WO1999026279A1

WO1999026279A1 - Exposure method and aligner

Info

Publication number: WO1999026279A1
Application number: PCT/JP1998/005184
Authority: WO
Inventors: Tetsuo Taniguchi
Original assignee: Nikon Corporation
Priority date: 1997-11-18
Filing date: 1998-11-18
Publication date: 1999-05-27
Also published as: JPH11150053A; AU1173499A

Abstract

An aligner such that the variation of image-forming characteristic which is caused by the absorption of the energy of illumination light in a projection optical system can be accurately corrected even if double exposure is performed while the pattern of a reticle is changed for every wafer. During a period TA, the pattern of a 1st reticle is exposed under an exposure condition A and, during a period TB, the pattern of a 2nd reticle is exposed under an exposure condition B. These exposures are alternately repeated. A coefficient which shows the model of the variation of the image-forming characteristic during the period TA and a coefficient which shows the model of the variation of the image-forming characteristic during the period TB are determined in accordance with the ratio between the applied energies under the exposure conditions A and B. The change ΔP of the image-forming characteristic during the periods TA and TB is estimated in accordance with the coefficients and the image-forming characteristic is corrected so as to cancel the change ΔP.

Description

Description Exposure method and apparatus

The present invention relates to an exposure method used for transferring a mask pattern onto a photosensitive substrate when, for example, manufacturing a semiconductor element, a liquid crystal display element, or a thin-film magnetic head using a photolithography technique, and The exposure apparatus is particularly suitable for use in performing multiple exposure. Background art

In a projection exposure apparatus such as a stepper used for manufacturing a semiconductor device or the like, it is known that imaging characteristics (distortion, best focus position, etc.) fluctuate due to absorption of illumination light for exposure by a projection optical system. Therefore, various correction methods have been proposed. One example of such a correction method is to measure the incident energy to the projection optical system, predict the amount of change in the imaging characteristics according to a previously determined 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 the correction mechanism described above.

In this regard, in recent years, in order to further improve the resolution, a modified illumination technology or a phase shifter that illuminates 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. Technology 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 changes greatly, and the fluctuation characteristics of the above-mentioned imaging characteristics change. As disclosed, a technique has been proposed in which a model of the fluctuation characteristic is used after being changed according to exposure conditions.

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 while the influence of the previous exposure condition remains in the projection optical system, and the imaging characteristics change discontinuously. For this reason, it is difficult to make a highly accurate prediction calculation using a model of the fluctuation characteristic of the imaging characteristic under a certain exposure condition. Even if a model of the fluctuation characteristics under these two exposure conditions is obtained in advance and a method of switching the model of the fluctuation characteristics when the exposure conditions are switched is used, the imaging characteristics are not changed when the exposure conditions are switched. May change discontinuously, which may leave a correction error in the imaging characteristics.

For this reason, for example, Japanese Patent Application Laid-Open No. 6-45217 proposes a method of suspending exposure under new exposure conditions until the influence of the previous exposure conditions is sufficiently reduced. Alternatively, as disclosed in Japanese Patent Application Laid-Open No. 7-94339, the discontinuity (offset) of the imaging characteristic at the time of the change is measured in advance, and when the imaging characteristic is switched, A method of adding the offset to the imaging characteristic after switching has also been proposed.

As described above, the conventional method of correcting the imaging characteristics uses a model of the fluctuation characteristics of the imaging characteristics obtained in advance according to the exposure conditions, and performs the offset correction when switching the exposure conditions. In this method, the amount of change in the image characteristics is predicted, and the image formation characteristics are corrected so as to offset the predicted amount of change. However, in the actual exposure process, for a while after switching the exposure conditions, the imaging characteristics under the two exposure conditions are mixed, and the offset of the imaging characteristics is temporarily considered. Even 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. This method uses two or more different layers on the same layer (sensitive layer) on the wafer.

This is a method in which (or two or more types) of reticle patterns are overlapped and exposed. Specifically, for example, when a pattern in which a dense pattern and an isolated pattern coexist is exposed on the same layer, the dense pattern and the isolated pattern have significantly different states of generation of diffracted light. The numerical aperture (NA), illumination conditions, exposure amount, and best focus position of the projection optical system are different. Therefore, the dense pattern and the isolated pattern are drawn on the first and second reticles different from each other, and the pattern of the two reticles is double-exposed under the optimum exposure conditions, for example, by using both of them. Compared to the method of exposing a pattern with mixed patterns under both intermediate exposure conditions Therefore, both patterns can be transferred with high resolution.

When applying this double exposure method, when using a photosensitive material (photoresist) that has a long time from exposure to development and a small change in characteristics as in the past, for example, one lot of wafers After exposing the pattern of the first reticle, the pattern of the second reticle can be exposed to the one lot of wafers. However, recently, in order to increase the resolution, excimer laser light such as KrF (wavelength: 248 nm) or ArF (wavelength: 193 nm) having a shorter wavelength is used as illumination light. It is being used. The chemically amplified resist, which is a photoresist suitable for excimer laser light, is suitable for forming fine patterns, but the chemical stability after exposure is not very good, so the time between exposure and development is reduced. In order to shorten the time, it is necessary to perform development processing within a certain time after 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 exchanged by exchanging the reticle. The reticle pattern must be exposed, and the exposed wafer must be sequentially transferred to a development process.

In this way, when performing double exposure of the two reticle patterns under the optimal exposure conditions for each wafer, the fluctuations in the imaging characteristics of the projection optical system due to the absorption of the thermal energy of the illumination light are corrected. In this case, the imaging characteristics fluctuate in such a tendency that the imaging characteristics under the 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 when replacing the reticle as in the conventional case. In addition, 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 this method to photosensitive materials such as chemically amplified resists that require a short resting time.

In view of the above, it is a first object of the present invention to provide a method capable of performing exposure in a desired imaging state when performing exposure while alternately switching a plurality of exposure conditions.

Further, the present invention can reduce the fluctuation of the imaging characteristic due to the absorption of the energy of the illumination light in the projection optical system, even when performing multiple exposure while exchanging patterns for each wafer. A second object is to provide an exposure method that can accurately correct.

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

An exposure method according to the present invention is an exposure method for exposing an image of a mask pattern on a substrate through a projection optical system under a predetermined exposure energy beam, wherein a plurality of different exposure conditions are sequentially applied to the substrate. When performing exposure while switching, the image forming characteristic of the projection optical system is corrected in accordance with the switching operation of the exposure condition.

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

In other words, for example, when the double exposure method is used, 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.

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, using the exposure time ratio makes it possible to accurately predict the average fluctuation amount of the imaging characteristics with a small amount of calculation. .

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

An example of 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. For example, since the transmittance or the like changes depending on the type of mask pattern, the amount of energy incident on the projection optical system changes.

When exposing under a plurality of different exposure conditions, the imaging characteristics of the projection optical system are corrected according to the ratio of the amount of the exposure energy beam incident on the projection optical system under each exposure condition. It may be. Since the ratio of the mixture of the imaging characteristics under a plurality of exposure conditions can be almost accurately estimated based on the ratio of the amount of incident energy, the amount of change in the imaging characteristics can be almost accurately estimated based on the ratio. Can be predicted. Next, an exposure apparatus according to the present invention is an exposure apparatus that exposes an image of a mask pattern onto a substrate via a projection optical system under a predetermined exposure energy beam. And an image forming characteristic correcting unit for correcting the image forming characteristic of the projection optical system in accordance with the switching operation of the exposure condition.

According to such an exposure apparatus of the present invention, the exposure method of the present invention can be used.

In addition, the exposure control unit stores in advance a model of a variation characteristic of an imaging characteristic due to absorption of an exposure energy beam of the projection optical system for each of a plurality of exposure conditions, and performs exposure by, for example, a double exposure method. In this case, as an example, based on the model obtained by averaging the models of the fluctuation characteristics according to the ratio of the irradiation energy amount under each exposure condition, the fluctuation of the imaging characteristics due to the irradiation of the exposure energy beam is corrected. It is desirable to do it. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a partial view showing a projection exposure apparatus used in an example of an embodiment of the present invention. FIG.

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

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

FIG. 4 is an explanatory diagram of how to obtain 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 the correction amount (change amount) of the imaging characteristic in the first part of FIG. BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 shows the projection exposure apparatus of this example. In FIG. 1, KrF (wavelength 24

8 nm), A r F (wavelength 1 9 3 nm), or F ₂ (irradiation Meiko IL as an exposure energy beam emitted from the exposure light source 1 consisting of an excimer laser light source 1 5 7 nm), etc. wavelengths, lighting The light is incident on a shaping optical system for shaping the cross-sectional shape of light, and an illuminance uniforming optical system 2 including a fly-eye lens for uniforming the illuminance distribution. Note that a mercury lamp, X-ray, or the like can be used as the exposure light source 1.

The exit surface of the illumination uniforming optical system 2 is the reticle to be transferred (reticle R in Fig. 1).

1) corresponds to the optical Fourier transform plane (pupil plane) for the pattern plane, and on the exit plane, a turret plate 3 on which various aperture stops for switching the illumination conditions for the reticle are formed is arranged. . That is, around the rotation axis of the turret plate 3, a circular aperture stop for normal illumination, a small circular aperture stop 3a for setting a small σ value as a coherence factor, and an annular illumination are provided. A ring-shaped aperture stop 3b for performing the operation, and a deformed stop composed of four apertures arranged around the optical axis are arranged. By rotating the turret plate 3 via the drive motor 3 g, the desired aperture stop It is configured so that it can be installed. An example of the turret plate 3 is disclosed in Japanese Patent Application Laid-Open No. 9-265554 (US Pat. No. 5,739,899). The illumination light IL emitted from the illuminance uniforming optical system 2 and having passed through a predetermined aperture stop in the turret plate 3 is further converted into an optical system 4 including a relay lens, a field stop, and the like, a mirror for bending the optical path 5 The illumination area on the pattern surface (lower surface) of the reticle R 1 is illuminated via the condenser lens 6. The optical system 4 also includes an integrator sensor 4a for branching a part of the illumination light IL and detecting the amount of the light, and a reflectance monitor 4b for detecting the amount of light reflected from the reticle side. . The image of the pattern in the illumination area of reticle R 1 is projected onto the surface of a semiconductor wafer (hereinafter simply referred to as “wafer”) W at a projection magnification of ^ (1 Z 4, 1/5, etc.) via projection optical system PL. Is transferred to the exposure area.

Note that an illumination system including the illuminance uniforming optical system 2 and the optical system 4 is disclosed in Japanese Patent Application Laid-Open No. Hei 5-2010 (U.S. Pat. No. 5,719,704). An illumination system including a rod-type optical integrator as described above may be used.

A variable aperture stop (not shown) is provided on the optical Fourier transform plane (pupil plane) for the pattern surface of the reticle R1 in the projection optical system PL. A pupil filter of a so-called light-blocking type that blocks illumination light in a predetermined area centered on the optical axis as disclosed in Japanese Patent Application Laid-Open No. HEI 6-124870. A phase filter or the like in which a phase member is formed can be installed as needed. Note that the exit surface of the uniform illumination optical system 2 is almost optically conjugate with the pupil plane of the projection optical system PL, and is substantially optically conjugate with the pupil plane of the projection optical system PL due to the rotation of the turret plate 3. When the light intensity distribution (secondary light source) in the plane is changed, the intensity distribution on the pupil plane of the projection optical system PL also changes. In addition, a chemically amplified resist is applied to the surface of the wafer W in this example, and the surface is held so as to coincide with the image plane of the projection optical system PL during exposure. 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 perpendicular to the plane of Fig. 1. Take and explain.

First, the reticle R 1 is sucked and held on the reticle holder 7, and the reticle holder 7 has three drive elements 8 (two in FIG. Only the driving element 8 appears. 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. The moving mirror on the reticle holder 7 and the external laser interferometer 10 measure the X coordinate, Y coordinate, and rotation angle of the reticle stage RST (reticle R), and the measured values are used as the reticle stage drive system 11, The reticle stage drive system 11 is supplied to the main control system 14 and controls the operation of the reticle stage RST based on the measured values and control commands from the main control system 14.

In addition, a reticle exchanging device 12 is installed near the reticle base 9, and a second reticle R 2 is placed on the first slider 13 of the reticle exchanging device 12, and the reticle exchanging device 1 2 Replaces the reticle R 1 on the reticle holder 7 with the second reticle R 2 via the second slider (not shown) and the first slider 13 according to a control command from the main control system 14. . Thereafter, reticle R 1 and reticle R 2 are alternately set on reticle holder 7 by reticle exchange device 12. Reticles R 1 and R 2 in this example are formed with a pattern for double exposure, which 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 fixed on a wafer stage WST for positioning the wafer W in the X and Y directions by, for example, a linear motor method. Have been. 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. The moving mirror on the sample stage 23 and the external laser interferometer 25 measure the X coordinate, Y coordinate, and rotation angle of the sample stage 23 (wafer W), and the measured values are used as the wafer stage drive system 26, The wafer stage drive system 26 is supplied to the main control system 14, and controls the operation of the wafer stage WST based on the measured values, control commands from the main control system 14, and the like.

At the time of exposure, while the reticle R 1 (or reticle R 2) and one shot area on the wafer W are kept stationary in a predetermined positional relationship, an image of the pattern of the reticle R 1 is projected via the projection optical system PL. After exposing the shot area, the wafer stage WST is stepped and the next shot area on the wafer W is moved to the exposure area, The operation of exposing the image of the pattern of the circle R1 is repeated in a step-and-repeat manner, and each shot area on the wafer W is exposed. As described above, the projection exposure apparatus of this embodiment is of a batch exposure type (stepper type), but the present invention is disclosed in Japanese Patent Application Laid-Open No. 6-291106 (US Pat. No. 5,721,608). The present invention can also be applied to a scanning exposure type projection exposure apparatus such as a step-and-scan method as disclosed in (1). In the case of the scanning exposure type, the reticle stage RST is also provided with, for example, a function of continuously moving in the Y direction. The scanning is performed synchronously with the projection magnification 3 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 during the above exposure, a slit image is obliquely placed on the side of the projection optical system PL at a measurement point on the surface of the wafer W near the exposure area. The projection optical system 27 that projects the light, and the reflected light from the surface of the wafer W is received and the slit image is re-imaged, so that the focus signal corresponding to the defocus amount from the image plane at the measurement point is obtained. An oblique incidence type focus position detection system (hereinafter, referred to as “AF sensors 27, 28”) composed of an output light receiving optical system 28 and is provided. The focus signals of the AF sensors 27 and 28 are supplied to the main control system 14 and the wafer stage drive system 26, and the wafer stage drive system 26 has the target value whose focus signal is set from the main control system 14. The operation of the Z repelling drive mechanism in the sample stage 23 is controlled so that the initial value becomes 0.

As described above, the illumination system and projection optical system including multiple optical components are incorporated into the exposure apparatus main body to perform optical adjustment, and the reticle stage and wafer stage are attached to the exposure apparatus main body to electrically and mechanically By connecting, the projection exposure apparatus of this example is manufactured.

Next, it is assumed that the projection exposure apparatus of this example exposes a certain circuit pattern image to a certain layer on the wafer W by a double exposure method. The circuit pattern image is, as an example, a butterfly image in which a dense pattern image 32 and an isolated pattern image 33 are mixed, as shown in FIG. 3 (a), and each pattern image is a light shielding pattern. Shall correspond. In this case, the dense pattern and the isolated pattern have mutually different optimal exposure conditions (such as the aperture stop of the illumination system, the numerical aperture of the projection optical system PL, and the exposure amount). The corresponding reticle pattern is decomposed into a first reticle R1 pattern shown in FIG. 3 (b) and a second reticle R2 pattern shown in FIG. 3 (d). In this case, the pattern of the first reticle R1 is formed by the dense pattern 32A and the light shielding pattern 33A for covering the isolated pattern, and the pattern of the second reticle R2 is The light-shielding pattern 32B for covering the dense pattern and the isolated pattern 33B are formed. Note that, for example, a phase shifter may be provided in the dense pattern 32A, and the reticle R_l may be a phase shift reticle.

When the pattern image of the first reticle R1 is exposed on the wafer W, the annular aperture stop 3b shown in FIG. 3 (c) is used as the aperture stop of the illumination system, and the second When exposing the pattern image of the reticle R2, an aperture stop 3a for a small σ value shown in FIG. 3 (e) is used as the aperture stop. Also, since the first reticle R 1 and the second reticle R 2 have different pattern abundances and, consequently, transmittances for the illumination light IL, the incident energy to the projection optical system PL depends on which reticle is used. Change.

As described above, in this example, the exposure is performed by the double exposure method, but also in this case, the projection optical system PL depends on 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 change gradually. Therefore, the projection exposure apparatus of this 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, a detection signal from an environment sensor 30 including an atmospheric pressure sensor, a temperature sensor, and a humidity sensor is supplied to a main control system 14 via a signal processing device 29, and the main control system 14 detects the detection signal. The atmospheric pressure, temperature, and humidity around the projection optical system PL are recognized from the 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 supplied to the main control system 14. . 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 to the projection optical system PL is obtained.

—Amount and transmittance of reticle R 1 (or R 2) (pattern abundance) can be measured. Even during the exposure, the exposure amount to the wafer W (incident energy to the projection optical system PL) is constantly indirectly monitored via the integrator sensor in the optical system 4. And the reflectance monitor of the optical system 4 can be used to determine the reflectivity of the wafer W, and thus the amount of illumination light reflected by the wafer W and returned to the projection optical system PL. .

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. If the reticle is non-telecentric, it is used to correct the projection magnification. Further, in FIG. 1, a lens element L 2 is inserted into a lens frame 20 via three driving elements 19 such as a piezo element which can be extended and contracted on a lens barrel 18 of the projection optical system PL. Is held, and a lens element L1 is held in the lens frame 22 via three drive elements 21 that can expand and contract in the Z direction on the lens frame 20. The expansion and contraction amounts of the driving elements 19 and 21 are controlled by an imaging characteristic control system 16. For example, by moving the lens elements L 2 and L 1 near the reticle R 1 in the direction of the optical axis AX and tilting them within a predetermined range, for example, the projection is performed. Correction of magnification, field curvature, symmetric distortion components, etc. can be performed.

Further, 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 is provided, and the imaging characteristic control section 16 includes: By controlling the pressure in the closed space 17 via the pressure variable section 15, for example, magnification, coma aberration, field curvature and the like can be corrected. In addition, when the fluctuation amount of the image plane (best focus position) of the projection optical system PL can be predicted, the main control system 14 detects the wafer stage drive system 26 by the AF sensors 27 and 28. By issuing a command to add the offset corresponding to the variation to the target value of the focus signal, the surface of the wafer W can follow the variation of the image plane. It is necessary to prepare a number of these correction mechanisms corresponding to the items (degrees of freedom) that need to be corrected. If the above aberrations cannot be corrected completely independently, the combination of the driving amount of each correction mechanism and the corresponding variation amount of aberration is represented by a simultaneous equation. By solving this simultaneous equation, the desired variation of the aberration is obtained. The drive amount of each correction mechanism for obtaining the amount (correction amount) may be obtained. Correction amount of each aberration (imaging characteristics) by main control system 14 In response to the setting, the imaging characteristic control system 16 determines the drive amount of the corresponding correction mechanism and drives each correction mechanism.

Next, a calculation method for estimating the amount of change in the imaging characteristics of the projection optical system PL due to the absorption of the illumination light IL will be described. The main control system 14 measures the energy amount of the illumination light IL incident on the projection optical system PL and the transmittance of the reticle R by the irradiation amount monitor 24 on the wafer stage WST. Also, when monitoring the energy amount of the light beam passing through the projection optical system PL with higher accuracy, the main control system 14 uses the detection signal from the reflectance monitor 4 b provided in the optical system 4 to Calculate the energy of the light beam reflected by the wafer W and returned to the projection optics PL. Further, the main control system 14 determines, for example, at what timing the illumination light IL is incident on the projection optical system PL based on the detection signal of the integrator sensor 4a in the optical system 4, and the illumination light IL for the projection optical system PL. Can be recognized. 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 represents the elapsed time t, and the vertical axis represents the variation Δ 結 of the imaging characteristics. I have. The solid curves 31A and 31B represent changes in the imaging characteristics when the reticles R1 and R2 are used, for example. The following describes the exposure conditions (illumination conditions, reticle pattern type, numerical aperture of the projection optical system PL, and the inside of the projection optical system PL) when exposing the patterns of the first reticle R 1 and the second reticle R 2. Filter type; presence / absence, exposure amount, etc.) are referred to as exposure condition A and exposure condition B, respectively. As shown by the curves 31 A and 3 IB in FIG. 2, after the irradiation of the projection optical system PL with the illumination light IL is started at time t0, the imaging characteristics gradually change, and the irradiation continues. As a result, the absorption and radiation are gradually balanced, and the imaging characteristics are saturated to a certain value. When the irradiation is stopped at time t1, the imaging characteristics gradually return to the original state. In this case, the illumination conditions are different under the exposure conditions A and B, and 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. 3 As shown by IB, the 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 ΔΡ of the imaging characteristic is, for example, the change amount (defocus amount) of the best focus position, the error of the projection magnification / 3, or the amount of distortion. The change amounts ΔP under the exposure conditions Α and B are respectively Let ΔΡ A and ΔΡ B. Then, assuming that the time constants under the exposure conditions A and B are τ A and B, respectively, and the saturation values of the variation ΔP under the exposure conditions A and B are PA and PB, respectively. The changes ΔPA and ΔΡ are each expressed as a function of time t as follows: The following model represents the variation between t O ≤ t ≤ tl with the irradiation start time t 0 as 0 in FIG.

△ PA-PA {1 _exp (— 1 A)} (1 A)

△ PPB {1— exp (− t Z B)} (1 B) In FIG. 2, the model of the fluctuation characteristic during t 1 <t is expressed as the change Δ ΡΑ, Let ΔΡΒ be PA 1 and PB 1, respectively, as an example. As a model of the fluctuation characteristic, a table such as a table for the elapsed time t may be used.

Δ P A = P A 1-exp {-(t-t 1) / τ A} (2 A)

Δ P B = P B 1exp {-(t-t1) / τB} (2B)

By successively substituting the elapsed time t from the irradiation start into these models, the main control system 14 obtains the change amounts △ ΡΑ and Δ の of the imaging characteristics at the elapsed time t. be able to. Since the cumulative incident energy during t 0 t ≤ t 1 is proportional to the elapsed time t, the elapsed time t can be made to correspond to the cumulative incident energy.

As described above, different models of the variation characteristics correspond to different exposure conditions one by one. In these cases, the saturation values PA and PB are coefficients (usually proportional to energy and change) that indicate how much variation (at saturation level) occurs for a given energy. The time constants τΑ and Β are coefficients that indicate the speed of change (how long it takes to reach saturation). In addition, since the saturation values ΡΑ, Β 及び and the time constants A, B vary depending on the amount of incident energy (illuminance) per unit time, their saturation values PA, PB, and the time constant τΑ, Β The value of It is stored 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 unit of the main control system 14 stores those environmental conditions. A variation characteristic model for obtaining the amount of change in the imaging characteristic with respect to the variation is also stored. Then, 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 incident energy amount to the imaging characteristic control system 16. In the imaging characteristic control system 16, imaging is performed via at least one of the driving elements 8, 15, 19, 21 so as to cancel the sum of the supplied changes in the imaging characteristics. Correct the image 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 imaging characteristics when performing exposure by the double exposure method in the projection exposure apparatus of this embodiment will be described. In this case, in this example, the operation of exposing the pattern image of the first reticle R 1 to each wafer in one lot under the exposure condition A is performed in order to shorten the time for putting the chemically amplified resist on the wafer. And the operation of exposing the pattern image of the second reticle R 2 under the exposure condition B are alternately repeated. In order to reduce the frequency of reticle replacement and increase the throughput of the exposure process, after exposing the first wafer, the reticle must not be replaced after the first wafer has been exposed. Exposure of the pattern image of the second reticle R2, and then exchanging the reticle to expose the pattern image of the first reticle R1, and thereafter, it is desirable to perform the reticle exchange at an intermediate point of the exposure of each wafer. .

As described above, when each wafer is sequentially exposed under the exposure conditions A and B, the imaging characteristics change when the fluctuation characteristics of the imaging characteristics under the two exposure conditions A and B are mixed at a constant rate. Can be considered. However, even if the projection optical system PL is irradiated in a state where the imaging characteristics of the exposure conditions A and B are mixed, the passing position of the illumination light IL is different between the exposure conditions A and B. Since the change characteristics are different from the change characteristics for exposure condition B, it is necessary to calculate the amount of change in the imaging characteristics using different coefficients (saturation value and time constant) for exposure conditions A and B. is there.

Fig. 4 shows the case where it can be considered that the imaging characteristics of exposure conditions A and B are mixed. FIG. 4 is an explanatory diagram of a method of determining a coefficient of a model indicating a variation amount of an imaging characteristic under exposure conditions A and B. In FIG. 4, a horizontal axis indicates a total irradiation energy under two exposure conditions A and B. The ratio ε of the amount of irradiation energy under the exposure condition B to the amount of energy. The irradiation energy is determined by the transmittance of the reticle and the illuminance on the reticle determined by the exposure conditions of the resist (per unit time with respect to the projection optical system PL monitored by the integrator sensor 4a in the optical system 4). Incident energy) and the irradiation time. If the irradiation energy amounts under the exposure conditions A and B are ∑EA and ∑EB, respectively, the ratio ε is as follows.

ε = ∑ ∑ Ε ノ (∑ Ε Α + ∑ Ε Β) (3)

The ratio ε can be calculated from the output of the dose monitor 24 in FIG. 1 and parameters (eg, the dose, the number of shots, etc.) that determine the exposure sequence. Alternatively, by setting the dose monitor 2 in the exposure area instead of the wafer W and actually performing the above-described double exposure operation, that is, by executing a dummy exposure sequence, the output of the dose monitor 24 is obtained. It is also possible to obtain from the actually measured value of the integrated value.

Also, the vertical axis on the left side of FIG. 4 represents the coefficient k A under the exposure condition Α, and the vertical axis on the right side represents the coefficient k B under the exposure condition B, and the solid curves 34 A and 34 B correspond to the coefficient k A and kB are shown. The coefficients k A and k B are, for example, proportional coefficients for obtaining the saturation values PA and PB (see equations (1A) and (1B)) described with reference to FIG. 2 from the illuminance of the illumination light. It is. In this case, the value k A o of the coefficient k A under the exposure condition A when the ratio ε is 0 is the value of the imaging characteristic in the case where one lot of wafers are continuously exposed only under the exposure condition A. The value of the coefficient k Β _{0 under} the exposure condition B when the ratio ε is 1 is the coefficient representing the fluctuation characteristics. Are coefficients representing the fluctuation characteristics of the imaging characteristics.

In the range of 0 <ε <1, where the double exposure is performed for each wafer, the effects of exposure conditions A and B are mixed, and in this example, exposure condition B changes even with the same irradiation energy as an example. Because the ratio is large, the curve increases as the ratio ε increases from 0.

As shown by 4, the coefficient k 露光 under the exposure condition Α gradually increases. Conversely, as the ratio ε decreases from 1, the coefficient k 露光 under the exposure condition Β gradually decreases as shown by the curve 34 Β. In this case, the ratio ε is 1 The value k Ai of the coefficient k A when it is close to, and the value k of the coefficient k とき when the ratio ε is close to 0 may be obtained in advance. For example, the value is a coefficient indicating the amount of change under exposure condition A when greatly affected by exposure condition B. The coefficients kA and kB in the range of 0 <ε <1 are two It can be regarded as an average coefficient in which the effects of exposure conditions A and B are mixed.

Curve 34A is approximately such that ε = 1 and the value of the coefficient k なる is larger by (kAi-kB.) With respect to the coefficient k 即ち, that is, kA for each ε _The value obtained by adding (kAi−kΑ0) · ε to ₀ may be obtained in association with the value. Similarly, the curve 34 Β is approximately such that ε = 0 and the value of the coefficient k Β is smaller than the coefficient k k by (kAo−k Bi), ie, k B for each ε . The value obtained by subtracting (k B. 1 k Bj · (1− ε) from the above may be obtained in association with the above. Calculating the values of the coefficients kA and k B in this way means that the coefficients are averaged here. Consider asking.

Alternatively, in order to more accurately obtain the characteristics of the curves 34A and 34B, an exposure experiment is performed while changing the ratio ε in a predetermined step from 0 to 1 in advance to confirm the values of the coefficients kA and kB. The characteristics of the curves 34A and 34B may be obtained as a function (for example, a quadratic function or the like) relating to the ratio ε, 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 kΑ and k より may be obtained from the ratio ε during the double exposure. The coefficients kA and kB in Fig. 4 are the coefficients for obtaining the saturation values PA and PB, but the time constants A and B are the same as the curves 3A and 3B in Fig. 4. You should ask for. 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 t S when the projection optical system PL in FIG. 1 is sufficiently cooled. The horizontal axis represents the elapsed time t, and the vertical axis represents 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 Δ Ρ of the imaging characteristic. In addition, since double exposure is performed while exchanging the reticle once per wafer, the exposure period TA under the exposure condition A and the exposure period TB under the exposure condition B alternate alternately at a substantially constant cycle. Has been repeated. In this case, the ratio ε of the irradiation energy amount between the exposure condition A and the exposure condition B is εX. At this time, The control system 14 obtains the values kAx and kBx of the coefficients kA and kB under the exposure conditions A and B at the ratio εx from the stored characteristics of the curves 34A and 34B in FIG. Similarly, the time constant coefficient is obtained from the ratio ε X.

Based on these coefficients, the main control system 14 determines the amount of change ΔΡΑ in the imaging characteristic under the exposure condition 期間 during the period Τ モデル by using the model of the variation characteristic represented by the equations (1Α) to (2Β), for example. The change amount Δ Ρ 像 of the imaging characteristic under the exposure condition Β of the period Τ で is sequentially calculated. In FIG. 5, the change amounts Δ 結 and ΔΡΒ of the imaging characteristics under the exposure conditions Α and Β are represented by solid-line curves 35 Α and 35 B, respectively. Even when the projection optical system PL is in a completely cooled state (t = tS), the optical path in the projection optical system PL varies depending on the exposure conditions A and B, so that the amount of change in the imaging characteristics ΔΡ (ΔΡΑ, ΔΡΒ) Are slightly different, and as a result, the correction amount C is also slightly different.

Then, in the period TA, the main control system 14 calculates the change amount もと calculated under the exposure condition A.

The value of the correction amount C is set so as to offset P A, and in the period TB, the value of the correction amount C is set so as to offset the change amount ΔP B calculated under the exposure condition B. Thus, even if the amount of change in the imaging characteristics differs under each exposure condition, the amount of change in the imaging characteristics can always be properly corrected.

Next, referring to FIG. 6, a set of coefficients kAx, kBx, etc. determined in FIG. 4 are determined for each exposure condition, even though the exposure conditions A and B are alternately switched. A description will be given of the fact that the required calculation is sufficient. FIG. 6 is an enlarged view of a portion corresponding to the first part of the curved line 35A of the exposure condition A in FIG. 5, and in FIG. Shows the actual amount of change Δ 結 A in the imaging characteristics, and the amount of change ΔΡΑ is large during the exposure period TA under the exposure condition A, and small during the exposure period TB under the exposure condition B. . Also, the curve 36A decreases in the period TC1 and TC2 at the boundary between the period TA and the period TB when the illumination light is not irradiated for the reticle exchange time or the wafer exchange time. Is shown. In FIG. 5, the portion where the variation ΔΡ is reduced is omitted. Ideally, it should be corrected according to the change of the solid curve 36A, but it is difficult to calculate exactly the state where the two exposure conditions A and B are mixed.

On the other hand, the dotted curve 37A in FIG. And the change amount Δ Ρ の of the imaging characteristic calculated according to the average coefficient of B and B (the inverse of this sign becomes the correction amount C of the imaging characteristic). In the period TB, the correction amount C of the imaging characteristic is determined based on the curve 35B in FIG. In this example, the exposure conditions A and B are switched at a sufficiently short interval compared to the time constant of the change in the imaging characteristics, so that the correction error (the difference between the curves 36A and 37A) is sufficiently small to cause a problem. No. As described above, according to this example, even when the reticle is replaced for each wafer and the exposure is performed by the double exposure method, the imaging characteristics based on the change in the incident energy to the projection optical system PL and the environmental conditions are changed. Can be corrected with high accuracy.

In FIG. 6, the pattern image of the first reticle R1 is exposed on the first wafer during the period TA (TA1) starting from the time point tS, and the first half of the next period TB is performed. In TB1, the pattern image of the second reticle R2 is exposed on the first wafer. Then, in the second half of the period TB, the pattern image of the second reticle R2 is exposed on the second wafer in the second period TB2, and in the first half of the next period TA, the second wafer is exposed in the second period TA2. On the other hand, the pattern image of the first 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 is exchanged in the middle of the exposure of each wafer.

For a device such as a semiconductor, a step for designing the function and performance of the device, a step for manufacturing a reticle based on the design step, a step for manufacturing a wafer from a silicon material, a pattern of a reticle by the exposure apparatus of the above-described embodiment. It is manufactured through a step of exposing the wafer to a wafer, a step of assembling devices (including a dicing step, a bonding step, and a package step), and an inspection step.

In the above embodiment, two reticles Rl and R2 are prepared for performing double exposure. For example, the first pattern and the left half of the reticle are provided on the right half of one reticle. Alternatively, the second pattern may be drawn in advance, and the first and second patterns 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 number of exposure conditions becomes three or more. In other words, in the case of three exposure conditions, FIG. A coefficient indicating the amount of change in the imaging characteristics may be obtained from the ratio of the amount of irradiation energy.

Further, it is also possible to simplify and apply the above embodiment. For example, if there is not much difference between the curve 35 A of the exposure condition A and the curve 35 B of the exposure condition B in FIG. 5, only one kind of an intermediate coefficient between the curves 35 A and 35 B is used. It is conceivable to calculate the amount of change Δ で and perform the same correction for both exposure conditions A and B. In this case, the calculation load of the main control system 14 is reduced, and the management of the coefficients is facilitated.

Also, adjust the imaging characteristics by approximating the curves 35A and 35B in Fig. 5 with straight lines.

As another example of simplification, when exposing under exposure condition A, the amount of change is calculated using the formula of exposure condition A of 100% (ε = 0 in Fig. 4), and exposure under exposure condition Β In such a case, the amount of change is calculated using a calculation formula where the exposure condition 1 is 100% (ε = 1 in Fig. 4), and the correction amount may be the sum of the two exposure conditions. In this method, the effects of the mixture of the two exposure conditions are ignored, and they are calculated independently of each other and simply added, without having to consider the Daraf in FIG. This method is effective when the influence of mixing 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. Industrial applicability

According to the exposure method of the present invention, since the imaging characteristics of the projection optical system are corrected in accordance with the switching operation of the exposure conditions, the exposure is performed when multiple exposures are performed while the plurality of exposure conditions are alternately switched. There is an advantage that a desired imaging state can be accurately maintained by suppressing a change in imaging characteristics without reducing the throughput of the process.

In addition, when correcting the imaging characteristics of the projection optical system according to the ratio of the respective exposure times of a plurality of exposure conditions, fluctuations in the imaging characteristics can be easily suppressed by performing a simple calculation. .

Also, 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, a mask (a wafer) is provided for each substrate (wafer). Such as performing multiple exposure while changing the pattern of the reticle) Even in this case, there is an advantage that the fluctuation of the imaging characteristic due to the absorption of the exposure energy beam in the projection optical system can be accurately corrected.

Further, when the exposure condition is any one 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 influence on the imaging characteristics is considered. Fluctuations in the imaging characteristics can be corrected more accurately. Further, 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, Variations in image 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.

Claims

The scope of the claims

1. An exposure method for exposing a substrate via a projection optical system with an image of a mask pattern, wherein when performing exposure by sequentially switching a plurality of different exposure conditions to a certain layer on the substrate, An exposure method, wherein the imaging characteristic of the projection optical system is corrected in consideration of switching.

2. When performing exposure under the plurality of different exposure conditions, the imaging characteristic of the projection optical system is corrected according to the ratio of the energy amount of the exposure beam incident on the projection optical system under each exposure condition. The exposure method according to claim 1, wherein:

3. The exposure method according to claim 2, wherein an imaging characteristic of the projection optical system is corrected according to a ratio of each exposure time of the plurality of exposure conditions.

4. A parameter for determining a change in the imaging characteristic is determined according to a ratio of an energy amount of an exposure beam incident on the projection optical system under each of the plurality of different exposure conditions. The exposure method according to item 2.

5. The exposure method according to claim 2, wherein the exposure beam incident on the projection optical system includes a reflected beam from the substrate.

6. The exposure method according to claim 1, wherein a plurality of the mask patterns are prepared, and an image of the plurality of mask patterns is exposed to a layer on the substrate under different exposure conditions.

7. After exposing each of the plurality of partial regions on the substrate using a first mask pattern of the plurality of mask patterns, the plurality of partial regions are exposed using a second mask pattern different from the first mask pattern. 7. The exposure method according to claim 6, wherein each of the partial regions is exposed.

8. The exposure condition includes a condition relating to illumination of the exposure beam on the mask pattern, a condition relating to the numerical aperture of the projection optical system, a condition relating to the type of the mask pattern, and a condition relating to an optical filter in the projection optical system. 2. The exposure method according to claim 1, comprising at least one of the following.

9. The illumination-related conditions include an energy intensity distribution in a plane substantially conjugate to a pupil plane of the projection optical system in an illumination system that irradiates the mask pattern with an exposure beam. 9. The exposure method according to claim 8, wherein:

10. The exposure method according to claim 8, wherein the mask pattern includes a phase shift pattern.

9. The exposure method according to claim 8, wherein the optical filter limits the exposure beam in a predetermined area near an optical axis of the projection optical system.

12. The exposure method according to claim 8, wherein the optical filter has a phase member in a predetermined region.

13. The exposure method according to claim 1, wherein the imaging characteristic includes at least one of magnification, curvature of field, distortion, and coma.

14. The exposure method according to claim 1, wherein the correction of the imaging characteristics includes a position adjustment of a part of lens elements of the projection optical system.

15. The exposure method according to claim 4, wherein the parameter includes a coefficient of a model function for determining a change in the imaging characteristic of the projection optical system due to the irradiation of the exposure beam.

16. A device manufactured by using the exposure method according to claim 1.

1 7. In an exposure apparatus that exposes a substrate with an image of a mask pattern via a projection optical system,

An exposure system for performing exposure by sequentially switching a plurality of different exposure conditions for one layer on the substrate,

An exposure apparatus, comprising: a correction system that corrects an imaging characteristic of the projection optical system in consideration of switching of the exposure condition.

18. The correction system corrects an imaging characteristic of the projection optical system according to a ratio of an energy amount of an exposure beam incident on the projection optical system under each of the plurality of different exposure conditions. The exposure apparatus according to claim 17, wherein

19. The exposure conditions include conditions relating to illumination of the exposure beam on the mask pattern, conditions relating to the numerical aperture of the projection optical system, conditions relating to the type of pattern of the mask, and optical filters in the projection optical system. The exposure apparatus according to claim 17, wherein at least one of the conditions is included.

20. In order to change the condition related to the illumination, the exposure pattern The exposure apparatus according to claim 17, further comprising an optical member that changes an energy intensity distribution in a plane substantially conjugate to a pupil plane of the projection optical system in an illumination system that irradiates a beam.

21. The exposure apparatus according to claim 17, wherein the correction system includes a driving system that drives a part of lens elements in the projection optical system.

22. The exposure apparatus according to claim 17, wherein the correction system includes a driving system that drives a mask having the mask pattern.

23. The exposure apparatus according to claim 17, wherein the correction system includes a pressure controller that adjusts a pressure in a closed space in the projection optical system.

24. An environment sensor for checking an environment in which the substrate is exposed, wherein the correction system corrects an imaging characteristic of the projection optical system in consideration of information obtained from the environment sensor. The exposure apparatus according to claim 17, wherein

25. The method according to claim 17, wherein the substrate is exposed while moving the substrate by a scanning method.