JP2762627B2 - Method for detecting misalignment between mask and wafer - Google Patents

Description

The present invention relates to a method for detecting a positional shift between a mask and a wafer,
The present invention relates to a mask and wafer position shift detection method applicable to an X-ray exposure apparatus.

[Technical environment]

In recent years, semiconductors have been trending toward higher integration as typified by DRAM, and the minimum line width of VLSI patterns is also approaching the region of microns to submicrons. In such a situation, in a conventional optical semiconductor exposure apparatus using ultraviolet g-rays and i-rays, the resolution limit by light wavelength is said to be about 0.5 μm. There is a strong demand for a next-generation exposure apparatus that can respond. As this next-generation exposure apparatus, an X-ray exposure apparatus is currently expected to be promising, and research and development are proceeding.

[Conventional technology]

Conventional technologies include, for example, B.Fay
The Journal of Vacuum Science
Technology (Journal of Vacuum Science Technolog
y) “Optical Alignment System for Sub-micron X-ray Lingraphy” (Vol. 16 (6) pp. 1954-1958. Nov / Dec. 1979)
ent System for Submicron X-ray Lithography), there is an alignment method using a linear Fresnel zone plate (LFZP).

 Here, the principle will be described with reference to the drawings.

FIG. 5 is a sectional view showing an alignment method using LFZP.

A linear diffraction grating 16 is engraved on the wafer 15, and a mask 17 faces the wafer 15 with a predetermined gap therebetween. An LFZP 18 having a focal length equal to the gap between the mask and the wafer is drawn on the mask 17.

FIG. 6 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 arranged, and when the distance from the center of the mark is r _n Here, f is the focal length, and λ is the wavelength of the laser used for alignment. Although the center stripe of the LFZP shown in the figure is transparent, the opposite configuration is also possible.

FIG. 7 is a plan view showing a linear diffraction grating of a wafer mark. The diffraction grating has a structure in which rectangles having the same size of the linear diffraction grating are arranged at equal intervals, and the diffraction angle is determined by the pitch P of the diffraction grating.

In FIG. 5, the parallel laser beam 19 incident from above the mask 17 is condensed by the LFZP 18, focuses on the surface of the wafer 15, and forms a slit-like image.

This imaged slit and the linear diffraction grating 16 on the wafer surface
Are aligned, the laser beam is diffracted and again LF
The light passes through ZP18 and becomes parallel light, which is detected as an alignment signal.

FIG. 8 shows a conventional LFZ disclosed in JP-A-55-43598.
FIG. 3 is a perspective view showing an automatic overlaying apparatus using an alignment method by P.

The automatic superposition apparatus shown in FIG. 8 includes a table 20, a piezoelectric transducer 21 for moving the table in the x direction, a wafer 15 mounted on the table and having a linear diffraction grating 16 pre-engraved thereon, It includes a mask 17 which is arranged opposite to the wafer 15 at a distance of several tens of μm and on which an LFZP 18 is previously drawn.

The apparatus further includes a laser light source 22;
Laser beam emitted from the laser beam is focused by a first lens 23 and is excited by a generator 24.
The light is reflected by the galvanometer mirror 26 driven by the second lens 27, passes through the second lens 27, and irradiates the LFZP 18 on the mask 17. With respect to the second lens 27, the irradiation position on the galvanometer mirror 26 and the irradiation position on the mask 17 are conjugate positions.

Further, the first lens 23 and the second lens 27 constitute an afocal optical system.

Therefore, the angle of incidence of the laser beam on the mask 17 can be changed by the galvanomirror 26, and the beam condensed by the LFZP 18 is applied to the wafer 15 as shown in FIG.
Scan above.

The diffracted light from the linear diffraction grating 16 on the wafer 15 is spatially separated from the incident light and detected by the detector 28.

The signal from the detector 28 and the signal from the generator 24 are input to the lock-in amplifier 29, and the phase difference signal d is obtained by detecting the phase difference between the two signals.

The position shift signal d is input to the power source 30 of the piezoelectric converter, and the table is moved according to the position shift, and the mask 17 and the wafer 15 are superposed.

[Problems to be solved by the invention]

The above-described conventional method for detecting the positional deviation between the mask and the wafer has a drawback that the reflected diffracted light intensity is weak and the S / N ratio is poor because the laser beam passes through the LFZP twice when entering and exiting the LFZP.

Further, there is a disadvantage that the intensity of the reflected diffracted light is remarkably reduced with respect to the gap deviation between the mask and the wafer, and the gap must be set with high accuracy before the positional deviation is detected.

[Means for solving the problem]

According to the method for detecting a positional shift between a mask and a wafer according to the present invention, the mask and the wafer are placed facing each other, a linear Fresnel zone plate having an opening at the center is provided on the mask, and a linear diffraction pattern is formed on the wafer. A grating, irradiating a laser beam to the linear diffraction grating on the wafer through an opening of the linear Fresnel zone plate, and reflecting and diffracting the linear Fresnel zone by the linear diffraction grating.
Imaging the laser light collimated by the zone plate with a lens and measuring the position of the image formed by the lens.

〔Example〕

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

 FIG. 1 is a block diagram showing one embodiment of the present invention.

The method for detecting the displacement between the mask and the wafer shown in FIG.
A mask 1 and a wafer 2 are set apart by a predetermined gap s, an LFZP 4 having a focal length equal to the gap s and having an opening 3 in the center is provided on the mask 1, a linear diffraction grating 5 is provided on the wafer 2, The beam diameter of the laser beam 6 is expanded by a beam expander 7, the laser beam 6 expanded by the beam expander 7 is collected by a lens 8, and the laser beam 6 collected by the lens 8 is The laser beam 6 reflected and diffracted by the linear diffraction grating 5 is collimated by the LFZP 4, and the laser beam 6 collimated by the LFZP 4
Through a rotary slit 9 on the image forming surface of the lens 8.
, The laser beam 6 passing through the rotating slit 9 is focused by the receiver lens 10, the photo sensor 11 is arranged at the focal point of the receiver lens 10, and the reference light source 12 and the reference photo are sandwiched by the rotating slit 9. A step of providing a sensor 13 and phase-detecting the output b of the photosensor 11 with the lock-in amplifier 14 using the output a of the reference photosensor 13 as a reference signal to obtain a position shift signal c.

In this embodiment, the half mirror 15 separates the incident laser beam from the reflected and diffracted laser beam.

FIG. 2A is a plan view of the LFZP4 shown in FIG.
FIG. 2B is a sectional view showing the operation of the LFZP4 shown in FIG.

Edges of each slit LFZP4 shown in FIG. 2 (a) is the distance from the center When r _n, (N = 1, 2, 3,...), And r _2m-1 <r <r _2m (m = 1,
2, ...) is opaque. Here, f is the focal length, and λ is the wavelength of the laser beam 6.

From the above equation, the focal length f = 40 μm, wavelength λ = 632.8 nm (He
-Ne laser), the width of the central opening of LFZP4 shown in FIG. 2 (a) is 2r ₁ = 1.08 μm.

Therefore, the laser beam 6 applied to the linear diffraction grating 5 on the wafer 2 must be focused to a beam diameter of about 10 μm.

FIG. 3 (a) shows the LFZP4 shown in FIGS. 2 (a) and (b).
Top view of LFZP4 _b , with the opaque part closest to the center removed
Figure 3 (b) is a sectional view showing the manner of operation of the LFZP4 _b shown in FIG. 3 (a).

The width of the central opening of LFZP4 _b is 2r ₃ = 17.53 μm,
The use of the LFZP4 eliminates the necessity of narrowing the laser beam 6 more narrowly. Further, the positioning accuracy of the mask 1 with respect to the laser beam 6 may be low.

Laser beam 6 scattered at the edge of linear diffraction grating 5
Is collimated by the LFZP4 having a focal length equal to the gap s between the mask 1 and the wafer 2, and the lens 8
Is imaged by

That is, the image of the linear diffraction grating 5 on the wafer 2 is enlarged and projected on the focal plane of the lens 8.

FIG. 4 is a schematic diagram showing a projection optical system including the LFZP 4 and the lens 8 on the mask 1.

The focal length f ₁ of the LFZP4, and the focal length of the lens 8 and f _2, magnification of the projection optical system is expressed by f ₂ / f _1.

For example, when the focal length of the LFZP4 is 40 μm and the focal length of the lens 8 is 80 mm, the magnification is 2000 times, and a minute displacement of the linear diffraction grating 5 can be easily observed.

In the present embodiment, since the lens 8 has a function of focusing the laser beam 6 irradiated on the linear diffraction grating 5 at the same time as forming an image of the laser beam 6 collimated by the LFZP4, The distance to 2 is arranged so as to be equal to the focal length of the lens 8.

However, it is also possible to use separate lenses for the lens for condensing the laser beam irradiated on the linear diffraction grating 5 and the lens for imaging the laser beam collimated by the LFZP4.

In this embodiment, the principle of position detection is applied to position detection of the enlarged and projected linear diffraction grating 5 image. The laser beam 6 passing through the rotating slit 9 is collected by the receiver lens 10,
It is detected by the photo sensor 11.

When the output b of the photosensor 11 is position-detected by the lock-in amplifier 14 using the output a of the reference photosensor 13 as a reference signal, the phase of the output b of the photosensor 11 changes in accordance with the displacement of the image of the linear diffraction grating 5. And lock-in amplifier 14
Outputs a displacement signal c.

Therefore, it is possible to detect positional deviation with high accuracy irrespective of the intensity of the diffraction light reflected by the linear diffraction grating 5.

In addition to the method using the scanning slit described in the present embodiment, it is also possible to detect the linear diffraction grating 5 image using a linear image sensor or a one-dimensional CCD.

The beam expander 7 is for expanding the diameter of the laser beam 6 and condensing it effectively by the lens 8.

Since the beam diameter on the mask 1 surface decreases as the magnification of the beam expander 7 increases, the magnification of the beam expander 7 is determined from the size of the opening 3 of the LFZP 4.

〔The invention's effect〕

In the method for detecting a positional shift between a mask and a wafer according to the present invention, a laser beam is applied to a linear diffraction grating through an opening of the LFZP, so that the laser beam only passes through the LFZP at the time of emission. Since a signal with a small decrease and a high S / N ratio can be obtained, there is an effect that a highly accurate position shift can be detected.

Further, since there is little change in the intensity of the reflected diffracted light with respect to the change in the gap between the mask and the wafer, there is no need to set the gap with high accuracy, and there is an effect that the apparatus becomes inexpensive.

[Brief description of the drawings]

1 is a block diagram showing one embodiment of the present invention, FIG. 2 (a) is a plan view of the LFZP shown in FIG. 1, and FIG. 2 (b) is an operation of the LFZP shown in FIG. 2 (a). 3 (a) is a plan view of the LFZP shown in FIGS. 2 (a) and 2 (b) with the central portion thereof removed, and FIG. 3 (b) is a view of FIG. 3 (a). FIG. 4 is a schematic view of a projection optical system composed of an LFZP and a lens, FIG. 5 is a cross-sectional view showing the principle of a conventional displacement detection method, and FIG. 6 is a cross-sectional view of the LFZP. FIG. 7 is a plan view showing the structure of a linear diffraction grating, and FIG. 8 is a perspective view showing an automatic overlaying apparatus using a conventional displacement detection method. 1,17 ... mask, 2,15 ... wafer, 3 ... opening, 4,18 ... LFZ
P, 5, 16: Linear diffraction grating, 6, 19: Laser beam, 7: Beam expander, 8: Lens, 9: Rotating slit, 10
… Receiver lens, 11… Photo sensor, 14,29… Lock-in amplifier.

Claims (3)

(57) [Claims] 1. A mask and a wafer are placed facing each other, a linear Fresnel zone plate having an opening in the center is provided on the mask, a linear diffraction grating is provided on the wafer, and a laser beam is applied to the linear beam. The linear diffraction grating on the wafer is irradiated through the opening of the Fresnel zone plate, and the laser light reflected and diffracted by the linear diffraction grating and collimated by the linear Fresnel zone plate is imaged by a lens. And measuring a position of an image formed by the lens and a position shift between the mask and the wafer. 2. The method according to claim 1, wherein the position of the image formed by the lens is measured by a linear image sensor. 3. The method according to claim 1, wherein the laser beam imaged by the lens is detected by a photo sensor through a scanning slit, and the output of the photo sensor is synchronously detected.