Seiko Epson Corp of Japan and JSR Corp of Japan have succeeded in making a low-temperature poly-Si thin-film transistor (TFT) using inkjet technology. The technology is intriguing because it provides all the advantages of organic TFTs, which are widely considered to be the next-generation Si TFT technology.
Si TFT used in liquid crystal display (LCD) panels and other applications is today manufactured with photolithography, requiring expensive equipment like vacuum pumps and cleanrooms. Process time is long and material utilization efficiency poor, boosting manufacturing cost. Organic TFTs, on the other hand, can utilize inkjet and other printing technologies to achieve low manufacturing costs, but because it uses a material other than Si, the resulting performance is not very satisfactory, including low electron mobility. Researchers have now developed a liquid Si material that makes it possible to manufacture Si TFTs using inkjet technology. The technology offers the promise of TFTs with performance sufficient to drive LCD panels or organic electroluminescent (EL) panels, and with prices lower than they are now.
The main component of the developed liquid is polysilane, with SiH2 bases connected in a normal chain. After coating the glass substrate with polysilane it is sintered at about 540Cto burn off the H2, leaving only the Si.
Polysilane has been known since the 1920s, but it was never used as a liquid because it was insoluble in organic solvents. Seiko Epson and JSR mixed cyclo penta silane (CPS), a base that polymerizes polysilane, to make it possible to dissolve it in an organic solvent. The boiling point of CPS is about 1,290C, and the boiling point of the toluene used as the organic solvent is about 110C, so both evaporate during sintering at about 540?.
Wetting Characteristics
The firms hope to go commercial with the technology within five or six years. According to Tatsuya Shimoda, deputy managing director and fellow, Corporate R&D at Seiko Epson, the key problem is that the wetting characteristics of the liquid on the glass substrate are poor.
The prototyped low-temperature poly-Si TFT shows an electron mobility of 6.5cm2/Vs, which is still insufficient in comparison to standard low-temperature poly-Si TFTs (see Fig). As Seiko Epson's Shimoda said, "The developed liquid shows extremely unusual wetting characteristics. When we tried coating it with an inkjet, control stability was bad and we ended up with some films about 300nm in thickness."
When the same liquid is used to make spincoated TFTs electron mobility rises to a high 108cm2/Vs, and switching characteristics are about the same as conventional TFTs. Spincoating requires a downstream patterning process using photolithography, however, so that inkjet printing is advantageous when it comes to cost in actual use.
Liquid Discharge 1fL
In April 2006, SIJTechnology Inc of Japan began offering prototype manufacturing and ink evaluation services.
With the inkjet technology from SIJTechnology, the ink discharge is only 1fL (femtoliter; femto is 10-15), a volume only 1/1,000th or less of standard systems. This is equivalent to a droplet diameter of 0.5um, and means that inkjet technology can now handle very fine patterns and 3D structures impossible with conventional inkjet technology implementations.
Kenshu Oyama, president of SIJTechnology, said, "Prior art could only handle line widths down to 50um, but our technology can go as low as about 3um." The technology can be used to form 3D structures because of the small droplet diameter, which ensures they dry immediately after contact.
by Takuya Otani
(July 2006 Issue, Nikkei Electronics Asia)
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