The advancement of semiconductor technology has led to the development of smaller and more efficient electronic devices. This progress is driven by the continuous scaling of integrated circuits, resulting in the introduction of lower technology nodes (such as 5nm, 3nm and 2nm). While these nodes offer numerous benefits, they also present several significant design challenges that were not present in older technology nodes. In this post, we will explore some of the biggest design challenges encountered in lower technology nodes.
One of the greatest challenges in lower technology nodes is increased process variability. As the dimensions of transistors shrink, the impact of manufacturing variations on circuit operation becomes more pronounced. Variability in critical dimensions, such as gate length and oxide thickness, can lead to significant performance variations among devices on . This poses a significant challenge for designers as they must either account for or mitigate process variability to ensure reliable and predictable circuit operation.
The Following graph depicts the process variations in three critical feature of the transistors: the channel length (Leff), the oxide thickness (Tox) and the threshold voltage (Vth) for a few generations of semiconductor processes.
To tackle process variability, designers employ techniques like statistical analysis, process-aware design, and design for manufacturability (DFM) methodologies. These approaches involve using statistical models to predict device performance variations, incorporating guard bands and design margins, and optimizing layouts to enhance manufacturing yield.
Guard bands and design margins are incorporated in the form of derates that model the impact of process variations on the timing. Examples of derates are:
LLE (Local Layout Effect) Derates: LLE derates model the abrupt transitions of the RX layer between two standard cells placed adjacent to each other, that may result in slowing down or speeding up some cells. These effects are only pronounced for 10nm and lower technology nodes.
OCV, AOCV and POCV derates: OCV refers to On-Chip Variations, AOCV refers to Advanced On-Chip Variations and POCV refers to Parametric On-Chip Variations. OCV is a flat derate methodology for every cell in the design and it tends to be optimistic for deep paths and pessimistic for shallow depth paths. AOCV mitigates that by incorporating some intelligence about the path depth and the distance travelled. POCV is a statistical derating approach. In older technologies, OCV or AOCV used to accurate enough for timing sign-off. However, in single digit technologies, designers need to use POCV which ensures a more accurate timing sign-off at the cost of added EDA complexity.
With the shrink in the semiconductor technology node, the total power dissipation has been increasing. Following graph shows the dynamic and leakage power trends with scaling. While the dynamic power has increased linearly with scaling the leakage power has increased exponentially.
In order to keep the design power dissipation in check, designers have come up with innovative techniques like clock gating, power gating, dynamic voltage and frequency scaling. This adds to the design complexities and might also impact the time to .
Lower technology nodes are characterized by higher power densities due to more transistors packed into a smaller area. This denser integration results in higher power consumption and greater heat generation, leading to significant challenges in heat dissipation. As power density increases, traditional cooling mechanisms such as air cooling become less effective.
Following graph shows the power density for Intel’s NOC processor over the generations. Power densities exceeded the air cooling limits, and designers need to use other techniques to allow the heat to dissipate from the devices.
Designers face the challenge of managing power dissipation and preventing thermal issues, such as overheating, which can degrade circuit performance and reliability. They employ various techniques like low-power design methodologies, power gating, dynamic voltage, and advanced cooling techniques like liquid cooling or heat pipes. While older technology nodes employed the use of clock gating and power gating, designs with single digit technology nodes need advanced techniques like DVFS to ensure circuit reliability.
Dynamic Voltage and Frequency Scaling (DVFS) also helps with power and thermal management of the device. There are temperature sensors placed in the design, particularly at locations which tend to have high switching activity, little to no clock gating etc. When the temperature sensor detects temperature higher than a certain threshold, it sends the signal to clock generator to reduce the clock frequency or the operating voltage to bring the temperature back to normal at the cost of lower performance. This is essential because high temperatures caused due to high switching can have profound impact on the circuit reliability by slowing down the standard cells, electromigration issues and voltage drop issues.
Additionally, careful floorplanning, power grid design, and placement of heat-sensitive components can help optimize heat dissipation in lower technology nodes.
With the shrinking of feature sizes in lower technology nodes, the current densities in interconnects increase, leading to higher susceptibility to electromigration. Electromigration is a gradual phenomenon where metal atoms move due to the high current density, resulting in degradation or failure of interconnects over time by creating opens or shorts. This poses a significant reliability challenge in lower technology nodes.
The graph below shows the scaling of the height and width of the M1 layer from 180nm process to 22nm process. The shrink in the cross-sectional area will be even more pronounced with technology scaling, thereby resulting in increased current densities, making the process more susceptible to failing electromigration.
ASIC designers must consider electromigration effects and design interconnect structures that can handle high current densities. Techniques like redundant wiring, proper sizing of interconnects, and current density-aware routing algorithms can help mitigate electromigration issues. Additionally, design rules and guidelines provided by foundries play a crucial role in ensuring electromigration reliability.
In lower technologies, designers need to limit the drive strength of the cells or use wider than minimum width interconnects for clock routing or use via ladders at the output of big drivers to limit seeing widespread electromigration issues in their design.
As technology nodes shrink, the proximity of closely spaced interconnects increases, leading to crosstalk and signal integrity issues. Crosstalk occurs when signals from one interconnect couple into adjacent interconnects, causing interference and signal degradation. Signal integrity problems such as delay, noise, and skew can significantly impact circuit performance and functionality.
To address signal integrity challenges, designers employ techniques such as shielded interconnects, spacing optimization, insertion of guard bands, and the use of low-k dielectric materials. Additionally, careful analysis of coupling capacitance and inductance effects, as well as comprehensive routing strategies, can help minimize crosstalk.
Lower technology nodes introduce new and more stringent design for manufacturing (DFM) constraints. These constraints are imposed by the manufacturing process itself and the limitations of lithography, etching, and other fabrication steps. As feature sizes reduce, traditional lithographic techniques encounter resolution limitations, resulting in issues like line-edge roughness (LER) and pattern distortion.
Designers need to be aware of these DFM constraints and incorporate them into the design process to ensure manufacturability and yield. Techniques like optical proximity correction (OPC), phase-shift masks (PSM), and source mask optimization (SMO) are employed to overcome lithographic challenges. Additionally, layout optimization, process-aware design, and design rule checking (DRC) methodologies help meet the stringent DFM requirements of lower technology nodes.
Parasitic effects play a crucial role in determining the performance and reliability of circuits in lower technology nodes. These effects include parasitic capacitance, resistance, and inductance introduced by interconnects, as well as parasitic capacitances and resistances in transistors and other devices. As the dimensions shrink, the impact of parasitic becomes more significant, leading to reduced signal integrity, increased power consumption, and slower circuit speeds.
Some examples of parasitic capacitance include fringe capacitance and overlap capacitance. These are quite difficult to model accurately and are becoming more and more significant in single digit technology nodes.
Technology scaling has helped scale the gate delays, but this has not resulted in a proportional decrease of the interconnect delays. The relative interconnect delays have rather increased and have become a significant part of the total path delay. Moreover, the RC interconnect delay scales differently in contrast to the standard cell delay. The associated variation of the RC delays across RC corners and the variation of the standard cell delays across PVT corners sometimes creates simultaneous setup-hold critical paths. Designers need to identify these paths early in the design cycle and use creative design techniques to be able to meet timing on such paths.
Designers employ advanced extraction tools and methodologies to accurately model and analyze parasitic effects during the design process. Techniques like parasitic-aware synthesis, optimization, and layout techniques can help minimize the impact of parasitics on circuit performance. Furthermore, accurate and detailed parasitic extraction is crucial for timing analysis, power analysis, and noise analysis in lower technology nodes.
Yield of a semiconductor process is a quantitative measurement of efficiency of the semiconductor manufacturer and it is the fraction of functional and error-free chips produced divided by the maximum number of chips that could have been produced on a single wafer.
A new process node often has a low yield to begin with. Semiconductor foundries strive to improve the design yield by analyzing the defects, improving the process- whether that be with respect to designing and upgrading the equipment or optimizing the use of chemicals, fine tuning the DRC decks to modify the design for better manufacturability, and having a tighter control over the environmental variables. The foundry and the design teams need to work closely to be able to isolate and reproduce the issue, and subsequently work closely with the EDA partners to update the signoff checklist or request for tool enhancements to eliminate the issue. The following graph highlights the improvement in yield for any given technology node over time.
As designers pack more devices on the same die, the overall run times explode. EDA tools or specifically the place and route engine is expected to solve multiple problems at once, thereby ensuring that the design is correct-by-construction to begin with.
Placement engine, for example, needs to come up with optimal placement while also working on power optimization by shortening the nets that have high switching activity and high capacitance. Trial CTS is run at placement stage to predict the clock latencies to the registers to perform concurrent clock and data optimization. Last but not the least, the placement needs to be IR-drop aware to avoid an explosion in the IR drop hotspots after the design has gone through clock tree synthesis and detailed routing.
Similarly, router needs to solve shorts, DRCs, timing, and even DFM violations. EDA tools are expected to work out of the box with minimal intervention from the designers to ensure a quick turn-around time.
In conclusion, lower technology nodes present several significant design challenges that were not as pronounced in older technology nodes. Process variability, power density and heat dissipation, electromigration and reliability, signal integrity and crosstalk, design for manufacturing (DFM) constraints, and parasitic effects are some of the key challenges designers face. Addressing these challenges requires the integration of advanced design techniques, analysis methodologies, and optimization approaches to ensure the successful realization of electronic devices in lower technology nodes.
