ZnO:Al Thin Film - Wet Etching

ZnO:Al Thin Film - Wet Etching

Material Name: ZnO:Al
Recipe No.: 10360
Primary Chemical Element in Material: Zn
Sample Type: Thin film
Uses: Etching
Etchant Name: None
Etching Method: Dry etching
Etchant (Electrolyte) Composition: ZnO:Al films were reactively sputtered on glass substrates (Corning, Eagle XG) in a vertical in-line sputtering system (VISS 300, by von Ardenne Anlagentechnik, Dresden, Germany). The system is typically under a base pressure of 6×10 exp(-4) Pa. Rotatable dual magnetron cathodes (RDM) with metallic Zn:Al tube targets (0.5 wt%) were operated at discharge power of 10 kW with mid-frequency excitation of 40 kHz to achieve a growth rate of up to 90 nm·m/min. The process pressure, Argon gas flow and substrate temperature were respectively 0.96 Pa, 200 sccm and 350°C as measured via heat radiation prior to deposition. The working point was controlled via plasma emission monitor (PEM) in the transition mode at PEM intensity of 35 % and average oxygen gas flow of 160 sccm. More details on PEM control can be found in literature. A related paper provides more detailed information on ZnO:Al film properties. As-grown ZnO:Al films with low resistivity of less than 4×10-4 O·cm were used for etching experiments followed in this study. ZnO:Al films were etched in aqueous solutions of diluted hydrofluoric (HF 1%) or hydrochloric (HCl 0.5%). As reference material we used a ZnO:Al film sputtered at low rate from ceramic tube targets in the same deposition system, as such ZnO:Al films could achieve excellent light trapping upon HCl etching in silicon based thin film solar cells.

The thicknesses were measured with surface profiler. The electrical properties of the films were investigated by 4-point probe and Hall effect measurement using van der Pauw method. The morphologies of as-deposited and etched ZnO:Al films were evaluated by atomic force microscopy (AFM, Nanoscope system from Veeco). Optical transmission and reflection of surface textured thin films were carried out with a double beam spectrometer equipped with an integrating sphere (Perkin Elmer Lambda). An index matching fluid (CH2I2) was used to avoid systematic measurement errors due to light scattering of the rough films during optical measurement for absorption determination.

We studied the ZnO:Al film properties after etching. Fig.1 shows the variation of thickness and sheet resistance as well as root mean square (RMS) roughness of ZnO:Al films after two-step etching first in HF (120 sec) and then in HCl. The values given at negative times correspond to the as-grown ZnO:Al film. The etch times correspond to the second etch step in 0.5 % HCl. Thickness (solid square) first decreases by about 150 nm by etching in HF solution for 120 s, and then decreases gradually from 720 nm to 550 nm with the increase of etching time in HCl solution. Sheet resistance (solid circle) increases with the decrease of thickness from about 3.8 Omega to 9 Omega. RMS roughness (solid triangle) rises from about 10 nm to about 90 nm by the first etching step. During the second etching step the RMS roughness shows a maximum of about 100 nm at 8 s and then decreases again for long etch times. In addition, the data of ZnO:Al film is shown, that was etched only by a single etching in HCl solution for 60 s. The sheet resistance is about 4.5 Omega due to its relatively high thickness (800 nm). The depth of large craters of these films is about 200 nm. Even though the films are etched for long time in HCl solution, the RMS roughness cannot be raised above 60 nm. Upon a longer etching time the films would suffer from holes that are etched to the glass.
Procedure (Condition): No data
Note: Highly transparent and conductive aluminum doped zinc oxide thin films (ZnO:Al) were reactively sputtered from metallic targets at high rate of up to 90 nm·m/min. For the application as transparent light scattering front contact in silicon thin film solar cells, a texture etching process is applied. Typically, it is difficult to achieve appropriate etch features in hydrochloric acid, as the deposition process must be tuned and the interrelation is not well understood. We introduce a novel two step etching method based on hydrofluoric acid. By tuning the etch parameters we varied the surface morphology and achieved a regular distribution of large craters with the feature size of 1-2 µm in diameter and about 250 nm in depth. Microcrystalline silicon single junction solar cells (µc-Si:H) and amorphous/microcrystalline (a-Si:H/µc-Si:H) tandem solar cells with high efficiency of up to 8.2% and 11.4%, respectively, were achieved with optimized ZnO:Al films as light scattering transparent front contact.
Reference: H. Zhu, et al., Novel etching method on high rate ZnO:Al thin films reactively sputtered from dual tube metallic targets for silicon based solar cells, Solar Energy Materials & Solar Cells 95 (2011) 964 - 968.

Figure 1: Variations of thickness and sheet resistance as well as RMS roughness of high growth rate ZnO:Al films upon etching: Negative etch times correspond to initial values before etching, the values at 60 s correspond to single step etching in HCl; all other values correspond to the second HCl step after a first HF etch.