Proximity X-Ray Lithography

The year 1972 witnessed the invention of proximity x-ray lithography by Hank Smith at Massachusetts Institute of Technology. Many companies (IBM, Canon, Nikon), universities (MIT and the University of Wisconsin), and governments (United States and Japan) spent well over a billion dollars on its development. The first attempts at production of x-ray aligners were made in the mid-1970s by AT T and MIT, and in the early 1980s by Micronix Partners, Hampshire Instruments, Nikon, and Karl Suss. X-ray step-and-repeat and step-and-scan systems were eventually made commercially available.  

The promise of x-ray lithography lies in the fact that it offers the shortest optical wavelength, and theoretically the highest resolution of all of the other optical lithographies. The minimum resolvable linewidth w in x-ray lithography is given by 

Absorber made of high atomic number material 

There are, however, some major limitations of x-ray lithography, and these include the difficulty of fabricating 1 x masks, the high cost of generating the x rays at sufficient power, penumbral and diffraction blurs that limit resolution (in the case of point sources), and overlay control.  

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Fig. 42. Diagram of a mask used for 1 1 proximity x-ray lithography. These thin-membrane masks, required for optimum transmission when using patterned metal absorbers, must remain free of distortion to maintain pattern fidelity during exposure.

X. M. Yang, R. D. Peters, T. K. Kim, P. F. Nealey, S. L. Brandow, M. S. Chen, L. M. Shirey, W. J. Dressick, Proximity x-ray lithography using self-assembled alkylsi-loxane films Resolution and pattern transfer. Langmuir, 2001,17,228. 

L. Malmqvist, L. Rymell, H.M. Hertz Droplet-target laser-plasma source for proximity X-ray lithography. Appl. Phys. Lett. 68, 2627 (1996)... 

Proximity-mode X-ray lithography, 75 159 Proxmire Amendments, 78 685 Prussian Blue, 74 534, 536-537 79 407 22 810 color, 7 332 Prussian blue systems... 

X-ray lithography offers one outstanding advantage over the other methods used for fabricating sub-micron dimensions. The x-rays are neither scattered in the resist nor are they diffracted, so very high aspect ratio resist patterns are produced, and there is no proximity effect as there is with electrons. 

L First manufacturing use of chemically amplified resists Plasma-developed resist first described X-ray proximity lithography demonstrated Bis-azide rubber resists introduced DNO-novolac resist for microelectronics introduced Photoresist technology first applied to transistor fabrication DNO-novolac resist patented by Kalle... 

Figure 5.6 Lithography by hard X-rays using a proximity gold mask. The X-rays produce a different solubility in etching solutions between the exposed and unexposed regions. This approach is typically used for the fabrication of polymer-based resist with high aspect ratio (a). After that the resist is used as a mold or for structural application (b). The proper choice of a functional material in a weakly or non-cross-linked state such as sol-gel in the xerogel state can offer several advantages for the preparation of functional devices in a one-or two-step process. Film patterning can...

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