Sussex-based Powerlase is making lasers that are expected to have an impact on the production of organic light emitting diodes (OLEDs).
Laser annealing is used in OLED production to convert easily-deposited amorphous silicon into polysilicon which makes better transistors because carrier mobility is higher.
“One of the processes our green lasers can do is thin-film silicon annealing to make low-temperature polysilicon,” Powerlase’s applications engineering manager Paul Harrison told
EW.
According to Harrison, LCD makers tend to use UV excimer lasers to do the conversion. “These are not as good as our green lasers,” he said.
“UV is typically absorbed into the top 25% of the 50-100nm silicon layer and the process relies on the layer’s thermal properties to push heat all the way through. This is a slow process. Green lasers are absorbed through the full depth of the layer.”
As well as being quicker, Harrison claims the equipment is cheaper and that crystals grown with green light are bigger leaving fewer crystal boundaries to inhibit carrier flow.
“With excimer lasers, growth is upwards,” he said. “With green lasers it is sideways, so the crystal can be larger than the beam.”
His argument is that OLEDs need better semiconductors because they are current driven and need five transistors per pixel, compared with LCDs that are voltage driven and have only one or two transistors per pixel.
Good quality polysilicon will also eventually allow logic circuits to be deposited around the edges of the display glass.
Powerlase, a spin-out from London’s Imperial College, has been making lasers in Crawley for the display industry since 2001.
Its intellectual property is in acousto-optically Q-switched side-pumped neodymium-doped yttrium aluminium garnet (YAG) lasers.
Pump energy comes from rows of semiconductor lasers and the output is 1,064nm infra-red, which can be converted to 532nm green using a frequency-doubling crystal.
“We do the highest power laser of its type,” claimed Harrison. “Copper and silicon both process better with green lasers.”
UV at 355nm can also be produced – originally the firm was set up to make extreme ultra-violet (EUV) sources for next-generation lithography – a market that has yet to appear.
“The biggest application we have is high-speed laser patterning of indium tin oxide for plasma display panels,” said Harrison.
“For PDPs, it is safe to say we are the market leader. Our technology is used to manufacture 60% of the world’s plasma screens. It has been adopted by Samsung and LG.”
In plasma displays, Powerlase competes against a wet etch process which patterns an entire 42inch panel in one step.
“Wet etch uses a huge chrome-on-glass mask that has to be replaced every six months,” said Harrison.
Instead, the laser process flashes a 40ns multi-MW laser pulse through a mask to expose one 1x1mm patch of ITO every flash. Up to eight lasers are used.
This might sound slow until Harrison explains that it is quicker then wet etch. “It pulses at 6kHz so it takes a few tens of seconds to write a 42inch panel,” he says.
Yield also improves with laser etch, claims Powerlase. Figures filtering back suggest wet etch had 80-85% yield compared with more than 97% for laser etch when a PDP company ran the two processes side-by-side during evaluation – although Harrison concedes wet-etch may be achieving more than 90% today.
To project the 1x1mm image, optics convert the Gaussian laser output to a top hat profile that illuminates the 3x3mm mask. This is then focussed down to produce the 1x1mm image.
PDP panels are mature technology, but Harrison says they are far from obsolete.
“Manufacturers need to push the cost down. We are looking at ways to pattern other conductive layers using lasers, and at patterning tin oxide which could be a cheaper alternative to ITO,” he says.