Layer Deposition: Thermal Oxidation and CVD. Rupesh Gupta IIT Delhi Supervisor: Dr. Chacko Jacob

Transcription

1 1 Layer Deposition: Thermal Oxidation and CVD Rupesh Gupta IIT Delhi Supervisor: Dr. Chacko Jacob

2 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 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)

4 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 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.

6 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 7 Modeling Oxidation

8 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

9 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 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

11 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

12 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

13 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

14 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 15 Common Oxidation Methods Dry Oxidation Wet Oxidation Si O 2 SiO 2 Si 2H2O SiO2 2H2 Slow Growth For films up to nm Faster growth due to high solubility of H 2 O in SiO 2 (Higher B, B/A values ) For thicker films

16 Oxidation Equipment 16 Quartz Tube Wafers Resistance Heating Flat temperature profile maintained using thermocouples

17 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 18 Factors Affecting Growth Kinetics Temperature B C 1 E1 exp( ) kt B A C 2 E2 exp( ) kt Crystal Orientation Pressure

19 Crystal Orientation (111) (110) Difference more obvious for thin oxides (100) Most ICs made with (100) Si orientation

20 Pressure 20 Increase in oxidation rate even at low temperatures

21 21 OUTLINE Thermal Oxidation and Model Factors Affecting Kinetics Future Trends: Oxidation o CVD and Model o Factors Affecting Kinetics o Future Trends: CVD

22 Future Trends: Oxidation 22

23 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 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 25 OUTLINE Thermal Oxidation and Model Factors Affecting Kinetics Future Trends: Oxidation CVD and Model o Factors Affecting Kinetics o Future Trends: CVD

26 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 27 Chemical Vapor Deposition Dielectrics Semiconductors Metals

28 Issues 28 Poor Step Coverage Conformal Coverage High electrical resistance Greater chances of mechanical cracking and failure

29 Space Filling 29

30 Process 30 Gas Stream Wafer Susceptor

31 31 Growth Kinetics Governed primarily by the steps 1. Mass Transfer 2. Surface Reaction

32 32 F h C C F k S C 1 g G S 2 S

33 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.

34 34

35 Atmospheric Pressure CVD 35 APCVD for epitaxial Si deposition SiCl H Si 4HCl A Cold Wall Reactor

36 36 OUTLINE Thermal Oxidation and Model Factors Affecting Kinetics Future Trends: Oxidation CVD and Model Factors Affecting Kinetics o Future Trends: CVD

37 37 Factors Affecting Growth Boundary Layer Effects Depletion Effects Unintentional Doping Pressure

38 Boundary Layer Effects 38 Continuous gas flow Diffusion of reactants Boundary layer Deposited film Silicon substrate A Detailed Picture of Deposition Process

39 39 Velocity Profile Gas flow Gas flow Boundary layer Boundary layer

40 Solution: Reactor Geometry 40 Effect important in Transfer limited regime Susceptor is slightly tilted to minimize effect.

41 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 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

43 Pressure 43

44 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 45 Pressure Lowering Diffusivity ( no.of collisions experienced by gas species ) 1 Diffusivity 1 P Total P Total h ( Diffusion Coeff.) g

46 46 Effect of Pressure Lowering

47 LPCVD Reactor 47 P Quartz tube Pump Gas inlet Stand up wafers Wafer boat A Hot Wall Reactor

48 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

49 PECVD Equipment 49 Inert Gas Process Gas RF Power Heated Plate Wafer By Products

50 Mechanism 50

51 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 52 OUTLINE Thermal Oxidation and Model Factors Affecting Kinetics Future Trends: Oxidation CVD and Model Factors Affecting Kinetics Future Trends: CVD

53 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 54 Challenges Contamination Like carbon from precursors Difficulty with alloys Unwanted reactions Controlling the exact composition of alloy

55 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

56 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 57 THANK YOU