VLSI Circuits Notes eBook
About this eBook
begin somewhere. In this chapter, we will introduce the first ideas of physical design by examining the concept of layout in more detail. This includes ideas such as design rules and interconnect routing. The details of designing CMOS circuits will be covered in the following chapters.
3.2 Masks and Layout Drawings Every material layer in an integrated circuit is described by a set of geometrical objects of specified shape and size. These objects are defined with respect to each other on the same layer, and also with reference to geometrical objects that lie on other layers, both above and below. Layout drawings relay this information graphically, and can be used to generate the masks needed in the fabrication process. Because of this relationship, we will take the viewpoint that a layout drawing represents the top view of the chip itself. When we visualize an integrated circuit, it appears as a set of overlapping geometrical objects. In a layout editor, each layer is described by using a distinct color or fill pattern that allows us to see the objects relative to each other. Once we get oriented to seeing an integrated circuit in this manner, it is a simple matter to construct transistors and route the interconnect lines as required. While classical schematic representations provide the topology of the network, the layout gives us the ability to modify the performance of a circuit. Performing the layout is therefore an intrinsic part of the design process.
Prof.Hansaraj Guhilot, KLE, Belagaum
When designing digital logic circuits in CMOS, the goals remain quite simple Design a circuit that implements the logic function correctly, and, Adjust the parameters to meet the electrical specifications. This is often more difficult than it sounds, particularly when we note that state-ofthe art VLSI chips can have several million MOSFETs with the associated interconnect lines. At the most basic level, we find that many problems arise when performing the layout of an integrated circuit. Some deal with the practical aspects of circuit operation, others originate from physical properties of the materials involved, and yet others are due to limitations in the fabrication processes. These all contribute to the techniques used in the physical design.
3.3 Design Rule Basics Design rules are a set of guidelines that specify the minimum dimensions and spacings allowed in a layout drawing. They are derived from constraints imposed by the processing and other physical considerations. Violating a design rule may result in a non-functional circuit , so that they are crucially important to enhancing the die yield. Limitations in the photolithography and pattern definition give rise to several critical design rules. Since these are strongly dependent on equipment used in the fabrication process, they tend to change with improving technology. The situation is complicated by the fact that physical phenomena and device design considerations also enter into the picture. In this section, we will examine some of the
design rules associated with a CMOS processing technology. .
3.3.1 Minimum Linewidths and Spacings