Why SiO 2? The extraordinary properties of SiO 2 are the basis of the success of MOS-technology

Transcription

1 Oxidation 1

2 Why? The extraordinary properties of are the basis of the success of MOS-technology Non-crystalline insulator Very high energy gap Φ M Easy to grow on Easy to integrate in a process E M Excellent interface between substrate Stable and insensitive to following process-steps Excellent scaling possibilities no real candidates for replacement E 0 1.0eV χ ox 9.0eV 3.0eV 4.9eV Φ S χ si E C E V 2

3 Properties of - : 3.12 Å, -O: 1.62 Å, O-O: 2.27 Å 10 nm : atomic layers 1.2 nm : only about 4 atomic layers Bonding angle : 110~180 ( 144 ) Dielectric constant: 3.9 Energy gap : 9 ev Density : 2.20 g/cm 3 Refractive Index : ~1.462 Dielectric strength MV/cm Tetrahedra structure θ Bridging Oxygen Oxygen licon 3

4 Application of Thermal Oxides 4

5 Transistor Process Flow (1970s) Al nmos Cross-section PSG poly n + n + p- N Clean p- p- Field Oxidation Oxide Etch p- p- Gate Oxidation Poly Dep. p- poly p- poly Poly Etch P + Ion Implant p- poly n + poly p- n + Annealing 5

6 Masking Oxide Much lower B and P diffusion rates in than that in can be used as diffusion mask Dopant 6

7 Pad Oxide Relieve strong tensile stress of the nitride Prevent stress induced silicon defects licon nitride Pad Oxide licon Substrate 7

8 Blanket Field Oxide Isolation licon Wafer Clean licon Dioxide licon Activation Area Field Oxidation Field Oxide licon Oxide Etch 8

9 Gate Oxide Thickness of oxide (T ox ) must closely match the specification of the MOSFET design T ox must be sufficiently uniform across the entire wafer, and from wafer to wafer, and from run-to-run Extremely low Q f and D it (Good / interfacial properties) E BD > 8MV/cm, pinhole free and negligible defects Sufficient long lifetime under normal operating High resistance to hot-carrier damage Resistance to boron penetration Refer to Dr. Hong Xiao V G V D > 0 Poly Gate Thin oxide n + Source Electrons p- n + Drain Substrate Source: Dr. PT Liu 9

10 Growth Mechanism Native oxide: surface has a high affinity for oxygen formed in the air or chemical cleaning process quality is bad and should be eliminated Å Thermal oxidation: Dry Oxidation ( solid) + O O ( solid) 2 2 Wet Oxidation (steam oxide) ( solid) + 2 H O O ( solid) + 2H

11 1000 o C + Dry Oxidation Original licon Surface licon Dioxide ( ) 55% 45% licon wafer () Thickness of silicon = 0.45 x (thickness of ) substrate substrate Oxidizing species diffuse through to / Source: Dr. PT Liu 11

12 Oxide Growth: Deal-Grove Model Deal-Grove Relation (linear-parabolic growth law): Linear Growth (Reaction-Limited) Regime B Oxide Thickness X = t A Diffusion-limited Regime X = B t t ox t 0.44 t ox Top oxide surface Original silicon surface Oxidation Time licon Oxide-silicon surface 12

13 Growth Mechanism Deal-Grove Relation (linear-parabolic growth law): 2 ox T + AT = B( t+ τ ) 2 Tox = Bt ox for t >> τ, t >> A 2 /4B Diffusion-controlled for (t+ τ) << A 2 /4B T ox B = ( t+τ ) A Reaction-controlled * DC x + Ax i i A= 2 D[ + ] B= τ = k h N B A, B : temperature, ambient composition, pressure and crystalline orientation τ is related to the initial oxide thickness s I C g Gas phase C g Gas phase Reaction control F 1 Concentration Diffusion control F 1 C 0 Concentration C 0 F 2 F 2 C* F 3 X 0 X 0 C* C i C i + F

14 <100> licon Dry Oxidation Oxide Thickness (micron) <100> licon Dry Oxidation 1200 C 1150 C 1100 C 1050 C 1000 C 950 C 900 C Oxidation Time (hours) 14

15 Interfacial Structure Dangling bond surface Interfacial trap impurity Oxygen vacancy Stretched bond 15

16 Interfacial Structure bulk-oxide count TEM boundary total Oxygen count sub-oxide count t ox D. Muller, et. al, Nature, 399, 758(1999) 16

17 Oxide Traps and Defects Mobile Ionic Charge K + Na + Oxide Trapped Charge Fixed Oxide Charge Interface Trapped Charge (Deal, 1980) Fixed Charge (Q f, N f ) (Positive): structural defects due to incomplete oxidation and stress, within 2.5 nm from / not electrically communicate with interface states Mobile Oxide Charge (Q m, N m ): Na +, Li +,K +,H + Oxide Trapped Charge (Q ot,n ot ): positive or negative due to hole or electron traps in borken -O bonds radiation, charge injection in high field Interface Trapped Charge (Q it,n it, D it ): structural defects due to incomplete oxidation at / donor or acceptor-like surface-potential dependent 17

18 / Interface Interface states: imperfect bonds Electrically interacted with channel carriers Assuming each dangling bond give rise to one interface state Impact on device characteristics threshold voltage ( V t ) carrier mobility ( G m ) reliability, oxide integrity and HCI( hot-carrier-injection) degradation Hydrogen annealing at o C is effective to passivate Q it at the very end step of process. 18

19 Deal-Grove Triangle Q f decreases with increasing oxidation temperature Dry Post oxidation annealing in N 2 or Argon ambient is needed to minimize Q f However, annealing should be kept within specific time period without causing Dry N 2 or Argon increase of Q f Fixed Charge, Q f 19

20 Conventional Furnace Equipment 20

21 Thermal Process Hardware Control System Gas Delivery System Loading System Exhaust System Process Tube 21

22 MFC Furnace System MFC MFC MFC Process Tube Control Valve Scrubber Regulator HCl N 2 吹除淨化氮氣 Exhaust Control System 在氧化製程中, 總是把氮氣當作鈍氣應用在系統閒置時 晶圓裝載 溫度提昇 溫度穩定和晶圓卸載等步驟中 乾式氧化步驟中也使用氯化氫,HCl or TCA (trichloroethane), 來減少氧化物中的移動離子, 使其成為不可移動的氯化物化合物, 並將界面電荷 (Interface state charge) 降至最低!! 22

23 Furnace Configuration Horizontal Tube Vertical Tube Center Zone Heating Coils Heaters Gas Flow Quartz Tube T ± 0.5 Flat Zone Tower several hundred wafers Distance Ramp up ~1 /sec 23

24 Dry Oxide Process Sequence Idle with purge N 2 flow Idle with process N 2 flow Wafer boat push in with process N 2 / flow Temperature ramp-up with process N 2 / flow Temperature stabilization with process N 2 / flow Oxidation with, HCl; stop N 2 flow Oxide annealing; stop ; start process N 2 flow Temperature cool-down with process N 2 flow Wafer boat pull out with process N 2 flow Idle with process N 2 flow Repeat process with next boat 24

25 Oxidation Recipe 25

26 Wet Oxidation Faster, higher throughput (H 2 O, HO species) Thick oxide, such as LOCOS Dry oxide has better quality Process Temperature Thickness Oxidation Time Dry oxidation 1000 C 1000 Å ~ 2 hr Wet oxidation 1000 C 1000 Å ~ 12 min Source: Dr. PT Liu 26

27 Effect of Oxidation Ambient Wet oxidation rate is much higher than dry oxidation rate because HO - or H 2 O diffuses much faster than in. 27

28 Pyrogenic Steam System Hydrogen Flame, 2 H H 2 O H 2 To Exhaust Thermal Couple Process Tube Paddle Wafer Boat Typical H 2 : ratio is between 1.8:1 to 1.9:1. 28

29 Outside Torch System (OTS) 29

30 Pyrogenic Wet Oxidation System MFC Process Tube MFC MFC MFC Wafers Burn Box Control Valves H 2 Process N 2 Purge N 2 Regulator Scrubbier Exhaust 30

31 Pyrogenic Oxide Process Sequence Idle with purge N 2 flow Idle with process N 2 flow Ramp with process N 2 Wafer boat push in with and process N 2 flows Temperature ramp-up with and process N 2 flows Temperature stabilization with and process N 2 flows Ramp turn off N 2 flow Stabilize flow Turn on H 2 flow, ignition, and H 2 flow stabilization 31

32 Pyrogenic Oxide Process Sequence (Cont.) Steam oxidation with and H 2 flows Hydrogen termination; turn off H 2 while keeping flow Oxygen termination; turn off, start process N 2 flow Temperature ramp-down with process N 2 flow Wafer boat pull out with process N 2 flow Idle with process N 2 flow Repeat process with next boat Idle with purge N 2 flow 32

33 Oxide Measurement Thickness Uniformity SEM, TEM, Profilermeter Color chart Spectrophotometry (Reflectometry) Ellipsometry C-V I-V, breakdown voltage C-V, oxide charge 33

34 Spectrophotometry (Reflectometry) Incident light 1 Interference 2 Human eye or photodetector t Dielectric film, n( λ) Substrate 34

35 Interference in Thin Films 180 o phase change λ 0 n 0 sinφ= n 1 sinβ λ 1 = λ 0 /n 1 n 2 > n 1 > n 0 =1 λ 2nx cosβ 2x cosβ = m λ = n m m = 1,2,3 : constructive interference m = 1/ 2,3 / 2,5 / 2 : distructive interference 35

36 Color Chart 36

37 Spectroreflectometry System Substrate Film Detectors UV lamp Reflectance (%) Constructive interference Destructive interference λ 1 λ 2 λ Wavelength (nm) 37