디스플레이용 비정질 InGaZnO 박막트랜지스터의 제작 및 특성에 관한 연구

Abstract

최근 비정질 InGaZnO 박막트랜지스터는 높은 이동도와 우수한 균일성, 투명성, 낮은 공정온도의 장점을 가지고 있어 TFT-LCD나 AM-OLED와 같은 능동구동 디스플레이 소자로써 각광을 받고 있다. 본 논문은 우수한 a-IGZO TFT를 제작하기 위해 a-IGZO 박막의 증착조건 최적화, 채널층의 증착조건과 annealing 및 구동환경이 TFT의 특성에 미치는 영향, 그리고 a-IGZO TFT의 불안정성에 대하여 살펴보았다. RF 스퍼터링을 이용하여 a-IGZO를 증착할 경우 반응기체 내 산소 함량과 RF power에 따라 박막의 조성과 광학적, 전기적 특성이 큰 영향을 받으며 10%의 산소비와 40 W에서 채널에 적합한 a-IGZO 박막을 얻을 수 있었다. 위 조건으로 제작한 a-IGZO TFT에서 전계효과 이동도(μSAT)와 subthreshold swing(S), ON/OFF 전류비가 2.66 cm2/V• s, 1.07 V/dec, ~107인 양호한 소자특성을 얻었고, annealing 시 17 cm2/V• s, 0.72 V/dec로 소자특성이 향상되었다. a-IGZO TFT에 광을 조사하면 OFF current와 S의 증가로 소자특성이 크게 열화된다. 스트레스 전압이나 온도, 광에 의한 a-IGZO TFT의 불안정성을 살펴보면 전압이 높을수록 VTH의 이동이 증가하고, 온도가 상승할수록 μSAT의 변화가 증가하며, 광을 조사하면 photo-generated carrier의 영향으로 S가 변화한다. 또한 a-IGZO와 a-Si:H의 불안정성을 비교한 결과, TFT-LCD의 화소동작을 모사한 NBTS 평가에서는 a-IGZO TFT가 상용 a-Si:H TFT보다 우수한 VTH 안정성을 나타내었고, 반면에 Cdown의 동작을 모사한 pulse stress 평가에서는 a-IGZO MIS capacitor에서 매우 취약한 C-V 안정성을 드러내었다. 실험결과 a-IGZO TFT를 상용화하기 위해서는 광에 대한 안정성을 확보하는 것이 무엇보다도 시급한 과제이며, 게이트 절연막 및 a-IGZO/절연막 계면의 특성을 향상시키기 위한 연구가 필요하다.|Recently, amorphous InGaZnO thin-film transistors (a-IGZO TFTs) have attracted considerable interest for applications to the active matrix display devices such as TFT-LCD and AM-OLED due to their high mobility, good uniformity, transparency, and low temperature processability. To fabricate quality a-IGZO TFTs, we examined the effect of various process parameters of a-IGZO deposition such as oxygen content, RF power, and annealing condition on the device parameters, and driving environments on TFT characteristics as well as instability of a-IGZO TFTs were investigated. For the deposition of a-IGZO by using RF sputtering, the oxygen content in reactive gases and RF power showed great influence on the chemical compositions, optical and electrical properties of a-IGZO films, and the best property of a-IGZO film for channel was obtained at 10% of oxygen ratio and 40 W of RF power. The a-IGZO TFT underwent above optimized process conditions showed excellent characteristics of a field-effect mobility (μSAT) of 2.7 cm2/V• s, a subthreshold swing (S) of 1.1 V/dec, and an ON/OFF current ratio of ~107. After annealing, the μSAT and SS were increased to 17 cm2/V• s and 0.72 V/dec, respectively. However, when a-IGZO TFTs were exposed to light, device characteristics were significantly deteriorated due to the increase in IOFF and S. The instabilities of a-IGZO TFTs by the bias stress voltage, temperature, and light illumination were investigated. The VTH shift increased with the bias voltage, while the μSAT variation increased with the temperature. The S was changed by light illumination due to the effect of photo-generated carriers. In addition, if comparing the instability of a-IGZO and a-Si:H, the NBTS (negative bias temperature stress) test simulating the TFT-LCD’s pixel operation showed that the a-IGZO TFT had greater stability than an commercial a-Si:H TFT in the threshold voltage characteristics. However, the pulse stress simulating Cdown’s operation revealed very lower C-V stability in a-IGZO MIS capacitor. According to the test results, improvement of the instability against light illumination for mass production of a-IGZO TFTs is still necessary, and therefore, further studies on the gate insulators and a-IGZO/insulator interfaces are required.; Recently, amorphous InGaZnO thin-film transistors (a-IGZO TFTs) have attracted considerable interest for applications to the active matrix display devices such as TFT-LCD and AM-OLED due to their high mobility, good uniformity, transparency, and low temperature processability. To fabricate quality a-IGZO TFTs, we examined the effect of various process parameters of a-IGZO deposition such as oxygen content, RF power, and annealing condition on the device parameters, and driving environments on TFT characteristics as well as instability of a-IGZO TFTs were investigated. For the deposition of a-IGZO by using RF sputtering, the oxygen content in reactive gases and RF power showed great influence on the chemical compositions, optical and electrical properties of a-IGZO films, and the best property of a-IGZO film for channel was obtained at 10% of oxygen ratio and 40 W of RF power. The a-IGZO TFT underwent above optimized process conditions showed excellent characteristics of a field-effect mobility (μSAT) of 2.7 cm2/V&#8226