1. Chapter 9 Thin Film Deposition

2. Introduction
The layers on top of the silicon substrate are usually deposited
Dielectrics
Silicon oxide, silicon nitride
Semiconductors
poly-Si or a-Si
Metals
95% Al/5% Si
Ti or W clad copper
Silicides (metal-silicon molecule)
Carbon

3. Characteristics of Deposition
Quality of deposition
Composition of the film
Contamination levels
Defect density
Pinholes, step coverage
Mechanical properties
Stress
Electrical properties
Conductivity
Optical properties
Reflectivity

4. Introduction Composition Contamination
May vary with deposition method and parameters
Composition control is very important when the material can have a range of compositions
Ratio of alloys and multilayer stacks of materials can change the chemical, electrical, optical, and mechanical properties of film.
Contamination
Unwanted moisture, undesired metals, incorporation of oxygen and halogens

5. Introduction
Defects
Pinholes and other structural defects must be minimized
often result from particles on the surface of the wafer

6. Introduction Other quality considerations Films must be stable
Particularly if there are further thermal or chemical procedures to be carried out on the wafer.
They must adhere to the substrate
They must have minimum stress

7. Introduction Uniformity of Thickness
The films must be uniform across the wafer and from wafer to wafer
Variations in thickness as in (b) can lead to high electrical resistance and localized heating
Can lead to cracking from thermal cycling and electromigration

8. Step Coverage Coverage of the side of the step
The ratio of the minimum thickness deposited on the side of the step divided by the thickness deposited on the top horizontal surface

9. Conformal step coverage
Refers to a step coverage of unity
Usually desired, but there are processes that rely on a step coverage of zero
Conformal step coverage of PECVD SixNy

10. Aspect Ratio Deep, narrow features with high ARs are harder to fill
PVD tantalum barrier layer with ~60% step coverage

11. SEM image showing poor step coverage (breadloafing) of metal 1 into a silicon contact. (Courtesy Analytical Solutions)  

13. Introduction Space-filling properties
Via hole or contact hole filling with metal
Filling spaces or gaps in shallow trenches or between metal lines
Voids in the film itself or between film and semiconductor
High contact or sheet resistance
Voids can lead to cracking of dielectrics

14. Two main categories of thin fim deposition
They are:
Chemical vapor deposition (CVD)
Physical vapor deposition (PVD)
Wafer is placed in a chamber and the constituents of the film are delivered in the gas phase to the surface where they form a film

15. Chemical Vapor Deposition
Reactant gases are introduced to the chamber
One or more than one gas may be used plus carrier gases (nonreactive gases)
In some cases, there is no gas source for a particular material so an inert carrier gas (Ar, N2) is bubbled bubble through a liquid source and the vapor is transported into the chamber.

16. Chemical Vapor Deposition
The system is designed so that the chemical reactions between the gases takes place on or very close to the wafer surface and not in the gas stream to produce the film
Particles produced in the gas stream rain down on the wafer surface and cause pinholes or low density films
CVD is used to deposit Si and dielectrics because of good quality films and good step coverage

17. Chemical Vapor Deposition
There are several variants of the process
Atmospheric pressure (APCVD)
Low pressure (LPCVD)
Plasma-enhanced (PECVD)
Most processes take place at elevated temperatures ( oC)
Increase reaction rate
Provide kinetic energy to allow reaction products to move along wafer surface
Increases film density and reduces pinholes and voids

18. Chemical Vapor Deposition

19. 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

20. 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

21. Issues in APCVD
Release of the reactants or reaction product from the surface
Defined by the “sticking coefficient”
Composition of surface changes sticking coefficient
Re-emission is important in coverage and filling
Reaction on the chamber walls
cold wall versus hot wall processes
Wafer surface topology
surface diffusion of reactants and byproducts

22. Model for APCVD Simple model for the two important processes
Mass transfer of reactants to wafer surface
Surface reactions
Equate these two steps under steady state conditions
The model looks very much like the model we developed for oxidation

23. APCVD The problem can be set up as follows
There are two fluxes of atoms: F1 and F2

24. APCVD
Flux from the gas phase is driven by the concentration gradient from the flowing gas to Si surface through a stagnant boundary layer
Laminar flow condition
It is given (in molecules/cm2/s) by hG is the mass transfer coefficient through the boundary layer

25. APCVD
Flux that is consumed by the reaction at the surface is if the reaction is a first order reaction. kS is the chemical reaction rate at the surface (cm/s)

26. 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

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

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

29. 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

30. APCVD

31. Epitaxial deposition of Si

32. 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)

33. 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

34. 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

35. APCVD

36. APCVD

37. 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

38. 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

39. 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

40. APCVD

41. 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.

42. 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.

43. 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