VLSI design vs working in a fab: what's the difference?

In one sentence: in job ads, VLSI usually means designing chips, and a fab is where chips are manufactured. Designers write the circuit on a computer, verify it and lay it out; the fab builds that design onto silicon wafers one layer at a time. One side spends the day with screens and simulation results, the other with tools, measurement data and a production line that never stops.

What is VLSI, and why do job ads use it?

VLSI stands for very large scale integration: putting a huge number of transistors on one chip. A single phone chip today holds more than ten billion. The term became the name of a whole field: a university course in "VLSI design" or a job ad for a "VLSI engineer" is usually about turning that many transistors into a chip that works.

Designing a digital chip roughly goes like this:

1. Specification: what the chip has to do, how fast, and how little power it may use.
2. Writing the circuit: describing what the circuit does in a hardware description language (HDL), which looks a bit like writing code.
3. Verification: running simulations again and again to confirm the circuit does the right thing. This often takes the largest share of the design work.
4. Synthesis and physical design: turning the circuit into logic gates, then deciding where each one sits on the chip and how the wires run.
5. Sign-off and tape-out: checking that nothing breaks the fab's design rules and that the layout matches the circuit, then handing the final design to the fab. That handoff is called tape-out.

What does a fab do?

After tape-out, the design is made into a set of photomasks: one glass plate for each layer of the pattern, like the negatives for a photo. Then wafers go through hundreds of steps in the fab: light prints the mask's pattern onto them, the unwanted parts are etched away, thin films are grown, the surface is polished, cleaned and measured, and the whole trip often takes weeks to a few months. Fab engineers look after how steadily those steps run: the tools, the recipes, the measurement data, and the tool that stops in the middle of the night. See What do the seven process areas of a chip fab do?

How are the two different?

	Chip design (VLSI)	Wafer fab
Where you work	An office, at a computer	Between the cleanroom and the office, often going into the cleanroom
Daily tools	Design software (EDA), simulation, code	Process tools, measurement instruments, control charts and a lot of data
Common degrees	Electrical, electronics (ECE), computer science	Chemical, materials, physics, chemistry, mechanical, electrical
Pace	Follows projects and tape-out dates	The tools run 24 hours, often with shifts or on-call duty
Cost of a mistake	A design error found after tape-out can mean new masks and another round, costing money and months	One wrong step can mean a whole lot of wafers is reworked or scrapped, and holds up the lots queuing behind it

That is the usual picture, not a rule: people at design companies visit fabs, and plenty of people in fabs spend the day writing code to analyze data. What each degree does inside a fab is in Which degrees work in a chip fab?

Where do the two sides meet?

Design and manufacturing do not work in separate worlds. They meet in at least four places:

• Design rules: the fab tells designers how narrow a line can be and how close two lines can sit. Designs that follow the rules are designs the fab can make.
• Tape-out: the moment the design is handed over is the line between the two sides.
• The pilot run: a new chip's first lots are its pilot run (in the game, on the R&D line). Designers read the test results; fab engineers look for the step that caused a problem.
• Yield: how many of the chips come out good. When yield is low, both sides have to work out whether the design sits too close to the limits or the process is not running steadily.

If a job ad says VLSI, is it always design?

Not always. Some ads use "VLSI" or "semiconductor" for test, packaging or even fab jobs. Before you apply, read what the job does every day: writing circuits in a hardware description language and running simulations is design; tuning recipes, looking after tools and reading measurements is the fab; writing test programs and reading every chip's result is test.

How to choose

• You like writing code, simulating on a computer and splitting a big problem into many small blocks: you lean toward design.
• You like things you can see and touch, solving problems on the floor and finding causes in data: you lean toward the fab.
• You like both: test, product engineering and process integration sit in between and need a bit of both sides every day.

In fab24hr

The main story of fab24hr, The life of a chip, starts after the design is handed over: a small hearing-aid company has designed its own chip for the first time, it has taped out and become a set of masks, and in chapters 1 to 5 you make it for the first time on the R&D line, reading the test results and working out which step caused a problem. After that it goes to the production fab, then a test-and-assembly house, and finally into someone's hands. A day in the fab lets you work in one of 31 roles on the R&D line, in the production fab or at a test-and-assembly house. The company, chip and people in the game are fictional; how design and manufacturing divide the work follows public, general sources, and every company does it differently.