Nanolithography by elastomeric scattering mask: An application of photolithographic standing waves

Gorgi Kostovski, Arnan Mitchell, Anthony Holland, Ernest Fardin, Michael Austin
2006 Applied Physics Letters  
An application is demonstrated for the much maligned standing wave in photolithography that is responsible for the sidewall corrugations in photoresist patterns. We demonstrate the realization of a polydimethylsiloxane ͑PDMS͒ scattering mask through the casting of these sidewall corrugations and their application as the masking components in an otherwise transparent bulk of PDMS. Photoresist structures with widths in the order of 80 nm are realized by the application of this mask, demonstrating
more » ... mask, demonstrating excellent correlation with the lateral depths of the sidewall corrugations. The continuity of the sidewall corrugations around corners makes both straight and curved lines realizable with equal simplicity. Contact photolithography has long struggled with the realization of nanometer scale patterning of photoresists due to difficulties in achieving conformal contact between its rigid masks and the photoresist surface. More sophisticated photolithographic techniques, including deep UV, electron beam lithography, and focused ion beam lithography have shown excellent results, although limited access to costly infrastructure has restricted their application. Elastomeric masks, however, as pioneered by Whitesides and co-workers, 1,2 have proven themselves to be more than capable of realizing nanometer scale structures in photoresist. These masks are developed and applied using standard photolithographic techniques and equipment. Simple implementations such as the light coupling 3 and transparent reflective masks 1 have demonstrated microscale patterning in both 1:1 and 2:1 pattern density ratios by the mechanisms of differential transmission and total internal reflection, respectively. More recently, the elastomeric phase mask has demonstrated both 90 ͑Ref. 2͒, and 50 nm ͑Ref. 4͒ resist features by using the recessed regions of the elastomeric mask to induce ⌸ radian phase shifts between regions of transmitted light, thus realizing intensity nulls beneath the sidewalls of the mask features. Traditionally elastomeric masks have been realized by casting master photoresist structures patterned by standard lithography. Unfortunately, lithography has long been troubled by a sidewall roughness that is not defined by molecular resolution. Rather, this sidewall roughness is caused by a periodic variation in light intensity through the resist that is the result of interference between the normally incident planar wave front and that reflected from the substrate/ resist interface. The result is a periodic corrugation of the resist sidewalls that is oriented perpendicular to the surface normal. Much effort has been expended in eliminating these corrugations. 5 The development of top and bottom antireflection coatings, the employment of postexposure bakes, and the inclusion of attenuating dyes in the photoresist are only some of the avenues that have been pursued. In this letter, we present a novel implementation of polydimethylsiloxane ͑PDMS͒ elastomer mask that is capable of realizing nanometer scale features comparable to those demonstrated by the elastomeric phase mask, 2,4 but which operates by a significantly different mechanism. We demonstrate that the mechanical flexibility of PDMS makes it uniquely suited to taking faithful casts of these lateral relief structures in photoresist, as has been demonstrated elsewhere on a much larger scale. 6 This allows us to exploit these corrugations as the mechanism by which the PDMS scattering mask operates. Thus, just as sloped sidewalls are required for the reflection mask to realize total internal reflection, and smooth vertical walls are required for the elastomeric phase mask to allow spatial overlap between neighboring out of phase wave fronts, the sidewall corrugations that appear in photolithography become the masking elements in this new implementation of elastomeric mask. The procedure employed to generate the nanoscale structures presented in this letter is illustrated in Fig. 1 . Master fabrication begins by depositing an ϳ200 nm thin film of the positive photoresist AZ5206E onto a polished silicon wafer by closed lid spinning at 3300 rpm for 30 s. This is patterned by contact photolithography using a quartz mask and a broadband Karl Suss MJB3 mask aligner. Developing the pattern realizes rectangular ribs of photoresist with prominent sidewall corrugations, shown in Figs. 2͑a͒-2͑c͒. A cast is then taken of this master structure by applying a 10:1 mixture of PDMS ͑Dow Corning 184 Silicone Elastomer Kit͒, curing it, and then separating it from the photoresist pattern to produce a thick 5 mm sheet of PDMS with surface recesses. These recesses correspond to the master photoresist ribs, on the sidewalls of which are the standing wave corrugations. This is the PDMS scattering mask. The scattering mask is applied by allowing it to conform to the surface of a fresh layer of AZ5206E photoresist and illuminating it with the same mask aligner that was used to create the master pattern. The scattering mask presents the normally incident planar wave front with three distinct paths to the resist below it, illuminating the photoresist with three distinct energies as in Fig. 1͑f͒ . Maximum energy is coupled by the PDMS that contacts the photoresist surface. Slightly less energy is received by photoresist directly below the PDMS recesses since incident light is presented with a PDMS/air and then air/photoresist interface. This energy dif-a͒ Electronic
doi:10.1063/1.2190899 fatcat:xaq6lhripnbezb2jpvszvweyjq