Title of the Invention

SOFT X-RAY LITHOGRAPHIC SYSTEM

Technical Field

This invention relates to a lithographic system for soft X-ray with use of a Fresnel zone plate (hereinbelow abbreviated to FZP).

More particularly, this relates to a lithographic system for forming a reduced image from a densely drawn planar pattern by transmission of soft X-ray and beam condensation with use of a

FZP.

Background Art Formation of IC (integrated circuits) patterns onto a target wafer in semiconductor industry has been achieved mainly by optical reduction exposure with use of light beam and optical lense system and, for recent desire of enhancing further integral density, conventional photo-lithography has reached its physical limit because of inevitable diffraction phenomenon. The titled soft X-ray lithography is expected to breakthrough the conventional technical limit, for merit of its nature that X-ray has decisively short wave length.

However, presently a suitable imaging system for the X-ray lithography having reductive or scale-down potential is not available and thereby adopted process in this art is so called the proximity exposure, which will reproduce only equi-dimensional copy of an original, for instance, a mask pattern. Therefore, it is not possible in principle to obtain a reduced copy, which

leads to follow that minimum line width to be transcripted from an original, or from a mask pattern, can not be finer than width of the original and actually resultant copied lines become contrary to the desire of obtaining a reduced, unblurred copy.

Disclosure of the Invention This invention is intended to advance the conventional art as noted and to render the reductive projection system which has been proved of its merit in photo- lithography to be feasible in soft X-ray lithography.

The present inventive imaging system comprises basically a soft X-ray source which is emissive like a point source, a mask pattern or an original to be transcripted, a spherically concaved re lector to converge efficiently soft X-ray onto a FZP, which is for focusing onto a resist plate or a target material. These components will be illustrated in drawings and explained in detail hereinbelow.

Making reference to the Fresnel zone plate (FZP), it has been known as an imaging element without use of a lense and become recently convenient in manufacture of a highly performable FZP system through application of holographic technology or electron beam.

Then, reference is made to basic concept of a FZP when approached by the holography with reference to Fig. 3, wherein a spherical wave diverging from PI and another spherical wave converging to P2 are assumed and, if a FZP pattern is recorded at the middle between PI and P2, this FZP will function as a lense having f=

1 a/2 with respect to the recording light (wave length thereof is j, assumed 4000 angstroms), but the same FZP will function with respect to soft X-ray (wave length thereof is assumed 8 angstroms) as a lense haveing f'= 4000/8X f= 500f = 250a. It 5 follows that manufacture of a FZP having a focus distance =100 mm under use of soft X-ray having a wave length = 8 angstroms leads to conclude a= 0.4 mm, which is a conveniently practicable size.

Further, it is to be noted here that obtainment of minimum 10 resolutable width W= 0.2 micro meter requires, according to formula (1) which will appear hereinbelow, that aperture size of the FZP should be 0.49 mm, which is feasible by the holographic technology.

15 Recommendable manufacture of the FZP is that a substrate plate capable of transmitting soft X-ray in use is deposited with gold(Au) to a suitable thickness and thereon a photoresist material is coated, and thereafter a target pattern of the FZP which will be copied is recorded onto the Au surface by known

20 ion etching process and therethrough the FZP is complete. If a substrate capable of transmitting soft X-ray is not available, it is allowed in place to include a support frame and to take a free¬ standing zone plate structure.

25 The FZP thus obtained is defined by a gathering of concentric 4 circles wherein the outmost circle pitch is equal to the resolutable width W and centripitally pitches become larger. The inventive embodiments, noted hereinbelow, will employ a FZP thus obtained.

The spherically concaved reflector for soft X-ray, a major constituent in embodying the invention, may be fabricated by sputtering onto a reflector substrate carbon and tungsten in turn with steps of several angstroms each layer up to build 15 or 16 layers.

Now making reference to optical analysis on a system comprising the FZP, the reflector and soft X-ray with respect to projecting a perodic line and space pattern, it is assumed with analogy of optical lense that F number(focal ratio to aperture) of the lense is denoted by F and that wave length in use is by λ(lumda), resolutable minimum line and space pitch (resolution width) is given as below;

W = 1.22F-λ ... (1) then, requirement of F number with visible light of 400 nm and soft X-ray of 8 angstroms is compared to which answer is that, with reference to the same resolultion width, F number of the soft X-ray is permitted to be 500 times over the visible light, which leads to suggest that the FZP may be of extremely small diameter for the X-ray. Therefore, should the resolution requirement be made more stringent to be 1/10, F number may be 50 times, which proves that employment of soft X-ray enables attainment of exceedingly high level of resolution.

In this invention, the X-ray is converged by a concaved reflector onto the FZP, which brings high utilization of the X-ray. An alternative method of imaging with use of concave mirror for reflection, but such a kind of mirror is required to have finished accuracy of more than 1/8 wave length which comes to such a difficult accuracy that the mirror surface should be finished to

be more than 1 angstrom, in the case of X-ray having about 8 angstroms.

Brief Description of Drawings Fig. 1 is a not-to-scale drawing, but useful to explain an arrangement of components involved in an inventive embodiment.

Fig.2 is to show a conventional proximity exposure process.

Fig. 3 is to show basic concept of manufacture of a FZP approached by holography. ■ These drawings are presented by way of illustration and therefore these should not be construed as limiting the invention.

Mode(s) for Carrying Out the Invention In Fig. 1, 1 is a X-ray source, 2 is a spherically concaved reflector for soft X-ray, 4 is a FZP, and an original or mask pattern 3 is placed at a suitable position between the reflector 2 and the FZP 4. 6 is a target substrate to copy IC pattern, for instance and thereon lying is a photoresist 5. In the above arrangement, the reflector 2 is disposed to project image of emitting zone of the source 1 onto surface of the FZP 4.

For illustration, reference is made to an example that an IC pattern lying on a 5 mm square chip is scaled down to 1/5 by simultaneous exposure.

As soft X-ray, a wave length 8.34 angstroms is obtained by use of alumimum characteristic K (alpha) line.

Assuming that focal distance f= 100 mm, attainment of a resolution width U= 0.2 micro meter requires, according to the

formula (1), that is; = 1.22x(F number of the FZP)X λ (1), that the FZP should have a diameter= 0.51 mm.

Further, distance between the FZP 4 and the resist surface 5 is assmumed to be 120 mm, for instance, which follows that distance between the FZP 4 and the mask pattern 3 should be 600 mm. And a required size of the mask pattern is 25 mm square to which the reflector is so set as to irradiate uniformly and image of the X-ray source is produced thereby.

Industrial Applicability As is illustrated above, copying target that minimum line width is 0.2 micro meter requires only on a mask pattern that the same should be 1 micro meter, which makes manufacture of the mask pattern so readily.

In the case of the conventional proximity process illustrated in Fig. 2, emitting X-ray, if not completely parallel, will diffract into underside of the pattern 3 and X-ray source, if not close to a point source, may render blurred periphery in shadow. Therefore, in the case of using a X-ray tube, it is necessary that the source 1 is r emoted as possible from the mask 3 and electron beam is aimed to a target with a narrow aperture as possible, which might need employment of a revolving anode and further lead to obtain SOR light as ideal source. In contrast to the conventional necessities, this invention is freed from restrictions with the source.

In addition, method of reductive projection which the present

invention bases on, is advantageous in apparatus aspect, for instance, mechanisms for controlling interval between the resist and the mask pattern and for accurate positioning thereof, and prevention of influence by heat.

Hereinabove, the present invention has been explained chiefly by application to the IC pattern copy, but this inventive system will be applied to microscopic engineering, whereby medical and biological studies desire to observe living celles under scope as they are alive. The electron microscopy will hardly fulfill this desire, to which this inventive X-ray lithography has potential to approach utilizing merit of reductive projection.