Making a mark

The International Technology Roadmap for Semiconductors (ITRS) identified pattern replication by means of nanoimprint lithography (NIL) as a contender for the 32nm technology node and beyond in 2010 (1).

Thanks in part to this, NIL is currently enjoying strong growth and attention within the semiconductor and related industries, such as data storage, life sciences and opto-electronics.

What is fascinating about NIL is that it offers a cost efficient pattern replication technique that can be used to develop nano sized devices. At the same time it can be employed for process simplification and high volume manufacturing of not-so-nano size features.

For example, NIL has can combine the ever-shrinking bit size on data storage media with novel hard disc manufacturing techniques, providing a huge market for the technique in the manufacture of hard disc drives (HDD).

HDDs are being utilised in more and more commercial applications because of their increasing reliability, size and low cost (2). Consumers demand higher data storage capacities for MP3, digital images and movies to name a few.

NIL is also the perfect manufacturing technique for processing low cost substrates for consumable fluidic devices such as those used in the life sciences sector, where there is an ever-increasing demand for low cost and rapid analysis/information retrieval systems.

Imprint technologies indeed have been employed by researchers and manufacturers seeking cures for SARS (3), the dangerous respiratory disease that caused an epidemic in Asia a couple of years ago.

The main aim of NIL - like other forms of lithography - is to create a pattern on a substrate. This is usually achieved either by a large-area imprint or by a step-and-repeat process similar to the stepper technology used in conventional semiconductor manufacturing techniques.

There are a variety of ways by which NIL can form the pattern. The most common are thermal imprinting/hot embossing, ultra violet curable imprinting and contact printing (figure 1).


Hot embossing NIL

Many of the hot embossing processes used in NIL are similar in operation and requirements to MEMS wafer bonding technology. The main challenges are - like with MEMs wafer bonding - control and uniformity of temperature and pressure over the substrate area and atmosphere control in the process chamber (pressure, gas etc).

In addition hot embossing technology requires active cooling and temperature ramp control due to the nature of the thermoplastic substrates.

Hot embossing NIL techniques have been used to achieve feature sizes on wafers as small as sub-100nm.

In the hot embossing process, the polymer is heated above its glass-transition temperature and high contact forces are applied. A polymer (often PMMA) is imprinted with a patterned stamp. The stamp is usually made of silicon or nickel for high aspect ratio structures.

Both the stamp and the substrate are heated to their relevant temperatures above the glass transition temperature (Tg) of the spin-on-polymer, thereby decreasing the polymer viscosity for the imprinting process.

Application of high contact force between stamp and polymer enables filling of the cavities of the stamp. After the imprinting step, the whole stack is cooled well below Tg to cure the patterned features.

The controlled separation of stamp and polymer is carried out at elevated but below Tg temperatures (figure 2). Substrates of up to 200mm in diameter have been successfully replicated with silicon masters. Once the pattern has been transferred into the resist, standard micro-fabrication techniques are employed to etch into the substrate.

The entire process is shown schematically in figure 3 - a imprint; b de-embossing; c de-scum; d etch (RIE, ICP); e deposition and release, f template or device; and g bonding (4).


UV-NIL
Ultra violet-based NIL is best suited for complementing semiconductor processes for both process simplification, as suggested by research body Sematech (5), and as a mix and match function, with UV-NIL used to apply the critical layer. One consideration with UV-NIL is that it is important to be compatible with existing stepper technology and overlay accuracy techniques.

The EV Group has developed a modified aligning system - EVG(r)620 - for use with EV-NIL and substrates up to 150mm in diameter. The system aligns the transparent stamp to the substrate as well as the imprinting and curing process.

The company is currently developing a system for 200mm and above processes for deployment in the second quarter of 2005.

In most cases, either quartz glass stamps (hard stamps) or PDMS stamps (soft stamps) are used for UV-NIL processes.

The process flow for UV-NIL is as follows: a monomer coated carrier substrate (eg silicon wafer) as well as the transparent stamp are loaded into the aligner and fixed by vacuum on their respective chucks.

After the optical alignment of the substrate and the stamp is defined, the stamp and substrate are brought into contact. (The alignment accuracy of state-of-the-art mask aligners used for fabrication of MEMS devices is in the sub-1µm range. However, the accuracy is forecast to be increased to the sub-50nm range to meet the demands of nanofabrication.)

An adjustable uniform contact force of up to 750N is applied to imprint the monomer with the stamp pattern. Furthermore, an adjustable vacuum contact is applied which ensures intimate contact between stamp and substrate. The curing process of the imprinted structures is accomplished by UV-exposure with broadband wavelength from 350nm to 450nm (4).

Contact printing
Contact printing NIL is similar in operation to UV-NIL. It is a precision aligned direct patterning technology that has great potential in life science applications (figure 4). Applications range from biological weapon detection systems to next generation solar cells.


Conclusion
It is estimated that there are currently 160 NIL systems in operation worldwide, mainly at universities and research and development centres. What all of these systems have in common is that their template/stamp manufacturing depends on progress in advanced lithography technologies to provide the high-density fine pitch nano scale features.

NIL is a replication method compared to the patterning approach by optical lithography tools. In optical lithography, the wavelength of the light source supported by various optical correction methods determines the minimum feature size.

All NIL approaches work in direct contact with the resist, solution and polymer. The main challenge with the technology at present is controlling defects. This will have to be overcome before NIL can achieve full commercialisation (6).

Moreover, successful demonstration of commercially available resists, equipment technology and metrology and the development of standards and roadmaps will be needed to take NIL from the laboratories into high volume manufacturing.

Achieving all this is one of the key aims of a recently founded nanoimprint lithography international consortium (7). Called NILCom, the new organisation intends to develop the infrastructure required to commercialise nanoimprint lithography.

Figure 1. Nanoimprint lithography technologies

Figure 2. Thermal imprint cycle

Figure 3. Heat embossing process schematic

Figure 4. Contact printing application (courtesy of IMI-CNRC)

References

(1) http://www.itrs.net/Common/2004Update/2004_07_Lithography.pdf; Page 23.

(2) A Hand, "Nanoimprint Suppliers Gathering Support"; Semiconductor International; 2/1/2005.

(3) CK Lee et al, "Status of MEMS in Taiwan: Report 2004"; The 10th World Micromachine Summit; 5/5/2004.

(4) S. Jakeway et al, "Transition of MEMS Technology to Nanofabrication"; International Conference on MEMS Nano and Smart Systems; 23/7/2003.

(5) W Trybula, "Sematech, ARMC and NANO"; Nanoprint and Nanoimprint Technology Conference; 2/12/2004

(6) M Nordan, "Which Nanolithography Platforms will succeed 193nm?"; Semi Nanoforum; 17/11/2004

(7) www.NILCom.org