by Neha K. Choksi

May 12, 2010 – Optical lithography methods are expensive, lack flexibility, and lack resolution for the sub-50nm node. Nanoimprint lithography (NIL) is considered an attractive alternative. NIL is a method of patterning in which a three-dimensional surface pattern of a stamp is transferred onto a moldable surface film on a substrate through mechanical contact. The resulting 3-dimensional pattern on the surface coating is then transferred to the underlying wafer by common semiconductor processing techniques.

Most imprint lithography vendors use spin on tools to create a 50nm thin-film surface coating before embossing. Molecular Imprints, however, has a different approach: Jet and flash imprint lithography (J-Fil). S.V. Sreenivasan, founder and CTO of Molecular Imprints, shared the company’s approach with attendees at the IEEE San Francisco Bay Area Nanotechnology Council seminar on April 20, 2010.

Because real world applications of nanolithography entail pattern complexity and density variations at both the macro and micro scale, this technique deposits the surface coating where it is most needed on the substrate by leveraging standard inkjet technology (see figure). After low-pressure embossing, the resist is UV cross-linked before the mask is separated from the surface, eliminating the use of solvent. The mask makes contact in the middle of the field with radial sweeping action to minimize bubble formation.

The key to this inkjet deposition method is a low-viscosity monomer resist and the use of small volume droplets (on the order of 1.5pL). Because the droplet density can be tailored to the desired pattern density, the fluid travels less distance during the stamping process. Also, the use of this "drop on demand" technique reduces the thickness and the variation of the residual layer that is inevitably left on the lower surfaces of the 3-D resist imprint. This reduces overall non-uniformities of the final pattern. The inkjet tool itself must be carefully calibrated, as variation exists even between same inkjet models.

Step-and-flash imprint lithography process. Benefits include throughput (room-temperature process) and lower capital cost (no track system). (Source: Molecular Imprints)

The company is careful to control the surface energy of the mask. The need to keep the surface energy low enough to allow easy release of the mask from the resist, but not so low that the mask surface itself will not consistently wet, must be carefully controlled. Initially, this technique leveraged a stepper technique for imprinting — the precise fluid deposition reduces the edge bead challenges enabling successful stitching of fluid fields. However, the company now has the option of whole substrate printing as well.

By leveraging its NIL technique for a variety of applications, the company aims to be a market leader in the field. Because the cost-sensitive magnetic storage industry requires a high-density of precise islands of magnetic material separated by non-magnetic material, their high-resolution, low-cost patterning technique of NIL is an attractive solution when compared to the costly 193nm optical lithography method. The company has also demonstrated 2.55nm linewidth roughness and 18nm half-pitch using their stepper patterning technique for non-volatile memory applications. In this case, NIL can be mixed and matched with mainstream photolithography to achieve the associated critical dimensions where necessary.

Furthermore, researchers at the University of Texas are exploring the use of NIL technology for biomedical applications. By leveraging the J-Fil technique to create well-controlled dot structures, researchers are able to study the impact of particle shape, aspect ratio, and size on the effectiveness of particles for targeted nanoparticle drug delivery to fight lung cancer. Because the process selectively deposits the material before patterning, the material waste is less than that of spin or roll-on techniques. This enables a lower cost per dose when compared to other techniques.

Despite its success, Sreenivasan concedes that imprint lithography is not yet ready for all applications. For example, silicon microprocessors have a low tolerance for defects thus making it an unlikely fit at this time. Regardless, Sreenivasan is optimistic that the company’s non-traditional approach to NIL will enable it to further lower cost and reach 10nm feature sizes for production solutions on the horizon.

Neha K. Choksi is an independent consultant based in Mountain View, CA. She has worked for a variety of MEMS companies including as director of product engineering at Silicon Microstructures and as a consultant focusing on commercialization and high volume production of MEMS devices. E-mail: Choksi [at] gmail.