MagScan Technology:

Electrostatic scanning of ion beams has been used since the beginning of commercial ion implant technology. It is easy to deflect an ion beam by placing a pair of deflector plates on opposite sides of the beam and applying an electric field to them. This approach, however, is limited to beams of low beam current due to space charge. The mass of the neutralizing electrons is so much less than that of the ions in the beam, an electric field strong enough to deflect the ion beam sweeps all the neutralizing electrons out of the beam immediately. A low current ion beam blows up a little but a high current beam blows up so much that this beam deflection technology simply can not be used.

Ions can also be deflected by a magnetic field. The advantage is that neutralizing electrons are trapped by the magnetic field rather than swept away. The reason magnetic beam scanning has generally not been used is the difficulty of creating a rapidly changing magnetic field. The self inductance and hysteresis of electromagnets makes the design of a high performance magnetic scanning system difficult.

The Ibis-1000 was the first implanter to incorporate a magnetic beam scanning system. The Ibis-owned design was developed in the early 1990's. Two developments were critical to the success of this approach. The first is the design of the scanner itself. The coils are designed to maximize field uniformity over a maximum possible width, and to minimize eddy currents. Self inductance is minimized by using as few turns in the coils as possible. This, of course, requires large currents which, in turn, requires flowing significant amounts of water through the coils. Typical required field strengths are 1 to 2 kG, and typical peak currents are 350 to 600A.

The second development is driven by the need to provide such currents with carefully tailored waveforms. Typical scanning frequencies are 100 to 200 Hz, and for best uniformity and beam utilization it is important to allow high frequency features on the scan waveform. The Ibis 1000, for example, uses Copley switching power amplifiers. The desired scan waveform is generated by a computer algorithm, and the actual field is measured by a single coil encircling the totality of the field. Thus the voltage in the coil is proportional to the time rate of change of the magnetic field (hence the name B-dot coil). This signal is then used in a servo loop to feed back the actual field value and correct the drive current so that the actual field matches the desired scan waveform.

The benefit of this approach is that a very high current ion beam (50 to 100mA) can be accurately controlled without compromising space charge neutralization. The approach is used on Ibis ion implanters for producing SIMOX-SOI wafers. The accuracy of the scan algorithm directly controls the thickness of the silicon overlayer, probably the most critical parameter for an SOI wafer.