basic vlsi layout design using microwind

Learn how to Design in Microwind Knowledge Required for VLSI Lab_Experiment 3

Basic VLSI Layout Design Using Microwind: Part 1 Manual Source: http://www.microwind.me/2009/08/hello-in-this-small-lab-tutorial-series.html Grab the Microwind Software/tool from here: http://www.microwind.net/downloads.php Please unzip it and find the .exe file. If you have clicked on the right icon (i.e., a black colored IC), you should be able to see this:

(Click on the image to enlarge and see clearly)

On the right side of the window, you can see a pallet. This is your basic tool. Using palette, you can lay different layers, for example, Polysilicon, Diffusion Layer, N-Well etc.

At the top of the screen you can see various icons. Let us explore each of them one by one.


To make use of any of the following palette tools, click on it, and then click on the layer to which you want to apply. 1. Vdd Supply. This is used to supply the necessary voltage required for the layout. We usually apply this to N Well (to avoid floating) and to pMOS. 2 Ground. As simple as ground. It grounds the region over which it is placed. 3 Ads Clock to the clicked region. You can specify the period, low time, high times and many other specifications. If you are a beginner, do not change the default values. Simply click and click on the region to be applied. Important note: Once you have applied clock to any node (point or region), the next time when you click on it to apply it to other region, it takes a period twice that of last click automatically. This happens to maintain binary counting. 4. Looks like an eye ? If you want to see voltage, current or few other parameters of the node, then this is used. Usually it is used to see the output during simulation. It can also be used in debugging the layout by checking behavior of a particular node. You can see few other buttons too. We shall come to them as and when we use them. The next set of palette tools are to be used in a bit different way. For example, if you need a polysilicon layer, then click on Red Icon with Polysilicon written on it. Come to the main area (Black: Substrate) hold the left mouse button, move the mouse holding left button. It is much easier than MS Paint. (Try it

now). 5. The metal 1 Layer. When you need to give metallic connection between two different parts of the layout, use this option. Please note that when you draw the metal layer over say a polysilicon, it doesn’t get connected (attached) automatically. You need to use an appropriate contact to do this. The following diagram illustrates various contacts.


6. Polysilicon. Enough kidding. Its simple.

7. P+ Diffusion. The substrate which we use here is P-substrate (Black Color). We are supposed to lay it under N-well. You can draw P+ diffusion first, but make sure that you cover it with N-well. The ‘+’ sign indicates that it is doped higher than the P substrate. I recommend drawing it on the top. It won’t make any difference wherever you draw, but it increases understand-ability of the circuit. Follow the convention everywhere.

8. N+ Diffusion. I suggest drawing it in the bottom. Its same as P+. But it doesn’t need any well. You can simply place it over P-substrate (Why? )

9. The much discussed N well is ready to be drawn now! You can lay it over P+ Diffusion. Make sure that it surrounds it completely. You can have a look at various layers in the following figure:


It was not much difficult. In fact the whole tool is pretty simple. You have covered a major part of it. If you are a bit tired, have a cup of tea. (If you know me, then have a courtesy to offer)

Now let us explore few tools which can enhance the layout and make it indeed a layout. I assume that by now, you have drawn at least some random blocks.

Lets have a look at the tools which are there in the top menu bar:


1. Opens already existing layout. If you have saved your layout somewhere, browse to that folder and you can open it.

2. Save the current layout. The layouts are stored with .MSK extension. If you want to store to a specific location go to File>Save As.

3. Its called draw box. When you click on this, then you can draw the last layer you selected from palette.

4. Your best partner, the gun! To erase, delete or remove a particular area, click on this icon and then click on that particular layer. Tip: If you want to remove only a selected portion, first click on shoot, then hold the left mouse button and select the layer which you need to selectively remove. To completely remove more than one layer, cover them keeping the left mouse button clicked.

5. Why to draw the same device again which you have already drawn? We have a copy tool. Click on this, then again highlight the area by holding the left mouse key and covering that area, release the mouse button. Safely place the area where you want to. (I suggest trying this by drawing a very simple layer)

6. Stretch (Resize)/Move tool. Yet another important tool. It is used if you want to move a certain layout from on place to another. Basically it may be contact or Vdd or Gnd etc. It can also be a layer or a complete device. If you want to move a complex layout, click on this, and again the same mouse holding trick will do it. Click once when you get satisfactory result. Resize: This is used when the design fails the design rules. You can select this and resize the layout. Click once when you get satisfactory result. Hint: If you want to resize horizontally, first click on the tool icon, go to the horizontal edge you want to resize and then draw an imaginary rectangle smaller than the edge such that it over laps the layer and substrate and resize. The same holds good for resizing vertically. (See image for illustration)

I strongly recommend you to try this by drawing a single layer. In the above figure, I am resizing the right edge of the polysilicon. You can see the rectangle in green. (gets drawn on its own when you drag the mouse by holding left key)

7. Zoom In (Maximize) the layout. If you want to view the layout in detail, use this tool. 8. Zoom out. I suggest using this when you are drawing a large layout. Zoom out to connect the

devices in proper alignment. 9. See All. As it says, it fits your layout to the screen. 10. It shows the electrical properties. (You will not be using it initially, so don’t worry) 11. Run Simulation: Click this when you are done. It shows the various input and output waveform.

(We will come to this in the next part) 12. Measures the distance of various layers, devices. If you know the design rule, use it repeatedly to

verify you adhere to them 13. 2-D cross section. As the name indicates, it shows the 2-Dimensional view of the selected area.

Hint: After connecting two layer, say polysilicon and metal, with a contact, you need to make sure that both the layers are connected. You will get erroneous results if they fail to connect properly. In such a case, click on this tool and then hold the right mouse button and drag it till you cover the contact. It clearly indicates if the layers are connected. Diagram illustrates the same.

14. This gives 3-D view of the layout. Try this out, looks good! J

15. DRC: Design Rule Check. It check various design rules and suggest corrections if the rules are violated. Hint: It is recommended to perform the DRC after laying each and every layer. This helps you to debug the circuit easily. Also try to use as less amount of diffusion or silicon as possible. Remember: In silicon real estate, every nano meter is precious. Not only that, it prevents unwanted capacitance and resistance

16. Add text. This is like commenting code in programming language. It helps to make your layout readable. Click on this tool and click where you want to add the text.

17. Connect Layers: Suppose you have drawn two layers, and want to interconnect (they are not connected by default) click on this and then on the layers.

18. This simulates MOS characteristics. 19. Show Palette. It happens. Sometimes, the palette disappears. (I don’t know why). To get it back,

click on this. 20. To scroll around the layout, to see various areas, use these arrows.

Microwind Tutorial Part 2: Design of an Inverter Welcome to the Part 2 of the basic layout tutorial. In the following section, I will be giving various shortcut methods, tips and tricks. We shall design an inverter. Our aim will is to adhere to the design rules using minimum possible area. I strongly recommend reading Part 1 thoroughly before trying this. It will make you familiar with the terms. Step 1: Open the Microwind. (Refer Part 1 for details).Before we get started, let’s brush up the basics of Lambda based design rules. I don’t suggest memorizing them, because we have them at hand. But you need to be familiar with the frequent rules (such as width of layers, spacing etc). However you can find the completed design rules built in the program. On the main tab, Click File>Property. A window will popup. Click on Detail of design rule button. You have them all in detail! (You should be able to see the following window). Scroll down. At the bottom you should be able to see the frequently used rules. (This is for 6 metal processes. You may see a smaller list if you have selected a different foundry. No need to worry. The basic layers remain the same.)

(Click on the image to enlarge)I will note down few of them which we will be using for this design. (Highlighted red). I suggest you to note down the same. The spacing shown is between the similar layers. The spacing between different layers may vary. But we always have DRC (Design Rule Check) to see if we are right. Step 2: We shall first draw p+ diffusion. We already know from the rules that it should be: minimum of 4 X 4. (Try drawing smaller than this and run DRC. You will become familiar with this.)But wait, if we draw a diffusion of 4 by 4, we won’t be able to make a proper transistor by laying polysilicon. (Minimum width of the polysilicon is 2 ). So, we need to resize it. Click on the resize tool. (Marked with red circle). Stretch the mouse with left button clicked on the horizontal edge. (Refer to Part 1, resizing the image). Stretch it for around say 8, and release the button. The actual minimum width required will be 12, (4 each for two contacts+ 4 ? spacing between each contacts= 8+4 = 12 . Note that the poly will be laid between the contacts. The minimum width of poly should be 2 . It comfortably lies between the contacts. Illustration follows.)

(Click on the image to enlarge) (Diffusion after stretching. Only partial diagram is shown) Step 3:Selecting polysilicon from the palette, draw the polysilicon layer exactly at the center of the diffusion. (Horizontal center. Poly should be vertical). Note that I have extended poly to 3. This is to comply with the design rule. (The extra poly surrounding the diffusion should be more than 3).

Step 4: Select the n-well from the palette and draw the N-well. (I love the color and pattern.) I have

drawn it arbitrarily. To make sure that we are following the design rules, click on DRC.

As you can see, the design rule says that “The extra nwell surrounding the diffp is less than 6.” You can directly click on the nwell and stretch it to a next level. Perform the DRC again to see if everything is fine before proceeding to the next step. One strange behavior of DRC is that, at the first click it points to the error, but even if the error is not corrected, the error message goes off. To recheck, you need to click again. One good thing about it is that, it also shows the scale. So that you can stretch or resize accordingly. Step 5:We are ready to draw the n diffusion now. Click on n+diffusion from the palette and draw it below p+. Keep the size same. (You can even keep it half of p+ diffusion, but since we have drawn the p+ diffusion of 4, we can not draw this diffusion less than that). Also maintain the distance between nwell and diffusion 6. Perform the DRC again. Your screen should look something similar to this:

Since in an inverter both the device gates are connected to each other, we need to extend the poly of the upper device to cover the lower device. We can also lay the poly from the palette. After this step the

layout should be like: Step 6:Select metal layer from the palette. Join the devices (Source and Drain). Select appropriate contact from palette and click on metal-diffusion interface. Note the minimum width of the metal is maintained to 3. While placing the contacts, the dotted rectangle’s left edge should coincide with the left edge of the

metal. Illustration: We need to verify the contacts. Sometimes, we see the contacts, but they are not really bound. To verify, click on 2-D View. (Circled Red). Hold the left mouse button and strike diagonally over the contact. (See

the dotted line on nmos). If everything goes fine, you will see the following window:

At present we are not bothered about other parameters, let us just see if there is a purple line, highlighted red. If you can see this, it means you have connected the layers successfully. If you are not able to see, then there is no need to worry. It’s not due to color blindness J close this window. Click on the connect layers button (top menu, underline yellow in last but one figure) and click on the layer you need to connect. Repeat the same for the other contact. Step 7:You have crossed the most rigorous part of the tutorial. If you wish to quit it now, I won’t stop. You have done 99% of it. Pat your back and let’s move ahead. From the palette, select Vdd. Click on nwell. Again from click on vdd and then on the source of pmos. Add a contact where you have added vdd. Apply Vss to the drain of the nmos in the same way. We need to apply the input to the inverter now. Select the clock from the palette and click on the poly connecting both pmos and nmos. We should be able to see the output. For the present layout, the output will be available at the metal (joining drain and source of the devices). Click on the Visible Node (looks like an eye in the palette) and then click on the

metal.You are done now! Step 8 (Final Step):Click on Run Simulation button. If you have followed the tutorial properly, you will

be able to see this: You can view various simulation parameters. See the voltage vs. voltage to view the transfer characteristics. I have uploaded the .MSK file over the following link. You can verify your design with mine and try it out.In the next part of the tutorial, we will be learning:

1. How to save time by using existing layout. 2. How to Copy, paste and invert

Basically the next tutorial will be focused on design of transmission gate. See this space for more learning. Till then goodbye.

basic vlsi layout design using microwind

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