Polysilicon-on-paper for printing low-cost circuits
Polysilicon can be made on paper and other low-temperature substrates by a process developer by researchers in The Netherlands and Japan. The same process has yielded CMOS transistors on a wafer.
Silicon has already been used as an alternative to organic and oxide semiconductor materials in inks for printing transistors on plastic substrates using the liquid cyclopentasilane (CPS).
However, getting CPS to turn into silicon, requires annealing at at least 350C which precludes the use of cheaper plastic (polyethylene terephthalate (PET, used in lemonade bottles) or polyethylene
naphthalate (PEN)) or paper substrates.
The actual process is UV curing to produce polysilane, followed by 350C annealing to form amorphous silicon.
After this, an excimer laser can be used to convert the a-Si to polysilicons, which has a higher mobility than a-Si.
The researchers, from Delft University of Technology and the Japan Advanced Institute of Science and Technology, found that they could go directly from UV-cured CPS to polysilicon using a single 28ns flash from a 308nm XeCl excimer laser without the annealing step.
With the team’s process, which has to be done in an stmosphere without oxygen, the maximum required temperature is 150C – compatible with the cheaper substrates. The laser processing temperature is actually lower, but 150C is needed to bake the photoresist when making transistors from the film.
Deposited on paper (a rather special paper for printed electronic – PowderCoat HD) it yields a 200nm film which has remained healthy over at least nine months.
Rather than use polysilicon-on-paper to prove transistors can be made from its polysilicon, the team used the oxidised surface of a silicon wafer.
Both p and n-channel transistors were made, achieving carrier mobility around 3cm2/Vs and on-off ratios of 104.
Transformation using multiple lower power laser pulses pushed mobility as high as 22cm2Vs, fast enough for UHF-RFID tags according to the team, although on-off ratio dropped to 102.
So far no way to combine high mobility and high on-off ratio has been reported.