Abstract: Review of bonding and interconnection methods in microelectronics

Abstract

This paper will review the major bonding and interconnection methods being employed in microelectronic manufacturing. Although there is a large number of variations of method, the physical processes for achieving a reliable interconnection can be reduced to two categories: solid phase joining and liquid phase joining. Solid phase joining is achieved as the name implies, without melting by the formation of regions of intimate metal−to−metal contact utilizing the application of energy. The two principle sources of energy are a combination of heat and pressure (thermocompression) and ultrasonic vibrations, and excellent papers have been written describing the physical processes occurring at the interface in each of these processes.1,2 Liquid phase joining is achieved by following a thin layer of nonferrous filler metal between the surfaces to be joined, and the physical processes of the wetting of the joint surfaces have been described in detail.3 By far the most widely used method for making interconnections in microelectronics involves soldering the semiconductor die to the header and joining individual areas of the circuit to the package leads using fine diameter wires. The die bond is generally achieved with high−strength gold−based eutectic solders; however, with increasing die size, the stresses due to thermal expansion mismatch increase until it becomes necessary to use low−strength, highly plastic lead−based solders in place of the gold solders to prevent the die from cracking.4 The wire bonds are achieved utilizing either thermocompression or ultrasonic bonding, and excellent reviews have been written summarizing the methods for manufacturing the wire bonds and for testing and evaluating their integrity.5,6 The major advantages of the die and wire bond interconnection system are its flexibility and simplicity, while its disadvantages relate to the high−labor dependence of the system both in terms of cost and reliability. Another interconnection method utilized where long−term reliability is required is beam lead bonding. In this technique, gold beams are formed leading from the device metallization, generally by plating, and thermocompression bonding is used to form the joint with the metallized substrate.7 More reliable (at high temperatures and current densities) multimetal gold systems 8 are used for the device metallization and special dielectrics for passivation, and the advantage of the over−all system is increased reliability particularly under harsh operating conditions. The disadvantages are increased cost (due to the extra processing steps involved), and lack of general availability. A third interconnection method involves creating bumps on the semiconductor device surface, flipping the chip, and simultaneously joining these bumps to the substrate. The most common method of joining employs lead−based soft solders,9 although thermocompression and ultrasonic bonding systems have also been employed in a number of development schemes. Recent emphasis has been on automating bump interconnection systems utilizing film carrier techniques.10 The advantages of the system involve potential lower costs for packages involving many bonds and the excellent reliability reported for solder joint system.11 The disadvantages include the complex metallurgy system required and the lack of inspectability. A fourth interconnection method is spider bonding employing ultrasonic bonding to join a standard aluminum metallized die to an etched or stamped aluminum ’’spider’’ which in turn is welded to a lead frame.12 The advantages of this method include the potential lower costs due to simultaneous bonding, and the simple metallurgy system. The disadvantages include lack of availability and the requirement for an additional joint.