A Method of Preparing Thin-Film Micro-PTC ThermistorsBased on BaTiO3 Using YAG-Laser Irradiation

Yuichi Sato, Toshiaki Kawamura, and Susumu Sato

Department of Electrical and Electronic Engineering, Akita University, Akita, 010-8502 Japan

SUMMARY

A technique is proposed for obtaining thin-film posi-tive temperature coefficient (PTC) microthermistors madeof BaTiO3-based material. In this technique, stacked layersof Ti and BaO thin films are irradiated in air by a YAG laserbeam, which creates BaTiO3 semiconductor microregionswith PTC characteristics. The experiments indicate that bysuitably choosing the optical power of the laser beam andits scanning rate over the stacked layers, it is possible, andin fact easy, to control the resistance of the resulting BaTiO3

semiconductor microregions. We found that the resistanceof the BaTiO3 semiconductor material created in this waymust be sufficiently low in order for the material to exhibitPTC characteristics. In this case, the material becomes athermistor, that is, its resistance increases rapidly withincreasing temperature. As the temperature increases, theresistance suddenly begins to increase starting at a certainthreshold temperature. In an attempt to control this thresh-old temperature, we modified the technique described inthis research by replacing some of the Ti sites with Zr. Thismodified technique led us to conclude that, just as in bulkBaTiO3, such control was indeed possible. When our tech-nique is used, high-temperature heating of the entire sub-strate is not necessary, either during the deposition of thethin film or after its deposition. In this way we confirm thatit is possible to create compound device modules by fabri-cating multiple thin-film microdevices on a single sub-strate, including PTC microthermistors. © 2002 WileyPeriodicals, Inc. Electron Comm Jpn Pt 2, 85(11): 25–31,2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecjb.1113

Key words: PTC thermistor; BaTiO3; YAG laser;microthermistor; thin film.

1. Introduction

When microscopic amounts of rare-earth elementimpurities, such as La or Ce, are added to barium titanate(BaTiO3), or when the BaTiO3 is processed so as to producea state of oxygen deficiency, its fundamental electricalcharacteristics, which are those of an insulator, are trans-formed into those of a semiconductor [1, 2]. A fundamentaland well-known feature of the measured temperature vari-ation of the resistance of BaTiO3 in this induced semicon-ducting state is its negative temperature coefficient, whichconverts into an extremely large positive temperature coef-ficient (PTC) as the temperature approaches the Curiepoint. Until now, most widely used thermistors with such atemperature variation make use of temperature compensa-tion, constant-temperature heating, and the like.

The recent proliferation of commercial electronicequipment has resulted in a corresponding need for varioushigh-functionality microelectronic devices. In particular,there is a need for PTC thermistors, either alone or ascomponents of hybrid device complexes along with othermicroelectronic units. The fabrication of these hybrid com-plexes involves large numbers of devices on the samesubstrate, which must be distributed according to somehigh-density array geometry. Manufacturing based on thin-film techniques involves direct deposition of thin films withdesirable characteristics on a starting substrate, after whichphotolithography is used to specify the arrangement of the

© 2002 Wiley Periodicals, Inc.

Electronics and Communications in Japan, Part 2, Vol. 85, No. 11, 2002Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J83-C-4, No. 4, April 2000, pp. 294–299

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devices. However, the juxtaposition of different kinds ofthin-film microdevices on one substrate leads to crowding.Furthermore, fabrication involving BaTiO3 thin films usu-ally calls for high-temperature heat treatments, either dur-ing the film deposition or afterwards [3, 4]. These heattreatments place strong constraints on the manufacturingprocess if some components of the hybrid device are notresistant to high temperatures.

Because the light from a focused laser beam has anextremely high energy density, laser-beam irradiation isoften used for materials processing functions such as per-foration and cutting. Laser beams are also commonly usedto induce evaporation, where the beam is used as a heaterfor an evaporation source, to modify material at a solidsurface, to activate impurity centers, such as doping, and soon [5–9]. This ability to deliver high energy densities tomicroregions is an additional feature of lasers that makesthem useful in a wide variety of applications over and aboveoptical communication, optical computing, and the like. Inthe past, several authors, in the course of fabricating thin-film microthermistors based on oxide materials from theMn-Cu system, started by depositing low-resistancestacked layers of Mn and Cu thin film at room temperature.After these layers were deposited on the substrate, theauthors attempted to induce oxide reactions by irradiatingmicroregions in air with the focused beam from a YAG(yttrium aluminum garnet) laser (wavelength 1.06 µm). Theeffectiveness of using a laser in this way was confirmed inRef. 10. Following this procedure eliminates the need toheat the entire body of the substrate to high temperatures,either while a thin film is being deposited or after itsdeposition. An advantage to this is that preexisting thin-filmdevices of various kinds need not be subjected to thermalprocessing, so that microdevices with arbitrary sizes andcharacteristics can be easily packed together at arbitrarypositions on same substrate.

In this paper we describe a method for obtainingthin-film micro-PTC thermistors made of BaTiO3 by firstdepositing low-resistance thin-film materials that can com-bine to form the compound on a substrate, and then irradi-ating microregions of the stacked layers in air with a YAGlaser, causing BaTiO3 to form. Specifically, we present aunified discussion of our investigations of PTC charac-teristics exhibited by BaTiO3 thin-film microregions withsemiconductor characteristics, which are made by stackingTi and BaO thin films and then irradiating with the beam ofa YAG laser to induce oxidation.

2. Experimental Methods

Table 1 shows the deposition and laser irradiationconditions for each stacked layer in turn. To start with, thinlayers of each BaTiO3 constituent were successively depos-

ited on a substrate to make a thin layer stack. The depositionof these thin layers was implemented by multitarget radio-frequency (RF) magnetron sputtering setups (JEOL, JEC-SP360M). Moreover, because Ba is an especially unstablematerial chemically, particulate BaO (3N purity) and me-tallic Ti (4N purity) were used for the targets. The substratewas made of quartz glass, and Ar was used as a sputteringgas. The Ti and BaO thin films were gradually deposited onthe quartz substrate in stacked thin films. The Ar gas pres-sure during sputtering was 2.7 Pa, the RF power a constant100 W, and no special heating of the substrate was used. Inorder to maintain a 1:1 ratio of Ti to Ba metal, the thicknessof the Ti thin film was set at 0.10 µm, the BaO thin filmthickness at 0.24 µm. Two views of the structure of thestacked layers are shown in Fig. 1. The Ti thin film isdeposited first, followed by a continuous BaO thin filmwhose area is smaller than that of the Ti film. In order tomake a structure with exposed portions of the Ti thin layeron the surface that can be used as electrodes, masks ofvarious shapes were attached to the top surfaces of the thinfilms during the deposition.

Following this, the deposited BaO/Ti thin film stackwas attached to an X–Y stage, and a beam expander andlens were used to focus down the YAG laser beam (wave-length 1.06 µm) to a spot 50 µm in diameter. Irradiation ofthe stacked layer took place in air at room temperature. Asshown in Fig. 1(a), the irradiated region where the BaO andTi thin films are overlapping has a width of 0.3 mm. As theX–Y stage moves at a constant speed, the laser beamirradiation point crosses this overlapping region only once.Moreover, in this work we ensured that the laser beam wasirradiating the material effectively whether we kept the laserbeam irradiation power constant and varied the scanningvelocity of the irradiation point or kept the scanning veloc-ity constant and varied the irradiation power.

It is well known that when some of the Ti sites inBaTiO3 are replaced by Zr or Sn, for example, or some ofthe Ba sites are replaced by Sr or Pb, for example, the Curiepoint of the material shifts either to lower or higher tem-

Table 1. Conditions for deposition of the BaO/Ti orBaO/Ti-Zr stacked films and YAG laser irradiation

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peratures, respectively. This is also true of the temperatureat which its resistance suddenly increases. Thus, in additionto performing the above-mentioned basic experiment, thatis, on the Curie-point shift when some of the Ti sites arereplaced by Zr, we also investigated the shift in the tempera-ture at which the resistance suddenly increases. To do so,we first deposited a fixed-composition Ti-Zr mixed thinfilm by simultaneously sputtering the Ti target and a Zrtarget (purity 3N), and then deposited an additional BaOthin film on top of it in the usual way, thereby forming astacked layer.

On each stacked layer, after converting the materialin a sample by irradiation with the YAG laser beam, wemeasured the temperature dependence of the sample’s re-sistance. In order to measure the temperature characteristicsof the resistance, we attached gold lead wires to the surfacesof the end sections of those portions of the Ti thin film thatwere exposed and not covered with the BaO thin film, andmeasured the resistance by the two-terminal method. Wethen placed the sample in an isothermal bath in order to varyits temperature controllably. In addition, we investigated itssurface structure using an optical microscope, and madeX-ray diffraction measurements in order to study the crystalstructure of the regions irradiated by the laser beam.

3. Measurement Results and Discussion

Prior to irradiation by the laser beam, the resistanceof the stacked-layer structure made by placing a BaO thinfilm on top of a Ti thin film, as measured by attaching lead

wires directly to the exposed Ti thin film, should reflect thelow resistance of the latter. When these stacked thin filmsare irradiated by the YAG laser in air, the stacks are locallyheated to high temperatures, which causes the oxygen inthe air to react with the Ti and BaO thin films. When thishappens, it is found that the resistance of the sample willchange by an amount that depends on how long the reactionlasts. As stated above, BaTiO3 that is oxygen deficientbecomes a semiconductor. Therefore, the longer the reac-tion continues, the more it increases the resistance of thematerial. Figure 2 shows photographs from an optical mi-croscope of the surfaces of samples generated in an experi-ment where the scanning velocity was varied from 5 to 100µm/s with a constant 1.4 W of optical irradiation powerfrom the laser beam; the widths of the converted regions arein the range from 40 to 50 µm. In Fig. 3 we plot theresistances of these samples measured at room temperature.Because the stacked thin films are irradiated by the laserbeam for a long time when the scanning velocity of the laserbeam is low, we postulate that the reaction is far advancedin these samples. Our results showed that the resistancebecame extremely high, around 107 Ω (corresponding to aresistivity of 103 Ω⋅cm), for a 5 µm/s scanning velocity, and

Fig. 1. Schematic diagram of the prepared samples.

Fig. 2. Optical microscope photographs of thelaser-irradiated samples.

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revealed a tendency for a sample’s resistance to increasewith decreasing scanning velocity of the laser beam. More-over, it seems that these resistances eventually saturate atan extremely high value for very low scanning velocities ofthe laser beam. The plots shown in Fig. 4 of the temperaturedependence of the resistance of a high-resistance samplereveal a characteristic negative temperature coefficient(NTC), that is, the resistance decreases with increasingtemperature. It is well known that oxygen-deficient BaTiO3

is a semiconductor with n-type conductivity even whenundoped by impurities. Because the activation energy ineach sample was around 0.15 eV in all cases, we argue thatthese NTC characteristics appear because of the donoractivation that accompanies the temperature rise.

Although the phenomenon that creates the PTC char-acteristics in BaTiO3, that is, that makes the resistanceincrease abruptly with increasing temperature, is the latter’sapproach to the Curie point, we found that these extremelyhigh values of the resistance of the sample in the neighbor-hood of the Curie point can completely obscure the PTCcharacteristics. For this reason, in order to obtain samplesin which PTC characteristics could be measured, we had tofabricate samples whose resistance at temperatures in thevicinity of the Curie point was low. With this in mind, wenext ran experiments on several low-resistance samples,which we obtained by fixing the laser beam scanning ve-locity at 50 µm/s and decreasing the irradiation power. InFig. 5 we plot the resistance of several of these samplesversus laser beam irradiation power at room temperature.We were able to obtain samples with resistances at roomtemperature, whose change in resistance was nearly propor-tional to the laser beam irradiation power. To explain thiswe claim that as the laser beam irradiation power becomessmall, the degree to which the material reacts and oxidizesbecomes small as well. However, when the irradiationpower is low it appears to be easy to create many regions inwhich the conversion is not complete, and a scatter appearsin the sample resistance values. In Fig. 6 we plot themeasured temperature dependence of the resistance for thesamples whose room-temperature resistances are low. Ac-cording to this figure, the lower the irradiation power on thesample from the laser beam, the more rapidly the resistanceincreases with increasing sample temperature and the moreobvious the PTC characteristics become. Moreover, inthese samples the temperature at which the sudden increasein resistance begins is approximately 120 °C, which isgenerally the same temperature as in bulk BaTiO3.

Once we found that we could obtain samples thatexhibit PTC characteristics by lowering the power of the

Fig. 3. Resistances of the samples as a function of thelaser beam scanning velocity.

Fig. 4. Temperature dependence of the resistances onthe laser beam scanning velocity.

Fig. 5. Resistances of the samples as a function of thelaser irradiation power.

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laser beam or speeding up the scanning rate, we made X-raydiffraction measurements in order to investigate the crystalstructure of these samples. Moreover, since the width of theconverted regions is only 50 µm after a single pass of thescanning laser beam, that is, extremely narrow, we scannedthe laser beam over many passes while shifting the irradia-tion spot sideways. This led to samples in which the area ofthe converted regions was large, which we then measured.Figure 7 shows the X-ray diffraction pattern for a samplefabricated with a laser beam power of 0.7 W scanned at avelocity of 50 µm/s. Outside the regions irradiated by thelaser beam, we detected in the diffraction background apeak related to the BaO thin film deposited on the top partof the stacked layer, and peaks related to BaCO3, whichBaO can easily convert to. On the other hand, the presenceof a diffraction peak related to BaTiO3 on this plot isevidence that a reaction has occurred between oxygen in theair and the Ti and BaO thin films to form BaTiO3, mediatedby the radiation from the YAG laser beam. However, acomparison of known diffraction peaks for BaTiO3 with

those on the plot indicates that a mixture of cubic and squarecrystals of BaTiO3 was obtained. Further investigations ofthe laser beam irradiation conditions are necessary to clar-ify this point.

It is known that replacing some of the Ti sites or Basites with another element in BaTiO3 causes the Curie pointshift either to lower or higher temperatures, respectively. Inthe same way, the technique described here can be used toreplace elements and change the Curie point, making itpossible to shift the temperature where the resistance sud-denly increases. For instance, we tried to shift this tempera-ture to lower values by replacing some of the Ti sites withZr. To do this, while depositing the Ti thin film we alsosputtered a Zr target at the same time, so that a Ti-Zr mixedthin film was deposited. We then deposited a BaO thin filmon top of this film in the usual way, thereby making astacked layer which was irradiated by the YAG laser beam.The resistance temperature characteristics of a sample madein this way are shown in Fig. 8. According to the figure,when some of the Ti sites are replaced with Zr, the tempera-ture at which the resistance suddenly increases is found toshift to lower values, with the amount of the shift clearlyproportional to how large the Zr replacement ratio was.These results indicate that by using the method describedhere we can make the same kind of element replacement inbulk sintered BaTiO3, and control the temperature where itsresistance suddenly increases by the same technique.

4. Conclusions

We have evaluated a method for fabricating a micro-PTC thermistor out of a microregion of BaTiO3 at anarbitrary position on a thin film by using heat alone, inwhich the microregion of BaTiO3 semiconductor with PTCcharacteristics is created by irradiating a stacked layer of Tiand Ba thin films with the focused beam of a YAG laser.

Fig. 7. X-ray diffraction pattern of the laser-irradiatedsample.

Fig. 8. Temperature dependence of the resistance of thesample in which Zr is substituted for a part of Ti.

Fig. 6. Temperature dependence of the resistances onthe laser irradiation power.

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Our results show that by adjusting the optical power andscanning velocity of the YAG laser beam it is possible toobtain BaTiO3 semiconductor thin-film microregions assmall as 50 µm, and we have confirmed that PTC charac-teristics are indeed present in samples with very low room-temperature resistance. Moreover, we have found that it ispossible to control the temperature at which the resistancesuddenly increases by replacing some of the Ti sites withZr. Our studies of this technique show that high tempera-tures are not necessary when depositing thin films on asubstrate, and that by applying heat treatment after deposi-tion to only a local region of the film it is possible to createthin-film micro-PTC thermistors. This suggests that it isnow feasible to easily fabricate compound modules madeup of many different kinds of thin-film microdevices to-gether on the same substrate.

REFERENCES

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AUTHORS

Yuichi Sato received his B.S., M.S., and Ph.D. degrees in electrical and electronic engineering from Tokyo MetropolitanUniversity, Akita University, and Tohoku University in 1985, 1989, and 1996. From 1989 to 1996 he was a research associate,and currently serves as a lecturer in the Department of Electrical and Electronic Engineering, Faculty of Engineering andResource Science, Akita University. He was a Visiting Fellow and worked on nitride semiconductors at Macquarie University,Sydney, from 1997 to 1998. He is active in research on semiconductor thin-film growth and devices.

Toshiaki Kawamura (photograph not available) received his B.S. and M.S. degrees in electrical and electronicengineering from Akita University in 1995 and 1997. He is currently working as an engineer at Nichikon Co., Ltd.

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AUTHORS (continued)

Susumu Sato received his B.S., M.S., and Ph.D. degrees in electronics from Tohoku University in 1964, 1966, and 1969.From 1969 to 1974 he was a research associate in the Department of Electronics, Faculty of Technology, Tohoku University. In1974 he became an associate professor in the Department of Electronics, Akita University, and currently serves as a professorin the Department of Electrical and Electronics Engineering. He was a research associate at Temple University, Philadelphia,Pennsylvania, from 1979 to 1980, where he worked on liquid-crystal fluorescent-display devices. He is active in research onsemiconductor devices, liquid-crystal devices, and optical sensing.

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