VLSI Digital System Design Input-Output Pads
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
Pad Size and Spacing Pad size Pad spacing Smallest to which a wire can be bonded 100 – 150 μm Pad spacing Smallest to which a bonding machine can bond 150 – 200 μm
Interdigitated Pads Allows more pads on die I-O circuitry
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 Deposit solder ball on I-O pad Heat die and board to reflow solder Solder surface tension positions die on board
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
Output Pad Latchup Prevention Separate the nMOS and pMOS transistors Separate the power supplies for I-O from internal logic Guard rings
Guard Rings Ohmic contacts to metal Collect minority carriers 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
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
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
Input Pad Electrostatic Discharge Protection X
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
Alternative Input Pad ESD Protection X
Punch-Through Device Built of closely-spaced source and drain diffusions No gate “Avalanches” at c. 50 V
Pull-Up or Pull-Down Resistor Long pMOS transistor Long nMOS transistor Connect gate to signal for IDDQ testing