Limits of Quantitative Microanalysis Using Secondary Ion Mass Spectrometry

Scanning Electron Microscopy Volume 1985 Number 2 Part II Article 7 2-19-1985 Limits of Quantitative Microanalysis Using Secondary Ion Mass Spectrometry Peter Williams Arizona State University Follow this and additional works at: https://digitalcommons.usu.edu/electron Part of the Biology Commons Recommended Citation Williams, Peter (1985) "Limits of Quantitative Microanalysis Using Secondary Ion Mass Spectrometry," Scanning Electron Microscopy: Vol. 1985 : No. 2 , Article 7. Available at: https://digitalcommons.usu.edu/electron/vol1985/iss2/7 This Article is brought to you for free and open access by the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Scanning Electron Microscopy by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. SCANNING ELECTRON MICROSCOPY/1985/I I (Pag e~ 553 - 56 1) 0586-5581/85$1 . 00+.0S SEM In c ., AMF O' Ha1te (Ch,{_cago) , IL 60666 USA LIMITS OF QUANTITATIVEMICROANALYSIS USING SECONDARY ION MASSSPECTROMETRY Peter Wi11 i ams Arizona State University Department of Chemistry Tempe, AZ 85287 ( Pap e r r ece iv ed April 10 1984, Completed manuscript re ceiv e d Fe bruar y 19 1985) Abstract Introduction The li mitations on secondary ion micr o The unique feature of secondary ion micro analytical perfor mance imposed by ionization analysis (SIMS) is that it combines microscopy probabilities, mass spectrometer transmission, with chemical specificity. The technique thereby re4ui rement s for standards and sputtering allows quantitative chemical analysis, in con artifa cts have been investigated . The trast to the more qualitative nature of non se nsitivity of a modern magnetic mass chemical imaging techniques (optical microscopy, sµec tromrte r for sputtered B+ from oxidized Si scanning electron microscopy). Performance is~ 10- i ons detected/atom sputtered . For thi s limits quoted for the technique usually include se nsiti vi ty , it is shown that ion microscopy of lateral (x-y) resolution better than 1 µJll, in a part-p er- million impurity is limited in lateral depth (z) resolution better than 10 nm, and reso lution to~ 1 µm. For a 1% impurity, lateral detection limits in the ppm to ppb range. The reso lution of ~ 30 nm is achi evabl e . Depth prospects for a three-dimensional chemical profile analysis at the ppm lev el requires sample microscopy are often discussed 9 • SI MS al so areas ~ 10 µm2. Isotope abundance determinations allows quantitative analysis with an accuracy in volumes~ 1 µm3 requir e the concentrat i on of better than 10% and precision of a few percent tne least-abundant i sot ope to be ~ 1%. 22, 23 However, each of these attainable limits is to a large extent exclusive of the others. With the increasing availability of sub-micron beam sizes, it is of interest to examine the interrelated performance limits of SIMS micro analysis. McHughl3, Levi-Setti and Fox9 and Slodzianl 6, 1 7 have discussed the interrelation ship between sample volume and detection limits earlier. The rel ati onshi p between these factors and the other performance features listed above has not been comprehensively addressed. The useful yield The ultimate limiting factor in secondary ion microanalysis is the amplitude of the secondary ion signal detectable from a given microvolume. This signal is determined by the ionization probability, Yi• of the sputtered atoms (molecules) and the transmission T of the secondary ion optics and mass spectrometer. Both parameters vary over a wide range; Yi is a sensitive function of the ionization potential or electron affinity of the sputtered species 18 , and of the chemistry of the sputtered surface 24, Key Words: Secondary ion mass spectrometr y , while T depends critically on instrumental detection li mits, quantitative accuracy, depth design 4. The optimum SIMS performance limits resolution, lateral resolution, ion microanalysi s quoted above are typically obtained for easily ionized species, sputtered from oxygenated or Address for correspondence: cesiated surfaces (for positive or negative ions, P. Williams, Dept. of Chemistry, Arizona State respectively), analysed with the highest University, Tempe, AZ 85287 , USA transmission secondary ion mass spectrometer, and Phone No. (602) 965-4107 with performance sacrificed in all degrees of 553 P. Willi ams freedom but the one to be optimized. (about 2 ppm), so that this represents a detec Because y and T are difficult to measure tion limit for quantitative microanalysis in this independently {each requires knowledge of the area for this particular instrument and species. other), they are usually combined into a quantity Figure 1 shows the interrelationship between termed the "useful yield", i.e., the ratio of the analysed area and detection limits for values of number of mass-analysed ions detected to the the useful yield spanning the range for most number of atoms of that species sputtered l 0 • elements and well-optimized secondary ion analy This quantity is measurable , most simply using sers. {The range of positive or negative i oni za ion-implanted samples where the number of atoms ti on probabilities is greater than 102, but sputtered from a given analysed area is given elements with low positive ion yields generally accurately by the implant dose. Table 1 shows have high negative ion yields, and vice versa, so useful yield data for high transmission SIMS that it is possible by choosing the higher yield instruments based on quadrupole {Atomika) and to reduce the range to about 102). It is clear magnetic {Came ca) secondary ion analysers. The that part-per-billion detection limits require data from the author's laboratory was obtained large sampled areas, while part-per-million lim with gas-phase oxygen flooding of the sample in its are only achievable in depth profiles in addition to the primary oxygen ion beam, to micron-scale areas with the highest possible assure complete saturation of the surface with instrument transmission and ionization oxygen. probability. ~ 162 C Table 1 0 3 Measured useful yield for B+ sputtered from ~ 10 ,o oxygenated silicon for quadrupole and magnetic L secondary ion analysers. <+-164 E Instrument Useful yield for B+ 0 ~ 105 Cameca I MS3f4 6 X 1 □- 3 6 4 4 :':'.16 Atomika a-DIDA 3 X 1 □- E 2 _j -7 Cameca I MS3f 2 X 1 □- 10 C {this work) 0 ·- -8 +> 1 0 u The data in Table 1 should be representative of (1) magnetic and quadrupole secondary ion analysers ~ 1 QsL...---'--_._ _ ___. _ ___, 2 1 1 2 optimized for good transmission. Notice in Q 16 16 10° 10 10 particular that without oxygen enhancement {or Beam Di a met er (microns) cesium enhancement for negative ions) useful yields drop precipitously -- by three orders of magnitude for boron in silicon for example25, Such a sacrifice of signal is unacceptable in any of the sample-1 imited analytical situations Fig. 1. Depth profile detection limits for described below. limited-area SIMS, calculated for a range of practical useful yield values, signal integration Limits of lateral resolution in depth profiles over 10 nm depth and minimumsignal 100 counts. The importance of the useful yield in microanalysis is displayed in a sample Limits of depth resolution calculation. Consider a requirement to obtain a depth profile of implanted boron in a single Depth resolution in sputtering analysis can transistor, some 3 x 3 microns in size, on an in principle be excellent -- recent studies show integrated circuit chip, to determine a junction that at least soi of sputtered atoms come from depth. pch data point will involve integration the outermost atomic layer 5 • However, if erosion of the B signal over a selected sputtered depth proceeds beyond the first layer, depth resolution interval. For adequate depth resolution, the is limited by the effects of ion beam mixing22. interval should be no greater than 10 nm. For Roughly speaking, ion beam mixing homogenizes adequate precision in determining the junction surface and subsurface material over a depth on depth, the minimumacceptable signal from a 10 nm the order of the primary ion range, R • depth interval should be no less than 100 counts Definitions of depth resolution vary depending gn ( 10% standard deviation). Given an instrument the analytical situation. For a delta-function with a useful yield for boron of ii, 104 boron feature broadened into a Gaussian, the resolution atoms must be sputtered from the analytical can be defined as twice the standard deviation of microvolume in order that 100 ions be detected. the Gaussian; an equivalent measure for a 104 boron atoms in a 3 x 3 micron x 10 nm broadened step function is the distance over microvolume is a concentration of 1 x 1017 B/cm3 which the signal varies from 16% to 84% of 554 Limits of Quantitative Microanalysis Using SIMS maximum. Alternatively, two features can be said epitaxy with individual layer thicknesses of 4.3 to be chemically resolved if the one contributes nm. The layers are almost resolved -- depth less than, say, 10% to the signal from the other; resolution as defined above is ~ 5 nm. Note that the resolution then depends on the relative for laterally homogeneous thin films such intensity of the signals from the two features. performance is not incompatible with sub-ppm Ion-beam mixing tails decay exponentially, with a detection limits, because large areas can be characteristi c length on the order of the primary sampled. ion range (i.e. , the signal decays by a factor of The prospects for a three-dimensional 10 over a distance ~ 2.303rR , where r is a chemical microscopy are again limited by the factor, typically between 0.5 pand 2, introduced useful yield. Figures 3 and 4 indicate the to corre ct for preferenti a 1 sputtering relationships between analysed area, or beam effects 22 ); two dilute features differing in diameter, and sampled depth for ana lyte concentration by a factor, K, will be chemically concentrations of 1 ppm and 1%.

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