Synergetic Effect of Aluminum and Mo/Al Etching in Phosphoric Acid-Based Etchant with Nitric Acid

Abstract

This study examined the etching behaviors and electrochemical characteristics of aluminum in phosphoric acid-based etchants. The etchant used for the research was composed primarily of phosphoric acid and nitric acid. Surface analysis of the etched aluminum sample revealed the existence of Al 2 O 3 , AlO(OH), and PO 4 3− . Other than the contribution of phosphate ions, the experimental results showed that hydrogen ions in the etchant influenced the etching rate substantially. Inclusion of nitric acid in the etchant caused a synergetic effect on the etching rate. Hence, this study proposes an etching mechanism in which hydronium ions and phosphate ions separately dissolve the passivated film. In the etchant, aluminum becomes passivated AlO(OH) and a dissolvable complex simultaneously. The adsorbed H (ads) produced on the Al surface is removed by nitric acid, which leads to acceleration in etching rate. In the study of electrochemical impedance measurements, an equivalent-circuit model with two capacitor elements has been established. In addition, this study investigated the etching behavior of Mo/Al. The dramatic change in open-circuit potential explained the different aspects for discrete etching durations.Materials used as interconnections in thin film transistor liquid crystal displays (TFT-LCDs) require low resistivity, high adhesion to glass, resistance to stress migration, and good patternability with proper taper angles. With the advantages of high electrical conductivity, low material cost, high adhesion, and good responses to patterning, aluminum and its alloys have been used widely in TFT-LCDs as data lines, gate materials, and source-drain electrodes. 1 Within manufacturing, aluminum has been widely used because the sputtering target cost is cheaper and is easier to handle than copper. To prevent the problem of hillock or whisker formation during thermal processing, certain metals such as Ti, Ta, Si, Cu, and Nd are embedded in Al alloys or multilayers. 2-6 Among these aluminum alloys, the addition of 2% Nd in aluminum has been applied extensively for metal deposition on glass within TFT processes. In addition, because Mo is capable of suppressing hillocks and improving taper angles simultaneously, metal-capped structures of Mo/Al and Mo/Al/Mo are the major construction in metal line processes. 7,8 Within industries, phosphoric acid-based etchants comprising phosphoric acid, nitric acid, acetic acid, and water are applied widely in aluminum alloy etching. In addition, Mo/Al stacked metal can be taper-etched easily with the phosphoric acid-based etchants. 9 Previous studies 10 showed that the dissolution rate of Mo increases in conjunction with nitric acid content. However, the adoption of Mo/Al and Mo/Al/Mo metal depositions in etching processes is complicating etching behavior. For example, the control of the taper angle is difficult because of the overhang structure and galvanic corrosion between Mo and Al. Although relevant studies have presented discussions on the kinetics and characteristics of alum...

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“…The wet chemical aluminum etching process using the Al Etch-M1 etchant a well-established for many electronics industries. 28 The functions of each component in this specic etchant are different, like the nitric acid used for alumina oxidation reaction; phosphoric acid removes the reaction products away from the surface of the substrate, and acetic acid is used as a diluent to maintain a low etching concentration. The primary mechanism of wet chemical aluminum etching reaction steps are as follows: 29 Anode:…”