Pulse DC Sputtered AZO Films for Low Resistivity
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Solution Overview
Problem
Current aluminum doped zinc oxide (AZO) TCO films face challenges in achieving optimal transparency, conductivity, and surface morphology for silicon tandem photovoltaic applications, with issues such as high resistivity, optical absorption, and chemical durability, particularly due to variations in deposition temperature and sputtering processes.
Innovation Solution
The development of AZO films using pulse DC sputtering at temperatures above 325°C, resulting in a columnar grain structure with small lateral grain size, independent resistivity of film thickness, low optical absorption, improved thermal stability, and enhanced chemical durability, achieved by sputtering an oxide target with 0.5-2 wt% Al2O3 composition and specific process conditions like high duty cycle pulsed DC voltage and low process pressure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional DC sputtering with metal targets is used to deposit AZO films, then the process is easier to control, but the films exhibit high resistivity and non-uniform properties due to target poisoning and hysteresis
Solution Approach 1:
The patent employs pulsed DC sputtering instead of continuous DC sputtering. The pulsed nature of the power supply periodically reverses the polarity, preventing target poisoning and hysteresis effects. This periodic action allows the target surface to be cleaned during reverse pulses, ensuring uniform film deposition and consistent resistivity across the film while maintaining ease of process control.
Solution Approach 2:
The patent changes the sputtering parameters by using pulsed DC power with specific duty cycles (e.g., 50% duty cycle) and frequency ranges (e.g., 1-100 Hz). It also optimizes other parameters such as substrate temperature (200-450°C), oxygen partial pressure (0.1-1 Pa), and power density (0.5-2 W/cm²). These parameter changes enable simultaneous achievement of low resistivity, uniform properties, and ease of manufacture.
2Reliability
If higher doping levels are used to reduce resistivity, then electrical conductivity improves, but optical absorption in the infrared increases due to increased plasma frequency
Solution Approach 1:
The patent optimizes the aluminum doping concentration to a specific range (0.5-2 wt% Al2O3 in the target) and controls the sputtering parameters to achieve optimal film stoichiometry. By precisely controlling these parameters, the patent achieves low resistivity films with minimized infrared absorption, finding the optimal balance between electrical conductivity and optical transparency.
Solution Approach 2:
The patent produces composite-like structures with controlled grain boundaries and phase distributions. The pulsed DC sputtering process creates films with specific microstructural characteristics that optimize both electrical and optical properties, effectively creating a composite material system with tailored properties for both conductivity and transparency.
3Reliability
If deposition temperature is increased to improve film density and reduce resistivity, then electrical properties improve, but surface roughness increases and chemical durability decreases
Solution Approach 1:
The patent identifies and implements an optimal deposition temperature window (200-450°C) where films achieve low resistivity without excessive roughness. Within this temperature range, the patent further optimizes other parameters including oxygen pressure, power density, and deposition rate to simultaneously achieve dense films with smooth surfaces and good chemical durability.
Solution Approach 2:
The pulsed DC sputtering process with controlled duty cycle and frequency creates periodic heating and cooling cycles during deposition. This periodic thermal action allows the film to develop dense structure for low resistivity while preventing excessive grain growth that would increase roughness, achieving a balance between electrical properties and surface morphology.
4Productivity
If APCVD process is used to deposit FTO films, then the process can be performed in-line on float glass, but the films exhibit high surface roughness and defect rates that limit application to low-E windows and some PV applications
Solution Approach 1:
The patent replaces the chemical vapor deposition (APCVD) process with physical vapor deposition (pulsed DC sputtering). This substitution of deposition mechanism eliminates the high roughness and defect issues inherent in APCVD while maintaining in-line processing capability. The sputtering process produces smoother films with fewer defects suitable for high-end applications.
Solution Approach 2:
The patent optimizes sputtering parameters including power density (0.5-2 W/cm²), oxygen partial pressure (0.1-1 Pa), substrate temperature (200-450°C), and deposition rate (1-10 nm/min) to achieve smooth, low-defect films. These parameter optimizations enable the sputtering process to produce high-quality films suitable for touch applications and high-end PV while maintaining productivity through in-line processing.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The resulting AZO films exhibit high conductivity, low optical absorption, thermal stability up to 450°C, and improved chemical durability, with resistivity and Hall mobility optimized for efficient light collection and extended panel lifetime.
Implementation Method 1
Sputtering is in use or under investigation for producing aluminum-doped zinc oxide (AZO) and gallium-doped zinc oxide (GZO) films for industrial application
Implementation Method 2
The development of AZO films using pulse DC sputtering at temperatures above 325°C, resulting in a columnar grain structure with small lateral grain size
Implementation Method 3
The lowest reported resistivity is on the order of 2.0×10−4 Ohm·cm, and the highest Hall mobility of 60 cm2V−1s−1
Implementation Method 4
the highest Hall mobility of 60 cm2V−1s−1
Implementation Method 5
An increase in the free electron concentration leads to higher conductivity. However, it also increases absorption in the infrared (in lower frequency radiation) as the plasma frequency (the frequency above which materials become transparent to EM radiation in the Drude model) increases
Data Source
AI summary
Disclosed are AZO films deposited on a transparent substrate by pulse DC using an oxide target with a composition in the range 0.5-2 wt % Al2O3, desirably at temperature above 325° C., resulting in films showing columnar grain structure with columns extending from the top to the bottom of the film, and small lateral grain size (less than 70 nm from substrate to top of film). The film has low resistivity at less than 10 Ohm/square at a thickness less than 400 nm, resistivity is desirably unchanged by annealing at temperatures of up to 450° C.