Ferroelectrics – Material Aspects BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications Nanomiser ®Flame Flow Controller Substrate Atomizing Solution Filter Fig. 1. Schematic of the CCVD system, the thin film NanoSpray combustion process Au, Cu, Ir, Ni, Complex oxides: (Ba,Sr)TiO , (Pb,La)(Zr,Ti)O , (La,Sr)CoO , Spinels, YBa , SrRuO Simple oxides: Al , SiO , ZnO, ZrO , Cr , Cu O, Fe , MgO, Mn , NiO, , RhO , RuO Substrates Used Single crystal ceramics: Si, sapphire, LaAlO , MgO, SrTiO , yttrium stabilized ZrO quartz Polycrystalline ceramics: SiC, Si , silica Applications Capacitors, resistors, catal tic applications, corrosion resistance, electronics, en ines, ferroelectrics, solar cells, fuel cells, optics, piezoelectrics, buffer la ers, superconductors, thermal barrier, thermal cont rol, and wear resistance Table 1. Partial list of materials deposited by CCVD 3. Depositions of ferroelectric thin films by CCVD Many ferroelectric materials, such as BST and PZT, have been deposited successfully by the CCVD technique. These ferroelectric thin film s are grown epitaxially on sapphire, single crystal MgO, and single crystal SrTiO (STO) substrates. 3.1 Depositions of BST thin films by CCVD and their properties Compared to polycrystalline or text ured thin films, epitaxial diel ectric thin films show higher dielectric breakdown and lower dielectric loss. Th erefore, epitaxial thin films are preferred for many applications, especially for high frequenc y microwave applications. Single layer BST and multilayer dielectric thin films have been successfully deposited on sapphire (both and orientations). Figure 2 shows typical plan view and cross sectional images on a single layer BST thin film of sapphire substrate by CCVD. The film is dense and smooth with uniform grains and thickness. Figure 3 shows an area detector XRD pattern and a (110) pole figure of a typical BST thin film on Ferroelectrics – Material Aspects Fig. 2. SEM (a) plan view and (b) cross sect ion images of typical BST thin films by CCVD Inter-digital capacitors (IDC) with an 8 BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications of the multilayer dielectric film appear as dots, aligning with (104) and (006) peaks of sapphire (the (006) plane is parallel to th e substrate surface), showing the multilayer dielectric film was grown epitaxially on the -sapphire substrate as single layer thin films. Fig. 4. Tuning and dielectric loss of a single layer BST film on sapphire substrate as a function of applied voltage The same IDC and CPW structures were fabricated on the multilayer thin films. The dielectric and microwave properties of a select ed multilayer thin film and a standard single layer film are summarized in Table 2. The multilayer thin film has a slightly lower capacitance at 1 MHz compared to the standard single layer film. However, its dielectric loss at 1 MHz and 0 V is about 0.005, which is much lo wer than that of the single layer thin film (0.028). The figure of merit (FOM), which is de fined as (tuning × capacitance)/loss tangent, of the multilayer film is about 3 times as high as that of the standard single layer film. The high FOM and low dielectric loss benefit th e applications for high frequency and high power microwave devices. directio Ferroelectrics – Material Aspects Fig. 5. (a) SEM image and (b) XRD pattern of a multilayer dielectric thin film Sample ID Capacitance and loss at 1 MHz S at 50 GHz (dB) 0 V 40 V Cp (pF) Cp (pF) Multilayer 1.14 0.005 0.90 0.0031.46 21.1 4820 Single layer 1.28 0.028 0.85 0.019- 33.3 1537 Table 2. Comparison of electrical properties , PTO) and lead zirconate (PbZrO , PZO) with different Zr/Ti ratios . It is well known that their Film Film directio BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications physical properties can be modified by changi ng the Zr/Ti ratio and substituting a part of Pb ion by tri-valent ions. Among the tri-valent dopants, lanthanum (La) has been found the most suitable element for increasing the de nsity and other physical properties of the Ferroelectrics – Material Aspects 202530354045505560 (degree) Sapphire (0006) PLZT 15/30/70 PLZT 20/30/70 PLZT 12/40/60 PLZT 17/40/60 PLZT 15/50/50 PLZT 17/50/50 (111) (110) 100 Fig. 7. XRD patterns of the PLZT thin films with various La contents an d Zr to Ti ratios by sapphire substrate with a PLT seed layer Pole figure measurements were performed on th e epitaxial films using (110) reflections. The pole figure of a PLZT 20/30/70 thin film is shown in Figure 8. As BST thin films on sapphire, the PLZT thin film shows six shar p dots of (110) poles with narrow density distributions, which is similar to that of th e PLT seed layer. There is no broadening or satellite found from the pole figure, suggestin g an excellent crystallinity. The PLZT films grew off the PLT seed layer and keep the cr ystallographic orientations. The orientation relationship between the PLZT thin film and sapphire substrate is PLZT (111)//sapphire (001) and PLZT [110]//sapphire [104]. Fig. 8. (110) pole figure of a PLZT 20/30/70 thin film on sapphire substrate with a PLT seed layer Figure 9 shows the SEM microgra phs of the PLZT thin films with different compositions. These PLZT thin films inherit the microstruc ture of the PLT seed layer. They contain uniformly distributed fine grains less than 100 nm in size. The film morphology is strongly influenced by the film composition. For the film with a Zr to Ti ratio of 50/50 (not shown in BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 11 the figure), the grains are not closely packed . Voids and pores were formed in this film. With the increase of Ti content and the decrease of Zr content, the film density increases and the grain size decreases. For the film PLZT 20/30/70, there is no pin hole formed. All of these films are crack free. It is also noticed th at particles were formed on these films, which may be attributed to poor atomization or high flame temperature. Further studies show that without the PLT seed layer the PLZT films deposited at the same conditions contain multiple out-of-plane orientations or are rand om. Pyrochlore phase was also formed at these Fig. 9. SEM images of PLZT thin films with different La contents and Zr to Ti ratios on sapphire substrates with a PLT seed layer, (a) PLZT 17/40/60 and (b) PLZT 20/30/70 The optical properties of these PLZT thin f Ferroelectrics – Material Aspects cell. Its cubic structure is related to that of perovskite (CaTiO ), but the TiO octahedra are tilted to produce a square planar environment for Cu . Cu atoms are bonded to the four oxygen atoms and the large Ca atoms are without bonds. 05001000150020002500 Wavelength (nm) Transmittance (%) PLZT 17/50/50 PLZT 17/40/60 PLZT 20/30/70 Fig. 10. Optical transmittance spectra of PLZT th in films with various La contents and Zr to Ti ratios grown on sapphire substrate with a PLT seed layer Subramanian and coworkers (Subramanian et al., 2000, 2002) prepared CCT based ceramics by sintering related powders. A diel ectric constant of higher than 10 was achieved at room temperature. The dielectric constant increases rapidly with the increase of temperature and at 450 C. Based on Subramanian’s work, Ra BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 13 2030405060 (degree) CCT (220) CCT (400) Fig. 11. XRD spectra of CCT thin films deposited at 1025 C on STO substrate with different The frequency and bias voltage dependence of di electric constant and quality factor of a 45 nm thick CCT film on STO substrates are shown in Figure 13. In the tested frequency range, dielectric constant decreases slightly while qua lity factor increases with the increase of bias voltage. For example, at 1 MHz and 0 V, the diel ectric constant and quality factor are 45,320 Ferroelectrics – Material Aspects and 730, respectively. When applying a bias vo ltage of 40 V, at the same frequency, the dielectric constant and quality became 44,950 an d 950, respectively. In tested voltage range, when frequency is lower than 1 MHz, dielec tric constant decreases gradually with the increasing of frequency. It shows a sharp de crease at the frequency of 10 MHz at all the tested voltages. However, quality factor increa ses rapidly with the increase of frequency in the tested voltage range. 04080120160200240280320360400440 Film thickness (nm) Fig. 12. Dielectric constant of CCT films on STO (100) substrates as a function of film Frequency (Hz) 20 V 40 V 20 V 40 V Fig. 13. Dielectric constant and quality factor of a CCT film, deposited at 950 C for 10 min on STO (100) substrate, as a function of frequency at different bias voltages 3.3.2 CCT thin films on -sapphire substrate ) has a hexagonal crystal structure with a = 4.759 Å and c = 12.99 Å. It is widely used to deposit ferroelectric material s for electrooptic applications and radiation hardened electronic components. Sapphire has a different crystal structure than LAO or BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 15 deposited onto sapphire at a temperature of 1025 C. Figure 14 (a) shows the SEM image of a 90 nm thick CCT film. The base layer of the film is dense and smooth with uniform and fine grains. There are a few hillocks on its surfac e. With the increase of film thickness, the films become rougher and grains become la 2030405060 (degree) Intensity (a.u.) (0006) CCT (220) CCT (222) CCT (400) CCT (310) 90 nm 630 nm Fig. 14. (a) SEM image of a 90 nm thick CCT film and (b) XRD spectra of CCT films with The dielectric constant of CCT films on -sapphire substrates, as shown in Figure 15, decreases with the increase of film thickness, which is similar to those of CCT on STO and LAO, but with lower values. The dielectric co nstant is about 930 for a 90 nm thick film. It decreases to about 235 when film thickness increases to 630 nm. 0100200300400500600700 Film thickness (nm) Fig. 15. Dielectric constant of CCT films on -sapphire substrates as a function of film Ferroelectrics – Material Aspects From the results of CCT films on different substrates, it is clear that CCT films on STO have the highest dielectric constant while the ones on c-sapphire substrates have the lowest. It is worthwhile to point out that the observation of the dielectric constant decreasing with increasing film thickness in this study is contra st to the fact found in other ferroelectric thin films such as Ba (BST) and CCT on platnized Si wafers. Conventionally the dielectric collapse is understood by assuming the existence of “dead layers” with severely depressed dielectric constants at the electrode-di electric interfaces. These dead layers act as parasitic capacitors in series with the “bulk-like” dielectrics. Hence the decrease in dielectric STO (100) 200 (220) + (400) (100:746) 11825 LAO (100) 180 (220) + (400) (100:892) 1188 -Sapphire 180 (220) + (222) + (400) (100:42:19) 450 BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 17 rapidly variable filters will be available to provide tactical capability. These existing issues can be overcome through reconfiguring transm ission and reception in a few microseconds using tunable filters. Low cost, low loss, high IIP , high speed, robust, and radiation-hard filters can enable wide adaptations. Planar lo w-voltage capacitor structures with improved power handling capability have been developed using BST films and used in the design and fabrication of tunable filters operated at freque ncies of 2, 6-20, and 30-45 GHz. In the past several years, Gimat has worked on materials development, as well as design, fabrication, and testing of tunable ferroelectric thin film based microwave devices. 4.1 RF MEMS filters with tunable bandwidth and tunable center frequency For this application, a CPW admittance inverter topology was employed to realize a Ka- band tunable filter. Two configurations, wideband (WB) and narrowband (NB), were designed, as shown in Figure 16. The electr onic realization of bandwidth control is accomplished by introducing a ground-to-gr ound connection through the inter-resonator gaps as shown in Figure 16 (b). For proof-of concept, perfect open and short connections were used at first. Figure 17 shows the me asured results of wideband and narrowband 3- pole filters without BST capacitors. Their band widths are 4% and 8% respectively, with a center frequency of 39.5 GHz. Fig. 16. 3-pole filter topologies: (a) wideband and (b) narrowband configuration 30364248 -60 -50 -40 -30 -20 -10 S21 and S11 (dB) Frequency (GHz) S11_NB S21_NB S11_WB S21_WB Ferroelectrics – Material Aspects Fig. 18. A 2-pole CPW admittance inverter tunable filter with MEMS switches 2530354045 Frequency (GHz) -40 -30 -20 -10 Insertion Loss in dB 37.33 GHz -2.334 dB 35.48 GHz -2.82 dB 32.97 GHz -10.39 dB DB(|S(2,1)|) WB 0v DB(|S(2,1)|) WB 1v DB(|S(2,1)|) WB 2v DB(|S(2,1)|) WB 5v DB(|S(2,1)|) WB 10v DB(|S(2,1)|) WB 15v DB(|S(2,1)|) WB 20v DB(|S(2,1)|) WB 30v DB(|S(2,1)|) WB 40v 2530354045 Frequency (GHz) -60 -50 -40 -30 -20 -10 Insertion Loss in dB 35.3 GHz -4.76 dB 30.8 GHz -17.48 dB DB(|S(2,1)|) NB 0v DB(|S(2,1)|) NB 1v DB(|S(2,1)|) NB 2v DB(|S(2,1)|) NB 5v DB(|S(2,1)|) NB 8v DB(|S(2,1)|) NB10v DB(|S(2,1)|) NB 15v DB(|S(2,1)|) NB 20v DB(|S(2,1)|) NB 25v DB(|S(2,1)|) NB 30v DB(|S(2,1)|) NB 40v DB(|S(2,1)|) NB 50v DB(|S(2,1)|) NB 60v Fig. 19. Measured insertion loss, S , of a 2-pole CPW admittance inverter filter with MEMS switches (a) at the up and (b) at the down state, each curve representing the insertion loss at one voltage level BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 19 RF microelectromechanical systems (MEMS) filt ers were also designed and fabricated, as shown in Figure 18. Two MEMS switches are us ed to realize ground-to-ground connection. One bias pad is used to activate the switches. When the MEMS are at the up state, the filter is wideband. A narrowband filter results when the switches are at the down state. The RF MEMS filter was tested up to 45 GHz. Figure 19 (a) presents the insertion loss of the filter. The filter has an insertion loss, S 14001600180020002200 -50 -40 -30 -20 -10 S11 & S21 (dB) Frequency (MHz) Insertion Loss, 0 V Return Loss, 0 V Insertion Loss, High Bias Return Loss, High Bias Fig. 20. (a) CDMA filter prototype, (b) CDMA Filter Response at 0V and high DC bias 4.3 X-band to Ku-band tunable filters Slow wave resonators (SWR) are sections of transmission lines periodically loaded with tunable BST capacitors. By changing simultaneo usly the capacitance of each BST device, the effective dielectric constant, thus, the electrical length of the resonator is changed so that a different resonance frequency is achieved. In addition, du e to the increased effective Ferroelectrics – Material Aspects Tx to antenna frequency fange Insertion loss Antenna to Rx frequency range Insertion loss Table 4. Typical specifications of a US PCS duplexer dielectric constant the resulting filters are more compact (smaller size) when compared to /2 or /4) filters. The change in the effective dielectric constant also results in a characteristic impedance change. A prototype of a 3-pole SWR filter with a dimension of 8.30 mm × 2.65 mm is shown in Figure 21. Figure 22 shows the microwave result s of a 2-pole and a 3-pole SWR filter. Both filters have the same tuning range, i.e. 4.15 GHz, which corresponds to a tunability of 48%. The 3-pole filter shows higher loss due to its narrower bandwidth and possibly more Fig. 21. A prototype of a 3-pole SWR filter with a total of 8 DC blocking capacitors 4.4 X-band back-to-back 4-pole band-pass filters A tunable back-to-back 4-pole filter was built on a flexible organic liquid crystal polymer (LCP) substrate, which consists of two open -loop resonators coupled to the two others through apertures lithographically opened in their common ground plane, resulting in a footprint size reduction of about 50% compared to typical open-loop resonator based filters. The filter frequency is tuned using BST capa citor chips, which are mounted and ribbon- BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 21 681012141618 Frequency (GHz) S21 (dB) S21 (0 V) S21 (10 V) S21 (20 V) S21 (30 V) 681012141618 Frequency (GHz) S11 (dB) S11 (0 V) S11 (10 V) S11 (20 V) S11 (30 V) -50 -40 -30 -20 -10 681012141618 Frequency (GHz) S21 (dB) S21 (0 V) S21 (10 V) S21 (20 V) S21 (30 V) -40 -30 -20 -10 681012141618 Frequency (GHz) S11 (dB) S11 (0 V) S11 (10 V) S11 (20 V) S11 (30 V) Fig. 22. Measured (a) insertion loss and (b) re turn loss of a 2-pole and (c) insertion loss and 89101112131415161718 S21 (dB) 89101112131415161718 S11 (dB) 0 V 5 V 10 V 20 V 30 V Ferroelectrics – Material Aspects Fig. 24. Prototype of the 4-pole tunable filter with mounted capacitors: oblique views of the (a) top resonators (1 and 4) and (b) bottom resonators (2 and 3) Fig. 25. Measured filter response without ca pacitors, compared to the simulated response BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 23 transition effects which are not de-embedded. BST capacitors are then mounted on the substrate and ribbon-bonded. A bias voltage of 0 to 30 V is applied to the four capacitors to tune the center frequency. Figure 26 presents the measured results. The measured insertion loss is also compared to the simulated ones with a serial resistance of 6 , an inductance of 0.6 nH, and a capacitance of 90 fF at 0 V and 54 fF at 30 V. An insertion loss of 5.4 dB at 9.1 GHz and 1.84 dB at 10.25 GHz is achieved, result ing in an analog tuning of 12.6% with a Fig. 27. Photograph of a ring filter 20222426283032343638404244464850 Frequency (GHz) -40 -35 -30 -25 -20 -15 -10 Return Loss (S11) & Insertion Loss (S21) in dB 33.7 GHz -2.321 dB 31.6 GHz -2.33 dB DB(|S(1,1)|) DB(|S(2,1)|) DB(|S(1,1)|) DB(|S(2,1)|) Ferroelectrics – Material Aspects insertion loss in pass-band is 2.3 and 2.0 dB at 0 and 30 V, respectively. The 3-dB bandwidth is 20% for both bias states. The fi lter tunes from 31.6 to 33.7 GHz, a 6.6% tunability. 4.6 Phase shifters In the meanwhile, phase shifters have also been developed using ferroelectric BST capacitors for frequencies ranging from L- to Ka-band. Fig. 29. Schematic of all-pass network BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 25 0.511.522.533.544.5 Frequency (GHz) -6.5 -5.5 -4.5 -3.5 -2.5 -1.5 Insertion Loss in dB 3 GHz -4.326 dB DB(|S(2,1)|) DB(|S(2,1)|) 05V DB(|S(2,1)|) 10V DB(|S(2,1)|) 15V DB(|S(2,1)|) 20V DB(|S(2,1)|) 25V DB(|S(2,1)|) 30V DB(|S(2,1)|) 35V 0.511.522.533.544.5 Frequency (GHz) -40 -35 -30 -25 -20 -15 -10 Return Loss in dB 3 GHz -11.53 dB DB(|S(1,1)|) DB(|S(1,1)|) 05V DB(|S(1,1)|) 10V DB(|S(1,1)|) 15V DB(|S(1,1)|) 20V DB(|S(1,1)|) 25V DB(|S(1,1)|) 30V DB(|S(1,1)|) 35V Fig. 30. (a) Insertion loss, S21 and (b) 11.522.533.544.55 Frequency (GHz) 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 Insertion Phase Shift in Degrees 3 GHz 376.3 Deg 05V (Deg) 10V (Deg) 15V (Deg) 20V (Deg) 25V (Deg) 30V (Deg) 35V (Deg) Fig. 31. Phase shift of the 3 GHz phase shifte r at different frequencies and bias voltages Ferroelectrics – Material Aspects 5. Conclusions As a summary, high quality epitaxial or textur ed ferroelectric and dielectric thin films, including BST (both single layer and nanostructured multilayer), PZT, and CCT, have been BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 27 Copel, M., Baniecki, J. D., Du ncombe, P. R., and Shaw, T. M. (1998). Compensation Doping of Thin Films. Ferroelectrics – Material Aspects Im, J., Auciello, O., Streiffer, S. K., and Krauss, A. R. (2000). Composition Control of BST and Other Ferroelectric Thin Films by CCVD and Their Properties and Applications 29 Pintilie, L., Boldyreva, K., Alexe, M., an d Hesse, D. (2008). Capacitance Tuning in Antiferroelectric Ferroelectric PbZrO Epitaxial Multilayers. J. Phys. Vol. 10, No.1, (January 2008), pp. 013003, ISSN 1367-2630 Polla, D. L. and Francis, L. F. (1998). Proc essing and Characterization of Piezoelectric Materials and Integration into Microelectromechical Systems. Ann. Rev. Mater. Sci. Vol. 28, No.1, (January 1998), pp. 563, ISSN 1531-7331 Prakash, B. S., Varma, K. B., and Maglione, M. (2008). Depositi on and Dielectric Properties of CCTO Thin Films Deposited on Pt/Ti/SiO Ferroelectrics – Material Aspects Takeuchi, I., Chang, H., Gao, C., Xiao, X. D., Downes, M. J., and Venkatesan, T. (1998). Combinatorial Synthesis and Evaluation of Epitaxial Ferroelectric Device Libraries.