BACKGROUND OF THE INVENTION
The present invention relates to an exposure method and an exposure apparatus that use EUV light (Extreme Ultra Violet light, extreme ultraviolet light). In particular, the present invention relates to an EUV exposure method and an EUV exposure apparatus that can accurately align a mask (reticle) in a short time.
In an exposure apparatus using an EUV light, an ultraviolet laser, an energy beam such as a charged particle beam, a position where a pattern formed on a reticle is projected onto a sensitive substrate (wafer) in order to perform alignment with high accuracy And the position of the wafer alignment sensor must be accurately grasped. For this reason, in an exposure apparatus using ultraviolet light, the mark detection light reflected by the reticle alignment mark on the reticle and the mark detection light incident via the projection optical system are reflected on the fiducial mark on the wafer stage. The image that is reflected by the mark, FM), and returned to the reticle position through the projection optical system is optically enlarged and imaged with a CCD image sensor, etc., and image processing is performed to obtain the relative positional relationship between the two. Yes.
However, in an exposure apparatus that uses EUV light, there is no substance that transmits light in the wavelength region of EUV light, so a multilayer film reflector is used as the optical element. The reflectance of such a multilayer-film reflective mirror is about 70% at the maximum. For this reason, as described above, the amount of light is significantly reduced before the light reflected by the FM of the wafer stage returns to the reticle through the projection optical system. Furthermore, in order to lead the image to an image sensor for image processing, it is necessary to add at least two reflecting mirrors, and the light amount further decreases. Therefore, performing alignment in this manner is not practical because the throughput is greatly reduced.
Therefore, in the EUV exposure apparatus, a method is used in which an aerial image sensor is prepared on a wafer stage and a reticle alignment mark is detected by the sensor (see, for example, Patent Document 1 and Patent Document 2). The aerial image sensor includes, for example, a slit (light transmission part) formed in the reference mark FM of the wafer stage and a photodetector arranged below the slit.
Incidentally, the reticle alignment mark formed on the reticle has a drawing position error of, for example, about ± 0.5 mm with respect to the outer shape of the reticle. When the reticle is set in the exposure apparatus, the reticle is taken out from the reticle transport case, transported to the chuck of the reticle stage by the reticle transport system, and electrostatically attracted onto the chuck. Considering such a position error of the transport system and accuracy of a detection system for detecting the reticle position on the chuck, the reticle alignment mark on the reticle stage is expected to have a position error of ± 0.5 mm or more. This position error includes a position error in the XY direction (on a plane perpendicular to the optical axis) and a rotation error around the Z axis (optical axis).
[Patent Document 1]
JP-A-8-78313 [Patent Document 2]
Japanese Patent Laid-Open No. 11-219900
[Problems to be solved by the invention]
In order to detect the image on which the reticle alignment mark having such a position error is projected by the aerial image sensor of the wafer stage, the detection sensitivity of the aerial image sensor is obtained by relatively scanning the reticle stage and the wafer stage at a low speed. It takes a long time.
At the time of detection, it is necessary to measure the relative distance between the position at which the reticle alignment mark is projected onto the wafer stage and the position of the wafer alignment sensor with a precision of several nanometers or less. For this purpose, it is preferable that the reticle alignment mark and the slit of the aerial image sensor have periodic lines with a sufficiently narrow pitch. However, when the rotation error of the image is large, a problem that the periodic structure cannot be detected occurs.
The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an EUV exposure method and an EUV exposure apparatus that can align a reticle with high accuracy in a short time.
[Means for Solving the Problems]
In order to solve the above problems, the EUV exposure method of the present invention is an exposure method using EUV light having a wavelength of 5 to 20 nm as a light source, and a mask (including a reticle) on which a pattern to be transferred to a sensitive substrate is formed. Is mounted on the mask stage, the pre-alignment mark is detected using pre-alignment light having a wavelength different from that of the exposure light, and the mask is pre-aligned, followed by fine alignment on the mask using the exposure light. And a step of finely aligning the mask by detecting a mark.
Before fine alignment, prealignment is performed using light having a wavelength different from the wavelength of the exposure light, and the reticle stage is moved to XYθ so that the fine alignment mark can be easily detected during fine alignment using the aerial image sensor. Positioning is performed for three degrees of freedom. Thereby, reticle fine alignment can be performed with high accuracy.
In the present invention, if the wavelength of the pre-alignment light is substantially the same as the wavelength used in the mask pattern defect detector, the pre-alignment mark can be detected with high contrast.
The EUV exposure apparatus of the present invention is an exposure apparatus that transfers a pattern formed on a mask (including a reticle) to a sensitive substrate using EUV light having a wavelength of 5 to 20 nm,
A pre-alignment mark and a fine alignment mark are formed on the mask, means for detecting the pre-alignment mark using pre-alignment light having a wavelength different from the wavelength of the EUV light, and the fine UV light using the fine UV light. Means for detecting an alignment mark.
In the present invention, the fine alignment mark detection means is mounted on a sensitive substrate stage on which the sensitive substrate is mounted, and is provided with a pattern plate provided with an opening (slit) that transmits EUV light; A light-receiving element that receives the EUV light, and the opening has the same periodic structure as an image in which the fine alignment mark is transferred to the sensitive substrate.
As described above, since a means for greatly reducing the amount of light is not used, a high resolution pattern can be detected on the sensitive substrate stage.
In the present invention, the exposure apparatus further includes a wafer alignment unit for detecting a position of the sensitive substrate (wafer), and an alignment mark detected by the wafer alignment unit is provided on the pattern plate. The reticle coordinates can be associated with the wafer coordinates.
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an example of an EUV exposure apparatus will be described with reference to the drawings.
FIG. 1 is a diagram schematically showing a configuration of an EUV exposure apparatus (four-projection system) according to an embodiment of the present invention.
The EUV exposure apparatus includes an illumination system IL including a light source. EUV light radiated from the illumination system IL (generally, light having a wavelength of 5 to 20 nm is used, specifically, light having a wavelength of 13.5 nm is used) is reflected by the folding mirror 1 to be reflected by the reticle 2. Is irradiated.
The reflective reticle 2 is sucked and held on the reticle stage 3 with the pattern surface facing downward in the direction of gravity. The reticle stage 3 has a stroke of 100 mm or more in the scanning direction (Y direction), has a minute stroke in a direction orthogonal to the scanning direction (X direction), and has a minute stroke in the optical axis direction (Z direction). . The position in the XY direction is monitored with high accuracy by a laser interferometer (not shown), and the position in the Z direction is monitored by a reticle focus sensor including a reticle focus light transmission system 4 and a reticle focus light reception system 5.
Below reticle stage 3, reticle pre-alignment sensor PA is arranged. The sensor is fixed to the exposure apparatus, and the position relative to the exposure apparatus is guaranteed with an accuracy of about several tens of nanometers. In FIG. 1, only one reticle pre-alignment sensor PA is depicted, but actually, one is arranged on the front side and the back side. The light source of the sensor PA emits ultraviolet rays having a wavelength of 248 nm.
The wavelength of light used for defect inspection of a general reticle pattern is also 248 nm. This wavelength can be said to be a wavelength selected so that the substance forming the multilayer film of the reticle and the substance forming the absorption layer have a sufficient optical contrast with respect to ultraviolet rays.
The EUV light reflected by the reticle 2 enters the lower projection optical column 14 in the drawing. This EUV light includes information on a circuit pattern drawn on the reticle 2. The reticle 2 is formed with a multilayer film (for example, Mo / Si or Mo / Be) that reflects EUV light, and is patterned on the multilayer film with or without an adsorption layer (for example, Ni or Al). .
The EUV light that has entered the optical barrel 14 is reflected by the first mirror 6, then sequentially reflected by the second mirror 7, the third mirror 8, and the fourth mirror 9, and finally perpendicular to the wafer 10. Is incident on. The reduction magnification of the projection optical system including these mirrors is, for example, 1/4 or 1/5. In this figure, there are four mirrors. A. It is effective to increase the number of mirrors to 6 or 8 in order to further increase. An alignment off-axis microscope 15 is disposed in the vicinity of the lens barrel 14.
The wafer 10 is placed on the wafer stage 11. The wafer stage 11 can move in a plane (XY plane) orthogonal to the optical axis, and the movement stroke is, for example, 400 to 500 mm. The wafer stage 11 can move up and down with a minute stroke in the optical axis direction, and the position in the optical axis direction (Z direction position) is monitored by a wafer autofocus sensor comprising a wafer autofocus light transmission system 12 and a wafer autofocus light reception system 13. Has been. The position in the XY direction of the wafer stage 11 is monitored with high accuracy by a laser interferometer (not shown). In the exposure operation, the reticle stage 3 and the wafer stage 11 are synchronously scanned at the same speed ratio as the reduction magnification of the projection optical system, that is, 4: 1 or 5: 1.
A reference mark (pattern plate) FM is formed on the wafer stage 11. An aerial image sensor is disposed on the reference mark FM. The aerial image sensor will be described later.
Next, the configuration of the reticle will be described.
2A and 2B are views for explaining the configuration of the reticle. FIG. 2A is a plan view of the whole, and FIG. 2B is a plan view showing a part thereof enlarged.
The reticle 2 in this example is square, and a square pattern region (oblique hatched portion) CP is formed at the center. Reticle alignment marks AM are formed on the left and right sides of the pattern region CP. Reticle alignment marks AM form a pair on the left and right with the pattern region CP interposed therebetween, and in this example, five pairs are formed.
As shown in FIG. 2B, the reticle alignment mark AM includes a large cross mark AM1 and a small periodic line mark AM2. The periodic line mark AM2 includes a periodic line mark whose lines are arranged in the vertical direction and a periodic line mark arranged in the horizontal direction. The mark portions (white portions) of both marks AM1, AM2 are portions where the reflective multilayer film is exposed by removing the absorption layer, and the surrounding portions (black portion) are portions covered with the absorption layer. .
The cross character AM1 is a mark (reticle pre-alignment mark) detected by the pre-alignment sensor PA. The mark has a line width of several hundred nm to several tens of μm, and can be detected by ultraviolet rays emitted from the pre-alignment sensor PA and having a wavelength of 248 nm.
On the other hand, the periodic line mark AM2 is a mark (reticle fine alignment mark) detected by an aerial image sensor described later. The period of the mark is close to the resolution of the projection optical system, and the line width is, for example, several tens to several hundreds nm. When this mark is projected on the wafer stage, it has the same periodic structure as the periodic line pattern of the slits of the aerial image sensor.
Next, the aerial image sensor formed on the reference mark FM will be described.
3A and 3B are diagrams for explaining the configuration of the reference mark FM. FIG. 3A is a plan view, and FIG. 3B shows an output waveform of the photodetector of the aerial image sensor. In FIG. 3B, the vertical axis indicates the light intensity, and the horizontal axis indicates the scanning position.
The aerial image sensor includes an opening (slit, light transmission part) S1 formed in the reference mark FM, and a photodetector (not shown) arranged below the slit. The slit S1 is composed of five periodic line patterns extending in the vertical direction and five periodic line patterns extending in the horizontal direction. The period of these patterns and the period of the fine alignment mark AM1 of the reticle alignment mark AM described above are the same on the wafer, and the line width is several tens nm to several hundreds nm.
The light that has passed through the slit S1 of the aerial image sensor is detected by a photodetector arranged downstream of the slit. By relatively scanning the reticle stage 3 and the wafer stage 11 and detecting a signal from the photodetector in synchronization with the scanning, the relative positional relationship between the reticle stage 3 and the wafer stage 11 can be known. The peak position in the signal waveform shown in FIG.
As shown in FIG. 3A, the reference mark FM is also provided with a cross mark S2 in which thick lines intersect. The cross mark S2 is a wafer alignment sensor calibration mark.
Next, a reticle alignment method will be described.
First, the reticle 2 is taken out from the reticle case by the reticle transport system, transported to the reticle stage 3, and fixed to the reticle holder by electrostatic adsorption. Next, reticle pre-alignment mark AM1 (see FIG. 2) formed on reticle 2 is detected by reticle pre-alignment sensor PA. Based on the detection result, the reticle stage 3 is moved in the X and Y directions and rotated around the Z axis to position the mark AM1. That is, before the reticle fine alignment, the reticle fine alignment mark AM2 is positioned so as to be detectable by the aerial image sensor, and the rotation error between the aerial image sensor and the image of the reticle fine alignment mark AM2 is reduced. Then, pre-alignment of the reticle 2 is performed. At this time, as described above, the position of the pre-alignment sensor PA is guaranteed with high accuracy for the exposure apparatus, and the reticle pre-alignment mark AM1 detected by the sensor is positioned with high accuracy for the exposure apparatus. it can.
After the pre-alignment is completed, the reticle stage 3 is moved, and EUV light is irradiated onto the reticle fine alignment mark AM2 (see FIG. 2) formed on the reticle 2. Then, an image obtained by projecting the light reflected by the mark AM2 onto the wafer stage 11 is detected by the aerial image sensor of the reference mark FM. At this time, since the pre-alignment of the reticle has been completed, the position of the image of the mark AM2 is a position that can be detected by the aerial image sensor, and the position of the mark AM2 that has passed through the slit S1 (see FIG. 3) of the aerial image sensor. The periodic structure of the image can be identified. Then, the reticle stage 3 and the wafer stage 11 are synchronously scanned to detect a signal from the photo detector, and the signal stage is positioned at a peak.
After the fine alignment, the wafer alignment sensor 15 measures the wafer alignment sensor calibration mark S2 (see FIG. 3) provided on the reference mark FM. Then, the reticle coordinate system and the wafer coordinate system are associated.
Thereby, the alignment of the reticle is finished, and the exposure process is started.
According to this method, ultraviolet light having a wavelength of 248 nm is used as light of the reticle pre-alignment sensor PA. Since ultraviolet rays have a higher resolution by a shorter wavelength than visible light, highly accurate alignment can be performed. Further, as described above, the light used for general reticle pattern defect inspection is also ultraviolet light (DUV light) having a wavelength of 248 nm, so that the alignment mark can be detected with sufficient optical contrast. Although the accuracy and contrast are inferior, visible light can be used as the light source of the pre-alignment sensor PA.
Further, although the intersecting line pattern is used as the wafer alignment sensor calibration mark, a periodic line pattern may be used for higher accuracy. For example, the aerial image sensor S1 may also be used. However, it is necessary that the period of the reticle fine alignment mark has a resolution that can be detected with visible light.
【The invention's effect】
As is clear from the above description, according to the present invention, since pre-alignment is performed using pre-alignment light having a wavelength different from the wavelength of exposure light, and then fine alignment is performed using exposure light, Alignment can be performed with high accuracy.
[Brief description of the drawings]
FIG. 1 is a drawing schematically showing a configuration of an EUV exposure apparatus (four-projection system) according to an embodiment of the present invention.
FIGS. 2A and 2B are diagrams illustrating the configuration of a reticle, in which FIG. 2A is a plan view of the whole, and FIG.
3A and 3B are diagrams illustrating the configuration of a reference mark FM, in which FIG. 3A is a plan view and FIG. 3B shows an output waveform of a photodetector of the aerial image sensor.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Folding mirror 2 Reticle 3 Reticle stage 4 Reticle focus light transmission system 5 Reticle focus light reception system 6 First mirror 7 Second mirror 8 Third mirror 9 Fourth mirror 10 Wafer 11 Wafer stage 12 Wafer autofocus light transmission system 13 Wafer auto Focus light receiving system 14 Projection optical column 15 Off-axis microscope