By Stuart Knight

High-quality process monitoring will become increasingly important as PV manufacturers drive towards grid parity, forecasts global control equipment group HORIBA.

HORIBA is one of the world’s leading developers of analysis and control products. Its devices are used across the complete spread of thin-film photovoltaic manufacturing processes as well as for conventional crystalline silicon PV making.

Analysis improves processes, speeds up throughput and increases yield, cutting production costs and maximizing cell efficiency. So HORIBA expects these technologies to be adopted ever more widely as the drive to produce more efficient solar cells at lower costs continues.

“Long term, demand for greater efficiency will grow, even up to and beyond grid parity,” says HORIBA’s European Division Manager Stuart Knight. “PV cell manufacturers need to control costs to compete for OEM and end user business. They also need to respond to demand from customers for more efficient products. These factors will drive manufacturers to search for ways to enhance production systems and eliminate waste. Control equipment is vital in this area. So we see it as becoming an ever more important factor in the PV manufacturing industry.” HORIBA points out that the forces currently shaping the developing PV manufacturing industry will be accelerated if political pressures are brought to bear, especially if the U.S. makes a major commitment to green energy.

In this article, the vast array of control equipment now available to PV cell manufacturers, covering conventional and thin-film processes, is presented.

For thin-film processes, HORIBA users typically apply its sensitive analysis equipment, especially a range of products based on the company’s leading-edge film thickness and film characterization technology.

Meanwhile, a suite of fluid control devices--also used in thin-film processes--an make a significant contribution to streamlining and developing new crystalline silicon production lines.

Even for conventional crystalline silicon manufacture, accurate measurement and control is crucial in setting up new production processes or streamlining existing ones. HORIBA’s background in scientific analysis is encouraging research and development departments to install its high-specification analysis tools to work on developing more efficient crystalline silicon PV making processes.

Film Deposition Process Control and Thin-film Analysis

Film deposition process control and thin-film analysis equipment lets users quickly evaluate and analyse film characteristics, including thickness, composition and degree of crystallization, to improve cell quality management and yield. Measurements are initially used to prove a process, typically offline in a lab using stand-alone inspection systems. When a process is proven, inline inspection systems can be used in the production process.

Products in this group are designed to support the formation of ideal films for solar cells by measuring the thickness and degree of crystallization in each film layer, and selecting the optimum deposition conditions. This enables effective cell quality management, and promotes more stable conversion efficiency, says HORIBA.

From the initial research and development work to production, HORIBA provides products for all thin-film processes. The UVISEL spectroscopic ellipsometer is a table-top system that achieves high accuracy and sensitivity in the measurement of film thickness and optical properties. Other instruments used for measuring material properties are an automatic film analyser for routine film thickness and optical constant measuring in both thin-film silicon and sensitized dyes production, and a glow discharge optical emission spectroscopy device for crystalline silicon and CIGS applications.

For quality control during PV production, HORIBA has responded to market demand for high performance with the new FF-2000 Series. It can perform non-destructive analysis of full-sized substrates across all thin-film processes thin-film silicon, CIGS and sensitized dyes. It features XY mapping and metrology tools. Sensors include a spectroscopic ellipsometer, a Raman spectroscopic analyser, a spectroscopic reflectometer, an X-ray Fluorescence (XRF) head and a four-point probe.

The accurate endpoint detection and plasma condition control is a vital part of homogeneous product quality in thin-film processes. In-situ process control is offered by monitoring plasma in the CVD chamber.

Fluid Control

Fluid control devices are used to control flow, pressure and chemical concentration. HORIBASTEC is the largest mass flow controller supplier in the world and has an international reputation for developing new fluid control technologies, and produces a wide range of flow, pressure and concentration control components. Accurate and repeatable results in this area are essential to the development and manufacture of solar cell technologies. Improved yield and quality allow better control over conversion efficiency.

HORIBA has developed a dedicated digital mass flow controller for solar cell manufacturing processes, the SEC N100 Series, which can be applied across all PV making processes for crystalline silicon, thin-film silicon, CIGS and sensitized dyes. It controls the flow of both process gases and cleaning gases. Multi-gas and multi-range functions reduce overall costs, for example by decreasing flow controller inventory.

The series provides analog/digital, DeviceNet and PROFIBUS communication functions that are compatible with solar cell manufacturing equipment. A high-end version offers high precision and high-speed response with an extensive line-up of models covering flow ranges from 1/10 of a cubic centimeter per minute up to 200 liters per minute. Multi-gas, multi-range functions let the user change the type of gas and the full-scale flow volumes. Dedicated software makes this easy to do without removing the mass flow controller from the gas panel or pipe.

Other fluid control equipment covers liquid and vapour delivery with a compact baking system and a bubbling system, both used in crystalline silicon and thin-film silicon PV processes, a residual gas analyser used in thin-film silicon and dye-sensitized PV processes, a gas concentration monitor used in thin-film silicon PV manufacturing and a liquid auto refill system for crystalline silicon and thin-film silicon PV manufacturing.

How Else Can Control Equipment Help?

There are four other areas where control and monitoring equipment can enhance the manufacturing process--chemical solution monitoring, impurity control, material analysis and water analysis/waste water monitoring.

Chemical Solution Monitoring

Chemical solution monitoring optimizes processes and increases cell supply stability and conversion efficiency. Real-time monitoring of chemical solution concentrations reduces waste. HORIBA’s chemical solution concentration monitors support the optimization of solar cell manufacturing processes through real-time measurement of chemical solution concentrations in cleaning and etching processes. This enables greater accuracy in chemical-based texture formation and the removal of impurities. That can contribute dramatically to improved conversion efficiency in crystalline silicon solar cells.

Alongside its CS-100 Series chemical solution monitors for components such as IPA/KOH or HF/HNO3, HORIBA has a range of other products suitable for chemical solution monitoring’s dissolved ozone/H2O Monitor, hydrofluoric acid monitor, HF/HCI concentration monitor, low concentration HF/HCI/NH3 monitor, carbon sensor conductivity meter, resistivity meter and IPA gas concentration monitor.

Impurity Control

Precise impurity control in raw materials and manufacturing processes increases the quality of solar cells. Solar grade silicon (SOF-SI) used in solar cells must have carbon and oxygen concentrations of just a few mass parts per million. Where manufacturing methods use graphite receptacles, carbon and oxygen concentrations must be accurately analyzed and strictly controlled. In other words, impurity control is essential in both material and manufacturing processes. HORIBA has a range of elemental analysis tools that provide high precision, repeatable analysis of carbon, oxygen, hydrogen and other elements--vital to quality control in solar cell manufacturing processes. Key products in this area are carbon analyzers and oxygen/nitrogen analyzers as well as hydrogen analyzers, an ICP emission spectrometer, energy dispersive X-ray analyzer and X-ray analytical microscope.

Material Analysis

Analysis and evaluation of solar cell components improves cell life and durability, HORIBA points out. It says this is true across materials ranging from semiconductors to dye-sensitised. Because solar cells comprise a variety of materials, there are a number of variables affecting cell life. Effective structural analysis and evaluation of component materials, including evaluation of optical characteristics, helps ensure stability in electrodes, electrolytes and dye-sensitised. HORIBA devices used for this material-oriented approach--such as its Raman spectroscopy system and photoluminescence spectrophotometer--contribute to increased conversion efficiency as well as improved life and durability of cells.

Other products suitable for material analysis are the near-infrared spectrofluorometer, energy dispersive X-ray analyser, cathodoluminescence spectrophotometer, fluorescence lifetime system, X-ray analytical microscope and glow discharge optical emission spectroscopy.

DI Water Analysis and Waste Water Monitoring

Recycling the large quantities of water used in solar cell manufacturing enables the preservation of valuable resources and the environment, HORIBA says. Water used for cleaning can be reused by building recycling systems. Monitoring waste water and disposal processes is also essential to protect the water environment. HORIBA has developed a wide range of advanced analysis and measurement technologies suitable for production processes involving pure and ultra-pure water, but their instruments are also used in recycling, waste water treatment and monitoring.

The company’s main products for waste water monitoring are the automatic total nitrogen/total phosphorous monitoring system and the fluoride ion monitor. These are applied across all PV manufacturing processes. Other products suitable for waste water monitoring across all manufacturing processes are an automatic COD monitoring system and the free fluoride ion monitor. Products for measuring water purity in cleaning processes across all PV manufacturing processes are the portable DO monitor, 2-channel resistivity meter and 2-channel conductivity meter.

HORIBA has a worldwide service network providing support in every significant PV manufacturing location across the globe.

Stuart Knight is European Division Manager at HORIBA (www.HORIBA.com).

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