Hafnium dioxide as a dielectric for highly-sensitive waveguide-coupled surface plasmon resonance sensors , Kunal Tiwari, Suresh C Sharma , and Nader Hozhabri Citation: AIP Advances 6, 045217 (2016); doi: 10.1063/1.4948454 View online: http://dx.doi.org/10.1063/1.4948454 View Table of Contents: http://aip.scitation.org/toc/adv/6/4 Published by the American Institute of Physics AIP ADVANCES 6, 045217 (2016) Hafnium dioxide as a dielectric for highly-sensitive waveguide-coupled surface plasmon resonance sensors Kunal Tiwari,1 Suresh C Sharma,1,a and Nader Hozhabri2 Department of Physics, University of Texas at Arlington, Arlington, Texas 76019, USA Nanotechnology Research Center, Shimadzu Institute, University of Texas at Arlington, Arlington, Texas 76019, USA (Received 22 December 2015; accepted 20 April 2016; published online 27 April 2016) Hafnium dioxide has been recognized as an excellent dielectric for microelectronics However, its usefulness for the surface plasmon based sensors has not yet been tested Here we investigate its usefulness for waveguide-coupled bi-metallic surface plasmon resonance sensors Several Ag/HfO2/Au multilayer structure sensors were fabricated and evaluated by optical measurements and computer simulations The resulting data establish correlations between the growth parameters and sensor performance The sensor sensitivity to refractive index of analytes is determined ∂θ SPR ≥ 470 The sensitivity data are supported by simulations, which to be Sn = ∂n also predict 314 nm for the evanescent field decay length in air C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4948454] Since the discovery of the surface plasmon excitations (SPEs) in the 1950s interest in the subject has led to the development of surface plasmon resonance (SPR) sensors for biological, chemical, and physical applications.1–11 A particular class of SPEs represents electromagnetic waves that propagate along a metal/dielectric interface (surface plasmon polaritons (SPPs)) and which decay exponentially with distance from and normal to the interface We have recently investigated a variety of waveguide-coupled Ag/Si3N4/Au multilayer structures for SPR sensor applications.6,12 In the search for new and improved waveguide dielectric materials, we have undertaken studies of the usefulness of hafnium dioxide (HfO2) as a dielectric to be used in the fabrication of Ag/HfO2/Au SPPs based sensors In comparison to the Si3N4 dielectric,13–17 hafnium dioxide offers several attractive properties For example, HfO2 is characterized with 4-6 times higher dielectric constant, optical transparency over much wider range of wavelengths (250-2000 nm), times higher density in the solid state, chemical stability, and very high melting point 27580C.18–22 The high dielectric constant and good optical transparency over wider wavelength range are useful for high sensitivity waveguide coupled bimetallic (WCBM) sensors working in VIS-IR range The surface plasmon ε (ε ′ +iε ′′ ) 1/2 ′ ′′ , where ω is the polaritons are characterized by a wavevector kSP = kSP + ik SP = ωc ε d+(εm′ +iεm′′ ) d m m angular frequency and c is the speed of light, ε d is an isotropic, real and positive dielectric constant of the dielectric medium and ε m = ε m ′ (ω) + iε m ′′ (ω) is the frequency dependent complex ′ dielectric constant of the metal.4,23,24 While the real part k SP of the wave vector determines the ′′ wavelength of the surface plasmon polaritons, the imaginary part ik SP accounts for SPPs damping as these excitations propagate along the metal/dielectric interface In a recent publication, we have presented detailed results on the usefulness of Si3N4 as a waveguide dielectric in Ag/Si3N4/Au structures.6,12 In its own right, the Si3N4 dielectric is an important material It is widely used by the microelectronics industry for oxidation masks, protection and passivation barrier layers, gate dielectric, etch-stop layer and inter-level insulator because of its high thermal stability, hardness, chemical inertness and insulating properties It is also transparent from about 250 to 900 nm with relatively large refractive index n = 1.91 @ λ = 632.8 nm However, much higher values of the a Author to whom correspondence should be addressed, Electronic mail: sharma@uta.edu 2158-3226/2016/6(4)/045217/5 6, 045217-1 © Author(s) 2016 045217-2 Tiwari, Sharma, and Hozhabri AIP Advances 6, 045217 (2016) dielectric constant, melting temperature, density, and range of wavelengths over which the material is optically transparent, make HfO2 an attractive dielectric for SPPs based sensors In this work, we have fabricated a series of Ag/HfO2/Au structures in class-100 clean room NanoFab and evaluated their performance by SPR measurements The SPR data are supported by MATLAB computer simulations In the fabrication of our samples, firstly, 37 nm thick Ag was deposited on quartz in AJA International thermal evaporator at base pressure of x 10−6 Torr with 1.3 Å/sec deposition rate It was followed by sputter deposition of 125 nm HfO2 by using hafnia target in a Kurt J Lesker LAB-18 sputtering equipment at base pressure of x 10−8 Torr, deposition rate of 0.18 Å/sec under Ar gas flow at 35 SCCM and 200 W RF The process pressure was kept at mTorr During sputtering, the substrate temperature was set at room temperature, although it could have risen to 40-50oC during sputtering of the HfO2 target Following deposition of the HfO2 dielectric layer, 28 nm Au (purity ≥ 99.999%) was deposited on top of the dielectric in AJA thermal evaporator at deposition rate of 0.7 Å/sec For the purpose of the determination of the actual thickness of each one of the layers of Ag, Au, and HfO2, a silicon wafer was mounted in the growth chamber next to the sensor device being fabricated The thickness of the layer deposited on the silicon wafer was measured by using KLA-TENCOR P-6 profilometer and also confirmed by using an Ocean Optics reflectometer During each subsequent deposition of HfO2 and Au layers, a freshly patterned silicon wafer was mounted next to the device under fabrication and the thickness of the layer deposited over the silicon wafer was measured as described above It is noteworthy that MATLAB simulations,6 which are extremely sensitive to film thickness, also confirmed the accuracy of our thickness measurements The refractive index of the HfO2 film was measured by using J A Woollam, VB-400 series ellipsometry equipment with scanning capabilities for wavelengths from 300-1700 nm The ellipsometry data obtained for HfO2 fils are shown in the inset in figure-1 and they provide a refractive index value of 2.042 @ λ = 630 nm This is in excellent agreement with published data for HfO2 films.19,21,22,25 The crystal structure of HfO2 thin film, deposited on silicon (100) single crystal wafer, is represented by XRD data shown in figure-1, where the dashed vertical lines represent the characteristic peaks for the HfO2 monoclinic phase.26 The film is polycrystalline and the Scherrer equation provides 8-10 nm for the average grain size in the film An SPR sensor is realized by optically coupling the Ag/HfO2/Au multilayer structure to high index prism (Edmund optics, N-SF11, n = 1.7847 at 632.8 nm), which was then used for SPR measurements by utilizing recently developed fixed-detector Kretschmann configuration optical system.12,27 This pump-probe system, shown in figure 2, uses 10-mW He-Ne laser (λ=632.8nm) laser to excite surface plasmon polaritons at sensor surface and another 2-W water cooled Spectra Physics Ar-ion laser (λ=514nm) to produce nanoparticle fluorescence on the sensor surface.12 The detection system consists of JY-Horiba’s 1250M Spectrometer equipped with a liquid nitrogen cooled CCD camera It is well known that the most commonly-used SPR FIG XRD data for HfO2 film deposited on (001) silicon substrate The vertical lines indicate monoclinic phases The inset shows ellipsometry data for the same film 045217-3 Tiwari, Sharma, and Hozhabri AIP Advances 6, 045217 (2016) FIG Fixed detector optical system for simultaneous measurements of SPR and photoluminescence experimental technique of angular interrogation (prism coupling) requires the exciting laser beam to be scanned over large angles at the prism base coated with a metal thin film It also requires relatively large and uniform sensor surface, as well as sufficiently large amount of the sample to be investigated The technique encounters two additional problems; (i) the shape/profile of the incident spot changes from circular to elliptical with increasing incident angle, and (ii) it becomes difficult to maintain the plasmon excitation spot fixed during the angular scans These problems are eliminated by using the fixed detector Kretschmann optical system.6,7,27 In this system, neither the prism nor the detector is rotated and the incident angles are scanned simply by steering the incident beam by using a translatable stage Figure-3 shows our measurements for: (1) 37 nm Ag/HfO2/28 nm Au sensors having three different thicknesses of the dielectric (120, 125 and 130 nm) and (2) measurements by using the 37 nm Ag/120 nm HfO2/28 nm Au for several analytes having refractive indices in the range 1.0 – 1.3772 We observe that both the reflectance at resonance and the FWHM of the resonance FIG SPR data measured by using three different hafnia sensors consisting of dielectric thicknesses of 120, 125, and 130 nm The SPR data for air, water, ethyl alcohol, and iso-propyl alcohol were measured by using 37 nm Ag/125 nm HfO2/28 nm Au sensor The inset compares the sensitivity of the Ag/ HfO2/Au, Ag/Si3N4/Au, and single metal (Au) sensors 045217-4 Tiwari, Sharma, and Hozhabri AIP Advances 6, 045217 (2016) are sensitive to the thickness of the dielectric For the range of the dielectric thickness grown (120-130 nm), the sensor with 120 nm thick dielectric between 37 nm Ag and 28 nm Au shows the highest sensitivity and resolution Therefore, this particular structure is used for the majority of our measurements and the resulting data are the subject of the discussion in the remainder of this manuscript As seen in the inset, the device is highly sensitive to changes in the refractive index; ∂θ SPR these data yield a sensitivity of Sn = ∂n ≥ 470 It should be mentioned that both the sensitivity and figure-of-merit of this particular Ag/HfO2/ Au sensor are somewhat inferior to those of the optimized Ag/Si3N4/Au sensor investigated in our recent work.6 However, our computer simulations predict that even the hafnia sensor structure can be further improved and, with the realization of an optimized dielectric thickness, a FWHM ≤ 0.230 can be obtained.12 This could not be accomplished due to unforeseen experimental difficulties in growing extremely high quality hafnia dielectric, which is required for obtaining extremely narrow SPR curves In this context, it should be mentioned that our computer simulations assume an “ideal” dielectric surface of the highest quality We suspect that the non-ideal nature of the dielectric surface (roughness) contributes to the broadening of the SPR curves Much additional work is needed to understand possible correlations between the quality of the dielectric material and the sharpness of the SPR data The inset in figure-3 also compares the sensitivity of the hafnia, as well as that of the siliconnitride sensors against the sensitivity of a single-metal (Au) sensor As it is well known, the single ∂θ SPR ≃ 760 metal Au sensor exhibits the highest sensitivity Sn = ∂n We further evaluate the accuracy of the sensor performance by carrying out MATLAB computer simulations of the SPR curves for analytes used in our measurements The computational details of the transfer matrix method used for these simulations have been discussed in an earlier publication.6 Figure-4 shows simulations for the Ag/HfO2/Au and Ag/Si3N4/Au sensors It is noteworthy that these simulations not use any adjustable parameters There is excellent agreement between SPR measurements and simulations of the reflectance at resonance and refractive index sensitivity However, computer simulations predict sharper resonance curves; we obtain sensitivities of approximately 43 and 420 and FWHM of about 0.23 and 0.28 for optimized structures (37 nm Ag/135 nm HfO2/28 nm Au and 35 nm Ag/150 nm Si3N4/28 nm Au sensors) Figure-5 compares simulated nature of the evanescent field and its decay in air and water for the hafnia sensor The evanescent electric field decays exponentially with distance increasing normal to and away from the surface and the decay lengths are 314 nm and 232 nm in air and water, respectively In conclusion, we have utilized surface plasmon resonance measurements and computer simulations to evaluate the usefulness of high-dielectric constant HfO2 as a waveguide material for SPR sensors A sensor fabricated with 37 nm Ag/ 125 nm HfO2/28 nm Au exhibits high sensitivity to FIG MATLAB simulations of SPR data for air and water by using parameter values for the Ag/HfO2/Au and Ag/Si3N4/Au sensors 045217-5 Tiwari, Sharma, and Hozhabri AIP Advances 6, 045217 (2016) FIG Electric field penetration into air and water from the surface of Ag/HfO2/Au sensor ∂θ SPR refractive index, Sn = ∂n ≥ 470 This is comparable to 570 recently reported for an optimized thickness of Si3N4 in Ag/ Si3N4/Au sensor.6 Our computer simulations predict that for an optimized thickness of the HfO2 dielectric (135 nm), it is possible to realize a sensor with evanescent field decay lengths of 314 nm in air and 232 in water, as well as obtain extremely narrow resonance curves with FWHM ≤ 0.230 The high refractive index (2.04) and very high melting temperature of HfO2 (27580C) make it an attractive dielectric material for high sensitivity surface plasmon resonance sensors We 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Singh and S C Sharma, Opt Laser Technol 56, 256 (2014) ...AIP ADVANCES 6, 045217 (2016) Hafnium dioxide as a dielectric for highly- sensitive waveguide- coupled surface plasmon resonance sensors Kunal Tiwari,1 Suresh C Sharma,1 ,a and Nader Hozhabri2... for the surface plasmon based sensors has not yet been tested Here we investigate its usefulness for waveguide- coupled bi-metallic surface plasmon resonance sensors Several Ag/HfO2/Au multilayer... applications.1–11 A particular class of SPEs represents electromagnetic waves that propagate along a metal /dielectric interface (surface plasmon polaritons (SPPs)) and which decay exponentially with distance