Microwave MCM-C Utilizing Low Loss LTCC and Photo-patterning Processes

Microwave applications at frequencies in excess of 1 GHz, such as wireless devices, are well suited to ceramic construction. Other circuit fabrication technologies do not exhibit the same balance of low loss, precise geometries, cost and reliability. In addition, ceramic technology has the ability to provide greatly enhanced functionality by integrating buried passive components. A further requirement stems from the increasing use of flip-chip technologies, which provide compact assemblies with good microwave characteristics suitable for volume production. Many alternative technologies fail to provide the appropriate interconnect density and, in the case of polymer circuits, require an expensive under fill process. This article describes a new low loss low temperature cofired ceramic (LTCC) technology that, when combined with a photo-patterned thick-film technology, offers outstanding interconnect density and microwave performance. The ability to combine the efficiency of cofired ceramic with multiple out photo processing leads to low cost systems solutions involving multichip module with cofired substrate (MCM-C) technology. Data are presented to show the outstanding performance available both as microstrip lines and other circuit components, extending the application of LTCC into the 30 GHz and higher range of frequencies.

Peter Barnwell, Michael P. O'Neill and Charles Sabo
Heraeus Inc., Cermalloy Division
West Conshohocken, PA

Charles Free
Middlesex University
London, UK

As frequencies increase toward 40 GHz for many applications, circuit technology and performance become increasingly critical. Line loss rises with frequency, making the loss characteristics of the technology particularly important. At the same time, dimensions decrease, making geometrical properties critical. Conventional technologies, with the exception of thin-film circuitry, struggle to meet these needs. However, thin-film technology, which is expensive to manufacture, is not suited to the newer volume requirements.

Lower microwave frequency circuits are often manufactured using conventional FR4 or similar laminate materials. Unfortunately, these materials exhibit excessive loss, which becomes particularly significant as frequencies rise. An additional problem is the poor dimensional stability of laminate materials, with both temperature and humidity having a significant effect on dimensions. Again, this problem becomes increasingly significant with increasing frequency. What is needed is a ceramic-based technology that can offer several advantages, including low dielectric loss; a precisely defined dielectric constant that is stable with frequency; precise, stable dimensions; excellent thermal management; integrated passive components; and low costs due to multiple out parallel processing.

Conventionally, ceramic technologies for high volume manufacture have used thick-film technology, sometimes in conjunction with LTCC. Unfortunately, the precision of the lines fabricated with thick-film technology, while capable of fabricating low loss lines, does not allow the fabrication of precise geometries for devices such as filters and couplers. Furthermore, conventional LTCC technology is high in dielectric loss (10-2) and relatively high in dielectric constant (typically 8). These problems are unfortunate since the parallel manufacturing technology and integrated components available with LTCC offer a very cost-competitive technology.

A family of photo-patterned thick-film materials has been developed to address the lack of precision in thick-film technology. These materials offer excellent microwave performance. In conjunction with this, a novel low dielectric constant, low loss LTCC material has been developed that offers outstanding capabilities when used with the photo-patterned materials.

Photo Thick-film Measurements

Photo-processed thick-film technology has been discussed extensively elsewhere1,2 and will not be discussed in detail here. It consists of an etchable high density gold or silver conductor and a photo-sensitive, low loss dielectric. A full program of tests on microstrip lines using this technology on various substrates was performed recently and the results are presented.

It was decided to investigate microstrip lines using 96 percent, 99.5 percent and 99.6 percent alumina. The 96 percent alumina was (as fired) Coors ADS96F, the 99.5 percent alumina was Coors roll compacted as fired and the 99.6 percent alumina was Coors, lapped to a surface finish of 10 min. It is particularly significant to compare the performance of these different materials because they vary significantly in cost: The 96 percent alumina is the least expensive with the 99.5 percent alumina being approximately twice the cost and the lapped 99.6 percent alumina at least 10-times the cost. The different performance of the materials on these different cost substrates is considered to be vital data for a microwave designer.

At the same time the loss of etched lines was measured, direct printed lines were measured to provide a performance comparison between the two technologies. All etched lines were fabricated using Heraeus KQ500 gold; printed lines were fabricated using KQ550 gold. Due to the fine line printing capability of the KQ550, it was possible to realize lines with widths very similar to those of the etched lines.

The majority of the work was carried out on 0.635 mm (25 mil) thick alumina with measurements up to 20 GHz. A microstrip line width of 0.635 mm was used to produce a 50 W impedance. Above this frequency, thinner substrates are needed to ensure correct microstrip propagation, therefore, a 0.25-mm (10 mil) substrate thickness and line width was adopted. Measurements were made up to 40 GHz with these substrate dimensions. The measurements used both a direct pass through and a meander line on a 2 in2 substrate. The substrate was then mounted in a Wiltron jig.

Photo Thick-film Performance

The results obtained from the measurements described are shown as plots of loss in decibels per millimeter. Figures 1, 2 and 3 show the results obtained with 96 percent alumina, 99.5 percent alumina and 99.6 percent lapped alumina, respectively. It can be seen immediately that, as expected, the etched lines show a significant performance advantage over the printed lines. However, this advantage is significant only above approximately 8 GHz and is relatively small for the 99.5 percent alumina. In all cases, the etched lines show excellent performance. Even on 96 percent alumina the etched lines show performance comparable with published figures for thin film.

These results could be interpreted as indicating that direct printed lines are acceptable as a general-purpose microstrip line technology at these frequencies. This capability is certainly the case for simple transmission lines. However, if these lines are used as part of a component, it is not just the loss that is significant, but also the geometrical precision. For example, if an edge-coupled filter is being fabricated, it is likely to require line widths and spacing of 75 mm or better. Even though it may be possible to achieve this goal by direct printing, the tolerance on the dimensions is likely to be on the order of 25 mm. This tolerance makes the performance of the resulting component subject to wide variations in bandwidth and center frequency. The etched line, on the other hand, has an edge tolerance of 1 mm and produces a far more stable and reproducible component.

Further conclusions can be drawn from these three plots. First, it can be seen that loss reduces with the higher purity, higher cost substrates. However, this improvement is a small difference only, making the lower cost substrates suitable for circuit fabrication. Second, the difference between the printed and etched line on the 99.5 percent material is smaller than with the other materials. This result is believed to be due to the nature of the substrate surface allowing high quality printing, but will be the subject of further evaluation. Figure 4 shows a plot of loss vs. frequency for the thin, 250 mm line on 0.25 mm thick, 99.6 percent lapped alumina. It can be seen immediately that the line loss rises steadily with increasing frequency but is linear, indicating no problems with dispersion. Again, the loss is higher in the printed line compared to the etched line. Finally, the loss at 20 GHz is only 20 percent higher than for the wider 0.635-mm line. These results indicate excellent low loss behavior of the microstrip line on a thin alumina substrate.

Low Loss LTCC

Although the classic microstrip line on alumina discussed previously is of great interest, it is somewhat limited in the fabrication of complex circuits. What is needed is the ability to build multilayer patterns, incorporate resistors and other passive components and, hence, realize a complete structure. While this goal can be achieved using thick-film multilayer technology, the use of an LTCC process allows complex, low cost structures to be built in high volume. The reduction in processing steps with LTCC is significant and multiple out circuits can be fabricated readily. Unfortunately, as noted, LTCC tends to offer a high loss, high dielectric constant material that is not suitable for microwave interconnect.

As a result of these limitations, a new low loss dielectric material has been developed based on the KQ dielectric material referenced earlier. This tape can be fabricated in various fired thicknesses from 75 mm. The material allows the ready fabrication of structures where the microstrip line can be fabricated on top of the tape to produce the thin dielectric material required for operation at higher microwave frequencies. The process can be taken a step further with the fabrication of lines buried within the tape to form stripline and similar structures. With the inclusion of general interconnect and passive components, a complex integrated high performance structure can be readily built.

The low loss tape has been measured at X-band (8 to 12 GHz), yielding a dielectric constant of 3.94 and a loss factor of 5 x 10-4. It can be seen that the required low loss is achieved together with the advantage of a low dielectric constant.

LTCC in Combination with Photo Thick Film

In order to realize the structures allowable with the low loss LTCC, compatible materials of appropriate performance are required. The photo-patterned KQ materials discussed earlier prove ideal for this task. Even with a low dielectric constant of 3.94, a 75 mm tape thickness requires a narrow 150 mm line width for a 50 W impedance. Conventional LTCC processes can realize such line widths, but with poor tolerance. KQ photo processing can readily achieve such dimensions with excellent edge acuity.

Structures combining low loss LTCC and KQ photo-processed conductors have been built, allowing the use of a thin tape material with good connection to the ground plane. Two alternative versions of the structure were fabricated - one with a fired tape thickness of 130 mm and one with a fired thickness of 260 mm. The line widths were designed to provide a 50 W impedance (using the 3.94 dielectric constant), resulting in line widths of 290 and 560 mm, respectively. Loss was measured using a comparison between a straight through line 50 mm long in comparison with a meander line of 110 mm. This procedure allowed for any losses in the interface with the Wiltron jig to be isolated.

Measurements of line loss are shown in Figure 5 . It can be seen immediately that the 290 mm line produces a loss of 0.039 dB/mm at 40 GHz, which is approximately 14 percent greater than the similar 250 mm line on 99.6 percent lapped alumina. The wider line on the thicker material produces a loss of 0.03 dB/mm at 40 GHz, 12 percent lower than on 99.6 percent alumina. These results are considered to be excellent and show the outstanding applicability of this technology to building microwave structures.

The small sizes that result allow the parts to be manufactured as multiple out circuits on a relatively large substrate. The photo-processing technique is also suitable for multiple out work, guaranteeing accurate alignment over a large surface area. These benefits offer mass production cost benefits similar to those found in integrated circuit and printed circuit board fabrication and make the technology ideal for economical high volume production as required by current telecommunications applications.

Conclusion

The performance of photo-processed thick-film conductors up to at least 40 GHz has been demonstrated and excellent results have been obtained. Using this knowledge, a low loss LTCC tape material with good microwave properties has been demonstrated. What is now needed is a fully integrated, cofireable LTCC system that allows the production of high performance components together with integrated passive components. The cost benefits resulting from such a structure will be considerable and research and development work is being directed into this area. The combination of these materials and processes will lead to the realization of compact, low cost, high performance microwave structures.

Acknowledgment

The authors would like to acknowledge Bill Lorelli and Jim Wood at Heraeus for their great assistance with LTCC structure fabrication. The support of Susan Munyon at Coors Ceramics has also been invaluable. Portions of this paper were presented at the International Conference and Exposition Exhibition on High Density Packaging and MCMs in Denver, CO, April 6-9, 1999.

References

1.         P.G. Barnwell and J. Wood, "A Novel Thick-film on Ceramic MCM Technology Offering MCM-D Performance," Proceedings of the Sixth International Conference on MCMs, Denver, CO, April 1997, pp. 48-52.

2.         P.G. Barnwell, C. Free, L. Chin and C. Aitchison, "A Novel Thick-film on Ceramic Microwave Technology," Proceedings of the 1998 Asia Pacific Microwave Conference, December 1998.

Peter Barnwell received his BSc and PhD degrees in applied physics from City University, London, and has worked in the hybrid microelectronics industry for more than 30 years. His technical specialization has concentrated on RF and microwave properties of thick-film technology. Barnwell has published more than 70 technical papers, including several at the annual International Microelectronics and Packaging Society (IMAPS) Conference in the US. He is currently president of IMAPS Europe. In addition, Barnwell is business manager for Advanced Materials and Telecommunications Applications at Heraeus Inc., Cermalloy Division, where he has established new technologies in the marketplace and interfaced with both circuit manufacturers and systems companies.

Michael P. O'Neill received his BS degree in industrial engineering from Drexel University and has worked in the electronic materials industry for 14 years. He has held various positions in research and development, technical service, market development and sales with DuPont Microcircuit Materials. Currently, he is sales and marketing manager for Thick Film and Advanced Materials with Heraeus Inc., Cermalloy Division. O'Neill has authored or co-authored a number of papers on thick-film and LTCC materials and applications and is a member of IMAPS.

Charles Sabo received his BS and MS degrees in metallurgical engineering from Lafayette College and Purdue University, respectively. He spent three years as a member of the technical staff at the Microelectronics Circuits Division of Hughes Aircraft Co., and has been with Heraeus Inc.'s Cermalloy Division for the past 11 years in the position of senior quality assurance engineer and senior research metallurgist. Currently, he is technical service manager for Advanced Packaging responsible for the company's KQ and LTCC product lines. Sabo has co-authored eight papers dealing with material aspects of microelectronic packaging and is a member of IMAPS.

Charles Free is a principal lecturer and leader of the Communications Research Group at Middlesex University in the UK. He is the author of 50 technical and scientific papers and his current research interests are in dielectric measurements at microwave frequencies, microstrip antennas, high frequency MCM design, microstrip phase shifters and digitally controlled microwave oscillators.