6.2.3 Summary to: 6.2 Si Oxide and LOCOS Process

Silicondioxide (SiO2) has been the "ideal" dielectric with many uses in chip manufacture
  • Gate oxide for Transistors
  • Dielectric in Capacitors
  • Insulation
  • Stress relieve layer
  • Masking layer
  • Screen oxide during Implantation
  • Passivation
Only recently (2007) is it replaced by "low k" and "high k" dielectrics, i.e. dielectrics with a dielectric constant either lower or larger than that of SiO2
"Low k" dielectrics (polymers, porous SiO2, ..; the ideal material has not yet been found) are used for intermetal insulation; low k is important here to keep the RC time constants small  
"High k" dielectrics (the present front runner is HfO2) will replace the gate oxides. They can be somewhat thicker than SiO2 without sacrificing capacity, while strongly reducing tunneling currents.  
SiO2 can be made in several ways:  
  • Dry thermal oxidation:
    2 Si  +  O2    Þ   2 SiO2
  • Wet thermal oxidation:
    Si  +  2 H2O   Þ    SiO2  +  2 H2
  • "Chemical Vapor Deposition"
    (next sub-chapter)
  • "Spin-on techniques
    (next sub-chapter)
  • "Anodic oxidation (presently not
    used in technology)
Dry oxidation is relatively slow but gives best oxide qualities as defined by:
  • Uniformity
  • thickness control
  • Break down field strangt
  • Interface quality
  • Reliability
Typical use: Highest quality gate oxid.
Wet oxidation is about 10 times faster; it is used whenever relatively thick oxides are needed.
Typical use: Field oxide.
The other methods are needed whenever there is no Si available for oxidation (e.g. intermetal dielectrics).
As long as the process is diffusion controlled (i.e. the time it takes oxygen to diffuse through the already formed oxide determines rates, the thickness increases protorional to t1/2  
For thin oxides the growth rate is reaction controlled and the thickness - time dependence becomes complicated.  
Growing oxide only locally ("LOCOS") was a key process for field oxides.  
locos process
Without a "buffer" oxide below the masking nitride, large mechanical strain develops, producing plastic deformation and thus dislocations around the oxide edges.  
These "Oxide edge dislocations" kill the transistor.  
Buffer oxides solve the problem, but create new problems: A "birds beak" develops, increasing lateral dimensions beyond the mask dimension.  
"LOCOS" is a good example for a universal feature of Si technology: Solutions to "old" problems create new problems. Solutions to the new problems... and so on. It follows:
  • Process complexity increases all the time.
  • New materials are needed all the time.
Multiple Choice questions to all of 6.2

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© H. Föll (Electronic Materials - Script)