非晶氧化銦鎵鋅(a-IGZO)薄膜電晶體差分放大電路之研究

Abstract

本研究目標在於開發非晶氧化銦鎵鋅(a-IGZO)薄膜電晶體差分放大電路。首先藉由改變SiOx鈍化層沉積與退火順序、後退火時間與主動層厚度，於玻璃基板上進行a-IGZO薄膜電晶體電性優化，並藉由此a-IGZO薄膜電晶體製作出電阻負載共源極放大器與電晶體負載共源極放大器。接著，再透過兩個相同負載的共源極放大器製作出差分放大電路，並利用理想電流源與電晶體電流源兩種不同的電流源進行驅動。最後，再將共源極放大器與差分放大器應用於聚偏氟乙烯(poly(vinylidene fluorid), PVDF)壓電觸覺感測，分析兩種放大器對於共模雜訊的抑制能力。 所採用的a-IGZO薄膜電晶體具交錯下閘極型結構，在製作上由於背通道鈍化層沉積時會導致薄膜電晶體臨界電壓與次臨界擺幅增加，因此實驗中先沉積SiOx背通道鈍化層後再進行退火製程，以修復沉積時對主動層與介電層造成之缺陷。優化後的主動層厚度與退火時間分別為22.5 nm和60 mins，此時a-IGZO薄膜電晶體之飽和載子遷移率達3.94 cm2V-1s-1、臨界電壓4.31 V、次臨界擺幅 0.22 V/dec和電流開關比為 8.5×108。 在放大器方面，藉著最佳參數a-IGZO薄膜電晶體製作出1 M電阻負載共源極放大器與幾何長寬比(W/L)active/(W/L)load為16的電晶體負載共源極放大器，在VDD為10 V時，電壓增益分別達6 V/V與3 V/V，截止頻率則為542.2 Hz與2.13 kHz，兩者電壓增益皆與理想計算值相近。相較於電阻負載共源極放大器，電晶體負載共源極放大器具較高的截止頻率，此乃因為串聯之負載電晶體的電容使整體電路的等效電容降低而得以提升。接著，利用兩相同結構的共源極放大器串接形成差分放大電路，並分別以理想電流源與電晶體電流源驅動下分析電路的低頻增益與截止頻率。在以理想電流源驅動的差分放大電路，當VDD為10 V時低頻增益與截止頻率兩值都與組成的共源極放大器相近；而以電晶體電流源驅動的差分放大電路，其截止頻率則都高於組成的共源極放大器，原因亦來自於串聯的驅動電晶體降低整體電路的等效電容值，進而提升截止頻率。 最後，分別將電晶體負載共源極放大器與以理想電流源驅動的電晶體負載差分放大電路應用於PVDF壓電觸覺感測上。結果顯示共源極放大器無法抑制共模雜訊，會同時放大雜訊與壓電訊號，在共模雜訊作用下訊雜比(signal to noise ratio, SNR)值約為186.2 (22.7 dB)；而所製作的差分放大電路則能有效的抑制共模雜訊，在共模雜訊作用下SNR值能提升至549.5 (27.4 dB)。

The goal of this research is to develop differential amplifiers based on amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs). First, the bottom-gate a-IGZO TFTs were optimized in terms of the sequence of processing steps, and the thickness and annealing time of the a-IGZO channel. Next, common-source amplifiers with either resistive loads or active loads were implemented using the optimized a-IGZO TFTs. Afterwards, two common-source amplifiers with identical loads were interconnected through via-holes to form the differential amplifier, which was driven by either an ideal current source or a transistor current source. Last, both common-source amplifiers and differential amplifiers were connected to PVDF piezoelectric tactile sensors, and their abilities to reject common-mode signals were analyzed. The a-IGZO TFT has an inverted-staggered bottom-gated structure. Because the deposition process of SiOx passivation layer can introduce defects in the a-IGZO channel and the HfO2 gate dielectric, causing an increase of the threshold voltage and subthreshold swing, the passivation layer was deposited followed by an annealing process of the a-IGZO channel. The optimal channel thickness and annealing time are 22.5 nm and 60 mins, respectively. The optimal a-IGZO TFT has an saturation mobility 3.94 cm2V-1s-1, threshold voltage 4.31 V, subthreshold swing 0.22 V/dec, and on/off current ratio of 8.5×108, respectively. The optimized a-IGZO TFTs were then integrated to form common-source amplifiers with either a resistive load of 1 M or an active load using a geometric aspect ratio, (W/L)active / (W/L)load, of 16. At a supplied voltage (VDD) of 10 V, the voltage gains are 6 V/V and 3 V/V, respectively, the cutoff frequencies are 542.2 Hz and 2.13 kHz, respectively. Both voltage gains were close to the theoretical values. Because the equivalent capacitance of the entire amplifier circuit with an active load is lowered due to the series connection of two TFTs, its cutoff frequency is higher than that of the amplifier with a resistive load. Next, differential amplifiers were formed by integrating two common-source amplifiers with identical loads. The amplifiers were either driven by an ideal current source or a transistor current source. For differential amplifiers driven by an ideal current source, both the low-frequency gain and the cutoff frequency are close to that of the individual common-source amplifier. For differential amplifiers driven by a transistor current source, the cut-off frequency is increased, because the driving transistor connected in series can reduce the equivalent capacitance of the entire circuit. Finally, the common-source amplifier and differential amplifier with an active load and driven by an ideal current source were connected to PVDF piezoelectric tactile sensors. The result shows that the common-source amplifier cannot reject the common-mode signal and amplifies the noise and signal simultaneously, giving a signal-to-noise ratio (SNR) of 186.2 (22.7 dB), while the differential amplifier can effectively reduce the common-mode signal, yielding a SNR of 549.5 (27.4 dB).