In X-ray lithography the choice of the optimal conditions depends very much on the class of applications that are addressed. In particular two main class of applications have caused the differentiation of X-ray lithography into High Resolution X-Ray Lithography (HRXRL), and the Deep X-Ray Lithography (DXRL). Though the principle is the same, applications, technology, equipments, masks, problems differ.
The main focus of HRXRL is high resolution nanopatterning. The objective is to overcome the resolution limitations of DUV lithography for main use in the microelectronics industry. In DXRL the target application is micromechanics and MEMS, with tall (hundreds of micrometers) and high aspect ratio structures (ratio between height and lateral size), but with resolution that in are in the range of micrometers. This requires the use of a much more penetrating X-ray radiation, with respect to the HRXRL, in order to expose thick resist film.
In deep X-ray lithography there are two main limitations to achieve high resolution patterning. One is related to the thickness of absorber, typically gold, where the required thickness is in the range of ~ 10-20 µm in order to provide a sufficient contrast. Therefore printing features at the sub-100 nm resolution would require structures in the absorber with aspect ratio in excess of 100, which is absolutely difficult to achieve. The second, more basic limitation, is that the photoelectrons generated by X-ray of 10 keV energy have correspondingly much higher spreading range in the resist up to 100-200 nm.