FreePDK15:Layout Tutorial 1

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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 line to the .mybashrc file in your home directory:

alias setup_freepdk15='source /afs/eos/lockers/research/ece/wdavis/tech/FreePDK15/cdslib/setup/'

The line defines an alias that gives a command to setup your environment to use the FreePDK15 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). If you regularly use machines that default to the tcsh shell (rather than bash), then you might want to also add the following line to the .mycshrc file in your home directory, which does the same thing for that shell:

alias setup_freepdk15 source /afs/eos/lockers/research/ece/wdavis/tech/FreePDK15/cdslib/setup/setup.csh

Before moving on, source your .mybashrc (bash) or .mycshrc (tcsh) file or log out and log back in.

Note to users outside NCSU: The scripts mentioned above is provided in $PDK_DIR/cdslib/setup/

Start the Cadence Design Framework

  1. Log in to a Linux machine. The setup for this tutorial is currently supported only on Linux machines.
  2. Create a directory to run this tutorial, called something like "layout1". Change to this directory.
  3. Type "add cadence2016" at the command prompt. This will add the Virtuoso tools to your search path.
  4. 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.
  5. Type "setup_freepdk15" at the command prompt. This will setup your directory by copying in various files that are needed to run the Cadence tools, including .cdsinit and cds.lib.
  6. Start the Cadence Design Framework by typing "virtuoso &" at the command prompt.

$ mkdir layout1
$ cd layout1
$ add cadence2016
$ add calibre2013.4
$ setup_freepdk15
$ virtuoso &

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

ArtistHspiceStartCIW freepdk.png

Another window that is very handy is the Library Manager, which allows you to browse the available libraries and create your own. To display this window, choose Tools -> Library Manager... from the CIW Menu.

ArtistHspiceStartLibMgr freepdk.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_FreePDK15". 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_pcell and nmos_pcell. 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_FreePDK15", cell "nmos_pcell", view "layout". Click "close" on the browser window. Then expand the parameters section of the create-instance dialog to look for parameters called nFins and Gate. Make sure these are set to 2 and A, respectively.

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. Make sure to place it exactly on the y-axis as shown below, which will make things easier later-on. Next, in the create-instance dialog box, change the cell from "nmos_pcell" to "pmos_pcell". Place the pmos roughly as shown below, also on the y-axis. Finally, hit "Escape" to stop adding instances.

FreePDK15 Inv1.png

Now, you will notice that you don't immediately see what is inside the nmos_pcell 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.

FreePDK15 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: ACT, NIM, GATEA, GATEB, and AIL1. The PMOS is like it, except that it uses layers PIM and NW and omits NIM. Note that there is nothing magical about this instance. 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 this instance, so that’s what we’ll do.

Note also the letters "drw", "net", or "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_pcell 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.

Next, place instances of three vias, as shown below. The top and bottom instances will be M1A_AIL2 and will connect to the active areas. Make sure that the AIL2 shapes on the via are perfectly centered inside the AIL1 shapes on the mos-devices. The middle instance will be M1A_GIL, which will eventually connect to the gate. Make sure that its M1A shape is perfectly aligned with the other two vias.

FreePDK15 Inv3.png


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. Double-click on one of the two M1004A errors (or single-click and hit "shift-H") to highlight the error in the layout viewer as shown. NOTE: This should work as described by default, but if it doesn't, 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.

FreePDK15 RVE.png FreePDK15 Markers.png

In this particular case, the M1A shapes are too close together. To fix this error, you'll need to move the instances further apart. It’s good practice to space the vias 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 vias 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. If you simply want to remove the error markers, choose Highlight->Clear Highlights… in the RVE, or click on the button that looks like an eraser.

Keep modifying your layout until the M1004A error disappears, and move the pmos_pcell cell to stay aligned with its via. To learn more about each design-rule, find the complete list of the current rules in the FreePDK15 Table of Contents). This page gives the name and value of every rule. Note that the rule names are slightly from what is displayed in the RVE, such as M1004A which appears as M1.4.

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

FreePDK15 Inv4.png

Now let's look at the remaining errors. Here we're using the name as they appear on the web-page (rather than the RVE), for easy reference.

  • ACT.9 - This is a rule to prevent latch-up. We'll need to create well-contacts later to solve these errors.
  • NW.4 and NIM/PIM.7 - These are minimum-area rules. We could fix them now by drawing larger NW/NIM/PIM boxes around the current ones. However, since we still have more NW, NIM, & PIM shapes to make (for the well-contacts), we can put these off until later to save effort.
  • GIL.2 - This rule shows that the M1A_GIL via is not entirely error-free. In particular, the GIL shape is not wide enough. The via was designed this way, because GIL is generally used for horizontal routing. When we paint shapes, we'll extend the GIL shape on the via to the right, to connect to the gate.

Before we start painting, we have to make some changes. First, we have enough space to add more fins on our transistors, so let's do that. Fortunately, our p-cells make that easy to do. Simply select each instance and select Edit->Basic->Properties (or press "q"). Select the Parameters tab on the properties dialog-box and change nFins to 3. Then click Apply or Ok. You will see the transistor shape update to include a third fin. Note that you can also select whether you want the gate shape to be drawn in GATEA or GATEB with the Gate parameter. Re-run DRC to make sure that you have not violated the ACT.2 rule. When done, it should look like the image below.

FreePDK15 Inv5.png

Finally, we'll need to add contacts for the drains. Add two instances of M1B_AIL2 as shown below. Make sure that they are centered on the AIL1 shapes in the mos-devices.

FreePDK15 Inv6.png


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

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

FreePDK15 Inv7.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.

Go ahead and create rectangles of GATEB to connect the other gate-lines, as well as a rectangle to connect the M1B vias as shown below. Finally, create a rectangle to extend the GIL shape to the right as shown below. Re-run DRC and fix any errors that you find (except for the ACT.9, NW.4, and NIM/PIM.7 rules, which we will get to next).

FreePDK15 Inv8.png

Other Ways to Create Shapes

Here are some other ways to create shapes that you may want to try:

  • Path - You can draw a path by selecting the entry layer and then choosing Create->Shape->Path (or press "p"). Then click wherever you like in the layout. A minimum-width path will be drawn. This is mostly convenient for higher levels of metal, which don't usually have different width rules for different routing directions.
  • Copy - You can copy a shape or instance with the following steps:
    1. Selecting the shape/instance
    2. Choose Edit->Copy (or press "c")
    3. Click on the shape/instance to set a reference point for the copy
    4. Drag the mouse to the location of the copy
    5. Click again to place the shape/instance

Create Well Contacts

Now let's solve those latch-up errors. Do that by creating the following instances, such that your layout matches the one below:

  • Add AIL1_N to connect to the n-well. Make sure that its ACT shape is exactly 96nm away from the PMOS ACT shape, to satisfy rule ACT.5.
  • Add AIL1_P to connect to the p-well. Make sure that the right side of its PIM shape is exactly aligned to the left side of the NIM shape on the AIL1_N instance, for maximum density.
  • Add two instances of M1A_AIL2. Make sure the AIL2 shapes are centered on the AIL1 shapes for the well contacts that you just made.

FreePDK15 Inv9.png

Now let's solve those remaining DRC rules. Create rectangles of NW, NIM, and PIM to match the layout below. Remember that the minimum width of NIM & PIM is 128nm (rule NIM/PIM.1) and that the NW shape should be 31nm away from the ACT shape in the p-well contact (rule ACT.7). Keep modifying your layout until there are no more 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".

FreePDK15 Inv10.png

Finally, 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.

FreePDK15 Inv11.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 “Create Label”. Leave all other options as they are.

LayoutTut1FreePDK15 Create Pins.png

Next, click the “Options…” button next to "Create Label". You will see another dialog box appear:

LayoutTut1FreePDK15 Display Pin.png

Set the height to 0.02 um and the "Layer Name" to "Same As Pin". Click OK. Make sure that you have M1A-drw selected as your entry layer.

Next, click on the layout where you want the vdd! 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 M1A-drw. Repeat this procedure for the gnd! and in pins. Before adding the out pin, make sure to change the entry layer to M1B-drw. When you are done, your layout should look like the one below (with NW, NIM, and PIM set invisible, for clarity).

Important Note: It is absolutely essential that you select the Create Label box when you create 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.

LayoutTut1FreePDK15 Final Layout.png

Congratulations! You have completed the tutorial. Save your design and take a screen-shot of your layout to turn in with your assignment.

ECE 546Students: Hand-in this image 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 can read through the Virtuoso Schematic Editor L User Guide that comes with the Cadence documentation. Start the documentation browser by typing

cdnshelp &

at the command prompt, and then selecting Virtuoso Layout Suite->Virtuoso Layout Suite L User Guide in the browser window that appears. This should display the pages that you select.

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 cdnshelp documentation browser offers many more links for you to learn about the Cadence Design Framework.