Steven T. Coyle

Multisource optimization of a column for electron lithography

The optimization of key parameters determining the performance of a multisource electron column is discussed. A 50 keV multisource test bed incorporating a photocathode and multiple modulated light beams has been developed and tested. The multisource test bed allows for a detailed evaluation of both the photocathode sources and the electron optics for sub-100 nm lithography applications and is designed to reduce electron–electron interactions. Results have been obtained using cesiated GaAs negative electron affinity and gold photocathodes at beam energies varying from 10 to 50 kV, allowing the experimental evaluation of key design parameters.

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 <0.8 ...

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.

Negative electron affinity group III-nitride photocathode demonstrated as a high performance electron source

The need for a high performance (low energy spread 3months per spot, emission stability <1%∕h) electron source continues as part of the development of new e-beam writing and inspection tools. We present measurements from a group III-nitride (indium gallium nitride) photocathode in a demountable vacuum system to measure energy spread, lifetime, and preliminary blanking effects. We show the results of cathodes operating in ultrahigh vacuum (UHV), high vacuum (HV), and oxygen-rich backpressures. Our results show InGaN has a longitudinal energy spread of <300meV in reflection mode, flat lifetimes of 60h per illuminated spot where the yield changes by <10%, and stable emission with typical recoveries within 99% of original photocurrent for all blanking periods and vacuum conditions tested (0.5to10min periods).

Basic constraints for a multibeam lithography column

A 50 keV multibeam test bed incorporating a photocathode and multiple modulated light beams has been developed and constructed. The column allows for a detailed evaluation of both the photocathode sources and the electron optics for sub-100 nm lithography applications and is designed to reduce electron–electron (e–e) interactions. The basic constraints determining the performance of the multibeam electron column are discussed. Results have been obtained using gold photocathodes at beam energies varying from 10 to 50 kV, allowing the experimental evaluation of key design parameters.

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.

Electron–electron interactions in multibeam lithography columns

We have used Monte Carlo simulations to evaluate electron–electron (e–e) interactions in multibeam lithography columns. For a linear array of 32 beams with variable length from 40 to 400 μm at the photocathode, the spot size and placement were calculated for total beam currents of up to 800 nA at the substrate. In general, the e–e interactions are reduced when the linear array size is increased, therefore reducing the total beam blur. However, this is not true for placement errors induced by e–e interactions, specifically when the absolute magnitude of the error at the edge of the array is considered. We have used a multibeam test bed, equipped with a magnification stack for high spatial resolution imaging of the photoemission, to evaluate experimentally the effect of e–e interactions.

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...

Progress toward a high-brightness photoemission source for multiple-electron beam lithography

A high-brightness photoemission source is being developed for use in a multiple-electron beam lithography system suitable for mask patterning. Cesium telluride (Cs2Te) photocathodes have achieved stable brightness of 2–3×106 A/cm2 Sr and current densities of 30–60 A/cm2. Magnesium photocathodes have achieved stable brightness of 1–3×106 A/cm2 Sr and current density of 40 A/cm2. Long term current stability of better than the required specification of 17%/5 min has been obtained at photoyields of ∼30 nA/mW.

Preliminary evaluation of surface plasmon enhanced light transmission with a scanning 257nm ultraviolet microscope

Surface plasmon enhanced illumination (SPEI) devices could be useful in multielectron beam lithography systems to meet the roadmap requirements for mask- and direct write applications by creating arrays of light beams, or in conjunction with a photoemitter to produce a very stable array of electron beams from a patterned photocathode. Preliminary experiments were performed with a scanning 257nm UV transmission microscope to evaluate the performance of SPEI devices. Light transmission enhancement relative to simple apertures greater than 200× were obtained with these devices utilizing apertures as small as 60nm diameter. The results will be presented in this article together with other possible advantages for e-beam applications.