The Advanced Lithography TechXPOT at this year’s SEMICON West will explore progress in extreme ultraviolet lithography (EUVL), its economic viability for high-volume manufacturing (HVM) and other lithography solutions that will address the march to 5nm and onward to 3nm.
As a prelude to the event, SEMI asked Neeraj Khanna, Global Head of the Patterning Customer Engagement Team at KLA-Tencor, a speaker at the TechXPOT, for insights about the readiness of inspection and metrology tools for EUVL applications at 5nm and 3nm. For a full list of speakers and program agenda, visit http://www.semiconwest.org/programs-catalog/lithography-5nm-and-below.
Neeraj Khanna, Global Head of the Patterning Customer Engagement Team at KLA-Tencor
SEMI: In general, how would you characterize the readiness of inspection and metrology tools intended for EUVL applications at 5nm and 3nm? In particular, what are some of the remaining research and development challenges that need to be addressed for each of these nodes?
Neeraj Khanna: KLA-Tencor is working closely with its customers to qualify and ramp EUV. Our suite of inspection, metrology and data analytic solutions are being implemented to enable EUV infrastructure readiness including, for example, new reticle and resist qualification, scanner qualification, and EUV ramp preparation. These EUV integration activities require process control systems that support a wide range of applications, including hotspot discovery, lithography modeling, focus/dose process window qualification, reticle print check, mask blank inspection, and process and tool monitoring.
As with any major technology inflection, it is critical to understand sources of process variation to enable ramp at optimal yield. For example, stochastics result in random pattern variations, which have a major impact on EUV yield. To manage stochastics, IC manufacturers are deploying process control solutions that support fast modeling of stochastic variations coupled with high-sensitivity, high-coverage wafer defect inspection. Another example is a methodology called hybrid scanner utilization whereby, when EUV scanners are implemented in production, they will only be used for a few layers, while all other layers will be patterned with 193i scanners. This technique requires tighter control and monitoring of overlay budgets.
SEMI: How are you able to achieve this tighter control and monitoring?
NK: To understand why hybrid scanner utilization requires tighter control and monitoring of overlay budgets, it’s important to outline how this differs from current scanner implementation. For critical layers in current process flows using 193i lithography, pattern layers for a given wafer are printed using the same stage/chuck on the same scanner. The overlay performance achieved using this lithography strategy is called dedicated chuck overlay (DCO). Use of a dedicated scanner and chuck for lithography reduces inter-scanner and inter-chuck distortion effects, resulting in DCO overlay error of less than 1nm.
When EUVL is first implemented in production, it will be used for a few layers – likely, cut masks and contacts with eventual migration to metal 1 layers. All other layers will be patterned with 193i scanners. This hybrid scanner operation eliminates any possibility of using a dedicated scanner and dedicated chuck to support tight overlay performance specifications. Instead, fabs will be forced to optimize mix-and-match overlay (MMO), with the overlay performance obtained using different scanners for printing different layers on a given wafer.
With overlay specifications for advanced DRAM and logic at ~2.5nm, fabs will need to implement strict 193i-to-EUV scanner matching strategies or risk consuming 60 to100 percent of the overlay budget on just MMO. To achieve tighter overlay control and monitoring required for MMO, fabs need to implement dense, in-field overlay error measurements that feed into scanner fleet management systems. KLA-Tencor’s ATL™ overlay metrology system supports a high measurement speed and the use of small in-die targets, enabling dense in-field overlay measurements with high accuracy. Our 5D Analyzer® data analytic and management system includes scanner fleet management capability that enables automatic product-based corrections to minimize MMO error, helping fabs reduce the risk to yield loss associated with a 193i-EUV mixed scanner implementation.
SEMI: What other challenges do you see coming to the fore at 5nm and 3nm?
NK: Overall, the 5nm and beyond design nodes will face challenges associated with new lithography technology, potential new device structures and smaller pattern pitches. IC manufacturers will require process control solutions that not only identify process windows, but also monitor patterning parameters and defectivity at multiple points to identify process shifts. To monitor dynamic processes at these advanced nodes, inspection and metrology tools will need to have both sensitivity to critical parameters/defects and robustness to process variation in order to provide IC engineers with smart feedback for efficient control of their processes.
SEMI: Could you elaborate on what will be required to monitor patterning parameters and defectivity at multiple points? How different will the techniques be at 5nm/3nm vs. at say, 7nm or 10nm?
NK: As an example of monitoring at multiple points, consider the transition to EUV lithography. With EUV, the cost per scan goes up dramatically. Thus, IC manufacturers will monitor parameters at multiple points to maximize yield and minimize risk:
- EUV reticle qualification requires inspection and metrology throughout the entire flow from mask blank manufacturing, to the mask shop, to the IC fab.
- For the advanced design nodes associated with EUV, wafer qualification requires monitoring and control of wafer defectivity, shape and geometry throughout the wafer manufacturing process. It also requires control of fab incoming wafer qualification while ensuring that fab-wide processes are meeting defect and shape standards necessary for printing smaller feature sizes.
- EUV resist characterization and qualification requires comprehensive lithography simulation, wafer defect inspection, and film thickness and uniformity measurements to help reduce development time and prepare the litho stacks for production ramp.
- As EUV scanners are ramping, fabs are faced with finding any unknown particle sources within the new scanner chambers. This situation is driving the need for tighter and more frequent PWP (particles per wafer pass) chamber monitors to ensure scanner cleanliness. In addition, EUV scanner qualification requires reticle front and backside particle checks, and hotspot discovery and process window qualification using optical wafer defect inspection and e-beam review.
- EUV process monitoring encompasses overlay error monitoring, focus/dose monitoring, critical dimension (CD) and 3D device shape monitoring, and continuous process window monitoring.
- EUV inline defect monitoring and tool monitoring is important for reducing baseline defectivity in the litho cell for faster ramp, and for early identification of litho excursions in production. A critical part of this monitoring strategy is inline after develop inspection (ADI) monitoring, which is defect inspection on patterned wafers after printing and development of the resist ADI. Inline ADI innovations that find yield-critical defects allow fabs to reduce process issues, prevent at-risk wafers early in the process, and enable rework when excursions are found in production.
As with past technology transitions, the implementation of multiple monitoring steps as part of a comprehensive process control strategy will be critical for a fab’s successful ramp of EUV.
Debra Vogler, SEMI