ELEC 7364 Lecture Notes Summer Si Oxidation. by STELLA W. PANG. from The University of Michigan, Ann Arbor, MI, USA

Transcription

1 ELEC 7364 Lecture Notes Summer 2008 Si Oxidation by STELLA W. PANG from The University of Michigan, Ann Arbor, MI, USA Visiting Professor at The University of Hong Kong The University of Michigan Visiting Prof. HKU p. 1 S. W. Pang Typical CMOS Cross-Section Planar Technology with Multiple Levels The University of Michigan Visiting Prof. HKU p. 2 S. W. Pang

2 Films on Si Dif f us i on Im p l a n t a t i o n Depo s i t i o n Mod i f i c a t i on R e a c t i on Fil m for m ation Pl a t i ng Spin c o a t i ng Phys i c a l Ch e m i c a l O x ida t ion Nit r i d a t ion Si l i c i d a t ion Pho t ol i t h ogra phy Etchi n g The University of Michigan Visiting Prof. HKU p. 3 S. W. Pang Typical SiO 2 Thickness The University of Michigan Visiting Prof. HKU p. 4 S. W. Pang

3 Trends for Gate Oxide Deposited Oxide The University of Michigan Visiting Prof. HKU p. 5 S. W. Pang Si Oxidation Consumption of Si to form SiO 2 - Some of the Si near the surface is used to grow Oxide - Si-SiO 2 Interface moves down as SiO 2 is grown Thermal Oxide: Amorphous; 2.27 g/cm3; E g =9 ev (Quartz is Crystalline) The University of Michigan Visiting Prof. HKU p. 6 S. W. Pang

4 Oxide Structure and Interface The University of Michigan Visiting Prof. HKU p. 7 S. W. Pang Dry and Wet Oxide Oxidation Temperature 900 to 1100 C Dry Oxide: Slower Growth Rate, Better Quality Si + O 2 SiO 2 Wet Oxide: Faster Growth Rate, Lower Quality Si + 2H 2 O SiO 2 + 2H 2 The University of Michigan Visiting Prof. HKU p. 8 S. W. Pang

5 Oxidation Kinetics - Deal-Grove Model Oxidizing Species: O 2, O, O +, O -, O 2+, 1. Transportation from Ambient to Wafer Surface 2. Diffusion from SiO 2 Surface to Si-SiO 2 Interface 3. Reaction at Si-SiO 2 The University of Michigan Visiting Prof. HKU p. 9 S. W. Pang Oxidation Rate - Flux Balance and Interaction at Interface x o 2 + Axo = B(t + τ) x o = A 2 [(1+ t + τ A 2 / 4B )1/ 2 1] where x o =oxide thickness; t = oxidation time A = 2D( k s h ) with D=Diffusion Constant; k s =Reaction Rate Constant; h=gas phase mass transfer B = 2DC * N 1 with C*=Concentration of Oxidizing Species in SiO 2 ; N 1 =Number of Oxidant Molecules. Since SiO 2 has 2.2x10 22 molecules/cm 3, for O 2 oxidation, N 1 = 2.2x10 22 molecules/cm 3 and for H 2 O oxidation, N 1 =4.4x10 22 molecules/cm 3 τ = x 2 i + Ax i B with xi =Initial Oxide Thickness The University of Michigan Visiting Prof. HKU p. 10 S. W. Pang

6 Oxidation Rate Constants For Short Oxidation Time Linear Region - B/A = Linear Rate Constant, depends on Reaction Rate at Interface (t +τ ) << A2 4B x o B (t +τ ) A B A = k C * s For Long Oxidation Time Parabolic Region N 1 - B=Parabolic Rate Constant, Oxidation limited by Diffusion t >>τ x o 2 Bt The University of Michigan Visiting Prof. HKU p. 11 S. W. Pang Oxidation Rate Varies with Time Both Rate Constants (B/A, B) increase with Temperature - Faster for Wet Oxidation compared to Dry Oxidation B/A (µm/hr) B (µm 2 /hr) (hr) 1000 C Dry C Wet C Wet The University of Michigan Visiting Prof. HKU p. 12 S. W. Pang

7 Oxidation Rate Constants B/A Linear Rate Constant - Reaction Rate at Interface B Parabolic Rate Constant - Diffusion The University of Michigan Visiting Prof. HKU p. 13 S. W. Pang Si Wafers in Oxidation Furnace Horizontal furnace The University of Michigan Visiting Prof. HKU p. 14 S. W. Pang

8 Color Chart for Si 3 N 4 Thickness The University of Michigan Visiting Prof. HKU p. 15 S. W. Pang Factors that Affect Oxidation - I Crystal Orientation - Oxidation Rate for (111) > (110) > (100) - B Little Dependence on Orientation since diffusion through oxide is not affected by Si orientation - B/A Oxidation Rate Ratio (111)/(100) ~ 1.68 Reaction Rate at Si-SiO 2 interface depends on number of Bonds available for Oxidation; (111) 1.18x10 15 cm -2 ; (100) 6.77x10 14 cm -2 - Less Crystal Orientation Dependence at Higher Temperature or Longer Time since the oxidation is limited by Diffusion in Parabolic region Dopant Effects - Dopants (B, As, P, ) Enhance Oxidation Rate - For B: B Piles up in SiO 2, weakens SiO 2 Structure and enhances Diffusion of Oxidant - For P: P Piles up in Si. Linear Rate Constant (B/A) becomes faster, not much effect on B - Heavy Doping causes E F to shift, which increases Vacancies in Si. This provides more Reaction Sites for Oxidation The University of Michigan Visiting Prof. HKU p. 16 S. W. Pang

9 Factors that Affect Oxidation - II H 2 O Effect (Dry vs. Wet Oxidation) - H 2 O Increases Oxidation Rate with Lower Quality Oxide and more Defects - Both B/A and B increase for Wet Oxidation - Faster Reaction Rate at SiO 2 -Si Interface - Larger Amount of Oxidant Allowed in SiO 2 : C* for H 2 O 3x10 19 cm -3 ; C* for O 2 5.2x10 16 cm -3 Halogen Effect (Cl 2 or F-based) % HCl Addition - Increases Oxidation Rate since HCl reacts with O 2 to from H 2 O - Converts Impurities in Si (e.g. Na, Fe, ) to form Volatile Products: Reduce Metallic Impurity Concentration and Defects in Oxide; Improve Breakdown Voltage and Lifetime The University of Michigan Visiting Prof. HKU p. 17 S. W. Pang Factors that Affect Oxidation - III Pressure Effect - High Pressure Oxidation atm, C Increases Vertical Growth Rate faster than Lateral Growth Rate Oxide can be grown at Lower Temperature (Temperature reduced by 30 C with increase of 1 atm) Reduce Birdʼs Beak Encroachment - Low Pressure Oxidation 10-2 to 1 atm, C Reduce Rate for Thin Oxide Higher Oxide Quality to grow at Lower Pressure than Lower Temperature Oxidation rate ~ P 0.8 The University of Michigan Visiting Prof. HKU p. 18 S. W. Pang

10 Faster Oxidation Rate at Higher Pressure The University of Michigan Visiting Prof. HKU p. 19 S. W. Pang Oxidation to Isolate Devices ~30% Volume Expansion when Oxide is Formed Defects Around Birdʼs Beak Recess Oxide to Reduce Topography The University of Michigan Visiting Prof. HKU p. 20 S. W. Pang

11 Dopant Distribution at Interface Dopant Re-Distribute until Chemical Potential is the same on each side of Interface Can Affect V T due to change in Channel Doping Segregation Coefficient Dopant in Si m = Dopant in SiO 2 Examples: (Dry Oxidation, 950 C; Left: B in Si; Right: P in Si) The University of Michigan Visiting Prof. HKU p. 21 S. W. Pang Dopants In Oxide and Near Interface a) m<1, slow oxide diffusion: e.g. B b) m<1, fast oxide diffusion: e.g. B with H c) m>1, slow oxide diffusion: e.g. P d) m>1, fast oxide diffusion: e.g. Ga The University of Michigan Visiting Prof. HKU p. 22 S. W. Pang

12 Thin Oxide (<30 nm) Grown at Lower Temperature, Lower Pressure, Rapid Thermal Oxidation (RTO), or Plasma-Assisted Oxidation Deal-Grove Model: OK for Wet Oxidation (τ=0); Faster Initial Growth Rate, Adjusted by having τ > 0 For Thin Oxide, Oxidation Rate decreases exponentially with thickness: dx ox dt = B 2x ox + A +C 1e From D-G Excess Rate Decay Length: L 1 ~ 5 nm; L 2 ~ 7 nm Si Surface with Additional Sites for Oxidation. These sites decrease exponentially into Si with decay length of ~3 nm. These sites enhance oxidation Blocking Layer SiO causes accumulation of Oxygen that can transform rapidly to SiO 2 x ox L 1 +C 2 e x ox L 2 The University of Michigan Visiting Prof. HKU p. 23 S. W. Pang Problems - Large Leakage Current (e.g. at 1V, 1x10-12 A/cm 2 for 3.5 nm, 10 A/cm 2 for 1.5 nm) - Defects in Oxide and Interface - Control of Oxide Thickness - Dopant Penetration from Doped Poly-Si Gate Approaches Ultra Thin Gate Oxide (<5 nm) - Nitrided Films (Oxide-Nitride, Oxide-Nitride-Oxide, Oxynitride, ) - High k Dielectrics (ε SiO2 =3.9; ε Si3N4 =7.5; ε Ta2O5 =25; ε TiO2 =50; ε SrTiOx3 =150, ) Equivalent Oxide Thickness (compared to SiO 2 with ε=3.9) For x eq = 3 nm and with ε SiO2 = 3.9, ε TiO2 = 50: x TiO2 = X eq ε TiO2 ε SiO2 = 38.5nm The University of Michigan Visiting Prof. HKU p. 24 S. W. Pang

13 Alternative Oxides Thermal Oxidation with NO, N 2 O, O 2 re-oxidation of oxynitrides HF/H 2 O Vapor - NO Oxynitride, Self-Limiting Growth since nitrogen rich oxide is a diffusion barrier - N 2 O Reduce to NO and O which grow oxide with N Close to Si -SiO 2 Interface: NO Nitrogen in Oxide; O Remove N in Oxide near Surface - O 2 Anneal of Oxynitride Forms SiO 2 at Interface with Oxynitride near Surface Plasma Assisted Oxidation - Remote Plasma to Activate O 2, N 2 O, N 2, NH 3, - Lower Thermal Budget - Typical Condition for Oxidation/Nitridation: 300 C, 300 mtorr, <1 min; Annealing at 900 C, 30 s - N Tends to Locate Close to Si-SiO 2 Interface with ~1 monolayer N (7x10 14 cm -2 ) after 90 s plasma exposure when remote plasma at higher pressure (~300 mtorr) and N 2 O is used - N Tends to be at Top Surface when N2 is used at lower pressure (~100 mtorr) - Effective in Lowering Leakage Current and Blocking B Diffusion from Poly-Si Gate The University of Michigan Visiting Prof. HKU p. 25 S. W. Pang Oxide-Nitride Stacks Advantages of Nitride - Interface Nitridation Lower Leakage Current - Bulk Nitride Allows Increased Physical Thickness - Top-Surface Nitride Block Dopant Diffusion from Poly-Si Gate Disadvantages of Nitride - Film Stability - Positive Charges and Defects in Nitride tend to vary when Bias is Applied - Worse Stability and Reliability of Nitride as Gate Dielectric The University of Michigan Visiting Prof. HKU p. 26 S. W. Pang

14 High k Dielectrics Allows Thicker Film to Lower Leakage Current - Fowler-Nordheim Tunneling Current Increases Exponentially with Reduced Thickness or Reduced Barrier Height Needs Thicker Film with Larger Barrier Height Needs Thermally Stable Film (most high k films form SiO 2 on Si) Needs to develop New Etching Techniques Silicates and Oxide of Hafnium and Zirconium - HfSiON, HfO 2, HfSiO,. The University of Michigan Visiting Prof. HKU p. 27 S. W. Pang Properties of Insulators The University of Michigan Visiting Prof. HKU p. 28 S. W. Pang