Nanoimprint Lithography (2010)

Lithography 496 both imprinting and etching are important in order to obtain a suitable resist profile without any residual layer. For some particular applications, the resist layer can be replaced by functional materials or omitted for an imprint directly into the substrate. Currently, two main types of nanoimprint methods are existing, i.e., hot embossing and UVbased nanoimprint lithography (UV-NIL). An overview of the most important nanoimprinting techniques and its related stamp materials is shown in figure PMMA, COC, PC) spin coated on a substrate or a polymer substrate. After pre-bake and heating up above the glass transition temperature of the polymer, the imprint is performed by applying a contact force on the stack consisting of template and substrate. Then, the template-substrate assembly is cooled down before the template release. Alternatively, the UV-based technique is based on the use of photo-curable resist, which can be e a s i l y de f or m e d a t r oom t e m p e r a t u r e a n d c u red by UV-light exposure. Thus the main difference between the two techniques are the material types of both template and resist, i.e., transparent templates and photosensitive resists for UV-NIL, but in some cases nontransparent templates and thermoplastic resists for thermal-NIL. For both hot embossing and UV-based processes, the template fabrication is one of the most critical issues because of its high cost. This depends on the required minimum feature size, the density of the features and the patterned area. In general, electron beam lithography and reactive ion etching are used to pattern silicon or quartz glass wafers. High resolution patterning over a large wafer area is time consuming but ideally, once the template has been produced, it can be used for many times. In practice, both silicon and quartz glass templates can be damaged due to process imperfection. Compared to hot embossing, UV-NIL is advantageous when multi-level patterning is needed due to the fact, that high precision alignment can be done much easier with processes performed at room temperature as there are no thermal gradients generated. Basically, the cycle time of UV-NIL is shorter than in hot embossing due to the absence of heating and cooling cycles (heating-up to the imprinting temperature and cooling down to the de-embossing temperature). In addition, both pattern placement accuracy and overlay alignment accuracy of UV-NIL are also inherently higher than in hot embossing processes, because of reduced size variations of templates and wafer materials due to the thermal cycles. It is known that with an increase of the wafer size, the wafer flatness decreases so that a high pressure has to be applied to assure a good imprinting uniformity. Typically, the maximum imprinting area is limited to about 25 mm 25 mm for UV-NIL processes, considering the stiffness and the thickness variations of quartz glass templates and silicon wafers. In particular cases, the step-and-repeat strategy can be applied to replicate the same small sized template many times over the whole wafer area. However, this requires a more sophisticated imprint tool and can not be applied to the general case of large wafer pattern replication. On the other hand hot embossing is superior over UV-NIL if polymer materials need to be imprinted directly. Thus, it is highly desirable to have a low cost solution for large area replications, with less risk of template damage but improved process latitude. One possibility is to use soft templates, which are referred to soft working stamps. The use of soft materials should allow for a more conformable imprinting over a large wafer area. Accordingly, soft and hard UV-NIL refers to the material hardness of the template material, hard for a quartz glass template and soft for a soft layer attached to a glass back plane (soft UV-NIL). Another advantage is that such soft working stamps can be easily obtained by casting and curing a liquid www.intechopen.com