Sputtering

Sputtering is a physical process whereby atoms in a solid target material are ejected into the gas phase due to bombardment of the material by energetic ions. Sputtering is largely driven by momentum exchange between the ions and atoms in the material, due to collisions. The process can be thought of as atomic billiards, with the ion (cue ball) striking a large cluster of close-packed atoms (billiard balls). Although the first collision pushes atoms deeper into the cluster, subsequent collisions between the atoms can result in some of the atoms near the surface being ejected away from the cluster. The number of atoms ejected from the surface per incident ion is called the sputter yield and is an important measure of the efficiency of the sputtering process. Other things the sputter yield depends on are the energy of the incident ions, the masses of the ions and target atoms, and the binding energy of atoms in the solid.

The ions for the sputtering process are supplied by a plasma that is induced in the sputtering equipment. In practice a variety of techniques are used to modify the plasma properties, especially ion density, to achieve the optimum sputtering conditions, including usage of RF alternating current, utilization of magnetic fields, and application of a bias voltage to the target.

The sputtered atoms, those ejected into the gas phase, are not in their thermodynamic equilibrium state. Therefore, they tend to condense back into the solid phase upon colliding with any surface in the sputtering chamber. This results in deposition of the sputtered material on all surfaces inside the chamber. This phenomenon is used extensively in the semiconductor industry to deposit thin films of various materials onto silicon wafers. It can also be used to apply thin coatings to glass for optical applications. It can also be performed at very low temperatures which makes it the ideal method to deposit gate, source, and drain metals for thin-film transistors, as well as contacts for PIN diodes. In fact the use of sputtering to deposit thin films on a substrate is probably the most important application of sputtering today.

One important advantage of sputtering as a deposition technique is that the deposited films have the same concentration as the target material. This might be surprising, since as we mentioned before the sputter yield depends on the atomic weight of the atoms to be sputtered. Therefore, one might expect one component of an alloy or mixture to sputter faster than the other components, leading to a higher concentration of that component in the deposited film. It is true that the components are sputtered at different rates, however, since only surface atoms can be sputtered, the faster sputtering of one element leaves the surface enriched with the other element, which effectively counteracts the difference in sputter rates resulting in deposited films with the same composition as the target. This contrasts sharply with evaporative techniques, in which one component often evaporates preferentially, resulting in a deposited film with a different composition than the source material.

Another application of sputtering is to etch away the target material. One such example occurs in Secondary Ion Mass Spectroscopy (SIMS), where the target sample is sputtered at a constant rate. As the target is sputtered, the concentration and identity of sputtered atoms are measured using Mass Spectroscopy. In this way the composition of the target material can be determined and even extremely low concentrations (~2*10^-6%) of impurities detected. Furthermore, because the sputtering continually etches deeper into the sample, concentration profiles as a function of depth can be measured.