Pulsed NIL vs Conventional Thermal NIL
The Pulsed-NIL patented process stems from the thermal Nanoimprint Lithography. While in the latter the heat for the thermal cycle is provided by the hot plates of a press, in Pulsed-NIL technology the stamp itself is the source of heat, thanks to a 2D integrated Joule’s heater buried below its patterned surface. High temperatures are obtained with a short and intense current pulse. Consequently, the polymer in contact with the stamp attains very low viscosity during an extremely short thermal cycle.
Pulsed-NIL uses heatable stamps, but at the same time relies on established concepts and know-how for thermal NIL. Compared to the patterned silicon stamps used in conventional NIL, a surface layer of distinct depth is modified to be electrically conductive. Therefore, it can be heated by a high voltage pulse by resistive heating. The main practical difference is the presence of electrical connections between tools and stamps, while most other aspects remain unaffected. A number of advantages derive from this simple working principle.
The lack of external heating and cooling enables a simpler machine concept and alignment due to reduced thermal expansion because heating is restricted to the surface of the stamp while its bulk remains at room temperature. Exposure to electrical current pulses leads to heating of the surface within microseconds, and after molding is completed, to rapid cooling with a few milliseconds by dissipation into the bulk of both, substrates of stamp and molded polymer substrate. The concept has already been extended to 100 mm round wafer substrates and simulations demonstrate that areas above 100 mm can be uniformly heated. The short pulses enable heating far beyond typical molding temperatures in thermal NIL, i.e. much higher than 200°C for typical thermoplastic polymers.
Due to high temperatures involved, molding can be quite fast, and due to the short pulses, degradation of the polymer is less crucial. Therefore the equipment has almost all advantages of thermal NIL (simple tool frame, pressure control for conformal printing with high pressure instead of capillary filling, hard stamp structures, opaque stamps, silicon as master material), but reduces the difficulties typically associated with it (thermal expansion, long imprint times for low residual layer thickness).
Pulsed-NIL competitive advantage
- High speed – high throughput: whatever rate at which robots will be able to feed the substrates to the Pulsed-NIL module, will never exceed that of the latter to imprint them.
- Almost seamless tiling of large surfaces in a step & repeat mode starting with small-size stamps. The Pulsed-NIL principle has demonstrated the capability to keep the width of seams between adjacent fields to less than 100 nm.
- Applicable to virtually all thermoplastic materials. Even those softening at high temperatures like Polyimide (Kapton®), PTFE (Teflon®)) or PEDOT:PSS, can be patterned. The short cycle prevents material degradation even at the high temperature attained in the cycle.
- Texture the surface of bulk plastic objects without introducing deformations.
- Low impact on dimensional stability of the stamp, by reducing thermal expansion effects (heating occurs just at the stamp surface).
- Possibility of origination multi-scale, multi-tier 3D structures (by consecutive addition of smaller and smaller features).
- Energy saving (>100-fold reduction of heat required by Pulsed-NIL compared to conventional thermal NIL for the same patterned area).
- Nanomanufacturing with patterning capabilities beyond the reach of current photolithographic techniques, i.e. sub-10 nm resolution in resists for pattern transfer into the underlying substrate.
- Imprint by shaping material instead of exposure and wet development, i.e. functional materials, 3D shapes, hybrid structures, including stamp copying.
- Hierarchical nanostructures. Upscaling towards higher throughput, larger area, automation and possible integration into future manufacturing lines.
The pulsed NIL very innovative approach circumvents a range of disadvantages typically associated with thermal NIL.
In particular, it addresses specific issues which will be difficult to tackle with existing machines and concepts:
- overlay between stitched areas: the current ULISS machine does not have this ready yet, but has provided a definitive prove that seamless stitching of patterns is technically achievable. Dedicated step&repeat (S&R) setups under development will enable the seamless nanopatterning of surfaces even of large areas, by stitching the pattern of smaller size dies, which will then be used as a counterpart for roll-to-roll (R2R) process where seams are almost unavoidable;
- printing on topography and preserving the initial shape: over-imposing a nanostructure on a pre-structured plastic surface, without causing the collapse or the distorsion of the existing structures, it is now possible, and will provide an important methododology to produce in a straightforward manner hierarchical patterning, biomimicking strategies, structures and functionalities present in nature.
- functional materials which need to maintain their physical-chemical properties after they are prepatterned, can be now a easily patterned without loosing their functionality, due to the shortness of the thermal cycle.
Thermal Nanoimprint Lithography (NIL) and why it needs improvements
The invention of Nanoimprint Lithography (NIL) in 1994 can be certainly listed among the main breakthroughs in nanotechnology. If the ability to reproduce nanostructures, down to the sub-10 nm scale, appears as the NIL’s most striking feature, the very affordable cost of entry-level tools and the relative simplicity of the process have undoubtedly contributed to its widespread use in research.
The possibility of parallel processing large-area substrates has extended its attractiveness to industry. The unique combination of the above features, i.e. extreme resolution, low cost, and large area parallel processing of thermoplastic thin films enables NIL to rival well-established mainstream lithographic technologies in a broad range of application fields, including photonics, MEMS, microfluidics, biomedical devices, renewable energy conversion systems, data storage and consumer electronics.
However, in the perspective of mass production the current throughput is suboptimal, and this may at the end reveal itself as the very NIL’s Achilles heel. In thermal NIL (T-NIL) indeed, considerable time is spent in the steps of heating, indentation (or cavities’ filling) and cooling. In this respect, ultraviolet nanoimprint lithography (UV-NIL) is today better placed to satisfy the needs of mass production, since low-viscosity photo-curable monomers or oligomers are employed as resists. Our focus here is on T-NIL and ways to increase its throughput, thus enhancing its competitiveness also with respect to UV_NIL.
In the standard implementation of T-NIL, heating and cooling steps typically require both more than 1 minute. This time mainly depends on engineering aspects of the equipment and represents an overhead on the total process duration. The features’ indentation time usually ranges between 1 and 10 min, and it is instead controlled by polymer rheology. Our work moves from the consideration that the duration of the thermal NIL cycle is determined by practical aspects more than fundamental physical limits. In fact each of the three phases of the process, i.e. heating, indentation, and cooling can be accomplished orders of magnitude faster than in the standard T-NIL implementation as it was demonstrated already in a laser assisted NIL process where the high temperature reached, estimated in 400 °C, ensures the fast melting of the polymer and the indentation of the tip into the resist . However the above approaches to ultrafast thermomechanical patterning are not easy to extend to areas larger than a few mm2, and in particular to the technologically relevant target of a full wafer.
The essential aspect, that can be “exported” and that enables these processes to run much faster than the standard thermal NIL process is that no “large” thermal capacity gets heated and only a small mass participates to the thermal cycle in addition to that of the thin resist film. Therefore, what needs to be avoided of the standard NIL process is involving in the thermal cycle the many kilograms of steel of the hot plates, since this results in very slow heating and cooling, and precludes the possibility of reaching much higher temperatures at which also the indentation step would occur much faster due to a much lower polymer viscosity. It has also to be mentioned that at each thermal cycle a large amount of thermal energy stored in the hot plates is wasted, increasing the energetic cost of the process. A way heating without heating plates is that the stamp itself provides the heat for melting the resist film.