Present status and problems of X-ray lithography

Abstract

Very intense technological efforts for increasing the density of integration ofsemiconductor devices have been made in the development of new and economical lithography methods for structures below2/am. The lithography methods used up to now in production are working with UV radiation in the region near 400 rim.The resolution limit due to Fresh'el diffraction allows the replication of structure dimensions down to 3--4 urn. A further improvement of the optical lithography (introduction ofdemagnifying projection printing instead of 1: 1 proximity exposure) probably will lead to powerful production machines enabling the reproduction of structures with dimensions down to 1 ,am.On the other hand, the expenditure increases rapidly when decreasing the resolution limits. For dropping the structure size below 1 tzm, the alternative lithography methods are electron-beam writing and X-ray lithography. Additionally, an application of these methods for larger structures could be furthered by an improvement ofthe yield. X-ray lithography is a direct further development of 1: 1 proximity printing, with the help of a mask, by drastically lowering the wavelen~h. This means the maintenance of a relatively simple process, also in the case ofvery small structures. But an important condition, if X-ray lithography is to be a practical method, is the availability ofan economical X-ray source in the wavelength region of1 nm with nearly parallel radiation. Because at this wavelength there are no problems with Fresn'el diffraction, and the distortions caused by the secondary electrons generated in the resist are much lower compared with electron-beam writing, structures down to 0.1/am are achievable with high aspect ratios. Although the very good replication features have been conf'gmed in many experiments, there are many problems to be solved, especially concerning the radition source, the resist-and the masktechnology.The application of X-ray lithography could mainly provide advantages in the mass production of VLSI devices with constant design.The domain of e-beam writing, besides mask production, could be the production of circuits with frequent changes in design (e.g., in the phase of circuit development).

Advances in lithographic techniques for precision nanostructure fabrication in biomedical applications

“…XRL can achieve resolutions of approximately 15 nm [ 42 ]. Unlike methods such as UVL, XRL can cope with large proximity gaps without causing diffraction or proximity effects until the required feature size reaches approximately 100 nm [ 43 ]. However, like EUVL, the short wavelength and consequently high energy of the photons cause blur, which limits its resolution [ 45 ].…”

“…Wavelength reduced to 193 nm or 248 nm [ 32 ] 65–130 nm [ 32 ] Improved resolution in comparison to traditional UV lithography [ 32 ] Shorter wavelengths are more easily reflected [ 33 ] Interference effects [ 32 ] Maximum total thickness variation is 0.5 µm [ 34 ] Low depth of focus [ 34 ] Extreme UV lithography Use UV light and mask to pattern a photoresist. Wavelength reduced to 13.5 nm [ 35 ] < 10 nm [ 36 ] Improved resolution in comparison to traditional UV lithography [ 36 ] Shorter wavelengths are more easily reflected [ 33 ] Low photon transmission efficiency [ 37 ] Defects in the photomask warp pattern [ 38 ] Secondary electrons cause blur [ 39 ] Increased stochastic pattern variations [ 40 ] X-ray lithography Use x-rays and mask to pattern a photoresist [ 41 ] 15 nm [ 42 ] Improved resolution in comparison to traditional UV lithography [ 42 ] Large substrate-mask distances do not cause diffraction/proximity effects until the feature width approaches 100 nm [ 43 ] Shorter wavelengths are more easily reflected [ 33 ] Masks are thin, fragile, and expensive [ 44 ] Secondary electrons cause blur [ 45 ] Electron beam lithography Use electrons to pattern a resist [ 46 ] > 10 nm [ 47 ] Precise control [ 48 ] Can pattern complex geometries [ 48 ] Beam can damage the substrate [ …”

“…Large substrate-mask distances do not cause diffraction/proximity effects until the feature width approaches 100 nm [ 43 ]…”