INSTITUTE OF PHYSICS PUBLISHING
JOURNAL OF MICROMECHANICS AND MICROENGINEERING
J. Micromech. Microeng. 14 (2004) 793–797
PII: S0960-1317(04)74919-0
Fabrication and testing of an integrated thermal flow sensor employing thermal isolation by a porous silicon membrane over an air cavity D N Pagonis, G Kaltsas and A G Nassiopoulou IMEL, NCSR ‘Demokritos’, PO Box 60228, 153 10 Aghia Paraskevi, Athens, Greece E-mail:
[email protected]
Received 16 January 2004 Published 19 April 2004 Online at stacks.iop.org/JMM/14/793 (DOI: 10.1088/0960-1317/14/6/005) Abstract In this work, an integrated thermal gas flow sensor was fabricated and evaluated using a new thermal isolation technique based on a porous silicon membrane over an air cavity in silicon. The fabrication process of a thermal gas flow sensor employing this type of micro-hotplate is described together with the theoretical estimation of the thermal distribution around the heater of the sensor. Experimental results of the thermal isolation achieved are shown. A flow evaluation of the sensor is presented and discussed. All the results obtained are compared with the corresponding ones for an identical sensor using a micro-hotplate made of porous silicon membrane but without the air cavity underneath. The improvement achieved by the air cavity is evaluated.
1. Introduction The major concern in all thermal sensors is the thermal isolation achieved on its sensing elements; a temperature rise up to 400–500 ◦ C and even above in a predefined area while keeping the substrate at room temperature can be a typical prerequisite for the design of such a sensor. Reasonably good thermal isolation from the silicon substrate can be achieved by growing a thick membrane locally on the silicon substrate, on which the active elements of the sensors are integrated [1–5]. In the following we will refer to this type of micro-hotplate as ‘porous silicon micro-hotplate’. In this technology, thermal isolation is due to the lower thermal conductivity of porous silicon (0.1–2 W m−1 K−1) compared to that of crystalline silicon (145 W m−1 K−1). The successful fabrication of various integrated thermal sensors employing porous silicon thermal isolation technology has been reported in the literature [6–8]. An improvement of this technology has been recently developed by the authors [9], based on the creation of an air cavity underneath the porous membrane. The even lower thermal conductivity of air (2.62 × 10−2 W m−1 K−1) compared to that of porous silicon assures a superior local thermal isolation on top of the porous membrane. 0960-1317/04/060793+05$30.00
By using this improved technique we fabricated and tested a thermal gas flow sensor which was compared with an identical sensor based on the conventional porous silicon thermal isolation technology. The thermal isolation achieved in both cases was estimated through a set of thermal simulations and compared with the values obtained experimentally by electrical measurements on the fabricated devices. The two sensors were tested under similar flow conditions and the results were compared.
2. Fabrication process The process flow for the improved porous silicon thermal isolation technology and the gas flow sensor are described elsewhere [8, 9]. An outline will be presented in the following sections. In all the cases p-type 100 oriented silicon substrates were used, having a resistivity of 1–2 cm. 2.1. Porous silicon over air cavity technology The process developed for porous silicon over cavity formation is a two-step electrochemical process. A porous silicon layer is initially formed on a predefined area on the substrate, by
© 2004 IOP Publishing Ltd Printed in the UK
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Figure 1. Typical SEM picture showing a cross-section of a 10 µm deep air cavity covered by a 10 µm thick porous membrane.
Figure 2. Top-view picture of a typical thermal flow sensor based on the porous silicon over air cavity technology.
anodization of monocrystalline silicon through an appropriate masking layer. More specifically, the sample is anodized in an HF:ethanol solution with a current density below a critical value Jps for electropolishing, which depends on the electrochemical solution used and the resistivity and the type of the silicon substrate [10]. Subsequently, in the next step, by increasing the current density above Jps, a cavity is formed underneath the porous layer by electropolishing of silicon. The mask used in order to define that area is a bi-layer of polycrystalline silicon and thermal oxide each having a thickness of 100 nm [11]. In the case of the conventional one step porous silicon thermal isolation technology, porous silicon is simply formed at the predefined area by proper anodization of the sample and the process is stopped at this level. In order to provide mechanical stability to the suspended porous silicon micro-hotplate over the air cavity, the surrounding area of p-type silicon has to be transformed into n-type by appropriate doping with phosphorous dopants, prior to anodization and the consequent electropolishing process [12]. The resulting n-type area is not made porous during the anodization, thus creating a 7.5 µm thick monocrystalline silicon supporting ring around the porous membrane. A scanning electron microscope picture showing the crosssection of a constructed porous membrane over an air cavity is presented in figure 1, where we can see the n-type silicon ring supporting the membrane. By using this process, 10 µm thick free-standing porous silicon membranes over a 10 µm air cavity were formed.
membrane while the ‘cold’ one lies on the silicon substrate. The final processing step is aluminium deposition (500 nm thick) and Al patterning in order to form the second part of the thermopiles and the necessary metal pads. A scanning electron microscope (SEM) picture showing the top view of the sensor is shown in figure 2. The output of the sensor is a differential measurement of the voltage developed at each thermopile under flow [8].
2.2. Thermal gas flow sensor fabrication The integration of the gas flow sensor on the suspended porous silicon membrane is a three-mask process and it is described in detail in [8]. TEOS oxide is first deposited on top of the membrane for electrical isolation while a 500 nm polysilicon layer is then deposited on top of the TEOS layer by low pressure chemical vapor deposition (LPCVD). The polysilicon layer is subsequently implanted by boron and patterned in order to form a heater on top of the membrane and the first part of two sets of poly-Si/Al thermopiles at each side of the heater. The ‘hot’ contact of the thermopiles lies on the 794
3. Thermal distribution around the heater on the micro-hotplate The estimated and measured maximum temperatures reached on the porous silicon over cavity micro-hotplate for a given power, will be presented in this section. The theoretical estimation was carried out by appropriate thermal simulations while the actual temperature reached on the micro-hotplate was calculated through a set of electrical measurements. The results are compared with the corresponding ones for a device fabricated by employing the conventional porous silicon micro-hotplate technology. 3.1. Expected thermal isolation on the constructed device A set of simulations using the commercial software package CoventorWare by Microprosm was carried out in order to theoretically estimate the expected thermal isolation provided by the porous silicon over air cavity micro-hotplate. The geometrical structure and dimensions used in the simulation were similar to those of the real sensor (figure 2). In more detail, the dimensions of the thermally isolated area were 1170 µm × 100 µm, the poly-Si heater’s length and width were set equal to 570 µm and 20 µm respectively, while a set of nine Al/p-type doped poly-Si thermocouples was assumed at each side of the heater. The hot contact of the thermopiles was set 20 µm away of the heater; each of the thermopiles had a width equal to 10 µm. A fixed room temperature (300 K) was set at the bottom of the structure while a power of 35 mW was applied to the heater. The conductivities of porous silicon and air were taken equal to 0.8 W m−1 K−1 and 0.0262 W m−1 K−1 respectively. The main goal of
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Figure 3. Simulated thermal distribution on the top of the sensor’s surface. The power applied to the heater is equal to 35 mW when the technology of poSi over cavity (10 µm poSi over 10 µm air cavity) is employed while a set of values (35 mW to 55 mW) was used for the conventional technology (40 µm thick porous silicon layer). For an applied power equal to 35 mW, the maximum temperature reached at the surface of the heater is equal to 384 K and 358.45 K accordingly.
the simulations was to determine the maximum temperature reached on the sensor’s surface. Figure 3 shows the predicted temperature distribution on the top of a sensor. The maximum temperature reached for the specific power applied is equal to 384 K. The corresponding temperature distribution on a 40 µm thick porous layer on silicon (optimum thickness for conventional porous silicon micro-hotplates [8]) for various power values applied to the heater (35 mW to 55 mW) is also shown for comparison. The predicted rate for the maximum temperature increase on the heater is equal to 1.67 K mW−1 for the conventional technology and 2.4 K mW−1 for the developed one. Obviously, the expected thermal isolation offered by the porous silicon/air-cavity technique is superior. 3.2. Measured temperature on the heater as a function of the applied power Electrical measurements were carried out on the constructed gas-flow sensors for both micro-hotplates to calculate the maximum temperature reached in each case on the heater for a given power. The following procedure was followed: initially, each device was placed on a prober station with an integrated hotplate; by increasing the hotplate’s temperature gradually, the heater’s resistance was monitored as a function of temperature as presented in figure 4. We should note that for the given Boron doping of poly-Si, the heater’s resistance is expected to increase linearly with the supplied power [8, 13]. The obtained resistance increase rate was measured equal to 1.635 per degree for both cases of isolation; it should be noted that this value is very close to that reported earlier in the literature [8]. In a second step, the resistance was measured as a function of the applied power on the device. The obtained curve for a device using porous silicon over air cavity micro-hotplate technology is presented in figure 5; the measured resistance increase rate for the specific device was equal to 3.69 mW−1, while
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Figure 5. Increase of the heater’s resistance as a function of the applied power for a device using a porous silicon over air cavity micro-hotplate.
the corresponding one for a device using the conventional porous silicon micro-hotplate (40 µm thick porous silicon layer on silicon) was measured equal to 2.69 mW−1. By combining the two obtained curves for each of the devices, one can plot the temperature increase of the heater as a function of the applied power as shown in figure 6. The increase rate for the maximum temperature reached on the device in both cases of thermal isolation can be deduced from the above data. These values are equal to 1.64 K mW−1 and 2.25 K mW−1 accordingly. It should be mentioned that the values experimentally determined are quite close to those obtained by simulations, as described in the previous section (1.67 K mW−1 and 2.4 K mW−1 respectively).
4. Electrical characterization of the sensor After the evaluation of the thermal isolation achieved on the constructed heater by using the porous silicon over air cavity micro-hotplate, the electrical characterization of the sensor 795
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Figure 6. Maximum temperature increase on the heater as a function of the applied power, for both cases of thermal isolation. The measured data are in accordance with the predicted ones through simulations.
Figure 8. Sensor output as a function of flow (gas velocity) for typical devices constructed with both thermal isolation technologies. Porous silicon over Air Cavity technology Porous silicon technology
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Figure 9. The recorded thermopile difference response as a function of the square root of the gas flow for typical devices constructed with both thermal isolation technologies.
4.2. Sensor response under gas flow was pursued. Initially, the thermopiles’ response as a function of the electrical power supplied to the heater was monitored; for a given geometry of a fabricated sensor, the response is mainly a function of the thermal isolation achieved on the device. Subsequently, the device was characterized for a given gas flow.
4.1. Static sensor characterization without gas flow The sensor response was obtained as the voltage difference of each set of thermopiles to an applied electrical power. A response of 0.110 V W−1 for each thermocouple was obtained for the sensor based on porous silicon over cavity micro-hotplate, while the response of a sensor employing the conventional porous silicon micro-hotplate was found to be equal to 0.081 V W−1. Figure 7 shows a typical thermopile response for various values of the supplied power, for the two types of micro-hotplates. 796
For these measurements, the sensors were initially mounted on a PCB base which was designed in such a way as to hermetically fit in an appropriate semi-circular tube. The experimental setup used for flow measurements has been described in detail elsewhere [13]. It consists of a network of gas tubes, three mass flow controllers (each for a different flow scale), a PC for data acquisition and the sensor fixture. All the flow measurements were made at room temperature, while pure nitrogen gas was used. Flows in the range of 0–2000 sc cm were used, which correspond to an average flow velocity of 0–9.43 m s−1 for the inlet dimensions of the specific sensor fixture. For the given dimensions of the semicircular tube, the value of the hydraulic diameter was calculated equal to D = 3 × 10−3 m [14], resulting in a Reynolds number Re = 1874, for a flow velocity of 9.43 m s−1, which is below the critical value for turbulent flow (Re = 2000). Thus, the gas flow over the sensor for the specific flow range is laminar. The data acquisition was accomplished using LabVIEW 4.0 software package.
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Figure 10. The thermopile difference response as a function of flow for various powers supplied to a sensor based on the conventional thermal isolation technology, in comparison with the corresponding one for a device constructed with the porous silicon over air cavity technology.
The sensor was operated at a constant temperature. In more detail, an initial power of 35 mW was supplied at the polysilicon heater, which corresponds to a temperature increase of about 80◦ above ambient on the heater’s surface. The drop of this temperature, induced by the flow of the gas, can be monitored by detecting the consequent drop of the sensor’s heater resistance. By monitoring in real-time the value of this resistance throughout the flow measurement, the applied power was adjusted, to keep the heater’s temperature constant. The recorded thermopile difference response as a function of flow is shown in figure 8. The thermopile difference increases linearly with the square root of the velocity (as shown in figure 9), thus the sensitivity of the flow sensor can be determined as Vdifference = 0.49 mV (m s−1 )−1/2 . S= √ U The sensitivity per heating power is equal to S Sn = = 14 mV (m s−1 )−1/2 W−1 . Pin In the same figure the response of a sensor using the conventional porous silicon micro-hotplate is also shown for the same power supplied. (The corresponding temperature increase on heater for this case is about 57◦ above ambient.) The sensitivity for this sensor is equal to Vdifference S = √ = 0.24 mV (m s−1 )−1/2 U or S Sn = = 6.84 mV (m s−1 )−1/2 W−1 . Pin Figure 10 shows the sensor output (thermopile voltage difference) as a function of flow for various powers supplied to this sensor in comparison with the one for the device using the porous silicon over air cavity micro-hotplate. As we
A monolithic thermal gas flow sensor was fabricated using a novel porous silicon–air cavity micro-hotplate. The thermal isolation achieved was measured and the obtained values agree with the predicted ones through simulations. The sensor was measured both without flow and under nitrogen gas flow. All the results obtained were compared with those acquired from an identical thermal sensor fabricated employing the conventional porous silicon micro-hotplate technology for thermal isolation and the improvement in sensor characteristics achieved by the air cavity underneath porous silicon was evaluated.
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