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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.

Figure 1. Top view of 3D thin film circuit containing three (3) metal layers and two (2) polyimide layers.

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 Basics

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.