Historical Development

An essential requirement for near field photolithography techniques such as ENFOL is to maintain a conformable photomask in intimate contact with the photoresist during exposure. The first work using conformable photolithography masks was reported by Smith in 1969 for fabricating surface acoustic wave (SAW) devices. Mechanical pressure was used to maintain contact and interdigital electrodes with 2.5 |im periods were replicated. In later work Smith demonstrated 400 nm linewidths on an 800 nm pitch using the same technique but this time using vacuum pressure to attain intimate contact between the mask and substrate. A distinct advantage of this system is the insensitivity of the linewidth to exposure time when operating in true intimate contact. In that paper Smith hints at the prospect of achieving higher resolutions - "Although no effort has been made to explore the ultimate limitation of the conformable mask photolithography, our observations lead us to speculate that narrower linewidths than reported here are probably possible, but may require the exercise of finer control over photoresist thickness and exposure parameters". These words turned out to be quite accurate, although were not realised until nearly a decade later, when White et al. demonstrated patterning of 150 nm features with a 157 nm laser source (White et al. 1984). A further important advantage of the conformable mask that is cited is the absence of damage or wear through repeated use (Smith et al. 2000), an improvement on the conventional photomasks which suffer degradation with use.

Goodberlet has used a modification of the conventional conformable mask, where the absorber is embedded into the mask substrate. His embedded amplitude mask (EAM) consists of a substrate material made from fused silica, with embedded Cr absorber patterns. There are a number of advantages of the having the absorber embedded: a flat mask would be expected to reduce local deformations around the absorber areas, the narrow regions between the absorber also improve waveguiding with their higher refractive index, and the planarity of the mask may protect the absorber and minimise particle contamination. The EAM mask has been used to reproduce 100 nm line and space structures, using a source with a wavelength of 220 nm. Pattern placement errors have been evaluated by Goodberlet by measuring the in-plane distortions of a double exposure with the mask misaligned slightly between the first and second exposure. An average displacement of 58 nm was observed over an area of 2 cm2. Attempts at multilevel alignment are also in progress. More recently Goodberlet (2002) has demonstrated patterning features down to 45 nm using this technique.

In another technique, light-coupling masks (LCM) for conformable contact photolithography have been developed by a group at IBM's Research Laboratory in Zurich (Schmid et al. 1998). The masks are made of an elastomeric material, similar to that developed for microcontact printing (Rogers et al. 1997), which allows for conformal contact. The mask consists of a surface relief pattern and relies on this pattern to induce local optical modes; this waveguiding action amplifies the intensity in the protruding mask regions, which results in contrast in the underlying resist (Paulus et al. 2001). Another contrast mechanism occurs when the wavelength is much smaller that the features on the mask; if the depth of the surface relief results in a phase change close to n the destructive interference that results at the mask edge results in a strongly unexposed region. The same effect is observed in near field phase shifting masks (PSMs). Results for LCM exposure demonstrate that 240 nm period is possible with a low surface relief pattern (70 nm deep) using 248 nm wavelength exposure. In this case the low surface relief and small size of the patterns means that the second exposure mechanism is not significant.

Fig. 17.1 illustrates the four possible mask configurations for near field lithography. A simulation study comparing the performance of these different techniques has been performed (Paulus etal.2000); whilst the contrast, depth of focus and minimum feature size are slightly improved for the embedded absorber and recessed absorber compared to the protruding absorber, the simplicity of mask fabrication makes the latter technique the best choice in many cases. An alternative to using a conformable mask with a rigid substrate is to use a rigid mask with a conformable substrate. Ono et al. have replicated 500 nm period structures with such an arrangement. They formed substrates from 1|im silicon diaphragms and performed a contact exposure with standard chrome on glass mask using vacuum pressure and a mercury arc lamp source. However, the use of conformable substrates limits the range of devices that can be fabricated. Near field schemes using conformable masks are much more prominent, and other experimental and simulation studies have appeared in the literature where resolution beyond the diffraction limits have been demonstrated (Haefliger and Stemmer 2004; Luo and Ishihara 2004).

Protruding absorber

Embedded absorber

Recessed absorber

Phase shift mask

Fig. 17.1. Illustration of the four possible configurations of near field masks, where dark features represent the absorber: (a) is the protruding absorber (b) embedded absorber (Goodberlet 2000), (c) recessed absorber (Schmid et al. 1998; Paulus et al. 2001), and (d) chromeless phase shift mask (Levenson 1993; Alkaisi et al. 1998)

Phase shift mask

Fig. 17.1. Illustration of the four possible configurations of near field masks, where dark features represent the absorber: (a) is the protruding absorber (b) embedded absorber (Goodberlet 2000), (c) recessed absorber (Schmid et al. 1998; Paulus et al. 2001), and (d) chromeless phase shift mask (Levenson 1993; Alkaisi et al. 1998)

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