LONDON Ė A team of researchers from the University of California Berkeley has a paper at the International Electron Devices Meeting (IEDM)that describes how to use an inkjet printer to print microelectromechanical system (MEMS) switches and proposes their use for flexible display backplanes.
The switch is of the electrostatically-operated cantilever beam type whereby a voltage on a gate electrode attracts the beam to close with a contact. The removal of the voltage allows the beam to spring back breaking contact. The total area of the device is about 1Ėmm by 1-mm although there is scope of scaling the manufacture.
Traditionally active-matrix switching for display backplanes has been done by amorphous silicon thin-film-transistors, although recently organic polymers have also been applied to the task. Both types of TFT suffer from some leakage current in the off-state and some resistance in the on-state. In their paper the UCB authors point out that their inkjet-printed MEMS switch has near ideal characteristics for the application of zero off-state leakage and on-state resistance of about 10 ohms.
The switch is made by repeated inkjet printing passes to build up conductive and insulator layers. A layer of polymethylmethacrylate (PMMA) is used as a sacrificial layer to separate the cantilever from the gate contact below.
The gate, drain and source electrodes are printed on to an oxidized silicon wafer using a silver particle ink. Cross-linked poly-4-vinylphenol (PVP) is baked at 180 degrees C for 30 min to form an insulating gate dielectric layer. The drain electrode is thickened using in the contacting region by multiple print cycles. The PMMA thickness is about 2-microns.
The source beam is about 100-micron wide by 550-micron long and printed on top of the sacrificial PMMA. The thickness of the beam is also built up by repeated printing at about 450-nm per printed later. The researchers report that they found the beam needed to be greater than 1.6-microns thick to avoid failure. The last manufacturing step is the selective removal of the PMMA using acetone at 50 degrees C.
The researchers conclude that the device has moderate switching delay of approximately 10 microseconds and that with dimensional scaling the performance should be suitablefor video-rate active-matrix display applications. The low thermal process budget also makes inkjet-printed MEMS technology a potential candidate for large-area systems on glass.
IEDM is taking place in Washington DC from Dec. 5 to 7.