Why chip people live in microns

You want to tell a colleague how wide a metal stripe is, using a unit you can say out loud.

That unit is the micrometer, a millionth of a meter. Type 20 into a form that expects millimeters and the strap arrives as 20 millimeters, a thousand times the width anyone said. Under the usual scale of 1000 database ticks per micrometer, that is 20,000,000 ticks instead of 20,000. The integers are large and legal. The neighboring block is buried. A human hair is tens of micrometers across. A comfortable metal line in an older chip process is often one or two micrometers. The integers underneath those spoken sizes are database units, the ticks the file actually stores.

The picture at the top of this page is an illustration, not a photograph, of a human hair set beside those lengths. The hair is marked at about 80 micrometers across. A closer mark shows 1 micrometer, one millionth of a meter. A still closer mark shows 1 nanometer, and a nanometer is a thousandth of a micrometer. Modern chip features sit in that range: small beside a hair, large enough that people would rather say micrometers than scientific notation. The illustration is a scale lesson. It is not a chip drawing, and it is not something this page redraws. Twenty millimeters would be a bar as wide as a small coin, and it would not fit beside that hair at all.

A chosen number of ticks per micrometer

The second picture leaves the illustration and shows a layout drawing of two straps.

The teal strap is thin. The navy strap is thicker, and in this drawing the navy strap is four times the teal strap's height: 1000 database units for the teal, 4000 database units for the navy. If one micrometer is 1000 database units, the teal strap is one micrometer tall and the navy strap is four micrometers tall. That mapping is a common choice. It is not a law of nature. Another project may store 2000 ticks per micrometer, or 10000, or something the process kit wrote into the file header. The same integers then mean different micrometers. The straps do not relabel themselves. Only the header, and the conversation around it, say which reading is real.

Hallway talk stays in micrometers. The file stays in ticks. A metal strap drawn at 0.480 micrometers is 480 ticks when the common mapping holds, and 960 ticks when the project uses 2000 ticks per micrometer. A via that is 0.170 micrometers on a side is 170 ticks under the common mapping. The editor paints a micrometer label because someone declared the scale. The status line can say 4.000 next to the navy strap and mean four micrometers, or it can say 4.000 and mean four of whatever unit the display is using. Read the unit suffix in the same glance as the number.

Nanometers show up when a gate or a film is smaller than a micrometer. 180 nanometers is 0.18 micrometers. Four micrometers is 4000 nanometers. Same length, two dialects. Pick one dialect per sentence, and pick the one the process document uses. Board people often live in millimeters and in mils. A mil is a thousandth of an inch, about 25 micrometers. A sentence that mixes mils, millimeters, and micrometers without converting is how a drill and a gate trade places.

Twenty that became millimeters

The power strap specified as 20 micrometers was typed into a board form that still expected millimeters, because that was the last tool on the desk. The wafer could not be used. The thin teal strap in the second picture is the size of a one-micrometer line under the common mapping. The navy strap is four. Neither of them is twenty millimeters.

The same class of miss happens when a process kit speaks nanometers and a person types micrometers. A 0.18 micrometer gate entered as 180, in a field that already means micrometers, becomes a 180 micrometer slab. A mechanical outline in millimeters, brought in as if the numbers were micrometers, shrinks a screw hole to a speck, or the reverse blows a gate up to package size.

Read the file header for ticks per micrometer before you believe the status line, and read the unit suffix in the same glance as the number. Then convert. The illustration at the top is the antidote to a naked number: eighty micrometers is a hair, one micrometer is the small mark, one nanometer is the smaller mark. The second picture is the same discipline on a drawing. One thousand ticks and four thousand ticks are one micrometer and four only while this file's header says so.

Convert the straps before you trust the status line

Open the file that holds the second picture and read the header before you read the straps. Under the common mapping, 1000 ticks per micrometer, the teal strap is one micrometer and the navy strap is four. Write those two spoken sizes next to the tick counts: 1000 and 4000. If the header says 2000 ticks per micrometer, the same integers are half a micrometer and two micrometers, and the status line that still says 4.000 is lying about the unit. The drawing does not relabel itself.

Do the same for the via that is 0.170 micrometers on a side. At 1000 ticks per micrometer it is 170 ticks. At 2000 ticks per micrometer the same spoken size is 340 ticks. A person who copies 170 into the second file has drawn a via of 0.085 micrometers and will not see it in the status line if the suffix is hidden. The hair in the illustration is the scale check for the spoken number: about 80 micrometers across, so the one-micrometer teal strap is a small fraction of that thickness, and a 20 millimeter strap typed by mistake would not fit in the frame at all.

A 0.18 micrometer gate is 180 nanometers, and it is 180 ticks only while the header says 1000 ticks per micrometer. Entering 180 into a field that already means micrometers builds a 180 micrometer slab. Say the dialect the process document uses, once, in the note that travels with the file, and convert the teal strap, the navy strap, and the 0.170 micrometer via into that dialect before you send the integers.