Chemical Purity of Tantaline vs Tantalum

ToF-SIMS characterization of Ta samples

Project No.:   COMF/

Analysis Performed:  17.09.01-24.09.01

Claimant:    Danfoss A/S - DK-6430 Nordborg - Att.: Bo Gillesberg

Report written by
Jiang Wei
Danish Polymer Centre,
Risø National Laboratory

Introduction

Tantalum has been used for surgical implant applications. Naturally, the purity of the material is of great concern. According to ASTM (USA) standard, very small amount of other elements (less than 0.1 wt%), i.e., Ni, Fe, Ti, W, Mo, Si, Ni, C, O, N, and H is allowed to be present in the bulk material. Since the level of the impurities is much lower than the detection limit of XPS, ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) could be a suitable method to detect these impurities, although ToF-SIMS is not a quantitative method.
     Tantaline has applied a CVD method (“CVD-Ta”) stainless steel. SEM analysis showed that the thickness of the coating is in the m range. There is no doubt that a good coating has been formed on the substrates. The purpose of ToF-SIMS analysis is to qualitatively characterize the composition of both outer layer of surface and bulk material of Tantaline’s Tantalum layer. Based on the analysis results, one may compare the purities of the Ta coating with the c.p. Tantalum reference.

Analysis method

Secondary ion mass spectra of the Ta reference and coatings were recorded with a TOF-SIMS IV (ION-TOF GmbH, Germany) time-of-flight secondary ion mass spectrometer in the mass range of 0-1000 m/z. Primary ions Ga+ generated by the Ga gun (working voltage: 25 kV) bombard the surface of samples. An area of 50x50 m was analyzed with a measuring or sputter time of 1500 seconds. To obtain the time (depth) profile, Ar+ generated from the sputter gun (working voltage: 3 kV) was used to sputter away the material. Preliminary measurements showed that bulk layer of Ta was reached using a sputter time of 1500 seconds. Flood gun for electron neutralization was used during measurements.  Using the software IONSPEC from ION-TOF GmbH, species (ions) were assigned after mass calibration.
 

Sample description and handling

The samples provided 1) Ta reference (quality defined by ASTM F560-98); Tantaline CVD-Ta coating on stainless steel disc.
 

Results

Mass spectra are shown in Figures 1-3. Apart from the spectrum shown in Figure 1, which was obtained from the first 10 seconds of sputtering, the others were accumulated from the last 10 seconds of a total measuring time of 1500 seconds. Therefore, spectra shown in Figures 2-3 represent the chemistry of the bulk materials. The assignment of most of ions and their relative intensities (normalized to the intensity of Ta for each sample) are given in Table 1. It should be noted that the intensities are not proportional to the concentrations of the metals in bulk material. The time (depth) profiles of the samples were presented in Figures 4 and 5. It has to be mentioned that the total sputter depth is not known, although the sputter time does correspond to the depth. However, the sputter depth could be measured by other analytical instruments, e.g., AFM.

     The most abundant species detected from the first 10 seconds are mainly Na, K, Ca and CmHn, which originate from the adsorbed hydrocarbons (contaminants). The intensities of Ta and TaO are quite low at this stage, suggesting that outer layer of surface consists of the contaminants. As an example, mass spectrum of Ta reference obtained from the first 10 seconds was shown in Figure 1. Since the nature of bulk material is of greater interest, the rest of this report will focus on the spectra obtained from the last 10 seconds. Figures 2-3 show no signals from CmHn, indicating that bulk material was analyzed.

Sample 1 (ASTM F560 Ta reference): Mass spectrum and time profile are shown in Figure 2 and Figure 4 , respectively. The most prominent ions are Na, K and Al, and the intensities of Na and K are saturated. Other impurities including Li, Ca, Si and Ti were also detected with relatively high intensities. The most abundant Ta species are Ta, Ta2 and Ta4. The time (depth) profile (Figure 4) showed that the intensities of Ta and TaO are more or less the same during the first 100 seconds, indicating that a thin oxide layer exists. The intensity of the TaO decreases with sputter time (depth) as expected.

Sample 2 (Tantaline CVD-Ta treatment on stainless steel disc): Mass spectrum and time profile are shown in Figure 3 and Figure 5, respectively, indicating that the sample is the pure than the reference. Very little impurities were detected for this sample. This may indicate the process under the specific conditions has a purifying effect. Figure 5 shows that the intensity of C2H5 originated from hydrocarbons decreased to the noise level within a sputter time of 100 s.
 

Table 1 summarized the analysis results from the samples. Comparing the intensities of impurities of Tantaline depositied tantalum on stainless steels and a reference sample of ASTM F560 Tantalum metal, Tantaline CVD-Ta treatment result in a lower contaminant levels than in the reference. These results could suggest that the nature of the substrates also somehow affect the purity of Ta coating.

Conclusions
 

ToF-SIMS has proved to be a suitable method to characterize the Ta samples and provides qualitative information of the quality of Ta coatings. 

     As expected, the contaminants hydrocarbons were found on the outer layer of the surfaces. TaO was detected with similar or slightly high intensity than Ta during the initial sputter process (typical sputter time: 100 seconds), indicating that a thin oxide layer exists.

     The results from this work suggest that Tantaline Treatment of stainless steel result in a very low level of impurities. The pruity is even lower than the Ta reference, suggesting that the deposition process probably has a refining nature.

Table 1. Comparison of normalized relative intensity of ions for a Tantaline Samples vs. Tantalum reference. The measuring time is 1500 seconds. However, to get the information of bulk material, only the intensity obtained from the last 10 seconds was used for calculation (normalization). 
 

 
 

Figure 1. Mass spectrum of Ta reference obtained from the first 10 seconds.

Figure 2. Mass spectrum of Ta reference obtained from the last 10 seconds.

Figure 3. Mass spectrum of Tantaline CVD-Ta on stainless steel obtained from the last 10 seconds.

Figure 4.  Time (depth) profile of tantalum reference.
 

Figure 5. Time (depth) profile of CVD Ta coating on stainless steel.