1. Field of the Invention
The present invention relates to an X-ray exposure apparatus, an X-ray exposure method, an X-ray mask, an X-ray mirror, a synchrotron radiation apparatus, a synchrotron radiation method and a semiconductor device, and more specifically, it relates to an X-ray exposure apparatus, an X-ray exposure method, an X-ray mask, an X-ray mirror, a synchrotron radiation apparatus, a synchrotron radiation method and a semiconductor device capable of employing X-rays of a shorter wavelength region than the prior art for exposure.
2. Description of the Background Art
In recent years, requirement for higher integration and refinement of a semiconductor device is becoming more and more strong. Therefore, necessity for forming a pattern of a semiconductor integrated circuit smaller than the prior art increases. Thus, an X-ray proximity exposure technique employing X-rays shorter in wavelength than exposure light having been employed in general as exposure light is watched with interest in a photolithographic working step.
FIG. 24 is a schematic diagram of a conventional X-ray exposure apparatus. Referring to FIG. 24, the X-ray exposure apparatus is formed by a synchrotron radiation source 101, X-ray mirrors 103, a heat removal filter 104, a beryllium window 105, a window 122 consisting of a silicon nitride film, an X-ray mask 106 and a vertical X-Y stage 123 for setting a semiconductor wafer 109. Radiation 102 generated in the synchrotron radiation source 101 passes through the X-ray mirrors 103, the heat removal filter 104, the beryllium window 105 and the window 122 consisting of a silicon nitride film and reaches the X-ray mask 106. In the X-ray mask 106, a circuit pattern to be transferred to the semiconductor wafer 109 is formed by an X-ray absorber. The radiation 102 passes through the X-ray mask 106, whereby this circuit pattern is transferred to resist applied onto the semiconductor wafer 109. Such an X-ray exposure apparatus is shown in NTT R & D Vol. 43, No. 6, p. 501 (1994), for example.
At this point, the radiation 102 is continuous spectral light having wavelengths over a wide range from the X-ray region to the infrared region. As to X-rays required in an X-ray exposure step of transferring the transfer pattern to the semiconductor wafer 109, on the other hand, only X-rays of a certain proper wavelength region are required. Therefore, the conventional X-ray exposure apparatus first utilizes the reflection characteristics of the X-ray mirrors 103 for absorbing/cutting shorter-wavelength X-ray components having wavelengths of not more than 0.7 nm. Then, when the radiation 102 is transmitted through the heat removal filter 104 consisting of beryllium, X-ray components whose wavelengths are longer than 1.5 nm are substantially entirely absorbed/cut by the heat removal filter 104 due to the characteristics of beryllium.
Thus, the radiation 102 is so adjusted that the wavelengths thereof are in the range of about 0.7 to 1.5 nm. Then, the radiation 102 is successively transmitted through the beryllium window 105 and the window 122 consisting of a silicon nitride film. At this time, heat is hardly generated in the beryllium window 105 and the window 122 consisting of a silicon nitride film. The space between the beryllium window 105 and the window 122 consisting of a silicon nitride film is filled with helium of the atmospheric pressure. Therefore, the beryllium window 105 serves as a partition between a vacuum region upstream the beryllium window 105 and an atmospheric pressure region on the downstream side. The heat removal filter 104 cuts unnecessary X-ray components, thereby suppressing heat generation of the beryllium window 105. Consequently, it is possible to keep mechanical strength of the beryllium window 105.
The window 122 consisting of a silicon nitride film fills the role of a partition between the region filled with helium and the atmosphere. When bringing it into an apparatus structure setting the vertical X-Y stage 123 in a helium atmosphere, the window 122 consisting of a silicon nitride film is unnecessary.
The X-ray mask 106 is formed with the circuit pattern to be transferred to the semiconductor wafer as hereinabove described. This circuit pattern is transferred by irradiating a prescribed region of the resist applied to the semiconductor wafer 109 with the radiation 102 through the X-ray mask 106.
In general, a heavy metal such as gold or platinum has been employed as the material for the surfaces of the X-ray mirrors 103 reflecting X-rays. This is because reflectance for X-rays of about 60% is obtained at wavelengths around about 0.7 nm, which are the wavelengths of X-rays employed for exposure also when relatively increasing an oblique-incidence angle of the radiation 102 with respect to the X-ray mirrors 103 to about 2°. Means of converging a larger quantity of X-rays by preparing X-ray mirrors having a large converging angle with such a material of gold or platinum is studied. The intensity of X-rays employed for exposure can be increased by thus converging a lager quantity of X-rays. Consequently, it becomes possible to obtain a high throughput in the exposure step.
It is also proposed to employ silicon carbide or fused quartz as the material for the X-ray mirrors 103. This silicon carbide can bring the reflectance for X-rays to an extremely high value of about 90% by relatively shallowly setting the oblique-incidence angle to about 1°.
While a beryllium thin film is proposed as the material for the heat removal filter 104 absorbing/cutting long-wave X-rays, a proposal for employing silicon nitride or a diamond thin film in an auxiliary manner is also made in addition. This is for an object of increasing efficiency of heat absorption and an object of attaining oxidation prevention of the beryllium thin film.
The X-ray mask 106 generally comprises a membrane consisting of silicon carbide or the like and an X-ray absorber formed on this membrane. At this point, silicon carbide is employed since absorbance for X-rays of about 0.7 nm to 1.5 nm in wavelength, which are X-rays employed for exposure, is relatively small.
Thus, gold, platinum, silicon carbide, fused quartz or the like is proposed as the material for the surfaces of the X-ray mirrors reflecting X-rays. Further, beryllium, silicon nitride, diamond or the like is proposed as the window material. At this point, any of these is on the premise of employment of X-rays having a peak wavelength of about 0.75 nm, generally regarded as most suitable, as exposure light.
At this point, the reason why it has been said that the X-rays having a peak wavelength of about 0.75 nm are suitable as the optimum exposure light is as follows:
That is, in principle, the resolution of an obtained optical image improves as employing X-rays having shorter wavelengths, and it is possible to form a fine pattern. As the wavelengths of the X-rays reduce, however, energy of the X-rays increases. Consequently, when the resist applied onto the semiconductor wafer 109 is irradiated with the X-rays in the exposure step, photoelectrons are generated in this resist. The kinetic energy of these photoelectrons increases as the energy of the X-rays incident upon the resist increases. The resist is sensitized by these photoelectrons. Consequently, it follows that the region of the resist sensitized by the photoelectrons generated in the resist increases as employing Short-wave X-rays. Consequently, it follows that the pattern formed on the resist is blurred due to influence by these photoelectrons. That is, it has been regarded that the range of these photoelectrons decides the resolution limit as such.
In consideration of the range of these photoelectrons, therefore, it has generally been said that the optimum peak wavelength of X-rays employed for exposure is about 0.75 nm.
Thus, it has been considered that the range of the photoelectrons decides the resolution limit, and hence it has generally been said that a pattern having a line width or a line space of not more than 100 nm cannot be formed through an exposure step employing X-rays having a peak wavelength of about 0.75 nm as described above.
In order to improve the resolution in the exposure step employing X-rays under such circumstances, there has been made a proposal for attaining higher resolution by employing a low-contrast mask, a phase-shift mask prepared by vertically tapering an absorber pattern, a mask subjected to optical proximity effect correction or the like. In any case, however, it has been difficult to remarkably improve the resolution.
Since the aforementioned problem of the range of photoelectrons is present, an idea of shifting the wavelengths of X-rays employed for exposure to a shorter wavelength region thereby attaining higher resolution has not generally been studied in the technical field of X-ray exposure performing transfer of a circuit pattern for a semiconductor device. When employing X-rays of a shorter wavelength region, the X-rays are readily transmitted through an X-ray absorber of an X-ray mask since the energy of the X-rays is larger than general. In order to attain a necessary contrast, therefore, it is conceivably necessary to increase the thickness of the X-ray absorber. In such a case, the transmission characteristics of the X-rays are deteriorated due to a waveguide effect when the X-rays are transmitted through a transfer pattern formed by the X-ray absorber having a large thickness, and hence there has been such a problem that the resolution of the transferred circuit pattern lowers. Thus, it has been regarded that refinement of the transfer pattern is difficult.
As an exposure technique with Short-wave X-rays, there is an example setting the exposure wavelength to about 0.3 nm in the field of a micromachine technique. However, it has thus employed the Short-wave X-rays for an object of performing high-aspect pattern working of forming a pattern of several microns with a height of about several 100 microns by increasing transmission ability of X-rays into resist. Further, a pattern size required in this field of the micromachine technique is larger than a required pattern size demanded in the aforementioned field of semiconductor devices by at least one digit to two digits. In addition, the thickness of the X-ray absorber of the X-ray mask is also larger than that employed in the field of the semiconductor devices. Further, a metal such as titanium is employed for a substrate of the X-ray mask. That is, the aforementioned technique belongs to a technical field absolutely different from the technical field of the present invention. Further, in relation to an exposure apparatus employing a point light source of an electron beam excitation type employing a palladium target, an exemplary experiment employing a mask prepared by forming an absorber on a substrate consisting of boron nitride by gold plating for performing exposure with X-rays of a wavelength region of 0.415 nm to 0.44 nm is reported. However, this technique also belongs to a technical field basically different from the present invention employing a synchrotron radiation source.
At this point, necessity for forming a fine pattern whose design rule is about 0.05 μm has recently become obvious following requirement for refinement and higher integration of a semiconductor device. In the aforementioned exposure step employing X-rays whose peak wavelength is about 0.75 nm, it is conceivably difficult to accurately form such a fine pattern whose line width or line space is at the level of 0.05 μm.
Therefore, the inventors have made various experiments and studies aiming at spreading the application limit of the X-ray exposure technique to a finer region and transferring a pattern of high resolution at a high speed (attaining a high throughput). Consequently, they have found it possible to employ X-rays of a shorter wavelength region than general for the X-ray exposure step as described later. However, the current X-ray exposure apparatus has been designed basically on the premise of employing X-rays whose peak wavelength is about 0.75 nm as exposure light, and hence it has been difficult to effectively use X-rays having wavelengths smaller than 0.7 nm, for example, as exposure light.
The present invention has been proposed in order to solve the aforementioned problems, and one object of the present invention is to provide an X-ray exposure apparatus capable of transferring a pattern of high resolution and capable of attaining a high throughput by spreading the wavelengths of X-rays employed for X-ray exposure to a shorter wavelength region than general.
Another object of the present invention is to provide an X-ray exposure method capable of transferring a pattern of high resolution and capable of attaining a high throughput by spreading the wavelengths of X-rays employed for X-ray exposure to a shorter wavelength region than general.
Still another object of the present invention is to provide an X-ray mirror employed for an X-ray exposure apparatus capable of transferring a pattern of high resolution and capable of attaining a high throughput by spreading the wavelengths of X-rays employed for X-ray exposure to a shorter wavelength region than general.
A further object of the present invention is to provide an X-ray mask employed for an X-ray exposure apparatus capable of transferring a pattern of high resolution and capable of attaining a high throughput by spreading the wavelengths of X-rays employed for X-ray exposure to a shorter wavelength region.
A further object of the present invention is to provide a synchrotron radiation apparatus applicable to an X-ray exposure apparatus capable of transferring a pattern of high resolution and capable of attaining a high throughput by spreading the wavelengths of X-rays employed for X-ray exposure to a shorter wavelength region.
A further object of the present invention is to provide a synchrotron radiation method applicable to an X-ray exposure apparatus capable of transferring a pattern of high resolution and capable of attaining a high throughput by spreading the wavelengths of X-rays employed for X-ray exposure to a shorter wavelength region.
A further object of the present invention is to provide a highly integrated semiconductor device manufactured with an X-ray exposure method capable of transferring a pattern of high resolution and capable of attaining a high throughput by spreading the wavelengths of X-rays employed for X-ray exposure to a shorter wavelength region than general.