Development of CVD technology and the pursuit of thin-film solar cells

Visiting the PLK plant in Ottawa, Illinois, in 1998

Development of CVD technology and the pursuit of thin-film solar cells

The beginning of CVD technology development

It started with anti-fog glass for airplane cockpits

Around 1940, countries around the world were racing to develop propeller aircraft. One of the biggest problems was that cockpit glass would fog up. For a pilot, foggy cockpit glass could be deadly. As a state-of-the-art glass manufacturer, Libbey-Owens-Ford Company (LOF; currently Pilkington North America Inc.) was working to address this problem by developing a fog-resistant glass. The result was an innovative technology that coated glass with tin oxide. Because tin oxide conducts electricity, it would heat the glass when energized – and thus prevent fogging. This technology was patented in 1942.


What is the CVD (Chemical Vapor Deposition) process?

Different processes for coating glass were being explored around the world. The CVD process involves introducing gases onto the glass surface and then energizing it to create chemical reaction that produces a film coating. NSG employed a thermal CVD process that used thermal energy. This method was considered simpler and less costly than other coating methods.

CVD process mechanism
CVD process mechanism

Development of a low-resistance film with the CVD process

Since the 1940s, many different approaches to coating glass have been explored around the world, resulting in methods such as the vacuum deposition method, the sputtering method, and the CVD process. While the vacuum deposition method created a low-resistance, high-conductivity indium tin oxide (ITO) film. Because indium was expensive, difficult to obtain in stable quantities and vulnerable to bending, there were demanded for a cheaper film coating technology.
Against this backdrop, in the late 1970s, NSG’s R&D department began to explore a CVD process that would create a film under atmosphere pressure conditions that had the low resistance of ITO without using indium.

Development of testing equipment for coating using the CVD process

NSG believed that coating condition was an important factor to increasing the conductivity of the generated film. Because it was difficult to conduct experiments to optimize the condition using existing equipment, the Company first decided to develop testing equipment that would allow it to freely change the conditions for coating.
In 1980, the testing equipment was completed. Despite repeated experiments, however, engineers were unable to reduce the electric resistance value. NSG engineers attended a lecture given by engineers visiting from the United States and learned about a unique gas separation coater. After obtaining more detailed information, NSG used this process with its testing equipment but was still unable to reduce the electric resistance value.
After repeated trial and error, engineers realized that there were disparities in the CVD conditions and five years after the start of development, finally achieved their goal for conductivity (10-4Ωcm).

Increased demand for thin-film solar cells and the launch of full-fledged operations

Around 1980, thin-film solar cells became the focus of much attention. In 1982, NSG provided a 10cm x 10cm square sample at the request of a U.S. company. About a month later, NSG suddenly received an order for a thousand 15cm x 15cm square pieces, and after successfully fulfilling the order, received another order for 30cm x 30cm square pieces. Solar cells of this size could not be produced with the existing equipment.
Predicting an expansion of the thin-film solar cell market, NSG management decided to build manufacturing facilities at the Sagamihara Plant. Having focused on developing coating technologies, however, NSG did not have the manufacturing technology to coat the film in float glass process. After determining that there was not enough time to develop this equipment in-house, top management and the R&D team decided to install existing solar cell plate glass manufacturing equipment with a proven track record.

Installation of equipment at the Sagamihara Plant

After signing the contract, the equipment was installed at the Sagamihara Plant in 1985. Since 1980, NSG had been conducting repeated experiments and improvements of its coater, which resulted in a proprietary and superior configuration. Using the installed manufacturing equipment and remodeling it with the Company’s coater resulted in products that fulfilled customer demands in terms of both function and film thickness. Six years of R&D efforts by NSG finally produced results.
With manufacturing capability in place, NSG began product development utilizing a wide variety of CVD process coating technologies in 1986. A new customer placed an order with the Company to supply SnO2 boards for calculators, with a request to enlarge the size to 60cm x 120cm in the future. The Company had to work at full pitch to develop and improve the equipment necessary to manufacture these products.
However, numerous issues with the production line arose with the start of mass production. Each problem was resolved one by one by the persons in charge and finally, NSG was able to produce according to plan and the business for SnO2 glass for calculators gradually grew.

Online CVD equipment and the expansion of operations

The decision to install online CVD equipment

Online CVD installation team and PLK personnel (at PLK in the U.S.)
Online CVD installation team and PLK personnel
(at PLK in the U.S.)

In 1994, NSG’s management proposed to obtain the license for online CVD process from Pilkington USA (PLK, formerly LOF). To produce a SnO2 film at the Sagamihara Plant, sliced glass was reheated after coming out of the float furnace. However, with the online CVD process, the film was produced by applying CVD coating on hot glass inside the float bath, or while “online.” By coating glass online, the manufacturing process became more efficient and a stronger film could be produced. No of the Japanese companies had this manufacturing process.
It was not easy to obtain a technical license from PLK, however, in 1997, NSG was able to sign a licensing agreement in after meeting with the senior management at PLK.

Shortly after that, NSG formed the team for installation of online CVD, and in 1998, the team visited PLK’s Ottawa factory in the United States. PLK proposed that NSG install the latest CVD equipment that PLK was building and after many discussions, the installation was approved by the NSG Executive Committee.

Installation of online CVD equipment at the Chiba Plant

Because the production of large sheets of solar cell glass would require upgrading of the existing production equipment, it was decided that online CVD equipment would be installed at the Chiba Plant’s No.1 Furnace, which had demonstrated expertise in online development. Initially, many employees were reluctant to install CVD equipment in the float bath because of the risk that the entire production line would have to be halted if there was a problem in the float bath. However, the management realized that competing in a new market required taking risks and incorporating new initiatives. In 1998, online CVD equipment was installed at the Chiba Plant on schedule.

CVD inauguration ceremony
CVD inauguration ceremony

Float bath line and online CVD

Float bath line and online CVD
Online CVDA deposition apparatus is installed on the float bath. Chemical reactions at high temperatures deposit highly durable metal oxide with greater efficiency.

Overcoming difficult conditions

The online CVD equipment at the Chiba Plant began operation to produce test products for the customers. There were many problems with the early products, such as the transmittance, but with quality engineering, NSG was able to control CVD gas conditions and SnO2 film absorption to successfully begin delivery of products that fulfilled the required specifications in 1999.
Both NSG and the customer sensed great potential for solar cells and worked together to develop high-performance solar cells. Increasing the transmittance, reducing the reflectance as well as forming the ideal surface morphology of the SnO2 film continued to be technical challenges in the production of thin-film solar cells, but the two companies brought together their R&D expertise to overcome these issues and mass produce high-quality products.

Installed in six factories in five countries around the world

Installed in six factories in five countries around the world

Online CVD today

In 2006, NSG and PLK merged to form the NSG Group. Online CVD equipment can now be found not only at the Chiba Plant, but at six plants in five different countries around the world, working to fulfill the diverse demands of each region.
Glass produced with the online CVD process is used not only for solar cells, but also for Spacia® low-E vacuum glass, Pair Multi Super® multi-layer glass and other architectural glass products for buildings, apartments, homes and many other applications. Furthermore, NSG TEC™, with a transparent conductive oxide (TCO) film, offers many different commercial applications, such as refrigerated display cases and touch panels.

With environmental issues becoming increasingly important worldwide, the use of glass for solar cells and energy-saving glass products, produced by NSG Group, is expected to increase significantly in the future.
With continuous development of the CVD process, NSG Group successfully created a 0.7mm glass with CVD film in January 2018. NSG Group will continue to develop a diverse range of products using the online CVD film coating process to contribute to better lifestyles and the environment.