On the following pages, there are several examples of Hierarchical Design.Īll designs are built up from very simple primitives, basically just 6 different types of elements. And each cell can contain instances of other cells, so it is possible to build up a very complex layout of simple, repeated elements. Then you can place a copy of the entire grouping of objects at multiple locations in your layout. A hierarchical CAD program allows you to group shapes together into “cells”, sometimes called “structures” or “blocks”. This would be very tedious and inefficient. But be aware that in the fundamental format, the layers will always be referenced by their number, so in data preparation for e-beam or mask writing, you'll need to know the actual data layer number(s) for the layers you wish to print.Ī very simple CAD program might let you define shapes as described above, but to build up a more complex layout, you would have to define each and every shape individually. In the actual data file, layers are assigned a numerical label, from 0-255 (but there are several different upper values in use.) Many CAD systems also allow you map meaningful names, such as 'Source', 'Drain' and 'Gate' to your layers (these would be layers you would find in a design for a Field Effect Transistor). Each shape within your drawing is assigned to exactly one layer. So our CAD drawings accommodate this by having multiple layers within each drawing. Multi-layerOccasionally something we want to build is simple enough to just be built with one layer of lithography, but more commonly, we need to build multiple layers, one after the other. For some mask shops, the price for a mask build may depend on how fine the placement grid you need for your design, with finer-spaced grids costing more. If you use an exposure tool with less resolution, all vertices will be rounded (snapped) to the nearest vertex in that instrument’s grid. Each vertex in your shape must be located at integer multiples of this fundamental grid. For our ebeam, this grid size is typically 1 nm or 0.125 nm. Grid-basedAll data points within a design are positioned on a fixed-spacing square grid of points in 2-dimensional space. In this figure, the left drawing will result in only the outline being exposed (if anything - it's possible the outline is too narrow to be exposed at all!), versus the right shape, in which the entire, filled polygon area will be exposed. A not uncommon beginner error is to define only outlines rather than areas, with the mistaken belief that the inside areas will somehow be understood to be filled-in. In the case of a photomask, this area will either be opaque (eg chrome) or transparent (no chrome) whereas in direct write, after exposure and development, the area in your cad will either have resist removed (positive resist) or resist remaining (negative resist).Īrea-basedWith your CAD drawing, you will be defining areas - most typically which areas of a photoresist (or e-beam resist) should be exposed and which areas should not be exposed. Shapes to be exposed on your mask or wafer have area. The shapes that make up your design are each defined as some number of 2-D coordinates, such as: (X1,Y1), (X2,Y2), (X3,Y3) etc. Shapes have X and Y dimensions, but no Z.

Two-dimensionalSince microelectronics CAD is based on the idea that the data will be used to generate a photomask, the shapes within a CAD layout are all two-dimensional shapes, such as squares and polygons. Let me explain each of these in greater detail:

The type of CAD we’re going to be doing here, which still forms the basis of all microelectronics designs in use in every fab in the world, has the following characteristics: Basic Concepts of CAD for Nanofabricationīefore we dig into the details of the layout procedure, I’m going to give a little background:

Layouteditor drawing