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How to regulate partial gas pressure

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How to regulate partial gas pressure

Some processes, such as reactive sputter processes, require the most constant possible incidence rates for the reacting gas molecules on the substrate being coated.

The “incidence rate” is the same as the “impingement rate” discussed in the page on Outgassing; it is directly proportional to the partial pressure. The simplest attempt to keep the partial pressure for a gas component constant is throughput by regulating with a flow controller; it does have the disadvantage that the regulator cannot determine whether, when and where the gas consumption or the composition of the gas in the vacuum chamber changes. The far superior and more effective option is partial pressure control using a mass spectrometer via gas inlet valves. Here the significant peaks of the gases being considered are assigned to channels in the mass spectrometer. Suitable regulators compare the analog output signals for these channels with set-point values and derive from the difference between the target and actual values for each channel the appropriate actuation signal for the gas inlet valve for the channel. A configuration of this kind has been realized to control six channels in the QUADREX PPC. Gas inlet valves matching the unit can also be delivered.

The gas used to measure the impingement rate (partial pressure) must naturally be drawn from a representative point in the vacuum chamber. When evaluating the time constant for a regulation circuit of this type it is important to take into account all the time aspects and not just the electrical signal propagation and the processing in the mass spectrometer, but also the vacuum-technology time constants and flow velocities, as illustrated in Figure 4.17. Pressure converters or unfavorably installed gas inlet lines joining the control valve and the vacuum vessel will make particularly large contributions to the overall time constant. It is generally better to establish a favorable S/N ratio with a large signal (i.e. through an inlet diaphragm with a large opening) rather than with long integration periods at the individual channels. Contrasted in Figure 4.18 are the effects of boosting pressure and lengthening the integration time on signal detectability. In depictions a, b and c only the integration period was raised, from 0.1 to 1.0 and 10 seconds (thus by an overall factor of 100), respectively. By comparison, in the sequence a-d-e-f, at constant integration time, the total pressure was raised in three steps, from 7.2 · 10^-6 mbar to 7.2 · 10^-5 mbar (or by a factor of just 10 overall).

Fig 4.17 Partial shares for overall time constraints

Fig 4.18 Improving the signal-to-noise ratio by increasing the pressure or extending the integration time

Maintenance

Cathode service life

The service life of the cathode will depend greatly on the nature of the loading. Experience has shown that the product of operating period multiplied by the operating pressure can serve as a measure for the loading. Higher operating pressures (in a range of 1 · 10^-4 to 1 · 10^-3mbar) have a particularly deleterious effect on service life, as do certain chemical influences such as refrigerants, for example. Changing out the cathode is quite easy, thanks to the simple design of the sensor. It is advisable, however, to take this opportunity to change out or at least clean the entire ion source.

Sensor balancing

Sensor balancing at the mass axis (often erroneously referred to as calibration) is done today in a very easy fashion with the software (e.g. SQX, Transpector-Ware) and can be observed directly in the screen. Naturally, not only the arrangement along the mass axis will be determined here, but also the shape of the lines, i.e. resolution and sensitivity (see the page on Specifications in mass spectrometry).

Cleaning the ion source and rod system

It will be necessary to clean the sensor only in exceptional cases where it is heavily soiled. It is usually entirely sufficient to clean the ion source, which can be easily dismantled and cleaned. The rod system can be cleaned in an ultrasonic bath once it has been removed from the configuration. If dismantling the system is unavoidable due to particularly stubborn grime, then the adjustment of the rods which will be required afterwards will have to be carried out at the factory.

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