Advanced e-beam lithography overview positioning and alignment complications from design to EBPG gpf workfile

Simplest case of direct write (without marker searches and alignment) just exposure at ‘some’ position holder centre alignment microscope general layout: array (n,m) of pattern centre positions pitch SRD = srd_x, srd_y (srd= step and repeat distance) job definition file type: *.JOB (or *.LAYOUT)

Marker search positioning & alignment manually with SEM (not in batch jobs!) any recognisable structure with known position automatically more accurate physical principle contrast in backscattered electron yield RECT or TOPO type POS or NEG tone best markers are rectangles, max size 100 µm crosses can be used, but aligning is harder default marker RECT POS 20,20 (holder)

marker search with image grabbing 1. the reference image (template image): an image of (another) marker of an artificial image obtained from e.g. pattern data 2. image of the marker (marker image) 3. offset between template image and marker image is calculated

Marker search positioning & alignment marker search parameters expected contrast CONTRA max radius ISRAD (default 50µm, max half scanfield) step ISXSTP, ISYSTP (default 30% marker size) keep free area (> 100µm) around marker confusing of search failure by other features searching = high exposure backscattered electrons

Marker search positioning & alignment prerequisites (use alignment microscope) loading of wafer, rotation < 0.2º position of first (reference) marker known next marker within search radius array of markers (≥2x2) preferable along x and y typical alignment accuracy 3 sigma = 30 nm due to stage movement 3 sigma = 10 nm obtainable with in-field alignment (which needs special .com-file) NB! wafer cq. mounting plate connected to holder with springs holder with springs to stage

Alignment positioning & alignment alignment is done in steps each step improves accuracy in most cases 2 steps are sufficient (global, layout and pattern markers): coarse and matrix at least 3 markers needed for correcting: rotation, shift and scaling at least 4 markers allow also keystone to be corrected.

Coarse-alignment positioning & alignment check on prerequisites improves effectiveness actual marker search with premarker

Matrix-alignment positioning & alignment 3 or 4 markers in orthogonal arrangement markers from one cell or from different cells more closely spaced markers = better overlay accuracy *.LAYOUT files: pattern should be within the marker field die-to-die in-die

extra flexibility with CJOB positioning & alignment pattern can be outside marker field markers not necessarily in orthogonal system several patterns and pattern layers can be exposed after one alignment But: in CJOB no array of markers possible

Problems positioning & alignment features close to the marker design larger free area around marker, at least 100 µm layers not well-aligned to each other ensure the centres of the patterns coincide. actions: 1) pattern sizes (in x and y) are equal, or 2) put layers in one design and process them together small alignment errors ASCII (accuracy) definition of patterns? drift problems? substrate well mounted? charging problems? resist heating problems? marker positions and pitch OK? use marker CENTRE, not a corner!

Proximity effect (1) complications the developed pattern is wider than the scanned pattern, due to the interactions of primary beam electrons with the resist and substrate → resist outside scanned pattern receives a non-zero dose

Proximity effect (2) complications Forward scattering: due to electron-electron interactions  deflect primary electrons by a small angle, leads to broadening of the beam [~1 … 10nm] depends on: - resist material resist thickness acceleration voltage

Proximity effect (3) complications Backscattering: electrons do not stop in the resist but penetrate in the substrate and still contribute to exposure by scattering back into the resist causing inelastic / exposing processes lateral range [~10 µm] depends on: substrate material acceleration voltage

Proximity effect (4) complications Consequence: dose variations pattern distortions Correction methods: Local Dose test critical dimension Ghost exposure Global Shape correction Dose correction

Proximity effect (5) complications Forward and backscattering can be approximated by a two- gaussian model:

Proximity correction (6) complications Dose correction EBPG offers infinite number of different relative doses in pattern; trial and error process Layout Beamer can handle backscatter dose in more analytical way with input from Monte Carlo calculation by program TRACER

Charging complications drain of electrons needed requires conductive layer otherwise unwanted beam deflections proven solutions: intermediate layer in multilayer resist system (e.g. Ge or Si) thin conductive Ge layer on top of resist and dry (plasma) removal before development metal underlayer like chromium of mask plates SnO2 or InSnO2 (transparent substrates) Elektra92 (conductive polymer) Some recipes available on request

Miscellaneous complications frequency out of range too high  smaller aperture, defocus too low  multiple pass writing (more sensitive to instabilities!) vibrations if writing frequency close to 800Hz from turbo instabilities may occur  multiple pass writing stitching take critical nano-details in centre of a scan field

Layout Beamer from design to EBPG gpf workfile Software 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

Layout Beamer from design to EBPG gpf workfile important commands demonstration