This paper describes a new and simple method from our Valincona to study the dynamics of wet corrosion of ITO films by monitoring the resistance of the film during corrosion. This method enables the study of 0.1 to 150 nanometer/ Etch rate between minutes. Three different states can usually be distinguished: (1) slow initial etching; (2) fast overall etching stage and (3) the slow etching stage at the end. The method was shown to be particularly suitable for studying phenomena at the end of the etching process, where isolated patches of film remain adhered to the substrate.
Due to its considerable electrical conductivity and optical transparency, indium tin oxide (ITO) is one of the most widely used transparent conductive oxides (TCO) for displays, touch panels, solar cells and other related applications. , patterning of ITO films is usually accomplished by photolithography, which includes an etching step that is mainly wet etching in industrial processes.
In wet etching studies of ITO, the procedure for evaluating etch rate is usually not explicitly mentioned. Since the focus of these studies is the total etch rate, the etch rate may be evaluated by dividing the film thickness by the total etching time, However, there is no mention of how to determine the total etching time, and the basic assumption for evaluating the etching rate is that the etching rate is constant during the etching process.
During the etching of thin films of ITO and other transparent conductive materials such as SnO2 and ZnO, the study of etching rate requires monitoring of the thickness or mass-related quantities of the film. The optical monitoring method can be ellipsometry, direct transmission and reflection measurement or by Grating structures measure transmission and reflection. Since for very thin films (50nm) direct measurement of transmission and/or reflection is not sufficient to assess thickness, the preparation of grating structures in ITO films requires an etching technique that avoids under-etching, as this is not possible in some applications. Shaping ITO films is not always possible and the application of this technology was not considered.
In addition, measuring the thickness of very thin ITO films is troublesome because the surface is rough and isolated ITO residues are formed on the substrate surface. In order to monitor the etching rate during the etching process, a resistance-related measurement technique is applied, which The technology does not require photolithography to prepare the sample, and the above problems may exist. In addition, conductivity is one of the most important properties of transparent oxide films such as ITO. Resistance is an easily measured parameter, which is similar to that obtained by the etching process. Directly related to the electrical properties of the device, the new method is not limited to ITO and can also be used to study the corrosion dynamics of various conductive films.
After describing the basic elements of the resistance monitoring method during etching, the shape of the resistance monitoring curves is discussed and it is shown that from these curves a more relevant etch rate can be obtained ITO. A method was designed to monitor the resistance during etching of ITO films in acidic etchants such as hydrochloric acid (HCl) and oxalic acid (H2C2O4). This monitoring was performed in the following manner: samples were taken out of the etching solution every minute, and To measure the resistance outside the solution, this method was deliberately chosen because measuring the resistance in situ when the membrane is in the etching solution will be affected by parasitic currents in the conductive etching solution. Before the measurement, the sample was placed in an ultrasonic bath in distilled water of 50C Clean 1html in 1 minute, rinse with distilled water and alcohol, and dry at 100℃. This procedure limits the minimum etching time to 1 minute, while the time required to remove the sample from the solution and wash in distilled water to stop the reaction is usually 7 second, which does not significantly increase the error in measuring the etching time, and no base is used to stop the reaction, ensuring that no additional reagents are adsorbed on the ITO surface during the measurement.
We measure the resistance of the sample with parallel contacts spaced 10 mm apart. This creates parallel 2 squares, so the sheet resistance is twice the measured resistance.In order to ensure stability and low contact resistance , a specially structured clamp was manufactured, which is made of a cantilever pulled by a rope with an adjustable length and can exert up to 4 kg of force on a tool that will The sample is pressed against two conductors made of tinned copper braided wires that are parallel 100 meters through PVC separators parallelism deviation 1%). The sheet resistance of the thin ITO film is high enough that It is also possible to use 4 point measurements, in which case the resistance is measured using the Jandel HM21 4 point probe tip equipped with tungsten carbide needles spaced 1 mm apart to simulate experimental conditions by applying and releasing pressure on the sample , a series of 20 measurements were performed on the same sample to test the repeatability of the 2 measurements. The average deviation of the resistance measurement of was better than 0.5%. This fixture allows fast measurement of resistance during the etching process.
corroded 25 and 175 nanometer ITO films of two different thicknesses in oxalic acid and hydrochloric acid. During the etching process, the resistance of the 25 nanometer ITO film increased from the initial value 98/ to infinity. The figure below shows the resistance during the etching process. Changes in the normalized reciprocal value of . The vertical axis in the figure represents R0/R, where R0 is the initial sheet resistance before etching, and R is the actual sheet resistance, measured during the etching process according to the method described in the previous section. The figure shows that strict degreasing of ITO films affects the initial etching behavior.
The total etching time tetch is the time measured from the start of the reaction to the point where the resistance is at least 10 times greater than that of the unetched film. Stop etching when the measured resistance of the sample exceeds 200 M. Since this criterion does not ensure that all material has been removed from the glass surface, special attention should be paid to the final stage of the etching process.
The curve in the figure shows the 3 stages of the etching process during the total etching time tetch : The initial etching cycle of the slow etching represented by t0 ( start time ) , in tb ( overall etching time ) The fairly rapid reduction of R0/R during , and the slow etching of at the end of tr ( remaining etching time ), start time t0 is defined as the time when the linear behavior starts, tr is defined as the time from the end of this linear part until R0/ The time elapsed since R is less than 10-7. Transitions between states can be determined by plotting the first and second order time numerical derivatives of R0/Rt.
Consider the etching rate de/dt ( units are nanometers / minute ), where e represents the thickness of a uniform isotropic film in a solution that is well stirred and also has a large excess compared to ITO molecules Etchant molecules, if there is no autocatalysis of reactants, then de/dt is a constant : In other words, e decreases linearly with time, and the initial thickness of the film is represented by e0, after a period of time in the etching solution , we get et. In the equations (1a) and (1b), the resistivity of the conductive film is considered to be independent of the thickness. This is generally not true in thin films because at small film thicknesses an increase in electron scattering effects at the film surface is observed due to the fact that the electron free paths are larger than the film thickness.This results in a deviation from the bulk resistivity, and larger resistivities are observed in films of ~300nm. An increase in resistivity is observed at smaller film thicknesses. The resistivity of ITO films deposited by evaporation is below 100 nanometers. 2 times was added because when the film becomes thinner after etching, the microcrystalline morphology and the inhomogeneity of the electrical connections between microcrystals may play a greater role.
At the end of the etching process, isolated ITO islands are observed on the glass. As long as the ITO crystallites are in contact with each other, the resistance has a finite value. In the case of isolated ITO crystallites, it is assumed that the resistance will be infinite, in the case of uniform islands In most cases, the transition is expected to be sharp : is similar to the transition from permeable to non-permeable. In most cases, no sharp transition from finite resistance to infinite resistance (10M) is observed. Therefore, a A simple percolation-based model cannot explain our results, and a possible explanation for our results is that there is ion conduction on the glass surface between isolated ITO islands. This may be caused by the diffusion of Na into the ITO film to form the NaInxOy compound during post-annealing after deposition. presents a new method to study wet corrosion kinetics of conductive films by monitoring resistance. The method has been tested on ITO films of 25 and 175nm. This new method provides insight into the etching process and can typically distinguish three distinct stages: : an initial stage with a very slow etch rate, an initial stage with a very slow etch rate, and an initial stage with a very slow etch rate for most of the film. A fast etch rate and a slow etch rate at the end. Etch rates based on recorded total etch time will often underestimate the associated etch rate: it is recommended to calculate the etch rate from the time linear phase in the R0/R curve. Furthermore, it is considered that this method is particularly suitable for studying phenomena at the end of the etching process.
