Amorphous InGaZnO Thin Film Transistors Operating Beyond 1

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Amorphous InGaZnO Thin Film Transistors,Operating Beyond 1 GHz Achieved by. Optimizing the Channel and Gate Dimensions, Yiming Wang Hanbin Wang Jiawei Zhang He Li Gengchang Zhu Yanpeng Shi Yuxiang Li Qingpu Wang Qian. Xin Zhongchao Fan Fuhua Yang and Aimin Song Senior Member IEEE. silicon amorphous indium gallium zinc oxide a IGZO has. Abstract Amorphous indium gallium zinc oxide a InGaZnO drawn much attention due to its high mobility typically 10 to. or a IGZO has already started replacing amorphous silicon in 100 cm2 Vs high transparency 80 in visible light region. backplane driver transistors for large area displays However mechanical flexibility and high uniformity 7 As such. hardly any progress has been made to commercialize a IGZO for a IGZO has started replacing amorphous silicon in display. electronic circuit applications mainly because a IGZO transistors. backplane drivers While most oxide semiconductors are n. are not yet capable of operating at GHz frequencies Here. nanoscale a IGZO thin film transistors TFTs are fabricated on type recent advances in p type oxide semiconductors such as. a high resistivity silicon substrate with a Ta2O5 gate dielectric SnO 8 9 10 11 CuxO 12 13 and NiO 14 15. Carrier mobilities up to 18 2 cm2V 1s 1 have been achieved By already resulted in fully oxide based complementary logic. optimization of the TFT channel length and contact overlap we gates 9 16 17 which are the fundamental building. are able to demonstrate current gain and power gain cut off blocks towards large scale integrated circuits ICs However. frequencies at 1 24 and 1 14 GHz respectively both beyond the 1 hardly any progress has been made in commercializing. GHz benchmark Such a performance may have implications in a IGZO for ICs largely due to the low operation frequencies. developing at least medium performance a IGZO TFTs based of a IGZO thin film transistors TFTs to date. circuits for low cost or flexible electronics, Extensive efforts have been made in order to improve the. speed of oxide semiconductor based TFTs 18 19 20,Index Terms thin film transistor a InGaZnO high. frequency current gain 21 22 23 Single crystalline ZnO ZnMgO TFTs on. GaAs substrates showed a current gain cut off frequency fT. of 1 75 GHz and a maximum oscillation frequency fmax of. I INTRODUCTION 2 45 GHz 24 Nanocrystalline ZnO TFTs on high resistivity. silicon substrates have also demonstrated a high frequency. T HERE have been rapid developments in oxide, semiconductors in the last decade 1 2 3 One of the.
important aspects of most oxide semiconductors is that. response fT 2 45 GHz fmax 7 45 GHz 21 High, performance was also achieved by depositing c axis aligned. crystalline IGZO on heated substrate using a sophisticated. the electron mobility remains largely the same even when the nanoscale 3D gating structure 25 However single. semiconductor changes from crystalline to amorphous phase crystalline materials require complex and expensive. because the isotropic s orbitals of metal atoms effectively deposition technique and hence are usually incompatible with. overlap to form conduction band in contrast to silicon low cost large area manufacturing Nanocrystalline materials. Polycrystalline silicon and single crystal silicon can have high can suffer from low yield issues due to the uncontrollable. carrier mobilities from 50 to 1000 cm2V 1S 1 Cut off grain boundaries In contrast a IGZO can be deposited by. frequencies beyond 2 GHz have been achieved in TFTs using industrial compatible radio frequency RF sputtering. mechanically transferred single crystalline silicon onto a technique and its amorphous nature ensures high uniformity. polyethylene substrate 5 6 To the best of our knowledge and hence high yield in high density IC manufacturing. there has not been report on GHz TFTs based on amorphous So far a IGZO TFTs using self alignment or quasi vertical. silicon This is because amorphous silicon has a carrier fabrication processes on flexible substrates showed. mobility greatly reduced from that of the crystalline silicon current gain cut off frequencies around 100 MHz 20 26. typically only 1 cm2V 1S 1 or below In contrast to amorphous 27 28 In 2015 a IGZO TFTs with a gate length of 1 5 m. This work has been supported by the Engineering and Physical Sciences demonstrated an improved current gain cut off frequency of. Research Council No EP N021258 1 National Basic Research Program 384 MHz on glass substrates 18 As frontend rectifiers. of China Nos 2016YFA0301200 and 2016YFA0201800 National a IGZO Schottky diodes on rigid and flexible substrates. Natural Science Foundation of China Nos 11374185 and 11304180 and. Fundamental Research Fund of Shandong University No 2016WLJH44 operating beyond a milestone value of 2 45 GHz have been. Yiming Wang Hanbin Wang He Li Gengchang Zhu Yanpeng Shi reported 23 29 However a IGZO TFTs operating at GHz. Yuxiang Li Qingpu Wang Qian Xin are with the Center of frequencies have not been demonstrated yet making them. Nanoelectronics and School of Microelectronics Shandong University unable to compete with ubiquitous silicon ICs that are. Jinan 250100 China e mail xinq sdu edu cn, Jiawei Zhang and Aimin Song are with the School of Electrical and typically clocked at a few GHz In this work by optimizing. Electronic Engineering The University of Manchester Manchester M13 the topological structures and shot channel effects we. 9PL U K e mail A Song manchester ac uk demonstrate high speed a IGZO TFTs beyond the 1 GHz. Zhongchao Fan and Fuhua Yang are with the Engineering Research benchmark The record fT and fmax of 1 24 GHz and 1 14 GHz. Center for Semiconductor Integration Technology Institute of. Semiconductors Chinese Academy of Sciences Beijing 100083 China show the potential of using a IGZO TFTs in transparent ICs. e mail zcfan semi ac cn fhyang semi ac cn, and microwave electronics devices with channel lengths of 360 nm 750 nm 1 2 m 5 m. II EXPERIMENTAL PROCEDURE, Fig 2 a Output characteristics and b transfer characteristics of the. smallest TFT with Lch 360 nm and Lov 150 nm, and overlap lengths of 150 nm 10 m 20 m in order to.
systematically study the effects of the dimensions As the most. important parameter that describes the high frequency. properties of TFTs the current gain cut off frequency in the. Fig 1 Schematic illustrations and SEM image of the IGZO TFTs a first approximation is given by. Schematic of the three dimensional perspective view of the device 5 5 0 7523 1. layout b Schematic of the cross sectional perspective view of the 63 63 89. device c Tilt view SEM image of the smallest TFT with Lch 360 nm where gm is the transconductance VGS is the gate voltage Vth. and Lov 150 nm is the threshold voltage eff is the effective mobility and the. The devices were fabricated on high resistivity 10 gate capacitance CG is the sum of the gate to source. k cm Si wafers coated with 100 nm thick thermally gate to drain and gate to channel capacitances 32 The. oxidized SiO2 Ti Pd 20 nm 30 nm gate electrodes were effective mobility is extracted from the device transfer. deposited using electron beam evaporation A 120 nm thick characteristics which are realistic reflection of the effective. Ta2O5 gate insulator was deposited by RF sputtering at room gate modulation ability of the TFTs after considering the. temperature RT The optimized deposition conditions for the influence of the contact resistance RC As described in Eq 1. Ta2O5 dielectric were as follows the RF power chamber it may appear that if reducing Lch a higher fT can be obtained. pressure Ar flow rate and O2 flow rate were maintained at 90 However in short channel devices eff becomes lower than. W 1 73 mtorr 6 sccm and 3 sccm respectively A the a IGZO intrinsic mobility 0 due to the contact resistance. 24 nm thick a IGZO film was immediately sputtered after becoming more and more dominant with decreasing Lch It is. Ta2O5 without breaking the vacuum at an Ar flow rate of 20 critical to optimize Lov in order to improve the cut off. sccm and RF power of 90 W A 3 a IGZO target was used frequency While increasing Lov reduces the contact resistance. with In2O3 Ga2O3 ZnO 1 1 1 mol Ti Pd 20 nm 30 nm by gate induced carriers at the contacts particularly in the. source and drain were formed using the electron beam linear transport region which improves fT 33 the parasitic. evaporator Finally a post annealing was performed at 200 C overlap capacitance due to the overlap shall also have an. in air for 30 min to improve the electronic properties of IGZO adverse effect on fT 26 In this study devices with various. 30 31 The patterning of sputtered Ta2O5 and a IGZO channel lengths 360 nm 750 nm 1 2 m and 5 m and. layers was achieved by electron beam lithography and lift off overlap lengths 150 nm 10 m and 20 m were fabricated. process which is more controllable than etching technique and The output and transfer characteristics of the smallest. involves fewer chemicals to cause potential contaminations device Lch 360 nm Lov 150 nm are shown in Figs 2 a. and b respectively The effective mobility threshold voltage. III RESULTS AND DISCUSSION and on off current ratio were found to be 1 18 cm2V 1s 1 1 84. Figures 1 a and b show the schematic diagrams of the V and 4 104 for the smallest device in comparison with. TFT structure used in this work A two finger bottom gate 18 2 cm2V 1s 1 2 84 V and 1 4 106 for the largest device. configuration was adopted A ground signal ground GSG with Lch 5 m and Lov 10 m It is noted from the output. layout compatible with co planar microwave probes was used characteristics in Fig 2 a that the device was not completely. as the contact pads for high frequency characterizations TFTs pinched off This is due to the short channel effect becoming. with different gate lengths were fabricated at the same more pronounced with decreasing channel length which can. condition The dimensions of the TFTs were determined using be explained by the charge sharing model and drain induced. scanning electron microscope SEM In Fig 1 c the SEM barrier lowering 34 35 This effect is inevitable for. image of the TFT shows a channel length Lch of 360 nm and short channel thin film transistors Nevertheless the on off. an overlap length Lov Lov GS Lov GD the sum of the ratio 4 104 is still sufficient for most logic applications. gate to source and gate to drain overlap lengths of 150 nm which typically requires an on off ratio higher than 1000 36. Each device had two finger gates with a width of 50 m In our experiment the short channel effect was observed in. resulting in a total gate width of 100 m There are in total 12 devices with Lch 1 2 m. Fig 3 Effective carrier mobility as a function of channel length for. different gate channel overlaps, Fig 5 High frequency properties of IGZO TFTs a Current gain cut off. frequency as a function of overlap length of the devices with channel. lengths of 0 36 m black line 0 75 m red line and 1 2 m blue line. respectively b Maximum available gain maximum available gain and. unilateral power gain of the smallest TFT at VDS VGS 7 V MAG exists. when stability factor K 1 c Small signal current gain H21 as a function. of frequency of the smallest TFT at different VDS and VGS d Simulated fT. compared with experimental results of the smallest TFT. Fig 4 Channel resistance and contact resistance as a function of overlap effective mobility can be expressed as. length with channel lengths of 360 nm and 5 m at VGS 5 V and 7 B 1 C 2. VDS 5 V respectively The results were obtained according to Ref 4 8 0 23 DE 563. by assuming the intrinsic field effect mobility of IGZO being 18 2 cm2 where Cox is the dielectric unit capacitance and HE is a. function of Lov describing the total contact resistance 26. As shown in Fig 3 the effective mobilities of the Considering the effective mobility as well as the parasitic. short channel devices dropped sharply because the total capacitance caused by the overlap length the current gain. resistance between source and drain is dominated by the cut off frequency is given by 26. contact resistance RC rather than the channel resistance Rch I KL OP RS. This is in contrast to long channel devices where Rch is much 3. 2 KL WS KL KY Z WS I KL H OP RS, higher than RC As a result in short channel devices the. effective carrier mobility is determined by both Lch and Lov as It shows that the cut off frequency is affected by both. shown in Fig 3 Based on Eq 1 therefore the improvement parasitic capacitance dependent term KL KY and. of TFT speed is non trivial since all three key parameters eff contact resistance dependent term I H Thus in order to. Lch and Lov are interplayed due to the influence of improve the high frequency performance in short channel. short channel effect and contact resistance When Lov is larger devices it is necessary to experimentally evaluate the. than the transfer length LT which describes the distance that influence of each factor. 63 carriers flow from the semiconductor into the electrode The small signal RF characterizations of the TFTs were. 37 the contact resistance remains almost constant However measured from 0 01 to 1 GHz with an input power of 10 dBm. when Lov LT the contact resistance becomes much larger by using a vector network analyzer connected to a. because it is determined not only by the overlap resistance two channel source measure unit through bias tees Cut off. region with a higher gate induced carrier concentration 33 frequencies including fT and fmax can be e. semiconductor changes from crystalline to amorphous phase Amorphous InGaZnO Thin Film Transistors Operating Beyond 1 GHz Achieved by Optimizing the Channel and Gate Dimensions Yiming Wang Hanbin Wang Jiawei Zhang He Li Gengchang Zhu Yanpeng Shi Yuxiang Li Qingpu Wang Qian Xin Zhongchao Fan Fuhua Yang and Aimin Song Senior Member IEEE T This work has been supported by the

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