Multilayer Ceramic Integrated Circuit (MCIC) Technology:
An Enabler for the Integration of Wireless Radio Functions
Hundreds of discrete components are used in the design and assembly of wireless radio products and the numbers are increasing to meet the market need for increased functionality. Manufacturing and cost issues are driving the need to develop technologies that can effectively integrate these components, achieving functions that meet performance requirements in a cost-effective way.
In this regard, the use of Low Temperature Cofire Ceramic (LTCC) Technology to make Multilayer Ceramic Integrated Circuits (MCIC) for high frequency wireless communications equipment has gained momentum in recent years. The advantage of using MCIC for integration of passive components, especially at 800 MHz and above, is discussed in an article written by David Wilcox, Vice President of Technology at Motorola, Inc., and published in the August 1999 issue of Advancing Microelectronics. Mr. Wilcox describes why Motorola is using MCIC technology to advance the integration of components to create and to support RF circuit functions such as transmit/receive switches, delay lines, filters, impedance matching VCO's and directional couplers for wireless portable communication devices. Such functions require the ability to integrate components such as capacitors, resistors, transmission lines, and inductor coils.
The MCIC thrust at Motorola represents a robust fabrication technology for creating a variety of components and products useful to the electronics industry. The MCIC technology might be viewed as the advanced process technology analogue enabling advanced electronic ceramic devices and systems, analogous to thin film process technologies that enables the semiconductor industry to create generation after generation of semiconductors with increasing volumetric efficiency.
The foundation of MCIC technology is the formation of LTCC "green" sheets or "tapes" that are subjected to a variety of operations when fabricating multilayer ceramic electronic devices and sub-systems as follows: a) formation of via holes and cavities using mechanical, laser, and e-beam techniques; b) thick film screen printing of powder metal conductors, dielectrics, magnetic and resistor patterns; and c) an ability to stack up the individual layers and laminate to form a composite three-dimensional structure.
It is typical in the high frequency wireless and radio frequency (RF) circuit design world to partition the MCIC's parts in such a manner as to prevent cross-talk between the digital and RF sections and between separate RF sections. There are several functions in the RF portion of a radio circuit that require low electrical losses. Examples of these are the need for matching circuits and filters with low electrical loss (high Q) critical to increase battery life or "talk" time. One of the most important elements in a high frequency circuit is the use of a helical transmission line to form a compact resonator. The Q of the resonator used in the design of these RF applications can be described as 1/Q = 1/Qm + 1/Qd where Qm = the inverse of metal conductor loss and Qd = the inverse of dielectric ceramic tape loss. Conductor loss is a significant factor in resonant structures for circuits operating above 800 MHz.
Motorola has built prototype MCIC's using Du Pont's #943 low dielectric constant LTCC green tape. Transmission lines are created by thick film screen printing of Du Pont's #6145 conductor material made from a silver powder. These new materials however, have not reduced the conductor loss to an acceptable level in circuits operating above 800 MHz. When screen printed and co-fired, Du Pont's powder metal conductor materials produce porous transmission lines and offers no real advantage due to the dominance of conductor loss acting against the lower dielectric constant #943 material.
THE RTI SOLUTION:
RTI owns certain patents and know-how relating to the formulation, processing, and manufacturing of ceramic and glass composites, which do not shrink upon co-firing to the degree of ordinary LTCC materials of this type. The slight shrinkage (< 2%) which is present can be controlled to a very tight tolerance of +/- 0.01%. RTI has trademarked this patented LTCC technology as ZERO SHRINK TECHNOLOGY (ZST™). The ZST™ LTCC system owned by RTI provides significant advantages and engineering degrees of freedom in manufacturing over other LTCC tape materials. Zero Shrink Technology Diagram
Because ZST™ exhibits a near ZERO shrinkage and ZERO shrinkage tolerance upon firing, precise feature locations are maintained in the X, Y, and Z axis and yield improvements of over 30% can be realized when compared to conventional materials systems. ZST™'s properties also allow for the creation of advanced circuits, which may not be built with traditional LTCC tape materials. Some properties include the embedding and co-firing of: discrete components such as ceramic chip capacitors for true passive integration, ceramic heat spreaders with integral heat pipes for thermal management (>2000 W/mK), and integral fired Alumina segments for combined thick and thin film metalization.
ZST™ has a very important contribution in the design of high frequency wireless and radio frequency (RF) circuits.
By embedding and co-firing solid metal wires and etched metal foils to create dense circuit conductors to control DCR losses for transmission lines, MCIC's built using ZST™ will enjoy a substantially increased Q, providing superior RF performance over Motorola's MCIC packaging.
Other advanced applications exist with ZST™ beyond the electronics area. These applications are best summarized by stating that with RTI's ZST™ articles can be formed in the green or unfired state and, when fired, the resultant article is essentially the same size and shape as the green article. Limitations caused by shrinkage present in traditional ceramics such as physical size, or firing distortion are eliminated as well as the costly machining associated with hard-fired ceramics. Further, RTI's ZST™ can be custom engineered to vary properties; dielectric constants from 3.7 to over 100, temperature coefficients of expansion 0 ppm to 120 ppm, and fired densities ranging from quite porous to totally hermetic.
RTI is prepared to work with a strategic alliance partner to manufacture and sell products based upon this ZST™ technology and know-how. Of particular interest is a ZST™ technology that can be used in electronic applications to manufacture MCIC's more advanced than those being produced by Motorola.
If the partners decided to manufacture and sell ZST™ LTCC "green" tape, we could sell this product in direct competition with Du Pont Company, Ferro Corporation, and Heraeus GmbH under a protection umbrella afforded by our patents. The prime manufacturing cost for ZST™ is approximately 0.4 cents per sq. in. The market price for LTCC tape, from Du Pont, ranges from 3.5 to 30 cents per sq. in. There is a lot of margin in this product. Additionally, we would be offering a far superior product. Note: The ZST™ LTCC "green" tape system is fully compatible with all Du Pont Company, Ferro Corporation, and Heraeus GmbH mixed metal systems.
RTI can also provide a turnkey manufacturing plant for fabrication of ZST™ LTCC green tape. This would include all manufacturing equipment, training of engineers technicians and production staff, process specifications, marketing training complete with strategies to penetrate the United States, Asian, and European markets, and an ongoing consultancy assistance program.
The establishment of a LTCC multilayer circuit manufacturing facility can also be provided with the added advantage of our patented ZST™ technology. Armed with this advantage, the new facility would not be a "me-too" producer and would therefore have substantial technical advantages not available to competitors.
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