### 6.1.3 Summary to: 6.1 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. |  |
|  | 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. |
Questionaire
Multiple Choice questions to all of 6.1

© H. Föll (Semiconductor Technology - Script)