- 學位論文
可撓性非晶氧化銦鎵鋅薄膜電晶體差分放大器之研究
Flexible a-IGZO Thin-Film Transistor-Based Differential Amplifier Circuits
摘要
本研究目標在開發可撓性非晶氧化銦鎵鋅(a-IGZO)薄膜電晶體差分放大器。研究中使用5.5 μm的聚醯亞胺軟板為基板，並利用聚醯亞胺封裝層使電路元件能位在中性面，降低機械應變下對元件、對電路的特性影響。首先在玻璃基板上進行電晶體特性與電路設計的測試，在完成鈍化層接觸孔的蝕刻後，對元件進行第一次350℃空氣退火，產生n型電晶體的特性，但由於電晶體的臨界電壓、接觸電阻過大，嚴重影響電路的表現，於是再進行二次退火來調整電晶體的特性，使電路特性能接近理論值。接著再將電路製程轉移到聚醯亞胺軟性基板上，由於封裝層固化會使元件經歷額外的熱退火效應，導致電晶體成為空乏型，不符合電路設計，在調整退火程序，取消350℃空氣退火，僅使用封裝層固化的溫度來退火元件，成功完成具封裝之可撓性非晶氧化銦鎵鋅薄膜電晶體差分放大器。 本研究的薄膜電晶體採用下閘極交錯型的結構，使用鉻當閘極、二氧化鉿當介電層、氧化銦鎵鋅當主動層、氧化銦錫當源極和汲極、二氧化矽當鈍化層，製程中在完成鈍化層接觸孔的蝕刻後，於空氣中進行350℃退火。在元件通道寬長尺寸為180 μm/30 μm時，可以得到元件特性如下：Vth, sat = 3.49 V、μsat = 5.46 cm2V-1s-1、S.S. = 0.24 V/dec；當元件在製作完成後，再經過330℃的氮氣退火以提升元件特性，所獲得的元件特性參數為：Vth, sat = 1.87 V、μsat = 9.12 cm2V-1s-1、S.S. = 0.12 V/dec。在此條件下製成的共源極放大器擁有15 dB的電壓增益以及1442 Hz的截止頻率、單階差分放大器有15.83 dB的電壓增益以及1315 Hz的截止頻率、二階差分放大器則有30.8 dB的電壓增益以及358 Hz的截止頻率。 接著在聚醯亞胺基板上製作元件，以矽基板當承載基板，在上面旋塗聚醯亞胺前驅物，並於475℃固化，接著沉積氮化矽以及二氧化矽作為緩衝層，再進行電晶體製作，最後進行固化封裝製程，同時對元件退火，完成後以機械性的方式分離矽基板，完成具封裝之可撓性差分放大器。此時通道寬長尺寸為180 μm/30 μm的電晶體特性如下：Vth, sat = 1.26 V、μsat = 7.05 cm2V-1s-1、S.S. = 0.12 V/dec，共源極放大器的電壓增益14.55 dB，截止頻率885 Hz；單階差分放大器電壓增益15.56 dB，截止頻率514 Hz；二階差分放大器電壓增益30.54 dB，截止頻率393 Hz。在電壓增益值上，製作於聚醯亞胺基板上的電路與玻璃上的結果接近，但在截止頻率上，製作於聚醯亞胺基板上的共源極放大器和單階差分放大器則低於玻璃上的電路，可能是因為軟板較不平整所致。 最後對於封裝的電晶體以及電路進行彎曲測試，元件在0.8 cm的曲率半徑下，張應力以及壓應力對於元件特性都沒有太大的改變，元件特性與平坦時相近，可以得知經過封裝後，元件的確接近中性面，不容易受到機械應變的影響。
並列摘要
In this study, flexible differential amplifier circuits based on amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) are developed. To minimize the influence of mechanical bending on the circuit performance, the circuits are fabricated on a 5.5-μm-thick polyimide (PI) substrate and then encapsulated with another polyimide layer of the same thickness to place them on the neutral plane. The TFTs and circuits are first fabricated on glass substrates for optimization. After the etching of passivation layer’s contact hole, TFT is annealed at 350℃ in air to get the n-type characteristic. However, the transistor is not perfect enough. Because of the larger threshold voltage and contact resistance, the characteristic of amplifier degrading a lot. To improve the TFT performance, post fabrication anneal at 330℃ in nitrogen ambient is adopted. Next, the fabrication process is transferred to the PI substrate. However, the extra heating effect introduced during the curing process of encapsulation layer causes the operation of the TFT changes from enhancement to depletion mode. To circumvent this issue, the process of annealing was changed. The 350℃ anneal in air is canceled which is replaced by the heating effect during the curing process. The encapsulated flexible a-IGZO TFT-based differential amplifier is successfully developed. The TFT has an inverted-staggered bottom-gate structure. E-beam evaporated Cr, atomic-layer deposited HfO2, rf-sputtered IGZO, e-beam evaporated indium tin oxide and plasma-enhanced-chemical-vapor-deposited SiO2 are used as the gate electrode, gate dielectric, active channel, source/drain electrodes and back-channel passivation, respectively. After the etching of contacr hole, annealing at 350℃ in the air. The TFT with a channel dimension of W / L = 180 μm / 30 μm exhibits a threshold voltage of 3.49 V, saturation mobility of 5.46 cm2V-1s-1, and subthreshold swing of 0.24 V/dec. After the post-fabrication anneal at 330℃ in nitrogen, the threshold voltage, saturation mobility and subthreshold swing are improved to 1.87 V, 9.12 cm2V-1s-1, and 0.12 V/dec, respectively. The voltage gain and cut-off frequency of the common source amplifier are 15 dB and 1442 Hz, respectively. Those of the single-stage differential amplifier are 15.83 dB and 1315 Hz. For two-stage differential amplifier, the corresponding values are 30.8 dB and 358 Hz. To fabricate flexible a-IGZO TFT-base differential amplifiers, we use Si wafer as the carrier substrate and spin PI varnish on it. Next, the PI varnish is cured at 475 ℃, followed by the deposition of SiNx and SiO2 buffer layers by PECVD. The amplifiers are then fabricated. After the circuit fabrication, another PI layer is spun on and cured as the encapsulation. Finally, the sample is debonded from the Si carrier substrate by mechanical force. The flexible a-IGZO TFT with a channel dimension of W / L = 180 μm / 30 μm has a threshold voltage of 1.26 V, saturation mobility of 7.05 cm2V-1s-1, and subthreshold swing of 0.12 V/dec. The voltage gain and cut-off frequency of the flexible common source amplifier are 14.55 dB and 885 Hz, respectively. Those of the flexible single-stage differential amplifier are 15.56 dB and 514 Hz. For the flexible two-stage differential amplifier, the corresponding values are 30.54 dB and 393 Hz. The voltage gains of the on-PI amplifiers are comparable to those of on-glass counterparts. The degraded bandwidth of the on-PI amplifiers may cause by the unflat of the flexible substrate. Finally, the influence of mechanical bending on the performance of the flexible TFT and amplifier circuits are investigated. For both outward (tension) and inward (compression) bending at a radius up to 0.8 cm, the performance of TFT and amplifier circuits are nearly unchanged, which confirms that flexible circuits are insensitive to mechanical bending events when they are located close to the neutral plane.