Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
1999-09-03
2001-04-24
Noori, Max (Department: 2855)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
Reexamination Certificate
active
06220102
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The invention relates to the fabrication of integrated circuit devices, and more particularly, to a method and apparatus to verify the adequacy or strength of the connection between die bonding and the lead frame during package assembly.
(2) Description of the Prior Art
In the design of semiconductor devices, a large number of metal interconnect lines are created. These interconnect lines can be located in one plane or they can be located on a number of planes that are superimposed within the device package and are part of this package. Contact and via openings are created in the insulating and dielectric layers that separate the interconnect metal structures. Depending on the process design and technology, metal structures can consist of small contact and via openings that range from about 0.3 to 1.0 um. Other metal structures can include wide metal lines ranging from about 1 micrometer to about 20 to 30 micrometer. Or metal bonding pads, which can be as large as about 50 to about 100 um. Bonding pads serve the function of connecting the input and output of the die to other electrical functions such as ground, power and signal functions.
For many of the advanced semiconductor devices, device signals such as ground, power and I/O signals require numerous bonding pads. With the increased density of components within a chip and with increased sophistication of the circuitry contained within the chip, further demands are placed on the number of bonding pads for each chip. For many designs, the number of bonding pads becomes the limiting factor on chip size and chip function.
In the field of semiconductor devices, producing simple, reliable, and inexpensive bonding pads is a primary concern of manufacturing. Bonding pads are wired to device elements located in the semiconductor die substrate and provide exposed contact regions of the die which are suitable for wiring to components external to the die. In one typical case, a bonding wire is attached to the bonding pad at one end and a portion of the lead frame at the other.
An example of a simple and inexpensive bonding pad is an exposed aluminum surface. A gold bonding wire can be bonded to this aluminum pad. An important concern in the creation of such a bonding pad is the pad reliability and its performance under various conditions of temperature. When ambient temperatures are less than approximately 150 degrees C., the physical attachment and the electrical connection between the gold wire and the aluminum pad are sufficiently reliable. However, when temperature rises above 150 degrees C., the bond rapidly degenerates due to the growth of gold and aluminum intermetallics. That is, the two metals start to diffuse between each other and begin forming aluminum-gold chemical compositions. As a result, porosity, delamination, and voiding occur within the bond. Further increasing the temperature tends to worsen this relationship, and the bond will eventually fail. Consequently, potential reliability problems prevent using the aluminum bonding pad under conditions where the ambient temperature is known to exceed 150 degrees C. Furthermore, even when the ambient temperature is less than approximately 150 degrees C., the aluminum bonding pad is susceptible to corrosion simply because it is exposed.
Aluminum can however grow a passivating oxide layer in air and is as a consequence protected against corrosion. Aluminum wiring used in semiconductors, however, contains copper, which does not have a passivating oxide, and the Al—Cu alloy used is more vulnerable to corrosion. The corrosion of aluminum wires is caused by several sources such as chlorine transported through the plastic packaging and the passivation materials, chlorine from the etching compounds and as etching by-products, phosphorous acid formed from excess phosphorous in the phosphosilicate glass, etc. Only a small amount of chlorine is required to cause severe local corrosion of the aluminum lines. Aluminum corrosion can, in addition, occur very quickly after metal etching.
Copper is electro-positive with respect to hydrogen and is not vulnerable to corrosion. However, in air copper oxide grows linearly with time, indicating the lack of a protective oxide. This lack of a passivating oxide makes copper more vulnerable to chemical corrosion. To avoid or minimize this corrosion, most applications of copper metalization involve some protective layer deposited on top of the copper.
A basic requirement for bonding pads is that they provide a maximum number of I/O interconnect locations. Intersection of wires that are used to make these I/O connections is thereby not desired (since these wires would now have to be electrically isolated further adding to the processing cost) which leads to an arrangement for the bonding pads around the periphery of the final package. Materials used for the bonding pads include metallic materials such as tungsten and aluminum while heavily doped polysilicon can also be used for contacting material. The bonding pad is formed on the top surface of the semiconductor device whereby the electrically conducting material is frequently embedded in an insulating layer of dielectric. In using polysilicon as the bonding pad material, polysilicon can be doped with an n-type dopant for contacting N-regions while it can be doped with p-type dopant for contacting P-regions. This approach of doping avoids inter-diffusion of the dopants and dopant migration. It is clear that low contact resistance for the bonding pad area is required while concerns of avoidance of moisture or chemical solvent absorption, thin film adhesion characteristics, delamination and cracking play an important part in the creation of bonding pads. For these reasons extra steps, such as the creation of a metal seed layer and diffusion barrier layers (of Ti or TiN) within the openings created for the deposition of the bonding pad, are often taken if metal (tungsten, aluminum) is used for the bonding pad.
One of the methods than that is used to improve circuit performance and circuit density is to mount semiconductor die in a package or substrate. The interconnects between the die and the substrate are made using die bonding whereby the die-bonds are directly connected to contact points on the package on which the die is mounted. This connection must, for obvious reasons, be dependable and of high quality. This leads to a need for test equipment that accurately tests and measures the bond strength and that can make measurements that relate to and identify the modes of failure that occur when the bonding wire detaches from the bonding pad. Two parameters have thereby been identified, in U.S. Pat. No. 4,055,992, as being relevant and of importance, that is the strength of the connection and the rate of aging of the connection. The bond pull strength is critical in determining both of these factors, the test apparatus must therefore be capable of measuring the force that is required to break the bond and at the same time observe the mode of failure. In view of the fragile nature of semiconductor die, it is difficult to position the die with respect to the test equipment and at the same time make contact with the bond wire while pulling the wire until failure occurs, all the while measuring the force that is being exerted on the wire. Prior Art has provided a number of methods for accomplished this. These Prior Art methods are highlighted below.
An apparatus for testing the bond strength between two laminated layers of material is taught by A. R. Mancini in U.S. Pat. No. 3,019,644. Mancini discloses the use of an electric motor and a force gauge to measure the stripping force of two laminated layers. The apparatus consists of a rigid frame having a common drive shaft connected to several structural elements including sprocket gears, sprocket chain, drive pulleys, idler pulleys, a bifurcated support member, hook and a spring scale. Both layers are slowly advanced by the mechanization in opposite directions until there is a failure; the spring scale
Ackerman Stephen B.
Noori Max
Saile George O.
Vanguard International Semiconductor Corporation
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