Development of ZnO based oxide semiconductors for flexible thin film transistor

Abstract

최근 연구가 가속화 되고 있는 산화물 반도체 기반의 박막 트랜지스터는 기존의 LCD 등의 디스플레이 제품의 구동소자로 쓰이는 실리콘 기반 트랜지스터를 대체할 수 있는 차세대 물질로 각광을 받고 있다. 특히 산화아연 (ZnO) 반도체는 높은 전자이동도, 낮은 생산가격, 투명도, 저온 공정이 가능하다는 장점을 지니고 있기 때문에 기존에 사용되던 비정질 실리콘과 폴리실리콘을 대체 가능할 것으로 예상된다. 이런 장점을 바탕으로 산화물 반도체는 플렉서블 전자소자 적용에 가장 적합한 물질로 판단되고 있다. 하지만, ZnO TFT를 상용화 하기 위해서는 신뢰성 문제를 반드시 극복해야 한다. TFT의 동작전압(VON) 변화는 전자소자의 오작동을 일으키게 되고, 특히 LCD 디스플레이의 트랜지스터는 negative bias에 의한 스트레스 시간이 positive bias의 그것에 비해 500배 이상 크기 때문에 negative bias에 의한 소자의 신뢰성 향상은 반드시 해결해야 할 과제이다. 본 연구에서는 이런 배경을 바탕으로 가장 bias 신뢰성이 높은 HZTO TFT를 개발하는데 그 목적을 뒀다. 이를 위해 우선, DC 마그네트론 스퍼터링 장비를 이용하여 비정질 IGZO TFT를 최적화하였고 산소 플라즈마 처리를 통해 채널과 SiNx 계면의 결함을 줄여 그 특성을 향상시킬 수 있었다. 더 나아가 S/D저항을 줄이기 위해 여러가지 금속을 S/D으로 도입하였고, TFT S/D series resistance, intrinsic field effect mobility (μFE-i), transfer length (LT), and effective contact resistance (RC-eff)를 추출하여 채널층과 S/D간의 관계를 해석했다. 또한, 저저항 배선으로 Cu S/D을 사용하여 우수한 특성을 얻을 수 있었다. 최적화된 실험 조건을 바탕으로 bias 신뢰성 향상을 위한 ZnO TFT를 제작하여 그 특성을 살펴보았다. 우선, PECVD로 SiO2 (10nm)를 SiNx위에 증착하여 ECR 산소플라즈마 처리된 SiNx와 그 특성을 비교해 보았다. SiO2 (10nm)를 증착한 TFT의 경우 채널층과 절연체 계면의 trap이 줄어들어 소자의 전이특성이 크게 개선되었고, bias 신뢰성 또한 월등히 향상되었다. 이어서 본 연구에서는 ZnO TFT에 산화력이 큰 Hf을 첨가하여 신뢰성을 향상시키는 시도를 해보았다. HfZnO TFT의 경우, negative 와 positive bias temperature instability (NBTI, PBTI) 테스트에서 모두 신뢰성이 향상되는 것을 보여주었고, 열처리 온도를 높일수록 신뢰성은 더욱 향상되었다. Hf의 도핑효과를 알아보기 위해, Hf이 채널 전체에 도핑된 경우, 계면에만 도핑된 경우, 채널의 bulk에만 도핑된 경우, 도핑되지 않은 경우의 4가지 소자를 제작했고, 그 특성을 비교해 보았다. 전이특성 비교와 신뢰성 테스트를 통해 Hf의 높은 산화력으로 인해 산소결핍에 의한 결함이 감소한 것을 알 수 있었고, TFT의 신뢰성 향상을 관찰할 수 있었다. 특히 본 실험을 통해 TFT의 신뢰성은 채널과 절연체의 계면에 의해 그 특성이 좌우된다는 것을 파악할 수 있었다. 이어서 본 연구에서는 산화주석(SnO) TFT에도 Hf을 첨가하여 그 특성을 비교했고, 신뢰성이 향상되는 것을 관찰했다. 문턱전압 이하 전류(subthreshold current)를 온도별로 측정하여 HfSnO TFT의 밴드갭 내부의 변화를 관찰했고, Hf의 첨가로 내부의 trap이 감소함을 알 수 있었다. ZnO TFT는 외부에 노출됐을 경우, 공기 중의 산소나, 수분에 의해 특성이 변하게 되고 이를 막을 수 있는 보호막을 필요로 한다. 수분보호막의 효과를 알아보기 위해 PEALD로 Al2O3, TiO2를 단일막 또는 다층막으로 형성하여 수분투과특성을 파악했고, 단일막, 완충막이 적용된 보호막, 다층막 순으로 수분투과특성이 우수해지는 것을 파악했다. 개발된 보호막을 TFT위에 증착했고 특성을 살펴본 결과, TFT의 전이특성 변화가 보호막 공정 중의 플라즈마에 의한 손상에 의해 생겨난 것을 알아냈으며, 우수한 수분투과특성을 지닌 보호막을 TFT에 적용했을 때 신뢰성을 향상 시킨다는 것을 파악했다. 또한 bias 스트레스에 의한 동작전압의 변화는 단순한 전하 이동에 의해서만 생기는 것이 아니라, 이온화된 수분이 채널 표면에서 흡/탈착됨에 따라서도 변화한다는 것을 밝혀냈다. 최종적으로 우수한 전이특성과 향상된 신뢰성을 가지는 비정질의 HfZnSnO(HZTO) TFT를 개발했다. HZTO TFT는 14.33 cm2/Vs 의 이동도와 0.609 V/decade의 subthreshold swing (SS), 109 이상의 전류 점멸비를 가지는 우수한 전이특성을 지니고 있었다. NBTI 테스트를 실시하여 TFT의 신뢰성을 살펴본 결과 동작전압의 변화가 감소했고, 특히 Hf의 첨가량이 많아질수록 신뢰성은 더욱 향상되었다. 본 연구에서 개발된 고신뢰성의 HZTO TFT를 통해 차세대 플렉서블 디스플레이 구동소자 적용의 가능성이 더욱 높아질 것으로 기대된다.|Recently, oxide semiconductor-based thin film transistors (TFTs) have attracted considerable attention as alternatives to silicon-based TFTs particularly for use in active-matrix TFT-based backplanes such as the active-matrix liquid crystal displays (AM-LCDs). In particular, zinc-based oxide semiconductors have attracted a tremendous amount of attention as an active channel material because they offer high mobility, low processing cost, high transparency, and low temperature processability compared to conventional amorphous silicon and polycrystalline silicon TFTs. These low-temperature oxide semiconductors can form the active material for transistors with a performance that significantly exceeds that of amorphous and poly-Si. Thus, this technology can be used for next generation flexible electronics. However, in order to efficiently manufacture ZnO-based TFTs, it is crucial to overcome the problems of voltage independent stability and reliability. Any shift in the turn-on voltage (VON) of the driving transistor in on- or off-bias stressed conditions will cause reduction in the output drain current, leading to device malfunction. In particular, in commercial AM-LCD devices, the total stress time of the negative gate bias is more than 500 times that of the positive gate bias. Thus, device degradation due to negative bias temperature instability (NBTI) is a critical issue that must be resolved. The primary focus of this thesis is development of highly stable thin-film transistors employing a HZTO channel layer. First of all, we fabricated a-IGZO TFTs under optimized conditions at room temperature using a DC magnetron sputtering system. Moreover, we focused on high performance a-IGZO TFTs with a SiNx gate insulator treated using electron cyclotron resonance (ECR) remote oxygen plasma. The interface treatment using O2 plasma improves the interface quality by lowering the interface trap density. Cu electrodes were investigated to obtain good ohmic characteristics in a-IGZO based TFTs. Specifically, we discussed the S/D series resistances and their effects on the TFT performance. The TFT S/D series resistance, the intrinsic field effect mobility μFE-i, transfer length LT, and effective contact resistance RC-eff were extracted by the well-known transmission line method (TLM) using a series of TFTs with different channel lengths. Second, in order to improve bias stability, we reported the fabrication of high performance ZnO TFTs with a SiNx gate insulator using SiO2 interlayer treatment. The interface treatment, which employs electron cyclotron resonance (ECR) O2 plasma and SiO2 interlayer deposition using plasma enhanced chemical vapor deposition (PECVD), improves the interface quality by lowering the interface trap density. Also, we report new hafnium zinc-oxide (HZO) semiconductor materials that have been developed for use as active-channel layers to resolve both negative and positive bias temperature instability (NBTI, PBTI) issues related to ZnO-based TFTs. These devices were fabricated with HZO films deposited in a two-step process in order to investigate bias stability improvement. For comparison, an HZO (the atomic ratio of Hf in HZO films : 0.8 at. %) film 10 nm thick was first grown on the dielectric, and then the intrinsic 30-nm-thick ZnO film was deposited onto the HZO film. A second sample was deposited in the reverse order. We found that the Hf in the HZO thin films acts to control the trap density in the interfacial layer by suppressing defect related oxygen vacancies. We also investigated the influence of hafnium doping on bias stability in SnOx TFTs. The addition of Hf slightly increases μFE and decreases SS values. In addition, Hf may play an important role in improving the bias stability of TFTs by reducing the total trap density, due to their high binding energy. The improvement in the VON shift may be due to reduction in total trap density resulting from the suppression of defects related oxidation state of the Sn ion caused by the high binding energies of Hf ions. Additionally, an instability issue for these types of devices is the well known surface sensitivity of metal oxides. For example, adsorption of molecules from the ambient such as O2, H2O, etc. can result in accumulation or depletion of the surface region and affect conductivity through the device. In this regard, we investigated changes in device characteristics for the ZnO and HZO TFTs using PEALD-deposited Al2O3 and TiO2 water vapor barrier films. By adopting a superior barrier, the device properties were improved, and the VON stability under the negative bias stress was considerably improved in the ZnO and HZO TFTs. It was shown that the negative VON shift during bias stress was due not only to charge trapping, but also from dynamic interactions between the exposed backchannel and the ambient atmosphere. Lastly, the final focus of this thesis research introduces an amorphous hafnium-zinc-tin oxide (HZTO) semiconductor material as a robust active channel layer for flexible application. HZTO TFTs exhibited good electrical properties with a field effect mobility of 14.33 cm2/Vs, a subthreshold swing of 0.609 V/decade, and a high ION/OFF ratio of over 109. Time dependence of the turn-on voltage (VON) shift in HZTO TFTs was reported under an on-current bias temperature stress measured at 60 oC. HZTO TFTs with 2.0 atomic % (Hf element) showed negligible VON shift, compared with -4 V shift in HZTO TFT with 0.5 atomic %. Elemental hafnium may play an important role in improving the bias temperature stability of TFTs due to its high oxygen binding energy. Based on our results, a-HZTO semiconductors are promising candidates as robust channel layers for next generation flexible electronics applications.; Recently, oxide semiconductor-based thin film transistors (TFTs) have attracted considerable attention as alternatives to silicon-based TFTs particularly for use in active-matrix TFT-based backplanes such as the active-matrix liquid crystal displays (AM-LCDs). In particular, zinc-based oxide semiconductors have attracted a tremendous amount of attention as an active channel material because they offer high mobility, low processing cost, high transparency, and low temperature processability compared to conventional amorphous silicon and polycrystalline silicon TFTs. These low-temperature oxide semiconductors can form the active material for transistors with a performance that significantly exceeds that of amorphous and poly-Si. Thus, this technology can be used for next generation flexible electronics. However, in order to efficiently manufacture ZnO-based TFTs, it is crucial to overcome the problems of voltage independent stability and reliability. Any shift in the turn-on voltage (VON) of the driving transistor in on- or off-bias stressed conditions will cause reduction in the output drain current, leading to device malfunction. In particular, in commercial AM-LCD devices, the total stress time of the negative gate bias is more than 500 times that of the positive gate bias. Thus, device degradation due to negative bias temperature instability (NBTI) is a critical issue that must be resolved. The primary focus of this thesis is development of highly stable thin-film transistors employing a HZTO channel layer. First of all, we fabricated a-IGZO TFTs under optimized conditions at room temperature using a DC magnetron sputtering system. Moreover, we focused on high performance a-IGZO TFTs with a SiNx gate insulator treated using electron cyclotron resonance (ECR) remote oxygen plasma. The interface treatment using O2 plasma improves the interface quality by lowering the interface trap density. Cu electrodes were investigated to obtain good ohmic characteristics in a-IGZO based TFTs. Specifically, we discussed the S/D series resistances and their effects on the TFT performance. The TFT S/D series resistance, the intrinsic field effect mobility μFE-i, transfer length LT, and effective contact resistance RC-eff were extracted by the well-known transmission line method (TLM) using a series of TFTs with different channel lengths. Second, in order to improve bias stability, we reported the fabrication of high performance ZnO TFTs with a SiNx gate insulator using SiO2 interlayer treatment. The interface treatment, which employs electron cyclotron resonance (ECR) O2 plasma and SiO2 interlayer deposition using plasma enhanced chemical vapor deposition (PECVD), improves the interface quality by lowering the interface trap density. Also, we report new hafnium zinc-oxide (HZO) semiconductor materials that have been developed for use as active-channel layers to resolve both negative and positive bias temperature instability (NBTI, PBTI) issues related to ZnO-based TFTs. These devices were fabricated with HZO films deposited in a two-step process in order to investigate bias stability improvement. For comparison, an HZO (the atomic ratio of Hf in HZO films : 0.8 at. %) film 10 nm thick was first grown on the dielectric, and then the intrinsic 30-nm-thick ZnO film was deposited onto the HZO film. A second sample was deposited in the reverse order. We found that the Hf in the HZO thin films acts to control the trap density in the interfacial layer by suppressing defect related oxygen vacancies. We also investigated the influence of hafnium doping on bias stability in SnOx TFTs. The addition of Hf slightly increases μFE and decreases SS values. In addition, Hf may play an important role in improving the bias stability of TFTs by reducing the total trap density, due to their high binding energy. The improvement in the VON shift may be due to reduction in total trap density resulting from the suppression of defects related oxidation state of the Sn ion caused by the high binding energies of Hf ions. Additionally, an instability issue for these types of devices is the well known surface sensitivity of metal oxides. For example, adsorption of molecules from the ambient such as O2, H2O, etc. can result in accumulation or depletion of the surface region and affect conductivity through the device. In this regard, we investigated changes in device characteristics for the ZnO and HZO TFTs using PEALD-deposited Al2O3 and TiO2 water vapor barrier films. By adopting a superior barrier, the device properties were improved, and the VON stability under the negative bias stress was considerably improved in the ZnO and HZO TFTs. It was shown that the negative VON shift during bias stress was due not only to charge trapping, but also from dynamic interactions between the exposed backchannel and the ambient atmosphere. Lastly, the final focus of this thesis research introduces an amorphous hafnium-zinc-tin oxide (HZTO) semiconductor material as a robust active channel layer for flexible application. HZTO TFTs exhibited good electrical properties with a field effect mobility of 14.33 cm2/Vs, a subthreshold swing of 0.609 V/decade, and a high ION/OFF ratio of over 109. Time dependence of the turn-on voltage (VON) shift in HZTO TFTs was reported under an on-current bias temperature stress measured at 60 oC. HZTO TFTs with 2.0 atomic % (Hf element) showed negligible VON shift, compared with -4 V shift in HZTO TFT with 0.5 atomic %. Elemental hafnium may play an important role in improving the bias temperature stability of TFTs due to its high oxygen binding energy. Based on our results, a-HZTO semiconductors are promising candidates as robust channel layers for next generation flexible electronics applications.