Research Of TOF-SIMS Secondary Ion Measurement Technology

TOF-SIMS(Time of Flight Secondary Ion Mass Spectroscopy, TOF-SIMS) is one of the most important methods for the surface analysis. It can be used to directly analyse varies solid samples and be applied in broad fields like bioscience, semiconductor technology and environmental sciences, etc. The advantage of TOF-SIMS is the adoption of the pulsed ion beam, which makes the loss of the sample much smaller than modern secondary ion mass spectroscopy. Hence, it fits perfectly the analysis of tiny geological sample. But the detect precision is not high, not only because of the insufficient account of the ion caused by the pulsed ion beam, but also for the sake of,the trace amounts of most elements in most geological samples.The secondary ion is the excited and charged ion realized by the process that sample target surface is bombarded by high-speed ions, which directly reflects the composition of the sample. The measurement of the secondary ion is one of the core technology of TOF-SIMS. Although the increase of the transmission efficiency of secondary ion has capability to improve the accuracy of TOF-SIMS, it makes the debugging of the spectrometry difficult and overlapped peak. A series of studies on secondary ion measurement technology have been conducted. tails are as follows:(1)Research and Development of time of flight mass analyser with Single Gridless Reflectron.The transmission efficiency of the secondary ion is the basis for high-precision analysis of TOF-SIMS. In order to make the current TOF-SIMS used for the qualitative analysis of substances and the surface element imaging, the spectrometry needs high resolution rather than high precision. However, the isotope determination in geology requires TOF-SIMS with high detection accuracy, precision means improving ion transmission efficiency. To solve this problem, we developed a time-of-flight mass spectrometry with gridless single reflection. The spectrometry uses a gridless mirror without the shield grid of traditional ion mirror.It also reduces the ion barrier, although it decreases the mass-resolving power. By using secondary-ion-multiple-focusing technology,ion is focused in different positions of the spectrometry and it reduces ion losses caused by high-energy divergence. The time delay separation technology used by a ion detector of high precision allows different ions differentiated amplification and thus helps to reduce quantization error of ion detection. After testing, the resolution of mass is 3700(FWHM, m/z = 107), but the transmission rate is reach to 38%, it could serve for improving the accuracy of TOF-SIMS and the subsequent application of isotopic analysis of geology.(2)Research on real-time detection method of secondary ion beam spot.Detecting the ion beam spot is an effective means of assessing TOF-SIMS ion optics system. TOF-SIMS Time of Flight mass analyzer adopts multiple ion-optical components. In the process of debugging instrument, it requires measuring ion beam spot heavily and repeatedly. Due to cumbersome and inefficient current methods for detecting secondary ion beam spot of TOF-SIMS, we have proposed a multi-angle cutting line phase surrounded by CLC on the basis of the law on knife edge. At 0°, 45 °, 90 °, 135 ° in four directions sharp knife edge "cutting" the ion beam,which gives rise to ion intensity curve. Finally we restore the ion beam spot beam shape and the core area to detect ion beam spot in real time. We compare the method with the current ion bombardment trace method with high acceptance and also use the method for TOF-SIMS secondary ion beam spot detection. The experimental result shows that adopting multi-angle cutting lines surroundding the mapping method can substantially restore ion beam spot and describe ion beam current core position. Compared to the ion bombardment trails, it does not require to disassemble the cavity and observe equipment. This method can be used for real-time detection of the beam spot of secondary ions and for the measurement of the contour of the core region and the peripheral with good effects.(3)Research on the approach of overlapped peak separationThe initial electron energy of the secondary ion in TOF-SIMS reaches 50 e V. The wider distribution of energy caused by increasing the transmission efficiency of secondary ion results in the overlapped peak. We would present the approach for the overlapped peak separation of TOF-SIMS. Taking advantages of the characteristic of TOF-SIMS, that is those spectral peaks with close mass number have close shapes, the approach determines the peak positions by the correlation algorithm between Gaussion curve and original mass spectrum, then selects the individual single peaks by the principle of “high amplitudes kept and low amplitudes left”, and acquires characteristic peak by stacking and normalizing the single peaks. The power spectral density function is constructed by the characteristic peak, and positions as well as amplitudes of the overlapped peak are determined in accordance with the minimum principle. Ultimately, the separation of overlapped peak is achieved. The appearance of asymmetric peak is a common phenomenon in TOF-SIMS. In view of varies symmetric overlapped peak, the result of the approach is compared with the results of Gaussian matching method and Lorentzian matching method respectively. While dealing with symmetric peaks with δ=0.2, they are basically consistent, but the effect of characteristic peak matching method is much better than the others while dealing with asymmetric peak with δ=1.6. The experimental results show that this method can be used to separate the 29 Si and 28Si1 H overlapped peak, with improved resolution from 0.371 up to 0.519 and to restore original shape information of the peaks.(4)The application test of TOF-SIMSThis chapter mainly focuses on the experimental study of the application of TOF-SIMS, including the relative content analysis experiments of copper alloy surface elemental and solid isotope analysis experiments of metallic silver. When detecting the relative content of TOF-SIMS, problems such as matrix effects and instrumental mass fractionation make TOF-SIMS unable to measure the element content directly and thus the measurement has to be done indirectly through calibrating the instrument. We analyze respectively 64Zn/120 Sn and 208Pb/120 Sn of reference material GBW02137 and GBW02140. The relative error of 64Zn/120 Sn is superior to 4.4% while the relative standard deviation RSD is less than 2.5%. In contrast, the relative error of 208Pb/120 Sn is superior to 20% while the relative standard deviation RSD is less than 5%. The analysis of isotope Ag is also be conducted. The result indicates that the relative error is less than 1%, and the RSD is 3.4% which is all better than the result of 208Pb/120 Sn in copper alloy.