The vacuum chamber set-up is described in§4.1 and shown in figure 4.1. The magnetron was equipped with a 3 in. (75 mm) Ti target and energised using a combination of the HiPIMS impulse unit and the pre-ionizing power supply, both described in §4.1.3. The pulse width and frequency of the discharge voltage pulse were fixed at 100µs and 100 Hz, respectively, and a constant average discharge powerPd = 100 W was used throughout all
experiments presented in this chapter. A simplified schematic of the experimental set-up is shown in figure 5.1. The gas inlets were located behind the magnetron with the Ar and O2 flow rates being set by separate, but identical mass flow controllers. The total working
gas pressure, ptotal, was varied from 0.4 to 1.6 Pa, with a proportionally constant oxygen
partial pressure, pO2 = 0.2ptotal.
A single cylindrical Langmuir probe was placed in the vacuum chamber and moved radially, parallel to the target surface. The housing of the magnetron was moved orthog- onally to the Langmuir probe to allow for measurements at different axial positions. The radial positionr = 0 was defined as the discharge centreline and the axial position of the target surface was defined asz = 0. The magnetic null is located at positionr= 0, z= 30 mm, as indicated in figure5.1. The probe position was varied betweenr= 0−40 mm, and z = 50−100 mm. To obtain time-resolved Langmuir probe measurements, the internal delay generator of the acquisition system was synchronized to the discharge pulse and used to step through the entire on-time and a portion of the discharge afterglow. A time range t = 0−300µs was used with a time-step, ∆t, of 1 µs with t = 0 defined as the on-set of the discharge voltage pulse. Each data point was recorded 5 times and each full I-V trace was recorded 5 times again before an average was found to produce a single I-V characteristic. The bias voltage of the probe was varied from−70 V to +20 V with a resolution of 0.05 V. In between data acquisition runs, a potential of −150 V was applied to the probe tip in order to sputter clean depositions from its surface.
5. PLASMA DYNAMICS IN A REACTIVE HIPIMS DISCHARGE
Figure 5.1. A schematic representation of the experimental set-up. The location of the magnetic null is displayed on the discharge centreline at an axial distance of 30 mm from the target surface.
The orbital-limited-motion (OML) theory [129,148] was used to infer positive ion den- sities (equation 4.10). In order to determine ni using the OML approach, the ionic mass,
Mi, must be known, or at least estimated. In pure Ar discharges, this is straightforward
as Ar+ is typically the dominant positive species. However, in a reactive discharge con-
taining multiple gases, the ion current is likely to consist of contributions from multiple positive species. Moreover, multiply-charged ions are also present in HiPIMS discharge.
The proportions of the constituent positive ions were estimated by means of energy- resolved mass spectrometry. An example of a mass spectrum obtained in an Ar/O2
discharge with ptotal = 0.4 Pa at a position along the discharge centre and 100 mm away
from the target surface is presented in figure 5.2. The measured signal intensities are normalized to Ar+. As seen in figure 5.2, the most abundant positive ions are identified
as Ar+, Ar2+, O+, O2+ and Ti+. Since the mass spectrum only shows the intensities
sampled from a set pass energy (5 eV in this case) and not the entire energy range, it is more accurate to calculate ion proportions using the integral values of the energy distributions. The energy distributions of the main positive ions are displayed in figure
5.3 alongside their corresponding fractions.
The average positive ion mass is obtained byMi =
P
ikMi wherek is the ion fraction
as shown in figure 5.3 and Mi is the ion mass of the ith species. The calculated time-
averaged fractions were 0.13, 0.27, 0.51, 0.06 and 0.03 for O+, O2+, Ar+, Ar2+ and Ti+, respectively, yielding an average ion mass, Mi ≈35 amu. This was the value used as the
5. PLASMA DYNAMICS IN A REACTIVE HIPIMS DISCHARGE
effective positive ion mass when calculating ion densities using the OML approach. The influence of negative ions on the positive ion flux is neglected due to the low α values measured in the times investigated (see chapter 6).
Although a time-averaged value of the effective positive ion mass is used here, it is worth noting that the positive ion composition is likely to vary both spatially and temporally. For instance, it is expected that during times of high discharge current density and at positions adjacent to the target surface, the metal ion fraction will be increased. Furthermore, dissociation of O2 molecules into O atoms is more likely at positions closer
to the target due to an increased probability of O2 molecules experiencing electron impact
collisions, resulting in a larger O+/O+2 ratio. However, for the relatively low discharge power densities employed here the ratio between metallic ions to background gas ions is expected to be low. Moreover, at typical substrate positions (e.g. axial distances of z = 50−100 mm, as investigated here) the O+/O+
2 ratio is also expected to be small.
Hence, the positive ion fractions displayed in figure 5.3 are sufficient to describe the average positive ion mass for the times and spatial positions explored in this chapter.
Figure 5.2. Positive ion mass spectrum obtained from an Ar/O2 discharge held at a
pressure of 0.4 Pa with the EQP orifice placed at a position r = 0 mm,
5. PLASMA DYNAMICS IN A REACTIVE HIPIMS DISCHARGE
Figure 5.3. The energy distributions of O+, O2+, Ar+, Ar2+and Ti+obtained from an Ar/O2 discharge held at a pressure of 0.4 Pa with the EQP orifice placed
at a position r= 0 mm,z= 100 mm in the chamber. The corresponding ion fractions are inset.