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The improvement of pyroelectric properties of PZT thick films on Si substrate by TiOx barrier layer
Infrared Physics & Technology 58 (2013) 51–55
Contents lists available at SciVerse ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
The improvement of pyroelectric properties of PZT thick films on Si substrate by TiOx barrier layer Q.X. Peng a, C.G. Wu a,⇑, W.B. Luo a, L. Jin a,b, W.L. Zhang a, C. Chen a, X.Y. Sun a a b
State Key Lab of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China Electronic Information Division, Ministry of Industry and Information Technology, Beijing 100036, China
h i g h l i g h t s " Effects of TiOx thickness on PZT pyroelectric properties are studied. " TiOx are prepared by thermal oxidation of melt Ti layer. " At 750 °C sintering temperature, TiOx layers are significant in blocking Si diffusion. " Pyroelectric coefficient of PZT thick films are enhanced by 70% with 400 nm TiOx thickness. " Two diffusion barrier mechanisms are presented.
a r t i c l e
i n f o
Article history: Received 18 July 2012 Available online 29 January 2013 Keywords: PZT TiOx Diffusion barrier EPD
a b s t r a c t The effects of TiOx diffusion barrier layer thickness on the microstructure and pyroelectric characteristics of PZT thick films were studied in this paper. The TiOx layer was prepared by thermal oxidation of Ti thin film in air and the PZT thick films were fabricated by electrophoresis deposition method (EPD). To demonstrate the barrier effect of TiOx layer, the electrode/substrate interface and Si content in PZT thick films were characterized by scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS), respectively. The TiOx barrier thickness shows significant influence on the bottom electrode and the pyroelectric performance of the PZT thick films. The average pyroelectric coefficient of PZT films deposited on 400 nm TiOx layer was about 8.94 109 C/(cm2 K), which was improved by 70% than those without diffusion barrier layer. The results showed in this study indicate that TiOx barrier layer has great potential in fabrication of PZT pyroelectric device. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to the excellent pyroelectric properties of lead zirconate titanium (PZT) thick film, it has been intensively researched and widely applied in pyroelectric sensors [1–3]. The integration of PZT thick films with Si-based substrate is of great technical importance because of the various advantages, such as fabricating pyroelectric devices using MEMS technology and the integration of these device with Si integrated circuits. Many methods, such as screen printing, EPD, composite film technology, and direct writing [4], have been used to grow PZT thick films. A high temperature sintering process is needed to transform the green body to ceramic film no matter what method are used to deposit the thick films. However, serious interdiffusion and chemical reaction usually happen during this sintering process [4]. The reaction productions (such as lead silicate), formed during sintering process, are detri⇑ Corresponding author. Tel./fax: +86 028 83202140. E-mail addresses: [email protected] (Q.X. Peng), [email protected] (C.G. Wu). 1350-4495/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2013.01.003
mental to the pyroelectric properties of the films [5,6]. Thus, the imperative problem to PZT thick films is the interdiffusion and chemical reaction happened during the high temperature sintering process [4]. Numerous material researchers have been devoting to this issue. In principle, there are two approved ways which could be effective in blocking the interdiffusion. The first one is to reduce sintering temperature by using sintering aid. Various of low melting temperature metallic oxides, such as LiBiO2, PbO–Cu2O, Pb3O4, Nb2O5 [7–9], were adopted to decrease the sintering temperature of ceramic. Nevertheless, the optimized sintering temperature of PZT thick films in most published results (i.e. for the films prepared on the basis of PZT powders) was still higher than the formation temperature of lead silicate compound (720 °C) [10,11]. The second way is to insert a diffusion barrier layer such as Al2O3 [12], ZrO2 [13], MgO [14], NiAl [15], between the bottom electrode and Si substrate, which has been found to be more effective to prevent the interdiffusion and reducing the formation of impurity phase. Compared to these reported barrier layers, TiOx has the
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Q.X. Peng et al. / Infrared Physics & Technology 58 (2013) 51–55
following advantages. Firstly, the stability of the bottom electrode could be enhanced by TiOx barriers because of its moderate thermal expansion coefficient nature [16]. Secondly, because of its chemical compatibility with the PZT composition, it would not produce melt element contaminant in the thick films [17]. However, the influence of TiOx diffusion barrier layer thickness on the pyroelectric properties of PZT thick films on Si substrate was not well studied. TiOx barrier layer was fabricated thermal oxidation of Ti thin film in air in this paper. The thickness effects of TiOx barrier on the microstructure and pyroelectric performance of the PZT thick films on SiO2/Si substrate were studied by SEM and dynamic pyroelectric measurements.
2. Experiment procedures
2.4. Microstructure and electric properties evaluation The surface topography and root mean square (RMS) roughness of Ti and oxidized TiOx were examined by atomic force scanning (AFM). The crystalline phase of the barrier layer was characterized by XRD. SEM and EDS were carried out to indentify the electrode/ substrate interface and Si contents in PZT thick films, respectively. For electrical measurements, Pt(100 nm)/Ti(40 nm) top electrode array was grown by DC sputtering to form capacitor structures. The area of each electrode pad is 1 mm2. The deposition parameters are as the same as the bottom electrode. After that, all the films were poled at 150 °C for 15 min under an applied electric field of 6 V/lm. Finally, the pyroelectric and ferroelectric properties of the samples were tested with pyroelectric coefficient (Pc) auto-measurement system [19,20]. The system has the advantages of high temperature stability (±0.002 °C), low noise level (14 nV) and low system relative error (3‰).
2.1. Diffusion barrier layer process TiOx diffusion barrier layer was fabricated by oxidizing the Ti metal layer. To obtain different TiOx thickness, Ti films with different thickness were deposited on SiO2/Si(1 0 0) substrate by DC magnetron sputtering at room temperature. The base vacuum of Ti sputtering is higher than 7.5 107 torr. The deposition was carried out at 1.5 103 torr argon. The sputtering voltage and current were 320 V and current 100 mA, respectively. The sputtering rate of Ti metallic layer was 12 nm/min. After deposition, The Ti layers were annealed in a furnace at 700 °C for 1 h to oxidize the Ti to TiOx immediately. By controlling the sputtering time, TiOx barrier thickness from 0 nm to 600 nm with interval value about 100 nm were prepared.
2.2. Bottom electrode deposition Two-hundred nanometer Pt bottom electrode was deposited on the TiOx diffusion barrier layer. The substrate was cleaned by ultrasonic wave cleaner and oxygen plasma to remove the inorganic and organic residual on titanium oxidation layer surface before deposition. The sputtering voltage of Pt deposition was 385 V. The other sputtering parameters, including gas, vacuum, current, were the same as the Ti metallic layer deposition.
2.3. PZT thick film preparation The PZT thick film was deposited by EPD method using PZT (Pb(Zr0.3Ti0.7)O3, Hayashi Chemical Industry Company, Japan, commercial grade)/ethanol powder suspension with 2 mol% lead germinate (PGO) sintering aid. EPD is a competitive method because of its capability of directly patterning the green films [18]. This method allows the absence of the following etching action for the ceramic thick films which may bring about negative impact on the material properties. Ethanol suspension with PZT, PGO and triethanolamine (TEA) were vibratory milling for 4 h. In order to charge the particles, the PH of the suspension was adjusted to five by diluting nitric acid. The suspension was then homogenized via ultrasonic dispersing and magnetic stirring vigorously. The charged PZT particles were drove toward the cathode by the direct voltage applied between the graphite anode and counter Pt/ TiOx cathode. The deposition rate was about 10 lm/min. 30 lm thick PZT films were deposited by EPD in 3 min. All the thick films were sintered at 750 °C for 1 h in furnace with rising rate 4 °C and cooling rate 2.5 °C per minute, respectively.
3. Results and discussion 3.1. Microstructure of the TiOx barrier layer Ti metal layers were thermal oxidized at 700 °C for 1 h in air atmosphere to obtain TiOx diffusion barrier layers. The XRD patterns of Ti and TiOx films were showed in Fig. 1. According to the XRD results, there is no diffraction peaks related with Ti or TiOx except Si (2 0 0) peak in the pre-oxidation specimen due to sputtering condition [21]. After oxidation, all the peaks in the XRD patterns can be indexed into anatase and rutile phase of TiOx. The oxidation result in this paper is disaccord with that composed of pure rutile TiO2 reported by F.F.C. Duval et al. [16]. In the oxidation process, the growing route of TiOx crystals was from the Ti/air interface to Ti/substrate interface. As time progresses, oxygen has to pass through the prior titanium oxide in order to reach the fresh interface to convert the Ti (or lower Ti) to TiOx (or higher Ti) [22]. The partially transformation of Ti layer to rutile TiOx may result from the decreased migration of oxygen caused by the high density of the sputtered Ti layer. The surface morphologies of the barrier layer before and after oxidation were also presented in Fig. 1. As shown in Fig. 1b, the RMS of barrier layer before oxidation was about 2.78 nm. The barrier layer surface roughness after oxidation shown in Fig. 1c increased to about 2.87 nm. There was no obvious change in the surface roughness of the films before and after oxidation. However, the surface grain size of the oxidized film increased from about 50 nm to 75 nm compared to the pre-oxidized one. The film thickness increased from 135 nm to 200 nm after oxidation according to surface profiler (Veeco dektak 150) measurement results. The expansion of grain size and thickness are resulted from the incorporation of oxygen into titanium [17,21].
3.2. Effects of TiOx thickness on the PZT thick films Thirty micrometer PZT thick films were then deposited by EPD on Si substrate with different TiOx barrier layer thickness ranging from 0 to 600 nm. Fig. 2 displays the cross sectional SEM images of the PZT/Pt/x nm TiOx/SiO2 (denoted Tx in the following text where x represents the TiOx thickness) samples sintered at 750 °C for 1 h. The SEM cross section image of PZT thick films deposited on Pt/SiO2/Si substrate without TiOx barrier (T0) was shown in Fig. 2a. It can be seen from the graph that there are many delaminated area between Pt and SiO2. A lot of irregular vacancies were also observed at the SiO2 surface indicating that some Si atoms may diffused into the Pt/TiOx layer and PZT film.
Q.X. Peng et al. / Infrared Physics & Technology 58 (2013) 51–55
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Fig. 1. XRD patterns and AFM surface morphology of Ti film and TiOx film oxidized at 700 °C.
Fig. 2. Cross sectional SEM of PZT thick films for different TiOx thickness.
As the TiOx thickness increased from 0 to 400 nm, the vacancies were eliminated and the TiOx/SiO2 interface became smoother and clearer gradually. As the thickness further increased, vacancies and micropores appeared again at the TiOx/SiO2 interface, which was formed to release thermal stress between the barrier layer and substrate. When the TiOx thickness reached up to 600 nm, the electrode structure was deteriorated and became even worse than the one without TiOx barrier. Two possible mechanisms were proposed to explain this phenomenon. Firstly, oxygen transferred through Pt layer and reacted with unoxidized titanium and low valence titanium ions (maybe randomly distributed in TiOx layer). This will change the volume of TiOx layers, and destroyed the multilayer structure consequently. Secondly, thermal strain will be accumulated at the Pt/TiOx/SiO2 layers during sintering and cooling processes because the thermal expansion coefficient mismatch between Pt (14.2 106/K), SiO2 (0.4 106/K) and TiOx (8.5 106/K) [23]. The different expansion and shrinkage rate of Pt/TiOx/SiO2 layers in the thermal treatment process tended to produce stress between interlayer, which would result in warping and delamination [24]. In order to analyze the effects of the diffusion barrier layer, Si content in PZT thick films were measured by EDS for each sample
with different TiOx thickness. The testing pots were about 1 lm above the Pt layer surface. Two random testing pots at the same depth level were selected to do the analyses. The average values
Fig. 3. Si content measurements in PZT films with different TiOx diffusion barrier layer thickness.
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of each Tx were plotted and the offset values were showed as error bars in Fig. 3. According to the results, it was found the Si content in sample T0 without TiOx diffusion barrier layer is about 45 wt%, which proved the Si diffusion phenomenon into PZT at sintering process. The Si content decreased gradually as the TiOx thickness layer increased. When the TiOx thickness was equal to or larger than 300 nm, almost no Si element could be detected by EDS. These results indicated that the barrier layer thicker than 300 nm could block the diffusion of Si into PZT thick film effectively. When TiOx diffusion barrier layer was inserted, the diffusion of Si into PZT thick films became more difficult because of the blocking effect and barrier effect of TiOx layer [25]. Firstly, the defects in TiOx/ SiO2 interface reduced the transmission paths and block the diffusion of Si element. Secondly, TiSi2 can form at the TiOx/SiO2 interface if the thermal treat temperature is higher than 600 °C [26]. The mobility of Si was probably weakened continuously as Si diffused step by step in the barrier layer if the barrier is thick enough. The Pc of PZT thick films were tested by dynamic method using the measurement system reported by [20]. The Pc is given by [19]:
Pc ¼
Ip AdT=dt
4. Conclusion A feasible way to produce TiOx diffusion barrier layer for PZT thick films sintered at temperature up to 750 °C was shown in this paper. The mixed crystallite TiOx including anatase and rutile could be obtained by thermal oxidation of Ti at 700 °C for 1 h. It has been shown that thick TiOx diffusion barrier layer could significantly suppress the interdiffusion. Moreover, it was found that TiOx layer with proper thickness could improve the bottom electrode characters and enhance the pyroelectric properties of PZT thick film. Whereas, the deterioration of the bottom electrode deposited on too thick TiOx barrier would lower the final measured Pc.
Acknowledgments This work has been supported by the National Natural Science Foundation of China (NSFC. 51102037) and the Fundamental Research Funds for the Central Universities from UESTC (Nos. ZYGX2010J030 and ZYGX2011J023).
ð1Þ
where A is the capacity area, Ip is the response current and dT/dt is the slope of temperature curve. The average Pc values and offset values of the Tx were calculated and plotted in Fig. 4. When the TiOx thickness was 0 nm, the average Pc and offset value was 5.29 109 C/(cm2 K) and 0.20 109 C/(cm2 K), respectively. The Pc of the PZT samples increased as TiOx became thicker at first. When the TiOx thickness was 400 nm, the average Pc of the PZT (T400) reached the maximal value (8.94 109 C/(cm2 K)), which was 70% larger than the one without TiOx barrier (T0). The Pc of T400 was almost equal to that of the PZT thick film fabricated on alumina by screen-printing [27]. Combined the EDS result in Fig. 3, the improvement of pyroelectric performance was result from the decreased Si diffusion. However, when the thickness of TiOx was larger than 400 nm, the Pc started to decrease. When the TiOx layer was increased to 600 nm, the average Pc decreased by about 14% compared to that of T400 group. The decrease of Pc may be attribute to the actual effect area loss inferred from the poor electrode structure. As shown in Fig. 2, more vacancies can be observed when the TiOx thickness is more than 400 nm. The appearance of vacancies undermined the bottom electrode might tear the integrity of the conductive layer, which leaded to the loss of actual effective area of the capacity structure. The decrease of effective electrode area will diminish the measured Pc in formula (1). Therefore, these Pc results of Tx (x > 400) should not represent exact pyroelectric properties of PZT thick film.
Fig. 4. Pyroelectric coefficient of PZT thick film species for different TiOx thickness.
References [1] M. Es-Souni, M. Kuhnke, A. Piorra, C.H. Solterbeck, Pyroelectric and piezoelectric properties of thick PZT films produced by a new sol–gel route, J. Eur. Ceram. Soc. 25 (2005) 2499–2503. [2] Q.L. Tan, W.D. Zhang, C.Y. Xue, J.J. Xiong, J. Liu, J.H. Li, T. Liang, Design, fabrication and characterization of pyroelectric thin film and its application for infrared gas sensors, Microelectron. J. 40 (2009) 58–62. [3] D. Akai, K. Hirabayashi, M. Yokawa, K. Sawada, Y. Taniguchi, et al., Pyroelectric infrared sensors with fast response time and high sensitivity epitaxial Pb(Zr, Ti)O3 films on epitaxial Y-Al2O3/Si substrates, Sens. Actuators, A-Phys. 130 (2006) 111–115. [4] R.A. Dorey, R.W. Whatmore, Electroceramic thick film fabrication for MEMS, J. ElectroCeram. 12 (2004) 19–32. [5] Sylvia Gebhardt, Lutz Seffner, Falko Schlenkrich, Andreas Schönecker, PZT thick films for sensors and actuator applications, J. Eur. Ceram. Soc. 27 (13–15) (2007) 4117–4180. [6] Hana Uršicˇ, Marko Hrovat, Darko Belavicˇ, Jena Cilenšek, Silvo Drnovšek, Janez Holc, Marina Santo Zarnik, Marija Kosec, Microstructural and electrical characterisation of PZT thick films on LTCC substrates, J. Eur. Ceram. Soc. 28 (2008) 1839–1844. [7] X.X. Wang, K. Murakami, O. Sugiyama, S. Kaneko, Piezoelectric properties, densification behavior and microstructural evolution of low temperature sintered PZT ceramics with sintering aids, J. Eur. Ceram. Soc. 21 (2011) 1367– 1370. [8] D.L. Corker, R.W. Whatmore, E. Ringgaard, W.W. Wolny, Liquid-phase sintering of PZT ceramics, J. Eur. Ceram. Soc. 20 (2000) 2039–2045. [9] B.H. Chen, L. Wu, Y.B. Chen, Piezoresponse of Nb-Doped PZT Ceramic and its application, ISAF, 16th IEEE, Nara, 2007, pp. 599–602. [10] H.Y. Chang, S.Y. Cheng, C.L. Sheu, Microwave sintering of ferroelectric PZT thick films, Mater. Lett. 62 (2008) 3620–3622. [11] F. Dauchy, R.A. Dorey, Patterned crack-free PZT thick films for microelectromechanical system applications, Int. J. Adv. Manuf. Technol. 33 (2007) 86–94. [12] S.H. Kim, C.E. Kim, Y.J. Oh, Influence of Al2O3 diffusion barrier and PbTiO3 seed layer on microstructural and ferroelectric characteristics of PZT thin films by sol-gel spin coating method, Thin solid films 305 (1997) 321–326. [13] Y. Jeon, D. Kim, K. No, S. Kim, J. Chung, Residual stress analysis of Pt bottom electrodes on ZrO2/SiO2/Si and SiO2/Si substrates for Pb(Zr, Ti)O3 thick films, Jpn. J. Appl. Phys. 39 (2000) 2705–2709. [14] N. Abdulbasit, H.K. Kim, Growth of highly oriented Pb(Zr, Ti)O3 films on MgObuffered oxidized Si substrates and its application to ferroelectric nonvolatile memory field-effect transistors, Appl. Phys. Lett. 73 (1998) 3941–3943. [15] B.T. Liu, C.S. Cheng, F. Li, L. Ma, Q.X. Zhao, Z. Yan, et al., Ni–Al diffusion barrier layer for integrating ferroelectric capacitors on Si, Appl. Phys. Lett. 88 (2006) 252903:1–252903:3. [16] F.F.C. Duval, R.A. Doreyk, R.H. Haigh, R.W. Whatmore, Stable TiO2/Pt electrode structure for lead containing ferroelectric thick films on silicon MEMS structures, Thin solid films 44 (2003) 35–240. [17] H.J. Nam, H.H. Kim, W.J. Lee, The effects of the preparation conditions and heat-treatment conditions of Pt/Ti/SiO2/Si substrates on the nucleation and growth of Pb(Zr, Ti)O3 films, Jpn. J. Appl. Phys. 37 (1998) 3462–3470. [18] D.Y. Lee, J. Shim, T.S. Kim, J.H. Park, Optimization of functional layers in piezoelectric thick film MEMS process, in: IEEE International Conference, Kaohsiung, 2011, pp. 64–67. [19] C.G. Pu, L. Xiang, Pyroelectric coefficient measurement for ferroelectric materials, Infrared and Laser Engineering 35 (2006) 771–776 (in Chinese).
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[24] J.G. Pepin, W. Borland, P. O’Callaghan, R.J.S. Yong, Electrode-based causes of delaminations in multilayer ceramic capacitors, J. Am. Ceram. Soc. 72 (1989) 2287–2291. [25] R. Rolls, R.D. Shaw, The influence of borate coatings on the high-temperature oxidation of iron, Corros. Sci. 14 (1974) 431–441. [26] R.Y. Hu, Z.L. Wang, Y.L. Liu, Thin film interaction between titanium and silicon, Res. Prog. See. 7 (1987) 252–256. [27] C.G. Wu, Y.S. Luo, Q.X. Peng, W.B. Luo, W.L. Zhang, Manufacture of large area porous PZT thick films and pyroelectric detector based on screen-printing technology, Infrared Laser Eng. 40 (2011) 1225–1229.
Contents lists available at SciVerse ScienceDirect
Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared
The improvement of pyroelectric properties of PZT thick films on Si substrate by TiOx barrier layer Q.X. Peng a, C.G. Wu a,⇑, W.B. Luo a, L. Jin a,b, W.L. Zhang a, C. Chen a, X.Y. Sun a a b
State Key Lab of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China Electronic Information Division, Ministry of Industry and Information Technology, Beijing 100036, China
h i g h l i g h t s " Effects of TiOx thickness on PZT pyroelectric properties are studied. " TiOx are prepared by thermal oxidation of melt Ti layer. " At 750 °C sintering temperature, TiOx layers are significant in blocking Si diffusion. " Pyroelectric coefficient of PZT thick films are enhanced by 70% with 400 nm TiOx thickness. " Two diffusion barrier mechanisms are presented.
a r t i c l e
i n f o
Article history: Received 18 July 2012 Available online 29 January 2013 Keywords: PZT TiOx Diffusion barrier EPD
a b s t r a c t The effects of TiOx diffusion barrier layer thickness on the microstructure and pyroelectric characteristics of PZT thick films were studied in this paper. The TiOx layer was prepared by thermal oxidation of Ti thin film in air and the PZT thick films were fabricated by electrophoresis deposition method (EPD). To demonstrate the barrier effect of TiOx layer, the electrode/substrate interface and Si content in PZT thick films were characterized by scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS), respectively. The TiOx barrier thickness shows significant influence on the bottom electrode and the pyroelectric performance of the PZT thick films. The average pyroelectric coefficient of PZT films deposited on 400 nm TiOx layer was about 8.94 109 C/(cm2 K), which was improved by 70% than those without diffusion barrier layer. The results showed in this study indicate that TiOx barrier layer has great potential in fabrication of PZT pyroelectric device. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Due to the excellent pyroelectric properties of lead zirconate titanium (PZT) thick film, it has been intensively researched and widely applied in pyroelectric sensors [1–3]. The integration of PZT thick films with Si-based substrate is of great technical importance because of the various advantages, such as fabricating pyroelectric devices using MEMS technology and the integration of these device with Si integrated circuits. Many methods, such as screen printing, EPD, composite film technology, and direct writing [4], have been used to grow PZT thick films. A high temperature sintering process is needed to transform the green body to ceramic film no matter what method are used to deposit the thick films. However, serious interdiffusion and chemical reaction usually happen during this sintering process [4]. The reaction productions (such as lead silicate), formed during sintering process, are detri⇑ Corresponding author. Tel./fax: +86 028 83202140. E-mail addresses: [email protected] (Q.X. Peng), [email protected] (C.G. Wu). 1350-4495/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.infrared.2013.01.003
mental to the pyroelectric properties of the films [5,6]. Thus, the imperative problem to PZT thick films is the interdiffusion and chemical reaction happened during the high temperature sintering process [4]. Numerous material researchers have been devoting to this issue. In principle, there are two approved ways which could be effective in blocking the interdiffusion. The first one is to reduce sintering temperature by using sintering aid. Various of low melting temperature metallic oxides, such as LiBiO2, PbO–Cu2O, Pb3O4, Nb2O5 [7–9], were adopted to decrease the sintering temperature of ceramic. Nevertheless, the optimized sintering temperature of PZT thick films in most published results (i.e. for the films prepared on the basis of PZT powders) was still higher than the formation temperature of lead silicate compound (720 °C) [10,11]. The second way is to insert a diffusion barrier layer such as Al2O3 [12], ZrO2 [13], MgO [14], NiAl [15], between the bottom electrode and Si substrate, which has been found to be more effective to prevent the interdiffusion and reducing the formation of impurity phase. Compared to these reported barrier layers, TiOx has the
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Q.X. Peng et al. / Infrared Physics & Technology 58 (2013) 51–55
following advantages. Firstly, the stability of the bottom electrode could be enhanced by TiOx barriers because of its moderate thermal expansion coefficient nature [16]. Secondly, because of its chemical compatibility with the PZT composition, it would not produce melt element contaminant in the thick films [17]. However, the influence of TiOx diffusion barrier layer thickness on the pyroelectric properties of PZT thick films on Si substrate was not well studied. TiOx barrier layer was fabricated thermal oxidation of Ti thin film in air in this paper. The thickness effects of TiOx barrier on the microstructure and pyroelectric performance of the PZT thick films on SiO2/Si substrate were studied by SEM and dynamic pyroelectric measurements.
2. Experiment procedures
2.4. Microstructure and electric properties evaluation The surface topography and root mean square (RMS) roughness of Ti and oxidized TiOx were examined by atomic force scanning (AFM). The crystalline phase of the barrier layer was characterized by XRD. SEM and EDS were carried out to indentify the electrode/ substrate interface and Si contents in PZT thick films, respectively. For electrical measurements, Pt(100 nm)/Ti(40 nm) top electrode array was grown by DC sputtering to form capacitor structures. The area of each electrode pad is 1 mm2. The deposition parameters are as the same as the bottom electrode. After that, all the films were poled at 150 °C for 15 min under an applied electric field of 6 V/lm. Finally, the pyroelectric and ferroelectric properties of the samples were tested with pyroelectric coefficient (Pc) auto-measurement system [19,20]. The system has the advantages of high temperature stability (±0.002 °C), low noise level (14 nV) and low system relative error (3‰).
2.1. Diffusion barrier layer process TiOx diffusion barrier layer was fabricated by oxidizing the Ti metal layer. To obtain different TiOx thickness, Ti films with different thickness were deposited on SiO2/Si(1 0 0) substrate by DC magnetron sputtering at room temperature. The base vacuum of Ti sputtering is higher than 7.5 107 torr. The deposition was carried out at 1.5 103 torr argon. The sputtering voltage and current were 320 V and current 100 mA, respectively. The sputtering rate of Ti metallic layer was 12 nm/min. After deposition, The Ti layers were annealed in a furnace at 700 °C for 1 h to oxidize the Ti to TiOx immediately. By controlling the sputtering time, TiOx barrier thickness from 0 nm to 600 nm with interval value about 100 nm were prepared.
2.2. Bottom electrode deposition Two-hundred nanometer Pt bottom electrode was deposited on the TiOx diffusion barrier layer. The substrate was cleaned by ultrasonic wave cleaner and oxygen plasma to remove the inorganic and organic residual on titanium oxidation layer surface before deposition. The sputtering voltage of Pt deposition was 385 V. The other sputtering parameters, including gas, vacuum, current, were the same as the Ti metallic layer deposition.
2.3. PZT thick film preparation The PZT thick film was deposited by EPD method using PZT (Pb(Zr0.3Ti0.7)O3, Hayashi Chemical Industry Company, Japan, commercial grade)/ethanol powder suspension with 2 mol% lead germinate (PGO) sintering aid. EPD is a competitive method because of its capability of directly patterning the green films [18]. This method allows the absence of the following etching action for the ceramic thick films which may bring about negative impact on the material properties. Ethanol suspension with PZT, PGO and triethanolamine (TEA) were vibratory milling for 4 h. In order to charge the particles, the PH of the suspension was adjusted to five by diluting nitric acid. The suspension was then homogenized via ultrasonic dispersing and magnetic stirring vigorously. The charged PZT particles were drove toward the cathode by the direct voltage applied between the graphite anode and counter Pt/ TiOx cathode. The deposition rate was about 10 lm/min. 30 lm thick PZT films were deposited by EPD in 3 min. All the thick films were sintered at 750 °C for 1 h in furnace with rising rate 4 °C and cooling rate 2.5 °C per minute, respectively.
3. Results and discussion 3.1. Microstructure of the TiOx barrier layer Ti metal layers were thermal oxidized at 700 °C for 1 h in air atmosphere to obtain TiOx diffusion barrier layers. The XRD patterns of Ti and TiOx films were showed in Fig. 1. According to the XRD results, there is no diffraction peaks related with Ti or TiOx except Si (2 0 0) peak in the pre-oxidation specimen due to sputtering condition [21]. After oxidation, all the peaks in the XRD patterns can be indexed into anatase and rutile phase of TiOx. The oxidation result in this paper is disaccord with that composed of pure rutile TiO2 reported by F.F.C. Duval et al. [16]. In the oxidation process, the growing route of TiOx crystals was from the Ti/air interface to Ti/substrate interface. As time progresses, oxygen has to pass through the prior titanium oxide in order to reach the fresh interface to convert the Ti (or lower Ti) to TiOx (or higher Ti) [22]. The partially transformation of Ti layer to rutile TiOx may result from the decreased migration of oxygen caused by the high density of the sputtered Ti layer. The surface morphologies of the barrier layer before and after oxidation were also presented in Fig. 1. As shown in Fig. 1b, the RMS of barrier layer before oxidation was about 2.78 nm. The barrier layer surface roughness after oxidation shown in Fig. 1c increased to about 2.87 nm. There was no obvious change in the surface roughness of the films before and after oxidation. However, the surface grain size of the oxidized film increased from about 50 nm to 75 nm compared to the pre-oxidized one. The film thickness increased from 135 nm to 200 nm after oxidation according to surface profiler (Veeco dektak 150) measurement results. The expansion of grain size and thickness are resulted from the incorporation of oxygen into titanium [17,21].
3.2. Effects of TiOx thickness on the PZT thick films Thirty micrometer PZT thick films were then deposited by EPD on Si substrate with different TiOx barrier layer thickness ranging from 0 to 600 nm. Fig. 2 displays the cross sectional SEM images of the PZT/Pt/x nm TiOx/SiO2 (denoted Tx in the following text where x represents the TiOx thickness) samples sintered at 750 °C for 1 h. The SEM cross section image of PZT thick films deposited on Pt/SiO2/Si substrate without TiOx barrier (T0) was shown in Fig. 2a. It can be seen from the graph that there are many delaminated area between Pt and SiO2. A lot of irregular vacancies were also observed at the SiO2 surface indicating that some Si atoms may diffused into the Pt/TiOx layer and PZT film.
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Fig. 1. XRD patterns and AFM surface morphology of Ti film and TiOx film oxidized at 700 °C.
Fig. 2. Cross sectional SEM of PZT thick films for different TiOx thickness.
As the TiOx thickness increased from 0 to 400 nm, the vacancies were eliminated and the TiOx/SiO2 interface became smoother and clearer gradually. As the thickness further increased, vacancies and micropores appeared again at the TiOx/SiO2 interface, which was formed to release thermal stress between the barrier layer and substrate. When the TiOx thickness reached up to 600 nm, the electrode structure was deteriorated and became even worse than the one without TiOx barrier. Two possible mechanisms were proposed to explain this phenomenon. Firstly, oxygen transferred through Pt layer and reacted with unoxidized titanium and low valence titanium ions (maybe randomly distributed in TiOx layer). This will change the volume of TiOx layers, and destroyed the multilayer structure consequently. Secondly, thermal strain will be accumulated at the Pt/TiOx/SiO2 layers during sintering and cooling processes because the thermal expansion coefficient mismatch between Pt (14.2 106/K), SiO2 (0.4 106/K) and TiOx (8.5 106/K) [23]. The different expansion and shrinkage rate of Pt/TiOx/SiO2 layers in the thermal treatment process tended to produce stress between interlayer, which would result in warping and delamination [24]. In order to analyze the effects of the diffusion barrier layer, Si content in PZT thick films were measured by EDS for each sample
with different TiOx thickness. The testing pots were about 1 lm above the Pt layer surface. Two random testing pots at the same depth level were selected to do the analyses. The average values
Fig. 3. Si content measurements in PZT films with different TiOx diffusion barrier layer thickness.
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of each Tx were plotted and the offset values were showed as error bars in Fig. 3. According to the results, it was found the Si content in sample T0 without TiOx diffusion barrier layer is about 45 wt%, which proved the Si diffusion phenomenon into PZT at sintering process. The Si content decreased gradually as the TiOx thickness layer increased. When the TiOx thickness was equal to or larger than 300 nm, almost no Si element could be detected by EDS. These results indicated that the barrier layer thicker than 300 nm could block the diffusion of Si into PZT thick film effectively. When TiOx diffusion barrier layer was inserted, the diffusion of Si into PZT thick films became more difficult because of the blocking effect and barrier effect of TiOx layer [25]. Firstly, the defects in TiOx/ SiO2 interface reduced the transmission paths and block the diffusion of Si element. Secondly, TiSi2 can form at the TiOx/SiO2 interface if the thermal treat temperature is higher than 600 °C [26]. The mobility of Si was probably weakened continuously as Si diffused step by step in the barrier layer if the barrier is thick enough. The Pc of PZT thick films were tested by dynamic method using the measurement system reported by [20]. The Pc is given by [19]:
Pc ¼
Ip AdT=dt
4. Conclusion A feasible way to produce TiOx diffusion barrier layer for PZT thick films sintered at temperature up to 750 °C was shown in this paper. The mixed crystallite TiOx including anatase and rutile could be obtained by thermal oxidation of Ti at 700 °C for 1 h. It has been shown that thick TiOx diffusion barrier layer could significantly suppress the interdiffusion. Moreover, it was found that TiOx layer with proper thickness could improve the bottom electrode characters and enhance the pyroelectric properties of PZT thick film. Whereas, the deterioration of the bottom electrode deposited on too thick TiOx barrier would lower the final measured Pc.
Acknowledgments This work has been supported by the National Natural Science Foundation of China (NSFC. 51102037) and the Fundamental Research Funds for the Central Universities from UESTC (Nos. ZYGX2010J030 and ZYGX2011J023).
ð1Þ
where A is the capacity area, Ip is the response current and dT/dt is the slope of temperature curve. The average Pc values and offset values of the Tx were calculated and plotted in Fig. 4. When the TiOx thickness was 0 nm, the average Pc and offset value was 5.29 109 C/(cm2 K) and 0.20 109 C/(cm2 K), respectively. The Pc of the PZT samples increased as TiOx became thicker at first. When the TiOx thickness was 400 nm, the average Pc of the PZT (T400) reached the maximal value (8.94 109 C/(cm2 K)), which was 70% larger than the one without TiOx barrier (T0). The Pc of T400 was almost equal to that of the PZT thick film fabricated on alumina by screen-printing [27]. Combined the EDS result in Fig. 3, the improvement of pyroelectric performance was result from the decreased Si diffusion. However, when the thickness of TiOx was larger than 400 nm, the Pc started to decrease. When the TiOx layer was increased to 600 nm, the average Pc decreased by about 14% compared to that of T400 group. The decrease of Pc may be attribute to the actual effect area loss inferred from the poor electrode structure. As shown in Fig. 2, more vacancies can be observed when the TiOx thickness is more than 400 nm. The appearance of vacancies undermined the bottom electrode might tear the integrity of the conductive layer, which leaded to the loss of actual effective area of the capacity structure. The decrease of effective electrode area will diminish the measured Pc in formula (1). Therefore, these Pc results of Tx (x > 400) should not represent exact pyroelectric properties of PZT thick film.
Fig. 4. Pyroelectric coefficient of PZT thick film species for different TiOx thickness.
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