How are guard rings used in IC layout to improve reliability and performance?
Guard rings improve the performance of silicon devices by isolating them from each other. They create a low resistance ring in the well or substrate around the device group. Guard rings improve the performance of silicon detectors by reducing electric fields at the edges of junctions, which helps in several ways:
Preventing Charge Buildup
The guard rings help prevent charge build up by other nearby devices. This ensures that the operation of the guarded device group is not influenced by unwanted charges.
Read also: Antenna Prevention Techniques in VLSI Design
Stabilizing Potential
Guard rings prevent fluctuating potentials from other devices from affecting the guarded group. By maintaining a stable potential, they enhance the reliability and performance of the devices.
Leakage Current Reduction
Guard rings reduce the leakage current in the main device, which enhances the detector’s performance. When guard rings are biased, they help in collecting carriers generated in the peripheral regions, preventing these carriers from increasing the leakage current in the main junction.
Read also: Guard rings, Wells, Deep N-well, Dummy devices – Analog Layout
High-Voltage Operation
Guard rings enable detectors to operate at higher voltages by reducing the electric field intensity at the junction edges. This reduction prevents breakdown and allows the detector to handle higher voltages without damage.
Spectroscopic Performance
Guard rings improve the spectroscopic performance of silicon detectors. When the guard rings are grounded, the extra peaks in the spectrum (caused by particles hitting the guard ring region) are minimized, resulting in a cleaner and more accurate measurement.
Guard rings ensure that each device operates independently and without interference from neighboring devices, leading to more consistent and reliable performance.
Why We Add Multiple Taps in Analog Circuit Layouts
When we design circuits, we try to keep the N-well isolated from the rest of the circuit. But during operation, charge can still build up in the N-well. I’ve learned that this build-up can affect the gate-source voltage (VGS) of P-channel devices, which in turn impacts how the circuit performs. To avoid this, we insert N+ “taps” in the layout. These taps help drain the excess charge and keep the N-well at the correct voltage.
Since the N-type material in the well is quite resistive, you and I need to add multiple taps across the N-well area to maintain proper control. The same idea applies to the P-type substrate—we insert P+ substrate taps to prevent charge from accumulating there too. According to the process design rule manual (DRM), there’s a maximum distance allowed between taps to avoid issues like latch-up. But in analog designs, we usually insert taps much more frequently to ensure everything runs reliably.
