NanoImprint Lithography: Ready For High Volume Manufacturing
The Sindre system enables industrial scale production of advanced microand nanometre structures for a wide range of applications, which have not been possible until now. Among the first new products that will reach the market are high brightness LED's for projection devices, and next generation hard disk drives based on discrete track recording (DTR), and bit pattern media (BPM) technology. Other market segments like multi-layer optical storage media, optical components and filters for communication devices and displays, high density interconnects, polymer electronics, and bioapplications will highly benefit as well. The initial wafer throughput of Sindre is targeted to 30-90 wafers/hour but is expected to reach even higher values once customer specific production processes are optimized and implemented on site.
The traditional way of NanoImprint Lithography employs a hard mould, preferably made of silicon, fused silica, or nickel. This mould is printed into a polymer layer that is either brought above its glass transition temperature, or a cross-linkable prepolymer, or a UV-curable monomer. The mould itself can be produced by high-resolution lithography tools based on electron beams, interference of light or other technologies. These tools generally have a low throughput but the advantage of NIL is that the mould can be used for many reproduction cycles by printing or even prior to printing by development of entire mould families via nickel electroforming.
Until now, hurdles related to hard stamps, hard press tools with rigid pistons, or master stamp-related cost of ownership discussions implied some drawbacks that prevented NIL technology's breakthrough from industrial scale production. Major issues have been:
(i) direct contact between hard master stamp material (i.e. silicon, nickel or quartz) and hard target substrate (i.e. semiconductor, quartz etc.) that implies risks regarding lifetime of the cost intensive master.
(ii) The wafer flatness of stamps and substrates can vary resulting in defects or breakage. (iii) Particle contamination can result in major defect areas around the particle and eventually lead to damage or breakage of the master stamp. (iv) Stamp manufacturing cost reduction by reducing the stamp and imprint area and using step and stamp tools are not competitive due to lack of throughput and the limited imprint area. Many devices are larger than 1 inch2.
What the industry needs is: (i) a tool that guarantees a maximum stamp lifetime of several thousand imprints per master to minimise stamp contributions to the cost of ownership model, (ii) a tool that can print at high pressures without damaging stamps or substrates, (iii) a tool that can print adaptively on non-flat surfaces or surfaces with particle contamination, (iv) a tool with high throughput that can print on large areas, preferably on wafer scale, (v) a tool that guarantees thin and homogeneous residual layers high fidelity pattern transfer, and (vi) a simple imprint process enabling easy downstream processing resulting in low costs of ownership.
Obducat has responded to these concerns and the solutions were successfully implemented in a series of Obducat's R&D and pre-production imprint tools that are widely distributed with a large installation base worldwide. The final step for industrialisation was to build the fully automated Sindre tool, that was designed around a proprietary IPS/STU imprint process cycle comprising two imprint steps and modules that are based on Obducat's soft-press technology.
Soft Press Technology
Obducat's imprint tools employ compressed gas for equal distribution of high pressure across the entire sample surface rather than rigid metal constructions never achieving the nanometre precision required for wafer scale reproduction of nanometre structures. The Soft Press technology allow the achievement of a homogeneous, sub-20 nm residual layer with a variation better than ± 5 nm. Moreover, Obducat's Soft Press technology is scalable up to any imprint area size opening the pathway for nanostructuring of large area devices i.e. flat-panel displays or high density interconnects on PCB boards.
The First Step
The first imprint step is a replication of a master stamp into an Intermediate Polymer Stamp called IPS. The IPS material is supplied as polymer liner on rolls and fed through the entire HVM-tool. Once an IPS has been manufactured at imprint head 1 it is transported to the second imprint head where the IPS pattern is printed once onto the final target substrate before it is discarded. Employing IPS stamps instead of hard stamps solves hurdles (i)-(iv) above and fulfils the requirements (i)-(v) since the soft material will not damage the master stamp or the substrate. It adapts to uneven surfaces such as epitaxially overgrown substrates or samples contaminated with particles. The IPS-process has a self-cleaning effect to the master stamp: loose particles are picked up with each IPS leaving clean master surfaces behind for the next IPS to be produced. This ensures an extended lifetime for master stamps, maintains high imprint quality, and reduces the cost of ownership. The STU-process is executed by printing the IPS once into a pre-heated, pre-polymer layer spincoated on the target substrate. The use of spincoating rather than complicated dispensing technologies in conjunction with Soft Press technology guarantees a homogeneous residual layer thickness < 20 ± 5 nm on wafer scale.
The IPS/STU process enables high fidelity pattern transfer without the necessity of planarisation layers or pattern transfer layers. Simple oxygen plasma ashing of the residual layer prior to subsequent downstream processing opens the polymer mask pattern. For all applications the resist itself or an additional hard mask are more than sufficient to enable the pattern transfer. Figure 4 shows a SEM micrograph of an IPS obtained from a nickel master stamp (top) and its STU-imprint result. The pattern is a typical DTRhard disk drive pattern that will enter the market in the near future. It was generated with an Obducat EBR 50kV TFE Electron Beam Recorder.
As already mentioned the flexible IPS can also be used to print on uneven surfaces such as epitaxially grown compound semiconductors that are commonly used for manufacturing light emitting diodes. It adapts to the curvature and major growth defects like spikes on the substrate are no hindrance in the application of this technology.
Figure 5 shows an IPS obtained from a silicon master stamp (top) as well as its imprint on an uneven epitaxially overgrown compound semiconductor substrate. The pattern shows a 12-fold symmetry photonic quasi-crystal pattern as it can be used for light extraction enhancement in coloured high-brightness LEDs. The structures are printed with a residual layer thickness of 15 +/- 5 nm on wafer-scale. Mesophotonics, UK, supplied the pattern.