GB2307303A

GB2307303A - Apparatus and method for projection exposure

Info

Publication number: GB2307303A
Application number: GB9622640A
Authority: GB
Inventors: Jae-Kwan Song; Sang-Young Moon; Kyoung-Shin Park; Ho-Jong Song
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 1995-11-14
Filing date: 1996-10-30
Publication date: 1997-05-21
Also published as: JPH09148244A; CN1152136A; DE19643952A1; KR970028856A; GB9622640D0

Description

2307303 APPARATUS AND METHOD FOR PROJECTION EXPOSURE The present invention

relates to an apparatus and method for projection exposure. It has particular application to an apparatus for projection exposure in which arrangement of multiple apertures enhances uniformity of light intensity and ensures a process margin, and to a corresponding method.

In the manufacture of a semiconductor device, the quantities of impurities which are implanted into fine regions of a silicon substrate should be precisely controlled. It is well known that patterns for defining the fine regions are formed by photolithography.

By photolithography, a film in which a pattern is to be formed, for example, an insulation film or a conductive film on a semiconductor substrate, is coated with a photosensitive film, and then a predetermined portion of the photosensitive film is exposed to light (e.g. ultra violet), electron beam, or X-ray. A portion which has high solubility with respect to a developer is then eliminated from the photosensitive film to form a photosensitive film pattern. Subsequently, a film where a pattern is to be formed is etched, using the photosensitive film pattern as a mask, to form various kinds of patterns such as an interconnection and an electrode.

In manufacture of a semiconductor device by photolithography, methods using a phase shift mask and an offaxis illumination method are most widely used for forming a pattern of a size smaller than the minimum size that can be 1 is formed by photolithography. The off-axis illumination method is regarded as a most effective method for manufacturing a device having integration equal to or higher than 256M DRAM.

The off -axis illumination method, in which only zero- and first- order diffracted light tilted by an aperture is incident on a wafer, enhances resolution and depth of focus. By the off -axis illumination method, comparing to a usual illumination method, resolution can theoretically be enhanced by 1.5 times. This is based on the following formula.

R1 = X 2 (NA) R2 = X 2 (NA+sine) where R1 indicates resolution with conventional illumination, R2 indicates the resolution with off-axis illumination, X indicates the wavelength of a light source, NA indicates numerical aperture, and e indicates the angle of incidence of light which is incident on the wafer. When both an identical wavelength and an identical numerical aperture are used, the resolution with off-axis illumination is approximately 1.5 times higher than that with conventional illumination.

FIG. 1 shows schematically a projection exposure apparatus for conventional illumination.

The projection exposure apparatus for conventional llumination comprises a light source (not shown) for generating light, a fly's eye lens 2 for controlling intensity of the light generated by the light source, an aperture 4a for transmission of incident light from the fly's eye lens in a predetermined form, a condenser lens 6 for writing the light which has passed through the aperture on a photomask, a 2 photomask 8 for selectively transmitting the incident light from the condenser lens, a projection lens 10 for controlling magnification of the light which passes through the photomask and for then focusing the light on a wafer, and a wafer 12 on which a pattern is formed by the light focused by the projection lens.

The zero order diffracted light which has passed through the photomask 8, is incident onto the projection lens 10 in parallel with the light axis (vertical incidence component).

+1st and -1st order diffracted light is incident onto the projection lens 10 at an angle of OrD (off-axis incident component). Accordingly, the zero-, +1st and -1st order diffracted light components interfere with one another on the wafer 12, and the distribution of incident light intensity is thereby changed. Owc designates the angle of incidence of light incident on the wafer 12.

The distribution of light intensity on the wafer 12 is shown in FIG. 2.

(a) of FIG. 2 shows the distribution of intensity of the light which is incident onto the projection lens 10 through a photomask T1 of FIG. 1. (b) of FIG. 2 shows distribution of light intensity on the wafer 12 of FIG. 1.

The light intensities on the wafer 12 of FIG. 1 are formed so that a maximum point of the highest light intensity is clearly distinguished from a minimum point of the light intensity of 11011. As the pattern of the photomask 8 is finer, the diffracted angle OrD becomes larger. Accordingly, when sinOrD is larger than the numerical aperture, diffracted light 3 other than the zero-order diffracted light deviates away from the projection lens 10. As a result, only the zero-order diffracted light reaches the wafer 12, so that interference does not occur in the wafer. Here, minimal resolution is as follows.

R =;L 2 MA where X represents the wavelength of the incident light, and NA represents the numerical aperture.

FIG. 3 shows schematically a conventional off-axis is illumination apparatus.

An exposure portion of t-he aperture 4b is different from the aperture 4a of FIG. 1, i.e., of a conventional illumination apparatus. The light transmission region of the aperture 4a of the usual illumination apparatus corresponds to a central portion of the condenser lens 6. However, the projection region of the aperture 4b of the off-axis illumination apparatus corresponds to an edge portion of the condenser lens 6.

Light diverging from fly's eye lens 2 passes through the aperture 4b, and then will be incident onto the edge portion of the condenser lens 6 and in parallel with the optical axis. Subsequently, the incident light is refracted according to the refraction index of the condenser lens 6 while passing through the condenser lens 6, and then the refracted light is incident onto the photomask 8 at an angle of 6r to the optical axis. The light incident onto the photomask 8 has a diffraction angle, which is determined by a distance of a pattern formed 4 on the photomask - The diffracted light of -1st or higher orders deviates away from the projection lens 10, because the numerical aperture of the photomask 8 is larger than sin20r. Accordingly, only the zero- and 1st order diffracted lights reach the wafer 12, and they interfere with each other to change the distribution of light intensity. The changed distribution of light intensity is shown in FIG. 4.

or is the refractive angle of light at the condenser lens, 0 rp is the refraction angle of light at the photomask, and OCO and 0 WP are angles of incidence of light incident on the wafer 12.

Referring to FIG. 4, (a) shows distribution of the intensity of the light which is incident onto a photomask 8, and (b) shows distribution of light intensity on the wafer 12. Here, a difference between a maximum point and a minimum point is large, and the distribution of light intensity is not uniform.

The off -axis illumination can focus only diffracted light having an effective component, so that both the resolution and the depth of focus are increased.

However, since a central portion of an aperture in the conventional offaxis illumination is blocked up, the light intensity is low. Here, more exposure time is necessary to increase in the light intensity, so that productivity is affected. It is also known that the conventional off-axis illumination is not good for formation of a contact pattern. That is, in order to secure an align margin for manufacturing a second generations or higher high-integrated device of 64M DRAM grade, a contact hole of 0.3gm or less is required. However, this cannot be performed by the above-mentioned conventional offaxis illumination.

In such conventional off -axis illumination, the resolution and pattern profile deteriorate in an edge portion of a line/space pattern due to the proximity effect. The depth of focus of both an isolation pattern and an oblique pattern is close to or lower than that of the conventional illumination.

is Accordingly, it is a first object of the present invention to provide an apparatus of projection exposure in which uniformity of light intensity can be enhanced.

It is a second object of the present invention to provide a method of projection exposure in which a fine pattern can be effectively formed by the apparatus.

In a first aspect of the invention, there is provided an apparatus of projection exposure comprising a light source, a fly's eye lens for controlling intensity of the light generated from the light source, apertures arranged at least more than two for diffracting the light to have passed through the fly's eye lens by twice or more times, a condenser lens for admitting the light which has passed the aperture, a photomask for selectively passing the incident light from the condenser lens, a projection lens for focusing the light which has passed the photomask and a wafer on which a pattern is formed by the light focused by the projection lens.

It is preferable that the remoter the aperture is located from the fly's eye lens, the smaller the light transmission 6 is region thereof is. It is further preferable that for the apertures, two or more quadrupole -shaped or annular- shaped apertures can be used, or a combination of apertures of each different shape can be used. Also, it is preferable that the aperture comprises eight or more light transmission regions.

In a second aspect of the invention, there is provided a method of projection exposure comprising the steps of:

diffracting the light generated from a light source by at least two apertures; obliquely entering the diffracted light to a photomask; and focusing the light which has passed the photomask on a waf er.

It is preferable that the light generated from a light source is partly blocked when passing each aperture.

According to aspects of the present invention, increase in the number of the apertures and aperture slits can enhance uniformity of light intensity. Thus, a mask pattern can be satisfactorily realized on the wafer. Further, reduction in changes of a pattern size and a profile can increase process margin.

Specific embodiments of the present invention are described in detail below, by way of example, with reference to the attached drawings, in which:

FIG. 1 shows schematically a conventional illumination apparatus; FIG. 2 shows distribution of light intensity on a wafer 7 is in a conventional illumination apparatus; FIG. 3 shows schematically a conventional off-axis illumination apparatus; FIG. 4 shows distribution of light intensity on a wafer in conventional off-axis illumination; FIG. 5 shows schematically a projection exposure apparatus according to an embodiment of the present invention; FIG. 6 shows distribution of light intensity on a wafer of the projection exposure apparatus according to an embodiment of the present invention; FIG. 7 comprises photographs by a scanning electron microscope showing contact patterns which are formed on a wafer using conventional off-axis illumination apparatus; and FIG. 8 comprises photographs by an electron microscope showing contact patterns which are formed on a wafer using a projection exposure illumination apparatus according to an embodiment of the present invention.

Referring to FIG. 5, a projection exposure apparatus according to an embodiment of the present invention includes a light source (not shown) for generating light, a fly's eye lens 32 for controlling the intensity of light which is generated by the light source, first and second apertures 34a and 34b f or transmitting light which is incident f rom the f ly, s eye lens 32 in a predetermined shape, a condenser lens 36 having a focal length of If,' for refracting incident light in parallel with the light axis after passing through the second aperture 34b so as to have a predetermined angle (Or Or O1r) 8 with respect to the light axis, a photomask 38 for selectively transmission of the incident light from the condenser lens 36, a projection lens 40 for focusing the light which was transmitted through the photomask 38 onto a semiconductor substrate by controlling the magnification of the light, and a wafer 42 on which a pattern is formed by the light focused by projection lens 40.

Oir and Olrp are the refraction angles of the first refracted light component and 02r and 62rp are the refraction angles of the second refracted light component with respect to the light axis at the photomask, and 0. and O.P are the angles of incidence of light incident at the wafer.

First, according to the embodiment of the present invention, two apertures between the fly's eye lens 32 and the condenser lens 36 are arranged in parallel with each other. A light transmission region of the second aperture 34b is smaller than that of the first aperture 34a. Accordingly, part of the diffraction components of the light which was transmitted through the first aperture 34a are blocked by the second aperture 34b, so that rectilinear propagation of light is increased.

Two (in the embodiment of the present invention) or more apertures can be used, such that the light transmission region is reduced for an aperture which is farther from the fly's eye lens 32, so that it is preferable that a part of diffracted components of the "front" aperture is interrupted by the "rear" aperture.

9 Here, two or more quadrupole apertures, two or more annular apertures, or a combination of a quadrupole aperture and an annular aperture can be used. In the case of two or more quadrupole apertures, the number of slits which the light can pass through may be eight or more.

The number of aperture slits, i.e., the projection regions, is twice or more as in conventional off-axis illumination.

The intensity of light transmitted by the aperture can be expressed as follows.

1 F21 = (sin2 N7r6) / (sin2 7r6)... (1) where 'IN" indicates the number of slits.

The phase difference of the light can be expressed by the following formula.

6 = d / X (sin61 + sin82 (2) As noted from the formulae (1) and (2), the main maximum (first intensity of light) is sharper as the number 'IN" of slits increases, and resolution is therefore enhanced. Here, N is significantly greater than 1 and A6 is significantly less than 1, so that the phase difference of the light is smaller as the number of slits increase. That is, although the focus changes, the phase difference of the light is still small, so that depth of focus is deeper than for the conventional offaxis illumination. Moreover, changes in pattern size and pattern profile according to the focus change are small, which enhances margin in a manufacturing process.

Light passes through more slits than in conventional quadrupole aperture, so that the probability of composite waves of the wafer having maximum or minimum intensity is very low. In embodiments of the present invention, the probability of composite waves having maximum intensity or minimum intensity is as follows.

is d12sin012 M)12 (1) d14sin614 MX14 (3) d13sine.3 MX13 (2) d23sinO23 MX23 (4) d76sinO76 = MX76 (27), d7.sin07. MX78 (28) where 11d11 represents a distance between slits.

By formulas (1) through (28), the intensity of the composite wave on a wafer is maximized when dsinO is an integral multiple of the wavelength. Accordingly, probability that an arbitrary point on the wafer has the maximum intensity of the compositive wave is 1/2 28. Likewise, when dsine is an integral multiple of X/2, the arbitrary point on the wafer has the minimum intensity of the compositive wave. Here, the probability is 1/2 28. (In the case of the conventional off axis illumination, the probability that an arbitrary point on the wafer has the maximum intensity or the minimum intensity of the compositive wave is 1/2 6.) The light intensity, which is formed by overlapping light components each having a different wavelength, is continuously distributed between the maximum and the minimum of the composite wave on the wafer. Accordingly, the intensity of light distributed on the wafer is wider and thus more uniform than that in the conventional off-axis illumination.

11 Accordingly, the pattern on the photomask is exactly transcribed onto the wafer, so that resolution of the contact pattern is also enhanced.

Referring to FIG. 6, (a) shows the distribution of the intensity of light which is incident on the photomask 38, and (b) shows the distribution of the light intensity on a wafer 42 of FIG. 5. The light intensities are distributed on the is maximum point, the minimum point and a region therebetween by zero-order and first order diffracted light. Accordingly, the distribution of the light intensities is more uniform than that for the conventional case.

FIGS. 7 and 8 are SEM photographs of a contact pattern having a size of 0. 35gm. According to embodiments of the present invention, a pattern on a mask can be transcribed exactly onto a wafer so that excellent resolution can be obtained.

In the apparatus of projection exposure and a method thereof according to the present invention, first, two or more apertures are arranged between a fly's eye lens and a condenser lens. Accordingly, part of the light diffracted when the light passes each of the apertures is blocked by the other aperture, so that rectilinear propagation of light is enhanced.

Second, the number of light transmission regions of an aperture may be greater than for a conventional aperture, so that both resolution and uniformity of light intensity are enhanced.

Third, changes of a pattern size and a profile according to a change of a focus may be reduced, thereby enhancing 12 process margin.

It should be understood that the invention is not limited to the illustrated embodiment and that many changes and modifications can be made within the scope of the invention by a skilled person in the art.

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Claims

CLAIMS:

is 1. An apparatus for projection exposure comprising:

light source; fly's eye lens for controlling intensity of the light generated by said light source; at least two apertures for diffreaction of light after passage through said fly's eye lens; a condenser lens for passage of light which has passed through said at least two apertures, wherein the apparatus is arranged such that only light which has passed through at least two of said at least two apertures is admitted to said condenser lens; a photomask for selectively passage of the light incident from said condenser lens; a projection lens for focusing light which has passed through said photomask; and such that a pattern is formed on a wafer to be exposed by the light focused by said projection lens.
2. An apparatus for projection exposure according to claim 1, wherein the more remote an aperture of said at least two apertures is from said fly's eye lens, the smaller is the light transmission region of said aperture.
3. An apparatus for projection exposure according to claim 1 or claim 2, wherein said two or more apertures have quadrupole shape or an annular shape.
4. An apparatus for projection exposure according to claim 3, wherein said at least two apertures comprise a combination of a quadrupole aperture and an annular aperture.

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5. An apparatus for projection exposure according to claim 1, wherein said apertures provide eight or more light transmission regions.
6. An apparatus for projection exposure as claimed in claim 5, wherein said at least two apertures comprise two quadrupole apertures.
7. An apparatus for projection exposure substantially as herein described with reference to Figures 5 and 6 and with or without reference to Figure 8 of the accompanying drawings.
8. A method of projection exposure comprising the steps of:

diffracting light generated from a light source by passage through at least two apertures; directing said diffracted light to a photomask for oblique incidence; and focusing the light which has passed through the photomask onto a wafer.
9. A method of projection exposure according to claim 8, wherein on pasage through each aperture, a part of said light is blocked.
10. A method of projection exposure as claimed in claim 8 or 9 using an apparatus for projection exposure as claimed in any of claims 1 to 7.

is