Controlling the B/Ti ratio of TiBx thin films grown by high-power impulse magnetron sputtering

Abstract

TiBx thin films grown from compound TiB2 targets by magnetron sputter deposition are typically highly over-stoichiometric, with x ranging from 3.5 to 2.4, due to differences in Ti and B preferential-ejection angles and gas-phase scattering during transport from the target to the substrate. Here, the authors demonstrate that stoichiometric TiB2 films can be obtained using high-power impulse magnetron sputtering (HiPIMS) operated in power-controlled mode. The B/Ti ratio x of films sputter-deposited in Ar is controllably varied from 2.08 to 1.83 by adjusting the length of HiPIMS pulses ton between 100 and 30 µs, while maintaining average power and pulse frequency constant. This results in peak current densities JT,peak ranging from 0.27 to 0.88 A/cm 2. Energy-and time-resolved mass spectrometry analyses of ion fluxes incident at the substrate position show that the density of metal ions increases with decreasing ton due to a dramatic increase in JT,peak resulting in strong gas rarefaction. With ton < 60 µs (JT,peak > 0.4 A/cm 2), film growth is increasingly controlled by ions incident at the substrate, rather than neutrals, as a result of the higher plasma dencity and, hence, electronimpact ionization probablity. Thus, since sputter-ejected Ti atoms have a higher probability of being ionized than B atoms, due to their lower first-ionization potential and larger ionization cross-section, the Ti concentration in as-deposited films increases with decreasing

Strategy for simultaneously increasing both hardness and toughness in ZrB2-rich Zr1−xTaxBy thin films

“…The B/Ti ratio was thus controlled by varying the field strength of external Helmholtz coils. Another approach, 38 also demonstrated for TiB2, but this time using high-power impulse magnetron sputtering (HiPIMS), is to increase the peak current density JT,peak per pulse by decreasing the HiPIMS pulse length. This results in strongly increased gas rarefaction leading to higher metal-ion densities in the discharge.…”

Where Is the Unmatched Transition Metal in Substoichiometric Diboride Line Compounds?

“…Growth took place in Ar (99.999% pure) discharges at a constant dc power of 100 W for 40 min with a target-to-substrate separation of 6.5 cm. The substrate temperature was maintained constant at Ts = 900 o C. The Ar pressure PAr is 20 mTorr with B ̅ axial = (B ̅ mag + B ̅ ext) set to 200 G and the ion to Ti flux ratio of 52 and the ion energy ~8 eV as described elsewhere [16]. B/Ti ratio in as-deposited thin films was determined from RBS analyses to be 1.9±0.04.…”

“…Currently, magnetron sputtering from a TiB2 compound target is the primary approach for depositing TiB2 thin films [9,10,11,12] although other techniques such as high-power impulse magnetron sputtering [13,14,15,16], and cathodic-arc evaporation have been addressed [17,18].…”

Microstructure and Materials Properties of Understoichiometric TiBx Thin Films Grown by HiPIMS

“…Few reports present understoichiometric TiBx thin films. [16,19,20] An emerging method for TiB2 growth is HiPIMS. [16,[19][20][21] [20] demonstrate hardness of HiPIMS-grown TiBx thin films with B/Ti ratio of 1.62 to 1.97 in the range of 39-45 GPa, and attribute this to film densification.…”

“…[6][7][8] While the most common method to grow TiB 2 is direct-current magnetron sputtering from compound targets, several reports show other approaches in choice of synthesis, including electroplating, arc-PVD, pulsed DCMS, RF-sputtering, and HiPIMS. [9][10][11][12][13][14][15][16] DCMS typically produce overstoichiometric material, with a B/Ti ratio in the range of 2.4 to 3.5. Previous work has shown that the excess B in these TiB 2 films move towards grain boundaries and form nanostructures, a mechanism that is well described.…”