The George Washington University School of Engineering and Applied Science Department of Electrical and Computer Engineering ECE122 – 30 Lab 5: Layout of a Buffer Ritu Bajpai October 1, 2008

Objective Our objective today is to learn to get the layout for our module, in L- Edit. For this we first design a buffer in S-Edit. Obtain it’s layout in L-Edit. Simulate the extracted view in T-Spice. 2

Buffer What is a buffer? A buffer is a circuit which does not affect the voltage of a circuit (voltage buffer) or current of a circuit (current buffer) yet alters the desired parameters to improve the performance of the circuit. 3

Why a buffer? For example a buffer can be used as an impedance transforming state at the output. It can be used to reduce the output impedance of a circuit while keeping the voltage response unchanged. Or a buffer can also be used as a wave shaping stage for digital circuits. 4

Layout Basics 1 Layout of the chip defines the various layers associated with the masks used in fabrication. The goal of layout process is to implement the design in a compact area while satisfying the design rules set by the foundry. 51. Analysis and design of digital integrated circuits by Hodges Jackson and Saleh.

Schematic Using S-Edit A buffer module is created using 2 SCMOS inverters, placed in series and terminated on 2 sides by input and output ports as shown below. 6

Creating a symbol Once you have your schematic ready, go to the symbol mode for your module in S- Edit and create a symbol for the buffer. Next you can set up a test bench to simulate your buffer and get the transient simulation results. 7

Test bench for the buffer

Adding input output pads to the buffer Instance your buffer into a new module and add pads to the inputs and outputs. I saved this module as Buffer_wPads 9

Exporting the design Export your design as a tpr file. 10

Exporting the design

Open L-Edit Replace Setup: C:\My Documents\Tanner EDA\Tanner tools v13.0\L-Edit and LVS\SPR\Lights\Layout\lightslb 12

Replace set up 13

SPR Setup in L-Edit Go to SPR->Setup Fill in the paths for the tech library and your netlist 14

SPR Place and Route in L-Edit Go to SPR->Place and Route, Hit Run 15

Layout Using L-Edit If it completes correctly you will see this: 16

Top view of layout with pad frame 17

Note the pads on the pad frame We see that there are total 4 pads on the pad frame. Two of them correspond to the pads that we attached at the input and the output of the buffer. Other two correspond to Vdd and Gnd. 18

Obtaining cross-sectional view of the layout Cross sections Process Definition File is C:\My Documents\Tanner EDA\Tanner Tools v13.0\L-Edit and LVS\SPR\Lights\Layout\lights.xst 19

Cross sectional view of the layout 20 You will not see the exactly same cross-sectional view as shown above because it depends on the co-ordinate you choose.

Cross-sectional view You can choose pick option from the cross-section window instead of defining the co-ordinate and use the cursor to choose the co-ordinate on top view where you want to see the cross-sectional view. 21

Homework Implement the following functions as a single module using only NAND gates from the SCMOS library. Test and layout. Extra points for optimal designs X = ((!A)B + A(!B)C) 22