratio εx.

Based on these coefficients, the main control system 14 determines the amount of change ΔP of the imaging characteristic under the exposure condition A during the period TA by using the model of the variation characteristic represented by the equations (1A) to (2B).
A, and the change amount Δ of the imaging characteristic under the exposure condition B in the period TB
PB is obtained by sequential calculation. In FIG. 5, the change amounts ΔPA and ΔPB of the imaging characteristics under the exposure conditions A and B are represented by solid-line curves 35A and 35B, respectively. Even if the projection optical system PL is in a completely cooled state (t = tS),
Since the optical path in the projection optical system PL differs depending on the exposure conditions A and B, the value of the change amount ΔP (ΔPA, ΔPB) of the imaging characteristic slightly differs, and as a result, the correction amount C also slightly differs. .

Then, the main control system 14 sets the value of the correction amount C so as to cancel the change amount ΔPA calculated under the exposure condition A in the period TA, and sets the value of the correction amount C under the exposure condition B in the period TB. The value of the correction amount C is set so as to cancel the calculated change amount ΔPB. Thus, even if the amount of change in the imaging characteristics differs depending on each exposure condition, the amount of change in the imaging characteristics can always be appropriately corrected.

Next, referring to FIG. 6, the exposure conditions A,
Despite the fact that B is switched alternately, a set of coefficients kAx, kB determined in FIG.
It will be described that the calculation for obtaining x and the like is sufficient. FIG. 6 is an enlarged view of a portion corresponding to the first portion of the curve 35A of the exposure condition A in FIG. 5, and in FIG.
A is the actual change amount ΔP of the imaging characteristics under the exposure condition A.
A, the change amount ΔPA is large during the period TA exposed under the exposure condition A, and small during the period TB exposed under the exposure condition B. Further, the fact that the curve 36A decreases in the periods TC1 and TC2 at the boundary between the period TA and the period TB indicates a state in which the illumination light is not irradiated corresponding to the reticle exchange time or the wafer exchange time. I have. In FIG. 5, the portion where the variation ΔP is reduced is omitted. Ideally, this solid curve 36A
Should be corrected in accordance with the change in the exposure conditions, but it is difficult to accurately calculate a state in which the two exposure conditions A and B are mixed.

On the other hand, a dotted curve 37A in FIG.
Indicates a change amount ΔPA of the imaging characteristic calculated according to the average coefficient of the exposure conditions A and B as shown in FIG. 4 (the inverse of this sign becomes the correction amount C of the imaging characteristic). Further, in the period TB, the correction amount C of the imaging characteristic is determined based on the curve 35B in FIG. In this example, since the exposure conditions A and B are switched at intervals sufficiently shorter than the time constant of the change in the imaging characteristics, the correction error (the difference between the curves 36A and 37A) is sufficiently small and does not pose a problem. As described above, according to this example, even when reticle exchange is performed for each wafer and exposure is performed by the double exposure method, the imaging characteristics of the projection optical system PL based on changes in incident energy and environmental conditions are also improved. The amount of change can be corrected with high accuracy.

In FIG. 6, during the period TA (TA1) starting from time tS, the first wafer
Is exposed, and the next period TB
In the first half of the period TB1, the second wafer is
Of the reticle R2 is exposed. Then, in a second period TB2 of the period TB, a pattern image of the second reticle R2 is exposed to the second wafer, and in a first half period TA2 of the next period TA, the second wafer is exposed to the second wafer. The pattern image of one reticle R1 has been exposed. Then, in the latter half period TA3, the pattern image of the first reticle R1 is exposed on the third wafer, and thereafter, the reticle exchange is performed in the middle of the exposure of each wafer.

In the above-described embodiment, two reticles R1 and R2 are prepared for performing double exposure. For example, the first pattern is provided on the right half surface of one reticle.
The second pattern is drawn on the left half surface,
And the second pattern may be alternately exposed. In the above embodiment, the exposure conditions to be switched are the two conditions A and B, but the same can be applied even when the exposure conditions are three or more. That is, in the case of three exposure conditions, FIG. 4 is a three-dimensional graph, and a coefficient indicating the amount of change in the imaging characteristics may be obtained from the ratio of the mutual irradiation energy amounts of the three exposure conditions.

The above embodiment can be simplified and applied. For example, when there is not much difference between the curve 35A of the exposure condition A and the curve 35B of the exposure condition B in FIG. 5, the change amount ΔP is calculated using only one kind of intermediate coefficient between the two curves 35A and 35B, and It is conceivable that the same correction is performed for both the conditions A and B. In this case, the main control system 14
Calculation load is reduced, and the coefficient management becomes easy.

The curves 35A and 35B in FIG. 5 may be approximated by straight lines to correct the image forming characteristics. As another example of simplification, at the time of exposure under exposure condition A, exposure condition A is set to 1
The change amount is calculated by a calculation formula of 00% (ε = 0 in FIG. 4), and the exposure condition B is set to 100% (ε in FIG.
= 1), the change amount is calculated, and the correction amount may use the sum of the two exposure conditions. In this method, 2
The effect of the mixture of the two exposure conditions is ignored, and the calculations are performed independently of each other and simply added, and there is no need to consider the graph of FIG. This method is effective when the influence of the mixture is small because the calculation load and the coefficient setting load are small.

It should be noted that the present invention is not limited to the above-described embodiment, and it is needless to say that various configurations can be adopted without departing from the gist of the present invention.

[0059]

According to the exposure method of the present invention, since the imaging characteristic of the projection optical system is corrected in accordance with the switching operation of the exposure condition, it is possible to perform multiple exposure while alternately switching a plurality of exposure conditions. In this case, there is an advantage that a desired imaging state can be accurately maintained by suppressing a change in imaging characteristics without lowering the throughput of the exposure process.

Further, when correcting the imaging characteristics of the projection optical system in accordance with the ratio of the respective exposure times of a plurality of exposure conditions, it is possible to easily suppress the fluctuation of the imaging characteristics by performing a simple calculation. Can be. Further, when a plurality of mask patterns are prepared and the images of the plurality of mask patterns are exposed to the same sensitive layer on the substrate under mutually different exposure conditions, 1
Even in the case of performing multiple exposure while exchanging the pattern of the mask (reticle) for each substrate (wafer), the advantage that the fluctuation of the imaging characteristics due to the absorption of the exposure energy beam in the projection optical system can be accurately corrected. is there.

If the exposure condition is any of the illumination condition of the exposure energy beam, the numerical aperture of the projection optical system, and the type of the mask pattern, the exposure condition having a large effect on the imaging characteristics is taken into consideration. Therefore, it is possible to more accurately correct the fluctuation of the imaging characteristic. Also, when performing exposure under a plurality of different exposure conditions, when correcting the imaging characteristics of the projection optical system according to the ratio of the amount of the exposure energy beam incident on the projection optical system under each exposure condition, the Variations in characteristics can be corrected more accurately.

Next, according to the exposure apparatus of the present invention, there is an advantage that the exposure method of the present invention can be used.

[Brief description of the drawings]

FIG. 1 is a partially cut-away configuration diagram showing a projection exposure apparatus used in an example of an embodiment of the present invention.

FIG. 2 is a diagram for explaining a state of a change in an imaging characteristic caused by illumination light absorption of a projection optical system.

FIG. 3 is an explanatory diagram of patterns of two reticles and corresponding exposure conditions when exposing by a double exposure method.

FIG. 4 is an explanatory diagram of a method of obtaining a coefficient of a model of a variation amount of an imaging characteristic when performing exposure by a double exposure method in the embodiment.

FIG. 5 is a diagram showing a correction amount (change amount) of an imaging characteristic under each exposure condition when performing exposure by a double exposure method in the embodiment.

FIG. 6 is an enlarged view showing a correction amount (change amount) of an imaging characteristic in a first part of FIG. 5;

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 exposure light source 2 illuminance uniforming optical system 3 turret plate R1, R2 reticle PL projection optical system W wafer 8, 19, 21 drive element RST reticle stage 12 reticle exchange device 14 main control system 15 pressure variable section 16 imaging characteristic control system 17 Closed space 23 Sample stage 24 Irradiation amount monitor WST Wafer stage 30 Environmental sensor

Claims (6)

[Claims] 1. An exposure method for exposing a mask pattern image onto a substrate via a projection optical system under a predetermined exposure energy beam, wherein the substrate is exposed while sequentially switching a plurality of different exposure conditions. An exposure method comprising: correcting an imaging characteristic of the projection optical system in accordance with a switching operation of the exposure condition. 2. The exposure method according to claim 1, wherein the image forming characteristic of the projection optical system is corrected according to a ratio of each exposure time of the plurality of exposure conditions. 3. The exposure method according to claim 1, wherein a plurality of the mask patterns are prepared, and images of the plurality of mask patterns are exposed to the same sensitive layer on the substrate under different exposure conditions. 4. The exposure according to claim 1, wherein the exposure condition is one of an illumination condition of the exposure energy beam, a numerical aperture of the projection optical system, and a type of a pattern of the mask. Method. 5. When exposing under a plurality of different exposure conditions, the image forming characteristic of the projection optical system is corrected according to a ratio of an amount of an exposure energy beam incident on the projection optical system under each exposure condition. 3. The exposure method according to claim 1, wherein: 6. An exposure apparatus for exposing a mask pattern image onto a substrate through a projection optical system under a predetermined exposure energy beam, wherein the substrate is exposed while sequentially switching a plurality of different exposure conditions. An exposure apparatus, comprising: an exposure control unit that performs the operation; and an imaging characteristic correction unit that corrects an imaging characteristic of the projection optical system in accordance with the switching operation of the exposure condition.