Technology Innovation Showcase: Metrology

Paula Doe, SEMI, San Jose, www.semi.org -- Semiconductor International, 5/18/2007

As expanding computing power increasingly makes new kinds of measurements possible, and shrinking chip geometries increasingly make them necessary, this year’s Technology Innovation Showcase highlights some ways to better control chemical composition on product wafers, wafer position in the chamber, and moisture levels in the gas distribution system.

The volunteer committee of equipment and materials veterans who selected these technologies to feature at SEMICON West 2007 aimed to help introduce innovations that will be useful to the industry, especially those from somewhat outside the mainstream. “Our goal is to showcase things that will have an impact,” said Ralph Kirk, director of Technology Programs at SEMI North America. “These crusty old industry guys go out and look for things that are important.” This year, they found ReVera (Sunnyvale, Calif.) with inline process control, CyberOptics Semiconductor’s (Beaverton, Ore.) on-board wafer vision system that allows a window into the process chamber, and GE Sensing’s (Billerica, Mass.) low-cost, fast-acting moisture sensors — all promising to make a difference in improving yields.

Improved throughput and better small-spot capabilities are starting to extend compositional metrology beyond nitrogen in logic gate dielectrics to patterned wafers, memory and implants. ReVera reports its second-generation X-ray photoelectron spectroscopy (XPS) tool VeraFlex is now being used for inline process control on product wafers. While device makers are primarily using it to control nitrogen levels in gate silicon oxynitride (SiON) films, they’re also developing inline applications for memory dielectrics and ion and plasma implant monitoring.

The XPS technique uses low-energy X-rays to generate a signal from the top 100 Å of the substrate, and directly identifies and quantifies the elements present by the characteristic photoelectrons they emit. It also measures film thickness, calculated by the attenuation of the signal from the known material underneath. But the first-generation tool worked only on monitor wafers.

The company has now figured out how to focus the X-ray beam down to a 50 × 50 µm box so that it can be used on patterned wafers while maintaining the precision and throughput needed for production-level metrology. The second-generation VeraFlex tool focuses the X-rays by first generating much higher X-ray fluxes than the previous system, then using compound, precisely shaped single-crystal X-ray optics to focus the beam on the metrology box. John Samuels, director of marketing, said that some VeraFlex customers are using the XPS measure as their primary metrology technique for SiON gate dielectrics for film thickness, as well as for nitrogen content. Also driving interest are more challenging memory cell dielectrics for both flash, DRAM and doped high-k/metal gate stacks.

XPS measurement of BF3 plasma implant shows both boron concentration (left) and SiO2 thickness (right). (Source: ReVera)

General surface contamination control is another new application area. STMicroelectronics (Geneva, Switzerland) has reported using ReVera’s in-fab XPS to monitor fluorine contamination in its metallization processes, as well as de-scum effectiveness in post-implant cleans. ReVera also believes it can directly measure high dopant concentrations in ultrashallow junctions, in addition to surface oxide and any contaminates present.

CyberOptics Semiconductor ’s wafer-like sensor goes where a wafer goes and sees what a wafer sees. The smart wafer makes it easier to teach the automation system to place the wafer exactly, increasing tool utilization by shortening the time needed for calibration or start-up after maintenance or a crash. The WaferSense Auto Teaching System (ATS) carries an onboard vision system, lights and a processor, and sends its data out by Bluetooth wireless technology to a personal computer.

“You can see what the wafer sees in the dark, on the back side of the tool, or with a dangerous robot that you don’t want to get near,” said Dennis Bonciolini, CTO of the Beaverton, Ore., company. “It gives you access to inaccessible locations and hand-off points. Some folks have cut teach time down from 8 to 2 hours,” he noted. “People have just come to accept long down times and buy the extra tools to make up for it. But less teach time means an increase in tool utilization and capacity.”

Wafer-like wireless sensor reports robot positioning in the chamber in real-time video. (Source: CyberOptics)

More precise wafer centering also starts to make more of a significant difference with ultrathin films in photoresist track tools, where differential heating from wafer misplacement on hot and cool plates can result in uneven resist coating. Chemical vapor deposition (CVD) tools can also produce drift in film thickness if the wafer is not centered.

Key to getting the system to work is wide bandwidth and fast processing, which capture the image in real time and return it quickly enough to see the offsets for teaching the robot. The onboard vision system works similarly to the military’s smart bombs, learning to recognize a circular feature on the target, like a support pin on the bake plate or a hole in the chuck. It takes an image of the target feature, measures the offset from the center of the target to its own geometric center with ±0.1 mm accuracy, and sends the results to the computer, tracking the changes in position in real time as adjustments are made to the robot. The teaching wafer will hold up in a vacuum and up to 50°C, or up to 120°C for five minutes if it is not in direct contact with the heat source.

The company also offers a gapping sensor for CVD and etch tools in a similar wafer-like wireless unit, which uses three sensors to measure capacitance to define a plane for uniform film deposition, or to precisely tilt the showerhead for atomic layer deposition (ALD) and other very thin films.

GE Sensing is introducing a low-cost, fast-acting trace moisture transmitter for gas distribution systems in fabs, allowing a facility to add more checkpoints to catch changes in moisture level sooner before they cause yield problems.

This new take on the old aluminum oxide-based sensor won’t have quite the accuracy of the more complex systems using tunable diode laser absorption spectroscopy, vibrating quartz crystal or phosphorous pentoxide systems to catch changes of 1-100 ppb in gas moisture content, but it won’t cost $20,000-$50,000 either.

Like traditional aluminum oxide sensors, the HygroTrace ppb moisture transmitter is small enough to thread onto the gas pipes close to the desired sample points, but it has a faster response time than the traditional technology.

The traditional aluminum oxide sensor is a porous metal oxide capacitor with a thin-film porous conductor on the top surface, explained GE Sensing’s moisture product manager John Kerney. It measures the change in capacitance as the water molecules adsorb and desorb onto the pure walls of aluminum oxide in response to the changing water vapor pressure in the sample. The new version heats up the sensor to drive off the water, then measures how fast the moisture readsorbs. This measurement is proportional to the moisture concentration in the sample gas. “Response time is faster than other aluminum oxide sensors by orders of magnitude, and equivalent to that of the more expensive sensors,” Kerney said. The company said the sensors are 80-90% smaller than others of the same response rate, and will cost 50-70% less, so a fab can afford to use more sample points to catch changes in moisture sooner, although at some cost in range and accuracy.

Key to getting the new approach to work was figuring out how to get the aluminum oxide to withstand much higher temperatures.