IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 5, MAY 2015
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Aerosol Deposition of Yttrium Iron Garnet for Fabrication of Ferrite-Integrated On-Chip Inductors Scooter D. Johnson, Harvey S. Newman, Evan R. Glaser, Shu-Fan Cheng, Marko J. Tadjer, Fritz J. Kub, Fellow, IEEE, and Charles R. Eddy, Jr. U.S. Naval Research Laboratory, Washington, DC 20375 USA We have employed aerosol deposition (AD) to deposit 39 µm thick polycrystalline films of yttrium iron garnet at room temperature onto sapphire at a rate of 1–3 µm/min as an initial investigation of utilizing AD for fabricating ferrite-integrated on-chip inductors. We characterize the structural and magnetic properties of the as-received starting powder, as-deposited film, and a pressed puck formed from the starting powder. Results show that the films are comprised of randomly oriented polycrystalline grains with structural and magnetic properties that closely resemble that of the starting powder. Results from coating a gold single-turn inductor show an increase in inductance of 79% up to ∼300 MHz without affecting the Q-factor. These results demonstrate AD as a promising technique for depositing thick ferrite films at high deposition rates for low-temperature fabrication of ferrite-integrated on-chip inductors. Index Terms— Aerosol deposition (AD), monolithic microwave integrated circuit, radio frequency integrated circuit (RFIC), room temperature deposition, thick film, yttrium iron garnet.
I. I NTRODUCTION CHALLENGE to miniaturizing devices for RF circuits, is reducing the footprint of the embedded inductors. One solution for reducing inductor size is to replace the air-core with a magnetic material [1]. This is accomplished by fabricating inductors on top of a single-plane or sandwiched between a double-plane of a material, such as Ni-Fe, Co-Zr-Ta, Co-Nb-Zr, or Co-Hf-Ta-Pd. These materials are chosen for their high permeability and low-melting temperature, which enables integration into a system-on-chip process by magnetron sputtering (MS) [2]–[5]. However, the low resistivity of these materials requires additional fabrication steps to decrease eddy-current loss and to insulate the plane from the conductor. The solution is to deposit an oxide or polyimide layer between the conductor and plane [4] and/or fabricate slotted planes [3]. There have also been efforts to utilize higher resistivity magnetic materials fabricated with a copper inductor deposited directly onto the magnetic plane. Some examples [6]–[8] are Y2.8 Bi0.2 Fe5 O12 , Ni0.4 Zn0.4 Cu0.2 Fe2 O4 , Co7 ZrO9 , (YLa)3 Fe5 O12 , and Y3 Fe5 O12 (YIG). While the processing steps are reduced by eliminating the need for an insulating layer, the high melting temperatures involved in the oxide deposition pose difficulty for incorporation into systemon-chip technology. The aerosol deposition (AD) is a room temperature thick-film deposition technique that accelerates a precursor of solid particles to a high velocity [9]. The high-velocity particles impact, fracture, and adhere to the target substrate forming a thick well-adhered polycrystalline film comprised of nanocrystallites. The deposition rate is typically several micrometer per minute. The film thickness can range
A
Manuscript received April 22, 2014; revised September 23, 2014; accepted November 4, 2014. Date of publication November 11, 2014; date of current version May 22, 2015. Corresponding author: S. D. Johnson (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2014.2369376
from a few micrometers to >100 μm and with densities close to the bulk density of the material (>90%). While already established techniques, such as MS, liquid phase epitaxy, and pulsed laser deposition have been utilized to deposit YIG materials, these techniques have comparatively slow growth rates (0.6 μm/h reported for MS [10]), are performed at elevated growth temperatures [11], and often require postannealing to crystallize the films [10], [12]. The primary advantage of AD is the ability to produce comparable films using a substrate and solid powdered precursor material at room temperature, thereby producing films with the same structural properties as the precursor. With these metrics in mind, the technological advantages of developing AD for integrating ferrite materials into current CMOS processing of system-on-chip technology becomes evident. Johnson et al. [13] report in detail the processing method behind AD and the characterization methods used in this paper. Those results provide a comparison of the as-deposited films to the sintered films. In this paper, we complement those results to further demonstrate the technological impact AD may have on integration of ferrite materials into system-on-chip technology. The results are divided into two parts: 1) we report structural and magnetic film properties in comparison to the precursor powder and a bulk-like puck made from the same material and 2) we demonstrate the AD technique for depositing YIG onto a single-turn inductor. In Section III, we compare the material properties and the inductor results for a better understanding of the effect of the AD deposited films on the inductor. Polycrystalline YIG films were deposited onto sapphire substrates and the fabricated test inductors at room temperature by AD using untreated YIG powder (Trans-Tech Inc., Adamstown, MD) with an average particle size of 0.5 μm. In this paper, the AD conditions were identical for the sapphire and inductor depositions. The resulting films were 39 μm thick. The settings for the AD system were as follows: 1) carrier gas = nitrogen (99.999% minimum purity);
U.S. Government work not protected by U.S. copyright.
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IEEE TRANSACTIONS ON MAGNETICS, VOL. 51, NO. 5, MAY 2015
2) scan speed = 0.65 mm/s; 3) nozzle-substrate distance = 7.5 mm; and 4) pressure difference P = 300 Torr. The films were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), vibrating sample magnetometry, and ferromagnetic resonance (FMR). Film density was estimated from the ratio of mass to volume as described previously [13]. Gold 2 μm thick single-turn inductors with a 20 μm line width and a 4.22 mm total line length (see Fig. 5) were fabricated onto an alumina substrate. The substrate was metalized with a Ti/W/Au stack, and was patterned using positive photoresist. A Transene TFA Au etchant, with an etch rate of ∼2.8 nm/s at 25 °C, was used to etch the top Au layer. The Ti/W stack was etched in a 1:1:10 solution of NH4 OH:H2 O2 :H2 O. Device isolation was verified using an ohmmeter. The inductors were connected to 50 coplanar probe pads to facilitate probing in a shunt configuration (our probe pad configuration was designed to allow both series and shunt inductors to be measured, however, only shunt inductors were produced in this experiment). Measurements of the reflection coefficient S11 and phase θ of the shunt inductor between 10 MHz and 10 GHz in steps of 10 MHz were carried out on a Cascade Microtech microwave probe station using a GGB Industries Model 40A GSG coplanar probe having a pitch of 150 μm. Data were collected using an Agilent Technologies Model N5245A PNA-X vector network analyzer. Calibration was performed at the probe tip using a GGB CS-5 calibration standard. The complex load impedance at the probe tip was )/(1 − S ), with Z = 50 obtained from Z 11 = Z 0 (1 + S11 0 11 (S /20) j θ 11 and S11 = 10 e . The inductance L, resistance R, and Q-factor of the inductor before and after coating with YIG were obtained from the complex load impedance Z 11 = R + j ωL in the form L = I m(Z 11 )/2π f , R = Re(Z 11 ), and Q = ωL/R.
Fig. 1. SEM images of an as-deposited film showing the morphology of the top surface of the film. (a) Agglomerated particles and porosity in the film is evident. (b) Fractured particles of
