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Imaging microanalysis of silicon nitride ceramics with a high resolution scanning ion microprobe
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Science 29 (1987) 300-316 North-Holland, Amsterdam
IMAGING MICROANALYSIS OF SILICON NITRIDE CERAMICS WITH A HIGH RESOLUTION SCANNING ION MICROPROBE
J.M. CHABALA, R. LEVI-SETTI 7’he Enrico Fermi Institute and Department of Physics, The University of Chicago, Chicago, IL 69637, USA
S.A. BRADLEY and K.R. KARASEK Allied Signal Engineered Materials Research Center, Des Plaines, IL 60017, USA Received
8 June 1987; accepted
for publication
8 July 1987
Samples of silicon nitride sintered with Y203 and MgO are examined by scanning ion microscopy and analyzed by secondary ion mass spectrometry. High lateral resolution elemental images of both fractured and polished ceramic surfaces are interpreted within the context of crystal morphology and intergranular composition. Advantages as well as limitations of the SIMS method are discussed.
1. Introduction
Using newly developed scanning ion microprobes, it is now possible to image and analyze, with submicron lateral resolution, the elemental composition and structure of ceramic materials. Advanced scanning ion microprobes (SIM), utilizing finely focused beams extracted from liquid metal ion sources [1,2], can produce images with 20 nm resolution. Coupled with secondary ion mass spectrometers (SIMS), these instruments are well suited to the study of microscopic chemical segregations in many materials. We have applied a microprobe of this kind (UC-HRL SIM), developed in collaboration between the University of Chicago and Hughes Research Laboratory, to a preliminary study of silicon nitride ceramics. Sintered silicon nitride is in advanced ceramic often used for high temperature applications. Due to the covalent nature of Si,N, bonding, the sintering process requires the addition of small amounts of oxides and the application of a nitrogen overpressure to prevent dissociation at temperatures above 1700 o C. Sintering agents such as MgO, Y203, and others, present in l-10 wt% concentrations, react with the silicon nitride phases, allowing sintering to occur at temperatures between 1600 and 1900“ C. The properties of the secondary phases which form along grain boundaries ultimately determine the 0169-4332/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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mechanical properties of the ceramic. Since this is particularly true for the high temperature creep properties, characterization of the grain boundary phases is very important when developing materials for high temperature engines. We have used the UC-HRL SIM to obtain mass spectra of silicon nitride ceramics sintered with MgO or Y,O, from small samples areas and to explore the feasibility of high resolution SIMS mapping of their constituent elements. Detailed descriptions of the UC-HRL SIM and its performance have been presented previously [3,4]. Briefly, the instrument is composed of a two-lens focusing column, a high-transmission secondary ion energy analyzer and transport system and an RF quadrupole mass filter for SIMS analysis. A channel electron multiplier detector operating in pulse mode collects the transmitted ions; a discriminator removes electronic background originating in the detector. The resulting one-detected-ion to one-pulse signal is suitably amplified and shaped for display on a cathode ray tube. Image acquisition time is adjusted to obtain adequate signal statistics. Images are recorded on Polaroid film in scans containing 1024 X 1024 picture elements (pixels); the camera settings are adjusted to maintain linear film response. Consequently, the gray scale for each micrograph varies and can be estimated from image feature areas and total counts. For this experiment, the gray scale ranges from O-4 (black-white) counts per pixel (e.g. fig. 4c) to O-40 counts per pixel (e.g. fig. 6b). The signal to noise ratio for SIMS performed with the UC-HRL SIM is on the order of lo4 for the elements and clusters analyzed here. Two additional channel electron multipliers, overlooking the target region, detect secondary electrons or ions for imaging of the surface topography and material contrast of a sample. In the present study, the UC-HRL SIM was operated with a 40 keV, 50 pA Ga+ probe focused to an area approximately 70 nm in diameter. All elemental maps were obtained sequentially over 20 x 20 pm2 areas (10 x 10 pm* for figs. 6c and 6d). Under these conditions, each map (512 s acquisition time) caused the erosion of approximately 40 nm of material (160 nm for figs. 6c and 6d). A number of factors characteristic of our instrument in particular, and of the SIMS method in general, have enabled a detailed analysis of the ceramic samples, in spite of their insulating properties. A thin gold coating (- 10 nm) was applied to all samples by plasma sputter deposition. This coating, which is rapidly sputtered away from the region being scanned, is adequate to prevent the electrical charging of the sample which often hampers SIMS analysis of insulators. We attribute the absence of charging in our instrument to two concurrent factors: (a) the gallium metal implant which allows the transport of excess surface charge to the conductive coating surrounding the field of view, and (b) the use of modest probe currents which, though small, are still adequate for microanalysis. Furthermore, the Ga+ probe is as effective as an O- probe in terms of either positive or negative secondary ion yields [5]. The
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high oxygen content of the ceramics studied also provides a constant positive ion yield enhancement: no decrease in elemental sensitivity occurs during in-depth sputtering, even without oxygen flooding to replenish surface oxygen coverage (a common SIMS practice 161).Adequate SIMS sensitivity exists for the visualization of grain boundary pockets at lateral resolution down to the probe size [7]. Unfortunately, as is true for all SIMS techniques, the exact characterization of these pockets is difficult.
2. SIMS microanalysis of yttria-doped silicon nitride The sintering of silicon nitride with yttria addition is attractive in view of the high temperature strength of the ensuing ceramic. At elevated temperature, the yttria reacts with the surface oxide of Si,N, and then densification occurs by a liquid phase sintering process; the resulting second phase fills the .interstices between &Si3N4 crystals. The composition and crystallography of these phases have been extensively studied [8-lo]. A variety of second phases such as Y,Si207, Y2SiOs, and Y,Si,O,N, have been observed, and in a recent investigation [ll] of the same ceramic studied here, YSiO,N was found. This latter phase is known to oxidize to Y,Si,07. The sample which we examined with the UC-HRL SIM was fabricated by a reaction bonding process which begins with the nitridation of compacted silicon powder at elevated temperature. Subsequently, the temperature is
Fig. 1. Scanning electron microscope image of the etched surface of an yttria-sintered (8 wt% Y203) Si,N, ceramic. The bright, protruding features correspond to the areas of the intergranular phase.
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increased while maintaining a nitrogen over-pressure, thus causing sintering to occur. The sintering aid level was 8 wt% Y,O,. An etched, conventionally prepared portion of this ceramic was imaged with a scanning electron microscope (SEM) (fig. 1). Intergranular pockets protrude from the easily discernible silicon nitride crystal matrix. For examination in the SIM, a 1 mm thick slice of this ceramic was ground by standard mineralogical procedures to a final polish with 0.5 pm alumina powder. After thorough cleaning with a series of solvents in a sonicating bath, the surface was coated with a thin Au film (- 10 nm) by sputter plasma deposition. A fracture surface was also Au-coated for SIM study. All measurements and images were obtained after the Ga+ probe had sputter-removed the trace oxides and surface contaminants remaining on the ceramic surfaces. 2. I. Mass spectra SIMS mass spectra were obtained at mass resolution m/Am = 250, by scanning over 20 x 20 pm2 areas of a fractured surface, to erode an ultimate depth of - 320 nm. Because the silicon nitride grain size is typically l-5 pm, spectra collected from such scan areas average over a large number of grains and grain boundary pockets. Figs. 2a and 2b show mass spectra over the interval O-100 amu for positive and negative secondary ions. These spectra were accumulated in 4000 s. To facilitate the interpretation of these spectra, an isolated Si,N, crystal and Y203 powder were independently analyzed. In the positive ion spectrum (fig. 2a), “Y and the Si isotopes are prominent, as expected. The intense peaks at masses 69 and 71 represent the isotopes of Ga, originating from the self-sputtering of implanted probe ions. The peaks at masses 70 and 72 are attributed to SiN,, being present with the same abundance configuration in the isolated Si,N, crystal spectrum and absent in the pure Y203 spectrum. In addition, the molecule Si2N contributes, to a smaller extent, to the signals at masses 70, 72, 73 and 74 (SiN, is a more probable Si,N, sputter fragment than Si,N). The expected “Sii4N3 and 28Si2gSi’4N peaks at mass 71 cannot be separated from the intense ‘lGa signal. Other prominent peaks occur at masses 23 (Na), 27 (Al, C,H,), 40 (Ca, Sic), 56 (Fe, Si,, CaO) and 90 (YH). There is apparent Li contamination (masses 6, 7). Unfortunately, the low mass resolution of the RF quadrupole mass filter does not allow the discrimination of possible molecular interferences except in cases where a characteristic isotopic structure may be evident. In the negative ion spectrum the most intense signal corresponds to 26CN. In SIMS, this molecule is detected with very high sensitivity and facilitates (in the presence of abundant nitrogen) the detection of carbon or vice versa. In the present spectrum, C and C2 are also independently detected. Other characteristic signals are those of 0, Cl, SiN, SiO, and SiO,. In these spectra, the dynamic range extends over at least four orders of magnitude.
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Fig. 2. SIMS mass spectra for an yttria-sintered (8 wt% YrOs) SisN, ceramic, obtained from 20 x 20 pm’ areas which contain representative fractions of each phase present. 4000 s accumulation time. To convert the plotted counts/channel to counts/s, divide the former by 3.9. A 40 keV, 50 pA Ga4 probe was used to obtain all spectra and images presented in this article. (a) Positive secondary ions. (b) Negative secondary ions.
Several consistent approaches have been developed for the quantification of SIMS data (see e.g. Werner [12]). By the use of reference standards to determine elemental sensitivity factors, which are sensitive to sample matrix effects, and by recourse to semi-empirical formalism [13], elemental concentrations can be determined to, at least, - 10% accuracy. Because the overall ion yields are affected by the nature of the matrix, SIMS quantification generally requires that the sample be homogeneous over the analyzed volume. In principle, quantification of heterogeneous mixtures is also possible, provided standards can be found for each constituent phase. The issue of quantification for ceramic samples such as those under investigation here is extremely
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challenging, due to the presence of segregated phases, some of which occur only in submicron domains and are the specific result of intermatrix interactions. Only the relative abundance of corresponding phases among different samples can be readily determined for these materials. In order to determine elemental sensitivity factors for the secondary phases observed, e.g., in reaction bonded silicon nitride ceramics, it is necessary to refer to the grain boundary pockets themselves for calibration purposes. Although the probe size of the UC-HRL SIM is smaller than that of many interstitial pockets, it is difficult to obtain significant spectra from such small surface areas because deep craters are rapidly sputter-formed which may extend to underlying Si,N, grains. On the other hand, individual Si,N, grains, which are readily visualized in the SIM, are large enough to permit mass spectrum determination, as mentioned above. A spectrum for the intergranular phases can then be obtained by subtracting a suitably normalized Si,N, spectrum from the total spectrum for a large area (such as those of fig. 2). To apply this procedure, the fractional area of the Si,N, grains must be determined by image processing techniques. The fractional area of the Si,N, grains (which is the same as their fractional volume in a finely interspersed mixture) in this ceramic is 83.48, obtained by averaging the crystal surface coverage for many separate fields of view. After the appropriate spectral subtraction, as outlined above, one finds that the intense mass 70 signal in fig. 2a, ascribed to SiN,/Si,N, originates from the nitride crystals rather than from the YSiOZN phase. Other conclusions can be drawn from this set of spectra. However, accurate atomic accounting, i.e., the quantitative association of detected ions to sample concentration, remains elusive without prior knowledge of mass-dependent ionization probabilities for a given matrix. Atomic accounting also requires the exhaustive summation of all normalized signals from sputtered molecular fragments. More effort must be devoted to compiling a catalogue of SIMS matrix properties for additional ceramics. 2.2. Elemental maps Fig. 3 shows elemental maps of a 20 X 20 pm2 area of a fractured surface, the topography of which is given in the ion-induced secondary electron (ISE) micrograph of fig. 3c. A comparison of the 89Y+ elemental distribution map (fig. 3a) with the 160- map (fig. 3b) shows a complete correspondence of the sharply defined image details, clearly demonstrating the association of Y and 0 in the intergranular phase. These images, however, do not provide a faithful representation of the size of the intergranular pockets because the sample surface results from a fracture which can occur preferentially along grain boundaries, as can be seen in fig. 3c. Thus, the secondary phase may partially or totally coat the exposed crystals. In addition, the presence of sharp edges is
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Fig. 3. SIMS elemental images of the fractured surface of an yttria-sintered (8 wt% Y203) SiaN, ceramic. All images are of the same area, each acquired in 512 s. (a) 89Y+ elemental map, indicating the distribution of the YBO,N intergranular phase. 3.0X106 counts displayed. (b) 160-, 2.0 x lo6 counts. (c) ISE image of the surface topography. (d) 28Si+, 9.2 X lo5 counts. The three-dimensional shading arises from surface edge effects. (e) 26CN-, 3.5X10” counts. (f) Resputtered 69Ga+ map, giving a detailed description of sample microstructure. To be compared with the SEM image, fig. 1. 8.0 X lo6 counts.
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known to enhance the secondary ion yields (edge effect), which depend on the orientation of the emitting surface relative to the incident beam direction, leading to a distorted signal intensity distribution [7]. The secondary ion escape probability increases for emission at angles away from the surface normal. The physical origin of this phenomenon is poorly understood. This effect is particularly noticeable in the *%i+ map of fig. 3d, which exhibits a definite correlation with the surface topography. This correlation masks the differences in Si intensity between the Si,N, crystals and their surrounding intergranular pockets. We attribute to edge effects the dramatic filamentary appearance of the 26CN- map (fig. 3e). Although carbon is not an expected constituent of this ceramic, the body of evidence presented by the mass spectra (see fig. 2b) and by many CN maps similar to that of fig. 3e strongly suggests that a nonnegligible concentration of carbon must be present in the ceramic interior. Fig. 3f is a map of the self-sputtered Ga following implant by the probe. This image provides a detailed description of the morphological microstructure of the sample; it, along with the ISE topography image (fig. 3c), are to be compared to the SEM image, fig. 1. Its features will be discussed, together with additional examples, in a separate section below. Although the analysis of a fracture surface provides a complete, albeit qualitative, compositional and structural description of the ceramic, it is desirable also to examine a polished section of the same, which should be free of SIMS artifacts caused by the surface topography. Accordingly, we show in fig. 4 selected maps obtained from a polished sample, prepared as described above. The cross section of the Y-rich grain boundary pockets is clearly outlined in the Y map (fig. 4a). A complementary image is obtained by mapping the self-sputtered Ga (fig. 4b). We note that whereas the Y map of the fractured surface (fig. 3a) contains a range of signal intensities, the Y map of the cross section (fig. 4a) shows an almost constant signal intensity throughout the interstitial areas. Similarly, the Ga map (fig. 4b), which outlines the areas occupied by the Si3N, grains, is monotonal. The lateral resolution of the Y and Ga images is - 0.1 pm. In the Si map of fig. 4c, the Si intensity in the intergranular pockets is reduced (by a factor of - 2) compared to that in the crystal areas, which is consistent with the stoichiometry of the Si content of YSi02N versus Si,N,. The reduced signal per pixel of this map causes the edges of the pockets to be less defined than in figs. 4a and 4b 171. The CN- map shows a diffuse distribution with sparse, strongly emitting spots 100-600 nm in size, the location of which does not correlate with the prominent structures found in the Y and Si maps. This indicates that carbon is indeed present in the crystal interior and that the filamentary structure observed on the fracture surface (fig. 3e) is not the result of preferential edge deposition of carbon contaminants from the environment. The diffuse carbon may be an instrumental artifact, while the source of the concentrated spots is not clear.
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Fig. 4. SIMS elemental images of the polished surface of the same silicon nitride ceramics as in fig. 3. All images are of the same area, each acquired in 512 s. (a) sgY+ elemental map. 7.1 X lo5 counts displayed. (b) Resputtered 69Ga+ map. For this ceramic, the areas of greatest intensity correspond to material with the largest atomic number density. 4.3 ~10’ counts. (c) ‘*Si+, 3.6 X 10’ counts. Two concentrations are discernible. The granular appearance is due to relatively low signal statistics, not background noise. (d) 26CN- image, revealing a uniform carbon contamination. 1.2 X lo6 counts.
Fig. 5a gives the intensity distribution of the tightly bound molecule, SiO; , which originates from the intergranular YSi02N, exactly mirroring the Y distribution (not shown) for this area, as expected. The SiN- map of fig. 5b dramatically demonstrates the difficulty of quantifying SIMS information. The SiN- intensity from the Si,N, crystals is about one-third that from the intergranular regions. However, from atomic counting calculations without regard to chemical matrix effects, one would expect the SiN- signal to be 4.6 times more intense from the crystals. Several factors combine to produce this discrepancy. First, the Si originating from Si,N, has a strong preference to be
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Fig. 5. Additional SIMS elemental images of the polished surface of the same silicon nitride ceramics as in figs. 3 and 4. Both images are of the same area, each acquired in 512 s. (a) 60SiO; distribution, identical to corresponding Y and 0 maps (not shown). 1.9 X 10' counts displayed. (b) 40SiN- image, 1.9 X lo6 counts. The intergranular areas are enhanced, contrary to initial expectations. See text.
sputtered as a part of SiN: or SiZN+ clusters, as shown by the large mass 70 peak in fig. 2a. As a trade-off, less SiN- emerges compared to that from YSiO,N. Second, the bound oxygen in the intergranular phase may enhance, in some unpredictable manner, SiN- emission and ionization. Other facts, which are also collected under the catchall banner “matrix effects”, include ionization variations due to density and different molecular binding patterns. 2.3. Resputtering of the implanted Ga A striking phenomenon, peculiar to the use of a gallium probe, is the differential resputtering or self-sputtering of the Ga atoms implanted during scans with the SIM. A dynamic equilibrium is reached between the rate of Ga ion implant and the resputtering rate, which are, respectively, functions of the stopping power of the material and of the sputtering yield of the matrix [14]. This effect causes image contrast in Ga maps of heterogeneous materials, such as that shown in figs. 3f and 4b. Additional high resolution examples of this phenomenon are shown in fig. 6. A comparison of the Ga+ maps of figs. 6b and 6d with the corresponding Y images (figs. 6a and 6c) shows that Ga is predominantly resputtered from the Si,N, crystals. This can be qualitatively understood, in part, in terms of the higher atomic number density of Si,N, versus that of YSi4N. The intrinsic Ga contrast, when combined with the dependence of the secondary ion yields on the surface topography, provide
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Fig. 6. Examples of image contrast in resputtered Ga images. Yttria-sintered (8 wt% Y203) Si,N4 ceramic, fractured surface. Each image acquired in 512 s. (a) “Y+ elemental map. 1.6 X lo6 counts displayed. (b) 69Ga+, same area as (a). 10.6 x lo6 counts. (c) 89Y+, larger magnification map. 1.1 x lo6 counts. (d) 69Ga+, same area as (c). 9.3 x lo6 counts.
images which have a three-dimensional appearance and outline with utmost detail the shape and distribution of the individual Si,N, grains.
3. SIMS microanalysis of MgO-doped silicon nitride Silicon nitride with MgO as a sintering aid has poor high temperature mechanical properties because the resulting grain boundary pockets are noncrystalline [15]. The composition of the gram boundary pockets is typically near the miscibility gap of the Si,N,O-SiO,-MgSiOs ternary [16]. Complementary work performed for this sample on a dedicated scanning transmission electron microscope showed the grain boundary pockets to have a Mg/Si ratio
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Fig. 7. Scanning electron microscope image of the etched surface of a MgO-sintered ceramic.
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of about unity. Three principal phases have been identified by traditional techniques: ,&Si,N, (the bulk), MgSiO,, and Si,N20. Amorphous MgSiO,N,, may also be present. The grain structure was irregular and ranged from 0.2 pm to about 2 pm in size; particles from the tungsten carbide milling media were found throughout. A SEM image of an etched surface of this ceramic is presented in fig. 7. Mass spectra of this ceramic were obtained from Au-coated fractured surfaces. The enhanced secondary ion signal from the rough surfaces facilitates the rapid acquisition of high signal statistics. For elemental mapping, in order to present a smooth, non-artifact producing surface to the ion probe, a 1 mm thick ceramic slice was ground to a final polish with a 1 pm diamond polish. Attempts to obtain a finer polish were unsuccessful because the resulting wear debris, perhaps containing WC particles, unavoidably caused micron-scale scratches. Again, the surface was cleaned in solvents, coated with Au, and sputter-cleaned ‘with the Ga+ probe. 3.1. Mass spectra SIMS mass spectra of this sample were obtained under conditions identical to those for the yttria-sintered ceramic. Fig. 8a gives the positive ion mass spectrum, accumulated in 4100 s. The spectral structure in the mass interval 69-73 is identical to that of the previous ceramic and again represents a signature for Ga-bombarded Si3N,. A comparison of this spectrum with that
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Fig. 8. SIMS mass spectra for a MgO-sintered Si,N, ceramic, obtained from 20X20 pm* areas which contain representative fractions of each phase present. (a) Positive secondary ions, 4100 s accumulation time. To convert the plotted counts/channel to counts/s, divide the former by 4.0. (b) Negative secondary ions, 2800 accumulation time. Conversion factor: 2.7.
of yttria-sintered ceramic (fig. 2a) reveals several significant changes other than the obvious shifts in Mg and Y signals. Strong 23Na and 39K signals indicate substantial alkali contamination within the bulk of the material. In addition, the large C,H, (mass 27) and, in the negative spectrum (fig. 8b), CN (mass 26) peaks both mark the presence of carbon. Tungsten (mass 184), because of its low sputtering yield, is difficult to detect with the SIMS method. Overall, the striking similarity between the two sets of ceramic spectra is the consequence of two factors: (1) for both ceramics, the secondary ion yields from the Si,N, crystals dominate those from the intergranular phases, and (2) molecular fragments containing Mg are often difficult to detect. For example,
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in fig. Sa, the 48Mg, peak is weak and the mass 40 peak, while containing some MgO, is more definitely identified as Ca (by examining the “footprint” of the isotopes of Ca). 3.2. Elemental maps Fig. 9 contains a set of elemental maps from one 20 X 20 pm* area of the polished surface of this magnesia-sintered ceramic. In order to better understand the elemental distribution in this material, one must closely compare the 28Si+ 24Mg+ and i60- images (figs. 9a, 9b and 9c, respectively). The Si image contains two distinct concentration levels. The Mg, originating from the sintering agent, has segregated into small (< 1 pm*), sharply defined domains (fig. 9b). Note that the intergranular areas which contain Mg also correspond to some, but not all of the regions with intense Si emission (fig. 9a). Evidently, of the three phases inherent in this ceramic, two give very similar, large Si signals, about three times as intense as that from the third, more abundant phase. The 0 map (fig. 9c) unequivocally reveals the composition of this ceramic. The intense areas in this map are associated with similar bright areas in the Si and Mg maps: this phase must contain Mg, Si, 0, and possibly N. Similarly, the intermediate intensity areas in the 0 image contain Si, 0, and possibly N. The large dark areas contain no 0 (these areas correspond to the dim Si regions) and are immediately identified as Si,N,. Within the Si-O-N system, silicon oxynitride, Si,N,O, is a common phase. A final clue to the composition of this ceramic: the 0 : 1: 3 oxygen intensity ratios between the three phases match the stoichiometry of the 0 content of Si,N, : Si,N,O : MgSiO, .
Computer analysis of MgO-sintered ceramic images gives the following fractional areas (volumes) for each phase: Si,N,, 85.8%; Si,N,O, 10.0%; MgSiO,, 4.2%. The intergranular phases are too small to permit the acquisition of statistically significant spectra. Matrix-induced anomalies in secondary ion yield are apparent in the Si image. The observed Si,N, : Si2N20 : MgSiO, :: 1: 3 : 3 intensity ratios are widely different from the 15 : 7 : 14 ratios predicted by atomic counting. In a like manner, the 0 : 1: 3 ratios detected in the SiO, map (fig. 9e) are not convincingly compatible with the predicted 0 : 1: 2 values. For both of these cases the presence of 0 within the intergranular areas has enhanced secondary ion emission. Whereas for the yttria-sintered ceramic the Ga map (fig. 4b) is an accurate, though qualitative, measure of atomic number density, this interpretation is not adequate for the magnesia ceramic. In the yttria ceramic the intergranular pockets are dark; in the magnesia ceramic they are bright (fig. 9d). The atomic number density for the three phases in this second ceramic are nearly identical, yet they exhibit markedly different Ga resputtering/ionization rates.
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Fig. 9. SIMS elemental images of the same SisN, + MgO ceramic as analyzed in fig. 8, polished surface. All images are of the same area, each acquired in 512 s. (a) *%ic elemental map, showing two levels of concentration. The black areas are contamination inclusions. 1.4 X lo6 counts displayed. (b) 24Mg+, 2.0X106 counts. (c) ‘60- elemental map, displaying three concentration levels. The intense areas correspond to the Mg distribution (b) and are attributed to the MgSiO, phase. The intermediate intensity areas correspond to the intense Si areas (a) and are attributed to the Si,N,O phase. 9.7 X lo5 counts. (d) Resputtered 69Gaf image, 11.9X lo6 counts. (e) 6oSiO;, 4.4 X lo4 counts. (f) 26CN-, 8.5 X 10’ counts.
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Additional model calculations must be performed before the questions of Ga dynamic equilibrium and sputtering properties are fully understood. Fig. 9f shows the CN- distribution for this ceramic (this image is shifted slightly upwards in comparison to the other maps in this figure). The strong carbon-emitting inclusions correspond to dark areas on the Si and Ga maps, and are attributed to WC fragments from the milling media. Again, the diffuse carbon may be an instrumental artifact. 4. Conclusion
With the UC-HRL scanning ion microprobe, it is possible, after minimal preparation, to image a ceramic’s surface topography along with its compositional morphology. In the examples presented here, breakup products of the Si,N, crystalline matrix (Si, SiN, SiN,) and of the intergranular phases (Si, 0, Mg, Y, Siq, C, C,, CN) could be mapped with signal statistics sufficient to reach the limiting resolution of the instrument (- 0.1 pm or better for the conditions of this experiment). Additional structural information is obtained by mapping the self-sputtered Ga implant. Surprisingly, a thin Au coating of the insulating ceramic surface is sufficient to prevent sample charging, thus allowing correlative elemental imaging by repeated scans of the same area. This ability may stem from the conductivity imparted to the surface layers by the Ga implant and from the use of low primary beam currents ( - 50 PA). In fact, the high detection efficiency of the UC-HRL SIM [3] permits the rapid acquisition (I 512 s) of 1024 x 1024 raster size elemental maps, at these probe currents, with satisfactory signal statistics. A distinct advantage of the SIMS method over electron energy loss spectrometry and other related methods is its sensitivity to low-Z elements (for example, signal/noise for Li is > 104), facilitating the analysis of a diverse range of materials. On the other hand, the problem of absolute quantification of SIMS signal has plagued researchers since the inception of the technique. In the near future, the incorporation of dedicated image analysis facilities into microprobe systems will enable precise studies of small, submicron regions, as well as make possible rapid archiving of spectra from calibration samples. This investigation suggests that high resolution SIM imaging microanalysis may become a powerful complement to existing techniques for the study of ceramic materials. Acknowledgements
The research performed at the University of Chicago was supported by the NSF Materials Research Laboratory and by the Allied Signal Engineered Materials Research Center. The authors wish to thank Y.L. Wang for technical assistance and useful discussion.
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References [l] R.L. Sehger, J.W. Ward, V. Wang and R.L. Kubena, Appl. Phys. Letters 34 (1979) 310. [2] R. Levi-Setti, G. Crow and Y.L. Wang, in: Secondary Ion Mass Spectrometry SIMS V, Eds. A. Benninghoven, R.J. Colton, D.S. Simons and H.W. Werner (Springer, New York, 1986) p. 132. [3] R. Levi-Setti, G. Crow and Y.L. Wang, Scanning Electron Microsc. 2 (1985) 132. [4] R. Levi-Setti, Y.L. Wang and G. Crow, Appl. Surface Sci. 26 (1986) 249. [5] F.G. Rudenauer, in: Secondary Ion Mass Spectrometry SIMS IV, Eds. A. Benninghoven, J. Okano, R. Shimizu and H.W. Werner (Springer, New York, 1984) p. 133. [6] G. Blaise and M. Bemheim, Surface Sci. 47 (1975) 324. [7] R. Levi-Setti, J. ChabaIa and Y.L. Wang, Scanning Microsc. Suppl. 1 (1987) 13. [S] O.L. Krivanek, T.M. Shaw and G. Thomas, J. Am. Ceram. Sot. 62 (1979) 585. [9] J.T. Smith and C.L. Quackenbush, Am. Ceram. Bull. 59 (1980) 529. [lo] C.C. Ahn and G. Thomas, J. Am. Ceram. Sot. 66 (1983) 14. (111 S.A. Bradley and K.R. Karasek, J. Mater. Sci. Letters (1987) in press. [12] H.W. Werner, Surface Interface Anal. 2 (1980) 56. (131 CA. Anderson and J.R. Hinthome, Anal Chem. 45 (1973) 1421. 1141 K. Wittmaack, Nucl. Instr. Methods B7/8 (1985) 779. (151 F.F. Lange, B.J. Davis and D.R. Clark, J. Mater. Sci. 15 (1980) 601. [16] D.R. Clark, N.J. Zaluzec and R.W. Carpenter, J. Am. Ceram. Sot. 64 (1981) 601.
Applied
Surface
Science 29 (1987) 300-316 North-Holland, Amsterdam
IMAGING MICROANALYSIS OF SILICON NITRIDE CERAMICS WITH A HIGH RESOLUTION SCANNING ION MICROPROBE
J.M. CHABALA, R. LEVI-SETTI 7’he Enrico Fermi Institute and Department of Physics, The University of Chicago, Chicago, IL 69637, USA
S.A. BRADLEY and K.R. KARASEK Allied Signal Engineered Materials Research Center, Des Plaines, IL 60017, USA Received
8 June 1987; accepted
for publication
8 July 1987
Samples of silicon nitride sintered with Y203 and MgO are examined by scanning ion microscopy and analyzed by secondary ion mass spectrometry. High lateral resolution elemental images of both fractured and polished ceramic surfaces are interpreted within the context of crystal morphology and intergranular composition. Advantages as well as limitations of the SIMS method are discussed.
1. Introduction
Using newly developed scanning ion microprobes, it is now possible to image and analyze, with submicron lateral resolution, the elemental composition and structure of ceramic materials. Advanced scanning ion microprobes (SIM), utilizing finely focused beams extracted from liquid metal ion sources [1,2], can produce images with 20 nm resolution. Coupled with secondary ion mass spectrometers (SIMS), these instruments are well suited to the study of microscopic chemical segregations in many materials. We have applied a microprobe of this kind (UC-HRL SIM), developed in collaboration between the University of Chicago and Hughes Research Laboratory, to a preliminary study of silicon nitride ceramics. Sintered silicon nitride is in advanced ceramic often used for high temperature applications. Due to the covalent nature of Si,N, bonding, the sintering process requires the addition of small amounts of oxides and the application of a nitrogen overpressure to prevent dissociation at temperatures above 1700 o C. Sintering agents such as MgO, Y203, and others, present in l-10 wt% concentrations, react with the silicon nitride phases, allowing sintering to occur at temperatures between 1600 and 1900“ C. The properties of the secondary phases which form along grain boundaries ultimately determine the 0169-4332/87/$03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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mechanical properties of the ceramic. Since this is particularly true for the high temperature creep properties, characterization of the grain boundary phases is very important when developing materials for high temperature engines. We have used the UC-HRL SIM to obtain mass spectra of silicon nitride ceramics sintered with MgO or Y,O, from small samples areas and to explore the feasibility of high resolution SIMS mapping of their constituent elements. Detailed descriptions of the UC-HRL SIM and its performance have been presented previously [3,4]. Briefly, the instrument is composed of a two-lens focusing column, a high-transmission secondary ion energy analyzer and transport system and an RF quadrupole mass filter for SIMS analysis. A channel electron multiplier detector operating in pulse mode collects the transmitted ions; a discriminator removes electronic background originating in the detector. The resulting one-detected-ion to one-pulse signal is suitably amplified and shaped for display on a cathode ray tube. Image acquisition time is adjusted to obtain adequate signal statistics. Images are recorded on Polaroid film in scans containing 1024 X 1024 picture elements (pixels); the camera settings are adjusted to maintain linear film response. Consequently, the gray scale for each micrograph varies and can be estimated from image feature areas and total counts. For this experiment, the gray scale ranges from O-4 (black-white) counts per pixel (e.g. fig. 4c) to O-40 counts per pixel (e.g. fig. 6b). The signal to noise ratio for SIMS performed with the UC-HRL SIM is on the order of lo4 for the elements and clusters analyzed here. Two additional channel electron multipliers, overlooking the target region, detect secondary electrons or ions for imaging of the surface topography and material contrast of a sample. In the present study, the UC-HRL SIM was operated with a 40 keV, 50 pA Ga+ probe focused to an area approximately 70 nm in diameter. All elemental maps were obtained sequentially over 20 x 20 pm2 areas (10 x 10 pm* for figs. 6c and 6d). Under these conditions, each map (512 s acquisition time) caused the erosion of approximately 40 nm of material (160 nm for figs. 6c and 6d). A number of factors characteristic of our instrument in particular, and of the SIMS method in general, have enabled a detailed analysis of the ceramic samples, in spite of their insulating properties. A thin gold coating (- 10 nm) was applied to all samples by plasma sputter deposition. This coating, which is rapidly sputtered away from the region being scanned, is adequate to prevent the electrical charging of the sample which often hampers SIMS analysis of insulators. We attribute the absence of charging in our instrument to two concurrent factors: (a) the gallium metal implant which allows the transport of excess surface charge to the conductive coating surrounding the field of view, and (b) the use of modest probe currents which, though small, are still adequate for microanalysis. Furthermore, the Ga+ probe is as effective as an O- probe in terms of either positive or negative secondary ion yields [5]. The
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high oxygen content of the ceramics studied also provides a constant positive ion yield enhancement: no decrease in elemental sensitivity occurs during in-depth sputtering, even without oxygen flooding to replenish surface oxygen coverage (a common SIMS practice 161).Adequate SIMS sensitivity exists for the visualization of grain boundary pockets at lateral resolution down to the probe size [7]. Unfortunately, as is true for all SIMS techniques, the exact characterization of these pockets is difficult.
2. SIMS microanalysis of yttria-doped silicon nitride The sintering of silicon nitride with yttria addition is attractive in view of the high temperature strength of the ensuing ceramic. At elevated temperature, the yttria reacts with the surface oxide of Si,N, and then densification occurs by a liquid phase sintering process; the resulting second phase fills the .interstices between &Si3N4 crystals. The composition and crystallography of these phases have been extensively studied [8-lo]. A variety of second phases such as Y,Si207, Y2SiOs, and Y,Si,O,N, have been observed, and in a recent investigation [ll] of the same ceramic studied here, YSiO,N was found. This latter phase is known to oxidize to Y,Si,07. The sample which we examined with the UC-HRL SIM was fabricated by a reaction bonding process which begins with the nitridation of compacted silicon powder at elevated temperature. Subsequently, the temperature is
Fig. 1. Scanning electron microscope image of the etched surface of an yttria-sintered (8 wt% Y203) Si,N, ceramic. The bright, protruding features correspond to the areas of the intergranular phase.
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increased while maintaining a nitrogen over-pressure, thus causing sintering to occur. The sintering aid level was 8 wt% Y,O,. An etched, conventionally prepared portion of this ceramic was imaged with a scanning electron microscope (SEM) (fig. 1). Intergranular pockets protrude from the easily discernible silicon nitride crystal matrix. For examination in the SIM, a 1 mm thick slice of this ceramic was ground by standard mineralogical procedures to a final polish with 0.5 pm alumina powder. After thorough cleaning with a series of solvents in a sonicating bath, the surface was coated with a thin Au film (- 10 nm) by sputter plasma deposition. A fracture surface was also Au-coated for SIM study. All measurements and images were obtained after the Ga+ probe had sputter-removed the trace oxides and surface contaminants remaining on the ceramic surfaces. 2. I. Mass spectra SIMS mass spectra were obtained at mass resolution m/Am = 250, by scanning over 20 x 20 pm2 areas of a fractured surface, to erode an ultimate depth of - 320 nm. Because the silicon nitride grain size is typically l-5 pm, spectra collected from such scan areas average over a large number of grains and grain boundary pockets. Figs. 2a and 2b show mass spectra over the interval O-100 amu for positive and negative secondary ions. These spectra were accumulated in 4000 s. To facilitate the interpretation of these spectra, an isolated Si,N, crystal and Y203 powder were independently analyzed. In the positive ion spectrum (fig. 2a), “Y and the Si isotopes are prominent, as expected. The intense peaks at masses 69 and 71 represent the isotopes of Ga, originating from the self-sputtering of implanted probe ions. The peaks at masses 70 and 72 are attributed to SiN,, being present with the same abundance configuration in the isolated Si,N, crystal spectrum and absent in the pure Y203 spectrum. In addition, the molecule Si2N contributes, to a smaller extent, to the signals at masses 70, 72, 73 and 74 (SiN, is a more probable Si,N, sputter fragment than Si,N). The expected “Sii4N3 and 28Si2gSi’4N peaks at mass 71 cannot be separated from the intense ‘lGa signal. Other prominent peaks occur at masses 23 (Na), 27 (Al, C,H,), 40 (Ca, Sic), 56 (Fe, Si,, CaO) and 90 (YH). There is apparent Li contamination (masses 6, 7). Unfortunately, the low mass resolution of the RF quadrupole mass filter does not allow the discrimination of possible molecular interferences except in cases where a characteristic isotopic structure may be evident. In the negative ion spectrum the most intense signal corresponds to 26CN. In SIMS, this molecule is detected with very high sensitivity and facilitates (in the presence of abundant nitrogen) the detection of carbon or vice versa. In the present spectrum, C and C2 are also independently detected. Other characteristic signals are those of 0, Cl, SiN, SiO, and SiO,. In these spectra, the dynamic range extends over at least four orders of magnitude.
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Fig. 2. SIMS mass spectra for an yttria-sintered (8 wt% YrOs) SisN, ceramic, obtained from 20 x 20 pm’ areas which contain representative fractions of each phase present. 4000 s accumulation time. To convert the plotted counts/channel to counts/s, divide the former by 3.9. A 40 keV, 50 pA Ga4 probe was used to obtain all spectra and images presented in this article. (a) Positive secondary ions. (b) Negative secondary ions.
Several consistent approaches have been developed for the quantification of SIMS data (see e.g. Werner [12]). By the use of reference standards to determine elemental sensitivity factors, which are sensitive to sample matrix effects, and by recourse to semi-empirical formalism [13], elemental concentrations can be determined to, at least, - 10% accuracy. Because the overall ion yields are affected by the nature of the matrix, SIMS quantification generally requires that the sample be homogeneous over the analyzed volume. In principle, quantification of heterogeneous mixtures is also possible, provided standards can be found for each constituent phase. The issue of quantification for ceramic samples such as those under investigation here is extremely
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challenging, due to the presence of segregated phases, some of which occur only in submicron domains and are the specific result of intermatrix interactions. Only the relative abundance of corresponding phases among different samples can be readily determined for these materials. In order to determine elemental sensitivity factors for the secondary phases observed, e.g., in reaction bonded silicon nitride ceramics, it is necessary to refer to the grain boundary pockets themselves for calibration purposes. Although the probe size of the UC-HRL SIM is smaller than that of many interstitial pockets, it is difficult to obtain significant spectra from such small surface areas because deep craters are rapidly sputter-formed which may extend to underlying Si,N, grains. On the other hand, individual Si,N, grains, which are readily visualized in the SIM, are large enough to permit mass spectrum determination, as mentioned above. A spectrum for the intergranular phases can then be obtained by subtracting a suitably normalized Si,N, spectrum from the total spectrum for a large area (such as those of fig. 2). To apply this procedure, the fractional area of the Si,N, grains must be determined by image processing techniques. The fractional area of the Si,N, grains (which is the same as their fractional volume in a finely interspersed mixture) in this ceramic is 83.48, obtained by averaging the crystal surface coverage for many separate fields of view. After the appropriate spectral subtraction, as outlined above, one finds that the intense mass 70 signal in fig. 2a, ascribed to SiN,/Si,N, originates from the nitride crystals rather than from the YSiOZN phase. Other conclusions can be drawn from this set of spectra. However, accurate atomic accounting, i.e., the quantitative association of detected ions to sample concentration, remains elusive without prior knowledge of mass-dependent ionization probabilities for a given matrix. Atomic accounting also requires the exhaustive summation of all normalized signals from sputtered molecular fragments. More effort must be devoted to compiling a catalogue of SIMS matrix properties for additional ceramics. 2.2. Elemental maps Fig. 3 shows elemental maps of a 20 X 20 pm2 area of a fractured surface, the topography of which is given in the ion-induced secondary electron (ISE) micrograph of fig. 3c. A comparison of the 89Y+ elemental distribution map (fig. 3a) with the 160- map (fig. 3b) shows a complete correspondence of the sharply defined image details, clearly demonstrating the association of Y and 0 in the intergranular phase. These images, however, do not provide a faithful representation of the size of the intergranular pockets because the sample surface results from a fracture which can occur preferentially along grain boundaries, as can be seen in fig. 3c. Thus, the secondary phase may partially or totally coat the exposed crystals. In addition, the presence of sharp edges is
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Fig. 3. SIMS elemental images of the fractured surface of an yttria-sintered (8 wt% Y203) SiaN, ceramic. All images are of the same area, each acquired in 512 s. (a) 89Y+ elemental map, indicating the distribution of the YBO,N intergranular phase. 3.0X106 counts displayed. (b) 160-, 2.0 x lo6 counts. (c) ISE image of the surface topography. (d) 28Si+, 9.2 X lo5 counts. The three-dimensional shading arises from surface edge effects. (e) 26CN-, 3.5X10” counts. (f) Resputtered 69Ga+ map, giving a detailed description of sample microstructure. To be compared with the SEM image, fig. 1. 8.0 X lo6 counts.
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known to enhance the secondary ion yields (edge effect), which depend on the orientation of the emitting surface relative to the incident beam direction, leading to a distorted signal intensity distribution [7]. The secondary ion escape probability increases for emission at angles away from the surface normal. The physical origin of this phenomenon is poorly understood. This effect is particularly noticeable in the *%i+ map of fig. 3d, which exhibits a definite correlation with the surface topography. This correlation masks the differences in Si intensity between the Si,N, crystals and their surrounding intergranular pockets. We attribute to edge effects the dramatic filamentary appearance of the 26CN- map (fig. 3e). Although carbon is not an expected constituent of this ceramic, the body of evidence presented by the mass spectra (see fig. 2b) and by many CN maps similar to that of fig. 3e strongly suggests that a nonnegligible concentration of carbon must be present in the ceramic interior. Fig. 3f is a map of the self-sputtered Ga following implant by the probe. This image provides a detailed description of the morphological microstructure of the sample; it, along with the ISE topography image (fig. 3c), are to be compared to the SEM image, fig. 1. Its features will be discussed, together with additional examples, in a separate section below. Although the analysis of a fracture surface provides a complete, albeit qualitative, compositional and structural description of the ceramic, it is desirable also to examine a polished section of the same, which should be free of SIMS artifacts caused by the surface topography. Accordingly, we show in fig. 4 selected maps obtained from a polished sample, prepared as described above. The cross section of the Y-rich grain boundary pockets is clearly outlined in the Y map (fig. 4a). A complementary image is obtained by mapping the self-sputtered Ga (fig. 4b). We note that whereas the Y map of the fractured surface (fig. 3a) contains a range of signal intensities, the Y map of the cross section (fig. 4a) shows an almost constant signal intensity throughout the interstitial areas. Similarly, the Ga map (fig. 4b), which outlines the areas occupied by the Si3N, grains, is monotonal. The lateral resolution of the Y and Ga images is - 0.1 pm. In the Si map of fig. 4c, the Si intensity in the intergranular pockets is reduced (by a factor of - 2) compared to that in the crystal areas, which is consistent with the stoichiometry of the Si content of YSi02N versus Si,N,. The reduced signal per pixel of this map causes the edges of the pockets to be less defined than in figs. 4a and 4b 171. The CN- map shows a diffuse distribution with sparse, strongly emitting spots 100-600 nm in size, the location of which does not correlate with the prominent structures found in the Y and Si maps. This indicates that carbon is indeed present in the crystal interior and that the filamentary structure observed on the fracture surface (fig. 3e) is not the result of preferential edge deposition of carbon contaminants from the environment. The diffuse carbon may be an instrumental artifact, while the source of the concentrated spots is not clear.
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Fig. 4. SIMS elemental images of the polished surface of the same silicon nitride ceramics as in fig. 3. All images are of the same area, each acquired in 512 s. (a) sgY+ elemental map. 7.1 X lo5 counts displayed. (b) Resputtered 69Ga+ map. For this ceramic, the areas of greatest intensity correspond to material with the largest atomic number density. 4.3 ~10’ counts. (c) ‘*Si+, 3.6 X 10’ counts. Two concentrations are discernible. The granular appearance is due to relatively low signal statistics, not background noise. (d) 26CN- image, revealing a uniform carbon contamination. 1.2 X lo6 counts.
Fig. 5a gives the intensity distribution of the tightly bound molecule, SiO; , which originates from the intergranular YSi02N, exactly mirroring the Y distribution (not shown) for this area, as expected. The SiN- map of fig. 5b dramatically demonstrates the difficulty of quantifying SIMS information. The SiN- intensity from the Si,N, crystals is about one-third that from the intergranular regions. However, from atomic counting calculations without regard to chemical matrix effects, one would expect the SiN- signal to be 4.6 times more intense from the crystals. Several factors combine to produce this discrepancy. First, the Si originating from Si,N, has a strong preference to be
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Fig. 5. Additional SIMS elemental images of the polished surface of the same silicon nitride ceramics as in figs. 3 and 4. Both images are of the same area, each acquired in 512 s. (a) 60SiO; distribution, identical to corresponding Y and 0 maps (not shown). 1.9 X 10' counts displayed. (b) 40SiN- image, 1.9 X lo6 counts. The intergranular areas are enhanced, contrary to initial expectations. See text.
sputtered as a part of SiN: or SiZN+ clusters, as shown by the large mass 70 peak in fig. 2a. As a trade-off, less SiN- emerges compared to that from YSiO,N. Second, the bound oxygen in the intergranular phase may enhance, in some unpredictable manner, SiN- emission and ionization. Other facts, which are also collected under the catchall banner “matrix effects”, include ionization variations due to density and different molecular binding patterns. 2.3. Resputtering of the implanted Ga A striking phenomenon, peculiar to the use of a gallium probe, is the differential resputtering or self-sputtering of the Ga atoms implanted during scans with the SIM. A dynamic equilibrium is reached between the rate of Ga ion implant and the resputtering rate, which are, respectively, functions of the stopping power of the material and of the sputtering yield of the matrix [14]. This effect causes image contrast in Ga maps of heterogeneous materials, such as that shown in figs. 3f and 4b. Additional high resolution examples of this phenomenon are shown in fig. 6. A comparison of the Ga+ maps of figs. 6b and 6d with the corresponding Y images (figs. 6a and 6c) shows that Ga is predominantly resputtered from the Si,N, crystals. This can be qualitatively understood, in part, in terms of the higher atomic number density of Si,N, versus that of YSi4N. The intrinsic Ga contrast, when combined with the dependence of the secondary ion yields on the surface topography, provide
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Fig. 6. Examples of image contrast in resputtered Ga images. Yttria-sintered (8 wt% Y203) Si,N4 ceramic, fractured surface. Each image acquired in 512 s. (a) “Y+ elemental map. 1.6 X lo6 counts displayed. (b) 69Ga+, same area as (a). 10.6 x lo6 counts. (c) 89Y+, larger magnification map. 1.1 x lo6 counts. (d) 69Ga+, same area as (c). 9.3 x lo6 counts.
images which have a three-dimensional appearance and outline with utmost detail the shape and distribution of the individual Si,N, grains.
3. SIMS microanalysis of MgO-doped silicon nitride Silicon nitride with MgO as a sintering aid has poor high temperature mechanical properties because the resulting grain boundary pockets are noncrystalline [15]. The composition of the gram boundary pockets is typically near the miscibility gap of the Si,N,O-SiO,-MgSiOs ternary [16]. Complementary work performed for this sample on a dedicated scanning transmission electron microscope showed the grain boundary pockets to have a Mg/Si ratio
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Fig. 7. Scanning electron microscope image of the etched surface of a MgO-sintered ceramic.
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of about unity. Three principal phases have been identified by traditional techniques: ,&Si,N, (the bulk), MgSiO,, and Si,N20. Amorphous MgSiO,N,, may also be present. The grain structure was irregular and ranged from 0.2 pm to about 2 pm in size; particles from the tungsten carbide milling media were found throughout. A SEM image of an etched surface of this ceramic is presented in fig. 7. Mass spectra of this ceramic were obtained from Au-coated fractured surfaces. The enhanced secondary ion signal from the rough surfaces facilitates the rapid acquisition of high signal statistics. For elemental mapping, in order to present a smooth, non-artifact producing surface to the ion probe, a 1 mm thick ceramic slice was ground to a final polish with a 1 pm diamond polish. Attempts to obtain a finer polish were unsuccessful because the resulting wear debris, perhaps containing WC particles, unavoidably caused micron-scale scratches. Again, the surface was cleaned in solvents, coated with Au, and sputter-cleaned ‘with the Ga+ probe. 3.1. Mass spectra SIMS mass spectra of this sample were obtained under conditions identical to those for the yttria-sintered ceramic. Fig. 8a gives the positive ion mass spectrum, accumulated in 4100 s. The spectral structure in the mass interval 69-73 is identical to that of the previous ceramic and again represents a signature for Ga-bombarded Si3N,. A comparison of this spectrum with that
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Fig. 8. SIMS mass spectra for a MgO-sintered Si,N, ceramic, obtained from 20X20 pm* areas which contain representative fractions of each phase present. (a) Positive secondary ions, 4100 s accumulation time. To convert the plotted counts/channel to counts/s, divide the former by 4.0. (b) Negative secondary ions, 2800 accumulation time. Conversion factor: 2.7.
of yttria-sintered ceramic (fig. 2a) reveals several significant changes other than the obvious shifts in Mg and Y signals. Strong 23Na and 39K signals indicate substantial alkali contamination within the bulk of the material. In addition, the large C,H, (mass 27) and, in the negative spectrum (fig. 8b), CN (mass 26) peaks both mark the presence of carbon. Tungsten (mass 184), because of its low sputtering yield, is difficult to detect with the SIMS method. Overall, the striking similarity between the two sets of ceramic spectra is the consequence of two factors: (1) for both ceramics, the secondary ion yields from the Si,N, crystals dominate those from the intergranular phases, and (2) molecular fragments containing Mg are often difficult to detect. For example,
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in fig. Sa, the 48Mg, peak is weak and the mass 40 peak, while containing some MgO, is more definitely identified as Ca (by examining the “footprint” of the isotopes of Ca). 3.2. Elemental maps Fig. 9 contains a set of elemental maps from one 20 X 20 pm* area of the polished surface of this magnesia-sintered ceramic. In order to better understand the elemental distribution in this material, one must closely compare the 28Si+ 24Mg+ and i60- images (figs. 9a, 9b and 9c, respectively). The Si image contains two distinct concentration levels. The Mg, originating from the sintering agent, has segregated into small (< 1 pm*), sharply defined domains (fig. 9b). Note that the intergranular areas which contain Mg also correspond to some, but not all of the regions with intense Si emission (fig. 9a). Evidently, of the three phases inherent in this ceramic, two give very similar, large Si signals, about three times as intense as that from the third, more abundant phase. The 0 map (fig. 9c) unequivocally reveals the composition of this ceramic. The intense areas in this map are associated with similar bright areas in the Si and Mg maps: this phase must contain Mg, Si, 0, and possibly N. Similarly, the intermediate intensity areas in the 0 image contain Si, 0, and possibly N. The large dark areas contain no 0 (these areas correspond to the dim Si regions) and are immediately identified as Si,N,. Within the Si-O-N system, silicon oxynitride, Si,N,O, is a common phase. A final clue to the composition of this ceramic: the 0 : 1: 3 oxygen intensity ratios between the three phases match the stoichiometry of the 0 content of Si,N, : Si,N,O : MgSiO, .
Computer analysis of MgO-sintered ceramic images gives the following fractional areas (volumes) for each phase: Si,N,, 85.8%; Si,N,O, 10.0%; MgSiO,, 4.2%. The intergranular phases are too small to permit the acquisition of statistically significant spectra. Matrix-induced anomalies in secondary ion yield are apparent in the Si image. The observed Si,N, : Si2N20 : MgSiO, :: 1: 3 : 3 intensity ratios are widely different from the 15 : 7 : 14 ratios predicted by atomic counting. In a like manner, the 0 : 1: 3 ratios detected in the SiO, map (fig. 9e) are not convincingly compatible with the predicted 0 : 1: 2 values. For both of these cases the presence of 0 within the intergranular areas has enhanced secondary ion emission. Whereas for the yttria-sintered ceramic the Ga map (fig. 4b) is an accurate, though qualitative, measure of atomic number density, this interpretation is not adequate for the magnesia ceramic. In the yttria ceramic the intergranular pockets are dark; in the magnesia ceramic they are bright (fig. 9d). The atomic number density for the three phases in this second ceramic are nearly identical, yet they exhibit markedly different Ga resputtering/ionization rates.
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Fig. 9. SIMS elemental images of the same SisN, + MgO ceramic as analyzed in fig. 8, polished surface. All images are of the same area, each acquired in 512 s. (a) *%ic elemental map, showing two levels of concentration. The black areas are contamination inclusions. 1.4 X lo6 counts displayed. (b) 24Mg+, 2.0X106 counts. (c) ‘60- elemental map, displaying three concentration levels. The intense areas correspond to the Mg distribution (b) and are attributed to the MgSiO, phase. The intermediate intensity areas correspond to the intense Si areas (a) and are attributed to the Si,N,O phase. 9.7 X lo5 counts. (d) Resputtered 69Gaf image, 11.9X lo6 counts. (e) 6oSiO;, 4.4 X lo4 counts. (f) 26CN-, 8.5 X 10’ counts.
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Additional model calculations must be performed before the questions of Ga dynamic equilibrium and sputtering properties are fully understood. Fig. 9f shows the CN- distribution for this ceramic (this image is shifted slightly upwards in comparison to the other maps in this figure). The strong carbon-emitting inclusions correspond to dark areas on the Si and Ga maps, and are attributed to WC fragments from the milling media. Again, the diffuse carbon may be an instrumental artifact. 4. Conclusion
With the UC-HRL scanning ion microprobe, it is possible, after minimal preparation, to image a ceramic’s surface topography along with its compositional morphology. In the examples presented here, breakup products of the Si,N, crystalline matrix (Si, SiN, SiN,) and of the intergranular phases (Si, 0, Mg, Y, Siq, C, C,, CN) could be mapped with signal statistics sufficient to reach the limiting resolution of the instrument (- 0.1 pm or better for the conditions of this experiment). Additional structural information is obtained by mapping the self-sputtered Ga implant. Surprisingly, a thin Au coating of the insulating ceramic surface is sufficient to prevent sample charging, thus allowing correlative elemental imaging by repeated scans of the same area. This ability may stem from the conductivity imparted to the surface layers by the Ga implant and from the use of low primary beam currents ( - 50 PA). In fact, the high detection efficiency of the UC-HRL SIM [3] permits the rapid acquisition (I 512 s) of 1024 x 1024 raster size elemental maps, at these probe currents, with satisfactory signal statistics. A distinct advantage of the SIMS method over electron energy loss spectrometry and other related methods is its sensitivity to low-Z elements (for example, signal/noise for Li is > 104), facilitating the analysis of a diverse range of materials. On the other hand, the problem of absolute quantification of SIMS signal has plagued researchers since the inception of the technique. In the near future, the incorporation of dedicated image analysis facilities into microprobe systems will enable precise studies of small, submicron regions, as well as make possible rapid archiving of spectra from calibration samples. This investigation suggests that high resolution SIM imaging microanalysis may become a powerful complement to existing techniques for the study of ceramic materials. Acknowledgements
The research performed at the University of Chicago was supported by the NSF Materials Research Laboratory and by the Allied Signal Engineered Materials Research Center. The authors wish to thank Y.L. Wang for technical assistance and useful discussion.
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