Characterizing Samples with High Resolution X-Ray Microanalysis

Advanced materials analysis in high tech industries including nanotechnology, life sciences, semiconductors and microelectronics needs ever increasing high-resolution analytical performance to allow analysts to understand the slight variations in the characteristics of the samples.

The term resolution is applied in two distinctly different aspects of SEM-based microanalysis. Resolution is generally defined as the system’s ability to resolve, or separate, two aspects of the analysis which are very close together. Spatial resolution in imaging and mapping will have an impact on the ability of the analyst to visibly separate two physically closely spaced items.

Improved energy resolution allows differentiation between two elements in a spectrum, whose peaks fall at closely spaced energies, as with light element peak separation.

A truly superior microanalysis system is capable of delivering high resolution in both areas with a single detector. This can be illustrated by the analysis of a semiconductor device using the TEAM™ EDS Analysis System with an Octane Silicon Drift Detector (SDD).

Materials Challenge

Since the size of the features of interest continues to move to the nanoscale, it is necessary to reduce the probe size of the electron beam to achieve the required spatial resolution for a successful analysis. This can be achieved by lowering the beam accelerating voltage (kV) and beam current, resulting in a smaller interaction volume in the sample.

However, this also leads to lower X-ray signal intensity and smaller spectral energy range with fewer peak options. Hence, for successful analysis the collection efficiency of the X-ray signal needs to be maximized with a larger detector sensor and improved geometrical solid angle. Furthermore, energy resolution at the low end of the spectrum becomes crucial for resolving the closely spaced element peaks.

Analytical Results

Mega-Resolution Phase Maps

A phase map from the cross section of a semiconductor device obtained using a TEAM™ EDS System with Octane Ultra SDD and an electron beam energy of 5keV is depicted in Figure 1. The data collection reveals the connection of a gold bond wire to an aluminum bond pad with the device structure underneath.

TEAM™software showing phase map of wire bond

Figure 1. TEAM™software showing phase map of wire bond

While the low beam energy results in a higher spatial resolution due to limited penetration depth and volume from which X-rays are generated, the significant peak overlaps between the heavier element L- and M-lines and the light element K-lines make elemental mapping and correct spectral deconvolution highly challenging from an energy resolution standpoint.

The phase map in Figure 1 reveals the regions with similar chemical composition instead of the pure elemental maps. This is a unique feature of the TEAM™ EDS System, enabling rapid identification of variations between regions without the need to overlay multiple elements. An example is illustrated in Figure 2, which shows the Si elemental map. This map shows all the regions where Si is present, but does not show the differences between the areas.

Si elemental map

Figure 2. Si elemental map

By extracting the Si maps from the distinct phases that are automatically identified by the TEAM™ software during phase map acquisition, users are able to easily indentify the glass particles present in the device encapsulation material and the silicon oxide dielectric layer in the device (Figure 3), the silicon nitride barrier layer (Figure 4), and the pure silicon substrate (Figure 5).

Si-O phase extracted from phase map

Figure 3. Si-O phase extracted from phase map

Si-N phase extracted from phase map

Figure 4. Si-N phase extracted from phase map

Si phase extracted from phase map

Figure 5. Si phase extracted from phase map

The investigation of the gold and aluminum rich phases reveals an interesting feature of the sample. The pure gold phase in Figure 6 clearly shows the gold bond wire, while Figure 7 reveals the aluminum bond pad and metal layers. However, a gold/aluminum intermetallic phase is also identified by the TEAM™ software (Figure 8). Quantification of the X-ray spectra from intermetallic phase reveals the composition to be 60 atomic percent aluminum and 40 atomic percent gold.

Au phase extracted from phase map

Figure 6. Au phase extracted from phase map

Al phase extracted from phase map

Figure 7. Al phase extracted from phase map

Au-Al phase extracted from phase map

Figure 8. Au-Al phase extracted from phase map

The intermixing of aluminum and gold is caused by the junction being exposed to elevated temperatures. This is a well-known failure mechanism in integrated circuits as the growth of intermetallic phases causes a volume reduction, leading to voiding at the gold-aluminum interface. The intermetallic is also a poor conductor, meaning there is increased electrical resistance in the bond connection.

By examining the high magnification map of the device structure in Figure 9, the exceptional light element sensitivity of the Octane Ultra detector at low acceleration voltages is even clearer. The size of the yellow aluminum metal structures are 1µm (upper) and 750nm (lower) and the nitrogen-rich passivation layers around the structures are clearly resolved.

TEAM™ software showing phase map of integrated circuit

Figure 9. TEAM™ software showing phase map of integrated circuit

High Resolution Imaging with Line Scan

While mapping provides a detailed picture of the sample and better understanding of the elemental distribution and phases, the X-ray line scan is an invaluable tool when very rapid acquisition is required, which is often the case with beam sensitive samples. The nitrogen signal from a line scan across an aluminum layer in the device is depicted in Figure 10. The width and step size of the upper passivation layer are 150 and 20nm respectively and the total acquisition time of the line scan was under 30 seconds.

Line scan across Al interconnect

Figure 10. Line scan across Al interconnect

The short acquisition time highlights the ability of a large area detector to record high quality spectra rapidly without any information loss and demonstrates the excellent light element sensitivity even at very fast acquisition times.

This is possible because of the resolution stability and superior detector electronics of the Octane SDD series, which has the fastest processing times on the market and guarantees high quality data at all acquisition speeds.

Recommended EDAX Solution

A large area SDD, such as the EDAX Octane Ultra, is ideal for performing high-resolution semiconductor microanalysis. The large detector active area and well-engineered geometrical design maximizes proximity with superior solid angle when compared to detectors of similar size, allowing the SEM operating conditions needed for optimal imaging resolution.

The resolution quality of the low energy spectrum coupled with the unique software functionality of TEAM™ EDS offers light element performance results never before possible with large area detector systems. By combining these two performance attributes, Octane SDDs maximised both spatial and energy resolution, providing the ultimate analytical results on highly demanding samples.

This information has been sourced, reviewed and adapted from materials provided by EDAX Inc.

For more information on this source, please visit EDAX Inc.

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