Using Polyimide In 3D Thin Film Multilayer Circuitry
Today, the cost and complexity of all platforms and systems in military and defense technologies are being challenged by the need for high functionality in smaller but less expensive architecture, especially in light of current national budget challenges. In the area of RF and microwave electronics it is becoming evident that a move from two dimensional (2D) planar circuitry architecture to three dimensional (3D) thin film multilayer technology can result in dramatic reduction of chip size, increase of operation speed, and lower total module costs.
Circuit miniaturization is achieved by stacking multiple layers of thin film circuitry and polyimide dielectric coatings to form 3D interconnected multilevel circuits that can be very useful in high frequency, microwave and millimeter wave applications. Moving to 3D construction saves significant 2D area, shortens signal paths to provide dramatic increases in speed and performance, and reduces power consumption. An example of such a device is shown in the cross-section picture of Figure 1.
Polyimide is used as the interlevel dielectric layer in between the thin film metal circuitry. Polyimide plays a critical role as the dielectric material due to its ability to planarize the topography of the metal layers and provide good electrical characteristics and fabrication process compatibility. Polyimide is a polymer material known for its thermal stability, good chemical resistance, and excellent mechanical properties.
Photosensitive polyimides are now commercially available and this offers simplified processing and lower cost because photoresist does not have to be used in order to image the polyimide. In fact, photosensitive polyimide behaves just like a photoresist in that it is applied at the substrate level using standard thin film photoresist processes and equipment. A 5 micron thick polyimide film is capable of resolving 1 mil (25 micron) features like a contact via. After application, exposure, and developing, thermal curing or imidization is used in order to convert the film into a robust cured polymer. The cured polyimide will then be able to withstand the essential operations of microelectronics fabrication which include baking, sputtering, plating, etching, dicing, die bonding, soldering, wirebonding and sealing. The associated cured film properties are shown in the accompanying table and are well suited for high performance electronics applications.
The cured polyimide films can be designed to be between 5 and 20 microns in thickness. The cured polyimide films possess favorable electrical properties for applications in RF and microwave electronics. The dielectric constant is low at 3.3 which allows for high radiation efficiency and high speed signal propagation. The dissipation factor is low as well at .001. The dielectric strength and volume resistivity are also favorably high for use as an insulating layer in today’s microelectronic devices.
Water absorption does affect the dielectric constant due to the higher dielectric constant of water but is avoided when devices are sealed in a package or housing.
Polyimide And Thin Film Together
The cured polyimide films demonstrate good adhesion to common substrates like alumina and aluminum nitride. Substrates can be polished so they are useful at high frequencies. The polyimide is also compatible with thin film metalizations such as gold for wirebonding, as well as nickel and palladium for soldering.
When the superior material properties of polyimide and the dimensional resolving power of thin film technology are combined, then 3D interconnected multilevel circuits become practical. The 3D interconnected multilevel construction allows for the realization of multifunction chips that combine analog, digital, and RF or microwave signals for breakthrough circuit performance. These chips can be used to improve the cost/size/performance of a vast array of military and defense electronics. In our experience we have seen designers obtain size reductions ranging from 3-15X.
The true benefits of this construction method comes from stacking circuitry on top of other layers of circuitry, separated but interconnected where needed through contact vias in the polyimide. This “3D integration” approach allows for circuits with multiple DC, power, and ground plane layers to be stacked on top of one another. Other thin film features such as copper or gold filled-vias and integrated laser trimmed sheet resistors can enhance functionality. A cross-section of such a device is shown in Figure 2.
Moving current designs to 3D thin film multilayer polyimide technology could allow for dramatic performance improvements for military and defense applications in many of the following areas:
- Missile Guidance and Control
- Electronic Warfare Suites
- Airborne and Ground Radar
- Combat Avionic Systems
- Electronic Sensor Systems
- Thermal Imaging
- Airborne Communication and Surveillance
- Countermeasures and Decoys Systems
- Space Based Infrared Systems
- Satellite Guidance and Communication Electronics
High Frequency Characterization
When 3D thin film multilayer polyimide circuits are used for microwave and millimeter applications, high frequency performance data become very important for the circuit design. Microstrip and stripline transmission lines are widely used in these circuits due to their compatibility with the circuit environment and ease of integration and fabrication. As is typical in 3D multilayer integration of RF and microwave components, stripline and shielded coplanar waveguide find more use because their physical structure favors multilayer interconnection. For the top level planar structures, such as matching networks, antennas and other high frequency components, microstrip lines are the transmission method of choice. Ground planes are also used to shield high frequencies and minimize unwanted interference. Passive components such as resistors, inductors, filters, etc. can also be easily integrated.
Incorporating all of these circuit features into one unit at high frequencies is not simple, but it is being done successfully in applications where the existing cost or performance equation needs to be inverted. Companies are successfully undertaking a methodical approach to building quick turn, cost effective prototypes, in order to build their design knowledge, feature by feature. The end result can be a new product that boldly sets a new standard in performance and size and feature set.
This article was written by Michael D. Casper, President, UltraSource, Inc. (Hollis, NH). For more information, Click Here .