illumination. The maximum amplitude and the formation speed of SRGs strongly depend on the polarization of the writing beams. Usually, SRGs of weak amplitude are formed uponssorspillumination, whereaspp,rlcp, and ±45° illumination leads to the formation of pronounced gratings.[50,62,131]If the illumination time is sufficiently long, some materials develop SRGs for all discussed illumination types.[51]
The underlying mechanism leading to SRG formation remains unresolved until today.[42,53]
Various models have been suggested to explain the origin of the driving force, involving thermal considerations,[54]models of gradient forces induced by isomerization pressure[55]
or optical fields,[56,57]mean-field theory,[58]and diffusion-based approaches.[59,60]None of them explains all the experimental findings of SRG formation. In the thermal model, SRGs result from gradients in temperature, which turn out to be negligibly small for the applied optical fields (∼10−4K). Moreover, part of the theoretical considerations apply to any ab-sorbing material and do not require the isomerization of azobenzene. Pressure gradients may arise during the isomerization process because thecis and trans form occupy a dif-ferent amount of free volume. The polarization dependence is not predicted correctly in such a model for all the optical fields of the various grating types. The optical-field gra-dient force model seems to solve this problem, since it reproduces experimentally found polarization features. Based on the assumption that the photoisomerization induces a spa-tially varying electric polarization, it suggests that the optical field exerts a force onto the medium. Saphiannikova et al. pointed out that the optically induced force density at a typi-cal writing intensity of 100 mW cm−2is about two orders of magnitude smaller than that of gravity.[136]Moreover, arguments exist that the forces exerted on polarizable media should depend on gradients in the light intensity and not on polarization.[33]Attractive forces be-tween the dipoles of aligned chromophores are taken into account in the mean-field ap-proach. Because it predicts an accumulation of the material in bright areas, it does not describe the phase shift of the optical and the surface relief grating observed in amorphous systems. Instead, the model can be applied to certain liquid-crystalline side chain poly-mers in which the interaction between chromophores is so strong that they form an in-phase relief grating.[137,138] Diffusion models are based on the assumption that the azobenzene chromophores perform an inchworm-like motion along their long axis. This describes the polarization dependency correctly and predicts a higher formation speed for systems with smaller molecules. SRG formation in polymers is excluded from these models, since the random motion of the backbone-connected moieties is not expected to allow for any net transport. Eventually, it is conceivable that more than one of the models applies, adding even more complexity to the mechanism of SRG formation.
Without further specification of the origin of the molecular force leading to SRG for-mation, the material transport can be described successfully with fluid-mechanics mod-els.[139,140] One has to assume, however, that the material is in a liquid or liquid-like state.
3.3 Athermal Photofluidization
A prerequisite for SRG formation or any other photo-induced macroscopic motion is the ability of an amorphous azobenzene-functionalized glass to flow like a liquid. A tempera-ture increase of the material caused by optical heating can be excluded as a source of such a phase transition. Even at intensities as high as 2 W cm−2, the photothermal temperature increase in azobenzene glass formers has been found not to exceed 15 K.[54,67,141] This is negligibly small as compared to the highTgof many glass formers. Instead, the liquid-like flow behavior can be attributed to another peculiarity of azobenzene-functionalized
materi-has entered the discussion about the flow mechanism of azobenzene-functionalized glass formers very recently.[79,142,143]Its existence was debated controversially in the early time, which can mainly be attributed to the discrepancy between the physical and the descriptive definition of the term “fluid”.[45]A reasonable definition of a photofluid will be given after briefly summarizing the underlying physics according to the current state of knowledge.
Upon absorption of a photon, an excitation energy of 2 to 3 eV is stored in an azobenzene moiety. In an isolated azobenzene molecule (in vacuum), most of this energy is dissipated into its low-frequency vibration modes upon electronic relaxation and isomerization within
∼0.4 ps,[142] raising its internal temperature to ∼1150 K.[112] In solution, the isomeriza-tion is hindered by intermolecular forces and the conversion takes place on a much longer timescale (&10 ps).[144] Here, the process is too slow to excite molecular vibrations and the energy can be transferred into cooperative motion of the adjacent molecules. Fang et al.
showed that in an azobenzene-based tethered self assembled monolayer a similar interaction allows for the activation of 30 degrees of freedom at 800 K, which are distributed over the nearest neighbors of the absorbing molecule.[142]Even though the situation changes in bulk materials, such high temperatures clearly exceed theTg of polymers or molecular glasses.
At the same time, the above temperature value does not correspond to an average temper-ature increase because the absorption events of single photons are separated in space and time even for high intensities. However, local barriers hindering the reorientation of the molecule can be overcome, resulting in a local glass transition. At ambient light conditions the efficiency of local fluidization stays below a critical value. Thus, the viscosity remains that of a solid.
The photofluidization of azobenzene-functionalized homopolymers and molecular glass-es in the bulk has been invglass-estigated by Vapaavuori et al.[45] They found that illumination with visible light leads to a strong band shift of the vibrational modes of the azobenzene moieties without affecting the Tg of the material. The changes in the molecular environ-ment of the azobenzene moieties upon illumination are similar to those measured during conventional heating of the material aboveTgin the absence of light. However, the optically induced motions and, therefore, the generation of free volume is very heterogeneous, occur-ring predominantly close to the azobenzene moieties. Consequently, the amorphous phase is maintained by the polymer backbone or the core of the molecular glass former, whereas the azobenzene-functionalized part experiences an effective temperature of up to 530 K.[45]This temperature is derived by means of comparing vibrational modes and does not correspond to the actual temperature of the material, which is essentially that of the photo-inactive bulk.
It is an effective quantity to indicate that the mobility of the photo-active groups is increased to such a degree that the amorphous glass former behaves like a liquid.
In summary, athermal photofluidization denotes the light-induced process of turning an azobenzene-functionalized material into a photomobile state, in which it has the properties of both a glass and a fluid. Although the macroscopic change in temperature is negligibly small, the viscosity decreases by many orders of magnitude.[44,142]Depending on the inten-sity of the incident light and the molecular structure of the material, amorphous azobenzene systems show either viscoelastic[44,143]or viscous[46]flow behavior. The interaction of the azobenzene moieties with the optical field is so efficient that even crystalline systems can be converted into photofluidic melts.[145] The capability of azobenzene materials to form SRGs at room temperature impressively proves the existence of the photofluidic state. In contrast to thermal melting, photofluidization occurs quasi-instantaneously throughout the whole illuminated volume.
4
From Photofluidization to Lithography — Concept and Theory
Switching azobenzene systems into the photofluidic state grants access to unexpected areas of application. The fabrication of coatings or resists with patterns of high complexity is of great importance for the lithographic manufacturing process of various devices. Optical, contact-free patterning of azobenzene-based resists, also denoted by “directional photofluid-ization lithography” (DPL), may even become an alternative to common optical lithographic techniques.[42] A new and different concept is introduced here, following the concept of well-established imprint techniques. It utilizes photofluidizable systems and the adhesive properties of flexible molds to replicate predefined structures from a master. The theoretical background discussed in this chapter addresses the flow characteristics of viscous fluids in confined geometries.
4.1 Introduction to Nanoimprint Lithography
Nanoimprint lithography (NIL) is a replication technique developed to overcome the draw-backs of high-resolution patterning methods.[146]For instance, e-beam lithography is capa-ble of generating structures with features as small as 5 nm, but its throughput is rather low.
Photolithography is fast, but its resolution is limited to∼30 nm and high facility costs arise from circumventing the optical diffraction limit.[147–149]The idea of NIL is to transfer a pat-tern from a rigid master — e. g., prepared from one of the techniques mentioned above — to a resist by molding. For this purpose, a resist material, which either cures upon UV-light irradiation or softens upon heating, is coated onto a suitable substrate.[150] Subsequently, the master is brought into contact with the resist and the pattern of the master is imprinted in the resist. In a final step, the master is removed and the imprinted structures are etched into the substrate. High costs are avoided because neither an expensive equipment nor a great amount of time is needed. These advantages in combination with the high resolution of more costly techniques makes NIL a good candidate for nanoscale mass production.[151]
In the early stages, NIL was also referred to as the “mold mask method”.[152]This is due to the fact that often a less rigid polymer mold is prepared from the master, which is used for the imprinting process instead. The advantage of this approach is that the master suffers less stress and that a larger number of materials becomes available for the imprinting process.
Despite the fact that the mold carries the inverse pattern, the principle does not differ from imprinting the master.
29
Because NIL is a direct contact method, problems can emerge from the interaction of the mold or the master with the resist, e. g., partial mold filling or breaking, sticking of the resist layer, or material shrinkage. To minimize such defects, various techniques have been developed in the past.[42]Three of the most important ones are discussed in the following.
T-NIL UV-NIL
etching p≈20 to 100 bar heating aboveTG
mold
substrate liquid resin mold
solid polymer substrate
viscous polymer
solid polymer crosslinked polymer
p≈0 to 5 bar UV- exposure cooling
liquid resin liquid resin
Figure 4.1: Scheme of the T-NIL (left) and UV-NIL (right) process. Both techniques transfer the profile of a mold to a thin polymer film on top of a substrate. Etching transfers the structure to the substrate below.
Thermal Nanoimprint Lithography(T-NIL) is based on the characteristic of thermo-plastic materials to have a significantly decreased viscosity when heated aboveTg. Initially, a thermoplastic is prepared as a thin, homogeneous film on a substrate, to which the relief pattern of the mold needs to be transferred (cf. Fig. 4.1). After placing the mold on top of the solid film, the resist is heated aboveTg and the thermoplastic begins to fill the cavities of the mold. Typical values for the resist viscosity during this step are 103 to 107Pa s.[153]
The mold is removed as soon as the resist layer has cooled down sufficiently to maintain the imprinted surface profile.
A clear advantage of this method is that a large number of polymers can be used. This is important, e. g., for the fabrication of optical elements.[154]However, the cooling step takes a crucial amount of time, which can be partially compensated by decreasing the processing temperature. Robust, rigid molds withstanding high pressures have to be used in this case.
The imprints are prone to mold-sticking defects due to the differing thermal expansion co-efficients of mold and resist. Yet, T-NIL has made a lot of progress, starting from the initial work of Chou et al.[155]in 1995 to now being capable of producing high-quality micro- and
4.2 ATHERMAL AZOBENZENE-BASED NANOIMPRINT LITHOGRAPHY 31