AN ULTRA-LOW POWER SWITCH ARRAY OF TEMPERATURE AND HUMIDITY SENSORS WITH DIRECT DIGITAL OUTPUT
Weibin Zhu, Yafan Zhang, Navid Yazdi
Evigia Systems Inc., Ann Arbor Michigan, USA ABSTRACT
This paper reports an ultra-low power cantilever switch array of temperature and humidity sensors with direct digital output. The sensors are designed to measure a temperature range of -40~150°C and a relative humidity range of 20~95%. The direct digital output precludes any need for power demanding analog-to-digital conversion circuitry. A redundancy is employed in the sensor design for the yield enhancement of low-cost sensor fabrication. The fabricated sensors have been tested and characterized in an environment chamber. Test results indicate that both temperature sensor array and humidity sensor array have the desired response temperature and humidity. KEYWORDS
Ultra-low power, digital switch, humidity sensor, temperature sensor. INTRODUCTION
It is essential to develop a low-cost and low-power consumption system to monitor accumulated temperature and humidity for storage and shipping conditions of items in various applications including supply-chain management of perishable goods, pharmaceuticals, chemicals, and fresh agriculture products. Despite the recent advances in low-power IC-based temperature sensors, the power requirement for continuous monitoring of temperature over a few years with a digital output goes beyond the energy capacity of miniature batteries. The existing IC-based temperature sensors provide about 30µW with analog output and 900µW with digital output [1, 2]. In the last three decades, MEMS humidity sensors have been employed with various transduction techniques including capacitive sensing using the change of dielectric constant of humidity sorbent polymer [3], resonant frequency measurement of a bimorphic diaphragm [4] and a bimorph strain measured by piezoresistors [5]. However, the power consumption of such techniques can be easily in a milliwatt range.
This paper reports an ultra-low power cantilever switch array of temperature and humidity sensors (Fig. 1) with direct digital output. Each of the cantilevers in the sensor array extracts the needed mechanical energy for its switching directly from the input sense parameters by converting the thermal energy or the environment moisture sorption to the needed mechanical switching. This switching action results in a direct digital output without any needed for internal power source. Thus the power consumption can be as low as in nanowatt range which is at least two orders of magnitude lower than the existing IC-based temperature sensors and MEMS humidity sensors. In the past, bimetallic effect has been employed in MEMS temperature sensors [6, 7]. The innovation of the present approach is in applying high-
density compact MEMS bimorph sensor switch arrays and the ability of monolithic integration with CMOS circuitry to directly impact system level parameters in power, size, and cost. Another novelty is a redundancy employed in the sensor design (Fig. 2) for the yield enhancement by choosing a set of these compact and simple sense elements from a large array that could be effectively fabricated without noticeable cost penalty.
Figure 1: Cross-section of an integrated system with multiple-sensors for environmental monitoring. The MEMS temperature and humidity sensors are fabricated with a process that is compatible with post-CMOS process.
SENSOR DESIGN
Both temperature and humidity sensors consist of an array of MEMS bimorph cantilever beams that are fabricated by two dissimilar thin films. The temperature sensor beams are constructed by layering two dissimilar thin metal films (Al/Au) and humidity sensor beams are built using a thin layer of metal film (Al) together with a thin layer of polyimide film (PI4104, HD MicrosystemsTM). The working principal of the temperature sensor is based on the mismatch between the coefficients of thermal expansion (CTE) of two metal films, and the humidity sensor is based on humidity absorption induced in volume expansion in polyimide film. In the event of temperature or humidity change, the cantilever beams deflect out-of-plane such that the free ends of the bimorph metal cantilevers make contact to the switch electrodes, acting as on-and-off switches.
In practice, the thin film metal layers have intrinsic stress, which results in initial deflection of the beams at room temperature and thus shifts the switching threshold of the sense element. This is an undesirable effect and controlling the thin films stresses in the manufacturing process is challenging. While minimizing the intrinsic
stress in our fabrication process, a new paradigm in yield enhancement of low-cost sensor fabrication has been employed by making a large-array of compact simple microsensors and front-end circuitry, and selecting a subset of the array (Fig. 2). The array selection can be performed at high-throughput and cost effectively since optical inspection and electrical contact measurements are adequate for the initial test. This is a major shift from the conventional sensor manufacturing where the tolerance of the manufacturing processes are tightened and post fabrication calibration are employed to perfect an individual sensor characteristics. This approach is enabled by the low-cost miniature fully-integrated sensor structure which makes addition of redundant sense elements to the array cost effective. SENSOR FABRICATION
All fabrication process steps were developed to be compatible with the post CMOS production. The process utilized multiple layers of PECVD oxide as sacrificial layers to form the cantilevers and top electrodes. A detailed process flow is shown in Figure 3. (a) LPCVD nitride is deposited on the CMOS wafer and Cr/Pt is patterned as interconnects, (b) Low temperature (100 °C) PECVD oxide is deposited and the thickness determined the switching gaps between the cantilever and bottom electrode. Via holes are opened in anchor areas using Pad Etch 1 solution. Al/Pt is subsequently refilled in this via holes. (c) Photo definable polyimide PI4104 is coated and patterned for humidity sensor only. The polyimide film is then cured at 375 °C. (d) 1.5 µm Al is deposited and patterned with lift-off process. (e) 0.5 µm gold is deposited and patterned for temperature sensor with lift-off process. (f) Another layer of PECVD oxide is deposited to cover the sensors and creates the gaps between top electrodes and cantilever. Via holes are etched with Pad Etch 1 solution on the top electrode areas. (g) Top electrodes are formed by gold plating. (h) Sensor arrays are released using Pad Etch 1 solution to remove the PECVD oxide sacrificial layers. Supercritical drying (CPD) is finally performed to obtain the suspended cantilever structure.
There are a few challenges in this fabrication
processes. (1) The intrinsic stress on each thin film has to be well monitored during the entire process. A post process temperature annealing step was introduced before
LPCVD nitride
CMOS wafer
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Figure 3: A fabrication process of bimorph cantilever beam array for both temperature sensors and humidity sensors.
Fabrication Yield Enhancement byusing redundant array elements &
reassigning switch threshold valuesS1
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Selected array elements withtarget process parameters
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Figure 2: The designed sensor array with a fabrication yield-enhancement by utilizing sensor arrays with redundancy.
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the final release of the device to relieve the intrinsic stresses as much as possible, and preclude sensor drift due to long term stress relaxation. (2) Further, the stress distributed across the wafer is generally not uniform. There exists slightly variation of intrinsic stress in different locations on the wafer. As a consequence, the initial deflection can be slightly different and the temperature threshold of each beam can be shifted. (3) The etch rate of various metals such as Cr and Al is small but not zero in Pad Etch solution. Therefore, the etch time in the final release step needs to be optimized. While a die level release is straight forward, the uniformity of the release on wafer level is challenging and require a very good circulation and agitation in the etch bath.
Figure 4 and Figure 5 show the SEM pictures of fabricated temperature and humidity sensor arrays. As shown in the pictures, the fabricated cantilevers are straight with a proper annealing before the released.
Top electrodes (Au)
Anchor
Figure 4: Bimetallic temperature sensor array
Top electrodes (Au)
Figure 5: SEM of humidity sensors with four cantilever arrays.
EXPERIMENTAL RESULTS The fabricated temperature and humidity sensors
have been tested and characterized in an environment chamber (Tenney BTRC) with various temperature and humidity conditions. Test results have been obtained. The temperature sensor and the humidity are wirebonded to different 64-pin package and tested separately. The sensors were tested in the environmental chamber with controlled temperature and humidity. The ON/OFF state of each cantilever switch is monitored and recorded.
Figure 6 shows the response (switch from OFF to ON) of the temperature sensors with respect to different lengths of cantilever beams. Similarly, Figure 7 shows the response (switch from OFF to ON) of the humidity sensors with respect to different lengths of cantilever beams as well. The relative humidity (RH) is controlled in a range of 20-90% at 40°C in the humidity test. Each cantilever switched back to its original state when the temperature or humidity is reduced below its threshold. The experimental results show that the actual temperature and relative humidity thresholds of each fabricated sensor are slightly larger than the design values. This is primarily because of the intrinsic stress of the metal layers and the Al metal layer being slightly etched in the Pad Etch solution in the final released.
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Temperature (°C)Figure 6: The thickness of Au/Al in this array is 0.5/1.5µm and the width is 50µm. The cantilevers with different lengths are closed at different temperatures as indicated in this figure.
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Relative Humidity (%)Figure 7: The humidity sensor is tested in the environmental chamber by varying humidity in a range of 20-90% at 40°C. The thickness of PI4104/Al in this array is 3/1.5µm and the width is 60µm. The cantilevers with different lengths are closed at different humidity.
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CONCLUSION In conclusion, an ultra-low power cantilever switch
array of temperature and humidity sensors with direct digital output has been designed, fabricated, and tested. Test results indicate that this type temperature and humidity sensors are capable of providing low-power solutions for many application challenges while improving overall performance. The fabrication process of these sensor arrays is robust enough to potentially achieve low-cost and high-yield. The uniformity of intrinsic stress of the metal films and of the final release etch can be further optimized to improve the yield on wafer level. More tests on the sensor array on various temperature ranges and humidity ranges are ongoing.
One of the main issues with humidity sensors is their temperature sensitivity. This sensitivity can be compensated electronically using analog or DSP approaches, but the power requirement is very demanding. One low-power viable solution is to create a unique humidity switching threshold by placing a humidity switch and a temperature switch in series. This approach is currently being explored and the results will be published in the future.
ACKNOWLEDGEMENT
The authors thank Dr. Robert Hower for his early contribution in all aspects of this project. The work was funded in part by a grant from the US National Science Foundation (Award Number 0956908).
REFERENCES [1] http://www.maxim-ic.com [2] http://www.fairchildsemi.com [3] G. Delapierre, et. al, “Polymer-based capacitive humidity sensor: characteristics and experimental results,” Sensors and Actuators, vol. 4, 1983, pp. 97-104. [4] A. Schroth, K. Sager, G. Gerlach, A. Haberli, T. Boltshauser, H. Baltes, “A resonant poliyimide-based humidity sensor,” Sensors and Actuators B, vol. 34, 1996, pp. 301-304. [5] R. Buchold, A. Nakladal, G. Gerlach, P. Neumann, “Design studies on piezoresistive humidity sensors,” Sensors and Actuators B, vol. 53, 1998, pp. 1-7. [6] K. Goldman, M. Mehregany, “Novel Micromechanical Temperature Memory Sensor,” Proc. 1995 IEEE International Conference on Sensors & Actuators(Transducers 95),, June 1995, pp. 132 – 135. [7] A. DeHennis, K.D. Wise, “An all-capacitive sensing chip for temperature, absolute pressure, and relative humidity,” Proc. 2003 IEEE International Conference on Solid-State Sensors, Actuators & Microsystems (Transducers’03), Boston, USA, June 2003, pp. 1860-1863.
