2003 Progress Report: Dry Lithography: Environmentally Responsible Processes for High Resolution Pattern Transfer and Elimination of Image Collapse using Positive Tone ResistsEPA Grant Number: R829586
Title: Dry Lithography: Environmentally Responsible Processes for High Resolution Pattern Transfer and Elimination of Image Collapse using Positive Tone Resists
Investigators: DeSimone, Joseph M.
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Richards, April
Project Period: November 1, 2001 through November 1, 2004
Project Period Covered by this Report: November 1, 2002 through November 1, 2003
Project Amount: $347,898
RFA: Technology for a Sustainable Environment (2001) RFA Text | Recipients Lists
Research Category: Sustainability , Nanotechnology , Pollution Prevention/Sustainable Development
Conventional lithography, the process by which integrated circuits (ICs) and memory devices are produced, requires more than 1 kg of organic solvent and aqueous waste to produce a single chip. As a consequence, the IC industry represents a significant obstacle to environmental sustainability. Therefore, there has been significant interest in developing alternative solvents and processes in this area. Carbon dioxide (CO2) has proven to be a promising candidate as an environmentally responsible alternative to aqueous and organic solvents in a range of processes. Workers in Dr. DeSimone’s laboratory recently have demonstrated the utility of CO2 in the lithographic process at 193 nm. Forces driving the IC industry, however, continue to push for smaller feature sizes by either adopting immersion technologies to push even greater circuit density from existing platforms (193 nm immersion lithography) or moving toward smaller wavelengths (157 nm lithography and 157 nm immersion lithography). Because of our successes in the work surrounding the development of CO2-based 193 nm negative tone photoresists, the objective of this research project is to develop positive tone resists for application in CO2-based 193 nm immersion lithography, 157 nm lithography, and 157 nm immersion lithography.
There also are a number of practical benefits for which liquid and supercritical CO2-based processes hold potential. As the IC industry moves toward larger wafers, spin coating in CO2 will alleviate the challenges of spin coating larger surface areas such as film uniformity. In addition, as feature sizes are reduced and aspect ratios increased, the problems associated with image collapse in the aqueous base development of resists can be eliminated by the use of CO2 rather than water because of its dramatically reduced surface tension.
There are some considerations in the development of a photoresist, which must be addressed. A positive tone image is obtained when the exposed areas of a resist film undergo a chemical change such that they are rendered more soluble in the developing medium ( i.e., the exposed regions are removed in the development step). To accomplish this in a CO2-based system, the following criteria must be met: (1) a photoresist must be synthesized that is soluble in liquid CO2 for purposes of deposition; (2) there must be a method to provide for the appropriate solubility contrast upon exposure to a specified wavelength of radiation; (3) it is necessary for the resist to exhibit minimal absorbance at the exposing radiation wavelength to allow for complete latent image transfer; (4) the resist must posses a sufficiently high glass transition temperature (Tg) to prevent image blurring at elevated temperatures during the lithographic process; and (5) the resist must exhibit sufficient etch resistance to allow for the image transfer to the substrate.
As we have reported previously, we have developed a contrastable monomer, providing the appropriate chemistry for a solubility switch by taking advantage of the well known pinacol rearrangement that consists of dehydration of an alcohol accompanied by a methyl migration to give the less polar keto form. Figure 1 illustrates the initial set of polymers we designed. These are synthesized from solution free-radical copolymerization of the contrastable monomer and Zonyl ® TM.
Figure 1. Contrastable CO2-Soluble Polymers Synthesized by Copolymerization of Zonyl ®TM and a Styrenic Monomer Containing a Pinacol-Like Pendant Group
These materials gave promising results concerning solubility and contrast chemistry. Because they exhibit low Tg values and are semicrystalline, we have focused on developing amorphous polymer structures that provide the sufficiently high Tg and the absorbance characteristics we require. Over the last year, workers in the DeSimone laboratory have developed methods of preparation and demonstrated the utility of a number of different fluoropolymer platforms for application in lithography.
The polymers in Figure 2 have been prepared and exhibit relatively high Tg values (around 140 to 150ºC for A and around 130ºC for B). Absorbances at 193 nm are approximately 0.1 μm-1 and approximately 0.005 μm-1 for A and B, respectively, and at 157 nm are approximately 2 to 3 μm-1 and approximately 0.15 μm-1 for A and B, respectively.
We have prepared a contrastable monomer and compounds to demonstrate the concept of a solubility switch. In addition, we have developed various photoacid generators tailored for such processes in a dry CO2 platform. We continue to investigate and have identified various backbone structures that hold potential for this application.
Figure 2. Potential Fluoropolymer Backbones for Application as a Positive Tone CO2-Based Photoresist
The films deposited in this manner then will be delivered for imaging experiments on our new ASML 5000/900 series 193 nm stepper, which is now up and operating. This tool provides us with fast and immediate results allowing for optimization of conditions for image transfer at a speed previously unattainable.
We will incorporate the contrastable monomer into the various polymer structures outlined above. In addition, we will engage in kinetic studies of the rearrangement process in the resulting polymers. Solubility changes in CO2 will be measured. This should give us a handle on a potential resist with the low absorbance and high Tg and etch resistance we are targeting. These materials will be tested further for solubility in CO2, with an eye toward spin coating in liquid CO2.