JP2614861B2 - Reflective X-ray mask - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention uses a light source in the range from soft X-ray to vacuum ultraviolet,
Reflective X used when transferring a fine pattern on a wafer
It relates to a line mask.

[Problems to be solved by conventional technology and invention]

As a conventional method for forming a fine pattern, an exposure method using ultraviolet light having a wavelength of about 400 nm has been used. However, when the pattern size is about 1 μm, the physical resolution limit due to diffraction and interference becomes a limit of 1 μm or less. It is not useful as a pattern formation method. As a resolution improvement measure, we aim to shorten the wavelength,
Exposure methods using i-line of 350 nm or 250 nm are being studied, but the resolution is limited to about 0.7 μm. Further, when the wavelength is further reduced, there is no lens material having an appropriate refractive index. For this reason, a proximity exposure method using soft X-rays has been proposed as a transfer method without using a lens. In this method, as shown in FIG. 7, soft X-rays having a wavelength of 4 to 20.degree.
A mask 2 on which a pattern for absorbing X-rays is formed and a wafer 3 coated with a resist sensitive to soft X-rays are installed in parallel at intervals of several tens of μm, and a soft X-ray flux 1 Is irradiated to transfer the pattern on the mask 2 onto the wafer 3. This method has been confirmed to be an excellent transfer technique having features such as high resolution and dust resistance. However, for the same-magnification projection, the mask is governed by the performance of an exposure machine that draws a mask pattern. The pattern transfer performance (resolution, alignment accuracy, etc.) by soft X-rays cannot be improved beyond the performance of the pattern. Also,
Because of proximity exposure, there is a problem that resolution is reduced by Fresnel diffraction.

That is, assuming that the gap between the mask 2 and the wafer 3 is s, the wavelength of the used light source is λ, the proportionality constant representing the degree of diffraction is K, and the pattern dimension is d, the following relationship is established.

d = K (s · λ) ^1/2 (1) Here, if a value corresponding to the resolution limit is given as the proportional constant K, the equation (1) gives the minimum pattern size that can be resolved.

For example, when K = 1.5, s = 10 μm, and λ = 10 °, from equation (1), d = 0.15 μm is the resolution limit. Actually, the gap S needs to be 20 μm or more in consideration of mask damage and the like, so that a very fine pattern cannot be formed.

The X-ray mask used for the proximity exposure method is
As shown in FIG. 8, an absorber 5 for absorbing soft X-rays and a permeable film 6 for transmitting soft X-rays are formed on a support substrate 4. Normally, a Si wafer is used for the support substrate 4, Au, Ta, W is used as a material of the absorber 5, and a permeable film 6 is made of Si, although it depends on the soft X-ray wavelength used. SiO ₂ ,
Si ₃ N ₄ , SiC, BN, Ti, and polyimide are mainly used. Figure 9 is a graph showing the relationship between the transmittance in the thickness of the absorption coefficient and 1μm of these materials for Al K _alpha ray (8.34Å). The transmittance is about 78% for the Si ₃ N ₄ film and the film thickness is 2
At μm, the transmittance is reduced to 62%, so it is necessary to make the film as thin as possible. For this reason, it is easy to be damaged and handling mishandle increases. In addition, since heavy elements such as Au, Ta, and W are formed on this thin film with a thickness of about 0.5 μm, it is difficult to maintain the overall stress balance, and warpage of the thin film and distortion of the pattern occur. Furthermore, since there is no direct escape path of the heat generated by the soft X-rays absorbed by the absorber or the permeable membrane, there is a problem that pattern distortion occurs due to thermal expansion of the absorber at the time of soft X-ray irradiation.

On the other hand, with the advancement of thin film formation technology in the semiconductor industry, materials composed of heavy elements and
Advances have been made in multilayer film formation technology in which layers are alternately stacked by 10 °, and as this application, a multilayer mirror having a reflectivity of several tens percent at direct incidence in the soft X-ray region has been developed. A pattern reduced to 1/5 of 4 μm has been reported as a reduction optical system using this multilayer film as a concave or convex mirror (Kinoshita et al., Proceedings of the 47th JSAP, p322, lecture number: 28p-ZF-15). Thus, by using a multilayer mirror, projection exposure in the X-ray region, which has been impossible in the past, can be realized, and a reduction projection exposure system using not only a transmission mask but also a reflection mask can be configured.

[Means for solving the problem]

The reflection type X-ray mask of the present invention comprises a multilayer film in which thin films composed of a substance mainly composed of a heavy element and a substance mainly composed of a light element are alternately formed on a substrate having a sufficient thickness. The present invention is characterized in that a layer in which a part of the multilayer film is covered with another substance is used as an absorption part, or a layer in which a part of the multilayer film is removed is used as an absorption part.

[Action]

The multilayer film increases the reflectance of soft X-rays and vacuum ultraviolet rays (hereinafter, these are represented by soft X-rays).

Further, the substrate does not need to transmit soft X-rays, so that a sufficiently thick substrate can be used. For this reason, the mask itself is less likely to be damaged, and warpage of the substrate and distortion of the pattern are less likely to occur. Furthermore, even if soft X-rays are absorbed during irradiation with soft X-rays, heat is conducted from the pattern portion to a substrate having a large heat capacity, so that thermal expansion distortion is reduced.

〔Example〕

FIG. 1 is a view showing a first embodiment of the reflection type X-ray mask of the present invention, in which a substance mainly composed of a heavy element and a substance mainly composed of a light element are alternately laminated on a substrate 7. It has a configuration in which a multilayer film is laminated, and a patterned intermediate layer 9 ′ made of a substance that absorbs soft X-rays is formed thereon. Here, the pitch d of the stacked body of the substance mainly composed of the heavy element and the substance mainly composed of the light element of the multilayer film 8 depends on the wavelength λ and the incident angle θ of the incident soft X-ray and the black reflection condition ( 2d sin ^θ = mλ: m is an integer). Also, the middle layer 9 '
Is set such that the contrast between the soft X-rays reflected by the multilayer film 8 and the soft X-rays passing through the intermediate layer 9 ′ becomes a sufficient contrast to form a pattern by reducing and projecting the pattern. Made with the thickness to give.

In order to manufacture such a reflective X-ray mask, as shown in FIG. 2A, an intermediate layer 9 and a resist are formed on a sample in which a multilayer film 8 is formed on a substrate 7 having good flatness in advance. Then, as shown in FIG. 2B, a resist pattern 10 'is formed by photolithography or electron beam, X-ray, or ion beam lithography. Then, the intermediate layer 9 is processed using the patterned resist 10 'as an etching mask to obtain the structure shown in FIG. 2 (c). Thereafter, if the resist 10 'is removed, the reflection type X-ray mask shown in FIG. 1 is obtained.

If a material having a large absorption is used as the intermediate layer 9, it can be formed with a thickness sufficiently smaller than the pattern width of the processed portion.

When Ta or W is used for the intermediate layer 9, it can be processed with high accuracy by dry etching using a chlorofluorocarbon-based gas (such as CBrF ₃ ). When Au is used, it can be processed by ion etching with an inert gas. . These heavy metals have wavelength
Since it can well absorb soft X-rays and vacuum ultraviolet rays from 10 ° to several 100 °, it can be used as an intermediate layer.

For the intermediate layer 9, SiO ₂ or Si ₃ N ₄ can be used. S
iO ₂ and Si ₃ N ₄ have an extremely large absorption coefficient at a wavelength of 20 ° or more. For example, 100% Si ₃ N ₄ transmits only 9% at a thickness of 0.1 μm, and passes through the 0.1 μm thick Si ₃ N ₄ film again even if the underlying multilayer film has a reflectance of 10%. The intensity of the soft X-ray emitted into a vacuum is 8 × 10 ⁻⁴ times the intensity of the light incident on the mask, and the intensity ratio of the multilayer film reflecting surface can be 100 times or more. Usually, it is sufficient that the contrast of a mask exposed with a resist is 10 times or more in transmittance. Si ₃ N ₄ and SiO ₂ can be processed with high accuracy using a Freon-based etching gas, for example, C ₂ F ₆ without causing a large pattern conversion difference by reactive ion etching.

Further, for the intermediate layer 9, a relatively light metal such as Al or Cr can be used. Al is frequently used as a wiring material for LSI, and Cr is used as a light shielding portion of a photomask. Both can be processed with high precision by dry etching using a gas containing chlorine, and have a large absorption coefficient as compared with the above-mentioned Si ₃ N ₄ and SiO ₂ , so that the thickness of the intermediate layer can be reduced.

Further, as the intermediate layer 9, a carbon (c) film or a polymer material containing many C can be used. C is 44.7
It has an absorption edge at Å, but has an extremely large absorption coefficient up to over 1000 で は at longer wavelengths. Therefore, it can be used as an intermediate layer for wavelengths from around 100 °. The C film can be processed with high accuracy by reactive ion etching using O ₂ , and a polymer material, for example, a resist can form a pattern of an intermediate layer only by an exposure and development process without an etching process.

The uppermost layer of the multilayer film as the reflection surface may be formed sufficiently thicker than the lower layer, and this layer may be patterned to be an intermediate layer. In this case, since the intermediate layer 9 can be formed as one layer of the multilayer film 8, the number of steps can be reduced.
In this case, as a multilayer film material, WC, Mo-Si, W-Si, T
If a high melting point metal such as i-Si, Ta-Si or Re-Si and a light element such as Al, Si, C or Cr are used, it can be easily processed by the above-described conventional etching process.

In the above embodiment, the intermediate layer is formed of a single substance, but may be formed of a composite layer. In particular, the intermediate layer may be formed of a substance that can be easily removed between the multilayer layer and the reflective surface. If the layers are interposed, the multilayer film substrate can be reused, and the mask manufacturing cost can be reduced.

As a method for forming the multilayer film, any of a method by vacuum evaporation, a method by ion beam sputtering, a method by radio frequency (RF) sputtering, and a molecular beam epitaxy (MBE) method can be used.

FIG. 3 is a view showing a second embodiment of the reflection type X-ray mask of the present invention, in which 7 is a substrate, and 8 'is a patterned multilayer film serving as a reflection surface for soft X-rays. . Here, the pitch of the patterned multilayer film 8 'is made to substantially satisfy the Bragg condition as in the first embodiment. In order to manufacture a reflection type X-ray mask having such a structure, as shown in FIG. 4A, a pattern 11 is formed on a multilayer film 8 laminated on a substrate 7, and this pattern 11 is used as a mask. Then, the multilayer film 8 is etched to obtain the structure shown in FIG. 4B, and then the pattern 11 is removed. The pattern 11 may be a resist pattern or the intermediate layer 9 'used in the first embodiment. When the intermediate layer 9 'is used, the pattern becomes a negative pattern.

As a configuration of the multilayer film as the reflection surface, if a material containing Mo, Ta, W, Ti, Cr as a heavy element layer and Si as a main element as a light element layer is selected, a Freon-based etching gas ( It can be easily and continuously etched by CBrF ₃ or the like. If SiO ₂ or Si ₃ N ₄ is used as the intermediate layer, etching can be performed with the same Freon-based etching gas.

The reflection type X-ray mask of FIG. 3 can also be manufactured by a process using lift-off as described below. As shown in FIG. 5 (a), a pattern 12 is previously formed on the substrate 7 with high accuracy, and a multilayer film is deposited thereon as shown in FIG. 5 (b). Next, when the pattern 12 is removed, the pattern 12 and the multilayer film 8 thereon are removed, and a reflection type X-ray mask composed of the patterned multilayer film 8 'shown in FIG. 5C is obtained.

In this method, it becomes difficult to form a multilayer film uniformly between the patterns as the pattern becomes finer.
The thickness of the multilayer film required for reflection is 1000 ° or less, and if the pattern width is 1 μm or more, the openings are large and uniform adhesion can be achieved.

 Here, the reflectance of the multilayer mirror will be described.

Fig. 6 shows a multilayer film of W-Si (W: 10Å, Si: 15Å, 30 layers)
Shows the calculated values of the reflectance characteristics in the range of the incident angle of 20 ° and the wavelength of 40 to 60 °. As you can see from the figure, the wavelength
At 50 °, a reflectance of 8% is obtained. This multilayer film has a thickness of 750 mm as a whole, and a thin film having such a thickness can be etched accurately.

For a Mo-Si multilayer film, the wavelength is 170, 4 mm, and the pitch is 95.0 mm (M
o: 38.2Å, Si: 56.8Å), and in the case of 20 layers, a reflectance of about 47% as a calculated value and about 68% as an experimental value have already been obtained near an incident angle of 15 ゜. (Troy W. Barbie Jr., Super Lattice and Microstructures, 1st
Vol. 4, No. 4, 1985, p. : Troy W.Barbee Jr, Superlatt
ices and Microstructures, Vol. 1, No. 4, (1985). As described above, when a multilayer film is used for the reflection surface, soft X-rays can be reflected with high reflectance. The present invention has been made in view of the knowledge of such a multilayer film and the feasibility of the X-ray reduction projection exposure method.

[Effects of the Invention] As described above, the present invention is a reflective surface using a multilayer film in which thin films of a substance mainly containing heavy elements and thin films of a substance mainly containing light elements are alternately formed in multiple layers, A part of the multilayer film is adhered with another substance, or a processed layer is used as an absorption layer, thereby forming a pattern to form a reflection type X-ray mask. It is possible to improve the pattern accuracy in ease of handling and to manufacture a mask at low cost.

Further, since the pattern portion is cooled by the sufficiently thick substrate, the thermal expansion of the pattern at the time of soft X-ray irradiation can be suppressed. Since the reflectivity of the multilayer film is more than 10%, the contrast can be sufficiently secured. Also, when used as a mask for reduction projection exposure, the pattern can be enlarged by the reduction ratio, and since the pattern has a step and a large edge contrast, the pattern defect can be easily detected. is there.

The reflective X-ray mask of the present invention can be designed for any wavelength from soft X-rays to vacuum ultraviolet rays by selecting a material according to the wavelength of the light source used.

When an X-ray reduction projection exposure apparatus is constituted by a combination of the reflection type X-ray mask of the present invention and a reduction optical system, a pattern of 0.1 μm or less can be formed in principle.

[Brief description of the drawings]

FIG. 1 is a view showing a first embodiment of the reflection type X-ray mask of the present invention, and FIG. 2 is a view for explaining a manufacturing process of the reflection type X-ray mask of the first embodiment. FIG. 3 is a view showing a second embodiment of the reflection type X-ray mask of the present invention, and FIG.
FIG. 5 is a view for explaining a manufacturing process of the reflection type X-ray mask of the second embodiment. FIG. 5 is a view for explaining another manufacturing process of the reflection X-ray mask of the second embodiment. FIG. 7 is a view showing a calculated value of the reflectance of the multilayer mirror, FIG. 7 is a view for explaining a conventional proximity exposure method, FIG. 8 is a view showing a transmission X-ray mask, FIG. is a diagram showing the absorption coefficient for Al K _alpha line of the material to be used for permeable membrane (8.34Å), the relationship between the transmittance at a thickness of 1 [mu] m. 1 ... Soft X-ray flux, 2 ... Transmission type mask, 3 ... Wafer,
4 ... support substrate, 5 ... absorber, 6 ... permeable membrane, 7 ...
Substrate, 8,8 '... multilayer film, 9,9' ... intermediate film, 10,10 '...
… Resist, 11,12 …… Pattern.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Nobuyuki Takeuchi 3-1, Morinosato Wakamiya, Atsugi-shi Nippon Telegraph and Telephone Corporation Inside Atsugi Telecommunications Research Institute (72) Inventor Nao Ishihara 3-1 Morinosato-Wakamiya, Atsugi-shi Nippon Telegraph and Telephone Inside the Atsugi Electric Communication Laboratory Co., Ltd. (72) Inventor Hideo Yoshihara 3-1, Morinosato Wakamiya, Atsugi-shi Nippon Telegraph and Telephone Corporation Inside the Atsugi Electric Communication Laboratory Co., Ltd. (56) References JP-A-61-168917 (JP, A) JP-A-62-48019 (JP, A) JP-A-55-101945 (JP, A) JP-A-60-84818 (JP, A) Nikkei Microdevice November 1986, PP. 47-48 Extended Abstracts of the 18th Confence on Solid State Devices and Materials, Tokyo, 1986, PP. 17−20

Claims (2)

(57) [Claims] 1. A multi-layer film in which thin films of a substance mainly composed of a heavy element and a substance mainly composed of a light element are alternately deposited on a substrate, and a light element material absorbing X-rays is formed on the multilayer film. A reflection type X-ray mask, wherein a pattern comprising: 2. The reflection type X-ray mask according to claim 1, wherein the light element material forming the pattern is silicon dioxide (SiO ₂ ), silicon nitride (Si 3 N 4), silicon (S
i) a reflective X-ray mask, which is made of aluminum (Al) or chromium (Cr).