Write short note on bipolar process.Ripunjay Tiwari
Ans. The pnp and npn are the basic active devices in the bipolar process. Chemically and polished treated wafers ready for use in the manufacturing process are provided to design engineer by industries. By using the batch processing technique, we have to fabricate in this wafer transistors of the structure shown in the fig. 1.17.
The sequence of steps is given below –
- Total oxidation of the wafer.
- First photomasking operation to define windows in the oxide for the diffusion of buried n+
- First diffusion to grow buried n+ layers as shown in fig. 1.18
(a), using the diffusant arsenic or antimony.
- Etching of the oxide from the entire surface.
- Growing of an n epitaxial layer, in which process the buried n+ layer diffuses a little both into the substrate and into the epilayer.
- Total oxidation.
- Second photomasking operation to open windows in the oxide for isolation diffusions.
- Second diffusion to provide isolating p layers and isolated n islands in the epi-layer, using the diffusant boron.
- Third photomasking operation to open windown in the oxide for base diffusion.
- Third diffusion to grow base p layers [see fig. 18 (b)], using the diffusant boron. The diffusion involves two stage predeposition and drive-in.
- Fourth photoasking operation to open windows in the oxide for emitter diffusion and ohmic contacts of the collectors.
- Fourth diffusion to grow n+ layers, using the diffusant phosphorous. The diffusion here is sometimes of the two stage type too. Emitter diffusion is shown in fig. 18(c).
- Fifth photomasking operation to define windows in the oxide for ohmic contacts.
- Total deposition of aliminium film onto the wafer as shown in fig. 1.18(d).
- Sixth photomasking operation to open windows in the photoresist for the interconnection pattern.
- Etching to thew aluminium film through the photoresist mask and removal of the remaining photoresist. Fig. 18(e) shows the etching step.
- Thermal treatment for firing of aluminium into silicon.