Photomask and projection exposure mechanism using the same

A photomask for receiving light for exposure from a light source and projecting the emitted light onto a resist film on a wafer through an optical system so as to pattern the resist film includes a plurality of light transmitting portions for transmitting the light for exposure through a converging portion, and a convex portion made of a transparent or translucent material. The convex portion is formed protruding into the side on which the wafer is provided in order to cover the light transmitting portions so that the light for exposure is emitted as transmitted light which can form an image on the imaging plane of the wafer by utilizing refractive effects.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a photomask (reticle) to be used in a 
photolithography system, and more particularly to a photomask suited to 
efficiently transfer various fine patterns into different positions on a 
wafer. 
2. Description of the Related Art 
The photolithography technology of a reduced projection exposure method has 
approached the resolution limit with the reduction of design dimensions 
and the fineness of processing patterns. There has been used a phase 
shifting method for improving the resolution limit. According to the phase 
shifting method, the resolution limit is theoretically improved by about 
twice the resolution limit as compared with a conventional method. The 
phase shifting method has been described in IEEE Trans. on Electron 
Devices, Vol. ED-31, No. 6, 1984, pp. 753 to 763, for example (see FIG. 
31). 
However, the phase shifting method has the following drawbacks. That is, 
the phase shifting method is difficult to apply to a complex fine pattern 
such as an actual LSI pattern. More specifically, the phase shifting 
method is easy to apply to a regular line and space pattern. However, the 
phase shifting method is hard to apply to an isolated fine pattern, an 
aperiodic pattern, a plurality of combined patterns and the like. 
For the plurality of combined patterns, there has been proposed a 
multistage phase shifting method described in Extended Abstracts of 51th 
Autumn Meeting of the Japan Society of Applied Physics (1990), pp. 491 and 
492, Lecture No. 27p-ZG-4,5, for example. For the isolated pattern, there 
has been proposed an auxiliary pattern method disclosed in Japanese 
Unexamined Patent Publication No. 62(1987)-67514, for example. However, 
the above-mentioned methods are not easy to apply in practice. 
There has also been known a phase shifting method in which a phase shifter 
243 is provided only around a transmitting portion (see FIG. 30). This 
phase shifting method is a so-called self-aligning type (ex. edge 
enhancement, rim, etc.) and is effective in the resolution of the isolated 
pattern. In FIG. 30, a self-aligning type of phase shifting reticle 
sequentially includes a glass substrate 241 as a reticle and the phase 
shifter 243 from the light source side to the wafer side. The phase 
shifter 243 is provided through a Cr (chromium) film 242. A method using a 
self-aligning type phase shifter has been described in Extended Abstracts 
of 51th Autumn Meeting of the Japan Society of Applied Physics (1990), pp. 
492, Lecture No. 27p-ZG-2, for example. Also in the self-aligning type of 
phase shifter, however, there is utilized the interference effects of 
light in a light transmitting portion. Consequently, the gradient of a 
light intensity is lowered, so that the self-aligning type of phase 
shifter is hard to apply to fine patterns. 
When the processing patterns are made finer so as to have almost the same 
size as an exposure wavelength or less, a quantity of light transmitted 
through a reticle light transmitting portion is decreased. Consequently, 
the effects of oblique incident components cannot be ignored for vertical 
incident components (see FIG. 23). FIG. 24 shows the arrangement of an 
optical system. In FIG. 24, light emitted from a light source 221 passes 
through a condenser lens 222. The convergent light is incident on a 
photomask 21 within the range of an estimated angle .theta.c based on a 
focal position (for example, a central position represented by a point P2 
of an aperture 141 as a light transmitting portion shown in FIG. 14). 
Then, the convergent light is emitted as transmitted light with an 
estimated angle .theta.p through a light transmitting portion 223, is 
projected onto the plane of a projection lens 224 in the direction of an 
optical axis (in the direction of an arrow A), and is finally projected 
onto the imaging plane of a wafer. FIG. 16 schematically shows an optical 
system. In FIG. 16, the reference numeral 225 denotes an imaging plane on 
the wafer. A waveform B denotes the light intensity of a projected image. 
In the case where the self-aligning type of waveform expands and the light 
intensity on the imaging plane is decreased. Consequently, a light 
intensity contrast (gradient) is lowered so that a resist pattern cannot 
be resolved. Since the phase difference between adjacent transmitted light 
is utilized, the phase shifting method is hard to apply to the isolated 
fine pattern, the aperiodic pattern and the plurality of combined 
patterns. 
SUMMARY OF THE INVENTION 
The present invention provides a photomask (reticle) wherein the 
above-mentioned drawbacks can be eliminated in fabricating an LSI pattern 
which appears in actual exposure steps. 
More particularly, the present invention provides a photomask for receiving 
light for exposure from a light source and projecting the emitted light 
onto a resist film on a wafer through an optical system so as to pattern 
the resist film which comprises a plurality of light transmitting portions 
for transmitting the light for exposure through a converging portion, and 
a convex portion made of a transparent or translucent material and formed 
protruding into the side on which the wafer is provided in order to cover 
the light transmitting portions so that the light for exposure is emitted 
as transmitted light which can form an image on the imaging plane of the 
wafer by utilizing refractive effects. 
In another aspect, the present invention provides a photomask for receiving 
light for exposure from a light source and projecting the emitted light 
onto a resist film on a wafer through an optical system so as to pattern 
the resist film which comprises a plurality of light transmitting portions 
through which incident light is transmitted, and a distributed refractive 
index lens provided in order to cover the light transmitting portions for 
emitting the incident light as transmitted light which can form an image 
on the imaging plane of the wafer by utilizing refractive effects. 
The present invention provides as a mechanism having a photomask a 
projection exposure mechanism comprising a converging system for gathering 
light for exposure from a light source, a plurality of light transmitting 
portions through which incident light passing through the converging 
system is transmitted, a photomask including a convex portion or 
distributed refractive index lens which presents refractive effects for 
forming the incident light from the converging system as transmitted light 
which can form an image on the imaging plane of a wafer, an optical system 
for projecting the transmitted light from the photomask onto the wafer, 
and a supporting portion for supporting the wafer such that a wafer 
imaging plane is provided in parallel with a plane perpendicular to the 
optical axis of the light transmitted from the light source through the 
optical system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE PRESENT INVENTION 
According to an embodiment of the present invention, a convex portion 
(spherical or aspherical surface) is fabricated by using a material which 
transmits light to the center of a photomask light transmitting portion, 
and more propagation components of transmitted light are gathered on a 
projection lens (or an optical system) by utilizing the refractive effects 
of the light. Consequently, it is possible to improve a light intensity 
and a light intensity contrast on an imaging plane. More specifically, 
resolution is improved so that fine patterns can be transferred. Since the 
convex portion is a self-aligning type, it is easy to apply to an isolated 
pattern and an aperiodic pattern and it can be utilized for an actual LSI 
pattern. 
The convex portion is provided on the center of the light transmitting 
portion and serves to gather on the projection lens (or optical system) 
more light propagation components in the vicinity of the transmitting 
portion by utilizing the refractive effects. More specifically, if the 
material and convex portion (having a curvature C or partial curvature Cp) 
of the transmitting portion are optimized for an optical system, the 
distribution of a light intensity of projected images can be improved. 
FIG. 24 conceptually shows an optical system in the vicinity of a reticle 
provided on a general projection exposure device. Light emitted from a 
light source 221 is gathered by a condenser lens 222 so as to form a focal 
plane in the vicinity of the lower end of a photomask (reticle) 241. At 
this time, there are determined by the optical system of the exposure 
device an estimated angle .theta.c on the condenser lens 222 side and an 
estimated angle .theta.p on the projection lens 224 side based on a focal 
position. An effective light intensity necessary for the optical system 
has an upper limit. FIG. 23 conceptually shows light transmitted through 
the reticle 241. In the case where a conventional reticle is used, 
vertical incident components 211 can mostly be transmitted through a 
transmitting portion 223. In the case where of a reticle having a convex 
portion 212 provided on the center of the transmitting portion 223 is 
used, oblique incident components 213 having an estimated solid angle 
.PHI. (a solid angle based on the width .DELTA. of the transmitting 
portion 223) corresponding to the curvature C (or partial curvature Cp) 
can be utilized for the projected images of the light. When the pattern 
size of the reticle is reduced, the above-mentioned effects further become 
marked. As compared with the prior art, there will be described with 
reference to FIGS. 13, 14, 15, and 25 to 29 a process in which the 
vertical and oblique incident components are incident on a mask through 
the condenser lens and are then projected from the projection lens onto a 
wafer through the convex portion. 
FIGS. 25 to 29 show the case where a self-aligning type of phase shifter 
200 is used according to the prior art. 
As shown in FIG. 29, in the case where the oblique incident components (1), 
(2) and (3) of convergent light incident on a mask 201 have greater angles 
than the estimated angle .theta.c, transmitted light components, which are 
to be projected as transmitted light onto a projection lens 224 and 
correspond to the incident components (1), (2) and (3), are projected into 
positions (1a), (2a) and (3a). While the components (1), (2) and (3) pass 
through a central position (represented by a point P1) on the aperture 202 
of a transmitting portion, only the component (3) contributes to the 
image-formation on the point P1. The reference numeral 203 denotes a Cr 
plane. 
According to an embodiment of the present invention, a convex portion 142 
is provided to cover the aperture 141 of a transmitting portion as shown 
in FIG. 14. Consequently, it is seen that all the oblique incident 
components (11), (12) and (13) of convergent light passing through the 
central position (represented by a point P2) on the aperture 141 are 
projected into the positions (11a), (12a) and (13a) on the imaging plane 
of a projection lens 143 and contribute to the image-formation on the 
point P2. The reference numeral 144 denotes a Cr plane. 
The foregoing characteristics depend on the refractive effects of the 
convex portion 142. Referring to a self-aligning type of phase shifting 
reticle 200 (see FIGS. 25 to 27 and 30) according to the prior art, light 
passes through points P1, P2 and P3 in a mask 201 and is then emitted as 
transmitted light having an estimated angle .theta.p (see FIG. 28). 
According to an embodiment of the present invention, the convex portion 
142 is provided. As shown in FIG. 13, light passing through points P4, P5 
and P6 on the aperture 141 of a mask 100 is emitted from the convex 
portion 142 at estimated angles .theta.p+.beta. (.beta.&gt;0) and 
.theta.p+.beta.' (.beta.'&gt;0). The components which contribute to the image 
formation on the points P4, P5 and P6 have greater angles by the angles 
.beta. and .beta.' as compared with the points P1, P2 and P3 shown in FIG. 
28. 
There will be described the vertical incident components of convergent 
light. In the case where the mask 100 for an embodiment of the present 
invention is used as shown in FIG. 9, the vertical incident components of 
light emitted from a light source 91 and converged through a projection 
lens 92 are incident on the convex portion 142 formed on the aperture 141 
of the mask 100. Then, the vertical incident components are emitted as 
transmitted light by refractive effects and are finally projected onto a 
wafer 93 through a projection lens 143. 
In this case, the vertical incident components passing through points P7, 
P8, P9, P10 and P11 on the aperture 141 are first projected onto points 
P12, P13, P14, P15 and P16 on the projection lens 143 so as to become 
reduced optical images 99, and are then projected onto points P17, P18, 
P19, P20 and P21 on the wafer 93. The points P7 and P11 are positioned in 
edge portions on the mask 100. The points P7 and P11 correspond to points 
P21 and P17 which are positioned in edge portions on the wafer 93, 
respectively. 
Thus, the vertical incident components are projected onto the wafer 93. The 
vertical incident components thus projected overlap with the oblique 
incident components projected onto the wafer 93 as shown in FIG. 14. 
Consequently, there is determined a relative light intensity I(X) on the 
wafer as an imaging plane. As a result, the relative light intensity I(X) 
can further be improved. 
As shown in FIG. 9, a light intensity contrast on the imaging plane of the 
wafer is defined by a gradient (dI(X)/dXw) on the points P21 and P17 in 
the edge portions of the wafer 93 (Xw is equal to X) . Taking the point 
P17 in the wafer edge portion as an example in FIG. 9, the coordinate 
position Xw of the light intensity contrast is defined by a gradient 
(dI(X)/dXw) on the point P17 as shown in FIG. 10. Taking the point P21 in 
the wafer edge portion as an example, the coordinate position Xw is 
defined by a gradient on the point P21. In FIG. 10, the reference 
designation H denotes the curve of a relative light intensity I(X) 
according to an embodiment of the present invention. The reference 
designations J and K denote the curves of a relative light intensity I(X) 
according to the prior art. As seen from FIG. 10, the gradient of the 
curve H in the wafer edge portion according to an embodiment of the 
present invention is greater than those of the curves J and K. 
Accordingly, the light intensity contrast can further be increased. 
Consequently, good patterning can be performed. 
FIG. 8 shows the approximate value of the area of a transmitted image in 
the case where an embodiment of the present invention is applied to a step 
of forming a contact hole having a diameter of 0.3 .mu.m on a wafer. The 
transmitted image on an optical system incident plane is an image which is 
projected onto a plane perpendicular to an optical axis (a light 
propagation direction). 
In FIG. 8, a light transmitting portion 82 is formed on a mask 81. The 
light transmitting portion 82 covers a square aperture having a side of 
0.3 .mu.m, and is a square having a side D. By way of example, the light 
transmitting portion 82 has a side D of 1.66 .mu.m and an area S1 of 
1.66.times.1.66=2.7.quadrature. .mu.m.sup.2. The top plane 83a of an 
optical system 83 is provided apart from the mask 81 at an interval 
L.sub.MP of about 5 cm. An image 84 projected and incident onto the top 
plane 83a has a size of 2.25 to 2.76 .mu.m.sup.2, for example. An imaging 
plane 85 on a wafer is provided apart from the optical system 83 at an 
interval L.sub.PW of about 1 cm. An image 86 projected onto the imaging 
plane 85 has a size of 0.09 to 0.11 .mu.m.sup.2 A .1/5reduction optical 
system is used for the optical system 83. 
From the foregoing, it is seen that the light intensity contrast is 
improved. 
In FIG. 10, there are compared the relative light intensities I(X) of a 
phase shifter (photomask) having the convex portion (shown by the curve 
H), a conventional mask having no phase shifter (shown by the curve J) and 
a conventional self-aligning type of phase shifter (shown by the curve K, 
see FIGS. 26 and 27). FIG. 10 shows data obtained by setting the thickness 
R of a resist film 80 on a wafer (Si substrate) 93 to about 1 .mu.m and a 
pattern width W to about 0.30 .mu.m as shown in FIG. 12. In FIG. 10, an 
axis of abscissas Xw (.mu.m) is the lateral coordinate of the resist film 
80 shown in FIG. 12. A center O is provided in the middle point of the 
pattern width W (a position W/2). FIG. 11 shows the comparison of the 
variation of a resist height R for performing development by a developer 
111 so as to form a contact hole 70 with that of the prior art (shown by a 
curve F). According to an embodiment of the present embodiment (shown by a 
curve E), it is seen that the contact hole 70 having a width of 0.3 .mu.m 
is formed. According to the prior art, it is seen that the resist film is 
removed at a height of about 0.5 .mu.m so that the contact hole is not 
formed. FIG. 15 specifically shows the state at the time of exposure. FIG. 
12 specifically shows the state at the time of development. In FIG. 12, 
the reference numerals 121 and 122 denote a non-exposure portion and an 
exposure portion, respectively. It is seen that the surface form of the 
resist film 80 is varied in the order of a, b, c, d and e with the passage 
of developing time. 
According to an embodiment of the present invention, examples of a convex 
portion are a square pattern 71 shown in FIG. 1, a rectangular pattern 61 
shown in FIG. 2, and a pattern 51 obtained by combining a square and a 
rectangle as shown in FIG. 5. 
In addition, an example of a distributed refractive index lens 101 is shown 
in FIGS. 4 and 21. 
In FIG. 3, a refractive index distribution conceptually shows the state of 
the distribution of a refractive index of which a maximum point 
(n.sub.max) is taken as a center. The minimum point (n.sub.O) of the 
refractive index corresponds to a lens end. 
According to an embodiment of the present invention, the curvature or a 
plurality of partial curvatures of a convex portion 142 is(are) basically 
defined by a sectional shape in a central position. As shown in FIG. 1, 
the convex portion 142 having a square pattern is almost symmetrical with 
respect to a center. The sectional shape of the convex portion 142 taken 
along a dotted line (X.sub.M line) has a curvature Cs and a width W (see 
FIG. 6). These values are varied. As shown in FIG. 2, the convex portion 
142 having a rectangular pattern is almost symmetrical with respect to 
central axes 1 and 2 (dotted lines). A central section on the long side 
(L) has a curvature C.sub.L and a width W.sub.L (see FIG. 22). A central 
section on the short side (S) has a curvature C.sub.S and a width W.sub.S 
(see FIG. 7). These values are varied. 
FIG. 5 shows a photomask pattern 51 obtained by combining a rectangle ABCD, 
and squares BEFC and FGHC. In FIG. 5, the basic symmetry axes of the 
pattern 51 include an axis 1 of a partial pattern 51a defined by a 
rectangle AEFD and an axis 3 of a partial pattern 51b defined by a 
rectangle EGHB. 
In this case, the convex portion 142 is almost symmetrical with respect to 
the axes 1 and 3. More specifically, the sectional shape of the convex 
portion 142 taken along the axis 1 has a width W1 and partial curvatures 
C.sub.LU and C.sub.LDS as shown in FIG. 17. The width W1 is almost equal 
to a line segment AE. 
The sectional shape of the convex portion 142 taken along the axis 3 has a 
width W3 and partial curvatures C.sub.SL and C.sub.SR as shown in FIG. 18. 
The width W3 is almost equal to a line segment EG. 
The sectional shape of the convex portion 142 taken along the axis 2 as a 
junction of the rectangle ABCD and the square BEFC forming the rectangle 
AEFD has a width W2 and a curvature C2 as shown in FIG. 19. The width W2 
is almost equal to a line segment BC. 
The sectional shape of the convex portion 142 taken along an axis 4 as a 
junction of the rectangle AEFD and the square FGHC has a width W3 and a 
curvature C3 as shown in FIG. 20. The width W3 is almost equal to a line 
segment HG. When the values of W2, W3, C2 and C3 are varied, there can be 
obtained a combination type of photomask having a desired convex portion 
142. 
Embodiments of the present invention will be described with reference to 
FIGS. 1, 2, 5 and 4 which are the contour maps and the sectional view of a 
reticle to which the present embodiments are applied. FIG. 1 shows a 
square pattern (contact pattern) 71 having a side according to a first 
embodiment of the present invention. A convex portion is formed on a 
reticle transmitting portion by a transparent or translucent material 
(SiO.sub.2, SOG or the like). The convex portion is defined by a curvature 
C or partial curvature Cp within the range of manufacturing errors. In 
FIG. 1, the convex portion is defined by the entire curvature C, for 
example. 
FIG. 2 shows an example of a simple rectangular pattern 61 according to a 
second embodiment of the present invention. If a length in a long side 
direction is represented by L and a length in a short side direction is 
represented by S, the central section of the convex portion 142 is defined 
by a sectional shape on the long side taken along an axis 1 (see FIG. 22) 
and a sectional shape on the short side taken along an axis 2 (see FIG. 
7). More specifically, the sectional shapes on the long and short sides 
are defined by curvatures C.sub.L and C.sub.S, respectively. 
FIG. 5 shows a pattern 51 obtained by combining two rectangular patterns 
according to a fourth embodiment of the present invention. The sectional 
shapes of the convex portion 142 taken along axes 1, 2 and 4 are shown in 
FIGS. 17, 19 and 20, respectively. Each sectional shape is defined by the 
curvature C or partial curvature Cp. By way of example, the sectional 
shapes of the convex portion 142 taken along the axes 1 and 3 shown in 
FIG. 5 are defined by the partial curvatures C.sub.LU and C.sub.LDS and 
the partial curvatures C.sub.SL and C.sub.SR, respectively. FIGS. 19 and 
20 show the sectional shapes of the convex portion 142 taken along the 
axes 2 and 4 shown in FIG. 5, respectively. 
FIGS. 3, 4 and 21 show the state of a pattern 91 according to a third 
embodiment of the present invention, in which a distributed refractive 
lens (of a plane type) 101 having the same optical effects as in the 
convex portion is provided on a light transmitting portion 102. The 
distributed refractive lens 101 serves to refract the transmitted light 
with a continuous refractive index distribution (n.sub.0 to n.sub.max) in 
similar manner to the convex portion. 
FIGS. 10 and 11 show the simulated results of a light intensity 
distribution on an imaging plane, which are obtained by using a photomask 
having the convex portion 142. The reference designation H denotes the 
results obtained by using the photomask having the convex portion 142 
according to embodiments of the present invention. The reference 
designation J denotes the results obtained by using a photomask having no 
phase shifter. The reference designation K denotes the results obtained by 
using a self-aligning type of phase shifter according to the prior art. 
As the conditions of exposure, a wavelength .lambda. is set to 365 nm, a 
numerical aperture NA is set to 0.45, and a coherent factor .sigma. is set 
to 0.50 in order to form a contact hole having a diameter of 0.30 .mu.m 
(an actual diameter is 1.5 .mu.m because a 1/5reduction optical system is 
used). A curve H denotes a relative light intensity I(X) obtained in the 
case where the convex portion 142 (see FIG. 1) is provided on a square 
mask having a size of 0.04 .mu.m. A curve J denotes a relative light 
intensity I(X) obtained by using a square mask which has a size of 0.30 
.mu.m and has no phase shifter. A curve K denotes a relative light 
intensity I(X) obtained by using a self-aligning type of mask in which a 
shifter width on the periphery of an aperture 202 is 0.06 .mu.m (having a 
phase difference of .pi.) and which has a square central light 
transmitting portion 300 having a side of 0.28 .mu.m (having a phase 
difference of 0). As described above, a 1/5reduction optical system is 
used for the optical system. 
As shown in FIGS. 10 and 11, the convex portion is formed on the center of 
the light transmitting portion of a fine exposure pattern, so that a light 
intensity on the imaging plane and a light intensity contrast on the 
resist plane of a wafer as a target position can be increased. 
Consequently, it is possible to enhance resolution in a lithography 
process. The light intensity and light intensity contrast are varied 
according to the curvature C or partial curvature Cp of the convex portion 
provided on the light transmitting portion. Their optimum values depend on 
the manufacturing environment and an exposure device to be used. 
According to the embodiments of the present invention, the light intensity 
and light intensity contrast of the projected image can be enhanced by 
forming the convex portion on the light transmitting portion of the 
photomask. Consequently, the resolution in the lithography process can be 
improved. Thus, the fine patterns of an actual LSI, in particular, an 
isolated pattern, an aperiodic pattern and a plurality of combined 
patterns can effectively be transferred onto the wafer.