Bassam Shamoun

Progress toward a raster multibeam lithography tool

A prototype raster multibeam optics is evaluated for 45 nm mask pilot lithography. A laser beam is split into 32 beamlets, which are modulated acousto-optically then focused onto a photocathode. This generates 32 electron beams which are scanned and focused on a writing surface. Photocathode photoyield and lifetime exceed 200 nA/mW and 100 h per spot, respectively. Photoyield recovers to within 1% after 1 min fully extinguished. The size of the laser spot is 300 nm, full width half maximum (FWHM) and the size of the corresponding electron beam spots is 50 nm (FWHM) at the writing plane. In the first demonstration of multiple electron beam lithography, 200 nm line/space arrays and 100 nm isolated lines were printed with an interim four-beam datapath. Preliminary experiments and a theoretical evaluation indicate that resist outgassing does not significantly reduce photocathode performance. An improved electron-optical column has also been designed, with a 35 nm pixel pitch, and a placement accuracy of <0.8u2002...

Cs halide photocathode for multi-electron-beam pattern generator

A unique approach to photocathode operation is described in this article. We utilize a relatively large bandgap CsBr photocathode material that under normal conditions would not photoemit with radiation energy less than the bandgap plus the work function. However, the material can be activated by proper UV illumination to obtain photoemission at wavelengths as long as 532nm. Photoyields as high as several hundred nA∕mW and current densities greater than 100A∕cm2 have been routinely obtained with lifetimes (50% degradation) well in excess of 200h at 257nm. The performance of the photocathode meeting all the requirements for a multi-electron-beam pattern generator will be presented.

Prototype raster multibeam lithography tool

A prototype raster multibeam lithography tool was constructed and is being evaluated for use as a mask writer at the 50 nm node. The photocathode illumination module (PIM) focuses a linear brush of 32 individually modulated laser beams into 300 nm full width at half maximum spots on the surface of a photocathode. The PIM has been module tested with satisfactory results and integrated into the prototype tool. A new electron-beam photocathode gun and column have also been integrated into the tool. The tool has generated a linear array of 32 electron beams which have been magnified and focused onto a YAG screen as well as demagnified and scanned across a knife edge. Preliminary results have produced 86 nm spots at the writing plane.

Assessment of thermal loading-induced distortions in optical photomasks due to e-beam multipass patterning

Thermal loading-induced distortion in the photomask during e-beam patterning has recently received special attention due to its significant contribution to overlay errors. Multipass e-beam writing, a strategy proposed to reduce the heating effects and associated distortions, was simulated using three-dimensional finite element models. Thermal responses of the photomask during multipass patterning were determined and global in-plane distortions were calculated. For the given system exposure conditions of 40 μC/cm2 at 50 keV, the average value of the 3σ pattern placement error due to the bulk heating of the photomask obtained from multipass writing was found to be ≈3.5 nm which is 28% lower than that of single pass writing. Parametric studies showed that thermal radiation has a large influence on the mask cooling.

Electron-electron interaction induced beam displacement in a multiple electron beam system

We have studied electron-electron (e-e) interaction induced beam displacement in multiple electron beam systems both experimentally and using trajectory simulation. A prototype 32-beam lithography system was used to record the beam displacements on chrome mask plates and the latter were directly measured by electron microscopy. Both experimental data and simulation results of demagnifying columns were consistent with a demagnification of the electron beam array with increasing current. While the simulation matched qualitatively the experimental data well, it predicted a smaller effect than measured. The overall demagnification was underestimated by 20%–30%. In an attempt to understand the underlying physical reason, we used a phenomenological model for the e-e induced beam displacement to fit the acquired data. The analysis suggested that the trajectory simulation underestimated the defocusing lens effect of the interacting electrons by about 1.5. The parametric expression derived from this model may be u...

Predictive and corrective model for bulk heating distortion in photomasks

Finite element (FE) numerical models were proposed to simulate and predict substrate thermal expansion in photomask substrates and were found to be computationally expensive and dependent on the mask-writing strategy. The present work describes a newly developed model that predicts and corrects for the substrate heating effects in the photomask. This prosed model provides a practical way of predicting in-plane distortions during real-time patterning that is not limited to nay writing strategy or pattern density distribution. The main advantage of this model is that it significantly reduces the computational time by using the linear superposition theory. By adopting the concept of linear superposition, pattern placement errors of mask substrate scan be determined at any time during writing using lookup tables from precomputed FE models. IF the thermal distortion of the substrate at the time during writing using lookup tables from, precomputed FE models. IF the thermal distortion of the substrate at the time of writing is known, beam deflection can be introduced to correct for the distorted substrate. The result predicted by the linear superposition FE model showed a difference of less than 10 percent compared with those predicted using a real-time calculated Fe mode, in a worst case scenario. The accuracy of the linear superposition FE model was found to be partially dependent on the size of the simulated patterning field. The results presented in this paper illustrate the effect of other parameters on the performance of the newly developed model, such as the shape of the patterning fields and pattern coverage uniformity. The overview of this work focuses on fused silica mask substrate materials.

Photomask in-plane distortion induced during e-beam patterning

As feature sizes decrease and the demand for throughput increases, the semiconductor industry must concentrate on pattern positioning accuracy and process efficiency. Thermomechanical distortions induced in the photomask during fabrication may act to constrain the desired range of operating conditions to meet the manufacturing requirements for pattern placement accuracy and throughput. 3D finite element heat transfer and structural models have been developed to determine the global in-plane distortions induced in the photomask during e-beam patterning. Results obtained from these models show that the thermal-induced distortions, caused by global heating, are significant. Whereas, distortions due to the mechanical loading, caused by resist in situ stress relief, are minimal and can be neglected.

Effects of material properties on patterning distortions of optical reticles

Bulk (or global) heating of photomasks due to e-beam energy deposition during patterning causes thermal expansion of the mask substrate and leads to pattern placement errors. Finite element calculations were performed to simulate the in-plane distortions (IPD) due to the single pass writing of a 6 in. X 6 in. optical reticle. Comparison studies were performed to identify the effects of material properties (such as thermal conductivity and the coefficient of thermal expansion) when pattering SiO2 and CaF2 substrates. Final IPD maps illustrate that thermal distortions of the CaF2 will need to be controlled in order to satisfy increasingly stringent error budgets.

A raster multibeam lithography tool for sub-100-nm mask fabrication utilizing a novel photocathode

A raster multibeam lithography tool is in Etec’s roadmap to meet the stringent requirements of sub 100 nm mask fabrication. The tool leverages the long experience obtained with the ALTA laser pattern generators and the high resolution capabilities of e-beam lithography. A photocathode controlled by acousto-optic modulated 257nm laser beams is utilized to generate 32 electron beams. The beams are accelerated at 50 KV in an electron column, demagnified and focussed on the mask or wafer substrate. The performance of the photocathode and other system components will be presented together with preliminary lithographic patterning.

Supercritical Vapor Explosions: Comparisons Between Thermodynamic and Mechanistic Models

The thermal interaction of certain molten materials such as Al 2 O 3 with water results in vapor explosions with very high (supercritical) pressures and propagation velocities. A quasi-steady-state analysis and a transient analysis of a supercritical vapor explosion in one-dimensional multiphase flow were applied to analyze experimental data of an Al 2 O 3 -water fuel/coolant interaction obtained from the KROTOS 26, 28, 29, and 30 tests. The shock adiabatic thermodynamic model and the TEXAS mechanistic model were used to perform this analysis. The predicted results of the initial vapor void fraction and explosion conversion ratio from both models, together with the estimated experimental results, for the KROTOS 26 test were compared.