Leonidas E. Ocola

Roughness study of a positive tone high performance SCALPEL resist

In this article we discuss the line edge roughness of positive chemically amplified resists exposed on the SCALPEL exposure system in terms of the image formation process. The image formation process for a SCALPEL exposure on a positive chemically amplified resist has been simulated using discrete models from exposure through development. Key parameters have been identified that enable image formation simulations without the need of detailed molecular models. Molecular models are needed though to obtain several of these parameters. Surface and line edge roughness, as measured by scanning electron microscopy and atomic force microscopy, have been simulated and compared to experimental results. Results are consistent with a “percolation network formation for diffusion-reaction development” model for chemically amplified resists.

Lithography for sub-60 nm resist nanostructures

As the semiconductor community continues to follow the Semiconductor Industry Association Roadmap, resist structures are being printed further into the nanometer domain. However, a persistent issue for successful sub-60 nm resist patterning is mechanical stability at high aspect ratios. The objective of this article is to understand what processing conditions facilitate processing resist nanostructures with useful aspect ratios for the fabrication of sub-60 nm transistors. We have found that, in aqueous based development and rinse, if the resist thickness is reduced, then the aspect ratio is sacrificed for the sake of resolution. The implication is that there is a resolution limit at which resist structures will have aspect ratios that are useful for device fabrication. We have also found that there are development effects that occur in the thick film regime that are not reproducible with thin films. The best resolution structures we have been able to print are lines of 28 nm in width using direct write e...

Space-charge effects in projection electron-beam lithography: Results from the SCALPEL proof-of-lithography system

In projection electron-beam systems resolution and throughput are linked through electron–electron interactions collectively referred to as space-charge effects. Hence, a detailed understanding of these effects is essential to optimizing the lithographic performance of a projection electron-beam lithography system. Although many models have been developed to describe one or more of the various aspects of the Coulomb interactions that occur in the beam, there is minimal experimental data available. We have performed a series of experimental measurements in the scattering with angular limitation projection electron-beam lithography (SCALPEL) proof-of-lithography system to characterize the space-charge effects for such an optical configuration. The results of those measurements have been compared to a combination of computer simulations and analytical models. The agreement between the models and experiments was good, within the limits of experimental error. We determined the exponent in the dependence of blu...

Negative chemically amplified resist characterization for direct write and SCALPEL nanolithography

Despite the common link of using high energy electrons as exposure probes in direct write and SCALPEL lithography, the imaging systems in both are different. The question arises then if an optimal process for direct write lithography will provide optimal results in SCALPEL. Characterization of a series of negative tone chemically amplified resists for use in direct write (at 50 kV) and projection electron beam (at 100 kV) nanolithography is reported. The current work has explored modifying the postapplied bake and postexposure bake temperatures and times to find an optimum process for direct write nanolithography and then verify it is applicable to SCALPEL. Results, using NEB-22A (Sumitomo Chemical Co.), show that this is possible. We have demonstrated that in direct write, the NEB-22A resist has excellent resolution (36 nm in 125-nm-thick resist, 75 nm in 500-nm-thick resist) and process latitude (>20%), with a sensitivity of about 14 μC/cm2 at 50 kV exposure. The etch resistance is 3 nm/s and is being u...

Monte Carlo study of high performance resists for SCALPEL nanolithography

Abstract The semiconductor community continues to push the limits of device dimensions by exploring new high-resolution lithography technology. As part of the SCALPEL lithography resist program, our goal is to be able to print sub-100 nm structures at doses that will permit high throughput, reduce wafer heating and still maintain good process latitude. Using 100 KV exposures on a SCALPEL tool, 100 nm structures were printed at exposure dose of 5.8 μC/cm 2 (and 80 nm isolated trenches at 5.4 μC/cm 2 ) in positive resists. In negative resists, isolated 100 nm were printed at a dose of 6.8 μC/cm 2 , and 80 nm structures at 7.2 μC/cm 2 were resolved as well. These results are well below the 10 μC/cm 2 minimum dose requirement for high throughput. Monte Carlo simulations were used as means to understand energy absorption mechanisms of these e-beam optimized resists, DUV and 193 nm resists. Atomic composition was found to factor in improved resist ionization. The resin (or low-Z elements) is found to account for more than 99% of ionization events during exposure.

Commercialization of SCALPEL masks

To investigate the viability of large scale manufacture of SCALPEL masks, key components of the SCALPEL mask process have been performed by commercial suppliers. SCALPEL mask blanks have been fabricated by MCNC to specifications supplied by Lucent and have been delivered, patterned, and utilized. Patterning, inspection, and metrology have been performed by DuPont Photomask and Photronics using the standard set of tools used for photomasks. A wet chemical pattern transfer process has been developed that is compatible with the processing tools in the mask shops and is extensible to the 0.1 μm generation of integrated circuits. SCALPEL masks that have been fabricated using these processes and tools exhibit excellent pattern fidelity and feature edge quality.

Technique for rapid at-wavelength inspection of extreme ultraviolet mask blanks

We have developed two new methods for at-wavelength inspection of mask blanks for extreme-ultraviolet (EUV) lithography. In one method an EUV photoresist is applied directly to a mask blank which is then flood exposed with EUV light and partially developed. In the second method, the photoresist is applied to an EUV transparent membrane that is placed in close proximity to the mask and then exposed and developed. Both reflectivity defects and phase defects alter the exposure of the resist, resulting in mounds of resist at defect sites that can then be located by visual inspection. In the direct application method, a higher contrast resist was shown to increase the height of the mounds, thereby improving the sensitivity of the technique. In the membrane method, a holographic technique was used to reconstruct an image of the mask, revealing the presence of very small defects, approximately 0.2 μm in size. The demonstrated clean transfer of phase and amplitude defects to resist features on a membrane will be ...

CMOS compatible alignment marks for the SCALPEL proof of lithography tool

SCALPEL alignment marks have been fabricated in a SiO 2 /WSi 2 structure using SCALPEL lithography and plasma processing. The positions of the marks were detected through e-beam resist in the SCALPEL proof of lithography (SPOL) tool by scanning the image of the corresponding mask mark over the wafer mark and detecting the backscattered electron signal. Single scans of line space patterns yielded mark positions that were repeatable within 30 nm 3σ with a dose of 0.4 μC/cm 2 and signal-to-noise of 16 dB. An analysis shows that the measured repeatability is consistent with a random noise limited response. The mark detection repeatability limit, that can be attributed to SPOL machine factors, was measured to be 20 nm 3σ. By using a digitally sequenced mark pattern, the capture range of the mark detection was increased to 13 μm while maintaining 36 nm 3σ precision. The SPOL machine mark detection results are very promising considering that they were measured under electron optical conditions that were not optimized.

Development of a Technique for Rapid At-Wavelength Inspection of EUV Mask Blanks

We have dramatically increased the sensitivity of a technique for the rapid inspection of EUV multilayer-coated mask blanks. In this technique an EUV sensitive resist is applied directly to a mask blank which is then flood exposed with EUV light and partially developed. Reflectivity defects in the mask blank results in mounds in a partially developed positive resists that appear as high contrast objects in a standard Nomarski microscope. The use of a higher contrast resist is shown experimentally to result in the creation of dramatically taller mounds. A simple model for the exposure and development of the resists has been developed and the predictions of the model compare well with the experimental results.

Alignment Mark Detection In CMOS Materials With SCALPEL E-Beam Lithography

A manufacturable process for fabricating alignment marks that are compatible the SCALPEL lithography system is described. The marks were fabricated in a SiO2/WSi2 structure using SCALPEL lithography and plasma processing. The positions of the marks were detected through e-beam resist in the SCALPEL proof of lithography (SPOL) tool by scanning the image of the corresponding mask mark over the wafer mark and detecting the backscattered electron (BSE) signal. Scans of 1 micrometers line-space patterns yielded mark positions that were repeatable within 20 nm 3(sigma) with a dose of 4 (mu) C/cm2 and signal-to-noise of 32 dB. An analysis shows that the measured repeatability is consistent with a random noise limited response combined with SPOL machine factors. By using a digitally sequenced mark pattern, the capture range of the mark detection was increased to 13 micrometers while maintaining 35 nm 3(sigma) precision. Further improvements in mark detection repeatability are expected when the SCALPEL electron optics is fully optimized.