ScanMaster II - Scan Amplifier
Protection & Assurance
The Brookhaven ScanMaster II is a plug compatible solid-state scan amplifier replacement for the vacuum-tube scan amplifier in certain Varian medium-current ion implanters. It develops extremely linear high-voltage triangular waveforms for sweeping the beam, and high-voltage DC for offsetting the beam. The three modules comprising the vacuum-tube amplifier are removed and replaced by the two modules comprising the ScanMaster II.
ScanMaster II Amplifier Module
ScanMaster II High-Voltage Power Supply Module
The X and Y generator cards in the beam scan controller are removed and replaced by Brookhavens X and Y generator cards. The Varian scan generator cards cannot be used with the Brookhaven amplifiers, and the Brookhaven scan generator cards cannot be used with the vacuum-tube amplifiers. The frequencies of the horizontal and vertical sweeps are the same as used in the vacuum-tube scan amplifier, 1019Hz and 117Hz respectively. The maximum voltages, used at 200KeV, are 6.5kVpp for the horizontal sweep, 9kVpp for the vertical sweep, and 8kV for DC offset. The triangular waveforms are generated on Brookhaven Cards in the Beam Scan Controller, go to the Digital Main Control for energy tracking purposes, back to the Beam Scan Controller, to the ScanMaster II, and then to the deflection plates.
A circuit board in the ScanMaster II called the Timing Control PCB receives one horizontal triangular waveform and one vertical triangular waveform from the Beam Scan Controller and manipulates these signals in a number of ways. On this board are the master gain adjustments, controls to match the amplitude of the X+ to the X- and the Y+ to the Y-, buffer amplifiers, preamplifiers, and inverting amplifiers. Also on this board are the circuits that produce signals called interlace and vertical steering. Other functions of this circuit board control the timing of the switching of the high-voltage relays. It is on this board that one triangular waveform becomes the two waveforms needed for the deflection plates.
In the case of the horizontal waveform, one signal is called the X+ drive signal and the other is called the X- drive signal. For the vertical, one is called the Y- drive signal and one is called the Y+ drive signal. These signals go to four amplifier circuit boards. The two horizontal amplifier PCB's are identical to each other. Each board has two amplifiers on it. One amplifier drives the primary of the signal transformer and the other amplifier drives the secondary.
Troubleshooting one bad channel is simplified because the two amplifier boards can be swapped with each other and if the problem follows one board, you know which board is bad. This is the same for the vertical circuitry, but the horizontal and vertical amplifier-PCBs cannot be swapped with each other. Both chassis have cover interlocks that remove power to the ScanMaster II, reducing the chance of hazard to technical personnel. The ScanMaster II has other troubleshooting aids built in. On the front panel are BNC's that provide the monitor signals to the outside world (1000/1). The signals to the plates are actively differentiated and the derivatives are also brought to the front panel.
The velocity of the beam across the wafer must be constant in order to obtain good dose uniformity. The ScanMaster II is a transformer-coupled amplifier and because a transformer has the nonlinear characteristics of an inductor, an innovative approach to develop the linear triangular waveforms required for ion implantation is incorporated. This is accomplished by taking a sample of the transformer drive signal, feed-forward, and a sample of the transformer output signal, high-voltage feedback, mixing them in the proper ratio, amplifying this correction signal, and driving the bottom of the secondary with it. An extremely linear signal for the plates is thereby created in real time.
The ScanMaster II incorporates other innovative circuits, one of which is interlace. Interlace is somewhat similar to what is known as dither in the vacuum-tube amplifiers. Another innovative circuit implemented in the ScanMaster II provides a 90-degree phase shift in the relationship of the horizontal and vertical circuits. The combined effect of the interlace circuit and the phase shift circuit is a major improvement in micro-uniformity. The high-voltage DC for the 7-degree bend is passed through the secondary of the signal transformer.
Since the beam has more than one position, high-voltage relays move the positive and negative supplies into and out of the different plate circuits. These relays are controlled by the timing circuits on the Timing Control PCB mentioned above. In an effort to extend the life of the relays, the current that would otherwise be flowing in their contacts when switched is turned off. Some of this current is from the high-voltage power supply. The output of the high-voltage power supply is controlled so that it goes from 8000V to about 50V in .3 msec.
This is accomplished on circuit boards called active high voltage shunts. There is a shunt for the positive power supply and a shunt for the negative power supply. There are 13 FETS in series that turn on in a kind of wave motion that shunts the power supply down to nearly zero volts when activated. The shunt also acts as a constant current load for the power supply. What is not obvious is that at maximum energy, each FET is dissipating about 3 watts! Therefore each FET has two heat sinks on it and there are two fans mounted on the side panel of the power supply with vents designed to maximize the airflow across the FETS. The time for one pass of the beam horizontally across the wafer is about 500usec. The time for a power supply to go from 8000V to 0V without the aid of a shunt is 50msec. If a FET fails, or if switching voltages are not below a certain threshold, for any reason, when told to switch, circuitry in the ScanMaster II prevents the switch. The relay-inhibit- alarm is then activated and the beam is gated to beam dump. Repeated activation of the relay inhibit alarm when gating the beam may be an indication of a failed shunt, or may be indicative of malfunctions in the implanter such as a de-focused beam remaining on the deflection plates when the relays are told to gate the beam.
There are other alarms that can be activated by supervisory circuits in the ScanMaster II.
Lost scan signals; lost positive or negative DC offset voltage; and lost 0 to 14V DC OFFSET INPUT signal. The ScanMaster II can also sense that the beam is not in the proper place at the beginning of an implant. When any of these faults are detected, an aural alarm is sounded, an LED is illuminated, and the beam is gated to beam dump.
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