SIMS Instrumentation: Secondary Ion Detectors

The most widely used SIMS instruments have as many as four detectors. These include an ion counting electron multiplier, a Faraday cup, and two ion image detectors. The following figure shows the arrangement of detectors. The ion counting electron multipliers are the most sensitive detectors. They must be protected from intense ion beams. The Faraday cup detector moves on a solenoid to cover the electron multiplier when the incoming ion signal is too high. High energy neutral species form by charge exchange when an ion beam strikes a surface. These neutrals contribute noise to the ion signal. If an electrostatic sector precedes the electron multiplier, the neutrals can be eliminated from the ion signal. Quadrupole mass analyzers also use electrostatic sectors or deflectors to minimize the contributions of high energy neutral species to the ion signal. The ion beam passes through a small hole in the electrostatic sector when the sector is deactivated. This path leads to dual microchannel plate and resistive anode encoder image detectors. The projector lenses bring an image of the sample into focus on the image detectors.

Electromagnetically active components are shown in blue. The ion beam trajectories (indicated in red) are greatly exaggerated in the lateral directions. In particular, the image detectors are smaller and the path through the electrostatic analyzer is narrower. The ions pass through a much smaller hole in the sector.

Electron Multipliers

An electron multiplier consists of a series of electrodes called dynodes, each connected along a resistor string. The signal output end of the resistor string attaches to positive high voltage. The other end of the string goes to the electron multiplier case and ground.

The dynode potentials differ in equal steps along the chain. When a particle (electron, ion, high energy neutral, or high energy photon) strikes the first dynode it produces secondary electrons. The secondary electrons are accelerated into the next dynode where each electron produces more secondary electrons. A cascade of secondary electrons ensues. The dynode acceleration potential controls the electron gain.

Electron multipliers can also be made from continuous dynode materials rather than discrete dynodes. This glassy material contains lead that provides conductivity comparable to the resistor chain in the discrete dynode electron multipliers.

Most SIMS measurements use electron multipliers operating with sufficiently high gain to produce a detectable pulse for every ion arrival. Pulse counting is the most sensitive ion detection method. Detector noise arises from stray ions and cosmic rays, but these signals are normally less than one count per second.

In order to detect both positive and negative ions, the electron multiplier case stands at ground potential. The output end of the resistor chain must operate at high positive potential. This requires that the output pulse be capacitively coupled to the detector electronics. The detector electronics require a recovery time (dead time) after an ion arrival before a second ion can be detected. The detector dead time limits the measureable ion arrival rate to around 1e6 counts per second. Thus the electron multiplier dynamic range extends from below 1 to about 1e6 ion counts per second.

Pulse counting detectors follow Poisson statistics which require that each ion arrives independently of all other ions. A measurement consists of counting ions for a fixed period of time and the result takes the form of a counting number, n. The standard deviation of the measurement is equal to the square root of the number of counts. The second equation shows the standard deviation relative to the signal.

Faraday Cups

A Faraday cup is just an electrode from which electrical current is measured while a charge particle beam (electrons or ions) impinges on it. The shape helps minimize loss of secondary electrons that would alter the current measurement. A deep cup with an electron repeller plate minimizes secondary electron loss.

Ion Image Detectors

Ion image detectors depend on microchannel plate electron multiplier arrays. These plates consist of large arrays of small channel electron multipliers. SIMS instruments typically use round arrays with about 2000 channels across a diameter. Each channel is 10 microns in diameter. Channels are located on 12 micron centers and the total array is 25 mm in diameter.

Each channel has dimensions of 10 x 400 micron. The channels are 7 degrees from perpendicular to the array surface. The voltage across single channel plate can produce gains has high as 1e5.

For SIMS use, two microchannel plates combine to easily produce gains of 1e6.

Two kinds of anode provide either direct visualization, or computer compatible ion position data. Two microchannel plates followed by a phosphor screen for visuallizing the electron cascade provides an easy way to monitor the secondary ion beam. SIMS analysts call this combination a dual microchannel plate (DMCP). As the electrons are accelerated into the phosphor anode, they generate more than one photon per electron. Thus, the anode provides additional gain.

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