Characterization of Temperature Gradients on MEMS Acceleration Sensors Procedia Engineering 168 ( 2016 ) 888 – 891 1877 7058 © 2016 The Authors Published by Elsevier Ltd This is an open access article[.]
Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 168 (2016) 888 – 891 30th Eurosensors Conference, EUROSENSORS 2016 Characterization of temperature gradients on MEMS acceleration sensors Cristian Nagela, Frederik Antea, Martin Putnikb, Johannes Classenb, Jan Mehnerc a Robert Bosch GmbH, Zentralbereich Forschung und Vorausentwicklung, Renningen, Germany b Robert Bosch GmbH, Automotive Electronics, Reutlingen, Germany c Technische Universität Chemnitz, Chemnitz, Germany Abstract MEMS acceleration sensors have been systematically analyzed across a wide temperature range between -40°C and +85°C Failure mechanisms such as offset drift induced by thermal mismatch of different materials are well understood However, in densely packed electronic devices (e g., smartphones) distances between neighboring components are very small Thus, power intensive components like microprocessors can heat up and create temperature gradients in their vicinity In this paper we introduce a measurement system to systematically investigate the influence of temperature gradients on MEMS sensors For precise investigation of their offset and sensitivity, the system can be rotated in the earth’s gravitational field Experimental results show that the z-axis offset is linearly dependent on out-of-plane temperature gradients whereas its sensitivity is nearly constant The in– plane axes are not affected Several hypothesis are still under investigation but currently a microfluidic effect caused by the temperature distribution of the gas within the sensor cavity is the most likely explanation © 2016 Published by Elsevier Ltd This © 2016The TheAuthors Authors Published by Elsevier Ltd is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: temperature gradient, MEMS, sensor, accelerometer, gyroscope, LGA, package Introduction Smartphones or tablets require MEMS (micro-electro-mechanical system) inertial sensors for various applications like orientation detection (portrait/landscape), compass functionality (with magnetometers) or activity tracking In the past years the sensors have been strongly miniaturized to reduce the production costs and to meet the size requirements of novel applications (e g., wearables) The influence of static temperature effects on the offset deviation due to thermal mismatch between the different materials such as silicon, solder alloy, mold compound, and adhesives has been systematically analyzed [1] Inertial sensors are usually calibrated under static homogeneous temperature 1877-7058 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference doi:10.1016/j.proeng.2016.11.298 Cristian Nagel et al / Procedia Engineering 168 (2016) 888 – 891 conditions, e g., for CE (consumer electronics) applications in the range of -40°C and +85°C However, as the integration density in complex electronic devices is continuously rising, distances between sensors and heat sources (CPU, GPU, …) become smaller In non-optimized devices this can lead to temperature gradients of approximately 0.2 K/mm [2, 3] In this paper we study the influence of thermal gradients on the performance of accelerometers and gyroscopes For a systematic analysis of these effects a dedicated measurement system was developed and used for characterization of standard CE inertial sensors from Bosch Sensortec Experimental setup The measurement equipment consists of two peltier elements which are heating up or cooling down copper inserts to the temperatures T1 and T2 Those temperatures are controlled using PT100 sensors and the temperature controller PRG RS H 400 from Peltron GmbH The sensor is soldered onto a test board, placed between those copper inserts and clenched with soft thermally conductive pads on the top side of the sensor and the bottom side of the test board (Fig 1) Thermally conductive pads compensate position tolerances as well as reduce the thermal resistance between the sensor/test board and the copper inserts The resulting temperature difference ΔT = T2 – T1 is effective along the zaxis of the sensor To investigate offset and sensitivity of the sensor the system is mounted onto a horizontal table to allow rotation in the earth’s gravitational field with up to 10°/s Figure 1: Measurement equipment for evaluation of thermal gradients on the sensor characteristics The system consists of two peltier elements to apply a homogeneous temperature or vertical temperature gradients along the z-axis of a sensor Experimental Results A BMI160 (3 x 2.5 x 0.83 mm³) of Bosch Sensortec with integrated MEMS accelerometer and gyroscope was chosen for evaluation Thermally conductive pads with a thickness of 300 μm are used to conduct the heat from the copper inserts to the sensor To reduce the thermal resistance the sensor is clenched between those pads at a defined compression force This setup causes deformation of the sensor and leads to offset deviations of all acceleration axes (Fig 2) Thus, four springs are used to compensate the stress induced by the thermal expansion of the setup and to ensure a constant force of 10 N during measurements This is the minimum force which is necessary to hold the upper copper insert in position All subsequent data are normalized to the offset value at this force The transient measurement shows the accelerometer offset as function of time for a temperature difference of K and K (Fig 3) The offset of the z-axis is instantaneously influenced by the presence of an external temperature 889 890 Cristian Nagel et al / Procedia Engineering 168 (2016) 888 – 891 gradient whereas the in-plane axes remain unaffected The z-offset deviation rises to its maximum as the temperature gradients increases up to K and it diminishes as the temperature gradient vanishes The in-plane axes stay at essentially mg This effect was also investigated with different temperature gradients ΔT between K and ± K (Fig 4) The offset of the z-axis shows a linear dependence on the temperature gradient, while the maximum offset deviation is well within the BMI160 specifications of 150 mg within lifetime The slope of the linear z-offset function is more than 20 times larger than the slope of the x- and y-axis The sensitivities of the x- and z-axis are not affected by the out-of-plane temperature gradients (Fig 5) The sensitivity of the y-axis cannot be measured because the system rotates around this axis in the gravitational field The evaluated range of temperature gradients clearly defines a maximum value for today’s smartphones without heat spreading optimizations [2] The heat conduction in the sensor drastically lowers the effective temperature gradient down to 0.5 K which leads to even smaller deviations for the z-axis Figure 2: Influence of the clenching force on the offset signal of a MEMS accelerometer Deviation of offset is strongly dependent on force Figure 3: Transient measurement of acceleration offset for different temperatures of the lower and upper copper inserts Only the offset of the z-axis depends on the temperature difference ΔT = T2 – T1 Figure 4: Offset signal as function of the temperature gradient across a MEMS inertial sensor All axes are normalized to mg at K The zacceleration axis shows a linear dependence on the temperature difference between upper and lower insert Figure 5: Accelerometer sensitivity as function of the temperature gradient across a MEMS inertial sensor The sensitivity of the yacceleration axis cannot be investigated in this mounting position Cristian Nagel et al / Procedia Engineering 168 (2016) 888 – 891 891 In addition, the MEMS gyroscope of the BMI160 has been investigated under application of different temperature gradients In the first step the gyroscope is measured without rotation to investigate the rate offset Fig shows that all angular rate axes are unaffected by the temperature gradient In the second step the system rotates around the yaxis to investigate the rate sensitivity (Fig 7) Neither rate offset nor rate sensitivity are changing over the temperature region Specification Figure 6: Normalized rate offset as function of the temperature gradient across a MEMS inertial sensor All channels are independent of the applied temperature gradient Rotation about y-axis Figure 7: Rate signal as function of the temperature gradient across a MEMS inertial sensor while the system rotates about the y-axis at 10°/s All channels are independent of the applied temperature gradient Conclusion A measurement equipment to characterize the influence of external temperature gradients on MEMS inertial sensors is presented The measurements show a linear dependence of the z-axis acceleration output as a function of the temperature gradient while all other acceleration and angular rate axes are not affected The thermal mismatch of expansion of the different materials can be excluded as root cause as only the z acceleration axis is affected The main contribution is caused by a molecular dynamic gas effect based on momentum transfer from the gas atoms to the MEMS structure which is called radiometric or Knudsen effect [4] This effect is driven by temperature gradients within the gas-filled sensor cavity and can be influenced by the MEMS layout, the cavity pressure or the gas type Today’s applications are not affected by this effect as very high temperature gradients (ΔT ˃˃ 0.5 K) on very short distances (d = 0.8 mm) must be applied on purpose to observe small deviations However, for high performance applications these effects may become relevant and deserve further systematic investigation References [1] N Yazdi, F Ayazi, K Najafi, Micromachined inertial sensors, Proceedings of the IEEE 86 (1998) 1640-1659 [2] V Chiriac, S Molloy, J Anderson, K Goodson, A Figure of Merit for Smart Phone Thermal Management, Electronics COOLING (2015) [3] Q Xie, J Kim, Y Wang, D Shin, N Chang, M Pedram, Dynamic thermal management in mobile devices considering the thermal coupling between battery and application processor, IEEE/ACM International Conference on Computer-Aided Design (2013) 242-247 [4] A Ketsdever, N Gimelshein, S Gimelshein, N Selden, Radiometric phenomena: From the 19 th to the 21st century, Vacuum 86 (2012) 16441662 ... deviations for the z-axis Figure 2: Influence of the clenching force on the offset signal of a MEMS accelerometer Deviation of offset is strongly dependent on force Figure 3: Transient measurement of. .. measurement of acceleration offset for different temperatures of the lower and upper copper inserts Only the offset of the z-axis depends on the temperature difference ΔT = T2 – T1 Figure 4: Offset signal... dependence of the z-axis acceleration output as a function of the temperature gradient while all other acceleration and angular rate axes are not affected The thermal mismatch of expansion of the