Surface waves measure stiffness of low-k materials

Surface waves measure stiffness of low-k materials
At the 90-nanometer node and below, a progression of low-k dielectric materials is being introduced for IC interconnects to achieve smaller wiring dimensions at higher circuit speed. Compared with traditional silicon-dioxide-based dielectric, these low-k materials reduce the effective line-to-line capacitance of the interconnect structure and therefore permit a smaller wiring pitch.

However, integration of low-k into the copper damascene fabrication process presents a number of challenges. While the electrical properties of low-k materials are superior to oxide, their mechanical and chemical properties are not.

One integration challenge is the low mechanical stiffness of these materials, quantified by the Young's modulus (ratio of the stress to the strain). Low-k film materials do not resist the physical stresses of processing and packaging as well as silicon dioxide, particularly when porosity is introduced into the low-k film, further weakening it.

For example, many low-k materials cause difficulties during copper chemical-mechanical planarization (CMP), because they tend to crack or delaminate. Some researchers believe that a threshold value of Young's modulus is required for a film to successfully pass the CMP step. In addition, dielectrics with low Young's modulus may degrade copper reliability, because they contribute to stress voiding during circuit operation.

Therefore, during candidate material selection and process optimization, methods are needed for measuring the stiffness of low-k films in order to characterize new materials and their deposition processes.

Currently nanoindentation is the most widely used technique for this purpose. The technique, which is destructive, works by measuring the mechanical resistance of a film sample as a tiny indenter tip is pressed into it. However, there are difficulties in applying nanoindentation to low-k materials, especially for typical dielectric-film thicknesses, which are below 0.5 micron. The measurement of the soft low-k film tends to be influenced by the very stiff silicon substrate beneath it, resulting in modulus readings that are too high.

Fortunately, modulus measurement using laser acoustics can provide a more accurate measure of the stiffness of these soft low-k films. Producers and users of low-k film materials have been exploring the use of laser acoustics to characterize these materials for several years.

The Advanced Metrology Systems unit of Philips Electronics has been engaged in collaborative research with leading industry R&D centers since 2000 to develop methods for modulus measurement using a proprietary laser-acoustics technology. The surface-acoustic-wave technology, first developed for copper film thickness metrology, permits excitation and detection of surface waves over a range of precisely controlled acoustic wavelengths, enabling more complete characterization of a film's elastic properties.

The surface-wave technique works by using beams from a pulsed laser, imaged into a striped pattern, to initiate an acoustic wave on the sample surface with wavelength equal to the spacing of the stripe pattern. A second probe laser detects the passing wave and its frequency spectrum is then calculated. Computer analysis of the frequency spectrum vs. the acoustic wavelength permits determination of stiffness-that is, the Young's modulus-as well as other elastic constants such as the Poisson's ratio.

In contrast to nanoindentation, the SAW method of modulus measurement maintains its accuracy for soft low-k films. In fact, the large acoustic mismatch between the soft film and the hard silicon substrate results in greater variability of the frequency spectrum vs. the acoustic wavelength, and therefore greater sensitivity of the measurement.

Furthermore, measurement with laser-induced surface waves opens a new window into the deposition processes for low-k materials. Because the surface-wave method is rapid (5 to 30 seconds per site) and nondestructive, has a small measurement spot (15 x 30 microns) and is highly repeatable, an automated system can be used to measure detailed uniformity maps of Young's modulus.

This full-wafer mapping capability allows the process engineer to more completely characterize the film material and detect mechanical nonuniformities that may be induced by the deposition process or subsequent processing. This is important because such nonuniformities may result in yield issues further down the line.

At the 90-nm node, most chip makers have adopted CVD-based carbon-doped oxide dielectrics. As the industry transitions to the next generation of low-k materials, which are likely to be less stable than those currently in widespread use, the ability to characterize modulus uniformity may become more important.

For future nodes, spin-on-based polymers may find more acceptance. All candidate materials, CVD or spin-on, will require some element of porosity, which can be difficult to control. Since porosity variations and polymer curing can both introduce variations in Young's modulus, stiffness metrology could become more important, both to characterize within-wafer stiffness uniformity and to monitor batch-to-batch variations in film processing.

Since the surface-wave method is nondestructive, it could be used to monitor low-k stiffness properties directly on product wafers if very frequent monitoring is required for future processes.

In all, process engineers are likely to find that the introduction of new generations of low-k materials comes with new characterization challenges, and mechanical-properties measurement is likely to grow in importance. Laser-acoustic measurement with surface waves provides an ideal method to both characterize and monitor the Young's modulus of new film materials.

Michael Gostein is chief technologist at Philips Advanced Metrology Systems (Natick, Mass.).

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