Polyimide Based Embedding Technology for RF Structures and Active Components

In the world of electronic circuitry, especially in devices for portable applications, there is a clear trend towards mechanically flexible circuits. In most of these applications compactness and weight are highly important factors. The compactness of the resulting circuit can be improved even more by integrating electronic components in the substrate, and not only on front- and backside. Thus the development of technologies for embedding electronic components (both passives, and RF structures as well as ultra thin, foldable chips) in flexible substrates is the logical next step.

This paper presents two polyimide (PI) based technologies of integrating components for flexible circuits: the ultra thin chip package (UTCP) and PI based RF structures.

UTCP is a technology for embedding ultra thin chips in polyimide substrates. Silicon chips thinned down to 20 - 30 µm are packaged in between two spin-on polyimide layers, resulting in a total thickness of only 50 - 60 µm. Chip, PI and metal layers are so thin that the whole package is even bendable. The embedded chips can be used as a package (e.g. the package can be solder assembled on interconnection substrates (PCB or flex) like a common component). Alternatively the ultra thin chip package is suitable for embedding in rigid or flex boards, replacing for example the naked die. The use of a thin chip package here has two main advantages. First, the alignment constraints for the embedded package are not as severe as for the embedded die: a fan out metallisation provides contacts with more relaxed pitches. Second, the “Known Good Die” (KGD) issue is solved as the packaged chip can be tested before embedding.

IMEC Gent is also developing a polyimide based technology for RF thin-film type resistors, capacitors and inductors, connected by RF microstrip lines. A suitable sequence of process steps has been established. The integrated RF structures on the thin film on flex consist of 3 metallization layers and will use microstrip transmission lines for the interconnection. The base substrates consist of spincoated PI on a rigid glass carrier. Sputtered NiCr is used as resistive material for the thin film resistors, spin-on PI as dielectric and Cu is sputtered for the metallization layers. Five different test vehicles have been designed and the first test structures are implemented. This technology realizes PI based RF structures that can either be incorporated in standard flexible substrates or assembled on a rigid or flex board, as a package.

This contribution mainly focuses on the technological side of both embedding technologies; but also reports on the first embedding tests with the UTCP, on the first reliability testing and on electrical characterization. This work was carried out under the EC IST funded project IP-SHIFT (contract number 507745).