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Direct-write inkjet printing for fabrication of barium strontium titanate-based tunable circuits
Thin Solid Films 515 (2007) 3820 – 3824 www.elsevier.com/locate/tsf
Direct-write inkjet printing for fabrication of barium strontium titanate-based tunable circuits T. Kaydanova ⁎, A. Miedaner, J.D. Perkins, C. Curtis, J.L. Alleman, D.S. Ginley National Renewable Energy Laboratory, 1617 Cole Blvd Golden CO 80401 USA Received 10 January 2006; received in revised form 28 September 2006; accepted 6 October 2006 Available online 14 December 2006
Abstract Tunable capacitors with up to 30% tuning and a loss tangent (tanδ) less than 0.002 at 1 MHz were fabricated from Ba0.6Sr0.4TiO3 (BST) films using inkjet-printed liquid metalorganic precursors. BST films of various thicknesses were produced by printing multiple stacks of the individual inkjet-printed layers. The dielectric constant of the printed films increased as a function of thickness. The largest dielectric constant, 1000, and the highest tunability, 30%, were measured on a 420 nm thick film, the thickest film studied in this work. Spray-printed silver contacts were employed and demonstrated good adhesion and good electrical contact to the inkjet-printed BST films. This also demonstrated proof of principle for directwrite printing of metal contacts onto BST films from metalorganic sources. © 2006 Elsevier B.V. All rights reserved. Keywords: Direct write; Inkjet; BST; Tunable circuits
1. Introduction Tunable dielectric materials, such as BaxSr1−xTiO3 (BST) and related composites [1] enable low-cost tunable electronics for high frequency GHz applications [2–4]. Compared to bulk or thick films, thin films of such tunable dielectrics offer the additional advantages of reduced operating voltages, lower power loss of the metal contacts and ease of circuit integration. Atmospheric pressure deposition approaches such as sol–gel and metalorganic decomposition (MOD) have demonstrated the potential for low-cost fabrication of tunable dielectric films [5–8]. In such conventional sol–gel and MOD processes, thin films are deposited by a variety of methods including dipcoating, spin-casting and spray-printing of the liquid metalorganic precursors. However, such liquid precursors are also amenable to inkjet printing, a low-cost direct-write deposition tool suitable for fabrication of medium-resolution circuits without any photolithographic processing. Using an inkjet printhead, thin films of BST could potentially be deposited with 20–100 μm spatial resolution [9]. In addition, precursor inks ⁎ Corresponding author. E-mail address: [email protected] (T. Kaydanova). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.10.009
can be formulated for a variety of the other key electronic materials in microwave circuits including conductors, insulators and semiconductors [10–12]. Thus, potentially the entire electronic circuit, as well as some packaging elements, could be deposited by direct-write inkjet printing. This fabrication approach could provide critical economical and environmental advantages due to its' inherent materials utilization efficiency, elimination of both the photolithographic and the chemical etch process steps, a low capital equipment cost and ease of integration with other necessary manufacturing processes. The goal of this work was to demonstrate the feasibility of the inkjet-printing approach for the complete fabrication of tunable microwave circuits. Hence, a very simple tunable device — a coplanar tunable capacitor with a thin BST film and silver contacts from liquid metalorganic precursors was fabricated and characterized [13]. Thin BST films with composition Ba0.6Sr0.4TiO3 were produced by inkjet-printing of a metalorganic precursor on MgO substrates followed by a combined pyrolysis–annealing step. Structural and electrical properties of the inkjet-printed BST films were studied as a function of film thickness. The electrical measurements were done using co-planar capacitors with evaporated metal contacts on inkjet-printed BST films. The largest dielectric constant,
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1000, highest tunability, 30%, at 9 V/μm, and a loss tangent less than 0.002, at room temperature and 1 MHz were obtained for the 420 nm thick film, the thickest used in the study. For some devices, silver electrodes were fabricated by spraypyrolysis of a metalorganic precursor followed by photolithography and chemical etching. At present, the geometrical perfection, specifically the precision of the edge positioning, of the inkjet-printed Ag lines is not sufficient to print the coplanar contact patterns with the required electrode spacing (∼ 10 μm) directly [14]. However we have previously demonstrated that the spray-printed silver coatings have physical properties similar to the inkjet-printed silver [15] from the same precursor. Thus, we used photolithographically defined spray-printed Ag contacts in place of inkjet-printed Ag electrodes for this initial feasibility study. The interdigitated capacitor with spray-printed contact fabricated on the 280 nm thick inkjet-printed BST film demonstrated tunability of 24% at 9 V/μm and a loss tangent less than 0.002. 2. Experimental details 2.1. Inkjet-printed BST films BST films were deposited by inkjet printing of the solution of 1.5 mM of Barium Neodecanoate [Ba(OOC10H19)2], 1 mM of Strontium Neodecanoate [Sr(OOC10H19)2] and 2.5 mM of neat liquid Titanium Butoxide [Ti[O(CH2)5)CH3]4] in 20 ml of butanol. The ink was filtered through 0.2 μm mesh filter prior to supplying it to the inkjet head. The primary components of the inkjet printing set-up were a stationary Microfab MikroJet™ glass-lined piezoelectric printhead with a 50 μm orifice, an X–Y translation stage with a 1 μm positional tolerance and a heated substrate holder capable of heating a 10 cm × 10 cm substrate to 400 °C. The piezoelectric printhead was driven by a computercontrolled waveform-generator with a maximum frequency of 20 kHz. The inks were supplied into the glass capillary of the printhead from syringe reservoirs via 3 mm outside-diameter tygon tubing. BST lines were deposited onto MgO substrates at 75 °C by moving the substrate at 20 mm/s while inkjet printing the precursor solution at 200 drops per second. Single layer rectangular patterns (patches) of BST precursor films were deposited by printing adjacent lines with an 80 μm center-tocenter spacing. Multiple layers were printed for thicker films. The BST precursor films were then pyrolized and annealed in a single thermal cycle by heating the printed films up to 1100 °C at 5 °C/min and holding them at this temperature for three hours in flowing oxygen under ambient pressure in a quartz-tube furnace. The thickness profile of the printed patterns was measured with a DekTak 3 profilometer. Θ−2Θ X-ray diffraction (XRD) was used both to identify the phase formed and to determine the crystalline quality of the annealed BST films. The XRD measurements were performed using Advanced Diffraction System Scintag X1 with Cu source. The surface morphology was studied by Atomic Force Microscopy (AFM) with Autoprobe CP from Park Scientific Instruments in contact mode
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using silicon Ultralever™ ULCT probe from Park Scientific Instruments. 2.2. Straight–gap capacitors with evaporated contacts Straight–gap capacitors with 4 μm thick evaporated Ti/Au/ Ag/Au contacts [13] were fabricated on the top surface of five and six-layer-thick BST films. The dielectric constant ε and tunability (ε(0V) − ε(V)) / ε(0V) of the inkjet-printed films were calculated from the Capacitance–Voltage (C (V)) measurements of the test capacitors using a conformal mapping technique [16]. The 1 MHz C (V) measurements were taken using an HP 4284A Inductance Capacitance Resistance (LCR) meter and a KEPCO BMP 1000 M Direct Current (DC) voltage source at room temperature. 2.3. Interdigital capacitors with spray-printed Ag contacts For the Ag metal contacts, Silver (Hexafluoroacetylacetonate)(1,5-cyclooctadiene) was dissolved in butanol [12] and spray-coated in air on a four-layer thick annealed BST film using a hand-held Vega 2000 airbrush. During spraying, the substrate was heated to 350 °C with a resistive heater. The conductivity of the sprayed silver layer was measured using a conventional four-probe technique. Interdigitated capacitor contacts [13] were fabricated from the spray-printed silver layer using photolithography to define the pattern followed by etching off the unwanted Ag in 30% concentrated nitric acid. C (V) measurements of the capacitor with spray-printed Ag contacts were performed as described above. 3. Results and discussion 3.1. Structural properties 3.1.1. Inkjet-printed BST After pyrolysis and annealing, the single-pass printed BST lines were ∼ 200 μm wide (Fig. 1) and 35 nm tall at the center. Hence, with an 80 μm center-to-center offset, the overlap between the adjacent lines in the patch patterns was ∼ 120 μm, thus providing good continuity of the printed BST film. The average thickness of a one-layer patch was ∼ 70 nm. The final average thickness of the printed patches was 280 nm, 350 nm and 420 nm for four, five and six layer films respectively,
Fig. 1. Optical microscope image of a 200 μm wide inkjet-printed BST precursor line.
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Fig. 2. Optical microscope image of the 420 nm (a) and 350 nm (b) thick BST films (dark gray areas) with the straight–gap test capacitors fabricated on them (light gray areas).
consistent with 70 nm/layer. The 420 nm thick film developed a network of cracks as seen in Fig. 2a, however the thinner 280 nm and 350 nm thick films were crack free (Fig. 2b). The surface of the inkjet-printed films, as characterized by AFM (Fig. 3), was relatively smooth with a root mean square (RMS) average roughness of 3.5 nm and grain sizes in the range of 50 nm to 300 nm. XRD Θ−2Θ spectra of the 420 nm and 280 nm films, (Fig. 4), show that the films were phase-pure BST. The films were primarily (100) oriented, as expected for BST on MgO, with a small fraction of grains with (110) and (111) orientations. The average size of the BST crystallites as calculated from full width at half maximum of the (100) lines [17] was 130 nm for the 420 nm thick film and 70 nm for the 280 nm thick film, in general agreement with the AFM data. The lattice constants for the 420 nm and 280 nm thick films were calculated from the position of the XRD peaks in Fig. 4 [17]. Both films had the same lattice constant, 0.3956 ± 0.0002 nm, corresponding to a Ba content of ∼ 56% [18] slightly less than the intended 60% Ba composition. This 4% deviation could simply be due to a slight weighing error of the precursor constituents.
interdigitated capacitor shown in Fig. 5 was formed by etching a photolithographically patterned spray-printed silver layer. These results with spray-coated silver together with previous work on inkjet-printed Ag on Si wafers [15] strongly suggest that high conductivity and good adhesion can be expected for inkjet-printed silver on BST. 3.2. Electrical properties 3.2.1. Inkjet-printed BST films For both the straight–gap capacitors with evaporated contacts and the interdigitated capacitors with spray-printed contacts, the loss parameter D (the ratio of active to reactive part of measured impedance) of the tested capacitors was below the
3.1.2. Spray-printed interdigitated electrodes The spray-printed silver layer deposited at 350 °C was 4 μm thick with resistivity ρ = 2 μΩ·cm, close to that of the bulk Ag. The good adhesion of the metal layer to the inkjet-printed BST film was confirmed with the Scotch tape pull test. The
Fig. 3. AFM image of the 280 nm thick inkjet-printed BST film.
Fig. 4. XRD Θ−2Θ patterns of the 420 nm and 280 nm thick BST films on MgO. The two curves on the graph are offset relative to the absolute intensity scale for the sake of the comparison convenience.
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represent a real trend in tunability of the BST films with thickness. There is also a possibility that the tunability and the dielectric constant of the BST films as-measured with these test capacitors depend upon the metal/dielectric interface [20]. Hence, we note that in the evaporated electrode, the Ti adhesion layer is in contact with the BST while in the spray-printed capacitors, silver directly contacts the BST. The increase of the effective dielectric constant with film thickness that we observed here for the inkjet-printed BST is common for thin BST films [8,19,21]. It is often attributed to the influence of the metal/BST interface [20,21]. In our films, it may also be due to the increasing grain size [22,23]. According to XRD data, the average grain size of the 420 nm thick film was almost two times greater than that of the 280 nm thick film (130 nm vs 70 nm respectively).
Fig. 5. A microscope image of a section of a spray-printed, photolithographically defined interdigitated capacitor.
measurement accuracy of the LCR meter (0.002). Thus the tanδ the loss tangent of the films at 1 MHz was less than 0.002. Fig. 6a shows that the inkjet-printed BST films are tunable i.e. the capacitance of the test capacitors was reduced by application of the DC bias. As expected, for a thicker film the capacitance of the straight–gap capacitor on a 420 nm thick film was larger over the whole range of tuning voltages than the capacitance of the same test capacitor on 350 nm thick film. The 280 nm thick film was tested using an interdigitated capacitor geometry resulting in a much larger effective electrode width and therefore its' capacitance was larger than that of the thicker films measured with straight–gap capacitors. The dielectric constant as-presented in Fig. 6b as a function of bias voltage were calculated from the C (V) curves of Fig. 6a taking the different capacitor geometries into account [16]. Strictly speaking this calculation is only correct at 0V. At higher voltages due to the planar construction of the electrodes the distribution of the bias field in the dielectric layer is not uniform, this results in non-uniform dielectric constant for the tunable dielectric. Thus the dielectric constant calculated at nonzero bias should be seen as an average or “effective” dielectric constant. The zero-bias dielectric constant ranged between 500 and 1000 and increased with the film thickness as can be seen by looking at the leftmost points in Fig. 6b. The maximum tunability of 30% of the dielectric constant at the maximum applied field of 9 V/μm was achieved on the thickest 420 nm thick film. A 20% tunability was measured for the 350 nm thick film with the electrodes of the same geometry. Note that a lower tunability is to be expected for a thinner film with a lower dielectric constant [19]. For the interdigitated capacitor on the 280 nm thick BST film, the tunability was 24%, 4% greater than that measured for thicker 350 nm film with straight–gap capacitor. We believe that this is likely due to the difference in the electrode geometry of the test capacitors [13] and does not
3.2.2. Spray-printed interdigitated electrodes The results of the electrical testing of the spray-printed interdigitated capacitor are represented by the topmost C (V) curve in Fig. 6a. As-expected for a BST capacitor, the capacitance decreases with increasing bias voltage. The dielectric constant of the 280 nm BST film (Fig. 6b) appears to be in line with what was measured for other films that were processed with
Fig. 6. Room temperature dielectric properties of BST films measured at 1 MHz. Panel (a): The C (V) curves of the test capacitors on inkjet-printed BST films of various thickness. Panel (b): The dependence of dielectric constant on the bias field for the inkjet-printed BST films of various thicknesses. The straight–gap capacitors are represented by solid lines, the interdigitated capacitor by dashed lines.
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evaporated contacts. Based on this, and taking into account the good tunability obtained with spray-printed electrodes, we conclude that a reasonable electrical contact was established directly between the spray-printed Ag and underlying BST film. Hence, along with the good conductivity of the silver layer and its' good adhesion to the BST film, we take this as a proof of principle demonstration for the viability of metal electrodes from liquid metalorganic precursors on BST thin films for tunable circuit applications. However, no direct comparison with evaporated contacts could be made at this point, since the spray-printed and evaporated capacitors had different electrode geometry as well as the differences in the thickness of the tested BST layers.
[3] [4]
[5] [6] [7] [8] [9] [10]
4. Conclusions Crystalline tunable Ba0.6Sr0.4TiO3 (BST) films of various thickness (280 nm, 350 nm and 420 nm) were produced by inkjet printing of a metalorganic precursor. The BST films were granular and had a smooth surface (3.5 nm RMS), however the 420 nm thick film had a developed network of microcracks. The dielectric constant of the BST films increased as a function of the film thickness. The highest tunability, 30%, was measured on the thickest, 420 nm, film which had a dielectric constant of 1000. The tunability of the thinnest, 280 nm, thick film was 24%. Interdigitated contacts produced by photolithographic patterning of spray-printed Ag coating demonstrated good conductivity, good adhesion and electrical contact to an inkjet-printed BST film. Taken together with the reasonable structural and dielectric properties of the inkjet-printed BST film, these results suggest that it should be possible to fabricate complete BST-based tunable circuit elements using sequential inkjet printing. References [1] L.C. Sengupta, E. Ngo, M.E. O'Day, S. Stowell, R. Lancto, IEEE Electron Device Lett. (1995) 622. [2] C.M. Carlson, T.V. Rivkin, P.A. Parilla, J.D. Perkins, D.S. Ginley, A.B. Kozyrev, V.N. Oshadchy, A.S. Pavlov, A. Golovkov, M. Sugak, D. Kalinikos, L.C. Sengupta, L. Chiu, X. Zhang, Y. Zhu, S. Sengupta, Materials Issues for Tunable RF and Microwave Devices Symposium, 30
[11] [12]
[13] [14]
[15]
[16] [17] [18] [19] [20] [21] [22] [23]
Nov.–2 Dec. 1999, Materials Research Society, Warrendale, PA, USA, 2000, p. 15, Boston, MA, USA. F.A. Miranda, R.R. Romanofsky, F.W. Van Keuls, C.H. Mueller, R.E. Treece, T.V. Rivkin, Integr. Ferroelectr. 17/1–4 (1998) 231. A. Kozyrev, A. Ivanov, V. Keis, H. Khazov, V. Osadchy, T. Samoilova, O. Soldatenkov, A. Pavlov, G. Keopf, C. Mueller, D. Galt, T. Rivkin, in: R. Meixner (Ed.), IEEE MTT-S International Microwave Symposium Digest, vol. 2, IEEE, New York, NY, 1998, p. 985. F. Yan, P. Bao, Z. Zhang, J. Zhu, Y. Wang, H.L.W. Chan, C.-L. Choy, Thin Solid Films 375/1–2 (2000) 184. J.J. Xu, A.S. Shaikh, R.W. Vest, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 36/3 (1989) 307. A.B. Catalan, J.V. Mantese, A.L. Micheli, N.W. Schubring, J. Appl. Phys. 76/4 (1994) 2541. S.B. Krupanidhi, C.-J. Peng, Thin Solid Films 305/1–2 (1997) 144. W.R. Cox, T. Chen, Optics Photonics News 12/6 (2001) 32. D.L. Schulz, C.J. Curtis, R.A. Flitton, H. Wiesner, J. Keane, R.J. Matson, K.M. Jones, P.A. Parilla, R. Noufi, D.S. Ginley, J. Electron. Mater. 27 (1998) 433. D.B. Wallace, W.R. Cox, D.J. Hayes, Direct-Write Technologies for Rapid Prototyping Applications, Academic Press, CA, 2002, p. 177. T. Rivkin, C.J. Curtis, A. Miedaner, J. Alleman, D.L. Schulz, D.S. Ginley, in: G.S. Mathad (Ed.), 198th International Symposium of the Electrochemical Society, The Electrochemical Society, Inc, Phoenix, Arizona, 2000, p. 80. T.V. Rivkin, C.M. Carlson, P.A. Parilla, D.S. Ginley, Integr. Ferroelectr. 29 (2000) 215. T. Rivkin, C. Curtis, A. Miedaner, J. Perkins, J. Alleman, D. Ginley, IEEE 29th Photovoltaics Specialists Conference, IEEE, New Orleans, LA, 2002, p. 1326. C.J. Curtis, D.L. Schulz, A. Miedaner, J. Alleman, T. Rivkin, J.D. Perkins, D.S. Ginley, Materials Research Society Symposium, vol. 678, 2002, Y8.6.1. S.S. Gevorgian, T. Martinson, P.L.J. Linnér, E.L. Kollberg, IEEE Trans. Microwave Theor. Tech. 44/6 (1996) 896. B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, Prentice Hall, Upper Saddle River NJ, 2001, p. 167. M. Adachi, J. Harada, T. Ikeda, S. Nomura, E. Sawaguchi, T. Yamada, Oxides; New Series, vol. III/16a, Springer-Verlag, New York, 1981, p. 296. M. Nayak, T.-Y. Tseng, Thin Solid Films 408/1–2 (2002) 194. O.G. Vendik, S.P. Zubko, L.T. Ter-Martirosyan, Appl. Phys. Lett. 73/1 (1998) 37. B. Panda, G.D. Nigam, S.K. Ray, Indian J. Pure Appl. Phys. 37/4 (1999) 318. A.S. Shaikh, G.M. Vest, J. Am. Ceram. Soc. 69/9 (1986) 682. H.N. Al-Shareef, D. Dimos, M.V. Raymond, R.W. Schwartz, C.H. Mueller, J. Electroceram. 1/2 (1997) 145.
Direct-write inkjet printing for fabrication of barium strontium titanate-based tunable circuits T. Kaydanova ⁎, A. Miedaner, J.D. Perkins, C. Curtis, J.L. Alleman, D.S. Ginley National Renewable Energy Laboratory, 1617 Cole Blvd Golden CO 80401 USA Received 10 January 2006; received in revised form 28 September 2006; accepted 6 October 2006 Available online 14 December 2006
Abstract Tunable capacitors with up to 30% tuning and a loss tangent (tanδ) less than 0.002 at 1 MHz were fabricated from Ba0.6Sr0.4TiO3 (BST) films using inkjet-printed liquid metalorganic precursors. BST films of various thicknesses were produced by printing multiple stacks of the individual inkjet-printed layers. The dielectric constant of the printed films increased as a function of thickness. The largest dielectric constant, 1000, and the highest tunability, 30%, were measured on a 420 nm thick film, the thickest film studied in this work. Spray-printed silver contacts were employed and demonstrated good adhesion and good electrical contact to the inkjet-printed BST films. This also demonstrated proof of principle for directwrite printing of metal contacts onto BST films from metalorganic sources. © 2006 Elsevier B.V. All rights reserved. Keywords: Direct write; Inkjet; BST; Tunable circuits
1. Introduction Tunable dielectric materials, such as BaxSr1−xTiO3 (BST) and related composites [1] enable low-cost tunable electronics for high frequency GHz applications [2–4]. Compared to bulk or thick films, thin films of such tunable dielectrics offer the additional advantages of reduced operating voltages, lower power loss of the metal contacts and ease of circuit integration. Atmospheric pressure deposition approaches such as sol–gel and metalorganic decomposition (MOD) have demonstrated the potential for low-cost fabrication of tunable dielectric films [5–8]. In such conventional sol–gel and MOD processes, thin films are deposited by a variety of methods including dipcoating, spin-casting and spray-printing of the liquid metalorganic precursors. However, such liquid precursors are also amenable to inkjet printing, a low-cost direct-write deposition tool suitable for fabrication of medium-resolution circuits without any photolithographic processing. Using an inkjet printhead, thin films of BST could potentially be deposited with 20–100 μm spatial resolution [9]. In addition, precursor inks ⁎ Corresponding author. E-mail address: [email protected] (T. Kaydanova). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.10.009
can be formulated for a variety of the other key electronic materials in microwave circuits including conductors, insulators and semiconductors [10–12]. Thus, potentially the entire electronic circuit, as well as some packaging elements, could be deposited by direct-write inkjet printing. This fabrication approach could provide critical economical and environmental advantages due to its' inherent materials utilization efficiency, elimination of both the photolithographic and the chemical etch process steps, a low capital equipment cost and ease of integration with other necessary manufacturing processes. The goal of this work was to demonstrate the feasibility of the inkjet-printing approach for the complete fabrication of tunable microwave circuits. Hence, a very simple tunable device — a coplanar tunable capacitor with a thin BST film and silver contacts from liquid metalorganic precursors was fabricated and characterized [13]. Thin BST films with composition Ba0.6Sr0.4TiO3 were produced by inkjet-printing of a metalorganic precursor on MgO substrates followed by a combined pyrolysis–annealing step. Structural and electrical properties of the inkjet-printed BST films were studied as a function of film thickness. The electrical measurements were done using co-planar capacitors with evaporated metal contacts on inkjet-printed BST films. The largest dielectric constant,
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1000, highest tunability, 30%, at 9 V/μm, and a loss tangent less than 0.002, at room temperature and 1 MHz were obtained for the 420 nm thick film, the thickest used in the study. For some devices, silver electrodes were fabricated by spraypyrolysis of a metalorganic precursor followed by photolithography and chemical etching. At present, the geometrical perfection, specifically the precision of the edge positioning, of the inkjet-printed Ag lines is not sufficient to print the coplanar contact patterns with the required electrode spacing (∼ 10 μm) directly [14]. However we have previously demonstrated that the spray-printed silver coatings have physical properties similar to the inkjet-printed silver [15] from the same precursor. Thus, we used photolithographically defined spray-printed Ag contacts in place of inkjet-printed Ag electrodes for this initial feasibility study. The interdigitated capacitor with spray-printed contact fabricated on the 280 nm thick inkjet-printed BST film demonstrated tunability of 24% at 9 V/μm and a loss tangent less than 0.002. 2. Experimental details 2.1. Inkjet-printed BST films BST films were deposited by inkjet printing of the solution of 1.5 mM of Barium Neodecanoate [Ba(OOC10H19)2], 1 mM of Strontium Neodecanoate [Sr(OOC10H19)2] and 2.5 mM of neat liquid Titanium Butoxide [Ti[O(CH2)5)CH3]4] in 20 ml of butanol. The ink was filtered through 0.2 μm mesh filter prior to supplying it to the inkjet head. The primary components of the inkjet printing set-up were a stationary Microfab MikroJet™ glass-lined piezoelectric printhead with a 50 μm orifice, an X–Y translation stage with a 1 μm positional tolerance and a heated substrate holder capable of heating a 10 cm × 10 cm substrate to 400 °C. The piezoelectric printhead was driven by a computercontrolled waveform-generator with a maximum frequency of 20 kHz. The inks were supplied into the glass capillary of the printhead from syringe reservoirs via 3 mm outside-diameter tygon tubing. BST lines were deposited onto MgO substrates at 75 °C by moving the substrate at 20 mm/s while inkjet printing the precursor solution at 200 drops per second. Single layer rectangular patterns (patches) of BST precursor films were deposited by printing adjacent lines with an 80 μm center-tocenter spacing. Multiple layers were printed for thicker films. The BST precursor films were then pyrolized and annealed in a single thermal cycle by heating the printed films up to 1100 °C at 5 °C/min and holding them at this temperature for three hours in flowing oxygen under ambient pressure in a quartz-tube furnace. The thickness profile of the printed patterns was measured with a DekTak 3 profilometer. Θ−2Θ X-ray diffraction (XRD) was used both to identify the phase formed and to determine the crystalline quality of the annealed BST films. The XRD measurements were performed using Advanced Diffraction System Scintag X1 with Cu source. The surface morphology was studied by Atomic Force Microscopy (AFM) with Autoprobe CP from Park Scientific Instruments in contact mode
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using silicon Ultralever™ ULCT probe from Park Scientific Instruments. 2.2. Straight–gap capacitors with evaporated contacts Straight–gap capacitors with 4 μm thick evaporated Ti/Au/ Ag/Au contacts [13] were fabricated on the top surface of five and six-layer-thick BST films. The dielectric constant ε and tunability (ε(0V) − ε(V)) / ε(0V) of the inkjet-printed films were calculated from the Capacitance–Voltage (C (V)) measurements of the test capacitors using a conformal mapping technique [16]. The 1 MHz C (V) measurements were taken using an HP 4284A Inductance Capacitance Resistance (LCR) meter and a KEPCO BMP 1000 M Direct Current (DC) voltage source at room temperature. 2.3. Interdigital capacitors with spray-printed Ag contacts For the Ag metal contacts, Silver (Hexafluoroacetylacetonate)(1,5-cyclooctadiene) was dissolved in butanol [12] and spray-coated in air on a four-layer thick annealed BST film using a hand-held Vega 2000 airbrush. During spraying, the substrate was heated to 350 °C with a resistive heater. The conductivity of the sprayed silver layer was measured using a conventional four-probe technique. Interdigitated capacitor contacts [13] were fabricated from the spray-printed silver layer using photolithography to define the pattern followed by etching off the unwanted Ag in 30% concentrated nitric acid. C (V) measurements of the capacitor with spray-printed Ag contacts were performed as described above. 3. Results and discussion 3.1. Structural properties 3.1.1. Inkjet-printed BST After pyrolysis and annealing, the single-pass printed BST lines were ∼ 200 μm wide (Fig. 1) and 35 nm tall at the center. Hence, with an 80 μm center-to-center offset, the overlap between the adjacent lines in the patch patterns was ∼ 120 μm, thus providing good continuity of the printed BST film. The average thickness of a one-layer patch was ∼ 70 nm. The final average thickness of the printed patches was 280 nm, 350 nm and 420 nm for four, five and six layer films respectively,
Fig. 1. Optical microscope image of a 200 μm wide inkjet-printed BST precursor line.
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Fig. 2. Optical microscope image of the 420 nm (a) and 350 nm (b) thick BST films (dark gray areas) with the straight–gap test capacitors fabricated on them (light gray areas).
consistent with 70 nm/layer. The 420 nm thick film developed a network of cracks as seen in Fig. 2a, however the thinner 280 nm and 350 nm thick films were crack free (Fig. 2b). The surface of the inkjet-printed films, as characterized by AFM (Fig. 3), was relatively smooth with a root mean square (RMS) average roughness of 3.5 nm and grain sizes in the range of 50 nm to 300 nm. XRD Θ−2Θ spectra of the 420 nm and 280 nm films, (Fig. 4), show that the films were phase-pure BST. The films were primarily (100) oriented, as expected for BST on MgO, with a small fraction of grains with (110) and (111) orientations. The average size of the BST crystallites as calculated from full width at half maximum of the (100) lines [17] was 130 nm for the 420 nm thick film and 70 nm for the 280 nm thick film, in general agreement with the AFM data. The lattice constants for the 420 nm and 280 nm thick films were calculated from the position of the XRD peaks in Fig. 4 [17]. Both films had the same lattice constant, 0.3956 ± 0.0002 nm, corresponding to a Ba content of ∼ 56% [18] slightly less than the intended 60% Ba composition. This 4% deviation could simply be due to a slight weighing error of the precursor constituents.
interdigitated capacitor shown in Fig. 5 was formed by etching a photolithographically patterned spray-printed silver layer. These results with spray-coated silver together with previous work on inkjet-printed Ag on Si wafers [15] strongly suggest that high conductivity and good adhesion can be expected for inkjet-printed silver on BST. 3.2. Electrical properties 3.2.1. Inkjet-printed BST films For both the straight–gap capacitors with evaporated contacts and the interdigitated capacitors with spray-printed contacts, the loss parameter D (the ratio of active to reactive part of measured impedance) of the tested capacitors was below the
3.1.2. Spray-printed interdigitated electrodes The spray-printed silver layer deposited at 350 °C was 4 μm thick with resistivity ρ = 2 μΩ·cm, close to that of the bulk Ag. The good adhesion of the metal layer to the inkjet-printed BST film was confirmed with the Scotch tape pull test. The
Fig. 3. AFM image of the 280 nm thick inkjet-printed BST film.
Fig. 4. XRD Θ−2Θ patterns of the 420 nm and 280 nm thick BST films on MgO. The two curves on the graph are offset relative to the absolute intensity scale for the sake of the comparison convenience.
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represent a real trend in tunability of the BST films with thickness. There is also a possibility that the tunability and the dielectric constant of the BST films as-measured with these test capacitors depend upon the metal/dielectric interface [20]. Hence, we note that in the evaporated electrode, the Ti adhesion layer is in contact with the BST while in the spray-printed capacitors, silver directly contacts the BST. The increase of the effective dielectric constant with film thickness that we observed here for the inkjet-printed BST is common for thin BST films [8,19,21]. It is often attributed to the influence of the metal/BST interface [20,21]. In our films, it may also be due to the increasing grain size [22,23]. According to XRD data, the average grain size of the 420 nm thick film was almost two times greater than that of the 280 nm thick film (130 nm vs 70 nm respectively).
Fig. 5. A microscope image of a section of a spray-printed, photolithographically defined interdigitated capacitor.
measurement accuracy of the LCR meter (0.002). Thus the tanδ the loss tangent of the films at 1 MHz was less than 0.002. Fig. 6a shows that the inkjet-printed BST films are tunable i.e. the capacitance of the test capacitors was reduced by application of the DC bias. As expected, for a thicker film the capacitance of the straight–gap capacitor on a 420 nm thick film was larger over the whole range of tuning voltages than the capacitance of the same test capacitor on 350 nm thick film. The 280 nm thick film was tested using an interdigitated capacitor geometry resulting in a much larger effective electrode width and therefore its' capacitance was larger than that of the thicker films measured with straight–gap capacitors. The dielectric constant as-presented in Fig. 6b as a function of bias voltage were calculated from the C (V) curves of Fig. 6a taking the different capacitor geometries into account [16]. Strictly speaking this calculation is only correct at 0V. At higher voltages due to the planar construction of the electrodes the distribution of the bias field in the dielectric layer is not uniform, this results in non-uniform dielectric constant for the tunable dielectric. Thus the dielectric constant calculated at nonzero bias should be seen as an average or “effective” dielectric constant. The zero-bias dielectric constant ranged between 500 and 1000 and increased with the film thickness as can be seen by looking at the leftmost points in Fig. 6b. The maximum tunability of 30% of the dielectric constant at the maximum applied field of 9 V/μm was achieved on the thickest 420 nm thick film. A 20% tunability was measured for the 350 nm thick film with the electrodes of the same geometry. Note that a lower tunability is to be expected for a thinner film with a lower dielectric constant [19]. For the interdigitated capacitor on the 280 nm thick BST film, the tunability was 24%, 4% greater than that measured for thicker 350 nm film with straight–gap capacitor. We believe that this is likely due to the difference in the electrode geometry of the test capacitors [13] and does not
3.2.2. Spray-printed interdigitated electrodes The results of the electrical testing of the spray-printed interdigitated capacitor are represented by the topmost C (V) curve in Fig. 6a. As-expected for a BST capacitor, the capacitance decreases with increasing bias voltage. The dielectric constant of the 280 nm BST film (Fig. 6b) appears to be in line with what was measured for other films that were processed with
Fig. 6. Room temperature dielectric properties of BST films measured at 1 MHz. Panel (a): The C (V) curves of the test capacitors on inkjet-printed BST films of various thickness. Panel (b): The dependence of dielectric constant on the bias field for the inkjet-printed BST films of various thicknesses. The straight–gap capacitors are represented by solid lines, the interdigitated capacitor by dashed lines.
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evaporated contacts. Based on this, and taking into account the good tunability obtained with spray-printed electrodes, we conclude that a reasonable electrical contact was established directly between the spray-printed Ag and underlying BST film. Hence, along with the good conductivity of the silver layer and its' good adhesion to the BST film, we take this as a proof of principle demonstration for the viability of metal electrodes from liquid metalorganic precursors on BST thin films for tunable circuit applications. However, no direct comparison with evaporated contacts could be made at this point, since the spray-printed and evaporated capacitors had different electrode geometry as well as the differences in the thickness of the tested BST layers.
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[5] [6] [7] [8] [9] [10]
4. Conclusions Crystalline tunable Ba0.6Sr0.4TiO3 (BST) films of various thickness (280 nm, 350 nm and 420 nm) were produced by inkjet printing of a metalorganic precursor. The BST films were granular and had a smooth surface (3.5 nm RMS), however the 420 nm thick film had a developed network of microcracks. The dielectric constant of the BST films increased as a function of the film thickness. The highest tunability, 30%, was measured on the thickest, 420 nm, film which had a dielectric constant of 1000. The tunability of the thinnest, 280 nm, thick film was 24%. Interdigitated contacts produced by photolithographic patterning of spray-printed Ag coating demonstrated good conductivity, good adhesion and electrical contact to an inkjet-printed BST film. Taken together with the reasonable structural and dielectric properties of the inkjet-printed BST film, these results suggest that it should be possible to fabricate complete BST-based tunable circuit elements using sequential inkjet printing. References [1] L.C. Sengupta, E. Ngo, M.E. O'Day, S. Stowell, R. Lancto, IEEE Electron Device Lett. (1995) 622. [2] C.M. Carlson, T.V. Rivkin, P.A. Parilla, J.D. Perkins, D.S. Ginley, A.B. Kozyrev, V.N. Oshadchy, A.S. Pavlov, A. Golovkov, M. Sugak, D. Kalinikos, L.C. Sengupta, L. Chiu, X. Zhang, Y. Zhu, S. Sengupta, Materials Issues for Tunable RF and Microwave Devices Symposium, 30
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