Use and Characterization of X-ray Lithography for Micromachining and
Nanostructure Fabrication

X-ray lithography is a process by which an image is transferred from an absorbing mask to a radiation-sensitive resist film (Fig.1). Alignment of x-ray mask to the substrate is required for multi-layer x-ray lithography applications and so called mix-and-match techniques, when different types of microlithography are used for different technological layers. The resulting latent image in the resist is then developed to produce a patterned resist mask which acts as a shield in the next step of microfabrication, when material is either removed from the substrate by etching through the resist mask or additional material is deposited.
Fig. 1 Schematic view of the proximity X-ray lithography process

Physically the process of exposure in X-ray lithography is the same as in ordinary optical lithography, but the wavelengths involved are much shorter (0.1 - 10 nm in X-ray lithography versus 150-450 nm in the optical case). The current position of X-ray lithography in the spectrum of microfabrication techniques is still a technique for tomorrow. However, demands for a high throughput lithography tool capable of resolution beyond 0.18 µm will almost certainly arise by the year 2001 [1], and X-ray lithography with its resolution limit of nearly 30 nm will always remain an attractive possibility and may even represent the last resort for planar technology.
While the use of X-ray lithography in commercial manufacturing of ULSI ICs is probably only a matter of time, its use for nanostructuring is already required in order to avoid the use of slow and expensive e-beam lithography in cases where large volumes of nanostructures are to be produced. Most of these nanostructures can only be of high scientific and commercial value, if a parallel method for their mass replication is used. Compared to a sequential e-beam writing, X-ray lithography at wavelengths of 1-1.5 nm with a synchrotron as a radiation source can gain about two orders of magnitude in throughput and also improve the process quality due to virtually nonexistent proximity effects.
Just as 1960s planar technology made a revolution in the electronics industry, the same technology is now bringing about revolutionary advances in the manufacturing of miniaturized tools and other micromechanical devices. LIGA [2], which stands for Lithographie Galvanoplastie Abformung , is a method for making such structures. The main feature of the method is that it allows the manufacture of structures with high aspect ratios (1:100) and with micron lateral resolution. A broad variety of materials can be machined, including metals, plastics, glasses, and ceramics.
In order to be able to expose the thick resist layers used in LIGA, X-rays hard enough (2-10 keV) to penetrate through the whole depth of the resist must be used. To have reasonable exposure times the intensity of the radiation is required to be as high as 0.5-1W/cm2. For these two reasons synchrotron radiation from storage rings with 1-2 GeV energy of electrons appears to be the only suitable source for LIGA exposures.
Another important property of collimated synchrotron radiation is its small (about 1 mrad) angular dispersion, which in an ideal case permits exposures with 1:1000 (width : height) aspect ratio. Use of tilted exposures can allow fabrication of quasi three-dimensional structures.
The BL811D beamline (Fig. 2) was designed to conduct the beam of X-rays from the bending magnet of the MAXII ring to the lithography set up. It has two Be windows for vacuum insulation as well as beam-shaping baffles and the usual equipment for pumping and pressure monitoring. A high speed X-ray scanner has been built and commissioned. One of the most important features of the scanner is its high scanning speed up to 200 mm/s. Maximum exposed area is 76 x 76 mm.

Fig. 2 Fragment of the BL811D beamline with exposure station (3-d solid modeling)

The scanner was initially designed as a micromachining exposure tool for lambda=0.1÷0.5 nm SR. However, use of flexible mechanical layout of the moving exposure chamber and the filtering mirror chamber made it possible to use the scanner both for micromachining and nanofabrication with only minor adjustments . In nanofabrication hard x-ray spectral components are filtered out by a chromium coated silicon mirror reflecting the beam at grazing incidence (1 -2 degrees). Remaining x-rays with lambda=1÷2nm make the exposure. The design of the scanner vacuum system allows operation in a flow of helium with pressure from 0.001 to 1000 mbar. Helium helps to decrease heat load on the mirror and x-ray mask. In the case of nanolithography the pressure of helium fixes mask and substrate in contact. The pressure is optimized for different applications to combine required x-ray absorption with adequate cooling of mask, substrate, and scanner optical components.

[1]   SAL(Suss Advanced Lithography)One Step Ahead Report Feb. 1997
[2]   E. Becker et al., Microelectronic Engineering 4, 35 (1986)