Interconnections in Integrated Circuits

Integrated circuits have their own unique form of wiring to connect together the micro-miniature components they contain.  This consists of extremely thin aluminium, etched to form a network of separate strips that create the electrical connections that are needed, called interconnections.  This is exactly like a printed circuit board, commonly used for assembly of electronic circuits, in which the connections are etched into films of copper plated onto an insulating board.  

In both cases, interconnections in a given layer cannot cross over each other without shorting out.  To give greater freedom, in printed circuit technology, wiring can be done on the front of the board and on the back, with connections through the board (called plated-thru holes or vias) to allow the possibility of cross-overs without shorts.  

Something very similar is done in IC technology.  Instead of using an insulating board to separate the wiring layers, this is done using a very thin film of silicon dioxide (glass) or alternatively, an insulating polymer.

In the most modern integrated circuit technology, copper interconnect is used, and there may be up to a dozen or more independent layers, each insulated from all others.

Drive circuit for an optical transmitter/receiver designed in CUED
 (optical micrograph)

The light green and light brown stripes both represent aluminium conductors.
Each stripe is about 20 micrometres in width, and the chip is about 5 mm square

These images were taken in the scanning electron microscope.  They are higher magnification views than can be obtained with the optical microscope.  You can clearly see the rectangular cross section of the interconnections (left and centre); the right hand image shows a hair-thin bond wire used to connect the chip to the outside world. 

David Holburn, 16 October 2006