Cadence IC Design ManualFor EE5518ZHENG Huan QunLin Long YangRevised onMay 2017Department of Electrical & Computer EngineeringNational University of SingaporeContents1 INTRODUCTION (4)1.1 Overview of Design Flow (4)1.2 Getting Started with Cadence (6)1.3 Using Online Help (8)1.4 Exit Cadence (8)2 SCHEMATIC ENTRY (9)2.1 Creating a New Design Library (9)2.2 Creating a Schematic Cellview (10)2.3 Adding Components to Schematic (11)2.4 Adding Pins to Schematic (12)2.5 Adding Wires to Schematic (13)2.6 Saving Your Design (14)3 SYMBOL AND TEST CIRCUIT CREATION (15)3.1 Creating Symbol (15)3.2 Editing Symbol (16)3.3 Building Test Bench (18)4 SIMULATING YOUR CIRCUIT (21)4.1 Start the Simulation Environment (21)4.2 Selecting Project Directory (21)4.3 Setup Model Library (22)4.4 Choosing the Desired Analysis (22)4.5 Setup Variables (23)4.6 Saving Simulation Data (24)4.7 Saving Output for Plotting (24)4.8 Viewing the Netlists (25)4.9 Running the Simulation (25)5 PHYSICAL LAYOUT (28)5.1 Layout vs Symbol of CMOS Devices (28)5.2 Starting Layout Editor (29)5.3 Vias (31)5.4 Changing the Grid (33)5.5 Inserting and Editing Instances (34)5.6 Drawing Shapes / Paths (35)5.7 Creating Pins (36)6 DESIGN VERIFICATION: DRC AND LVS (38)6.1 Performing DRC (38)6.2 Performing LVS (40)6.3 Performing PEX (41)7 POST‐LAYOUT SIMULATION (45)7.1 Simulation the Extracted Cell View (45)8 CONCLUSION (46)1INTRODUCTIONThis manual describes how to use Cadence IC design tools. It covers the whole design cycle, from the front-end to the back-end, i.e., from the pre-layout design to the post-layout design.The manual aims to provide a guide for fresh users. Following the manual, users can start doing analog IC design even though the users don’t have any knowledge of the tools.An inverter is used to illustrate the whole cycle of analog IC design, and Cadence Generic 45nm (cg45nm) kit is the technology library used for implementing the inverter. The method stated in the manual can be applied to other type of analog circuit design.1.1Overview of Design FlowFigure 1 shows a typical analog IC design flow.The design flow starts from schematic entry with the Cadence schematic capture tool –Schematic Editor. Devices or cells from the cg45nm or other libraries are used to build your circuit. Your design is hierarchical; therefore higher level schematics also incorporate cells which you have already developed. The schematics which you enter at this stage therefore typically consist of a number of base library cells and also lower level cells designed yourself.These are described in Sections 2 and 3 of the manual.When you have finished designing a particular circuit, you need to simulate it to ensure that it works as expected. It would be unlikely that your circuit works as expected at the first time so you have to repeat the cycle to improve the circuit, as shown in Figure 1, until the circuit works satisfactorily. This must be done for each sub-circuit of your design and then for the top level design. How to simulate and view the performance of simulation results are presented in Sections 4 of the manual.When the performance of the circuit is satisfactory, it is ready to start the physical design or layout of the circuit. The layout starts with the cell or device placement. Once the cells have been placed, routing can be carried out. Routing connects the cells/device of the design.After finishing placement and routing, the layout has to go through the Design Rule Check (DRC) with rule decks provided by PDK provider, to ensure that there is no design rule violation in the layout. The layout has to be rectified accordingly to the rules’ requirement till it passes DRC.Upon a successful DRC, it is Layout-versus-Schematic (LVS) check, to assure that all connections in the layout are correct. The layout has to be amended accordingly to the schematic If LVS doesn’t pass. DRC has to be done whenever layout is changed. The process is repeated until the LVS passes.Figure 1. Analog IC Design FlowThe next step is parasitic extraction (PEX) to get the extracted view of the circuit, which is used for post–layout simulation. The extracted view includes the parasitic effects in both the instances/devices and the required wiring interconnects of the circuit.Following DRC, LVS and PEX, it is post-layout simulation. The post-layout simulation is essential to make sure that the circuit with the extra parasitic parameters functions well and still meet the design specifications. If the performance of the post-layout simulation is not acceptable, back to the stage of schematic entry to check the circuit. Basically, re-design the circuit is necessary. Repeat the whole flow until the results of the post-layout simulation meet the design specifications.If everything is satisfactory, the next stage is GDSII Generation. It generates a file which depicts the low level geometry of layout. GDSII format is industry standard format suitable fora semiconductor company to fabricate and manufacture the chip of layout. This is briefed inthe last section of the manual.1.2Getting Started with CadenceUpon logging into your account, you will be brought to the Linux Desktop Environment.Right click on the desktop and click Open Terminal to open a “window” on the desktop. This window is the Linux command line prompt at which you can run Linux commands. After running a Linux command, this window also shows the output of the command.The following steps show how to start Cadence with cg45nm kit.A.Create a working directory - project (it can be any name as you like) with thecommand:mkdir projectwhere mkdir is Linux command and the project is the directory name;B.Enter the working directory with the command:cd projectwhere the cd is the Linux command;C.Type the followings commands to do the environment setup for using Cadence Generic45nm PDK.cp /app11/cg45nm/USERS/cds.lib .cp /app11/cg45nm/USERS/assura_tech.lib .cp /app11/cg45nm/USERS/pvtech.lib .D.Start cadence in the working directory – project with the following command:virtuoso &where virtuoso is the command to start Cadence IC design tool.Now, Cadence tools are successfully started. Keeps only the Command Input Window (CIW) which is shown in Figure 2.Figure 2. CIW WindowDo not close this CIW and try to keep it in view whenever you are using Cadence. Error messages and output from some of the tools are always sent to the CIW. If something doesn't appear to be working, always check the CIW for error messages. In addition, the CIW allows the user great control over Cadence by interpreting skill commands which are typed into it.E.In the CIW, select Tools Library Manager. The Library Manager pop up as inFigure 3. The Library Manager is where you create, add, copy, delete and organizeyour libraries and cell views.Figure 3. Library Manager WindowYou can see that the library gpdk045 appears in the Library column of the librarymanager.Now, you have started Cadence tool and loaded the cg45nm kit successfully. There are some documents in /app11/cg45nm/ gpdk045_v4_0/docs, and you can always refer to these documents for the information such as devices, device models, DRC rules and others related to cg45nm kit.Next time, you need only to repeat the steps B and D, for launching Cadence virtuoso and doing your project.1.3Using Online HelpCadence provides a comprehensive online manuals for all Cadence tools. You can launch the online help by typing the following command at the Linux prompt.cdnshelpThis invokes the online software manuals. Alternately, there is a help menu on each Cadence window. Manual which is related to that window related will pop-up once clicking on the help button.1.4Exit CadenceTo exit Cadence, just click on the cross sign X or File Exit in CIW. It is necessary to exit Cadence when it is not in use. Your library file would be locked or cannot edited next time if Cadence was not exited properly.2SCHEMATIC ENTRYNow that Cadence is running, you are almost ready to start entering schematics. However, you must first create a library which will be used to store all the parts of your design. Then, schematic can be created in the library.2.1Creating a New Design LibraryA.In the Library Manager window, select File→New→Library. New Library formpops up as shown in Figure 4.B.In the New Library form referring to Figure 4, key in your design library name(example: test) in the field of Name, and then click Ok.C.Click Ok in the pop-up window - the Technology File for New Library, referring toFigure 5.D.Choose gpdk045 in the Attach Library to Technology Library form, referring toFigure 6, and then click Ok.Figure 4. New Library FormFigure 5. Technology File for New Library FormFigure 6. Attach Library to Technology File FormA new library, named test, should appear in your Library Manager window.2.2 Creating a Schematic CellviewA.In Library Manager, select the Library where you would like to create a schematic. Then,select File→New→Cell View.B.Set up the New File form as Figure 7Figure 7. Create CellViewC.Click OK when done. A blank schematic window for the "inv" (your cell name)schematic appears.Explore the functions available by putting your mouse over the toolbar and fixed menu icons.In addition, note that some of the menu selections have alphabets listed to the right of them. These are bind-key or shortcut-key definitions which are very useful in the long run.Test them out during the schematic drawing in subsequent steps.2.3Adding Components to SchematicFigure 8 shows the schematic which you are going to patch, and the property of each component is listed in Table 1.Figure 8. Inverter CircuitTabel 1. Component Properties of Figure 8: Inverter CircuitComponents Library Name Cell Name PropertiesPMOS gpdk045 pmos1v l:45nm w:120nm (default size)NMOS gpdk045 nmos1v l:45nm w:120nm (default size)Here is the example on how to add component instances by placing cell views from libraries. Type “i” bind-key or select Create Instance in the schematic window or click on the menu bar to display Add Instance form. Then in the Add Instance window, select gpdk045as Library, choose the NMOS transistor by selecting nmos1v in Cell and also choose symbol as View, as shown in Figure 9.Figure 9. Add Instance FormSimilarly, add the pmos1v into the schematic. As an example, here we just keep all theparameters as default.If you place a component with the wrong parameter values, select the component and type “q” bindkey or use the Edit→Properties→Objects command to change the parameters. Use the Edit→Move command or type “m” if you place components in the wrong location.2.4Adding Pins to SchematicYou must place I/O pins in your schematic to identify the inputs and the outputs. A pin can be an input, output or an input-output (bi-directional) pin.Type “p” or select Add →Pin from inv Schematic Window or click the Pin fixed menuicon in the schematic window. The Add Pin form appears as Figure 10.Figure 10. Add Pin FormClick Hide and move you cursor to the Schematic Window. Place pins at the correct places and click right mouse key to rotate the pin if necessary.Add pins according to Table 2, paying attention to the direction.Table 2. Pin Names and Direction of invPin Names DirectionVin InputVout OutputVDD, GND Input-OutputCaution: Do not use the add component form to place schematic pins.2.5 Adding Wires to SchematicAdd wires to connect the components and pins in the design.A.Type “w” or select Add →Wire (narrow) in Schematic Window or click (narrow)fixed menu icon.B.In the schematic window, click on a pin of one of your components as the first pointfor your wiring. A diamond shape appears over the starting point of this wire.C.Follow the prompts at the bottom of the design window and click left mouse key onthe destination point for your wire.D.Continue wiring the schematic. When done wiring, press Esc with your cursor in theschematic window to cancel wiring.2.6Saving Your DesignCheck the design to ensure that it is correct and save the design.A.Click the Check and Save icon in the schematic window.B.Observe the CIW output area, for the information of the check and save action.3SYMBOL AND TEST CIRCUIT CREATIONSymbols are useful when creating designs as it is impractical to show every transistor on the top level schematic. Instead, the symbols of cells are created in order to instantiate them in the higher level schematics and make them more readable (i.e. hierarchical designs). Create a symbol for your design so you can place it in a test circuit for simulation.3.1Creating SymbolA.In the inv schematic window, select Create → Cellview → From Cellview. CellviewFrom Cellview pops up as shown in Figure 11.Figure 11. Cellview From Cellview FormB.Click OK in the Cellview From Cellview form. The Symbol Generation Options formappears as Figure 12. Enter the information listed in Table 3 for the symbol.Table 3: Pin SpectificationsLeft Pins : VinRight Pins : VoutTop Pins: VDDBottom Pins: GNDFigure 12. Symbol Generation Options FormC.Click OK in the Symbol Generation Options form. A window with a symbol createdautomatically by the tools pops up, referring to Figure 13.Figure 13. Symbol Generated AutomaticallyD.Observe the CIW output pane and note the messages stating Adding ‘CDFinformation ...’.3.2Editing SymbolYou can modify the symbol to have a more meaningful shape for easy recognition.A.Move your cursor over the symbol, until the entire green rectangle is highlighted. Clickleft to select it.B.Click Delete icon in the symbol window to delete the green rectangle.C.Select Create→Shape→Polygon. Follow the prompts at the bottom of the symbol, anddraw the triangle shown in Figure 14.D.Type “m” or click Move icon in the symbol window, move the pins to the finaldestination.E.Select [@partName], and use Edit→Properties→Object to change it to inverter asshown in Figure 14.Figure 14. Edit Object Properties FormF.Save your edited symbol view. The final symbol is shown in Figure 15.Figure 15. Symbol of inv3.3Building Test BenchTo test the inverter that you have just built, you need to create a test bench. This test bench will also be used during the post-layout simulation.Creating an inv_test schematic cellview with the below information, following the steps listed in Section 2 – SCHEMATIC ENTRY. The test bench is as shown in Figure 17.Library Name : testCell Name : inv_testView Name : schematicLibrary Name Cell Name Propertiestest inv_testanalogLib Vdc VDDanalogLib vpulse Referring to Figure 16analogLib gnd GNDanalogLib cap 1f FFigure 16. Vpulse FormFigure 17. Test Bench – inv_test for inv CircuitNote:There are wire names Vin and Vout in Figure 17. These can be created by clicking on Create Wire Name on the inv_test schematic window. Key in Vin Vout in the Names field of the Add Wire Name form, and then click Hide. Moving your mouse to the schematic window, click the wire where you want it to be named in the same sequence as typing the names in the Names field.4SIMULATING YOUR CIRCUITBefore starting the simulation, make sure that the schematic (inv_test) is open, then perform the following steps.4.1Start the Simulation EnvironmentIn your schematic window, select Launch →ADE L. The Analog Design Environment (ADE) window appears as shown in Figure 18.Figure 18. ADE Window4.2Selecting Project DirectoryIn the ADE window, select Setup→Simulator/ Directory/ Host. A Choosing Simulator form appears as Figure 19. In the Project Directory blank, type in /var/tmp/(desired folder name) to save your simulation files in the /var/tmp directory on the local server. Click OK to confirm.Figure 19. Choosing Simulator/Directory/Host FormAs each user account has a limited quota, this helps to conserve memory space in your account and prevents you from exceeding your account quota. However, note that contents in this folder is deleted periodically every 30 days automatically.4.3Setup Model LibraryIn the ADE window, select Setup Model Libraries. The Model Library setup form appears. Double click the column of section, and then click the down arrow to choose tt which is typical N and P model parameters. The model library setup for the inv_test circuit is shown in Figure 20. Click ok on the setup form to finish the settings.The information of models can be found in/app11/cg45nm/gpdk045_v4_0/docs/gpdk045_pdk_referenceManual.pdf.Figure 20. Model Library Setup for inv_test4.4Choosing the Desired AnalysisIn the ADE window, click the Choose Analyses icon . The Choosing Analyses form appears. Cadence ADE is able to run several types of simulations consecutively. You are then able to view the signals from different simulations at the same time. In this example, we will do transient analysis, so we shall setup transient analyses through the ADE as Figure 21.Figure 21. Setup for Transient Analyses4.5Setup VariablesThere is a variable, VDD, in the inv_test circuit. We need to set a value to it before starting simulation.In the ADE window, click Variables. Enter the name as the variable name VDD, then set the valueas 1.1, and finally click Ok. Please take note that 1.1v is the nominal voltage for this technology.Figure 22. Editing Design Variables4.6Saving Simulation DataThe simulation environment is configured to save all node voltages in the design by default. In larger designs, where saving all of the data requires too much disk space, you can select a specific set of node to save. Following steps show you how to select terminals to save.A.In the ADE window, select Outputs→Save All.B.The Keep Options form appears. Do not modify the form at this time. However, if youneed to save less data, under the first option “Select signals to output”, Click “selected”.4.7Saving Output for PlottingSelect the signals that you would like to observe.A.Select Outputs→To Be Plotted→Select On Design.B.Note that if you click on wires / nets, voltage signals are selected. If you click onconnection nodes, currents flowing through that note and into the component are saved.C.Follow the prompts at the bottom of the schematic window. Click on the output wireslabeled with Vout and Vin (select the wire that you want to monitor).D.Press Esc with your cursor in the schematic window when finished.Now you have set up the simulation environment which as shown in Figure 23. You can save the simulation state. This saves all the information such as the Model Path, outputs, analyses, environment options, and variables so that you do not need to set these parameters the next time again.Figure 23. ADE window with completed settingsIn the ADE window, select Session→Save State. Tick Cellview and then click OK. You can recall your settings by selecting Session→Load State.4.8Viewing the NetlistsSometimes, you need to view the netlist of your circuit or design. You can do so through the ADE, select Simulation→Netlist→Create / Display / Recreate.If there are any errors encountered during this step, check the messages in the CIW and retrace your steps to see that all data was entered properly.4.9Running the SimulationSelect Simulation→Netlist and Run to start the simulation or click on the Run Simulation icon in the Simulation Window. After the simulation is done, a waveform window will pop up showing the simulation results as Figure 24.Click on the waveform window to separate Vin and Vout.You can create a horizontal or vertical marker by clicking Marker on the waveform window. For example, creating a horizontal marker on Figure 24 with put Y Postion at 0.5*VDD=550mV, and then zoom in. The waveform window will look like Figure 25. Delays of the inverter could be found from the reading on the marker.Figure 24. Output of SimulationFigure 25. Waveform with Marker.Explore the icons on the toolbar as well as the various items on the menu. Try to add markers as that is something that will be used often during your simulations. You can also update the titles and labels on your plot to make them easy to read or more meaningful, if necessary.*Quick Tip : Shortcuts “a” and “b” to place a delta marker where you observe the difference between two points. What does shortcuts “v” and “h” do?There are many other functions available in the calculator tool, explore and play around with them.By now, you have finished pre-layout simulation (schematic level simulation). Next, you need to draw the layout of the inverter circuit and then do post-layout simulation to check your circuitperformance.5 PHYSICAL LAYOUTBy now, you should know how to create and simulate your circuit. Once the performance of your design is satisfactory, the next step in the process of making an integrated circuit chip is to create a layout. What is a layout? A layout is basically a drawing of the masks from which your design will be fabricated. Therefore, layout is just as critical as specifying the parameters of your devices.Before we get into the layout, first you need to understand the design rules for layout. Design rules give guidelines for generating layouts. They dictate spaces between wells, sizes of contacts, minimum spacing between a poly and a metal, and many other similar rules.Design rules are essential to any successful layout design, since they account for the various allowances that need to be given during actual fabrication and to account for the sizes and the steps involved in generating masks for the final layout. Note that the layout is very much process dependent, since every process has a certain fixed number of available masks for layout and fabrication.You may find more details on the Design Rules Manual (DRM):/app11/cg45nm/gpdk045_v_4_0/docs/gpdk045_drc.pdf5.1 Layout vs Symbol of CMOS DevicesIn this section, we look at only three devices: nmos1v and pmos1v. Check the process document, you can find the information for other devices.Figure 26 shows the nmos1v device. From layout view, you can see that the terminal B is the black background of the layout window.Figure 26. Layout vs Symbol of NMOSFigure 27 shows the pmos1v device, which looks similar to NMOS device but with P type implant (orange-stripe layer) and N-well (purple surrounding layer). G D SBFigure 27. Layout vs Symbol of PMOS5.2Starting Layout EditorNow we are going to create a new layout in the cell “inv” in “test” library.A.In Library Manager, select File→New→Cellview ... A Create New File form pops up.B.Select "test" as Library Name; enter "inv" as Cell Name, "layout" as View Name.C.Choose Open with Layout XL, and then click OK.Figure 28. Create Cellview – LayoutUseful layerselectionfeatureFigure 29. Layout WindowCell "inv" with "layout" view in library "test" will be created. It is opened up automatically, followed by inv schematic window, as shown in Figure 29. The layout editor contains two main sub-windows, namely the Layers sub-window on the left and Layout Editing window on the right. Notice the Layers sub-window on the left side of the layout view. This sub-window displays the fabrication layers defined in the technology. You can find the cross sectional profile in the process documents.Each layer is represented by a different color and pattern for easier differentiation. The black background on the right can be interpreted as the p-substrate of the wafer.To hide a layer, use the middle scroll button to click on a layer. To disable a layer from use, use the right mouse button.You might notice that some layer names appear more than once in the Layers sub-window. For example, Metal1 appears two times: one as Metal1 drawing, the other as Metal1 pin. Metal1 drawing is a layer with drawing purpose, and such layers with drawing purposes will be fabricated in the mask. The pin layers are symbolic layers and serve to indicate position of I/O pins and define net names. Such layers are not part of the mask layout and will not be fabricated.5.3ViasVias are used to connect between layers, much like those used in PCB design.There are different types of vias for different layer pairs. Normally a via is only for connecting two successive layers, e.g., Metal 1 and Metal 2. In case there is a metal jump between more than two layers, via stacking is required.In the layout window, click Create→Via or type “o” to bring up the via menu. Place the vias on the layout editing window, you can observe the layers that are involved in each type of via. Experiment with the different modes and configurations in the via menu to create arrays and stacks of vias as well. For example,A.Click on Create→Via, the Create Via window pops up as figure 30 shows.B.Choose M1_PO under Via Definition, and click on the layout window to place it andthen press Esc button to stop the placing. You can change the number of Rows and Columns on the Create Via form.C.To view the layers of M1_PO, click to select it first and then press Shift + f key. Observethe via appears different.D.To check the layers used in via M1_PO, select it and then click Edit→Hierarchy→Flatten as shown in figure 31. Click OK on the pop-up form shown in Figure 32.E.Now, you can separate the layers and check layers’ property to find out the layers’ name.Via M1_PO connects layers Metal 1 and Poly as shown in Figure 33.Try to explore different options (Rows, Columns, Stack, etc.) under via menu by yourself, this will be very helpful for layout drawing.Figure 30. Create Via windowsFigure 31. Edit ViaFigure 32. Flatten FormFigure 33. Via M1_POThe M1_PSUB and M1_NWELL contacts are substrate and n-well contacts that are used to connect the bulks of the NMOS and PMOS respectively. For the inverter circuit used in this manual, the bulks of the NMOS and PMOS need to be connected to ground (GND) and VDD respectively.5.4Changing the GridIn Figure 29, the black window on the right is the layout editing window. The position of the cursor in layout editing window is indicated by the coordinate showed on the top right corner of the window after X: and Y:. The unit here is "µm". Move your cursor around the editing window and see the X: Y: values change with step size 0.1. Change the step size to 0.005 as that is the minimum step size for this technology.From Layout Editing window pull down menu, select Options →Display... change "X Snap Spacing" and "Y Snap Spacing" to 0.005 then click on "OK". Now move the cursor around the editing window again, you will see the X: Y: values change with step size 0.005.There are raw grid and fine grid (as small dots) on the window background. If you cannot clearly see the raw grids, from pull down menu select Window →Zoom out by 2In addition to pull down menu and bind key "z", "Zoom Out" is also listed in the picture tool bar to the left of the window. Find it and try it out.Also you may use up, down, left, and right arrows to move around the design window. You will need to use "Zoom in" and "Zoom out" and those arrows many times throughout your design process. So it's not a bad idea to practice them a little bit now.To save and close the cell view, from Virtuoso Editing window, Select Design →Save.。
