Stefan Hien

Collapse behavior of single layer 193- and 157-nm resists: use of surfactants in the rinse to realize the sub-130-nm nodes

In this study we determined the dimension dependent onset of pattern collapse for different 193 and 157nm resist platforms, and explored production relevant techniques to suppress pattern collapse. Test structures were designed and implemented to generate well-defined capillary forces on beams of resist during drying. X-ray and 193nm (using alternating phase shifting masks) lithography were used to print test structures and patterns of dense lines with critical dimensions as small as 100 nm. The collapse behavior was quantified in terms of the critical aspect ratio for collapse as a function of the spacing between structures. The resist platforms exhibited different collapse behavior at line widths of greater than 150 nm, but at line widths of 100 nm and less, all of the resist structures collapsed with aspect ratios greater than 3. A principal conclusion from this work is that changes in resist chemistry or formulation alone will not be sufficient to solve the collapse problem at the 100 nm node and below. The most effective strategy to suppress the resist collapse is to reduce the capillary forces that act on the structures during drying. For 193 nm resists, collapse behavior was quantified for a number of surfactants that were added to the rinse liquid. We demonstrate that with a simple modification of the final rinse and drying process, we could increase the critical aspect ratio from 4.2 to 5.2 at a spacing of 110 nm for a champion resist. This means, for example, that we can image 110 nm dense lines with the surfactant rinse at a thickness of 575 nm whereas without surfactant we were limited to 460 nm. The results are interpreted in terms of the contact angles of rinse liquids on the resists and their respective surface tensions.

Acid diffusion analysis in the chemically amplified CARL resist

The demand for smaller device dimensions in microlithography drives the need to understand and control diffusion during photoresist processing. In advanced chemically amplified systems the lithographic performance is strongly influenced by diffusion of acid and base additives. Diffusion parameters and photoacid generation efficiencies were quantitatively evaluated using an established in situ photometric method employing a pH-sensitive organic dye[1,2]. This method does not require expensive inspection tools and allows estimations of the lithographic performance aside the production environment. A kinetic model for the post-exposure bake (PEB) has been proposed and transferred to transport properties. The experimental data for this model have been obtained from UV/VIS spectroscopy measurements. Rough estimations of effective diffusion lengths are based on molecular reaction dynamics. The influence of the process conditions are discussed in greater detail. Results are used to improve the lithographic performance of the dual-wavelength CARL^(R) [3] resist system presently evaluated at Infineon Technologies.

Ultrathin film imaging at 157 nm

In future lithography the 157 nm wavelength is expected to succeed the 193 nm wavelength in 2004. So an early CARL resist for sub 100 nm resolution was developed at Infineon Technologies within the German BMBF project Laserbasierte Ultraprazisionstechnik - 157 nm Lithographie. Common 248 and 193 nm resist materials have a high absorbance (7- 12 μm-1) A main challenge at this short exposure wavelength is the development of a transparent base polymer or the imaging has to be done alternatively with ultra thin films. In contrast to a high transparency of the polymer a high quantum yield for the photo chemicals is required. CARL is a bilayer resist system developed by Siemens/Infineon Technologies. A modified CARL version is presented here for exposures at 157 nm, consisting of a silicon free top resist (Si free CARL) as thin imaging layer. A separate silylation step of the structured top resist after exposure and wet development provides a high etch resistance in the dry development step and allows imaging of ultra-thin films with a film thickness of ca. 50 nm. An oxygen plasma is taken to transfer these top resist structures into the up to 300 nm thick underlying Novolac type bottom resist. In dry development. The bottom resist itself provides high etch resistance also for aggressive substrate etch processes.

Attenuating phase-shifting mask at 157 nm

An attenuating phase shifting mask has been designed, fabricated, and tested at 157 nm. It consists of two layers, a metal attenuator and a transparent phase shifter. The metal, platinum, was chosen for its chemical and radiation stability. The phase shifter was a commercial spin-on glass. A single step of pattern transfer has been implemented, which significantly simplifies the fabrication process of the mask. The lithographic advantage in increased depth of focus was demonstrated for 130-nm spaces and contacts, and it was found to agree with numerical simulations.

Negative-tone resist for phase-shifting mask technology: a progress report

With the objective to make smaller device structures at a given illumination wavelength the semiconductor industry is more and more trying to implement Phase-Shifting Masks (PSMs) as resolution enhancement technique for DUV lithography. However, with positive photoresists there is a phase edge problem. Using negative resists is the easiest approach to solve the phase edge problem. This is one of the reasons why negative resists are becoming more and more attractive for leading edge lithography. Therefore, we are developing a novel negative resist with 248/193 nm crossover capability. Most experiments were done in imitation of the CARLR bilayer resist process. The goal was to use established resist techniques and polymer materials, and just to change the generators and additives to get tone reversal. Using a photoacid generator (PAG) as additive leads to positive tone. In contrary with a photobase generator (PBG) and thermoacid generator (TAG) combination in a negative tone behavior is observed. Comprehensively, this blending concept allows the use of similar working polymers in both, positive and negative resists. The generator efficiencies were studied as well as the diffusion behavior of resist components during resist processing. Especially, process factors like baking conditions were investigated with the objective to control diffusion and limit resist outgassing in a high activation energy resist platform. Furthermore, in adaptation of the CARLR process, a separate liquid silylation step was integrated and investigated for various process conditions. In our paper we will discuss the characteristics and the lithographic capabilities of the novel methacrylate based negative resists. First promising results are based on DUV (248/193 nm) and ebeam exposures. Recent results with our positive version indicate the same outstanding possibilities. We expect a similar performance for the negative pendant in the near future.

Novel fluoro copolymers for 157-nm photoresists: a progress report

Several fluoro-substituted polymers consisting of acid cleavable methacryoic or cinnamic acid tert.-butyl ester compounds copolymerized with maleic acid anhydride derivatives were synthesized by radical copolymerization. Vacuum ultraviolet transmission measurements of the samples reveal absorbances down to 5micrometers -1 despite of the strongly absorbing anhydride moiety which serves as silylation anchor for the application of the Chemical Amplification of Resist Lines (CARL) process, one of the promising approaches for sub-90nm pattern fabrication. Some of the samples exhibit resolutions down to 110nm dense at 157nm exposure using an alternating phase shift mask. The feasibility of the CARL principle including the silylation reaction after development has been demonstrated with selected fluorinated polymer samples.