Energy Stability and Beam Transmission Efficiency of High Voltage Power Supply for Medium Current Ion Implanter
Ion implantation precisely introduces dopant atoms into semiconductor wafers to create the electrical regions that form transistors and other devices. Medium current implanters deliver beam currents in the microampere to milliampere range, suitable for many doping applications. The high voltage power supply that accelerates the ions determines the implantation energy, which affects the dopant depth distribution. Energy stability and beam transmission efficiency are critical parameters that determine the implantation accuracy and the system productivity.
The ion implantation process begins with generating ions of the desired dopant species in an ion source. The ions are extracted from the source, accelerated through a potential difference, mass analyzed to select the desired species, and directed to the wafer where they implant into the crystal lattice. The implantation depth depends on the ion energy, with higher energies producing deeper implants. The dopant dose, the number of ions implanted per unit area, depends on the beam current and the implantation time.
Energy stability refers to the constancy of the ion energy during implantation. Variations in the acceleration voltage cause variations in the ion energy, which affect the depth distribution of the implanted dopants. For semiconductor devices where junction depths must be controlled to nanometer precision, energy variations can cause significant device parameter variations. The energy stability requirement depends on the implantation energy and the depth precision required, with typical specifications requiring voltage stability better than 0.1 percent.
Sources of energy instability include power supply ripple, voltage drift, and transient variations during beam setup or changes in operating conditions. Ripple at the power line frequency or switching frequency modulates the ion energy, causing energy spread in the beam. Long term drift causes the energy to change over the course of an implant, affecting the depth profile. Transients during current changes or load variations can cause temporary energy excursions.
The high voltage power supply design for energy stability includes precision voltage regulation, low ripple design, and fast response to load changes. The voltage reference determines the long term accuracy and should have low temperature coefficient and aging rate. The feedback control loop maintains the output voltage at the set value, with the loop gain determining the rejection of disturbances. Filtering reduces ripple at the output, with the filter design trading off ripple reduction against response speed.
Beam transmission efficiency measures the fraction of generated ions that successfully reach the wafer. Losses occur at multiple points in the beam path including extraction from the source, transport through the accelerator tube, mass analysis, and scanning to the wafer. The transmission efficiency affects the implantation time for a given dose, directly impacting the system throughput. Low transmission wastes ion source capacity and may cause unwanted implantation of lost ions on apertures and other surfaces.
The acceleration voltage affects the beam transmission through several mechanisms. Higher voltages produce higher ion velocities, reducing the space charge effects that can cause beam blowup. However, high voltage can also cause lens effects in the accelerator tube that affect the beam focusing. The voltage level affects the magnetic field required for mass analysis, as the magnetic rigidity of the ions depends on their momentum. Optimization of the voltage for transmission must consider these interacting effects.
Space charge neutralization affects the beam transmission, particularly at higher currents. The positive charge of the ion beam creates electric fields that can defocus the beam, causing losses. Neutralization by electrons, either from the residual gas or from dedicated neutralizers, cancels the space charge. The neutralization effectiveness depends on the gas pressure and the beam parameters. The high voltage affects the neutralization by determining the ion velocity and the time available for electron capture.
Beam scanning spreads the ion beam over the wafer surface to achieve uniform implantation. Electrostatic scanning uses varying voltages on deflection plates to sweep the beam. The scanning waveform and frequency affect the implantation uniformity and the beam transmission. The scanning voltages must be coordinated with the acceleration voltage to ensure the beam reaches all parts of the wafer without striking the scanning plates or other apertures.
Measurement of energy stability uses techniques including beam energy analysis with electrostatic analyzers, depth profiling of implanted test samples, and direct measurement of the acceleration voltage. Beam energy analyzers measure the energy spread of the beam, indicating the ripple and noise contributions. Depth profiling by secondary ion mass spectrometry or similar techniques reveals the actual implantation profile, from which energy variations can be inferred. Voltage measurement with calibrated dividers and precision voltmeters directly characterizes the power supply stability.
