A temperature microsensor for biological investigations

Microelectronic Engineering 57–58 (2001) 787–792 www.elsevier.com / locate / mee

A temperature microsensor for biological investigations a, a b c d M. Zaborowski *, P. Grabiec , T. Gotszalk , E. Romanowska , I.W. Rangelow a

b

Institute of Electron Technology, Al. Lotnikow 32 /46, Warsaw, Poland Institute of Microsystem Technology, Wroclaw University of Technology, ul. Janiszewskiego 11 /17, Wroclaw, Poland c Department of Plant Physiology, Warsaw University, ul. Krakowskie Przedm. 26 /28, Warsaw, Poland d Institute of Technical Physics, University of Kassel, Heinrich-Plett-Str. 40, Kassel, Germany

Abstract A microprobe for contact temperature measurements is described. It consists of a silicon cantilever with a Ni thermoresistor deposited at the unfixed end. Heat capacity of the probe and mechanical stress in Si is discussed together with contact detection methods between the sensor and a plant tissue sample. A resistance versus temperature curve is presented.  2001 Elsevier Science B.V. All rights reserved. Keywords: Temperature sensor; Microsensor technology; Micromachining; Ni thermoresistor

1. Introduction Living plants need a continuous energy supply (light absorption, oxidation of organic compounds). The main source of the chemical energy used by cells of a living plant is adenosine triphosphate (ATP). Uncoupled reactions of the ATP synthesis cause a free energy release in form of heat. The amount of heat formed during these processes can be evaluated as changes of temperature, i.e. efficiency of the energy transformation. This topic is significant in investigations connected to nature environment protection. The purpose of this work was to develop a microsensor suitable for temperature measurements of botanical living tissues and cells. The temperature changes of 0.18C have been expected. A number of macroscopic probes have been used for the purpose of temperature measurements of small botanical samples. We used a thermistor probe in our first attempts, however this device showed several disadvantages as self-heating effect during long- and medium period D.C. measurements (Fig. 1) and a considerable high influence of the probe on the temperature of the measured object. Therefore a much smaller microsensor was developed and a suitable A.C. signal acquisition method was chosen. * Corresponding author. Tel.: 1 48-22-716-5992; fax: 1 48-22-716-5991. E-mail address: [email protected] (M. Zaborowski). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00490-7

 2001 Elsevier Science B.V. All rights reserved.

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M. Zaborowski et al. / Microelectronic Engineering 57 – 58 (2001) 787 – 792

Fig. 1. Temperature of a thermistor probe itself kept in air after switching on.

2. Physical considerations Our calculations have been focused on a silicon cantilever of 20 mm thickness with a small thermosensitive receptor placed at the unfastened end (similar to the AFM sensor [1]). The heat capacity of the cantilever as a function of its dimensions is shown in Fig. 2. The cantilever works as a heat drain in the case of temperature differences and therefore influences a measured sample. This influence can be estimated by a comparison of heat capacity of both: the probe and the sample. Since

Fig. 2. Heat capacity of 20 mm thick silicon cantilevers as a function of their width and length. The right-sided scale unit is a theoretical pea ( pisum sativum) leaf cell heat capacity, average cell dimensions are 45 3 45 3 70 mm 3 .


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Fig. 3. Maximum stress in a Si cantilever pressed at the unfixed end by a force equal to 0.01 N, directed perpendicularly to the surface.

detailed data of heat capacity of a pea leaf tissue are not available, we assume in first approximation that they are made of water — see right-sided scale in Fig. 2. A cantilever width of several tens of micrometers is small enough to measure the temperature of individual plant cells without significant influence on the sample temperature. Mechanical properties of the cantilevers generally become worse for geometrical dimensions good for low heat capacity. Calculated maximum stress values in Si created by a rather small force (0.01 N ¯ 1 G) acting perpendicularly to the beam surface are presented in Fig. 3. Relatively wide ( . 100 mm) or short cantilever should be applied in order to prevent the cantilever damage. We measured maximum forces expected during the measurements of plant leaf temperatures by means of a microprobe (Fig. 4). Scratching a surface of a sample with a metal wire of diameter of 200 mm produced forces up to 0.08 and 0.14 N, for parallel and perpendicular directions, respectively.

3. Sensor technology Temperature microsensors were manufactured according to the flow chart presented in Table 1. A cross-section of the chip is shown in Fig. 5. A sputtered nickel meander-shaped line has been applied as a thermosensitive resistor. The 150 mm wide cantilevers after freon plasma etching are presented in Fig. 6. The chips were assembled by glueing to flexible supports and by ultracompression bonding. Fig. 7 displays a typical thermoresistor characteristic. The signal from the thermosensor was processed outside of the chip. A touch of a botanical tissue by the cantilever could be detected in two ways. Firstly piezoresistors manufactured at the cantilever surface allowed for measurements of a force acting on the microbeam

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Fig. 4. Forces necessary to scratch a pea leaf top surface measured by means of a dynamometer in two directions with respect to the leaf axis of symmetry.

[2]. Secondly, direct galvanic connection of the sensor and the tissue could be detected. An electric resistance across a pea leaf was found to be of order of 10 MV in the case of a slight trace left by a probe in a top surface. Extremely soft touching without any visible signs produced a too high resistance (over 200 MV). Leafs with removed top layer cells, so-called epidermis, showed a repeatable resistance from 1 to 3 MV, which was not dependent on the pressing force.

4. Conclusions and summary A micro-dimensional temperature sensor was successfully developed. It fits properly to a family of physical value microsensors manufactured using Si micromachining technology. The sensor allows for precise contact measurements of the temperature of biological living tissues. The microsensor is small enough to measure a real temperature of individual cells of a pea leaf, which is the subject of our interest. The detection of a contact between the microprobe and a sample by means of piezoresistors Table 1 n 1 layer diffusion for contacts to the bulk Si substrate (n-type; k100l; r 5 3–5 Vcm) p 1 diffusion for contacts to p-type piezoresistors Boron implantation and diffusion for piezoresistors Silicon nitride masking for Si wet etching (back side) Deposition and definition of Ni thermoresistors Deposition and definition of metal connections (Al) Anisotropic wet etching of silicon (back side) Front side plasma etching of Si membrane and cutting out the final microprobe shape


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Fig. 5. Schematic cross-section of the microprobe chip with essential details.

works properly in the case of any sample. The additional direct galvanic detection method is useful for the investigation of leafs with removed epidermis. The microsensor chips have to be bonded to a flexible foil in order to minimise the pressing force between the cantilever and the sample. The

Fig. 6. Two versions of the thermoresistor sensor.

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Fig. 7. Relative resistance versus temperature for: (A) magnetron sputtered Ni thermoresistor; (B) evaporated Ni thermoresistor; (C) earlier thermistor probe.

temperature measurement should be carried out under an optical microscope (magnification: 20 3 to 100 3 ). Acknowledgements Authors wish to acknowledge Dr. P. Dumania for productive discussions. The work was partially supported by the State Committee for Scientific Research in Poland under Grant No: 8T10C 024 17. References [1] I.W. Rangelow, T. Gotszalk, P. Hudek, F. Shi, P.B. Grabiec, P. Dumania, Proc. SPIE 2881 Micromachining and Microfabrication Conf., Austin (TX), 1996, p. 14. [2] T. Gotszalk, N. Abedinov, W. Barth, E. Lorenz, P. Hudek, T. Debski, P. Janus, P. Grabiec, M. Zaborowski, Fabrication and Properties of Micromechanical Calorimeter with Piezoresistive Detection Sensor and Resistive Microheater, Conf. Micro- and Nano-Engineering 99, 23–25.09.1999, Rome.