Diffractive phase-shift lithography photomask operating in proximity printing mode Giuseppe A. Cirino*1, Ronaldo D. Mansano2, Patrick Verdonck3, Lucila Cescato4, and Luiz G. Neto5 1
CCET, Federal University of Sao Carlos, Rod. Whashington Luis, Km 235 - 13565-905, Sao Carlos, SP, Brazil 2 PSI, University of Sao Paulo, Av. Prof. Luciano Gualberto, 158, trav. 3 - 05508-970, Sao Paulo, SP, Brazil 3 IMEC, Kapeldreef 75, B-3001 Leuven, Belgium 4 IFGW, Campinas State University, 13.083-970, Campinas, SP, Brazil 5 EESC, University of Sao Paulo, Av. Trabalhador Sao-carlense, 400 - 13560-970, Sao Carlos, SP, Brazil *
[email protected]
Abstract: A phase shift proximity printing lithographic mask is designed, manufactured and tested. Its design is based on a Fresnel computergenerated hologram, employing the scalar diffraction theory. The obtained amplitude and phase distributions were mapped into discrete levels. In addition, a coding scheme using sub-cells structure was employed in order to increase the number of discrete levels, thus increasing the degree of freedom in the resulting mask. The mask is fabricated on a fused silica substrate and an amorphous hydrogenated carbon (a:C-H) thin film which act as amplitude modulation agent. The lithographic image is projected onto a resist coated silicon wafer, placed at a distance of 50 µm behind the mask. The results show a improvement of the achieved resolution – linewidth as good as 1.5 µm - what is impossible to obtain with traditional binary masks in proximity printing mode. Such achieved dimensions can be used in the fabrication of MEMS and MOEMS devices. These results are obtained with a UV laser but also with a small arc lamp light source exploring the partial coherence of this source. ©2010 Optical Society of America OCIS codes: (310.6845) Thin film devices and applications; (070.7345) Wave propagation; (150.1135) Algorithms; (110.5220) Photolithography; (220.4000) Microstructure fabrication
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Received 17 Jun 2010; revised 6 Jul 2010; accepted 7 Jul 2010; published 20 Jul 2010
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Received 17 Jun 2010; revised 6 Jul 2010; accepted 7 Jul 2010; published 20 Jul 2010
2 August 2010 / Vol. 18, No. 16 / OPTICS EXPRESS 16388
1. Introduction The huge importance of the lithography on the fabrication of micro devices is well known. For most applications binary photolithography masks are employed. These masks consist of a transparent plate, covered by a patterned opaque film that acts as a light intensity modulating agent. The phase shift mask technique, introduced in the beginning of the eighties, employs a transparent thin film that modulates the phase of the transmitted light in order to compensate the diffraction effect of the edges of the mask features [1–4]. This technique has been improved since then [5–7]. A phase shift of an incoming wavefront can be implemented both by adding a transparent thin layer with adjustable thickness on certain regions of the substrate, or by removing a specific amount of material from the same transparent region of the substrate. The first method imposes a π-phase delay in the wavefront, whereas the second one imposes a phase advancing by π radians. In this way, a phase shift mask can be considered a diffractive optical element capable of modulating both phase and amplitude (or intensity) of light [8–10]. Proximity printing has been a very popular lithographic technique for microelectronic fabrication, because of its simplicity and low cost. Although the resolution achieved by this technique is far too large for today’s microelectronic fabrication, it is still very suitable for MEMS and MOEMS devices, with dimensions which typically range from several micrometers to several millimeters. Many authors have been exploring this technique, in several fields of MEMS and MOEMS applications. Dentinger and associates studied the influence on the final resolution for thick resists exposed in proximity mode [11]. Kathleen investigated modeling and characterization of the resolution of proximity printing x-ray lithography [12]. Meyerhofer and associates investigated the application of proximity printing in the copy of chrome masks, and shown its potential for replicating 10X step-and-repeat recticles [13]. The fabrication of microlens arrays by exploring the diffraction of light and proximity exposure has been demonstrated by Lin and associates [14], Ping and associates [15], and Yang and associates [16]. Lin and associates fabricated optical waveguides by this same technique [17]. Schwalm and associates have simulated the proximity printing lithography in a model, which takes into account the effects of wave propagation inside a resist layer, diffraction, standing waves and the profile degradation beyond certain limits for the proximity distance between mask and wafer [18]. Meliorisz and associates also presented simulation results for proximity printing lithography, and compared scalar with rigorous electromagnetic computations [19,20]. Teschke and associates have developed an interesting approach to implement halftone phase masks for analog lithography by using kinoform computer-generated holograms. This approach is derived from the interferometric phase contrast technique [21]. The resolution, W, of the proximity exposure technique is limited by the diffraction effects that occur at the mask pattern edges [22, 23], and given by
W = γ λdg
(1)
where dg denotes the gap between the mask and wafer under processing, λ is the operating wavelength and γ is a parameter which depends on the lithography process (typically 1< γ
