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Impact of RF-GD-OES in practical surface analysis
Spectrochimica Acta Part B 58 (2003) 1573–1583
Review
Impact of RF-GD-OES in practical surface analysis K. Shimizua, H. Habazakib, P. Skeldonc, G.E. Thompsonc,* a
University Chemical Laboratory, Keio University, 4-1-1 Hiyoshi, Yokohama 223-8521, Japan b Graduate school of Engineering, Hokkaido University, N13-W8, Sapporo 060-8628, Japan c Corrosion and Protection Center, University of Manchester Institute of Science and Technology, Manchester M60 1QD, UK Received 10 April 2003; accepted 4 June 2003
Abstract The capabilities of RF-GD-OES for depth profiling analysis are illustrated based on the recent results from the authors’ laboratories. Through the analysis of a hard disk, a thin anodic alumina layer with a delta function marker layer, and a 5-nm thick air-formed oxide film on sputter deposited stainless steel, it is demonstrated that RF-GD-OES has enormous potential for depth profiling analysis of ultra-thin layers, less than 10-nm thick, as well as films of several tens of microns thickness. The remarkable features of RF-GD-OES arise from the nature of RF-GD sputtering where samples, conducting or non-conducting, are sputtered stably with Arq ions of very low energy (-50 eV) and a current flux of the order of ;100 mA cmy2. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Surface analysis; RF-GD-OES; Depth profiling; Thin film
1. Introduction Through the clear realization of the enormous potential of GD-OES for depth profiling analysis of ultra-thin films, less than 10-nm thickness, the world of practical surface analysis, which had been fairly static in the 1980s and 1990s, may be changed drastically w1x. Utilizing the high sputtering rates of more than 1 mm miny1, GD-OES has been used widely for depth profiling analysis of relatively thick films, in the tens of microns range, such as galvanized or painted steels w2,3x. It was, however, employed *Corresponding author. E-mail address: [email protected] (G.E. Thompson).
only occasionally for depth profiling analysis of thin films, less than 1-mm thick, although some encouraging results were reported in previous literatures. For example, Mn-enrichment in an outermost surface region, ;5 nm, of an electrochemically cleaned and annealed steel was disclosed successfully by Ohashi et al. w4x as early as 1979 using a modified Grimm-type GD source. Subsequently, thin passive films, a few nanometers thick, formed on iron in boric acid–buffered solutions, were analyzed successfully by Berneron and Charbonneir w5x, generating GD-OES depth profiles that are of quality comparable to those of AES. Further, thin oxide films ;30-nm thick formed on stainless steels by oxidation in air at 873, were analyzed by Suziki et al. w6x. More
0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547(03)00133-2
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recently, Hodoroaba et al. w7x have analyzed various thin multilayer sandwiches by RF-GD-OES and HFM plasma SNMS. Unfortunately, however, the potential of GD-OES for ultra-thin film analysis has not been explored fully or to the point where it is generally accepted in the surface analysis community that GD-OES is a potential alternative to AES, SIMS or even to angle-resolved XPS, but without lateral resolving power. In the present review, it is demonstrated, using recent examples from the authors’ laboratories that RF-GD-OES has enormous potential, not only for the analysis of thick films, but also for the analysis of ultra-thin layers, conducting or non-conducting, less than 10-nm thick w1x, where AES and SIMS have dominant roles. It is also demonstrated that the significant features of RF-GD-OES, enabling such analysis, arise from the nature of RF-GD sputtering where samples, both conducting and non-conducting, are sputtered very stably from the commencement of analysis with Arq ions of very low energies -50 eV and high current densities of the order of ;100 mA cmy2 w8,9x. The low Arq energies ensure film sputtering proceeds without significant formation of altered layers, which is a pre-requisite for successful depth profiling analysis of ultra-thin films at high depth resolution w10–12x. The high current densities allow film sputtering to proceed at very high rates, above 1 mm miny1, thereby extending the limit of film thickness for analysis to 100 mm. Thus, RF-GDOES covers a very wide thickness range from the first nanometre to depths of several tens of microns from the surface. Further, RF-GD-OES analysis is undertaken rapidly, simply and reproducibly, with excellent depth resolution and sensitivities for detection of most elements in the Periodic Table. The results presented here were obtained using a Jobin Yvon RF-5000 GD-OES instrument with a Marcus-type discharge lamp, operating at constant pressure in argon and with constant applied power. Details of the analysis are given in relevant sections, as appropriate. In GD-OES analysis, material or process-related elements or impurities are all predicted in advance. Therefore, the wavelengths for detection of all related elements, 48 elements maximum are pre-selected at the time of
analysis; unexpected impurities or contaminants are not normally detected. 2. Depth profiling analysis of a hard disk The first example of analysis is that of a hard disk for magnetic data storage w13,14x. Generally, the disks consist of layers, both conducting and non-conducting, of widely differing thicknesses from a few tens of nanometres to several tens of microns. Further, the disks contain numerous elements, as well as various ‘process-related impurities or contaminants that are important to follow during the production processes. From an estimate in 2000, over 1 billion disks are produced worldwide each year, growing at approximately 20% per annum. However, production losses of 60– 70% are common, because of the extreme delicacy of the structures and the need for high quality. Thus, reduction of production losses, through strict quality control in real production lines, is highly necessary. Beside such practical importance, the disks with multi-layers of widely differing thicknesses that contain various elements, are highly suited to demonstrate the true ability of RF-GD-OES for depth profiling analysis of surface regions, from the first nanometre to depths of several tens of microns rapidly, simply and with excellent depth resolution and sensitivities. Most disks are produced over aluminium–magnesium alloy or glass substrates. Here, an aluminium-based disk, supplied from Fujitsu Company, Japan, was employed, since the multi-layered structure of the disk can be revealed directly and precisely, through transmission electron microscopy of ultramicrotomed sections. The multi-layered structure observed may then be compared closely with the RF-GD-OES depth profile. Briefly, the disk was fabricated in the following manner. An aluminium disk, 1-mm thick and 3 inch diameter, was initially zincate treated to allow electroless-deposition of amorphous Ni–P layers ;12-mm thick. The disk was then polished mechanically to an average roughness of ;1 nm, leaving an amorphous Ni–P layer ;10-mm thick, to ensure sufficient surface hardness. Over the mirror-polished disk, chromium, cobalt–chromium
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magnetic alloy and diamond-like carbon (DLC) layers, each 10–30-nm thick, were successively deposited by sputtering. Finally, a thin, carbonbased lubricant layer was applied. Ultramicrotomed sections of the hard disk were prepared in the usual way, using an RMC ultramicrotome and a diamond knife w15x. The ultramicrotomed sections, ;10-nm thick were examined in a TECNAI F20 field emission, high resolution transmission electron microscope operated at 200 kV. The disk was depth profiled at an argon pressure of 680 Pa and a power of 40 W. The area analyzed was 4 mm diameter and the sampling time was 0.01 s for the first 15 s of analysis, when the thin lubricant layer, DLC, Co–Cr and Cr layers were analyzed. Subsequently, the sampling time was 0.1 s. Fig. 1 shows a transmission electron micrograph of an ultramicrotomed section of the hard disk. The aluminium substrate is observed at the bottom of the micrograph, and contains 4.5 wt.% magnesium. Over the aluminium substrate, the Ni–P layer, approximately 10-mm thick is observed as a dark band of material. Selected area electron diffraction analysis confirms that the Ni–P layer is amorphous. The Ni–PyAl interface appears rough, due to the zincate treatment applied to the aluminium substrate to facilitate subsequent electrolessdeposition of the Ni–P layer. However, such roughness is of crucial importance to ensure excellent adhesion between the aluminium substrate and the Ni–P layer. The Cr, Co–Cr magnetic alloy and DLC layers are present at the surface, but these layers are too thin to be revealed clearly at this magnification. Fig. 2 shows a high-magnification image of the boxed area shown in Fig. 1, with the Cr, Co–Cr magnetic alloy and DLC layers now revealed clearly. Over the Ni–P layer, the Cr layer is observed as a light band of material, 10–15-nm thick, and exhibits a columnar structure, which is the main cause of surface roughness. Then the Co–Cr magnetic layer is observed as a dark band of material, 15–20-nm thick. The DLC layer, 10– 12-nm thick, is readily evident over the Co–Cr layer. The DLC layer is applied to protect the Co– Cr magnetic layer from scratching during disk drive operation and from corrosion. A thin lubricant layer is expected at the outermost surface, but
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Fig. 1. Transmission electron micrographs of an ultramictotomed section of a hard disk.
the layer appears to have been detached from the surface with embedding resin during sectioning and is not observed. In the present example, where a large number of elements are of relevance in different regions and at various depths from the surface, the GDOES profile has been separated into three profiles indicated in Figs. 3 and 4. The very high speed of GD-OES analysis is evident, taking only approximately 130 s to traverse through the 10-mm thick Ni–P layer. Furthermore, a prolonged vacuum pumping is not necessary in GD-OES depth profiling analysis. Thus, the total time required for the analysis, the time required to generate a profile after the sample had been placed at the sample
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Fig. 2. A high magnification image of the boxed area of Fig. 1, revealing DLC, Co–Cr and Cr layers over the amorphous Ni–P.
holder, was only approximately 10 min in the present example. The profiles in Fig. 3a and b are relatively noisy during the first 15 s of analysis
Fig. 3. RF-GD-OES depth profile of the disk.
due to the shorter sampling time of 0.01 s to obtain high depth resolution and hence to reveal the thin Cr, Co–Cr, DLC and lubricant layers. In the aluminium substrate, magnesium is detected readily. At the Ni–PyAl interface, Zn, Fe and Na impurities that are residues from the zincate treatment, are disclosed successfully. In the Ni–P layer,
Fig. 4. As Fig. 3, but showing a near-surface region of the disk that reveals the Cr, Co–Cr, DLC and lubricant layers.
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the PyNi ratio is constant throughout the film thickness. Further, nitrogen, lead and hydrogen impurities are detected. Nitrogen arises from the ammonia used to adjust the pH of the plating bath. Lead results from the presence of a small amount of lead acetate, usually a few ppm that was added to the plating bath to improve the leveling of the deposit. Finally, hydrogen arises from the atomic hydrogen generated during electroless deposition of Ni–P, which subsequently diffused into the deposit and is probably present as fine bubbles. Fig. 4 shows the GD-OES depth profile in the near-surface region. GD-OES took only approximately 1 s to traverse through the lubricant, DLC, Co–Cr and Cr layers. The Cr and Co–Cr layers are revealed clearly. Further, small amounts of iron and hydrogen impurities are disclosed in the Cr layer; these are the main impurities in the Cr sputtering target. Interestingly, the carbon profile exhibits two peaks. Further, a sharp sodium peak is observed between these peaks. Considering that sodium is a typical contaminant between the processes, it is evident that the inner and outer peaks correspond to the DLC and lubricant layers, respectively. Unlike SIMS, the so-called ‘matrix effect’ is insignificant in GD-OES analysis. Depth profiles are readily quantified through calibration against known reference materials w2x. Various schemes for quantitative analysis have been developed for both RF and DC powered sources and are built in commercial instruments w2x. Thus, the profile of Fig. 4 can be readily quantified as shown in Fig. 5. The accuracy of analysis is improving continuously through recent introduction of DC bias voltage correction and hydrogen corrections in RF analysis w16x. 3. Depth resolution in RF-GD-OES depth profiling From the previous example, it is evident that the depth resolution of RF-GD-OES is sufficiently high to disclose thin surface layers, a few nanometres thick. In order to assess the depth resolution of RF-GD-OES, however, high quality specimens, where thin layers of highly uniform thicknesses are present over highly flat substrates, are required.
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Fig. 5. As Fig. 3, but after quantification.
Fortunately, amorphous alumina films, -1-mm thick, grown over electropolished, high purity aluminium substrates, by anodic oxidation in appropriate electrolytes, approximately closely to the ideal situation. Further, the films with delta function marker layers can be prepared readily, cheaply, routinely and with very high degree of precision w1,17,18x. Furthermore, of specific importance here, the locations of delta function marker layers can be revealed directly by transmission electron microscopy of ultramicrotomed sections as now shown. A typical example is given in Fig. 6. Over the highly flat aluminium substrate, present at the bottom of the micrograph, a 360-nm thick film is evident as the dark band of material of highly uniform thickness. At a depth of 40 nm, a darker, narrow line, ;2 nm width is observed clearly; the line represents the region of chromium enrichment and runs parallel to the highly flat metalyoxide interface. Full details of the specimen preparation procedures are given elsewhere w17,18x. Although the region indicated is very limited compared with the area analyzed by RF-GD-OES, which is approximately 4 mm diameter, the amorphous nature of the oxide and its growth kinetics ensure that such uniformity of the film thickness and flatness of the metalyoxide interface, along with the location of the band of chromium enrichment,
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Fig. 6. Transmission electron micrograph of an ultramicrotomed section of a 360-nm thick anodic alumina film with a delta function marker layer associated with chromium enrichment.
are maintained over the macroscopic specimen surface w19,20x. Fig. 7 shows the RF-GD-OES profile of the film shown in Fig. 6. The profile was recorded at an argon pressure of 680 Pa and a power of 40 W where highest depth resolution was realized. For clarity, only the aluminium and chromium profiles are indicated. The signal intensity of aluminium is fairly stable and constant through the analysis of the oxide film. The subtle periodic variation or waviness, observed in the aluminium profile is not due to variations in aluminium concentrations in the film, but arises from an optical interference effect w21x. As sputtering proceeds and the metaly oxide interface is reached, the aluminium profile exhibits a very sharp increase over a narrow region equivalent to a width of ;6 nm. After the sharp
increase, the signal intensity of aluminium continues to increase, although at progressively decreasing rates, until a steady value is reached. This ‘rounding-off’ in the aluminium profile is due to micro-roughening of the exposed aluminium substrate by sputtering. The delta function marker layer, associated with the chromium enrichment is revealed clearly as a narrow peak, with a full width at half-maximum height (FWHM) of ;5 nm, at a position corresponding to the expected depth of 40 nm. The excellent depth resolution of RF-GDOES is thus demonstrated clearly. For the specimen of the quality employed here, degradation of depth resolution due to the original surface roughness is insignificant. Consequently, the depth resolution is determined by the following factors w11x: (1) uniformity of the sputtering rate over the 4 mm diameter crater; (2) sputter-induced micro-roughening of the surface; (3) atomic mixing due to collision of Arq ions and possible redeposition of sputtered materials from the plasma; (4) stability of the sputtering rate through the analysis of the oxide layer. In RF-GD-OES depth profiling, the samples are sputtered with Arq ions of energy of -50 eV. Based on the typical Arq penetration length, 0.1 nmy100 eV for Cu for example w22x, atomic mixing and formation of thin
Fig. 7. RF-GD-OES depth profile of the film shown in Fig. 6. The metalyoxide interface is shown by the solid line.
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altered layer are considered insignificant. Further, the samples are bombarded alternately by Arq ions and electrons during analysis and charge compensation takes place in situ w23x, allowing stable sputtering of the highly insulating alumina film. Therefore, factors (3) and (4) are not important. Further cross-sectional TEM and AFM examination of the sputter craters have shown that micro-roughening is totally absent w18x. Thus, the depth resolution here is determined mainly by the shape of the sputtered crater, which is known to be strongly influenced by the Ar pressure. At a given applied power, an optimum Ar pressure generates a so-called flat-bottom crater and the highest depth resolution is achieved. Increase or decrease of Ar pressure from the optimum results in rapid degradation of depth resolution, with the production of convex (round-bottom) or concave (deep-sided) craters w2x. In practice, an optimum Ar pressure can be found readily and quickly by checking the sharpness of the transition of the profiles of relevant elements at appropriate interfaces with Ar pressure w24x. The search is greatly assisted by the speed of the analysis associated with the high sputtering rates and the absence of an ultra-high vacuum. Under the condition of analysis employed, the sputtering rate was uniform over the 4 mm crater w18x. Examination of the residual film thickness over the crater at a sputtered depth of approximately 300 nm indicated that the sputtering rate was constant to within 1%, or better, in the central region of the crater of approximately 3.4 mm diameter. As the position moves towards the edge of the crater, however, the sputtering rate increased gradually. Even at the immediate periphery of the crater edge of approximately 0.1 mm width, where the sputtering rate was greatest, it was only approximately 8% higher than the central region. Successful revelation of the Cr-marker layer, ;2-nm thick, as the narrow peak of FWHM of ;5 nm at a depth of 40 nm is apparently due to uniform sputtering over the 4 mm crater. The depth resolution degrades with the interfacial depth due to the increased sputtering rate near the edge of the crater, generally called the ‘edge effect’ w18x. In other words, it is evident that the distinct advantage of RF-GD-OES, associated with its unique ability
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to sputter films, both insulating and conducting, with Arq ions of low energy (-50 eV), without significant atomic mixing and altered layer formation, should be realized fully for depth profiling analysis of ultra-thin films of thicknesses less than 10 nm where degradation of depth resolution due to the edge effect is insignificant. 4. Depth profiling analysis of an air-formed oxide film on stainless steel The potential of RF-GD-OES for depth profiling analysis of ultra-thin films is now demonstrated, with close comparison to SIMS, through the analysis of an air-formed oxide film, -5-nm thick, grown over a thin AISI type-304 stainless steel film, approximately 0.3-mm thick that was sputter deposited over a mirror-polished silicon wafer w25x. Other examples are shown elsewhere w1x. A thin, sputter deposited stainless steel film, rather than bulk stainless steel sheet, was used to obtain a clean, reproducible and highly flat surface such that roughness-induced degradation of depth resolution is minimized. Further, it has been shown through XPS analysis that the air-formed oxide film has a duplex structure w26x, consisting of an outer Fe2O3 layer and an inner layer enriched with Cr2O3. Nickel in the metal is not oxidized and, therefore, is not present in the oxide film. Because of these features, the air-formed oxide film on sputter deposited stainless steel film could be an ideal standard to demonstrate the potential of RFGD-OES for depth profiling analysis of ultra-thin layers of several nanometers thickness. The original mirror-like appearance of the silicon wafer was retained after sputter deposition of the thin stainless steel film, suggesting that the flatness of the sputter deposited stainless steel surface is comparable with that of the silicon substrate as intended. AFM examination has shown, however, that the surface is not flat at the microscopic level as shown in Fig. 8a; the scanned area is 400=400 nm. Noting that the height scale is magnified considerably compared with the lateral scale, the surface generally is reasonably flat, but it comprises of a series of hills and valleys, varying in height by up to approximately 5 nm. Surface roughness of this magnitude is unavoid-
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Fig. 8. (a) AFM image of the surface of the stainless steel sputter deposited over the mirror-polished silicon wafer; (b) transmission electron micrograph of an ultramicrotomed section of the stainless steel film and its air-formed oxide film, showing the growth of amorphous oxide layer of uniform thickness of approximately 4.8-nm.
able, since it is related to the columnar structure of the sputter deposited film. Fig. 8b shows a transmission electron micrograph of an ultramicrotomed section of the stainless steel film and its air-formed oxide film. Here, the stainless steel film was deposited on an oxide coated, highly flat aluminium substrate, prepared by anodic oxidation of an electropolished aluminium, in order to allow
sectioning using an ultramicrotome and a diamond knife. The ultramicrotomed sections, of 5–10-nm thickness, were prepared in the usual manner and examined in a TECNAI F20 field-emission, high resolution transmission electron microscope operated at 200 kV. The stainless steel substrate is observed at the bottom of the micrograph. Crossed lattice fringes of spacing of 0.202 nm, correspond-
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Fig. 9. RF-GD-OES (a) and SIMS (b) depth profiles of the air-formed oxide film on the sputter deposited stainless steel film.
ing to the {110} planes of a-Fe, are observed. Over the slightly rounded stainless steel surface, due the columnar structure, the air-formed oxide film is observed clearly. The film is largely amorphous and the thickness, approximately 4.8 nm, is highly uniform. RF-GD-OES and SIMS depth profiles are shown in Fig. 9a and b, respectively. RF-GD-OES depth profiling was carried out at an Ar pressure of 608 Pa and a power of 40 W; the sampling time was 0.002 s. SIMS depth profiling was carried out at ULVAC-PHI Research Laboratory, Kawasaki, Japan, using a Dynamic SIMS PHI ADEPT 1010 instrument. A Csq primary ion beam, with an energy of 500 eV, current 50 nA and an incident angle of 60o, was employed. Further, the 133 Cs56Feq, 133Cs52Crq and 133Cs58Niq secondary ions were monitored throughout analysis in order to minimize the matrix effect w27x. The duplex nature of the air-formed oxide film, -5-nm thick, is revealed clearly in both profiles. Firstly, and concerning the RF-GD-OES depth profile, the signal intensity of iron increases immediately after the start of the analysis. However, the signal intensities of chromium and nickel increase only after a time delay. The delay time is longer for nickel than for chromium. The initial plateau
region observed in the iron profile, where neither chromium nor nickel is present, is associated with the outer Fe2O3 layer. The chromium profile exhibits a peak located at the metalyoxide interface; the peak is associated with the inner layer enriched with Cr2O3. Further, the half-height position of the leading edge of the nickel profile is located at the metalyoxide interface, determined independently from the iron profile, indicating that nickel is not present in the oxide film. A small peak, observed in the nickel profile beneath the metalyoxide interface, is associated with the enrichment of the unoxidized nickel in the metal beneath the oxide film. From the RF-GD-OES depth profile, along with XPS results, it appears that the oxide growth on sputter-deposited stainless film involves preferential oxidation of iron and chromium, with a duplex structure developed by the faster migration of Fe3q ions relative to Cr3q ions w25x. Turning to the SIMS depth profile, it is evident that the 133Cs56Feq, 133Cs52Crq and 133Cs58Niq profiles are very similar to the Fe, Cr and Ni distribution profiles of RF-GD-OES, except for the characteristic, anomalous transition in the signal intensity in the SIMS profile at the very early stage of sputtering. The close agreement between the RF-GD-OES and SIMS depth profiles confirms
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that RF-GD-OES is well-suited for depth profiling analysis of ultra-thin films, less than 10 nm. Further, and of importance, it is evident that the matrix effect has been reduced considerably through monitoring CsMq clusters, yielding a ‘quasi-genuine’ SIMS depth profile.
and conducting or non-conducting, and their interfaces, which all play key roles in modern surface technologies, ranging from electronics to corrosion protection of metals.
5. Conclusion
Thanks are due to Mr Y. Uchida of Horiba Company, Tokyo, Japan, for provision of time on a Jobin Yvon RF-5000 GD-OES instrument.
Successful depth profiling analysis of ultra-thin films, less than 10-nm thick, extends greatly the power of RF-GD-OES in the field of practical surface analysis. It is the only technique which allows quantitative depth profiling analysis of surface regions, both conducting and non-conducting, from the first nanometre to depths of several tens of microns. Considering the very high speed of analysis, excellent depth resolution and sensitivity, the relatively low cost of the instrument, and ready operation and maintenance of the instrument, without the requirements for highly experienced scientists, RF-GD-OES may be employed, for the first time in the history of surface depth profiling analysis, for quality control of various surface enhanced industrial products in real production lines. These unique features of RF-GD-OES, that are not seen with other traditional depth profiling techniques, such as AES and SIMS, arise from the nature of RF-GD sputtering where samples, both conducting and non-conducting, are sputtered very stably with Arq ions of low energies (-50 eV) and high current fluxes of the order of ;100 mA cmy2. Like other sputter depth profiling techniques, such as AES, XPS and SIMS, RF-GD-OES has limitations. The sputtering in the RF-GD plasma occurs in an isotropic manner over a 4 mm (or 2 mm) diameter sampling area. Consequently, surface imaging and micro-spot analysis are not possible. Further, chemical information is not obtained. Even with these limitations, the versatility, simplicity and accuracy, in terms of depth resolution, sensitivity and reproducibility, make RF-GD-OES an indispensable tool in the field of practical surface analysis. When the RF-GD-OES results are compared with information from other techniques, the outcome is an important understanding of surfaces, surface films, thin or thick
Acknowledgments
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Review
Impact of RF-GD-OES in practical surface analysis K. Shimizua, H. Habazakib, P. Skeldonc, G.E. Thompsonc,* a
University Chemical Laboratory, Keio University, 4-1-1 Hiyoshi, Yokohama 223-8521, Japan b Graduate school of Engineering, Hokkaido University, N13-W8, Sapporo 060-8628, Japan c Corrosion and Protection Center, University of Manchester Institute of Science and Technology, Manchester M60 1QD, UK Received 10 April 2003; accepted 4 June 2003
Abstract The capabilities of RF-GD-OES for depth profiling analysis are illustrated based on the recent results from the authors’ laboratories. Through the analysis of a hard disk, a thin anodic alumina layer with a delta function marker layer, and a 5-nm thick air-formed oxide film on sputter deposited stainless steel, it is demonstrated that RF-GD-OES has enormous potential for depth profiling analysis of ultra-thin layers, less than 10-nm thick, as well as films of several tens of microns thickness. The remarkable features of RF-GD-OES arise from the nature of RF-GD sputtering where samples, conducting or non-conducting, are sputtered stably with Arq ions of very low energy (-50 eV) and a current flux of the order of ;100 mA cmy2. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Surface analysis; RF-GD-OES; Depth profiling; Thin film
1. Introduction Through the clear realization of the enormous potential of GD-OES for depth profiling analysis of ultra-thin films, less than 10-nm thickness, the world of practical surface analysis, which had been fairly static in the 1980s and 1990s, may be changed drastically w1x. Utilizing the high sputtering rates of more than 1 mm miny1, GD-OES has been used widely for depth profiling analysis of relatively thick films, in the tens of microns range, such as galvanized or painted steels w2,3x. It was, however, employed *Corresponding author. E-mail address: [email protected] (G.E. Thompson).
only occasionally for depth profiling analysis of thin films, less than 1-mm thick, although some encouraging results were reported in previous literatures. For example, Mn-enrichment in an outermost surface region, ;5 nm, of an electrochemically cleaned and annealed steel was disclosed successfully by Ohashi et al. w4x as early as 1979 using a modified Grimm-type GD source. Subsequently, thin passive films, a few nanometers thick, formed on iron in boric acid–buffered solutions, were analyzed successfully by Berneron and Charbonneir w5x, generating GD-OES depth profiles that are of quality comparable to those of AES. Further, thin oxide films ;30-nm thick formed on stainless steels by oxidation in air at 873, were analyzed by Suziki et al. w6x. More
0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0584-8547(03)00133-2
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recently, Hodoroaba et al. w7x have analyzed various thin multilayer sandwiches by RF-GD-OES and HFM plasma SNMS. Unfortunately, however, the potential of GD-OES for ultra-thin film analysis has not been explored fully or to the point where it is generally accepted in the surface analysis community that GD-OES is a potential alternative to AES, SIMS or even to angle-resolved XPS, but without lateral resolving power. In the present review, it is demonstrated, using recent examples from the authors’ laboratories that RF-GD-OES has enormous potential, not only for the analysis of thick films, but also for the analysis of ultra-thin layers, conducting or non-conducting, less than 10-nm thick w1x, where AES and SIMS have dominant roles. It is also demonstrated that the significant features of RF-GD-OES, enabling such analysis, arise from the nature of RF-GD sputtering where samples, both conducting and non-conducting, are sputtered very stably from the commencement of analysis with Arq ions of very low energies -50 eV and high current densities of the order of ;100 mA cmy2 w8,9x. The low Arq energies ensure film sputtering proceeds without significant formation of altered layers, which is a pre-requisite for successful depth profiling analysis of ultra-thin films at high depth resolution w10–12x. The high current densities allow film sputtering to proceed at very high rates, above 1 mm miny1, thereby extending the limit of film thickness for analysis to 100 mm. Thus, RF-GDOES covers a very wide thickness range from the first nanometre to depths of several tens of microns from the surface. Further, RF-GD-OES analysis is undertaken rapidly, simply and reproducibly, with excellent depth resolution and sensitivities for detection of most elements in the Periodic Table. The results presented here were obtained using a Jobin Yvon RF-5000 GD-OES instrument with a Marcus-type discharge lamp, operating at constant pressure in argon and with constant applied power. Details of the analysis are given in relevant sections, as appropriate. In GD-OES analysis, material or process-related elements or impurities are all predicted in advance. Therefore, the wavelengths for detection of all related elements, 48 elements maximum are pre-selected at the time of
analysis; unexpected impurities or contaminants are not normally detected. 2. Depth profiling analysis of a hard disk The first example of analysis is that of a hard disk for magnetic data storage w13,14x. Generally, the disks consist of layers, both conducting and non-conducting, of widely differing thicknesses from a few tens of nanometres to several tens of microns. Further, the disks contain numerous elements, as well as various ‘process-related impurities or contaminants that are important to follow during the production processes. From an estimate in 2000, over 1 billion disks are produced worldwide each year, growing at approximately 20% per annum. However, production losses of 60– 70% are common, because of the extreme delicacy of the structures and the need for high quality. Thus, reduction of production losses, through strict quality control in real production lines, is highly necessary. Beside such practical importance, the disks with multi-layers of widely differing thicknesses that contain various elements, are highly suited to demonstrate the true ability of RF-GD-OES for depth profiling analysis of surface regions, from the first nanometre to depths of several tens of microns rapidly, simply and with excellent depth resolution and sensitivities. Most disks are produced over aluminium–magnesium alloy or glass substrates. Here, an aluminium-based disk, supplied from Fujitsu Company, Japan, was employed, since the multi-layered structure of the disk can be revealed directly and precisely, through transmission electron microscopy of ultramicrotomed sections. The multi-layered structure observed may then be compared closely with the RF-GD-OES depth profile. Briefly, the disk was fabricated in the following manner. An aluminium disk, 1-mm thick and 3 inch diameter, was initially zincate treated to allow electroless-deposition of amorphous Ni–P layers ;12-mm thick. The disk was then polished mechanically to an average roughness of ;1 nm, leaving an amorphous Ni–P layer ;10-mm thick, to ensure sufficient surface hardness. Over the mirror-polished disk, chromium, cobalt–chromium
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magnetic alloy and diamond-like carbon (DLC) layers, each 10–30-nm thick, were successively deposited by sputtering. Finally, a thin, carbonbased lubricant layer was applied. Ultramicrotomed sections of the hard disk were prepared in the usual way, using an RMC ultramicrotome and a diamond knife w15x. The ultramicrotomed sections, ;10-nm thick were examined in a TECNAI F20 field emission, high resolution transmission electron microscope operated at 200 kV. The disk was depth profiled at an argon pressure of 680 Pa and a power of 40 W. The area analyzed was 4 mm diameter and the sampling time was 0.01 s for the first 15 s of analysis, when the thin lubricant layer, DLC, Co–Cr and Cr layers were analyzed. Subsequently, the sampling time was 0.1 s. Fig. 1 shows a transmission electron micrograph of an ultramicrotomed section of the hard disk. The aluminium substrate is observed at the bottom of the micrograph, and contains 4.5 wt.% magnesium. Over the aluminium substrate, the Ni–P layer, approximately 10-mm thick is observed as a dark band of material. Selected area electron diffraction analysis confirms that the Ni–P layer is amorphous. The Ni–PyAl interface appears rough, due to the zincate treatment applied to the aluminium substrate to facilitate subsequent electrolessdeposition of the Ni–P layer. However, such roughness is of crucial importance to ensure excellent adhesion between the aluminium substrate and the Ni–P layer. The Cr, Co–Cr magnetic alloy and DLC layers are present at the surface, but these layers are too thin to be revealed clearly at this magnification. Fig. 2 shows a high-magnification image of the boxed area shown in Fig. 1, with the Cr, Co–Cr magnetic alloy and DLC layers now revealed clearly. Over the Ni–P layer, the Cr layer is observed as a light band of material, 10–15-nm thick, and exhibits a columnar structure, which is the main cause of surface roughness. Then the Co–Cr magnetic layer is observed as a dark band of material, 15–20-nm thick. The DLC layer, 10– 12-nm thick, is readily evident over the Co–Cr layer. The DLC layer is applied to protect the Co– Cr magnetic layer from scratching during disk drive operation and from corrosion. A thin lubricant layer is expected at the outermost surface, but
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Fig. 1. Transmission electron micrographs of an ultramictotomed section of a hard disk.
the layer appears to have been detached from the surface with embedding resin during sectioning and is not observed. In the present example, where a large number of elements are of relevance in different regions and at various depths from the surface, the GDOES profile has been separated into three profiles indicated in Figs. 3 and 4. The very high speed of GD-OES analysis is evident, taking only approximately 130 s to traverse through the 10-mm thick Ni–P layer. Furthermore, a prolonged vacuum pumping is not necessary in GD-OES depth profiling analysis. Thus, the total time required for the analysis, the time required to generate a profile after the sample had been placed at the sample
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Fig. 2. A high magnification image of the boxed area of Fig. 1, revealing DLC, Co–Cr and Cr layers over the amorphous Ni–P.
holder, was only approximately 10 min in the present example. The profiles in Fig. 3a and b are relatively noisy during the first 15 s of analysis
Fig. 3. RF-GD-OES depth profile of the disk.
due to the shorter sampling time of 0.01 s to obtain high depth resolution and hence to reveal the thin Cr, Co–Cr, DLC and lubricant layers. In the aluminium substrate, magnesium is detected readily. At the Ni–PyAl interface, Zn, Fe and Na impurities that are residues from the zincate treatment, are disclosed successfully. In the Ni–P layer,
Fig. 4. As Fig. 3, but showing a near-surface region of the disk that reveals the Cr, Co–Cr, DLC and lubricant layers.
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the PyNi ratio is constant throughout the film thickness. Further, nitrogen, lead and hydrogen impurities are detected. Nitrogen arises from the ammonia used to adjust the pH of the plating bath. Lead results from the presence of a small amount of lead acetate, usually a few ppm that was added to the plating bath to improve the leveling of the deposit. Finally, hydrogen arises from the atomic hydrogen generated during electroless deposition of Ni–P, which subsequently diffused into the deposit and is probably present as fine bubbles. Fig. 4 shows the GD-OES depth profile in the near-surface region. GD-OES took only approximately 1 s to traverse through the lubricant, DLC, Co–Cr and Cr layers. The Cr and Co–Cr layers are revealed clearly. Further, small amounts of iron and hydrogen impurities are disclosed in the Cr layer; these are the main impurities in the Cr sputtering target. Interestingly, the carbon profile exhibits two peaks. Further, a sharp sodium peak is observed between these peaks. Considering that sodium is a typical contaminant between the processes, it is evident that the inner and outer peaks correspond to the DLC and lubricant layers, respectively. Unlike SIMS, the so-called ‘matrix effect’ is insignificant in GD-OES analysis. Depth profiles are readily quantified through calibration against known reference materials w2x. Various schemes for quantitative analysis have been developed for both RF and DC powered sources and are built in commercial instruments w2x. Thus, the profile of Fig. 4 can be readily quantified as shown in Fig. 5. The accuracy of analysis is improving continuously through recent introduction of DC bias voltage correction and hydrogen corrections in RF analysis w16x. 3. Depth resolution in RF-GD-OES depth profiling From the previous example, it is evident that the depth resolution of RF-GD-OES is sufficiently high to disclose thin surface layers, a few nanometres thick. In order to assess the depth resolution of RF-GD-OES, however, high quality specimens, where thin layers of highly uniform thicknesses are present over highly flat substrates, are required.
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Fig. 5. As Fig. 3, but after quantification.
Fortunately, amorphous alumina films, -1-mm thick, grown over electropolished, high purity aluminium substrates, by anodic oxidation in appropriate electrolytes, approximately closely to the ideal situation. Further, the films with delta function marker layers can be prepared readily, cheaply, routinely and with very high degree of precision w1,17,18x. Furthermore, of specific importance here, the locations of delta function marker layers can be revealed directly by transmission electron microscopy of ultramicrotomed sections as now shown. A typical example is given in Fig. 6. Over the highly flat aluminium substrate, present at the bottom of the micrograph, a 360-nm thick film is evident as the dark band of material of highly uniform thickness. At a depth of 40 nm, a darker, narrow line, ;2 nm width is observed clearly; the line represents the region of chromium enrichment and runs parallel to the highly flat metalyoxide interface. Full details of the specimen preparation procedures are given elsewhere w17,18x. Although the region indicated is very limited compared with the area analyzed by RF-GD-OES, which is approximately 4 mm diameter, the amorphous nature of the oxide and its growth kinetics ensure that such uniformity of the film thickness and flatness of the metalyoxide interface, along with the location of the band of chromium enrichment,
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Fig. 6. Transmission electron micrograph of an ultramicrotomed section of a 360-nm thick anodic alumina film with a delta function marker layer associated with chromium enrichment.
are maintained over the macroscopic specimen surface w19,20x. Fig. 7 shows the RF-GD-OES profile of the film shown in Fig. 6. The profile was recorded at an argon pressure of 680 Pa and a power of 40 W where highest depth resolution was realized. For clarity, only the aluminium and chromium profiles are indicated. The signal intensity of aluminium is fairly stable and constant through the analysis of the oxide film. The subtle periodic variation or waviness, observed in the aluminium profile is not due to variations in aluminium concentrations in the film, but arises from an optical interference effect w21x. As sputtering proceeds and the metaly oxide interface is reached, the aluminium profile exhibits a very sharp increase over a narrow region equivalent to a width of ;6 nm. After the sharp
increase, the signal intensity of aluminium continues to increase, although at progressively decreasing rates, until a steady value is reached. This ‘rounding-off’ in the aluminium profile is due to micro-roughening of the exposed aluminium substrate by sputtering. The delta function marker layer, associated with the chromium enrichment is revealed clearly as a narrow peak, with a full width at half-maximum height (FWHM) of ;5 nm, at a position corresponding to the expected depth of 40 nm. The excellent depth resolution of RF-GDOES is thus demonstrated clearly. For the specimen of the quality employed here, degradation of depth resolution due to the original surface roughness is insignificant. Consequently, the depth resolution is determined by the following factors w11x: (1) uniformity of the sputtering rate over the 4 mm diameter crater; (2) sputter-induced micro-roughening of the surface; (3) atomic mixing due to collision of Arq ions and possible redeposition of sputtered materials from the plasma; (4) stability of the sputtering rate through the analysis of the oxide layer. In RF-GD-OES depth profiling, the samples are sputtered with Arq ions of energy of -50 eV. Based on the typical Arq penetration length, 0.1 nmy100 eV for Cu for example w22x, atomic mixing and formation of thin
Fig. 7. RF-GD-OES depth profile of the film shown in Fig. 6. The metalyoxide interface is shown by the solid line.
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altered layer are considered insignificant. Further, the samples are bombarded alternately by Arq ions and electrons during analysis and charge compensation takes place in situ w23x, allowing stable sputtering of the highly insulating alumina film. Therefore, factors (3) and (4) are not important. Further cross-sectional TEM and AFM examination of the sputter craters have shown that micro-roughening is totally absent w18x. Thus, the depth resolution here is determined mainly by the shape of the sputtered crater, which is known to be strongly influenced by the Ar pressure. At a given applied power, an optimum Ar pressure generates a so-called flat-bottom crater and the highest depth resolution is achieved. Increase or decrease of Ar pressure from the optimum results in rapid degradation of depth resolution, with the production of convex (round-bottom) or concave (deep-sided) craters w2x. In practice, an optimum Ar pressure can be found readily and quickly by checking the sharpness of the transition of the profiles of relevant elements at appropriate interfaces with Ar pressure w24x. The search is greatly assisted by the speed of the analysis associated with the high sputtering rates and the absence of an ultra-high vacuum. Under the condition of analysis employed, the sputtering rate was uniform over the 4 mm crater w18x. Examination of the residual film thickness over the crater at a sputtered depth of approximately 300 nm indicated that the sputtering rate was constant to within 1%, or better, in the central region of the crater of approximately 3.4 mm diameter. As the position moves towards the edge of the crater, however, the sputtering rate increased gradually. Even at the immediate periphery of the crater edge of approximately 0.1 mm width, where the sputtering rate was greatest, it was only approximately 8% higher than the central region. Successful revelation of the Cr-marker layer, ;2-nm thick, as the narrow peak of FWHM of ;5 nm at a depth of 40 nm is apparently due to uniform sputtering over the 4 mm crater. The depth resolution degrades with the interfacial depth due to the increased sputtering rate near the edge of the crater, generally called the ‘edge effect’ w18x. In other words, it is evident that the distinct advantage of RF-GD-OES, associated with its unique ability
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to sputter films, both insulating and conducting, with Arq ions of low energy (-50 eV), without significant atomic mixing and altered layer formation, should be realized fully for depth profiling analysis of ultra-thin films of thicknesses less than 10 nm where degradation of depth resolution due to the edge effect is insignificant. 4. Depth profiling analysis of an air-formed oxide film on stainless steel The potential of RF-GD-OES for depth profiling analysis of ultra-thin films is now demonstrated, with close comparison to SIMS, through the analysis of an air-formed oxide film, -5-nm thick, grown over a thin AISI type-304 stainless steel film, approximately 0.3-mm thick that was sputter deposited over a mirror-polished silicon wafer w25x. Other examples are shown elsewhere w1x. A thin, sputter deposited stainless steel film, rather than bulk stainless steel sheet, was used to obtain a clean, reproducible and highly flat surface such that roughness-induced degradation of depth resolution is minimized. Further, it has been shown through XPS analysis that the air-formed oxide film has a duplex structure w26x, consisting of an outer Fe2O3 layer and an inner layer enriched with Cr2O3. Nickel in the metal is not oxidized and, therefore, is not present in the oxide film. Because of these features, the air-formed oxide film on sputter deposited stainless steel film could be an ideal standard to demonstrate the potential of RFGD-OES for depth profiling analysis of ultra-thin layers of several nanometers thickness. The original mirror-like appearance of the silicon wafer was retained after sputter deposition of the thin stainless steel film, suggesting that the flatness of the sputter deposited stainless steel surface is comparable with that of the silicon substrate as intended. AFM examination has shown, however, that the surface is not flat at the microscopic level as shown in Fig. 8a; the scanned area is 400=400 nm. Noting that the height scale is magnified considerably compared with the lateral scale, the surface generally is reasonably flat, but it comprises of a series of hills and valleys, varying in height by up to approximately 5 nm. Surface roughness of this magnitude is unavoid-
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Fig. 8. (a) AFM image of the surface of the stainless steel sputter deposited over the mirror-polished silicon wafer; (b) transmission electron micrograph of an ultramicrotomed section of the stainless steel film and its air-formed oxide film, showing the growth of amorphous oxide layer of uniform thickness of approximately 4.8-nm.
able, since it is related to the columnar structure of the sputter deposited film. Fig. 8b shows a transmission electron micrograph of an ultramicrotomed section of the stainless steel film and its air-formed oxide film. Here, the stainless steel film was deposited on an oxide coated, highly flat aluminium substrate, prepared by anodic oxidation of an electropolished aluminium, in order to allow
sectioning using an ultramicrotome and a diamond knife. The ultramicrotomed sections, of 5–10-nm thickness, were prepared in the usual manner and examined in a TECNAI F20 field-emission, high resolution transmission electron microscope operated at 200 kV. The stainless steel substrate is observed at the bottom of the micrograph. Crossed lattice fringes of spacing of 0.202 nm, correspond-
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Fig. 9. RF-GD-OES (a) and SIMS (b) depth profiles of the air-formed oxide film on the sputter deposited stainless steel film.
ing to the {110} planes of a-Fe, are observed. Over the slightly rounded stainless steel surface, due the columnar structure, the air-formed oxide film is observed clearly. The film is largely amorphous and the thickness, approximately 4.8 nm, is highly uniform. RF-GD-OES and SIMS depth profiles are shown in Fig. 9a and b, respectively. RF-GD-OES depth profiling was carried out at an Ar pressure of 608 Pa and a power of 40 W; the sampling time was 0.002 s. SIMS depth profiling was carried out at ULVAC-PHI Research Laboratory, Kawasaki, Japan, using a Dynamic SIMS PHI ADEPT 1010 instrument. A Csq primary ion beam, with an energy of 500 eV, current 50 nA and an incident angle of 60o, was employed. Further, the 133 Cs56Feq, 133Cs52Crq and 133Cs58Niq secondary ions were monitored throughout analysis in order to minimize the matrix effect w27x. The duplex nature of the air-formed oxide film, -5-nm thick, is revealed clearly in both profiles. Firstly, and concerning the RF-GD-OES depth profile, the signal intensity of iron increases immediately after the start of the analysis. However, the signal intensities of chromium and nickel increase only after a time delay. The delay time is longer for nickel than for chromium. The initial plateau
region observed in the iron profile, where neither chromium nor nickel is present, is associated with the outer Fe2O3 layer. The chromium profile exhibits a peak located at the metalyoxide interface; the peak is associated with the inner layer enriched with Cr2O3. Further, the half-height position of the leading edge of the nickel profile is located at the metalyoxide interface, determined independently from the iron profile, indicating that nickel is not present in the oxide film. A small peak, observed in the nickel profile beneath the metalyoxide interface, is associated with the enrichment of the unoxidized nickel in the metal beneath the oxide film. From the RF-GD-OES depth profile, along with XPS results, it appears that the oxide growth on sputter-deposited stainless film involves preferential oxidation of iron and chromium, with a duplex structure developed by the faster migration of Fe3q ions relative to Cr3q ions w25x. Turning to the SIMS depth profile, it is evident that the 133Cs56Feq, 133Cs52Crq and 133Cs58Niq profiles are very similar to the Fe, Cr and Ni distribution profiles of RF-GD-OES, except for the characteristic, anomalous transition in the signal intensity in the SIMS profile at the very early stage of sputtering. The close agreement between the RF-GD-OES and SIMS depth profiles confirms
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that RF-GD-OES is well-suited for depth profiling analysis of ultra-thin films, less than 10 nm. Further, and of importance, it is evident that the matrix effect has been reduced considerably through monitoring CsMq clusters, yielding a ‘quasi-genuine’ SIMS depth profile.
and conducting or non-conducting, and their interfaces, which all play key roles in modern surface technologies, ranging from electronics to corrosion protection of metals.
5. Conclusion
Thanks are due to Mr Y. Uchida of Horiba Company, Tokyo, Japan, for provision of time on a Jobin Yvon RF-5000 GD-OES instrument.
Successful depth profiling analysis of ultra-thin films, less than 10-nm thick, extends greatly the power of RF-GD-OES in the field of practical surface analysis. It is the only technique which allows quantitative depth profiling analysis of surface regions, both conducting and non-conducting, from the first nanometre to depths of several tens of microns. Considering the very high speed of analysis, excellent depth resolution and sensitivity, the relatively low cost of the instrument, and ready operation and maintenance of the instrument, without the requirements for highly experienced scientists, RF-GD-OES may be employed, for the first time in the history of surface depth profiling analysis, for quality control of various surface enhanced industrial products in real production lines. These unique features of RF-GD-OES, that are not seen with other traditional depth profiling techniques, such as AES and SIMS, arise from the nature of RF-GD sputtering where samples, both conducting and non-conducting, are sputtered very stably with Arq ions of low energies (-50 eV) and high current fluxes of the order of ;100 mA cmy2. Like other sputter depth profiling techniques, such as AES, XPS and SIMS, RF-GD-OES has limitations. The sputtering in the RF-GD plasma occurs in an isotropic manner over a 4 mm (or 2 mm) diameter sampling area. Consequently, surface imaging and micro-spot analysis are not possible. Further, chemical information is not obtained. Even with these limitations, the versatility, simplicity and accuracy, in terms of depth resolution, sensitivity and reproducibility, make RF-GD-OES an indispensable tool in the field of practical surface analysis. When the RF-GD-OES results are compared with information from other techniques, the outcome is an important understanding of surfaces, surface films, thin or thick
Acknowledgments
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