The influence of X-ray fluorescence on LIGA sidewall tolerances

Abstract

Fluorescence radiation during LIGA X-ray exposure and the impact of the resulting secondary dose on feature sidewall tolerances are examined using numerical methods. The study addresses fluorescence emitted by the substrate of the PMMA resist and focuses on the tolerances obtained for resists patterned with a range of feature sizes. New models of the secondary dose and subsequent feature development are presented and discussed. These models are coupled such that the evolving feature geometry during development depends on the local total dose, as well as the development temperature and the transport of PMMA fragments away from the dissolution front. We find that sidewall tolerances for typical exposure and development conditions are less than 0.1 micron for a metallized silicon substrate, but exceed several microns for thick titanium, copper and nickel substrates. We also find that these tolerances are very sensitive to the feature geometry, development temperature and the magnitude of the primary dose. IntroductionSecondary radiation is a well-known source of dimensional error in the LIGA micro-fabrication process [1][2][3][4]. During X-ray exposure of the PMMA resist, some primary photons from the synchrotron source are absorbed in the PMMA substrate. This absorption produces both highenergy photoelectrons and fluorescence X-rays. Photoelectrons emitted from the substrate are known to affect PMMA adhesion, but these are generally absorbed over a very small distance and so do not strongly affect structure accuracy. In contrast, fluorescence photons are absorbed over large distances, several millimeters in some cases. Since fluorescence is emitted isotropically, a portion of this radiation is absorbed in masked regions of the PMMA. During subsequent development, this increased dose may result in substantial dissolution of feature sidewalls, leading to dimensional errors between the patterned mask absorber and the final plastic or electroformed metal part. Secondary radiation emitted from the mask membrane may similarly contribute to sidewall dissolution and the accompanying degradation of accuracy [4].Previous computational studies of the LIGA process have addressed in some detail the two-dimensional distribution of the increased dose due to both fluorescence and photoelectrons [2][3][4][5]. However, there has been relatively little study of the effects of this distribution on the twodimensional history of development [5]. Generally, the final sidewall profile has been estimated based on contours of the total dose. This approach provides fair agreement with measured results under limited conditions, but clearly does not address the varying times each portion of the sidewall is exposed to developer. It also does not take into account that the development rate of PMMA varies smoothly with the dose down to 0.2 kJ/cm 3 or less [4].To help understand and quantify LIGA sidewall tolerances, we have developed coupled models addressing both exposure and development. The one-dimensional multiwavelength model of primary X-ra...

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

“…Vertical profiles of bright and shadow-region primary doses are computed using LEX-D, a onedimensional multi-wavelength model describing the spectrum of the synchrotron, x-ray transmission through any beam filters, transmission through the mask, and transmission and absorption in the PMMA resist [9]. The electron dose distributions, as presented here, are also calculated using this code.…”

Final-part metrology for LIGA springs, Build Group 1.

“…2pm over the 1500pm structure height) sidewall profile with a larger footing on the bottom, is typical, resulting partly from the ultra-deep x-ray lithography process. While the primary dose in the PMMA resist is uniform, which would result in a practically straight sidewall profile, there are secondary radiation effects leading to increased dose on the bottom of the PMMA, which reflects in more material in the electroplated structures [2,[19][20][21]. Also, during electrodeposition, the PMMA resist tends to swell, affecting the top of the mold most because it is exposed to the electrolyte the longest and is not directly attached to the substrate.…”

LIGA-based microsystem manufacturing:the electrochemistry of through-mold depostion and material properties.

“…Since the substrate is exposed to x-rays during the exposure of the PMMA resist, metallization layers between the substrate and resist (such as those commonly used as plating bases, like copper) can re-emit the x-ray radiation (e.g., by fluorescence), damaging the bond at the resist-metallization interface and provoking adhesion failures during subsequent processing. 82 Since this damage can lead to the failure of the entire LIGA mold, alternative plating bases that do not behave in such a way are thus of interest.82 At the Forschungzentrum Karlsruhe in Germany, an approach using Ti02 as a plating base has been developed and successfully empl~yed.~' Other suitable plating base materials have been recommended, but electrodeposition processes where good nucleation and adhesion to the base still need to be developed for these materials. 82 In this section, the challenges in attaining good uniformity in deposit morphology and/or deposit alloy composition over various length scales during the LIGA electrodeposition process step was emphasized.…”

“…82 Since this damage can lead to the failure of the entire LIGA mold, alternative plating bases that do not behave in such a way are thus of interest.82 At the Forschungzentrum Karlsruhe in Germany, an approach using Ti02 as a plating base has been developed and successfully empl~yed.~' Other suitable plating base materials have been recommended, but electrodeposition processes where good nucleation and adhesion to the base still need to be developed for these materials. 82 In this section, the challenges in attaining good uniformity in deposit morphology and/or deposit alloy composition over various length scales during the LIGA electrodeposition process step was emphasized. In practice, an equally important aspect in electrodeposition for LIGA is the subject of electrolyte aging and maintenance.…”