How can you mitigate the impact of process variations on IC design?

?? Mitigating Process Variations in IC Design: ??? - Statistical Techniques: Monte Carlo simulations for precision. ?? - Design Margins: Safety margins for worst-case scenarios. ?? - Process Control: Tight controls enhance consistency. ??? - Feedback Mechanisms: Real-time adjustments via in-situ monitoring. ?? - Adaptive Techniques: Dynamic voltage scaling and circuitry for optimization. ?? - Robust Circuit Design: Redundancy and error correction for resilience. ?? - Post-Fabrication Tuning: Trim or calibrate for fine-tuning. ??? - Testing and Characterization: Thorough testing for understanding IC response. ?? - Advanced Modeling: Use tools for precise process predictions. ?? ??Bolster IC designs, minimize variations, ensure reliability.??

Process variations come in 3 forms. 1 Wafer level variations, these are the resistivity of components or the gain of the transistors set up by parameters like oxide thickness and etching tolerances. 2 The mismatch of individual components to other components. These are caused by 2 different effects. a) resolution effects, or fuzzy edges, this is particularly true for smaller components. b) process slopes, no process is flat, though the larger the wafer generally the flatter the process. Wafer surface looks more like a topographical map than a smooth surface. These combine to create the mismatch. 3) stress related mismatch, hard to quantify and hard but not impossible to accommodate.

Anything which relies on transistor response. In my experience process variation does impact everything apart from steady state signals. If these variations are not modelled and considered properly, it directly affects the yield and one end up wasting lot of $$$. What gets impacted is an endless list with a few basic ones listed as below:- 1. Parasitics 2. Transistor response 3. Tolerance to various parameters like fluctuation in power network. 4. Noise sensitivity of the circuit. 5. Timing failures 6.Physical Design Verification ( DRC) checks. ...etc..etc

Design margins can help you to a certain extent but making smart design choices and choosing self calibrating architecture goes a long way.

In almost all IC’s price wins, so the design approach should reflect that. In most cases making the components bigger and interdigitating them improves the matching, however it increases the area. Running Montecarlo to establish the minimum margin to meet specification is key, running PVT on its own is fraught with problems, running MC on its own and you could miss a cliff-edge parameter. The number of sigma you design for is variable dependent on the complexity of the chip, and horribly the larger the chip the grater the margin need to be. The application also has an impact. A PMIC due to its size could be 4 to 6 sigma, where a 100% tested op-amp may only need 2~3 sigma.

IC design/engineering is not the first branch to address process variation to improve yield using statistical modelling. Mechanical stack designs has been using 3-Sigma,5-Sigma based statistical coverage models since decades. IC design and EDA vendors have to work a lot better in improving the toolset to consider advanced statistical modelling , like Brenderizing the Statistical tolerance ( Sigma used in POCV) to make the pessimism optimal and close to realism.

At first thought “Process Compensation” sounds like a complex technique. It is not. In fact in order for it to work correctly it should be as simple as possible. An example is a constant gain amplifier. Instead of a high gain amplifier with complex feedback, a simpler amplifier that relies on ratios of process parameters can be used. 1 Resistive degenerated common source amplifier with a resistive load of similar type. 2 Common source amplifier with a same type transistor in a common gate configuration as load. 3 Common source amplifier biased with a constant-gm bias, and using a resistive load of the same type used in the bias circuit. Same applies for any other circuit. It requires always going back to the basics and seeking simplicity.

Other process compensation techniques to mitigate process variations by design may include device area scaling to reduce threshold voltage (VT) variations among adjacent transistors. VT reduces inversely with the MOS transistor gate area which is determined by the product of the transistor's gate width (W) and Length (L). If the device are is quadrupled, the sigma variation in VT will be reduced by a factor of two. This is known as Pelgrom’s law.

Sometimes we can correlate process parameters with adjusting trim targets. For example a current limit can vary by +/- 30% due to resistor tolerance, however we can reduce that to 1/3rd if we set the part for the resistor sheet rho. The wafer RT data may be a viable source of this value.

Above all techniques used to compensate for process variations effects, the choice of an intrisically robust architecture is a key point. At the end, the complexity of compensation and calibration stuff (and its impact on yield) does not result in better results than a wide architecture that is little sensitive to variations and that requires less stuff to compensate for.

We may want to consider do we trim prepackage or post package. Sometimes post package time is seen as the holy grail, since any package shift has already happened, however there are times where there is not the availability of pins to reconfigure to access that data. Sometimes we need a second temperature point to help with accuracy, so we may do a double trim where we store the wafer sort data (normally hot) and use it to optimize the room temperature trimming. The problem with this is we do not know how much the hot data is off, once it is in the package. If at all possible design the parts to need only a room temperature trim. Sometimes that means extra complexity, but this is normally saved in cheaper trim.

A great deal of process variation impact can be mitigated by the following techniques: - choosing self calibrating architectures - sizing devices appropriately, if possible use multiple unit instances to ensure matching - attention to layout placement and orientation of the process sensitive devices during placement. - Avoid placement at the die edge to reduce mechanical stress There a lot more techniques, above mentioned are just few common examples. But in the end there will be some process variation which we will habe to live with, and thats where the design margins come handy