Printing methods are becoming important in the fabrication of flexible electronics. A transfer printing method
has been developed for the fabrication of organic thin-film transistors (OTFT), capacitors, resistors and inductors onto
plastic substrates. The method relies primarily on differential adhesion for the transfer of a printable layer from a
transfer substrate to a device substrate. A range of materials applications is illustrated, including metals, organic
semiconductors, organic dielectrics, nanotube and nanowire mats, a patterned inorganic semiconductor and graphene.
Transfer printing can be used to create complex structures including many disparate materials sequentially printed onto
the flexible substrate, with no mixed processing steps performed on the device substrate. Specifically, the fabrication
and performance of model OTFT devices consisting of a polyethylene terephthalate (PET) substrate, gold (Au) gate and
source/drain electrodes, a poly(methyl methacrylate) (PMMA) dielectric layer and either a pentacene (Pn) or a poly(3-
hexylthiophene) (P3HT) organic semiconductor layer will be presented. These transfer printed OTFTs on plastic outperform
non-printed devices on a Si substrate with a SiO2 dielectric layer (SiO2/Si). Transfer printed Pn OTFTs on a
plastic substrate have exhibited mobilities of 0.237 cm2/Vs, compared to non-printed Pn OTFTs on a SiO2/Si substrate
with mobilities of 0.1 cm2/Vs. Transfer printed P3HT TFTs on a plastic substrate have exhibited mobilites of 0.04
cm2/Vs, compared to non-printed P3HT TFTs on a SiO2/Si substrate with mobilities of 0.007 cm2/Vs.
Specular X-ray reflectivity (SXR) can be used, in the limit of the effective medium approximation (EMA), as a highresolution
shape metrology for periodic patterns on a planar substrate. The EMA means that the density of the solid
pattern and the space separating the periodic patterns are averaged together. In this limit the density profile as a
function of pattern height obtained by SXR can be used to extract quantitative pattern profile information. Here we
explore the limitations of SXR as a pattern shape metrology by studying a series of linear grating structures with
periodicities ranging from 300 nm to 16 &mgr;m. The applicability of the EMA is related to the coherence length of the Xray
source. For our slit-collimated X-ray source, the coherence length in the direction parallel to the long axis of the
slit is on the order of 900 nm while the coherence along the main axis of the beam appears to be much greater than
16 &mgr;m. Limitations of the SXR pattern shape metrology are discussed and examples of determining quantitative
pattern profiles provided.
Directly patterning dielectric insulator materials for semiconductor devices via nanoimprint lithography has the
potential to simplify fabrication processes and reduce manufacturing costs. However, the prospect of mechanically
forming these materials, especially when they are highly porous, raises concerns about their physical integrity. We
report the direct imprinting of 100 nm parallel line-space patterns into a high modulus poly(methylsilsesquioxane)-based
organosilicate thin film that is capable, in its non-patterned form, of meeting the ultra-low dielectric constant
requirement of k ≈ 2.3. Immediately after imprinting a (5 to 10) % shrinkage in the pattern height of the partially
vitrified patterns relative to the mold is quantified using X-ray reflectivity. Nanoscopic pores with an average diameter
of approximately 2.2 nm are then generated in the patterns at high temperatures, through the volatilization a second
phase porogen, while the material simultaneously vitrifies into a glassy organosilicate network. Pattern shape changes
upon vitrification are also quntified and indicating that a 12 % reduction in the pattern height of the porogen-loaded
imprint is observed with very little change in the pattern width. For a imprint without the added porogen, the shrinkage
is still anisotropic in the height direction, but reduced approximately by 4 %. Our results show that nanoporous low-k
patterns can be replicated via nanoimprint lithography with very little loss in the pattern quality.
The thermal embossing form of nanoimprint lithography is used to pattern arrays of nanostructures into several different polymer films. The shape of the imprinted patterns is characterized with nm precision using both X-ray scattering and reflectivity techniques. By studying the time dependent response of the pattern shape at temperatures near the glass transition temperature, we are able to perceive large levels of residual stress induced by the imprinting process. The large shear fields that result as the viscous polymer flows into the mold leads to residual stresses. At elevated temperatures in the freestanding structures (once the mold has been separated from the imprint), there is an accelerated reduction in pattern height in the reverse direction from which the material originally flowed into the mold. Two factors that influence this residual stress include the molecular mass of the polymer resist and the amount of time the pattern is annealed at high temperature in the presence of the mold.
To address several of the challenges associated with nanoimprint lithography, new measurement techniques that can correlate the physical structure of an imprinted nanostructure with the materials used and the imprinting conditions are critical for optimizing imprint processes. Specular X-ray reflectivity (SXR) is a widely used technique to quantify the thickness, density, and roughness of the non-patterned films. Here we extend the applicability of SXR to imprinted nanostructures by characterizing the pattern height, the line-to-space ratio as a function of pattern height, the residual layer thickness, and the fidelity of pattern transfer.
Controlling the thickness and uniformity of the unpatterned, residual layer is a critical challenge to sub-50 nm patterning with nanoimprint lithography (NIL). While nanometer level uniformity is essential, there is currently a lack of metrological capability for residual layer characterization. Specular X-ray reflectivity (SXR) is a versatile and widely used metrology to quantify the thickness, density, and roughness of thin smooth films. Here we extend specular X-ray reflectivity (SXR) to measure the thickness of the residual layer with sub-nm resolution. In addition to the residual layer thickness, X-ray reflectivity also reveals detailed information about the pattern height, the line to space ratio, and the relative line width variations of the pattern as a function of the pattern height.