Metal-oxide (MO) thin-film transistors (TFTs), such as those based on indium-gallium-zinc
oxide (IGZO), have been widely used as integrated drivers and switches in flat-panel
displays. In addition, low-temperature processed MO TFTs enable the implementation of very
thin, lightweight, and ultra-flexible integrated circuits. However, donor-defects induced by
oxygen deficiency or hydrogen (H) in the channel region of a TFT can lead to variations and
negative shifts in the turn-on voltage (V
ON).The circuit design based on MO TFTs is sensitive
to variations in TFT parameters such as V
ON and mobility, which may lead to undesirable
reductions in noise margins for digital circuits and output offset in analog differential amplifiers.
Furthermore, the specific application of flexible circuits, su...[
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Metal-oxide (MO) thin-film transistors (TFTs), such as those based on indium-gallium-zinc
oxide (IGZO), have been widely used as integrated drivers and switches in flat-panel
displays. In addition, low-temperature processed MO TFTs enable the implementation of very
thin, lightweight, and ultra-flexible integrated circuits. However, donor-defects induced by
oxygen deficiency or hydrogen (H) in the channel region of a TFT can lead to variations and
negative shifts in the turn-on voltage (V
ON).The circuit design based on MO TFTs is sensitive
to variations in TFT parameters such as V
ON and mobility, which may lead to undesirable
reductions in noise margins for digital circuits and output offset in analog differential amplifiers.
Furthermore, the specific application of flexible circuits, such as the analog front-end (AFE)
system for bio-potential acquisition, is plagued by the hybrid integration of off-substrate
capacitors or resistors and large circuit footprints. This dissertation explores approaches to
address these issues.
Fluorination is investigated as a potential solution to passivate the donor-defects of MO
TFTs on the flexible substrate within a given thermal budget. The fluorinated IGZO TFTs
exhibit suppressed apparent short-channel effects and better uniformity of V
ON with reduced
negative shift after laser lift-off from a glass carrier substrate. The enhanced performance of
fluorinated TFTs is attributed to the ability of fluorine to resist against H-induced degradation.
Accompanying a reduction in process temperature from 400 to 300 ℃, the channel current
of IGZO TFT is found to change from one insensitive to the size of the conductive source/drain
(S/D) regions to one suppressed with decreasing size of the regions. The different behavior is
attributed to the distinct donor-species responsible for the formation of the thermally induced
S/D regions at the two process temperatures. Such sensitivity, often undesirable, can be reduced
by completing a donor drive-in thermal treatment before the patterning of the S/D electrodes.
Employing the developed low-temperature TFT technology, a range of digital and analog
circuit building blocks are designed, fabricated, and characterized. Owing to the improved
uniformity of V
ON after fluorination, 2-4 decoders consisting of fluorinated IGZO TFTs
exhibit higher gain, wider noise margins, more tightly distributed transition voltage, and larger
output swing. In addition, the enhancement characteristics of fluorinated IGZO TFTs, i.e., >
0, reduce the error of current replication in a current mirror.
The developed flexible IGZO TFT technology is also applied to realize an AFE system for
bio-potential signals acquisition. The AFE system consists of three monolithically integrated
constituent components: a bias-filter circuit with a bio-compatible low cut-off frequency of ~1
Hz, a 4-stage differential amplifier offering a large gain-bandwidth product of ~955 kHz and
a notch filter exhibiting over 30 dB suppression of the power-line noise. Respectively built
using conductive IGZO electrodes with thermally induced donor-agents and enhancement-mode
fluorinated IGZO TFTs with exceptionally low leakage current, both capacitors and
resistors with significantly reduced footprints have been realized. Defined as the ratio of the
gain-bandwidth product of an AFE system to its area, a record-setting figure-of-merit of ~86
kHz/mm
2 has been achieved. This is about an order of magnitude larger than the
kHz/mm
2 of the nearest benchmark. Requiring no supplementary off-substrate signal-conditioning
components and occupying an area ~11 mm
2 , the stand-alone AFE system has
been successfully applied to both electromyography (ECG) and electrocardiography (EMG). A
1×5 electrode array with monolithically integrated AFE in-pixel is also fabricated to collect the
spatial distribution of ECG and EMG.
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