Semiconductor devices for display and memory application

Transcription

1 Semiconductor devices for display and memory application Chungnam National University April 18, 2014 Gawon Lee 1

2 Contents 1. Semiconductor Engineering Lab. 2. Oxide Thin Film Transistors 2.1 Introduction 2.2 Thin Film Transistor (TFT) 2.3 Oxide TFT for Display application 3. NVM: SONOS Flash Memory 3.1 Introduction 3.2 Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) Flash Memory 3.3 SONOS Technology 2

3 Semiconductor Engineering Laboratory 학생현황 : 석사 5 명 / 박사 4 명 (2014 년 4 월 ) 홈페이지 : cnu.ac.kr/~semieng, gawon@cnu.ac.kr 주요연구분야차세대디스플레이셀구동용박막트랜지스터연구차세대고성능플래쉬메모리연구이종접합태양전지 3

4 Oxide TFT Research Field Fabrication Analysis Stability TFT Structure Schottky contact adoption to suppress back channel Channel Engineering New cation adoption (Ti) Gate Insulator High-k dielectric (Al2O3) S/D Contact Homogeneous contact using AZO TCO Interface control Passivation High-k dielectric New Merged Device Using nano-particle Characterization Tool Development 1/f Noise Analysis 도입 Pulsed IV method Simulation Feedback to mechanism model Potential distribution Dominant current path Bias Stability Hump Analysis focusing on ion movement Illumination Stability V Th focusing on oxygen vacancy (V O ) model 4

5 Flash Memory Research Field Fabrication Analysis Stability SONOS Structure 2D structure 3D structure Retention/Reliability Degradation Mech. Tunneling Ox. Engineering N2 implant adoption Trapping layer High-k dielectric New NVM Device Using nano-particle Transparent flexible NVM Characterization Tool Development Charge pumping method 1/f Noise Analysis Simulation Interface trap effect Structure effect (Rounding effect in 3D) 5

6 Flexible transparent Display/Device Application: Oxide Thin Film Transistors 6

7 Introduction: Flexible Transparent Display 7

8 Introduction: Flexible Transparent Display Flexible display : cell phone, PDA, laptop, e-book,wearable lightweight, low power consumption, bendable 8

9 Introduction: Flexible Transparent Display 얇고가볍고전력소비효율이높고깨지지않는투명하고유연한특성이중요해짐. Notebook, monitor, digital TV market, E-book application 현재 display 소자에사용되고있는 a-si, poly-si 의단점을극복할수있는 (High Temp. Process, Low Performance, 낮은광투과도등 ) 새로운전자소자개발필요성극대화 9

10 Introduction: Short History of Thin Film Transistors Julius Edgar Lilienfeld US "Method and apparatus for controlling electric current" first filed in Canada on (1930), describing a device similar to a MESFET US "Device for controlling electric current" filed on , a thin film MOSFET US "Amplifier for electric currents" filed on , solid state device where the current flow is controlled by a porous metal layer, a solid state version of the vacuum tube US "Electrolytic condenser" filed on , Electrolytic capacitor From Thin Film Transistor Technology Past, Present, and Future, The Electrochemical Society Interface (Spring 2013) 10

11 Introduction: Short History of Thin Film Transistors From Transparent Electronics: From Synthesis to Applications, Wiley 11

12 Introduction: Short History of Thin Film Transistors From Transparent Electronics: From Synthesis to Applications, Wiley 12

13 Introduction: Short History of Thin Film Transistors 트랜지스터발명가,Bardeen,Schockley,Brattain 게르마늄단결정으로만든최초의 BJT 13

14 Thin Film Transistor : Definition TFT: 절연성기판위에형성된 Field Effect Transistor A voltage applied at the gate controls the flow of electrons (resistance) from the source to the drain; a positive gate voltage attracts electrons to the bottom surface of the semiconductor layer and creates a conduction channel. When a voltage difference is applied between the two connector wires, electrons enter at one end (the source) and exit at the other (the drain), resulting in a current along the channel. Figure from 14

15 Thin Introduction Film Transistor: Structure - A common substrate is glass, since the primary application of TFTs is in liquid crystal displays while the conventional substrate of transistor is silicon wafer. But any substrate can be used (glass, polymer sheets, steel foils..), which causes flexible electronics but limits the process temperature and deposition method. - By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be made completely transparent 15

16 Thin Introduction Film Transistor: Structure A thin-film transistor (TFT) - Special kind of field-effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate. 16

17 Introduction Thin Film Transistor: Manufacture Topside resist backside exposure Cr-gate has been patterned on the glass substrate PECVD gate oxide a-si:h channel and nitride stopper layers have been deposited. Nitride etching and resist stripping Chrominu sputtering CrSi2 formation. 17

18 Introduction Thin Film Transistor: Manufacture - Material Silicon : amorphous silicon, microcrystalline silicon, poly-silicon. Compound semiconductors: cadmium selenide, metal oxides such as zinc oxide Organic materials - Process Temperature Substrates cannot withstand high annealing temperatures, the deposition process has to be completed under relatively low temperatures. - Thin Film Deposition/Formation Process Chemical vapor deposition (CVD, ALD) Physical vapor deposition (Sputtering, Evaporation, MBE) Solution-coating process (Printing) 18

19 Thin Film Transistor for Display Application Active Matrix Liquid Crystal Display (AMLCD) - Switching transistor/diode - 19

20 Thin Film Transistor for Display Application Structure of TFT-AMLCD : Color TFT Panel 20

21 Thin Film Transistor for Display Application Appendix) TFT-LCD 제작용유리기판사이즈 Need for Uniformity and Low Temperature Process 21

22 Thin Film Transistor for Display Application OLED Structure Need for High mobility and Uniformity 22

23 Thin Film Transistor for Display Application AM OLED Structure 23

24 Oxide TFT for Display Application A-Si TFT LTPS TFT Oxide TFT Semiconductor Amorphous Si Poly Si Oxide Process (TFT) 4~5 mask 5~9 mask 4~5 mask Mobility <1cm 2 /V-s 50~150cm 2 /V-s 1~80 cm 2 /V-s TFT uniformity Good Poor Good Cost Cost/Yield Low/High High/Low Low/High Process Temp. ~250 o C >250 o C RT~300 o C Display Mode LCD, OLED LCD, OLED (Small size) LCD, OLED Substrate Glass Glass Glass, Plastic 경제성, 성능그리고신뢰성이모두확보되는기술이필수적 Si 을근간으로하는 TFT 의경우낮은이동도가문제 ( 대형 TFT 의성능을만족시킬만큼높지않다 ) Si 자체가가시광 [ 가시광선의에너지 (~3.1eV)] 영역을흡수 ( 차단 ) 하기때문에화면의휘도가낮아짐 유기물사용의경우아직도재료및공정등이초기기술상태이며향후에도신뢰성확보가쉽지않을것으로예상 Oxide TFT 는현재의 AMLCD 및 AMOLED 에서사용되고있는 Si-based TFT 를대체하여고성능소자제작가능 24

25 Oxide TFT for Display Application Source: IDTechEx 25

26 Oxide TFT for Display Application LTPS and Oxide TFT Backplane Manufacturing Capacity Source: 2013 NPD DisplaySearch TFT LCD Process Roadmap Report 26

27 Oxide TFT for Display Application Material Binary compound - ZnO - SnO 2 - In 2 O 3 Ternary compound - IZO, ZTO, IGO - AZO, GZO (TCO) ZnO가가장안정적인특성. In2O3가 bixbyite 구조로 octahedral site를공유함으로인해 high-mobility의특성을보임 3족원소인 Ga, Al을첨가하여 carrier concentration control Quaternary compound - IGZO(In 2 O 3 -Ga 2 O 3 -ZnO) - SGZO(SnO 2 -Ga 2 O 3 -ZnO) IGZO 는상온에서 Amorphous 구조이나, High-mobility 특성을나타냄. - bixbyite 구조 : 산소일부가부족한구조, Oxygen vacancy 의농도가높기때문에 ionic conductivity 가높다 27

28 Oxide TFT for Display Application Material: ZnO The ZnO channel TFTs exhibit better performance (higher field-effect mobility) than organic and a-si:h TFTs Resource availability (Low cost material) Non-toxicity Direct, Wide bandgap energy (3.4eV) -> High transmittance High exciton binding energy(60 mev) Low temp./inexpensive growth Easily etched (acids and alkalis) Thermal, chemical, Radiation stability Current Problems N-type only because of Oxygen vacancy and Zinc interstitial High performance TFTs with stability and uniformity 29

29 Oxide TFT for Display Application Appendix) Zinc interstitial in ZnO Zn Zn - Zn - Ec Zn 2n Zn + +e - Zn + Zn ++ +e - Zn 2n Zni ++ +2e - n-type Ev Eg=3.36eV Zni=2.9eV 30

30 Oxide TFT for Display Application Appendix) Oxygen vacancy in ZnO Vo O 2 는높은휘발성에의해 Vacancy 생성 Zn O O O 2-2 ( ZnO 2 gas) Vo 2 e Zn dangling bonds each contributing 1/2 electron to a neutral vacancy 31

31 Oxide TFT for Display Application Material: a-igzo + Gallium Covalent bond length Ga-O = 1.92 Å Zn-O = 1.97 Å No deformation of the ZnO lattice High concentration of Ga + Indium Indium-gallium-zinc oxide Indium-zinc oxide Heavy metal cations Lower processing temperature, Good uniformity / reproducibility High mobility 32

32 Oxide TFT for Display Application Material: a-igzo 33

33 Oxide TFT for Display Application Material: a-igzo Improvement of optical characteristic and resistivity Low temperature deposition and good film smoothness, uniformity (amorphous oxides have more uniform structures and smoother surfaces) High performance (In spite of the amorphous structure, these oxides have relatively high electron mobility because Gallium Indium) 34

34 Nonvolatile Memory: Silicon-Oxide-Nitride-Oxide-Silicon Flash Memory 35

35 Introduction: Standard semiconductor memories Referred From Doo Seok Jeong, et. al., Emerging memories: resistive switching mechanisms and current status, Rep. Prog. Phys. 75 (2012) 36

36 Introduction: Storage capacitor of NVM s Image from 37

37 Introduction: NAND Flash Roadmap Image from 38

38 SONOS Flash Memory : Structure 39

39 SONOS Flash Memory : Basic Mechanism NOR vs. NAND type Flash Memory Similar Structure with DRAM Structure (Higher Density) Picture from Samsung Flash Seminar 40

40 SONOS Flash Memory : Basic Mechanism NAND Type: Program Method Picture from Samsung Flash Seminar 41

41 SONOS Flash Memory : Basic Mechanism NAND Type: Read Method Picture from Samsung Flash Seminar 42

42 SONOS Flash Memory : Basic Mechanism NAND Type: Program/Erase Method Picture from Samsung Flash Seminar 43

43 SONOS Flash Memory : Structure 44

44 SONOS Tech: Bandgap-Engineered (BE) SONOS - Tunnel oxide is replaced by triple layers of ultrathin ONO - Good data retention (charges are blocked by the total ONO barrier) - High Erase speed (easy tunneling through thin bottom oxide) Referred from Szu-Yu Wang et al., Reliability and Processing Effects of Bandgap-Engineered SONOS (BE-SONOS) Flash Memory and Study of the Gate-Stack Scaling Capability, IEEE Transactions on Device and Materials Reliability, Volume 8, Issue 2, June 2008 Page(s):

45 SONOS Tech: BE-SONOS : Gradual Variation of relative S/N ration (Tapered Bandgap Nitride Layer) Key Idea : The small barrier heights between nitride and the surrounding oxides have rendered the charges that enter nitride slip away easily and result in a poor charge-trapping efficiency. In addition, the deep trapping levels of standard nitride also make it difficult to catch carriers. 46

46 SONOS Tech: TANOS (SANOS) Band Diagram of SONOS and SANOS TaN Gate AL2O3 SiN Tunnel Oxide N + N + High-K dielectric for blocking oxide Low-voltage operation Possible to use thicker tunnel oxide High work function of TaN decrease of electron injection from gate during FN erase operation 47

47 SONOS Tech: 3D SONOS Technology 3D Structure: Technology Direction of NAND flash 48

48 SONOS Tech: 3D SONOS Technology BiCS Structure (2007, VLSI Symp.) P-BiCS Structure (2009, VLSI Symp.) 49

49 SONOS Tech: 3D SONOS Technology TCAT Structure (2009, VLSI Symp.) 50

50 Summary 1. Oxide Thin Film Transistor (Most promising candidate for next-generation display) Low Temperature Process/High Performance/Transparent/Low cost Material: IGZO and ZnO Issue: Bias/Optical Stability is required to be improved 2. SONOS (Promising candidate for NVM device) Scaling Limits of FG Flash Memory Tunneling Oxide Thickness/GCR/Floating Gate Interference (Crosstalk) SONOS Technology Trade-off between P/E speed and Data retention Gate Stack Optimization (TANOS, BE-SONOS, etc.) New Structure (3D structure) 51