Prints of light

Electronic printing process could lead to transparent solar panels that are thin enough to be used as tint on windows. Siobhan Wagner reports.

A technique that allows electronic circuits to be printed on a wide array of surfaces could lead to new products such as transparent solar panels that are thin enough to be used as tint on windows.

The technology from Semprius, a University of Illinois spin-out, uses a rubber stamp-like method to bond circuits to substrates.

With this new approach, circuits are first formed on a semiconductor wafer using conventional wafer processing techniques.

Then a special wet etching chemical process slices chips off the surface of the host wafer. The chips can range in size from 10 to 100 times thinner than the wafer depending on the application.

‘We’re unaware of anyone who has sliced them in this way for the purpose of electronics,’ said one of the developers, Prof John Rogers of the University of Illinois.

A silicone rubber stamp picks up the chips and bonds them to a substrate coated with a thin polymer adhesive, which is used in conventional electronics.

But Rogers said there are ways to do the printing without an adhesive layer. ‘The silicone rubber has very low surface energy, so when it makes contact with the chiplet there is some adhesion there but it is very weak,’ he said. The intrinsic differences in the material properties between the rubber and the final device substrate, he said, allow transfers to occur without an adhesive layer.

The simple stamping method can transfer tens of thousands of individual silicon ribbons or device components, such as transistors and diodes, to a range of conventional or unusual substrates.

Rogers said a conventional substrate such as glass could be used for LCD displays. The process would allow transistors to be placed directly on to an LCD back panel, which could increase pixel response times for the display and eliminate motion blur.

He said one of the unconventional substrates his group is researching is rubber. ‘We have ways of integrating silicon devices on pieces of rubber, like a rubber band, so they can be stretched and formed in ways that enable you to integrate electronics within the body,’ he said. ‘We’re looking at a variety of biomedical applications.’

The choice of substrate is almost limitless because the fabrication of the silicon wafer is completely separated from the final device substrate. This means that high-temperature processing, high resolution and high registration lithography patterning can be used, as well as cleaning processes using strong acids and bases and other chemicals.

Rogers said that his team is still perfecting the printing process.

‘The two things we are concerned about are the yield and the registration,’ he said. ‘The yield is defined by the fraction of the chips picked up on the stamp and printed on the substrate. The registration is defined by how well the chiplets are positioned relative to one another in their printed state compared to the state they were fabricated on the source wafer.’

Rogers said with the tools they are using, yields are in the order of 99.9 per cent, but that figure needs to be improved for applications such as displays. ‘We are very encouraged because there are many ways we can improve it,’ he said.

The placement accuracy is currently about one micron. This, said Rogers, is good enough for all the applications including displays. ‘Yet we still need improved yield.’

Semprius has a number of working prototype displays it developed with a team of industrial partners that cannot yet be named. ‘They will go public in a couple of months,’ said Rogers.

The company has also developed a number of working prototypes for solar modules that are designed to reduce the cost per watt. The modules look similar to standard solar panels, but Rogers’ team at Illinois is exploring new ideas for the solar industry, such as transparent panels that are thin enough to be used as tint on windows.

‘Semprius is interested in these ideas, but in the short term the most immediate opportunity is creating a low-cost replacement for the solar panels people are already installing,’ said Rogers.