Beam Cross Section Distribution Measurement System of High Voltage Power Supply for Medium Current Ion Implanter
Medium current ion implanters occupy a critical position in semiconductor manufacturing, providing the ion doses required for threshold voltage adjustment, well formation, and other essential doping processes. The high voltage power supply that accelerates ions to their implantation energy fundamentally determines the beam characteristics, with the spatial distribution of beam current across the cross section being particularly important for achieving uniform dopant incorporation across wafer surfaces. A comprehensive beam cross section distribution measurement system enables characterization and optimization of the beam profile, ensuring the implant uniformity specifications demanded by advanced semiconductor device fabrication.
The ion beam generated in an implanter originates from an ion source where dopant atoms are ionized, extracted, and formed into a beam. This beam then passes through a mass analysis magnet that selects ions of the desired mass to charge ratio, eliminating unwanted ion species from the beam. The analyzed beam enters an acceleration column where the high voltage power supply imparts the final energy to the ions. Throughout this transport, the beam expands due to space charge repulsion between ions and may be focused or shaped by various electrostatic and magnetic elements. The resulting beam cross section at the target position determines the implantation uniformity across the wafer.
Beam cross section measurement systems employ various techniques to characterize the spatial distribution of beam current. Multiwire beam profile monitors utilize arrays of horizontal and vertical wires that intercept a small fraction of the beam current. By measuring the current on each wire, the system reconstructs the beam profile in both transverse directions. The wire spacing determines the spatial resolution of the measurement, with finer spacing providing more detailed profile information but intercepting more total beam current. The wires are typically made of refractory materials to withstand the beam power loading, and may be moved through the beam to sample different positions.
Scanning wire or scanning slit systems provide an alternative approach where a single wire or narrow slit moves through the beam while the transmitted or intercepted current is recorded as a function of position. This scanning approach can provide higher spatial resolution than fixed multiwire arrays and avoids the need for multiple independent current measurements. The scanning speed must be fast enough to characterize the beam profile before significant changes occur, yet slow enough to achieve adequate signal to noise ratio in the current measurement.
Faraday cup arrays offer absolute current measurement capability by collecting ions in electrically isolated cups that capture all incident charge. Arrays of Faraday cups distributed across the beam cross section directly measure the local current density at each cup position. Faraday cups provide accurate absolute current measurement but intercept the beam completely at the measurement locations, making them more suitable for characterization during setup rather than continuous monitoring during production implants. Secondary electron suppression using biased electrodes in front of the cups ensures that all collected charge is properly attributed to the incident ions.
The high voltage power supply characteristics influence the beam cross section distribution through several mechanisms. Voltage stability affects the ion energy and the focusing characteristics of electrostatic lenses in the beamline. Voltage ripple at frequencies comparable to the transit time of ions through the optical system can cause time varying focusing that blurs the beam profile. The acceleration voltage also determines the space charge effects in the beam, with higher energy beams having reduced space charge expansion due to the greater ion velocities.
Beam current fluctuations from the ion source or extraction power supplies translate directly into variations in the beam profile. Higher beam currents increase space charge forces that expand the beam, while lower currents allow tighter focusing. The relationship between total beam current and beam size depends on the focusing strength of the beamline elements and the degree of space charge compensation from electrons in the beam path. Measurement of the beam profile as a function of total current enables characterization of this relationship for process optimization.
The measurement system must operate in the high voltage environment of the implantation deck, presenting significant engineering challenges. Electronic systems for signal amplification, digitization, and transmission must either operate at high voltage potential or communicate across high voltage isolation barriers. Fiber optic transmission of measurement signals provides excellent electrical isolation while enabling high bandwidth data transfer. Alternatively, the measurement signals may be transformed to ground potential through isolation transformers or optically coupled isolators.
Real time beam profile monitoring enables active control of the beam characteristics during implantation. Feedback systems can adjust focusing elements, beamline steering, or source parameters to maintain the desired beam profile despite drifts or disturbances. The control bandwidth must be appropriate for the timescales of the disturbances being corrected, with slower thermal drifts requiring less bandwidth than faster electrical fluctuations. Integration of the measurement system with the implanter control system enables automated optimization of beam parameters for different implant recipes.
The beam profile data supports process development and troubleshooting beyond real time control. Historical beam profile records enable correlation of implant uniformity with beam characteristics, supporting identification of factors affecting process performance. Comparison of beam profiles for different ion species, energies, or currents provides insight into the behavior of the beamline under various operating conditions. This characterization capability supports optimization of implanter setup procedures and maintenance of consistent process capability over time.
Calibration of the beam profile measurement system ensures accurate and reproducible results. The position calibration relates the measurement channels to physical positions in the beam cross section, typically determined by moving a known feature through the measurement plane and recording the response. The current calibration ensures that the measured currents accurately represent the actual beam current, verified by comparison with absolute Faraday cup measurements or the total beam current measured at the beam stop. Regular calibration verification maintains confidence in the measurement accuracy throughout the system lifetime.
