Lining up flexible electronics
Scientists in the USA have developed a dual-tone photo-resist for novel multiwavelength photolithography. This is said to overcome the problems of poor registration and alignment when patterning multiple layers on polymers and metal foils for flexible electronics.
A team at the University of Texas has engineered a photoresist that deposits two patterns instead of one, enabling manufacturers to control the alignment between them, as well as save money and time in processing steps.
The resist works by capturing negative and positive tone latent images simultaneously at different wavelengths by a single combination of light exposures. The first image (positive tone) is then developed and etched, and the negative latent image is later brought out by flood exposure to UV light and heating from 80-120ºC.
Conventional photolithography is a repetitive, sequential process using single wavelengths of light – for multi-layer structures it is only possible to expose, develop and etch one pattern at a time, requiring a different photoresist each time.
George Powch of Versatilis, Shelbourne, USA, the company that is licensing the photoresist from the University of Texas, notes, ‘Conventional photolithography is fine for semiconductor wafers as you have good registration and alignment between process steps. If you try to do anything on a flexible substrate, you have a devil of a time maintaining alignment as the underlying material shrinks, stretches, etc’.
This is because polymeric and metal foil substrates vary in coefficient thermal expansion, dimensional stability and in their reaction to semiconductor patterning, unlike rigid silicon wafers.
The dual-tone resist is made using commercially available materials – novolak polymer and DNQ to create the positive tone, with a 4-4’-bis(azidophenyl) sulfone crosslinker added for the negative tone.
Dr Grant Wilson at the University explains that thermal activation is essential to bring out the negative pattern, ensuring it is not activated by wavelength alone. This is because commonly used dry plasma etching works by exposing the material to short wavelengths, and so etching the positive pattern would inadvertedly activate the negative areas.
On a wavelength
Versatilis originally developed the idea of multiwavelength photo-lithography as part of its work for the USA’s Defense Advanced Research Projects Agency.
Powch expands, ‘[The Agency’s] concept was to make high performing transistors on large area flexible substrates for applications like phased array antennae – they could be rolled up into soldiers’ backpacks. Printing is a lower resolution process, so you get wide, low mobility semiconductor channels, limiting the applications to low-switch speed devices.
Photolithography is cheap at micron-scale – if existing exposure tools can be made to handle flexible media, the capital investment for high performing flexible electronics goes down’.
Versatilis has also created a proprietary coloured mask that combines the different wavelength filters and patterns into one, removing the need to sequentially swap different wavelength filters in the exposure tool.
Powch says the technology is now available for interested partners to take forward, eventually into roll-to-roll manufacturing and perhaps even to the nanoscale. He adds, ‘This technique would be good for top gate, but not bottom gate, transistors. We [also] have some novel, proprietary top gate TFT structures that lend themselves particularly well to this process’.
The technique could also be used to reduce masking and photoresist steps for fabrication on semiconductor wafers.
Professor Henning Sirringhaus, of the Optoelectronics Group at the University of Cambridge, UK, notes that it is ‘an interesting technique, which is inspired by the recently developed self-aligned imprint lithography. The latter has been devised by Hewlett Packard Labs to overcome the costly aligning process by using a 3D mask impressed with multiple patterns.
Sirringhaus adds, ‘The [Versatilis] technique could potentially overcome the distortion and registration issues. [However], with poly-crystalline silicon [circuits] there are other issues such as process temperature limitations.’
Further information: Versatls
Materials World Magazine, 01 Sep 2009
A team at the University of Texas has engineered a photoresist that deposits two patterns instead of one, enabling manufacturers to control the alignment between them, as well as save money and time in processing steps.
The resist works by capturing negative and positive tone latent images simultaneously at different wavelengths by a single combination of light exposures. The first image (positive tone) is then developed and etched, and the negative latent image is later brought out by flood exposure to UV light and heating from 80-120ºC.
Conventional photolithography is a repetitive, sequential process using single wavelengths of light – for multi-layer structures it is only possible to expose, develop and etch one pattern at a time, requiring a different photoresist each time.
George Powch of Versatilis, Shelbourne, USA, the company that is licensing the photoresist from the University of Texas, notes, ‘Conventional photolithography is fine for semiconductor wafers as you have good registration and alignment between process steps. If you try to do anything on a flexible substrate, you have a devil of a time maintaining alignment as the underlying material shrinks, stretches, etc’.
This is because polymeric and metal foil substrates vary in coefficient thermal expansion, dimensional stability and in their reaction to semiconductor patterning, unlike rigid silicon wafers.
The dual-tone resist is made using commercially available materials – novolak polymer and DNQ to create the positive tone, with a 4-4’-bis(azidophenyl) sulfone crosslinker added for the negative tone.
Dr Grant Wilson at the University explains that thermal activation is essential to bring out the negative pattern, ensuring it is not activated by wavelength alone. This is because commonly used dry plasma etching works by exposing the material to short wavelengths, and so etching the positive pattern would inadvertedly activate the negative areas.
On a wavelength
Versatilis originally developed the idea of multiwavelength photo-lithography as part of its work for the USA’s Defense Advanced Research Projects Agency.
Powch expands, ‘[The Agency’s] concept was to make high performing transistors on large area flexible substrates for applications like phased array antennae – they could be rolled up into soldiers’ backpacks. Printing is a lower resolution process, so you get wide, low mobility semiconductor channels, limiting the applications to low-switch speed devices.
Photolithography is cheap at micron-scale – if existing exposure tools can be made to handle flexible media, the capital investment for high performing flexible electronics goes down’.
Versatilis has also created a proprietary coloured mask that combines the different wavelength filters and patterns into one, removing the need to sequentially swap different wavelength filters in the exposure tool.
Powch says the technology is now available for interested partners to take forward, eventually into roll-to-roll manufacturing and perhaps even to the nanoscale. He adds, ‘This technique would be good for top gate, but not bottom gate, transistors. We [also] have some novel, proprietary top gate TFT structures that lend themselves particularly well to this process’.
The technique could also be used to reduce masking and photoresist steps for fabrication on semiconductor wafers.
Professor Henning Sirringhaus, of the Optoelectronics Group at the University of Cambridge, UK, notes that it is ‘an interesting technique, which is inspired by the recently developed self-aligned imprint lithography. The latter has been devised by Hewlett Packard Labs to overcome the costly aligning process by using a 3D mask impressed with multiple patterns.
Sirringhaus adds, ‘The [Versatilis] technique could potentially overcome the distortion and registration issues. [However], with poly-crystalline silicon [circuits] there are other issues such as process temperature limitations.’
Further information: Versatls
Materials World Magazine, 01 Sep 2009
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