26 Sep 2018
Cost-Effective Manufacturing of High-Quality ITO Thin Films on 300 mm CMOS for Next Generation Display Technologies
Owing to its exceptional optical and electrical properties, indium-tin oxide (ITO) is nowadays the material of choice for providing transparent electronic conduction in nearly all flat panel displays used in TV screens, laptop computers and mobile phones, regardless whether they are based on liquid-crystal display (LCD) or organic light-emitting diode (OLED) technologies. However, emerging applications of wearable electronics and augmented reality have very demanding technical requirements in terms of brightness, response time, contrast ratio, energy efficiency and advanced packaging density which cannot be met by these display technologies.
The new micro-LED technology is currently considered as one of the most promising solutions that would offer the levels of performance required by next generation display market applications. Whether these displays have to be fabricated by hybridization of micro-LED arrays with the CMOS active matrix using the flip-chip technology, or by monolithic hybrid integration, successful realization of micro-LED based devices requires front end semiconductor manufacturing approaches and handling standards for the thin film processes involved. For the transparent conductive ITO layers this means an absolute control of particles and defects, single wafer traceability, and compatibility with standard 12” manufacturing technology. Moreover, improved heating uniformity is required, especially for achieving uniformly high optical transmission and low ITO resistivity.
In this work, we investigate the physical properties of high-quality ITO layers deposited on 12” Si and Si/500 nm-thermal-SiO2 wafers in an industrial multi-chamber CLUSTERLINE® 300E magnetron sputtering system, the process modules of which are attached to a central transfer module equipped with vacuum robot. The ITO layers with thickness ranging from 5 to 100 nm were deposited by DC sputtering at a pressure of 2.8×10-3 mbar (base pressure 4×10-8 mbar) from an In2O3/SnO2 90/10 wt. % long-life target (~1500 kW·h) with a diameter of 400 mm. The optimum electrical conductivity and optical transmission were obtained by adding a small amount of O2 into the Ar sputtering gas (e.g. 0.75% O2 for 1 kW sputter power). Both as-deposited and post-annealed ITO layers were investigated. The post-annealing treatment was performed in a dedicated high-pressure degassing module at a temperature of 300°C.
The optical and electrical properties of the deposited ITO thin films set a new benchmark of quality for cost-effective manufacturing of transparent conductive layers on 300 mm CMOS, and can therefore help manufacturers meet exacting standards for next generation display technologies. Thickness uniformity of the ITO layers sputtered on the 12” wafers measured with a 5 mm edge exclusion using a spectral ellipsometer was below ±3%. Absolute roughness measured by means of atomic force microscopy (AFM) was ~0.1 nm. The average sheet resistance of the post-annealed 25 nm ITO layers measured by means of the four-point probe technique was 70.4 Ω/sq. (electrical resistivity 176 µΩ⋅cm) with a uniformity of ±3.4%. Optical transmission of the 25 nm ITO layers measured on 2” sapphire wafers attached at various locations on the 12” carrier wafers was 97.7%. High volume manufacturing was demonstrated by BKM fabrication flow exceeding 45 wafers/hour using multiple ITO PVD chambers and post-annealing units.