Studies on fabrication of high density MCM-D substrate by aluminum anodization techniques

High density and high reliability MCM-D substrates attract much attention in the microelectronics packaging area. In this paper, we present recent studies on aluminum thin film anodization process to fabricate high density MCM-D substrate. With this technology four layers MCM-D substrates deposited on glass bases are successfully fabricated. The glass plate is selected as heat sink base of the substrate. Aluminum anodization equipment is developed to prepare porous aluminum oxide thin film dielectrics. Aluminum interconnect strips are formed in each thin film layer and aluminum via posts which link adjacent layers are prepared by selective anodization method. Kelvin structure and comb patterns structure are designed and fabricated to test via posts resistance and the insulation resistance in surface layer and in between adjacent layers. The resistance tests meet the GJB test standard. Peripheral and array structures are designed for flip chip interconnection. Three main steps are involved in the process. Firstly, aluminum thin film is deposited on a glass substrate. Secondly, photoresist is coated and developed to make the anodization mask. The aluminum strips and via posts are protected by the photoresist mask. Then the aluminum film is selective anodized in the electrolyte to porous anodic alumina, thereby creating conductance strips and via posts in this step. The above three steps are repeated until the required number of layers are achieved. In the traditional etch process to make MCM-D substrate, the etch process in multilayer lead to nonplanarity etch steps. The metal film coverage of the steps is crucial to conductance interconnect and reliability. The selective anodization processes overcome the nonplanarity problem. Conductance strips and via posts and insulation dielectric layers are formed in the same plane. The interconnection and reliability of substrate are improved greatly. In the fabricated MCM-D substrate, the minimum sizes of conductive line and via posts are 20 μm and 40 × 40μm respectively. The resistance of the layer insulation dielectrics is higher than 10⁹ Ω. There was no residual aluminum thin film and short current between conductance strips. Aluminum between strips was totally anodized to insu- lated porous anodic alumina. The brevity of process and good planarity of structure make MCM-D substrate more reliable and suitable for the high density and reliability microelectronic systems.