Layout grids: display grid, database unit, and manufacturing grid

You want every corner of a shape to land on a point the factory has agreed to print.

“On the grid” needs a noun. Display dots, database ticks, and the manufacturing step are three different papers, and a corner can sit on one while missing another. A pad that looks clean on dots 0.5 micrometers apart can still be 2 nanometers off a 5-nanometer factory step. The writer then rounds it, and a space drawn at 0.18 micrometers comes back as 0.178 and fails. The drawing looked aligned. The file was rejected.

The picture at the top of this page makes one of those papers the subject. Orange dots sit on a regular pitch, in rows and columns. A teal rectangle and a navy rectangle lie on that pitch: each edge runs from dot to dot. You can put a finger on a corner and find a dot under it. Nothing in the picture is a ruler. The dots themselves are the claim. If the edges meet the dots, the rectangles agree with this particular paper, and that paper happened to be a multiple of the factory step, so those two would have survived the rounding the gold rectangle would not.

Landing corners on the dots you can see

The second picture keeps the same kind of dots and moves the rectangle off them.

The gold rectangle is still a box, an axis-aligned rectangle stored as left, right, bottom, and top. Its edges do not meet the orange dots. They fall in the gaps. The fill looks as solid as the teal and navy rectangles at the top of the page. The offense is the coordinates. A viewer who only checks color will pass it. A viewer who checks corners against the dots will stop.

Those orange dots are a display grid, the paper the editor is willing to show you. It is often coarse on purpose, so the dots stay visible when you are drawing a pad or a strap. Under that paper is a finer step, the database unit: one integer tick in the file. The file can store a point between the orange dots whenever the tick is finer than the dot pitch. That is what the gold rectangle is doing. It is legal as stored integers, and it is off the paper you can see.

The third paper is the one the factory publishes. A manufacturing grid might be 5 nanometers, or 1 nanometer, or some other pitch in the process kit. Your display dots might be 0.5 micrometers apart because that is comfortable to click. A corner can sit on every display dot and still miss the manufacturing grid, if the display pitch is not a multiple of the factory pitch. The reverse happens too. A corner can miss the orange dots, like the gold rectangle, and still be a whole number of manufacturing steps. On-grid always needs a noun. On which grid?

Which paper the factory will enforce

A pad corner looked clean against the dots at the zoom used for assembly. Those dots were 0.5 micrometers apart. The mask shop's grid was 5 nanometers. The corner was 2 nanometers off that fine pitch, which is invisible against a 0.5 micrometer dot and illegal to the writer. The tool rounded the corner to the nearest legal step. The space to the neighbor, drawn at 0.18 micrometers, came back as 0.178 micrometers and failed the minimum space. The rectangles at the top of the page would have survived that check, because their edges already sat on the displayed pitch and that pitch was a multiple of the factory step. The gold rectangle in the second picture is the one that would have moved.

Snap is how you aim at dots while you draw. It does not choose which paper is real. You set snap to the manufacturing grid when you are creating geometry the factory must accept. You might set a coarser snap for a floorplan, then check the result against the fine grid before you stream the file out. Off-grid is the name for a miss against the grid that matters. A miss against the display dots is a warning. A miss against the manufacturing grid is the defect.

Before you trust a corner, name the three pitches in one sentence: the dots on the screen, the database tick, and the factory step. Turn the display dots on and zoom until you can see whether an edge cuts a gap the way the gold rectangle does. Then read the coordinate and divide it by the manufacturing step. A remainder means the writer will move that edge, and the space to the neighbor will not be the number you drew. Leave a shape in the drawing only when you can say which of the three grids it satisfies.

Three pitches, one corner

The dots on the screen were 0.5 micrometers apart. The mask shop’s grid was 5 nanometers. The corner was 2 nanometers off that fine pitch, invisible against a 0.5 micrometer dot and illegal to the writer. The tool rounded it. A space drawn at 0.18 micrometers came back as 0.178 micrometers and failed. The rectangles at the top of the page survive because their edges already sit on a displayed pitch that is a multiple of the factory step. The gold rectangle in the second picture is the one that would move.

Before you trust a corner, name the three pitches in one sentence: the dots on the screen, the database tick, and the factory step. Read the coordinate and divide by the manufacturing step. A remainder means the writer will move that edge, and the space to the neighbor will not be the number you drew. Snap aims at dots. It does not choose which of the three papers is real.