Multichip modules are an advanced packaging concept in which multiple integrated circuits (ICs) are directly attached to a substrate such as a multilayer printed circuit board. In a typical implementation, the printed circuit board substrate material is either polymer or ceramic, and the attachment is either by epoxy or by solder.
Once the integrated circuits are physically attached to the substrate, they are electrically connected to the substrate. The most common approach for achieving electrical connection is wire bonding. Wire bonding uses elevated temperature and mechanical force and/or vibration to connect a microscale wire between two bond pads. The wire typically attaches first to a bond pad on the integrated circuit, then to a bond pad on the substrate itself.
In one implementation of the multichip module concept, each integrated circuit is epoxied or soldered into a recessed cavity in the top surface of the substrate. Wire bonds electrically connect the integrated circuits with bond pads provided on the top surface of the substrate.
In the lowest cost and most manufacturable implementation, however, the top surface of the substrate is planar so that the integrated circuits are epoxied or soldered to the same substrate surface on which the bond pads are provided. A drawback to this approach is that the epoxy or solder process introduces contaminants on the substrate surface, particularly on the bond pads, and this degrades wire bond adhesion. Poor wire bond adhesion reduces bond strength during pull testing (see, e.g., MIL-STD-883H), and degrades mechanical reliability and overall assembly yield.
Plasma cleaning may be applied to the module after integrated circuit attachment, to remove the contaminants before wire bonding takes place. This greatly increases the mechanical strength of the wire bonds and process repeatability. However, semiconductor manufacturers strongly discourage the use of plasma cleaning with CMOS integrated circuits and related IC technologies. The problem is that plasma cleaning can induce an accumulation of excessive charge on the surface of an integrated circuit that, in the case of CMOS and related technologies, is harmful to reliability and functionality. Experimental results have demonstrated that excessive charge induced by plasma cleaning may typically be expected to destroy the functionalities of CMOS integrated circuits. This is unfortunate because CMOS (which has been a dominant integrated circuit technology for the last 30 years) is expected to continue its technological dominance for years to come.
It is desirable in view of the foregoing to provide for the application of plasma cleaning to an MCM without producing harmfully excessive charge on its constituent ICs.