Surface engineering opportunities
Surface Engineering assumes modification of surface properties of materials by means of application of coatings or surface treatments. Recent advances in nanotechnology opened an opportunity to modify surfaces in a nanoscale range. Nanoscale surface engineering has found use in many fields. It is driving miniaturisation of electronics and medical devices, fabrication of new sensors and actuators as well as in the creation of new classes of materials and devices.
A few years ago Molecular Vapour Deposition (MVDR) technology was introduced to commercial markets by Applied Microstructures as a surface engineering alternative to solution based methods used in many laboratories for a last decade. These methods include immersion of sample into a sequence of solvent-based solutions. MVD technology, as a vapour based technique, provides solvent-free, low material waste and a environmentally friendly processing method. It also proved to produce high quality nanocoatings due to its unique capability of tightly controlling the process environment.
Nanocoatings, in the form of organic monolayer films, have been commonly used to reduce the surface energy in micro-structures to improve their performance and reliability. Most notably, failures attributed to ‘release’ and ‘in-use’ stiction, which is the adhesion of compliant micromechanical structure surfaces in close proximity, can be reduced by orders of magnitude using a few angstrom-thick film. These antistiction coatings are self-assembled monolayers (SAMs), which can uniformly coat complex structures including high-aspectratio comb drives in MEMS inertial sensors, areas under the mirrors in MEMS displays, membranes of MEMS microphones, and face plates of MEMS inkjet nozzles.
Stiction is a term that has been applied to the unintentional adhesion of compliant microstructure surfaces when restoring forces are unable to overcome interfacial forces such as capillary, van der Waals and electrostatic attractions. The large surface-area-to-volume ratios of surface and bulk micromachined components bring the role of stiction into the foreground, as adhesion of these microparts to adjacent surfaces is a major failure mode for MEMS.
Conventionally, solution-based (wet processing) techniques have been used to apply these films, however, the liquid deposition process in manufacturing is extremely difficult to apply due to particulation problems caused by the high sensitivity of the reaction to environmental humidity. Anti-stiction layers deposited from a vapor phase have proved to enhance yield in MEMS devices by eliminating capillary stiction phenomena, which in turn allows simplification and reduced cost of MEMS packaging. Thin and conformal adhesion layers, in the form of metal oxides, deposited in-situ prior to organic low surface energy coatings, allow for the expanded range of useable MEMS materials (metals, glasses and polymers) and provide for enhanced mechanical, thermal and chemical stability. Recently developments in MVD deposition technology has transitioned from mainly R&D labs and pilot production into high volume production facilities for the manufacturing of MEMS displays, microphones and inkjet print heads.
Another exciting area of MVD surface engineering is emerging technologies, and mainly Nanoimprint Lithography (NIL). NIL is maturing as an alternative to optical lithography, at least (and for now) in niche market applications, like hard disk drives, LEDs, photonics, micro optics, and microfluidics. Lux Research believes that nanoimprint tool business could grow to nearly US$235 million by 2010.
One of the most promising areas of NIL is Data Storage, where current technology approaches the magnetic media limit. Conventional magnetic media consist of magnetic grains, each one free to assume its own magnetisation state. The signal to noise ratio of the media is roughly proportional to the number of magnetic grains per bit. The superparamagnetic limit is reached at the point when grain becomes so small (<8 nm) that thermal energy alone can flip its magnetisation direction. Today’s disk drives have densities of 100 Gb/in2 with spacing of 86 nm. Increasing capacity to 1Tb/in2 would require spacing of 27 nm, and 10 Tb/in2 – 9 nm. For 500 Gb/in2 density and beyond alternative to optical lithography technologies such as thermally assisted recording (TAR) and patterned media (PM) are being considered as likely routes.
Patterned Media (PM) has single-domain magnetic elements having a uniform well-defined shape and specific location on a disk. The PM master mold is fabricated using e-beam writing. Then, the mold is used to replicate a pattern to a number of individual disks. For example, one of the methods is in some ways similar to the digital video disk (DVD) manufacturing process (Fig 1), except that a UV-cured liquid resist is used instead of relying on thermal softening of a solid polymer.
Another promising application of NIL, replication of micro optics (diffractive optical elements, phase masks, Bragg gratings, microlens arrays) has proved to be very useful in driving down the cost of manufacturing optical components.
Microlenses and microlens arrays are finding applications mainly in the domain of optical microsystems, including optical interconnects, biomedical instruments, optical data storage and optical communications. NIL techniques open a route for fast and inexpensive prototyping of microlens arrays without compromising optical quality.
Another important potential application for NIL is high-brightness LEDs, which are enhanced with photonic crystals. A photonic crystal is a periodically repeating structure made of two materials of different dielectric constants, which can provide so called photonics band gap. As the dimensions of 2D photonic crystals are in the order of nanometers, NIL nanolithography is an obvious choice.
NIL, as a printing technology, requires mechanical contact between mould and resist (polymer material), thus resist adhesion to the mould is one of the challenges for nanoimprint lithography. When intimately contacted, resist tends to be pulled from the substrate and remain on the mould. This creates a defect, which affects not only the particulate substrate, but all other consequently printed substrates because of an air gap formed between a mold and a substrate. The main approach to overcoming this problem it is to apply a low surface energy coating to the mold surface, either in the solution phase or vapour phase. Such low surface energy coating would drastically reduce adhesive forces between mold and resist materials.
The NIL replication process can be used for a fast fabrication of plastic microfluidic devices. Large numbers of complex microchannels are manufactured from a single master which have been created using high precision optical or e-beam lithography. This reduces the cost of such microfluidic device, and hence allows the final device to be disposable.
Numerous coatings have been utilised as a low surface energy release layer for NIL. Amorphous fluoropolymers, for example Teflon AF, made by DuPond Fluoroproducts, Cytop, manufactured by Asahi Glass, or Optool DSX of Daikin Industries (20% solution in HFE) can be deposited by spin-coating or dipping.
Unfortunately, deposition of these coatings in a liquid phase into nm-feature structures is not always conformal, reproducible or defect-free.
Self-assembled monolayers (SAMs) are considered an ideal solution for NIL because of the self-assembling nature of these films. The SAMs thickness is equal to the length of the molecule of precursor use, thus thickness uniformity is excellent. SAMs can be deposited from a liquid or vapour phase. Films deposited from a vapour phase have fewer aggregates of the silane molecules on the surface, because of vacuum process capability to accurately controlling moisture environment. Moreover, vapour is more effective than the solution in penetrating into the nanoscale gaps of the mould, thus providing much lower defect densities.
Applied Microstructures, Inc. offers equipment and technology (Molecular Vapour deposition – MVDTM) for SAM deposition from a vapour phase for variety of applications including Nanoimprint Lithography. Fig. 8 shows simplified schematics of the MVD-100 tool, which includes vacuum chamber, integrated plasma source (for sample surface pre-treatment with oxygen plasma), and three vapour delivery lines configured for accurate delivery of reactive vapours.
Applied Microstructures first tool, the MVD-100 is a low cost-solution for R&D and small production needs, while MBD-150 tool, has capabilities to process an entire cassette (25) of 8” wafers with an automatic loading and thus provides capabilities for high-volume manufacturing.
In Fig. 10, a comparison of liquid and vapour deposited films as measured by AFM are shown. In the micrographs, the z-scale was 10nm. It can be observed that the liquid immersion film has embedded particulates as shown in the red highlights, whereas the MVD (vapour) deposited film was smooth and defect free. The peaks in the thickness graphs were attributed to particulation from excessive water. The uncontrolled source of water exists from ambient moisture. In contrast, particulates can be eliminated in the MVD process by accurately controlling the partial pressure of water which hydrolyses the precursor. This results in improved pattern fidelity during imprinting. The mould surface energy is greatly reduced as a result of the formation of a densely packed perfluorinated monolayer which reduces resist adhesion to the mould.
The surface coverage of the film is determined by measuring the hydrophobicity against other published references. Goniometric measurements (by a Ramé-Hart Inc.’s Advanced Goniometer) show water static contact angles of ~110o and are very repeatable from process run to run, as shown in Fig. 11. On a smooth silicon surface, the high contact angles correlate to a low surface energy of ~ 3µJ/m2 which is desirable for a good release between the mold and resist. The SAM thickness measurements performed with a Gaertner ellipsometer with at an incidence angle of 70o @ 632.8nm. show an average monolayer thickness of ~1.2 nm, which closely matches to the thickness of a continuous molecular layer. Another advantage of a vapour process for release layers is the high level of conformality which can be achieved. The self-assembling and self-limiting characteristics of the deposition process help to maintain excellent critical dimension (CD) control of the mould pattern. Therefore, a faithful replication of the mould pattern can be generated in the resist. As shown in Fig. 12, CDs were measured on a CD SEM from feature of dense line in range from 38 nm to 125 nm. The results show an excellent correlation with no major deviations taking in account linewidth and linewidth roughness.
For an imprint application, repeatability was evaluated by monitoring mold release force and subsequent pattern inspection of the pattern for more than 150 imprints. In Fig. 11, the optical images of a patterned mould (right image) and without (left image) of the MVD release layers are shown. The test trend provides a good indication of the repeatability of the release force required during the imprint process. With the vapour deposited SAM release layer, the release force between the mold and the resist was reduced to from over 20 to 15 Newtons compared to uncoated moulds. The effectiveness of a MVD release layer on mould release performance was also observed by the mold pattern cleanliness. Excellent uniformity and repeatability of the imprint critical features were achievable with a MVD processed release layer.
Moulds made of Si or quartz materials are well suited as stamps, particularly because the thermal expansion coefficient of the stamp is often identical to the substrate to be structured. Moreover, due to high density of natural bonding sites (hydroxyl bonds) on Si and quartz surfaces silane based released layers can form durable covalent bonding with the surface. For many NIL schemes, however, other materials are used. For example, Ni is widely used for industrial NIL applications, where higher level of robustness is required. The most common surface of oxidised Ni is the (100) surface of NiO, which is built up from Ni2+ cations and O2- anions arranged in a sodium chloride structure. Due to the highly ionic character of the Ni-O bonds the covalent linkage to the silane group is chemically difficult. Moreover, the surface state of the nickel is highly dependant on the plating process and bath stock solution. Adhesion layers of SiO2 and TiO2, deposited by e-beam evaporation, have been used to improve bonding of silane-based release layers to Ni stamps.
Fig 14. clearly indicates that friction force of the release layer depends on the underlying substrate surface. Silicon and titanium oxide adhesion layers as thin as 5nm improve the density of organic layers, which manifests itself in reduction of surface energy. Recently, Applied Microstructures has widened the range of materials which can be deposited using MVD technology. For example, alumina (Al2O3) and titania (TiO2) can now be deposited by MVD in-situ, without breaking a vacuum in the same chamber, where organic release layers are deposited. Thus, nanolaminates comprised of oxide adhesion layers and organic low surface energy layer can be deposited in a single automatic sequence.
For thermal embossing of NIL, the thermal stability of release layers is very important. Coatings should withstand high temperatures (150-400 C) and considerable pressures (up to 100 lb/cm2). It was demonstrated that MVD engineered nanolaminates, comprised of oxide adhesion layers and organic release layers provide necessary thermal stability, as show on Fig 15.
Durability of the coatings against mechanical impact was estimated using IPA wipe tests, as shown on Fig 16. Coatings on both, Si and Ni substrate materials show good stability for up to 40 wiping cycles.
Conclusions
We believe that Nanoimprint Lithography (NIL) can be an invaluable manufacturing solution for a number of emerging technology applications including data storage, optics and microfluidics. These applications usually do not require stringent alignment and overlay registration, which continue to be a considerable challenge for NIL.
Moreover, NIL has proven to be the only cost-effective solution for the creation of sub-100nm feature devices, which makes it an obvious choice for high volume low cost applications including data storage, disposable lab-on-a-chip devices, and micro optics.
The technological challenges of minimising defects associated with the withdrawing the imprint mold from the substrate material that it has imprinted can be effectively overcome by the application of low surface energy release layers to the mold prior to the imprinting process.
MVDR (Molecular Vapour Deposition) technology provides a convenient and cost-effective way to apply these extremely thin and durable layers in a vapour phase.
The combination of organic layer deposition with inorganic adhesion layers deposited in-situ allows implementing this technology to a wider range of mold/stamp materials.
Therefore, NIL systems implemented with MVD and its associated unique characteristics can produce parts with uniform, exact and repeatable surfaces resulting in a highly enhanced yielding process. Since its recent introduction in 2004, MVD technology has been widely adopted into high volume manufacturing of MEMS displays, inkjet nozzles and microphones.
MVD tools are currently used extensively in nanoimprint technology development in many universities and nanofabrication facilities around the globe and it will surely play an important role in NIL’s transition from development to high volume manufacturing stage in many additional applications.
