November 1999

A worker loads a printed circuit board into a Veeco Industrial Measurement Division MXR x-ray fluorescent measuring tool, incorporating its innovative optical collimator technology.

Continued advances in microelectronics manufacture -- including increasing clock speeds, reductions in device size, impedance and capacitance factors, and thermal management issues -- have intensified the demand on the packaging and interconnect segments of the industry to provide smaller, faster, and more electrically and thermally conductive interconnect schemes. Sub-100-micron interconnect structures used in ball grid arrays (BGAs), flip chips, and wire bonding techniques on wafers, packages, and substrates are common now, and these structures will continue to decrease in size and increase in density.

This trend creates the need for better opaque film metrology tools. A new focusing method enables x-ray fluorescent (XRF) measuring tools, commonly used to determine the thickness and composition of interconnect metallizations in microelectronic and data storage devices, to measure in much smaller areas. Long the preferred tool for such measurements in the electronics industry, this method relies on the principle that any element, when exposed to a source of high-intensity x-rays, will emit x-rays or fluoresce at energy levels unique to that element. To obtain a measurement, the XRF system uses an x-ray source to produce a spectrum that is directed at a sample to induce the fluorescence.

Since the XRF is used to measure very small sample areas, small x-ray beams are required. Erroneous measurements will be obtained if the beam size is larger than the sample, or if the beam is mispositioned in such a way that its perimeter extends beyond the edge of the sample. Most XRFs use collimators, essentially pinhole apertures, to direct the beam. The collimator blocks all but a very small fraction of the generated x-rays, passing through only those traveling in a path coincident with the opening. They emerge in a cone-like beam whose initial diameter is equal to the diameter of the collimator's opening.

The XRF detects x-ray emissions from the sample and converts them to electronic pulses. Each pulse is then sorted according to its energy level and delivered into a memory location, or channel, in the XRF's multichannel analyzer, which also counts the number of pulses stored in each channel. A computer uses the data to generate a frequency distribution or histogram, displaying channel-number or energy-level information along the x-axis and number of pulses along the y-axis. To arrive at a measurement level, the instrument can either compare the spectral data from a sample to a previously stored calibration spectrum or use fundamental parameters to evaluate the sample. By these methods the instrument can calculate the thickness and composition of the sample.

XRF is a powerful tool but has until recently been taxed to provide fast and precise results on structures smaller than 100 microns, because the collimators used have been mechanical. With mechanical collimation, as the machined openings become smaller to resolve smaller interconnect structures, less of the primary beam can reach the sample. Since XRF systems are counting devices, their precision is proportional to the number of x-rays generated at the sample and subsequently fluoresced back from the sample to the detector. Thus, smaller beams mean reduced precision, longer measurement times, and reduced throughput.

Enter optical collimation

Recently a new method has been devised to improve the precision of XRF instruments. Optical collimation, as it is called, rather than governing the flow of primary x-ray beam photons, redirects them down to a point. This is accomplished by a device called a focusing element that utilizes a monolithic polycapillary optic in conjunction with a beam input array and an exit filter to deliver unprecedented x-ray intensities in areas as small as 50 microns. Count-rate gains on the order of 100 times and precision gains of an order of magnitude can be achieved with this new technology. The use of a focusing element allows for a micro-beam x-ray analysis of structures as small as 50 microns.

Typical applications for the new technology include analysis of solder-bump composition (Sn-Pb), three-layer under-bump metallurgy thickness measurement (such as Au/Ni-P/Cu and Cu/Cr/Ti), wire-bond-pad metallurgy (Au/Ni/Cu) thickness measurement, ball-grid-array and flip-chip metallurgy, package-to-substrate interconnects, and current-carrying metallurgy.

When the Industrial Measurement Division of Veeco Inc., of Ronkonkoma, NY, first integrated the optical collimator into its XRF tools, the company faced a challenging task in commercializing this technology. The biggest obstacle was the fact that the optical collimator is considerably more complicated from a mechanical standpoint, yet Veeco wanted the new instrument to be exactly the same size as the old one. The primary complexity is the need for two-axis positioning of the focusing element. At the same time engineers wanted to fit the case inside the existing product structure because of tight space requirements in the clean rooms where the devices are normally used and because of the need to reduce design and manufacturing costs.

Veeco engineers selected the R201XY roller slide positioning stage from Del-Tron Precision Inc. of Bethel, CT, because it was the smallest linear motion device they could find that fit their requirements. This stage easily fit within the confines of the existing product packaging. The stage also meets the accuracy requirements of the application without difficulty. It incorporates a spring-loaded micrometer drive that allows precise repeatable adjustments with low friction and zero backlash. The slides provide accuracy to 0.0001 in./in. of travel and repeatability of 0.0001 in. More than 60 models support load capacities to 160 lb.

The device also features a positive locking capability consisting of a steel shim and an extended micrometer bracket secured by a screw mounted to the side of the stage carriage. This allows the user to lock the position of the carriage during use. Locking micrometer heads are also available to lock the micrometer setting. Del-Tron makes more than 60 models of its ball slide positioner with load capacities of up to 60 pounds. These slides can be used for gauging and positioning light and medium loads; applications include measuring instruments and optical assemblies.

Veeco incorporated the slide into its MXR XRF metrology tool that delivers 40 to 300 times the x-rays and 10 times the precision of mechanically collimated XRF systems with equivalent beam sizes. The detection column of the MXR instrument features an electrically cooled solid-state detector for optimum sensitivity required for thin (100-500-angstrom) depositions, multilayer metal stacks, and elemental peak overlap applications. Veeco has also introduced a VXR instrument that utilizes vacuum technology to extend the elemental measuring range of the MXR to elements from aluminum to scandium, in addition to the titanium-to-uranium range of the MXR. This instrument also incorporates an evacuated-conduit design that allows measurement of the sample in the air while retaining the accuracy of vacuum measurement. Since chamber evacuation is not required, throughput is dramatically improved. Both of these instruments have experienced excellent success in the market, demonstrating the validity of this new technology.

For more information, contact Ed Keane at Del-Tron Precision, Inc., 5 Trowbridge Drive, Bethel, CT 06801; (203) 778-2727; fax: (203) 778-2721.

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