Low temperature co-firing ceramic (LTCC) composition for microwave frequency

[00004] The present invention relates to a dielectric ceramic composition for high frequency with high dielectric constants, high Q values, and good temperature coefficients in the resonant frequencies. More specifically, the present invention relates to a low temperature co-firing ceramic composition for microwave frequency that has a sintering temperature as low as ≦900° C. and can be co-fired with an electrode such as silver (Ag).

[00005] There have been recent remarkable developments in communication systems that use microwave frequencies with a range of 300 MHz to 300 GHz in mobile communications such as wireless telephone sets and car-phones, satellite broadcastings, satellite communications and the like. With the development of personal portable communications, miniaturization of terminals has become generalized. Thus, laminated parts are increasingly required to accomplish miniaturization, high light weighting, and surface mounting of terminal parts. Particularly, passive elements such as filters, duplexers, resonators and antennas, which are essential parts of communication equipment, were difficult to miniaturize. However, miniaturized thick film laminated chip elements were obtained with the use of low temperature sintering materials that can be co-fired with an electrode, such as silver (Ag) or copper (Cu). High dielectric constant and high frequency are required for miniaturization of parts because the wavelength of microwaves within dielectric ceramics is inversely proportional to the square root of a dielectric constant, and the frequency. Generally, a dielectric constant (ε_r) is inversely proportional to a quality factor (Q) value, and dielectric materials having a high dielectric constant are required for miniaturization of parts. However, as the frequency in use is increased to ≧1˜2 GHz, the parts are sufficiently miniaturized. Thus, materials having dielectric constants with a range of 20˜40 are now actively applied to a variety of chip antennas, chip filters and the like.

[00006] The representative examples of low temperature sintering dielectric ceramics for high frequency, which have been developed, include CuO and V₂O₅ containing the BiNbO₄ system, of which the Q·f₀ value is 18,300 GHz, the dielectric constant is 43, and the temperature coefficient of resonant frequency is +38 ppm/° C. These types of dielectric ceramics have sintering temperatures as low as 875° C., but have very high temperature coefficients of resonant frequency. Thus, they are impossible to apply to actual parts. Therefore, study of adding various additives to the BiNbO₄ system to stabilize the temperature coefficient of resonant frequency is now in energetic progress (See, H. Kagata, Jpn. J. Appl. Phys., 31, p.3152, 1992). Bismuth (Bi) reacts with electrodes when a BiNbO₄ system is in laminated sintering, causing a characteristic deterioration of the BiNbO₄ system (see, K. B. Shim, J. Mater. Sci., 35, p.813, 2000). Thus, a demand is rising for low temperature sintering dielectric ceramics materials that do not include Bi.

[00007] The object of the present invention is to provide solutions to the aforementioned problems in the art and provide a low temperature co-firing ceramic composition for microwave frequency that, (i) can be sintered at a low temperature (≦900° C.); (ii) does not react with a dielectric ceramic material when co-firing with an electrode such as silver (Ag); (iii) have dielectric constants with a range of 20˜40, and have a high quality factor (Q) value applicable to parts; (iv) as well as have a stable temperature coefficient of resonant frequency.

[00008] The object of the invention is achieved by providing a dielectric ceramic composition represented by the following formula 1:
Ca[(Li_1/3Nb_2/3)_1-xTi_x]O_3-δ+y(B₂O₃.ZnO.SiO₂.PbO)  (1)
wherein x is molar ratio and 0≦x≦0.3; y is wt % ratio and 0≦y≦15; and δ is a variable varying with the value of x and 7/60≦δ≦1/6.

[00011] The dielectric ceramic composition according to the present invention has dielectric constants of 23.4 to 46.2; a Q·f₀ value of 6,300 to 17,240 GHz; a temperature coefficient of resonant frequency (TCF) of −72.4 to +64.3 ppm/° C., which can be controlled to 0 ppm/° C. with the constitution change; and can be used as low temperature co-firing ceramics (LTCC), which is applied to dielectric ceramics parts for microwave frequency.

[00012] The properties of the LTCC composition for microwave frequency according to the present invention vary depending on the amount of TiO₂ to be contained in Ca[(Li_1/3Na_2/3)_1-xTi_x]O_3-δ and the amount of B₂O₃.ZnO.SiO₂.PbO to be added. As the molar fraction of TiO₂ is increased from 0 mole % to 30 mole %, and the amount of B₂O₃.ZnO.SiO₂.PbO to be added is increased from 10 wt % to 15 wt % at a sintering temperature of 900° C., the dielectric constants are gradually increased from 23.4 to 46.2, and the Q·f₀ values are gradually decreased from 17,240 to 6,300 GHz. Also, as the molar fraction of TiO₂ and the addition amount of B₂O₃.ZnO.SiO2.PbO are increased, the temperature coefficient of resonant frequency is gradually increased from −72.4 to 64.3 ppm/° C. By this, the temperature coefficient of resonant frequency may be controlled. In particular, when the composition, in which TiO₂ is contained in the amount of 20˜25 mole % and B₂O₃.ZnO.SiO2.PbO added in the amount of 10˜12 wt %, is sintered at 900° C., an excellent LTCC composition for microwave frequency may be prepared wherein the dielectric constants are 35.5˜42.4, the Q·f₀ values are 9400˜14250 GHz, and the temperature coefficient of resonant frequency is −10˜+10 ppm/° C.

[00013] The present invention is explained in detail through the examples given below. However, the examples presented here are for illustrative purposes only and should not be construed as limiting the invention.

[00014] CaCO₃, Li₂CO₃, Nb₂O₅, TiO₂ were weighed in a constitution ratio (x, mole ratio) indicated in Table 1, and mixed. The mixtures were calcined for 2 hours at a temperature of 850° C., and then milled. B₂O₃.ZnO.SiO₂.PbO was added to the mixtures in a given weight ratio (y, wt % ratio) indicated in Table 1, and then mixed. Subsequently, PVA aqueous solutions were added as forming additives. The mixtures were pressed into cylinder-type specimens having a diameter of 10 mm and a thickness of 5 to 6 mm. The cylinder-type specimens were heated for 1 hour at a temperature of 600° C. to remove organic binders and then sintered for 0.1˜3 hours at a temperature of 800˜1000° C. in air.

[00015] Both sides of the sintered specimens were thoroughly polished in a parallel fashion and introduced into a waveguide. Dielectric constants, Q values and temperature coefficients of the resonant frequency of the specimens were measured at a frequency range of 7˜10 GHz and temperature range of 20˜80° C., according to a dielectric resonator method. The microwave dielectric properties of each specimen are shown in Table 1.

[00002]

TABLE 1
High Frequency Dielectric Properties of
Ca[(Li1/3Nb2/3)1−xTix]O3−δ + y(B2O3.ZnO.SiO2.PbO)
Sample	Constitution	δ	Addition	Sintering	Dielectric	Q · f0	τf
No.	X (mole)	(mole)	y (wt %)	Temperature (° C.)	constant (εr)	(GHz)	(ppm/° C.)
1	0.00	1/6	0	1150	29.6	40000	−21.0
2			10	900	23.4	17240	−72.4
3			12		27.0	15620	−60.3
4			15		28.2	13240	−47.8
5	0.05	19/120	0	1150	32.4	29800	−4.5
6			10	900	24.4	16280	−60.6
7			12		29.4	15140	−52.4
8			15		31.7	11940	−44.5
9	0.10	3/20	0	1150	34.6	27200	−2.3
10			10	900	26.3	15910	−52.6
11			12		32.1	14690	−45.2
12			15		34.8	10610	−40.4
13	0.15	17/120	0	1150	36.8	26600	−1.7
14			10	900	29.9	15120	−39.0
15			12		34.2	13240	−37.7
16			15		37.5	9700	−35.9
17	0.20	2/15	0	1150	38.6	26100	0
18			10	900	35.5	14250	−10.0
19			12		38.8	11170	−9.3
20			15		40.3	7430	−8.6
21	0.25	1/8	0	1150	41.5	24900	12.3
22			10	900	40.4	11440	8.0
23			12		42.4	9400	10.0
24			15		45.0	7100	26.0
25	0.30	7/60	0	1150	44.7	22500	20.0
26			10	900	41.3	10260	31.0
27			12		43.9	8280	41.0
28			15		46.2	6300	64.3

[00016] The present invention relates to a dielectric LTCC composition for microwave frequency that has high dielectric constants, high Q values, and stable temperature coefficients of the resonant frequency, and that has a sintering temperature as low as ≦900° C. which can be co-fired without reacting with an electrode such as silver (Ag). The compositions are applied for their excellent characteristics required for commonly used cellular and PCS phones in the range of 800 MHz˜1.9 GHz, and laminated chip filters and antennae in the range of IMT 2000 of 2 GHz, which will be used hereafter.