Issue

Plated leadframe metrology

04/01/2002

Optical collimation improves capabilities for Au/Pd/Ni films

BY FRANCIS C. REILLY

A new metrology approach to metal film thickness measurement can be applied to the plated layers on leadframes. The technique, micro-beam X-ray fluorescence (MXRF), combines the fundamental metrology method of X-ray fluorescence with a new X-ray beam generation solution.

MXRF: What Is It?

MXRF is a new approach to metal film thickness determination. It employs a device called an optical collimator that bends X-rays to form an intense spot. These are directed at a sample to induce fluorescence. X-ray fluorescence yields information about what elements are present and how much of each is there. This information is used to calculate film stack thickness and composition.

The mechanism of X-ray fluorescence is shown in Figure 1. An incoming X-ray photon strikes an electron orbiting the atomic nucleus. That electron is ejected from the atom, and an electron from a higher energy orbital replaces that electron. This is known as electron cascading. As that electron drops into the lower orbital, it releases energy in the form of an X-ray photon. This process is called fluorescence. The energy of that X-ray photon is equal to the potential energy difference between the higher and lower energy orbitals. That value is a characteristic of the specific atom, and it allows the element to be identified. The relative intensity of the X-ray fluorescence at different energies allows a determination of the thickness and composition of the materials present. The thickness and composition are determined by comparing the relative intensities of each element to "known" values (thickness standards) or to mathematical values (fundamental parameters). Using thickness standards is called empirical calibration.

Current Plating Metrology Challenges

The use of a tri-layer metal system, in which a thin layer of gold (Au) deposited on a palladium (Pd) structural layer is placed on a nickel (Ni) barrier layer over a copper (Cu) substrate, is rapidly gaining acceptance for plating in high-density packaging applications (Figure 2). Reduction of gold content reduces per-unit costs. In addition, palladium is harder, and in this application, offers better mechanical reliability.

Figure 1. X-ray fluorescence.Click here to enlarge image

While manufacturers are adopting this material system, it presents two significant challenges for film metrology measurement. The first is the thickness of the films. In general, the metal system employs a protective gold deposition of 2 to 10 nm on a palladium layer of 10 to 50 nm over a 0.3 to 1.0 µm thick nickel layer.

The second challenge is the measurable area of the sample. Usually, working areas are pads that are 100 µm or smaller. Although XRF metrology is the method of choice for leadframe metal system thickness measurement, conventional XRF tools have not been capable of delivering sufficient X-ray flux in sub-100 µm areas to measure nanometer range films with adequate precision. Thus, the two trends of thinner layers and smaller areas typically prevent conventional metrology tools from meeting the requirements in this application.

New Collimation Approach

One solution to the challenges of thin-film, small-area metrology is an MXRF tool using an optical collimator to significantly increase the resulting X-ray flux. A consistent, controlled, and well-defined X-ray beam geometry is critical to measurement accuracy, and the process of shaping the primary beam geometry is called collimation. There are two principal methods of collimation: mechanical and optical.

Figure 2. Leadframe metal system with three metal layers above a copper substrate.Click here to enlarge image

Mechanical Collimation: A mechanical collimator is essentially a pin-hole aperture. A typical mechanical collimator assembly consists of a metal block featuring collimators of various dimensions. A single collimator is aligned directly beneath the path of the primary beam X-rays. X-rays pass through the aperture and emerge in a resultant beam with an initial diameter equal to the diameter of the collimator and then the beam fans out to a larger diameter. This resultant beam is targeted at the sample.

Conventional XRF metrology systems employ a mechanical aperture to govern the resultant beam cross-sectional area (Figure 3a). Because primary beam X-ray output (X-rays emanating from the tube) is constant as a function of the cross-sectional area, decreasing the size of the mechanical element decreases the resultant beam flux and, consequently, the tool's precision. There are many applications for which this precision is still sufficient, so the mechanical collimation approach is appealing for these because it is currently less expensive than optical collimation.

Table 1. MXRF tool capability for Au, Pd, and Ni thickness measurements. Samples were measured three times each by three different operators, and the variation was compared to the allowed tolerance to calculate the %R&R (repeatability and reproducibility). A value of 20 percent or less is considered good and 10 percent or less is excellent.Click here to enlarge image

Optical Collimation: Optically collimated XRF tools employ an optical element to shape and direct primary beam X-rays. Primary beam X-rays incident upon the surface section of the optical element propagate through the system and emerge in a convergent beam of very high intensity. Optically collimated systems capture and transmit a substantially higher quantity of the primary beam X-rays produced (100 to 1,000 times that of mechanically collimated systems). A higher intensity beam produces significantly higher X-ray count rates from the sample.

Figure 3. a) Mechanical collimation of an X-ray beam diminishes the total flux delivered to the sample. b) Optical collimation of an X-ray beam focuses the energy on a small spot, rather than blocking part of the beam to get a small spot size.Click here to enlarge image

MXRF tools use the optical element to capture primary beam X-ray output, redirecting its path to sub-100 µm areas at the sample interface (Figure 3b). The higher X-ray flux results in measurement precision gains of one to two orders of magnitude in this kind of system compared to conventional mechanically collimated XRF tools.

Tool Performance

The data shown in Table 1 summarizes the capability of the optically collimated MXRF tool for leadframe applications. The suitability of any tool for a particular application is commonly analyzed by the use of a "Gauge R&R" (repeatability and reproducibility) analysis. In the examples, three operators performed three sets of ten measurements each on samples using the same MXRF tool. Overall sample measurement deviation must fall within specified tolerance (%R&R) for tool performance to be considered capable of controlling a process.

Conclusion

The MXRF is a suitable option for measuring the thickness of plated metal layers on leadframes. The new optical collimation scheme allows enough X-ray flux in the small measurement areas to measure the nanometer scale films found in current plating technology. This metrology capability enables continuing advances in advanced leadframe metallization schemes.

Francis C. Reilly, director of sales and marketing, NeXray, 105 Comac Street, Ron kon koma, NY 11779; 631-738-9300; Fax: 631-738-9329; E-mail: [email protected].

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