JPH054604B2 - - 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, are becoming 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 around 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. [Prior art] As a conventional technology, for example, Japanese Patent Publication No. 55-43598
As shown in the above publication, there is an optical alignment method using a linear Fresnel zone plate (LFZP). The conventional mask-to-wafer alignment method uses a condensing lens called LFZP that uses light diffraction as the mask mark, and a linear diffraction grating as the wafer mark. This positional deviation detection method is described in the Journal of B. Fay et al.
Vacuum Science Technology Vol.16(6)pp1954
−1958Nov/Dec1979 “Optical Alignment”
System for Submicron X-ray Lithography”
has been reported. Here, the principle will be explained with reference to the drawings. FIG. 5 is an explanatory diagram showing an alignment method using LFZP. A diffraction grating 15 is engraved on the wafer 14, and a mask 16 is opposed to the wafer 14 with a predetermined gap apart. An LFZP 17 whose focal length is designed to be equal to the gap between the mask and the wafer is drawn on the mask 16. 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 rn.

[Problem that the invention seeks to solve]

The conventional method for detecting misalignment between a mask and a wafer described above requires an optical system including two lenses and a vibrating mirror, which has the drawbacks of making the device complex, making it difficult to miniaturize, and making it expensive. . 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. Furthermore, since the positional deviation detection range is narrow, about 2 μm at most, there is a drawback that the burden of pre-alignment becomes heavy. [Means for solving the problem] In the method for detecting misalignment 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 a linear Fresnel zone plate is provided on the wafer so that the misalignment between the mask and the wafer is detected. Two different wide diffraction gratings are provided adjacently, and a laser beam is irradiated to the linear Fresnel zone plate on the mask,
The method includes detecting reflected and diffracted light from the diffraction grating on the wafer with two detectors, and determining a difference between the outputs of the detectors. [Example] Next, an example 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. First diffraction gratings 5 having different pitches and having a wide width w are provided adjacent to each other in the direction of the axis The first-order reflected diffraction light from the diffraction grating 4 is detected by the first detector 7, and the second
The first-order reflected diffraction light from the diffraction grating 5 is detected by a second detector 8, and the difference between the outputs of the first detector 7 and the second detector 8 is determined. The diffraction angle θ due to the diffraction grating of pitch P is θ
It is expressed as = sin ^-1 (nλ/P) (n = ±1, ±2, ...). Therefore, the pitch of the first diffraction grating 4
Since P ₁ and the pitch P ₂ of the second diffraction grating are different,
The respective first-order diffraction angles θ ₁ and θ ₂ are also different, and 2
The two reflected and diffracted lights are spatially separated. FIG. 2 is a plan view showing an alignment mark used in the method of detecting a positional deviation between a mask and a wafer shown in FIG. Figure 2 a shows the mask mark LFZP.
3, and the focal length is designed to be equal to the gap S between the mask 1 and the wafer 2. Figure 2b shows the first diffraction grating 4 and the second diffraction grating of the wafer mark, each having a length l equal to the length l of LFZP3.
The width w of the diffraction grating is approximately half the width of the LFZP3, which is several tens of micrometers, and the pitches are _P1 and _P2 . In the figure, the first diffraction grating 4
Although the second diffraction grating 5 and the second diffraction grating 5 are in contact with each other, it is effective even if they are separated by a minute distance. 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. 1 from the first diffraction grating 4 on the wafer 2
A first detector 7 that detects the second-order reflected diffraction light, a second detector 8 that detects the first-order reflected diffraction light from the second diffraction grating 5 on the wafer 2, and the first detector 7 a first amplifier 9 that amplifies the output a of the second detector 8 and generates a first sense signal c; a second amplifier 10 that amplifies the output b of the second detector 8 and generates a second sense signal d; from the second sense signal d to the first sense signal d.
a subtracter 11 that subtracts the sense signal c of , and generates a position deviation signal e; an adder 12 that adds the first sense signal c and the second sense signal d, and generates a reference signal f; A divider 13 divides the deviation signal e by the reference signal f to generate a normalized positional deviation signal g. The method for detecting misalignment between a mask and a wafer shown in FIG. 1 detects misalignment in the direction of the axis x. FIG. 4a is a graph showing the relationship between the positional deviation of the mask 1 and the wafer 2 in the direction of the axis x, the output a of the first detector 7, and the output b of the second detector 8.
When the slit imaged by the LFZP 3 is on the first diffraction grating 4, the first reflected diffraction light from the wafer 2 is detected by the first detector 7, and the LFZP
The slit imaged by 3 is the second diffraction grating 5
When it is above the wafer 2, the first-order reflected diffraction light from the wafer 2 is detected by the second detector 8. Therefore, the waveforms of the output a of the first detector 7 and the output b of the second detector 8 in FIG. 4a appear to have approximately the width w of the diffraction grating. A positional deviation signal e is obtained by amplifying these signals and determining the difference. FIG. 4b is a graph showing the positional deviation signal. The slit imaged by LFZP3 is the first
At the boundary between the diffraction grating 4 and the second diffraction grating 5, the positional deviation signal e becomes 0, indicating that the mask 1 and the wafer 2 are correctly overlapped. Directness near the 0 point is good. Also, the positional deviation signal e is ±w
Therefore, if a servo system is constructed using this signal, it is possible to pull in to the 0 point if the initial positional deviation is within ±w. For example, if the width w of the diffraction grating is designed to be 20 μm, automatic superposition using an alignment servo is possible if the pre-alignment accuracy is about 40 μm. This level of accuracy can only be achieved with mechanical accuracy based on orientation flats. The wider the width w of the diffraction grating, the wider the positional deviation detection range, but it is desirable that the area occupied by the mark be small.
It is appropriate that the combined area is equal to the area of LFZP3. Although the signal strength of the output a of the first detector and the output b of the second detector changes due to the influence of the resist etc., the gap deviation between the mask 1 and the wafer 2 is normalized by the divider 13. Therefore, the sensitivity to positional deviation does not change regarding the normalized positional deviation signal g. Therefore,
By configuring a servo system using the normalized positional deviation signal g obtained by this signal processing system, the servo gain will not change due to fluctuations in signal strength due to gaps or the effects of registers, etc. Therefore, stable servo can be realized. 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. [Effects of the Invention] The method for detecting misalignment between a mask and a wafer of the present invention uses two lenses and a vibrating mirror to scan a laser beam to obtain a misalignment signal, but instead of scanning a laser beam using two lenses and a vibrating mirror to obtain a misalignment signal, the method detects misalignment from two adjacent diffraction gratings. 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 of the positional deviation detection signal can be increased without being affected by the frequency of the vibrating mirror. Furthermore, since a wide diffraction grating can be used, the positional deviation detection range can be widened, and pre-alignment is no longer necessary, which simplifies the apparatus and improves throughput.

[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 the conventional alignment method, and Figure 6 a is a graph showing the positional deviation signal.
FIG. 6b is a plan view showing the structure of the LFZP, FIG. 6b is a plan view showing the structure of the diffraction grating, and FIG. 7 is a perspective view showing an automatic alignment device 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, two wide diffraction gratings with different pitches are provided adjacent to each other on the wafer, and a laser beam is directed onto the mask. A mask and a wafer comprising: irradiating the linear Fresnel zone plate, detecting the reflected diffracted light from the diffraction grating on the wafer with two detectors, and determining the difference between the outputs of the detectors. positional deviation detection method.