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How Strong are Computer Chips Anyway?

Jeffrey Florando
Materials Science & Engineering
Stanford University
May 2001

As our society increasingly relies on microelectronic chips in everything from computers to cell phones to music players, the reliability of these chips becomes increasingly important. Even though the chip itself is very complex, it is essentially composed of many thin layers, called films, on a thick silicon wafer, called the substrate. Typically, "thin" refers to films that have a thickness on the order of a few microns or less (about 1/100 the size of a human hair). To ensure that the chips do not fail in service, chip designers use parameters that are dictated by the properties of the films. Therefore, the designer must have accurate property values in order to maximize the chip's performance. One important property for the reliability of the chips is the strength of the films. Strength is typically defined as the material’s ability to resist permanent deformation. My research involves developing a testing method to accurately measure the strength and other mechanical properties of thin films.

The speed of microelectronic chips is increasing every year. This speed can mainly be attributed to the shrinking dimensions of each component of the chip. As these dimensions decrease, the percentage of surface area increases, which gives rise to some unique properties. One such property is that the same material can be two to five times stronger in its thin film form than in its bulk form. Due to this difference, it is necessary that we develop testing methods that can accurately measure these properties in thin films.

Currently, there are various methods to measure thin film mechanical properties; however, most of these techniques either fail to provide the fundamental data needed or require very complex processing and testing techniques. Therefore, we have developed a microbeam bending technique as a simple method for studying the necessary mechanical properties of thin films.

A silicon beam, which is fixed at one end, is fabricated using standard semiconductor processing techniques. The beam has dimensions on the order of tens of microns, and a copper film is deposited onto it. This bi-layer beam is then deflected using a nanoindenter, which can very accurately apply load and displacement. This machine can measure loads and displacements on the order of micro-Newtons of force, and tenths of nanometers of displacement. This would be like measuring the force exerted on your palm by an object 1000 times lighter than a feather, and measuring a displacement which is 100,000 times smaller than a human hair. We then use the deflection data to calculate the strength of the film.

Microbeam bending in itself is not a new technique. Our method is very similar to previous studies, except that we use triangular-shaped beams instead of the standard rectangular-shaped beams. The advantage of the triangular beam is that due to its geometry, the entire surface of the beam deforms the same amount, unlike the rectangular geometry where most of the deformation occurs at the fixed end. Deformation is typically defined as a change in a material’s shape (not to be confused with deflection, which is the amount of displacement from a fixed point). The changing width of the triangular beam allows for the deformation to be uniformly distributed over the surface. This uniform deformation allows us to accurately extract the film properties, as well as produce more consistent results.

With this testing technique we can determine how processing parameters affect the mechanical response of the films. Specifically, the semiconductor industry is interested in how thermal cycles, passivating layers, and film thickness affect the strength of the film.

Chips can heat up hundreds of degrees during operation and then cool down to room temperature when not in use. This heating and cooling is known as thermal cycling, and these cycles can cause the internal structure, or microstructure, of the film to change. Generally, changing the microstructure can greatly influence how the film behaves. We can use our beam-bending technique to determine the extent of this change by heating the sample and measuring how the room temperature strength evolves with the changing microstructure. For instance, our results show that copper films that have been thermally cycled are weaker than as-deposited films.

Passivating layers have become an increasingly important issue in the semiconductor industry. Copper, which is now replacing aluminum as the new interconnect metal, readily reacts with air to form an oxide. Unfortunately, unlike aluminum, which forms a very protective oxide, copper's oxide allows oxygen to continue to penetrate and react, especially at the higher temperatures experienced during chip operation. Excess amounts of reacted copper can greatly reduce the speed of the chip. Therefore, a protective passivating layer is deposited on top of the copper, which will not only serve to protect the copper from reacting, but also helps to strengthen the film. This amount of strengthening can be measured using our beam-bending technique.

As the dimensions of the chips continue to decrease, the films must become thinner. Typically, the strength of a film increases as the thickness decreases; however, the amount of strengthening and the mechanisms for strengthening are still not fully understood. Our technique can be used to determine the amount of strengthening and may provide some insight into possible mechanisms. By knowing the amount of strengthening and the mechanisms, chip manufacturers will have a better understanding of how materials behave.

This beam-bending technique is not only simple to use, but it provides data that may lead to a better understanding of how thin films deform. By understanding how different processing parameters affect the strength of the films, manufacturers will be able to better optimize their chip design.