Deposition Rate Monitor Is Non-Intrusive, Material-Specific

One of the most important parameters in all physical vapor deposition (PVD) processes is the deposition rate. Controlling this process using an in situ, real-time deposition rate monitor has been difficult because the available systems used for this purpose employ the principles of quartz crystal microbalance (QCM), electron impact emission spectroscopy (EIES), or mass spectrometry (MS). The sensors in EIES and MS deposition controller systems use a thermionic emitter and therefore can only operate in a high-vacuum environment. Although the QCM sensor has no such pressure limitations, it is not material-specific and is considered to have limited usefulness in reactive evaporation or sputtering processes.

All of these techniques are intrusive and may perturb the procedure because they require the installation of a physical sensor inside the deposition chamber, which can disturb or contaminate the process and system.

Luxtron Corp. (Santa Clara, Calif.) has developed its Atomicas deposition controller, which it says offers a non-intrusive optical probing technique that allows the measurement of the PVD vapor flux without any disturbance or contamination of the actual process. According to the company, the system is the only material-specific deposition controller capable of operating in high gas pressure environments such as those found in sputtering and reactive evaporation systems.

At low deposition rates, the system exhibits a very high sensitivity for many elements. For elements such as copper, indium and cesium, the system can be used to control a deposition rate as low as 0.1 Å/sec over an extended time period. Unlike the Atomicas controller, all other deposition controllers sample the vapor flux at a single point (point sampling) away from the substrate. This is due to the physical size of the sensor and shadowing effects. The Luxtron system, on the other hand, samples the vapor flux over a finite length (linear sampling) in front of the substrate, obtaining considerably more accurate deposition rate control.

The Atomicas deposition controller provides a non-intrusive way to accurately determine deposition rate without compromising the process taking place inside the chamber. (Source: Luxtron)

Since AAS tends to be highly discriminating, the new controller is absolutely material-specific. When multi-source co-deposition processes take place, film composition can be precisely controlled with the use of multiple units, each one characterized to be sensitive only to a specific element. According to the company, the system also works equally well in ultrahigh vacuum or environments of any gas pressure.

Because the installation of the Atomicas system typically requires properly located optical viewports in the process chamber, some pre-planning in chamber design may become necessary. Also, coatings on the viewports, originating from the evaporated systems, may create problems. The platform's COPACT system is capable of compensating for viewport coating, but only to a certain degree. There are, however, special vacuum probes using fiber-optic feedthroughs for existing chambers that do not have optical viewports. Like almost all other optical instrumentation, the Atomicas platform does not perform very well when there are only very low signal levels.

Another problem is that, because of the instrument's sensitivity, a long path length for the probing optical beam may saturate the absorption signal at a high deposition rate for some elements. Thus, for high-rate deposition processes in a large chamber, proper shielding may be required to limit the absorption path length. Another disadvantage is that the platform can only detect free atoms, not molecules, ruling out its application to many optical coating processes that involve the evaporation of oxides and fluorides.

For additional information on inspection, measurement and test, go to www.semiconductor.net/imt.