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A Versatile Micro-Scale Silicon Sensor/Actuator with Low Power Consumption

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A Versatile Micro-Scale Silicon Sensor/Actuator with Low Power Consumption
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  A Versatile Micro-Scale Silicon Sensor/Actuator with Low Power Consumption A. Y. Kovalgin, J. Holleman, G. Iordache MESA +  Research Institute University of Twente Enschede, The Netherlands e-mail: a.y.kovalgin@utwente.nl  Abstract   —We designed a CMOS compatible hot-surface silicon device operating at a power down to sub-   W  . It has a pillar-shaped structure with a nano-size (10-100 nm) conductive link between the electrodes separated by a SiO 2  layer. The device is capable of maintaining a m-size hot-surface area of several hundred degrees centigrade due to non-radiative recombination of carriers in a thin (13 nm) poly silicon surface layer. Such a device can be used as a light source, a heat source, as well as a sensitive detector of light and heat. As a direct application, we demonstrate the feasibility to perform as an adsorption-desorption sensor, and as a unit for activating chemisorption/decomposition ( i.e.  micro-reactor). I.   I  NTRODUCTION  There is a great demand on simple and cheap devices, which when integrated on a chip, can allow actuation and/or in situ characterization of physical and chemical processes on micro/nano scale. This means the activation of surface chemical reactions (e.g. thermo- and photo-activated) and monitoring the occurrence of such reactions (e.g. adsorption, chemisorption, decomposition, combustion, desorption, etc.)  by means of embedding a variety of sensors into a multifunctional lab-on-chip. As an important application example, chemical sensors based on electrical conductivity changes and utilizing “micro hot-plates” are of interest [1]. In such sensors, the change in the resistance of a thin conducting layer with adsorption or desorption of specific gases is monitored. These sensors require elevated temperatures to operate in the range between 200 and 600 o C. As another example of a hot-surface device, a gas sensor for hydrocarbons (butane, methane, propane, etc.) can be considered, i.e. the so-called Pellistor [2]. Such a sensor is  based on the catalytic combustion of hydrocarbons, which  provides an extra amount of heat resulting in a higher local temperature. This temperature increase can be monitored by the change of the electrical resistance of the device. One significant drawback of conventional hot-surface devices is their relatively high power consumption. The commercial devices use e.g. a platinum wire as a heating element and require a power of about 100 mW. Other types of hot-surface devices realized more recently on suspended membranes employ a meander-like pattern of a conductive layer as platinum or poly-silicon and still require about 20-40 mW [3]. In such a design, an increase of thermal resistance of the electrical leads is important to minimize the heat losses. However, an increase of the thermal resistance can only be achieved by shrinking the cross-sectional area of the leads, which consequently increases their electrical resistance and results in undesired heat losses in the leads. This work extends the previously published idea of making micro-scale hot-surface devices based on generation of heat due to non-radiative recombination of carriers in a thin (13 nm) poly-Si surface layer [4]. The result is that an extended poly-Si area instantly becomes hot and the vertical heat losses through the massive mono-Si pillar are decreased. Finally, this leads to an increased efficiency of the device. II.   D EVICE D ESIGN A  ND R  EALIZATION    A.    Design Our device resembles a micron-sized volcano with a nano-sized conductive link in the center (Fig. 1 and Fig. 2). The link of 10-100 nm in size is formed between two electrodes separated by a thin SiO 2  layer in the centre of the  pillar-shaped structure (Fig. 1). To create the link, the insulating SiO 2  layer between the electrodes is locally destroyed by a two-step electrical stress, during which the insulator melts locally [5]-[7]. The link is called antifuse , which means that insulator becomes conductive after fusing contrary to a conventional  fuse . Because of the pillar-shaped structure with the conductive link-antifuse in the centre, the device is called a Pillar-Shaped Antifuse or a  PSA-device .  B.    Realization The process flow was already described in [4]. Briefly, an LPCVD silicon nitride film was first deposited on a  phosphorus-doped (100)-oriented silicon wafer with a This work was financially supported by the EU project SAFEGAS (GRD1-1999-10849) 1225 0-7803-9056-3/05/$20.00 © 2005 IEEE.  resistivity of 2-5 Ω⋅ cm. After patterning the nitride (2 × 2 µ m squares), approximately 1- µ m high silicon pillars covered with the nitride caps were formed on the wafer surface using isotropic CF 4 -O 2  plasma etching. The following LOCOS oxidation at 1050 o C provided sharpening the pillars and formation of a 400-nm silicon oxide for the thermo-electrical isolation. Further, the silicon nitride was etched off, and a thin gate oxide was grown onto the silicon pillar. Then a 13-nm thick phosphorous-doped (~10 20  at/cm -3 ) α -Si layer was deposited, followed by the LPCVD of a 9-nm thick silicon nitride layer for passivation of the device surface. C.   Coating procedure Using the spin-on-glass technology, the PSA-devices were coated with a porous Spin-On-Glass (SOG) layer, approximately 350 nm thick. This step was done to explore the feasibility of the device to monitor heat-enhanced adsorption and desorption processes in the porous layer. Briefly, the SOG-solution was prepared and applied to the substrate by spinning. During the next drying step there was a reduction by more than 50% in weight and volume of the material. Yet the film remained adherent and continuous and maintained complete surface coverage. It has been shown that, up to one micron of thickness, all shrinkage was taken up in the dimension perpendicular to the surface and not in the plane of the substrate. Next, further heat treatment was applied for hardening the layer. Although the micro porosity of silica is not removed entirely until 1000 o C, it may already  behave as an oxidant barrier or passivation coating at 600 o C. As the processing temperatures we used are below 400 o C, the obtained SOG film is a rather porous material. III.   P HYSICAL P ROPERTIES  The realized PSA-devices showed a diode-like behavior due to depletion effects in the mono-silicon bottom electrode [4]. It was confirmed that the devices were capable of generating a sufficient heat for the successful actuation of  physical and chemical processes on the surface. From the diode-like behavior, two mechanisms of heat generation can  be considered. If a  positive  bias is applied to the upper poly-Si electrode, the device acts as a resistor. The heat is locally dissipated in the centre and further distributed in lateral and vertical directions accordingly to thermal conductivities of the materials used. The lateral heat transport is limited due to the small thickness of the poly layer. The vertical heat transport and, therefore, the heat losses downwards are enhanced because the massive silicon pillar is close to the heat dissipation centre. In this regime, the efficiency of the heat generation is low due to the vertical losses. If a negative   bias is applied to the upper poly-Si electrode, the carrier recombination occurs along an extended surface area (~ 2 µ m) of the thin upper electrode [4]. The result is that the entire area instantly becomes hot and the vertical heat losses through the pillar are decreased due to the extended size of the heat dissipation area. The efficiency of the heat generation is increased compared to the first mechanism. It appears that, when current is applied, the PSA-devices emit light in the visible-near IR region (Fig. 3), which can be used e.g. to enable photo-activated processes. The spectra show a clearly distinguishable emission peak at about 700-720 nm. To verify a thermal srcin of the peak, we have simulated the emission spectra defined as a product of the calculated emissivity ε  ( λ   ,T  ) and pure black body radiation. 0.0E+005.0E-091.0E-081.5E-082.0E-08600 700 800 900 1000 1100 Wavelength, nm    I  n   t  e  n  s   i   t  y ,      W   /  n  m  50 uA 20 uA 15 uA 10 uA   Fig. 3:  Light emission spectra from a pillar-shaped device versus applied current. monosiliconbottom electrodesiliconoxidesiliconoxide polysilicon top electrodesilicon nitridepassivation link   Fig. 1:  Schematic cross-sectional view of the device Fig. 2:  SEM top-view image; the cavity is etched in the centre enabling seeing the bottom mono-Si electrode. 1226  The emissivity was calculated for the following stack of three thin layers: a 10-nm thick SiO 2 , a 13-nm thick poly-Si electrode, and a 9-nm thick silicon nitride passivation layer on top. The data to calculate the optical properties of silicon at elevated temperatures were taken from [8]-[10]. The optical properties of the other materials used were assumed to be independent on temperature. For the modeled stack of layers, the calculated emissivity ε  ( λ   ,T  ) decreased from 0.43-0.48 to 0.29-0.34 for the wavelength increased from 700 nm to 1000 nm in the temperature range 25-600 o C. The simulated spectra exhibited a gradual increase of the intensity with increasing the wavelength without showing any shoulder on it. From this, one can conclude that the emission spectra can not be attributed to the thermal emission and optical properties of the materials used. Most likely such a significant shift to higher energies (720 nm corresponds to 1.72 eV instead of the expected value of 1.1 eV) can be related to the recombination of hot car riers in the 13-nm thick upper silicon electrode or with increasing the  bandgap width due to the thinning of the electrode [11]. IV.   S ENSING P ROPERTIES  It is important to note that, during all the experiments on sensing, a constant current mode was used.  A.   Thermo-resistor and light sensor As mentioned, the PSA-devices exhibit a diode-like  behavior, i.e. a very high resistance when a negative bias up to hundred Volts is applied to the top electrode. This high resistance is caused by the depletion effects in the mono-silicon bottom electrode. The conductivity of such a negatively-biased device can strongly be influenced by temperature due to the temperature-dependent generation of new carriers in the depleted pillar. It appears that the link  can be used as a sensitive thermo-resistor with the sensitivity of about 0.7 Volts per degree centigrade [4]. Regarding the practical applications, such a device can monitor exothermic and endothermic processes on the surface. The same matters causing the sensitivity to temperature can also result in a drop of the resistance during the illumination with light (Fig. 4). As expected, the sensitivity to light decreases with increasing temperature. This is due to the fact that both heat and light provide extra carriers and therefore can independently suppress the depletion effects. In Fig. 4 one can notice that the response to light is hard to observe if the temperature exceeds 125 o C.  B.    Physical adsorption During the experiments on measuring the adsorption the surface temperature was not sufficiently high for chemisorption/decomposition of the precursors. The devices were able to sense the adsorption/desorption cylces in the  porous SOG layer, which affected the surface temperature.  Namely, after the precursor vapour has been introduced into the chamber containing air, it could penetrate into the pores and adsorb or condense there. Such an adsorption increased mass of the layer, which led to cooling the surface. This caused the resistance increase. As a consequence, an extra  power (i.e. higher voltage) should be applied to maintain the same current. This extra power increased the surface temperature and enhanced the desorption, which caused again cooling the surface and again required applying an extra power to maintain the same current. Such a self-amplifying mechanism resulted in a very sharp increase of the bias after introducing the precursor vapor. Similar effects were observed for different precursor vapours, i.e., water, acetone, ethanol, and 2-propanol. A high response to water vapour (Fig. 5) points out that the temperature is insufficient to suppress the physical adsorption of water. The average dissipated power is still in the µ W range. C.   Chemisorption/decomposition With further increasing the dissipated power, one can expect a suppression of the physical adsorption due to the higher temperature. Indeed, the devices become insensitive -100-80-60-40-2000 4 8 12 16 Time, hours    B   i  a  s ,   V  o   l   t  s   Fig. 5:  Device response (adsorption-desorption self-oscillations) on 0.95% of H 2 O vapor (never off during the test) in air; standby power 0.5 µ W, peak power 5-8 µ W. -100-80-60-40-2000 100 200 300 400 Time, sec    B   i  a  s ,   V  o   l   t  s light on25 o C70 o C125 o Clight off    Fig. 4:  Sensitivity to light at different substrate temperatures during a constant current stress of 0.1 µ A. 1227  to water vapor at dissipated powers in the range of 10-100 microwatts. An increase of the power beyond this value leads to even higher surface temperature probably enabling chemisorption of the precursors. Fig. 6 shows different responses to ethanol vapor at a varying applied power. Similar results were obtained for acetone and 2-propanol vapor. To check if the thermal conductivity change plays a role, we have tested chloroform vapor. At elevated temperature chloroform has 3 times lower thermal conductivity than air [12]. No effect was observed. The fact that the peaks are directed upwards is due to cooling the surface, which can probably be caused by an endothermic reaction occurring inside pores due to the thermal decomposition of the precursor vapor. In this sense, the devices enable monitoring e.g. the chemisorption.  D.   Selectivity to different precursors An important result is that the tested precursors (i.e. vapors of ethanol, acetone, and 2-propanol) exhibit different responses depending on applied powers. Plotting the response peak intensity (expressed in percents of the applied  power) versus applied power, one can obtain different curves for the tested precursors (Fig. 7). This rather promising result makes it possible to distinguish between several precursors in a mixture when an array of such sensors operating at various powers is used. V.   C ONCLUSIONS  The fabricated PSA-devices exhibit interesting sensing  properties and, combined with extremely low operating  power in a µ W/sub-mW range, can give rise to a number of  practical applications. We have demonstrated the feasibility to perform as a temperature-, adsorption-, desorption sensor, and as a device for activating chemisorption/decomposition of the precursors, i.e. a micro-reactor. R  EFERENCES   [1]   A. Hulanicki, S. Geab and F. Ingman, “Chemical Sensors: Definitions and Classification”, Pure&Appl. Chem ., Vol. 63 ,  No. 9, pp. 1247-1250, 1991. [2]   M. Gall, “Si-Planar-Pellistor array, a detection unit for combustible gases”, Sensors and Actuators B: Chemical, Vol. B16, No. 1-3, p. 260-264, 1993. [3]   Cs. Dücs ő , M. Ádám, P. Fürjes, M. Hirschfelder, S. Kulinui, and I. Bársony, “Explosion-proof monitoring of Hydrocarbons by Micropellistor”, in Eurosensors 2002 – Book of Abstracts, J. Saneistr and P. Ripka, Editors, p. 525-526, 2002. [4]   A.Y. Kovalgin, J. Holleman, G. Iordache, “A micro-scale hot-surface device based on non-radiative carrier recombination”  , Proc. of the 34th European Solid-State Device Research Conference, IEEE, R.P. Mertens and C.L. Clayes, Editors, p. 353-356, 2004. [5]   V. E. Houtsma, “Gate Oxide Reliability of Poly-Si and Poly-SiGe CMOS Devices”, Ph.D. Thesis, Enschede, 1999. [6]   A. Y. Kovalgin, J. Holleman, A. van den Berg, “On the feasibility of using antifuses as low-power heating / detecting elements in pellistor-type gas sensors” in   Sensor Technology 2001: Proceedings of the Sensor Technology Conference 2001, M. Elwenspoek, Editor, p. 107-112, Kluwer Academic Publishers, 2001. [7]   A.Y. Kovalgin, J. Holleman, A. van den Berg, “A novel approach to low-power hot-surface devices with decoupled electrical and thermal resistances” in Eurosensors 2002 – Book of Abstracts, J. Saneistr and P. Ripka, Editors, p. 31-32, 2002. [8]    N.M. Ravindra, S. Abedrabbo, W. Chen, F.M. Tong, A.K. Nanda, and A.C. Speranza, “Temperature-dependent emissivity of silicon-related materials and structures”, IEEE Trans on Semiconductor Manufacturing, Vol. 11, No. 1, pp. 30-39, 1998. [9]   G.E. Jellison, Jr. and D.H. Lowndes, “Optical absorption coefficient of silicon at 1.152 µ  at elevated temperatures”, Appl. Phys. Lett., Vol. 41 No. 7, pp. 594-596, 1982. [10]   G.E. Jellison, Jr. and F.A. Modine, “Optical functions of silicon at elevated temperatures”, Appl. Phys. Lett., Vol. 76, No. 6,    pp. 3758-3761, 1994. [11]   A.T. Fiory and N.M. Ravindra, “Light emission from silicon: some  perspectives and applications”, Journal of ELECTRONIC MATERIALS, Vol. 32, No. 10, pp. 1043-1051, 2003. [12]   Handbook of Chemistry and Physics, 73 rd  edition, D.R. Lide, Editor. CRC Press, Inc., p. 6-172, 2000. 0.05.010.015.020.025.030.035.00.00.40.81.21.6 Applied power, mW    R  e  s  p  o  n  c  e ,   %   o   f  p  o  w  e  r  2% Ethanol 8% Acetone 1.4% 2-Propanol   Fig. 7:  The response peak (i.e. the peak shown in Fig 6) intensity calculated in % of the applied power and plotted versus applied power for different precursor vapors in air. -50-40-30-20-10200600100014001800 Time, sec    B   i  a  s ,   V  o   l   t  s  1.3 mW 0.8 mW 0.4 mW 0.22 mW 0.16 mW 0.05 mW onoff    Fig. 6:  Device response (chemisorption-decomposition) after introducing 2% ethanol vapor in air into the chamber; a power in the sub-mW range is applied; on  — ethanol flow is turned on, off   — ethanol flow is turned off 1228
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