New Tags Use Crystal, Not Silicon

By Jonathan Collins

Nov. 4, 2005—Replacing a silicon RFID chip with a piezoelectric crystal, Carinthian Tech Research (CTR) of Villach, Austria, has developed new passive RFID tags able to tolerate and monitor temperatures in engineering and industrial applications.

Instead of storing data on silicon, CTR's system is based on the principle of using surface acoustic waves (SAWs) across a crystal to identify an RFID inlay. Using the crystal has the advantage of being able to operate in temperatures that silicon-based tags can not match. While RFID tags using silicon are limited to operating at temperatures no higher than 200° Celsius (392° Fahrenheit), CTR's SAW tags can operate at up to 400°C (752° F).

SAW tags can also operate in environments as low as -55° C (-67° F). According to CTR, this is lower than the temperature tolerated by both silicon-based passive tags, which must be above -40° C (-40° F) to operate, and silicon-based active tags, where the greater power needed to send signals in cold temperature limits the effectiveness of batteries to -30°C (-22° F).

When a tag needs to be protected, the crystal can be enclosed in a circular, stainless steel housing to create what CRT calls its monopole tag.

CTR’s tags can be utilized wherever items to be scanned are used in extreme temperature conditions or not easily accessible, such as inside continuously operating furnaces or in motion (for example, rotating engine parts).

The SAW tag uses a lithium niobate crystal in place of a silicon chip. Each crystal is read-only and transmits a unique ID number encoded onto the crystal by means of an etching process. Like a tag made with a silicon chip, a SAW tag's crystal is bonded to a metallic antenna that receives a signal from an RFID interrogator (reader).

The crystal expands and contracts according to its temperature. That variation in length also changes the way the tag processes any signal it receives. The hotter the crystal, the longer the crystal becomes, and the more delay there is in receiving the signal and transmitting it back to the interrogator. In cold temperatures, the reverse occurs.

An algorithm running in software on CTR's reader calculates the temperature according to the signal delay. CTR maintains its system is accurate to a temperature resolution of 1 percent and an accuracy of ±1° C (±1.8° F).

According to CTR, there are particular applications where the temperature capabilities of SAW-based RFID are a valuable asset. "We don't want to be in competition with silicon-based RFID," says Heimo Mueller, a sales manager at CTR. "SAW addresses a niche market for customers with specific applications."

CTR's system operates at 2.4 GHz in order to ensure a longer read range—up to 10 meters—than that of lower-frequency systems. The SAW tag itself measures 2 millimeters by 6 millimeters, but for applications where the tag needs to be protected, the crystal can be enclosed in a circular stainless steel housing, 9 millimeters wide by 4 millimeters high, to create what the company calls its monopole tag. SAW tags can also be mounted inside a metal surface with just the top of a monopole antenna left exposed. Where less protection is required, the SAW inlay can be constructed with the crystal and attached antenna mounted on an alumina substrate, hermetically covered with a ceramic cap.

CTR originally designed its RFID system to identify engine measurement equipment for AVL, a privately owned company that develops power train systems with internal combustion engines. AVL is utilizing CTR's system to track the tools used to measure temperature inside combustion engines.

According to CTR, SAW tags are priced individually at €100 ($120) per tag, though the price drops to a few euros each in quantities of more than 100,000. Interrogators cost €6,000 ($7,200) apiece. A SAW demo kit consisting of a reader, an antenna, various tags and software is also available for €6,690 ($8,000), excluding tax and shipment.