450 kV Ultra-Stable High-Voltage Technology for Electron-Beam Lithography Systems
Electron-beam lithography at sub-3 nm nodes demands acceleration voltages around 450 kV with stability better than 1 ppm over minute-long exposures and ripple below 500 mV peak-to-peak to prevent chromatic aberration and overlay degradation that would otherwise destroy pattern fidelity. Achieving this performance requires a cascade of specialized high-voltage engineering techniques that address every conceivable source of instability from component aging to acoustic vibration.
The core generator employs a parallel-fed Cockcroft-Walton multiplier driven by a 45 kHz resonant inverter operating from a precisely regulated 60 kV intermediate stage. Stability begins with a primary voltage reference using an oven-controlled buried-zener diode exhibiting less than 0.3 ppm/°C drift, followed by a temperature-compensated capacitive divider that provides feedback with 0.1 ppm resolution after 24-hour warm-up. Active regulation maintains output within ±2 V over hour-long periods despite load currents varying from 100 µA during writing to less than 5 µA during stage moves.
Ripple suppression combines multiple complementary mechanisms: the inherent low ripple of the parallel-fed multiplier topology, passive LC filtering using vacuum-compatible polypropylene capacitors with negligible piezoelectric coefficients, and active cancellation whereby a low-voltage amplifier injects counter-phase ripple derived from high-bandwidth sensing at the column entrance. The residual 500 mV specification is routinely achieved with greater than 6 dB margin.
Thermal management uses a closed-loop oil circulation system with ±0.02 °C stability at the multiplier tank, eliminating thermoelastic deformation of ceramic insulators that previously contributed several volts of drift through piezoelectric effects. All high-voltage components are immersed in pressurized SF6 at 4 bar to raise partial discharge inception well above operating stress while providing excellent heat transfer.
Mechanical isolation employs a two-stage approach: the entire high-voltage tank sits on an active pneumatic isolation platform that attenuates floor vibration above 2 Hz by more than 40 dB, while internal multiplier columns use carbon-fiber support rods with matched thermal expansion to the ceramic rings, preventing stress-induced voltage coefficients. Acoustic noise from cooling pumps is mitigated through anechoic enclosures and flexible bellows connections.
Long-term stability against component aging is maintained through periodic in-situ calibration using a precision resistive divider permanently installed in the tank. Every 500 hours of operation, the system executes an automated self-calibration sequence that adjusts digital compensation coefficients to nullify drift from capacitor dielectric absorption and resistor TCR changes.
Grounding and shielding architecture prevents microphonic effects from external electromagnetic interference. The accelerator column is surrounded by a mu-metal magnetic shield and a superconducting persistent-current loop that cancels residual 50/60 Hz fields to below 10 nT, while all control signals enter the tank through fiber-optic penetrations with greater than 180 dB common-mode rejection.
Cable stabilization addresses the frequently overlooked contribution of triboelectric and piezoelectric charging in high-voltage coaxial cables during stage motion. Special low-noise cable with graphite-impregnated semiconductive layers and stress-relieved construction exhibits less than 200 mV charge generation during 500 mm/s stage slews.
These techniques collectively deliver 450 kV with 0.8 ppm hour-to-hour stability and less than 300 mV ripple in production systems writing 2 nm features, enabling overlay performance below 1.5 nm 3-sigma and critical dimension uniformity that supports high-volume manufacturing of the most advanced logic devices.
