This work employs the idea of maintaining a hot surface by means of dissipating power at a nano-scale conductive link. The link is created between two polysilicon electrodes separated by a dielectric (a capacitor-like structure). From modelling, a link of 10 nm in diameter should be possible to maintain the surface temperature ranging between 750 and 1150 K within the surface diameter of 2 μm by absorbing a 3.3 mW of electric power. The devices can also be designed in such a way that the hot surface area is reduced to a sub-μm-size hotspot. The main advantage of the proposed idea is decoupling the electrical resistance and thermal resistance of the device. In this paper, two device structures based on antifuse technology are described. Both the thermo-electrical properties and feasibility to perform as a Pellistor-type gas sensor are discussed

Berg, A. van den

Holleman, J.

Kovalgin, A.Y.

English

Kovalgin, Alexeij Y.

van den Berg, Albert

University of Twente Research Information

A NOVEL APPROACH TO LOW-POWER HOT-SURFACE DEVICES WITH DECOUPLED ELECTRICAL AND THERMAL RESISTANCES  A.Y. Kovalgin, J. Holleman, A. van den Berg MESA+ Research Institute, University of Twente P.O. Box 217, 7500 AE Enschede, The Netherlands E-mail: A.Y.Kovalgin@el.utwente.nl   Summary - This work employs the idea of maintaining a hot surface by means of dissipating power at a nano-scale conductive link. The link is created between two polysilicon electrodes separated by a dielectric (a capacitor-like structure). From modelling, a link of 10 nm in diameter should be possible to maintain the surface temperature ranging between 750 and 1150 K within the surface diameter of 2 µm by absorbing a 3.3 mW of electric power. The devices can also be designed in such a way that the hot surface area is reduced to a sub-µm-size hotspot. The main advantage of the proposed idea is decoupling the electrical resistance and thermal resistance of the device. In this paper, two device structures based on antifuse technology are described. Both the thermo-electrical properties and feasibility to perform as a Pellistor-type gas sensor are discussed.  Keywords – gas sensor; nano-hotspot; antifuse  Subject category – Applications  I. INTRODUCTION Hot-surface devices have received much attention nowadays because of its utilization for Pellistor-type gas sensors [1] and micro reactors. One significant drawback of the Pellistor is its relatively high power consumption. This work employs the idea of maintaining a micro-size hot surface-area by means of dissipating power at a nano-scale conductive link created between two micro-scale (polysilicon) electrodes separated by a dielectric. It appears from the numerical modelling and experimental measurements that the link can perform as both the surface-heating element and heat-detecting device [2]. The latter is due to the fact that electrical resistance of the link is a sensitive measure of its temperature. From modelling, the link can locally be heated up to the melting point of Si (1415 oC) by absorbing electric power in the range of 2-5 mW. Additionally, the link of 10 nm in diameter is able to maintain a non-uniform hot surface within a diameter of a few µm. This gives rise to a number of practical applications, for example, in low-power Pellistor-type gas sensors. On the other hand, the devices can be designed in such a way that the hot surface area is reduced to a sub-µm-size hotspot, which makes it applicable to the variety of scanning probing techniques.  Apart from the low power consumption, the main advantage of the proposed concept is decoupling the electrical resistance and thermal resistance. In conventional hot plate devices, the heat is obtained either from the filament heater or suspended polysilicon meander. In such a design, an increase of thermal resistance of the electrical leads is an important issue due to the necessity to minimize the power loss through the leads. Indeed, an increase of the thermal resistance can only be achieved by shrinking the cross-sections, which gives rise to the increasing electrical resistance resulting in undesired extra power dissipation. In this paper, a new concept is proposed with independent control of both the resistances. Decoupling becomes possible because the electrical resistance is now determined by the link resistance only and has no influence on the thermal resistance due to the nano-scale link dimensions. In other words, the devices are designed in such a way that one can manipulate the heat flow in the bulk of the device without affecting its electrical resistance.  In this paper, we demonstrate our first results with respect to design, modelling, and practical realisation of the hot-surface devices with decoupled electrical and thermal resistances.  II. DESIGN ASPECTS The decoupling concept has been realised by two structures, both of which are described and discussed below. Structure 1 is designed in such a way that the hot surface is thermally isolated from the bottom silicon electrode only by means of a 500-nm LOCOS oxide (Fig 1a). This ensures a reduced hot area needed for scanning technique. The attention is paid to pre-determination of the link location on the surface. To improve the feasibility of the device as a gas sensor, a larger hot area has to be exposed to the gas ambient. For enlarging the area, Structure 2 with improved thermal isolation has been proposed (Fig 1b).   The conductive link between the electrodes can be established either conventionally by means of high-resolution lithography tools or by employing the antifuse technology [3], which is fully compatible with silicon technology. The antifuse is a conductive link of 5-50 nm in size caused by electrical breakdown due to tunnelling a constant current stress between two electrodes separated by a dielectric. Finally, applying a well-defined programming 88M2C3The 16th European Conference on Solid-State Transducers | September 15-18, 2002, Prague, Czech RepublicApplicationscurrent, which is higher then the initial stressing current, enlarges the link. It appears from our research that as the link is once created at certain value of the programming current, the size and electrical properties of the link cannot be changed under a current stress below the programming value [3].      monosiliconbottom electrodesiliconoxidesiliconoxide  polysilicon top electrodesilicon nitridepassivation link   silicon nitride  suspended  membranepolysilicon electrodesair air link silicon nitridepassivation  Fig 1. Schematics of the proposed structures.  Structure 1 (Fig 1a), which has already been made, consists of two electrodes: rather thick mono-Si pillar-type bottom electrode and very thin poly-Si top electrode (passivated). The surrounding oxide plays an important role for thermo-electrical isolation and confines the area of antifuse formation. The process flow employs standard LOCOS oxidation and CVD technology. As the first step, an LPCVD silicon nitride film was deposited on a phosphorus-doped (100)-oriented silicon wafer with a resistivity of 2-5 Ω⋅cm. After patterning the nitride (2×2 µm squares), approximately 1-µm high silicon pillars, covered with the nitride caps, were formed on the wafer surface using isotropic CF4-O2 plasma etching. The following LOCOS oxidation at 1050 oC provided sharpening the pillars and formation of a 400-nm silicon oxide for thermo-electrical isolation of the pillars. Further, the silicon nitride was etched away, and a thin gate oxide was grown on the silicon tip surface. Then, a 13-nm thick in-situ phosphorus-doped a-Si layer (~1020 at/cm-3) was deposited on the wafer surface, followed by LPCVD of a 9-nm thick silicon nitride layer for passivation of the device surface.   In still not realized practically Structure 2 (Fig 1b), both electrodes are designed from thin poly-Si layers. The suspended membrane provides improved thermal isolation. No steps towards pre-determined-in-space antifuse formation are yet made.  III. MODELLING Fig 2 represents modelled surface-temperature gradients for Structure 1 and Structure 2. For Structure 2, a link of 10 nm in diameter is able to maintain the surface temperature ranging between 750 and 1150 K within the surface diameter of 2 µm by absorbing a 3.3 mW of electric power. For Structure 1, equivalent in size link provides much sharper temperature profile at higher power consumption. Modelled temperature profiles exhibit the possibility to induce thermo-chemical reactions on micro-scale.  a)  300500700900110013000.0 0.2 0.4 0.6 0.8 1.0Distance from the link, micronsTemperature, K Structure 1: power 3.9 mW Structure 2: power 3.3 mW b)  Fig 2. Temperature gradients for a 10-nm link (modelled with SILVACO’s ATLAS).  An important characteristic is a power density per unit surface area, needed to increase the surface temperature in one degree (SPDD). From the modelling of Structure 2, SPDD mainly depends on two parameters: poly-Si electrode thickness and membrane diameter. It appears that SPDD gradually increases with poly-Si thickness. However, it becomes less with increasing the radius of poly-Si plate. This is due to the competition between lateral heat transfer conditions in the poly-Si layer (which are favourable for a thicker poly) and vertical heat losses through the membrane material and air. For example, for a 200-nm thick poly-Si electrode with a diameter of 26 µm one can obtain the same SPDD as for a 100-nm thick poly-Si electrode placed on the membrane with a diameter of 16 microns.  IV. EXPERIMENTAL MEASUREMENTS A. Electrical Characterization Electrical properties have only been measured for the devices with Structure 1. First, the devices were fused by applying a fixed current in the range between –10 nA and –100 nA (negative biasing of poly-Si electrode) to ensure a breakdown of the oxide layer at the tip. The applied power was in the range of microwatts. During further re-programming, the power was increased up to 1-10 mW.   The change of the device resistance (mainly the link resistance) with increasing applied power 89M2C3The 16th European Conference on Solid-State Transducers | September 15-18, 2002, Prague, Czech RepublicApplications(positive bias is applied to poly-Si electrode) is shown in Fig 3. At near-zero power the resistance falls in the range of hundreds of MOhms (in some cases it can be as high as a few GOhms) depending on the value of programming power. A gradual increase of power up to the programming value leads to a sharp decrease of the resistance. The resistance decrease can hardly be explained in terms of a heat enhanced carrier generation in the link material. As simulated in [2], the decrease already starts at near-room temperature. It appears that the electric fields at the link edges can control the passage of carriers through the link. The smaller the link the more dramatic the field influence.  Once realized dependence of the resistance on power can be multiply repeated, provided that the programming power is not exceeded. Exceeding the programming level causes re-programming of the link, which results in a rather noisy behaviour (Fig 3). In the range of relatively low powers (0.5-1.5 mW), the link resistance always decreases after re-programming. This can be attributed to gradual enlarging the link due to its re-melting. For higher powers (> 3 mW), the link resistance is always higher after re-programming. This effect can be explained by an irreversible change in the material quality (diffusion, etc.) caused by high temperature, occurring either in the link itself or in the electrodes. It is important to bear in mind that primary the link material is composed due to a nano-scale explosion involving both silicon and oxide.   0.E+002.E+044.E+046.E+048.E+041.E+050.E+00 1.E-03 2.E-03 3.E-03 4.E-03Dissipated Power, WDevice Resistance, Ohm  first sweeping second sweepingre-programming  Fig 3. Measured antifuse resistance versus applied power; at 1 mW the link re-programming can clearly be seen.  Stability tests have been carried out for some of the fabricated devices. Namely, the antifuses were programmed at 1 mW and then kept at that power for 16 hours meaning that its cores were at or near the melting point of Si (1415 oC). For some devices, instability during the first 2 hours has been observed, followed by reasonably stable behaviour. However, since the devices would normally operate at much lower temperatures, we expect only minor instabilities due to thermal degradation. An important result is that the pillar antifuse with the same type of electrode doping behaves similar to a diode (Fig 4). Negative biasing results in much higher device resistance compared to positive biasing. There are several reasons for such a diode-like behaviour, for example, depletion-related effects in the lowly doped silicon pillar, intrinsic link properties, etc. Similar matters probably cause the sensitivity of the device resistance to substrate temperature at negative biasing (Fig 4). Namely, due to a low concentration of free carriers in the sharp silicon pillar (or inside the link), the resistance becomes a sensitive measure of the temperature. The extra heat supplies new carriers, which leads to the resistance drop.   1.E-091.E-081.E-071.E-061.E-051.E-04-100 -50 0 50Bias, VoltsAbsolute Current, A25 o C70 o C150 o C125 o C  Fig 4. I-V characteristics of pillar antifuses at different substrate temperatures; bias is applied to poly-Si electrode.  B. Generation of Heat The following experimental results, obtained for Structure 1, exhibit its feasibility to perform as a hot-surface device. Fig 5 represents a surface SEM image of the device programmed at 1 mW. The round spot at the centre corresponds to the conductive link. One can estimate the link diameter of about 70 nm.     Fig 5. SEM image of the device after programming the link at 1 mW.  After a gradual increase of power up to 4.5 mW, the surface area significantly deteriorates compared to 90M2C3The 16th European Conference on Solid-State Transducers | September 15-18, 2002, Prague, Czech RepublicApplicationsthat depicted in Fig 5. At even higher powers, the surface temperature is high enough to sustain the evaporation of both the poly-Si layer and nitride film (Fig 6). The underlying mono-Si pillar can clearly be seen in the centre of the circle.     Fig 6. SEM image of the device after applying a high power in the range of tens of mW.  Fig 7 depicts the IR signal, emitted from the device, versus increasing power stress, which is a measure of the device temperature. The IR emission has been detected by means of an IR spectrometer. It was necessary to integrate the IR signal in the whole detectable range between 1,1 and 1,7 µm due to the low intensity of the emission.  0246810120 5 10 15 20Applied bias, Volts Averaged IR signal, arb. un. Fig 7. IR emission measured for Structure 1.  C. Experiments on Sensing Sensitivity of the pillar antifuses with respect to different sources of energy has been investigated. It appeared that a pillar antifuse was able to perform as a combined nano-scale heat detector and light sensor. The matters causing the sensitivity of the device resistance to the external heat at negative biasing were discussed earlier. The same process, namely the generation of new carriers, could also result in a dramatic drop of the antifuses resistance during the light illumination (Fig 8).   Because of the ability of antifuses to generate and detect heat, we have investigated its performance as Pellistor-type gas sensors. First promising results have already been obtained in this direction. In Fig 9, one can observe a significant response of the device after introducing acetone vapour into the measuring chamber. It is important to bear in mind that no catalytic layer aiming at decreasing the operating temperature and increasing the sensitivity was deposited on the device surface.   -100-80-60-40-200200 200 400 600 800Time, secBias, Volts Applied current: -2 µAlight onlight offlight on Fig 8. Influence of light on device resistance.  -100-80-60-40-2000 500 1000 1500Time, secBias, VoltsApplied current: -11 µAacetone on     150     330     680 acetone off     170     350     700 Fig 9. First experiments on sensing of acetone vapour. V. CONCLUSIONS In this work, we have proposed a novel approach to decoupling electrical and thermal resistances. The device structure based on the antifuse technology has been intensively studied. The measured hot-surface pillar antifuses exhibit the feasibility to perform as combined nano-scale heat sensors and light detectors. VI. ACKNOWLEDGEMENTS This work is financially supported by the EU project GRD1-1999-10849. VII. REFERENCES [1] M. Gall. Sensors and Actuators B (Chemical) B4, 533-8 (1991). [2] A. Y. Kovalgin, J. Holleman, A. van den Berg. In “Sensor Technology 2001”, ed. M. Elwenspoek, Kluwer Academic Publishers, p. 107-112, (2001).  [3] V. E. Houtsma, Ph.D. Dissertation, University of Twente, Enschede, The Netherlands, (1999).  91M2C3The 16th European Conference on Solid-State Transducers | September 15-18, 2002, Prague, Czech RepublicApplications

A novel approach to low-power hot-surface devices with decoupled electrical and thermal resistances

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A novel approach to low-power hot-surface devices with decoupled electrical and thermal resistances

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