The Faraday Cup is arranged in the process chamber and beam line, corresponding to an ion beam shooting position.
The Dose Control Systems for an ion implantation system of the type in which the wafers to be implanted are mounted around the periphery of a disk which rotates and also moves in a radial direction relative to an ion beam to expose successive sections of each wafer to the radiation. Francis William Aston's first mass spectrographs used photographic film to detect ions passed through the instrument, and photoplates continue to be used for ion detection in spark-source and glow discharge ionization instruments today. An earlier column discussed the operation of electron multipliers used as detectors in mass spectrometry (MS) (1).
In this column, as well as the next installment, we describe two detector systems used in MS — the Faraday cup and position-sensitive array detectors.
Because the detection is based solely on the charge, FC-based detectors exhibit no mass discrimination, which is an advantage in high precision measurements.
The water cooled copper made body includes a molybdenum cone for a high melting point and a low sputtering effect.
For each positive ion that enters the faraday, an electron is drawn from ground through the current meter to neutralize the positive charge of the ion. The control system senses beam current which passes through one or more apertures in the disk and is collected by a Faraday cup. The metal cup (Figure 1 is a photograph and Figure 2 is a schematic) is placed within a vacuum system to intercept a beam of charged particles (electrons or ions). The Electron MultiplierElectron multipliers are probably the most common means of detecting ions, especially when positive and negative ions need to be detected on the same instrument. The Photomultiplier or Scintillation CounterIn the photomultiplier (or scintillation counter) the ions initially strike a dynode which results in electron emission. Before entering the molybdenum cone, the beam is going  through a grounded disk and a biased disk. The Biased disk (power supply not included) as well as a permanent magnet are for the secondary electron suppression. The magnetic field stops outside secondary electrons from entering and secondary electrons produced inside from exiting.


This current is integrated to obtain a measure of charge which is compared with a calculated value based upon the desired ion dosage and other parameters. Farnsworth (2), who also invented analog television, and in his later years, a device that claimed to provide controlled fusion. An electron suppression plate surrounds the ion entrance aperture and keeps secondary electrons emitted from the impacted surface within the confines of the device.
Coupled with modern electronics, the Faraday cup is singularly useful in producing high precision measurements in isotope ratio mass spectrometers.
The charge on each particle (approximately 1.6 ? 10-19 C) is passed to the metal on neutralization of the impacting ion. Errors in the current measurement are reduced with the addition of an electron suppressor plate to the cup, as shown in Figure 1. These electrons then strike a phosphorous screen which in turn releases a burst of photons. The resultant controls the number of incremental steps the rotating disk moves radially to expose the adjacent sections of each wafer. Analog television has been replaced by digital transmission, and controlled fusion remains firmly entrenched in the future.
Array detectors (which can include arrays of miniaturized Faraday cups) are used not only as detectors for dispersive-beam instruments, but also as developmental tools for characterizing the position and cross section of a beam of ions as it traverses an instrument.
The cup is an element in a circuit; the current flow through the circuit can be very accurately measured and is directly proportional to the number of ions that have been intercepted by the Faraday cup. The suppressor plate reduces losses because of backscattering of the incident ions and also reduces the probability of escape for secondary electrons that may be released on ion impact. The photons then pass into the multiplier where amplification occurs in a cascade fashion - much like with the electron multiplier. This process is continued usually with two or more traverses until the entire surface of each wafer has been implanted with the proper ion dosage.


However, the electron multiplier became an extraordinarily useful device and is widely used in mass spectrometers.
A current of 1 nA in the circuit corresponds to the arrival of several billion singly charged ions per second at the Faraday cup. Commercial FC detectors may have a weak magnetic field to prevent secondary electrons from leaving the Faraday cup (4), and they may operate with a slight positive bias on the impacted surface to reduce secondary electron emission. The main advantage of using photons is that the multiplier can be kept sealed in a vacuum preventing contamination and greatly extending the lifetime of the detector. Despite this, the electron multiplier detection process is subject to a mass-discrimination effect (3). As expected, the limit of detection for a Faraday cup depends on the sensitivity of the electrometer in the circuit that it is connected to. Additionally, because the detector produces a signal for both fast-moving ions and neutral particles, it also produces detector "noise" unrelated to the mass-selected ions. The current passes through a circuit resistor, and the generated difference in voltage is measured (V = IR).
Even relatively simple circuits and low cost amplifiers can provide a 10 mV signal for a picoampere of input current. Revisiting equation 1 above reveals that measuring microvolts corresponds to a few thousand ions. The ability to avoid a scanning mass analysis is also advantageous, as we will describe shortly. The noise associated with the electronics necessary for "amplification" of the weak signal (usually involving a high-ohm resistor) is compensated for by a measurement time that can last several hundred seconds.



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