低溫多晶矽製程之類比積體電路設計與實現

Abstract

與傳統的非晶矽(amorphous silicon)薄膜電晶體(thin film transistor)相比較，低溫多晶矽(low temperature poly silicon)薄膜電晶體相對具有較高的電子遷移率(mobility)、較低的臨界電壓(threshold voltage)、和較高的生成穩定度，這些特性皆有利於顯示面板上的系統整合，進而使面板達到小巧、高穩定度和高解析度的特點。也因為這些特點，使得低溫多晶矽技術被視為最理想技術之一，期能應用在可攜式系統上，像是數位相機、手機、個人數位助理等，進而達到系統面板(system-on-panel)之應用。此外，系統面板之應用，在未來也會因為較低製作成本和較短的產品生成時間而一步一步的被實現。在過去幾年內，為了達到系統面板之應用，一些面板週邊的電路，像是直流-直流轉換器(DC-DC converter)、暫存器(register)、驅動電路(driver circuit)和數位至類比轉換器(DAC)等皆已被整合在玻璃基板上。另外，一些應用於不同方向且整合在玻璃基板上的出色研究也相繼被提出來，像是中央處理器(CPU)、記憶體、帶隙參考電壓電路(bandgap reference circuit)和射頻識別(RFID)標籤解調器(demodulator)。 雖然在低溫多晶矽製程中，利用加大多晶晶格來增加了元件的效能，但是這也在液晶面板上，造成了元件和元件間的隨機變異。而不規則的晶界、閘極絕緣界面的缺限、和在通道上的不完整離子摻雜也對低溫多晶矽的薄膜電晶體的電性特性造成了變異。一些像是熱載子應力、負偏壓溫度不穩定性和一些可靠度的問題已經證實，多晶膜電晶體的不穩定性是比單晶的矽製程金氧半場效電晶體還要更為嚴重。此外，在低溫多晶矽製程上的元件特性變異，相較於互補式金氧半導體製程而言也是更為嚴重。因此在實現低溫多晶矽製程之積體電路設計上，這些元件的變異性也是必需被考慮的。 在第二章中，本論文提出了玻璃基板上具有伽瑪校正(gamma correction)功能之六位元折疊(folded)電阻串數位至類比轉換器。藉著折疊電阻電路(folded R-string circuit)、分割的數位解碼器(segmented digital decoder)和重新排序之解碼電路(reordering decoding circuit)，所提出之數位至類比轉換器與傳統的電路比較，可以有效的降低約六分之一的面積。 在第三章中，本論文提出了兩個具有位準轉換功能並包含伽瑪校正數位至類比轉換器之類比輸出緩衝器。利用了運算放大器(OPAMP)、解碼器、電阻串、開關和具有3-V伽瑪校正之數位至類比轉換器，所提出之第一個類比輸出緩衝器可以直接驅動5-V的液晶面板(liquid crystal panel)，而不用重新設計適用於5-V伽瑪校正之數位至類比轉換器。 本論文第四章提出了一應用於觸碰式面板之玻璃基板上讀出電路。所提出之電路利用了開關-電容(switched-capacitor)技巧，來加大因為觸碰式面板的電容變化而造成的電壓差，並且也採用了相關性雙採樣(correlated double sampling)之技巧來降低因為製程變異而導致的偏移(offset)。所提出之電路最低可判別的電壓差為40 mV，並且可利用4位元的數位輸出來分辨觸碰到之面積。 第五章提出一應用於觸碰式面板之玻璃基板上具有數位校正(digital correction)功能的讀出電路。此電路包含了轉導(Gm)放大器、計數器(counter)和數位校正電路。所提出之電路只需要小部份之類比電路，並利用數位校正電路來補償低溫多晶矽製程造成的較大製程變異和較差的元件特性。跟第四章所提之電路相比較，本章所提之電路具有較低的電路複雜度、較低的功率消耗和較簡單的補償方式，但是相對而言，也因此需要較大的電路面積和會有較低的操作頻率。 第六章則是總結了本篇論文的主要結果，並且提出一些未來可以改善或新應用的建議。

Compared with conventional amorphous silicon (a-Si) thin film transistors (TFTs), some characteristics, such as higher carrier mobility, lower threshold voltage, and higher stability, of low temperature poly silicon (LTPS) TFTs can achieve compact, highly reliable, and high resolution for system integration within a display panel. For these features, LTPS technology is conceived as one of most desirable technology to accomplish realization of system-on-panel (SOP) application for portable systems, such as digital camera, mobile phone, personal digital assistants (PDAs) and so on. In addition, SOP application will be implemented step by step in the future to reduce the fabrication cost and shorten the product lead time. In the past few years, some peripheral circuits of display panel, like DC-DC converter, register, driver circuits, and digital-to-analog converter (DAC), had been integrated on glass substrate for SOP application. Furthermore, some remarkable advances had also been implemented on glass substrate, such as central processing unit (CPU), memory, bandgap reference circuit, and demodulator for RFID tags. Although using LTPS process can enlarge poly-grain size to improve the device performance, it usually accompanies a random device-to-device variation on LCD panel. The harmful effects of irregular grain boundaries, gate-insulator interface defects, and incomplete ion-doping activation in thin poly-silicon channels result in the variation on electrical characteristics of LTPS TFTs. Some properties such as hot carrier stress (HCS), negative bias temperature instability (NBTI), and reliability issues have been proved that the instability of polysilicon TFTs is more serious than that of single-crystalline silicon MOSFETs. In addition, the device characteristic variations in LTPS technology are also quite larger compared with CMOS technology, so the effect of device variation must be considered for on-panel circuit design. In chapter 2, a 6-bit folded R-string DAC with gamma correction on glass substrate is designed and verified in 3-贡m LTPS technology. By using the folded R-string circuit, segmented digital decoders, and reordering decoding circuit, the area of the new proposed DAC circuit can be effectively reduced to about one sixth of the traditional one. In chapter 3, two analog output buffers with level shifting function on glass substrate for panel application is designed and fabricated in a 3-贡m LTPS technology. By using OPAMP, decoder (decoder 2), R-string (R1, R201-R237), switches (MR01-MR37), and DAC with 3-V gamma correction parameters, the new proposed analog output buffer I can drive 5-V liquid crystal panel without re-designing the DAC with 5-V gamma correction parameters. In chapter 4, a new on-panel readout circuit for touch panel applications is designed and fabricated in a 3-贡m LTPS technology. The switched-capacitor (SC) technique is applied to enlarge the voltage difference from the capacitance change of touch panel, and the correlated double-sampling (CDS) technique is also employed to reduce the offset owing to process variation. The minimum detectable voltage difference of the proposed circuit is 40 mV, and the different touch area can be identified by the 4-bit digital output. In chapter 5, a new on-panel readout circuit with digital calibration, which contains transconductance amplifier, counter, and digital correction circuit, for touch panel applications is designed and verified in a 3-贡m LTPS technology. In the proposed circuit, only a small amount of analog circuitry is required and larger variation and worse device characteristics in LTPS process can therefore be compensated by the digital calibration circuit. Compared with proposed circuit in chapter 4, the proposed circuit in chapter 5 shows lower circuit complexity, lower power consumption, and easier calibration methodology. On the contract, the proposed circuit in chapter 5 also presents larger layout area and lower detection speed. Chapter 6 summarizes the main results of this dissertation. Some suggestions for the future works are also addressed in this chapter.