Basics E-beam lithography Introduction of concepts Pattern handling Hardware issues Miscellaneous

Main issues Introduction of concepts Resist Dose [µC/cm²] Pattern From design to beam stepping Electron Beam Pattern Generator Holder/substrate Positioning Beam Exposure

Electron Beam Pattern Generator Introduction of concepts Source Field Emission Gun Beam column spot, scanning beam Stage/holder  substrate, stepping, controls

Substrate holder Introduction of concepts Beam current measurement Calibrations (spot size, focus) Orientation reference (x,y)

E-beam definitions Introduction of concepts Dose (material, kV) µC/cm² Acceleration voltage kV (20kV, 50kV, 100kV) Spot size dspot 3 … 175 nm Beam current Ispot (~dspot²) 300 pA … 317 nA Beam step size BSS Resolution Res Beam step frequency BSF 500 Hz … 100MHz

From pattern to e-beam writing process pattern handling Hardware Stage movement Beam deflection Fracturing Main field (scanfield, block) Trapezium

Large structures pattern handling pattern divided into main fields substrate move for each next main field main field exposure by beam deflection each main field: beam @ center main field height compensation deflection calibrated to stage movement Writing order of main fields

Trapezium pattern handling T = pixel exposure time BSF (MHz) = 1/T(µs) Ispot(nA) 1 = 0.1 * ------------------------------- ~ ---------- Q(µC/cm²)*BSS²(µm²) dspot²

Trapezium-wise beam stepping in main field pattern handling Each (xi,yi) is the start of a new trapezium

EBPG CHARACTERISTICS pattern handling Trapezium resolution 0.08 nm ……… 0.5 nm Main field resolution 0.16 nm ……… 1.0 nm Main field deflection º max field size = 1048576*main resolution (20bits) º size range: 167.8 …1048.576 µm @ 100kV Trapezium deflection º BSF: 500 Hz – 100 MHz º BSS = 1,2,3,4,5,…,16383 * trapezium resolution º max field size = 16384 * trapezium resolution (14bits) º size range: 1.31 … 4.525 µm @ 100kV

Filling the pattern with spots pattern handling e-beam is stepped filling areas use dspot=1.2–1.5*BSS lines use minimal 4-5 spots in linewidth narrowest lines º take smallest spot º define linewidth of 1 BSS (often 1.25nm)

Intermezzo: dose considerations primary and secondary contributions primary dose º lateral range: spot diameter + little effect from forward scattering in resist º gaussian distribution º partial overlap of adjacent spots secondary electrons lateral range: 5 nm around primary electron path backscattered electrons (proximity) º lateral range: many µm º on Si, at 100 kV typically 50 µm … different dose settings for plane, line and spot

Intermezzo: dose considerations primary and secondary contributions

JOB TIME pattern handling beam-on time Q(µC/cm²)*A(mm²) 1 1 T(s) = 10* --------------------------- ~ --------- ~ --------- Ispot(nA) BSS² dspot² to minimize beam-on º split coarse and fine pattern layers, but keep small (e.g. 1 µm) overlap º calibration per layer takes 3 to 5 minutes overheads º main settling 50µs/trapezium º stage movements º big pattern files (max 1GB!)

Software and computer overview pattern handling Design Autocad/DesignCAD/LDM-file/L-Edit/Other Output formats DXF/ GDSII/ CIF/ TXL/ other formats Conversion LayoutBeamer  gpf pattern data Gpf pattern data  exposure EBPG  Cview inspection Computer Layout Beamer PG5000+ USER ---------------------- pegasus ---------------------- EPIC-ALFA (design) (job) pegasus.kavli.tudelft.nl epic-alfa.kavli.tudelft.nl cad/&KN-lab pg/pg5000@Delft or PG5200 EPIC-BETA epic-beta.kavli.tudelft.nl pg/pg5200@Delft

Around the stage hardware issues BSED: Back Scattered Electron Detector locating marker spot optimization and focussing Laser: height measure-ment  beam focus on substrate

Substrate holder hardware issues Cup: current measurement, x-y reference Markers: spot characteristics

Beam current measurement hardware issues Faraday cup

Calibration markers hardware issues spot optimisation (focussing) spot size measurement ‘zero’ height reference reference in (x,y) position marker search

Spot size measurement hardware issues dmeas² = dspot² + dedge² dedge = 20nm for very good marker but often > 30nm; depends on marker edge itself and possible contamination dmeas² = a.Ispot + b (small spots)

Spot size adjustment hardware issues FEG: high brightness electron source BSF too high  need for defocus discrete steps of 20nm/bit in FL@400 µm aperture adds quadratically to initial spot dadjust² = dmeas² + ddefocus² unround spot if a few bits real big spots: 4/3*measured size defocus to dmeas or adjust by [+ -]<bits>

Height adjustment hardware issues Beware: Transparent substrate Reflectivity topology Flatness: 1µm/mm  height error ~ 1.5µm  broadening of the beam with ~ 7.5nm (at 400µm aperture) Calibration marker at ‘zero’ height Adjustment range; ± 100 µm

Height level hardware issues final lens image of spot on substrate Substrate NOT at constant height should be < 1µm/mm Height compensation during exposure: spot size –FF scaling rotation  Final aperture MUST be well aligned

E-beam threats miscellaneous Charging e-drain required  conductive layer* Contamination work clean! handle holders carefully and with gloves Vibrations turbo: around 800Hz  writing strategy!* don’t be around during writing Thermal stability Keep door closed * In more detail in Advanced E-beam Litho

Limitations miscellaneous Resist shot noise: N ± √N swelling secondary electrons: 8nm extra Position accuracy laser interferometer: 0.6nm marker location: 30nm drift (0.1 µm/hr possible) main field overlay stitching: 60nm