Sparkling Atoms Point to Voids

Even a cursory glance at the International Technology Roadmap for Semiconductors (ITRS) readily shows that the damascene process — the filling of high-aspect-ratio vias and lines with copper — will prove to be difficult in the immediate future. The detection of subtle fill defects or voids in these features continues to be a very serious problem in search of a solution. However, this may soon be resolved with the use of X-ray fluorescence (XRF).

XRF has always proven useful for quick qualitative and quantitative analysis. It is a very attractive technology for use in composition and thickness analysis because it accommodates solid samples, metals and insulators, and production wafers. When it is properly calibrated, it can monitor metal film thickness between 0.2 nm and several microns with up to 0.01% precision, depending on which elements are being measured. In a grazing incidence configuration (total reflection X-ray fluorescence, or TXRF), it provides fast, nondestructive, full-wafer monitor capabilities for wafer metal contamination control.

In XRF, incident X-rays are applied to eject inner-shell electrons from the sample's atoms through the photoelectric effect. As a result, the atom releases an X-ray emission with the parent atom's energy characteristics with an intensity proportional to the element's amount. Conventional instruments use a stationary X-ray target, usually composed of elemental chromium, copper, rhodium, silver, molybdenum, tungsten or gold. The spectrum of X-rays directed at the sample includes the characteristic tube energy (K, L, M lines) combined with broadband Bremsstrahlung background. XRF can provide information from an analytical depth of 1-300 µm, depending on the incident X-ray radiation's energy and the characteristic fluorescence's escape depth.¹

Philips Analytical (Almelo, Netherlands) is working on a nondestructive technique that will detect voids in vias and damascene lines. The system is expected to be particularly well-suited to instances where there is a cluster of vias or lines that may have bad filling. The technique is XRF-based and uses a small, 25 µm spot to cause the copper material to fluoresce. Because X-ray fluorescence, particularly of copper, is very efficient, it almost becomes possible to count atoms. In actuality, when using XRF the user is counting atoms per square centimeter.

Fitted spectra used for analysis, measured in air. The peaks are colored to visualize the different samples and to show the peak. Shown here are two measurements on die on different areas of the wafer. The peak to the left shows one that is well filled; the one on the right is not. (Source: Philips Analytical)

The technique, according to Philips, has been further developed to provide the capability to reveal whether a specific structure has a good or bad fill. The system is being planned as a modular measurement head solution or as a capability that can be built into a stand-alone metrology system.

The new tool has required considerable software development to support its capabilities and make it practical to operate. The user selects the area that he wishes to inspect; because the system knows what the filling should be, it is capable of predicting what the signal should be and making measurements from die to die on that same position. It then compares and predicts what the fill is in those different areas from die to die on the wafer, providing a complete map of the entire wafer.

At this stage, there do not appear to be any disadvantages to the method, which, depending on the level of monitoring that the user wishes to apply — nine points on a wafer, for example — can be relatively fast. Obviously, should the user wish to monitor the entire wafer, the technique would take additional time. However, the measurement time per spot is on the order of ~1 sec, so if the user is only measuring nine spots, for example, wafer loading and unloading could have more of an impact on throughput than the operation of the XRF system itself.

Although the tool does not yet have a name or model number, alpha results appear to indicate that it should have a considerable impact on the need for detection of voids in the copper damascene process. According to Philips, the new system is expected to go into beta testing very soon.