How we make semiconductors

The raw material for a 'chip', semiconductor product or Integrated Circuit (IC) is silicon - the basic constituent of sand, and one of the earth's most abundant materials. Silicon is a 'semiconductor' - under some conditions it conducts electricity and under other conditions it doesn't. That can make it act as an on/off switch, and the basic circuit that does this is a transistor. Transistors are the basic building blocks of electronic circuits, and the on/off action is what we understand as the I and 0 of the digital equipment. IC processing combines millions of transistors on a chip, allowing us to make phenomenally complex circuits.

The first essential process is purifying the silicon, since any flaws or unwanted impurities will make circuits unusable. The near 100% purity silicon is melted, and a perfect silicon 'seed' crystal is lowered into it. The result is a long rod (ingot) of single-crystal silicon with a diameter (for the latest equipment generations) of 300 mm. This work is done by specialist manufacturers, who then accurately saw the ingot bare wafers (substrates) and deliver them to us, the semiconductor manufacturer.

A chip is manufactured in layers, with perhaps as man as two dozen layers containing millions and sometimes over a billion transistors in a single microchip. The basic manufacturing process is simple in concept, but very complex in its details. Circuits must be built with incredible precision to switch quickly and exactly each and every time. Getting the signals to the next transistors in line is itself a major issue, needing metal tracks across the chip that must be insulated from each other and from other circuits.

In the Front-End operation, a blank wafer, typically silicon, is the start of the process. Essentially, semiconductor manufacturing uses chemicals to create circuits by adding and removing layers on the wafer. In this way, the interconnected layers that form the circuit within the microchip are built and replicated across the wafer. Although the exact processing depends on the technology being used and the complexity of circuits, there are several major processing stages.

An insulating silicon dioxide (sand) layer is first grown onto the silicon wafer. Then, the electrical characteristics of the individual transistors are altered by diffusing or ion implanting precise amounts of impurities (dopants) into the silicon.

These impurities are what make silicon so useful, and they can be placed wherever desired on the wafer using photolithography 'masks'. The principle is the same as printing a photograph from a negative. The image of the chip is patterned in several layers. For each layer, the image is exposed on a photo resist across the wafer, and developed. The areas not covered by the photo resist can then be etched away using a plasma or chemical etchant, and/or electrically modified by adding dopants.

The interconnections between transistors are made by metallization. This process also makes the bonding pads that connect the chip itself to the package leads, and so to the rest of the world. The whole wafer is then - if necessary - planarized by Chemical Mechanical Polishing (CMP), given a protective layer, and tested to ensure the circuits work as they should.

In the Back-End operation, devices are then assembled using four steps. 1) 'Die preparation' cuts the wafer into individual integrated circuits or dice. 2) 'Die attach' attaches the die to the support structure (e.g. the lead frame) of the package. 3) 'Bonding' connects the circuit to the electrical contacts of the package, and so to the outside world. 4) 'Encapsulation' (usually by plastic molding) give physical and chemical protection to the circuit.

Subsequent steps that give the package its final form and appearance vary from package to package. Steps like marking and lead finish give the product its own identity and improve reliability.

Once assembled, the integrated circuit is ready to use. However, many things can go wrong and make a device fail: the die may have wafer fab-related defects or may crack during assembly, or the bonds may be poorly connected or not connected at all. So, before being shipped to the customer, assembled devices must first be electrically tested.

Surface mounted devices are then normally packed in tapes with pockets, while the tape is wound around a reel. Reels, trays or tubes are boxed and labeled to customer requirements for shipping.