Sintering, Microstructure, and Dielectric Properties of Copper Borates for High Frequency LTCC Applications

Abstract

New ceramic materials based on two copper borates, CuB2O4 and Cu3B2O6, were prepared via solid state synthesis and sintering, and characterized as promising candidates for low dielectric permittivity substrates for very high frequency circuits. The sintering behavior, composition, microstructure, and dielectric properties of the ceramics were investigated using a heating microscope, X-ray diffractometry, scanning electron microscopy, energy dispersive spectroscopy, and terahertz time domain spectroscopy. The studies revealed a low dielectric permittivity of 5.1–6.7 and low dielectric loss in the frequency range 0.14–0.7 THz. The copper borate-based materials, owing to a low sintering temperature of 900–960 °C, are suitable for LTCC (low temperature cofired ceramics) applications.

Novel copper borate ceramics with lithium-based sintering aids for LTCC terahertz applications

“…34–36 The copper borates under investigation generally fulfil the theoretical conditions for attaining a low dielectric permittivity. 20 Consistent with the dumped harmonic oscillator model, 33 the transition from the microwaves region to submillimetre waves will entail a small change in the real part of the dielectric permittivity ( ε ′), but a quite marked increase in its imaginary part ( ε ′′), and consequently in the loss tangent (tan δ = ε ′′/ ε ′). 13,22,33,37 This effect was confirmed experimentally for the commercial Ferro A6M LTCC material ( and tan δ = 0.003 at 200 GHz and , tan δ = 0.012 at 1 THz) 33 and in our previous studies for the Zn 4 B 6 O 13 ceramics ( and tan δ = 0.005 at 90–140 GHz, and and tan δ = 0.007 at 1 THz) 13 and the doped Zn 2 SiO 4 ceramics ( and tan δ ≤ 0.005 at 90–140 GHz, and and tan δ = 0.008 at 1 THz).…”

“…This effect was previously revealed for the undoped copper borate ceramics. 20 Fig. 5d illustrates good cooperation between the AgPd based internal conductor layer and the ceramic layer based on Li 2 WO 4 doped copper borate in a test LTCC structure.…”

“…The components of the copper borate ceramics, CuB 2 O 4 and Cu 3 B 2 O 6 , were synthesized via the solid state reactions of the starting reagents H 3 BO 3 and CuO (pure reagents, ≥99%, Sigma Aldrich), mixed in stoichiometric ratios. The two-stage calcination processes were carried out in covered crucibles and comprised the decomposition of boric acid at 200–400 °C for 2 h and synthesis at 700 °C for 5 h. 20 After the syntheses and ball-milling (350 rotations per min) of the products, the resulting powders were mixed in weight proportions of 3 : 1, selected based on the preliminary study as the most advantageous ratio. Then three compositions based on 75 wt% (86.1 mol%) CuB 2 O 4 –25 wt% (13.9 mol%) Cu 3 B 2 O 6 with the addition of 1.9 mol% Li 2 WO 4 , 5 mol% Li 2 WO 4 , 5.7 mol% LiBO 2 and 5.1 mol% Li 2 CO 3 as the sintering aids were prepared by ball-milling (Pulverisette 5, Fritsch, Germany) in isopropyl alcohol for 8 h and dried.…”

“…This effect was previously revealed for the undoped copper borate ceramics. 20 Dielectric properties in the THz and GHz range Fig. 6a and b compare the dielectric permittivity and loss tangent at 20 1C in the frequency range of 0.2-2.7 THz for the undoped ceramics and the ceramics doped with various Li-based additives.…”

“…In our previous study, two copper borates -CuB 2 O 4 and Cu 3 B 2 O 6 were indicated as novel and promising materials for LTCC substrates with good dielectric behaviour in the THz frequency range. 20 Since the sintering temperature of the undoped copper borates is in the 950-960 1C range, the limit for co-sintering ceramic layers with cheap commercial Ag thick film pastes, it is beneficial to find appropriate additives to lower this temperature without degrading the dielectric properties. The Li-containing sintering aids, such as LiF, Li 2 CO 3 , Li 2 WO 4 , and LiBO 2 , have been successfully used for suppressing the sintering temperatures of other ceramics, [21][22][23][24][25][26] including microwave substrates.…”