RF voltages

The octopole configuration is similar to the quadrupole and hexapole in affording a good ion guidance system in its all-RF mode. The poles are connected in two pairs of four nonadjacent rods, and each pair is connected to an RF voltage supply, 180° out of phase with each other. The octopole is even more efficient than the hexapole as an ion guide. 

Through the application of DC and RF voltages to an assembly of four parallel rods, ions can be filtered along their central axis and mass measured to give a mass spectrum. In the all-RF mode, the assembly is used as a guide for ions of all m/z values. 

The quadrupole ion-trap, usually referred to simply as the ion-trap, is a three-dimensional quadrupole. This type of analyser is shown schematically in Figure 3.5. It consists of a ring electrode with further electrodes, the end-cap electrodes, above and below this. In contrast to the quadrupole, described above, ions, after introduction into the ion-trap, follow a stable (but complex) trajectory, i.e. are trapped, until an RF voltage is applied to the ring electrode. Ions of a particular m/z then become unstable and are directed toward the detector. By varying the RF voltage in a systematic way, a complete mass spectrum may be obtained. 

A critical issue in scaling up a process is the uniformity in deposition rate and material quality. In general, once the deposition rate is constant within 5% over the whole substrate area, the material properties also do not vary much. After fine-tuning the power and gas flow rates, operators still may face in homogeneity issues. These can be caused by local changes in temperature, RF voltage, and gas composition, due to various causes. As an example, it has been reported that improper attachment of the substrate to the grounded electrode results in a local decrease of the deposition rate [150, 151]. 

FIG. 9. Relation between dc self-bias voltage Vj,. and applied RF voltage for different gases (Ar. H2. SiHa) and gas mixtures 0.1 < ISiHal/tlHTl-FlSiHal) < 0.9. Note that the slope is independent of the gas used. 

Comparing published data on the relation between the dc self-bias and RF voltage between Rauf and Kushner [180] and our group [151] shows that the proportionality constant (slope) in the GECRC is nearly twice the one observed in the ASTER reactor. In other words, the ASTER reactor is less asymmetric than the... 

The frequency effects are studied in the cylindrical geometry (R = 0.08 m, L = 0.027 m) at a constant power of 25 W, which corresponds to a volume power density 46 mW cm . The pressure is 120 mTorr with 45% SiHa and 55% H2. It is found that the RF voltage at this power scales with the frequency as Vrfv p = C, with C a constant. Because the induced displacement current increases with the... 

In RF discharges one has to compensate for the RF voltage component across the probe sheath. This is done with a pickup element, situated close to the probe tip, which should be in the plasma bulk as well. Compensating electronics within the probe is usually designed to work at a prescribed RF frequency, and cannot be used at other frequencies. 

The substrate is radiatively heated by heaters that are placed outside the vacuum. A backing plate ensures a laterally homogenous temperature profile. In the same chamber also PECVD can be carried out. The backing plate then is the grounded electrode, and the RF voltage is applied to the counter electrode. 

For a linear system like a probe circuit, the response fq(f) (RF field created by the coil) to the excitation v(t) (RF voltage produced by the spectrometer) is expressed by a convolution relation... 

One method of acquiring a mass spectrum is the mass selective instability scan. As the RF voltage increases, the ions with lowest m/z become unstable and are ejected through small holes in the end cap to hit a detector. As the RF voltage is further increased, heavier ions become successively unstable and are ejected, thus yielding a mass spectrum. 

By applying the supplementary RF voltage during injection (without scanning the RF voltage on the ring electrode) it is possible to prevent certain ions from being trapped. 

The supplementary RF voltage may also be set to contain all frequencies but a narrow band, corresponding to a narrow m/z range, forcing all ions outside this range to leave the trap. This mode of operation is usually referred to as ion isolation, and is the first step when performing MS/MS (see below). 

Ramping of both DC and RF voltages in a simultaneous fashion produces resonant or stable trajectories (Fig. 11.9) for ions of sequential... 

Fig. 11.10. Diagram illustrating the inner surfaces of the primary components of a Paul (3D) quadrupole ion trap. Ions generated by an external source are injected into the trap through an aperture in one of the end caps. Scan functions for isolating ions in the trap, exciting the mass selected ions to induce unimolecular dissociation, and ejecting ions from the trap (for detection) are implemented through the application of DC and RF voltages to the ring electrode.

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