It can not be overstated that the continued health of the chip industry hinges on profitable nanometer production, which depends on yield ramp and yield gap closure. The widening yield gap -- the difference between actual and predicted yield -- and its impact on profitability has far-reaching implications. This fundamental challenge has never been more increasingly critical as we face expanding parametric process variations. Volume diagnostics, along with statistical analysis and underlying root-cause accuracy, provide a proven, productive, and predictable path to profitability. This is indeed a feedback path of monumental value. Why? There are several reasons – both technical and economic. While profitable yield is by no means a new concern, the need for a consistently effective solution moving forward has never been more urgent. Several years ago IBS published the results of a study that showed yield ramps are slowing and design-process defects are growing. At each new process node, the time needed to reach nominal yields doubled – not by days, or weeks, but months.
The “Holy Grail” to Profitable Yield
At the epicenter of nanometer yield challenge are increasing defects due to systemic design-process interactions. There are areas in which design features and process limitations are so close to the edge that they cross the line, resulting in subtle defects – typically categorized as timing, transition, or small delay defects (SDD). Process technology advances, including changes in materials and aspect ratios of wires and vias, have introduced an increasing number of resistive bridges and vias, resulting in resistive shorts and opens. And more aggressive reticle enhancement techniques introduce more physical complexity and more inaccuracy in modeling. Traditional “in-line” yield diagnostic methodologies based on physical diagnostic analysis (PDA), including optical scanning and defect classification, do not adequately account for broader scale design-process interactions. While process control and in-line inspection techniques can minimize process variability, these techniques have no knowledge of design, and therefore they cannot productively or predictively identify design-process defects. And this predictability is the “Holy Grail” to profitable yield. Economic Influences and Implications of a Continuing Yield Gap
In addition to the technological challenges, there are macro-economic influences -- a consumer driven industry with decreasing product windows, increasing volumes, and relentless price pressures which require foundries and IDMs to optimize yield in a much narrower time frame. With macro-economic influences as they are, IC products do not reach expected yields during product lifetimes. This is unsustainable! Foundries are trying to copye by expanding the number and type of design rules. However, these too add to the overall complexity. Simply put, physical analysis (i.e. precision failure analysis) and verification are not keeping pace with the increasing number of design-process interactions resulting in systemic yield loss.
Modeling Accuracy in Diagnostics and Tighter DFM Links is the Key
So how do we resolve this? As indicated earlier, the answer has been with us for some time, but fundamentals remain absent from some of today’s attempts at implementation. Volume diagnostics and statistical analysis are a proven, more intelligent and comprehensive method for identifying the root cause of design-process defects. This methodology is also more effective at expediting the improvement of modeling and design rules to ensure profitable yield. Along with a more comprehensive methodology, the necessary underlying accuracy in defect modeling, indentification, and classification enable an essential feedback path in the neural network, or learning system. This system should necessarily include a broad range of fault models, algorithms and heuristics to isolate faults. And to ensure a closed loop learning network, we need increasingly tighter links to Design-for-Manufacturing (DFM) for improved design rule, modeling, and ATPG advancements. With greater accuracy in diagnostics, we can achieve productivity and predictability in physical analysis and physical design … and more meaningful strides will be made toward closing the yield gap and returning to more profitable production.