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Sputter Deposition of Indium Tin Oxide: To Pulse or Not to Pulse
Posted October 30, 2023 by Gayatri Rane
Sputter deposition characteristics of indium tin oxide (ITO) from compound targets for applications as a transparent conductive oxide (TCO) have been widely investigated to find deposition processes that optimize film properties. While production of coatings by magnetron sputtering is strongly influenced by the specifics of the coating equipment (such as target-substrate distance, magnetron design and gas inlets) and process-dependent factors (such as target material, process gas partial pressure and vacuum state), ITO deposition in particular is strongly affected by the target discharge voltage1,2. Several studies have shown that depositions performed at lower discharge voltages lead to higher quality films with low defect content2,3.
The impact on film quality was attributed to the effect of O- ion bombardment. Other studies have shown that O- ions formed during sputtering ITO targets can damage sensitive films as they get accelerated to the substrate and bombard the growing films with energy (-eV) equivalent to the target voltage (V)1. Reducing the target voltage leads to a reduction in the energy of the bombarding O- ions.
In this blog, pulsed DC is investigated as a means to study its impact on discharge voltage and the effect on the film properties.
Advanced Energy’s low-impedance (low-Z) Pinnacle® DC power supply delivers full power below 400 V and is widely deployed for the deposition of high-quality ITO films at low voltages. We compared the baseline ITO deposition film properties with film properties resulting from depositions with pulsed power. From the wide variety of pulsed power supplies AE makes, those tested include the AE Solvix® LF (pulsing up to 30 kHz) and AE Solvix HF (pulsing up to 350 kHz) units – which are typically used as bias supplies - and Ascent® SMS AP (pulsing up to 150 kHz) technologies.
Depositions were carried out on a system with a planar rectangular (740 cm2) ITO (90/10) target at power densities of 1.4 W/cm2 and 2.7 W/cm2 in an in-line glass coater. Argon (Ar) gas was used for the process and the oxygen flow was optimized to obtain low resistivity ITO films. Care was taken to deposit films with a target thickness of 100 nm on glass substrates for comparability of film properties.
The higher power density (2.7 W/cm2) processes across all the power supplies tested in this study resulted in a lower power-normalized dynamic deposition rate (DDR/P). Additionally, the higher power density depositions yielded inferior quality films with higher electrical resistivity, higher compressive stresses (GPa as compared to low MPa for the 1.4 W/cm2 processes) and lower UV-Vis transmittance.
Results for the depositions carried out at the lower power density (1.4 W/cm2), which yielded better quality of films compared to the higher power density depositions, are in the figures below. Of note, the trends of the results remain the same for both the power densities.
Figure 1 (clockwise): Dynamic deposition rate (DDR), electrical resistivity (100 nm film), discharge voltage, and mean transmittivity (between 400 to 1000 nm), as a function of pulsing frequency for the different power supplies tested: Solvix HF; Solvix LF; SMS Ascent® AP15 and Pinnacle® low-Z units
The effect of pulsing during the deposition process are summarized below:
Figure 2: SEM images of 100 nm ITO films deposited with straight DC (left) and with 100 kHz pulsed DC (right). Minor inhomogeneity can be seen at the film surface for film deposited with high-frequency pulsing, as shown in the right figure.
The results show that the film properties deteriorated with increasing discharge voltage, as has been already published. Since the target quality deteriorated with use, comparisons on the performance between the different power supplies cannot be conclusively derived from the results. However, the trends suggest no particular benefit of high-frequency pulsing on the film properties. Low-frequency pulsing (below 20 kHz) offers minor improvements in the film properties and might help mitigate arcing and related damages. Future studies will explore the impact of arcing on the films.
To conclude, in terms of overall film properties, straight DC deposited films nevertheless delivered superior ITO film properties and pulsing did not lead to drastic improvements based on the data collected in this study.
References:
The impact on film quality was attributed to the effect of O- ion bombardment. Other studies have shown that O- ions formed during sputtering ITO targets can damage sensitive films as they get accelerated to the substrate and bombard the growing films with energy (-eV) equivalent to the target voltage (V)1. Reducing the target voltage leads to a reduction in the energy of the bombarding O- ions.
In this blog, pulsed DC is investigated as a means to study its impact on discharge voltage and the effect on the film properties.
Advanced Energy’s low-impedance (low-Z) Pinnacle® DC power supply delivers full power below 400 V and is widely deployed for the deposition of high-quality ITO films at low voltages. We compared the baseline ITO deposition film properties with film properties resulting from depositions with pulsed power. From the wide variety of pulsed power supplies AE makes, those tested include the AE Solvix® LF (pulsing up to 30 kHz) and AE Solvix HF (pulsing up to 350 kHz) units – which are typically used as bias supplies - and Ascent® SMS AP (pulsing up to 150 kHz) technologies.
Depositions were carried out on a system with a planar rectangular (740 cm2) ITO (90/10) target at power densities of 1.4 W/cm2 and 2.7 W/cm2 in an in-line glass coater. Argon (Ar) gas was used for the process and the oxygen flow was optimized to obtain low resistivity ITO films. Care was taken to deposit films with a target thickness of 100 nm on glass substrates for comparability of film properties.
The higher power density (2.7 W/cm2) processes across all the power supplies tested in this study resulted in a lower power-normalized dynamic deposition rate (DDR/P). Additionally, the higher power density depositions yielded inferior quality films with higher electrical resistivity, higher compressive stresses (GPa as compared to low MPa for the 1.4 W/cm2 processes) and lower UV-Vis transmittance.
Results for the depositions carried out at the lower power density (1.4 W/cm2), which yielded better quality of films compared to the higher power density depositions, are in the figures below. Of note, the trends of the results remain the same for both the power densities.
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Figure 1 (clockwise): Dynamic deposition rate (DDR), electrical resistivity (100 nm film), discharge voltage, and mean transmittivity (between 400 to 1000 nm), as a function of pulsing frequency for the different power supplies tested: Solvix HF; Solvix LF; SMS Ascent® AP15 and Pinnacle® low-Z units
The effect of pulsing during the deposition process are summarized below:
- High-frequency pulsing led to a reduction in the DDR.
- Sheet resistivity of the films increased with pulsing frequency.
- Film transmittivity was not drastically affected by pulsing.
Figure 2: SEM images of 100 nm ITO films deposited with straight DC (left) and with 100 kHz pulsed DC (right). Minor inhomogeneity can be seen at the film surface for film deposited with high-frequency pulsing, as shown in the right figure.
The results show that the film properties deteriorated with increasing discharge voltage, as has been already published. Since the target quality deteriorated with use, comparisons on the performance between the different power supplies cannot be conclusively derived from the results. However, the trends suggest no particular benefit of high-frequency pulsing on the film properties. Low-frequency pulsing (below 20 kHz) offers minor improvements in the film properties and might help mitigate arcing and related damages. Future studies will explore the impact of arcing on the films.
To conclude, in terms of overall film properties, straight DC deposited films nevertheless delivered superior ITO film properties and pulsing did not lead to drastic improvements based on the data collected in this study.
References:
- Negative ions in reactive magnetron sputtering. T. Welzel and K. Ellmer. Vakuum; 2013, 25-2, 52-56.
- Comparison of the large-area reactive sputter processes of ZnO:Al and ITO using industrial size rotatable targets. V. Linss. Surface and Coatings Technology; 2016, 290, 43-57.
- Exploration into Sputtered Indium Tin Oxide Film Properties as a Function of Magnetic Field Strength. P. Morse and T. Strait. SVC Tech conference proceedings; 2016, 121-125.
Gayatri Rane
Advanced Energy
As an R&D scientist at AE, with past experience in sputtering and thin film analysis, I am involved with the research activities of the Customer Solutions Lab in Karlstein am Main, Germany.
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