Electron Beam Lithography (EBL) and Raith E-LINE

EBL

Optical lithography resolution is fundamentally bounded by the wavelength of light. Even with deep-UV excimer sources and immersion techniques, the minimum resolvable feature size is:

\[l_m = k_1 \frac{\lambda}{\mathrm{NA}}\]

where \(\lambda\) is the exposure wavelength, \(\mathrm{NA} = \bar{n}\sin\theta\) is the numerical aperture, and \(k_1\) encodes process-dependent factors. Pushing below ~70 nm with optical methods requires extreme engineering trade-offs. Electron beam lithography (EBL) sidesteps this limit entirely: the de Broglie wavelength of an electron accelerated to \(V\) volts is:

\[\lambda_e = \frac{h}{\sqrt{2m_e eV}} \approx \frac{1.23\ \mathrm{nm}}{\sqrt{V\ [\mathrm{V}]}}\]

At 30 keV, \(\lambda_e \approx 7\ \mathrm{pm}\), five orders of magnitude smaller than UV light. In practice EBL resolution is not diffraction-limited but scattering-limited, yet can reach 4–10 nm, far beyond what any optical system can achieve.

Working Principle

A heated filament (tungsten or lanthanum hexaboride \(\mathrm{LaB_6}\)) emits electrons by thermionic emission. These are accelerated to energies between 200 eV and 100 keV and focused into a narrow beam by a series of magnetic lenses. Electrostatic or electromagnetic deflection coils steer the beam across the sample; a beam blanker (the electron analogue of an optical shutter) switches exposure on and off. A pattern generator converts the CAD file and dose map into real-time deflection commands.

For writing large areas the stage moves mechanically, while fine positioning within each write field is handled by the beam deflection system. High-end systems use an interferometric stage, a laser beam reflected off a mirror on the stage, to track position with nanometre precision, far beyond the micron-level resolution of stepper motors.

Because electrons scatter strongly off gas molecules, the entire column operates under high vacuum (\(\sim 10^{-6}\ \mathrm{mbar}\) in the write chamber; \(\sim 10^{-9}\ \mathrm{mbar}\) in the electron gun column to prevent arc discharge and filament damage).

Electron–matter interaction

When the primary beam hits the resist and substrate, electrons lose energy through two types of scattering:

  • Inelastic scattering: energy transfer to the material, generating secondary electrons (energy \(< 50\ \mathrm{eV}\)) within the first few tens of nanometres below the surface. These low-energy electrons are the primary agents of resist chemistry.
  • Elastic scattering: momentum transfer with atomic nuclei, changing the electron trajectory. Some electrons are redirected backward and escape as backscattered electrons (energy up to several keV). When these re-enter the resist from below, they expose it at a distance from the original beam spot — the root cause of the proximity effect.

The combined interaction volume is often visualised as a pear shape: a narrow column near the entry point that broadens into a bulb deeper in the substrate. The shape depends on accelerating voltage and substrate density (Monte Carlo simulations are the standard tool for predicting it).

Counterintuitively, the primary beam at 30–100 keV does not directly expose the resist. Its energy is far too high to drive the polymer chemistry. Exposure is caused by the backscattered and secondary electrons, whose energies (eV–keV range) match the bond energies of the resist.

Increasing the accelerating voltage sends the interaction volume deeper and narrows it laterally, reducing the proximity effect and improving resolution. However, at very high voltages the substrate must be sufficiently conductive to avoid charge build-up, which would deflect the beam.

Resists

As in optical lithography, EBL resists are radiation-sensitive polymer films applied by spin coating and baked to remove solvent before exposure:

  • Positive resists: electron exposure cleaves the polymer chains into shorter fragments that become soluble in the developer. PMMA can reach ~11 nm resolution while specialised resists with smaller monomer units (e.g. LOR) can reach ~4 nm.

  • Negative resists: monomers are cross-linked by electron exposure, becoming insoluble. The unexposed regions are removed during development. Negative resists generally offer lower resolution (~100 nm) because the cross-linking reaction is spatially less confined than chain scission.

In both cases, resist thickness trades off against resolution: a thin resist produces sharper, better-defined features but limits subsequent processing (for example, a resist thinner than the metal to be evaporated will prevent a clean lift-off).

Undercut and angle-resolved evaporation

In EBL the backscattered electrons, arriving from below, expose the resist more at the bottom than at the top. This produces a undercut profile, i.e. a wider base than opening, which is actually advantageous for lift-off: the discontinuity between the film on the resist and the film on the substrate ensures clean separation when the resist is dissolved. By combining two polymer layers of different sensitivity, a pronounced suspended mask with a large undercut cavity can be engineered. Combined with directional deposition (e-beam evaporation) at multiple angles, this allows the formation of multi-material junctions without ever breaking vacuum.

Raith E-LINE EBL — Step-by-Step Protocol

The Raith E-LINE PLUS is an EBL system built around a Gemini technology-based ultra-high-resolution electron optical column. It can be used both for lithography and SEM imaging. It presents a Gemini column and multiple detectors: InLens detector (backscattered electrons, sensitive to surface chemistry) and Secondary detector (secondary electrons, sensitive to topography). A load lock allows sample exchange without venting the main chamber.

The system runs two software modules that must be started in order: Raith EO (electron optics control, turn on first and never turn off) and ELine PLUS (lithography and pattern software, turn on second, turn off first).

The column operates at \(P_\mathrm{col} \sim 10^{-9}\ \mathrm{mbar}\) and the chamber at \(P_\mathrm{cha} \sim 10^{-5}\ \mathrm{mbar}\).

Sample loading

  • Place the sample in a clamp on the sample holder,
  • press the 9th icon on the righ of the ELine PLUS interface (Load/Unload sample) and then "Sample exchange via load lock" → "Load sample", a procedure of loading will start, follow the step-by-step instructions carefully,
  • once loaded the sample, set "Gun" → "EHT on" → 5 kV to do SEM imaging, → 20 kV to do EBL,
  • set "Aperture" → 30 µm.

SEM imaging and column setup

  • Use the joystick to move across the sample,
  • adjust "Brightness" and "Contrast" (black vs. white dynamic range), then "Focus" (optimise until sharp, go slightly past the optimum, then come back to top), – to adjust the focus
  • adjust "Stigmation" (corrects beam astigmatism) if moving the focus in and out the,
  • adjust "Aperture alignment" if, using "Focus wobble" (Ctrl + Tab) the image oscillates up and down or right and left when not in focus,
  • gor fine movements use Ctrl + Tab which reduces noise by narrowing the scan area and increasing integration time.
  • to save images use Ctrl + E, add annotations with "Annotation" if sample metadata is needed.

Lithography — I. Current measurement

  • In "Sample Holder Map" navigate to the Faraday cup position with Ctrl + dx on the map,
  • press the 4th icon on the righ of the ELine PLUS interface (XY→UV), then "Positions" → "Faraday cup",
  • in "Stage Control" set WD = 10 mm (Absolute mode) and click "Adjust".
  • bring the beam into focus, zoom into the Faraday cup hole and select "Specimen current monitor" from the hidden menu, then check "Spot",
  • press the 5th icon on the righ of the ELine PLUS interface (Beam current), then "Measure", the beam current should be of hundreads of pA.

Move with the joystick along the borders of the sample to avoid inadvertently exposing the edges.

Lithography — II. Coordinate change (stage → CAD): 3-point alignment

  • Navigate to the sample on the map with Ctrl + dx.
  • locate the scratches or alignment markers at the sample corners and zoom in to fine features,
  • find a fine feature in the the bottom-left corner of the sample and focus well,
  • press the 4th icon on the righ of the ELine PLUS interface (XY→UV), then "Origin correction", in U and X write the coordinates you read at the bottom-right of the ELine PLUS interface, save them pressing the pipetta and then "Adjust". In this way this point will become (U=0, V=0),
  • (this point can be skipped) press "Angle correction", write in the first set of U and X the coordinates just written in "Origin correction" → press pipetta → press "Adjust". Find a fine feature in the the bottom-right corner of the sample, focus well, write in the second set of U and X the coordinates you read at the bottom-right of the ELine PLUS interface, save them pressing the pipetta and then "Adjust",
  • press "3-points alignment", write in the first set of U and X the coordinates just written in "Origin correction" → press pipetta → check bandierina → press "Adjust". Find a fine feature in the the bottom-right corner of the sample, focus well, write in the second set of U and X the coordinates you read at the bottom-right of the ELine PLUS interface, save them pressing pipetta, checking bandierina and then "Adjust". Find a fine feature in the the upper-right corner of the sample, focus well, write in the third set of U and X the coordinates you read at the bottom-right of the ELine PLUS interface, save them pressing pipetta, checking bandierina and then "Adjust". (It should appear the word "Focus!" appears in the toolbar. If it doen't, press dx → "Enable focus correction" no capito)
  • finally, go with the fulimino, one a the time, on the three point chosen to check if the focus is well done. If it is not, adjust the focus without moving the joystick → press "Adjust".

Lithography — III. Coordinate change (CAD → beam): writefield alignment

The writefield is the area within which the beam exposes without the stage moving.

  • Press the 3th icon on the righ of the ELine PLUS interface (Monitor), press "Images", set WF = 100µm, 100×100 µm → press "Execute". Take an image with a recognisable feature at the centre,
  • press "Widefield alignment procedure" → Manual → WF = 100µm, feature size 25µm. Bring the feature to the center with Ctrl + sx.
  • repeat and record a 10×10 µm image, then align with a 5 µm window.

Lithography — IV. Exposure

  • Press the 5th icon on the righ of the ELine PLUS interface and set the dose,
  • set the step size and press the calculator icon adjacent to dwell time,
  • this modifies the parameters, verify that the writing speed is \(\leq 10\ \mathrm{mm/s}\),
  • go to "File" → "New positionlist",
  • press the first icon on the righ of the ELine PLUS interface, import the CAD file as .gds dragging the file in "Positionlist",
  • press "Times" to shows the estimated exposure time,
  • move the position of the layer to where exposure is wanted, the starting coordinates refer to the bottom-left writefield,
  • press "Start" to begin exposure.

After exposure

  • Press the 9th icon on the righ of the ELine PLUS interface (Load/Unload sample) and then "Sample exchange via load lock" → "Unload sample", a procedure of unloading will start, follow the step-by-step instructions carefully,
  • proceed with development.