Ho-Seob Kim

Electron‐beam microcolumns for lithography and related applications

Lithography with an array of miniaturized scanning electron‐beam columns presents one of the most promising high‐throughput possibilities for fabrication of devices with feature sizes less than 100 nm. With scanning electron beams no mask is required and the necessary resolution and alignment of overlay structures are realizable. With arrays of microcolumns, the lithography throughput of a single column can be multiplied. The approach can also be used for a number of lithography related applications such as metrology, inspection, testing, etc. We review the status of the microcolumn program and discuss opportunities and challenges of this approach to high‐throughput nanolithography and related applications. Special emphasis is given to lithography in the 100 nm regime.

Experimental evaluation of a 20×20 mm footprint microcolumn

A miniaturized 1 kV electron beam column with a 20×20 mm square footprint for application in arrayed lithography was developed. The actual beam forming optics measured from the electron emitter to the last electrode in the beam focusing Einzel lens is only 3.5 mm in length. The electron source is a miniaturized, high brightness (120 μA/sr), low heating power (<1.5 W) Zr/O/W Schottky field emitter that provides a stable beam current with <1%/h current fluctuations. A custom designed, ultralow profile (0.8 mm high) annular microchannel plate (MCP) detector is fitted into the working distance, which can be varied between 1 and 5 mm, between the Einzel lens and the sample. The MCP provides a high gain, up to 3×104, detector for secondary and backscattered electron detection from solid samples. The beam is scanned over the sample using a prelens double octupole deflector for large field size, ≥100 μm, at low distortions and low deflection aberrations. Using a computer controlled digital pattern generator, patt...

An electron-beam microcolumn with improved resolution, beam current, and stability

We have built and tested a 1 keV electron‐beam microcolumn that focuses 1 nA of beam current into a 10 nm full width half‐maximum beam diameter at a working distance of 1 mm. The electron source is a miniaturized Zr/O/W Schottky field emitter with 150 μA/sr angular emission current density operating at about 1800 K at a distance of only 100 μm from a silicon membrane extractor electrode. The actual microcolumn is 3.5 mm long assembled mainly from silicon membrane electrodes. Improved einzel lens design and fabrication allowed the operation of this beam focusing element in the accelerating mode. Spherical and chromatic aberrations were reduced by factors of about 2–3, respectively, as compared to the retarding lens mode. Excellent beam current stability with less than 1% variation over several hours has been observed.

Emission characteristics of ultrasharp cold field emitters

We have examined the field emission characteristics of oxygen‐processed and thermal‐field buildup W 〈111〉 tips. Good emission angular confinement was found to correlate with the global geometry of the tip. Emission stability was related to the atomic arrangement at the apex. This phenomenon is described by the different driving forces for atomic surface diffusion. We also showed that tip apexes can be engineered to form flat (111) facets at the tip end for improved stability.

Miniature Schottky electron source

A miniature Schottky electron source has been developed and evaluated for applications in a new generation of scanning tunneling microscope aligned field emission microcolumns. Both the physical dimensions and the heating power of this source have been significantly reduced from a conventional source of the same kind. Operating parameters for such a source in a microcolumn environment in terms of emission characteristics, suppressor operating range, etc., have been evaluated. Test results show that very good emission stability at ≥100 μA emission current over several hours, and axial angular current densities in excess of 100 μA/sr can be obtained. Energy distributions have been measured using a carefully calibrated analyzer, and the results show a full width at half‐maximum of 0.4 to 0.76 eV for a 0.3 μm radius Schottky source operating over an angular current density range of 1 to over 100 μA/sr. A significant change in the shape of the energy distribution was observed over this range of operation, indi...

Sub‐40 nm resolution 1 keV scanning tunneling microscope field‐emission microcolumn

Recent advances in microcolumn design and fabrication have led to a significant improvement in the resolution obtained with a new 3.5 mm long microcolumn. At an electron energy of 1 keV and 2 mm working distance, the beam diameter, measured by the signal rise‐time while scanning over a sharp edge, is 40 nm and the resolution observed in scanning transmission electron microscope images is about 30 nm. These experimental results agree well with results of the electron‐optical modeling. All lenses and apertures in the column are made from silicon membranes and assembled using a novel ultrahigh vacuum compatible, multilayer anodic bonding technique. The scanning tunneling microscope aligned field‐emission source uses an oxygen processed, 50 nm radius, cold 〈111〉 W field‐emission tip. Good emission stability for scanning electron microscope operation has been achieved with a few percent of root‐mean‐square current fluctuation over about 30 min. With periodic tip flashes, the system has been operated reliably o...

Oxygen processed field emission tips for microcolumn applications

An oxygen induced sharpening process of field emitter tips, W〈111〉, for use in a scanning tunneling microscope aligned field emission microcolumn system has been developed. The sharpening process which depends on processing temperature and oxygen pressure can be used to control tip radius accurately with reliability and reproducibility. The measured tungsten removal rate was ∼13 A/min at a processing temperature of ≂1650 K and at an oxygen pressure of ≂4×10−5 Torr. The process is primarily intended for more accurate control of the tip radius and hence performance of newly etched tips, although damaged or blunt tips can also be resharpened in situ with this process. Favorable emission characteristics of the oxygen processed tips have been observed with microcolumn operation: (1) reasonably stable emission current, (2) low extraction voltage, (3) reproducible threefold symmetric emission patterns, and (4) small emission angle.

Performance of Zr/O/W Schottky emitters at reduced temperatures

Experimental measurements of emission stability and energy distributions from a Schottky emitter have been conducted at a tip temperature range from 1330 to 1800 K. The changes of emission properties have been observed at reduced tip temperatures. Noise fluctuations of the probe current increase with decrease of the tip temperature at a constant extraction voltage. The work function of the Schottky emitter increases with decrease of tip temperature. The energy distribution measurements show that the energy width at a given temperature increases with increasing angular emission current density. The energy width also increases with decreasing tip temperature at a given angular emission current density. The results indicate that the energy broadening is mainly contributed by electron tunneling. A comparison of the measured energy width with the theoretical predictions is discussed.

Energy distributions of field emitted electrons from carbide tips and tungsten tips with diamondlike carbon coatings

We have measured the energy distributions of electrons field emitted from tungsten carbide, HfC〈100〉, and ZrC〈100〉 tips, and tungsten field emitters with diamondlike carbon coatings. Multiple‐peaked energy distributions were observed from instability induced emission sites on the carbide tips. Energy distributions of electrons field emitted from the diamondlike carbon coated tungsten tips were broader than those from metal tips. They also showed a shift towards lower energies with increases in the emission current.

New three dimensional simulator for low energy (∼1 keV) electron beam systems

In this article, we describe the models and the results of a new three dimensional (3D) lithography simulation programs for low energy (∼1 keV) electron beam systems. Monte Carlo simulation was performed to obtain the energy intensity distribution in e-beam resists, and the models we have used were tabulated Mott data for elastic scattering, Moller and Vriens cross sections for inelastic scattering, and modified Bethe equation plus discrete energy loss for energy loss. The energy intensity in poly(methyl) methacrylate was calculated with the exposure simulation program with various pattern shapes. In the development simulation program, the 2D or 3D resist profile could be implemented. The ray tracing model and the Neureuther equation were used for the development simulation. The simulated developed depths as a function of energy were compared with experimental results developed by Rishton and Schock. The maximum deviation from the experimental results was 12.4 nm (6%) at 2500 eV, and all the data were wit...