Objective With the ongoing advancement of optical systems, there has been a growing demand in recent years for precision optical components across various cutting-edge research fields, including EUV lithography lenses, synchrotron radiation X-ray mirrors, and strapdown inertial navigation laser gyro resonators. Ion Beam Polishing (IBP) technology is characterized by its ability to remove complex shapes with excellent stability, absence of edge effects, non-contact non-destructive processing, and high precision. It is commonly employed as the final finishing process for high-precision optical components. While there exist various optimization schemes for the current ion beam shaping machining paths and their velocity distributions, there are still instances where the machine tool's dynamic performance cannot meet the requirements of the optimized machining schemes when processing components with large gradient errors. We introduce a novel Pulsed Ion Beam (PIB) machining technique to overcome the limitations associated with current ion beams in the processing of high-precision optical components. This method not only offers ultra-high removal resolution but also significantly reduces the demands on machine tool dynamics, prevents the formation of extra removal layers, and adeptly achieves precise dwell times at each machining point on the component.
Methods This article proposes a new PIB processing method, which adjusts the frequency of the pulse power supply to adjust the period of PIB, and controls the duty cycle to control the duration of the pulse beam current in a single period. It can achieve accurate and controllable material removal in the area that does not require processing by turning off the ion beam current in the non-processing area (Fig.1). Intelligent planning of machining paths using ant colony algorithm (Fig.9). Using ZYGO interferometer to measure the final processing results.
Results and Discussions The stability and linearity of PIB have been confirmed (Fig.2), with its removal resolution demonstrated to achieve material removal of 0.33 nm using just 5 pulses. The machining capabilities of traditional IBF and PIB in addressing gradient errors were compared through simulations. The results indicated that when the wavefront gradient of the surface shape error exceeds 0.5 λ/cm, the PIB offers a pronounced advantage in shaping (Fig.6). The implementation of the ant colony algorithm cut ineffective processing paths by 57% (Fig.9). Ultimately, the new processing strategy enabled the acquisition of surfaces with sub-nanometer precision. Following three stages of processing, the RMS error was reduced from 343.438 nm to 0.552 nm (Fig.15).
Conclusions This study introduces a new generation of ion beam processing techniques. Compared to traditional IBF methods, the PIB offers superior material removal resolution. By comparing the amounts of material removed with the same sputtering time but varying duty cycles, the PIB system's outstanding stability and linearity in material removal were confirmed. Additionally, five pulses were applied at a frequency of 1 Hz and a 10% duty cycle to sputter hafnium oxide thin films. The comparison of film thicknesses before and after processing confirmed that PIB achieves a sub-nanometer removal resolution of 0.066 nanometers per pulse. Simultaneously, the ACO algorithm was employed to optimize and plan the PIB machining paths, reducing ineffective paths by 57.7%. Ultimately, this processing strategy was used to fabricate an actual monocrystalline silicon mirror, achieving a sub-nanometer precision optical surface of 0.552 nm. This verifies the superior performance of the PIB processing strategy and system in achieving high-precision optical surfaces. It represents a more flexible, accurate, and efficient ion beam processing technique.
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