JPH0547631A - Method and apparatus for semiconductor exposure - Google Patents

Abstract

PURPOSE:To conduct a relative alignment of a mask and a wafer with high accuracy at low cost in an X-ray reduction exposure method by detecting a wafer mark on the back of the wafer by a mark detector, by detecting a reference mark on the mask by an X-ray mark detector and by correcting preliminarily the misalignment of the optical axes of the two detectors. CONSTITUTION:A slit opening 26 on a photo detector 25 is located near the position where the image of a mask reference mark 23 is formed. Scanning an X-Y stage 7, the position where the center of the image of the mask reference mark and the center of the slit opening 26 coincide with each other is searched. When this position is found, the coordinates of the X-Y stage is stored. Nextly, the position of the stage where the center of a wafer reference mark 30 coincides with the central axis of a wafer back surface detector 11 is searched and when this position is found, the coordinates of the X-Y stage is stored. The shift between the two coordinates is regarded as the axis offset value of the detector 11. The position of a wafer mark 10 is adjusted for the offset value and the X-Y stage 7 is moved by the predetermined amount from the arrangement position of a mask pattern and then exposure is conducted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a relative alignment method between a mask and a wafer in an X-ray reduction exposure technique used in an exposure process for manufacturing a semiconductor integrated circuit and an apparatus therefor.

[0002]

2. Description of the Related Art In a conventional semiconductor manufacturing process, an exposure apparatus such as a stepper using ultraviolet light is used to print a circuit pattern of a mask or a reticle on a wafer.
In recent years, the minimum pattern width of the circuit has been required to be smaller than 0.2 μm with the high integration of the semiconductor circuit, and the stepper using the ultraviolet light cannot be resolved due to the limitation of the lens system. Is becoming.

For this reason, an X-ray exposure method has been proposed in which X-rays having a wavelength shorter than that of ultraviolet light are used as an exposure light source. The X-ray proximity exposure method, which has been researched for a long time using a 1 × mask, is an exposure method that projects a mask pattern on a wafer like a shadow using a very thin membrane-like mask. It is difficult to manufacture a membrane mask free from dimensional distortion, and it has not yet been put to practical use.

On the other hand, recently, as an X-ray reduction exposure method in which a magnifying mask can be used, an exposure method using a reflection optical system as shown in FIG. 1 has been proposed (for example, JP-A-63-63).
No. 311315). That is, the magnifying reflection mask 1 is irradiated with soft X-rays having a wavelength of about 130Å emitted from a synchrotron radiation ring or the like, and the soft X-rays reflected from the reflection pattern portion on the magnifying reflection mask 1 are reflected on three aspherical surfaces. Mirror 2,
An image is formed on the wafer 6 via the reduction optical system composed of 3 and 4 and the bending mirror 5. In this case, the field that can be imaged without aberration in the reflection reduction optical system has a ring shape with a radius r = 12 mm and a width d = 1 mm as shown in FIG.
The entire surface of the wafer cannot be exposed at one time. for that reason,
The wafer 6 is placed on the XY stage 7 as shown in FIG.
The speed ratio between the magnifying reflection mask 1 and the XY stage 7 is accurately maintained at a mask magnification ratio, for example, 5: 1, and the magnifying reflection mask 1 and the XY stage 7 are mechanically simultaneously scanned to expose one field. .. After the exposure for one field, the XY stage 7 is two-dimensionally moved as in the conventional stepper, and the exposure for one field is performed again. The entire surface of the wafer is exposed by repeating such scanning exposure and stage movement. The condenser mirror 8 in FIG. 3 is for effectively collecting the soft X-rays emitted from the X-ray source and illuminating the reflection mask 1. Further, since the soft X-ray having a wavelength of about 130 Å is absorbed in the atmosphere, the optical system portion is installed in the vacuum container 9.

In the semiconductor manufacturing process, a so-called mask-wafer alignment operation is required to accurately superimpose the circuit pattern to be printed on the circuit pattern formed in the previous printing process. The alignment method of the X-ray reduction exposure method is disclosed in JP-A-63-312638. In this example, a technique similar to that of a conventional stepper is shown in which alignment marks on a mask and alignment marks on a wafer are aligned while being simultaneously observed through the reflection reduction optical system using visible light as illumination light. .. However, unlike the lens optical system of the stepper, the reflection reduction optical system has an upper limit of the numerical aperture (NA) of at most about 0.1 due to the restriction of "vignetting" of light rays by the reflecting mirror.
It has a drawback that sufficient resolution is not obtained for visible light and high alignment accuracy cannot be expected. In this case, if the illumination light for alignment is soft X-rays similar to the exposure light, a high resolution can be obtained, so good alignment accuracy can be expected, but a resist is applied on the wafer at the time of exposure. Since the soft X-ray illuminating light for the light is almost absorbed by the resist layer, it does not reach the alignment mark on the wafer surface, and the X-ray reflection signal of the intensity level necessary for the alignment work cannot be obtained.

On the other hand, as an alignment method which is not restricted by the soft X-ray illumination light by the resist layer, there is a method of detecting an alignment mark provided on the back surface of the wafer.
No. 115164. That is, as shown in FIG. 4, the wafer mark 10 is provided on the back surface of the wafer, and this is detected by the back surface mark detector 11 installed on the bottom surface of the wafer. In this case, the reference mark 12 on the mask is imaged on the same surface as the front surface of the wafer through the exposure optical system 13, and the mask mark image is also detected by the back surface mark detector 11, so that the exposure optical system 13 can be operated. It is possible to perform relative alignment between the mask and the wafer through them. However, when this back surface alignment method is applied to the X-ray reduction exposure method, the back surface mark detector 11 is a standard optical detection method using visible light, and a reference on a mask formed through an exposure optical system is used. The X-ray image of the mark cannot be detected. Although the above problem can be solved by using the X-ray detection method for the back surface mark detector, it is obvious that the detector becomes extremely complicated and expensive.

As described above, the alignment method in the conventional X-ray reduction exposure apparatus cannot be expected to have very high accuracy or requires an extremely expensive apparatus.

[0008]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a highly accurate and inexpensive relative mask / wafer alignment method that can be applied to the X-ray reduction exposure method.

[0009]

SUMMARY OF THE INVENTION The above object is to detect a wafer mark on the back surface of a wafer by a back surface alignment method by a conventional optical back surface mark detector using visible light while forming it through an exposure optical system. The position of the X-ray image of the reference mark on the mask to be detected is detected by a dedicated X-ray mark detector,
This can be achieved by calibrating the optical axis shift amount between both detectors in advance.

[0010]

The X-ray detector for detecting the mask reference mark is provided on the XY stage on which the wafer is loaded, while the reference back surface mark for calibration is provided on the XY stage, and the XY stage position where the mask reference mark position is detected. The relative positional relationship of the back surface mark detector with respect to the exposure optical system is known from the difference between the XY stage positions where the reference back surface mark position is detected, and based on the result, the back surface wafer mark detected by the back surface mark detector thereafter. The distance to move the wafer from the position to the desired exposure position is calculated, and exposure is performed while actually positioning.

[0011]

Embodiments of the present invention will be described in detail below with reference to FIG. A mask alignment mark 21 is provided at the end of the magnifying reflection mask 1, and the mask position is adjusted so that this mark coincides with the central axis of a mask alignment detector 22 fixed to the apparatus main body. Attach to the body. Further, a rectangular mask reference mark 23 having a width of 5 μm and a length of 150 μm is provided at the end of the mask 1. These marks and other circuit pattern portions are made of a reflective multilayer film and reflect the soft X-rays 24 emitted from the synchrotron radiation source. Mask reference mark 2
The soft X-rays reflected by 3 are aspherical reflecting mirrors 2, 3 and 4
It is reduced to ⅕ by the reflection type reduction projection optical system consisting of and is imaged on the semiconductor type X-ray light receiving element 25 via the bending mirror 5. The surface of the light receiving element 25 has a width of 1 μm and a length of 2
It is covered with a light-shielding film 27 having a slit opening 26 of 0 μm, and when the projected image of the mask reference mark 23 exactly coincides with the slit opening 26, the highest level light receiving signal is generated. The light receiving element 25 is fixed on the upper surface of the end portion of the XY stage 7 for moving the wafer 6.
The XY stage 7 is moved to search for a position where the received light signal level reaches a peak as described above. The coordinates of the XY stage are always detected by the laser interferometer 28, and when the stage position where the received light signal level reaches a peak is found, the coordinates are stored in a control circuit (not shown).

The wafer 6 is adsorbed on the XY stage 7. A wafer mark 10 for backside alignment is provided on the backside of the wafer 6, and a hole 29 is formed in the wafer loading center of the XY table 7 so that the backside mark detector 11 provided under the XY stage can detect this mark. I am open. A hole is also provided in the table portion directly below the light receiving element 25. As shown in FIG.
A mark 30 having the same shape as the wafer mark 10 for rear surface alignment on the lower surface of the wafer is also formed on the lower surface of the above, immediately below the slit opening 26, and the position can be detected by the rear surface mark detector 11. Here, since the thickness h of the light receiving element 25 is the same as the thickness of the wafer and is mounted in the same plane, the back surface mark detector 11 has the same focal plane as the mark 3
It is possible to detect 0 and the wafer mark 10. Similarly, the circuit pattern on the mask can be exposed on the wafer at the focal position where the mask reference mark 23 is imaged on the upper surface of the light receiving element 25.

Now, a procedure for calibrating the optical axis of the exposure optical system including the mask and the optical axis of the back surface mark detector in the above apparatus calibration will be described below.

First, the XY stage 7 is operated so that the slit opening 26 on the light receiving element 25 is positioned in the vicinity of the image formation position of the mask reference mark 23, and then the XY stage 7 is slightly vibrated and scanned to make the mask. A position where the center position of the reference mark image coincides with the center of the slit opening 26 and the received light signal has a peak is searched for. Further, the XY stage coordinates (Mx, My) at that time are stored and stored in the control circuit.
In the stage position, the back surface mark detector 11 is arranged so that the wafer reference mark 30 on the lower surface of the light receiving element 25 is positioned directly above the back surface mark detector 11, but it is slightly displaced due to temperature change or the like. In some cases, the XY stage 7 is slightly vibrated and scanned again to find a stage position where the center position of the wafer reference mark 30 coincides with the center axis of the wafer backside mark detector 11, and the stage coordinates (Wx, Wy) at that time are searched. ) Is stored in the control circuit as described above. Since the stage coordinates (Mx, My) at the time of calibrating the mask reference mark position and the stage coordinates (Wx, Wy) at the time of calibrating the back surface mark position do not completely match,
The shift amount between the two coordinates is used as the axis offset value of the wafer back surface detector 11.

After the offset value of the detector 11 is obtained as described above, after the position of the wafer mark 10 on the back surface of the wafer is detected by the back surface mark detector 11, the position is corrected by the offset amount. Further, by displacing the XY stage 7 by an amount determined from the arrangement position of the mask pattern and performing the exposure, the circuit pattern on the mask can be exposed at a desired position on the wafer. For example, when the offset value is 0 and the stage displacement amount is 0, the pattern of the mask reference mark 23 is exposed immediately above the wafer mark 10.

In the above description, only the alignment of one axis is described, but it is obvious that the alignment of three axes, that is, X, Y, and θ is actually required as in the conventional stepper. For this purpose, as shown in FIG. 7, three-axis rear surface mark detectors 11-1, 11-2, 11-3 are provided, and correspondingly, mask reference marks, slit openings, semiconductor light receiving elements, and Three sets of wafer reference marks may be provided on the back surface of the semiconductor light receiving element. In this case, when the offset in the θ direction is generated, the mask is rotated and mounted on the apparatus, specifically, the detection axis of the mask alignment detector 22 needs to be corrected, as in the conventional stepper. Is.

A second embodiment will be described with reference to FIG. In this example, a simple slit plate 31 is provided on the stage instead of the semiconductor light receiving element 25. Also in this case, the thickness of the slit plate 31 is the same as that of the wafer, and the upper surface has the above-mentioned thickness of 2
A light shielding film 33 having a slit opening 32 similar to that of 6 is provided. There is a hole 34 just below the slit opening 32 of the slit plate 31, and when the X-ray image of the mask reference mark 23 is located at the center of the slit opening 32 as in the case of the first embodiment. The maximum amount of X-rays passes through the opening and is received by the X-ray receiver 36 fixed on the stationary base 35 of the XY stage 7. Further, as in the first embodiment, the back surface mark detector 11 is provided below the XY stage to detect the wafer reference mark 30 provided on the lower surface of the slit plate 31, but in the case of this embodiment, X Since the line light receiver 36 and the back surface mark detector 11 mechanically interfere with each other,
The wafer reference mark 30 is provided at a distance of about 1 cm from directly below the slit opening 32.

The calibration method in this embodiment is exactly the same as that in the first embodiment, but a high-sensitivity X-ray receiver that is not restricted by the thickness can be used, so that a high alignment accuracy is obtained. Can be expected. Further, since the X-ray receiver 36 is fixed on the base, there is an advantage that the signal cable that easily picks up electric noise does not have to move together with the XY stage. However, in the case of this example, unlike the case of the first embodiment, since the backside wafer mark is detected at a position about 1 cm away from the center of the field of the exposure optical system, the influence of so-called Abbe error is caused. It becomes easy to receive.
In order to reduce the above-mentioned offset of about 1 cm, which causes the Abbe error, a method of bending the X-rays passing through the slit aperture 32 once with a multilayer film reflecting mirror or the like and then guiding the X-rays to the X-ray receiver may be considered. ..

In the above two embodiments, the X-ray receiver is placed on the stage side to see the relative positional relationship between the mask reference mark on the mask side and the mask calibration mark on the stage. It is obvious that X-rays may be irradiated from the mask calibration mark) and the X-ray receiver may be placed on the mask side.

Further, as a mask calibration mark on the stage, a multilayer film reflection pattern may be provided in place of the transmission type slit opening to detect X-ray reflected light.

In the above description, the shape of the mask reference mark and the mask calibration mark is described as a single pattern, but this may be a plurality of diffraction grating patterns, dot patterns, Fresnel patterns, or the like. That is clear.

[0022]

As described above, according to the present invention, since the mask position can be calibrated using the X-ray illumination light and the exposure optical system can be used, extremely high accuracy can be expected, and the X-ray dedicated to the mask calibration can be expected. Since it can be used as a light receiver, a simple and inexpensive device configuration can be realized.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an X-ray reduction exposure method.

FIG. 2 is a diagram showing a field shape in an X-ray reduction exposure method.

FIG. 3 is a diagram showing a configuration example of an X-ray reduction exposure apparatus.

FIG. 4 is a diagram illustrating a back surface alignment method.

FIG. 5 is a diagram showing a first embodiment of the present invention.

FIG. 6 is a diagram showing an X-ray receiving unit in the first embodiment.

FIG. 7 is a diagram showing an example of a detection configuration of XYθ axes.

FIG. 8 is a diagram showing a second embodiment of the present invention.

[Explanation of symbols]

1 ... Enlarged reflection mask, 6 ... Wafer, 7 ... XY stage,
10 ... Wafer mark, 11 ... Back surface mark detector, 12 ...
Mask reference mark, 25 ... Semiconductor X-ray light receiving element, 26 ...
Slit opening, 30 ... Wafer reference mark, 36 ... X-ray receiver.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Eiichi Seya 1-280 Higashi Koikeku, Kokubunji, Tokyo Inside Central Research Laboratory, Hitachi, Ltd. (72) Inventor Soichi Katagiri 1-280 Higashi Koikeku, Kokubunji, Tokyo Hitachi Central Research Laboratory (72) Inventor Kozo Mochichi 1-280, Higashi Koikekubo, Kokubunji, Tokyo Inside Hitachi Central Research Laboratory

Claims (6)

[Claims] 1. An exposure method for projecting and exposing a figure on a mask onto a sample using a projection optical system, a step of positioning a mask at a predetermined position of the projection optical system, and loading the sample on a stage. Moving the image projection plane of the projection optical system and detecting the stage position; and, via the projection optical system, the reference mark on the mask and the mask calibration mark provided on the moving stage. Position calibration step of detecting that the relative relative positional relationship is in a predetermined positional relationship and storing the stage position at that time, a wafer reference mark provided on the stage, and a back surface mark detection provided on the lower surface of the stage. Wafer position calibration step of detecting that the relative positional relationship with the device is in a predetermined positional relationship and storing the stage position at that time; Based on the difference between the two stage positions when calibrating the wafer position, the step of knowing the spatial positional relationship among the mask, the projection optical system, and the wafer position detector, and the wafer mark provided on the backside of the wafer are detected as the backside mark. And a pattern on the mask is projected and exposed at a desired wafer surface position by correcting the stage movement amount based on the spatial positional relationship after the detection by a device. 2. An exposure apparatus for projecting and exposing a figure on a mask onto a sample by using a projection optical system, means for positioning the mask at a predetermined position of the projection optical system, and loading the sample on a stage. Means for moving the image projection plane of the projection optical system and detecting the stage position, and optical of the reference mark on the mask and the calibration mark for the mask provided on the moving stage via the projection optical system. Position calibration means for detecting that the relative relative positional relationship is in a predetermined positional relationship and storing the stage position at that time, a wafer reference mark provided on the stage, and a back surface mark detection provided on the lower surface of the stage. Wafer position calibrating means for detecting that the relative positional relationship with the device is in a predetermined positional relationship and storing the stage position at that time; A means for knowing the mutual spatial positional relationship between the mask, the projection optical system, and the wafer position detector based on the difference between the two stage positions during the wafer position calibration, and the wafer mark provided on the back surface of the wafer for detecting the back surface mark. And a means for projecting and exposing a figure on the mask at a desired wafer surface position by correcting the stage movement amount based on the spatial positional relationship after the exposure is detected by a device. 3. The exposure method according to claim 1, wherein the light used for the exposure is a soft X-ray having a wavelength of 30Å to 150Å. 4. The light used for the exposure is a soft X-ray having a wavelength of 30Å to 150Å, and the detection light used in the mask position calibration step is the same soft X-ray as the exposure light. The exposure method according to claim 1, characterized in that 5. A method of detecting a relative positional relationship between a reference mark on a mask and a calibration mark for a mask on a stage in the mask position calibrating step is a method of detecting a reference mark on a mask on a calibration mark for a mask by a projection optical system. 4. The exposure method according to claim 3, wherein an image is formed on the image, and the degree of overlap between the mask reference mark and the image is based on a signal detected by a light receiving element provided on the lower surface of the mask calibration mark. 6. The light receiving element is a semiconductor sensor having the same thickness as that of a wafer, and an opening mask calibration mark is formed on the upper surface of the semiconductor sensor, and a wafer reference mark is formed on the lower surface of the semiconductor calibration mark. The exposure method according to claim 4, wherein the exposure method is provided in the.