JPH04269749A - Photomask and its manufacture - Google Patents

Abstract

PURPOSE:To improve the accuracy of pattern transfer of a photomask for phase shift by constituting the phase shifter of a material having low refractive index. CONSTITUTION:A photomask M1 for phase shift has a phase shifter 2 comprising low-refractive index material such as fluorine-contained resin or the like. The phase shifter 2 is formed by applying a fluorine-contained resin on the surface of the photomask M1 by spin coating and then etching the resin.

Description

[Detailed description of the invention]

0001

[Field of Industrial Application] The present invention relates to a photomask used in the manufacturing process (wafer process) of semiconductor integrated circuit devices and its manufacturing technology, and particularly to a technology that is effective when applied to a phase shift photomask. It is.

0002

[Background Art] As the miniaturization of semiconductor integrated circuits progresses and the design rules for circuit elements and wiring become submicron order, integrated circuit patterns on photomasks are printed using light such as i-rays (wavelength 365 nm). In the photolithography process of transferring images onto semiconductor wafers, a decrease in pattern accuracy becomes a serious problem.

For example, a photomask M as shown in FIG.
When transferring a pattern consisting of the light-transmitting regions P1, P2 and the light-shielding region N1 formed on the wafer onto a wafer, a pair of light-transmitting regions P1,
As shown in FIG. 7, the two lights immediately after passing through each of P2 have the same phase, so on the wafer, the two lights overlap each other at the boundary, as shown in FIG. They interfere and strengthen each other. As a result, as shown in FIG. 9, the contrast of the projected image of the pattern on the wafer decreases and the depth of focus becomes shallow, resulting in a significant drop in transfer accuracy in the case of fine patterns. .

[0004] As a means to improve such problems, a phase shift technique that prevents a decrease in the contrast of a projected image by changing the phase of light passing through a photomask is attracting attention.

For example, in Japanese Patent Publication No. 62-59296, a transparent film (phase shifter) is provided on one side of a pair of light transmitting regions sandwiching a light shielding region on a photomask, and two A phase-shifting photomask has been disclosed that weakens the intensity of light at a boundary between two lights on a wafer by inverting the phases of the lights.

The above phase shift photomask is shown in FIG.
A light transmitting area P1 on a photomask M1 as shown in FIG.
When transferring the pattern consisting of P2 and the light-shielding area N1 onto a wafer, a phase shifter 2 made of a transparent film having a predetermined refractive index is provided on either one of the pair of light-transmitting areas P1 and P2 to block the light transmission. Immediately after passing through regions P1 and P2, the phases of the two lights are reversed (Fig. 1
1), the film thickness of the phase shifter 2 is adjusted as shown in FIG. As a result, on the wafer, as shown in FIG.
Since the two lights interfere with each other at the boundary and weaken each other, the contrast of the projected image of the pattern is improved, as shown in FIG.

Furthermore, in Japanese Patent Application Laid-Open No. 62-67514, a second minute light transmitting region is provided around the first light transmitting region on the photomask, and a second light transmitting region is provided in one of the above light transmitting regions. A phase shift photomask has been disclosed which improves pattern transfer accuracy by providing a phase shifter and mutually inverting the phases of two lights transmitted through the pair of light transmitting regions.

[0008] Furthermore, in Japanese Patent Application Laid-Open No. 2-140743, a phase shifter is provided in a part of the light transmitting region, and the phases of two lights transmitted through a portion with the phase shifter and a portion without the phase shifter are mutually inverted. A phase shift photomask has been disclosed that improves pattern transfer accuracy.

Conventionally, these phase shift photomasks have been manufactured by the following method. First, a glass substrate (mask blank) with a thin Cr film deposited on its entire surface.
After spin-coating an electron beam resist on top and forming a latent image of an integrated circuit pattern on the electron beam resist using an electron beam lithography device, the exposed portion of the electron beam resist was removed with a developer and exposed. A photomask is created by etching the Cr film.

Next, spin-on glass (Spin On Gl), which is a phase shifter material, is applied to the entire surface of the photomask.
After depositing a transparent thin film, such as (as), the thin film is left in predetermined light-transmissive areas of the photomask by etching the thin film using lithographic techniques.

In order to mutually invert the phases of the light transmitted through the light transmission region provided with a phase shifter and the light transmitted through the light transmission region not provided with a phase shifter, the wavelength of the light is set to λ, and the refractive index of the phase shifter is set. The film thickness (d) of the phase shifter is set to satisfy the relationship of d=an integer multiple of λ/2(n-1), where n is the thickness (d) of the phase shifter. For example, when the wavelength of light is 365 nm (i-line) and the refractive index of spin-on glass, which is a phase shifter material, is 1.5, the thickness of the phase shifter is set to 365 nm or an integral multiple thereof.

0012

However, according to the studies of the present inventors, the conventional phase shift photomask and its manufacturing method have the following problems.

First, when spin-on glass, which is a phase shifter material, is spin-coated onto a photomask, there are localized areas where the thickness of the spin-on glass becomes uneven due to differences in the surface of the photomask. There is a problem in that the desired phase shift effect cannot be obtained at certain points.

Furthermore, since the spin-on glass that is the phase shifter material is made of the same silicon oxide (SiO2) as the quartz glass that is the photomask material, it is difficult to etch the spin-on glass spin-coated onto the photomask. There is a problem that a suitable etching selectivity cannot be obtained. Therefore, in the actual manufacturing process, it is essential to provide a thin film for an etching stopper under the spin-on glass.

[0015] Furthermore, a common problem with phase shift photomasks and ordinary photomasks is that the light that has passed through the photomask during exposure may damage the surface of the wafer or the surface of the objective lens on the optical path connecting the photomask and the wafer. reflected by
Furthermore, because so-called multiple reflections (stray light), which are re-reflected on the surface of the photomask, occur, even areas on the wafer that are originally non-exposed are exposed to some extent, resulting in a difference in the pattern transferred onto the wafer. There is a problem that accuracy decreases.

The present invention has been made in view of the above-mentioned problems, and its purpose is to provide a technique for suppressing a decrease in pattern transfer accuracy caused by variations in the film thickness of a phase shifter. .

Another object of the present invention is to provide a technique for shortening the manufacturing process of a phase shift photomask.

Another object of the present invention is to provide a technique for suppressing deterioration in pattern transfer accuracy caused by multiple reflections (stray light) of light transmitted through a photomask.

The above and other objects and novel features of the present invention will become apparent from the description of this specification and the accompanying drawings.

[0020]

[Means for Solving the Problems] In the phase shift photomask of the present invention, the phase shifter is constituted by a transparent thin film made of a low refractive index material such as a fluororesin.

[0021]

[Operation] The wavelength of light incident on a medium with a refractive index of n becomes longer as n becomes smaller, according to the following formula λ=λ0/n (λ: wavelength in the medium, λ0: wavelength of incident light). That is, by forming the phase shifter with a low refractive index material, the wavelength of light transmitted through the phase shifter becomes longer, and the optical path length becomes shorter accordingly. This reduces the difference in optical path length between the light transmitted through the light transmission region provided with the phase shifter and the light transmitted through the light transmission region not provided with the phase shifter. Also, the variation in the phase difference between the two lights becomes smaller.

Furthermore, by constructing the phase shifter with a low refractive index material, multiple reflections (
Since stray light) is reduced, it is possible to suppress a decrease in pattern transfer accuracy caused by this.

[0023]

Embodiment FIG. 1 is a sectional view of a main part of a phase shift photomask M1 of this embodiment, and FIG. 2 is a plan view thereof. This phase shift photomask M1 is a reticle on which an original image of an integrated circuit pattern, for example, five times the actual size, is formed for transferring a predetermined integrated circuit pattern onto a wafer.

The phase shift photomask M1 is made of transparent synthetic quartz glass having a refractive index of about 1.47, for example.
A pair of light transmitting regions P1 and P2 surrounded by a light shielding region N1 are formed on the main surface of the light transmitting device. The light-shielding region N1 is constituted by a light-shielding film 1 made of, for example, Cr.

In order to form the light transmitting regions P1, P2 and the light shielding region N1, a glass substrate (mask blank) on which a light shielding film 1 such as Cr is deposited on the entire surface using, for example, sputtering is prepared, and the main surface thereof is After spin-coating an electron beam resist on the substrate, the glass substrate is irradiated with an electron beam using an electron beam lithography device, the exposed portion of the electron beam resist is removed with a developer, and the exposed light shielding film 1 is etched.

A phase shifter 2 made of a transparent thin film is provided in one (P1) of the pair of light transmitting regions P1, P2 of the phase shift photomask M1. Conventionally,
The phase shifter 2 was made of a silicon oxide material such as spin-on glass, but the phase shifter 2 of this embodiment is made of a fluororesin having a lower refractive index than spin-on glass.

An example of a fluororesin suitable for use in the phase shifter 2 is "CYTOP" (trade name, fluororesin manufactured by Asahi Glass Co., Ltd.). This fluororesin has a refractive index of 1.34, which is much lower than that of spin-on glass (refractive index≈1.5). In addition, it has excellent transmittance of approximately 95% for i-line (wavelength: 365 nm) and approximately 80% for KrF excimer laser (wavelength: 248 nm), and is also capable of spin coating with a film thickness of 1 μm or less. be.

Therefore, in order to form the phase shifter 2 in the light transmitting region P1 using the above fluororesin, this fluororesin is spun over the entire surface of the photomask on which the light transmitting regions P1, P2 and the light shielding region N1 have been formed. After coating and baking to harden, etching is performed. Since the fluororesin has high activity against oxygen-based gases, reactive ion etching using oxygen-based gases is preferable for the etching. According to reactive ion etching using this oxygen-based gas, fluorine-based resin can be etched with a high selectivity to quartz glass, which is a photomask material, so silicon oxide-based materials such as spin-on glass can be etched. Unlike the conventional technique in which a phase shifter is created by using the same method, a thin film for an etching stopper is not required.

Light transmission region P1 provided with phase shifter 2
and the light transmission area P2 without phase shifter 2.
In order to mutually invert the phases of the light transmitted through the phase shifter 2, the wavelength of the light is λ, the refractive index of the phase shifter 2 is n, and the phase shifter 2
The film thickness (d) may be set to satisfy the relationship of d=an integral multiple of λ/2(n-1). For example, the wavelength of light is 36
5 nm (i-line), and the refractive index of the fluororesin constituting the phase shifter 2 is 1.34, the film thickness of the phase shifter 2 is 5 nm.
It may be 37 nm or an integral multiple thereof.

By the way, when the above-mentioned fluororesin, which is the material of the phase shifter 2, is spin-coated on a photomask, the film thickness (d) of the phase shifter 2 is changed from the above-mentioned level due to the difference in level between the light-transmitting region P1 and the light-shielding region N1. As a result, the phase difference between the two lights transmitted through the pair of light transmitting regions P1 and P2 may deviate from 180 degrees.

However, since the refractive index of the fluorine-based resin is much lower than that of silicon oxide-based materials such as spin-on glass, the wavelength of the light transmitted through the phase shifter 2 made of this fluorine-based resin differs from that of silicon oxide-based materials. The wavelength is longer than the wavelength of the light transmitted through the phase shifter 2 made of material, and the optical path length is correspondingly shorter.

Therefore, the phase shift photomask M1 of this embodiment has a light transmitting region P1 provided with the phase shifter 2.
and the light transmission area P2 without phase shifter 2.
Even if the difference in the optical path length of the light transmitted through the pair of light transmission regions P1 and P2 is shorter than before, and there is variation in the film thickness of the phase shifter 2, the 180 degree difference in the phase difference between the two light transmitted through the pair of light transmission regions P1 and P2 It is possible to reduce the deviation from the

That is, according to the phase shift photomask M1 of this embodiment, the pattern transfer accuracy is improved compared to the conventional phase shift photomask in which the phase shifter is made of a silicon oxide based material such as spin-on glass.

FIG. 3 shows the optical system of the reduction projection exposure apparatus 3 equipped with the phase shift photomask M1 of this embodiment.

On the optical path connecting the light source 4 and the semiconductor wafer 5, a collimator lens 6 and a phase shift photomask M are provided.
1 and an objective lens 7 are arranged. The collimator lens 6 converts the light emitted from the light source 4 into parallel light, which enters the phase shift photomask M1. Pair of light transmitting regions P1, P of phase shift photomask M1
The two lights L1 and L2, whose phases have been reversed by passing through the wafer 2, are combined into one by the objective lens 7, and are incident on the surface of the semiconductor wafer 5 as a spot-shaped light beam.

At this time, a portion of the lights L1 and L2 is reflected by the surface of the semiconductor wafer 5 and the surface of the objective lens 7, and enters the lower surface of the phase shift photomask M1. However, since a phase shifter 2 with a low refractive index is provided on the bottom surface, the re-reflection rate of light on the bottom surface of the phase shift photomask M1 is lower than that of the conventional phase shifter provided with a phase shifter made of silicon oxide material. It is lower than that of a phase shift photomask.

That is, according to the phase shift photomask M1 of this embodiment, the influence of multiple reflections (stray light) can be reduced, so the pattern transfer accuracy is improved compared to the conventional phase shift photomask. .

[0038] Above, the invention made by the present inventor has been specifically explained based on examples, but the present invention is not limited to the above-mentioned examples, and can be modified in various ways without departing from the gist thereof. Needless to say.

The phase shift photomask of the above embodiment has a phase shifter formed on a light shielding film such as Cr. For example, as shown in FIG. The light shielding film 1 may be formed on the transparent film. In this case, a region on the transparent film where the light shielding film 1 is not provided becomes the phase shifter 2.

Furthermore, instead of forming a light-shielding region using a light-shielding film such as Cr, as shown in FIG. The light-shielding region N1 can also be formed by forming a repeating pattern. In this case, by depositing a transparent film with a low refractive index on the surface of the quartz glass and etching this transparent film using electron beam lithography, the light transmitting region P1, the light shielding region N1, and the phase shifter 2 are formed in the same process. can be created at the same time.

The low refractive index transparent film constituting the phase shift is not limited to the above-mentioned fluororesin.

[0042]

[Effects of the Invention] Among the inventions disclosed in this application, the effects obtained by the typical inventions will be briefly explained as follows.
It is as follows.

According to the phase shift photomask of the present invention in which the phase shifter is made of a low refractive index material, patterns due to variations in the film thickness of the phase shifter and multiple reflections (stray light) of light transmitted through the photomask can be formed. Deterioration in transfer accuracy can be suppressed.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a main part of a phase shift photomask according to an embodiment of the present invention.

FIG. 2 is a plan view of essential parts of this phase shift photomask.

FIG. 3 is a diagram showing an optical system of a reduction projection exposure apparatus equipped with this phase shift photomask.

FIG. 4 is a sectional view of a main part of a phase shift photomask according to another embodiment of the present invention.

FIG. 5 is a sectional view of a main part of a phase shift photomask according to another embodiment of the present invention.

FIG. 6 is a sectional view of a main part of a photomask without a phase shifter.

FIG. 7 is a diagram showing the amplitude of light immediately after passing through this photomask.

FIG. 8 is a diagram showing the amplitude of light on a wafer.

FIG. 9 is a diagram showing the intensity of light on a wafer.

FIG. 10 is a sectional view of a main part of a conventional phase shift photomask.

FIG. 11 is a diagram showing the amplitude of light immediately after passing through this phase shift photomask.

FIG. 12 is a diagram showing the amplitude of light on a wafer.

FIG. 13 is a diagram showing the intensity of light on a wafer.

[Explanation of symbols]

1. Light shielding film 2 Phase shifter 3. Reduction projection exposure device 4. Light source 5 Semiconductor wafer 6 Collimator lens 7 Objective lens L1 light L2 light M1 Photomask for phase shift M2 Photomask N1 Light shielding area P1 Light transmission area P2 Light transmission area

Claims (4)

[Claims] 1. A phase shift photomask, wherein a phase shifter made of a transparent thin film is provided on one of a pair of light transmitting regions, and the phases of two lights transmitted through the pair of light transmitting regions are mutually inverted. A photomask characterized in that the phase shifter is made of a low refractive index material. 2. The photomask according to claim 1, wherein the phase shifter is made of a fluororesin. 3. The method of manufacturing a photomask according to claim 2, wherein the phase shifter is formed by spin-coating a fluororesin on the surface of the photomask and then etching the fluororesin. 4. The method of manufacturing a photomask according to claim 3, wherein the fluororesin is etched by reactive ion etching using an oxygen-based gas.