GDSII

GDSII is the file you actually hand to the foundry — the final, complete picture of your chip, written down as pure geometry. Imagine peeling your finished layout apart into transparent sheets, one per fabrication layer (this metal, that via, the polysilicon, each implant), and on every sheet tracing the exact outlines of every shape: a rectangle here, an L-shaped wire there, a tiny square for a contact. GDSII is the binary stencil book that holds all of those traced polygons. Everything upstream — floorplan, placement, the clock tree, routing, timing signoff, DRC and LVS — exists to produce this one clean file. When it leaves your hands, the design is done; the foundry needs nothing from you but these shapes.

Mechanically, GDSII (the GDS II Stream Format, a binary format from the early 1980s) stores a hierarchy of cells: a cell can hold raw polygons and paths, each tagged with a layer number and a datatype, and it can also reference other cells, placing them by position, rotation, and mirroring, even in repeated arrays. That hierarchy is why a memory with millions of identical bit-cells stays a manageable file size — you define the cell once and instance it everywhere, rather than re-drawing every copy. Crucially, GDSII carries only geometry plus those layer numbers; it does not record nets, timing, or rule intent. The foundry reads it through a layer map that says 'layer 31 datatype 0 is Metal-1,' and uses the shapes on each layer to build the photomask for that step.

Because the format is decades old, it has limits — 16-bit-era assumptions, integer coordinates on a database grid (commonly 1 nm or finer), and files that balloon at advanced nodes. Its modern successor, OASIS (the Open Artwork System Interchange Standard), encodes the same idea far more compactly and is now common for leading-edge tapeouts, but 'GDSII' remains the everyday word for 'the layout you ship.'