An Introduction to IC Design Under Linux
The latest freely-available version of SPICE is 3f4. Linux-ready sources for SPICE3f4 are available at ftp://ftp.sunsite.edu/pub/Linux/apps/circuits/spice3f4.tar.gz. Compilation and installation instructions are in spice3f4.readme. Note that you must have at least version 1.14.5 of BASH in order to compile. When we compiled ours, we made only one change. We removed the -dWANT_X11 from the conf/linux file, and so did not get the X interface but did get smaller binaries. Since we are using Sigview to look at the the outputs, we don't need this interface anyway. If you don't feel like compiling, an a.out format binary is available at ftp://ftp.eos.ncsu.edu/pub/vlsi/software/Linux/spice3/.
Sigview, developed by Lisa Pickel at MCNC, is an X11-based program for viewing the output of various simulators, including SPICE. By the time this article appears, it should be available at ftp://ftp.sunsite.edu/pub/Linux/apps/circuits/sigview31.tgz. After untarring this file, cd sigview and edit the Makefile. As noted in the instructions at the top, you might need to define SIG_DEV_FILE and SIG_TMP_PATH to point to appropriate places on your system. If they are already set correctly, you can simply use the included (a.out format) executable. If not, a simple make will do the job. You can then place the executable in $CAD_HOME/bin.
Before getting into the details of designing with Magic, it's very helpful to have a basic understanding of the physical structure of an IC and the process by which it's constructed. There are many different types of process technologies—CMOS, GaAs, bipolar and BiCMOS are common ones. Today, CMOS (Complementary Metal Oxide Semiconductor) is the dominant technology for high-density, low-power, low-cost digital circuits; accordingly, we'll focus our examples on designing for CMOS. However, the concepts presented apply equally well to any process.
ICs are constructed by depositing, implanting or growing various materials in patterned layers on a silicon base. Several layers are stacked vertically atop one another; each is separated from the layer immediately above and below by a thin layer of insulating material. It's important to note that a layer can contain many separate objects—for example, it can contain thousands of wires, none of which touch one another.
How do IC manufacturers form the patterns? Typical manufacturing processes are similar to spray-painting, in that they coat the entire IC surface with the material used to make the layer; this obviously precludes the formation of distinct shapes or patterns. If you've ever used a template to spray-paint letters onto a poster or wall, you know that the solution to this problem is to get a piece of shielding, called a mask, cut out the areas you want to paint, and spray through the resulting holes. IC process engineers do the same thing on a vastly smaller scale.
Each step in the fabrication process uses a different mask. Suppose we have a chip with two layers of metal wiring. We'll call the layer closest to the silicon base “m1” and the layer closest to the top “m2”. Because m1 and m2 are different layers, m2 wires can cross over m1 wires without making electrical contact. This is like a high road using an overpass to cross a low road. The left side of Figure 1 illustrates, in both an aerial and cross-sectional view, a vertical m2 wire overlapping a horizontal m1 wire. Note that the two are electrically separate. Suppose now that we want to create a point where a signal can cross from m1 to m2. To build this structure, we need a mask for m1, a mask for m2, and a mask for vias. A via is a hole in the insulating layer, which separates m1 from m2 and allows m2 to flow downward and contact m1, forming an electrical connection. This is shown on the right side of Figure 1; the black square is the via, which connects the two layers. Transistors are more complicated, requiring more masks and different materials.
Magic makes work easier for the designer by hiding much of this underlying physical complexity. Rather than work with mask layers directly, you manipulate abstract layers which implicitly represent one or more masks. For example, to create an m1 to m2 via, rather than draw three shapes on separate mask layers (the m1 layer, the hole layer and the m2 layer), you simply draw one shape on the abstract “via” layer, which has all three mask layers “built-in”. There are also abstract layers having a one-to-one correspondence to mask layers; m1 and m2 are examples. In general, an abstract layer can represent an arbitrary number of mask layers, permitting a great simplification in design.
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