- Email: [email protected]
Fabrication of planar thermocouples for real-time measurements of temperature profiles in polymer melts
Sensors and Actuators A 58 (1997) 179–184
Fabrication of planar thermocouples for real-time measurements of temperature profiles in polymer melts D. Debey a,U, R. Bluhm b, N. Habets b, H. Kurz a a
Institute of Semiconductors II, Technical University of Aachen, Sommerfeldstraße 24, D-52074 Aachen, Germany b Institute of Plastic Processing, Technical University of Aachen, Aachen, Germany Received 24 July 1996; revised 7 January 1997; accepted 14 January 1997
Abstract Thin-film thermocouples are promising candidates for real-time temperature-profile measurements with high spatial resolution. They are interesting for mass production because the production costs of these elements are rather low. Miniaturized thermocouple elements with 10 measuring points per 2 mm have been fabricated by standard photolithography and etch technology. Sensors with different metal combinations on various substrates are tested with respect to their sensitivity and speed. The best sensitivity is achieved with an Au/Al metallization on polyimide substrates. The time constant of the thermocouple response is 0.15 s on polyimide and 0.5 s on ceramic substrates. These thermocouples are applied to investigate the cooling processes of plastic materials within the cavity of an injection mould. Keywords: Miniaturization; Planar thermocouples; Spatial resolution; Response time
1. Introduction Thin-film thermocouples have been developed to measure temperature distributions of polymer melts in the cavity of an injection mould. The measurement principle is based on the Seebeck effect. Already in 1821 Seebeck discovered the so-called ‘thermal voltage’ in the contact area of two different metals [1]. Over a wide range of temperatures, the voltage between two different metal pairs increases linearly with the temperature. The Seebeck effect allows an easy and accurate temperature measurement to be made. For the determination of temperature distributions in an injection mould, temperature sensors with a high spatial resolution (several measuring points per millimetre) and a short response time are necessary. For this application, planar arrays of integrated thinfilm thermocouples have been developed. The selection of appropriate metal pairs and the final cheap and reliable fabrication process for arrays of micron-scale thermocouples are described. 2. Determination of temperature distributions in polymer melts In injection moulding a hot plastic melt is injected into a cold female mould. In the mould the material cools down U
Corresponding author. Tel.: q49 241 807 793. Fax: q49 241 888 246. E-mail: [email protected]
very quickly. Cooling rates of about 20 K sy1 are achieved in the centre of the melts, whereas the rates of the marginal layers can be as high as 1000 K sy1. These extreme cooling conditions are desirable from an economic perspective, allowing reduction of production costs by shorter cycle times. On the other hand, rapid and inhomogeneous cooling strongly influences the properties of the moulding, e.g., by creating high residual stress or modifying specific properties of the plastic (molecular orientation, formation of crystalline structures with specific size and shape). These conditions lead to a varying morphology over the cross section of the moulded part (e.g., very small crystallites in the boundary layer and big ones in the centre). Therefore, knowledge of the temperature distribution in the mould cavity is of great importance scientifically as well as economically. Fig. 1 shows the influence of inhomogeneous cooling velocities and shear forces on the morphology of polypropylene. For process prediction and optimization, simulation programs of the process have been developed. Detailed temperature distributions are calculated by these programs. In order to improve the reliability of these calculations, a process and materials data base relevant for the production process is necessary. The measurement of material data during the process is very important, because the material characteristics will be largely influenced by the morphology (Fig. 1) [2]. As the moulded plastic parts usually possess a wall thickness of 1–5 mm and because of the cooling velocity effect as
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3. General design conditions for sensors based on the Seebeck effect
Fig. 1. Different crystalline phases of polypropylene melt due to inhomogeneous temperatures during cooling behaviour.
explained above, temperature sensors have tofollowing requirements: Small size High spatial resolution (several measuring points per mm) Short response time Mechanical stiffness to withstand the conditions of the injection process. (Due to the high viscosity of polymer melts, the pressure required inside the cavity is several 100 bar). For monitoring of mould temperatures, infrared sensors are increasingly employed today. Such measuring systems detect the heat radiation (infrared spectral range) emitted by the plastic melt. The response time of the sensors is almost exclusively determined by the electronic data processing and is typically about 10 ms. The main disadvantage of infrared sensors is their limited spatial resolution. Due to excellent transmission properties of most plastics in the monitored infrared range, the thermal radiation is taken from a rather large volume in front of the sensor. Therefore the measured temperature values are average values of the sampled volume [3,4].
For measurements with high spatial resolution, thermocouple elements are considered to be appropriate because of their local measuring principle. To meet the extreme requirements in injection moulding, we developed arrays of miniaturized thermocouple elements for measurements at five measuring points per millimetre. In the following, the layout of the sensor array will be presented. The geometry of the sensors and their integration into the mould is depicted in Fig. 2. The sensor is designed for a 2 mm thick plate moulding and provides 10 measuring points. As the temperature along the cross section of the moulding (y-coordinate in Fig. 2) is to be determined, the sensor is mounted vertically to the cavity surface. The injected melt flows around the sensor. The thermal voltage can be measured at contact pads outside of the cavity. In order to guarantee sufficient mechanical stability of the sensor, the thermocouple elements are fabricated by thin-film technology on a solid substrate. The substrate must fulfil several requirements. First, the necessary mechanical stiffness and strength must be guaranteed. Secondly, the heat capacity of the sensor must be low to improve the response time of the sensors. A further important property for the substrate material is a low heat conductivity to prevent a quick heat exchange across the substrate, which would result in an artificial thermal equilibration between the measuring points. Additionally, sufficient thermal stability up to 3008C is needed. Various substrate materials with low thermal conductivity have been tested: polyimide (125 mm thick), zirconium oxide (500 mm thick) and boron nitride (300 mm thick), respectively. The low thermal conductivity results in good thermal isolation and guarantees high spatial resolution in temperature measurements. We have chosen polyimide substrate for the first tests. Its mechanical stability is sufficiently good to withstand the high pressure of the polymer melt and
Fig. 2. Temperature sensor integrated in the injection mould.
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Fig. 3. Sensor on polyimide foil with Au/Al contacts.
its thermal conductivity is low enough for high-resolution measurements of temperature profiles over the cross section of the mould. For the second test series, we have chosen ceramic substrates because of their better mechanical stability, but their thermal conductivity is higher than that of polyimide (polyimide, 0.17 W mKy1; zirconium oxide, 1.4 W mKy1; boron nitride, 25 W mKy1). Fig. 3 shows a sensor fabricated on a polyimide substrate. The sensor dimensions are 19 mm=11.5 mm. The Figure depicts the complete sensor including the measuring points (lower right corner) and contact pads (left side). For the contact between external wires and the sensor, small foil plugs with 20 junction points are used. The metal structures are 50 mm wide in the measuring area and 200 mm wide in the contact area. The fabrication of these structures will be explained in the following section.
4. Fabrication The fabrication process of Al/Au sensors starts with the preparation of the polyimide foil. It is cleaned in acid solution and dried in nitrogen atmosphere at 1008C. After the etch treatment, the polyimide has a roughness of the order of 70 nm, as illustrated by the atomic force micrograph of Fig. 4. The Au/Al metallization is now evaporated onto the foil. The thickness of the metal films is chosen to be 300 nm. At this
Fig. 4. Atomic force microscope analysis of a cleaned polyimide foil.
181
thickness, the substrate topography is fully covered and conductor lines without interruption are obtained. The gold layer is deposited by electron-beam gun evaporation at a pressure ˚ sy1. As a of 5=10y6 torr, with an evaporation rate of 3 A sticking layer, a thin 10 nm chromium layer is evaporated in situ. The metal film is patterned by optical lithography with a positive photoresist [5]. The resist is exposed at a wavelength of 365 nm with a flux of 100 mJ cmy2. The photoresist structure is then transferred into the gold layer either by dry or wet etching. Either argon-ion sputtering at an energy of 750 eV [6] or wet etching with a mixture of KIqI solution for the gold layer and Ce(NH4)2(NO3)qCH3COOH solution for the chromium layer is performed [7]. After removal of the photoresist either in an O2 plasma [8] or with hot acetone at 508C, aluminium is deposited by electron-beam gun evaporation at 5=10y6 torr with an evaporation rate of ˚ sy1. The Al sensor strip is again defined by optical 4 A lithography; pattern transfer from the resist into the Al layer is achieved by wet chemical etching with an HNO3/H3PO4/ H2O mixture. The Au lines are resistant against this etch solution. Finally, the photoresist is removed by O2 plasma.
5. Calibration of the sensors in an oil bath As the miniaturized sensors are not standard thermocouple elements, it is necessary to calibrate them before use. The calibration also allows the determination of the influence of the miniaturization and the influence of the chromium sticking layers between substrate and conduction lines on the thermal voltage. The calibration has been performed in a sealed oil bath (Fig. 5), heated with a hot plate. Its temperature has been measured with a commercial Ni–NiCr thermocouple element. The calibration has been performed for each of the thermocouples on the substrate individually and for different metal combinations. The contact between sensor and transducer of the measurement equipment is realized with a foil plug and by copper cables. Therefore, inside the plug a second thermovoltage occurs. To take this into account, the temperature of the plug has to be measured additionally in order to create a correct reference point for the temperature scale.
Fig. 5. Sensor calibration in an oil bath.
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Fig. 6. Thermocouples with different metal combinations on polyimide foil.
Fig. 7. Time response of Au/Al temperature sensor on different substrates.
Table 1 Thermovoltage of different metal couples (combinations) on polyimide foils
It is mainly determined by the heat capacity and the heat conductivity of the substrate. The heat capacity is determined by the mass to be heated and thus by the size and thickness of the substrate material. The step response of the sensors has been measured in the oil bath following a sudden change of the temperature. The measurements have been performed in the following way: after preheating of the oil bath to 2008C, the sensor at room temperature has been dipped quickly into the bath. The time evolution of the Seebeck voltage has then been recorded. Fig. 7 shows the time response of the Au/Al sensors for different substrates. In theory, the temperature rise should be described by an exponential function qsq0qD q[1yexp(yt/t)] 1, which is confirmed for the thermocouple on boron nitride. The quickest response time could be observed for the polyimide sensor due to its smaller thickness. After a fast response at the beginning (rise time: 0.15 s) leading up to 60% of the final temperature, the response of the sensor becomes much slower. This is explained by a change of the heat-transfer coefficient between the oil and the sensor, caused by small air bubbles brought into the oil during immersion of the sensor. The curve shows clearly the importance of a high heat coefficient between the sensor and the object to be measured. Sensors which have been reinforced by a steel spring for mechanical stabilization warm up particularly slowly. This is shown by measurements on a sensor on boron nitride. The rise time of the temperature is found to be 1.1 s as compared to 0.5 s for the sensor without mechanical stiffness (Fig. 7). Their response behaviour is too slow to analyse the injection-moulding process. Another problem arising with reinforced polyimide as a substrate is the heat transfer between the measurement points and the plug. Because of the high heat conductivity of the metal, the end temperature in the oil bath could not be measured (Fig. 7). An improvement of the response behaviour for substrates with a higher mechanical stiffness could only be reached by reducing the substrate thickness in a small area around the measurement points.
Metal configuration
Uth [mV Ky1] (literature)
Uth [mV Ky1] (measured)
Au/Ni Pt/Ni Au/Al Au/Sn
20.5 14.5 3.3 2.7
21.9 9.3 3.13 5.8
Fig. 6 shows the measured thermal voltages versus the temperature difference between the plug and the bath temperature. The data have been fitted with a linear regression procedure. The good agreement between the data points and the regression line confirms a linear dependence between the thermal voltage and the temperature over the whole temperature range of the measurements. For the material combinations (Au/Al, Au/Sn) with low thermal voltage, a temperature difference between 10 and 15 K is necessary to measure a voltage. This is explained by the electrical resistance of the whole measurement equipment (resistance of the plugs and the electrical leads). Table 1 lists the Seebeck coefficients obtained by the regression fits. The coefficients are compared to theoretical values taken from Refs. [1,9]. A good agreement can be found for Au/Ni and Au/Al pairs. For the Pt/Ni thermocouple, the measured values are one third lower than those found in the literature because a 3 nm Ti sticking layer has been used. In the case of the metal combination Au/Sn we have used a sticking layer of 10 nm Ni and also a protection layer of 10 nm Ni on top as a barrier against oxygen and moisture. For this reason, the measured thermovoltage value is different from the theoretical one. In general it has been found that the thermal voltage depends significantly on the sticking/protection layer. The miniaturization effect (reduced geometrical dimensions of the sensor structures) is negligible.
6. Response time of the new sensors For the time-resolved application of the sensors in injection moulding, the temporal response behaviour has to be known.
1 This behaviour has been observed for most of the sensors. Deviations are explained by the limited speed of the heat transfer in the oil bath itself and by changes of the thermal coupling between sensor and oil bath.
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8. Conclusions A technology for realizing miniaturized thermocouple elements consisting of different metal pairs (Au/Al, Au/Ni, Pt/Ni, Au/Sn) and substrates (polyimide, boron nitride, zirconium oxide) has been presented. With photolithographic techniques, it is possible to realize a spatial resolution down to five measuring points per millimetre, which is necessary to measure temperature profiles inside a cavity during the injection moulding of plastic. We have demonstrated application of sensor arrays for the temporal profiling of the temperature distribution in an injection mould for polymer processing. With the help of the measured temperature profiles, it is possible to calculate material parameters like the thermal diffusivity more accurately. These data help to improve temperature calculations with modern simulation programs like CADMOULD [10].
Acknowledgements Fig. 8. Time response for two measured points during cooling behaviour.
7. Measurements in the injection mould Fig. 8 shows the first results of an Au/Al sensor in an injection mould when the polymer is injected into the cavity. A polymer melt (polypropylene) with a temperature of 2808C has been injected into a mould with a surface temperature of 308C. In the mould, the plastic cools down very rapidly due to efficient heat transfer between the plastic and the mould. In the diagram, the measured temperature evolution for two different measuring points is displayed. After the plastic melt has reached the sensor in the injection phase, the temperature rises rapidly. At this point, a clearly improved response time is found in comparison to the oil-bath tests. The maximum temperature value is measured after 0.25 s, and the 1/e rise time is determined to be 0.1 s. The measured peak value of 2608C corresponds closely to the expected value as the polymer is injected at a temperature of 2808C. This allows us to argue that the rise time of 0.1 s represents an upper limit for the response time of the sensor for the conditions of the moulding injection. The faster response compared to the oil-bath tests is due to a faster heat transfer between plastic melt and sensor because of the high pressure inside the cavity. After the cavity has been filled completely, the plastic cools down in the cold mould. This happens at different sensor positions with a varying cooling velocity, according to the distance of the measuring point from the walls. At the end of the injection phase, the temperature in the boundary layer is slightly higher compared to the centre of the mould cavity. This is caused by the high shear stress near the surface of the cavity during the injection phase, which leads to additional heating. In the cooling phase, the boundary layer shows a higher cooling velocity, because of the close proximity to the cold mould.
This research program was financed by the AiF (Arbeitsgemeinschaft industrieller Forschungsvereinigungen). We thank the AiF and the BmWi (German Federal Ministry of Economy) for their support.
References [1] L.V. Ko¨rtve´lyessy, Thermospannung, Thermoelement Praxis, VulkanVerlag, Essen, 1987, pp. 16–28. [2] R. Bluhm, Improved temperature control in the injection moulding process, Ph.D. Thesis, Technical University of Aachen, Augustiner Buchhandlung-Verlag, Aachen (1996), pp. 37–53 fs. [3] R. Bluhm and W. Michaeli, Measuring Temperature in the Mould Cavity, Vol. 10, British Plastics and Rubber, Caterham, UK, 1994, 5–9. [4] K. Obendrauf, C. Kukla and G.R. Langecker, Schnelle Temperaturmessung mit IR-Fu¨hlern, Kunststoffe, Carl Hansa-Verlag, Munich, ED 83, December 1993, pp. 971–974. [5] G. DeWitt Ong, Modern MOS Technology, Photolithography, McGraw-Hill, Singapore, 1986, Ch. 8, p. 175. [6] C.J. Mogab and S.M. Sze, Dry Etching, VLSI Technology, McGrawHill, Singapore, 1983, Ch. 8, p. 319. [7] J.L. Vossen and W. Kern, Thin Film Processes, Academic Press, New York, 1978, Table 14, p. 465. [8] B. Meusemann, Reactive sputter etching and reactive ion milling — selectivity, dimensional control and reduction of MOS–interface degradation, J. Vac. Sci. Technol., 16 (1979) 1886–1891. [9] H. Franke, Thermoelektrizitaet, Lexikon der Physik, Vol. 3, Franckh’sche Verlagshandlung, Stuttgart, 1969, pp. 1685–1687. [10] CADMOULD — Software for the Simulation of the Injection Moulding Process, User Manual, Institute of Plastic Processing, Technical University of Aachen, 1995.
Biographies Denise Debey was born in Eupen, Belgium. She works as a technical assistant at the Institute of Semiconductor Tech-
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nology II of the Technical University of Aachen. She developed the technology of the presented sensors. Ru¨diger Bluhm was born in Bad Oldesloe, Germany. He studied mechanical engineering at the Technical University of Aachen. From 1991 to 1996 he worked as a research assistant at the Institute of Plastic Processing (IKV) in the injection-moulding department. He finished his Ph.D. thesis on improved temperature control during the injection moulding process in May 1996. Since June 1996, he has been working at BASF in Ludwigshafen. Norbertus J.M. Habets was born in Maastricht, The Netherlands. He is a student at the Technical University of Aachen in solid-state electronics. He joined the group for sample preparation.
Heinrich Kurz was born in Austria in 1943, and received his Ph.D. at the University of Vienna in 1971. From 1971 to 1980 he was scientific staff member of Philips Research Laboratories in Hamburg, Germany. There, he was engaged in research on optical storage and data processing. He joined the group of N. Bloembergen at Harvard University as research associate from 1981 to 1984. During this time, his scientific interest was directed to the interaction of ultrashort laser pulses with semiconductors. Since 1985 he has been professor of electrical engineering at the Technical University of Aachen. Since that time his research has included basic considerations of novel concepts for ultrafast switching processes at ultrasmall dimensions. The two main topics are femtosecond laser processes and nanoelectronic devices.
Journal: SNA (Sensors and Actuators A)
Article: 1470
Fabrication of planar thermocouples for real-time measurements of temperature profiles in polymer melts D. Debey a,U, R. Bluhm b, N. Habets b, H. Kurz a a
Institute of Semiconductors II, Technical University of Aachen, Sommerfeldstraße 24, D-52074 Aachen, Germany b Institute of Plastic Processing, Technical University of Aachen, Aachen, Germany Received 24 July 1996; revised 7 January 1997; accepted 14 January 1997
Abstract Thin-film thermocouples are promising candidates for real-time temperature-profile measurements with high spatial resolution. They are interesting for mass production because the production costs of these elements are rather low. Miniaturized thermocouple elements with 10 measuring points per 2 mm have been fabricated by standard photolithography and etch technology. Sensors with different metal combinations on various substrates are tested with respect to their sensitivity and speed. The best sensitivity is achieved with an Au/Al metallization on polyimide substrates. The time constant of the thermocouple response is 0.15 s on polyimide and 0.5 s on ceramic substrates. These thermocouples are applied to investigate the cooling processes of plastic materials within the cavity of an injection mould. Keywords: Miniaturization; Planar thermocouples; Spatial resolution; Response time
1. Introduction Thin-film thermocouples have been developed to measure temperature distributions of polymer melts in the cavity of an injection mould. The measurement principle is based on the Seebeck effect. Already in 1821 Seebeck discovered the so-called ‘thermal voltage’ in the contact area of two different metals [1]. Over a wide range of temperatures, the voltage between two different metal pairs increases linearly with the temperature. The Seebeck effect allows an easy and accurate temperature measurement to be made. For the determination of temperature distributions in an injection mould, temperature sensors with a high spatial resolution (several measuring points per millimetre) and a short response time are necessary. For this application, planar arrays of integrated thinfilm thermocouples have been developed. The selection of appropriate metal pairs and the final cheap and reliable fabrication process for arrays of micron-scale thermocouples are described. 2. Determination of temperature distributions in polymer melts In injection moulding a hot plastic melt is injected into a cold female mould. In the mould the material cools down U
Corresponding author. Tel.: q49 241 807 793. Fax: q49 241 888 246. E-mail: [email protected]
very quickly. Cooling rates of about 20 K sy1 are achieved in the centre of the melts, whereas the rates of the marginal layers can be as high as 1000 K sy1. These extreme cooling conditions are desirable from an economic perspective, allowing reduction of production costs by shorter cycle times. On the other hand, rapid and inhomogeneous cooling strongly influences the properties of the moulding, e.g., by creating high residual stress or modifying specific properties of the plastic (molecular orientation, formation of crystalline structures with specific size and shape). These conditions lead to a varying morphology over the cross section of the moulded part (e.g., very small crystallites in the boundary layer and big ones in the centre). Therefore, knowledge of the temperature distribution in the mould cavity is of great importance scientifically as well as economically. Fig. 1 shows the influence of inhomogeneous cooling velocities and shear forces on the morphology of polypropylene. For process prediction and optimization, simulation programs of the process have been developed. Detailed temperature distributions are calculated by these programs. In order to improve the reliability of these calculations, a process and materials data base relevant for the production process is necessary. The measurement of material data during the process is very important, because the material characteristics will be largely influenced by the morphology (Fig. 1) [2]. As the moulded plastic parts usually possess a wall thickness of 1–5 mm and because of the cooling velocity effect as
0924-4247/97/$17.00 q 1997 Elsevier Science S.A. All rights reserved PII S 0 9 2 4 - 4 2 4 7 ( 9 7 ) 0 1 3 8 9 - 7
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3. General design conditions for sensors based on the Seebeck effect
Fig. 1. Different crystalline phases of polypropylene melt due to inhomogeneous temperatures during cooling behaviour.
explained above, temperature sensors have tofollowing requirements: Small size High spatial resolution (several measuring points per mm) Short response time Mechanical stiffness to withstand the conditions of the injection process. (Due to the high viscosity of polymer melts, the pressure required inside the cavity is several 100 bar). For monitoring of mould temperatures, infrared sensors are increasingly employed today. Such measuring systems detect the heat radiation (infrared spectral range) emitted by the plastic melt. The response time of the sensors is almost exclusively determined by the electronic data processing and is typically about 10 ms. The main disadvantage of infrared sensors is their limited spatial resolution. Due to excellent transmission properties of most plastics in the monitored infrared range, the thermal radiation is taken from a rather large volume in front of the sensor. Therefore the measured temperature values are average values of the sampled volume [3,4].
For measurements with high spatial resolution, thermocouple elements are considered to be appropriate because of their local measuring principle. To meet the extreme requirements in injection moulding, we developed arrays of miniaturized thermocouple elements for measurements at five measuring points per millimetre. In the following, the layout of the sensor array will be presented. The geometry of the sensors and their integration into the mould is depicted in Fig. 2. The sensor is designed for a 2 mm thick plate moulding and provides 10 measuring points. As the temperature along the cross section of the moulding (y-coordinate in Fig. 2) is to be determined, the sensor is mounted vertically to the cavity surface. The injected melt flows around the sensor. The thermal voltage can be measured at contact pads outside of the cavity. In order to guarantee sufficient mechanical stability of the sensor, the thermocouple elements are fabricated by thin-film technology on a solid substrate. The substrate must fulfil several requirements. First, the necessary mechanical stiffness and strength must be guaranteed. Secondly, the heat capacity of the sensor must be low to improve the response time of the sensors. A further important property for the substrate material is a low heat conductivity to prevent a quick heat exchange across the substrate, which would result in an artificial thermal equilibration between the measuring points. Additionally, sufficient thermal stability up to 3008C is needed. Various substrate materials with low thermal conductivity have been tested: polyimide (125 mm thick), zirconium oxide (500 mm thick) and boron nitride (300 mm thick), respectively. The low thermal conductivity results in good thermal isolation and guarantees high spatial resolution in temperature measurements. We have chosen polyimide substrate for the first tests. Its mechanical stability is sufficiently good to withstand the high pressure of the polymer melt and
Fig. 2. Temperature sensor integrated in the injection mould.
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Fig. 3. Sensor on polyimide foil with Au/Al contacts.
its thermal conductivity is low enough for high-resolution measurements of temperature profiles over the cross section of the mould. For the second test series, we have chosen ceramic substrates because of their better mechanical stability, but their thermal conductivity is higher than that of polyimide (polyimide, 0.17 W mKy1; zirconium oxide, 1.4 W mKy1; boron nitride, 25 W mKy1). Fig. 3 shows a sensor fabricated on a polyimide substrate. The sensor dimensions are 19 mm=11.5 mm. The Figure depicts the complete sensor including the measuring points (lower right corner) and contact pads (left side). For the contact between external wires and the sensor, small foil plugs with 20 junction points are used. The metal structures are 50 mm wide in the measuring area and 200 mm wide in the contact area. The fabrication of these structures will be explained in the following section.
4. Fabrication The fabrication process of Al/Au sensors starts with the preparation of the polyimide foil. It is cleaned in acid solution and dried in nitrogen atmosphere at 1008C. After the etch treatment, the polyimide has a roughness of the order of 70 nm, as illustrated by the atomic force micrograph of Fig. 4. The Au/Al metallization is now evaporated onto the foil. The thickness of the metal films is chosen to be 300 nm. At this
Fig. 4. Atomic force microscope analysis of a cleaned polyimide foil.
181
thickness, the substrate topography is fully covered and conductor lines without interruption are obtained. The gold layer is deposited by electron-beam gun evaporation at a pressure ˚ sy1. As a of 5=10y6 torr, with an evaporation rate of 3 A sticking layer, a thin 10 nm chromium layer is evaporated in situ. The metal film is patterned by optical lithography with a positive photoresist [5]. The resist is exposed at a wavelength of 365 nm with a flux of 100 mJ cmy2. The photoresist structure is then transferred into the gold layer either by dry or wet etching. Either argon-ion sputtering at an energy of 750 eV [6] or wet etching with a mixture of KIqI solution for the gold layer and Ce(NH4)2(NO3)qCH3COOH solution for the chromium layer is performed [7]. After removal of the photoresist either in an O2 plasma [8] or with hot acetone at 508C, aluminium is deposited by electron-beam gun evaporation at 5=10y6 torr with an evaporation rate of ˚ sy1. The Al sensor strip is again defined by optical 4 A lithography; pattern transfer from the resist into the Al layer is achieved by wet chemical etching with an HNO3/H3PO4/ H2O mixture. The Au lines are resistant against this etch solution. Finally, the photoresist is removed by O2 plasma.
5. Calibration of the sensors in an oil bath As the miniaturized sensors are not standard thermocouple elements, it is necessary to calibrate them before use. The calibration also allows the determination of the influence of the miniaturization and the influence of the chromium sticking layers between substrate and conduction lines on the thermal voltage. The calibration has been performed in a sealed oil bath (Fig. 5), heated with a hot plate. Its temperature has been measured with a commercial Ni–NiCr thermocouple element. The calibration has been performed for each of the thermocouples on the substrate individually and for different metal combinations. The contact between sensor and transducer of the measurement equipment is realized with a foil plug and by copper cables. Therefore, inside the plug a second thermovoltage occurs. To take this into account, the temperature of the plug has to be measured additionally in order to create a correct reference point for the temperature scale.
Fig. 5. Sensor calibration in an oil bath.
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Fig. 6. Thermocouples with different metal combinations on polyimide foil.
Fig. 7. Time response of Au/Al temperature sensor on different substrates.
Table 1 Thermovoltage of different metal couples (combinations) on polyimide foils
It is mainly determined by the heat capacity and the heat conductivity of the substrate. The heat capacity is determined by the mass to be heated and thus by the size and thickness of the substrate material. The step response of the sensors has been measured in the oil bath following a sudden change of the temperature. The measurements have been performed in the following way: after preheating of the oil bath to 2008C, the sensor at room temperature has been dipped quickly into the bath. The time evolution of the Seebeck voltage has then been recorded. Fig. 7 shows the time response of the Au/Al sensors for different substrates. In theory, the temperature rise should be described by an exponential function qsq0qD q[1yexp(yt/t)] 1, which is confirmed for the thermocouple on boron nitride. The quickest response time could be observed for the polyimide sensor due to its smaller thickness. After a fast response at the beginning (rise time: 0.15 s) leading up to 60% of the final temperature, the response of the sensor becomes much slower. This is explained by a change of the heat-transfer coefficient between the oil and the sensor, caused by small air bubbles brought into the oil during immersion of the sensor. The curve shows clearly the importance of a high heat coefficient between the sensor and the object to be measured. Sensors which have been reinforced by a steel spring for mechanical stabilization warm up particularly slowly. This is shown by measurements on a sensor on boron nitride. The rise time of the temperature is found to be 1.1 s as compared to 0.5 s for the sensor without mechanical stiffness (Fig. 7). Their response behaviour is too slow to analyse the injection-moulding process. Another problem arising with reinforced polyimide as a substrate is the heat transfer between the measurement points and the plug. Because of the high heat conductivity of the metal, the end temperature in the oil bath could not be measured (Fig. 7). An improvement of the response behaviour for substrates with a higher mechanical stiffness could only be reached by reducing the substrate thickness in a small area around the measurement points.
Metal configuration
Uth [mV Ky1] (literature)
Uth [mV Ky1] (measured)
Au/Ni Pt/Ni Au/Al Au/Sn
20.5 14.5 3.3 2.7
21.9 9.3 3.13 5.8
Fig. 6 shows the measured thermal voltages versus the temperature difference between the plug and the bath temperature. The data have been fitted with a linear regression procedure. The good agreement between the data points and the regression line confirms a linear dependence between the thermal voltage and the temperature over the whole temperature range of the measurements. For the material combinations (Au/Al, Au/Sn) with low thermal voltage, a temperature difference between 10 and 15 K is necessary to measure a voltage. This is explained by the electrical resistance of the whole measurement equipment (resistance of the plugs and the electrical leads). Table 1 lists the Seebeck coefficients obtained by the regression fits. The coefficients are compared to theoretical values taken from Refs. [1,9]. A good agreement can be found for Au/Ni and Au/Al pairs. For the Pt/Ni thermocouple, the measured values are one third lower than those found in the literature because a 3 nm Ti sticking layer has been used. In the case of the metal combination Au/Sn we have used a sticking layer of 10 nm Ni and also a protection layer of 10 nm Ni on top as a barrier against oxygen and moisture. For this reason, the measured thermovoltage value is different from the theoretical one. In general it has been found that the thermal voltage depends significantly on the sticking/protection layer. The miniaturization effect (reduced geometrical dimensions of the sensor structures) is negligible.
6. Response time of the new sensors For the time-resolved application of the sensors in injection moulding, the temporal response behaviour has to be known.
1 This behaviour has been observed for most of the sensors. Deviations are explained by the limited speed of the heat transfer in the oil bath itself and by changes of the thermal coupling between sensor and oil bath.
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8. Conclusions A technology for realizing miniaturized thermocouple elements consisting of different metal pairs (Au/Al, Au/Ni, Pt/Ni, Au/Sn) and substrates (polyimide, boron nitride, zirconium oxide) has been presented. With photolithographic techniques, it is possible to realize a spatial resolution down to five measuring points per millimetre, which is necessary to measure temperature profiles inside a cavity during the injection moulding of plastic. We have demonstrated application of sensor arrays for the temporal profiling of the temperature distribution in an injection mould for polymer processing. With the help of the measured temperature profiles, it is possible to calculate material parameters like the thermal diffusivity more accurately. These data help to improve temperature calculations with modern simulation programs like CADMOULD [10].
Acknowledgements Fig. 8. Time response for two measured points during cooling behaviour.
7. Measurements in the injection mould Fig. 8 shows the first results of an Au/Al sensor in an injection mould when the polymer is injected into the cavity. A polymer melt (polypropylene) with a temperature of 2808C has been injected into a mould with a surface temperature of 308C. In the mould, the plastic cools down very rapidly due to efficient heat transfer between the plastic and the mould. In the diagram, the measured temperature evolution for two different measuring points is displayed. After the plastic melt has reached the sensor in the injection phase, the temperature rises rapidly. At this point, a clearly improved response time is found in comparison to the oil-bath tests. The maximum temperature value is measured after 0.25 s, and the 1/e rise time is determined to be 0.1 s. The measured peak value of 2608C corresponds closely to the expected value as the polymer is injected at a temperature of 2808C. This allows us to argue that the rise time of 0.1 s represents an upper limit for the response time of the sensor for the conditions of the moulding injection. The faster response compared to the oil-bath tests is due to a faster heat transfer between plastic melt and sensor because of the high pressure inside the cavity. After the cavity has been filled completely, the plastic cools down in the cold mould. This happens at different sensor positions with a varying cooling velocity, according to the distance of the measuring point from the walls. At the end of the injection phase, the temperature in the boundary layer is slightly higher compared to the centre of the mould cavity. This is caused by the high shear stress near the surface of the cavity during the injection phase, which leads to additional heating. In the cooling phase, the boundary layer shows a higher cooling velocity, because of the close proximity to the cold mould.
This research program was financed by the AiF (Arbeitsgemeinschaft industrieller Forschungsvereinigungen). We thank the AiF and the BmWi (German Federal Ministry of Economy) for their support.
References [1] L.V. Ko¨rtve´lyessy, Thermospannung, Thermoelement Praxis, VulkanVerlag, Essen, 1987, pp. 16–28. [2] R. Bluhm, Improved temperature control in the injection moulding process, Ph.D. Thesis, Technical University of Aachen, Augustiner Buchhandlung-Verlag, Aachen (1996), pp. 37–53 fs. [3] R. Bluhm and W. Michaeli, Measuring Temperature in the Mould Cavity, Vol. 10, British Plastics and Rubber, Caterham, UK, 1994, 5–9. [4] K. Obendrauf, C. Kukla and G.R. Langecker, Schnelle Temperaturmessung mit IR-Fu¨hlern, Kunststoffe, Carl Hansa-Verlag, Munich, ED 83, December 1993, pp. 971–974. [5] G. DeWitt Ong, Modern MOS Technology, Photolithography, McGraw-Hill, Singapore, 1986, Ch. 8, p. 175. [6] C.J. Mogab and S.M. Sze, Dry Etching, VLSI Technology, McGrawHill, Singapore, 1983, Ch. 8, p. 319. [7] J.L. Vossen and W. Kern, Thin Film Processes, Academic Press, New York, 1978, Table 14, p. 465. [8] B. Meusemann, Reactive sputter etching and reactive ion milling — selectivity, dimensional control and reduction of MOS–interface degradation, J. Vac. Sci. Technol., 16 (1979) 1886–1891. [9] H. Franke, Thermoelektrizitaet, Lexikon der Physik, Vol. 3, Franckh’sche Verlagshandlung, Stuttgart, 1969, pp. 1685–1687. [10] CADMOULD — Software for the Simulation of the Injection Moulding Process, User Manual, Institute of Plastic Processing, Technical University of Aachen, 1995.
Biographies Denise Debey was born in Eupen, Belgium. She works as a technical assistant at the Institute of Semiconductor Tech-
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nology II of the Technical University of Aachen. She developed the technology of the presented sensors. Ru¨diger Bluhm was born in Bad Oldesloe, Germany. He studied mechanical engineering at the Technical University of Aachen. From 1991 to 1996 he worked as a research assistant at the Institute of Plastic Processing (IKV) in the injection-moulding department. He finished his Ph.D. thesis on improved temperature control during the injection moulding process in May 1996. Since June 1996, he has been working at BASF in Ludwigshafen. Norbertus J.M. Habets was born in Maastricht, The Netherlands. He is a student at the Technical University of Aachen in solid-state electronics. He joined the group for sample preparation.
Heinrich Kurz was born in Austria in 1943, and received his Ph.D. at the University of Vienna in 1971. From 1971 to 1980 he was scientific staff member of Philips Research Laboratories in Hamburg, Germany. There, he was engaged in research on optical storage and data processing. He joined the group of N. Bloembergen at Harvard University as research associate from 1981 to 1984. During this time, his scientific interest was directed to the interaction of ultrashort laser pulses with semiconductors. Since 1985 he has been professor of electrical engineering at the Technical University of Aachen. Since that time his research has included basic considerations of novel concepts for ultrafast switching processes at ultrasmall dimensions. The two main topics are femtosecond laser processes and nanoelectronic devices.
Journal: SNA (Sensors and Actuators A)
Article: 1470