A New Type of Cross-Linked Resist for Small Pattern Roughness

Toru Yamaguchi, Masao Nagase, Kenji Yamazaki, and Hideo Namatsu
Device Physics Laboratory

In the fabrication of high-performance nanodevices, such as single electron transistors, even nanometer-order roughness produces large variations in device characteristics because device sizes are less than 10 nm. We have clarified that roughness is due to polymer aggregates 20-30 nm in size, which are contained in resist materials [1]. In resist films containing these aggregates, the polymers surrounding aggregates dissolve faster and the aggregates can be extracted from the resist surface one by one since the polymer density is higher inside the aggregates (the aggregate extraction development [2]). As a result, many aggregates remain embedded on the pattern sidewall and they are responsible for pattern roughness [Fig. 1(a)].
To reduce roughness, aggregate extraction development must be suppressed. To achieve this, we have developed SAGEX (suppressed aggregate extraction development) resist [3]. In SAGEX resist, the polymer aggregates are thermally cross-linked with each other before electron-beam exposure. In the exposed regions, the resist dissolves uniformly over the whole surface since cross-linking slows down the dissolution rate of the surrounding polymers to the point where it is the same as that of the aggregates themselves. As a result, the aggregate extraction development is suppressed and the resulting patterns have smaller roughness.
Figure 2 shows surface roughness on the exposed region observed by an atomic force microscope (AFM). In conventional resists, the surface becomes rougher because of the appearance of the aggregates on the surface. In contrast, in SAGEX resists, no aggregates can be observed and the surface roughness is drastically reduced to less than one-third compared to that with conventional resist film. This proves that the aggregate extraction development is actually suppressed in SAGEX resist. These results show that SAGEX resist is very effective for nanodevice fabrication.
[1] T. Yamaguchi et al., Appl. Phys. Lett. 71 (1997) 2388.
[2] T. Yamaguchi et al., Proc. SPIE 3333 (1998) 830.
[3] T. Yamaguchi et al., Jpn. J. Appl. Phys. 38 (1999) 7114.


Fig. 1. Formation process of resist patterns.

Fig. 2. AFM images of the exposed resist.