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Semiconductors 101, unit 7

How chips are made: the big picture

A wafer goes through hundreds of steps over several months in a fab. Look at the whole map first; the next units zoom in one area at a time.

Goals

A chip is built layer by layer: deposit a material, polish it flat on some layers, print a pattern, then etch away what is not needed or implant atoms, and repeat dozens of times; cleaning and measuring happen between the steps.
Each layer goes roughly once around the loop: once the pattern is printed, it is etched or implanted; not every layer is implanted or polished. The print is measured first, the resist is stripped after etch or implant, and cleaning and measuring happen between the steps.

Step by step

  1. Build in layers: transistors in the silicon first, then metal wiring stacked above.

    WhyA chip is like a building: the transistors at the bottom are switches, and the wiring above connects them. Between the transistors and the wiring, small, deep holes filled with metal make the connection up and down; these holes are called contacts.

    If notSwitches with no wiring between them stay separate and can't form a circuit that computes; a hole that isn't etched to the bottom or isn't filled leaves the layers unconnected.

  2. Give each layer one round: deposit a film, polish if needed, print the pattern, measure (redo if wrong), etch or implant, strip the resist, clean, then measure a few wafers.

    WhyThere must be material before anything can be cut, and resist covers what stays so only the open areas are etched or implanted.

    If notA surface that isn't polished flat stays uneven, so the next layer's pattern prints blurry and lines up poorly.

  3. Repeat the round dozens of times, hundreds of steps in all, and keep every step steady.

    WhyA complex chip has dozens of layers, and each one goes back to litho to print its own pattern.

    If notIf each step is just slightly short, hundreds of steps multiplied together cost a lot of yield (the share of chips that come out good) in the end.

  4. Group tools that do the same job into areas (this course follows the game's seven; real fabs split them in different ways), and move the wafers back and forth among them.

    WhyTools doing the same job are easier to keep in tune side by side, and people can split the work.

    If notWith the same kind of tool scattered around, settings are hard to keep matched and work is hard to split; besides, each layer comes back to the same kind of tool, so the route was never a straight line.

  5. Plan the quantity on the day of wafer start (going onto the line): a lot takes weeks to months from there to finished.

    WhyThere are hundreds of steps, and most of the time goes to waiting in line at tools, not to processing.

    If notUnless some part-finished wafers were kept waiting for this, wafers added halfway must start again from the first step, which means another weeks to months.

Key ideas

Design and photomasks come before the fab

A fab builds from a drawing. Engineers first design the circuit on computers and simulate and check it many times. When it's final, the design is handed over; this is called tape-out. The design is then made into a set of photomasks, at least one for each layer's pattern, like stencils for printing the circuit. Once the wafers and the photomasks are both at the fab, a batch of wafers goes onto the production line, which is called a wafer start, and hundreds of steps over weeks to months begin.

Seven process areas, each with one job

Tools that do the same job sit together, and that is an area. Wet clean washes off dirt too small to see. The furnace slides more than a hundred wafers into a very hot tube to grow an oxide layer that doesn't conduct, or to heat implanted atoms into place (annealing). Thin film lays down a new layer of material. Litho coats the wafer with light-sensitive photoresist and shines light through a photomask to print the pattern. Etch cuts away what the pattern doesn't protect. CMP (chemical mechanical polishing, also called planarization) polishes the surface flat. Implant shoots atoms such as boron or phosphorus into the silicon to decide where it conducts.

A chip is built like a building, floor by floor

At the bottom of a chip are the transistors, built into the silicon; above them, layer after layer of very fine metal wires connect the switches. Each layer has a different shape, so each one needs its own round: add a layer of material, polish it flat if needed, print its pattern with litho, etch or implant through it, clean, then start the next. Complex chips need dozens of patterned layers (a dozen or more metal layers, plus the vias between them and the transistor layers below, the finest printed in two or three passes), so a wafer comes back to litho dozens of times, and it all adds up to hundreds of steps, over a thousand for the most complex.

Then test, cut, package, and test again

Building the circuit on the wafer, the transistors and the wiring above them alike, is what the industry usually calls the "front end"; testing, cutting and packaging the finished wafers at a test-and-assembly house is the "back end". Inside a fab, engineers sometimes use the same two words for the transistor half and the wiring half of the process, so read them in context. First, very fine needles touch the contacts of every chip on the finished wafer, power it up and test it; this is called wafer sort, done at the fab, the test-and-assembly house or a dedicated test house, and bad chips caught now don't cost money to package. Then the wafer is cut into separate chips, and each goes into a package that protects the thin, brittle chip and brings its contacts to the outside. Each packaged chip is tested again, because packaging can damage it too and some tests can only be done fully once it is packaged; only the good ones ship.

Common misconceptions

People sayDuring its months in the fab, a wafer is being processed the whole time.
In factFrom wafer start to a finished wafer, most of the time isn't spent being processed but waiting in line at the tools. The tools are expensive and every wafer stays in the line that long, which is why fabs run 24 hours a day, with people working in shifts.
People sayWhat comes out of a fab is a pile of chips ready to use.
In factA fab ships whole wafers, with the chips still joined together and not yet tested one by one. Only at a test-and-assembly house are they tested, cut apart and packaged into parts that can go into a product.

Look: machines to take apart

Read

Further

Play

Start at chapter 2, "Station by Station": one hands-on step in each of the seven process areas →

Think

  1. Why is a chip built by repeating "deposit, print, etch" layer after layer rather than all at once? Try explaining it with house building or another everyday example.
  2. A wafer spends months in the fab. Over such a long process, where do you think the biggest risk lies, and how could it be caught early?

Teaching materials: slides

The key ideas, diagram, machines and questions of this unit as a slide deck, ready to project or to put into your own handouts.

Download slides (PPTX)271 KBDownload slides (PDF)300 KB

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Cite this article"Semiconductors 101, unit 7：How chips are made: the big picture" fab24hr, updated . https://fab24hr.com/en/learn/process-flow/