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Fall 2008: EE5323 VLSI Design I using Cadence

This tutorial has been adapted from EE5323 offered in Fall 2007. Thanks to Jie Gu, Prof. Chris Kim and Satish Sivaswamy of the University of Minnesota for creating & updating this tutorial. Thanks are also due to NCSU wiki for parts of the layout section.

Setting up your Account

1. Setting up the Process Design Kit (PDK):
1.1 Copy the file NCSU-FreePDK45-1.2.tar from /home/class/siva0024 to your home directory by using the command from your home directory:
        cp /home/class/siva0024/NCSU-FreePDK45-1.2.tar .  
1.2 Extract the archive using the command
        tar -xvf NCSU-FreePDK45-1.2.tar 
1.3 You should now be able to see a directory called FreePDK45 in your home directory. This directory contains an open-source, Open-Acess based PDK for the 45nm technology node and the predictive technology model which you will be using throughout this course.
1.4 Go to ~/FreePDK45/ncsu_basekit/cdssetup
        cd ~/FreePDK45/ncsu_basekit/cdssetup
Here, you will have to modify 2 files - cds.lib and setup.csh using vi, nano or any of your favorite editors as follows:
1.5 Add the following line at the end of your cds.lib
        DEFINE freepdk_cells $PDK_DIR/osu_soc/lib/freepdk45_cells 
1.6 Modify setup.csh as follows:
	1.6.1 Comment out the line which defines the environment variable PDK_DIR. Lines can be commented out by adding a # at the beginning. 
1.6.2 Set the PDK_DIR variable to the root directory of the FreePDK distribution
setenv PDK_DIR /home/class/use_your_username_here/FreePDK45
1.6.3 Comment out the line which defines the environment variable CDSHOME
1.6.4 Set the CDSHOME variable to the root directory the Cadence installation
setenv CDSHOME /home/vlsilab/cadence/ic611
1.6.5 Save and exit the file. Your process design kit is setup and ready to be used now.
1.7 Create your temporary Cadence work directory
        mkdir cds_ncsu
cd cds_ncsu
Always run Cadence from this directory to avoid cluttering up your workspace
1.8 Copy setup.csh(the file you modified in step 1.6) into this directory
        cp ~/FreePDK45/ncsu_basekit/cdssetup/setup.csh . 
Source this script using the following command
        source setup.csh
This script copies the files needed by Cadence and initializes the environment.
1.8 Invoke Cadence by typing virtuoso &. This should bring up the command interface window and library manager. You are now ready to design circuits in Cadence.

1. Example: Design and Simulation of an Inverter

This example will help you familiarize yourself with Cadence. This will show the most important commands and steps used when working with schematics in Cadence. As an example, you will design a simple inverter and simulate the delay of it. Before starting with the design example, there are a couple things worth mentioning: The following picture shows the schematic of an inverter, which is ready for netlist extraction. The following section explains how to draw it in Cadence.

Inverter Schematic

1.1 Create a library for your new design

Cadence Virtuoso is shown in the following picture


From Virtuoso,

        Type a new name(I used "sample" and this is what I will refer to from here on).        
        Select the "Attach to an existing technology library" option, click OK. 
        Then in the popup window, select NCSU_TechLib_FreePDK45 and click OK 
In Virtuoso command window, you will see the following messages (among other warnings) if everything is OK
        Created library "sample" as "/home/class/your_user_name/cds_freepdk/sample"        
        Design library 'sample' successfully attacjed to technology library 'NCSU_TechLib_FreePDK45'

1.2 Create a new cell

From the Virtuoso command window,
        In the popup window, select "sample" (or the name of the library you created in step 1.1).
        Cell name "inverter". view name "schematic" and open with "Schematics L". Click Ok
        Click "Always" if "Upgrade License" warning message shows up. Virtuoso schematic editor opens up. 
        Remember that anytime you get an Upgrade license warning, select the "Always/Yes" option everytime.

1.3 Design your Circuit

1.3.1 Place Components

For this inverter, you will need an nmos transistor, a pmos transistor, power and ground nodes.
From Schematic window,
Create->Instance Add Instance window opens. Click on "Browse". Now the Library Browser window open. These windows are shown in the following picture

Adding Components

Make sure the Library in the library browser is set to NCSU_Devices_FreePDK45. Use the library browser window and click on NMOS_VTL, then select the symbol view.
You can now enter the width and length of the transistor in the Add Instance window. Use the same procedure for PMOS transistors. Also, from the library Basic, you can get the symbols for VDD and GND (they define the net names for the power and ground nodes)
If you make any mistake, you can always use
Edit->Delete or
Edit->Rotate or
Edit->Move or
To change the properties of some of the components:
Edit->Properties->Objects Select the NMOS_VTL transistor and make sure the width and length are set to 90.0n M and 50.0n M respectively. If not, you can change the properties of the transistor.
Similarly, set the width and length of the PMOS_VTL transistor to 180.0n M and 50.0n M.

1.3.2 Connect Components

Connect the components as shown in the schematic above by using

1.4 Add Pins

By adding pins, you can identify the I/O ports of the schematic. At a later stage, you can also use pins as connection points for hierarchical designs. To learn more about this, see the section about creating symbols .
Type the pin name, such as VIN, select the direction as "input", and place it in the schematic. Do the same for VOUT, selecting the direction as "output".

1.5 Generate Netlist

In the schematic window:
        Design->Check and Save
        There should be no errors. 
        Launch->ADE L
The window of Cadence Analog Design Environment will show up.
        Setup->Simulator/Directory Host
        Set the simulator to hspiceD

A new window will pop up showing the generated HSPICE netlist. You may save this file by clicking the menu bar:
File->Save As
Specify the full path name and file name in the Save As window. For example, if we want save the file into a folder named "simulation" under folder "~/cds_ncsu", we should type in "~/cds_ncsu/simulation/inverter.sp". And the file name is "inverter.sp". If you do not provide a path before file name, the file will be saved under the folder where you start your Cadence.

Now we have the netlist ready for editing and simulating. Open the netlist in an editor and comment out the lines that are highlighted in the above picture. You can comment lines in a SPICE netlist by using *. Save the netlist and exit the editor.
Create another SPICE file named "runinv.sp" as below for simulation. It should contain the part of the SPICE file inverter.sp that you commented out in the previous step. You also need to include the models for the transistors and the generated netlist file "inverter.sp".
. Finally you provide the values for the input and VDD.

**Test inverter
.TEMP 25
+            ARTIST=2
+            INGOLD=2
+            MEASOUT=1
+            PARHIER=LOCAL
+            PSF=2
+            POST
.inc '/home/class/your_user_name/FreePDK45/ncsu_basekit/models/hspice/tran_models/models_nom/NMOS_VTL.inc
.inc '/home/class/your_user_name/FreePDK45/ncsu_basekit/models/hspice/tran_models/models_nom/PMOS_VTL.inc 
.include inverter.sp

v1 vdd! 0 1.1v
v2 vin 0 pwl 0ns 0 1ns 0 1.02ns 2.5 2ns 2.5 2.02ns 0 2.5ns 0
.tran 0.01n 3ns
Save the file “runinv.sp” and Exit the editor. The reason for using a separate file for simulation is because later on your circuit netlist may change, but the simulation file can be kept the same.

1.6 HSPICE simulation

The "runinv.sp" file includes the circuit netlist, technology models, supply voltages, input signal timing and simulation time. The setup for simulation is now complete. At the Linux command type in
         hspice runinv.sp
If you have followed all the steps correctly, you will see the following message:
Using: /home/vlsilab/synopsys/hspice_2007.03-SP1/hspice/linux/hspice
 ****** HSPICE - Z-2007.03-SP1 32-BIT (May 25 2007) 18:56:14 09/03/2008 linux
  Copyright (C) 2007 Synopsys, Inc. All Rights Reserved.
  ***** job concluded
1 ****** HSPICE - Z-2007.03-SP1 32-BIT (May 25 2007) 18:56:14 09/03/2008 linux
 ** test inverter
 Init: hspice initialization file: /home/vlsilab/synopsys/hspice/hspice.ini
 lic: Release hspice token(s)
HSPICE job runinv.sp completed.
The important message is the “job concluded”. If the job is “aborted”, you will have to debug the errors in your netlist. For detailed HSPICE command and syntax, the readers are referred to the HSPICE manual on the class webpage.

1.7 Use scope to view results

To view results, we have to start scope. For this, at the linux prompt type in scope Synopsys' CosmosScope opens. From the menu bar,
In the Opened window, select "HSPICE (*.tr*,*.ac*,*.sw*,*.ft*)” for “Files of type” and then select the "runinv.tr0" file. Clicking Open brings up the results browser window.

In the results browser window, double click "v(vin)" and "v(vout)" to plot the waveforms of the signals.

You can measure the propagation delay by
1) Drag the v(vout) signal into the same window as v(vin) signal;
2) Select both "vin" and "vout" signals using control key;
3) Click the second but last button on the tool bar;
4) Drag the start and end circles to the location you want to measure;

1.8 Working with Symbols

If you want to use your design in other schematics, you need to create a symbol for it. This is equivalent to the use of sub-circuits in HSPICE. Using hierarchy in your project makes it easier to organize.

1.8.1 Create a new symbol

Save the schematic before you create its symbol:
File->Check and Save
Create->Cellview->From Cellview, click OK.
A new popup window titled Cellview From Cellview opens up. Make sure that the Library name and Cell Name fields refer to the names of your cell and the new library you created earlier in the tutorial. From View Name should be schematic and To View Name should be symbol. Tool /Data Type should be set to schematicSymbol. Click on OK.
A Symbol Generation Options window opens up. Click on OK.

A new window will open with the symbol view. By default, the symbol shape is a rectangle, but you can change it. Since this design is an inverter, we will draw a triangle and put a small circle at the output. To do this, you will want to delete the green rectangle, draw the new shape, and move the terminals to new positions. Use Create->Shape to draw a triangle and place a circle. There are several shapes available: line, rectangle, circle, etc. You will also need to change the Selection box (the red rectangle), which defines the limits of the symbol. This can be done by stretching the Selection box. Figure below shows an example of the inverter symbol:

Don't forget to check and save.

1.8.2 Use the symbol in other schematics

Create a new schematic, using the instructions described in Create a new cell. Give a name such as test_inverter.
You place this symbol in the new schematic in the same way that you placed any other components, with:
This time change the Library to the library you created in this tutorial and click on inverter. Your symbol should be here. Now, you can define power supplies in the new schematic. If you place new VDD and GND components, they will be implicitly connected to the correspondent VDD and GND components that are inside the inverter.

To move in the hierarchy, select the inverter, and then:
Edit->Hierarchy->Descend Edit
You can choose the schematic or the symbol for editing.
To return to the previous schematic, use:

2. Layout of the Inverter

This example will help you create a layout of the inverter that you created in the first example. There are many considerations to take into account when deciding how to do a layout. This is NOT an example on layout techniques but more of a generalized example to help you get familiar with Virtuoso and laying out some basic components.

The following picture shows the layout of the inverter. The rest of the section explains how to make each of the separate components in Virtuoso.

2.1 Create layout view of the inverter

In the Virtuoso command window, Click on File->New->Cell View. In that select the library you created in the schematic design entry example. In the Type field, select "layout". The Open with application should be automatically set to "Layout L". Click on Ok. Click "Always/Yes if there is any license upgrade message".
A Layout editing and a LSW(Layer Selection Window) will open up. You are now ready to layout the inverter you designed earlier.

2.2 Layout Components for your circuit

You will create circuit components by painting shapes on the layout editor. You will use the LSW to select appropriate layers. The LSW is shown in the following picture.

You can draw a shape by first selecting the layer from the LSW, then Create->shape->.... You will be drawing rectangles for the most part. Note that this is similar to how you created symbols in Section 1.8.

2.2.1 Laying out an NMOS transistor

In this section, you will learn how to layout an NMOS transistor in this process. Before going into detail about the layout, it is recommended that open up the design rules of the process in another browser window. You can get the design rules of the FreePDK45 process from here. You will need to refer to the design rules later to fix errors in your layout.

An NMOS transistor uses the following layers: pwell, active, nimplant, poly, metal1 & contact.. You can start designing the transistor by selecting "active" on the LSW and drawing a rectangle on the layout editor. Note also the letters "drw", "net", and "pin" next to each entry in the LSW. These are the purposes of a shape. The purpose is used to indicate special functionality of a shape. We will be using "drw" for now.
At this point, do not worry about drawing the rectangle to exact dimensions and just draw a rectangle of any arbitrary dimension. You can zoom in and out of the editor by using the zoom buttons located on the top menubar of the editor. You can also draw a box around the area you want to zoom in by holding on to your right mouse click button. When you release the button, the area you selected will be zoomed in.
Once you draw the rectangle, you can select it and press "q" or Edit->Basic->Properties to change the dimensions of the rectangle.
. You can move objects around by pressing "m" or Edit->move. Readers are strongly encouraged to get familiar with other keyboard shortcuts as this will reduce design time later on.
The default units are "user units" and are in microns. For our inverter, the NMOS transistor has a width of 90n M. So, make sure that the active layer that you have just now drawn is also 90n M in width. Then you repeat the same process but by selecting the "nimplant" layer from the LSW. The active and nimplant layers must overlap and these form the Source and Drain diffusion regions of the NMOS transistor you are trying to create. The following pictures illustrate these two steps.

The next step is to draw the gate of the transistor. This is done by selecting "poly" in the LSW and drawing another rectangle to form the gate. Make sure that the length of the gate is set to 50n M to create the transistor that we used for the schematic. Now, we have formed the channel and the source and drain diffusion regions. The next step is to draw a pwell outside the NMOS transistor. Use the same procedure as described earlier to draw the pwell. The resulting transistor is shown in in the following picture

We still have to create contacts for the source, drain and the body terminals of the transistor. You can create contacts to the active layer by selecting "contact" from the LSW and painting a rectange on the Active layer. Paint two contacts for the Source and Drain regions of the transistor. Now you need to create a body terminal. To do this, click on Create->via or press "o". In the window that opens up, make sure that the Technology Library is set to NCSU_TechLib_FreePDK45 and select "PTAP" in the Via Definition field. Place the PTAP connection in contact with the pwell you have drawn. You can press Shift+F to reveal the details inside the PTAP connection.
Draw metal1 rectangles over the contacts so that you can connect the terminals to other signals in your circuit. Your NMOS transistor is now ready. The final transistor should look like the following picture.

You can create the PMOS transistor similarly. It uses the following layers:nwell, active, pimplant, poly, metal1 & contact. Make sure the dimensions of the PMOS transistor match that used in the schematic.

2.2.2 Layout of inverter

Now that you have a PMOS and an NMOS transistor, you are ready to draw a layout of the inverter. Make the rest of the connections by using a poly to connect the gates of the two transistors together. Connect the drains of the PMOS and NMOS transistors using metal 1. Also connect the NTAP and source of the PMOS transistor and the PTAP and source of the NMOS transistor together using metal 1.
Now you need to connect a metal 1 layer to the poly gate. To do this, click on Create->via and select "M1_Poly" as the via type. Attach it to the gate and connect a metal 1 rectangle to it.
Your inverter should now look like this.

2.3 Performing the DRC of the inverter

So far, we have drawn rectangles to create transistors and connected them to form an inverter. We now need to verify that the layout has passed all the design rules of the process. To do this, first save the layout and then click on
Calibre->Run DRC from the menu bar on the layout editor. The DRC form appears. Then click on Run DRC.
You can ignore any warning messages you get. When CAlibre finishes the DRC, it opens up 3 windows: A Calibre results window, Calibre interactive (which is the main DRC form) and a DRC summary report. You can check whether you layout passed all design rules by looking at the Results window.

2.3.1 Viewing and Correcting Errors in DRC

In this section, you will be shown how to find out errors in the layout and to fix them. The following picture shows the DRC results on the inverter that was laid out earlier in this section.

The presence of a red check mark in the DRC results window indicates that the layout failed the DRC check. The results window give information on what design rule was violated, where it was violated and how many instances have violated the rule.
In the topmost level in the results window, it says "Cell Inverter - 1 Result". This means that there is one instance of a design rule being violated somewhere in the layout. Underneath that, it says "check Poly.6" 1 Result. This is the design rule being violated.
In the message window below, it says that " Minimum spacing of field poly = 0.075". This means that somewhere in the design, we have 2 pieces of poly that are closer than 0.075 microns and this violates the design rule of the process.
The following picture shows a zoomed in version of our poly connections.

You can observe that there is a small gap between the poly connecting the gates of the two transistors and the M1_Poly contact. This resulted in the layout failing the DRC check. You can fix this error by moving the M1_Poly to the right so that gets in contact with the poly connecting the gates.
The following picture shows the results of the DRC when you fix this problem and re-run DRC.

Note that when you are working through this tutorial you may get other errors from DRC. You can get more information about design rules from the link posted earlier in this section or from the NCSU wiki page linked at the bottom of this page. The DRC results window will let you know what rules you have violated. So, in conjunction with the set of design rules in the wiki you can fix those errors.

2.4 Create Pins

Lastly, we need to create pins so that 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

Next, click the "Display Pin Name Option..." button. You will see another dialog box appear.

Set the height to 0.05 um and the layer to metal-1 dg. Click OK.

Next, click on 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 rectange does not matter as long as it only covers area that is already covered by metal1-dg. Re-run DRC to make sure the layout passes all design rules.

2.5 Performing an LVS check of the inverter

The next step is to perform a layout versus schematic(LVS) check. To perform an LVS check choose Calibre->Run LVS.... The LVS form appears as shown below.

Make sure you select the "Export from layout viewer" option under the Layout tab and "Export from schematic viewer" under the Netlist tab. Under the Outputs tab, set the svdb directory to svdb_inverter. Then click on "Run LVS" button. If LVS runs sucessfully, then you will see the following window with a smilie.

Click on "Transcript" tab in Calibre Interactive - LVS to see the log file. The LVS report is also opened and is shown below.

2.6 Extract Parasitics

To perform a Parasitic Extraction(PEX), choose Calibre->Run PEX.... The PEX form will appear. You can stick to the default options for now. Click on "Run PEX" button. If PEX runs sucessfully, you will be able to view the PEX Report file - inverter.pex.report and also the PEX netlist - inverter.pex.netlist, which is the extracted netlist from the layout along with parasitic capacitances and resistances.
Click on "Transcript" to view the log file. The main HSPICE netlist, inverter.pex.netlist contains only the intentionally designed devices.
Filenames with .pex and .pxi extensions are included in inverter.pex.netlist. It is OK if you do not understand the .pex and .pxi files. The top-level netlist file however, needs to be understood.
Once you have the extracted netlist, you can simulate it to see how it performs as opposed to the simulation results from the schematic. Refer to Section 1.6 for more information on simulation.

2.7 Layout tips

This tutorial is less focused on the techniques used for layout and is geared mainly towards introducing the reader to tools and the steps involved in creating a layout/design. Nevertheless, here are some pointers to do layout.

More Information

This tutorial is a watered down version of the more elaborate tutorial developed at NCSU. Interested readers are referred to NCSU FreePDK Wiki for more details.