Comparison of SEM-EDS , Micro-XRF and Confocal Micro-XRF for Electric Device Analysis

In general, scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS) has been used in laboratories or factories for the analysis of the element distribution in the samples such as electric components. However, SEM-EDS requires sample preparation, which usually is difficult and takes a long time. On the other hand, micro X-ray fluorescence (μ-XRF) can be utilized for the elemental analysis underneath the surface of the samples thanks to the high penetrating power of X-rays. Furthermore, the depth distribution of elements in the samples can be acquired by the latest confocal XRF. In this paper, the information regarding the depth direction in small electrical and electronic components is compared for SEM-EDS, μ-XRF and the confocal XRF. At the same time, we give a brief report on not only the development of confocal XRF equipment but also some ideas as to the depth profile of the samples. [DOI: 10.1380/ejssnt.2013.133]


I. INTRODUCTION
Recent development of nanotechnology is advancing the high functionality and the downsizing of electrical components.X-ray fluorescence spectrometry (XRF) is used for quality control of materials and failure analysis of structure inside small electrical components [1], as it is easy and rapid analytical method in the laboratory.
In particular, scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS) and micro Xray fluorescence (µ-XRF) analyzer [2,3] are generally used for the elemental mapping of a small part.SEM-EDS has the feature to measure the elemental distribution inside a material with a couple of micrometers spatial resolution, but the sample must be embedded in resin and must be polished to expose the part.This preparation is very complicated and time-consuming because it needs to grind down the sample surface in the accuracy of several micrometers.On the other hand, in the case of µ-XRF, elemental analysis inside the sample is possible without * Electronic address: shintaro.komatani@horiba.com the sample pretreatment, and elemental distribution can be measured with a lateral resolution of 10 µm.However, depth profile information can not be obtained.On the other hand, recently developed new technique, a confocal XRF [4][5][6][7][8], has a possibility for depth elemental profiling.We therefore evaluated whether the effective information inside a small electric component could be obtained by using confocal XRF [4][5][6][7][8].
In this paper, the results of internal depth elemental analysis of a thermistor (buried in the resin) were compared for three different methods; when analyzing small electronic components, it is necessary to get to know the features of each method in terms of depth analysis.Optimization and combination of analytical methods for small samples are also discussed.

A. Instruments
In the SEM-EDS analysis, an SEM (S-3400, Hitachi High-Technologies Corporation) and an EDS (EMAX Energy X-Max N150 , HORIBA Ltd.) were used to obtain the  For the µ-XRF, an X-ray micro analytical scope, XGT-7200 (HORIBA Ltd.,) was used.This instrument has 10 µm spatial resolution with a mono capillary lens [9][10][11][12] and a Rh target X-ray tube (50 kV, 1,000 µA).The sample chamber was in vacuum.The sample stage was operated at 4 µm step in the directions of X-Y, and the mapping analysis results with the resolution of 256×512 (1.024 mm×2.048mm) were obtained.
A confocal XRF [13] designed by Osaka City University was applied.In the optical system of this confocal XRF, the X-ray incidence angle was 45 degrees and the X-ray takeoff angle was 45 degrees.An X-ray tube with Rh target (50 kV, 500 µA) and a silicon drift detector (SDD) were used for this system.The sample chamber was in vacuum.The depth resolution of this system was 10.9 µm at 17.4 keV, and 56.0 µm at 1.5 keV.
In the present experiment, in order to perform elemental mapping analysis from two dimensions, X-axis (horizontal to a sample) and Z-axis (depth direction), the sample stage was controlled to move along the directions of X-Z at 4 µm step, and obtained the images in the range of several hundred micrometers.The origin of Z-axis was determined as the Z position where scattered X-rays appeared.

B. Sample
In order to compare SEM-EDS, µ-XRF and the confocal XRF for depth analysis, we measured a small thermistor embedded in resin.A thermistor is a temperature sensor that utilizes the temperature-dependent resistivity change of metal oxide sintered compact.Figure 1(a) is the schematic view of this thermistor.This thermistor has two Pt wires on which Pt electrodes are attached, and metal oxide sintered compact consists of Mn, Ni, and Co is wrapped around the electrode.A glass layer (Si, Zn, and Ca are contained) covers the sensor for electrical insulation.
This thermistor was embedded in a resin column with about 25 mm diameter and 15 mm height (Fig. 1(b)).The resin surface with the sample was ground until the metal oxide sintered compact appears for SEM-EDS analysis.Figure 1(c) is the top view image using an X-ray Fluoroscopic inspection system (SOFTEK CO., WORK-   This is because the incident electron beam has very small penetration depth so that the buried materials in the resin or the sintered compact were not excited to emit characteristic X-rays.

A. Elemental Mapping by SEM-EDS
The penetration depth of electron beam is generally approximated as the electron diffusion length, and in this case the penetration depth to the resin is several microns [14].Therefore, in order to analyze the materials embedded in resin by using SEM-EDS, it is necessary to scrape off the sample surface by several microns to expose the portion to be analyzed.This preparation process is very complicated.c) and (d) are the elemental mapping images of Si-Kα, Mn-Kα, and Pt-Lα, respectively.Similar to SED-EDS, µ-XRF was able to observe Mn in the metal oxide sintered compact which is exposed to the surface, and distribution of Si included in the neighboring glass is clearly visible.Furthermore, Pt-Lα from the two Pt electrodes inside the thermistor was mapped clearly.In addition, not only the exposed ends of both Pt wires but also the central portion which was buried in the resin (about 100 µm) was observed.

B. Elemental Mapping by µ-XRF
Since µ-XRF enables elemental analysis of materials inside a sample, it is not necessary to expose the sample surface like SEM-EDS case.However µ-XRF is not able to obtain the depth profile images.

C. Elemental Mapping by Confocal XRF
By using confocal XRF, we performed the elemental mapping analysis of X-Z cross sections at the portions of lines (1), ( 2) and (3) in Fig. 4. Figures 5(a [13].The lower the energy, the larger the resolution.

D. Depth profiling using Confocal XRF
In order to evaluate the depth analysis capability of confocal-XRF, depth analysis of a point on the glass portion (point (4) of Fig. 4) was performed.The normalized intensity of X-rays from the line (1) in Fig. 7 (f) was plotted for 6 elements (Al, Si, K, Ca, Ti and Zn) in Fig. 9.The low energy X-rays like Al-Kα and Si-Kα starts to be detected at about 10 µm scanning distance, and then high energy X-rays like K-Kα, Ca-Kα, Ti-Kα and Zn-Kα follow from about 40 to 50 µm scanning distance.This result suggests that higher energy X-ray fluorescence is from the deeper position of the Z-axis and thus the depth  resolution of high energy X-rays is small.The profile of Zn-Kα intensity is tailing to deep position of the Z-axis than other elements.This may mean that high energy X-ray like Zn-Kα from the deep position was not absorbed by the sample.

IV. CONCLUSIONS
We compared three methods, SEM-EDS, µ-XRF and confocal XRF, by measuring the element distribution inside a thermistor embedded in the resin.SEM-EDS enabled high spatial resolution (several microns) elemental mapping, but depth profiling requires scraping off the sample surface in order to expose the object.Because the scraping process requires an accuracy of several microns, this sample preparation must be very complicated and difficult.
With µ-XRF, Pt-Lα (9.4 keV) from Pt wire at 100 µm depth was mapped with a spatial resolution of 10 µm.However, depth profiling was not feasible by the conventional µ-XRF.
Confocal XRF enabled depth profiling of Pt-Lα (9.4 keV) from Pt wire that locates approximately 100 µm depth from the sample surface.The depth resolution was between 10 µm and 50 µm.We can conclude that confocal XRF is very useful for the depth analysis of buried material.

FIG. 1 :FIG. 1 :
FIG. 1: The thermistor analyzed in this work.(a) Structure of the thermistor, (b) FIG.1: The thermistor analyzed in this work.(a) Structure of the thermistor, (b) photograph of the resin column in which the thermistor is embedded, (c) top view image of the thermistor with the X-ray fluoroscopy, and (d) side view image.

Figures 2 (FIG. 4 :
Figures 2(a) to (d) show the results of SEM-EDS mapping analysis of the thermistor (Fig. 1(c)) embedded in resin.Figure 2(a) is backscattered electron image, and Figs.2(b), (c) and (d) are elemental mapping images of Si-Kα, Mn-Kα and Pt-Lα.Mn at the central part (metal oxide sintered compact) of the thermistor and Si (included in the glass) around the sintered compact are clearly visible.One of the two Pt wires was also observed, but Pt electrode buried in the metal oxide sintered compact and the other Pt wire embedded in resin were not observed.

Figures 3 (
Figures 3(a) to (d) are the results of mapping analysis of the thermistor (Fig. 1(c)) embedded in resin with µ-XRF.Figure 3(a) shows transmitted X-ray image, and Figs.3(b), (c) and (d) are the elemental mapping imagesof Si-Kα, Mn-Kα, and Pt-Lα, respectively.Similar to SED-EDS, µ-XRF was able to observe Mn in the metal oxide sintered compact which is exposed to the surface, and distribution of Si included in the neighboring glass is clearly visible.Furthermore, Pt-Lα from the two Pt electrodes inside the thermistor was mapped clearly.In addition, not only the exposed ends of both Pt wires but also the central portion which was buried in the resin (about 100 µm) was observed.Since µ-XRF enables elemental analysis of materials inside a sample, it is not necessary to expose the sample surface like SEM-EDS case.However µ-XRF is not able to obtain the depth profile images.

FIG. 5 :FIG. 5 :
FIG. 5: Depth profiling images of (a) Pt M and (b) Si K with the confocal FIG.5: Depth profiling images of (a) Pt M and (b) Si Kα with the confocal XRF along X-Z plane at the line (1) in Fig. 4.
images along the line (2) of Fig. 4. Images of Co-Kα, Mn-Kα, Ni-Kα, Pt-Lα, Si-Kα and Zn-Kα are shown.It was found that two Pt electrodes are buried inside at a depth of about 80 µm in the metal oxide sintered compact that contains Co, Mn and Ni.In addition, it was suggested that the Pt wires are coated with glass material that consists primarily of Si and Zn.Figures 8(a), (b) and (c) are X-Z elemental mapping images of Pt-Lα, Pt-Lα and Pt-M along the line(3) of Fig. 4. It was confirmed that the Pt wire locates at the depth of 80 µm.Although Figs. 8(a), (b) and (c) are of the same Pt wire, the sizes of Pt image area are different.This is mainly because of the difference of resolution of confocal XRF for different X-ray energies as the actual spatial resolutions (15 µm at Pt-Lα (11.1 keV), 17 µm at Pt-Lα (9.4 keV) and 48 µm at Pt-M (2.05 keV)) were already determined by another party

FIG. 8 :FIG. 9 :
FIG. 8: Depth profiling images of (a) Pt L , (b) Pt L and (c) FIG.8: Depth profiling images of (a) Pt Lα, (b) Pt Lα and (c) Pt M with the confocal XRF along X-Z plane at the portion of line (3) in Fig. 4.