Tutorial:Layout Tutorial

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In this tutorial you will go through creating an Inverter layout while performing design-rule checks (DRC). This tutorial assumes that you have logged in to an EOS machine and are familiar with basic UNIX commands.

Create Aliases to Setup Your Environment

Before you start this tutorial, add the following lines to the .mycshrc file in your home directory:

alias setup_freepdk45 source /afs/eos/lockers/research/ece/wdavis/tech/FreePDK45/ncsu_basekit/cdssetup/setup.csh
alias setup_pycell425 source /afs/eos/lockers/research/ece/wdavis/setup/py425_setup.csh

The first line defines an alias that gives a command to setup your environment to use the FreePDK45 design-kit with the Cadence tools. You can set up other design-kits with other commands (such as "add cadence_cdk", which sets up the Cadence Design Kit for the MOSIS technologies). The second line defines as alias for a command to setup the Synopsys PyCell environment, which we will use for Parameterized Cells (P-Cells) in Virtuoso.

Before moving on, either source your .mycshrc file or log out and log back in.

Note to users outside NCSU: The setup.csh script mentioned above is provided in $PDK_DIR/ncsu_basekit/cdssetup/setup.csh.

Start the Cadence Design Framework

  1. Log in to a Linux or Solaris machine.
  2. Create a directory to run this tutorial, called something like "layout1". Change to this directory.
  3. Type "add calibre2013.4" at the command prompt. This will add calibre (the tool we use to run design-rule checks) into your search path.
  4. Type "add cadence2013" at the command prompt. This will add the Cadence tools to your search path. Be sure you add this after Calibre. Otherwise, the command "add calibre" will give an error.
  5. Type "setup_pycell425" at the command prompt. This will add the Synopsys PyCell tool to your environment.
  6. Type "setup_freepdk45" at the command prompt. This will setup your directory by copying in various files that are needed to run Calibre and the Cadence tools, including .cdsinit, lib.defs, cds.lib, and .runset.calibre.drc.
  7. Start the Cadence Design Framework by typing "virtuoso &" at the command prompt.

Important Note: The order in which you add cadence2013 and freepdk45 to your environment is important. Unfortunately, the scripts aren't advanced enough to be used in any order. Many students' difficulties have been solved simply by sourcing the setup scripts in the right order.

% add calibre2013.4
% add cadence2013
% setup_pycell425
% setup_freepdk45
% virtuoso &

The first window that appears is called the CIW (Command Interpreter Window).

ArtistHspiceStartCIW freepdk.png

Open the library manager by selecting Tools->LibraryManager. This window allows you to browse the available libraries and create your own.

FreePDK libManager.png

Create Layout View of an Inverter

Create New Library

In the Library Manager, create new library called mylib. Select File->New->Library. This will open new dialog window, in which you need to enter the name and directory for your library. By default, the library will be created in the current directory. After you fill out the form, it should look something like this:

FreePDK createLib.png

Click OK. Next, you will see a window asking you what technology you would like to attach to this library. Select "Attach to an existing technology library" and click OK. In the next window, select "NCSU_TechLib_FreePDK45". You should see the library "mylib" appear in the Library Manager.

Create New Layout View

Next, select the library you just created in the Library Manager and select File->New->Cell View.... We will create a layout view of an inverter cell. Simply type in "inv" under cell-name and "layout" under view. Click OK or hit "Enter". Note that the "Application" is automatically set to "Layout L", the layout editor.

FreePDK newFile.png

Alternatively, you can select the "Layout L" tool, instead of typing out the view name. This will automatically set the view name to "layout".

Click Ok. You may see a warning about upgrading the license. Simply click Ok to ignore this warning. After you hit "OK", the Virtuoso screen will appear as shown below.

FreePDK Editor LSW.png

Now you are ready to draw objects in the Virtuoso window. In this section you learn to place copies of other cells: pmos_vtl and nmos_vtl. These cells are parameterized cells (or p-cells) which change their features when you change their parameters.

In Layout Editor select Create->Instance, or simply hit "i". This will pull up the "Create Instance" dialog box. Next, click "Browse" on the screen that appears and select the library "NCSU_TechLib_FreePDK45", cell "nmos_vtl", view "layout". Click "close" on the browser window. Then scroll down in the create-instance dialog to look for a parameter called Width. Make sure this is set to 0.09.

Next, move the cursor into the layout editor window. You should see a small instance at the tip of your cursor, as shown below.

LayoutTut1FreePDK placeInst1.png

You may want to zoom in before placing the instance. To do that, right-click and drag a box around the origin, as shown below. When you release the button, you should see that the instance is much larger.

LayoutTut1FreePDK placeInst2.png

Next, place the NMOS transistor so that your layout looks like the window below. Next, in the create-instance dialog box, change the cell from "nmos_vtl" to "pmos_vtl" and the width to 0.18. Place the pmos roughly as shown below. Finally, hit "Escape" to stop adding instances.

FreePDK Inv1.png

Now, you will notice that you don't immediately see what is inside the nmos_vtl symbol. You can fix this by hitting Shift-F to display all levels of hierarchy. (You can also do this by going to the Virtuoso Options menu, choosing Display and setting Display Levels from 0 to 32) To switch back, hit CTRL-F, or set the Display Levels back to 0 from the Options menu.

You may want to adjust your view so that it looks nicer. To zoom in, right-click and drag a box around the area you want to zoom in. Alternatively, you can hit "f" to "fit" the entire design in the window, or SHIFT-Z and CTRL-Z to zoom in and out by factors of 2.

Use the commands above to show the layout as below.

FreePDK Inv2.png

Now, look now at the "Layers" box on the left side of the layout window (which was referred to as the "Layer Selection Window" or LSW in older versions of Virtuoso). This box shows you the names of the layers that are "valid" (meaning that you can manipulate them). You can figure out which layers are part of the NMOS cell by making them visible and in-visible. To toggle a layer’s visibility, click the "V" check-box to the right of the layer's name. You can make all layers visible with the "AV" button, and no layers visible with the "NV" button. Sometimes, you may need to hit F6 to "redraw" the Virtuoso window after you’ve changed the visible layers. You can use the "Used Layers Only" check-box to limit the number of layers listed to include only those layers that are used in the current layout.

Note that even if you make all layers invisible, you may still see some shapes. This is because not all layers are "valid". Shapes in invalid layers cannot be altered and are always visible. To make all layers valid, you can right-click on the "Used Layers Only" label and choose Edit Valid Layers…. In general, it is recommended that you not set all layers as valid, because this clutters up the Layer list with many unused layers.

Using this approach, you should be able to figure out that the NMOS uses the following layers: pwell, active, nimplant, poly, metal1, contact, and text. The PMOS is like it, except that it uses layers pimplant and nwell instead of pwell and nimplant. Note that there is nothing magical about the p-cells. You could paint these shapes manually in the current cell-view, and it would make no difference whatsoever to the tool. However, it’s much less effort to use the p-cells, so that’s what we’ll do.

Note also the letters "drw", "net", and "pin" next to each entry in the layer list. These are the purposes of a shape. The purpose is used to indicate special functionality of a shape. We will discuss these more in later tutorials. For now, remember that "drawing" is the purpose that indicates that a shape will appear in the mask layout. You will sometimes see "drawing" abbreviated as "drw", and sometimes "dg".

Selecting and Moving Layout

By default, if you simply drag out a region while holding down the left mouse button (Button-1), whatever is within the box will be selected and highlighted in white.

  • Drag a box over the nmos you just instantiated. When you release the mouse button, whatever is "selected", in this case the nmos cell, will be highlighted.

Once you have selected an object (that is, an instance or a shape) you can do lots of things with it.

  • For example you can move it by typing the m hot-key. You can move layout up/down/left/right one grid at a time by clicking at the selection and moving the mouse. Try it.

You can also select objects by clicking on them.

  • Clicking the left mouse button once on an instance or shape selects it.

If you didn’t place your NMOS and PMOS cells exactly as illustrated above, try moving them now until they are.


To perform a Design Rule Check (DRC), choose Calibre->Run DRC…. The DRC form appears, as shown below. Then click "Run DRC". If you do not see the window appear, or if you get an error, then it's possible that you didn't type "add calibre" as instructed above. You will need to exit Virtoso, log out, and log back in, setting up your environment in the correct order.

FreePDK DRC.png

Viewing DRC Errors

You can learn about the errors by clicking on the rule in the Results Viewing Environment (RVE) window that pops up after DRC is complete. Click on an error and hit "shift-H" to highlight the error in the layout viewer as shown. NOTE: In order for Shift-H to work as described here, in the DRC RVE window, choose Setup->Options..., select "Zoom cell view to highlights by 0.7", and click "OK". You should only need to do this once. Your choice will be saved for the next time that you log in.

FreePDK RVE.png FreePDK Markers.png

In this particular case, the transistor wells are too close together. Fix this error by moving up the pmos. It’s good practice to space the NMOS and PMOS transistors by the smallest amount allowed in order to make the layout as dense as possible. You can draw temporary rulers by hitting "k" and dragging a ruler. You can clear the rulers by hitting "Shift-K". These rulers can help you to draw dense layout much faster than you would by constantly running DRC.

Move the PMOS and re-verify until there are no DRC errors. You can re-run DRC by simply clicking on "Run DRC" in the DRC Form window. You will be asked if you want to overwrite the layout file (inv.calibre.gds). Click Ok. Virtuoso is exporting a file to Calibre every time you run DRC. Note that you will need to save your layout each time you run DRC. Otherwise, the check will run on the last layout you saved.

Keep modifying your layout until there are no errors. You will know that there are no errors when there are no red boxes in the RVE. Alternatively, you can look in the file inv.drc.summary. When the layout is "DRC Clean", the last line of this file should read "TOTAL DRC Results Generated: 0".

To learn more about each design-rule, follow the links the the "Tool Tips" section of the course web-page, under "Design Rules".

If you simply want to remove the error markers, choose Highlight->Clear Highlights… in the RVE.

Once you are done, your layout should look like the one below:

FreePDK InvPre.png


We are now going to "paint" a piece of poly to connect the pmos and nmos devices together. We do this by creating a shape, in this case, a rectangle.

  • Select the poly layer in the layer list by left-clicking on it.
  • Hit “r” to draw a rectangle and draw the poly area.
  • Hit “Escape” to stop drawing rectangles.
  • Your layout should look like this:

FreePDK Inv3.png

Another type of shape that you can create is a paths. Connect the drain nodes of the NMOS and PMOS transistor as follows:

  • Select the metal1 layer in the layer-list by left-clicking on it.
  • Hit “p” to create a path.
  • Set the Width to 0.065 in the dialog box. (If you do not see the dialog box, then you can adjust the width after you draw the path by selecting it and hitting "q" to edit the properties. Set the width to 0.065 in the properties.)
  • Click on one end of the path, and double click to end the path.
  • Hit “Escape” to stop drawing paths.
  • Your layout should look like this:

FreePDK Inv4.png

If you don’t like the way your drawing turned out, you can select a shape and delete it with the delete key, or you can hit “s” (for stretch), and click on one of the sides of a path or rectangle to stretch it into the position that you like.

Also, you may want to run DRC checks periodically to make sure you're making progress in good direction. It’s also a good idea to save occasionally, by selecting File->Save.

Add Vias

Next, we need to add contacts (also called vias). First, we should add contacts to the poly and M1 shapes we just created, in order to make the layout more compact.

Create an M1_POLY via by choosing Create->Via... or simply hitting "o". You should see the Create Contact pop-up appears, as shown below. Set the "Via Definition" to "M1_POLY". The other options should be set correcly by default. Clock the "Rotate" button once to rotate it 90 degrees. Place it in the center of the poly shape that connects the two transistor gates.

Here are some more tricks you can use to make your layout as dense as possible:

  1. Create nwell and pwell rectangles between the transistors that touch. This avoids the spacing rule for wells, allowing a much denser layout.
  1. Mirror the pmos_vtl P-Cell across the X-axis in order to move its contacts further away from the M1_POLY contact. This will allow you to move the transistor and contact P-Cells closer together. Do this by selecting it and choosing Edit->Basic->Properties... (or hit "q"), click the Attribute tab and set the Rotation to MX. Then click Ok.
  2. Turn off the drain contacts in both the nmos_vtl and pmos_vtl P-Cells. Do this by going to the cell properties (as in the last step) and clicking the Parameter tab. Then de-select the parameter diffContactRight.
  3. Add M1_N and M1_P contacts that are rotated by 90 degrees to replace the ones you just removed.

These steps should result in a layout that is much more dense than before. Next, create strips of metal1 for VDD and GND. We typically make these shapes as horizontal bars across the top and bottom, and therefore call them “supply rails”. We then need to connect the rails to the source nodes of the transistors. Create these rails now, and make your design look like the one below. Again, try to make the layout as compact as possible and the supply rails as thin as possible, running DRC as often as needed to learn the design rules.

LayoutTut1FreePDK Contact Popup.png FreePDK Inv5.png

Next, we need to add contacts to wells, which serve as the bulk node of the transistors. Transistors do not have well-contacts by default, because they take up so much room. Several transistors can often share the same well-contact. In this class, we will require that every gate (that is, NOT, AND, OR, XOR, etc.) has at least one contact to each well.

Next create an NTAP via and place it near the PMOS transistor. Likewise, create a PTAP via and place it near the NMOS transistor. When you are done, your layout should look approximately like the one below.

LayoutTut1FreePDK Add Contacts.png

To finish our layout, we may also want to add some active shapes in between the NTAP contact and pmos_vtl P-Cell, as shown below. This will allow us to make a more compact layout than we would be able to make without these shapes. Do the same between the PTAP contact and nmos_vtl P-Cell. We also need to connect these NTAP and PTAP cells to the power rails. Create metal1 rectangles to connect these contacts to the rails. Make the layout as dense as possible. When you are done, your layout should look similar to the one below.

LayoutTut1FreePDK AddActM1.png

Create Pins

Lastly, we need to create pins so that the nodes in our layout have names that are human-readable. Create these pins by selecting Create->Pin…. You should see a dialog box appear, like the one below. Type the names vdd!, gnd!, in, and out in the “Terminal Names” text-box as shown below. Select “Display Pin Name”. Leave all other options as they are.

LayoutTut1FreePDK Create Pins.png

Next, click the “Display Pin Name Option…” button. You will see another dialog box appear:

LayoutTut1FreePDK Display Pin.png

Set the height to 0.05 um and the layer to metal1-dg (drawing). Click OK.

Next, click on the layout where you want each pin to be placed. You will need to click three times: twice to create a rectangle for the pin, and a third time to place the label. The shape of your rectangle doesn’t really matter, as long as it only covers area that is already covered by metal1-dg. When you are done, your layout should look like the one below.

Important Note: It is absolutely essential that you select the Display Pin Name box to create a label for each pin. The label must be in the same layer as the metal shape and must overlap the shape. This is necessary to pass Calibre LVS. This is not needed to finish Layout Tutorial #1; however, if you do not get into this habit now, then you will not be able to finish Layout Tutorials #2 and #3.

LayoutTut1FreePDK Final Layout.png

Congratulations! You have completed the tutorial. Save your design and select File->Print to print out a copy of your layout.

ECE 546Students: Hand-in this print-out of your layout. Make sure that your layout is as dense as possible. Points will be deducted for layout that is larger than necessary.

Further Reading

If you would like to learn more about the layout editor, you view the Cadence documentation. Start the documentation browser by typing

cdsdoc &

at the command prompt, make sure that IC6.1.1 is selected in the Active Library pull-down box at the top, and then select Virtuoso Layout Editor->Virtuoso Layout Suite L User Guide in the browser window that appears. This should start an HTML browser that displays the table of contents for the tutorial.

If you find that you cannot view the figures correctly in the web browser, you can browse to the documentation directory in...


...where you will find PDF files for all of these documents. The cdsdoc documentation browser offers many more links for you to learn about the Cadence Design Framework.