This article reports the main problem and the corresponding solution of the co-evaporation of Bi2Te3 and Sb2Te3 films for the fabrication of Peltier elements. This main problem consists in the control of the deposition rates of the two elements:
Bi or Sb and Te, which have very different vapor pressures. The control of the deposition ratio was achieved by means of a PID controller, which permitted the fabrication of thin-film Peltier elements that produce a temperature gradient in the order of 2C between their hot and cold junctions, when measured at free air conditions.Fundação para a Ciência e a Tecnologia (FCT) - SFRH/BD/18142/2004.Agencia de Inovação (MPYROM)

Correia, J. H.

Couto, Carlos

Gonçalves, L. M.

Rocha, J. G.

Universidade do Minho: RepositoriUM

Control of the Deposition Ratio of Bi2Te3 andSb2Te3 in a Vacuum Evaporator for fabrication ofPeltier ElementsL. M. Gonc¸alves, J. G. Rocha, J. H. Correia and C. CoutoIndustrial Electronics Department, University of Minho,Campus de Azurem 4800-058 Guimaraes, Portugal.Email L. M. Gonc¸alves: lgoncalves@dei.uminho.ptAbstract—This article reports the main problem and the cor-responding solution of the co-evaporation of Bi2Te3 and Sb2Te3films for the fabrication of Peltier elements. This main problemconsists in the control of the deposition rates of the two elements:Bi or Sb and Te, which have very different vapor pressures. Thecontrol of the deposition ratio was achieved by means of a PIDcontroller, which permitted the fabrication of thin-film Peltierelements that produce a temperature gradient in the order of2oC between their hot and cold junctions, when measured atfree air conditions.I. INTRODUCTIONThere are many industrial processes that use thin-film tech-nology: optical coatings of filters and lenses, microelectronics,microsensors, etc. A simple way to deposit thin films isachieved by evaporation. The material to be deposited is heateduntil it reaches the vapor state. The substrate (optical filter orlens, for example) is placed above the material to be deposited.The atoms of the material that constitute the thin-film willthen adhere to the substrate surface. In many cases, only thethickness of the deposited thin-film is important, so the controlcan be done in a simple way: the electrical power of the heatermust be enough to start the evaporation and it must be switchedoff when the desired thickness is achieved.For the fabrication of thin-film Peltier elements, instead ofsingle element substances, they are used compounds [1]. Un-fortunately, the deposition of compound substances like Bi2Te3and Sb2Te3 is not as simple as the elementary substancesdescribed above. This phenomenon is described along thisarticle, in its references and in related literature.Commercial Peltier devices are usually fabricated on atransversal (cross-plane) configuration as is shown in figure 1.In theory, this configuration could be reduced for micro-devicefabrication, but the conventional fabrication processes are notscalable to the micrometer range. Using a lateral (in-plane)configuration (figure 2), thin-film techniques can be used toscale down the thermoelectric coolers and generators to micro-device dimensions [2]. In the present work, planar thin-filmtechnology will be used to fabricate such devices.The Peltier element consists in pn semiconductor junctionswhere the n-type semiconductor is the Bi2Te3 and the p-type semiconductor is the Sb2Te3. Tellurium alloys are well-established low-temperature thermoelectric materials and areFig. 1. Cross-plane (transversal) Peltier cooler.Fig. 2. In-plane (lateral) Peltier cooler.widely employed in conventional thermoelectric generators andcoolers [3]. Different deposition techniques can be used to ob-tain Bi2Te3 and Sb2Te3 thin-films. Thermal co-evaporation [4],co-sputtering [5], electrochemical deposition [6], metal-organicchemical vapor deposition [7] and flash evaporation [8] aresome examples.The objective of the work is to fabricate the Bi2Te3/Sb2Te3junctions, from the Bi, Te and Sb elementary substancesindividually using a co-evaporation technique. In this case,both the thickness of the film and the evaporation rate ofeach element must be well controlled. In order to do that, wedeveloped a control setup consisting on a digital PID controller,sensors and actuators.The choice of the co-evaporation method is due mainlybecause it is inexpensive, simple and reliable, when comparedto other techniques that need longer time periods to prepare thestarting material or require more complicated and expensivedeposition equipments.The developed setup allowed the achievement of verypromising results in the fabrication of planar thin-film Peltierelements.II. THIN-FILM CO-EVAPORATIONBismuth, tellurium and antimony have large differences intheir vapor pressures. These differences would result in acompositional gradient along the film thickness, when Bi2Te3and Sb2Te3 films are directly evaporated from the compounds.This composition gradient effect is also reported in litera-ture [9]. The problem of composition gradient can be over-come, though, by the use of co-evaporation [4], where Bi andTe are evaporated in a vacuum chamber, from two molybdenumboats, whose temperatures are independently controlled. Asimilar procedure was adopted for Sb2Te3 deposition from twoindependent Sb and Te sources.Figure 3 shows a schematic diagram of the co-evaporationsystem used in the fabrication of the films. It consists in aSubstrateBi/Sb TeControllerSensorsMassMolybdenumboatsFig. 3. Co-evaporation system.vacuum chamber where the pressure reaches 10−6 mbar. In itsinterior two evaporation boats are placed: the first one holdsthe Bi or the Sb samples and the second one holds the Tesample. The boats are separated by a metallic wall and areheated by means of an electric current ranging between 100 Aand 200 A, which is supplied by the controller. At the topof the chamber are placed the substrate where the depositionis made and two mass sensors. The objective of the metallicwall that separates the two boats is to avoid the deposition ofthe material of the left boat in the right sensor and vice-versa.Table I shows the evaporation temperatures of the materialsthat were used in the fabrication of the Peltier elements, atthe pressure of 10−6 mbar. The Two sensors were used toTABLE ITEMPERATURE OF EVAPORATION OF THE MATERIALS AT 10−6 mbar OFPRESSURE.Material Temperature (oC)Bi 410Sb 345Te 207monitor the deposition rate of Bi/Sb and Te. Each rate wasmaintained at a fixed value, through independent control ofthe power applied to each molybdenum evaporation boat. Theevaporation rate must be between 0.2 nm/s and 0.4 nm/s forgood film properties.III. CONTROL SETUPBasically, the control of the evaporation rate is made byregulating the voltage and consequently the current that isapplied to the boats. In order to implement this control, it isnecessary to sense the deposited material, read the output ofthe sensors and act in the voltage applied to the boats. In thefollowing subsections, these items will be described with greatdetail.A. SensorsThe sensor uses the resonant frequency of an exposed quartzcrystal to sense the mass of deposited films attached to itssurface. The presence of mass of the deposited film on thesurface of the crystal decreases its resonant frequency. Therelationship between the mass of the film and the frequency ofthe sensor crystal is given by [10]:h =(NQdQπdFZf)arctan[Z tan(π(fQ − ffQ)], (1)where the terms used in the equation are defined as:h - film thickness;NQ - frequency constant for AT-cut quartz crystal (1.668×1013 Hz.A˚);dQ - density of quartz (2.648 g.cm−3);dF - density of film material;Z - Z-Factor of film material;fQ - frequency of crystal prior to depositing any film on it;f - frequency of loaded crystal.The Z-factor of the material can be calculated using theequation:Z =√dQSQdFSF, (2)where the terms used in the equation are defined as:dQ - density of quartz;SQ - shear modulus of quartz (2.947× 1011 g.cm−1.s−2);dF - density of film;SF - shear modulus of film.The contacts to the crystal are gold plated to ensure lowresistance and avoid oxidation. Measuring the frequency of thesignal generated on the oscillator, the thickness is calculatedeach four seconds. The evaporation rate is obtained by thechange of thickness. The crystal sensors are placed in suchway that they receive exactly the same quantity of materialthat reaches the substrate. The crystal sensors are water cooled(around 20oC) to minimize the influence of temperature onoscillating frequency.B. Readout deviceThe readout circuit of the sensors is shown in figure 4.It consists basically in a series resonant crystal oscillatorconfiguration and uses two cascaded inverters, which form anamplifier. Each inverter has a DC biasing resistor which biasesC SensorR RuCFig. 4. Readout circuit of the mass sensors.the inverter in the active region, i. e. halfway between the logic“1” and “0” states. In this region, the inverters act as highgain inverting amplifiers, introducing a phase shift of 180oin the signal. The output of the circuit is connected to themicrocontroller, which counts the number of pulses in a fixedtime interval, obtaining in this way the mass of the materialdeposited in the sensor.C. ControllerThe controller was implemented in a Philips SemiconductorsP89C51RD2 microcontroller. Basically, it reads the output ofthe two sensor circuits and the phase of the power line and actin the switching angle of two triacs (one for each evaporationboat). Figure 5 shows the circuit that performs the phasedetection of the power line voltage.PowerLine+5VR2R1R1uCFig. 5. Circuit that performs the phase detection of the power line voltage.Figure 6 shows the triac switching circuit, where the switch-ing angle is controlled by the microcontroller (uC) by meansof an optocoupler. This circuit controls the voltage applied tothe primary winding of a power transformer. The secondary ofthe transformer is directly connected to the evaporation boat.+5VuC+15 VPowerLineTransformerFig. 6. Power switching circuit.The algorithm used in the controller is a well-known PID.Nevertheless, the control of the evaporation ratio of a materialhas particular characteristics: below its vacuum evaporationtemperature nothing happens and above this temperature, ev-erything happens very quickly. In order to perform a goodevaporation ratio control, the temperature inside the boats mustbe approximately equal to the evaporation temperature of thematerial, and change only some tenths of degree Celsius. Inorder to obtain this behavior, the output of the control algorithmwas offsetted, i. e., without PID actions, the output of thecontroller will be the one that produce approximately theevaporation temperature of the material. With the introductionof this offset, the control algorithm worked fine, allowing thefabrication of the desired films.Figure 7 shows a picture of the circuit.Fig. 7. Picture of the circuit.D. ActuatorsThe output of each circuit represented in figure 6 is con-nected to the primary winding of a transformer. The secondarywinding of each transformer is directly connected to the re-spective molybdenum boat placed inside the vacuum chamber.Two transformers of 220 V/6 V , 1.2 kV A were used, whichpermits a ranging of the current in the boats up to 200 A,depending on the switching angle of the triacs.IV. EXPERIMENTAL RESULTSFirst of all, a test to the controller and the power electronicswas performed. Figure 8 shows the waveform obtained at theevaporation boat terminals, for a switching angle of 100o.−4−2 0 2 4 0  10  20  30  40  50t (ms)V (V)Fig. 8. Waveform at the evaporation boat terminals for a triac switching angleof 100o.Figures 9 to 11 show the closed loop control performanceof the system, for the deposition of the Bi2Te3 film. Thegraphs for the deposition of Sb2Te3 are similar, except thatthe electrical power applied to the evaporation boat containingthe Sb sample is a little bit smaller than the one of Bi.Figure 9 shows the evaporation rate of Te and Bi during the filmfabrication. As it is possible to see, there are small oscillationsaround a mean value.Evaporation rate TeBi 13 5 0  1000  2000  3000t (s)(A˚/s)Fig. 9. Evaporation rate of Te and Bi during the fabrication of the Bi2Te3film.Figure 10 shows the film thickness deposited at each sensoras a function of time. As it can be seen (and calculated),the control response is highly linear for both materials. ThePearson product moment correlation coefficients are 0.999975and 0,999977 respectively, i. e., very close to 1.Figure 11 shows the electrical power supplied to eachevaporation boat. From figures 9 to 11 referred above, it ispossible to conclude that the controller is suitable to controlwith precision the evaporation ratio and the thickness of a co-evaporation process.Figure 12 shows a picture of the fabricated device, whichconsists in four pairs of Peltier junctions. Each thermoelectricTeBiFilm thickness (nm) 0 1000  2000  3000800600 400 200t (s)Fig. 10. Mass of Bi and Te deposited at each sensor as a function of time.TeBi 180 200 220 240 260 0  1000  2000  3000Boat power (W)t (s)Fig. 11. Electrical power supplied to each evaporation boat.leg has 2.3 µm of thickness and the metal contacts have athickness of 0.5 µm.Figure 13 shows a cross-section of an homogeneous Bi2Te3film with 700 nm of thickness, fabricated in a previousexperiment.Table II shows the composition of the fabricated films,obtained by energy dispersive X-ray spectroscopy (EDX), andtheir respective Seebeck coefficients. Te and Bi/Sb contentTABLE IIPERCENTAGE OF ATOMS OBTAINED FOR EACH FILM AND RESPECTIVESEEBECK COEFFICIENT.Material Bi/Sb (%) Te (%) Seebeck(%) (%) µV/oCBi2Te3 39.83 60.17 -189Sb2Te3 41.49 58.51 140shows that the composition of both types of films is close tostoichiometry. An X-ray diffraction (XRD) analysis was alsoperformed. That analysis revealed that the structure of the filmsis polycrystalline. The Seebeck coefficients were measured byconnecting one side of the film to a heated metal block at afixed temperature and the other side to a heat sink at roomtemperature. The voltage generated in these known conditionswas then measured.Figure 14 shows a thermal image of one of the fabricatedPeltier elements, with an applied electric current of 5 mA.Fig. 12. Picture of the fabricated device.Fig. 13. SEM photograph of a homogeneous n-type 700 nm thick Bi2Te3film.Figure 15 shows a graph of temperature gradient versus dis-tance, obtained along the line drawn in figure 14. The obtainedtemperature gradient was close to 2oC. This test was performedin a natural convection (still air) environment, resulting ina surface temperature map of the device as represented infigure 14.V. CONCLUSIONSPeltier elements formed by junctions of Bi2Te3 and Sb2Te3were fabricated using a co-evaporation technique. This tech-nique is advantageous relatively to other possible ones becauseit is inexpensive, simple and reliable. The main difficulty of theco-evaporation consists in the control of the deposition rates ofthe two materials, which must be proportional between themin order to obtain a composition with the desired properties.The control of the evaporation rate was made by using aPID controller, which revealed very efficient. As result, thin-films of Peltier elements were obtained and it was reached thedifference of temperatures between hot and cold junctions inthe order of 2oC.REFERENCES[1] L. M. GONC¸ALVES, C. COUTO, P. ALPUIM, D. M. ROWE AND J. H.CORREIA,Fig. 14. Thermal image of the Peltier element.T (  C)o∆1.000.500.00−0.50−1.001.921.680.24 0.48 0.72 0.96 1.2 1.44Distance (mm)Fig. 15. Mapping of the temperatures along the line of figure 14.Thermoelectric microstructures of Bi2Te3/Sb2Te3 for a self-calibratedmicro-pyrometer, Sensors and Actuators A (2006), article in press.[2] G. MIN AND D. M. ROWE, Cooling performance of integrated thermo-electric micro-cooler, Solid-State Electron. 43 (1999) 923-929.[3] D.M. ROWE, CRC Handbook of Thermoelectrics, CRC Press, 1995.[4] H. ZOU, D.M. ROWE AND S.G.K. WILLIAMS, Peltier effect in a co-evaporated Sb2Te3(p)-Bi2Te3(n) thin-film thermocouple, Thin Solid Films408 (2002) 270.[5] H. BO¨TTNER, J. NURNUS, A. GAVRIKOV, G. KU¨HNER, M. JA¨GLE,C. KU¨NZEL, D. EBERHARD, G. PLESCHER, A. SCHUBERT AND K.H. SCHLERETH, New thermoelectric components using microsystemtechnologies, J. Microelectromech. Syst. 3 (2004) 414.[6] J. R. LIM, G. J. SNYDER, C. K. HUANG, J. A. HERMAN, M. A.RYANAND AND J. P. FLEURIAL, Thermoelectric micro-device fabricationprocess and evaluation at the jet propulsion laboratory, 21st InternationalConference on Thermoelectronics, Long Beach, U.S.A., August 25 29(2002) 535-539.[7] A. GIANI, A. BOULOUZ, F. PASCAL-DELANNOY, A. FOUCARAN, E.CHARLES AND A. BOYER, Growth of Bi2Te3 and Sb2Te3 thin-films byMOCVD, Mater. Sci. Eng. B 64 (1999) 19-24.[8] A. FOUCARAN, A. SACKDA, A. GIANI, F. PASCAL-DELANNOY AND A.BOYER, Flash evaporated layers of (Bi2Te3 Bi2Se3)(n) and (Bi2Te3Sb2Te3)(p), Mater. Sci. Eng. B 52 (1998) 154-161.[9] L. W. DA SILVA AND M. KAVIANY, Miniaturized thermoelectric cooler,ASME International Mechanical Engineering Congress and Exposition,New Orleans, U.S.A., November 17 22 (2002) 1-15.[10] C. LU AND O. LEWIS, Investigation of film-thickness determination byoscillating quartz resonators with large mass load, J. Appl. Phys. 43(1972) 4385.

Control of the deposition ratio of Bi2Te3 and Sb2Te3 in a vacuum evaporator for fabrication of Peltier elements

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Control of the deposition ratio of Bi2Te3 and Sb2Te3 in a vacuum evaporator for fabrication of Peltier elements

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