ML709
Can anybody tell who manufactured these opamps?
We often have seen this 709 design. This one is again a little different but quite similar.
709 B? A second revision?
The process to built such an old semiconductor is easy to understand. It is similar to the process involved in the LM306 (
https://www.richis-lab.de/Opamp09.htm).
Mask 1 builds the buried n+ structures that later are used to connect the collector of the transistors.
n epi forms a uniform n layer on top of the buried n+ structures.
Mask 2 forms trenches in the n epi that isolate the active areas against each other.
Mask 4 forms the (p doped) base areas of the npn transistors and the resistors. It looks like this mask worked reversed. The color of the 4 is a little greyish and this color is everywhere except on top of the base areas and the resistors.
Mask 5 forms the highly n doped emitter areas and the connectors to the buried n+ structures.
Mask 6 generates vias.
Mask 8 forms the metal layer.
Done!
https://www.richis-lab.de/Opamp34.htm