Low-Temperature Flexible Sensors Use Ink-Jet Patterning
Anne L. Fischer
Flexible electronics are used in supermarket labeling and in identification tags and could be used in large sensors for scanning cargo or packaging. As flexible electronics find their way into new applications, manufacturers are faced with problems that include high-cost processing, brittle materials, large-area scaling and more.
Active-matrix amorphous silicon image sensor arrays are the workhorses for large-area sensor applications such as x-ray imaging. These sensors include a layer that is usually 1 μm thick, which is twice as thick as the thin-film transistor backplane. The problem is that the thicker the sensor layer is, the more susceptible it is to cracking and breaking. Currently these arrays all are fabricated on glass, but a group of researchers at Palo Alto Research Center in California, has been working with sensor arrays fabricated by plasma-enhanced chemical-vapor deposition on glass and on flexible polyethylene naphthalate substrates. The amorphous silicon sensor structure consisted of a bottom-metal electrode followed by a 70-nm n layer, an i layer, a 10-nm p layer and a 55-nm indium-tin-oxide top electrode. According to William S. Wong, senior member of the research staff at the center, the goal was to get the backplane to perform to specifications normally seen for high-temperature devices on glass.
Two cross-sectional views of the sensor stack are shown (the top one created by electron microscopy). TFT = thin-film transistor.
An added challenge was to keep the sensor’s dark current from increasing as the thickness decreased. Tse Nga Ng, postdoctoral researcher at the center, indicated that, because thinner sensor films could lead to increased current leakage, the problem became finding a film thick enough so as not to jeopardize performance but thin enough to reduce mechanical stress. The sensor performance was found to be a strong function of the i-layer thickness and deposition temperature and was independent of the substrate (flex or glass) used. The group evaluated the process with varying i-layer thicknesses at different temperatures and found that the optimum thickness is ∼600 nm at a temperature of 150 °C. At this thickness and temperature, the leakage current is acceptable, and the loss of photogenerated charge from carrier recombination at defects is minimized.
A flexible amorphous silicon sensor array was fabricated at 150 °C and patterned by ink-jet digital lithography on polyethylene naphthalate. Photo provided by Dupont-Teijin.
Concurrently, they wanted to find a low-cost jet-printed patterning process that would define device structures on the multiple layers. Photolithography is the traditional patterning process, but it has suffered from misalignment problems. Conventional photomasks are rigid and align poorly to distorted flexible substrates. The distortion is the result of the thermal expansion coefficient mismatch between the device layers and the flexible substrate. By replacing the traditional photolithography method with ink-jet printing, the researchers locally aligned electronic masks before the print-patterning process (by digitally imaging alignment marks on the flexible backplane surface) and improved layer-to-layer registration in fabricating the flexible sensor array.
This current research is in its last year as a National Institute of Standards and Technology Advanced Technology Plan project in which the researchers worked with Varian Medical Systems of Palo Alto to develop novel sensor materials. The next step in this research is to develop large-area image sensors for cargo scanning, with the goal being to reduce the cost of large-area panels that will be used in US ports of entry.
Applied Physics Letters, Aug. 6, 2007, 063505.
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