In the world of 3D ICs, where features are becoming finer and submicron accuracy and precision is more important than ever to maintain intra-wafer uniformity throughout the wafer or die stacking process flow, process control by means of metrology and inspection is more important than ever. The industry offers a number of non-destructive options – optical, X-ray, scanning acoustic microscopy – targeted to specific markers in the process flow for maintaining wafer uniformity such as determining total thickness variation (TTV) of the temporary bond/debond adhesive; determining the depth of through silicon vias (TSVs), and measuring remaining silicon thickness (RST) during the TSV reveal step. However, only one company has endeavored to provide the capabilities to measure all of these markers in one system: Fogale Nanotech. During my visit to the Fogale Nanotech’s company headquarters, in Nimes, France, as part of Europe in 3D Part 2, I had the opportunity to see the latest generation tool – the TMAP Dual 3D 300A – to learn about what makes it tick, and find out why they have dubbed it “the Swiss army knife of 3D IC metrology and inspection.”
What makes the TMAP unique, explained Gilles Fresquet,VP, Fogale Nanotech and director of its Optical and Semiconductor Division, is that it’s a multi-sensor tool, combining microscopy techniques with a temporal-mode interferometer and a spectrometer in the same optical path. “We call it the Swiss army knife of metrology and inspection because it combines full field and single spot metrology techniques combined with white light and infrared microscopy for inspection in one tool and one optical path,” said Fresquet.
He explained the significance of integrating temporal mode interferometry along with spectrometry using an example of measuring TTV. In the case of a bonded wafer pair, when using optical detection with spectrometry only, you can detect peaks but have no idea what layer they are coming from. “With temporal mode interferometry you can detect peaks and determine what layer they are coming from because the distance between the two peaks can be measured,” he said.
Another example of the new tool’s capabilities, which I got to see first-hand in the R&D lab, is RST measurement for TSV reveal. Fresquet explained that when you have a bonded wafer pair, the carrier thickness can affect the uniformity of the TSV reveal. Variation in silicon thickness might prevent all the TSVs from being revealed. Using a combination of full field interferometry, phase shift, and z-scan method, Fogale’s system can simultaneously measure the height of the Cu nail, measure the surface roughness at nanometer level, and determine the RST before the reveal.
This latest tool is the second in a generation of tools targeting the WLP and 3D markets. The previous version, while still a “Swiss army knife”; could be considered the ‘executive’s model’ compared with the ‘engineer’s model’. The latest “blade” in the 300A is the temporal mode interferometer, explained Fresquet.
Fogale’s systems that target the 3D IC market represent the culmination of technologies developed for wafer level packaging (WLP), MEMS, LEDs, power applications and microfluidics. The processes used to manufacture microfluidics for inkjet printer heads are similar to 3D IC, noted Fresquet, although rather than stacking only 2 wafers, they stack 3. Lessons learned in developing a 200mm tool for high volume manufacturing in this market were leveraged in developing the 3D IC tool. All the technologies that have been integrated into TMAP Dual 3D 300A were first proven in the field as stand-alone tools. The result is a full process control solution for 3D IC TSVs including:
- TSV etching: CD and Depth
- Temporary Bonding: Thickness, TTV, Glue thickness uniformity, RST, and defect/void detection
- Backside thinning: edge trim, RST and roughness measurement, cracks defect review.
- TSV reveal: Height and co-planarity
- Stacking: die to wafer stacking process control
Fogale exercises a ‘lab to fab’ strategy; starting with small tools in R&D and low volume manufacturing, and then developing HVM systems to address the market at the right time. This way they can focus on the technology and bring it to full maturity before integrating it into a high-volume system. The added value is what’s inside the system, not the platform itself, noted Fresquet. The company also focuses on each applications space individually. “We have tools in the field for fan-out wafer level packaging (FOWLP), but 3D IC customers aren’t in manufacturing mode yet,” said Fresquet. “It’s important to address the right market at the right time.” The company also has 80-100 tools installed in HVM MEMS fabs, and has a small market (3-5 tools) in compound semiconductor manufacturing. “There’s a lot of overlap in niche markets, and we’ve built a business of this,” noted Fresquet. Fogale has R&D and university customers, as well as HVM manufacturing customers. “We even have process equipment suppliers who purchase Fogale tools to do process control in their applications labs,” said Fresquet.
Another unique quality of Fogale, is that while most suppliers integrate components from other companies, Fogale develops its own technology for its systems. In fact, in markets where it’s more profitable to sell components rather than systems, they sell the components. In semiconductors, it makes more sense to sell the whole system, so that is the business model for that market space.
Because of this unique combination of developing both the sensors and the systems for high precision optical metrology and inspection, Fogale has developed a reputation as a company that can solve problems. Customers who have procured a tool for its measurement capabilities, have come back to tell Fresquet about what it can do beyond its initial intended purpose, such as detecting cracks in thin wafers that has driven technologists to increase the thickness of the wafer. For some special niche applications where there are no solutions available, customers will come to Fogale for the solutions. The answer is almost always “yes, we can develop that.” This kind of interactive relationship helps Fogale determine its technology roadmap.
Thanks to the growing need from customers, there are many items on the roadmap related to inspections. Automatic inspection and defect review capabilities is next on the agenda. Stay tuned. ~ F.v.T.
