JPH05190421A - Alignment method of substrate - Google Patents

Abstract

PURPOSE:To eliminate the speckle of a video signal for alignment use and to eliminate an irregularity in the intensity of irradiation in an alignment operation which uses a beam of pulsed light as a beam of irradiation light. CONSTITUTION:A reticle 2 and an alignment mark on a wafer 5 are irradiated with a beam of pulsed laser light from an irradiation system 1; beams of reflected light are made incident on a CCD camera 9 by using a mirror 8. An electronic shutter at the CCD camera 9 is controlled in such a way that it is opened only when the beam of pulsed laser light is emitted. Video signals obtained by using the CCD camera 9 are stored in a memory circuit 11a at a signal processing circuit 11; the image of the alignment mark is obtained by the prescribed number of video signals stored in the memory circuit 11; and the relative dislocation between the wafer 5 and the reticle 2 is detected by using the images. A reticle stage 2a and a wafer stage 4 are moved by means of a main control device 12 and a drive stage system 6; the dislocation is eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a substrate aligning method 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 an apparatus for printing a substrate such as a semiconductor wafer, a step-and-repeat type projection exposure apparatus (hereinafter referred to as a "stepper") has become mainstream due to the miniaturization of circuit patterns and the enlargement of wafers. Is becoming. Various methods have been devised and implemented for aligning a reticle and a substrate such as a wafer (hereinafter referred to as “alignment”) in an exposure apparatus typified by this stepper.

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

[0004]

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

In the above-mentioned TTL method, generally, the alignment illumination light is obtained from the same light source as the exposure light. In an excimer laser stepper using an excimer laser as an exposure light source, a case where alignment illumination light is obtained from an excimer laser illumination system by this TTL alignment will be described.

Laser light has a high spatial coherence, and if it is applied to a wafer or the like as it is, specifications and interference fringes will occur. For this reason, the beam is oscillated or the phase of the speckle / interference fringe is changed for each pulse by the rotating diffusion plate, and the effect of the speckle / interference fringe is eliminated by the integration effect by irradiating this pulse multiple times. ..
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, there arise problems of the detection timing and the fetching time of the photoelectric detection portion of the reflected light. For example, considering the case where an image pickup means such as a CCD camera conforming to NTSC is used for the photoelectric detection unit, the exposure (accumulation) time on one screen is
1/60 second, 1/30 second in the case of frame accumulation.
Now, assuming that the pulse rate of the excimer laser is 2000 Hz, the number of pulses irradiated during this accumulation time is about 3 in the case of field accumulation and 6 to 6 in case of frame accumulation.
The number of pulses is 7 and it is impossible to remove interference fringes and speckles sufficiently.

FIG. 5 is an NTSC compliant frame storage CCD.
It is a time chart of conventional detection when a camera is used. ST is a measurement start signal, and the capturing of the video signal is started from the next VD signal. VD is a vertical synchronizing signal, and LS is a laser output pulse that is emitted so that the light amount is uniform in each field. IM is C
It shows the contents of the memory that captures the video signal output from the CD camera through the control unit. As can be seen from this figure, in the conventional method, only one pulse (six pulses in this time chart) of the laser output pulse is used to obtain one image, which means that the high spatial coherency of the laser beam is obtained. It is not possible to obtain an image without speckles or interference fringes caused by.

In order to solve this, a CCD camera is installed.
There is a method in which alignment is performed by a long-time exposure operation in which the exposure time is / 30 seconds or more.

FIG. 6 is a time chart when the CCD camera is operated in the long-time exposure mode in which the exposure time is 1/30 seconds or more. At ST, measurement start and stop are controlled. This makes it possible to obtain an image without speckles and interference fringes illuminated by dozens of laser output pulses.

However, in this case, since long-time exposure is performed, background noise increases due to the influence of the dark current of the CCD, and the obtained image has a poor S / N ratio, and good measurement accuracy is obtained. I can't get it.

The present invention has been made in view of the problems of the above-mentioned prior art, and in alignment of a substrate such as a wafer using pulsed light as irradiation light, alignment marks having no speckles and no illuminance unevenness are provided. It is an object of the present invention to provide a substrate alignment method capable of highly accurately aligning the substrate by obtaining an image.

[0013]

In order to achieve the above object, the substrate alignment method of the present invention is a video signal of an image pickup means for receiving the reflected light from the alignment mark of the substrate irradiated by the pulsed light. In the substrate alignment method, which comprises the step of detecting the positional deviation of the substrate by the step of: and the step of moving the substrate based on the detected positional deviation, the imaging means synchronizes with the emission of the pulsed light. By generating an intermittent video signal by opening the electronic shutter, storing a predetermined number of the video signals by the memory means, and adding the detected video signal, the position of the alignment mark is detected and compared with a predetermined reference position. The position shift is detected.

The video signal of the addition reference mark which does not move relative to the image pickup means is added to the video signal,
With this, it is preferable to correct the error at the time of addition.

[0015]

According to the method of the present invention, the image pickup means receives the reflected light of the alignment mark and generates the video signal only when the pulsed light is irradiated, thereby reducing the influence of noise such as dark current. A predetermined number of video signals are stored in the memory circuit without being received, and an image of the alignment mark without speckle and illuminance unevenness is obtained.

Therefore, the displacement of the substrate can be detected with high accuracy.

[0017]

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

FIG. 1 is a block diagram showing an embodiment. Illumination light emitted from an illumination system 1 which is an 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. The wafer 5, which is the substrate on 4, is exposed.

The wafer stage 4 is reciprocally moved by a stage drive system 6 which is a drive means in a direction along each of two mutually orthogonal axes (X axis, Y axis) in a plane perpendicular to the plane of the drawing. One axis (Z
Axis).

Reticle stage 2 holding the reticle 2
Also, a is reciprocally movable in the directions along the X axis and the Y axis by the stage drive system 6, and is also rotatable around 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 the main controller 12 described later.

An alignment detection system 7 for detecting the positional relationship of alignment marks which are alignment marks provided on the reticle 2 and the wafer 4, respectively, has a function of temporally changing the wavefront of illumination light to suppress speckles. A part of the illumination light from the illumination system 1 including is used as alignment illumination light, and the alignment mark of the reticle 2 and the alignment mark of the wafer 5 are detected from the reflected light of the mirror 8 by the CCD camera 9 which is an image pickup means.

In the actual reduction projection exposure apparatus, a detection system similar to the alignment detection system 7 is provided at a symmetrical position with respect to the optical axis of the projection lens system 3, but it is omitted in FIG.

A video signal from the CCD camera 9 of the alignment detection system 7 is input to the signal processing circuit 11 via the control unit 10. The electronic shutter of the CCD camera 9 is opened only for a short time including the emission of the pulse laser of the illumination system 1 to generate a video signal.

The signal processing circuit 11 stores the obtained video signal in the memory circuit 11a, and the memory circuit 11a as a memory means adds a plurality of video signals to obtain an image of the alignment mark.

The signal processing circuit 11 detects the image formation state on the light receiving surface of the CCD camera 9 of the image in which the alignment mark of the wafer 5 and the alignment mark of the reticle 2 are superposed on the basis of this image, and forms the image. Main controller 12 outputs a signal representing the amount of deviation between wafer 5 and reticle 2 according to the state.
Output to.

The sync signal generator 13 generates a sync signal for the detection system and a sync signal for the laser, and sends the sync signal to the control unit 10, the signal processing circuit 11, the main controller 12, the laser controller 14 and the drive system 6. give. The synchronization signal generator 13 is a signal processing circuit 11
Alternatively, it may be included in the main controller 12.

The pulse laser emits light for several tens of nanoseconds, and images obtained at other times become background noise such as dark current and stray light. Therefore, the pulse laser and the electronic shutter of the CCD camera are used. By synchronizing with and eliminating the charge accumulated except when the laser is radiated, and adding the video signals obtained only when the laser is radiated on the memory, there is no speckle / illuminance unevenness and there is little background noise. An image with a good S / N ratio can be obtained.

FIG. 2 is a time chart of this embodiment.

SH is an electronic shutter ON timing, and LS is a laser output pulse. Make the laser output while the electronic shutter is ON. IM is CCD camera 9 to control unit 1
This shows the contents of the memory that has fetched the video signal output through 0, and when fetching, it is fetched to the memory circuit 11a while performing addition with the memory itself. At this time, the data in the memory is also normalized so that it will not overflow.

After the required number of pulses have been taken in, measurement is performed from the obtained image, and based on the measurement result, the wafer stage 4 and / or the reticle stage 2a are moved to align the reticle 2 and the wafer 5.

It is also possible to implement the present invention using a plurality of image memories.

In this case, the image of each field is sequentially fetched into the memories 1 ... When the acquisition of the required number of pulses is completed, the images in each memory are added, and the images obtained by this addition are measured. The reticle 2 and the wafer 5 are aligned based on the measurement result.

As a result, an image without laser speckles and interference fringes can be obtained, and good measurement accuracy can be obtained without being affected by background noise due to dark current or the like.

In this embodiment, the image pickup means is a CCD camera conforming to NTSC, but it may be a camera conforming to other standards such as PAL, or a camera operating at its own timing instead of a camera conforming to these standards. Absent. Further, it may be a scanning type position detecting element such as a one-dimensional line sensor.

Next, the second embodiment will be described.

When the image processing is performed by using the CCD camera, the synchronizing signal CCD camera side generated in the processing section is synchronized or the signal processing timing of the image processing section is matched with the synchronizing signal on the CCD camera side. However, in this case, a pull-in means such as a PLL is used for synchronization, and a jitter of the pull-in causes each video signal to be shifted by a fraction of a pixel. Therefore, in the case of such a configuration, it is not possible to obtain a highly accurate image due to the influence of this jitter by simply adding the video signals as described above.

For this reason, an addition reference mark is placed in the visual field of the video signal, and when the addition is performed, the addition reference mark is measured instead of the pixel reference, and the jitter is added to each video signal for the measured value. Addition is performed in consideration of the resulting shift.

FIG. 3 is a block diagram showing this embodiment, in which the addition reference mark 15 is illuminated by the illumination light of the light source 16 and the reflected light is made incident on the CCD camera 9. The light source 16 is a stable CW light source such as a DC-lit LED. It is desirable that the measurement unit has a memory for each captured image. Note that, except that the addition reference mark 15 and the light source 16 are used, they are the same as those in the first embodiment, and are therefore denoted by the same reference numerals as those in FIG.

FIG. 4 shows an image having an addition reference mark according to this embodiment. For a normal NTSC compliant camera,
Since the jitter in the horizontal direction becomes a problem rather than the effect of the jitter in the vertical direction, marks are included in the vertical direction. It is preferable that the addition reference mark is composed of a plurality of marks because high-precision measurement is required. It is also possible to make the mark pitch finer and approach the CCD resolution limit to measure the position by moiré with the CCD.

[0041]

Since the present invention is configured as described above, it has the following effects.

It is possible to detect the positional deviation of the substrate with high accuracy by an image without speckles and illuminance unevenness, and to perform positioning with respect to the reference position with high accuracy. Therefore, highly accurate semiconductor exposure printing can be performed.

[Brief description of drawings]

FIG. 1 is a block diagram showing the present embodiment.

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

FIG. 3 is a block diagram showing a second embodiment.

FIG. 4 is a diagram showing an image of an alignment mark according to a second embodiment.

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

FIG. 6 is a time chart of another conventional example.

[Explanation of symbols]

 1 Illumination system 2 Reticle 2a Reticle stage 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 11a Memory circuit 12 Main controller 13 Synchronous signal generator 14 Laser controller 15 Addition reference mark 16 Light source

Claims (2)

[Claims] 1. A step of detecting a positional shift of the substrate by a video signal of an image pickup means for receiving reflected light from a position alignment mark of the substrate irradiated with pulsed light, and the step of detecting the positional shift based on the detected positional shift. In the method for aligning a substrate, which comprises the step of moving the substrate, the image pickup means generates an intermittent video signal by opening an electronic shutter in synchronization with the emission of the pulsed light, and the memory means predetermines the video signal. A method of aligning a substrate, wherein the position of the alignment mark is detected by storing and adding the numbers and comparing the displacement with a predetermined reference position. 2. A video signal of an addition reference mark that does not move relative to the image pickup means is added to the video signal, and an error during addition is corrected by the video signal of the addition reference mark. The method for aligning a substrate according to claim 1, wherein the substrate is aligned.