Fabrication of composite x-ray masks by micromilling

Abstract

An important aspect for the development of micromanufactured components and systems is to reduce the time and cost required to reach the prototype stage. At present, this development typically spans several years. Any fabrication approach which would reduce the cost and time-to-prototype would allow for the more rapid development of design concepts and the more rapid evolution of the design cycle. Direct fabrication of masks for X-ray lithography, by mechanical micromilling, is one potential avenue for rapid, lower cost development.The key process requirements for the fabrication ofa typical X-ray mask involves the selection ofboth substrate and absorber materials. The substrate must provide a mechanically stable support for the patterned absorber without introducing excessive attenuation ofthe X-ray flux that ultimately reaches the resist surface. Frame supported, thin membranes (such as SiC, C, Si3N4, Si) are most often used as well as low atomic number bulk materials (Be). The choice of elemental composition and thickness for the absorber will be largely determined by the resist sensitivity and the X-ray wavelength used. Many process steps are required in order to define the final absorber pattern geometry and will generally involve either additive or subtractive processes.Mechanical micromilling techniques may be used with either a single bulk material which serves the dual role of both substrate and absorber or with a composite structure consisting of a thin gold layer deposited on a thick, low atomic number bulk substrate. Single material masks ofaluminum and graphite have been investigated . A composite mask of graphite with a thin layer of sputtered gold has also been investigated. The paper will report on the developmental work for both types of masks and will give results for synchrotron X-ray exposures using these masks. Problems associated with using micromilling as an X-ray mask fabrication method will also be presented.

An experimental investigation of micro-machinability of copper 101 using tungsten carbide micro-endmills

“…Micromachining utilizes miniature milling, drilling and turning tools as small as 10 mm in diameter to produce micro-scale features. Although geometric and material capabilities of micromachining have been demonstrated by various researchers [5][6][7][8][9][10][11][12][13][14], industrial application of micromachining has been hindered by the lack of experience and knowledge on the micro-machinability of materials.…”

Precision of micromilled x-ray masks and exposures

“…Test exposures were made with varying thicknesses of commercially available (Goodfellow) graphite sheets. It was found that a thickness of less than 250 micrometers gave sufficient transmission of the x-rays to allow for timely exposures [Coane and Friedrich (1996)]. …”