PRECISION TEMPERATURE MEASUREMENT

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1 PRECISION TEMPERATURE MEASUREMENT Herbert Chelner, CEO and Chief Scientist Dr. Robert A. Mueller, President and General Manager Abstract Micron Instruments is best known for its world-class semiconductor strain gages and pressure transducers. Micron also sells an extremely small and highly accurate semiconductor temperature sensing element, made from the same materials but doped differently than the strain gage. As such, the semiconductor temperature sensing element offers many of the compelling benefits as the semiconductor strain gage. The ST N ( is a Silicon Semiconductor "N"-doped bulk type Temperature Sensing element that develops a large resistive change with temperature. This thermo-resistive device is etched from a solid piece of doped material and has a minimum of molecular slippages and/or dislocations resulting in a highly reliable device. When used as recommended, a signal resolvable to F is possible. Semiconductor Strain and Temperature Gages Semiconductor strain gages were discovered during the transistor era and became commercially available early in the 1950's. These gages may be homogeneous or diffused or silicon on sapphire (SoS) deposited. Diffused gages have a variety of problems that limit useful life and affect performance. SoS deposited gages have a much lower gage factor and are less corrosion resistant. There are two main elements from which semiconductor gages are made. These elements are Geranium and Silicon and they can be P or N doped. At Micron Instruments, the P doped (Boron) Silicon gage is selected for the basic strain sensing element, and the N doped Silicon is used for the temperature sensing element. Silicon gages have been proven to be more stable, more sensitive, and more corrosion resistant than Geranium gages. Micron Instruments Miniature N-doped Silicon Semiconductor Temperature Sensing element

2 Micron s homogeneous silicon temperature sensing element, the ST N, is a thermo-resistive device that has an ultra-fast response time. Further, the ST N s silicon composition offers high reliability, corrosion resistance, and a high measurement output. This N doped crystal is only long, wide, and thick (0.9mm by 0.1mm by 0.01 mm). This small mass allows the sensing element to have a very fast response to temperature even in a poor thermal transmitter and heat capacity fluid like air. The ST N has a very large change in resistance of 20 ohms per F (36 ohms per C) about ambient. With a repeatable resistance versus temperature curve, represented by a second order polynomial, it can be used with a microprocessor to read temperature accurately, or used in a resistive bridge configuration, and produce a linear change with temperature of as much as 10 millivolts per degree F (18 millivolts per degree C) between -65 F and 185 F (-80 C and 75 C). Figure 1. The ST N has a gage factor of less than 10. When bonded to a material that has an expansion of ten micro inches per inch, the ST N will change the linear output by about 2.0%, or two degrees per hundred degrees. The output will still be repeatable and, if in a bridge circuit, will still be linear assuming the modulus of elasticity is relatively linear. 2

3 Figure 2. Precision Temperature Measurement Applications Like the thermistor, the ST N is a device that changes its electrical resistance with temperature. However, unlike typical thermistors, the silicon semiconductor temperature (SST) device is capable of large changes in resistance for a small change in temperature high output. In applications requiring very fast response time, the small mass of the ST N makes it an alternative, for some applications, to other Resistance Temperature Devices (RTDs). The ST N is an excellent choice for the manufacturing of heat loss flow meters, measuring hot spots in circuit board or the temperature of components during operation, as a feedback control for heaters or coolers such as air conditioners or temperature chambers and etc. 3

4 A recommended Wheatstone bridge completion circuit for the ST N is shown below in Figure 3. Figure 3. Wireless Measurement With an ambient resistance of 5,000 Ω, the ST N is an excellent choice for use in wireless systems, requiring minimum energy to activate data acquisition and communication for passive RFID transceivers. Gaging Services Applying a gage as small as the ST N can be challenging. If you have a precise temperature measurement application and would like assistance gaging and verifying some prototype articles, contact us to get started. Contact Us. Micron Instruments has already worked with innovative companies on optimal selection, placement, and processing of semiconductor strain gages and temperature devices for high frequency, high pressure, and high-temperature applications. If you d like to discuss 4

5 your application or design, please contact us for a free, confidential consultation either by (Sensors@MicronInstruments.com) or phone ( ). 5