Ultra-High Spatial Resolution Chemical Mapping of Bulk Semiconductor Specimen

Typically elemental distribution maps with ultra-high resolution are acquired using a transmission electron microscope equipped with an EDS detector (TEM EDS). However, the elemental mapping of nanoscale features is also possible using a field emission gun scanning electron microscope and an EDS detector (SEM EDS), provided that the detector offers a high solid angle and collection efficency.

In order to resolve fine features using a SEM the beam divergence and diameter must be kept to a minimum. This can be acheived by keeping the probe current in a low range of between a few tens to a few hundreds of picoamperes. The weak X-ray fluorescence generated at such low probe currents makes these conditions challenging for EDS, which can be overcome using an EDS system optimized for high X-ray collection and fast processing. 

The unique Bruker XFlash^® FlatQUAD is an annular EDS detector with four SDD segments which is inserted from the side of the SEM to sit between the sample and the pole piece. This sample-detector geometry provides a higher collection efficiency, with an increased input count rates and faster measurement speeds, than conventional EDS detectors. 

XFlash^® FlatQUAD's optimized geometry, with a very high solid angle of 1.1 sr, makes it highly sensitive - allowing EDS analysis in the low pA range. This sensitivity makes XFlash^® FlatQUAD the ideal detector for the analysis of low X-ray yield samples, such as FIB lamella and nanoparticles on a grid, as well as for the analysis of beam-sensitive samples, such as non-conductive specimens and biological materials.   

In the example below, The elemental composition of a M0 layer of a 7 nm node FinFET device from a commercial CPU was measured. EDS SEM using the XFlash^® FlatQUAD was able to resolve features of 6-9 nm on the bulk specimen. The peak overlaps between the silicon (Si), tungsten (W) and hafnium (Hf) at around 1.7 kV were automatically deconvoluted using the ESPRIT software to display the chemical distribution in the EDS maps correctly. 

Figure 1. Ultra-high resolution SEM EDS map acquired at 5 kV and 1.1 nA with a measurement time of only 2 minutes with a high output count rate of 309,000 cps. Using the FlatQUAD, acquisition times are at least 25 times shorter than when using a conventional EDS detector. 

Figure 2. The same region as above, mapped at higher magnification (field of view: 250 nm) at 5kV and 1.1 nA probe current. Under such gentle conditions, FlatQUAD gives high output count rate of 305,000 cps enabling fast measurement times of only 89 seconds for this map.