Theodore M. Bloomstein

A Nanoparticle Convective Directed Assembly Process for the Fabrication of Periodic Surface Enhanced Raman Spectroscopy Substrates

Surface enhanced Raman scattering was discovered over 30 years ago, when it was noted that the usually weak molecular Raman scattering cross section was increased by orders of magnitude in the vicinity of metal surfaces. [ 1 , 2 ] Despite the early reports of single-molecule detection, [ 3 ] the promise of the technique as the basis for portable chemical sensors has not been fully realized. The reason for this gap between the science and the engineering of SERS lies in the formidable nanofabrication challenges it poses, which is the need to prepare large numbers of very small yet highly controlled “hot spots” as the sensing device. The need for “hot spots” arises because the SERS enhancement is composed of an electromagnetic effect and a chemical (or resonance) enhancement, with the electromagnetic effect being responsible for the majority of the enhancement. The “hot spots” are the manifestations of this fi eld enhancement, occurring only for select plasmonic materials. [ 4 ] Modeling studies of the electromagnetic effect indicate that dimerized plasmonic metal structures offer a signifi cantly higher electromagnetic fi eld enhancement than isolated structures, with the maximum fi elds occurring in the gap between the structures. [ 5 , 6 ] The fi eld enhancement dependence on the gap size is highly nonlinear: it becomes signifi cant when the gap is less than ∼ 10 nm, and then rises steeply with decreasing gap size. The importance of the small gap size on signal strength has been confi rmed experimentally. [ 7 , 8 ] While the SERS effect may be very high in a localized volume of the order of a few nm 3 , the probability that target analyte molecules adsorb there is very small. [ 9 ] Consequently, practical SERS-based sensors require the engineering of a very large number of such “hot spots” over areas that are at least a few mm 2 . Small interparticle spacing with a large number of metal particles has been realized for SERS experiments performed in nanoparticle solutions, which have been “activated” by electrolyte-induced aggregation. [ 10 ] However, solution-based

Outlook for 157-nm resist design

We have measured the transparencies of a number of a candidate resist materials for 157 nm, with an emphasis on determining which chemical platforms would allow resist to be used at maximum thicknesses while meeting requirements for optical density. Assuming an ideal resist optical density of 0.4, our findings show that all existing commercially available resists would need to be < 90 nm thick, whereas specialized hydrocarbon resists could be made approximately 120 nm thick, and new resists based on hydrofluorocarbons, siloxanes, and/or silsesquioxanes could be engineered to be used in thicknesses approaching 200 nm. We also assess the tradeoff between these thicknesses and what current information exists regarding defects as a function of resist thickness.

22-nm immersion interference lithography

Immersion interference lithography was used to pattern gratings with 22-nm half pitch. This ultrahigh resolution was made possible by using 157-nm light, a sapphire coupling prism with index 2.09, and a 30-nm-thick immersion fluid with index 1.82. The thickness was controlled precisely by spin-casting the fluid rather than through mechanical means. The photoresist was a diluted version of a 193-nm material, which had a 157-nm index of 1.74. An analysis of the trade-off between fluid index, absorption coefficient, gap size and throughput indicated that, among the currently available materials, employing a high-index but absorbing fluid is preferable to using a highly transparent but low-index immersion media.

Patterning of sub-50 nm dense features with space-invariant 157 nm interference lithography

We have implemented a space-invariant interference lithography tool for 157 nm F2 lasers, capable of creating dense line and space patterns with a spatial period of 91 nm. No gratings or curved optics are required, allowing a simple and inexpensive tool for resist and process development at 157 nm. Initial patterning of several commercial and experimental resists has resulted in high contrast features with little line edge roughness and good cross-sectional profiles, indicating that the fundamental performance of acid-catalyzed resists patterned at 157 nm may meet lithography requirements for sub-50 nm features.

Photoresist outgassing at 157 nm exposure

Contamination of optical elements during photoresist exposure is a serious issue in optical lithography. The outgassing of photoresist has been identified as a problem at 248nm and 193nm in production because the organic films that can be formed on an exposure lens can cause transmission loss and sever image distortion. At these exposure energies, the excitation of the photo acid generator, formation of acid, and cleavage of the protecting group are highly selective processes. At 157nm, the exposure energy is much higher (7.9 eV compared to 6.4 eV at 193nm) and it is known from laser ablation experiments that direct laser cleavage of sigma bonds occurs. The fragments formed during this irradiation can be considered as effective laser deposition precursors even in the mid ppb level. In this study, methods to quantify photoresist outgassing at 157 nm are discussed. Three criteria have been set up at International SEMATECH to protect lens contamination and to determine the severity of photoresist outgassing. First, we measured film thickness loss as a function of exposure dose for a variety of materials. In a second test we studied the molecular composition of the outgassing fragments with an exposure chamber coupled to a gas chromatograph and a mass spectrometer detector. Our third method was a deposition test of outgassing vapors on a CaF2 proof plate followed by analysis using VUV and X-ray photoelectron spectroscopies (XPS). With this technique we found deposits for many different resists. Our main focus is on F- and Si- containing resists. Both material classes form deposits especially if these atoms are bound to the polymer side chains. Whereas the F-containing films can be cleaned off under 157nm irradiation, cleaning of Si-containing films mainly produces SiO2. Our cleaning studies of plasma deposited F-containing organic films on SiO2 did not indicate damage of this surface by the possible formation of HF. Despite that we strongly recommend engineering measures to overcome contamination by resist, such as optimizing the purge flow between the final lens element and wafer surface or utilization of a lens pellicle.

Standard Chemical Thermodynamic Properties of Alkylnaphthalene Isomer Groups

The chemical thermodynamic properties of alkylnaphthalene isomer groups for C10H8 and C11H10 in the ideal gas phase have been calculated from 298.15 to 1000 K from tables of Stull, Westrum, and Sinke. In the absence of literature data on all isomers of higher isomer groups, the properties of isomers of C12H12 to C14H16 have been calculated using Benson group values. A new Benson group value for the 1,8‐dimethyl steric hindrance has been calculated from recent experimental data. The increments in isomer group properties per carbon atom have been calculated to show the extent to which thermodynamic properties of higher isomer groups may be obtained by linear extrapolation. Equilibrium mole fractions within isomer groups have been calculated for the ideal gas state from 298.15 to 1000 K. Values of C○p, S°, ΔfH°, and ΔfG° are given for all species from C10H8 to C14H16 with energy units of joules for a standard state pressure of 1 bar.

Experimental measurements of diffraction for periodic patterns by 193-nm polarized radiation compared to rigorous EMF simulations

Polarization dependent diffraction efficiencies in transmission through gratings on specially designed masks with pitch comparable to the wavelength were measured using an angle-resolved scatterometry apparatus with a 193 nm excimer source. Four masks - two binary, one alternating and one attenuated phase shift mask - were included in the experimental measurements. The validity of models used in present commercially available simulation packages and additional polarization effects were evaluated against the experimental scattering efficiencies.

Thermomechanical distortions of advanced optical reticles during exposure

If optical lithography is to be extended into the 157 nm regime, controlling mask-related distortions will be a necessity. Thermomechanical distortions during exposure could be a major source of pattern placement error, especially if alternative materials such as CaF2 or MgF2 are employed. Full 3D finite element heat transfer and structural models have been developed to simulate the response of the reticle during both full-field and scanning exposure systems. Transient and periodic steady-state temperature distributions have been determined for typical exposure duty cycles. Corresponding in-plane and out-of- plane thermal distortions have been identified for both fused silica and calcium fluoride substrates. Under equivalent exposure conditions, the distortions in the CaF2 are significantly higher.

Mechanical distortions in advanced optical reticles

Finite element models have been developed and refined to simulate the mechanical distortions associated with mask blank fabrication, pattern transfer, and exposure clamping. By modeling the substrate with layers associated with the mask fabrication process and then by prestressing specified layers, the resulting out-of-plane and in-plane distortions of the mask blank have been determined. Etching procedures were subsequently simulated to assess the pattern transfer distortions associated with both dark and bright field masks. Investigations included substrate materials which have acceptable optical transmission for wavelengths below 180 nm. Additional mechanical distortions associated with clamping the reticle into the exposure mount have also been considered.

Vectorial effects in subwavelength mask imaging

Ultra high numerical aperture (NA) enables extension of ArF lithography for the 45 technology node and beyond. The resulting changes in design rules drives feature sizes on the mask into the sub-wavelength regime. As 2-beam imaging techniques (off-axis illumination and alternating phase shift mask) are required for strong resolution enhancement in low-k1 lithography, traditional scalar and paraxial approximations used for optical image modeling are no longer valid in the ultra high NA regime. Vector and thick-mask based models are required to account for topographic effects and large angles of incident light at the reticle plane in ultra-high NA systems. Although vector-based imaging theory is well understood, experimental validation is required to ensure the appropriate topographical and optical parameters are being used. To address these issues, finite-difference time-domain rigorous electromagnetic simulation are compared to experimental measurements of the polarization dependent diffraction efficiencies on advanced optical reticles. Based on these results, the impact of mask induced polarization to vectorial imaging latitude is assessed. The impact of polarization purity, mask absorber profile, and Fresnel effects through the pellicle on process window and OPC are also discussed.