Wet etching mechanism and crystallization of indium–tin oxide layer for application in light-emitting diodes

Abstract

In the article, we describe the etching mechanism of indium-tin oxide (ITO) film, which was wet-etched using a solution of hydrochloric acid (HCl) and ferric chloride (FeCl 3 ). The etching mechanism is analyzed at various etching durations of ITO films by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), and selective area diffraction (SAD) analysis. In comparison with the crystalline phase of SnO 2 , the In 2 O 3 phase can be more easily transformed to In 3+ and can form an inverted conical structure during the etching process. By adjusting the etching duration, the residual ITO is completely removed to show a designed pattern. This is attributed to the negative Gibbs energy of In 2 O 3 transformed to In 3+ . The result also corresponds to the finding of energy-dispersive X-ray spectroscopy (EDS) analysis that the Sn/In ratio increases with increasing etching duration.

In Situ Real-Time Monitoring of ITO Film under a Chemical Etching Process Using Fourier Transform Electrochemical Impedance Spectroscopy

“…The decrease of R ct should be partially ascribed to the increase of electrode area ( C dl ) (see Figure A–D), but primarily to the electrochemical activation of ITO because the R ct C dl dropped more severely than the C dl increased. Being analogous to the observation reported by Su et al, the EDS analysis of the partially etched ITO thin films in Table S1 shows an increase of Sn/In ratio near the surface region as the etching proceeded . Although a higher rate constant of dimethylferrocene redox couple (DMFc/DMFc + ) was observed on ITO electrodes that had been manufactured to be intrinsically higher in Sn/In ratio, it is not yet clear if the activated electrochemical kinetics is related to the atomic composition of Sn and In near the ITO surface.…”

“…As one of the transparent conducting oxide (TCO) materials, indium tin oxide (ITO) film has been widely used in a variety of applications, such as liquid crystal displays (LCD), , light-emitting diodes (LEDs), , thin-film transistors (TFTs), , and thin-film photovoltaics (PV), , due to its good electrical conductivity and optical transparency . The majority of the aforementioned usages require the patterning of ITO film electrodes which is easily accomplished by a chemical wet etching process because of its high throughput and low cost. , Among many of the examples using patterned ITO electrodes are light-emitting diodes (LEDs) with ITO layers or patterns, ,, LCD devices, and numerous bioanalytical sensing platforms, − including 3D interdigitated ITO electrode array (IDA) − and enzyme-modified ITO microelectrode array . In consideration of its versatility as an electrode material and the huge scale of its industrial uses, it is undeniable that a thorough understanding of electrical and electrochemical aspects of an ITO electrode in relation to its etching process is necessary for further development of relevant devices and the material itself.…”

“…Along with the extensive utilization of thin film ITO electrodes in numerous devices, the wet chemical etching process of the ITO film has also received considerable attention in terms of its physical properties, (electro)­chemical properties, mechanism, kinetics, and so on. − In general, the wet etching of ITO has been performed under acidic aqueous solutions, such as phosphoric acid, aqua regia, , oxalic acid, − and solutions based on halogen acids. ,,,,− In particular, halogen acid or its mixture with ferric chloride is considered as one of the most widespread chemical etchants, and there even exist some commercialized products of HCl and FeCl 3 solution, such as ITO etchant TE-100, CE-100, and CE-200 (Transene, U.S.A.). Consecutive works conducted by Van den Meerakker demonstrated that undissociated halogen acid molecules (HXs) participate in the chemical dissolution of ITO, and the addition of I 2 or Fe 3+ as oxidizing agents significantly increases the ITO etch rate with improvement of etched edge characteristics.…”

“…Consecutive works conducted by Van den Meerakker demonstrated that undissociated halogen acid molecules (HXs) participate in the chemical dissolution of ITO, and the addition of I 2 or Fe 3+ as oxidizing agents significantly increases the ITO etch rate with improvement of etched edge characteristics. They also proposed a chemical mechanism of the ITO etching process in HCl solution containing FeCl 3 . , Su et al examined the change of the Sn/In ratio during the ITO etching under FeCl 3 –HCl solution, and suggested the wet etching mechanism in the aspect of the crystalline structure …”

“…10,11,21,25,30−32 In particular, halogen acid or its mixture with ferric chloride is considered as one of the most widespread chemical etchants, and there even exist some commercialized products of HCl and FeCl HCl solution, and suggested the wet etching mechanism in the aspect of the crystalline structure. 10 In spite of several efforts focusing on the examination of the chemical etching of ITO, there have been few investigations of the real-time electrochemical change of the ITO film during the process. Although Brumbach et al reported the acid activation phenomenon with ITO samples and differences in the kinetic parameters from voltammograms, 32 the observed electrochemical change was a point-to-point comparison that had only resulted from the very initial stage (<10 s) of acid treatment.…”