Who actually reads a layout?

You want a heater on a chip to come back from the factory still connected.

Several people and several programs have to agree on the same drawing before any metal is etched. You color the drawing so your eyes can separate the films. Those colors are your glasses. The next reader may be a script with no eyes at all. The picture at the top of this page is one crop, shown four ways, from a real sky130 block named encoder_top. You see filled colors, outlines, names, and a numbers table. The polygons are the same in each view, and there are four readers. The colors help a person parse the stack. The numbers are what a script sees. The colors are not in the file.

A design-rule check is one of those readers. It measures widths, spaces, and how far one shape must cover another. It does not know which lines you meant as construction scratches. Leave construction lines on a metal layer, because they helped while you stretched a bus, and the check treats them as metal. It can report hundreds of spacing violations. A team can spend a day repairing metal that was never supposed to ship. In that same report a real error sits unread: a via whose metal cover is too small. The heater comes back open. The check was right about the lines you left behind. The people read the wrong rows and missed the via.

Four ways to read one crop

The filled view paints local interconnect teal, the first metal navy, the second metal gold, vias orange and mauve, and the well green. Filled shapes make the stack readable at a glance. The paint lives in the editor.

The outline view is the same crop, hollow. Edges and overlaps get easier to see when a solid fill no longer hides a skinny crossing. People flip to outlines when a short might be sitting inside a shape that looked clean and finished.

The names view still shows the geometry, with words a person can read: the well, the local interconnect, the two metals, the via from local interconnect to the first metal, the via from the first metal to the second. A mask shop will tell you their screen will not match your palette. Names travel better than a private set of colors. Teal on your monitor is not a layer number on theirs.

The numbers view is a table of counts. In this window, layer 67, datatype 0, a second tag beside the layer number, holds 256 shapes and 1024 points, and layer 68, datatype 0, holds 126 shapes. The crop runs from (20.0, 26.5) to (30.0, 34.5) micrometers, about ten by eight. A script reads those integers and the layer and datatype pairs. It never saw teal. That table is the closest of the four to the stream file of cells, layers, and integer points. After you have said what the stream is, the short name is GDSII. Every panel is that same sky130 crop of encoder_top.

The filled panel, larger

The second picture is that filled panel, larger. Teal is local interconnect. Navy is the first metal, met1, layer 68 in the numbers above. Gold is the next metal, met2. Orange squares are vias from the local interconnect up to met1. Mauve squares are vias from met1 up to met2. Green is the well. Same crop as the four-panel picture at the top. A colleague who hides the via layer may still see teal, navy, and gold and imagine a connection the squares were supposed to make. A script reading the numbers does not imagine the squares. If a via is missing, the count can drop and a cover check can say so. A person who only loved the filled colors can miss it.

The heater failure lives in that gap between readers. Construction lines, left on metal, flooded a design-rule check with hundreds of spacing complaints. The via that failed that cover was one line among them. Miss the cover and the cut can land partly off the landing. The heater connection opens on the wafer. The filled picture can still look complete, because a via that is slightly off its metal is a small square, and the eye forgives a small square. The numbers do not forgive it.

A reader for every shape

Before you add a shape, name who it is for. A note for a person belongs on a layer the factory will not print. A via belongs to the connectivity reader and to the enclosure reader. Dummy metal for polish belongs to a density reader, and it will look like noise to someone hunting a short. A pin belongs to a netlist. Add a shape with no reader in mind, and every reader will do something with it you did not plan. The construction lines were a note to yourself that you stored as metal. Metal readers, all of them, took the note seriously.

Layout versus schematic is another reader. It is not the design-rule check. It extracts devices and joins from overlap and vias, then compares that list to the schematic. People shorten layout versus schematic to LVS once the job is clear. A screenshot of the filled panel can look finished while that comparison still finds an open. The design-rule check, shortened to DRC once the words are clear, can be clean of spacing errors and still say nothing about whether the heater net matches the sheet.

A mask writer is a reader too. It shoots the shapes on the layers you mark for a photomask, including construction lines if you left them there. It receives polygons. It does not receive your teal. Treat the numbers panel as the copy that will be measured, and use this second picture to explain the stack to a person. If the four readings disagree with what you remember drawing, believe the numbers, then fix the shapes. The heater never sees the palette.