Design and Realization of Capacitive Sensor Readout Circuit in LTPS Technology

• Main Authors: Lin, Yu-Ta, 林佑達
• Other Authors: Ker, Ming-Dou
• Format: Others
• Language: zh-TW
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/33089706727969712311

碩士 === 國立交通大學 === 電子研究所 === 99 === Low temperature polycrystalline silicon (LTPS) thin-film transistors (TFTs) have been widely investigated as a material for portable systems, such as digital camera, mobile phone, personal digital assistants (PDAs), notebook, and so on, because the electron mobility of LTPS TFTs is about 100 times faster than that of the conventional amorphous silicon TFTs. Furthermore, LTPS technology can achieve slim, compact, and high-resolution display by integrating the driving circuits on peripheral area of the display. This technology will also become more suitable for realization of System-on-Panel (SOP) applications. With SOP technique, circuits for various functions can be integrated on glass substrate to become value-added displays. Also, the input display technology creates opportunities for new applications such as a scanner for recording of text or images for on-line shops and touch-sensing circuits for detecting the position of finger or pen. Recently, integrating touch panel into glass substrate has attracted much attentions because of the aforesaid advantages. Touch panels used in mobile applications are mainly resistive or capacitive. Although resistive touch panel can achieve low cost with rarely malfunction, it has some drawbacks including serious glare, low transmittance, and single touch only. On the other hand, capacitive touch panel can realize multi-touch functionality easily which allows user to operate information instruments more intuitively. Because most LTPS TFTs are based on excimer laser crystallized poly-Si, random orientation of poly-Si grains, grain size variation, and incomplete termination of grain boundaries would lead to a quite large threshold voltage variation of TFT device which contributes to serious impact on the accuracy of analog circuits. In this work, a new readout circuit for capacitive sensor on glass in LTPS fabrication process has been proposed. The switch capacitance (SC) technique is used to compensate the threshold voltage variation effect. Different values of the sensed capacitance can be judged by ADC. In this way, the overall resolution for touch panel can be enhanced by interpolation method.