Issue



LTCC Integration of Passives


02/01/2004







Performance Results of a K80 Dielectric Paste

BY LIANG CHAI, AZIZ SHAIKH AND VERN STYGAR

Low-temperature cofired ceramic (LTCC) technology was initially conceived as a technology to bring together the functions of interconnect, package and integrated components. As a ceramic packaging option, LTCC extends the capability of traditional hybrids using multilayer thick film materials on rigid ceramic substrates such as alumina and aluminum nitride. Substrates with higher layer counts and true three-dimensional features can be realized using LTCC technology. Passive components such as capacitors, resistors, inductors, powder/signal couplers/dividers, microwave filters and transmission line structures can be built into the LTCC structure.

Capacitors can be fabricated into the ceramic substrate using a layer of LTCC as dielectrics. The capacitance of the parallel plate configuration is determined by the dielectric constant (k) of the LTCC, overlapping area of the conductors and the thickness of the LTCC layer. LTCC generally has a low dielectric constant, which is required for packaging and substrate applications. The dielectric constant is 5.9 (± 0.2) for A6 LTCC over the frequency range from near DC up to 100 GHz. Higher capacitance density must be achieved using screen-printed dielectric pastes with higher dielectric constant. The dielectric paste option not only can be incorporated with high volume manufacturing, but also provides a higher capacitance density, resulting from the higher k of the dielectric paste and thinner screen-printed dielectric thickness.

Experiment

Capacitor test parts were made using Ag conductors and 5 mil (green) A6S LTCC tape. The test pattern has 4 quads, and each quad consists of three sets of the following size of the capacitors: 10 × 10; 20 × 20; 40 × 40; 50 × 50; and 100 × 100 mil² (0.25 × 0.25; 0.5 × 0.5; 1.0 × 1.0; 1.27 × 1.27; and 2.5 × 2.5 mm², respectively). It should be noted that the above sizes are the conductor areas in the green state (before firing). The capacitor dielectric was printed 5 mil (0.13 mm) larger than the two parallel conductor layers in all four directions. For example, the dielectric layer is 15 × 15 mil² (0.38 × 0.38 mm²) for 10 × 10 mil² (0.25 × 0.25 mm²) conductor.

To fabricate the capacitor, the bottom conductor layer was printed on to the LTCC tape first. After printing the capacitor paste on the first conductor layer, the top conductor layer was printed on the capacitor dielectric layer. To achieve above 30-µm fired thickness of the capacitor dielectric layer, the capacitor paste was double printed.

In the green stage, the overall size of the capacitor test part is 3" × 3" (7.6 cm × 7.6 cm), and the overall thickness is 50 mil (1.27 mm), consisting of ten 5 mil green tape layers. The single capacitor layer was buried in the center of the substrate. "Dimpling" in the capacitor pad area of the substrate was observed when the capacitor layer is on the surface. Cavities were made to expose the terminations of the conductors for probe access.

The parts were fired at 850°C, using a standard profile. The electrical properties of the capacitor were tested up to 10 MHz using a HP 4192A impedance analyzer. Data at 1 kHz is reported.

Results

Interface of Capacitor and LTCC.
Optical microscopic photos show that the capacitor dielectric has no noticeable interaction with the A6 LTCC, which indicates that the conductor layers act as barrier layer and prevent the interaction of the capacitor dielectric and LTCC (Figure 1). The interaction zone only occurs where the capacitor dielectric extends over the conductor layers in contact with A6 LTCC.


Figure 1. This optical microscopic photo of the capacitor cross section shows no delamination or interaction between the dielectric layer and the LTCC.
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A SEM study confirmed that there is no noticeable interaction between the high-k dielectric and LTCC. LTCC compositions, Ca and Si, are confined in the LTCC body, while the major high-k dielectric compositions, Ba and Ti are in the capacitor layer. Ag diffusion into high-k dielectric was not detectable.

Capacitance, Dissipation Factor and Dielectric Constant.
The capacitance and dissipation factor of the capacitors were recorded directly from the impedance analyzer. A slight frequency effect was observed (Figure 2). As the frequency increases, the capacitance (C) tends to decrease and the dissipation factor (DF) tends to increase. No self-resonate frequencies of the designed capacitors were detected at frequency up to 10 MHz. Data at 1 kHz are reported and used for further calculation.


Figure 2. Capacitance and dissipation factor of a 100 × 100 mil² (green) capacitor.
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The dielectric constant of the capacitor dielectric was calculated from the measured dielectric thickness and overlapping conductor area of the capacitor. The average thickness of the capacitor dielectric is 37.5 µm. The conductor areas are closely associated with those calculated using the typical tape shrinkage. For example, a 100 × 100 mil² (2.5 × 2.5 mm²) capacitor is about 85 × 85 mil² (2.2 × 2.2 mm²) using the X and Y shrinkage rate of 15 percent. An average of dielectric constant of 80 is derived based on the above calculation when a silver internal conductor (33-398) is used. A slight change in the dielectric constant is observed when a different conductor is used due to the interaction of the inorganic additives in the conductor and the high k dielectric. A capacitance density of 17.5 pF/mm² (the slope) can be derived from the plot of capacitance as a function of the conductor area, with an average dielectric thickness of 37.5 µm. This represents a significant increase in capacitance density. A typical capacitance density is about 0.54 pF/mm² when a 5-mil (127 µm) A6 LTCC tape is used.


Figure 3. Capacitance and dissipation factor of K80 dielectric as a function of temperature.
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Temperature Coefficient of Capacitance.
The temperature coefficient of the capacitance (TCC) of the K80 dielectric was determined using a temperature chamber from -55° to 125°C. A solderable silver surface conductor (33-391) must be used for the conductor layer of the capacitor. The leads soldered to the capacitor terminations can then be connected to the cartridge of the temperature chamber. As mentioned previously, the conductor may have an effect on the properties of the capacitor. However, the conductor effect on TCC should be insignificant because TCC is a differential. Figure 3 shows that this dielectric has a better than ± 10 percent range (X7P), close to the X7F (± 7.5 percent) temperature characteristic. The dissipation factor tends to be flat at temperatures greater than room temperature, but increases significantly below room temperature.


Table 1. The K80 dielectric paste was successfully integrated into the A6 LTCC system, and showed temperature characteristics better than X7P.
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Conclusion

A k80 dielectric paste was integrated into the A6 LTCC material system. At a dielectric thickness of 37.5 µm, capacitance density of 17.5 pF/mm² can be achieved. This high-k capacitor dielectric has better than X7P temperature characteristics (Table 1).

Acknowledgement

The authors would like to acknowledge the contributions of Cristina Lopez, Hector Miranda, Eric Ness, Jerry Aguirre and Paul Garland.

References

For a complete list of references, please contact the authors.

Editor's Note: Copyright 2003 by IMAPS. Reprinted with permission from the 36th International Symposium on Microelectronics Proceedings, Nov. 16-20, Boston, MA.

LIANG CHAI, (formerly with Ferro Corp.) R&D director, may be contacted at Technic Inc., 300 Park East Drive, Woonsocket, RI 02895. AZIZ SHAIKH, director of technology, and VERN STYGAR, West Coast sales manager, may be contacted at Ferro Electronic Material Systems, Ferro Corp., 1395 Aspen Way, Vista, CA 92083; (760) 305-1000; shaikha@ferro.com and stygarv@ferro.com.