The Hindu Business Line : Making of a chip

Amazing, isn’t it, how thousands of lines of complex circuitry are scratched on thin silicon wafers. The technique is similar to one that was used by lithographers and cartographers hundreds of years ago.

Back then, they would coat a copper plate with a thin film of wax and scratch the image — such as a map — on the wax. Then they would pour nitric acid onto the plate. Acid would get into all the places where the wax has been etched away an d would eat into the copper, creating lines and ridges. Then, after washing all the wax away, they would pour ink into the lines.

After that it is but simple matter to press paper onto the plate and make copies.

In chip making, circuits are etched on silicon pretty much in the same way. Silicon ingots are sliced into extremely thin wafers. The wafer surfaces are made into silicon dioxide by subjecting the wafers to extreme heat and gas.

The wafer is then coated — like wax in the olden days — with a substance called ‘photoresist’ which becomes soluble when exposed to light.

The circuit that is to be etched on the silicon is first cut in ‘masks’, which are like stencils.

Just as ink passes through the cuts in stencils, light passes through the circuits cut in the masks.

In chip making, ultra violet light is passed through the masks and focuses onto the photoresist-coated silicon, through a series of image-diminishing lenses. That is how the image of a big circuit is created on a tiny silicon wafer.

Modern techniques use reflecting mirrors instead of lenses.

The exposed areas of the wafer turn into a gooey layer of photoresist, which is chemically washed away.

This leaves ridges of silicon dioxide on the wafers. Between the ridges are lines of the circuit, on which metal is atomically deposited.

A microprocessor chip may contain twenty such circuit-bearing layers, with interconnects, forming one huge circuit. Atoms of metal are deposited on the lines to conduct electricity.

The finer the light, the finer the image — as we all know. ‘Finer light’ means light of smaller wavelength. Ultra violet light, which cannot be seen by the naked eye, is of shorter wavelength.

Modern fabs use light radiating plasma, which emit light of wavelength as small as 13 nanometres. With finer lines you could have bigger circuits, capable of more functions.

Making a chip is an extremely complex business, involving some 300 steps. Hundreds of identical circuits are etched on a single sheet of wafer simultaneously — something like your reflection in a bank of tiny mirrors.

Each area of the wafer bearing a circuit is individually cut, given a protective coat, and tested. Only then they are ready for use in electronic devices.