Printed Barium Strontium Titanate capacitors on silicon
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
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