Glyph

Mask Layout Glossary

A mask layout is the geometric description of every patterned layer of a chip or photomask: closed polygons drawn on numbered layers, organized into a hierarchy of reusable cells, and exported as a GDSII (or OASIS) stream file that a mask shop or lithography tool can read directly.

What is a mask layout?

A mask layout is not a picture — it is exact, machine-readable geometry. Each layer in the layout corresponds to one step of the fabrication process (a metal, an implant, an etch), and every shape on that layer becomes part of the pattern that gets transferred to a physical mask or, in maskless lithography, exposed directly. Designers draw with polygons, paths, and cell instances rather than pixels, because the downstream tools need exact vertex coordinates, not an image.

The layout sits at the start of a longer process chain: design (drawing the layers in a tool like Glyph) → mask (transferring the pattern to a photomask or reticle, or in maskless systems skipping this step) → resist (coating the wafer or substrate with a light- or beam-sensitive film) → exposure (patterning the resist through the mask, or by direct-write). Errors caught at the design stage — with design-rule checks — are far cheaper to fix than errors caught after fabrication. For the physics and chemistry of the resist step, see NANYTE's photoresist glossary; for a maskless alternative to the mask step, see maskless lithography.

Because a real layout can contain millions of shapes, every mainstream layout format and tool relies on hierarchy — defining a device once as a cell and placing it many times — rather than drawing every instance's geometry out in full.

A–Z glossary

AREF (array reference) — A GDSII element that places a regular rectangular grid of instances of one cell in a single record, storing only the origin, row/column counts, and spacing rather than one placement per copy. See SREF and AREF.

Boolean operation — Union, difference, intersection, or XOR applied to two or more polygons to combine or carve shapes, rather than editing vertices by hand. See boolean operations.

Boundary — The GDSII element type for a filled, closed polygon — the most common way a layer's geometry is represented. See the GDSII record catalogue.

Bounding box — The smallest axis-aligned rectangle that fully encloses a shape, cell, or selection; used for quick spatial queries, fit-to-view, and coarse overlap tests before exact geometry is checked.

Cell (also structure) — A named, reusable collection of shapes and instances of other cells. Cells are GDSII's unit of hierarchy: define a transistor or waveguide once, then place it repeatedly. See cells and hierarchy.

Chamfer / fillet — Corner-editing operations that replace a sharp polygon vertex with, respectively, a straight angled cut or a rounded arc — common for manufacturability or optical mode-matching.

Database unit — The integer unit every GDSII coordinate is stored in; a UNITS record defines its size in user units and in metres. See database units.

Datatype — A secondary integer tag on a boundary or path, paired with its layer number, used to further subdivide geometry drawn on the same physical layer (for example, distinguishing a drawn shape from a simulation-only marker).

DRC (design-rule check) — An automated check of a layout against a process's minimum-width, minimum-spacing, and minimum-area rules, run before tapeout to catch violations that would fail fabrication. See design-rule check.

Flatten — Expanding every SREF/AREF instance in a cell (or hierarchy) into its literal, top-level shapes, removing the reference structure. Often required before DRC or export to formats without hierarchy.

GDSII — The dominant binary stream format for hierarchical IC and photomask layout, in use since the late 1970s. See the full GDSII file format reference.

Hierarchy — The nested structure formed when cells place instances of other cells, letting a layout reuse a design at many scales instead of duplicating its geometry everywhere it appears.

Layer — A numbered plane of geometry corresponding to one fabrication step (a metal, an implant, an etch, and so on); every shape belongs to exactly one layer (and datatype). See layers.

Layer/datatype pair — The (layer, datatype) tuple that GDSII actually uses to key geometry; two shapes with the same layer number but different datatypes are tracked separately.

Mask (also photomask) — A physical plate, typically patterned chrome on quartz, that blocks or transmits light during optical lithography to transfer a layout's geometry onto a wafer's resist layer.

OASIS — A newer SEMI-standard (P39) stream format for the same class of hierarchical layout data as GDSII, designed for much smaller file sizes on dense, large-scale designs. See GDSII vs OASIS.

Offset — Growing or shrinking a polygon's boundary outward or inward by a fixed distance, commonly used to add spacing margins or compensate for a process bias.

Path — The GDSII element type for an open polyline with a width, used for wires, waveguides, and routing traces rather than filled closed regions.

Photomask — See mask.

Polygon — A closed shape defined by an ordered list of vertices; the fundamental drawing primitive of a mask layout, whether drawn directly or produced by a boolean, offset, or trace operation.

Reticle — A photomask used in step-and-repeat or step-and-scan exposure systems, often carrying one or more die images that get stepped across the wafer; the terms mask and reticle are frequently used interchangeably.

Snap — Constraining a drawn point to a grid, an existing vertex or edge, or another geometric reference, so layouts land on exact, repeatable coordinates instead of arbitrary ones.

SREF (structure reference) — A GDSII element that places one instance of a named cell at a position, with optional rotation, mirroring, and magnification. See SREF and AREF.

Stipple / hatch — A fill pattern (diagonal lines, dots, etc.) used to render a layer on screen so overlapping layers stay visually distinguishable; a display convention, not part of the exported geometry.

Tapeout — The point at which a layout is finalized and sent off for mask-making or fabrication — the design-side deadline that DRC and review are meant to precede.

Text — A GDSII element carrying a string label at a point, used for annotation (pad names, cell labels) rather than as physically patterned geometry in most flows. Glyph also supports flattening text into real boundary polygons; see fonts.

User unit — The human-scale unit (typically micrometres) a GDSII file's coordinates are expressed in once scaled by the database unit, as distinct from the raw integer database units stored on disk.

Via — A layer (and its geometry) representing a vertical connection between two other layers, such as two metal interconnect levels in an IC, or two waveguide layers in a photonic circuit.

Waveguide — A patterned structure, typically a path or a boundary on a dedicated layer, that confines and guides light in a photonic circuit — the optical analogue of a metal trace.

KLayout and other product names mentioned above are trademarks of their respective owners, used for identification only; no affiliation or endorsement by Glyph is implied.

Updated 2026-07-12

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