X-ray fluorescence (XRF) is a powerful quantitative and qualitative tool ideally suited to analyzing film thickness and composition, determining elemental concentration by weight of solids and solutions, and identifying specific and trace elements in complex sample matrices. XRF analysis is used extensively in almost all industries, including metal finishing and refining, semiconductors, telecommunications and microelectronics, food, pharmaceuticals, cosmetics, agriculture, plastics, rubbers, textiles, fuels, chemicals, and environmental fields. The wide deployment of XRF analysis tools in industry is the result of XRF's ability to perform noncontact, nondestructive tests with speed and precision, combined with a low cost of ownership compared to other measurement techniques.

In metal finishing, XRF tools are primarily used to perform incoming analyses and process control checks on metal film thickness and composition, to perform incoming and pre-plate verification of the metal substrate, and to ensure the compliance of purchased or fabricated parts to the European Union's Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances Directive (RoHS) standards.

Principle of Operation

By definition, fluorescence is a process whereby a material absorbs energy and almost instantaneously releases energy in a characteristic form (see Figure 1). In XRF analysis, an incoming X-ray photon strikes an electron orbiting the atomic nucleus. The electron breaks free of its orbit and leaves the atom. An electron from a higher energy orbital replaces the freed electron (electron cascading). As it drops to the lower orbital, the electron releases energy (fluoresces) in the form of an X-ray photon, the energy of which is equal to the potential energy difference between the higher and lower energy orbitals.

When a sample is exposed to an X-ray source with a known, constant intensity, it will fluoresce characteristic X-rays at a rate (intensity) proportional to the concentration of the element(s) present in the sample. Two pieces of information are obtained from this analysis: the types of elements present, and the amount of each element present. It is therefore possible to use XRF to quantitatively measure the thickness of the sample, as well as its elemental composition.

Compared to other measurement techniques, the XRF method offers several benefits. It is much faster (10-30 seconds) compared to the relatively long setup and analysis times (10-60 minutes) often required by classical spectroscopic methods such as inductively coupled plasma (ICP), direct current plasma (DCP) and atomic absorption (AA). Additionally, while ICP, DCP and AA typically require a highly skilled technician to obtain reasonable and reproducible results, XRF is easy to use and does not require an experienced operator.

The XRF technique also offers significant advantages over typical eddy current and magnetic induction measurement methods. Unlike eddy current devices, XRF is not susceptible to the resultant electroplating finish's conductivity. The plated deposition's conductivity is not constant; it is a function of the bath chemistry and changes over time within a specific bath or between different tanks. Conductivity can often vary by as much as 40%. As a result, eddy current measurements can be dramatically biased. Additionally, XRF is not susceptible to the magnetic permeability fluctuations that are common between various ferrous substrates. Magnetic gauges often obtain significantly different plating thickness measurements on different steel bases or even different locations on the same steel base. For this reason, XRF is often the preferred technique, especially in multi-user and multi-application environments.

For WEEE/RoHS applications, XRF is much faster than ICP, and it allows analysis of areas as small as 2 mils. This smaller spatial resolution (the size of the area being analyzed) provides increased sampling capabilities and allows users to more easily screen for non-compliance. In addition, XRF is well suited to the production/assembly environment because it is easy to use. Once a calibration recipe has been established, go/no-go WEEE/RoHS verification can be obtained in several minutes without any operator intervention or interpolation.

XRF Tools

XRF tools typically incorporate five features:

• A source of high-intensity X-rays
  
  
• A collimation apparatus to define the X-ray beam size
  
  
• X-ray beam targeting and sample positioning mechanisms
  
  
• X-ray detection, processing and analyzing electronics
  
  
• Algorithmic tools for determining the thickness and/or composition of the sample

The XRF tool uses a high-intensity X-ray tube to generate X-rays. These primary beam X-rays are directed through a collimator and emerge in a single, tightly focused beam with a specific cross-sectional geometry. The beam impinges the sample material and induces fluorescence from the elements present (see Figure 2, p. 35). A detector then senses X-ray emissions from the sample and converts them into a series of pulses, with the amplitude of each pulse proportional to the energy of each incident X-ray.

An amplifier shapes the pulses into workable electronic signals, and the digitized pulses are sorted according to energy level and stored in a multi-channel analyzer (MCA). The number of pulses stored in each channel are counted to generate a frequency distribution (histogram), displaying channel numbers along its X axis (corresponding to energy level) and counts (the number of pulses detected at each energy level) along its Y axis. This histogram is known as a spectrum or pulse height analysis (PHA), and is the raw data from which thickness and composition determinations are made. The XRF instrument uses a combination of mathematical techniques to further refine the raw spectral data into a pure spectrum, which is correlated with a calibration model to produce a measurement result.

XRF instruments are typically available in either handheld or benchtop models. Handheld devices are well suited for analyzing large, bulk samples, which would be difficult or impractical to place in a benchtop chamber. They are also used to perform point-of-process analyses on incoming purchased inventory and to verify a material type prior to plating, as well as to perform at-process analyses of plated materials.

Benchtop XRF tools can be used to obtain micro-beam measurements, in which small areas (under ½ in.) need to be monitored, or to analyze film thickness on irregular or extremely small parts. Benchtop models can also be automated to allow the measurement of many samples simultaneously or in-depth profiling of individual samples.

Precise Analyses

XRF tools are a staple of modern metal finishing plants. They provide precise, rapid, non- contact, nondestructive film thickness and composition measurements of plating depositions and substrate metals, and they are an essential tool for verifying RoHS elements as specified in the new EU directive on hazardous substances.