Tutorial:Layout Tutorial - Northeastern University

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Tutorial:Layout TutorialIn this tutorial you will go through creating an Inverter layout while performing design-rule checks (DRC). Thistutorial assumes that you have logged in to an COE or ECE machine and are familiar with basic UNIXcommands.Create Aliases to Setup Your Environment% tcsh%source cadence setup.cshrc% source setup hspice new.csh% icfb &The first window that appears is called the CIW (Command Interpreter Window).Open the library manager by selecting Tools- LibraryManager. This window allows you to browse theavailable libraries and create your own.

Create Layout View of an InverterCreate New LibraryIn the Library Manager, create new library called mylib. Select File- New- Library. This will open newdialog window, in which you need to enter the name and directory for your library. By default, the library willbe created in the current directory. After you fill out the form, it should look something like this: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 ViewNext, select the library you just created in the Library Manager and select File- New- Cell View. We willcreate a layout view of an inverter cell. Simply type in "inv" under cell-name and "layout" under view. ClickOK or hit "Enter". Note that the "Application" is automatically set to "Layout L", the layout editor.

Alternatively, you can select the "Layout L" tool, instead of typing out the view name. This will automaticallyset the view name to "layout".Click Ok. You may see a warning about upgrading the license. Simply click Ok to ignore this warning. Afteryou hit "OK", the Virtuoso screen will appear as shown below. In addition, the LSW window (Layer SelectionWindow), which shows various mask layers, will automatically pop up.Now you are ready to draw objects in the Virtuoso window. In this section you learn to place copies of othercells: pmos vtl and nmos vtl.Select NCSU TechLib FreePDK45(Library) - nmos vtl(Cell) - layout (View). The You will see thenmos vtl layout cell. Select whole cell by dragging whole area and copy (press ‘C’ button and select selectedare) . Place in the layout editor. It’s channel length is 50nm and width is 90nm. X and Y axis scale unit is μm.This cell is smallest cell in this library. You can adjust the size of transistor by strechingpressing ‘s’)(Edit- Stretch orand editing using edit functions( move, copy, stretch, delete)It is composed of following layers: pwell, active, nimplant, poly, metal1, contact, and text.

Draw NMOS and PMOS transistor. Finally the transistor looks like this.

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 pwelland nimplant.You could paint these shapes manually in the current cell-viewNote 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 discuss these more in later tutorials. Fornow, remember that "drawing" is the purpose that indicates that a shape will appear in the mask layout. You willsometimes see "drawing" abbreviated as "drw", and sometimes "dg".Selecting and Moving LayoutBy default, if you simply drag out a region while holding down the left mouse button (Button-1), whatever iswithin 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 ata 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 theyare.DRCRefer to Design Rules in http://www.eda.ncsu.edu/wiki/FreePDK45:ContentsTo perform a Design Rule Check (DRC), choose Calibre- Run DRC . The DRC form appears, as shownbelow. Then click "Run DRC". If you do not see the window appear, or if you get an error, then it's possible thatyou didn't type "add calibre" as instructed above. You will need to exit Virtoso, log out, and log back in, settingup your environment in the correct order.

Viewing DRC ErrorsYou can learn about the errors by clicking on the rule in the Results Viewing Environment (RVE) window thatpops up after DRC is complete. Click on an error and hit "shift-H" to highlight the error in the layout viewer asshown. 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.

In this particular case, the transistor wells are too close together. Fix this error by moving up the pmos. It’s goodpractice to space the NMOS and PMOS transistors by the smallest amount allowed in order to make the layoutas dense as possible. You can draw temporary rulers by hitting "k" and dragging a ruler. You can clear the rulersby hitting "Shift-K". These rulers can help you to draw dense layout much faster than you would by constantlyrunning DRC.Move the PMOS and re-verify until there are no DRC errors. You can re-run DRC by simply clicking on "RunDRC" 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 yourlayout 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 nored boxes in the RVE. Alternatively, you can look in the file inv.drc.summary. When the layout is "DRCClean", the last line of this file should read "TOTAL DRC Results Generated: 0".

You will notice that there is one rule that you cannot satisfy by moving around the P-Cells (rule Metal.4, whichrequires the minimum area of metal1 to be 0.00845). We will fix this error later on.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:PaintingWe are now going to "paint" a piece of poly to connect the pmos and nmos devices together. We do this bycreating a shape, in this case, a rectangle.Select the poly layer in the LSW 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:

Another type of shape that you can create is a paths. Connect the drain nodes of the NMOS and PMOS transistoras follows:Select the metal1 layer in the LSW 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 widthafter you draw the path by selecting it and hitting "q" to edit the properties. Set the width to 0.065 in theproperties.)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:

If you don’t like the way your drawing turned out, you can select a shape and delete it with the delete key, oryou can hit “s” (for stretch), and click on one of the sides of a path or rectangle to stretch it into the position thatyou like.Also, you may want to run DRC checks periodically to make sure you're making progress in good direction. It’salso a good idea to save occasionally, by selecting File- Save.Next, create strips of metal1 for VDD and GND. We typically make these shapes as horizontal bars across thetop and bottom, and therefore call them “supply rails”. We then need to connect the rails to the source nodes ofthe transistors. Create these rails now, and make your design look like the one below. Again, try to make thelayout as compact as possible and the supply rails as thin as possible, running DRC as often as needed to learnthe design rules.

Add ViasNext, we need to add contacts (also called vias) 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 canoften 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.Create an NTAP via by choosing Create- Via. or simply hitting "o". You should see the Create Contact popup appears, as shown below. Set the "Via Definition" to "NTAP". The other options should be set correcly bydefault. Place it as close as possible to the PMOS transistor. Likewise, create an instance of the PTAP cell andplace it as close as possible to the NMOS transistor. Again, try to make the layout as dense as possible. We willneed to connect these NTAP and PTAP cells to the power rails. Create metal1 rectangles to connect thesecontacts to the rails. When you are done, your layout should look approximately like the one below.

Next, add a gate-connection in metal1, with a metal1-to-poly via. Do this by choosing Create- Via. again andset the contact type to M1 POLY. Position the via as shown below.

All of the Metal.4 rules should be passing, now, except for the POLY via that we just created. To fix this error,extend the metal1 shape to the left of the contact by 65nm, as shown below. That will fix the error.To finish our layout, we may also want to add some active shapes in between the NTAP contact and pmos vtlP-Cell, as shown below. This will allow us to make a more compact layout than we would be able to makewithout these shapes. Do the same between the PTAP contact and nmos vtl P-Cell.

Create PinsLastly, we need to create pins so that the nodes in our layout have names that are human-readable. Create thesepins 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 otheroptions 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 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 tocreate a rectangle for the pin, and a third time to place the label. The shape of your rectangle doesn’t reallymatter, as long as it only covers area that is already covered by metal1-dg. When you are done, your layoutshould look like the one below.Important Note: It is absolutely essential that you select the Display Pin Name box to create a label for eachpin. The label must be in the same layer as the metal shape and must overlap the shape. This is necessary to passCalibre 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.

Congratulations! You have completed the tutorial. Save your design and select File- Print to print out a copy ofyour layout.If you would like to learn more about the layout editor, you view the Cadence documentation. Start thedocumentation browser by typingcdsdoc &

Tutorial:Layout Tutorial 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 COE or ECE machine and are familiar with basic UNIX commands. Create Aliases to

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