Nanoimprinting lithography was initiated as an alternative way to achieve nanoscale structures with high throughput and low cost. We have developed a UV-nanoimprint process to fabricate 34x34 crossbar circuits with a half-pitch of 50 nm (equivalent to a bit density of 10 Gbit/cm2). Our resist was of a single layer, which required fewer processing steps than any bi-layer process, but yielded high quality results. By engineering the surface energy of the substrate, we also eliminated the problem of trapped air during contact with the mold due to non-conformal contact such that it spreads the resist and expels trapped air. Resist adhesion to the gaps between features in the mold during mold separation is a challenge that becomes more severe as the pitch size shrinks. We have improved the resist adhesion to the substrate by applying a monolayer of surface linker molecule on the substrate surface. The surface linker bonded the resist to the substrate surface chemically and produced fine imprinted patterns at 30 nm hp.
Nanoimprint lithography is a contact-lithography technology invented in 1996 as a low-cost alternative to photolithography for researchers who need high resolution patterning. Initially perceived as a trailing-edge technology for low-cost device fabrication, it has been recently demonstrated to achieve sub-10 nm resolution and alignment, which equal or surpass even the most advanced photolithography today. At Hewlett-Packard, we have successfully used it to fabricate switchable molecular memory arrays with a dimension of 65 nm half pitch. Nanoimprint has been placed on the International Technology Roadmap for Semiconductors (ITRS) as a candidate for next-generation lithography (NGL) for insertion in the 32 nm node in Y2013. The switch from using light to using contact to pattern will indeed bring new challenges, the most important of which are alignment and the 1x mask/template. For alignment, one imprint tool maker has achieved alignment of +/-7 nm 3 sigma using Moire patterns. For template fabrication, the lack of OPC and other sub-resolution features produced large savings in patterning, but it is nearly cancelled out by the need for more aggressive inspection because of the smaller tolerable defect size. The two combined to make the predicted cost of nanoimprint template to be similar to photomasks for 45-nm half pitch. At 32-nm half pitch, EUVL masks do not have complicated sub-resolution features and are predicted to be cheaper than comparable nanoimprint templates provided that the former’s defect levels can be reduced to what is required for economical manufacturing. In both cases, the challenges are not insurmountable and solutions are being actively pursued. However, if nanoimprint is indeed the disruptive technology to photolithography, it needs to take its initial aim at the low-end market rather than mount a frontal challenge at semiconductor manufacturing, which is the high-margin customers that photolithography will pursue and protect at all cost. The recent development in nanotechnology will lead to the commercialization of a new class of nanoscale devices requiring a high-resolution lithographic technique that does not have all the functionalities of photolithography. This approach will provide an initial customer base for nanoimprint to develop and improve and position it to challenge photolithography in the distant future.
We have utilized the nanoimprint lithography process described this paper to fabricate a rewritable, nonvolatile memory cell with an equivalent density of 6.4 Gbits/cm2. The architecture of the circuit was based on an 8x8 crossbar structure with an active molecular layer sandwiched between the top and bottom electrodes. A liftoff process was utilized to produce the top and bottom electrodes, made of Pt/Ti bilayers. The active molecular layer was deposited by the Languir-Blodgett technique. We proposed the use of a new class of nanoimprint resist formulated by dissolving a polymer in its monomer, such as poly(benzyl methacrylate) dissolved in benzyl methacrylate (~8%/92% wt). The new resist enabled us to achieve Pt /Ti lines of 40 nm in width and 130 nm in pitch, as described in this paper. Our overall nanofabrication process has the advantages of relatively low temperature (~70°C) and pressure (~500 psi or 4.5 MPa), both of which are critical to preserving the integrity of the molecular layer.
Organic bilayer light emitting devices have been constructed using rubrene-doped poly(2-methoxy-5(2'-ethylhexyloxy)-p- phenylenevinylene) (MEH-PPV) as the emissive material and a novel hybrid compound consisting of pyridine and oxadiazole -- 2,5-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5- yl]pyridine (PDPyDP) -- as an electron transport layer. Although such structures exhibited a higher external quantum efficiency than those in which the electron transport layer was 1,3-bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazol-5- yl]benzene (OXD-7) they degraded relatively quickly, both in storage and under an electrical stress. Considerable improvements in the device lifetimes were obtained by the insertion of a copper phthalocyanine layer between the anode and the MEH-PPV layer and a tris (8-hydroxyquinoline) aluminum (Alq3) layer between the PDPyDP layer and the Al electrode.
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