SIMS investigation of MoS2 based sputtercoatings

Applied Surface Science 179 (2001) 269±274

SIMS investigation of MoS2 based sputtercoatings C. Heinischa, K. Piplitsa, F. Kubelb, A. Schintlmeisterc, E. P¯uÈgerd, H. Huttera,* a

b

Institute of Analytical Chemistry, University of Technology, Vienna, Getreidemarkt 9/151, A-1060 Vienna, Austria Institute of Mineralogy, Crystallography and Structural Chemistry, University of Technology, Vienna, Vienna, Austria c Metallwerke Plansee GmbH., A-6000 Reutte, Austria d CSEM (Centre Suisse d'Electronique et de Microtechnique), CH-2007 NeuchaÃtel, Switzerland

Abstract Several multicomponent lubrication thin ®lms consisting of TiAlN and molybdenum disul®de (MoS2) were manufactured by magnetron cosputtering using different process parameters. The resulting thin ®lms were analyzed by SIMS depth pro®ling, electron probe micro analysis (EPMA) and X-ray diffraction. EPMA measurements are used to determine the concentration of major elements of the thin ®lm and to calculate relative sensitivity factors (RSFs) for SIMS quanti®cation of the depth pro®les. Whereas the concentration of titanium, aluminum and nitrogen nearly conforms the stoichiometric composition of TiAlN2, the concentration of molybdenum and sulfur are almost equal and do not comply the anticipated ratio of stoichiometric MoS2 in all analyzed lubrication ®lms. X-ray diffraction has been used to proof the amorphous nature of the ®lm but also shows a signi®cant difference in the crystalline structure between a pure TiAlN ®lm and layers with cosputtered MoS2. While the TiAlN/MoS2 cosputtered ®lms are completely amorphous the TiAlN layer includes cubic TiN in an otherwise amorphous ®lm. SIMS depth pro®ling indicates a constant amount of all analyzed elements through the ®lm except sulfur which decreases from the surface to the bottom of the layer. # 2001 Published by Elsevier Science B.V. PACS: 61.10.-i; 81.15.Cd; 81.40.Pq; 82.80.Ms Keywords: MoS2; TiAlN; SIMS

1. Introduction Friction and abrasion of objects with moving parts cause costs at a magnitude of 5% of the gross national product every year. Only the tribologically generated energy ``losses'' are estimated to occur a rate exceeding 1018 J/a in the US, which is approximately the annual energy output of 27 large electric power plants.

*

Corresponding author. Tel.: ‡43-1-58801-15120; fax: ‡43-1-58801-15199. E-mail address: [email protected] (H. Hutter).

Friction is a general phenomenon which converts kinetic energy into heat which arises at the contacting surfaces of components. The amount of heat depends on the normal force, the relative velocity of the components and the coef®cient of friction. To reduce the coef®cient of friction there are two possible methods. The ®rst and most frequently used method is lubrication by liquids (like oil) or solids (like graphite or molybdenum disul®de). The second method is to generate a suitable permanent coating of the components. Such an ideal solid lubricant-coating would have a very low friction coef®cient coupled with high wear resistance. It has thus been imagined to include

0169-4332/01/$ ± see front matter # 2001 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 2 9 4 - X

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clusters of a lubricating material dispersed within a hard matrix. Such a composite coating might provide both a wear resistant surface and some solid lubrication, with new pockets of lubricant being exposed as the ®lm wears down. Molybdenum disul®de (MoS2) is well established as a low friction coating in space industry as well as in vacuum technology. However, the lubrication performance of MoS2 is deteriorated rapidly if MoS2 coatings are used in wet atmosphere. This process destroys the Mo±S±S layer system by oxidation of the molybdenum layers (monoatomic Mo layer between two biatomic sulfur layers). The oxidation is strongly in¯uenced by humidity [1]. To allow the usage of MoS2 layers for other applications, this oxidation process has to be hindered or at least reduced. Using MoS2 in a hard matrix of TiAlN2 could hinder the oxidation process and improve the wear resistance of the material making it to an ideal solid lubricant. Secondary ion mass spectroscopy is a powerful tool to detect nearly all elements with a high analytical sensitivity. Additionally, this method provides easy sample preparation as well as the possibility of depth pro®ling. The sputter process due to the bombardment of the sample with (usually reactive) primary ions allows to determine the vertical distribution of every measured mass during sputter removal of the sample material from the surface. X-ray diffraction has proven to be a powerful method for analyzation of polycrystalline solid or powdered specimens. The sample is placed in a monoenergetic, collimated beam. Appropriately oriented grains diffract the X-rays or neutrons into a detector. By sweeping the angle of incidence and detection, a spectrum of diffraction peaks corresponding to the crystal lattice spacings is produced. The measured lattice spacings are compared with spacings of known

compounds to identify the crystalline phases and can also be used to re®ne the unit cell parameters. 2. Experimental details Four TiAlN/MoS2-coatings produced with different process parameters (see Table 1) on silicon substrates are prepared by magnetron co-deposition [2]. All compositions are determined by electron probe micro analysis (EPMA) which has been validated as reference method [3]. The thinnest TiAlN2-coating was determined with 1.25 mm by pro®lometer measurements (Dektak IIa). Therefore, all coatings provide suf®cient thickness for proper EPMA analysis. 2.1. EPMA-analysis The EPMA instrument used was an ARL SEMQ equipped with the Voyager II X-ray Quantitative Microanalysis 2100/2110 EDX system from Noran instruments. The detector was a SiLi detector with a Norvar window which allows analysis of light elements down to boron (accelerating voltage: 15 kV, takeoff angle: 52.58, scanned area: 100 mm  100 mm). Each investigated sample was analyzed three-fold on areas positioned at the sample diagonal and as the ®nal quantitative result the mean value of these measurements was calculated. One sample was de®ned as reference which was further used to establish a set of relative sensitivity factors (RSFs) for SIMS quanti®cation. 2.2. SIMS-analysis The SIMS instrument used throughout this investigation was an enhanced CAMECA IMS 3f [4,5]. An

Table 1 Process parameters used for manufacturing the samples Sample

Cr ion etch

Process parameters

A B C D

1 1 1 1

5 h, 5 h, 5 h, 5 h,

Stage Stage Stage Stage

2508C, TiAlN ˆ 8 kW  2 sources, bias ˆ 75 V; pressure control TiAlN ˆ 4 kW  2; MoS2 ˆ 2 kW  1, bias ˆ 75 V; pressure control TiAlN ˆ 4 kW  2; MoS2 ˆ 2 kW  1, bias ˆ 0 V; flow control (N2 and Ar) TiAlN ˆ 4 kW  2; MoS2 ˆ 3 kW  1, bias ˆ 0 V; flow control (N2 and Ar)

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additional primary magnet was installed to use a CAMECA Fine Focus Cs‡ ion source as well as a duoplasmatron source (for O2‡ ions). The secondary ion optics provide a one to one mapping from the sample surface to a channel plate mounted behind the secondary magnet allowing to record 2-D images. The resolution of the instrument utilized in our laboratory is about 2 mm lateral and 20 nm vertical. For all measurements a Cs‡ primary ion beam (primary energy: 5.5 kV, primary ion current: 100 nA) was applied to sputter the sample. Negative M secondary ions from a circular area of about 150 mm in diameter were accepted. 2.3. X-ray diffraction analysis The layers are analyzed by an Philips X'Pert diffraction system using Cu Ka. The analyzed angle (2Y) was 708 with a step range of 0.028. The X'Pert line software was used for identifying the substance by modeling. 3. Results and discussion 3.1. Film properties The properties of TiAlN ®lms also strongly depend on the exact ratio of Ti, Al and N. It was shown that best mechanical properties are obtained for a Ti to Al ratio between 1±1.2 [6,7]. Especially increasing Al contents lead to a rapid deterioration in mechanical stability whereas the high-temperature oxidation resistance increases with Al content [8]. For Ti to Al equals approximately 1, lower nitrogen content causes a decrease in hardness, whereas an increase of nitrogen content results in decreasing surface roughness or a more compacted surface coating [9]. For reactive sputtering processes, it was already shown, that a stoichiometric composition of TiAlN2 is obtained at a certain critical nitrogen ¯ow rate. An increase in ¯ow rate leads to no detectable changes in the chemical composition [10]. Titanium to aluminum ratios vary from 0.91 to 0.94. According to [8] this will lead to a worse mechanical stability. The ®lm thickness plays another important role in increasing the wear resistance. Increasing the MoS2

Fig. 1. REM picture of sample D. Film thickness of 2 mm.

input shifts the molybdenum to sulfur ratio closer to the stoichiometric composition and also increases the ®lm thickness. Samples A, B and C have a ®lm thickness of 1.25 mm, sample D has a thickness of 2 mm because of the higher MoS2 input. The results where con®rmed by SEM (see Fig. 1). Commercial available molybdenum disul®de contains approximately 10% oxygen. This explains the high oxygen content in the samples containing MoS2 while the sample without MoS2 does not contain oxygen (see Table 2). 3.2. SIMS measurement techniques The main disadvantage of SIMS analysis is the restricted accessibility of quanti®cation. One method is to compare SIMS results obtained from the sample with results from a sample with known similar composition. Relations between measured intensities and concentrations can be established using RSFs. The problem is the extreme variation of ionization probability, that depends strongly on the sample composition [11]. One of the most dif®cult elements for SIMS detection is nitrogen, whose sensitivity is low under cesium bombardment as well as under oxygen bombardment. A possible workaround is the detection of the corresponding nitrides instead, especially of CN or alternatively AlN , or CsN‡ (with Cs‡ ions from the primary beam) ions.

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Table 2 EPMA results, average of three measures Sample

Ti [at.%]

A B C D

24.57 17.02 15.29 10.22

   

0.16 0.22 0.15 0.18

Al [at.%]

Mo [at.%]

S [at.%]

N [at.%]

27.38 18.68 16.15 11.24

<0.5 15.99  0.78 15.19  0.78 15.60  0.74

<0.5 15.94  0.31 18.08  0.31 19.55  0.32

48.05 21.45 13.01 21.62

   

0.08 0.08 0.08 0.07

3.3. Unique signal Often, one SIMS signal origins from different molecular ions with the same mass (possible molecular ions for signal 28 for a system consisting of H, C, O, Al and Si are, e.g. 12 C16 O, 27 Al1 H or 28 Si). In general, larger molecular clusters are often not relevant according to less formation probability and therefore smaller signal intensity for this species. Suf®cient intensity means that count rates should exceed 100 counts per second Ð otherwise a very long counting time must be chosen to gain representative statistics. The ®rst requirement is met by respective unique strong signals for all main elements. If one signal intensity is mainly caused by a speci®c molecule with one limiting element, a direct relationship between measured intensities and concentrations of the limiting element can be established.

   

O [at.%] 0.16 3.24 3.41 3.25

<0.5 10.35  0.33 22.28  0.33 21.77  0.54

current also in¯uences intensity ratios and consequently RSFs. For this reason, the primary ion current was manually altered and obtained information about secondary ion intensities. As assumed, variations of the primary beam does not in¯uence secondary ion ratios. However, decreasing the primary ion current below 20 nA, secondary ion intensity ratios and therefore RSFs are affected.

3.4. SIMS standard Electron probe microanalysis (EPMA) can be used as reference quanti®cation method for this system. Homogeneity of TiAlN-coatings is not con®rmed by SIMS depth pro®le (Fig. 2a and b). Especially sulfur is decreasing from the surface to the bottom of the layer. Titanium, aluminum and molybdenum are enriched at the interface. So an ``average'' count rate is calculated to determine the quantity of the elements. 3.5. Measurement stability Especially one measurement parameter, i.e. the primary ion current is known to vary slightly during SIMS long-time measurements. Lower primary ion current results in lower secondary ion intensities. Hence, it was investigated if a varying primary ion

Fig. 2. (a) SIMS-depth pro®le of sample C. Sulfur decreases from the surface to the bottom of the layer; (b) SIMS-depth pro®le of sample D. Higher MoS2 input results in an increase of ®lm thickness.

C. Heinisch et al. / Applied Surface Science 179 (2001) 269±274

3.6. Matrix effects One conspicuous disadvantage of SIMS analysis is the often perceived strong matrix dependency of measured signals. A variation of one main element means a variation of the matrix, that can obviously in¯uence all other signal intensities and hence prohibits direct comparison of two samples with different matrices. Matrix effects express themselves by a nonlinear relationship between secondary ion intensity ratios and the respective concentration ratios. The none matrix-dependent relationship for our system is de®ned by the RSF. In practice, concentrations of elements in the samples are accessible by the approved EPMA method. Within the observed concentration ranges, linear relationships between count rates and concentrations for the elements titanium, aluminum, molybdenum, nitrogen and oxygen were found (see Fig. 3). The constant RSF show matrix independence and therefore the capability of SIMS to be applied as quanti®cation method for this system.

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present, concentrations for all other samples could be calculated. As a signi®cant amount of carbon was measured this method does not provide adequate results. A second possibility is to calculate the titanium content using a linear ®t and use the RSFs to calculate the concentration of the rest of the elements. As titanium can be calculated with low errors in this concentration range this method provides satisfying results. To compare SIMS results, all investigated TiAlN/ MoS2 samples are quanti®ed with the approved EPMA method. As expected, increasing difference between the reference and sample cause decreasing accuracy of SIMS analysis. Obvious errors for nitrogen, molybdenum and sulfur up to 20 relative percent for

3.7. Quanti®cation At least, SIMS quanti®cation is carried out to determine whether SIMS can be applied for the analysis of such systems. To reduce measurement condition in¯uences, SIMS quanti®cation is carried out analyzing a reference sample (sample B) together with an unknown sample. This reference was selected to establish a set of RSFs. With those set of RSFs and the assumption that no obvious trace elements are

Fig. 3. Linear correlation between counts and quantity proofs matrix independence of the RSFs.

Fig. 4. (a) X-ray analysis of the samples B, C, D and a Si[1 1 1] reference; (b) identi®cation of TiN(1 x) by modeling.

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samples with different oxygen contents are expected. Nevertheless, the absolute deviation is approximately only 3 at.% in each case. However, more accurate results are achieved quantifying the elements titanium and aluminum. The relative deviation is less than 5%, which means an absolute deviation of less than 1 at.%. 3.8. X-ray diffraction Sample A, which should be used as a reference for the other samples show additional peaks in the X-ray diffraction diagram (see Fig. 4a). The substance identi®ed by calculation in correlation with the quantitative EPMA results was cubic TiN with lattice distance Ê . As pure TiN is known to have a lattice of 4.1889 A Ê the crystalline substance is distance of 4.2380 A understoichiometric TiN(1 x). Sample B, C and D do not have a crystalline fraction. The crystal structure of sample A formed during manufacturing. The manufacturing parameters defer from the other samples in the temperature used. The 2508C temperature is responsible for the crystallization process (see Table 1). Peak broadening shows the formation of understoichiometric cubic TiN microcrystals in the TiAlN layer. 4. Conclusion  SIMS is capable for the quantification of the main elements in TiAlN/MoS2 cosputtered layer with an accuracy of 3 at.%.  X-ray diffraction shows that the whole system is Xray amorphous except the sample without cosputtered MoS2. In this sample, a crystallization process was initiated by higher temperature used during sputter deposition and understoichiometric cubic TiN have formed.

 Friction coefficients of 0.2 and less have been measured for these layers. It was assumed that friction properties of the material were based on crystalline MoS2. This assumption was wrong.  Commercial available molybdenum disulfide contains approximately 10% oxygen. This explains the high oxygen content in the samples containing MoS2. Molybdenum oxide also has friction properties. The friction process is not fully understood yet and further investigations have to be done.

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