1. A. Transport of Reactions to Wafer Surface in APCVD
Transport of reactants by forced convection to the deposition region
Transport of reactants by diffusion from the main gas stream to the wafer surface
Turbulent flow can produce thickness nonuniformities
Depletion of reactants can cause the film thickness to decrease in direction of gas flow
Adsorption of reactants on the wafer surface

2. APCVD B. Chemical reaction Removal of chemical byproducts
Surface migration
Site incorporation on the surface
Desorption of byproducts
Removal of chemical byproducts
Transport of byproduct through the boundary layer
Transport of byproducts by forced convection away from the deposition region

3. APCVD At steady state – if two fluxes are equal
The growth rate of the film, v (cm/s), is
Where N is the number of atoms incorporated into the film per unit volume
For single composition film, this is the density

4. Mole fraction
The mole fraction in incorporating species in the gas phase where CT is the concentration of all molecules in the gas phase

5. Two limiting cases for APCVD model
Surface reaction controlled case (kS<<hG)
Mass transfer or gas-phase diffusion controlled case (hG<<kS)

6. APCVD Both cases predict linear growth rates
but they have different coefficients
There is no parabolic growth rate
Surface reaction rate constant is controlled by Arrhenius-type equation (X=Xoe-E/kT)
Quite temperature sensitive
Mass transfer coefficient is relatively temperature independent
Sensitive to changes in partial pressures and total gas pressure

7. APCVD

8. Epitaxial deposition of Si

9. Epitaxial deposition of Si
Slopes of the reaction-limited graphs are all the same
activation energy of about 1.6 eV
This implies the reactions are similar; just the number of atoms is different
There is reason to believe that desorption of H2 from the surface is the rate limiting step
In practice
epitaxial Si at high temperatures (mass transfer regime)
poly-Si is deposited at low temperatures (reaction limited, low surface mobility)

10. Deposition of Si Choice of gas affect the overall growth rate
Silane (SiH4) is fastest
SiCl4 is the slowest
Growth rate in the mass transfer regime is inversely dependent on the square root of the source gas molecular weight
Growth rate is dependent on the crystallographic orientation of the wafer
(111) surfaced grow slower than (100)
Results in faceting on nonplanar surfaces

11. APCVD In the preceding theory, assumed hG and Cs were constants
Real systems are more complex than this
Consider the chamber where wafers lie on a susceptor (wafer holder).
Stagnant boundary layer, S, is not a constant, but varies along the length of the reactor
Cs varies with reaction chamber length as reaction depletes gases

12. APCVD

13. APCVD

14. Effects
Changes the effective cross section of the tube, which changes the gas flow rate
Increasing the flow rate reduces the thickness of the boundary layer and increases the mass transfer coefficient
Reduces gas diffusion length
To correct for the gas depletion effect, the reaction rate is increased along the length of the tube by imposing an increasing temperature gradient of about 5—25oC

15. APCVD
Sometimes we wish to dope the thin films as they are grown (e.g. PSG, BSG, BPSG, polysilicon, and epitaxial silicon).
Addition of dopants as gases for reaction
AsH3, B2H6, or PH3.
Surface reactions now include
Dissociation of the added doping gases
Lattice site incorporation of dopants
Coverage of dopant atoms by the other atoms in the film

16. APCVD
Another problem, common in CMOS production, is unintentional doping of lightly doped epitaxial Si when depositing them on a highly doped Si substrate.
Occurs by diffusion because of the high deposition temperatures (800—1000oC)
Growth rate of the deposited layers is usually much faster than diffusion rates (vt >> √Dt), the semi-infinite diffusion model can be applied

17. APCVD

18. Mass transport on to deposited films
Atoms can outgas or be transported by carrier gas from the substrate into the gas stream and get re-deposited downstream
The process is called autodoping
Empirical expression to describe autodoping
C*S is an effective substrate surface concentration and L is an experimentally determined parameter
As film grows in thickness, dopant must diffuse through more film and less dopant enters gas phase.

19. Autodoping
Autodoping from the backside, edges, or other sources usually results in a relatively constant level.
This is because the source of dopant does not diminish as quickly but is at a much lower level.

20. APCVD
The left part of the curve arises from the out-diffusion from the substrate
The straight line part arises from the front-side autodiffusion
The background (constant) part is from backside autodoping

21. In APCVD
It is critical to deliver the same gas flows to all the wafers in order to produce the same growth rates
Wafers placed side-by-side
In LPCVD
Mass transfer coefficient is higher as the diffusion distance of reactants is increased.
the boundary layer thickness slightly increased as P decreases
Reactions are limited by ks where adsorption on wafer surface is key limiting step.
Wafers can be stacked parallel to one another
Note that this arrangement will provide a higher throughput

23. LPCVD

24. PECVD
There are some cases where temperature requirements (thermal budget) will not allow high-temperature depositions
E.g., depositing Si3N4 or SiO2 after Al
APCVD and LPCVD do not produce good quality films ~ 450oC
Reaction produced in a plasma ignited using an inert gas between two electrodes
The sample may be heated
Usually between oC

25. PECVD

26. PECVD
Ideally suited to rapidly varying the film composition and properties during deposition
Apply a potential and an AC signal (13.56 MHz) across a low pressure of the inlet gases
The processes that occur are complicated and very difficult to model
The resultant products are very far from thermodynamic equilibrium
Generates high concentration of particulates
Pinhole density is a problem if chamber is not routinely cleaned.

27. PECVD
Oxide
Oxynitride
Microfluidic Channels formed by PECVD

28. CVD
LPCVD
PECVD

29. Polysilicon LPCVD Deposited by thermal decomposition of silane (SiH4)
Deposition temperature range °C
SiH4 (vapor) = Si (solid) + 2H2 (gas)
A typical set of deposition parameters:
Temperature: 620°C
Pressure: torr
SiH4 flow rate ~ 250sccm
Deposition rate = 8-10nm/min
Doping of film
In-situ (during deposition) by the addition of dopant gases such as phophine, arsine, and diborane
Often doped after deposition by diffusion or ion implantation
Typically highly doped to achieve low resistance interconnections
ohm-cm can be obtained in diffusion-doped polysilicon.

30. Silicon dioxide Variety of methods (PECVD, LPCVD, APCVD)
Index of refraction is used to determine the quality.
Can be doped or undoped
Oxide doped with 5-15% by weight of various dopants can be used as a diffusion source.
PECVD oxide
SiH4 (gas) + 2N2O (gas)  SiO2 (solid) + 2N2 (gas) + 2H2 (gas)
Temperature: °C
Deposition rate of 900nm/min is achievable
Not a good step coverage
Controllable film stress (compressive)
Contains hydrogen (SiOH)
Used for deposition over metals
LPCVD oxide using dichlorosilane
SiCl2H2 + 2N2O  SiO2 + 2N2 + 2HCl
Temperatures ~ 900°C
Good step coverage
Compressive stress used to stress-compensate LPCVD nitride

31. Oxide LPCVD oxide using tetraethylorthosilicate (TEOS) (liquid source)
Si(OC2H5)4  SiO2 (solid) + 4C2H4 (gas) + 2H2O (gas)
Deposited at temperatures between °C
Excellent uniformity and step coverage

32. Silicon nitride Variety of methods (PECVD, LPCVD, APCVD)
Can be used as an oxidation mask (LPCVD)
An excellent barrier to moisture and sodium contamination (PECVD)
PECVD nitride
SiH4 (gas) + NH3 (gas)  SixNyHz (solid) + H2 (gas)
Deposition temperature: °C
Deposition rate of 20-50nm/min
Not a good step coverage
Controllable film stress
Changes after high temperature anneal due to H2 out-diffusion
LPCVD nitride using dichlorosilane
2SiCl2H2 (gas) + 4NH3 (gas)  Si3N4 (solid) + 6H2 (gas)
Deposition temperatures: °C
Good step coverage
Tensile stress
Better resistivity (1016 ohm-cm) and dielectric strength (10 MV/cm) compared to PECVD nitride films ( ohm-cm and 1-5MV/cm)