Improvements of on-membrane method for thin film thermal conductivity and emissivity measurements

Sensors and Actuators A 117 (2005) 203–210

Improvements of on-membrane method for thin film thermal conductivity and emissivity measurements A. Jacquota,∗ , G. Chenb , H. Scherrera , A. Dauschera , B. Lenoira a

Laboratoire de Physique des Mat´eriaux, UMR CNRS-INPL-UHP 7556, Ecole Nationale Sup´erieure des Mines de Nancy, Parc de Saurupt, F-54042 Nancy, France b Mechanical Engineering Department, Massachusetts Institute of Technology, Cambridge, MA 02139, USA Received 23 February 2004; received in revised form 17 June 2004; accepted 18 June 2004 Available online 22 July 2004

Abstract A numerical simulation and various test-structure configurations were developed which enable the measurement of the in-plane thermal conductivity and emissivity of on-membrane thin films in a versatile and accurate manner. The simulation takes into account the twodimensional heat transfer in the membrane. Consequently, shorter membrane can be used and strained materials can be measured. The numerical simulation along with the new test-structures is used to understand the convective heat transfer at small scales and to quantify it. The measurement method is tested on various materials ranging from ceramics, metals to polycrystalline silicon. © 2004 Elsevier B.V. All rights reserved. Keywords: Thermoelectricity; Thermal conductivity; Emissivity; Thin films; Measurement

1. Introduction The thermal conductivity measurement of thin films has received particular attention in the recent years. This research endeavor is necessary to understand the heat conduction at very small scales especially in superlattices [1] and to feed realistic data into microdevices simulation tools [2]. Up to now, the most widely used measurement method is the so-called 3␻ method [3–5]. The 3␻ method is very accurate when measuring the thermal conductivity in the cross-plane direction of films but is far less accurate when the in-plane thermal conductivity is intended to be measured [6]. Nevertheless, the engineering of thermoelectric planar microgenerators, radiation thermopiles and microcoolers require in particular the measurement of the in-plane thermal conductivity of the materials the micro-devices are made of [2,7,8]. Furthermore, the thermal conductivity should be measured together with the emissivity and the heat transfer coefficient of convection because of the high surface to volume ratio of these devices. ∗

Corresponding author. Tel.: +33 3 83 58 41 70; fax: +33 3 83 57 97 94. E-mail address: [email protected] (A. Jacquot).

0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.06.013

Among the others measurement methods available, the method developed by V¨olklein and co-workers [9–11], called the V¨olklein method in the next paragraphs, is the most appropriate to obtain the in-plane thermal conductivity of thin films and the emissivity of very large membranes. Basically a thin metallic strip is prepared on the membrane, the strip being used as a heater and as a thermometer. Heat is generated by Joule effect using dc current. The change of resistance of the heater with the temperature is used to determine its temperature. The temperature rise is a function of the thermal conductivity of the membrane, the heating power and geometry of both the membrane and the heater. Nevertheless, this method requires the use of membranes with a large length-to-width ratio for the measurement results to be accurately interpreted mainly because of the interpretation tools used up to now. It would be of interest to extend the capability of this technique to small membrane sizes because it is for example difficult to make large membranes with strained materials. In this paper, a numerical simulation is presented, which enables the V¨olklein method to be more versatile and accurate. In particular, the effects of the finite membrane

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size and the heat losses through the metallic heater on the measurement accuracy are investigated. The capability of this technique to measure the heat transfer coefficient of convection at small scales is also investigated experimentally and the results interpreted by a convective to diffusive heat transfer regime. New test-structures configurations are described and used to measure the thermal conductivity and emissivity of SiNx /SiO2 /SiNx and Cr/Au/Cr multilayers as well as the thermal conductivity and emissivity of undoped, boron-doped and phosphorous-doped polycrystalline silicon films. The study of the dielectric multilayer has been motivated to test the accuracy of our measurement and our data reduction schemes since this composite has been already measured by V¨olklein [10]. The thermal conductivity of the metallic multilayer was measured to check the validity of the Wiedemann–Franz law since the thermal conductivity of metallic micro-heaters has to be known in the measurement method of V¨olklein. Polycrystalline silicon was investigated because this material is widely used in thermopiles. It is especially well suited to figure out to which extend the thermal conductivity can be affected by the grain size. Before presenting and discussing the results, the measuring principle and the data analysis of the V¨olklein method are briefly reviewed.

2. State of the art 2.1. Principle of V¨olklein’s method For the measurement of both the thermal conductivity and emissivity of thin membranes, two measurements are needed. They are obtained by measuring in vacuum the average temperature rises (T1 and T2 ) of two heaters (bolometers) of width g laid on two membranes (1, 2) of width (l1 + g, l2 + g) and of length (b1 , b2 ), supported on a silicon frame (Fig. 1). The heater temperature rises are a function of the heating powers (N1 and N2 ) and of thermal conductances (G1 and G2 ) of the test-structures. The heating powers are readily obtained by measuring the electrical resistance of the heater and the electrical current passing through it. The thermal conductance is related to the thermal conduction through the membrane and the heat radiated

by the membrane. The thermal radiation to conduction ratio increases when the membrane width increases. Thus, the membrane thermal conductivity and emissivity can be extracted from the measurements of the thermal conductance of two membranes having different widths. For achieving this goal, formula are needed to link the thermal conductance to the temperature rise and the heating power. These were derived first by V¨olklein and co-workers [9–11] and are presented in the next paragraph along with the derivation hypotheses. 2.2. Calculation of the temperature rise The starting point of the derivation of V¨olklein and coworkers [9–11] for calculating the temperature rise of the heater is to solve the heat transfer equation: ∂2 ∆T (x, z) − µ2 ∆T (x, z) = 0 ∂x2

(1)

where T(x, z) is the difference of temperatures between the membrane at the location (x, z) and the silicon frame which is assumed to be at room temperature, T0 , and µ2 =

8εσB T03 + h λd

(2)

In this equation, ε, σ B , h, λ and d are the membrane emissivity, the Stephan–Boltzmann constant, the heat transfer coefficient for convection, the thermal conductivity and the thickness of the membrane respectively. Note that h is equal to zero when the measurement is made in vacuum. From Eq. (1), the simplifying assumption that the heat flow within the membrane is directed only from the bolometer to the heat sink in the x direction (Fig. 1) is clearly made as pointed out by V¨olklein himself [12]. This is only true when the membrane length is far larger than its width. If we assume that these hypotheses are fulfilled and the measurement to take place in vacuum, the temperature rise ∆T1 of the bolometer on the membrane i averaged over its length is given by:
 2 Ni Ni υbi ∆Ti = 1− = tanh (3) Gi GCRi + GRBi υbi 2

Fig. 1. Schematic drawing of V¨olklein’s test-structures: The measurement is made on a large (a) and narrower membrane (b) to get both its thermal conductivity and its emissivity.

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with:




GCRi = 2λdbi µ coth µ

li 2

and υbi = 2



if µ is known. This value can be obtained by solving by dichotomy the following equation:



GRBi = 8εB σB T03 gbi

(GCRi + GRBi )bi 4(λB dB + λd)g

(4) (5)

(6)

GCRi represents the thermal conductance of the membrane resulting from thermal conduction and radiation and GRBi is the radiation conductance of the heater. The others parameters εB , λB and dB are the emissivity, the thermal conductivity and the thickness of the bolometer, respectively. If λB dB  λd, i.e. when the thermal conductance of the heater is far larger than that of the membrane, Eq. (6) is well approximated by:  υbi (GCRi + GRBi )bi ≈ (7) 2 4λB dB g The thermal conductivity of the heater can be evaluated [9] from its electrical resistivity, ρB , using the Wiedemann– Franz-law: λB =

π2 kB2 T 3ρB q2

(8)

where kB , and q denote the Boltzmann’s constant and the elementary charge, respectively. Similarly, the emissiviy of the heater can be calculated from the Woltersdorff’s equation: εB =

2R0 d ρB (1 + R0 dB /ρB )2

(9)

where R0 =

µ0 c0 2

205

(10)

with c0 and µ0 being the velocity of light in vacuum and the magnetic permability. 2.3. Data reduction steps The thermal conductivity λ and emissivity ␧ of the membranes can be obtained from Eqs. (4) and (2), respectively

GCR1 b1 coth(µ(l1 /2)) = GCR2 b2 coth(µ(l2 /2))

(11)

GCR1 and GCR2 are obtained from the experimental temperature rises T1 , T2 and the heating powers N1 , N2 measured on the two membranes by using Eq. (3). Nevertheless, the calculation of GCRi is not straightforward since vbi /2, in the second member of Eq. (3), is a function of GCRi itself. For this reason, an iterative process is used to calculate GCRi by using the Eqs. (3) and (7) initialized with: 
 υbi (0) Ni bi /∆Ti = (12) 2 4λB dB g

3. Experimental 3.1. Test-structures fabrication The thermal conductivity and emissivity of SiNx /SiO2 / SiNx , Cr/Au/Cr, undoped, boron and phosphorus doped polycrystalline silicon have been measured (Table 1) using the test-structures of Fig. 2a, b and the new test-structure c, respectively. The microfabrication of the test-structure displayed in Fig. 2a is straightforward. A SiNx /SiO2 /SiNx sandwich layer is deposited on a 1 0 0 oriented silicon wafer. The silicon nitride is deposited at 800 ◦ C by low-pressure chemical vapor deposition (LPCVD) and is silicon-rich to decrease its residual stress [13,14]. The silicon oxide is grown by LPCVD at low temperature mperature oxide), the residual stress being a strong function of the deposition temperature [15]. An etch window is opened through the dielectric multilayer on the back of the wafer by plasma etching using CF4 /O2 reactive gas. An Au(100 nm)/Cr(20 nm) sandwich layer is deposited by e-beam evaporation on the front side of the wafer and patterned by lift-off technique to make the micro-heater. Then, anisotropic etching of the silicon wafer with potassium hydroxide (KOH) leads to the formation of a membrane on the front side of the wafer. The Au/Cr multilayer and LPCVD silicon nitride are not etched in KOH. The fabrication of the test-structure in Fig. 2b requires only the deposition of the film to be measured on the backside of the test-structure shown in Fig. 2a. The thermal conductivity and emissivity of the film can be easily obtained when the thermal conductivity of the dielectric sandwich

Table 1 Physical properties of materials measured by V¨olklein’s method Materials Low stress LPCVD 200 nm SiNx /400 nm SiO2 /200 nm SiNx E-beam evaporated 20 nm Cr/100 nm Au/20 nm Cr Undoped LPCVD 350 nm thick poly-Si Phosphorous doped LPCVD 350 nm thick poly-Si (n type; n = 5 × 1020 cm−3 ) Boron doped LPCVD 350 nm thick poly-Si (p type; n = 8.6 × 1019 cm−3 )

Thermal conductivity (W m−1 K−1 ) 2.3 140 12.8 14.8 14.2

Emissivity 0.18 <0.1 0.55 0.55 0.55

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Fig. 2. Test-structures for measuring the thermal conductivity and the emissivity of (a) the bare membrane, (b) a film deposited on the back of the membrane, (c) a film on the top of the membrane.

layer is known. The fabrication procedure used to make the test-structures described in Fig. 2b is useful to measure any film that can be deposited at a temperature low enough for the heater to be not damaged or the deposition chamber to be not contaminated. It is not possible to use the previous fabrication procedure to measure the thermal properties of the polycrystalline silicon, because of the rather high deposition temperature needed (600 ◦ C). The metal of the micro-heater would contaminate the LPCVD chamber. In this particular case, we used the test-structure described in Fig. 2c where the polycrystalline silicon film is deposited on the front side of the test-structure. The polycrystalline silicon film is then patterned by wet etching so that to be side by side with the micro-heater. The wet etching is preferred to the plasma etching because of its higher selectivity. The details of the fabrication procedure of the test-structure and of the silicon film investigated are given elsewhere [2,16]. 3.2. Measurement procedure

Fig. 3. Top view of the grid for a 2D numerical simulation of the thermal conductivity measurement applied to a thin film on top of a membrane, using the V¨olklein method as described in Fig. 2c.The boundary conditions are adiabatic at the symmetry line and isothermal elsewhere. Zone 2 is covered by the measured film while membrane is uncovered in zone 1.

In order to use the metallic strips as temperature sensors, it is necessary to calibrate them carefully. The structures are first annealed at 400 K in air on a hot plate in a closedcopper box equipped with a thermocouple located as close as possible from the metallic strip. The annealing time is long enough for the electrical resistance of the metallic strips to not change over time. After this procedure, the electrical resistances are measured as a function of the temperature in the range 300–400 K using current reversals to subtract the thermal voltages in the steady-state regime. From this curve, the temperature rise of the bolometer can be estimated. The measurement of the thermal conductance of membranes are done in vacuum at a pressure less than 3 × 10−5 Torr. A Wheatstone bridge is used to measure the average temperature rise of each metallic strip. The bridge is first balanced with a small voltage typically 10 times smaller than the one needed to get a temperature rise of the metallic strip of 1 K. The voltage across the bridge along with the voltage measured on a reference resistance of 10  in serial with the sample is used to extract both the temperature rise and the heating power at a voltage higher than the one use to balance the bridge. The temperature rises measured on the largest dielectric membranes are typically of about 1 K for a heating power of 100 ␮W.

technique based on the finite volume method [5]. The general problem should be solved by using a three-dimensional grid. However, it can be reduced to a 2D problem by assuming that the heat flows in the plane of the membrane only. For symmetry reasons, the calculations can be restricted to one quarter of the membrane size (Fig. 3). The membrane is divided into three zones to be able to make calculations in the configuration of the Fig. 2c. The radiation and convection are taken into account through the emissivity (ε) and the heat transfer coefficient for convection (h), respectively. A linearized approximation for the radiation losses has been used. As a consequence, the calculation validity is limited to small temperature difference between the membrane and the ambient temperature. The heater finite thickness and the heat losses along the heater are taken into account. The boundary condition is such as the temperature of the silicon frame is constant and set to room temperature.

4. Numerical simulation

5.1. Effect of finite membrane size

We have solved the heat transfer problem of the teststructures described previously by a numerical simulation

The numerical simulation described previously has been used to study the effect of a finite membrane length.

5. Results and discussion

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Fig. 4. V¨olklein’s data analysis failure at small membrane length to width ratios. The calculations are done for (a) a bare SiNx (200 nm)/SiO2 (400 nm)/SiNx (200 nm) membranes and (b) for a membranes covered on the back by a Cr(20 nm)/Au(100 nm)/Cr(20 nm) multilayer. The thermal conductivity and emissivity used in these calculations are reported in Table 1 and the test-structure configurations in Fig. 2a and c. The membranes width are 1.6 mm. Solid line: average temperature rise of the heater calculated using Eq. (3) with a 100 nm thick heater. The heating is adjusted to have a 1-K temperature rise of the heater. Dotted line: calculated using Eq. (3) with a 200 nm thick heater. Dashed line: calculated with a heater 100 nm thick by numerical simulation at the same heating power. Dash-dotted line: calculated by numerical simulation with a 200 nm thick heater.

The average temperature rises of bolometers calculated using Eq. (3) and by the finite volume method are plotted for different membrane sizes (Fig. 4) with g = 20 ␮m, dB = 100 and 200 nm and λB = 200 W m−1 K−1 . Calculations have been done on two membranes with a large difference of thermal conductivity to figure out if it influences the effect of the finite membrane size on the heater temperature rise. The first kind of membrane is composed of SiNx (200 nm)/SiO2 (400 nm)/SiNx (200 nm) layers 1.6 mm wide and 800 nm thick (Fig. 4a). The second kind of membrane is composed of SiNx /SiO2 /SiNx layers covered on the back side by Cr(20 nm)/Au(100 nm)/Cr(20 nm) multilayer (Fig. 4b). In each case, the membranes length varies from 1.6 to 16 mm. The thermal conductivity of the dielectric and metallic multilayer along with the emissivity of the dielectric and the stacked metallic and dielectric multilayers used in the calculation are mentioned in Table 1. The heating power is adjusted in order to obtain a temperature rise of 1 K whatever the membrane size is, when using Eq. (3) and when the heater thickness is 100 nm. The same heating power is used in the numerical simulations whatever the heater thickness is. As seen in Fig. 4a, the temperature rises change significantly with the heater thickness if the calculations are done on the bare SiNx /SiO2 /SiNx membrane only. The deviation increases when the membrane length to width ratio is decreased. The changes are minor for the metallic multilayer. The main reason is that the thermal conductance of the bare membrane is about 10 times smaller than the one covered with the Au/Cr multilayer. In this case, the heat flow along the heater cannot be neglected compare to the heat flow in the membrane. The temperature rise calculated by Eq. (3) and by the finite volume method tends to be equal for large membrane length-to-width ratios whatever the heater thickness and the membrane thermal conductivity. The difference increases to about 60% for a square shaped membrane. From

theses results, it is clear that V¨olklein’s formulas are not suitable for small membrane length-to-width ratios. At small length to width ratios, the heat flow along the heater has to be accounted in addition by the numerical simulation. It is especially true when the membrane thermal conductance is low. Useful information can be obtained by the numerical simulation about the sensitivity of the emissivity and thermal conductivity measurements to the membrane size. This is demonstrated with test-structures made of two sets of SiNx /SiO2 /SiNx membranes of different sizes. The numerical simulation has been used to extract their thermal conductivity and emissivity from the temperature rises obtained experimentally in vacuum and in a black environment. The thermal conductivity of the membrane in the numerical simulation is adjusted to obtain the experimental temperature rise at the same heating power while the emissivity in the numerical simulation varies from 0 to 1. The thermal conductivity that matches the experimental temperature rise is plotted as a function of emissivity for membranes 1.0 mm × 10 mm and 1.6 mm × 16 mm in size (Fig. 5a) and membranes 0.5 mm × 1 mm and 1 mm × 2 mm in size (Fig. 5b). The thermal conductivity and the emissivity of the membrane are given by the crossing point of these two curves. The thermal conductivity and emissivity measured on the membranes 1.0 mm × 10 mm and 1.6 mm × 16 mm (0.5 mm × 1 mm and 1 mm × 2 mm in size) are 2.3 W m−1 K−1 and 0.18 (2.45 W m−1 K−1 and 0.125), respectively. The values of the thermal conductivity are very similar to results obtained by V¨olklein on membranes made with similar materials: 2.4 W m−1 K−1 [10]. Nevertheless, the emissivity is significantly lower than that reported in [10], especially the emissivity measured on the set of membranes of small size. The lack of accuracy is not so surprising when looking carefully to the results of Fig. 5. One can notice that the wider the membrane is, the larger the change of thermal

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Fig. 5. Thermal conductivity and emissivity measurement of SiNx (200 nm)/SiO2 (400 nm)/SiNx (200 nm) membranes. Measurement made on (a) membranes 16 mm × 1.6 mm and 10 mm × 1.0 mm in size and (b) membranes 2 mm × 1 mm and 1 mm × 0.5 mm in size. The thermal conductivity of the membrane is adjusted in the numerical simulation at each value of the emissivity for the temperature rise to be equal to the one measured for each membrane. The curves intersections give the emissivity and thermal conductivity of the membranes.

conductivity when the emissivity varies from 0 to 1. It does mean that the emissivity measurement is more accurate with the membranes 1.0 mm × 10 mm and 1.6 mm × 16 mm in size than with the membranes 0.5 mm × 1 mm and 1 mm × 2 mm in size. Moreover, the emissivity measurement accuracy will be lower with membranes having high thermal conductivity. 5.2. Heat transfer of convection measurement The numerical simulation offers also the possibility to extract the heat transfer coefficient of convection following a procedure similar to that previously described. This is demonstrated here with the same sets of membrane sizes presented before with measurements performed in air. The value of the emissivity is set to 0.18 in the numerical simulation. The apparent heat transfer coefficient of convection and the apparent thermal conductivity of the membrane extracted from the crossing point are 32.6 W m−1 K−1 and 34.9 W m−2 K−1 and 6.4 W m−1 K−1 and 57.2 W m−2 K−1 for the membranes 1.0 mm × 10 mm and 1.6 mm × 16 mm in size and 0.5 mm × 1 mm and 1 mm × 2 mm in size, respectively. These last values are in good agreement with those reported by V¨olklein [10]. Our results indicate that the heat transfer in air increases when the membrane size is decreased. Furthermore, the apparent thermal conductivity is far larger than the real thermal conductivity of the membrane measured in vacuum and decreases when the membrane size decreases. These observations can be qualitatively understood as follow. It is well known that at the small scale it is more difficult for the convective heat transfer to occur. This can be clearly seen through an analysis of the Grashof number [17,18], Gr : Gr =

ρ2 ga β(T − T0 )(l + g)3 µ2

(13)

where ρ, ga , β and T are the density, the acceleration due to gravity (9.81 N kg−1 ), the coefficient of thermal expansion for air and the average temperature of the membrane.

The smaller the Grashof number is the less likely the convective motion is. Consequently, a decrease of the membrane size (l + g) will impede the heat transfer by convection and for very small membrane, the heat will flow by conduction only. The apparent increase of the heat transfer coefficient cannot be explained by an increase of the convective motion above and below the membrane but is interpreted by a decrease of the average distance between the heater and the cold silicon frame. The decrease of the apparent thermal conductivity of the membrane when decreasing the membrane size is related to an increase of the heat conduction through the membrane to the heat conduction through air ratio because the surface to volume ratio of the membrane is decreased. 5.3. Material property measurements 5.3.1. Thermal conductivity measurements The in-plane thermal conductivity measurement of metallic multilayer and polycrystalline silicon with a high level of accuracy is an illustration of the new possibilities offered by the fabrication of the new test-structures and numerical simulations. The measurement results performed on Cr(20 nm)/Au(100 nm)/Cr(20 nm) multilayer are reported in Table 1. The thermal conductivity of the metallic multilayer is 140 W m−1 K−1 . Based on the resistivity measurement (5.23 × 10−8  m), the thermal conductivity of the metallic multilayer should be 140.5 W m−1 K−1 according to Eq. (8). These results confirm the validity of using the Wiedemann–Franz law to predict the thermal conductivity of thin metallic films as suggested experimentally by von Arx and Baltes [19]. Results obtained on polycrystalline silicon are also reported in Table 1. The thermal conductivity of undoped finegrained polycrystalline silicon (12.8 W m−1 K−1 ), having a grain size of about 50 nm, is very low compared to that of the single crystal (149 W m−1 K−1 ). This result confirms previously published data [20]. The thermal conductivity of the doped polycrystalline silicon is a little bit larger than that of the undoped material because of the subsequent annealing

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step required to activate the dopant. It can be pointed out that the thermal conductivity of the n-type layer is slightly higher than the one of the p-type layer. The difference is equal to the electronic contribution to the thermal conductivity of the phosphorus layer, the mobility of the boron-doped layer being far smaller. Due to it rather low thermal conductivity, finegrained polycrystalline silicon is a promising thermoelectric materials [16].

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Acknowledgements This work was supported by the Jet Propulsion Laboratory (contract 1217092) and DoD/ONR MURI (N00014-971-0516). The authors would like to thank K. Myazaki and D. Borca-Tasc¸iuc for helpful discussion.

References 5.3.2. Emissivity measurements The emissivity results performed on polysilicon films at room temperature are reported in Table 1. The value is 0.55, whether the films are doped or undoped. It is not surprising that the emissivity of our films do not depend on the carrier concentration. In fact, it is well known that the photons with a wavelength higher than 6 ␮m will interact with phonons and not with free-carriers [21]. Therefore, it is not surprising that the emissivity of our films do not depend on the carrier concentration since it is unlikely that there are a lot of photons with a short wavelength at room temperature (Wien’s displacement law). More surprising is the large emissivity value which is close to the intrinsic value of bulk silicon (0.67) [21,22]. Normally, the spectral emissivity of thin slabs should tend to zero when their thickness are decreased [23]. A large emissivity could have been explained by interference effect [24]. However, the rather large electromagnetic wave length distribution of a black body at 300 K may rule out the effect of interference on the emissivity. A possible explanation could be a stronger interaction between electromagnetic waves and the material through the fine-grained microstructure and consequently, to an increase of the extinction coefficient of the refractive index.

6. Conclusions The thermal conductivity and emissivity of thin films have been measured with an unprecedented level of accuracy and confidence by V¨olklein’s method. The data analysis has been improved by numerical simulation. The measurements can now be extended on membranes with small length-towidth ratios. The heat transfer around the membrane was investigated through the measurement of an apparent heat transfer coefficient of convection and an apparent thermal conductivity. The experimental results suggest a transition from convective to diffusive heat transfer when the size of the membrane is decreased. The use of new test-structures combined with the numerical simulation have been applied on selected materials. From the results performed on a metallic strip, we confirmed the validity of the Wiedemann–Franz law to predict the thermal conductivity. The analysis carried on fine-grained polycrystalline silicon layers revealed interesting features. Their thermal conductivity are for example one order of magnitude smaller than that of the single crystal and their emissivity do not depend on the carrier concentration.

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and a Guggenheim Fellowship, and serves on the editorial board four journals. H. Scherrer was born in 1941 in France. He is Professor at the Henri Poincar´e University in Nancy (France). Since 1985, his research activities are mainly focused in the field of thermoelectricity. He is the head of the thermoelectric team in the Laboratoire de Physique des Materiaux. He is the President of the European Thermoelectric Society and a member of the board of the International Thermoelectric Society, of the Thermoelectric Academy of the Ukraine and of the Refrigeration Academy of Saint Petersburg. He is an author or co-author of more than 100 publications and proceedings and of seven book chapters.

Biographies A. Jacquot was born in Saint-Di´e, France, in 1974. From 1998 to 2003 he was a doctoral student under the joint guardianship of the Polytechnical Institute of Lorraine, France, and the Martin-LutherUniversity of Halle-Wittenberg, Germany. From 2001 to 2002 he was an assistant researcher at the University of California in Los Angeles in the framework of his Military Service. He obtained his Ph.D. degree in material science and engineering from the Polytechnical Institute of Lorraine in 2003. His current research interests are the measurement and improvement of thermoelectric properties of nanocomposites and superlattices. He is an author or co-author of more than 20 publications and proceedings. G. Chen received his B.S. and M.S. from Huazhong University of Science and Technology (China) in 1984 and 1987, respectively, and his Ph.D. from UC Berkeley in 1993. He taught at Duke University (1993–1997) and UCLA (1997–2001) and is currently a professor at MIT. His research interests are focused on nanoscale transport phenomena, particularly thermal energy transport, and their applications in energy and information technologies. He is a recipient of the NSF Young Investigator Award

A. Dauscher was born in Strasbourg, France in 1958. She received her Ph.D. degree in Sciences from Louis Pasteur University in Strasbourg (France) in 1987. During 10 years, she worked as student and then as a CNRS researcher in the field of heterogeneous catalysis in Strasbourg. In 1993, she joined the research team on thermoelectric materials in the Laboratoire de Physique des Materiaux in Nancy. Her current research interest is focusing on the preparation of both thermoelectric thin films, by pulsed laser deposition, and new thermoelectric bulk materials, besides the microstructural and physical characterizations of the samples. B. Lenoir was born in Rambervillers, France in 1965. He received the ESSTIN (Graduate School at Nancy, France) engineering degree in physics in 1990 and the Ph.D. degree in material science and engineering from the National Polytechnics Institute of Lorraine in 1994. He is now an As´ sistant Professor at the Ecole Nationale Sup´erieure des Mines de Nancy in the Laboratoire de Physique des Materiaux. His research activities are mainly concerned with the synthesis, the structural and physical characterization of thermoelectric materials prepared in the bulk or thin film form. He is an author or co-author of more than 70 papers and proceedings.