1 Layer Deposition: Thermal Oxidation and CVD Rupesh Gupta IIT Delhi Supervisor: Dr. Chacko Jacob
2 OUTLINE Thermal Oxidation and Model o Factors Affecting Kinetics o Future Trends: Oxidation o CVD and Model o Factors Affecting Kinetics o Future Trends: CVD
3 Silicon Oxide Si is unique as its surface can be easily passivated with oxide layer. SiO 2 layers adhere well block diffusion of impurities can be easily patterned and etched are excellent insulators (gate dielectrics)
A Comparison 4 Oxidation Deposition Done on Si Good process control Electrically perfect Si- SiO 2 interface When underlying film is not Si (back end applications) Not used for layers<10nm Not as electrically perfect (not used for dielectrics)
5 Thermal Oxidation Oxidation process occurs at Si/SiO 2 interface by inward diffusion of oxidant. New interface is constantly forming and moving downward into the Si substrate. Si-Si bonds broken, Si-O bonds formed. Process involves volume expansion because of room needed for oxygen atoms.
Volume Expansion 6 Ambient O 2 / H 2 O SiO 2 Silicon Si O 2 SiO 2 Si 2H O H 2 SiO2 2 2 Flat and Narrow Transition Region Compressive Stress
7 Modeling Oxidation
The Deal Grove Model 8 Showed that over a wide range of conditions, the growth followed a linear parabolic law. Still used today to model planar oxidation. Cannot explain kinetics in shaped surfaces mixed ambients very thin oxides < 20nm oxides grown on heavily doped substrates
Three Step Process 9 C g C s C o C i F 1 F 2 F 3 SiO 2 Si F 1 h g C g C s F 2 D Co Ci t ox F k C 3 s i h g : mass transfer coeff. D: oxidant diffusivity k s : rate constant for oxidation reaction
10 Rigorous Solution Henry s Law: C 0 = HP S P S = partial pressure of oxidant Use: C S = N/V and PV = NkT C 0 = H (kt.c S ) => C S = C 0 /HkT
C* = Concentration in the solid oxide which would be in equilibrium with the partial pressure in the bulk of the gas (P G ) 11 C* = HP G F 1 = h(c*-c 0 ) ; h = h G /HkT Steady state: F 1 = F 2 = F 3 C i 1 C * ks kst h D ox CO Ci1 kst D OX
Solution of Deal Grove Model 12 F F 3 k C N S i 1 dt dt OX t 2 ox At ox B t A 1 1 2DC* 2D B ks h N1 X 2 i AX B i B C 1 E1 exp( ) kt B A C 2 E2 exp( ) kt
Rate Limiting 13 The slowest step out of oxidant diffusion and interface reaction will determine the overall process rate. The resultant oxide growth rate is t 2 ox At ox B t
Rate Limiting Steps 14 t ox t ox C G C G C S C* C* C i C S C i Reaction Controlled Regime Diffusion Controlled Regime B t ox t t 2 ox Bt A
15 Common Oxidation Methods Dry Oxidation Wet Oxidation Si O 2 SiO 2 Si 2H2O SiO2 2H2 Slow Growth For films up to 100-200 nm Faster growth due to high solubility of H 2 O in SiO 2 (Higher B, B/A values ) For thicker films
Oxidation Equipment 16 Quartz Tube Wafers Resistance Heating Flat temperature profile maintained using thermocouples
17 OUTLINE Thermal Oxidation and Model Factors Affecting Kinetics o Future Trends: Oxidation o CVD and Model o Factors Affecting Kinetics o Future Trends: CVD
18 Factors Affecting Growth Kinetics Temperature B C 1 E1 exp( ) kt B A C 2 E2 exp( ) kt Crystal Orientation Pressure
Crystal Orientation 19 1.68 (111) (110) Difference more obvious for thin oxides (100) Most ICs made with (100) Si orientation
Pressure 20 Increase in oxidation rate even at low temperatures
21 OUTLINE Thermal Oxidation and Model Factors Affecting Kinetics Future Trends: Oxidation o CVD and Model o Factors Affecting Kinetics o Future Trends: CVD
Future Trends: Oxidation 22
23 Challenges 2D and 3D effects have become increasingly important in small structures. Accurate prediction of shapes of oxides grown on non planar structures and stresses generated.
24 Challenges Increasing use of low temperature processes to control impurity diffusion. Oxides grow slowly at low temperatures. To obtain relatively thick layers: High pressure oxidation Deposited SiO 2 films. Ideal properties of Si/SiO 2 interface preserved by first growing a thin thermal oxide and then depositing SiO 2 on top.
25 OUTLINE Thermal Oxidation and Model Factors Affecting Kinetics Future Trends: Oxidation CVD and Model o Factors Affecting Kinetics o Future Trends: CVD
26 Chemical Vapor Deposition Reactant gases are introduced into the deposition chamber Chemical reaction between the these gases on the substrate surface produces the film
27 Chemical Vapor Deposition Dielectrics Semiconductors Metals
Issues 28 Poor Step Coverage Conformal Coverage High electrical resistance Greater chances of mechanical cracking and failure
Space Filling 29
Process 30 Gas Stream 1 7 2 6 3 4 5 Wafer Susceptor
31 Growth Kinetics Governed primarily by the steps 1. Mass Transfer 2. Surface Reaction
32 F h C C F k S C 1 g G S 2 S
Solution 33 Two limiting cases: 1. Diffusion controlled v h g 2. Reaction controlled v k S Growth rates in both regimes is linear with time. Reaction always occurs at the growing surface The diffusion is through a gas region of constant thickness not a growing solid region as in oxidation.
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Atmospheric Pressure CVD 35 APCVD for epitaxial Si deposition SiCl H Si 4HCl 4 2 2 A Cold Wall Reactor
36 OUTLINE Thermal Oxidation and Model Factors Affecting Kinetics Future Trends: Oxidation CVD and Model Factors Affecting Kinetics o Future Trends: CVD
37 Factors Affecting Growth Boundary Layer Effects Depletion Effects Unintentional Doping Pressure
Boundary Layer Effects 38 Continuous gas flow Diffusion of reactants Boundary layer Deposited film Silicon substrate A Detailed Picture of Deposition Process
39 Velocity Profile Gas flow Gas flow Boundary layer Boundary layer
Solution: Reactor Geometry 40 Effect important in Transfer limited regime Susceptor is slightly tilted to minimize effect.
41 Depletion Effects Source gas depletion occurs down the length of susceptor as reactant gases are consumed. Important in Reaction limited regime Solution: 5-25 temperature gradient imposed along the chamber to compensate. Alternatively the gas can be injected straight down from above.
42 Unintentional Doping Common when depositing a lightly doped epitaxial Si film on a highly doped Si substrate. 1. Outdiffusion Solid State Diffusion 2. Autodoping Addition of dopant atoms to gas stream Evaporation from wafer frontside, backside, other wafers or susceptor
Pressure 43
LPCVD 44 Atmospheric pressure systems have some major drawbacks: If operated at high T (transfer controlled) the wafers must be placed horizontally If operated at low T (reaction controlled) the deposition rate is low. No restriction on wafers Both result in low throughput Solution: Operate at low pressure in reaction controlled limited regime
45 Pressure Lowering Diffusivity ( no.of collisions experienced by gas species ) 1 Diffusivity 1 P Total P Total h ( Diffusion Coeff.) g
46 Effect of Pressure Lowering
LPCVD Reactor 47 P Quartz tube Pump Gas inlet Stand up wafers Wafer boat A Hot Wall Reactor
48 Plasma Enhanced CVD There may be restrictions on temperatures the substrate can be exposed to At low T, APCVD and LPCVD proceed at low deposition rates Solution: Using plasma source in addition to thermal source
PECVD Equipment 49 Inert Gas Process Gas RF Power Heated Plate Wafer By Products
Mechanism 50
51 Advantages of PECVD 1. Lower processing temperatures Low film stress 2. High deposition rates 3. Good film adhesion to the wafer 4. Excellent gap-fill and conformal coverage
52 OUTLINE Thermal Oxidation and Model Factors Affecting Kinetics Future Trends: Oxidation CVD and Model Factors Affecting Kinetics Future Trends: CVD
53 Future Trends: CVD Ever increasing aspect ratio Metal thickness not shrinking as fast as lateral dimension Difficult coverage and filling Solution: Atomic layer deposition technique Aspect Ratio h w
54 Challenges Contamination Like carbon from precursors Difficulty with alloys Unwanted reactions Controlling the exact composition of alloy
Summary 55 Oxidation Volume Expansion Linear Parabolic Model Factors Affecting Kinetics T, P: + Effect Crystal orientation: (111) > (110) > (100) Future Trends and Challenges Advanced gate dielectric materials 2D and 3D effects
Summary 56 CVD T Reaction controlled T Transfer controlled Factors Affecting Kinetics BL, Depletion, P: - Effect PECVD: Low T Future Trends and Challenges Aspect Ratio Contamination Alloys
57 THANK YOU