Photolithography importance; BJT structure

Importance of photolithography

Photolithography defines the shape and position of every region and layer of the IC, so it fixes the minimum feature size, the transistor density, the layer-to-layer alignment and the yield. It transfers the same mask pattern to all dies on a wafer at once, making mass production possible. For a BJT, separate lithography steps define the buried layer, the isolation, the base region, the emitter region, the contact windows and the metal; each is aligned to the previous one.

Final structure of the BJT inverter

The circuit is an npn transistor with a collector load resistor RL to Vcc and a base resistor Rin. On the chip, RL and Rin are made as diffused (for example p-type, made with the base diffusion) or polysilicon resistors, each with a metal contact at both ends.

        E             B               C      (Al contacts)
        |             |               |
 ---SiO2|----SiO2-----|-----SiO2------|--SiO2--
   +----------------------------+   +---------+
   |  +------------+            |   |   n+    |
   |  | n+ emitter |   p base   |   | (coll.  |
   |  +------------+            |   | contact)|
   +----------------------------+   +---------+
            n- epitaxial layer (collector)
   ===========  n+ buried layer  ===========
            p-type substrate (0 V)

• The n+ emitter is diffused inside the p base, which is diffused into the n- epitaxial collector.
• The n+ buried layer reduces collector resistance.
• The n+ collector contact brings the collector to the surface.
• SiO2 insulates the surface between contacts; aluminium connects E to 0 V, C to Vout and to RL (to Vcc), and B to Rin (to Vin).
• Isolation: p+ isolation walls reaching down to the grounded p-type substrate surround the device, so reverse-biased p-n junctions separate it from its neighbours.