So how do you actually make electrical contact to tiny microchips? Wirebonding is the standard industrial technique. For large chips (e.g. complex processors) fabricated in foundries, the process is fully automated. However in small research labs, a manual wirebonder is used, as each chip tends to be different.
I was rather enamoured by various artsey and atmospheric pictures available on the internet of people’s wirebonding endeavours, so I thought I’d try and capture a few of my own:
The process is as follows: First the chip (the dark coloured rectangle) is glued down onto a chip carrier (the large light coloured surrounding piece). The carrier may vary in design depending upon how many contacts are needed to be made and what apparatus that the chip is to be mounted within. Specifically, for very low temperature experiments it is important to have the chip in good themal contact with a metallic chip carrier (preferably Copper). A more specialised chip carrier for use in a dilution refrigerator is shown below:
The chip carrier is held firmly in the wirebonder vice, and then the wirebonder tip (which is like a needle with the metallic wire threaded through) is brought down into contact with the surface of the bonding pads on the carrier. The tip undergoes ultrasonic agitation, and is sometimes heated, which mimics a welding of the wire to the the metallic tracks below. As the tip is then moved by the user the wire is pulled through the needle and brought down again to form the second bond to the chip, at which point the bonder also cuts the remaining end of the wire. You end up with a small, neat loop of wire between the chip and your chip carrier.
Most of the time Sometimes, the wire comes unthreaded from the needle-tip.
It’s awkward to rethread, the hole through which you pass the thread is around the back of the needle, inclined at a 45 degree angle, and virtually invisible. It is necessary to use extremely fine tweezers to grab hold of the tiny thread. Here is a picture showing the rethreading process: