How is a wafer cut into chips? Backgrinding and dicing, step by step

To turn a wafer into separate chips, a test-and-assembly house first grinds the wafer thin from the back, then cuts it apart along the narrow gaps left between the chips (the scribe lines). This step is called wafer dicing, and one wafer often yields over a thousand chips. There are two main ways to cut: with a very thin diamond blade, or with a laser.

Where does dicing fit in the process?

After the chips are made in a wafer fab, the whole wafer goes to a test-and-assembly house (see "What is an OSAT?"). The rough order is: test first, then thin and cut, then die attach, wire bonding and molding, and finally final test. This article covers the "thin and cut" part; for how the chip is packaged after that, see "What is chip packaging?". Some packages are built across the whole wafer and cut only at the end (see "What is fan-out packaging?").

Step by step: from one wafer to many chips

Step 1: Know which chips are good

• What: before cutting, every chip is powered up and tested through the fine needles of a probe card (wafer sort), and the results are recorded by position as a wafer map.
• Why: once cut, a thousand chips look exactly alike; the map is what tells good from bad.
• Otherwise: bad chips get packaged too, and that money is wasted. For how testing works, see "How are chips tested?" and "What is a probe card?".

Step 2: Put protective tape on the front

• What: protective tape goes on the side with the circuits, and the wafer is turned face down.
• Why: the next step presses on the wafer and runs water over it the whole time; the circuits on the front can be scratched, and silicon dust and water can seep in.
• Otherwise: one scratch or one spot of dirt on the front, and those chips are ruined.

Step 3: Grind the back thin

• What: a grinding wheel set with diamond grains spins under running water and wears the silicon on the back away, layer by layer: coarse grinding first for speed, then fine grinding for flatness. The wafer starts at a little over 700 micrometers thick (a micrometer is a thousandth of a millimeter) and is often ground down to somewhere between a few tens of micrometers and about a hundred, roughly a tenth of where it started, about as thin as a sheet of paper.
• Why: thin chips make small, thin packages that fit in phones and earbuds; heat has less silicon to cross, so it gets out more easily; and to stack several chips, each one has to be thinner still (see "What is a TSV?").
• Otherwise: too thick, and the package won't fit. But too thin is a problem as well: silicon is hard and brittle like glass, and the thinner it gets, the more easily it bends and cracks. So when a wafer has to be very thin, it is often bonded to another wafer or a sheet of glass that holds it up while it is ground.

Why not make the wafer thin from the start? Because in the wafer fab it goes through hundreds of steps and in and out of high heat, and a thin wafer would warp or break. So it is ground only at the end.

Step 4: Remove the damage grinding leaves behind

• What: just under the ground surface there is a layer of tiny cracks you can't see, plus leftover stress (forces inside the material pulling against each other). Polishing or etching then removes that thin layer.
• Why: tiny cracks are where a chip breaks most easily, and the stress also makes a thinned wafer bend.
• Otherwise: the chips are more fragile and can crack the next time they are under force.

Step 5: Mount the wafer on dicing tape in a frame

• What: a sheet of sticky tape goes on the back of the wafer, its outer edge stretched across a metal frame, and the protective tape on the front comes off.
• Why: after cutting, the chips have to stay neatly in place so they still match the wafer map; the frame gives machines something to hold.
• Otherwise: the chips scatter as soon as they are cut, and there is no telling any more which ones were good.

Step 6: Cut along the scribe lines

The chips sit in a grid like a chessboard, with narrow strips between them that hold no circuits. These are the scribe lines, roughly a few tens of micrometers to a hundred wide, about the width of a hair. Small structures for measurement (test keys) and alignment marks also go here, and they are cut away with the strip. The machine first uses a camera to find the scribe lines and square the wafer up; a cut that strays into the circuit ruins that chip. There are three common ways to cut:

• With a blade: the blade is a very thin disc with diamond grains around its edge, only a few tens of micrometers thick, spinning at more than ten thousand turns a minute. Each pass of the wafer under the blade makes one cut, all the way through the wafer and only slightly into the tape; when one direction is done, the wafer turns 90 degrees and the other direction is cut. Pure water runs over it the whole time to cool the blade and wash the silicon dust away. It is the most mature, most widely used and cheapest method. The limits: the blade really does touch the silicon, so small pieces can break off the edges, the blade dulls with use, and the thinner the wafer, the more easily it cracks.
• By burning with a laser: a laser runs along the scribe line and heats the material until it melts and turns to gas. Nothing touches the wafer, so the edges chip less, which suits thin wafers; a laser is also often used to open up the top few layers of the scribe line before a blade cuts the rest. The limit: the cut is left with a band of heat-damaged material and spattered debris along its sides, and the chips can end up less able to take force.
• By marking a layer inside, then stretching: silicon lets certain infrared light through almost as if it were clear. With this kind of laser focused inside the wafer, a row of tiny altered spots is made along the scribe line, with almost nothing showing on the surface; when the tape is then stretched outward, the wafer splits along that row, like scoring a cracker before snapping it. No water is needed and almost nothing is cut away, so there is almost no debris. The limit: the focus has to be very precise.

Some processes do it the other way round: they cut halfway down from the front first, then grind from the back, and when the grinding reaches the cuts, the chips come apart on their own. That way nobody has to cut a wafer that is already very thin and brittle.

Step 7: Clean the wafer and check for chipping

• What: the cut wafer spins fast under a spray of water to wash off the remaining silicon dust, then is dried; next it is checked for chipped edges, cracks and crooked cuts.
• Why: silicon dust left on the chips dirties the pads, and a chip with a corner broken off into the circuit can't be used.
• Otherwise: bad chips travel on with the rest, nobody finds out until they are packaged, and the packaging money is lost too.

Step 8: Stretch the tape and take only the good chips

• What: the tape is stretched outward to widen the gaps between chips a little; some tapes lose most of their stickiness under ultraviolet light. The die bonder reads the wafer map: a fine pin pushes one good chip up from underneath, a suction tip above picks it off and places it on the board, and the bad ones stay on the tape.
• Why: if the gaps are too narrow, picking one chip scrapes its neighbor; if the tape holds too tight, a thin chip can crack as it is pulled off.
• Otherwise: good chips get damaged, or bad ones get packaged too. For how the chip is then connected to the outside, see "What is wire bonding?" and "What is flip chip?".

What about chipping and the cutting water?

• Chipping: a small piece breaks off the edge of a chip as it is cut. It happens more when the blade is dull or the cut is too fast. The fixes are ordinary ones: replace blades by how much they have been used, cut more slowly, and choose a suitable blade. How much chipping is acceptable is set in a standard beforehand; a chip where it reaches the circuit is never used.
• Water: the water is purified to take impurities out. Once used, it carries silicon dust and has to be treated before it can be let out (see "Where does a chip fab's used water go?").

In fab24hr

Tap "Clock in", choose "A day in the fab", tap "Test-and-assembly house" on the "Your role today" card and swipe to the assembly process engineer; you get a shift by the time on your device. The morning shift is at the dicing saw: Guo, the technician, shows you magnified photos and asks you to tap the chip whose nick cuts into the circuit. Then you decide whether to change to a new blade now (the machine stops for about twenty minutes) or slow the cutting down and keep going; at the die bonder, you decide whether to keep the chips nicked only at the edge.

Chapter 11 of the main story, The life of a chip, "The Ball Lifted", also starts next to the dicing saw: Shu-Hui, the line leader, tells you the "Glowmere" wafers have been cut, and only the chips wafer sort marked good go on. Tap the die bonder on the assembly line and its note says chips that failed wafer test are not picked. To watch a blade cut, one pass at a time, open the wafer dicing saw and tap "See it work".

There is no backgrinding machine and no laser dicing machine in the game. The test-and-assembly house, the "Glowmere" chip, the people and the downtime are fictional; how backgrinding and dicing are done follows public, general sources. For other jargon, see Chip fab jargon in plain English.