Elsevier

Thin Solid Films

Volume 589, 31 August 2015, Pages 111-114
Thin Solid Films

Printed Barium Strontium Titanate capacitors on silicon

https://doi.org/10.1016/j.tsf.2015.04.087Get rights and content

Highlights

  • Inkjet printing of Barium Strontium Titanate films

  • Deposition on silicon substrate

  • Inkjet printed silver top electrode

  • First ever BST films thinner than 1 μm RF functional variable capacitor that has required no lithography

Abstract

In this paper, we show that Barium Strontium Titanate (BST) films can be prepared by inkjet printing of sol–gel precursors on platinized silicon substrate. Moreover, a functional variable capacitor working in the GHz range has been made without any lithography or etching steps. Finally, this technology requires 40 times less precursors than the standard sol–gel spin-coating technique.

Introduction

Barium Strontium Titanate (BST) films have been studied for more than 20 years especially for their tunable properties [1], [2], [3], [4], [5]. Indeed, BST exhibits a large variation of dielectric constant when an external DC electric field is applied. Moreover, BST can handle large radio frequency (RF) power as high as 30 dBm [6]. Besides, BST composition can be chosen in order to avoid any hysteretic behavior, which in turn reduces dielectric losses and simplifies the associated electronics [1], [7]. Consequently, BST-based materials are nowadays the best tunable dielectric candidates to fulfill the tough requirements of tunable RF varactors for impedance matching. Industrial solutions have been recently proposed. Note that we do not discuss here the other technical solutions for impedance matching, which are based respectively on microsystems and on CMOS varactors [6]. Technologically speaking, BST thin-films can be deposited by several means, namely Pulsed Laser Deposition (PLD) [8], sputtering [3], sol–gel [9] or MOCVD [10]. Each of these techniques is mostly compatible with microelectronic standards and requires photolithography and etching steps. Deposition of BST thick-films has been demonstrated by screen printing allowing fully additive patterning [11], [12], tape casting [13] and spin-coating [14]. The reported studies on BST thick-film fabrication employ solutions with solid ceramic particles. Although film quality is not as good as the one of BST thin-films, such methods are expected to drastically reduce processing cost [11], [15].

A promising alternative deposition means is inkjet printing, enabling to quickly process small series of devices of both thin and thick-films. In the case of BST, this technique has been already investigated in the literature. In 2007, Kaydanova et al. printed 280 to 420 nm thick Ba0.6Sr0.4TiO3[16]. BST was deposited on MgO substrate and interdigitated (IDT) structures were used to show that this printed BST was tunable at 1 MHz. In 2013, Nikfalazar et al. showed that IDT structures of copper–fluorine-co-doped Ba0.6Sr0.4TiO3 printed on alumina were able to work at 12 GHz [17]. Also, the same team proved that BST can be printed on a bottom electrode made of Cr/Ni/Au, which was deposited and patterned by lithography on alumina substrate [18]. Recently, functional RF varactors [19] and phase shifters [20] were fully fabricated by inkjet printing of BST thick-films and silver electrodes. In this paper, we show that functional BST varactors in the GHz-frequency range based on Metal–Insulator–Metal (MIM) structure can be processed on silicon wafers by inkjet printing of BST thin-films without any lithography step. This work suggests that inkjet printing technology can be used to efficiently process low-cost and large area BST RF varactors on silicon with no need of complex clean room facilities.

Section snippets

Experimental details

Inkjet printing process lies on the equilibrium of the printing equipment, the ink rheological and physico-chemical properties such as viscosity and surface tension, and the substrate surface properties. The surface energy of the substrate must be higher than the ink surface tension for appropriate wetting. Moreover, roughness and porosity of the substrate also influence the ink spreading.

Printing was realized on a Dimatix DMP-2831 printer with 21 μm-side square nozzles. Nozzles have individual

Results

BST solution viscosity is around 1 mPa·s, defined by the solvent viscosity, which is one order of magnitude lower than the range recommended for the Dimatix printer. In order to obtain a correct ink ejection, the actuation voltage was set to 13 V and the nozzles were kept at ambient temperature. The BST droplet ejection versus time is illustrated in Fig. 1. The droplet diameter is 24 μm, corresponding to a volume of 7.2 pl. In the second photograph of Fig. 1, a satellite droplet appears because of

Discussion

In the case of dense and smooth substrates, the interaction between the ink and the substrate is driven by their respective surface energies. The average surface energy of Pt is 2.7 J·m 2[23], which is much higher than the surface tension of BST solvent (25 · 10 3 J·m 2). Hence, BST droplets greatly spread on Pt surface and a 30% larger area was measured compared to the 1 mm2-initial BST pattern. Moreover, the edges of the BST printed patterns exhibit a yellowish ring due to lower BST thickness.

Conclusion

In this paper, we showed that it is possible to process 65 nm-thick BST thin films by inkjet deposition on silicon substrate without any use of lithography and etching. BST has been used to realize RF capacitors that are functional at 2 GHz, with most of its characteristics similar to what is obtained with standard BST films processed in clean rooms. More specifically, its relative dielectric constant reaches 67 at 2 GHz without DC bias. Besides, the developed technology infers using 40 times less

References (28)

  • M.W. Cole et al.

    Structure–property relationships in pure and acceptor-doped Ba1–xSrxTiO3 thin films for tunable microwave device applications

    J. Appl. Phys.

    (2002)
  • B. Noren

    Thin film barium strontium titanate (BST) for a new class of tunable RF components

    Microw. J.

    (2004)
  • K. Entesari et al.

    RF MEMS, BST, and GaAs varactor system-level response in complex modulation systems

    Int. J. RF Microwave Comput. Aided Eng.

    (2008)
  • N. Setter et al.

    Ferroelectric thin films: review of materials, properties, and applications

    J. Appl. Phys.

    (2006)
  • Cited by (9)

    • Hybrid inks for 3D printing of tall BaTiO<inf>3</inf>-based ceramics

      2021, Open Ceramics
      Citation Excerpt :

      The concept has been further improved by adjusting the rheological properties of the ink to print 2D lines of BiT [26]. BaTiO3 is selected for its high applicability as an electroceramic, e.g. as lead-free piezoelectrics [27], capacitors [28] or high dielectric permittivity materials [25], and the recent growing interest in additive manufacturing of barium titanate [6,29–31]. Shape control is often crucial for such electroceramics, e.g. for ultrasound transducers the desired shape can vary from 2D structures [32] to annular [33] or fractal [34] configurations or curved surfaces for focusing the ultrasound [35].

    • Structural and dielectric properties of Bi-doped barium strontium titanate nanopowders synthesized by sol-gel method

      2017, Journal of Materials Research and Technology
      Citation Excerpt :

      Hence, a good trade-off between dielectric permittivity and loss is obtained in which can be ideal for room temperature tunable microwave applications [8]. BST nanostructures have been synthesized by various techniques such as metal-organic decomposition (MOD) [9], sputtering [10], pulsed laser deposition (PLD) [11,12], metal-organic chemical vapor deposition (MOCVD) [13], hydrothermal [14,15] and sol–gel methods [16–18]. Among them, sol–gel process offers important advantages over other techniques due to its simplicity, low cost, excellent composition control, high homogeneity at molecular level, high purity, low processing temperature, feasibility of producing complex materials and easy to dope other elements in a homogenous manner [19].

    • Dielectric and impedance spectroscopic investigation of the Ba<inf>0.3</inf>Sr<inf>0.7</inf>Ti<inf>0.873</inf>Zr<inf>0.097</inf>Mn<inf>0.03</inf>O<inf>3</inf> ceramic system

      2016, Ceramics International
      Citation Excerpt :

      This flexibility makes Ba1−xSrxTiO3 an efficient candidate material for electronic circuit technology. Furthermore, Ba1−xSrxTiO3 can withstand high radio frequency (RF) power up to 30 dBm, which enhances its suitability for applications in communication systems [4–7]. BaTiO3 (Ferroelectric) forms a continuous solid solution with BaZrO3 (Paraelectric) in the form of BaZrxTi1−xO3 with ABO3-perovskite structure due to charge similarity and close ionic radii of Zr4+ (0.860 Å) and Ti4+ (745 Å).

    View all citing articles on Scopus
    View full text