Computer Aids for VLSI DesignSteven M. RubinCopyright © 1994Appendix B: Caltech Intermediate FormatB.1 GeometryB.2 HierarchyB.3 ControlComputer Aids for VLSI DesignSteven M. RubinCopyright © 1994Appendix B: Caltech IntermediateFormatCaltech Intermediate Format (CIF) is a recent form for the description of integrated circuits. Created by the university community, CIF has provided a common database structure for the integration of many research tools. CIF provides a limited set of graphicsprimitives that are useful for describing the two-dimensional shapes on the different layers of a chip. The format allows hierarchical description, which makes the representation concise. In addition, it is a terse but human-readable text format. CIF is therefore a concise and powerful descriptive form for VLSI geometry. Each statement in CIF consists of a keyword or letter followed by parameters and terminated with a semicolon. Spaces must separate the parameters but there are no restrictions on the number of statements per line or of the particular columns of any field.Comments can be inserted anywhere by enclosing them in parenthesis. There are only a few CIF statements and they fall into one of two categories: geometry orcontrol. The geometry statements are: LAYER to switch mask layers, BOX to draw a rectangle, WIRE to draw a path, ROUNDFLASH to draw a circle, POLYGON to draw an arbitrary figure, and CALL to draw a subroutine of other geometry statements. The control statements are DS to start the definition of a subroutine, DF to finish the definition of a subroutine, DD to delete the definition of subroutines, 0 through 9 to include additional user-specified information, and END to terminate a CIF file. All of these keywords are usually abbreviated to one or two letters that are unique.B.1 GeometryThe LAYER statement (or the letter L) sets the mask layer to be used for all subsequent geometry until the next such statement. Following the LAYER keyword comes a single layer-name parameter. For example, the command: L NC;sets the layer to be the nMOS contact cut (see Fig. B.1 for some typical MOS layer names). NMnMOS metalNPnMOS polysiliconNDnMOS diffusionNCnMOS contactNInMOS implantNBnMOS buriedNGnMOS overglass CMFCMOS metal 1CMSCMOS metal 2CPGCMOS polysiliconCAACMOS activeCSGCMOS selectCWGCMOS wellCCCMOS contactCVACMOS viaCOGCMOS overglassFIGURE B.1 CIF layer names for MOS processes.The BOX statement (or the letter B) is the most commonly used way of specifying geometry. It describes a rectangle by giving its length, width, center position, and an optional rotation. The format is as follows: B length width xpos ypos [rotation] ;Without the rotation field, the four numbers specify a box the center of which is at (xpos, ypos) and is length across in x and width tall in y. All numbers in CIF are integers that refer to centimicrons of distance, unless subroutine scaling is specified (described later). The optional rotation field contains two numbers that define a vector endpoint starting at the origin. The default value of this field is (1, 0), which is a right-pointing vector. Thusthe rotation clause 10 5 defines a 30-degree counterclockwise rotation from the normal. Similarly, 10 -10 will rotate clockwise by 45 degrees. Note that the magnitude of this rotation vector has no meaning. The WIRE statement (or the letter W) is used to construct a path that runs between a set of points. The path can have a nonzero width and has rounded corners. After the WIRE keywordcomes the width value and then an arbitrary number of coordinate pairs that describe the endpoints. Figure B.2 showsa sample wire. Note that the endpoint and corner rounding are implicitly handled. FIGURE B.2 A sample CIF "wire" statement. The statement is: W25 100 200 100 100 200 200 300 200; The ROUNDFLASH statement (or the letter R) draws a filled circle, given the diameter and the center coordinate. For example, the statement: R 20 30 40;will draw a circle that has a radius of 10 (diameter of 20), centered at (30, 40). The POLYGON statement (or the letter P) takes a series of coordinate pairs and draws a filled polygon from them. Since filled polygons must be closed, the first and last coordinate points are implicitly connected and need not be the same. Polygons can be arbitrarily complex, including concavity and self-intersection. Figure B.3 illustrates a polygon statement. FIGURE B.3 A sample CIF "polygon"statement. The statement is: P 150 100 200 200 200 300 100 300 100200;B.2 HierarchyThe CALL statement (or the letter C) invokes a collection of other statements that have been packaged with DS and DF. All subroutines are given numbers when they are defined and these numbers are used in the CALL to identify them. If, for example, a LAYER statement and a BOX statement are packaged into subroutine 4, then the statement: C 4;will cause the box to be drawn on that layer. In addition to simply invoking the subroutine, a CALL statement can include transformations to affect the geometry inside the subroutine. Three transformations can be applied to a subroutine in CIF: translation, rotation, and mirroring. Translation is specified as the letter T followed by an x , y offset. These offsets will be added to all coordinates in the subroutine, to translate its graphics across the mask. Rotation is specified as the letter R followed by an x , y vector endpoint that, much like the rotation clause in the BOX statement, defines a line to the origin. The unrotated line has the endpoint (1, 0), which points to the right. Mirroring is available in two forms: MX to mirror in x and MY to mirror in y. Mirroring is a bit confusing, because MX causes a negation of the x coordinate, which effectively mirrors about the y axis! Defining subroutines for use in a CALL statement is quite simple. The statements to be packaged are enclosed between DS (definition start) and DF (definition finish) statements. Arguments to the DS statement are the subroutine number and a subroutine scaling factor. There are no arguments to the DF statement. The scaling factor for a subroutine consists of a numerator followed by a denominator that will be applied to all values inside the subroutine. This scaling allows large numbers to be expressed with fewer digits and allows ease of rescaling a design. The scale factor cannot be changed for each invocation of the subroutine since it is applied to the definition. As an example, the subroutine of Fig. B.4 can be