What is Metrology?

Metrology is the set of tools and techniques that allow control of the deposition and fabrication processes. Metrology (measurements) is an integral part of any manufacturing process, and of any quality control procedure, where the characteristics of the final product are compared against a parameter benchmark.

Each of the steps in the fabrication of a photovoltaic panel is carried out under very different conditions and requires the use of different characterization tools, depending on the parameter to be measured, the environmental conditions, and the geometry of the problem. For example, the techniques and toolset used to control or characterize a fabrication line based on glass substrates (batch process) are different than those used for flexible substrates in a roll-to-roll configuration, where the deposition is continuous over several kilometers of roll.

Metrology tools can be used after a single layer or complete device is finished, and can be also be used during the deposition process to monitor various parameters providing real time information to control the process.

Benefits of Metrology in Manufacturing Processes

The benefits of metrology used in a production environment can be summarized into two main categories: production cost reduction and quality assurance.

The determination of key parameters at different stages of a solar panel manufacture and the ability to maintain their values between pre-established limits is crucial to assure the end-user that the finished photovoltaic panel will provide the promised output power.

CIGS thin film

The use of metrology tools allows for a rational utilization of resources during fabrication and quality control after the product is finished.

Determination of layer composition, thickness and uniformity as the material is deposited may be used, through a feedback mechanism, to control the deposition “knobs” (power to material sources, speed of the substrate under the sources, temperature of the sources, etc.) to maintain the values within the acceptable manufacturing limits that will result in an efficient device. A lack of these metrology-based controls mean that if the parameters drift away from the optimal values, and outside the acceptable limits, the time, materials and energy used for the process will be wasted.

After the solar cells/modules are fabricated, metrology is needed in order to characterize their performance. By measuring the current and voltage characteristics obtained under illumination, it is possible to determine the maximum power the devices can provide under different levels of illumination, and how efficiently they convert sunlight into electricity. By measuring the current generated when filtering the light, it is possible to determine which parts of the solar spectrum are better utilized, and if particular layers of the device are performing in a sub or super standard way.

Of course, metrology is integral to the whole scientific research and development process, where the use of different techniques and correlation of results allows for an understanding of the influence of deposition parameters on the properties of the finished layers, and the subsequent implementation of modifications in the fabrication process that allow for improved solar panels with higher efficiency, longer lifetime and better performance.

Performance Measurement Standards

Establishing standard performance measurements to fairly compare different modules and PV types

  • The Performance Measurement Standards project is driven by the PV industry, including CIGS manufacturers and equipment suppliers. PVMC is working directly with the manufacturers in order to test and down-select light-soaking and temperature coefficient determination protocols, as well as test modules under low irradiance conditions, in order to better predict outdoor performance ratios. The final objective is to collaboratively design a test protocol that allows CIGS manufacturers the ability to predict and rate module performance and allow a significant comparison between different CIGS technologies, c-Si and CdTe modules.

Metrology for Manufacturing

Establishing in-line metrology for thickness and composition control for stack layers

  • The throughput of a CIGS-based PV manufacturing line is tied to the capability to control the process so as to predict the performance of the final product. PVMC seeks to enable low-cost and high-yield manufacturing via an integrated process control system. This system should be able to monitor composition and thickness of the majority of the layers that compose the stack. Goals of this project are system demonstration, feasibility evaluation of process control analytics and, eventually, the deployment of a full-scale demo system at a manufacturing facility. For this purpose, a monitoring system based on spectroscopic reflectometry installed in the sputtering and evaporation chambers available at the Pilot Line of the PVMC facility in Halfmoon, NY will be used to gather in-line data from the deposited layers. Spectroscopic ellipsometry is being used to develop optical models that, together with the database, will form the base of the monitoring/process control software.