ELEC451 Integrated Circuit Engineering Using Cadence's Virtuoso Layout Editing Tool Contents Contents 1. General 2. Creating and Working On a Layout o 2.1 Undoing/Re-doing an Action o 2.2 Display Options o 2.3 Layers o 2.4 Drawing Layers o 2.5 Connecting Rectangles and Layers o 2.6 Selecting/Deselecting Rectangles o 2.7 Checking Dimensions o 2.8 Editing Rectangles o 2.9 Adding Pins o 2.10 Printing a Layout o 2.11 Saving a Design 3. Verifying that Layout Design Rules are Met (DRC) 4. Extracting a Circuit from your Layout 5. Comparing Layout to Schematic(LVS) 1. General Virtuoso Layout Editor is also a graphical layout tool. It allows you to define your integrated circuit in terms of geometrical objects on each layer of the chip (actually you are drawing the masks for each layer).. The "layout'' of a user cell is stored in the same library as the schematic for that cell, and under the same cell name, but under a different "view''. To start a layout for an existing cell in Virtuoso, select File->Open from the icfb window menu. Now change the Library Name to the one you want, then click on the cell you want (or type in its name), change the view to "layout'', then click OK.
What you really need to know about using Layout Editor is covered in the CAD assignments themselves, but for a better understanding, or as a reference guide, please read the following sections. 2. Creating and Working On a Layout For the following discussion, remember that Cadence likes to do again what it just did last. Once you take one action, a dialog box pops up and you get placed in that "mode'', then can easily repeat that same action on another object using the mouse. To escape a "mode'', hit the ESC key, or click Cancel in the corresponding popped-up dialog box (if it is still visible). If in doubt, the mouse-action and prompt message lines at the bottom of the screen will always tell you what will happen next. 2.1 Undoing/Re-doing an Action You can undo any change by typing in a u, clicking the undo icon, or using the corresponding Edit menu item. You can actually undo a series of actions by repeatedly clicking undo. The depth of this undo capability is set under Options->User Preferences in the icfb main window. If you accidentally undo something you didn't want undone, just do a "redo'' (type key U, or use the Edit menu). 2.2 Display Options There are only a couple of important options here - you get to them under Options-> Display... You can choose both a minor and major grid spacing in microns: I suggest that these should be set to values 0.02 and 0.10 respectively. The X and Y snap spacings should be set to the minor grid spacing (0.02 microns recommended here). Lastly, turn on Pin Names in the list of items that will be displayed. 2.3 Layers Layer types are defined and chosen in a separate "layer selection window'' (LSW). You can define which layers are visible or selectable using the middle and right mouse buttons, along with the AV, NV, AS, NS buttons in the LSW, but I think it is easier to leave them all selectable and all visible (the default).
The layers come up in default colours. You can change to the colours I have defined by selecting from inside the LSW Edit->Display Resource Editor, then loading my_layers.drf. 2.4 Drawing Layers You can add one or more rectangles on each layer by clicking on the desired layer in the LSW, clicking the "Rectangle'' tool inside the main drawing window, then dragging the mouse on the drawing surface to define the rectangle. You can also choose to draw polygons, but it is much simpler to stick with using only rectangles (polygons are easily made by abutting rectangles). Like most actions, this rectangle-draw action can be taken either by selecting the menu item (Create->Rectangle), selecting the icon from the toolbar at the left (Rectangle icon), or using an equivalent keyboard key (in this case the r key). The status line at the top of the window is important. It reports your cursor's current X, Y coordinates, and the dx and dy values tell you how big the rectangle you are drawing is. When done drawing, hit ESC to escape the drawing mode. 2.5 Connecting Rectangles and Layers You connect rectangles on the same layer by just abutting them (remember all you are doing is drawing the processing mask for that layer, so if there is no gap between two rectangles, they define a single connected region). To connect between layers, you need CONTACTS or VIAS. Contacts are used to connect metal1 down to the lower layer of diffusion or polysilicon. Vias are used to connect from metal1 to a higher layer of metal2, or from metal2 to a higher layer of metal3. 2.6 Selecting/Deselecting Rectangles To select an existing rectangle, hover over it with the mouse until it is outlined in a highlighted yellow, then left-click on it until you see it highlighted in solid white. As with Composer for schematics, I don't really recommend selecting single objects (hovering and typing a shortcut key is easier). To deselect a rectangle, click in an empty region of the screen - the highlighting will disappear to let you know it is no longer selected.
To select a group of adjacent rectangles, click and drag to define an outline box with the mouse. When you release the mouse, all rectangles inside the box are selected as a group. 2.7 Checking Dimensions You can use "rulers'' to check your drawn dimensions. Type the k key to bring up a Create Ruler dialog box (this also positions one end of the ruler), and click to turn OFF the Keep Ruler checkbox, then click on the screen to complete your ruler - it will disappear since we have deselected the "keep ruler'' option. Now click on the edge of any rectangle, and then move to the opposite edge to view its distance. Clicking again finishes and erases the ruler. Do this as many times as need to measure different distances. Hit ESC to escape from "drawing-rulers'' mode. If you get any rulers that don't disappear, you can always erase them using Window->Erase All Rulers (or use the K key). 2.8 Editing Rectangles You can move, copy, resize(stretch) and delete from the Edit menu, but most of these have shortcut keys. The actions are described below. To undo any action type the "u" key. There is more than one way to do the following actions - I describe two alternatives for most. 2.8.1 Deleting a Rectangle Hover the mouse over the rectangle until it is dash-highlighted, then type the DEL key with your other hand. Clicking on anything else after this point would cause it to be deleted too (note the action-message line at the bottom of the screen) Hit ESC to escape this delete mode. Alternative: You can also click to select the rectangle first, then delete it. 2.8.2 Moving A Rectangle Hover the mouse over the rectangle until it is highlighted, then type the m key with your other hand. Now move the mouse to reposition the rectangle. You can only make purely-vertical or purely-horizontal moves in a single move (this helps
to keep things aligned). Since you are now in "move" mode, you can now click on ANY rectangle without typing m again to move it. Hit ESC to escape from this mode. Alternative: You can also click to select the rectangle first, then drag it using the mouse when the mouse pointer turns into a 4-way pointer. You can also click the Move toolbar icon before clicking on the rectangle. 2.8.3 Resizing (Stretching) A Rectangle Hover the mouse over the edge of the rectangle until it is highlighted, then type the s key with your other hand. Now move the mouse to reposition the rectangle's edge, and click to complete. You can now click on any other edge of any rectangle without typing s again. Hit ESC to escape from this mode. Note: If you select the whole rectangle first before typing s, then you are actually selecting the rectangle for a move, not a stretch. 2.8.4 Copying A Rectangle Hover the mouse over the desired rectangle until it is highlighted, then type the c key with your other hand. Now move the mouse to position the copy, and click to complete. You can now click on any other rectangle to copy it without typing c again. Hit ESC to escape from this mode. Alternative: You could also click to select the whole rectangle first before typing c, or click the Copy toolbar icon before clicking on the rectangle. 2.9 Adding Pins You will need to add connection points on your layout that will correspond to terminal names on the corresponding schematic symbol. Use Create->Pin menu item, or type ctrl-p key to bring up the Create Symbolic Pin dialog box. Type in the name of the Pin (case-sensitive), select the desired layer type (match it to the metal on your layout), and type in the pin dimension you want (it will create a square pin), then click to position the pin on top of the proper connection
point in your layout. Click again to drop the pin label where you want it. If you wish, you can change the size of the label before placing the pin using the Display Pin Name Options button. 2.10 Printing a Layout Select Design->Plot from the main Virtuoso menu, and set up for printing as described under "Using the Cadence Design System''. You will need to click on the Plot Options button in the dialog box that appears in order to choose the printer type, plus other options. 2.11 Saving a Design You can save a design from the Design menu. You can also save the design under a different name (for later editing to make a different version, say), but note that the current file name stays what it was. So if you want to save to a new name, and edit that newly-named design, you will first have to open it by using Design->Open. This is different from most editors and programs you may be used to. 3. Verifying that Layout Design Rules are Met (DRC) At any time, you can invoke a Design Rule Checker (DRC) to check whether you have violated any design rules for the integrated circuit process. From the Verify menu, select DRC, and just accept the default settings. The entire layout will be checked to make sure that all rules have been met (for example that contacts are the right size, that the gate poly extends past the diffusion, etc. etc.) Any errors are highlighted with flashing markers. By selecting Verify-> Markers->Explain, you can click near a marker to get an explanation of what rule was violated, then you can fix it. You can also get rid of all such markers from the same menu. 4. Extracting a Circuit from your Layout Once the layout has been DRC'd and checks OK, you can extract the MOS circuit that corresponds to the layout. From the Verify menu, select Extract, and just accept the default settings. An extracted "view'' of the cell you had layed out will be added to your library, and can be opened from Design->Open. The extractor will have inferred MOS transistors from your layout, and will superimpose these on top of your layout. To
extract parasistcs as well, you first need to pick "Parasitics'' under the Set Switches button. 5. Comparing Layout to Schematic(LVS) Once the layout has extracted into a transistor circuit, you can compare it any earlier-drawn schematic using the Layout-Versus-Schematic tool (LVS). From the Verify menu, select LVS, and make sure the cell names are correct in the left and right sides of the popped up LVS form, then click the Run button. If the LVS is successful, you must then look at the output (click the Output button) to see if the schematic and layout agree. If not, there is a design error in one of them that needs to be fixed.