George A. Riley
“Embossing” is an ancient technical term in the English language, traceable to the period 1350 – 1400, and used by Chaucer in its present sense. The technical meaning, creating a raised design by pressing a master die with the negative of the design against something malleable, has not changed over the ensuing six-and-a-half centuries. What has changed is the scale of the design features. Today’s embossing patterns may have features measured in nanometers.
Nano-embossing can rapidly pattern large areas with feature sizes that previously required far slower and more costly electron-beam or ion-beam patterning. These archaic technologies now may pattern the nano-scale features on the master die. The master die then replicates the features onto whole wafers, if only a single level of patterning is needed, or at chip size when several layers with an accurate overlay are required.
Nano-embossing has for several years been a closely followed laboratory development. Now, production nano-embossing equipment is commercially available from Suss MicroTec. Initial nano-embossed products include optoelectronic devices such as gratings and lens arrays, electro-mechanical devices such as SAW filters, and biotech devices, such as membrane chips for filtering bacteria or cells in the biomedical, pharmaceutical, and food industries.
Two related techniques are currently available in Suss production equipment: hot embossing, and cold embossing. Hot wafer embossing is most suitable for single-level imprinting. Hot embossing of wafers uses the master die as the wafer stamp. Wafers are first coated with a thermo-plastic polymer, that is, a polymer that softens when heated. The wafer is then positioned in a controlled-atmosphere stamping chamber. The stamp is brought down with pre-determined pressure and temperature to soften the polymer and simultaneously form the image on the whole wafer. The stamp is cooled to harden the image. As an example, the molded polymer may be used as a resist for lift-off patterning of metallization. Figure 1 shows interdigitated aluminum fingers, suitable for a surface-acoustic wave (SAW) filter.
Figure 1. SEM of 400 nm wide interdigitated fingers after aluminum liftoff. 
Hot embossing has been demonstrated using a standard SET FP-150 flip chip aligner-bonder.  However, the Suss SB6/8e substrate bonder is optimized for this operation. Since the hot embossing process is comparable to thermo-compression bonding, the SB6 does not need to be radically changed. The SB6e has controlled heating and cooling cycles for both the top and bottom heater, and accommodates multiple stamp and wafer sizes.
Cold embossing uses ultraviolet (UV) curing of optically desirable materials to produce a variety of embossed integrated-optic and micro-optic devices, such as lens arrays. The starting materials are thin films of UV-curable materials with desirable optical properties. The films may be deposited onto silicon, gallium arsenide, or similar wafers. The UV-transmissive patterned master template then is pressed against the film, imprinting the pattern, which is UV cured before pressure is released. Cold embossing allows more accurate overlay features. Cold embossing may be done on one or both sides of the wafer. Double-sided embossing allows micro-molding. Figure 2 shows a laser lens array formed by cold embossing.
Figure 2. UV-cast laser lens array in wafer-scale replication. Photo courtesy CSEM.
Since UV exposure is a precision optical process, Suss provides the MA6 Mask Aligner, optimized for wafer level cold embossing. The MA6 is capable of printing resists of less than 0.1micron thickness. Resolution of the embossing depends solely on the stamp resolution.
To make both hot and cold full-featured embossing available in one piece of equipment, Suss offers a dedicated machine, the NPS 200 Nano-Patterning Stepper. The NPS 200 is a step-and-repeat machine, combining the capabilities for hot or cold embossing at either the wafer or the chip level. The NPS 200 has been optimized for cost-effective production of devices of micro or nanometer scale features, with sub-20 nm embossing resolution. It is also the machine of choice for precisely aligned multi-layer applications.
Nano-embossing has emerged from the laboratory to offer relatively low cost patterning of nano-scale features over large areas. Nano-embossing will have wide application in MEMS, optoelectronics, biomedical, and related micropackaging applications. Suss equipment for micro and nano embossing now makes practical low cost production of a large variety of nanoscale products.
FOR MORE INFORMATION
 “Step and Stamp Imprint Lithography using a Commercial Flip Chip Bonder,” T. Haatainen, Jouni Ahopelto, Gabi Gruetzner, Marion Fink, Karl Pfeiffer, SPIE 25th Annual Symposium of Microlithography, Emerging Lithographic Technologies IV, Santa Clara, CA, USA, 2000.
 Centre Suisse d’Electronique et de Microtechnique SA CSEM, www.csem.ch