High‐Resolution Patterning of Molecular Glasses Using Supercritical Carbon Dioxide

Abstract

Continuous advances in the semiconductor and microfabrication industries are based on formation of increasingly smallscale devices. Photolithography, the process by which small features are patterned into a thin organic film (photoresist) by UV or electron-beam (e-beam) irradiation, is the principle process by which small features are formed. To produce the planned sub-50 nm features required for next-generation devices remains a challenge due to the increased importance of pattern perfection and elimination of problems such as pattern collapse in small, high-aspect-ratio features. At these very small dimensions, conventional development in an aqueous base can cause small, dense lines to collapse inward due to high surface tension. These and other problems will remain significant challenges unless new approaches to lithography are pursued. Supercritical carbon dioxide (scCO 2 ), which is known for its properties of high diffusivity and zero surface tension, is an ideal alternative to aqueous bases as an advanced development solvent and can help to realize the potential of sub-50 nm patterning.Supercritical CO 2 development is a powerful alternative process to aqueous-base or organic-solvent development for high-resolution patterning from both an environmental and practical point of view. In practice, scCO 2 is a single-phase fluid that exists above the critical temperature and pressure of CO 2 (T = 31°C, P = 7.4 MPa). Supercritical CO 2 as a solvent can provide environmental benefits, which result from a reduction in solvent and water waste, and it may also serve as a platform where harmful solutes are easily separated from the solvent in question (via depressurization). scCO 2 has been shown to be an ideal solvent for processing structures on the nanoscale. [1,2] In terms of performance, scCO 2 is a solvent with zero surface tension and high diffusivity. This leads to better development characteristics compared to liquid solvents for dense, high-aspect-ratio features. However, scCO 2 tends to be a very poor solvent for polymers, such as those that current photoresists are comprised of; though certain fluoropolymers and silicones have been shown to be soluble at moderate supercritical conditions. [3,4] This would necessitate the incorporation of groups, such as fluorinated side chains, into photoresist materials to effect scCO 2 solubility. Despite the advantages of scCO 2 development, there are several reasons that make the incorporation of fluorine in scCO 2 developable resists undesirable. Usually the large quantities of fluorine present in scCO 2 -soluble polymers degrade plasma-etch resistance and are undesirably expensive. Also, due to their persistent nature, fluorinated compounds are coming under increased scrutiny [5] and their use needs to be phased out. Finally, although scCO 2 -soluble polymeric resists based on fluoropolymer platforms have been studied in recent years, [6,7] no features smaller than 100 nm have been demonstrated. Regardless of exposure method, these materials have not shown sufficien...

3D Micro‐ and Nanostructures via Interference Lithography

“…[48,53] sc CO 2 development for the negative type molecular-glass photoresist has been suggested as a new process step to alleviate pattern collapse in densely packed, high-aspect-ratio structures. [54] However, supercritical drying of some positive photoresists resulted in the formation of cracks. Replacement of water with pentane or hexane, which have a lower surface tension (73.05 mN m -1 , 13.72 mN m -1 , and 18.43 mN m -1 for water, pentane, and hexane respectively at 20°C [44,55] ) has been proposed as an alternative method.…”

Physical Vapor Deposition of Molecular Glass Photoresists: A New Route to Chemically Amplified Patterning

“…In the last decade it has been shown that small molecules with glassy properties, so called molecular glasses, can serve as resist materials as well, [2][3][4][5][6] and recently, thermally stable star-shaped molecules with high glass transition temperatures have been introduced. [7,8] Usually, the necessary resist films needed for high resolution imaging are deposited by casting from solution via spin coating or doctor blading techniques, depending on application. One of the disadvantages of these solution-based methods is that solvents may contain dust or other high boiling residues, which causes a rougher surface or leaves impurities in the film.…”

Molecular Glass Resists as High‐Resolution Patterning Materials

“…Fully t-BOC-protected hexa(hydroxyphenylbenzene)-mp was developed in scCO 2 upon e-beam exposure to produce 50 nm negative tone feature sizes with a 3:1 aspect ratio. [34] Because of behavior as a one phase fluid, scCO 2 has proven [34] to alleviate pattern collapse in densely packed, high-aspect-ratio resist features. Recent work also reports that calix [4]resorcinarene derivatives up to 2000 g mol À1 can be developed in scCO 2 with very high dissolution rates.…”