JP2881062B2 - Substrate alignment method and apparatus - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for aligning a substrate such as a wafer using pulsed light as illumination light in a semiconductor exposure apparatus.

[0002]

2. Description of the Related Art In the field of semiconductor wafer baking equipment,
Due to the miniaturization of circuit patterns and the enlargement of wafers, these days, step-and-repeat type projection exposure apparatuses (hereinafter, referred to as
“Steppers” are becoming mainstream. Various methods have been devised and implemented for positioning of a reticle and a substrate such as a wafer (hereinafter, referred to as “alignment”) in an exposure apparatus represented by a stepper.

Conventionally, in the case of the TTL system, an illumination light source for alignment (for example, a laser beam) is irradiated from above the reticle, and the reflected light from the reticle mark and the wafer mark is detected by a photoelectric detecting section. The relative position shift between the wafer and the reticle has been detected by obtaining the center of the mark pattern from the video signal.

[0004]

However, according to the above-mentioned conventional method, when the illumination light for alignment is a pulse light such as a pulse laser, speckles are generated in the above-mentioned video signal and accurate measurement is performed. Becomes impossible. The details are described below.

In the TTL system described above, generally, illumination light for alignment is obtained from the same light source as the exposure light source.
A case will be described in which an illuminating light for alignment is obtained from an excimer laser illuminating system in this TTL type alignment in an excimer laser stepper using an excimer laser as an exposure light source.

[0006] If the laser beam has high spatial coherence and is irradiated onto a wafer or the like as it is, speckles and interference fringes will occur. For this purpose, the phase of the speckle / interference fringe is changed for each pulse by vibrating the beam or by a rotating diffuser, and the pulse is irradiated a plurality of times, thereby eliminating the influence of the speckle / interference fringe by an integration effect. . In the case of wafer exposure, several tens to several hundreds of pulses are required to remove interference fringes and speckles.

However, in the case of alignment, the detection timing of the reflected light by the photoelectric detection unit and the time for taking in the reflected light pose a problem. For example, considering the case where an NTSC-compliant CCD camera is used for the photoelectric detection unit, the exposure (accumulation) time for one screen is 1/60 second in the case of field accumulation.
In the case of frame accumulation, it is 1/30 second. Now, assuming that the pulse rate of the excimer laser is 200 Hz, the number of pulses irradiated during this accumulation time is approximately 3 pulses in the case of field accumulation, and 6 to 7 pulses in the case of frame accumulation.
In this case, interference fringes and speckles cannot be sufficiently removed. In addition to this, the above-mentioned CCD camera has low ultraviolet sensitivity, and the reflected light from the wafer is largely absorbed by the resist, so that the amount of light received on the light receiving side of the alignment light, which is the reflected light, is too small or too small. It becomes a problem.

FIG. 5 shows an NTSC-compliant frame storage CCD.
9 shows a time chart of conventional detection when a camera is used. ST is a measurement start signal, and capture of a measurement image is started by the next VD signal. VD is a vertical synchronizing signal, and LS is a laser output pulse which is irradiated so as to have a uniform light amount in each field. IM indicates the contents of the memory that captures the video signal output from the CCD camera through the control unit. As can be seen from this figure, in the conventional method, a single image is illuminated with only a few pulses (6 pulses in this time chart) of a laser output pulse. In this case, the high spatial coherency of the laser light is obtained. Therefore, it is impossible to obtain an image without speckles and interference fringes.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the related art. In positioning a substrate using pulsed light as illumination light, an image pickup means for generating a positioning video signal is reduced to 1/100. By adopting a long-time exposure that operates with an exposure time of 30 seconds or more, without causing speckle in the video signal, a clear positioning video signal can be obtained by adjusting the amount of light received by the imaging means. It is another object of the present invention to provide a method and an apparatus for aligning a substrate which can detect a positional shift of the substrate with high accuracy and perform high-accuracy alignment.

[0010]

In order to achieve the above object, a method of aligning a substrate according to the present invention is directed to a method of aligning a substrate with an image of an imaging means for receiving reflected light from an alignment mark of the substrate irradiated by pulsed light. Detecting the position shift of the substrate by a signal, and moving the substrate based on the detected position shift, wherein the exposure time of the imaging means eliminates speckle of the video signal. In the method of aligning the substrate is set to be longer than necessary, if the amount of light received by the imaging means during the exposure time does not reach a predetermined value, the exposure time is set longer, Conversely, when the light amount exceeds a predetermined value, the light emission amount of the pulse light source is reduced.

Further, the substrate positioning apparatus of the present invention comprises:
Illuminating means having a light source for generating pulsed light for irradiating the alignment mark on the substrate; imaging means for receiving a reflected light from the alignment mark to generate a video signal; and A signal processing circuit for detecting a displacement, a driving unit for moving the substrate according to an output of the signal processing circuit, a light amount detector for monitoring a light amount received by the imaging unit, and an output of the light amount detector. A light control mechanism that adjusts the light emission amount of the light source based on the exposure time of the imaging unit is set to be longer than necessary to eliminate speckles of the video signal. And

[0012]

According to the method of the present invention, the exposure time of the image pickup means is adjusted in order to receive the reflected light of a sufficient number of pulses to prevent the occurrence of speckle in the video signal due to the reflected light from the alignment mark. By setting the length to a necessary length or more, the occurrence of the speckle is prevented, and when the amount of light received by the imaging unit is insufficient and a clear video signal cannot be obtained, the exposure time is further extended. In this way, a necessary light amount is ensured, and conversely, if the light amount received by the imaging means is excessive, a clear video signal is obtained by reducing the light emission amount of the pulse light source.

A light amount detector for monitoring the amount of reflected light,
If a light control mechanism for controlling the light emission amount of the pulse light source based on the output of the light amount detector is provided, it is easy to adjust the exposure time of the imaging means and the light emission amount of the light source.

[0014]

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

FIG. 1 is a block diagram showing one embodiment. Illumination light emitted from an illumination system 1 which is illumination means using a pulse laser as a light source passes through a reticle 2 and a projection lens system 3 and a wafer stage 4. The wafer 5 as the upper substrate is exposed. The illumination system 1 has a function of temporally changing the wavefront of the illumination light to suppress speckle. The wafer stage 4 has two axes (X) orthogonal to each other in a plane perpendicular to the paper surface by a stage driving system 6 as a driving means.
Reciprocating in directions along each of the axis and the Y axis), and rotated about one axis (Z axis) perpendicular to the plane. Reticle stage 2 holding reticle 2
a is also reciprocally movable in the directions along the X axis and the Y axis by the stage drive system 6, and is rotatable about the Z axis. That is, the stage drive system 6 adjusts the relative positions of the reticle stage 2a and the wafer stage 4 by a main controller described later.

An alignment detection system 7 for detecting a positional relationship between alignment marks, which are alignment marks provided on the reticle 2 and the wafer 3, respectively, includes an illumination system 1
Is used as alignment illumination light, and an alignment mark of the reticle 2 and an alignment mark of the wafer 5 are detected from the reflected light of the mirror 8 by a CCD camera 9 as an imaging means. In an actual reduction projection exposure apparatus, a detection system similar to the alignment detection system 7 is provided at a symmetric position with respect to the optical axis of the projection lens system 3, but is omitted in FIG.

CCD camera 9 in alignment detection system 7
Is input to the signal processing circuit 11 via the control unit 10. Based on the video signal, the signal processing circuit 11 outputs a CC of the superimposed image of the alignment mark of the wafer 5 and the alignment mark of the reticle 2.
An image formation state on the light receiving surface of the D camera 9 is detected, and a signal representing a shift amount between the wafer 5 and the reticle 2 according to the image formation state is output to the main control device 12. The stage drive system 6 is controlled by an output signal of the main controller 12, and the reticle stage 2a and the wafer stage 4 are relatively moved to align the reticle 2 with the wafer 5. The synchronization signal generator 13 generates a synchronization signal for the detection system and a synchronization signal for the laser.
1. The synchronization signal is given to the main controller 12, the laser controller 14, and the stage drive system 6. Note that the synchronization signal generator 13 is used for the signal processing circuit 11 or the main controller 1.
2 may be included.

The illumination system 1 includes a dimming mechanism 1 for alignment.
5, a driving circuit 16 for driving the light control mechanism 15.
Is controlled by a control signal from the main controller 12
Drive.

The light amount detector 17 detects the amount of light incident on the CCD camera 9, and the main controller 12 controls the laser controller 14 or the dimming device so that the light amount becomes appropriate for the detection system based on a signal from the light amount detector 17. The mechanism 15 is controlled. In this embodiment, the light amount detector 17 is provided in the alignment detection system 7, but may be provided in the illumination system 1.

FIG. 2 is a time chart of this embodiment.
The CD camera 9 is operated in a long exposure mode in which the exposure time is 1/30 second or more. The output of the laser pulse and the accumulation of the image, which is a video signal, are started from the VD signal following the measurement start signal ST. When the light amount and the number of pulses reach an appropriate value, the laser pulse output and the accumulation of the image are terminated by the laser output stop signal SP. The image obtained in this way is taken into the measurement image memory IM with the next VD signal. This makes it possible to obtain an image free from speckles and interference fringes illuminated by several tens to several hundreds of laser output pulses.

Next, control of the number of pulses will be described. FIG. 3 shows an example of a sequence flow in the first measurement shot. Normally, the wafer exposure system has the number of pulses required for speckle removal as a parameter in advance,
Similarly, the measurement system has the number of pulses required for speckle removal as a parameter in advance.

Although the number of pulses is the same as that of the wafer exposure system,
Further, a value calculated based on the number of pulses of the wafer exposure system may be used. Further, the number of pulses required for speckle removal at the time of alignment may be obtained in advance on the apparatus and may be input.

In a first step S1, laser pulse output and image storage are started by a measurement start signal. At this time, monitoring of the light amount by the light amount detector 17 is also started at the same time. If the amount of light exceeds a predetermined value on the monitor before the number of pulses required for speckle removal is reached (second step S2), the light intensity is measured by the dimming mechanism 15 with the laser power equal to or more than the number of pulses required for speckle removal. Is adjusted so as to be suitable for the above, and after the accumulated image is discharged, the process is repeated from the first step again. If the predetermined light amount has not been reached even when the number of pulses required for speckle removal has been reached, exposure is continued until the light amount reaches the predetermined light amount (third step S3). Thereafter, the laser pulse output is stopped, and the obtained image is stored in the memory (fourth step S4).
Perform measurement. In the second and subsequent shots, dimming and measurement are performed using the number of laser pulses and the laser power obtained in the first shot to achieve high throughput.

In the above-described embodiment, the light amount of the alignment illumination light is monitored using the light amount detector 17 in order to obtain the light amount suitable for measurement by the number of pulses required for speckle removal. Monitoring of the light amount by the light amount detector 17 is not always necessary.

FIG. 4 shows an example of a sequence flow when the light quantity detector 17 is not provided. First step S1
Up to 1 is the same as that of FIG.
Since the light quantity to the camera 9 cannot be monitored, the laser pulse output is stopped when the number of pulses reaches the specified number in the second step S12, and an image is captured. The prescribed number of pulses in this case is always equal to or greater than the number of pulses required for speckle removal, and is assumed to be stored in advance as a parameter. If there is no problem with the amount of light in the obtained image, measurement is performed using this image. If the light quantity is insufficient, the number of pulses is increased and the process is performed again from the first step S11. The number of pulses to be increased at this time is determined by calculating a shortage from the light amount of the image obtained first. If the amount of light is too large (for example, if the obtained image is saturated), the light control mechanism 15
The laser power is adjusted to reduce the amount of light incident on the CCD camera 15, and the operation is performed again from the first step S11. After the second measurement shot, dimming and light control are performed based on the number of laser pulses and laser power obtained in the first measurement shot.
Measure and measure high throughput.

Further, if the bleaching of the resist by the measurement light becomes a problem during the wafer measurement, a laser pulse is irradiated until the effect of the bleaching does not appear in the measurement, and then the above measurement is performed.

The first measurement shot does not necessarily need to be a measurement shot, and may be performed using, for example, an alignment mark of a shot other than the measurement shot in global alignment. In this case, the above-mentioned second measurement shot is the first measurement shot of the actual alignment.

The above is the case where the alignment mark on the wafer is measured. However, the same can be applied to the case where the alignment mark on the stage reference mark 4a is measured. Furthermore, taking advantage of the fact that the same image can be obtained stably for the stage reference mark image, the illuminance unevenness of the obtained image or the distribution of the power spectrum in the spatial frequency domain is checked for the presence or absence of speckle, and the pulse By repeating this while changing the number, the number of pulses required for speckle removal can be checked.

In the above embodiment, the light amount detector 17 is provided in the alignment detection system 7, but may be provided in the exposure system as described above. In this case, the ratio between the light amount of the light amount detection unit in the exposure system and the light amount reaching the CCD camera 9 is measured in advance, and the light amount is determined in consideration of this ratio.

In this embodiment, the imaging device is a CCD camera conforming to NTSC. However, a camera conforming to other standards such as PAL may be used, and a camera operating at an independent timing instead of a camera conforming to these standards may be used. Absent. Further, a scanning position detecting element such as a one-dimensional line sensor may be used.

[0031]

Since the present invention is configured as described above, the following effects can be obtained.

By using a clear video signal without speckles, the displacement of the substrate can be detected with high precision, and the positioning of the substrate can be performed with high precision.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an embodiment of the present invention.

FIG. 2 is a time chart of the present embodiment.

FIG. 3 is a diagram showing an example of a measurement sequence flow for setting an exposure time and a light amount.

FIG. 4 is a diagram showing another example of the measurement sequence flow.

FIG. 5 is a time chart of a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Illumination system 2 Reticle 3 Projection lens system 4 Wafer stage 5 Wafer 6 Stage drive system 7 Alignment detection system 8 Mirror 9 CCD camera 11 Signal processing circuit 15 Light control mechanism 17 Light intensity detector

Claims (2)

(57) [Claims] A step of detecting a position shift of the substrate based on a video signal of an imaging means for receiving reflected light from an alignment mark of the substrate irradiated by the pulse light; and detecting the position shift of the substrate based on the detected position shift. Moving the substrate, wherein the exposure time of the imaging means is set to a length or more necessary to eliminate speckles of the video signal, wherein the imaging means If the amount of light received during the exposure time does not reach a predetermined value, the exposure time is set longer, and conversely, if the light amount exceeds a predetermined value, the light emission amount of the pulse light source is increased. A method of aligning a substrate, characterized in that the position is reduced. 2. An illumination unit having a light source for generating pulsed light for irradiating an alignment mark on a substrate; an imaging unit for receiving a reflected light from the alignment mark to generate a video signal; A signal processing circuit that detects a displacement of the substrate based on the signal; a driving unit that moves the substrate according to an output of the signal processing circuit; a light amount detector that monitors a light amount received by the imaging unit; A light control mechanism for adjusting the light emission amount of the light source based on the output of the detector, wherein the exposure time of the image pickup means is set to be equal to or longer than a length necessary to eliminate speckle of the video signal. A substrate alignment device, comprising: