JPH04212155A - Photomask and its production and exposing method - Google Patents

Abstract

PURPOSE:To offer a proximity exposing method for resolving the micropattern of the resolution limit of the conventional proximity exposing method or below and photomask design production using it. CONSTITUTION:The distribution of the amplitude and phase of photomask transmitting light is calculated so as to obtain a desired light intensity distribution on a substrate placed at a fine interval from a photomask. A constitution that a micro light shielding pattern and a phase sifter are formed on the photomask, and the calculated distribution is actualized, is adopted. The pattern transfer of high resolution is performed by using the conventional proximity exposing machine.

Description

[Detailed description of the invention]

0001

FIELD OF THE INVENTION The present invention relates to a photomask, a manufacturing method thereof, and an exposure method for obtaining high resolution performance in a proximity exposure method, etc., in which a photomask and a substrate such as a wafer or printed board are exposed with a small gap therebetween. .

0002

2. Description of the Related Art There are three exposure methods for transferring a desired pattern on a photomask onto a substrate such as a wafer or a printed board: a projection exposure method, a contact exposure method, and a proximity exposure method.
It can be broadly divided into Consider resolution, yield, exposure speed, and equipment price as criteria for evaluating the performance of each exposure method. The resolution is better when a finer pattern can be transferred, and the yield is better when the probability of pattern transfer defects is smaller.

Projection exposure methods using a lens system or mirror system have good resolution and yield, but are expensive because of the complicated device configuration, and the exposure speed is relatively slow because the photomask and substrate are moved. The contact exposure method, in which the photomask and substrate are brought into close contact with each other for exposure, has good resolution, high exposure speed, and low cost equipment, but the photomask and substrate can be damaged due to contact, resulting in low yields.

In the proximity exposure method, which exposes the photomask and the substrate by leaving a small gap, the equipment is low cost, the exposure speed is fast, and the yield is good because there is no damage caused by contact between the photomask and the substrate. is diffracted, and high resolution cannot be obtained.

[0005] Furthermore, as a prior art, Japanese Patent Publication No. 1-518
No. 25 is known. That is, this conventional technology includes
“A lithography mask has multiple transparent and opaque elements (halftones) smaller than the lithography resolution.
It has been shown that lithographic methods including lithography methods can improve resolution. Furthermore, it has been shown that an arbitrary complex amplitude transmittance distribution can be realized by using a phase shifter that shifts the phase of transmitted light in combination with a finite number of phase shifters and halftones.

[0006]

Problems to be Solved by the Invention The photomasks used in the above-mentioned conventional proximity exposure method have a problem in that the resolution decreases due to light diffraction. This problem will be explained using FIGS. 2 and 5. FIG. 2 is a cross-sectional view of a conventional photomask for exposing an isolated light-shielding pattern, in which a light-shielding pattern 12 is formed on a photomask 11. As shown in FIG. The horizontal axis x is the coordinate indicating the position on the substrate 13, and x1 and x2 are the coordinates of the boundary of the light shielding pattern 12. g is the gap between the photomask and the substrate. FIG. 5 shows the distribution of the light intensity I on the substrate 13 when the photomask 11 is irradiated with the exposure illumination light 10 for exposure. When a positive resist is applied onto a substrate, exposed and developed, a pattern remains on the substrate in areas where the light intensity I is below a certain threshold. A solid line 31 is a light intensity distribution when the illumination light 10 is parallel coherent light (when the viewing angle is zero), and the light intensity increases at the center of the pattern due to the influence of the diffracted light. No matter how the light intensity threshold value is set, the boundary of the resolution pattern cannot be set at x1 and x2. The one-dot chain line 30 is the light intensity distribution when the viewing angle 57 in FIG.
Although a desired pattern having a border of 2 is obtained, the gradient of the light intensity distribution near the light intensity threshold for resolution is very small. If this gradient is small, the change in pattern line width due to variations in exposure amount and process variations becomes large, resulting in a problem that a stable resolved pattern line width cannot be obtained.

FIGS. 7A and 7B are diagrams showing the contour line distribution of light intensity on the exposed substrate when a rectangular light-shielding pattern 35 is formed on the photomask. The length of the short side of the rectangle is the same as the pattern width d in the one-dimensional case,
35 also indicates the boundaries of the rectangle. Figure 7(a) shows the case where the viewing angle is zero, and no matter how the light intensity threshold is set, the circuit pattern shape changes greatly, and the resist pattern when the threshold is 0.4 is as shown by diagonal lines. Become. Figure 7 (
b) is a case where the viewing angle is increased as in the one-dimensional case, but it still has the problem that a desired circuit pattern shape cannot be obtained.

[0008] Furthermore, in Japanese Patent Publication No. 1-51825,
Regarding the challenge of improving the resolution of proximity exposure by reproducing the wavefront in reverse and imaging it on the substrate,
Not considered.

[0009] The purpose of the present invention is to solve the above problems.
To provide a photomask, a method for manufacturing the same, and an exposure method, which can accurately resolve minute patterns that cannot be resolved using a proximity exposure method or the like, and can perform exposure with small changes in pattern dimensions.

0010

[Means for Solving the Problems] In order to achieve the above object, in the present invention, a photomask is designed such that a desired light intensity distribution can be obtained on a substrate. One of the design methods is as follows. Let λ be the illumination light wavelength, and let g be the gap between the photomask and the substrate during exposure. When a desired transfer pattern is irradiated with parallel coherent light of wavelength λ, the complex amplitude distribution of light on a virtual plane z away from the circuit pattern in the optical axis direction is calculated by a computer using a light diffraction formula, etc. Calculate and find. Note that when using a formula for light diffraction, it is preferable to use an approximate formula for Fresnel diffraction or Fraunhofer diffraction. Then, a photomask may be formed that provides transmitted light with a phase distribution and an amplitude transmittance distribution that are equal to the complex conjugate of this complex amplitude distribution. That is, a photomask is used in which both the phase of transmitted light and the effective light amplitude transmittance change continuously or discretely. In actual exposure, using the exposure system shown in FIG. 9, the wavelength λ
The pattern is transferred onto the substrate by irradiating this photomask with illumination light whose main wavelength component is .

[0011]

[Operation] The operation according to the present invention will be explained with reference to FIG. FIG. 10A shows the propagation state of the wavefront of light when parallel coherent light is irradiated onto a photomask 60 having a desired transfer pattern. FIG. 10(b) shows the propagation state of the wavefront of light when the photomask 62 according to the present invention is irradiated with parallel coherent light for exposure. Figure 10(a)
The distance between the photomask 60 and the substrate 61 in FIG. 10B is the same as the distance between the photomask 62 and the substrate 63 in FIG. 10(b). 70
74 and 80 to 84 represent wavefronts with equal phase, and the thickness of the lines represents the amplitude of the light. for example,
The wavefront 71 immediately after transmission in FIG.
2 indicates that the phase of the pattern portion is significantly delayed and the amplitude is small. As the wave propagates, the amplitude of the portion corresponding to the pattern portion, such as the wavefront 74, increases, and the phase delay decreases. Wavefront 81 immediately after passing through the photomask
The photomask 62 is formed so that the wavefront has the same amplitude as the wavefront 74 but has an inverted phase. The wavefront propagates between the photomask 62 and the substrate 63 in the opposite direction to that in case (a), and the wavefront 82
, 83 are wavefronts having the same amplitude and inverted phase as the wavefronts 73 and 72, respectively. The wavefront 84 on the surface of the substrate 63 is a reproduction of the original wavefront 71 immediately after passing through the photomask 60. That is, on the substrate 63, the amplitude of light in the portion corresponding to the light shielding portion is zero, and the amplitude and phase are uniform in the transmitting portion. In an ideal case where the wavefront 81 is the inversion of the wavefront 74, the original intensity distribution of light immediately after passing through the photomask 60 can be reproduced no matter how large the gap between the photomask and the substrate is.

0012

EXAMPLE An example of proximity exposure using a photomask according to the present invention will be described with reference to FIG. FIG. 1 is a sectional view showing a method (embodiment) for realizing a photomask according to the present invention. Reference numeral 1 indicates parallel coherent light, 2 a photomask, 3 a minute chrome thin line pattern formed on the photomask, 4 a phase shifter, and 5 a substrate. This photomask is obtained as follows. Exposure illumination light 1 to a photomask 11 having a desired pattern 12 shown in FIG.
Let 0 be parallel coherent light. At this time, the amplitude distribution of the light immediately after passing through the photomask has a distribution 15 shown in FIG. 3(a). Substrate 13 separated from photomask 11 by gap g
When the complex amplitude distribution of the above diffracted light is obtained and its complex conjugate is taken, the solid line amplitude distribution 16 shown in FIG. 3(b) and the solid line phase distribution 18 shown in FIG. 3(c) are obtained. x indicating position on board
By dividing the axis into sufficiently fine intervals and discretizing the amplitude distribution 16 into three stages in each region, an amplitude distribution 17 shown by a dotted line is obtained. The part with transmittance of 1/3 has a ratio of 1:2 between the aperture and the light shielding part as shown in Fig. 3(d), and the part with 2/3 has a ratio of 2:1 as shown in Fig. 3(e). A thin line pattern 3 shown in is obtained. The x-axis division interval is selected so that this thin line pattern is not resolved. This thin line pattern 3 can be formed using chromium or the like by ordinary exposure, development, and etching processes. Further, the phase distribution 18 is similarly discretized to obtain a phase distribution 19 shown by a dotted line. To achieve this distribution, a phase shifter made of a transparent material that has a certain refractive index difference with respect to air is provided. The phase of the opening where the phase shifter is located will be delayed. Possible phase shifter materials include SiO2 and photoresist materials. The thickness of the phase shifter that gives the transmitted light a delay of the unit of discretization of the phase distribution is calculated, and the phase shifter is stacked in three layers with the thickness as a unit to form the phase shifter 4 shown in FIG. 1. FIG. 4 shows a light intensity distribution 25 on the substrate 5 when the photomask is exposed by irradiating parallel coherent light. The light intensity at the central portion corresponding to the light-shielding pattern is almost zero, and a resolved pattern whose boundaries are the original pattern boundaries x1 and x2 as shown by the dotted line is obtained. In addition, the light intensity distribution near the light intensity threshold for resolution has a steep slope, and changes in the resolved pattern width due to variations in exposure amount or variations in the development process are small. on the other hand,
The thin line pattern 3 used in this embodiment functions as a diffraction grating, and noise consisting of high spatial frequency components may occur in the light intensity distribution on the outer substrate (not shown in FIG. 4). However, the noise can be canceled by increasing the viewing angle 57 of the illumination system. Although not shown in the diagram,
Even if the viewing angle is increased to such an extent that the noise is canceled out, the light intensity distribution in FIG. 4 hardly changes.

The method of calculating the photomask used for proximity exposure of the same one-dimensional isolated light-shielding pattern is not limited to the one described above. This will be explained using FIG. 6. until now,
The amplitude and phase distribution of the light on the substrate surface separated by a minute gap was calculated by assuming that the light immediately after passing through the desired pattern had a uniform amplitude and phase. A method for obtaining a light intensity distribution with better contrast is that the amplitude distribution of light immediately after passing through the desired pattern is not the amplitude distribution 15 in FIG. Assuming that the amplitude distribution 33 has increased contrast, the complex amplitude distribution of light on the substrate surface is calculated. When exposure is performed using a photomask calculated in this way, a light intensity distribution with a larger slope near the light intensity threshold value for resolution is obtained, and fluctuations in pattern line width are reduced. Furthermore, it is only necessary that the light intensity distribution on the substrate during exposure be a desired distribution, and its phase does not matter. In the previous embodiment, the amplitude and phase distribution of the light on the substrate were calculated assuming that the phase of the light irradiated onto the desired pattern was uniform, but it is clear that any phase distribution may be given. By appropriately providing a phase distribution, the formation of a photomask can be made easier and the light intensity distribution on the substrate during exposure can be made more desirable.

An example of proximity exposure of a rectangular pattern in a two-dimensional case will be described with reference to FIG. Figure 8 (a
), consider a photomask 42 on which a desired rectangular pattern 41 is formed, and a virtual surface 43 separated from the photomask by an exposure gap g and partitioned into a lattice. When this photomask 42 is irradiated with parallel coherent light 40, the amplitude distribution and phase distribution of light within each grating on the virtual plane 43 are calculated. The amplitude distribution is discretized into three stages, and all gratings with zero transmittance are used as light shielding parts, and the transmittance is 1
The /3 lattice is replaced with a pattern in which the ratio of the shaded area shown by diagonal lines to the blank transparent area is 2:1 as shown in Fig. 8(b),
The grating No. 3 is replaced with a pattern in which the ratio of the light-shielding portion to the transmitting portion is 1:2 as shown in FIG. 8(c), and the grating having a transmittance of 1 is all opened. The size of the grid is determined in advance so that these patterns are so small that they cannot be resolved. FIG. 8(d) is a top view of the thin line pattern 44 on the photomask obtained as a result. Figure 8(d),(e
) and (f), the dotted lines indicate the boundaries of the original rectangular light-shielding pattern 41 in FIG. 8(a). On the other hand, the phase distribution is reversed and then discretized in a certain discretization unit, and the grating that delays the phase is replaced with a thicker phase shifter. FIG. 8(e) shows the phase shifter arrangement at that time, viewed from above the photomask. Reference numeral 45 represents a portion without a shifter, and 46, 47, and 48 represent regions having thicknesses of one, two, and three times the unit shifter thickness, respectively. FIG. 8(f) shows the contour line distribution of light intensity on the substrate separated by a gap g when the photomask having the thin line pattern 44 and the phase shifter arrangement is exposed by irradiating parallel coherent light. Similar to FIG. 7(b), the shaded area is the area where the light intensity is lower than the resolution light intensity threshold.
A resist pattern close to the original pattern has been obtained. Furthermore, since the contour line density near the light intensity threshold for resolution is large and the gradient of the light intensity distribution is large, line width variations are reduced. In this case as well, the viewing angle needs to be increased slightly because the light is diffracted in the direction perpendicular to the thin line and high-frequency noise is generated in the light intensity distribution.

Another embodiment of realizing the amplitude transmittance distribution and phase distribution of light in the photomask according to the present invention will be described with reference to FIG. First, in the case of amplitude transmittance, as shown in FIG.
The film is exposed and developed with an appropriate exposure distribution as shown in the figure. The exposure dose distribution may be realized by, for example, direct laser writing and changing the dose depending on the location. Regarding the phase distribution, similarly, after applying a photosensitive material whose refractive index of light after development changes depending on the exposure amount, exposure and development may be performed in the same manner. According to this embodiment, since the distribution of amplitude transmittance and phase modulation degree is replaced with the exposure amount, there is an effect that a continuous distribution can be easily realized.

Another embodiment of realizing an amplitude transmittance distribution of light in a photomask according to the present invention will be described with reference to FIG. FIG. 12 is a cross-sectional view of a photomask 92 having several layers of translucent films 93, and it is also possible to realize a distribution of light transmittance by stacking translucent films. When the phase delay is affected by the film thickness, the thickness of the phase shifter may be calculated by taking this into consideration.

An example of the proximity exposure method will be explained with reference to FIG. In FIG. 9, light from a high-pressure mercury lamp light source 50 is collected by a reflecting mirror 51 into a rod lens group 52,
The collimator lens 54 converts the light emitted from each rod lens into
, the photomask 55 is irradiated with parallel light. The number of rod lenses that emit light is limited by the aperture 53 immediately after the rod lens group, and the maximum inclination angle 57 of the irradiated light, called the viewing angle, is changed. When the diameter of the aperture 53 is made small, the light distribution becomes only parallel light perpendicular to the photomask, resulting in coherent illumination with zero viewing angle.

Next, the principle of the present invention will be explained based on FIG. That is, as shown in FIG. 10(b), the photomask 2 (
62) and the substrate 5 (63), the wavefronts propagate in the opposite direction to the case of FIG.
The wavefront has the same amplitude as 2 and the phase is inverted. Substrate 5 (
63) The wavefront 84 on the surface is a reproduction of the original wavefront 71 immediately after passing through the photomask 60. That is, on the substrate 5 (63), the amplitude of the light in the portion corresponding to the light shielding portion is zero, and the amplitude and phase are uniform in the transmitting portion. In an ideal case where the wavefront 81 is the inversion of the wavefront 74, the original intensity distribution of light immediately after passing through the photomask 60 can be reproduced no matter how large the gap between the photomask and the substrate is. In this way, even if the transferred pattern becomes fine, transfer exposure can be performed with high resolution without being affected by diffraction.

An embodiment in which a large area is transferred at once by proximity exposure will be described with reference to FIG. FIG. 13 shows a case where a pattern is transferred onto a substrate 103 using a large-sized photomask 100 according to the present invention. Since the photomask 100 itself bends due to gravity, the gap g1 at the center portion 102 is smaller than the gap g2 at the end portions 101, but the distribution of this gap can be determined by calculation. Therefore, when calculating the complex amplitude of light on a virtual surface assuming that a photomask having a desired pattern is irradiated with parallel coherent light, the gap between the photomask and the virtual surface may be changed depending on the position on the photomask. Exposure using a photomask calculated as in this example has the effect of realizing a desired light intensity distribution over the entire substrate surface without being affected by the deflection of the photomask.

Finally, an embodiment in which the photomask according to the present invention is applied to projection exposure will be described with reference to FIG. FIG. 14 is a cross-sectional view showing a case in which a photomask 110 is irradiated with exposure illumination light 113 and a desired pattern is transferred onto a substrate surface with steps using a projection exposure system consisting of an entrance pupil 115 and a projection lens 114. Assuming that the upper surface 111 of the step is in focus, the lower surface 112 of the step will be defocused if exposed using a conventional photomask. Therefore, the complex amplitude distribution of light on the surface 111 when an image is formed on the surface 112 is calculated. The height g of the step is regarded as the proximity gap, and is derived by the method used in the present invention. The complex amplitude distribution of the light immediately after passing through the photomask that realizes the complex amplitude distribution of this light on the surface 111 is inversely calculated from the transfer function of the projection lens system, and projection exposure is performed using the photomask 110 according to the present invention. . According to this embodiment, there is an effect that exposure can be performed while eliminating the influence of defocus caused by steps.

Next, the principle of the present invention will be explained more specifically with reference to FIG.
Explain using. That is, FIG. 15A shows the propagation situation of the wavefront of light when it is assumed that a desired one-dimensional light shielding pattern 12 (hereinafter referred to as desired pattern) is irradiated with parallel coherent light 70 of wavelength λ. FIG. 15(b) shows a photomask (hereinafter referred to as an imaging mask) 62 (2) according to the present invention.
) represents the propagation situation of the light wavefront when parallel coherent light 1 of the same wavelength λ is exposed in the opposite direction. Figure 15 (
The distance between the desired pattern forming surface 60 and the virtual surface 61 in a) is
Image-forming mask 62 (2) and exposed substrate 6 in FIG. 15(b)
It is the same as the interval 3, which is g. As shown in FIG. 15(a), on the desired pattern forming surface 60, the position coordinate
Let the position coordinates xi be taken above, and the coordinates of both ends of the desired pattern be taken as x1 and x2. 71 to 74 and 81 to 84 represent wavefronts with equal phase, and the thickness of the lines represents the amplitude of the light. For example, the desired pattern 12 in FIG. 15(a)
A wavefront 71 immediately after transmission shows that the amplitude of the light shielding part is zero and the amplitude phase of the transmitting part is uniform, and the wavefront 72 shows that the phase of the center part is significantly delayed and the amplitude is small. As the wave propagates, the amplitude of the central portion, like the wavefront 74, increases and the phase delay decreases.

The complex amplitude immediately after passing through the imaging mask 62 (2) is formed so that the phase of the complex amplitude on the virtual plane is inverted. Since the phase of the central portion of 74 is delayed, when the phase is inverted, the phase of the central portion of 81 advances. Since the propagation directions of the light are opposite, the delay in FIG. 15(a) and the advance in FIG. 15(b) correspond in the figure, and the wavefront 74 is the same as the wavefront 1.
It has the same shape as 05. Therefore, the wavefront of the light transmitted through the imaging mask 5 propagates in the opposite direction to that in the case (a), and the wavefront 84 on the surface of the exposed substrate 63 reproduces the wavefront 71 immediately after passing through the desired pattern 12. That is, on the exposed substrate 63, the amplitude of the light in the portion corresponding to the light-shielding portion is zero, and the light has a uniform complex amplitude in the portion corresponding to the transmitting portion. In this way, by using the image-forming mask, the influence of diffraction can be eliminated, and the intensity distribution of light immediately after passing through the desired pattern can be reproduced.

Next, proximity exposure for transferring a one-dimensional light-shielding pattern using the image-forming mask according to the present invention will be explained in more detail with reference to FIGS. 15 to 18. The gap g and pattern width d=x2-x1 are the same as in the case of FIG. 5 using a conventional photomask, but the wavelength λ is the same as in FIG.
The case is a little different. A minute light-shielding pattern 3 and a phase shifter 4 are formed on the imaging mask 62(2). A method for manufacturing this imaging mask will be explained based on FIG. 16.

When a desired pattern (one-dimensional light-shielding pattern 12 shown in FIG. 15) is irradiated with parallel coherent light of wavelength λ, the complex amplitude distribution uo(xo) immediately after the pattern passes through the pattern,
Using the wavelength λ and the gap g as input data 161, the computer 16 calculates the complex amplitude distribution of light on the virtual surface 61 separated by the gap g.
Calculate according to 2. The phase is positive in the direction in which the light travels. The pattern dimension d is less than or equal to the resolution limit R of proximity exposure expressed by Equation 1, and Fresnel approximation usually holds.

The resolution limit R of proximity exposure is expressed by equation 1, where λ is the exposure wavelength and g is the distance between the mask and the substrate.

[0026]

[Math 1]

[0027] Here, k1 is a constant and is approximately 1.4.

Then, the complex amplitude ui(x
i) is expressed by Equation 2 using the formula of one-dimensional Fresnel diffraction.

[0029]

[Math 2]

[0030] k is a wave number and k=2π/λ. Depending on the gap, wavelength, and pattern dimensions, the Fraunhofer diffraction equation may be used. Take the complex conjugate of the calculated complex amplitude ui(xi) and set the coordinates representing the position on the imaging mask as xm
The complex amplitude distribution um(xm) of the light transmitted through the imaging mask as
is obtained. The amplitudes of um(xm) and ui(xi) are equal, and the signs of the phases are reversed. If the amplitude of um(xm) is a(xm) and the phase is φ(xm), then Fig. 15
In the case of the desired pattern 12 shown in FIG. 17(a), the amplitude distribution 16 shown by the solid line and the phase distribution 18 shown by the solid line shown in FIG. 17(b)
is obtained. Since it is difficult to form an imaging mask that provides such a continuously changing distribution to the transmitted light, an approximation is performed. First, the xm axis is divided into sufficiently small intervals p. The amplitude distribution 16 and phase distribution 18 are sampled in each divided region. Next, when the amplitude distribution is quantized into three stages, an amplitude distribution 17 shown by a dotted line is obtained, and when the phase distribution is similarly quantized using π/6 as a unit angle, a phase distribution 19 shown by a dotted line is obtained. Further, the amplitude transmittance is replaced with the aperture ratio distribution using the minute light shielding pattern 3. The part with zero transmittance is the light shielding part 3, and the part with transmittance 1/3 is the part 26 in FIG. 17(c).
The 2/3 part is replaced with a pattern where the ratio of the light shielding part to the transmitting part is 1:2, such as 27, and the transmittance is 1. All parts shall be open. The division size p is selected so that these patterns are so small that they cannot be resolved. A light shielding pattern distribution as shown in 3 in FIG. 17(c) is obtained.

It should be noted that the number of quantization stages is not limited to three, but can be approximated to a maximum of nine with sufficient accuracy.

The phase distribution 19 is realized by a phase shifter 4 made of a transparent material having a certain refractive index difference with respect to air, as shown in FIG. 17(c). The thicker the phase shifter 4 is, the more the phase of transmitted light is delayed. If we calculate the thickness of the phase shifter that gives the transmitted light a delay of quantization unit π/6 of the previous phase distribution, and form a phase shifter pattern with the thickness as a step, we will obtain the phase shifter 4 shown in FIG. 17(c). Become. 28
a is a portion that advances the phase of the transmitted light by π/6, 29 is a portion that does not change the phase, and 28b is a portion that is delayed by π/6.

Note that the quantization unit is not limited to π/6, but may be in the range of π/12 (or more) to π/4 (or less).

As described above, as shown in FIG.
64.

In the photomask creation unit 164 to which the image-forming mask data 163 is input, the minute light-shielding pattern 3 is subjected to ordinary electron beam drawing, development, and development using chromium or the like.
It is formed by an etching process, and the phase shift pattern 4 is realized by a phase shifter (by changing the thickness) made of a transparent material that has a certain refractive index difference with respect to air. Incidentally, as the phase shifter material, SiO2 or a photoresist material can be considered. To form a multi-step phase shifter, a resist coated on a photomask substrate is coated with
There is a method of obtaining a thickness distribution by providing an exposure dose distribution corresponding to the phase distribution and developing after exposure. Further, phase shifters of unit thickness may be formed in an overlapping manner.

As described above, the image forming mask 6 according to the present invention
2(2) can be produced.

Imaging mask 2 manufactured in this way
(62) is installed in the proximity exposure apparatus shown in FIG. ,
A regular circuit pattern is transferred and exposed onto the exposed substrate 5 at high resolution.

The light intensity distribution on the exposed substrate 5 when the viewing angle is zero is 32 as shown in FIG. The light intensity at the center is almost zero, and the resolution light intensity threshold Ith=0.2
5, a resolution pattern having boundaries x1 and x2 as shown by the dotted line can be obtained. The light intensity distribution near x1 and x2 has a steep slope, and high contrast can be obtained. The image-forming mask 2 (62) of this example generates high-order diffracted light by the minute light-shielding pattern 3, and noise consisting of high spatial frequency components is generated in the light intensity distribution on the outer substrate, which is not shown in FIG. comes out. However, the size and resolution of the pattern remaining are not affected. This noise is canceled out by increasing the viewing angle 57 shown in FIG. At this time as well, the light intensity distribution in FIG. 18 remains almost unchanged.

An imaging mask for a two-dimensional pattern can be designed in a similar manner. The imaging mask for transferring the same rectangular light-shielding pattern 35 as shown in FIG. 7 will be described in more detail with reference to FIGS. 19, 20, and 21. As shown in FIG. 19, when a desired rectangular pattern 41 is irradiated with parallel coherent light 40 of wavelength λ, the amplitude distribution and phase distribution of light on a virtual plane 43 separated by a gap g are calculated by a computer 1.
62. That is, the coordinates on the desired pattern forming surface 42 are (xo, yo), the complex amplitude of the light immediately after passing through the desired pattern forming surface 42 is uo (xo, yo), and the virtual surface 43
Let the upper coordinates be (xi, yi) and the complex amplitude on the virtual plane 43 be ui(xi, yi). A two-dimensional Fresnel diffraction formula when using Fresnel approximation is expressed by Equation 3.

[0040]

[Math 3]

As in the one-dimensional case, the virtual surface 43 is divided by the grid interval p, and the complex amplitude distribution ui(x
i, yi) is calculated by the computer 162, and the complex conjugate is taken by inverting the sign of the phase to obtain the complex amplitude distribution um(xm, ym) of the light transmitted through the imaging mask. Furthermore, calculator 1
62 quantizes the amplitude distribution and phase distribution in the same way as in the one-dimensional case, and calculates the minute pattern and phase shifter pattern.
A minute light shielding pattern 44a shown in FIG. 20(a) and a phase shifter pattern shown in FIG. 20(b) are obtained. A rectangle 46a in FIG. 20 indicates the boundary of the original rectangular light-shielding pattern 35 in FIG. In FIG. 20(b), 47a is the thinnest part of the shifter and corresponds to 28a in FIG. 17(c), and 48a and 49a correspond to 29 and 28b, respectively. This imaging mask 2
FIG. 21 shows the contour line distribution of light intensity on the exposed substrate 5 at a distance of g when exposing parallel coherent light of wavelength λ to (62). Rectangle 46a is the original rectangular light shielding pattern 3
This is the boundary of 5. The resist pattern when the light intensity threshold value is 0.3 is shown by diagonal lines. Although the pattern is rounded at the corners and some undulations remain, it is clear that the reproducibility of the pattern has improved compared to FIG. 7(b). Furthermore, the contour line density near the resolved light intensity threshold is large, and high contrast is obtained. In this case as well, light is diffracted in the direction perpendicular to the thin line, producing high-frequency noise in the light intensity distribution, but this can be canceled out by slightly increasing the viewing angle.

FIG. 22(a) shows a minute light-shielding pattern 44b of an imaging mask for transferring two adjacent rectangular patterns 46b with the same gap and the same wavelength, and FIG. 22(b) shows a phase shifter pattern. . The dimensions of the short sides of the rectangle are the same as those of the rectangle shown in FIG. Figure 2
The longitudinal direction of the minute light-shielding pattern in Figure 2(a) is
) is orthogonal to 47b, 48b, and 49b of the rectangular phase shifter pattern shown in FIG. 22(b) correspond to 28a, 29, and 28b in FIG. 17, respectively. FIG. 23(b) shows a light intensity contour map on the substrate (viewing angle is zero) when exposing using this mask. For comparison, a case where a conventional photomask is used (the viewing angle is the same as in FIG. 7(b)) is shown in FIG. 23(a). The resist pattern indicated by diagonal lines in FIG. 23A is for a light intensity threshold of 0.4, and as shown in the figure, three patterns that are completely different from the desired pattern remain. In FIG. 23(b), like in FIG. 7, the corners are chipped and rounded, and a bulge is seen in the center, but the desired pattern is almost reproduced, and the contour line density near the resolved light intensity threshold is large and the contrast is high. It has been obtained.

In the embodiments of FIGS. 15 to 23, the wavelength λ=4
05nm, gap g=120μm, pattern width d=4
．． This is a case where the pitch is 5 μm and the pitch p is 1.5 μm. The visual angle at this time is preferably about 0.5°. It is shown that the same imaging mask pattern can be used with different wavelengths and gaps by using the similarity law. The coordinates of number 2 of one-dimensional Fresnel diffraction are √(g
λ), and the dimensionless dimension Xo=xo/√(gλ
), Xi=xi/√(gλ). Furthermore, Uo(xo
), Ui(xi) are defined by Equations 4 and 5.

[0044]

[Math 4]

[0045]

[Math 5]

At this time, if the phase term ejkg in Equation 2 is ignored and rewritten, the relationship shown in Equation 6 holds true.

[0047]

[Math 6]

In Equation 6, Ui(xi) representing the complex conjugate of the complex amplitude transmittance of the image forming mask depends only on Uo(xo) and does not depend on the gap g and the wavelength λ. Therefore,
If the dimensionless desired pattern shapes are the same, the minute light-shielding pattern of the image forming mask and the phase shifter pattern will also be similar, and from Equation 5, only the dimension needs to be multiplied by √(gλ). Furthermore, since the non-dimensional viewing angle Θ=(√(g/λ))·θ, if the viewing angle during exposure is selected so that the non-dimensional viewing angle Θ is equal, a similar light intensity distribution on the substrate can be obtained.

The method of calculating an image forming mask for transferring a certain pattern is not limited to the method described above. In the above embodiment, the complex amplitude distribution of light on the virtual plane was calculated with respect to the complex amplitude of light immediately after transmitting the desired pattern, assuming that the amplitude and phase of the transmitting portion were uniform. A certain change may be given to this amplitude and phase distribution. For example, in the case of a one-dimensional light-shielding pattern, in order to obtain a light intensity distribution with better contrast, the contrast of the amplitude distribution of light immediately after passing through the desired pattern is increased near the edges of the light-shielding pattern, as shown in FIG. When exposed using the image-forming mask calculated in this way, the gradient of the light intensity distribution near the resolution light intensity threshold becomes larger, and high contrast can be obtained. Further, due to the approximation, errors may occur in the pattern dimensions obtained after exposure. In this case, there is a method of modifying the original pattern dimensions used for calculation. For example, when the resist pattern size becomes smaller than desired, it is preferable to largely modify the original pattern size and perform the calculation. Furthermore, it is clear that calculations may be performed by giving an arbitrary phase distribution, assuming that the phase of the light immediately after passing through the original pattern is not uniform. By properly providing a phase distribution, it is possible to make the formation of a photomask easier,
The light intensity distribution on the substrate during exposure can be made more desirable.

In the above embodiments, the sampling interval p is constant, but there is also a method of making it variable to improve calculation efficiency. For example, it is preferable to reduce the sampling interval in a portion of the imaging mask where the complex amplitude transmittance changes greatly (such as a corner), and increase the sampling interval in a portion where the change is small. Further, in the above embodiment, the number of amplitude quantization stages is 3 and the phase quantization unit angle is π/6, but it is thought that the approximation accuracy improves as the number of stages increases or as the unit angle decreases. However, increasing the number of stages means reducing the dimensions of the minute light-shielding pattern, and decreasing the unit angle means increasing the number of stages of the phase shifter, making it more difficult to manufacture the mask. Furthermore, special fair 1
As shown in Japanese Patent No. 51825, there is also a method of using a phase shifter with a finite number of stages and approximating an arbitrary complex amplitude transmittance by a vector sum of complex amplitudes.

The method for creating an image forming mask is not limited to the light shielding pattern and phase shifter. In order to realize the amplitude transmittance distribution, as shown in FIG. 25, a photosensitive material 91 whose light transmittance changes after development depending on the exposure amount, such as a silver salt photosensitive material or a photopolymer, is coated on a mask substrate 90, and then this material is coated on a mask substrate 90. The photosensitive material 91 is exposed to illumination light having an appropriate distribution as shown in the figure and developed. The exposure dose distribution may be realized by, for example, direct writing with an electron beam or laser and changing the dose depending on the location. Regarding the phase modulation rate distribution, similarly, after applying a photosensitive material whose refractive index of light after development changes depending on the exposure amount, exposure and development may be performed in the same manner. According to this embodiment, since the distribution of amplitude transmittance and phase modulation degree is replaced with the exposure amount, there is an effect that a continuous distribution can be easily realized. FIG. 26 also shows semitransparent films 93a and 93 on the mask substrate 92.
A case is shown in which the amplitude transmittance distribution of light is realized by forming the elements b in an overlapping manner. If a phase delay is affected by the film thickness, the thickness of the phase shifter may be calculated taking this into consideration. The semitransparent film can be formed by etching one layer at a time starting from the thickest film, or by sputtering or CVD using a lift-off method.
The thin films of D may be stacked one layer at a time.

An advantage of using an image-forming mask is that even if the gap between the image-forming mask and the substrate to be exposed is not uniform, focused exposure can be performed over the entire surface. Considering the expected gap change during exposure, the gap g for light diffraction calculation is changed depending on the location. For example, the substrate 103 (5) to be exposed is placed on the table 104 over a large area at once.
When transferring to the image forming mask 1 as shown in FIG. 27(a),
Since 00 itself bends due to gravity, the gap g in the center
1 is smaller than the end gap g2. This gap distribution can be determined in advance by measurement using a measuring means or by calculation. In this way, the influence of deflection of the imaging mask 100 can be eliminated. In addition, Fig. 27 (
Even when there is a step on the substrate 103(5) as in b), the complex amplitude distribution of light on the virtual plane may be similarly obtained using g3 and g4.

Substrate 11 with steps formed by reduction projection exposure method
A similar method can be applied when transferring a pattern onto 6. FIG. 28 shows a case where the circuit pattern of the imaging mask 110 is transferred to a substrate 116 with steps by an imaging system consisting of an imaging lens 114 and a pupil 115. Using a conventional photomask, when surface 111 is in focus, surface 112 will be defocused by g. In this case, surface 11
The complex amplitude distribution of the light on the surface 111 that is imaged on the surface 111 is calculated. The height g of the step is regarded as a gap, and the complex amplitude distribution um (xm, ym) on 111 is calculated by the method described above. By scaling the coordinates of this complex amplitude distribution by a reciprocal multiple of the magnification of the imaging system, the complex amplitude distribution of the light transmitted through the imaging mask 110 is obtained. However, in order to form an image on 112, the viewing angle of the illumination light must be made small to some extent, and a highly coherent illumination system is required. In this way, the influence of defocus caused by the step difference can be eliminated.

[0054]

According to the present invention, the image-forming mask can be configured as described above, so that by just slightly changing the coherence of the illumination system of the conventional proximity exposure device, the proximity exposure using the conventional photomask can be improved. The effect is that microcircuit patterns below the resolution limit can be transferred with good reproducibility and high contrast. Further, it has the effect of making it possible to transfer onto a substrate with steps.

Further, according to the present invention, it is possible to easily design and manufacture an imaging mask.

Further, according to the present invention, it is possible to form a desired minute resist pattern, which has been impossible in the past, accurately and with small variations in pattern line width simply by using a near-coherent illumination system.

Furthermore, according to the present invention, resolution performance similar to that of contact exposure, in which contact between the imaging mask and the substrate is a problem, can be achieved by proximity exposure, resulting in high throughput, low cost, and high yield. , and has the effect of realizing exposure with high resolution.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing an example of proximity exposure using an image-forming mask of the present invention.

FIG. 2 is a cross-sectional view showing exposure using a conventional photomask.

FIG. 3 is a diagram showing the amplitude distribution and phase distribution of light and a thin line pattern.

FIG. 4 is a light intensity distribution diagram on a substrate.

FIG. 5 is a light intensity distribution diagram on a substrate.

FIG. 6 is a diagram showing the amplitude distribution of light.

FIG. 7 is a diagram showing a contour line distribution of light intensity distribution on a substrate in a conventional photomask.

8A is a diagram showing a photomask having a desired pattern irradiated with coherent light; FIGS. 8B and 8C are diagrams showing a thin line pattern in a grating; and FIG. e) is a plan view of the phase shifter arrangement, (f)
1 is a diagram showing a contour line distribution of light intensity distribution on a substrate.

FIG. 9 is a sectional view showing a proximity exposure system according to the present invention.

FIG. 10 is a diagram for explaining the principle of a photomask according to the present invention.

FIG. 11 is a sectional view showing a photomask using the photosensitive material according to the present invention.

FIG. 12 is a cross-sectional view showing a photomask using a translucent film according to the present invention.

FIG. 13 is a sectional view showing a large photomask and a substrate according to the present invention.

FIG. 14 is a sectional view showing a projection lens system according to the present invention.

FIG. 15 is a diagram showing the principle of an imaging mask according to the present invention.

FIG. 16 is a diagram showing an example of a method for manufacturing an image forming mask according to the present invention.

FIG. 17 is an explanatory diagram of a one-dimensional imaging mask according to the present invention.

FIG. 18 is a diagram showing a light intensity distribution on a substrate to be exposed when using an image-forming mask according to the present invention.

FIG. 19 is an explanatory diagram for designing an imaging mask for rectangular pattern transfer according to the present invention.

FIG. 20 is a diagram showing an example of a light shielding pattern and a phase shifter pattern of an image forming mask for rectangular pattern transfer according to the present invention.

FIG. 21 is a diagram showing a contour line distribution of light intensity on a substrate to be exposed when the image forming mask according to the present invention is used.

FIG. 22 is a diagram showing an example of a light shielding pattern and a phase shifter pattern of an image forming mask for adjacent pattern transfer according to the present invention.

FIG. 23 is a diagram showing a contour line distribution of light intensity on a substrate when a conventional photomask is used and the same contour line distribution of an image forming mask for adjacent pattern transfer of the present invention.

FIG. 24 is a diagram showing the amplitude of light transmitted through a desired pattern when contrast is emphasized.

FIG. 25 is a cross-sectional view for explaining a method of manufacturing an imaging mask by controlling the amount of exposure to a photosensitive material according to the present invention.

FIG. 26 is a cross-sectional view for explaining a method of manufacturing an imaging mask by obtaining an amplitude transmittance distribution using a semitransparent film according to the present invention.

FIG. 27 is a diagram for explaining proximity exposure when the gap is not constant according to the present invention.

FIG. 28 is a diagram for explaining a case where reduction projection exposure is performed on a substrate with a step according to the present invention.

[Explanation of symbols]

1... Parallel coherent light, 2, 62... Photomask (imaging mask), 3... Chrome thin line pattern (light shielding pattern)
, 4... Phase shifter, 5, 63... Substrate for exposure, 12... Desired pattern, 60... Desired pattern formation surface, 61... Virtual plane, 71-74... Equal phase plane at the time of calculation, 81-84... At the time of exposure Isophase surface.

Claims (13)

[Claims] Claim 1: The effective amplitude transmittance of light has a continuous or discrete distribution including halftone values other than 0 and 1, and the phase of transmitted light also has a continuous or discrete distribution. A photomask characterized by: Claim 2: The phase distribution of transmitted light is π/12 or more π/
2. The photomask according to claim 1, wherein the photomask varies in units of 4 or less. 3. Calculate the distribution of the complex conjugate of the complex amplitude of light on a virtual plane separated by a small gap when a desired transfer pattern is irradiated with single-wavelength parallel coherent light using a light diffraction formula. The distribution of transmitted light is obtained by calculating the complex amplitude distribution, or a distribution obtained by discretizing it, or a distribution obtained by discretizing the amplitude, phase, or both into a finite number of steps in a certain unit. A method for manufacturing a photomask, the method comprising manufacturing a photomask. 4. The method of manufacturing a photomask according to claim 3, wherein the dimensions of the desired pattern used in the calculation are modified or the amplitude and phase distribution of the light immediately after passing through the desired pattern is varied. 5. The method of manufacturing a photomask according to claim 3, wherein the complex amplitude distribution of the light is calculated using a Fresnel diffraction equation using Fresnel approximation. 6. The method of manufacturing a photomask according to claim 3, wherein the finite number of stages of amplitude quantization is 3 or 5, and the phase quantization unit is π/12 or more and π/4 or less. 7. A claim characterized in that a photosensitive material is coated, exposed while controlling the exposure amount distribution, and then developed to give transmitted light a desired phase distribution or effective amplitude transmittance distribution. 4. The method for manufacturing a photomask according to 3 or 4. 8. Production of a photomask according to claim 3 or 4, characterized in that a desired effective amplitude transmittance distribution is given to transmitted light by stacking semitransparent films having a certain transmittance. Method. 9. A photomask is irradiated with parallel coherent light or highly coherent light similar to parallel light, so that the light transmitted through the photomask has a certain complex amplitude distribution, so that the light intensity distribution is such that a desired circuit pattern can be obtained on the substrate. An exposure method characterized by exposing to light. 10. The exposure method according to claim 9, wherein the exposure method is a proximity exposure method in which a minute gap is provided between the photomask and the substrate. Claim 11: The effective amplitude transmittance of light has a continuous or discrete distribution including halftone values other than 0 and 1, and the phase of the transmitted light also has a continuous or discrete distribution. A photomask is irradiated with parallel coherent light or highly coherent light similar to parallel light, and the light transmitted through the photomask is exposed to have a certain complex amplitude distribution so as to obtain a light intensity distribution to obtain a desired circuit pattern on a substrate. An exposure method characterized by: 12. The exposure method according to claim 11, wherein the exposure method is a proximity exposure method in which a minute gap is provided between the photomask and the substrate. 13. The complex amplitude distribution of the light transmitted through the photomask is the distribution of the complex conjugate of the complex amplitude of the light on the virtual plane separated by the minute gap when it is assumed that the desired circuit pattern is irradiated with single-wavelength parallel coherent light. 13. The exposure method according to claim 10 or 12, wherein the exposure method is the same.