Calculation and experimental determination of the structure transfer accuracy in deep x-ray lithography

Abstract

Deep x-ray lithography with synchrotron radiation is the key microfabrication process in the LIGA technology. Micro-components with a height of some up to several mm can be manufactured with sub- precision. The pattern transfer accuracy is governed by technological constraints like thermal deformation of the mask as well as by various physical effects, e.g. diffraction, emission of photo- and Auger electrons, fluorescence radiation, radiation scattering and divergence of the synchrotron radiation beam. A computer program has been developed to investigate the significance of these effects to the dose distribution in the resist material, which in turn determines the lateral structure resolution and the quality of the resist structures. In this paper the models used for the calculation and the calculation procedure are introduced and the weight of the different contributions with respect to transfer accuracy is investigated. It is shown that beam divergence and diffraction are much less important than the image blur caused by photoelectrons. Fluorescence radiation emitted from the mask membrane or the substrate contributes to the dose deposition in the resist if mask membrane or substrate consists of high-atomic-number material. Scattering of radiation is negligible for resist layers which are less than some mm thick. The calculations are compared with measurement results for different substrate materials, synchrotron radiation sources and resist heights. A good agreement was found between calculated dose distributions and measured resist side wall profiles. This allows a partial compensation of the above-mentioned accuracy limiting effects early in the mask design phase.

Fundamental limitations of LIGA x-ray lithography: sidewall offset, slope and minimum feature size

“…The electron dose distributions, as presented here, are also calculated using this code. Figure 4 shows a comparison between doses computed using LEX-D with the half-space kernel (curves) and the results of a Monte Carlo calculation (symbols) for an exposure performed at DCI [5]. The two results agree within about 15% for all spatial positions within the resist.…”

Large batch dimensional metrology demonstrated in the example of a LIGA fabricated spring

“…The height of the part also complicates the dimensioning since techniques used in the semiconductor or silicon MEMS area are not directly applicable. With one exception [3], past work involving metrology in the LIGA process has mainly focused on thermal mask distortions and resulting errors [4], polymethylmethacrylate (PMMA) offsets and overall patterning accuracy [5][6][7][8], and PMMA swelling during processing and electroforming [9]. The presented work dimensionally quantifies lots of released direct LIGA metal parts which comprise all of the above process effects and relates them to part specifications.…”

Considerations for the Deep X-ray Lithography with the SU-8 Resist.

“…However, the resist heights are large (500 µm -1500 µm) and the small beam divergence, which is always present, has an influence on the final structure. [5,7] The spectral distribution of the synchrotron radiation depends upon the beam energy and ring radius. The spectrum is continuous and the shape is a function of the characteristic wavelength which in Angstroms is given by the equation, The spectra of DCI with windows and filters, determined from these equations, are shown in Fig.…”

“…This result holds for resist layers with a thickness of more than some 10 µm. [5] Fig . 12 shows the the calculated isodosis lines with the inclusion of diffraction, divergence and secondary electron effects.…”

“…Therefore, the necessity of the lithography with X-rays is not only to achieve very high aspect ratio resist structures, as X-rays are extremely penetrating, but to also transfer mask patterns with very high resolution into the resist. [4,5] Polymethylmethacrylate (PMMA) is the resist of choice for deep X-ray lithography. It is a positive tone resists and the action of X-rays is chain scission, i.e.…”