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Introduction to 2-input NAND Gate Layout
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Today we'll explore the layout design of a 2-input NAND gate. As you know, NAND gates are significant because they can be used to create any other logic gate. Can anyone tell me how the NAND gate operates?
The output is low only when both inputs are high!
Exactly! This characteristic impacts how we layout the design. Now, who can describe how NMOS and PMOS transistors are arranged in this layout?
The NMOS transistors are in series for the pull-down network, and PMOS transistors are in parallel for the pull-up network.
Correct! Remember the mnemonic 'NAND Pulls Down', which helps to recall that NMOS is in series. Let's proceed to how we maintain efficiency in the layout.
Design Rule Checks and Layout Efficiency
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When designing our layout, we must adhere to several design rule checks. Can anyone explain why these design rules are essential?
They ensure that the layout can be manufactured correctly without errors!
Exactly! Remember the acronym DRC - Design Rule Check. What are some common rules we need to follow?
We need to consider minimum width and spacing for metal and diffusion layers.
Right! And minimizing parasitic capacitance is also crucial for performance. Keeping that in mind, how would you ensure efficient routing?
By stacking transistors and sharing diffusion regions as much as possible!
Excellent! Let's summarize this part: efficient layouts reduce area and parasitics, boosting performance.
LVS and Verification techniques
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Upon completing our layout, we need to run LVS checks to ensure correctness. What does LVS stand for?
Layout Versus Schematic!
Exactly! Why is LVS so critical in the verification process?
It verifies that the layout matches the intended schematic connections, catching any discrepancies.
Right! Remember: 'No Match, No Fabrication' is a way to remember the importance of getting a clean LVS report. Let's move forward to post-layout simulation.
Post-Layout Simulation and Delay Analysis
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Once we have DRC-clean and LVS-verified layouts, we perform post-layout simulations. What is the purpose of these simulations?
To account for parasitic resistances and capacitances for realistic performance predictions!
Exactly! Parasitic extraction allows us to measure propagation delays accurately. Why do you think post-layout delays are generally higher compared to pre-layout?
Because we include additional elements like parasitic capacitance and resistance.
Exactly! Accurate delay measurements are crucial for ensuring that the design meets speed requirements. Let's recap: post-layout simulations help verify functionality and performance efficiency.
Introduction & Overview
Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.
Quick Overview
Standard
In this section, students learn to design a full-custom layout for a 2-input NAND gate, applying principles of transistors organization, interconnections, design rules, and verification techniques. The emphasis is on performance-driven layout, DRC, LVS checks, and post-layout simulation for accurate delay analysis.
Detailed
In this segment of the lab module 7, students engage in designing a full-custom layout for a 2-input NAND gate using CMOS technology, after acquiring foundational knowledge from earlier modules. The process includes understanding the architecture of the NAND gate, specifically its series connection of NMOS transistors and parallel connection of PMOS transistors. Key layout considerations address efficient area usage through transistor stacking, power and ground routing, and input/output placement. Students must adhere to rigorous design rules to avoid layout violations and perform essential verifications like Design Rule Check (DRC) for geometric correctness and Layout Versus Schematic (LVS) for connectivity checks. Additionally, the importance of post-layout simulation, including parasitic analysis and delay characteristics, is highlighted to ensure that the manufactured circuit meets functional and performance specifications.
Audio Book
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Create New Layout Cell View
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Create a new layout cell view for your NAND gate:
- Library: mylib
- Cell Name: nand2
- View: layout (ensure it's the layout view for the same cell name as your schematic for LVS).
Detailed Explanation
In this step, you are required to create a new layout cell in your design software. You start by specifying the library you will use, which is 'mylib'. Then, you name this specific layout cell 'nand2' to represent the 2-input NAND gate. Lastly, you need to ensure that the view selected is for the layout, not the schematic. This is important because it distinguishes between the design views you are working with and ensures that your layout is directly linked to the corresponding schematic when verifying connections later on using LVS (Layout Versus Schematic).
Examples & Analogies
Think of this step like creating a new folder on your computer for a specific project. Just like you name the folder to reflect the project's name, here you’re naming the layout cell to represent the NAND gate design. Selecting the correct folder type (layout vs. schematic) is like ensuring you're in the right directory to access or save your project files.
Draw the Layout - Strategic Placement
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Begin by drawing VDD and GND metal1 rails at the top and bottom of your layout area.
N-Well: Draw the nwell region large enough to contain your two PMOS transistors and their n_diffusion well contacts.
NMOS Placement (Series): Draw two n_diffusion rectangles for MN1 and MN2. These can be placed such that their drains are shared (abutted) to save area and contacts. Draw two separate vertical poly stripes crossing these diffusions for inputs A and B. The source of one NMOS connects to GND, the source/drain interface connects to the other NMOS, and the drain of the second NMOS connects to the output Y.
PMOS Placement (Parallel): Draw two p_diffusion rectangles for MP1 and MP2 inside the nwell. These should be separated. Draw two separate vertical poly stripes crossing these diffusions for inputs A and B. Both PMOS sources connect to VDD, and both PMOS drains connect to the output Y.
Detailed Explanation
This segment outlines how to create the physical layout of the NAND gate. You start by laying out the power rails (VDD for positive voltage and GND for ground) in the designated positions. Then, you draw the N-well, which contains the PMOS transistors. The positioning of the NMOS transistors is crucial; they are drawn in a series configuration where their drains share a common connection to save area and minimize parasitic capacitance. You will also add vertical stripes of polysilicon (a material used for the gate connections) on top of the diffusions for the inputs. After placing the NMOS, you will position the PMOS transistors in parallel and ensure that their connections maintain functionality according to the NAND logic structure.
Examples & Analogies
Imagine you are organizing furniture in a room. You start by placing the main power outlets (VDD and GND) where they will be most effective, similar to how you set up the power rails. The N-well region is like creating an area in the room specifically for a unique piece of furniture (the PMOS transistors). Then, arranging the tables (NMOS transistors) in a way that they share space, just as you would place two side tables close to each other to save space, represents how we arrange the NMOS transistors. Finally, placing couches (PMOS transistors) parallel to the side tables completes the room layout while ensuring everything flows well together.
Add Contacts and Routing
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Add contacts: Place diffusion_contacts to connect source/drain diffusions to Metal1, and poly_contacts to connect polysilicon gates to Metal1.
Add Well/Substrate Contacts: Crucially, add n_diffusion (with contacts) connected to VDD within the N-well, and p_diffusion (with contacts) connected to GND in the P-substrate region. Place these sufficiently close to the active devices.
Routing (Metal1): Draw metal1 lines to connect all components according to the NAND2 schematic:
- Connect gates to their respective input pins (A, B).
- Connect NMOS source to GND rail.
- Connect PMOS source to VDD rail.
- Connect the series NMOS drain-to-source interface.
- Connect the parallel PMOS drains and the series NMOS drain to the common output Y trace.
Detailed Explanation
In this part of the design process, you focus on connectivity by adding contacts. These contacts allow different parts of the layout to electrically connect, resembling how you might use connectors to join wires. The diffusion contacts link the source and drain of the NMOS and PMOS transistors to the metal layer used for routing connections. Additional well and substrate contacts are critical because they connect the associated regions of the layout to the power supply, which is necessary for operation. Following that, you will draw metal lines (Metal1) to establish electrical connections between various components in the layout, ensuring they correspond to your design’s schematic representation.
Examples & Analogies
Think of this step like wiring a circuit board. Just as you connect components using wires (or traces), here you are placing contacts that act like connectors that join different parts together in your layout to create an operational circuit. The well and substrate contacts work similarly to grounding your board; they ensure that everything functions correctly by providing appropriate voltage references where needed.
Layout Pins and Finalization
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Create layout pins (on metal1) for A, B, Y, VDD, GND. Frequently save your design.
Detailed Explanation
In this final step of the layout design, you need to create layout pins that serve as contact points for the input and output terminals of the NAND gate. These pins are crucial because they define where connections will occur when the layout is fabricated, linking the physical structure to the abstract schematic representation. Additionally, it is essential to save your design frequently to avoid any loss of progress, as layout work can be complex and time-consuming. Saving at regular intervals ensures you can always revert to the last clean version if needed.
Examples & Analogies
Consider this step like placing labels on a circuit board to indicate where the wires should connect. These labels (layout pins) mark the points of interaction between different elements of your layout. Just as a builder would save progress on a blueprint to prevent losing valuable work, you save your layout regularly to safeguard against unexpected interruptions or errors.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Full-Custom Layout: Custom designs requiring attention to transistor arrangement and routing.
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Design Rules: Specifications that ensure manufacturability and minimize layout errors.
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DRC and LVS: Verification techniques that confirm geometrical and connectivity integrity.
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Post-Layout Simulation: Simulations accounting for parasitic effects for realistic delay analysis.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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A NAND gate layout with NMOS in series and PMOS in parallel facilitates efficient operation in digital circuits.
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Utilizing shared diffusion for adjacent transistors in the layout reduces area and parasitic capacitance.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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A NAND gate can be a tricky mate, outputs low when both are great!
📖 Fascinating Stories
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Imagine two friends (inputs) pushing down a seesaw; together they make it go down (output low).
🧠 Other Memory Gems
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DRC: Dimensions, Rules, Checks - Keep designs on track!
🎯 Super Acronyms
LVS
- Layout verus Schematic
- verify that they match!
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: NAND Gate
Definition:
A digital logic gate that outputs low only when both of its inputs are high.
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Term: NMOS Transistor
Definition:
A type of MOS transistor that is typically used in the pull-down network of CMOS circuits.
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Term: PMOS Transistor
Definition:
A type of MOS transistor generally utilized in the pull-up network of CMOS circuits.
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Term: Design Rule Check (DRC)
Definition:
A verification step to ensure that a layout meets specified geometric design rules.
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Term: Layout Versus Schematic (LVS)
Definition:
A process that checks the connectivity of a layout against the original schematic.
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Term: Parasitic Extraction
Definition:
The process of identifying and quantifying unintended resistances and capacitances in a layout.