JPH0548929B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a method for detecting misalignment between a mask and a wafer;
In particular, the present invention relates to a method for detecting misalignment between a mask and a wafer that can be applied to an X-ray exposure apparatus.

[Technological environment]

In recent years, semiconductors, as exemplified by DRAM, have become increasingly highly integrated, and the minimum line width of VLSI patterns is moving from microns to submicrons. Under these circumstances, conventional optical semiconductor exposure equipment that uses ultraviolet g-line and i-line rays has a resolution limit due to the wavelength of the light.
Since it is said to be about 0.5 μm, there is a strong desire for next-generation exposure equipment that can handle patterns of 0.5 μm or less. Currently, as this next generation exposure equipment,
X-ray exposure equipment is seen as promising, and research and development is progressing.

[Conventional technology]

As for conventional technology, for example, there is the X-ray stepper "MX-1600" by Micronics, Inc., which was introduced in the April 1986 issue of Nikkei Micro Devices.The mask and wafer alignment in "MX-1600" is as follows. A condensing lens called a linear Fresnel zone plate (LFZP) that uses light diffraction is used as the mask mark, and a linear diffraction grating is used as the wafer mark. This alignment method is described in the Journal of B. Fay et al.
Vaeuum Science Technology Vol.16(6)
pp.1954−1954.Nov/Dec 1979 “Optical
Alignment System for Submicron X-ray
The principle is explained here with reference to the drawings.

FIG. 5 is an explanatory diagram showing an alignment method using LFZP.

A diffraction grating 15 is marked on the wafer 14, and a mask 16 is opposed to the wafer 14 at a distance of a predetermined gap. The mask 16 has a focal length equal to the gap between the mask and the wafer.
LFZP17 is depicted. FIG. 6a is a plan view showing the structure of the LFZP of the mask mark.
LFZP has a structure in which stripes of various widths and intervals are lined up, and the distance of the stripes from the center of the mark is r _o . It is expressed as Here, f is the focal length and λ is the wavelength of the laser used for alignment. Although the central stripe of the LFZP shown in the figure is transparent, the opposite configuration is also possible. Further, FIG. 6b is a plan view showing the diffraction grating of the wafer mark. The diffraction grating has a structure in which rectangles of equal size are arranged at equal intervals, and the diffraction angle is determined by the pitch, P, of the diffraction grating.

In FIG. 5, a parallel laser beam 18 incident from above a mask 16 is condensed by an LFZP 17 and focused on the surface of a wafer 14 to form a slit-shaped image. When this imaged slit and the diffraction grating 15 on the wafer surface are aligned, the laser beam is diffracted, passes through the LFZP 17 again, becomes parallel light, and is detected as an alignment signal.

FIG. 7 is a perspective view showing an automatic overlaying apparatus using the conventional LFZP alignment method disclosed in Japanese Patent Application Laid-Open No. 55-43598.

The automatic stacking device shown in FIG.
9, a piezoelectric transducer 20 for moving the table in the x direction, a wafer 14 mounted on the table and on which a diffraction grating 15 has been engraved in advance, and a piezoelectric transducer 20 placed opposite to the wafer 14 with a distance of several tens of μm and preliminarily formed. A mask 16 on which an LFZP 17 is drawn is included. The device further includes a laser light source 21, and the collimated laser beam emitted from the laser light source 21 is focused by a first lens 22 and is focused by a vibrating mirror 25 driven by a motor 24 excited by a generator 23. The second
LFZP1 on the mask 16 passes through the lens 26 of
7. Regarding the second lens 26, the irradiation position on the vibrating mirror 25 and the irradiation position on the mask 16 are paired. Furthermore, the first lens 2
2 and the second lens 26 constitute an afocal optical system. Therefore, the angle of incidence of the laser beam on the mask 16 can be changed by the vibrating mirror 25, and the beam focused by the LFZP 17 scans over the wafer 14, as shown in FIG. The diffracted light from the diffraction grating 15 on the wafer 14 is spatially separated from the incident light and detected by the detector 27. Signal from detector 27 and generator 2
The signal from 3 is input to the phase detector 28, and a positional deviation signal h is obtained by detecting the phase difference between the two signals. This positional deviation signal h is input to the power source 29 of the piezoelectric transducer, and the stage is moved in accordance with the positional deviation, thereby overlapping the mask 16 and the wafer 14.

By configuring the optical system and control system as described above, a closed-loop automatic superposition device can be realized.

The aforementioned X-ray stepper "MX1600" is equipped with four channels of such an optical system, with three channels detecting the amount of deviation in the xyθ directions, and the remaining one channel correcting the transfer magnification.

[Problem that the invention seeks to solve]

The above-described conventional method for detecting misalignment between a mask and a wafer requires an optical system including two lenses and a vibrating mirror, which has the disadvantage that the device becomes complex, difficult to downsize, and expensive. The manufacturing process also had the disadvantage of being time consuming as it required complicated optical axis adjustment of the optical system.

In addition, since the positional deviation signal is obtained by phase-detecting the signal from the detector and the drive signal of the vibrating mirror, the sampling frequency of the positional deviation signal is determined by the frequency of the vibrating mirror, so the sampling frequency is low. There were flaws.

[Means for solving problems]

In the method for detecting positional deviation between a mask and a wafer of the present invention, a mask and a wafer are placed facing each other, a linear Fresnel zone plate is provided on the mask, and areas on the wafer corresponding to the linear Fresnel zone plate are aligned with each other in pitch. Two linear diffraction gratings with different values are arranged in series with their centers shifted by a reduction distance, and a laser beam is irradiated onto the linear Fresnel zone plate on the mask to detect the reflection from the linear Z diffraction grating on the wafer. The method includes detecting diffracted light with two detectors and determining the difference between the outputs of the detectors.

〔Example〕

Next, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view showing an embodiment of the present invention.

The method for detecting the positional deviation between a mask and a wafer shown in FIG.
A first diffraction grating 4 and a second diffraction grating 5, each having a different pitch and a length half that of the LFZP 3, are placed in series in a region on the wafer 2 corresponding to Place them side by side. The LFZP 3 on the mask 1 is irradiated with a laser beam, the first-order reflected diffraction light from the first diffraction grating 4 on the wafer 2 is detected by a first detector, and the second diffraction grating 5 is detected by a first detector. The first-order reflected diffraction light from
the first detector and the second detector.
It consists of calculating the difference between the outputs of the detectors.

The diffraction angle θ by the diffraction grating of the pitch P is expressed as θ=Sin ⁻³ nλ/p (n=±1, ±2, . . . ). Therefore, since the pitch P 1 of the first diffraction grating ₄ and the pitch P 2 of the second diffraction grating 5 are different, the respective first-order diffraction angles θ ₁ and _{θ 2 are} also different, so the two reflected diffraction lights are spatially separated.

FIGS. 2a and 2b are plan views showing alignment marks used in the method for detecting misalignment between a mask and a wafer shown in FIG. 1.

Figure 2a is the mask mark LFZP3,
The focal length is designed to be equal to the gap S between the mask 1 and the wafer 2. FIG. 2b shows the first diffraction grating 4 and the second diffraction grating 5 of the wafer mark, each having the length LFZP3 1/2, the width w of the diffraction gratings being equal, and the pitches P ₁ and P It is ₂ .
The centers of the first diffraction grating 4 and the second diffraction grating 5 are shifted by a minute interval ΔD, but the minute interval ΔD is comparable to or smaller than the width w of the diffraction grating.

FIG. 3 is a block diagram showing a signal processing system in the method for detecting misalignment between a mask and a wafer shown in FIG. a first detector 7 that detects the reflected diffraction light from the first diffraction grating 4 on the wafer 2;
a second detector 8 that detects the reflected diffracted light from the second diffraction grating 5 above; and a second detector 8 that amplifies the output a of the first detector 7 and generates a first sense signal c. 1 amplifier 9, a second amplifier 10 that amplifies the output b of the second detector 8 and generates a second sense signal d, and a second amplifier 10 that amplifies the output b of the second detector 8 to generate a second sense signal d, a subtracter 11 that subtracts the first sense signal c and the second sense signal d to generate a reference signal f, an adder 12 that adds the first sense signal c and the second sense signal d to generate a reference signal f, and positional deviation signal g normalized by dividing by the reference signal f;
, and a divider 14 that generates .

The method for detecting misalignment between a mask and a wafer shown in FIG. 1 detects misalignment in the direction of the axis x.
Since the first diffraction grating 4 and the second diffraction grating 5 are shifted by a minute distance ΔD in the direction of the axis x, the signal a of the first detector and the signal b of the second detector have a spatial phase difference. produces ΔD. Figure 4a shows mask 1 and wafer 2.
The positional deviation in the direction of the axis x and the output a of the first detector
It is a graph showing the relationship between the output b of the second detector and the output b of the second detector. The output a of the first detector and the output b of the second detector draw equal waveforms with a phase difference ΔD, but the design is such that the two waveforms intersect when the positional deviation becomes 0. There is. The output a of the first detector and the output b of the second detector are output to the first amplifier 9.
The signal is then amplified by the second amplifier 10 to the required signal strength. FIG. 4b shows the first sense signal c
3 is a graph showing a positional deviation signal e obtained by calculating the difference between the first sense signal d and the second sense signal d. An S-shaped waveform is obtained,
The linearity is good near the zero point of positional deviation, and mask 1
Since the signal is proportional to the amount of positional deviation of the wafer 2, a closed-loop automatic overlay servo system can be constructed. The maximum positional deviation detection range is equal to the minute interval ΔD.

The signal intensities of the output a of the first detector and the output b of the second detector vary depending on the gap difference between the mask 1 and the wafer 2, the depth of the grooves in the diffraction grating, the effects of the resist, etc. Since the positional deviation signal e obtained in step 11 is normalized by the divider 13, the slope of the positional deviation signal near the 0 point, that is, the sensitivity to positional deviation does not change. Therefore, if a servo system is constructed using the normalized positional deviation signal g obtained by this signal processing system,
Since the servo gain does not change due to variations in signal strength due to gap deviation, groove depth of the diffraction grating, resist, etc., stable servo can be achieved. Further, when the reference signal f obtained by the adder 12 is at a high level, it indicates that the reference signal f is within the positional deviation detection range.

In this embodiment, two independent detectors are used as the detectors, but it is even better to use a two-split detector because the characteristics of each detector are the same.

〔Effect of the invention〕

The method of detecting misalignment between a mask and a wafer of the present invention uses two diffraction gratings whose centers are shifted by a minute distance, instead of scanning a laser beam using two lenses and a vibrating mirror to obtain a misalignment signal. By calculating the difference between the signals and using it as a positional deviation signal, an optical system including two lenses and a vibrating mirror is not required, which has the effect of simplifying the apparatus and reducing its cost.

Further, there is an effect that the sampling frequency for positional deviation detection can be increased without being affected by the frequency of the vibrating mirror.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing one embodiment of the present invention;
Figure a is a plan view showing the structure of LFZP, which is a mask mark used in the method for detecting misalignment between a mask and a wafer shown in Figure 1, and Figure 2 b is a plan view showing the structure of the LFZP, which is a mask mark used in the method for detecting misalignment between a mask and a wafer shown in Figure 1. FIG. 3 is a block diagram showing the signal processing system; FIG. 4 a is a graph showing the relationship between the amount of positional deviation and the detector output;
Figure b is a graph showing the positional deviation signal, Figure 5 is an explanatory diagram showing a conventional example, Figure 6a is a plan view showing the structure of the LFZP, Figure 6b is a plan view showing the structure of the diffraction grating, FIG. 7 is a perspective view showing an automatic superimposition apparatus using a conventional alignment method. 1, 16... mask, 2, 14... wafer,
3, 17...LFZP, 4...first diffraction grating, 5
... second diffraction grating, 6, 18 ... laser beam, 7 ... first detector, 8 ... second detector,
11... Subtractor, 12... Adder, 13... Divider, 22... First lens, 23... Generator, 25... Oscillating mirror, 26... Second lens, 28... Phase detection vessel.

Claims (1)

[Claims] 1. A mask and a wafer are placed facing each other, a linear Fresnel zone plate is provided on the mask, and two linear diffraction gratings with different pitches are placed on a region of the wafer corresponding to the linear Fresnel zone plate at a reduced distance. irradiate the linear Fresnel zone plate on the mask with a laser beam, detect the reflected diffraction light from the linear diffraction grating on the wafer with two detectors, A method for detecting misalignment between a mask and a wafer, characterized by determining a difference in output from a detector.