By Ed Korczynski, Sr. Technical Editor
In-the-know attendees at SEMICON West at a Thursday morning working breakfast heard from executives representing the world’s leading memory fabs discuss manufacturing challenges at the 4th annual Entegris Yield Forum. Among the excellent presenters was Norm Armour, managing director worldwide facilities and corporate EHSS of Micron. Armour has been responsible for some of the most famous fabs in the world, including the Malta, New York logic fab of GlobalFoundries, and AMD’s Fab25 in Austin, Texas. He discussed how facilities systems effect yield and parametric control in the fab.
Just recently, his organization within Micron broke records working with M&W on the new flagship Fab 10X in Singapore—now running 3D-NAND—by going from ground-breaking to first-tool-in in less than 12 months, followed by over 400 tools installed in 3 months. “The devil is in the details across the board, especially for 20nm and below,” declared Armour. “Fabs are delicate ecosystems. I’ll give a few examples from a high-volume fab of things that you would never expect to see, of component-level failures that caused major yield crashes.”
Ultra-Pure Water (UPW)
Ultra-Pure Water (UPW) is critical for IC fab processes including cleaning, etching, CMP, and immersion lithography, and contamination specs are now at the part-per-billion (ppb) or part-per-trillion (ppt) levels. Use of online monitoring is mandatory to mitigate risk of contamination. International Technology Roadmap for Semiconductors (ITRS) guidelines for UPW quality (minimum acceptable standard) include the following critical parameters:
- Resistivity @ 25C >18.0 Mohm-cm,
- TOC <1.0 ppb,
- Particles/ml < 0.3 @ 0.05 um, and
- Bacteria by culture 1000 ml <1.
In one case associated with a gate cleaning tool, elevated levels of zinc were detected with lots that had passed through one particular tool for a variation on a classic SC1 wet clean. High-purity chemistries were eliminated as sources based on analytical testing, so the root-cause analysis shifted to to the UPW system as a possible source. Then statistical analysis could show a positive correlation between UPW supply lines equipped with pressure regulators and the zinc exposure. The pressure regulator vendor confirmed use of zinc-oxide and zinc-stearate as part of the assembly process of the pressure regulator. “It was really a curing agent for an elastomer diaphragm that caused the contamination of multiple lots,” confided Armour.
UPW pressure regulators are just one of many components used in facilities builds that can significantly degrade fab yield. It is critical to implement a rigorous component testing and qualification process prior to component installation and widespread use. “Don’t take anything for granted,” advised Armour. “Things like UPW regulators have a first-order impact upon yield and they need to be characterized carefully, especially during new fab construction and fit up.”
Photoresist filtration has always been important to ensure high yield in manufacturing, but it has become ultra-critical for lithography at the 20nm node and below. Dependable filtration is particularly important because industry lacks in-line monitoring technology capable of detecting particles in the range below ~40nm.
Micron tried using filters with 50nm pore diameters for a 20nm node process…and saw excessive yield losses along with extreme yield variability. “We characterized pressure-drop as a function of flow-rate, and looked at various filter performances for both 20nm and 40nm particles,” explained Armour. “We implemented a new filter, and lo and behold saw a step function increase in our yields. Defect densities dropped dramatically.” Tracking the yields over time showed that the variability was significantly reduced around the higher yield-entitlement level.
Airborne Molecular Contamination (AMC)
Airborne Molecular Contamination (AMC) is ‘public enemy number one’ in 20nm-node and below fabs around the world. “In one case there were forrest fires in Sumatra and the smoke was going into the atmosphere and actually went into our air intakes in a high volume fab in Taiwan thousands of miles away, and we saw a spike in hydrogen-sulfide,” confided Armour. “It increased our copper CMP defects, due to copper migration. After we installed higher-quality AMC filters for the make-up air units we saw dramatic improvement in copper defects. So what is most important is that you have real-time on-line monitoring of AMC levels.”
Building collaborative relationships with vendors is critical for troubleshooting component issues and improving component quality. “Partnering with suppliers like Entegris is absolutely essential,” continued Armour. “On AMCs for example, we have had a very close partnership that developed out of a team working together at our Inotera fab in Taiwan. There are thousands of important technologies that we need to leverage now to guarantee high yields in leading-node fabs.” The Figure shows just some of the AMCs that must be monitored in real-time.
The only way to manage all of this complexity is with “Big Data” and in addition to primary process parameter that must be tracked there are many essential facilities inputs to analytics:
- Environmental Parameters – temperature, humidity, pressure, particle count, AMCs, etc.
- Equipment Parameters – run state, motor current, vibration, valve position, etc.
- Effluent Parameters – cooling water, vacuum, UPW, chemicals, slurries, gases, etc.
“Conventional wisdom is that process tools create 90% of your defect density loss, but that’s changing toward facilities now,” said Armour. “So why not apply the same methodologies within facilities that we do in the fab?” SPC is after-the-fact reactive, while APC is real-time fault detection on input variables, including such parameters as vibration or flow-rate of a pump.
“Never enough data,” enthused Armour. “In terms of monitoring input variables, we do this through the PLCs and basically use SCADA to do the fault-detection interdiction on the critical input variables. This has been proven to be highly effective, providing a lot of protection, and letting me sleep better at night.”
Micron also uses these data to provide site-to-site comparisons. “We basically drive our laggard sites to meet our world-class sites in terms of reducing variation on facility input variables,” explained Armour. “We’re improving our forecasting as a result of this capability, and ultimately protecting our fab yields. Again, the last thing a fab manager wants to see is facilities causing yield loss and variation.”