Embedded temperature metrology design reduces wafer contamination
BY CHRIS ANDERSON
FREMONT, Calif.—For years, engineers at SensArray (www.sensarray.com), a maker of process optimization tools for the semiconductor industry, heard from their customers how much they wanted a wireless temperature measuring device that could run within a lithotrack to measure the real-time temperatures of the hotplates and heating elements—without having to manually access the wafer.
The wait is over. In July, SensArray released its Integrated Wafer wireless temperature metrology system, a robotic analysis product that reduces contamination-control risks, could impact the way process managers collect critical data from their lithotracks, and impact how planned maintenance of fab production machines is managed.
Integrated Wafer is designed with a temperature metrology system within the wafer, gathering data through the entire semiconductor manufacturing process without disruption. It provides static and dynamic temperature measurement for critical processes for routine monitoring, assessing the effect of robotic positioning on temperature, diagnosing thermal problems, detecting variations in temperatures between brake plates, and quantifying the role of air-cooling and ventilation mechanisms.
"This technology allows users to gather [temperature] data without ever having to open up the equipment chambers," says SensArray spokesperson Heather Reed.
Before the Integrated Wafer technology, Reed explains, "each time users opened them [chambers], they risked wafer contamination. They also had to run a clean cycle after removing the wire measurement wafers. With this product, none of that is required."
The development of the Integrated Wafer took nearly two years, incorporating disparate technologies. The patented design is less than 0.5-mm thick, with the temperature sensors, power source and data collection system embedded in either a 300-mm or 200-mm silicon wafer.
Integrated Wafer produces a thermal mass and dynamic response equivalent to that of a product wafer. The 0.5-mm profile enables users to gather static and dynamic temperature data during production surveys that range from 15° C to 145° C, without equipment modification or production disruption.
"What we had before is a just a wafer that was thermal coupled," says Earl Jensen, who heads the Integrated Wafer development team. "The problem is if you have 29 sensors you'd have more than 60 wires coming out of the thing."
That meant in order to use the old measurement equipment, the track would need to be taken off line, the machine opened up, and the wafer placed on the hot plate. "Then they [engineers] would put the skins back on and wait half an hour to stabilize and get 'back to normal' to make the measurements, then reverse the whole thing and take it all apart again," Jensen notes. "So, to do a quick measurement was an hour or two minimum, and could take as much as a half day."
SensArray says the Integrated Wafer is deployed robotically from a base station that provides for communications and battery charging. The base station for the 300-mm version is built into a FOUP, and the 200-mm base station is located within a standard wafer cassette. Once the survey process is complete, the base station manages the downloading and communication of wafer data.
Being able to collect data without shutting the production track down is critical, SensArray claims, not just for clean productivity and the money saved by keeping the equipment running, but for being able to collect temperature information that reflects actual operating conditions during production rather than the simple snapshot that was provided from older metrology methods.
Now, where a planned maintenance measurement might once have occurred every couple of weeks with the old measurement system, manufacturers can collect temperature data on their lithotrack when they wish.
"We have heard from some customers that they are running the Integrated Wafer through every day and some are even doing it once per shift," says Reed.
Because the Integrated Wafer is a silicon wafer with the instrumentation machined into the surface, it has comparable heating properties as the other wafers moving through production and can give manufacturers a clear view of how each of the hotplates within the track is heating the wafers. But creating a wireless product that carries a power source, sensors and microprocessors, all capable of withstanding temperatures of up to 150° C, involved numerous design challenges.
Jensen and his team sourced technologies that were being used for disparate applications ranging from reading and transmitting tire pressure in real time, to processors that required extremely low power consumption. "We took the parts that we needed and used them out of context," Jensen says. "And we also had to take these parts beyond their spec limits, since they would need to perform at high temperatures."
For the power source, Jensen chose an ultra-thin battery that was developed to power a tire pressure sensor for automobiles and transmit warnings if the pressure falls outside a predetermined range. "We thought this battery would be good since it was designed to be inside the tire, and tires get hot when they are used," Jensen says. But the battery also needed to be thin, and this one—at just 10 mils thick—fit the bill.
Also important were processors that could both take the heat and use very little power. For this, SensArray turned to Texas Instruments and processor technology it developed in the 1970s but shelved. "There wasn't a need for a low-power application back then, but now it is a growing market for them with handheld devices," he says.
The 300-mm Integrated Wafer has 64 temperature sensors and the 200-mm size has 52 sensors; data from the wafer is compatible with SensArray's Thermal MAP Analysis software. In all, the company claims the Integrated Wafer can provide comprehensive temperature surveys lasting as long as 20 minutes and collecting up to 52 KB of data before needing to be recharged or have the data downloaded.
The company plans to introduce similar products for stepper, prober and etch processes. Reed says the company hopes to have beta sites set up for a prober version during the fourth quarter of this year.
SensArray also has its sights set on collecting more than just temperature data. "Temperature is just the most commonly measured thing, but it is not the only thing we can measure," says Jensen. "There is humidity and how much the wafer is tilting, calibration and wafer handling—the list is endless and will be expanded as customers decide that there is a need for measuring other parameters."