Attenuating Type Phase Shifting Mask and Method of Manufacturing Thereof

1. Field of the Invention

The present invention relates to an attenuating type phase shifting mask, and more particularly, to a pattern of attenuating type phase shifting mask and a method of manufacturing thereof.

2. Description of the Background Art

Recently, high integration and miniaturization of the semiconductor integrated circuit has been developed rapidly. Accordingly, fine processing of the circuit pattern formed on the semiconductor substrate has been developed rapidly. Particularly, photolithography has been recognized broadly as basic technology in formation of the pattern. Although various developments and improvements have been carried out so far for photolithography, miniaturization of the pattern is still on going and the need for improvement of the resolution of the pattern has been more and more increased.

Generally, the limit of resolution R (nm) in photolithography utilizing the demagnification exposure method can be represented as follows:

where .lambda. is a wavelength (nm) of light used, NA is the numerical aperture of a lens and k.sub.1 is a constant depending on resist process.

As can be seen from the above expression, values of k.sub.1 and .lambda. should be decreased and the value of NA should increased in order to improve the limit of resolution. In other words, the constant depending on resist process should be decreased, while the wavelength should be shortened and NA should be increased. However, it is difficult technically to improve the light source and lenses, and by shortening the wavelength and increasing NA, the depth of focus .delta. (.delta.=k.sub.2..lambda./(NA).sup.2) of light becomes shallower causing decrease of the resolution.

Referring to FIGS. 16(A), 16(B), and 16(C), a cross section of the mask, an electric field of exposure light on the mask and intensity of exposure light on a wafer when a conventional photomask is utilized will be described.

Referring to FIG. 16(A), a cross sectional structure of the photomask will be described. A metal mask pattern 20 made of chromium or the like is formed on a quartz glass substrate 10. Referring to FIG. 16(B), the electric field of exposure light on the photomask matches the mask pattern. As to the intensity of exposure light on the wafer, however, it is noted as shown in FIG. 16(C) that especially when the fine pattern is utilized, beams of the light transmitted through the mask intensity with each other at adjacent pattern images where those beams of light overlap with each other due to diffraction and interference of light. Consequently, the difference of the light intensity on the wafer is reduced resulting in decreased resolution.

In order to solve the above-mentioned problem, the phase shifting exposure method utilizing a phase shifting mask has been proposed in, for example, Japanese Patent Laying-Open Nos. 57-62052 and 58-173744.

Referring to FIGS. 17(A), 17(B) and 17(C), the phase shifting exposure method utilizing the phase shifting mask disclosed in Japanese Patent Laying-Open No. 58-173744 will be described.

FIG. 17(A) shows a cross section of the phase shifting mask. FIG. 17(B) shows the electric field of exposure light on the photomask. FIG. 17(C) shows the light intensity of exposure light on the wafer.

Referring to FIG. 17(A), the phase shifting mask includes a chromium mask pattern 20 formed on a glass substrate, and at every other aperture of mask pattern 20, a phase shifter 60 formed by a transparent insulating film such as a silicon oxide film is provided.

Referring to FIG. 17(B), as to the electric field of exposure light on the photomask provided by the light transmitted through the phase shifting mask, phases of the exposure light are alternately reversed by 180.degree.. In other words, beams of light are canceled with each other at adjacent pattern images where beams of light overlap with each other due to interference. Accordingly, as shown in FIG. 17(C), the resolution of pattern images can be improved, because of the sufficient difference of the intensity of exposure light on the wafer.

Although the above-mentioned phase shifting mask is very effective for a periodic pattern such as lines and spaces, the mask cannot be set appropriately in case of a complicated pattern because arrangement or the like of phase shifters is very difficult.

In order to solve the above-mentioned problem, an attenuating type phase shifting mask is disclosed in, for example, "JJAP Series 5 Proceedings of 1991 International Micro Process Conference, pp. 3-9" and in Japanese Patent Laying-Open No. 4-136854. The attenuating type phase shifting mask disclosed in the Japanese Patent Laying-Open No. 4-136854 will be described below.

FIG. 18(A) shows a cross section of the above-mentioned attenuating type phase shifting mask 500. FIG. 18(B) shows the electric field of exposure light on the mask. FIG. 18(C) shows the intensity of exposure light on the wafer.

Referring to FIG. 18(A), phase shifting mask 500 includes a phase shifting pattern 300 which is a predetermined exposure pattern, including a quartz substrate 10 through which the exposure light is transmitted, a transmitting portion 100 formed on a main surface of quartz substrate 10 for exposing the main surface of quartz substrate 10, and a phase shifter 200 for shifting the phase of the exposure light transmitting therethrough by 180.degree. relative to the phase of the exposure light transmitting through said transmitting portion 100.

Phase shifter 200 is an absorption type shifter film of a double-layered structure including a chromium layer 20 having 5-20% transmittance of exposure light, and a shifter layer 30 for converting the phase of the exposure light transmitting through transmitting portion 100 by 180.degree..

The transmittance of exposure light of phase shifter 200 is set to 5-25%, which is appropriate for lithography, since the thickness of the resist film after development is adjusted according to the transmittance as shown in FIG. 19.

The electric field on the mask of the exposure light transmitting through the phase shifting mask having the above-mentioned structure is as shown in FIG. 18(B). As to the intensity of the exposure light on the wafer, the light intensity at the edge of the exposure pattern is inevitably 0 as shown in FIG. 18(C), since the phase of the exposure light is reversed at the edge of the exposure pattern. Consequently, there is provided sufficient difference between the light intensity corresponding to the transmitting portion 100 and that corresponding to the phase shifter 200, so that the resolution can be improved.

The above-mentioned conventional technology, however, has the following disadvantages.

FIG. 20(A) shows positions of the attenuating type phase shifting mask placed in the exposure apparatus and a blind 70 for determining the exposure region of the exposure apparatus. FIG. 20(B) shows the electric field of the exposure light directly under the attenuating type phase shifting mask and blind 70. FIG. 20(C) shows the light intensity on the exposed material of the light transmitted through the attenuating type phase shifting mask and blind 70. FIG. 20(D) shows the region exposed by the light transmitted through the attenuating type phase shifting mask and blind 70.

Referring to FIG. 20(A), the region other than a chip pattern forming region (Lc) of the attenuating type shifting mask is covered with absorption type shifter film 20 in which pattern processing is not conducted. In a demagnification projection, exposure apparatus blind 70 which interrupts the light for determining the exposure region is provided at a prescribed position under the attenuating type phase shifting mask.

The aperture of blind 70 may have any width so long as it allows exposure of the chip pattern region, and therefore the width may be the same as the chip pattern region (Lc). However, since the position of blind 70 is controlled by the distance of about 1000 .mu.m (about 1 mm) and blind 70 is not positioned on the same plane of focus as the attenuating type phase shifting mask, the edge portion of blind 70 is not well focused out of focus. Thus, as shown in FIG. 20(A), the width of aperture of blind 70 (Lb) is set to about 1000 .mu.m wider than the chip pattern region (Lc) so that blind 70 does not overlap the chip pattern region (Lc).

Accordingly, in an ordinary photomask utilizing an interrupting film of chromium, for example, on the chip pattern, only 1/1000 of the light can be transmitted through chromium at most, so that the light passing through the gap between the chip pattern region and blind 70 will not expose the resist film on the semiconductor wafer.

In case of the attenuating type phase shifting mask, however, 5-20% of the exposure light passes through the gap between the chip pattern region and blind 70 as indicated by portion A of FIG. 20(B) because the transmittance of the absorption type shifter film serving as a material of the chip pattern is about 5-20%. Consequently, referring to FIG. 20(C), a region having the light intensity of I' which is 5-20% of the transmitted light I.sub.0 results between chip pattern region Lc and blind 70 as can be seen from the distribution of intensity of the light transmitted through the attenuating type phase shifting mask and through blind 70. Thus, referring to FIG. 20(D), there is formed a region 50 having the light intensity (I') and having the width of Ld of 5-20% around chip pattern region 30 (Lc.times.Lc).

When patterns of the attenuating type phase shifting mask are demagnified and transferred onto the semiconductor wafer utilizing the demagnification projection exposure apparatus having the above-described structure, exposure proceeds successively at a pitch of Lc which is the size of the chip pattern FIG. 21 shows the exposed regions on the semiconductor wafer when the wafer is exposed using the attenuating type phase shifting mask having the chip pattern of the size (Lc.times.Lc), utilizing the demagnification projection exposure apparatus.

In this case, since exposure proceeds at a pitch of Lc longitudinally and transversely, there is a region 50 having the light intensity (I') of 5-20% (I') as described above around the chip pattern provided by one exposure shot. This region 50 overlaps an adjacent region provided by another exposure shot. As exposure is repeated successively, region 50 is exposed overlapped with adjacent three regions 50 at each corner of the exposure region. As a result, the exposed region comes to include region 31 each of which is exposed with the light having the appropriate intensity I.sub.0 plus the intensity I', which is 5-20% of I.sub.0, and region 32 each of which is exposed with the light having the intensity I.sub.0 plus three times the intensity I'.

In regions 31 and 32 which are exposed in such overlapped manner, when a positive resist film, for example, is exposed, the thickness of the resist film decreases after development. On the other hand, when the absorption type shifter film having a high transmittance is used, the resist film is completely exposed so that the resist film is lost by development.

An object of the present invention is to provide an attenuating type phase shifting mask having such patterns that prevent exposure of the region around a normal exposure region, and more specifically prevent exposure of regions adjacent to the exposure region when exposure is carried out successively with the mask moved.

In order to achieve the above object, the attenuating type phase shifting mask according to one aspect of the present invention includes an attenuating type phase shifting pattern formed at a predetermined position on a photomask substrate; and an attenuating type auxiliary phase shifting pattern having a transmitting portion and a phase shifter formed on a predetermined position at the periphery of said attenuating type phase shifting pattern. Also, the attenuating type auxiliary phase shifting pattern includes patterns which are smaller than the limit of resolution of the exposure apparatus.

According to the attenuating type phase shifting mask, images cannot be formed on the semiconductor wafer by the light transmitted through the attenuating type auxiliary phase shifting pattern because the pattern is smaller than the limit of resolution of the exposure apparatus. Further, since beams of the light transmitted through the transmitting portion and beams of the light transmitted through the phase shifter overlap with each other and are in reverse phases, those beams cancel with each other due to interference, so that the light intensity on the semiconductor wafer can be reduced.

According to the present invention, values of the planar area (S.sub.0) of the transmitting portion, the planar area (S.sub.H) of the phase shifter, and transmittance T of the phase shifter are set such that the substantial light intensity on the exposed material resulting from light intensity on the exposed material of the light transmitting through the transmitting portion and the light intensity on the exposed material of the light transmitting through the phase shifter overlapped and canceled with each other is not more than 3% of the light intensity before transmitting through said transmitting portion and said phase shifter.

Accordingly, the light intensity on the semiconductor wafer can be controlled by adjusting the intensity of light transmitting through the phase shifter and the intensity of light transmitting through the transmitting portion.

More preferably, according to the present invention, in the attenuating type auxiliary phase shifting pattern, the ratio of the planar area (S.sub.0) of the transmitting portion to the planar area (S.sub.H) of the phase shifter, which is represented by S.sub.0 /S.sub.H, is set approximately equal to .sqroot.T of the transmittance (T) of the phase shifter.

According to the attenuating type phase shifting mask of the present invention, since the ratio S.sub.H /S.sub.0 is set as described above, the light intensity on the semiconductor wafer can be set not more than 3% of the light intensity before transmitting through the transmitting portion and the phase shifter.

More preferably, according to the present invention, the attenuating type auxiliary phase shifting pattern is provided at the entire periphery of the attenuating type phase shifting pattern.

Accordingly, even when a plurality of portions on the semiconductor wafer are to be exposed utilizing the attenuating type phase shifting mask, exposure can be carried out without exposing other regions.

More preferably, according to the present invention, the attenuating type phase shifting pattern is rectangular and the attenuating type auxiliary phase shifting pattern is provided in the vicinity of each of four corners of the attenuating type phase shifting pattern.

Therefore, when the semiconductor wafer is to be exposed successively in order utilizing the attenuating type phase shifting pattern, satisfactory exposure can be carried out because no region is exposed more than once by the light transmitting through the periphery of the attenuating type phase shifting pattern.

More preferably, according to the present invention, the planar shape of the transmitting portion of the attenuating type auxiliary phase shifting pattern is rectangular.

Since the phase shifter is provided integrally on the photomask substrate and the transmitting portion is in the form of aperture, the phase shifting photomask can be adhered on the substrate so that the attenuating type phase shifting mask having a highly reliable structure can be provided.

More preferably, according to the present invention, the transmitting portions and the phase shifters of the attenuating type auxiliary phase shifting pattern are linear and arranged alternately.

Thus, the pattern can be lines and spaces so as to reduce the cost of the attenuating type phase shifting pattern.

In order to achieve the above object, a method of manufacturing an attenuating type phase shifting mask includes the following steps. First, an attenuating type phase shifter film having a light transmittance of 5-20% is formed on a transparent substrate for shifting a phase of transmitted light by 180.degree..

After that, a resist film including an attenuating type phase shifting pattern region and an attenuating type auxiliary phase shifting pattern region formed at a predetermined position at the periphery of the attenuating type phase shifting pattern region is formed on the attenuating type phase shifter film. Then, the attenuating type phase shifter film is etched using the resist film as a mask.

The attenuating type auxiliary phase shifting pattern has a pattern having a resolution smaller than the limit resolution of the exposure apparatus.

Preferably, forming the attenuating type phase shifter film includes the steps of forming a semi-light shielding film having a light transmittance of 5-20% and forming a phase shifter film shifting a phase of transmitted light by 180.degree..

More preferably, forming the semi-light shielding film includes the step of forming a chromium film, and forming the phase shifter film includes the step of forming a silicon oxide film.

According to the method of manufacturing an attenuating type phase shifting mask, any images cannot be formed on the semiconductor wafer by the light transmitted through the attenuating type auxiliary phase shifting pattern because the resolution of the pattern is smaller than the limit of resolution of the exposure apparatus. Further, since beams of light transmitted through the transmitting portion and beams of light transmitted through the phase shifter portion overlap with each other and are in reverse phases, those beams cancel with each other due to interference. Thus, the light intensity on the semiconductor wafer can be reduced.

More preferably, forming the attenuating type phase shifter film includes the step of forming a film of a kind selected from a group comprising chromium oxide, chromium nitride oxide, molybdenum silicide oxide, and nitride oxide of molybdenum silicide.

Thus, the number of manufacturing steps of the attenuating type phase shifting mask can be reduced because the attenuating type phase shifter film is formed of one kind of film, so that the manufacturing cost of the attenuating type phase shifting mask can be reduced.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

FIG. 1(A) shows an attenuating type phase shifting mask according to the first embodiment viewed from the pattern formation surface, and FIG. 1(B) is a cross sectional view along line X--X of FIG. 1(A).

FIG. 2 shows patterns of an attenuating type auxiliary phase shifting pattern of the first embodiment.

FIG. 3(A) is an enlarged cross sectional view of the attenuating type auxiliary phase shifting pattern, FIG. 3(B) is a cross sectional view showing an electric field of the exposure light directly under the photomask substrate, FIG. 3(C) shows the electric field of the exposure light on the semiconductor wafer, and FIG. 3(D) shows the intensity of the exposure light on the semiconductor wafer.

FIG. 4 is a graph showing the relationship between the ratio of the planar area (S.sub.0) of the transmitting portion to the planar area (S.sub.H) of the phase shifter (S.sub.H) and the light intensity ratio on the wafer.

FIG. 5 is a graph showing the relationship between transmittance of the phase shifter and the value of a representing the range of the planar area of the phase shifter.

FIG. 6(A) shows the attenuating type phase shifting mask of the first embodiment when viewed from the pattern formation surface, FIG. 6(B) is a cross sectional view showing positions of the attenuating type phase shifting mask and a blind, FIG. 6(C) shows the electric field of the exposure light directly under the photomask substrate, and FIG. 6(D) shows the light intensity on the semiconductor wafer.

FIGS. 7-11 are sectional views showing the first through fifth steps of manufacturing the attenuating type auxiliary phase shifting pattern according to the first embodiment of the invention.

FIG. 12 shows patterns of an attenuating type auxiliary phase shifting pattern of the second embodiment.

FIG. 13(A) shows an attenuating type phase shifting mask of the third embodiment when viewed from the pattern formation surface, FIG. 13(B) shows patterns of an attenuating type auxiliary phase shifting pattern of the third embodiment, and FIG. 13(C) shows patterns of an attenuating type auxiliary phase shifting pattern of the fourth embodiment.

FIG. 14 is a first view illustrating a problem occurred in respective embodiments.

FIG. 15 is a second view illustrating a problem occurred in respective embodiments.

FIG. 16(A) is a cross sectional view showing a structure of a conventional photomask, FIG. 16(B) shows the electric field of the exposure light on the mask, and FIG. 16(C) shows the intensity of the exposure light on the wafer.

FIG. 17(A) is a cross sectional view showing a structure of a conventional phase shifting mask, FIG. 17(B) shows the electric field of the exposure light on the mask, and FIG. 17(C) shows the intensity of the exposure light on the wafer.

FIG. 18(A) shows a structure of a conventional attenuating type phase shifting mask, FIG. 18(B) shows the electric field of the exposure light on the mask, and FIG. 18(C) shows the intensity of the exposure light on the wafer.

FIG. 19 is a graph showing the relationship between transmittance and the thickness of the resist film after development.

FIG. 20(A) is a cross sectional view showing positions of the conventional attenuating type phase shifting mask and the blind, FIG. 20(B) shows the electric field of the exposure light directly under the photomask substrate, FIG. 20(C) shows the intensity of the exposure light on the semiconductor wafer, and FIG. 20(D) shows the state of exposure on the semiconductor wafer.

FIG. 21 shows problems encountered when the conventional attenuating type phase shifting mask is used.

The first embodiment of an attenuating type phase shifting mask according to the present invention will be described below.

FIG. 1(A) shows an attenuating type phase shifting mask 1 of the present embodiment when viewed from pattern formation side. FIG. 1(B) is a cross sectional view along line X--X of FIG. 1(A).

In attenuating type phase shifting mask 1, an attenuating type phase shifting pattern 2, which is a square region, is formed approximately at the center on a photomask substrate 4. An attenuating type auxiliary phase shifting pattern 3 is formed at the entire periphery of the attenuating type phase shifting pattern 2.

An attenuating type phase shifting pattern 2 is formed by a phase shifter portion 24 and a transmitting portion 27. Phase shifter portion 24 is formed by chromium film 2a having a transmittance of 5-20% and SiO.sub.2 film 2b providing phase shifting of 180.degree..

Referring to FIG. 2, an attenuating type auxiliary phase shifting pattern 3 will be described below. Attenuating type auxiliary phase shifting pattern 3 is formed by processing a phase shifter portion 34 formed by chromium film 3a having a transmittance of 5-20% and SiO.sub.2 film 3b providing phase shifting of 180.degree. into a pattern of a size smaller than the limit of resolution of the exposure apparatus. In FIG. 2, a planar area of phase shifter portion 34 is represented by S.sub.H and a planar area of a square transmitting portion 37 is represented by S.sub.0.

Referring to FIGS. 3(A) to 3(C), the light intensity on the semiconductor wafer of the exposure light transmitting through attenuating type auxiliary phase shifting pattern 3 will be described.

Referring to FIG. 3(A), phase shifter 34 and transmitting portion 37 are formed into predetermined shapes on photomask substrate 4 with the planar area of the phase shifter 34 represented by (S.sub.H) and the planar area of transmitting portion 37 represented by (S.sub.0).

Referring to FIG. 3(B), the electric field of the exposure light directly under photomask substrate 4 has the transmittance and the phase thereof converted according to patterns formed. As to the electric field on the exposed material of the exposure light transmitted through transmitting portion 37 and phase shifter 34, when optical images are projected through a demagnification lens, as shown in FIG. 3(C), the electric field on the exposed material of the exposure light transmitted through transmitting portion 37 has an approximately constant value as shown by curve f, while the electric field on the exposed material of the exposure light transmitted through phase shifter 34 also has an approximately constant value as shown by curve g similarly to curve f, because the patterns are below the limit of resolution and images are overlapped with each other.

Thus, as shown in FIG. 3(D), the intensity of the exposure light on the exposed material is reduced, since electric fields shown by curve f and curve g are canceled with each other.

If absolute values of electric fields shown by curve f and curve g are equal, the light intensity on the exposed material can be minimized.

The magnitude of electric fields shown by curve f and curve g can be determined respectively according to the relationship between planar area (S.sub.H) of phase shifter 34, planar area (S.sub.0) of transmitting portion 37, and transmittance (T) of phase shifter 34.

In FIG. 4, abscissa indicates the S.sub.0 /S.sub.H ratio, and ordinate indicates the ratio I/I.sub.0 (%) of the light intensity on the wafer.

The solid line represents the case when the transmittance of the phase shifter film is T=0.075 (7.5%), while the dotted line represents the case when the transmittance of the phase shifter film is T=0.12 (12%).

As can be seen from FIG. 4, the minimum value is obtained with S.sub.0 /S.sub.H =0.25 when the phase shifting film having the transmittance T=7.5% is used, while the minimum value is obtained with S.sub.0 /S.sub.H =0.35 when the phase shifter film having the transmittance T=12.0% is used, and the I/I.sub.0 ratio becomes not more than 0.1% in either case.

Accordingly, in order to attain the ratio I/I.sub.0 of not more than 3%, the relationship between planar area (S.sub.0) of phase shifter 34, planar area (S.sub.H) of the transmitting portion 37, and transmittance (T) of the phase shifter 34 should satisfy the following relationship:

The acceptable range of the value .sqroot.T will be described.

FIG. 5 is a graph in which abscissa indicates transmittance T% of the phase shifter 34, and ordinate indicates the value a when the range of planar area (S.sub.0) of phase shifter 34 is set to (1-a) S.sub.0 .about.(1+a) S.sub.0.

Since there is a correlation between S.sub.0 and S.sub.H, the value of S.sub.H changes according to the value of S.sub.0.

Consequently, the ratio S.sub.0 /S.sub.H can be represented by the following equations: ##EQU1##

Therefore, it holds: ##EQU2##

Next, attenuating type auxiliary phase shifting pattern 3 will be described in case, for example, where i-line (.lambda.=365 nm) is used as the light source and the demagnification exposure apparatus having 1/5 of demagnification ratio of the lens with NA=0.54 and k.sub.1 =0.4 is used.

Referring to FIG. 2, square patterns are formed at a pitch of 2.0 .mu.m with the size of an aperture of transmitting portion 37 being 1.0 .mu.m.times.1.0 .mu.m. The attenuating type auxiliary phase shifting pattern 3 is formed to have the width of 1500 .mu.m at the entire periphery of the attenuating type phase shifting pattern 2, as shown in FIG. 1.

Phase shifter 34 has a two-layered structure including a SiO.sub.2 film 2b controlling the phase and a Cr film 2a controlling the transmittance, of which phase shifting angle is set to 180.degree. and the transmittance is set to 12%.

The limit of resolution of the above-mentioned demagnification projection exposure apparatus is set to 0.4 .mu.m on the semiconductor wafer, while the limit is set to 2.0 .mu.m on the photomask which is five times that on the semiconductor wafer. Thus, attenuating type auxiliary phase shifting pattern 3 is sufficiently smaller than the limit of resolution.

Under the above-mentioned conditions, S.sub.0 /S.sub.H equals 0.33, and a equals 0.25 when transmittance is 12% according to FIG. 5, so that when we substitute these values, equation (4) would be as follows:

Since the value of .sqroot.T equals 0.346, the relationship of equation (4) can be satisfied and the relationship of equation (2) can be almost satisfied.

Exposure utilizing attenuating type phase shifting mask 1 having the above-described structure will be described below.

FIG. 6(a) shows an attenuating type phase shifting mask 1 wherein an attenuating type auxiliary phase shifting pattern 3 satisfying the above conditions is formed at the entire periphery of attenuating type phase shifting pattern 2.

FIG. 6(b) is a sectional view showing the position of attenuating type phase shifting mask 1 relative to a blind 7 of the exposure apparatus. FIG. 6(c) shows an electric field of the light directly under a photomask substrate. FIG. 6(d) is a graph showing the light intensity on the exposed material.

Beams of light transmitted through attenuating type auxiliary phase shifting pattern 3 do not form any images on the semiconductor wafer since the pattern is smaller than the limit of resolution, and at the same time, beams of light transmitted through the transmitting portion and beams of light transmitted through the phase shifter are canceled with each other resulting in the light intensity of not more than 3% of the light intensity before transmitting through the pattern, so that regions other than the exposure region are not exposed.

Thus, exposure can be carried out satisfactorily even in the case of successive exposure as before.

Next, description will be made on a method of manufacturing the attenuating type phase shifting mask with referring to FIGS. 7-11. Referring to FIG. 7, chromium film 23a having a thickness of about 200 .ANG. is formed on a quartz glass substrate 4. SiO.sub.2 film 23a having a thickness of about 4000 .ANG. is formed on chromium film 23a. An electron beam resist film (e.g. ZEP-810) 25 having a thickness of about 5000 .ANG. id formed on SiO.sub.2 film 23b.

Referring to FIG. 8, EB (Electron Beam) lithography is conducted on electron beam resist film 25 using a variable shaped electron beam exposure apparatus (e.g. NEC JBX-7000MV, 6AIII). EB lithography is executed as below in order to obtain an accurate attenuating type auxiliary phase shifting pattern.

More particularly, an exposure pattern of the attenuating type auxiliary phase shifting pattern is set to have dimensions smaller than those of the finished pattern, or a dosage of electron beams directed to the attenuating type auxiliary phase shifting pattern region is made greater than that directed to the attenuating type phase shifting pattern region, or both steps are carried out. For instance, attenuating type phase shifting pattern region 200 is exposed at a dosage of electron beams within the range of 8-10 .mu.c/cm.sup.2. In order to obtain apertures each having dimensions of 1 .mu.m.times.1 .mu.m .quadrature. at intervals of 2 .mu.m through exposure of attenuating type auxiliary phase shifting pattern region 300 by EB lithography, an exposure pattern should have dimensions of 0.8 .mu.m.times.0.8 .mu.m .quadrature.. Electron beams are directed at a dosage of 10-12 .mu.c/cm.sup.2 in an overdosed manner, whereby apertures having dimensions of 1 .mu.m.times.1 .mu.m .quadrature. can be obtained. Electron beam resist film 25 is developed after EB lithography is completed, and then a predetermined pattern is obtained.

Referring to FIG. 9, SiO.sub.2 film 23b is etched using electron beam resist film 25 having the predetermined pattern formed thereon as a mask. Etching is conducted with a magnetron RIE apparatus using CHF.sub.3 +O.sub.2 (CHF.sub.3 : O.sub.2 =90:10) as an etching gas with an RF power of 200 W, a magnetic field of 100 G, and a gas pressure of 50 mtorr. Whereby a SiO.sub.2 film 2b having a predetermined shape of pattern is formed at attenuating type phase shifting pattern region 200, and a SiO.sub.2 film 3b having a predetermined shape is formed at attenuating type auxiliary phase shifting pattern region 300.

Referring to FIG. 10, chromium film 23a is etched using again electron beam resist film 25. As in the case of above-mentioned SiO.sub.2 film, etching is conducted with the magnetron RIE apparatus using Cl.sub.2 +O.sub.2 (Cl.sub.2 : O.sub.2 =80:20) with an RF power of 100 W, a magnetic field of 100 G and gas pressure of 50 mtorr. Thus, chromium film 2a of a predetermined shape is formed at attenuating type phase shifting pattern region 200, and chromium film 3a of a predetermined shape is formed at attenuating type auxiliary phase shifting pattern region 300. Next, referring to FIG. 11, electron beam resist film 25 is removed, and thus attenuating type phase shifting mask 1 according to the present embodiment is completed.

In this embodiment, although a two-layer structure of SiO.sub.2 film for controlling phases and chromium film for controlling a transmittance is employed, a single layer film made of one kind of material selected from a group comprising chromium oxide, chromium nitride oxide, nitride carbide oxide of chromium, molybdenum silicide oxide, and nitride oxide of molybdenum silicide may be used to control the phase and transmittance to predetermined values. In this case, chromium oxide and the like can be thinner to have a film thickness of about 1200-1600 .ANG. compared with the above mentioned two-layer structure, thereby facilitating the formation of the phase shifting pattern.

The second embodiment according to the present invention will be described.

In the first embodiment, square patterns each having the size of 1.0 .mu.m.times.1.0 .mu.m have been described as patterns for the attenuating type auxiliary phase shifting pattern. In such square patterns, the phase shifter film is provided integrally so that it has a superior adhesion to the photomask substrate. However, manufacturing the photomask, the electron beam lithography is time consuming since a large number of transmitting portions should be formed. Especially, the time taken for manufacturing becomes longer in the lithography system utilizing a variable shaped beam, since it depends on the shape of patterns.

In order to shorten the time taken for lithography, attenuating type auxiliary phase shifting pattern 3 is formed to have lines and spaces with transmitting portions 37 and phase shifters 34 being linear and arranged alternately as shown in FIG. 12. In this case, the pattern 3 is formed at the entire periphery of attenuating type phase shifting pattern 2 with the width of the transmitting portion being 0.5 .mu.m, as in the first embodiment. The cross sectional view along line Z--Z of FIG. 12 is similar to the cross sectional view shown in FIG. 3(A).

The value of S.sub.0 /S.sub.H in the attenuating type phase shifting mask having the above-mentioned structure is 0.35 satisfying .sqroot.T=.sqroot.0.12.apprxeq.0.33, so that equations (2) and (4) can be satisfied.

Therefore, the similar effect can be obtained as in the first embodiment by utilizing the attenuating type phase shifting mask according to the second embodiment.

An attenuating type phase shifting mask according to the third embodiment of the present invention will be described.

In the above-described first and second embodiments, the attenuating type auxiliary phase shifting pattern is provided at the entire periphery of the attenuating type phase shifting pattern. However, if the transmittance of the absorption type shifter film forming the phase shifter is small, for example, 5-8%, exposure of regions other than the desired regions once by the light transmitted through the phase shifter may be acceptable. Thus, when the transmittance of the phase shifter is small, problem arises only at the region 32 shown in FIG. 21.

Accordingly, the attenuating type auxiliary phase shifting pattern are provided only in the vicinity of four corners of the attenuating type phase shifting pattern in the present embodiment.

FIG. 13(A) shows an attenuating type phase shifting mask 1 according to the third embodiment when viewed from the pattern formation surface, where an attenuating type auxiliary phase shifting pattern 3 is provided only in the vicinity of four corners of attenuating type phase shifting pattern 2.

In the present embodiment, the phase shifter of the attenuating type phase shifting pattern is formed of an absorption type shifter film having the transmittance T of 7.5%, and the phase shifter is formed with the length of one side being 2000 .mu.m at each of the four corners of the shifting pattern. In each phase shifter, square patterns having apertures of 0.9 .mu.m.times.0.9 .mu.m each has the transmitting portion 37 are formed at a pitch of 2.0 .mu.m.

Under the above-mentioned conditions, S.sub.0 /S.sub.H equals 0.25, and a equals 0.3 if the transmittance is 7.5% according to FIG. 5, and when we substitute these values, equation (4) would be as follows:

Since .sqroot.T is 0.27, equation (4) can be satisfied and also equation (2) can be almost satisfied.

When exposure is carried out utilizing attenuating type phase shifting mask 1 according to the third embodiment, even if the amount of exposure light is four times the most suitable amount of exposure light, reduction of the thickness of the resist film corresponding to four corners of the attenuating type phase shifting pattern is not observed.

The fourth embodiment according to the present invention will be described.

In an attenuating type phase shifting mask 1 according to the fourth embodiment, patterns formed on the attenuating type auxiliary phase shifting pattern in the third embodiment are changed into lines and spaces, as shown in FIG. 13(C), with transmitting portions 37 and phase shifters 34 being linear and arranged alternately. The width of transmitting portion 37 is 0.5 .mu.m and the pitch is 2.0 .mu.m. Since S.sub.0 /S.sub.H equals 0.25 satisfying .sqroot.T=.sqroot.0.075=0.27, equations (2) and (4) can be satisfied.

In the fourth embodiment, similar effect can be obtained as in the third embodiment.

In the above-described embodiments, dimensions of the peripheral pattern of the exposure region sometimes become smaller as shown in FIG. 14 or narrower as shown in FIG. 15 during exposure of the attenuating type auxiliary phase shifting pattern. This is because the amount of electron beams directed to the peripheral pattern portion of the exposure region is insufficient due to the proximity effect of exposure using electron beams.

In order to avoid such a problem, a dimensional bias (+0.1-0.3) is added to the data of the electron beam exposure at the time of exposing the peripheral pattern with electron beams.

Rectangular and linear shapes have been employed for the attenuating type phase shifting pattern, but circular or polygonal shapes may be utilized to achieve a similar function and effect as long as those shapes satisfy S.sub.0 /A.sub.h nearly equal .sqroot.T.

Although the attenuating type auxiliary phase shifting pattern has been provided at the periphery of the chip in the above embodiment, the attenuating type auxiliary phase shifting pattern may be provided within the chip if the light shielding portion is necessary, by applying the similar method.

The exposure method utilizing the attenuating type phase shifting mask according to the above-described embodiments can be utilized effectively in the manufacturing process of DRAM, SRAM, flash memory, ASIC, microcomputer, and a semiconductor device such as GaAs of 4M, 16M, 64M, or 256M. Further, such a method is sufficiently applicable to a semiconductor device or a manufacturing process of a liquid crystal display.

According to the attenuating type phase shifting mask of the present invention, beams of light transmitting through the attenuating type auxiliary phase shifting pattern do not form any images on the semiconductor wafer since the pattern is smaller than the limit of resolution. Since beams of light transmitted through the transmitting portion and beams of light transmitted through the phase shifter overlap with each other and the phase of the light transmitted through the phase shifter is reversed, those beams of light are canceled with each other, whereby the light intensity on the semiconductor wafer can be reduced. Therefore, exposure of the portion other than the exposure region is prevented, the quality of exposure at the time of manufacturing the semiconductor device is improved, and the yield of manufacturing the semiconductor device can be improved.

Also, by setting values of planar area (S.sub.0) of the transmitting portion, and planar area (S.sub.H) of the phase shifter, and transmittance (T) of the phase shifter of the attenuating type auxiliary phase shifting pattern to predetermined values it becomes possible to control the light intensity on the semiconductor wafer by adjusting the intensity of the light transmitting through the phase shifter and the intensity of the light transmitting through the transmitting portion.

On the other hand, by setting S.sub.O /S.sub.H, which is a ratio of planar area (S.sub.0) of the transmitting portion to planar area (S.sub.H) of the phase shifter of the attenuating type auxiliary phase shifting pattern, to be approximately equal to the value of .sqroot.T of transmittance (T) of the phase shifter, the light intensity on the semiconductor wafer can be controlled to be not more than 3% of the light intensity before transmitting through the pattern.

Further, the auxiliary phase shifting pattern is provided at the entire periphery of the attenuating type phase shifting pattern. Thus, even if a plurality of portions are exposed successively utilizing the attenuating type phase shifting mask, other regions are not exposed so that satisfactory exposure can be carried out.

Further, the auxiliary phase shifting patterns are provided only in the vicinity of four corners of the attenuating type phase shifting pattern having a rectangular shape.

Thus, when exposure is carried out successively in order on the semiconductor wafer utilizing the attenuating type phase shifting pattern, any region is not exposed more than once by the light transmitting through the periphery of the attenuating type phase shifting pattern, so that satisfactory exposure can be carried out.

The shape of patterns having apertures in the transmitting portion formed on the attenuating type auxiliary phase shifting pattern is rectangular. Thus, adhesive property of the phase shifting pattern to the photomask substrate is improved so that the attenuating type phase shifting mask having a highly reliable structure can be provided.

The transmitting portions and the phase shifters formed on the attenuating type phase shifting pattern are linear and arranged alternately. Thus, the attenuating type auxiliary phase shifting pattern can be formed easily so as to facilitate manufacturing of the attenuating type phase shifting mask.

According to a method of manufacturing an attenuating type phase shifting mask, any images cannot be formed on a semiconductor wafer by the light transmitted through an attenuating type auxiliary phase shifting pattern because the resolution of the pattern is smaller than the limit of resolution of the exposure apparatus. Further, since beams of light transmitted through a transmitting portion and beams of light transmitted through a phase shifter portion overlap with each other and are in reverse phases, those beams of light cancel with each other due to interference, whereby the light intensity on the semiconductor wafer can be reduced. This prevents exposure of portions other than exposure region during conducting exposure, improves exposure state during manufacturing of a semiconductor device, and further improves an yield of manufacturing of the semiconductor device.

More preferably, forming the attenuating type phase shifter film includes the step of forming a film of a kind selected from a group comprising chromium oxide, chromium nitride oxide, nitride carbide oxide of chromium, molybdenum silicide oxide, and nitride oxide of molybdenum silicide.

Thus, the number of manufacturing steps of the attenuating type phase shifting mask can be reduced because the attenuating type phase shifter film is formed of one kind of film, so that the manufacturing cost of the attenuating type phase shifting mask can be reduced.

Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.