Quadrupole mass spectrometer

As a diagnostic technique, quadrupole mass spectrometry is a powerful and versatile tool and is capable of performing in situ time-resolved measurements of the densities and energy distributions of positive and negative ions as well as neutral species with microsec- ond resolution. Being mass-selective, this technique is particularly useful in understanding the plasma chemistry in complex environments containing many different species, such as those found in reactive magnetron sputtering. For a detailed treatment of quadrupole mass spectrometry as applied to reactive plasmas, the reader is referred to [138] and references therein.

4.2.2.1 Principles of operation

The quadrupole mass spectrometer (QMS) used in the work presented in this thesis was the EQP300 manufactured by Hiden Analytical Limited (Warrington, UK). A schematic representation of the EQP300 probe and its main components is shown in figure 4.10. The instrument consists of several main parts which include the sampling office, ion focusing optics, an energy analyser, a quadrupole and an ion-counting secondary electron multiplier (SEM). For neutral gas analysis, an electron emitting filament located behind the orifice is used to ionize sampled residual gas. Regardless of the ion source, the sampled ions are then focused through the instrument via a system electromagnetic lenses. The ions are passed through the 45◦ electrostatic energy analyser, which selects ions with respect to their energy and passes them through to the quadrupole mass analyser. The quadrupole separates ions based on their mass-to-charge ratio (m/z) such that only ions of a particular m/z value reach the detector. As the measurement relies heavily on the ion trajectory being unobstructed, the QMS and detector must be housed inside a vacuum chamber and held at low pressure. The operating pressure of the SEM detector is ≤ 5 × 10−4 _{Pa and therefore the QMS requires differential}

pumping separate from the main vacuum chamber. Here a combination of a rotary pump (Edwards E2M2) and turbomolecular pump (Pfeiffer Balzers TMU-064) was used to ob- tain the required operating pressure, achieving an ultimate vacuum pressure of∼10−5Pa.

4.2.2.2 EQP components

A schematic diagram showing the major components of the EQP300 is given in figure

4.10 and brief overview of the function of each is provided below.

Sampling orifice: A small sampling orifice located at the front of the probe is necessary to maintain a sufficiently low operating pressure inside the EQP whilst allowing for a pressure in the range of 10−1 Pa to be maintained in the processing chamber. In the

4. EXPERIMENTAL APPARATUS

Figure 4.10. Diagram of the EQP300 with the major components and ion optics la- belled.

experiments presented here, a orifice diameter of 100 µm was used, which allowed for high signal readings without compromising the integrity of the EQP vacuum chamber. The use of a sampling orifice also implies that any ion sampling occurs as a line-of-sight measurement of ion flux, rather than density.

Extractor: Located directly behind the sampling orifice is the extractor electrode, to which a negative potential is applied to attract positive ion species. The polarity of Vext

(and other ion optics) is reversed for negative ion sampling.

Lens1: With the extractor electrode, lens1 forms an electrostatic lens to focus the sampled ions through the ionisation cage containing an electron-emitting filament and into the axis drift tube. Hamers et al. [139] demonstrated that the focal length of the electrostatic lens is dependent upon ion energy and if inappropriate potentials, Vext and

Vlens1, are applied to the ion optics chromatic aberration of the ion beam occurs.

Filament: The electron-emitting filament is not used for ion sampling modes, however, it is energised for residual gas analysis (RGA) mode. Electrons are generated via thermionic emission from the filament, typically made from tungsten or thoriated iridium for oxygen gas mixtures. Ions are subsequently formed via electron impact ionisation and channelled through the EQP by the electrostatic lenses.

4. EXPERIMENTAL APPARATUS

Figure 4.11. The front-end of the EQP300, including ion sampling components, drift tube and energy analyser. The power supply references are given, where open circles represent a voltage source. Electrodes supplying the filament and cage used in RGA mode are not shown.

that determines the energy of the ions entering the energy analyser. The axis potential is set by potentials separately applied to two electrodes: Vaxis+Ven (as shown in figure4.11)

where Vaxis =−40 V and is kept constant and Ven can be varied (note that the polarity

is reversed for negative ion sampling). The quad, vertical and horizontal electrodes form a quadrupole lens which usually does not require alteration, but can be tuned to control beam alignment and correct for any beam astigmatism. Lens2 works to transform the divergent beam in the drift tube to a parallel beam at the entrance of the energy analyser.

Energy analyser: Ions enter the energy analyser with an energy determined by the initial kinetic energy of the ion, Ei, and the energy gained in the drift tube, giving a total

ion kinetic energy:

Ek =Ei−q(Vaxis+Ven) (4.13)

for ions with a charge of q. The transmission energy of the energy analyser, EEA, is

determined by its geometry and the potential applied to its plates, Vpl. For the Hiden

EQP energy analyser, EEA = 5.5qVpl and is independent of the ion mass. For successful

transmission, the kinetic energy of the ion at the entrance of the energy analyser must equal EEA, hence

Ei−q(Vaxis+Ven) = 5.5qVpl

Ei = q(5.5Vpl+Vaxis+Ven).

4. EXPERIMENTAL APPARATUS

For 5.5Vpl = −Vaxis, it is straightforward to determine the energy of the incident ion

since successful transmission occurs for Ei = qVen. As Vaxis = −40 V is kept constant,

Vpl is kept constant at a potential of 7.27 V. For singly-charged ions, Ei = Ven and the

recorded energy distribution does not require correction (assuming chromatic aberrations are suppressed). For energy distributions of multiply-charged ions, the measured value of Ven must be multiplied by the corresponding ion charge.

Focus: The focus lens decelerates the ions exiting the energy analyser and focuses the beam into the quadrupole mass filter.

Quadrupole: As the energy analyser is responsible for separation of ions with respect to their kinetic energies, the quadrupole mass filter is responsible for ion separated with respect to their charge-to-mass ratios (m/z). A schematic of a quadrupole mass filter is shown in figure 4.12. A quadrupole consists of four parallel rods driven by RF and DC, with opposite rods held at the same potential. The time-dependent electric field between the rods results in ions only of a selected m/z to have stable trajectories, with other ions being lost to the rods or chamber walls.

Figure 4.12. Cross-section (a) and schematic diagram (b) of a cylindrical quadrupole mass filter, after [138]. Ions travel in thezdirection and are deflected in thex andy directions by the electric field generated via the application of the same RF and DC bias to opposite rods.

The mass filter system has a number of electrodes associated with it. V∆m de-

termines the mass resolution at low mass, Vres is used for high mass and Vtrans is a

virtual potential which determines the transit energy of ions through the quadrupole. Vtransis set to 3 V and it is recommended by the manufacturer that this remains constant.

Detector: The EQP is fitted with an off-axis continuous dynode electron multiplier, or secondary electron multiplier (SEM), operating in pulsed mode. To accelerate the ions exiting the mass filter, a potential is applied to the 1st dynode. Impinging ions cause the emission of secondary electrons, which are subsequently accelerated through the

4. EXPERIMENTAL APPARATUS

continuous dynode by a high positive potential set by the multiplier. Multiple impacts cause a cascade-like process which has the effect of amplifying the ion-count signal. It is worth noting here that SEM detectors are subject to degradation and amplification decreases over time. This is remeidied by increasing the multiplier potential. This process and other aspects of tuning the EQP and acquiring data are outlined in appendix A.