Process layers versus drawing layers

You draw a small square where a hole should be, and the factory needs a larger opening than the square you typed.

The derived strap stays where it was if you move only the drawn strap and do not run the recipe again. The small square lives on a drawing layer, the sheet you edit by hand. A process layer is a sheet a recipe builds from that drawing. The recipe sizes a shape, runs a boolean against other sheets, and writes the result somewhere new. You enter the drawn square once. The grown square, the enclosure, the cut that actually reaches silicon: those can be derived, and they should land on their own sheet so a later change to the drawing can be rebuilt instead of being patched by hand. The picture at the top of this page is the drawn pair you can see before any recipe runs: a small square inside a larger square, the cut you typed and the metal around it.

After the move, regenerate, and look at the corner. The fat path has to follow the thin one. If it does not, the etch will follow the sheet you did not edit.

Growing a shape onto a new sheet

The second picture is that growth along a route.

The gold path is thin and bent, a centerline with a width, the shape you drew. The green path follows the same bend and is fatter. It sticks out past the gold on the outside of the corner and along the legs. The green is the grown version: the recipe took the gold route, sized it, and wrote a new sheet. Green here can stand for a diffusion or a well that is supposed to be larger than the drawn line, or for any sized copy you do not want to maintain by hand. If you change the gold bend and run the recipe again, the green bend should follow. If you nudge the green bend by hand, the next recipe run will put it back, or you will be left with two truths.

Sizing is a number of ticks on each side, or a number of micrometers you then convert to ticks. A drawn path 2 micrometers wide, grown by 1 micrometer on every side, becomes 4 micrometers wide. That is the relationship the second picture is built to show: the green ribbon is twice the gold ribbon’s width, on a matching bend. A boolean then decides what survives. Metal AND the grown cut keeps only the overlap, which is how a via opening is clipped to the metal that must enclose it. Metal sized up, then stored on a new sheet, is an enclosure check’s input. The output sheet matters as much as the verb. Write the result onto the metal sheet and you have permanently mixed a derived shape into the drawing. Write it onto a process sheet and the drawing stays editable.

The square at the top of the page is the input you protect. Read the larger square as metal you also drew, leaving a frame around the cut on every side. That frame is an enclosure: metal still surrounding the hole after the process has wandered a little. If one micrometer is 1000 database units, a common choice, the inner square might be 8 micrometers on a side and the outer square 12, so the frame is 2 micrometers. A recipe that grows the cut by a few tenths of a micrometer still fits inside that frame. A recipe that grows the cut by 3 micrometers on a side will eat the frame. You want to see that on the derived sheet, while the square you typed stays the size you entered.

A hand edit on the sheet the recipe owns

The fat green path was derived from the thin gold path on a Friday and saved. On Monday someone moved the gold bend to clear a neighbor and did not run the recipe again. The green path still followed Friday’s route. The etch followed the green, because the process sheet was what the deck treated as real. The device on the screen, the gold path, and the device in silicon, the green path, disagreed by the Monday move. A second person, trying to help, widened the green path by hand. The next regeneration deleted that help and restored a growth of the old gold route, then a later fix regenerated from the new gold route and the hand widening vanished. Two afternoons were spent arguing with a sheet that was never the drawing.

Keep the drawn cut and the drawn path on sheets you edit. Let size and boolean write process sheets, and name those sheets so nobody sketches on them. When the gold path moves, regenerate the green path and look at the corner in the second picture’s terms: the fat path still framing the thin one, on the same bend. When the drawn square changes, regenerate the enclosure and look at the frame around it. If the frame is gone, the growth is too hungry or the metal drawing is too small. Fix the drawing or the recipe. Do not patch the derived sheet and hope the next run agrees with you.