3.6 Summary

3.6.1 Summary to: 3. Thin Films

Semiconductor technology is almost synonymous with thin film technology.
Chip cross-section
A thin film is always adhering to a substrate and (at least originally) continuous.  
Thin films may still be found in the product or may have been "sacrificed" during the making of the product.  
An IC is a study of thin films in and on the Si substrate.
The same is true for pretty much every semiconductor product.  
         
Thin always means "thin" relative to some intrinsic (internal) length scale. Examples are:  
  • Dimensions dx, y, z
  • Grain size dgrain
  • Lattice constants a0
  • λ radiation
    (light, IR, UV)
  • Absorption depths
  • Mean free path
    lengths.
  • Diffusion length
  • SCR width dSCR
  • Debye length dDebye
  • Critical thickness
    dcrit
    for electrical
    break down
  • Critical thickness dtu
    for tunneling
Structural length scales.
Wavelength and Interaction length scales.
Transport parameter length scales.
Electrical scales.  
         
There are many thin film applications outside of semiconductor technology:  
Optical, electrical, chemical, mechanical, magnetic technologies use thin films.  
         
Thin films have other spatial properties besides their thickness.
Thin film composite
Interface roughness and surface roughness R defined by their "root mean square":
 
R  =  1
N 		   N
Σ
i=1		 |zi|
 
     
Thin films adhere to their substrate.
Adhesive pull test
A direct measure of adhesion is the interfacial energy γAB between film A and substrate B.  
The phase diagram provides some guideline. Complete miscibility=good adhesion, (eutectic)) decomposition=(?) low adhesion. Calculations of γ are difficult.  
Full adhesion can only be obtained for films grown on a substrate. Adhesion energies can be measured.  
     
Generally, there will be stress σ and strain ε in a thin film and its substrate.  
Stress and strain in thin films
can be large and problematic!
A major source of strain is the difference of the thermal expansion coefficients α.  
 
εTF   =   ∆T · ∆α
     
σTF   =  Y · ∆T · ∆α
 
         
Stress in thin film may relax by many mechanisms, and this might be good or bad:
  • Cracking or buckling.
  • plastic deformation.
  • Viscous flow.
  • Diffusion.
  • Bending of the whole system (Warpage).
  
Stress relaxation mechanisms
Warpage can be a serious problem in semiconductor technology.  
         
Deposition of a thin layer must start with a "clean" substrate surface on which the first atomic / molecular layer of the film must nucleate.
First steps in thin layer nucleation
There are many possible interactions between the substrate and "first" incoming atoms.
As the interaction energy goes up we move from "some" absorption to physisorption (secondary bonds are formed) to chemisorption (full bonding)  
The sticking coefficient is a measure of the likelihood to find an incoming atom in the thin film forming.  
Immobilization by some bonding is more likely at defects (=more partners). The initial stage of nucleation is thus very defect sensitive.  
   
Simple surface steps qualify as efficient "defects" for nucleation.  
Firsdt steps in thin layer formation
Small deviations from perfect orientation provide large step densities. Nucleation therefore can be very sensitive to the precise {hkl} of the surface  
Intersections of (screw) dislocation lines with the surface also provide steps.  
This may cause grain boundaries and other defects in the growing layer.  
Scanning probe microscopy gives the experimental background  
         
There is always a nucleation barrier that has to be overcome for the first B-clusters" to form on A  
Nucleation Wetting angle
the three involved interface energies, all expressed in the "wetting angle", plus possibly some strain are the decisive inputs for the resulting growth mode.
  • Frank - van der Merve: Smooth layer-by-layer growth
  • Vollmer - Weber: Island growth
  • Stranski - Krastonov: Layer plus island growth
  
Growth modes
 
     
Epitaxial layers are crucial for semiconductor technology.
Misfit dislocations
Misfit of lattice constants will produce strained layers upon epitaxial growth; strain relief happens by the formation of misfit dislocations.
Misfit dislocations must be avoided at all costs!  
Below a usually rather small critical thickness dcrit of the the thin layer no misfit dislocations will occur.  
Rule of thumb:
0.5 % misfit   ⇒ dcrit ≈10 nm 		   
     
The internal structure of thin films can be anything known from bulk materials plus some (important!) specialities.  
a-Si: Micro electronics
a-Si:H: Solar cells, LCD displays
µc-Si:H: Solar cells
       
Properties of thin films can be quite different from that of the bulk material.  
Much better in thin films
  • Electrical break-down field strength of dielectrics.
  • Critical current densities in conductors.
The reason can be differences in length scales.  
Semiconductor technology relies to some extent on superior thin film properties.  
Exercise 3.6-1
All Questions to 3
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© H. Föll (Semiconductor Technology - Script)