Time-averaged negative ion energy distributions

The time-averaged energy distributions of different negative species were measured for a number of different conditions by varying the discharge parameters. Figure 7.2 shows an example of the temporal variation ofVd(t) andId(t) for the following discharge parameters;

p= 0.33 Pa, pO2/ptotal = 0.2, Pd = 100 W,τ = 100 µs and f = 100 Hz.

The mass spectra of both positive and negative ions obtained under the discharge conditions given above are presented in figures7.3a and7.3b, respectively. When recording the positive and negative mass spectra, the static energy analyser was tuned to monitor constant pass energy of 2 eV and 3 eV, as this was found to be the most probable energy for Ar+ and O−, respectively. From figure 7.3b, O− appears to be the most abundant of the negative species in the discharge while little to no oxygen compound species were detected in the negative ion mass spectra. The reason for this is due to the energy-

7. NEGATIVE ION ENERGY DISTRIBUTIONS IN REACTIVE HIPIMS

Figure 7.2. The voltage-current-time characteristic for the HiPIMS of Ti in an Ar/O2

discharge as described in the text. The peak charge density is approxi- mately 0.52 A cm−2.

selective nature of the instrument and can be made clear by examining the time-averaged energy distributions of the oxygen compound negative ions which are given in figure 7.4. From the energy distributions of TiO−, TiO−₂ and TiO−₃ as illustrated in figure 7.4, it is observed that these species are detected predominantly at high energies and are hence not present in the negative ion mass spectrum where the pass energy is fixed at 2 eV. Table 7.1 shows a list of the negative ion species for which energy distributions are shown in figure 7.4, with their correspondingm/z values.

From figure 7.4, O− is observed to be the most abundant negative species across the entire energy range (0 < E < 800 eV) at the position of the orifice, which corroborates with previous studies performed by other authors regarding negative ions in reactive magnetron sputtering where oxygen is employed as the reactive gas [20,117].

7. NEGATIVE ION ENERGY DISTRIBUTIONS IN REACTIVE HIPIMS

Figure 7.3. Mass spectra of positive (a) and negative (b) ions generated in reactive HiPIMS of Ti in an Ar/O2 mixture wherepO2/ptotal= 0.2. The EQP was

set to sample at a constant pass energy of 3 and 2 eV for the positive and negative mass spectrum, respectively.

Figure 7.4. Measured energy distributions of O−, O−₂, TiO−, TiO−₂ and TiO−₃ in re- active HiPIMS of Ti in an Ar/O2 mixture where pO2/ptotal = 0.2. The

7. NEGATIVE ION ENERGY DISTRIBUTIONS IN REACTIVE HIPIMS Species Mass-to-charge ratio (amu/z) O− 16 O−₂ 32 TiO− 64 TiO−₂ 80 TiO−₃ 96

Table 7.1. List of detected negative species alongside their corresponding mass-to- charge ratios (amu/z).

Also similar to results presented by other authors [20,110,116], the shape of the O energy distribution reveals three main ion populations; low, medium and high energy. The ions contributing to the low energy peak (E <70 eV) are either generated at the target surface and experience energy loss due to collisions or are formed in the plasma bulk via the post-ionisation of sputtered particles. It is also possible that low energy negative ions are formed inside the target sheath or pre-sheath via electron attachment or dissociative electron attachment processes rather than at the target surface and are hence accelerated by only a fraction of the cathode potential. However, the low electron density within the sheath makes the formation of negative ion species inside the sheath unlikely. It should be noted that very low energy O−ions formed in the plasma bulk go undetected by the EQP. The formation mechanism of these O− ions is typically dissociative electron attachment involving background oxygen molecules (i.e. O2+ e−→O−+ O). As a consequence, O−

ions formed via this mechanism have temperatures close to that of the background gas and therefore possess insufficient energy to overcome the potential barrier at the grounded orifice. The potential barrier is on the order of the local electron temperature (typically 1−4 eV, as shown in chapters 5 and 6). In order to investigate O− ions formed in this way, different methods such as laser-aided photodetachment are employed, as in chapter

6.

The medium-energy peaks (70 eV < E < 420 eV) originate from accelerated clusters (O−₂, TiO−, TiO−₂ etc...) dissociating as a result of collisions in the plasma bulk en route to the EQP orifice. Upon dissociation, the energy of the cluster is shared between its fragments according to the ratio of the fragment masses. This effect is most evident in the energy distribution of TiO−₂, where a smaller peak at E ∼ 410 eV is observed alongside the higher energy peak atE ∼495 eV (see figure7.4). The energy ratio of these two peaks is 0.83, which is almost identically equal to the mass ratio of TiO2 to TiO3

(MTiO2/MTiO3 ≈80/96), suggesting that the lower energy peak in the TiO

−

2 distribution

is a result of TiO−₃ molecules dissociating into TiO−₂ and O during transportation to the orifice. There is also a small peak centred about E ∼334 eV, which is equal to the target potential applied by the pre-ionizer during the pulse off-phase (evident in figure 7.2).

7. NEGATIVE ION ENERGY DISTRIBUTIONS IN REACTIVE HIPIMS

processes and subsequently accelerated through the cathode sheath. As can be seen in figure 7.4, TiO−₂ and TiO−₃ ions are only observed at high energies corresponding approximately to the average target potential during the on-time, suggesting that such ions are produced exclusively at the target surface and not in the plasma bulk. Moreover, their narrow energy distribution compared with the high-energy O− ion population also suggests that TiO−₂ and TiO−₃ ions are released from the target by ion-assisted desorption rather than by physical sputtering.