Localized temperature stability of low temperature cofired ceramics

[0001] This invention was made with Government support under contract no. DE-AC04-94AL85000 awarded by the U.S. Department of Energy to Sandia Corporation. The Government has certain rights in the invention.

[0002] The present invention relates to low temperature cofired ceramics and, in particular, to a low temperature cofired ceramic having localized temperature stability.

[0003] Low temperature cofired ceramic (LTCC) is a multilayer 3D packaging, interconnection, and integration technology. One of the advantages of LTCC is the ability to integrate passive components, such as capacitors, resistors, inductors, resonators, and filters, in the LTCC via cofiring processes. For LTCC modules with embedded resonator functions targeting high frequency applications, the temperature coefficient of resonant frequency (τ_f) is a critical parameter. A low τ_f, preferably close to 0 ppm/° C., is desirable to achieve temperature-stable resonator functions for radio and microwave frequency (RF and MW) applications. With a low τ_f resonator, the temperature compensation that requires additional mechanical and electrical circuits to achieve temperature stable resonator function can be eliminated. See C. Wang and K. A. Zaki, IEEE MMT-S Intern. Microwave Sump, Digest 3, 1041 (1993); and U.S. Pat. No. 5,302,924 to H. Johnson and A Tunumen. For filter applications, a low τ_f enables efficient use of bandwidth for maximum communication capacity.

[0004] There are multiple approaches to achieve temperature stable resonator functions. One common engineering method to achieve a near zero τ_f is to form solid solutions of different dielectric ceramics that have opposite τ_f. The method has been applied to many MW dielectrics for resonator applications as well as LTCC base dielectrics for integration of resonator functions. See W. Wersing, Solids State and Mat. Sci. 1, 715 (1996); R. C. Kell et al., J. Am. Ceram. Soc. 56, 352 (1973); S. Kucheiko et al., J. Am. Ceram. Soc. 79, 2739 (1996); I. M. Reaney, and D. Iddles, J. Am. Ceram. Soc. 89, 2063 (2006); S. Dai et al., J. Am. Ceram. Soc. 85, 828 (2002); H. Jantunen, J. Am. Ceram. Soc. 85, 697 (2002); H. Kapta et al., Jpn. J. Appl. Phv. 31, 3152 (1992); Y. Choi et al., Materials Letters 58, 3102 (2004); and U.S. Pat. No. 6,841,496 to Kim. The other practice is to form a hybrid layered structure incorporating alternating layers with opposite τ_f, either by sequential sintering or bonding processes. See N. Alford et al., IEE Proc-Sci. Meas. Technol. 147, 269 (2000); and L. Li and X. M. Chen, J. Am. Ceram. Soc. 89, 544 (2006).

[0005] Although there are commercial LTCC dielectrics with low τ_f, (e.g., Hereaus CT2000 with τ_f<10 ppm/° C.), most of existing LTCC dielectrics possess a τ_f in the range −50 to −80 ppm/° C. FIG. 1 shows τ_f data of several main stream LTCC dielectrics. It is expected that for integrated RF circuits using any of these LTCCs, the impact of τ_f will be reflected in a drift of resonant frequencies of embedded resonators over the device's operating temperatures. This drift results in extra design limitation and/or performance compromise.

[0006] Therefore, a need remains for an LTCC having localized temperature stability.

[0007] The present invention is directed to a multilayer low temperature cofired ceramic module having localized temperature stability, comprising a layer comprising a low temperature cofired ceramic having a temperature coefficient of resonant frequency; a passive component disposed on the low temperature cofired ceramic layer; and a compensating material having a temperature coefficient of resonant frequency opposite to that of the low temperature cofired ceramic, disposed on at least one side of the passive component. For example, the compensating material can be disposed as a layer, filled via, or cavity insert on at least one side of the passive component. The compensating material can comprise a glass, a ceramic filler, such as alumina, and titania or titanate. For example, the compensating material can comprise between 5 to 90 wt % of a low temperature cofired ceramic glass, between 0 to 90 wt % of ceramic filler, and between 0 to 90 wt % of titania or titanate. The multilayer low temperature cofired ceramic module can further comprise at least one additional compensating material having a different temperature coefficient of resonant frequency, disposed on another side of the passive component.

[0008] The low temperature cofired ceramics having localized temperature stability of the present invention enable zero or near zero τ_f embedded resonators by incorporating τ_f compensating materials locally into a multilayer LTCC structure. Chemical interactions can be minimized and physical compatibility between the τ_f modifiers and the host LTCC dielectrics can be achieved. Therefore, a stripline (SL) ring resonator with near zero τ_f can be fabricated in a non-zero τ_f commercial LTCC that is temperature compensated.

[0009] As an example of the present invention, the formation of proper τ_f compensating materials and subsequent successful cofiring of the materials in a DuPont 951 LTCC was demonstrated and SL resonators that have a near zero τ_f can be achieved. The compensation of the τ_f is realized locally inside a LTCC structure where the resonators reside. Therefore, existing LTCC material sets and processes can remain unchanged while incorporating the τ_f compensation materials.

[0017] For high frequency applications, a ceramic module preferably has a high dielectric constant to enable a small resonator size, low dielectric loss to enable a high quality factor and narrow bandwidth, and a small temperature coefficient of resonant frequency to provide a temperature-independent resonance frequency. This is because many of the elements used in multilayer ceramic integrated circuits, such as filters and resonators, need to be very stable under drifts in temperature. The temperature coefficient of resonant frequency τ_f is a device parameter which can be expressed in basic material properties:
τ_f=+½τ_∈−α  (1)
where τ_∈ is the temperature coefficient of dielectric constant and α the coefficient of thermal expansion (CTE). For LTCC dielectrics, α is typically between 4-10 ppm/° C., therefore τ_∈ dominates τ_f.

[0018] From eq. (1) it is clear that τ_f and τ_∈ have opposite signs. To compensate the negative τ_f of LTCC, materials with negative τ_∈, and thus positive τ_f, are necessary. Table 1 lists dielectric constant ∈, τ_∈, and normal sintering temperatures of candidate ceramics that could serve as compensating materials for LTCC dielectrics.

[0019]

TABLE 1
Potential compensating materials.
		Dielectric	τε	Sintering
	Materials	constant	(ppm/° C.)	Temp (° C.)
	TiO2	110	−750	~1200
	CaTiO3	130	−1600	~1400
	SrTiO3	285	−2400	~1550

• [0020] A. J. Moulson and J. M. Herbert, Electroceramics, Chapman & Hall, p 234, 1990.

[0021] The present invention is directed to a means to adjust temperature coefficient by cofiring locally in the multilayer LTCC structure with a compensating dielectric with τ_f having opposite sign to that of host LTCC dielectric. The invention enables temperature compensation only in that portion of the LTCC structure where the resonator functions resides, without affecting other embedded functions where such implementation is not necessary or is undesirable. The invention can be applied to any existing LTCC with a proper selection and development of a cofireable compensating dielectric. A major advantage of this invention is that it enables the use of existing LTCC systems, such as cofireable conductors and other functional materials, without the need to revamp an entire cofireable LTCC material system.

[0022] The present invention uses τ_f compensating materials and the cofiring of such materials in a LTCC module. FIG. 2a shows a schematic side-view illustration an uncompensated LTCC module comprising a passive component 11 embedded between LTCC layers 12 and 13. For example, the passive component 11 can be a filter or resonator. FIGS. 2b-g show some exemplary embodiments of the present invention. FIG. 2b shows a layer of compensating material 14 disposed on one side of the passive component 11 embedded between LTCC layers 12 and 13. As shown in FIG. 2c, one or more intervening LTCC layers 12′ can separate the compensating material 14 from the passive component 11, so long as the electric field from the passive component 11 penetrates through the intervening LTCC layer 12′ and the compensating material 14 to an electrical ground plane, for example, on the bottom surface of the module. Therefore, both the τ_f of the intervening LTCC layer and the τ_f of the compensating material can contribute to provide a near-zero overall τ_f at the passive component. FIG. 2d shows layers of compensating material 14 and 15 disposed on both sides of the passive component 11 embedded between LTCC layers 12 and 13. A different compensating material can be used on each side of the passive component. As shown in FIG. 2e, one or more intervening LTCC layers 12′ and 13′ can separate the compensating materials 14 and 15 from the passive component 11. FIG. 2f shows a layer of compensating material 14 on one side and an air gap 16 on the other side of the passive component 11 embedded between LTCC layers 12 and 13. Of course, the compensating material need not disposed as a layer on the passive component, but can be disposed on the passive component, for example, as a filled via 17 or an insert in a cavity in the LTCC layer, as shown in FIG. 2g.

[0023] As an example of the present invention, compensating glass-ceramic compositions in paste form were cofired in a commercial LTCC. Evidence of both physical compatibility, measured by matched shrinkage, and chemical compatibility, characterized by material interdiffusion, is provided to demonstrate the cofireability. Stripline ring resonators in LTCC were fabricated and characterized to quantify the effect of temperature compensation.

[0024] Several commercial LTCC glasses were evaluated. The main requirement is that the chemical constituents and glass softening point T_g of the LTCC match the onset temperature of shrinkage of the compensating material. One LTCC glass, designated as V-glass, was used as an example of the present invention. V-glass consists essentially of Al₂O₃, BaO₃, CaO, PbO, MaO and SiO₂. V-glass has density of 2.77 g/cm³, coefficient of thermal expansion of 5.5 ppm/° C., dielectric constant of 7.3 and glass transition temperature T_g=625° C. Table 2 lists the formulations of candidate compensating materials. For this example, the weight % of V-glass was fixed at 55%. The base formulation was 55 wt % V-glass+45 wt % Al₂O₃. To demonstrate the invention, portions of the Al₂O₃ were replaced by different titanates at several weight percentages to form 5′ a series of compositions. The mixture of V-glass, Al₂O₃ and titanate was co-milled using Al₂O₃ media to a median particle size 2.0 to 2.2 μm. For dielectric property measurement pellets at φ12.5×1 mm from dry co-milled powder with binders were pressed using a 2-step process: a uniaxial press at 5000 psi, followed by an isostatic press at 30000 psi. The pellets were sintered at 850° C./30 min in air. Au was sputtered on polished pellets to form electrodes.

[0025] Pastes of selected compensating materials were made using an organic vehicle ESL441 combined with thinner ESL401 (both from Electro-Science Labs) and a 3-roll mixing step. The pastes were screen printed on green LTCC tapes below and/or over conductor lines to form temperature compensation layers.

[0026]

TABLE 2
Formulation of compensating materials.
	V-glass	Al2O3	TiO2	CaTiO3	SrTiO3
Composition	(wt %)	(wt %)	(wt %)	(wt %)	(wt %)
Base	55	45
TO10	55	35	10
TO30	55	15	30
CTO10	55	35		10
CTO20	55	25		20
STO10	55	35			10
STO20	55	25			20

[0027] 10 mil DuPont 951 LTCC tapes were used with this exemplary material. Conductors included DuPont 5738 Au for via fill, 5734 Au for internal lines and 6143 Ag/Pd for post fired external metallization. SL ring resonators at a designed base resonance of 1.5 GHz were fabricated in 4-layer 951 panels. All panels were laminated under a standard process of 3000 psi isostatic pressure at 70° C. for 10 minutes. Panels were sintered at 850° C./30 min in air on Al₂O₃ setters. Sub-miniature version A (SMA) edge connectors were soldered to resonator panels to access the embedded ring resonator for resonant frequency measurement.

[0028] Capacitance of the compensating materials was measured on sintered pellets using an HP4194A impedance/gain-phase analyzer at 1 MHz inside a temperature chamber. The τ_f was calculated from
τ_f=−½(τ_c+α)  (2)
where τ_c is the temperature coefficient of capacitance and α the CTE.

[0029] Shrinkage data of the compensating materials, in the form of cylinders at φ6.25×5 mm, and the 951 dielectric, in the form of 5×5 mm stack of 20 layer tapes, were measured using a Netzsch DIL 402 dilatometer. A scanning electron microscope (SEM) was used to analyze microstructure and interface of the cofired materials.

[0030] The SL resonators were placed inside a temperature chamber and characterized using an Agilent E5062A network analyzer from −50 to 80° C. The resonant frequency f_r was obtained as the minimum frequency of the reflection coefficient (S11) of the ring resonator. τ_f was calculated using the following equation,

[0031] $\begin{array}{cc}{\tau }_f=\frac{1}{f_{\mathrm{ro}}}\left(\frac{\Delta f_r}{\Delta T}\right)& \left(3\right)\end{array}$
where f_ro is the resonant frequency at 20° C., Δf_r is the change of resonant frequency over −50 to 80° C., and ΔT equals to 130° C.

[0032] Archimedes bulk density and dielectric constant of the sintered compensating materials are listed in Table 3. Both TO10 and TO30 showed a lack of adequate densification. FIG. 3 displays normalized capacitance, scaled to the capacitance value at 20° C., of compositions CTO10, CTO20, STO10 and STO20 over the temperature range −50° C. to 80° C. τ_c and calculated τ_f of these compositions are also listed in Table 3.

[0033]

TABLE 3
Density and dielectric properties of compensating materials. τf is
calculated from τc using eq. (2) assuming an α of 7 ppm/° C.
	Archimedes	Dielectric
	bulk density	constant	τc	τf
Sample	(g/cc)	(1 MHz)	(ppm/° C.)	(ppm/° C.)
Base	3.19	7.8	190	−98.5
TO10	N/A
TO30	2.77
CTO10	3.20	9.7	83	−44.9
CTO20	3.15	12.0	−58	25.5
STO10	3.21	9.6	10	−8.7
STO20	3.30	12.2	−240	116.7

[0034] The base dielectric has a τ_c of 190 ppm/° C. and a τ_f of −98.5 ppm/° C. Therefore, without the addition of titanates, the base dielectric has a higher τ_f than the 951 LTCC. With the addition of 10 wt % of titanates, the CTO10 and STO10 still show positive τ_c, and thus negative τ_f. Obviously, the polarity of τ_f of these two compositions is the same as that of the 951 LTCC. Therefore, these compositions were not expected to adjust the τ_f towards zero, and were eliminated from further consideration. With a higher weight percentage of titanates, the CTO20 and STO20 reach a τ_f of 25.5 ppm/° C. and 116.5 ppm/° C., respectively. Considering that the 951 LTCC has a τ_f of −69 ppm/° C., the STO20 is expected to be more effective for τ_f compensation, and was thus down-selected for a cofiring study and for the fabrication of proof-of-concept SL ring resonators.

[0035] FIG. 4 shows shrinkage curves of STO20 and 951 along the Z-direction (thickness of the tape) at a heating rate 5° C./min from 500 to 850° C. The onset temperature of shrinkage of STO20, measured from a shrinkage-versus-temperature plot, is 741° C. Among all evaluated LTCC glasses this onset temperature of V-glass based STO compositions represents the closest match to the onset temperature 766° C. of 951 LTCC. The proximity of these onset temperatures provides a solid physical base for cofiring of STO20 in 951 LTCC.

[0036] To validate cofireability of STO20 with 951, the STO20 paste was screen printed on a 3″×3″ 951 single layer tape in a square ring pattern and sintered using a normal profile, as shown in FIG. 5A. FIG. 5B shows an optical image of a fractured cross section of the cofired structure. No deformation, either in 951 or in STO20, is visible, indicating a well matched sintering behavior. A close look at the STO20-951 interface by SEM is also presented in FIG. 5C. A relatively clean interface exists between the two cofired materials, suggesting no or minimum interaction and interdiffusion between the two.

[0037] The normalized resonant frequency versus temperature and correspondent τ_f data of 5 resonators are shown in FIG. 6: a resonator in 951 LTCC; two duplicated resonators with a single STO20 print on one side of the resonator line (STO20 1 layer); and two duplicated resonators with STO20 printed on both sides of the resonator line (STO20 2 layers). The inserts are cross section images that show details of the layered structures.

[0038] The 951 LTCC possesses a τ_f at −68.9 ppm/° C. With the incorporation of single layer STO20, the τ_f drops to 29.1 and 37.1 ppm/° C. for the two evaluated samples. The variation in τ_f data may come from the actual thickness changes of the STO20 layer between different resonator builds. Overall, a single STO20 layer cofired next to the resonator conductor line reduces the τ_f value of the non-compensated 951 LTCC approximately in half.

[0039] For the two resonators where the conductor lines are surrounded by the STO20 dielectric layers, one shows a τ_f of 5.1 ppm/° C. and the other a τ_f of −7.1 ppm/° C. The incorporation of two STO20 layers brings the t_f close to zero. Again, the variation of τ_f of the two samples may reflect the thickness variation of the printed STO20 layers. The fact that one resonator possesses a slightly positive τ_f and the other a slightly negative τ_f suggests that with a tight control on the thickness of the STO20 layer resonators with a true near zero or at zero τ_f are achievable.

[0040] The present invention has been described as a method for localized temperature stability of low temperature cofired ceramics. It will be understood that the above description is merely illustrative of the applications of the principles of the present invention, the scope of which is to be determined by the claims viewed in light of the specification. Other variants and modifications of the invention will be apparent to those of skill in the art.