Download VLSI Digital System Design

VLSI Digital System Design
Input-Output Pads
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Input-Output Pad Design
• I-O pad design is highly specialized
– Requires circuit design experience
– Requires fabrication process understanding
• Choose already-characterized library from:
– Fabrication vendor
– Third-party library vendor
– In-house group
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Pad Size and Spacing
• Pad size
– Smallest to which a wire can be bonded
– 100 – 150 μm
• Pad spacing
– Smallest to which a bonding machine can bond
– 150 – 200 μm
100-200 ? m
50-200 ? m
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Interdigitated Pads
• Allows more pads on die
I-O circuitry
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IBM C4 I-O Pads
• Bonding wires restrict I-O pads to die edge
• IBM C4 I-O pads can also be on interior of chip
1.Deposit solder ball on I-O pad
2.Heat die and board to reflow solder
3.Solder surface tension positions die on board
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Latchup
• Latchup occurs when voltage excursion outside
VSS < v < VDD
• Most likely at I-O pad
– Large transistors
– High current
– Inductance of bonding wire
– Connection to external circuitry
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Output Pad Latchup Prevention
• Separate the nMOS and pMOS transistors
• Separate the power supplies for I-O from
internal logic
• Guard rings
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Guard Rings
• Ohmic contacts to metal
– p+ diffusion in p-substrate
– n+ diffusion in n-well
• Collect minority carriers
– Injected into substrate when
drain diodes are forward-biased
• Rings should be continuous diffusion
– No crossovers
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Double Guard Rings
• Surround nMOS transistor by:
– p+ connection to VSS, surrounded by
– n-well with n+ connection to VDD
• Surround pMOS transistor by:
– p+ ring connected to VDD, surrounded by
– n-well with n+ connection to VSS
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Use Smaller Transistors in Parallel
• Minimize gate
– Reduce RC delay
– I-O pad transistors often have long gates
• Improve avalanche breakdown characteristics
• Parallel metal-transistor connections
– Minimize metal migration
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Input Pad
Electrostatic Discharge Protection
•
Input
pad
R
X
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Input Pad ESD Protection
• If not VDD <= X <= VSS,
one of the clamp diodes turns on
– Use double guard rings
• Resistor R limits current in clamp diode
– 200 Ω <= R <= 3000 Ω
– Tub resistor
• For n-well process: p-diffusion
• C is input capacitance
– Speed of signal limited by RC
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Alternative Input Pad ESD
Protection
Input
pad
X
R
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Punch-Through Device
• Built of closely-spaced
source and drain diffusions
– No gate
• “Avalanches” at c. 50 V
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Pull-Up or Pull-Down Resistor
• Long pMOS transistor
• Long nMOS transistor
• Connect gate to signal for IDDQ testing
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