High-Voltage Feedback for Excimer Laser Beam Stability
Excimer lasers, operating in the deep ultraviolet spectrum, are critical tools for semiconductor lithography, material processing, and medical applications. Their output beam stability, encompassing both energy per pulse and spatial pointing, is paramount for process reproducibility. The electrical excitation mechanism of these lasers, a fast high-voltage pulsed discharge across a halogen gas mixture, is intrinsically linked to beam performance. Implementing active high-voltage feedback control transforms the power supply from a simple trigger into the core actuator for stability enhancement.
The discharge in an excimer laser is a complex plasma event. The high-voltage pulse, typically in the range of 15 to 40 kilovolts with rise times in the tens of nanoseconds, must be applied with precise timing and shape to ensure uniform gas breakdown and efficient energy transfer. Variations in gas composition, temperature, and electrode condition from shot to shot cause fluctuations in impedance, leading directly to changes in output pulse energy. A closed-loop feedback system addresses this by treating the high-voltage pulse itself as a controllable variable. A photodiode or beam sampler captures a small fraction of the laser output immediately after each pulse. This energy reading is digitized and compared to a setpoint value by a fast controller.
The corrective action is applied to the subsequent laser pulse. The most common method is to modulate the charging voltage of the primary storage capacitor in the pulsed power module (PPM). The controller adjusts the target voltage for the capacitor charger, a high-voltage DC power supply, based on the error from the previous pulse. This requires a charger with excellent linearity and a bandwidth sufficient to settle to a new voltage within the inter-pulse period, which can be as short as a few milliseconds for high-repetition-rate lasers. The relationship between charging voltage and output energy is not perfectly linear and can exhibit hysteresis, necessitating adaptive algorithms in the controller that learn and compensate for the laser's dynamic response.
Spatial beam pointing stability is also influenced by discharge symmetry. Asymmetric erosion of the electrodes or localized gas density variations can cause the plasma channel to shift slightly, deflecting the optical beam. Advanced systems employ spatial beam profilers that analyze the far-field or near-field intensity distribution. If a drift in centroid position is detected, the feedback system can make fine adjustments to the trigger timing or voltage distribution between multiple pre-ionization or main discharge circuits to recenter the plasma. This may involve controlling multiple independent high-voltage switches or pulse-forming networks with sub-nanosecond timing resolution.
The high-voltage power supplies and switches in this feedback loop operate under extreme stress. The rapid, high-current switching subjects components to significant di/dt and dv/dt stresses, leading to electromagnetic interference that can corrupt sensitive feedback signals. Comprehensive shielding, star-point grounding, and the use of fiber-optic data links for control signals are essential. Furthermore, the feedback system must be intelligent enough to distinguish between normal stochastic noise and systematic drift, avoiding over-correction that could induce instability. It often includes filtering algorithms and validation checks against internal pressure and temperature sensors to provide context for the optical measurements.
Integrating this high-voltage feedback extends beyond energy stabilization. It enables advanced modes of operation, such as energy ramping or complex pulse trains where each pulse has a predetermined, different energy. The power supply system, under digital command, becomes a programmable energy source. This is crucial for applications like microlithography, where dose control across a scanning slit must be extremely uniform, or for medical procedures requiring precise ablation depth control. The synergy between ultrafast optical sensing, real-time digital processing, and high-speed high-voltage actuation is what transforms a naturally jittery gas discharge into a precision optical tool, pushing the limits of feature resolution and process yield.
