Electric VLSI Design System Tutorials from CMOSedu.com (Return)

Tutorial 2 – Layout and simulating the IV curves of PMOS and NMOS devices

In this tutorial we’ll layout and simulate the operation of NMOS and PMOS transistors using the setups, and C5 process, from Tutorial 1.

To begin, use Electric to open the jelib we used in tutorial 1, tutorial_1.jelib (right click to save to C:\Electric).

Save this library as tutorial_2.jelib, again in C:\Electric.

Next save the file C5_models.txt to C:\Electric for SPICE simulations.

This file contains the MOSFET models for the C5 process.

When finished create new cell, a schematic called NMOS_IV, see below.

Notice the R_divider group from the first tutorial is still present in this library.

Next go to the Components window and select/place the NMOS Node, circled, as seen below.

Fill the window and then edit the Nodes properties.

Let’s set the width of the device to 10 and leave the length at 2.

Hit OK when finished.

Next select (Ctrl-click is very useful for selection) the text (the width and length) on the Node.

While Nodes/Arcs can be rotated using Ctrl+J or Edit -> Rotate text must be rotated via its properties as seen below.

Rotate and move both the width and length of the MOSFET until you get the following.

Notice how the grid alignment was reduced to 0.25 so that the device’s length (here 2) could be move slightly away from center.

Change the grid alignment back to 0.5.

Next, select the NMOS Node (the symbol) and go to Tools -> Simulation (Spice) -> Set Spice Model.

We get the following.

Edit the SPICE-model text using Ctrl+I

From the C5_models.txt file above we see that the NMOS model name is NMOS and the PMOS model’s name is PMOS (easy to remember).

Change the name to NMOS and Rotation to 270 as seen below.

Move the SPICE name to the position seen in the following.

From this point on we can simply copy this Node to avoid going through these steps every time we place an NMOS device.

Repeat these steps for the PMOS device.

The cell name should be PMOS_IV.

The result is seen below.

These symbols are useful for general design but in this tutorial we want to have access to all of the transistor’s terminals including the device’s body.

In this PMOS_IV cell select the PMOS Node then go to the menu Edit -> Change or simply press C (the corresponding bind key).

The Window seen below will pop up.

Change the Node to a 4-port device as seen (hit Apply then Done).

Notice that the body (here the n-well) is next to the source of the PMOS device.

Since it’s common to draw the body and source at the top of the device, for a PMOS, and the drain and the bottom let’s use Edit -> Mirror -> Up <--> Down to flip the device so the source/body are at the top of the symbol.

Below we’ve also moved the PMOS SPICE model name.

Let’s repeat this (except for Up <--> Down mirroring) for the NMOS_IV cell.

Next let’s make layout views for these cells. Create these two cells now (Cell -> New Cell)

Go to the NMOS_IV{lay} view as seen below.

Go to the components menu on the left and select/place the nMos Node in the lower right corner of the menu, as seen below.

This is the layout of an NMOS device consisting of a p-well, active, n-select (to dope the active n-type), and poly (the pink).

Notice the highlight box indicating the Node is selected.

Next add nAct Nodes as seen below.

These will connect to the MOSFET to form the source and drain of the device.

They also provide a connection for metal1 Arcs to the MOSFET S/D.

Next add the metal1 to contact to poly1 as seen below.

This Node is used for connecting metal1 to the MOSFET’s gate terminal.

Finally, let’s place the body connection for the NMOS device, the pWell Node seen below.

Zoom out as needed.

We are now ready to connect the Nodes together with Arcs.

To size the Nodes use Edit -> Properties -> Object Properties (Ctrl+I or, if you have my Key Bindings from the first tutorial, just press lowercase Q)

Go to the nMos Node and change its Width to 10.

Next, with the nMOS Node still selected, go to Tools -> Simulation (Spice) -> Set Spice Model… and set the SPICE model name to NMOS (if you don’t do this we can’t simulate the layout)

The result is seen below.

Next select both of the nAct Nodes.

To select both first select one and then, while holding down on the Shift key, select the other.

Edit their properties and the following Window will open.

Change the X size to 10, the same width as the MOSFET.

Note that you can change each nAct Node individually but we want you to learn the tool so you can do things quickly ;-)

The last step, before wiring the Nodes together, is to change the X-size of the pWell Node.

Let’s set the width to 10 as well, see below.

Next, using the left mouse button select the top port on the nMos Node as seen below.

Move the mouse up to top nAct Node and RIGHT click on it.

The result is seen below. An N-Active Arc was added between the Nodes.

At this point move the nAct Node down to match the following.

DRC your layout, F5, after you’ve moved the Node to ensure no design rule errors.

Next let’s show what would happen if we had selected the nAct Node first, see below.

Moving the cursor up and RIGHT clicking on the nMos Node results in the following.

Notice that the width of the Arc changed and that both ends extend beyond the red dots.

If we select the bottom nAct Node and move it to the left we get the following.

If we keep moving this Node the n-Active Arc will eventually move as well.

This behavior is related to the Arc’s properties, see below.

Selecting Rigid will make the Arc move when we move the nAct Node.

Pressing Ctrl+Z (undo) until we get back to the point where we just added the N-Active Arc and then selecting this Arc results in the following.

Notice how the Arc extends beyond the poly1 (the pink).

This is an issue that will cause design rule errors (Active spacing error). We’ll discuss in greater detail shortly.

With this Arc active use the left arrow to move the Arc over until it aligns with the nMos Node, see below.

Instead of using the arrows you can use the mouse to move the Arc over. Notice the bottom nAct Node moves too.

Move the bottom nAct Node up into place (mirror of the top nAct Node) and DRC the layout.

There are 5 DRC errors. Pressing the > key we get the following, that is N-Active TOUCHES the transistor Node.

To fix this error select the bottom N-Active Arc as seen below.

It may be useful to press Ctrl+click to cycle through the possible selections.

Change this Arc’s properties (Ctrl+I) so that the End Extension is “Neither end” as seen below.

The result is seen below. Now the layout passes DRCs.

Let’s connect the gate of the MOSFET to the poly1 to metal1 contact Node by first selecting the left poly1 port on the transistor, as seen below.

Moving the cursor over to the metal1-poly1 contact and RIGHT clicking gives the following.

DRC the resulting layout to verify no DRC errors.

Let’s not forget about the ERC Well Check (Tools -> ERC -> Checks)

To view the related setups go to File -> Preferences -> Tools -> Well Check to see the following (setup from Tutorial 1)

When we perform a Well check on the above layout we expect to get two errors:

one because there is no well contact in the p-well surrounding the NMOS device and

a second because the isolated pWell Node isn’t connected to ground, see below.

Moving the pWell Node over so that it overlaps the nMOS p-well layer, as seen below, takes care of the first error.

Perform a DRC and ERC well check after moving this Node to show no DRC errors and only a single ERC error.

To let Electric know that we are going to only connect the substrate to ground, move your cursor below the selected pWell Node and RIGHT click to get the following.

At this point you should know how to select the added metal1 Arc above and how to change its width and end extension.

What we are going to do is “Export” the connection to the p-substrate via the pWell Node and the metal1 Arc we just added.

Only Nodes can be exported so we can’t export the metal1 ARC.

This means that we can Export the highlighted Pin above or the pWell Node.

Let’s export the Pin seen above. If it’s not highlighted then use Ctrl+click until the Pin is selected.

Next, with the Pin selected, go to the menu and select Export -> Create Export (or just use Ctrl+E)

Set the Export name to “gnd” as seen below. Leave the Export characteristics at unknown.

It’s important (when we do an NCC) to use lowercase gnd to represent ground (to match the symbol we’ll use for ground in a schematic shortly).

We’ll also use lowercase vdd to make the power symbol NCC (aka LVS) correctly.

The result is seen below. Now when we perform a well check we don’t get errors (see Electric Message window below).

Let’s add metal1 Arcs for the other 3 MOSFET terminals and export as seen below.

Make sure to DRC and ERC-Check Wells when you are done.

Also ensure you are exporting the Pins of the metal1 Arcs.

If you try to add an Arc connection to the S/D regions and N-Active Arcs are used go to the menu and select metal1, which is circled below.

Save your library, tutorial_2.jelib.

We can follow the same sequence of actions to lay out the PMOS_IV cell.

Let’s provide an abridged set of instructions to speed things up.

Go to the PMOS_IV{lay} view cell and add the Nodes seen below (labeled with Annotation text found in the Misc menu)

Change the width of the nMos Node to 10 and the x-size of the remaining Nodes to 10 with the exception the metal1-poly1 contact Node.

Next select the pMos Node and go to the menu, Tools –> Simulation (Spice) -> Set Spice Model… and set the PMOS’s SPICE model to PMOS.

Important if we want to simulate the layout (and we do!)

The result of these actions is seen below where the SPICE model name has been circled since it’s hard to see.

If we perform a DRC check on this layout we get the following error (the nWell isn’t tall enough).

Change the y-size of the nWell Node so that it’s 6.

You may have additional well errors if you’ve placed your Nodes too close together.

All of these errors disappear after the Nodes are connected together.

Let’s connect them together now, export the Pins, and delete the Annotation text as seen below.

DRC and ERC Well Check your design.

There shouldn’t be any errors.

Note that we could add some confusion by naming the Arcs (d, g, s, and w) above (by selecting the Arc and editing its properties using Ctrl+I) instead of exporting the Pins at the end of the Arcs.

This may sound silly now but when you get going fast, and are used to naming Arcs so that simulation data is easier to view (so you know what you are probing), this is a common mistake.

We are ready to simulate.

Go to the left Component menu and select the arrowhead under Misc to access Spice Code.

Add the Spice code seen below while ensuring you are using multiple lines and the text size is increased to 2.

The cell now looks like the following.

Note that if you haven’t saved the C5 models to C:\Electric as discussed at the beginning of this tutorial the simulation we’re about to do won’t run (correctly).

Going to the menu Tools -> Simulation (Spice) -> Write Spice Deck

results in the following (again, assuming, as mentioned in Tutorial 1 that you have LTspice setup correctly) when you plot the source current of the PMOS device

or using Electric’s probe which should run after you’ve closed LTspice (of course you have to use the Explorer to select the right signals).

Adding similar Spice Code to our NMOS_IV{lay} view results in

With simulation results from LTspice

and using Electric’s probe

The last step is to finish up the schematics so the layout and schematic NCC without errors.

Copy the Spice code from NMOS_IV{lay} to NMOS_IV{sch} and change the text’s size from 2 to 0.5.

Repeat for the PMOS_IV cells.

Go back to the NMOS_IV{sch} cell and select/place the ground symbol from the Component menu on the left, see below.

The ground Node symbol is a little large, relative the MOSFET symbol, so let’s shrink it.

First turn the Grid on by pressing Ctrl+G or Window -> Toggle Grid

With this ground Node active go to Edit -> Size -> Interactively or just press Ctrl+B

Adjust the size so that the ground Node is smaller and its single port at the top of the symbol stays on Grid, see below.

Next turn off the grid and add wire Arcs to get the following.

We need to export Pins to make the schematic/layout NCC correctly

To see the Pins let’s draw a box around everything, as seen below.

If this doesn’t work make sure that you have the Select Objects button (circled below) enabled (not the Select Area)

Using Export -> Create Export (or better yet Ctrl+E) export the appropriate Pins to match the layout, see below.

Remember to use Ctrl+click for ease of selection.

Make sure, as in all of these tutorials, that you leave the Export Characteristics at “unknown”

Note, again, we could confuse ourselves if we named the Arcs with these names instead of Exporting the Pins

Verify the above schematic matches the layout (perform an NCC)

At this point you can also simulate this schematic (since this is easy go ahead and do it).

Repeat these steps for the PMOS schematic view.

The end result is seen below.

Again verify that this schematic matches the layout using NCC.

Also, again, this schematic can be simulated (go ahead and do this).

This is the end of the second tutorial.

For your reference the final jelib (Electric library) used in this tutorial is located in tutorial_2.jelib (right click to save to C:\Electric).  

These first two tutorials have covered most of the basics of using Electric.

We are now ready to design, layout, and simulate more complicated CMOS circuits.

Return