JP2756862B2 - Exposure equipment - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure apparatus used for manufacturing semiconductor elements such as ICs and LSIs, and more specifically, to a step-like repetition exposure apparatus (commonly called a stepper). The present invention relates to improvement of a superposition function (commonly called alignment).

[Prior Art] The miniaturization and high integration of semiconductor devices (elements) are unavoidable, and they are constantly evolving. DRAM (Dynamic Random Access Memory), which plays a leading role in miniaturization, has already entered the sub-micron area at the product level, and at the research level, it has patterns with an accuracy of less than half micron (0.5 μm). Is being discussed.

The exposure apparatus called a stepper that emerged in the 256Kbit DRAM era is the main model in the production of 1M to 4Mbit DRAMs, and it is predicted that it will not surrender to future ultra-fine devices.

Speaking of miniaturization, resolution is always a matter of debate, but on the other hand, overlay accuracy is also more important than resolution.
The required accuracy of the superposition is about 1/3 to 1/5 of the resolving power.

The overlay accuracy can be largely divided into the following two elements.

Magnification, distortion component Alignment component The subject of the present invention is an alignment component.

The stepper that emerged with the off-axis alignment system has been proposed many improvements since then,
TTL alignment systems have become mainstream. The stepper alignment systems proposed so far can be roughly classified into the following three.

TTL ON AXIS system: The feature is that the alignment light is the same as the exposure light, and the reticle and wafer can be observed simultaneously.

TTL NON AXIS system: The alignment light is different from the exposure light, but the alignment light passes through the projection lens. Simultaneous observation of the reticle and wafer is difficult.

Off-axis system: An alignment microscope is arranged completely separate from the projection lens.

Of these, the off-axis type stepper has many indirect error factors that are involved in the relative alignment between the reticle and the wafer, and the time from the alignment to the exposure and the moving distance are long, so that the temporal change of the error component is large. High overlay accuracy cannot be obtained. For this reason, it is a method that is rarely used at present.

On the other hand, for the resolving power, according to the Rayleigh equation Re = k × (λ43NA), the exposure wavelength is set to the g-line (wavelength 43
The resolution Re has been improved by increasing the numerical aperture (NA) of the projection lens while keeping it fixed at 6 nm.

However, as is evident from the Rayleigh equation DOF = ± λ / 2NA ² , the depth of focus DOF decreases as the NA increases, while the design and manufacture of the projection lens have reached the limit.
Therefore, in order to support the future sub-micron generation, the exposure wavelength has to be shortened.

At present, the i-line (wavelength 365 nm) stepper is in the stage of practical use, and the next generation is a KrF excimer laser (wavelength 2 nm).
Excimer steppers with a light source of 48 nm) are promising.

However, the practical use of an excimer stepper has necessitated a review of the alignment system described above. This is because the glass material that transmits light of the KrF excimer laser (wavelength 248 nm) is slightly limited to quartz and fluorite, and it is very difficult in design to correct chromatic aberration for light other than the exposure wavelength.

FIG. 5A shows the axial chromatic aberration characteristics of a conventional g-line lens. FIG. 6 (b) shows the axial chromatic aberration characteristics of a conventional quartz single glass lens for excimer laser. In both graphs, the horizontal axis indicates wavelength, and the vertical axis indicates axial chromatic aberration.

In the case of the g-line lens shown in FIG. 5 (a), it is usually possible to design a combination of glass materials so that the characteristic curve contacts at a zero point at a target wavelength. On the other hand, in the excimer lens shown in FIG. 5B, since there is no degree of freedom of the glass material, the excimer lens becomes a substantially straight line crossing at one point of the target wavelength.

When, for example, a HeNe laser (wavelength 633 nm) is selected as the alignment light for the g-line lens, the axial chromatic aberration is about tens of μm. In contrast, for example, an Ar laser (wavelength 500 nm) is used as non-exposure
If is selected, the axial chromatic aberration reaches the order of mm. From this reality, as long as the longitudinal chromatic aberration characteristic curve cannot be improved, an excimer lens (stepper)
The non-exposure alignment system is difficult to realize.

Based on this recognition, as a proposal for improving the above-mentioned disadvantages of the off-axis alignment system, a system using a dummy wafer (Japanese Patent Laid-Open No. 1-120820) and a system using a chuck loaded with a recording material (Japanese Patent Laid-Open No. 1-2863).
09).

[Problems to be Solved by the Invention] However, in the method using a dummy wafer, a dummy wafer is required in addition to a normal wafer for printing an actual element pattern. Therefore, there are the following problems.

1) It takes time to create a dummy wafer.

2) The number of normal wafers is reduced by the number of dummy wafers, or a place for collecting dummy wafers is required in addition to the place of normal wafers.

3) A procedure of collecting the dummy wafer after measurement and loading the normal wafer is required, and there is a decrease in throughput and a change over time such as a baseline length during that time.

Further, the method using a chuck on which a recording material is mounted has the following problems since the recording material is mounted in the apparatus and the recording material is arranged outside the outer diameter of the wafer.

1) A procedure and means for erasing an image formed on a recording material are required.

2) Since the recording material is arranged at a position distant from the position where the actual element pattern of the normal wafer is printed, a difference occurs in the measured values such as the baseline length between the normal wafer and the recording material due to yawing or pitching of the stage. .

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above-described problems of the related art, and has as its object to greatly improve alignment accuracy in an exposure apparatus of an off-axis alignment system without using a chuck on which a dummy wafer or a recording material is mounted.

[Means and Actions for Solving the Problems] In order to achieve the above object, the present invention provides an exposure apparatus that projects a pattern on an original plate (reticle) through a projection optical system onto a substrate to be exposed (a photosensitive layer on a wafer). In the apparatus, a stage means for moving the substrate to be exposed, and through the projection optical system, to form an image of a specific mark on the original plate as a latent image on a photosensitive layer at a predetermined position of the substrate to be exposed, Illuminating means for illuminating only the specific mark, detecting means for detecting the image of the specific mark formed as a latent image on the photosensitive layer, and based on a detection output of the detecting means,
Control means for controlling the movement of the stage means.

At present, photosensitive materials containing photochromic materials have been developed. According to this, a latent image (an image in which changes in the refractive index and transmittance of the photosensitive layer due to light irradiation are detected under a microscope such as white light) (for example, Japanese Patent Application Laid-Open No. 61-114529) is easily formed. I can do it. In addition, the substrate to be exposed has some blank portions where the actual element pattern is not printed.

According to the present invention, a latent image is formed in a blank portion of a substrate to be exposed, and an error between an exposure position and an off-axis microscope position can be detected and corrected without using another medium such as a dummy wafer or a recording material. Thereby, the alignment accuracy of the off-axis system is improved.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows main components of a stepper, which is an exposure apparatus according to an embodiment of the present invention, and its overall arrangement.

In FIG. 1, reference numeral 1 denotes a photo original (hereinafter, referred to as a "reticle"), and reference numeral 2 denotes a semiconductor substrate (hereinafter, referred to as a "wafer") which is a substrate to be exposed.

The light beam 31 emitted from the excimer laser 30 illuminates the reticle 1 through the illumination optical system 3. This light beam reaches the wafer 2 via the projection lens 4, whereby the pattern on the reticle 1 can be transferred to the photosensitive layer on the wafer 2. Reference numeral 31 denotes an interface for controlling the driving of the excimer laser 30 from the CPU 91.

 An excimer illumination system will be briefly described.

The beam directed upward by the mirror 41 reaches the masking image plane via the incoherent optical system 32, the fly-eye lens 33, the condenser lenses 34a and 34b, and the mirror 35.
36 is a masking blade, 37a is a first masking imaging lens, 38 is a mirror, and 37b is a second masking imaging lens.

The reticle 1 is supported by a reticle holding table 11. The reticle holder 11 is supported by a moving stage (not shown).

The wafer 2 is exposed while being vacuum-sucked by the wafer chuck 21. FIG. 1 shows that the wafer 2 is a second support member.
The state supported by 22 is shown. Wafer chuck
The stage 21 is movable in each axis direction by the stage 5.

The stage 5 is supported on a stage base 50. The stage 5 includes a Y stage 51, an X stage 52 above the Y stage 51, a fine moving stage 54 for leveling and moving in the Z direction on the X stage 52, a rotating (θ) fine moving stage 55 above the fine moving stage 54, (Z) A fine movement stage 56 is provided. Fine movement stage 54 for leveling and Z direction movement
Are moved up and down by three piezoelectric elements (piezo elements) 53.

Wafer chuck 21 is mounted on Z fine movement stage 56. On the leveling and Z fine movement stage 54, mirrors 85 serving as references for the position coordinates of the stage system are mounted in the X and Y directions, respectively. The mirror 85 reflects the laser beam emitted from the laser interference length measuring device 86. Thus, the position of the stage and the traveling distance can be known. 87
Is a receiver for converting an optical signal into an electric signal.

On the other hand, on the leveling and Z fine movement stage 54, θ
A coarse movement mechanism 57 and a Z coarse movement mechanism 58 are configured. Further, the second support member 22 is placed thereon.

Above the reticle 1, a reticle optical system 6 is arranged. The reticle optical system 6 is an optical system that handles light having the same wavelength as the exposure light. Therefore, by releasing the switching mirror 41, the light beam 31 emitted from the excimer laser 30 is supplied to the reticle optical system 6 via the mirrors 42 and 43.

The reticle optical system 6 is a binocular optical system having two objective lens systems. The reticle optical system 6 allows the reticle 1
The target mark formed in the small area above is irradiated. This makes it possible to transfer the target mark onto the wafer 2 as a latent image. In the illumination by the reticle optical system 6, an incoherent optical system 40 is arranged between the mirrors 42 and 43 so as to have the same illumination σ as the illumination system 3 and to reduce illuminance unevenness. In addition,
Illumination σ refers to the ratio of the numerical aperture NA ₁ of the illumination system to the numerical aperture NA of the projection lens (σ = NA ₁ / NA). By changing this σ, the resolution of exposure changes the depth of focus.

The laser beam entering the reticle optical system 6 passes through a beam splitter 44, a relay lens 45, and an objective lens 46,
The target mark which is bent downward by the total reflection prism 47 and is arranged at a predetermined position of the reticle 1 is illuminated. Thereby, the image of the target mark is transferred to the wafer 2 via the projection lens 4. Reference numeral 48 denotes a CCD for observing a target mark on the reticle 1. The output of the CCD 48 is input to the CPU 91 via the interface 61.

Above the stage 5, an off-axis microscope 7 is arranged adjacent to the projection lens 4. The off-axis microscope 7 is a monocular microscope that handles non-exposure light (white light). Its main role is to detect the relative position between the internal reference mark 70 and the alignment mark on the wafer. The off-axis microscope 7 has the following configuration.

The objective lens 71 and the relay lens 72 magnify and project the wafer pattern and project the magnified image on the image plane 74. Electa lens 77,7
Numeral 8 functions as a low-magnification erector lens when both are inserted on the optical axis, and as a high-magnification erector when the erector lens 78 retreats, and projects an aerial image of the imaging surface 74 onto the light receiving surface of the CCD 79.

An optical fiber 25 guides light from a light source (not shown). Light emitted from the optical fiber 25 becomes illumination light for illuminating the wafer 2 via the illumination lens 26 and the beam splitter 73. Similarly, an optical fiber 27 guides light from a light source (not shown). Light emitted from the optical fiber 27 illuminates a reference mark 70 via an illumination lens 28.

The beam splitter 75 is disposed such that the pattern surface of the reference mark 70 and the image forming surface 74 have the same optical path length with respect to the light receiving surface of the CCD 79. Therefore, the reference mark 70 is also projected and imaged on the light receiving surface of the CCD 79 by the eleectors 77 and 78.

It is desirable that the off-axis microscope 7 has a focus detection function. This can be achieved by, for example, performing signal processing for blur amount detection on the output signal of the CCD 79.

The control circuit 9 is used to control each of the above-described components. The CPU 91 issues a command to each element according to the determined sequence software, and determines the next procedure by judging data from each element. The arithmetic circuit 92 is used for arithmetic processing that requires high speed and high accuracy, such as calculating the relative position between the reticle 1 and the wafer 2 mainly from the stage coordinates and the detection result of the off-axis microscope. The storage circuit 93 is used to store the measurement data and the calculation data. A partial detailed structure will be described later.

FIG. 2 shows a reticle 1 used in the exposure apparatus of the present embodiment.
FIG. In the figure, 101 indicates a transferable area of the projection lens 4, and 102 indicates an actual element pattern area. Precision wafer alignment marks 103L and 103 are located on the left and right of the scribe line area in contact with the actual element pattern area 102.
R is located. A rough alignment mark 104 is arranged above the scribe line area. Further, the correction marks 105L and 105R are arranged at the very end of the left and right transferable areas on the X axis. The precision wafer alignment marks 103L and 103R are for use in the next step.

FIG. 3A is a plan view of a wafer and a wafer chuck in the exposure apparatus of the present embodiment.

In FIG. 1, patterns formed in a previous process are arranged on a wafer 2. Marks corresponding to the alignment marks 103L, 103R, and 104 described in FIG. 2 are formed in portions corresponding to the scribe lines between the shot areas. Among them, the pattern pre-alignment mark is appropriately selected such that the coarse alignment mark 110 belonging to the 34th shot and the coarse alignment mark 111 belonging to the 42nd shot are used. Also,
AGA (Advanced Glob)
al Alignment), which is indicated by hatching
A mark belonging to an AGA shot [7, 13, 19, 35, 38, 41, 57, 63, 69] is used.

As shown in the drawing, there is a blank portion on the wafer 2 where no actual element pattern is formed in the periphery. For example, portions indicated by reference numerals 113, 114, 115, and 116 in the figure. The correction mark 105L, 1 described in FIG.
By projecting and exposing 05R, for example, a mark 122 is formed as a latent image.

FIG. 3B shows a state in which the precision alignment mark 117 and the correction mark 122 are observed in the field of view of the off-axis microscope 7. In this embodiment, the reticle is designed so that the two types of marks have the same shape. 130 is a frame screen of the CCD 79, and 131 is a reference mark 70 built in the off-axis microscope 7.

Assuming that FIG. 3B shows the state of observing the precision alignment mark 117, the mark 132 is the mark 103L on the reticle, and the mark 133 is the mark 103 on the reticle.
R, respectively, are created by projection exposure. If FIG. 3B shows the state of observing the correction mark 122, the mark 132 is the mark 105L on the reticle, and the mark 133 is the mark 105R on the reticle.
Thus, each is made by projection exposure.

In this case, the precision alignment mark 117 and the correction mark 122 are both marks existing on the reticle 1 at distant positions (the mark 103L and the mark 103R,
An adjacent mark is formed by sending the stage and exposing twice for the mark 105R and the mark 105R that is separated from the mark 105R).
This technique is proposed in Japanese Patent Application Laid-Open No. 63-013329 by the present applicant.

Next, an operation procedure of the exposure apparatus of the present embodiment will be described. The procedure is shown in the flowchart of FIG. The operation of each unit will be described with reference to FIGS.

In a stepper, a large number of wafers are continuously subjected to alignment exposure processing.

Referring to the flowchart of FIG. 4, when the exposure process for a predetermined reticle 1 is started in [Procedure 150], first, in [Procedure 151], the wafer 2 is exchanged, and the wafer 2 is transferred to the second support member 22. Is set. The replacement of the wafer 2 is performed in a state where the second support member 22 shown in FIG.

In [Step 152], the left and right pattern pre-alignment marks 110 and 111 on the wafer 2 are detected. For this reason,
First, the stage is, for example, the right pre-alignment mark 11
0 (FIG. 3) moves so as to enter the low-magnification field of view of the off-axis microscope 7, focuses, and detects the position of the mark 110. Then, the stage position at that time is stored. Note that focus detection is performed by the focus detection function of the off-axis microscope 7, and driving is performed by the Z coarse movement mechanism 59. The accuracy is coarse because it is low magnification. After detecting the position of the mark 110, the stage moves to the left pre-alignment mark 111, and performs focusing, position detection, and coordinate storage similarly to the right.

By this procedure 152, data for roughly understanding the coordinate relationship of the pattern engraved on the wafer 2 with respect to the device coordinates using the optical axis of the off-axis microscope 7 (in this case, the reference mark 70) as the origin and the XY stage as the coordinate axis. Is obtained.

In [Procedure 153], a coordinate error is calculated by the arithmetic circuit 92 based on the data obtained above, and the correction amounts ΔX, ΔY, Δθ of XYθ are calculated.

In [Procedure 154], the XY stage sets the correction amounts ΔX and ΔY
, The movement to the next target position (the position where the procedure 156 is performed) is started. The θ coarse movement mechanism 58 has a θ correction amount Δθ
Is driven to be zero.

In [Procedure 155], the second holding means 22 descends,
At the same time, the Z mechanism including the wafer chuck 21 is raised to transfer the wafer 2.

In [Procedure 156], AGA preliminary measurement is performed. In this step, precise alignment marks of four shots, for example, the thirteenth, nineteenth, sixty-third, and fifty-seventh shots are read by the off-axis microscope 7. Mainly, the residual error of θ is read, and the rotation is corrected using the θ fine movement stage.

In [Procedure 157], the correction marks 105L and 105R on the reticle 1 are exposed and transferred to a blank portion having no actual element pattern. Whether or not to perform this process for all of the four blank portions 113, 114, 115, and 116 without an actual element pattern depends on whether to check only the baseline length or whether to check the magnification and orthogonality of the stage. And can be set arbitrarily. Also, this process is performed on wafer 1
Whether it is performed for each wafer or once for several wafers can be arbitrarily set in consideration of the stability over time of the magnification and orthogonality of the baseline and the stage and the required accuracy.

In [Procedure 158], shot numbers 13, 19, 35, 38, 41, and 5
AGA is performed on the precision alignment marks of each shot of 7, 63, 69. At this time, whether to detect marks on only one side of the corresponding shot or marks on both sides is determined by
It can be set arbitrarily according to the required accuracy.

In [Procedure 159], AGA is performed on a correction mark formed in a blank portion having no actual element pattern.

In [Step 160], [Step 158] and [Step 15]
[9] The corrected array coordinates at the time of wafer exposure are determined from the alignment detection data obtained by the off-axis microscope and the stage position data obtained at each shot of the AGA.
From the data obtained in [Procedure 159], the baseline length, the magnification component and orthogonality component of the stage, and the magnification component of the projection lens can be calculated. The magnification component of the lens is not the focus of this embodiment. From the data obtained in [Procedure 158], the X-axis and Y-axis components of the shot array in the wafer, the magnification components in the X and Y directions, and the chip rotation direction can be calculated. From the above results, the array coordinates at the time of exposure are determined.

When the correction is performed up to the die rotation, a procedure for rotating and correcting θ for the die rotation is inserted after [Procedure 160], and the step exposure in the next [Procedure 161] is performed in a stepwise manner. do it.

In [Procedure 161], step exposure of the wafer is performed according to the array coordinates calculated in [Procedure 160].

In [Procedure 162], the exposed wafer is transferred from the chuck to the second holding means, and the wafer is collected.

In [Step 163], when the next unexposed wafer is waiting, the process returns to [Step 151], and when there is no waiting wafer, the sequence is completed.

[Modification of Embodiment] Modification 1 In the flowchart of FIG.
With 57, these two steps can be placed anywhere under the following conditions:

Steps 157 and 159 may be placed anywhere between steps 155 and 160.

Step 159 may be arranged several times. For example, another procedure may be inserted between the first time and the second time of the procedure 159.

Modification 2 In the present embodiment, the peripheral portion of the wafer is described as an example of the empty space other than the portion where the actual element pattern is to be printed, but if there is an empty space between shots as well as the peripheral portion, A latent image may be formed at that location (for example, on the margin of a shot portion for a test chip or on a scribe line) and detected.

Modification 3 In the embodiment, the description has been given of the off-axis type alignment system. However, the present invention is applicable to all systems except the alignment system of the divider ion axis and is effective.

Modification 4 As described in the embodiment, the magnification of the projection lens can be measured in the process of executing the present invention. In this case, good magnification accuracy includes a detection error of the microscope and a feed error of the stage. Therefore, it is somewhat impossible to correct the magnification by one detection.

However, factors that change the magnification of the lens are the atmospheric pressure and the environmental temperature, and cannot change rapidly. Therefore, it is possible to improve the accuracy by calculating the lens magnification every time in the procedure 160 in FIG. 4 and averaging the data of the plural times, and it is appropriate to correct the lens magnification using the average value. . It is a known technique that the lens magnification can be corrected by moving a part of the projection lens in the optical axis direction. Alternatively, in the case of an excimer quartz monochromatic lens, it has already been proposed that magnification can be corrected by shifting the exposure wavelength.

[Effects of the Invention] As described above, by forming a latent image in an empty space other than a portion where a real element pattern of a wafer is to be printed, it is possible to eliminate the time lapse which is a disadvantage of the off-axis alignment system without using another medium. Specific means for correcting the change error component can be provided, and the alignment accuracy can be greatly improved.

Further, the present invention makes it possible to detect a magnification error of the projection lens at the same time, and to improve the accuracy of the magnification distortion component by feeding back the detection error to the correction means.

[Brief description of the drawings]

FIG. 1 is a sectional view of an overall system of an exposure apparatus according to one embodiment of the present invention, FIG. 2 is a plan view of a reticle used in the exposure apparatus of the above embodiment, and FIG. FIG. 4 is a plan view of a wafer, a wafer chuck, and a latent image forming portion used in the exposure apparatus. FIG. 4 is a flowchart for explaining the operation of the exposure apparatus of the above embodiment. FIG. 5 is a g-line lens and an excimer lens. FIG. 1: reticle, 2: wafer, 3: illumination system, 4: projection lens, 5: stage, 6: reticle microscope, 7: off-axis microscope, 9: control circuit.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H01L 21/027

Claims (1)

(57) [Claims] 1. An exposure apparatus for projecting and exposing a pattern on an original plate to a photosensitive layer on a substrate to be exposed through a projection optical system, comprising: stage means for moving the substrate to be exposed; In order to form an image of a specific mark as a latent image on a photosensitive layer at a predetermined position on the substrate to be exposed, an illuminating unit that illuminates only the specific mark on the original plate, and the latent image formed on the photosensitive layer as a latent image An exposure apparatus comprising: a detection unit that detects an image of a specific mark; and a control unit that controls movement of the stage unit based on a detection output of the detection unit.