Facilitating Temperature Sensor Success
BY CHRIS SEYMOUR
Temperature sensors are usually consigned to supporting roles in semiconductor manufacturing and packaging. But, as in movies, how supporting roles perform can be the difference between success and failure. When problems with temperature accuracy, repeatability, or stability arise, the spotlight turns and remains fixed on temperature components.
In recent years, continuous and rapid advancements in the semiconductor industry have thrust temperature sensors into the spotlight. Traditional sensors have struggled to keep pace, and often have been the weakest link in new or advanced processes. For example, many of today’s applications, such as wafer and integrated circuit (IC) test and bonding applications, require an extremely tight temperature tolerance, and therefore, an extremely accurate sensor. During the past decade, the only way to make a sensor more accurate was to rely on tighter material property controls. Over the years, this has resulted in the use of purer and more homogenous elemental metals, which are often more costly and less readily available. This approach has only taken the industry so far. Process drift, unachievable levels of accuracy, and increasing costs drove the need to abandon this approach and search for more effective ways to improve sensor accuracy.
Figure 1. The plug-and-play technology provides linking of the application to the calibration lab.
Two recent developments have emerged and converged to overcome the limitations of traditional sensors, and perhaps more importantly, to bring additional options and benefits to processes. These developments include the use of smart-sensing technology and the development of the IEEE 1451.4 smart-sensing technology architecture standards. Today, many engineers involved in thermal processes are familiar with both developments, but have only caught a glimpse of their potential.
Smart sensors achieve high-level accuracy not through the use of purer materials, but by putting their known characteristics to work. Specifically, four error values, known from sensor calibration, are transferred into a compatible temperature controller during installation. The controller takes these four offset points, connects them with three straight-line segments, and then performs a high-order curve fit to correct known errors. This process improves the sensor’s accuracy because it knows the error limitations at specific temperatures and replaces previously assumed tolerance windows with exact information. The result is less process variation, better efficiency, and improved yield.
The other development, the IEEE 1451.4 standards, is also capturing the attention of the industry. The standards define the parameters for plug-and-play analog sensors, their interface to existing instrumentation, and the use of embedded transducer electronic data sheets (TEDS) to convey a sensor’s error values automatically.
IEEE 1451.4 standards contribute to smart sensing as Ford’s assembly line eased automotive production. The standards not only provide a universal format for smart-sensing information and the hardware it uses, they also propel the quick adoption of smart sensing by eliminating adaptability concerns - making the technology mainstream.
Still, users and potential users of smart sensors see only half of the technology’s potential. IEEE 1451.4-compliant instrumentation offers previously unobtainable levels of accuracy. But because the sensors no longer rely on the purity or composition of materials to achieve accuracy, the industry is free to use sensors constructed from virtually any alloy. That freedom means alloys can be selected to achieve other benefits, such as stability, robustness, homogeneity, or low price and availability.
Table 1. Calibration information increases accuracy.
This higher level of smart-sensing technology opens the door to an unlimited variety of new sensors - those that can meet modern design goals. If a smart thermocouple can carry its entire voltage table information, and this information can be communicated to a controller to correct known errors, it is no longer forced to be a standard thermocouple, such as Type J or K. Instead, alternative metals that are more accurate, stable, and available can be used. For example, engineers specifying a type of thermistor may have struggled between epoxy-coated units and glass-coated units. Epoxy-coated thermistors offer the best accuracy, but also a low-temperature rating. Conversely, glass-coated thermistors aren’t very accurate, but offer a high-temperature rating. Using smart-sensing technology allows the engineer to make a glass-coated thermistor as accurate as an epoxy-coated thermistor.
Smart sensing contributes to diagnostics and failure prediction. Sensors change and degrade in a repeatable and predictable way. Therefore, it is simple to enable sensors to communicate their status and health, via instrumentation, to operators and maintenance officials. This function allows users to “ask” a sensor if it is functioning properly before starting a batch or process, or schedule maintenance and downtime in a more cost-effective manner.
Combined, the benefits of smart sensing, especially with IEEE 1451.4 standards offering form and function, are transforming sensors from a limiting factor within new or advanced processes, to an enabling component worthy of their own spotlight.
CHRIS SEYMOUR served as sensor strategic marketing manager for Watlow. For more information, please contact Watlow Electric Manufacturing Co., 12001 Lackland Road., St. Louis, MO 63146; 1-800-4-WATLOW.