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Oxidation protection of copper surfaces using self-assembled monolayers of octadecanethiol
Applied Surface Science 252 (2005) 400–411 www.elsevier.com/locate/apsusc
Oxidation protection of copper surfaces using self-assembled monolayers of octadecanethiol David A. Hutt *, Changqing Liu Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK Received 20 October 2004; received in revised form 14 January 2005; accepted 16 January 2005 Available online 9 February 2005
Abstract Self-assembled monolayers (SAMs) of alkanethiols adsorbed onto clean surfaces of face centred cubic (fcc) metals have been studied extensively for their ability to control the chemical functionality of the surface and as a means of preventing the oxidation and corrosion of the substrate metal. However, in many cases it has been found that on reactive substrates such as copper, it is difficult to prepare SAMs without the incorporation of some oxygen into the structure. In this work, self-assembled monolayers of octadecanethiol (ODT) were formed on copper foil substrates using a series of etching treatments to remove the native oxide layer prior to deposition of the ODT coating from a modified solution. X-ray photoelectron spectroscopy was used to analyse the SAMs and showed that monolayers with no detectable oxygen content could be produced. The effect of exposing the samples to air at different temperatures was monitored to examine the rate of the oxidation process, which was found to vary strongly with temperature. Samples stored at room temperature were found to oxidise relatively quickly, while those kept in a refrigerator were slower. Storing samples in a freezer dramatically reduced the oxidation of the copper, such that samples kept for 10 weeks still did not show any clear evidence of oxygen incorporation. # 2005 Elsevier B.V. All rights reserved. Keywords: Self-assembled monolayers; Alkanethiol; Copper; Temperature; Oxidation; Octadecanethiol
1. Introduction Self-assembled monolayers (SAMs) of alkanethiols adsorbed onto surfaces of face centred cubic (fcc) metals such as gold, silver and copper have been studied extensively in recent years for their ability to produce surfaces with controlled chemical function* Corresponding author. Tel.: +44 1509 227658; fax: +44 1509 227648. E-mail address: [email protected] (D.A. Hutt).
ality that can be used for further applications such as cell growth and substrate patterning [1–12]. In addition, the potential of these materials to prevent the oxidation of reactive substrates such as copper, silver and nickel has been identified and a number of investigations of corrosion inhibition have also been conducted [13–26]. In many of these studies, the alkanethiol monolayers are found to produce effective barriers to the penetration of corrosive chemicals to the substrate metal surface and to limit the oxidation of the substrate metal. However, it is generally found
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.019
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that producing oxygen free SAMs on reactive metal substrates, such as copper and nickel, is problematic. The motivation for this study was the investigation of the application of these materials to the area of electronics manufacture where solder interconnection is widely used to form joints between components and copper contacts on the printed circuit board (PCB). In order to enable the molten solder to wet the copper surfaces, a flux is usually used that contains organic or inorganic acids that etch away oxide layers exposing active metal which can be efficiently wetted. It is desirable to reduce the amount of flux used in the soldering process as the corrosive nature of the residues can be detrimental to the long-term reliability of the product and the complete removal of flux is highly attractive, especially for applications such as optoelectronics where clean assembly methods are required [27]. In order to reduce the amount and activity (acidity) of the flux required in the soldering process, great efforts are made to reduce the oxidation of the copper components after manufacture, often through the use of preservative coatings of organic compounds such as benzotriazoles and imidazoles [28–30], or by coating with layers of noble metals (e.g. electroless nickel coated with immersion gold). These coatings allow the components and printed circuit boards to be stored in air for many weeks or months without significant oxide build-up, thereby maintaining their solderability for high yield manufacture. As part of an investigation of fluxless soldering techniques the application of self-assembled monolayers to the preservation of oxide free copper surfaces was conducted, using wetting balance solderability tests (reported elsewhere [31,32]) to determine the effectiveness of the preparation and storage techniques. The aim was to examine if SAM coatings could be formed on copper surfaces that had been etched to remove the native oxide layer and could then prevent the oxidation of the copper surface over a period of time, such that the material remained solderable without flux. Of particular interest was the ability of SAMs to form a barrier that was not too strong to prevent the action of the molten solder and high temperature from displacing it, thereby enabling a strong metallurgical joint to be created. In contrast to the many studies reported in the literature that used evaporated metal films and carefully cleaned glassware, one of the aims of this study was to determine
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the applicability of these techniques to an industrial environment where polycrystalline copper foils would be used and high levels of cleanliness would not be readily available at reasonable cost. In this work a method of forming SAMs of octadecanethiol (ODT) onto oxide free copper foil surfaces was developed and found to produce samples with no detectable oxygen content, in contrast to many other studies presented in the literature that indicated residual oxygen levels after thiol adsorption. Furthermore, in order to extend the time for which samples were solderable, a number of storage environments were tested that demonstrated that low temperature produced a significant reduction in the rate of oxidation of the underlying copper.
2. Experimental 2.1. Test coupon preparation The overall method for the deposition of the SAM coatings onto the copper surfaces was similar in principle to that used by others [26] and consisted of a number of etching treatments to remove the native oxide layer followed by exposure of the test piece to the thiol solution. For these experiments, copper test coupons 0.25 mm thick (Goodfellow, 99.9%) ranging in size from 20 mm 20 mm to 20 mm 50 mm were used. To begin with, the copper was etched in a commercially available aqueous microetch solution for around 5 min to create a uniform surface finish and to remove any surface contamination from the foil. After this, the sample was rinsed with water and a further etch in dilute hydrochloric acid (15 ml of 37% HCl in 250 ml water) for 10–15 min was used to remove the thin layer of oxide that formed after the microetch and rinsing treatments. Following the removal of the sample from the HCl solution, it was rinsed rapidly (approximately 3–5 s) in isopropanol (2-propanol, Fisher > 99.5% purity) and then immersed immediately into the SAM coating solution for around 60 min. The SAM coating solution consisted of 1-octadecanethiol (Aldrich, 98%, used as received) dissolved in isopropanol at a concentration of 5–10 mM with a further addition of 40 ml per litre of glacial acetic acid (Fluka, >99.8%). The acetic acid was included to act as a final etchant (similar to an organic flux) to any small quantities of oxide that
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formed during the isopropanol rinse. As mentioned above, one of the aims of these experiments was to investigate the applicability of these coatings to a large-scale manufacturing environment and therefore, no special precautions were taken to clean the glassware, which was either new or had simply been washed after earlier experiments. In addition, no degassing of the thiol solutions was undertaken. Following removal of the sample from the thiol solution, it was rinsed with isopropanol before drying with a cool air blower. Test pieces were then stored in air in a range of environments such as the open laboratory (20–25 8C, approximately 50%RH), a refrigerator (approximately 4 8C, 30%RH) or a freezer (approximately 30 8C). 2.2. Surface analysis using X-ray photoelectron spectroscopy (XPS) The XPS spectra were acquired using a Vacuum Generators ESCALAB machine. Each sample was mounted on a metal stub using conductive adhesive, so that no charge correction was required. All spectra were recorded using unmonochromatised Al Ka radiation from the X-ray source, with a 908 electron take-off angle to maximise the depth of material analysed. A survey scan was recorded first for each tested sample (A3 slit, 60 eV pass energy), followed by the collection of higher-resolution spectra of Cu 2p (A4 slit, 25 eV pass energy, 15 scans), S 2p (A4 slit, 40 eV pass energy, 50 scans), C 1s (A4 slit, 25 eV pass energy, 50 scans) and O 1s (A4 slit, 40 eV pass energy, 50 scans). The total collection time was limited to around 1 h to minimise X-ray damage [33,34]. In order to extract peak area and position information, the raw data for the high resolution spectra were fitted using XPS Peak version 4.1 software [35] without smoothing. Symmetrical Gaussian-Lorentzian (fixed: 30% Lorentzian for C 1s, 50% for Cu, O and S) curves were used and in some cases, given the noise level of the data, it was necessary to fix the widths and positions of some peaks to enable the software to find an optimum fit. These fits are included in the figures presented here as solid lines and should be used only as a guide to the eye for peak position and size identification. For relative changes in the quantities of elements with time, peak area ratios to Cu were used to avoid day-to-day variations in the
instrument performance and were based on the area under the fitted curves rather than the raw data.
3. Results 3.1. Surface analysis of freshly prepared SAMs of ODT on copper XPS was used to investigate the surface composition of SAM coated copper foil test pieces immediately after preparation. Fig. 1 shows XPS spectra recorded for samples left in the laboratory air for different periods of time. The 0 days labelled spectra in Fig. 1 represent the data obtained for a sample exposed to the laboratory ambient for around 30 min. It was found that using the experimental procedure described, that freshly prepared samples showed no evidence of O 1s peaks around 531 eV in the XPS spectrum (Fig. 1a) and that the C 1s peak at 285 0.1 eV (Fig. 1b) could be fitted by a single symmetrical curve, characteristic of a similar bonding environment such as that expected for the 18 C atoms of the ODT molecule [1]. The copper 2p region of the spectrum (Fig. 1c) showed only two single peaks with the 2p3/2 component located at 932.8 0.1 eV, which could be assigned to Cu(0) or Cu(I), and no additional features around 942 eV that are normally attributed to the presence of Cu(II) species. Unfortunately, the S 2p signal intensity was very low due to the inherently small percentage of S in the monolayer, the attenuation by the C chains and the limited spectrometer sensitivity, so that the spectra therefore had to be recorded using high sensitivity settings that prevented the resolution of the 2p1/2 and 2p3/2 components. Nevertheless, the S peak around 162.5 eV could still be identified, which was indicative of the S adsorbed onto the Cu surface as a thiolate [14,36,37]. The data did not indicate the presence of any oxidised S species, such as sulfonate at a binding energy of 167 eV, which was in agreement with the absence of O from the samples. 3.2. Effect of storage at room temperature ODT coated copper samples were stored in the laboratory in air at room temperature for different periods of time before they were analysed using XPS
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Fig. 1. XPS spectra of ODT SAMs on copper as a function of storage time in air at room temperature: (a) O 1s region, (b) C 1s region, (c) Cu 2p region, and (d) S 2p region. Data have been offset in the y direction for clarity; solid lines represent the backgrounds and curves used for fitting.
to determine the extent of any oxidation. Fig. 1 also shows the XPS data obtained for these samples. It is apparent that at room temperature over a period of 3 weeks that a significant amount of O was incorporated into the samples. After only 4 days a clearly observable O 1s peak formed, that could be fitted by a
single symmetrical curve centered at 530.2 eV, similar to that observed by Laibinis and Whitesides [14] for freshly prepared samples. With extended exposure to air at room temperature the O 1s peak grew in intensity and broadened to incorporate a second component at higher binding energy. Again, this is in agreement with
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the work of Laibinis and Whitesides, however, in contrast to their work for a similar air exposure time, this extra peak was only a small proportion of the overall peak size. Examination of the S 2p spectra over this time period did not show any significant changes in structure, despite the clear incorporation of O into the samples described above. No peaks indicative of S oxidation to sulfonate were observed, however, the noise level within the spectra makes it impossible to say absolutely, that no oxidised S species were present. In support of the limited amount of oxygen observed in the spectra, the Cu 2p spectral region did not show any significant changes in structure for samples stored up to 3 weeks in air and no additional features attributable to Cu(II) species could be identified. As the exposure time increased, the C 1s spectra did show some changes, the most notable of which was the overall shift to lower binding energy by 0.3–0.4 eV of the C 1s peaks for 1 week and 3 weeks air exposure. In addition, for the sample exposed to air for 3 weeks, an extra component at a slightly higher binding energy became apparent, that could perhaps be associated with the methylene group bonded to a S species that had oxidised to sulfonate. 3.3. Effect of storage at low temperature Fig. 2 shows XPS spectra obtained from samples stored in a refrigerator and freezer for different periods of time. For the samples stored in the refrigerator, there was a slower build-up of oxygen compared to samples kept at room temperature. In agreement with the data obtained for room temperature samples, the data could be fitted with two symmetrical peaks approximately 1.5 0.1 eV apart. However, for these samples, the relative size of the peaks was different with the higher binding energy component making a significant contribution to the overall peak envelope even at short exposure times. Within the limitations of the S data, there was no indication of any oxidised S species, even after 5 weeks exposure to air in the refrigerator. The C 1s peaks for 1 and 5 weeks air exposure remained symmetrical, but were both shifted to lower binding energy, similar to the behaviour displayed by the samples stored at room temperature.
In stark contrast to the samples stored in the laboratory and refrigerator, the samples stored in the freezer showed little evidence of an O peak in the O 1s region of the XPS spectrum, even after 10 weeks. This absence of oxidation was supported by the S 2p spectra which showed no evidence of sulfonate formation and there were no additional features in the Cu 2p spectra. The C 1s peak remained symmetrical and centred at the same binding energy as the freshly prepared sample for all air exposure times. Fig. 3 shows the Cu LMM Auger region of the survey scans obtained for these samples, which were not recorded at high resolution, but still show trends in the shape of the curves as a function of air exposure time. The spectra for the freshly prepared sample and the sample stored in the freezer for 10 weeks were very alike, supporting the XPS data that indicated no oxidation or changes in the sample condition upon storage in the freezer. Both samples displayed a strong peak around 568.5 0.5 eV indicative of Cu(0) or Cu(II) [18]. Based on the resolution of the spectrometer, it was not possible to identify which of these two oxidation states was contributing to the peak, however as there was no indication of Cu(II) in the Cu 2p XPS spectra and no oxygen species were detected, this peak is thought to be due to Cu(0). Of more importance in the Auger data for the freezer stored sample is the absence of features due to Cu(I), that are normally located around 2.4 eV above the Cu(0) peak [18], again, indicating that no oxidised Cu species were present. For samples that included oxygen as a result of exposure to air at room temperature for 3 weeks or in the refrigerator for 5 weeks, spectra were obtained that showed an additional feature at 571 0.5 eV, attributed to Cu(I). Fig. 4 summarises the XPS data using the peak area ratios of O/Cu, C/Cu and S/Cu for all samples. Peak areas were determined from the fitted curves rather than the raw data and ratios to Cu were used in order to remove instrument performance variations that may have occurred over the time scale of the experiments (care must be taken when comparing the ratios, as different instrument operating parameters were used for different elements; see Section 2). Both the C/Cu and S/Cu data showed little or no change (within errors) as the storage time increased independent of the storage environment. However, for the O/Cu data
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Fig. 2. XPS spectra for SAMs of ODT on copper after storage in air in a refrigerator and freezer: (a) O 1s region, (b) C 1s region, and (c) S 2p region. Data has been offset in the y direction for clarity, curves and backgrounds used to fit the data are included as solid lines.
an increase in the O/Cu ratio could be observed which, in agreement with the raw data presented above, was faster for the room temperature stored samples than the ones stored in the fridge. For samples stored in the freezer, no increase in the O/Cu ratio was observed during the 10 weeks of the experiments.
4. Discussion 4.1. Preparation of oxide free SAMs The results presented above indicate that using the sample preparation technique described here, self-
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Fig. 3. Cu LMM Auger region of the XPS survey spectrum for SAMs of ODT on copper exposed to air for different periods of time at different temperatures. Spectra have been offset in the y direction for clarity.
assembled monolayers of ODT can be prepared with no evidence of oxygen incorporation at the highly reactive copper substrate. This is in contrast to other studies presented in the literature, in which the monolayers were usually formed on freshly evaporated copper films or onto etched/reduced copper foils, where residual oxygen signals were often observable in XPS data. Similar to the methods described in this paper, a number of solution based pre-treatments to copper foils and single crystal substrates have been presented in the literature for SAM preparation. Feng et al. [26] used a series of chemical etches and rinses to remove the oxide layer prior to exposing the sample to the thiol solution. They focussed on nitric acid as the final etching step, followed by several rinses prior to immersing the sample in the thiol solution. Aramaki and co-workers [22–24] used an alternative approach to etching based on the electrochemical reduction of the copper to remove the oxide layer prior to thiol adsorption. In order to exclude oxygen during the preparation process, they conducted all treatments in a nitrogen inerted cell. The work presented here differs from these other studies in a number of ways: first, a
Fig. 4. XPS peak area ratios as a function of exposure time to air for samples stored at different temperatures: (a) O 1s/Cu 2p3/2, (b) S 2p/ Cu 2p3/2, and (c) C 1s/Cu 2p3/2. Peak areas are derived from fitted curves. XPS spectra were collected using different spectrometer conditions and therefore no comparison between figures should be made. Due to the noise level of the raw spectra and uncertainty in curve fitting, error limits to the ratios were estimated to be: C/Cu 5%, S/Cu 10% and O/Cu 10%.
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quantity of acetic acid was added to the thiol solution with the intention that this would act as an in situ etchant and remove any small amounts of oxidation prior to adsorption of the thiol head group. Further control experiments are required to demonstrate that the acetic acid plays a key role in removing oxygen from the copper surface, however samples prepared in solutions without this addition, did not perform as well in fluxless solderability tests. Isopropanol was used as a solvent in this work in contrast to most other studies that employ ethanol, as this was found to dissolve the ODT more readily and is a commonly used industrial solvent. Initially, it was thought that the acetic acid or the isopropanol solvent would be incorporated into the monolayer due to the interaction of the oxygen groups with the highly active copper surface, however the XPS spectral evidence did not indicate that this was the case. Ron et al. [38] investigated the effect of solvent on the adsorption of thiols onto copper surfaces with varying levels of oxidation and suggested that ethanol was a poor solvent for the formation of high quality monolayers as it was difficult to displace from the reactive copper with the thiol species. The branched isopropanol molecule used here is less likely to be incorporated into the SAM than a linear species due to steric hindrance. In addition to the changes in the thiol solution, the other steps in the process were also important. Based on fluxless solderability measurements, the rinse step between the HCl etch and the SAM solution was found to be significant: using an additional water rinse led to poor quality samples, presumably due to the rapid oxidation of the copper surface through reaction with the water. A rapid isopropanol rinse was found to be the most effective means to transfer between the aqueous HCl solution and the thiol solution. Of particular interest to the wider application of SAMs, was the observation that the use of bulk foil materials still enabled monolayers of good quality to be produced and that an extensive glassware cleaning regime was not necessary. 4.2. Oxidation of SAM coated copper Based on the XPS data, the oxidation of the samples in this study followed a similar mechanism to that observed by Laibinis and Whitesides [14] and by Feng et al. [26]. In their work, a single O 1s peak was
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observed immediately upon preparation of the SAM, which grew slightly in intensity and then incorporated a second peak at higher binding energy as oxidation proceeded. They attributed the initial O 1s peak to Cu2O, while the second peak was assigned to Cu(OH)2 species which was supported by examination of the Cu 2p region of the XPS spectrum that initially showed no features due to Cu(II), but became more evident as the exposure time to air increased. By using the Cu LMM Auger data, the presence of Cu(I) species at low air exposure was confirmed. In the present work, a similar development of the O 1s peak was observed, however the rate of growth of these peaks appeared to be slower: after 500 h (3 weeks) the major component was still the low binding energy one. This can be attributed partly to the absence of oxygen from the samples at the start of the aging process and to the longer chain molecules (ODT) used in this study. However, it is likely that the absence of oxygen from the surface at the beginning led to SAMs that were better ordered and with more consistent coverage, such that there were fewer pin holes through which oxygen and water molecules could diffuse to interact with the copper surface. The data presented above appears to indicate that the incorporation of oxygen into the samples only involves the oxidation of the copper substrate. Little evidence of sulfonate formation was observed during the timescales of the experiments as the S 2p XPS spectra only showed a single peak at a position characteristic of thiolate species. This seems unexpected, as in many other studies of thiols adsorbed on copper, the S head group was observed to oxidise to a sulfonate over similar time spans as those investigated here. As the S and O XPS spectra were recorded using the same spectrometer settings, it was possible to compare the O and S signals in order to make an estimate of the amount of O present, assuming that it was all located at the S/Cu interface. Allowance was made for the attenuation of the original O and S signal intensities by the overlaying C chains based on the electron escape depths determined by Laibinis et al. [39] and by applying the XPS atomic sensitivity factors of the two elements. A final ratio of O:S of 5 (1):1 was obtained for samples stored for 3 weeks in air at room temperature. These calculations showed that a substantial amount of oxygen was present at the interface after long air exposure times and in
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particular, sufficient oxygen atoms per S atom were available to form a complete monolayer of sulfonate species. The fact that sulfonate species were not observed as additional S 2p peaks in the XPS spectra indicates that the copper oxidises in preference to S at these relatively low oxygen exposures. As the C/Cu and S/Cu peak area ratios showed no obvious change throughout the air exposure periods it would appear that the SAM was still intact on the surface and bearing in mind the significant amount of oxygen present, it seems unlikely that patches of the surface became oxidised while others maintained a SAM coating. This data therefore appears to be consistent with the model proposed by Laibinis and Whitesides [14] of a continuous SAM layer on top of oxidised copper which, depending on the amount of oxidation, varies progressively from Cu(II) at the surface, to Cu(I) and then Cu(0) towards the bulk of the substrate. Only after longer air exposure times, when the Cu surface has become more substantially oxidised and the rate of oxidation is slowed due to the limited rate of diffusion into the bulk, does the thiolate to sulfonate reaction become significant. Further support for this model of copper oxidation below the SAM may come from the C 1s spectra presented here, which showed shifts to lower binding energy with extended oxidation time. These observations were very consistent for both room temperature and fridge exposed samples and were only noted for samples that had been oxidised for around 1 week or longer. There do not appear to be any other documented observations of this behaviour for SAMs on copper, however considering the low level of oxygen required to initiate this, in other studies this shift may already have taken place. A detailed investigation of the reason behind these shifts was beyond the scope of this work, however in other studies shifts have been observed for supported metal clusters and related to binding energy reference level changes due to the introduction of an insulating layer between overlayer and substrate that prevents rapid charge transfer during the photoemission process [40,41]. The presence of a continuous copper oxide film between the SAM and the copper substrate could generate a similar insulating layer that may account for the binding energy shifts observed here and would again be consistent with the model of Laibinis and Whitesides [14].
As described in the introduction, one of the motivations for this research was the development of SAM coatings as a means of preserving oxide free copper surfaces for fluxless soldering. In order to test the fluxless solderability of the materials, wetting balance tests were conducted, a procedure that is commonly applied in the electronics manufacturing industry to assess components before printed circuit board assembly [42–45]. The results of this work are reported elsewhere [31,32], but in brief, the experiment involves the immersion of the test piece into a bath of molten solder, while simultaneously monitoring the forces acting on it. In these experiments, a nitrogen atmosphere around the solder bath was used to prevent the rapid re-oxidation of the surfaces as they were heated. A close correlation between the fluxless solderability and the level of oxygen in the samples was observed: freshly prepared samples showed excellent solderability, but those that were kept at room temperature for 24–48 h were less wettable. Samples stored in the refrigerator could remain solderable for several days and those stored in the freezer for 10 weeks still retained their fluxless solderability, displaying rapid wetting and a high wetting force. It was interesting to note that the wetting balance test provided a very rapid and sensitive measure of the oxidation of the copper surface that enabled rapid screening of different sample preparation techniques and often indicated degradation of the samples before XPS showed a significant signal due to oxygen incorporation. 4.3. Effect of storage temperature on copper oxidation Of particular note from the experiments presented here, was the effect of temperature on the rate of oxidation of the SAMs/Cu substrate on exposure to air. Storing samples in a freezer prevented the copper from oxidising for many weeks. However, wetting balance tests revealed that on removal of the samples from the freezer and subsequent storage at room temperature, they became unsolderable at a rate similar to samples stored immediately in air. Initially, the influence of the humidity level difference between the storage environments was investigated by carrying out fluxless wetting balance tests of samples stored at room temperature in the laboratory (50%RH) and in a
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desiccator (20%RH). However, these tests showed no obvious differences between the samples [31,32]. The effect of temperature on the oxidation is thought to be due to a number of factors: first, the rate of reaction of the oxygen/water vapour with the copper is likely to be slowed as a result of the low temperature. Evidence for a reduction in the oxidation rate of bare copper upon storage in the freezer was observed during fluxless solderability tests of etched copper samples that were not SAM coated. These samples maintained fluxless solderability several days longer than if they were stored at room temperature [32], however the increase in solderability lifetime was not as dramatic as that observed for SAM coated samples. The main reason for the significant reduction in oxidation of the SAM coated samples stored in the freezer is likely to be the increased order within the ODT SAM at low temperature. This can be inferred from studies of the photo-oxidation of SAMs on gold and silver, where longer chain length has been shown to dramatically decrease the rate of photo-oxidation of the thiol head groups to sulfonates [46,47]. In these studies, the SAM was thought to provide a barrier to the penetration of the oxidising species to the S atoms and for chain lengths greater than 10 C atoms, a substantial reduction in the rate of photo-oxidation was observed, which was attributed to greater ordering of the chains through the maximisation of the van der Waals forces at room temperature. For SAMs on copper, several studies have investigated the effects of chain length on oxidation and corrosion protection. Jennings et al. [15] used impedance and capacitance measurements to investigate the rate of oxidation and corrosion of copper substrates as a function of the monolayer chain length and observed that for samples stored at room temperature in air, there was a significant slowing of the oxidation with increasing chain length. However, of particular note from their work was the observation that SAMs prepared from long chain molecules that included an ether linkage in the chain, did not display the same resistance behaviour as straight chain thiol molecules of the same length: rather, the coating resistance performance mirrored that of a shorter chain molecule with a similar bulk material melting point. The deduction from this was that the interactions that determine the melting point of a material play a significant role in determining the order and penetrability of the monolayer. This is in agreement with other work by Cooper and Leggett
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[48,49] that demonstrated that for SAMs composed of molecules of the same methylene chain length, much slower rates of photo-oxidation were observed for carboxylic acid terminated molecules compared to methyl terminated molecules. This was attributed to the relatively strong hydrogen bonding interaction at the tail group that increased the order in the underlying carbon chains thereby providing a more substantial barrier layer. In the present study, it is proposed that by reducing the temperature of the ODT SAM on Cu in a refrigerator or freezer, that the chains in the SAM can become more ordered due to the reduction in thermal energy capable of disrupting the van der Waals forces between them. With a more densely packed SAM, the penetration of oxygen/water vapour from the air to the Cu interface is restricted, such that there is a limited quantity of oxygen present at the interface and the rate of oxidation is therefore reduced. At present, insufficient data is available to determine accurately the rates of oxidation as a function of temperature, however by cooling in a freezer, a step change in the rate of oxidation appears to take place, that is, the oxidation process is almost completely terminated. This may indicate that a more significant change in the structure of the layer occurs at low temperatures, possibly through a twodimensional liquid to solid ‘‘crystallisation’’ or by ‘‘freezing’’ of the monolayer chain positions, similar to the glass transition seen in polymers and suggested previously by Cooper and Leggett [48]. A number of experimental and theoretical investigations of the structure of SAMs on gold surfaces have been carried out [e.g. 50–53] that have revealed a variety of phase transitions which are dependent on the chain length and temperature. These studies have identified alterations to the orientation and conformation of the chains over similar temperature ranges to those of interest in this work. Such structural changes could significantly influence the ability of a SAM to prevent the diffusion of oxygen/water vapour and subsequently lead to a sharp variation with temperature of the substrate oxidation rate, as observed here.
5. Conclusion This work has identified a method for depositing SAMs of octadecanethiol onto copper surfaces leading
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to freshly prepared materials with no detectable oxygen inclusion. This has been achieved using simple aqueous solution based etching treatments to remove the native copper oxide and through the use of a modified thiol solution that included a quantity of acetic acid to act as a final in situ etchant. The ability of these monolayers to prevent the oxidation of the underlying copper during storage in air at different temperatures was investigated and at low temperatures the rate of oxidation of the samples was found to decrease dramatically. Storage of test pieces in a freezer at 30 8C prevented any detectable oxidation of the copper even after 10 weeks. This was attributed to the greater ordering of the methylene chains at low temperature through the limited disruption of the van der Waals interactions resulting in a monolayer that presented a more substantial barrier to oxygen/water vapour penetration. The results indicate that the early stages of oxygen incorporation into the samples involve oxidation of the copper substrate in preference to the S head group and are consistent with a model proposed by Laibinis and Whitesides [14] that involves the SAM remaining on top of a copper surface that progressively oxidises. Further work needs to be conducted to measure the rate of oxidation in detail to provide a clearer understanding of the thermal stability of SAMs that may further widen the range of application of these useful materials.
Acknowledgments The authors thank Dr I. Mathieson for assistance in collecting the XPS data and the Engineering and Physical Sciences Research Council for financial support under grant no. GR/RO4157. The technical support of Shipley Europe Limited (now Rohm and Haas Electronic Materials Europe Limited), Celestica Limited and Nortel Networks is also gratefully acknowledged.
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D.A. Hutt, C. Liu / Applied Surface Science 252 (2005) 400–411 [29] N.R. Sorensen, F.M. Hosking, Advances in electronic packaging, in: Proceedings of the International Electronic Packaging Conference (InterPack), vol. 10, 1995, pp. 1101–1105. [30] R.L. Opila, H.W. Krautter, B.R. Zegarski, L.H. Dubois, G. Wenger, J. Electrochem. Soc. 142 (1995) 4074–4077. [31] C. Liu, D.A. Hutt, Circuit World 29 (2002) 19–23. [32] C. Liu, D.A. Hutt, Submitted to IEEE Trans. Components Packaging Technol. [33] A.J. Wagner, S.R. Carlo, C. Vecitis, D.H. Fairbrother, Langmuir 18 (2002) 1542–1549. [34] R.G. Nuzzo, L.H. Dubois, D.L. Allara, J. Am. Chem. Soc. 112 (1990) 558–569. [35] R. Kwok, Department of Chemistry, Chinese University of Hong Kong, XPSPEAK Version 4.1 software. [36] J. Huang, J.C. Hemminger, J. Am. Chem. Soc. 115 (1993) 3342–3343. [37] M.J. Tarlov, D.R.F. Burgess, G. Gillen, J. Am. Chem. Soc. 115 (1993) 5305–5306. [38] H. Ron, H. Cohen, S. Matlis, M. Rappaport, I. Rubinstein, J. Phys. Chem. B 102 (1998) 9861–9869. [39] P.E. Laibinis, C.D. Bain, G.M. Whitesides, J. Phys. Chem. 95 (1991) 7017–7021.
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Oxidation protection of copper surfaces using self-assembled monolayers of octadecanethiol David A. Hutt *, Changqing Liu Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire LE11 3TU, UK Received 20 October 2004; received in revised form 14 January 2005; accepted 16 January 2005 Available online 9 February 2005
Abstract Self-assembled monolayers (SAMs) of alkanethiols adsorbed onto clean surfaces of face centred cubic (fcc) metals have been studied extensively for their ability to control the chemical functionality of the surface and as a means of preventing the oxidation and corrosion of the substrate metal. However, in many cases it has been found that on reactive substrates such as copper, it is difficult to prepare SAMs without the incorporation of some oxygen into the structure. In this work, self-assembled monolayers of octadecanethiol (ODT) were formed on copper foil substrates using a series of etching treatments to remove the native oxide layer prior to deposition of the ODT coating from a modified solution. X-ray photoelectron spectroscopy was used to analyse the SAMs and showed that monolayers with no detectable oxygen content could be produced. The effect of exposing the samples to air at different temperatures was monitored to examine the rate of the oxidation process, which was found to vary strongly with temperature. Samples stored at room temperature were found to oxidise relatively quickly, while those kept in a refrigerator were slower. Storing samples in a freezer dramatically reduced the oxidation of the copper, such that samples kept for 10 weeks still did not show any clear evidence of oxygen incorporation. # 2005 Elsevier B.V. All rights reserved. Keywords: Self-assembled monolayers; Alkanethiol; Copper; Temperature; Oxidation; Octadecanethiol
1. Introduction Self-assembled monolayers (SAMs) of alkanethiols adsorbed onto surfaces of face centred cubic (fcc) metals such as gold, silver and copper have been studied extensively in recent years for their ability to produce surfaces with controlled chemical function* Corresponding author. Tel.: +44 1509 227658; fax: +44 1509 227648. E-mail address: [email protected] (D.A. Hutt).
ality that can be used for further applications such as cell growth and substrate patterning [1–12]. In addition, the potential of these materials to prevent the oxidation of reactive substrates such as copper, silver and nickel has been identified and a number of investigations of corrosion inhibition have also been conducted [13–26]. In many of these studies, the alkanethiol monolayers are found to produce effective barriers to the penetration of corrosive chemicals to the substrate metal surface and to limit the oxidation of the substrate metal. However, it is generally found
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.019
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that producing oxygen free SAMs on reactive metal substrates, such as copper and nickel, is problematic. The motivation for this study was the investigation of the application of these materials to the area of electronics manufacture where solder interconnection is widely used to form joints between components and copper contacts on the printed circuit board (PCB). In order to enable the molten solder to wet the copper surfaces, a flux is usually used that contains organic or inorganic acids that etch away oxide layers exposing active metal which can be efficiently wetted. It is desirable to reduce the amount of flux used in the soldering process as the corrosive nature of the residues can be detrimental to the long-term reliability of the product and the complete removal of flux is highly attractive, especially for applications such as optoelectronics where clean assembly methods are required [27]. In order to reduce the amount and activity (acidity) of the flux required in the soldering process, great efforts are made to reduce the oxidation of the copper components after manufacture, often through the use of preservative coatings of organic compounds such as benzotriazoles and imidazoles [28–30], or by coating with layers of noble metals (e.g. electroless nickel coated with immersion gold). These coatings allow the components and printed circuit boards to be stored in air for many weeks or months without significant oxide build-up, thereby maintaining their solderability for high yield manufacture. As part of an investigation of fluxless soldering techniques the application of self-assembled monolayers to the preservation of oxide free copper surfaces was conducted, using wetting balance solderability tests (reported elsewhere [31,32]) to determine the effectiveness of the preparation and storage techniques. The aim was to examine if SAM coatings could be formed on copper surfaces that had been etched to remove the native oxide layer and could then prevent the oxidation of the copper surface over a period of time, such that the material remained solderable without flux. Of particular interest was the ability of SAMs to form a barrier that was not too strong to prevent the action of the molten solder and high temperature from displacing it, thereby enabling a strong metallurgical joint to be created. In contrast to the many studies reported in the literature that used evaporated metal films and carefully cleaned glassware, one of the aims of this study was to determine
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the applicability of these techniques to an industrial environment where polycrystalline copper foils would be used and high levels of cleanliness would not be readily available at reasonable cost. In this work a method of forming SAMs of octadecanethiol (ODT) onto oxide free copper foil surfaces was developed and found to produce samples with no detectable oxygen content, in contrast to many other studies presented in the literature that indicated residual oxygen levels after thiol adsorption. Furthermore, in order to extend the time for which samples were solderable, a number of storage environments were tested that demonstrated that low temperature produced a significant reduction in the rate of oxidation of the underlying copper.
2. Experimental 2.1. Test coupon preparation The overall method for the deposition of the SAM coatings onto the copper surfaces was similar in principle to that used by others [26] and consisted of a number of etching treatments to remove the native oxide layer followed by exposure of the test piece to the thiol solution. For these experiments, copper test coupons 0.25 mm thick (Goodfellow, 99.9%) ranging in size from 20 mm 20 mm to 20 mm 50 mm were used. To begin with, the copper was etched in a commercially available aqueous microetch solution for around 5 min to create a uniform surface finish and to remove any surface contamination from the foil. After this, the sample was rinsed with water and a further etch in dilute hydrochloric acid (15 ml of 37% HCl in 250 ml water) for 10–15 min was used to remove the thin layer of oxide that formed after the microetch and rinsing treatments. Following the removal of the sample from the HCl solution, it was rinsed rapidly (approximately 3–5 s) in isopropanol (2-propanol, Fisher > 99.5% purity) and then immersed immediately into the SAM coating solution for around 60 min. The SAM coating solution consisted of 1-octadecanethiol (Aldrich, 98%, used as received) dissolved in isopropanol at a concentration of 5–10 mM with a further addition of 40 ml per litre of glacial acetic acid (Fluka, >99.8%). The acetic acid was included to act as a final etchant (similar to an organic flux) to any small quantities of oxide that
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formed during the isopropanol rinse. As mentioned above, one of the aims of these experiments was to investigate the applicability of these coatings to a large-scale manufacturing environment and therefore, no special precautions were taken to clean the glassware, which was either new or had simply been washed after earlier experiments. In addition, no degassing of the thiol solutions was undertaken. Following removal of the sample from the thiol solution, it was rinsed with isopropanol before drying with a cool air blower. Test pieces were then stored in air in a range of environments such as the open laboratory (20–25 8C, approximately 50%RH), a refrigerator (approximately 4 8C, 30%RH) or a freezer (approximately 30 8C). 2.2. Surface analysis using X-ray photoelectron spectroscopy (XPS) The XPS spectra were acquired using a Vacuum Generators ESCALAB machine. Each sample was mounted on a metal stub using conductive adhesive, so that no charge correction was required. All spectra were recorded using unmonochromatised Al Ka radiation from the X-ray source, with a 908 electron take-off angle to maximise the depth of material analysed. A survey scan was recorded first for each tested sample (A3 slit, 60 eV pass energy), followed by the collection of higher-resolution spectra of Cu 2p (A4 slit, 25 eV pass energy, 15 scans), S 2p (A4 slit, 40 eV pass energy, 50 scans), C 1s (A4 slit, 25 eV pass energy, 50 scans) and O 1s (A4 slit, 40 eV pass energy, 50 scans). The total collection time was limited to around 1 h to minimise X-ray damage [33,34]. In order to extract peak area and position information, the raw data for the high resolution spectra were fitted using XPS Peak version 4.1 software [35] without smoothing. Symmetrical Gaussian-Lorentzian (fixed: 30% Lorentzian for C 1s, 50% for Cu, O and S) curves were used and in some cases, given the noise level of the data, it was necessary to fix the widths and positions of some peaks to enable the software to find an optimum fit. These fits are included in the figures presented here as solid lines and should be used only as a guide to the eye for peak position and size identification. For relative changes in the quantities of elements with time, peak area ratios to Cu were used to avoid day-to-day variations in the
instrument performance and were based on the area under the fitted curves rather than the raw data.
3. Results 3.1. Surface analysis of freshly prepared SAMs of ODT on copper XPS was used to investigate the surface composition of SAM coated copper foil test pieces immediately after preparation. Fig. 1 shows XPS spectra recorded for samples left in the laboratory air for different periods of time. The 0 days labelled spectra in Fig. 1 represent the data obtained for a sample exposed to the laboratory ambient for around 30 min. It was found that using the experimental procedure described, that freshly prepared samples showed no evidence of O 1s peaks around 531 eV in the XPS spectrum (Fig. 1a) and that the C 1s peak at 285 0.1 eV (Fig. 1b) could be fitted by a single symmetrical curve, characteristic of a similar bonding environment such as that expected for the 18 C atoms of the ODT molecule [1]. The copper 2p region of the spectrum (Fig. 1c) showed only two single peaks with the 2p3/2 component located at 932.8 0.1 eV, which could be assigned to Cu(0) or Cu(I), and no additional features around 942 eV that are normally attributed to the presence of Cu(II) species. Unfortunately, the S 2p signal intensity was very low due to the inherently small percentage of S in the monolayer, the attenuation by the C chains and the limited spectrometer sensitivity, so that the spectra therefore had to be recorded using high sensitivity settings that prevented the resolution of the 2p1/2 and 2p3/2 components. Nevertheless, the S peak around 162.5 eV could still be identified, which was indicative of the S adsorbed onto the Cu surface as a thiolate [14,36,37]. The data did not indicate the presence of any oxidised S species, such as sulfonate at a binding energy of 167 eV, which was in agreement with the absence of O from the samples. 3.2. Effect of storage at room temperature ODT coated copper samples were stored in the laboratory in air at room temperature for different periods of time before they were analysed using XPS
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Fig. 1. XPS spectra of ODT SAMs on copper as a function of storage time in air at room temperature: (a) O 1s region, (b) C 1s region, (c) Cu 2p region, and (d) S 2p region. Data have been offset in the y direction for clarity; solid lines represent the backgrounds and curves used for fitting.
to determine the extent of any oxidation. Fig. 1 also shows the XPS data obtained for these samples. It is apparent that at room temperature over a period of 3 weeks that a significant amount of O was incorporated into the samples. After only 4 days a clearly observable O 1s peak formed, that could be fitted by a
single symmetrical curve centered at 530.2 eV, similar to that observed by Laibinis and Whitesides [14] for freshly prepared samples. With extended exposure to air at room temperature the O 1s peak grew in intensity and broadened to incorporate a second component at higher binding energy. Again, this is in agreement with
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the work of Laibinis and Whitesides, however, in contrast to their work for a similar air exposure time, this extra peak was only a small proportion of the overall peak size. Examination of the S 2p spectra over this time period did not show any significant changes in structure, despite the clear incorporation of O into the samples described above. No peaks indicative of S oxidation to sulfonate were observed, however, the noise level within the spectra makes it impossible to say absolutely, that no oxidised S species were present. In support of the limited amount of oxygen observed in the spectra, the Cu 2p spectral region did not show any significant changes in structure for samples stored up to 3 weeks in air and no additional features attributable to Cu(II) species could be identified. As the exposure time increased, the C 1s spectra did show some changes, the most notable of which was the overall shift to lower binding energy by 0.3–0.4 eV of the C 1s peaks for 1 week and 3 weeks air exposure. In addition, for the sample exposed to air for 3 weeks, an extra component at a slightly higher binding energy became apparent, that could perhaps be associated with the methylene group bonded to a S species that had oxidised to sulfonate. 3.3. Effect of storage at low temperature Fig. 2 shows XPS spectra obtained from samples stored in a refrigerator and freezer for different periods of time. For the samples stored in the refrigerator, there was a slower build-up of oxygen compared to samples kept at room temperature. In agreement with the data obtained for room temperature samples, the data could be fitted with two symmetrical peaks approximately 1.5 0.1 eV apart. However, for these samples, the relative size of the peaks was different with the higher binding energy component making a significant contribution to the overall peak envelope even at short exposure times. Within the limitations of the S data, there was no indication of any oxidised S species, even after 5 weeks exposure to air in the refrigerator. The C 1s peaks for 1 and 5 weeks air exposure remained symmetrical, but were both shifted to lower binding energy, similar to the behaviour displayed by the samples stored at room temperature.
In stark contrast to the samples stored in the laboratory and refrigerator, the samples stored in the freezer showed little evidence of an O peak in the O 1s region of the XPS spectrum, even after 10 weeks. This absence of oxidation was supported by the S 2p spectra which showed no evidence of sulfonate formation and there were no additional features in the Cu 2p spectra. The C 1s peak remained symmetrical and centred at the same binding energy as the freshly prepared sample for all air exposure times. Fig. 3 shows the Cu LMM Auger region of the survey scans obtained for these samples, which were not recorded at high resolution, but still show trends in the shape of the curves as a function of air exposure time. The spectra for the freshly prepared sample and the sample stored in the freezer for 10 weeks were very alike, supporting the XPS data that indicated no oxidation or changes in the sample condition upon storage in the freezer. Both samples displayed a strong peak around 568.5 0.5 eV indicative of Cu(0) or Cu(II) [18]. Based on the resolution of the spectrometer, it was not possible to identify which of these two oxidation states was contributing to the peak, however as there was no indication of Cu(II) in the Cu 2p XPS spectra and no oxygen species were detected, this peak is thought to be due to Cu(0). Of more importance in the Auger data for the freezer stored sample is the absence of features due to Cu(I), that are normally located around 2.4 eV above the Cu(0) peak [18], again, indicating that no oxidised Cu species were present. For samples that included oxygen as a result of exposure to air at room temperature for 3 weeks or in the refrigerator for 5 weeks, spectra were obtained that showed an additional feature at 571 0.5 eV, attributed to Cu(I). Fig. 4 summarises the XPS data using the peak area ratios of O/Cu, C/Cu and S/Cu for all samples. Peak areas were determined from the fitted curves rather than the raw data and ratios to Cu were used in order to remove instrument performance variations that may have occurred over the time scale of the experiments (care must be taken when comparing the ratios, as different instrument operating parameters were used for different elements; see Section 2). Both the C/Cu and S/Cu data showed little or no change (within errors) as the storage time increased independent of the storage environment. However, for the O/Cu data
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Fig. 2. XPS spectra for SAMs of ODT on copper after storage in air in a refrigerator and freezer: (a) O 1s region, (b) C 1s region, and (c) S 2p region. Data has been offset in the y direction for clarity, curves and backgrounds used to fit the data are included as solid lines.
an increase in the O/Cu ratio could be observed which, in agreement with the raw data presented above, was faster for the room temperature stored samples than the ones stored in the fridge. For samples stored in the freezer, no increase in the O/Cu ratio was observed during the 10 weeks of the experiments.
4. Discussion 4.1. Preparation of oxide free SAMs The results presented above indicate that using the sample preparation technique described here, self-
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Fig. 3. Cu LMM Auger region of the XPS survey spectrum for SAMs of ODT on copper exposed to air for different periods of time at different temperatures. Spectra have been offset in the y direction for clarity.
assembled monolayers of ODT can be prepared with no evidence of oxygen incorporation at the highly reactive copper substrate. This is in contrast to other studies presented in the literature, in which the monolayers were usually formed on freshly evaporated copper films or onto etched/reduced copper foils, where residual oxygen signals were often observable in XPS data. Similar to the methods described in this paper, a number of solution based pre-treatments to copper foils and single crystal substrates have been presented in the literature for SAM preparation. Feng et al. [26] used a series of chemical etches and rinses to remove the oxide layer prior to exposing the sample to the thiol solution. They focussed on nitric acid as the final etching step, followed by several rinses prior to immersing the sample in the thiol solution. Aramaki and co-workers [22–24] used an alternative approach to etching based on the electrochemical reduction of the copper to remove the oxide layer prior to thiol adsorption. In order to exclude oxygen during the preparation process, they conducted all treatments in a nitrogen inerted cell. The work presented here differs from these other studies in a number of ways: first, a
Fig. 4. XPS peak area ratios as a function of exposure time to air for samples stored at different temperatures: (a) O 1s/Cu 2p3/2, (b) S 2p/ Cu 2p3/2, and (c) C 1s/Cu 2p3/2. Peak areas are derived from fitted curves. XPS spectra were collected using different spectrometer conditions and therefore no comparison between figures should be made. Due to the noise level of the raw spectra and uncertainty in curve fitting, error limits to the ratios were estimated to be: C/Cu 5%, S/Cu 10% and O/Cu 10%.
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quantity of acetic acid was added to the thiol solution with the intention that this would act as an in situ etchant and remove any small amounts of oxidation prior to adsorption of the thiol head group. Further control experiments are required to demonstrate that the acetic acid plays a key role in removing oxygen from the copper surface, however samples prepared in solutions without this addition, did not perform as well in fluxless solderability tests. Isopropanol was used as a solvent in this work in contrast to most other studies that employ ethanol, as this was found to dissolve the ODT more readily and is a commonly used industrial solvent. Initially, it was thought that the acetic acid or the isopropanol solvent would be incorporated into the monolayer due to the interaction of the oxygen groups with the highly active copper surface, however the XPS spectral evidence did not indicate that this was the case. Ron et al. [38] investigated the effect of solvent on the adsorption of thiols onto copper surfaces with varying levels of oxidation and suggested that ethanol was a poor solvent for the formation of high quality monolayers as it was difficult to displace from the reactive copper with the thiol species. The branched isopropanol molecule used here is less likely to be incorporated into the SAM than a linear species due to steric hindrance. In addition to the changes in the thiol solution, the other steps in the process were also important. Based on fluxless solderability measurements, the rinse step between the HCl etch and the SAM solution was found to be significant: using an additional water rinse led to poor quality samples, presumably due to the rapid oxidation of the copper surface through reaction with the water. A rapid isopropanol rinse was found to be the most effective means to transfer between the aqueous HCl solution and the thiol solution. Of particular interest to the wider application of SAMs, was the observation that the use of bulk foil materials still enabled monolayers of good quality to be produced and that an extensive glassware cleaning regime was not necessary. 4.2. Oxidation of SAM coated copper Based on the XPS data, the oxidation of the samples in this study followed a similar mechanism to that observed by Laibinis and Whitesides [14] and by Feng et al. [26]. In their work, a single O 1s peak was
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observed immediately upon preparation of the SAM, which grew slightly in intensity and then incorporated a second peak at higher binding energy as oxidation proceeded. They attributed the initial O 1s peak to Cu2O, while the second peak was assigned to Cu(OH)2 species which was supported by examination of the Cu 2p region of the XPS spectrum that initially showed no features due to Cu(II), but became more evident as the exposure time to air increased. By using the Cu LMM Auger data, the presence of Cu(I) species at low air exposure was confirmed. In the present work, a similar development of the O 1s peak was observed, however the rate of growth of these peaks appeared to be slower: after 500 h (3 weeks) the major component was still the low binding energy one. This can be attributed partly to the absence of oxygen from the samples at the start of the aging process and to the longer chain molecules (ODT) used in this study. However, it is likely that the absence of oxygen from the surface at the beginning led to SAMs that were better ordered and with more consistent coverage, such that there were fewer pin holes through which oxygen and water molecules could diffuse to interact with the copper surface. The data presented above appears to indicate that the incorporation of oxygen into the samples only involves the oxidation of the copper substrate. Little evidence of sulfonate formation was observed during the timescales of the experiments as the S 2p XPS spectra only showed a single peak at a position characteristic of thiolate species. This seems unexpected, as in many other studies of thiols adsorbed on copper, the S head group was observed to oxidise to a sulfonate over similar time spans as those investigated here. As the S and O XPS spectra were recorded using the same spectrometer settings, it was possible to compare the O and S signals in order to make an estimate of the amount of O present, assuming that it was all located at the S/Cu interface. Allowance was made for the attenuation of the original O and S signal intensities by the overlaying C chains based on the electron escape depths determined by Laibinis et al. [39] and by applying the XPS atomic sensitivity factors of the two elements. A final ratio of O:S of 5 (1):1 was obtained for samples stored for 3 weeks in air at room temperature. These calculations showed that a substantial amount of oxygen was present at the interface after long air exposure times and in
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particular, sufficient oxygen atoms per S atom were available to form a complete monolayer of sulfonate species. The fact that sulfonate species were not observed as additional S 2p peaks in the XPS spectra indicates that the copper oxidises in preference to S at these relatively low oxygen exposures. As the C/Cu and S/Cu peak area ratios showed no obvious change throughout the air exposure periods it would appear that the SAM was still intact on the surface and bearing in mind the significant amount of oxygen present, it seems unlikely that patches of the surface became oxidised while others maintained a SAM coating. This data therefore appears to be consistent with the model proposed by Laibinis and Whitesides [14] of a continuous SAM layer on top of oxidised copper which, depending on the amount of oxidation, varies progressively from Cu(II) at the surface, to Cu(I) and then Cu(0) towards the bulk of the substrate. Only after longer air exposure times, when the Cu surface has become more substantially oxidised and the rate of oxidation is slowed due to the limited rate of diffusion into the bulk, does the thiolate to sulfonate reaction become significant. Further support for this model of copper oxidation below the SAM may come from the C 1s spectra presented here, which showed shifts to lower binding energy with extended oxidation time. These observations were very consistent for both room temperature and fridge exposed samples and were only noted for samples that had been oxidised for around 1 week or longer. There do not appear to be any other documented observations of this behaviour for SAMs on copper, however considering the low level of oxygen required to initiate this, in other studies this shift may already have taken place. A detailed investigation of the reason behind these shifts was beyond the scope of this work, however in other studies shifts have been observed for supported metal clusters and related to binding energy reference level changes due to the introduction of an insulating layer between overlayer and substrate that prevents rapid charge transfer during the photoemission process [40,41]. The presence of a continuous copper oxide film between the SAM and the copper substrate could generate a similar insulating layer that may account for the binding energy shifts observed here and would again be consistent with the model of Laibinis and Whitesides [14].
As described in the introduction, one of the motivations for this research was the development of SAM coatings as a means of preserving oxide free copper surfaces for fluxless soldering. In order to test the fluxless solderability of the materials, wetting balance tests were conducted, a procedure that is commonly applied in the electronics manufacturing industry to assess components before printed circuit board assembly [42–45]. The results of this work are reported elsewhere [31,32], but in brief, the experiment involves the immersion of the test piece into a bath of molten solder, while simultaneously monitoring the forces acting on it. In these experiments, a nitrogen atmosphere around the solder bath was used to prevent the rapid re-oxidation of the surfaces as they were heated. A close correlation between the fluxless solderability and the level of oxygen in the samples was observed: freshly prepared samples showed excellent solderability, but those that were kept at room temperature for 24–48 h were less wettable. Samples stored in the refrigerator could remain solderable for several days and those stored in the freezer for 10 weeks still retained their fluxless solderability, displaying rapid wetting and a high wetting force. It was interesting to note that the wetting balance test provided a very rapid and sensitive measure of the oxidation of the copper surface that enabled rapid screening of different sample preparation techniques and often indicated degradation of the samples before XPS showed a significant signal due to oxygen incorporation. 4.3. Effect of storage temperature on copper oxidation Of particular note from the experiments presented here, was the effect of temperature on the rate of oxidation of the SAMs/Cu substrate on exposure to air. Storing samples in a freezer prevented the copper from oxidising for many weeks. However, wetting balance tests revealed that on removal of the samples from the freezer and subsequent storage at room temperature, they became unsolderable at a rate similar to samples stored immediately in air. Initially, the influence of the humidity level difference between the storage environments was investigated by carrying out fluxless wetting balance tests of samples stored at room temperature in the laboratory (50%RH) and in a
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desiccator (20%RH). However, these tests showed no obvious differences between the samples [31,32]. The effect of temperature on the oxidation is thought to be due to a number of factors: first, the rate of reaction of the oxygen/water vapour with the copper is likely to be slowed as a result of the low temperature. Evidence for a reduction in the oxidation rate of bare copper upon storage in the freezer was observed during fluxless solderability tests of etched copper samples that were not SAM coated. These samples maintained fluxless solderability several days longer than if they were stored at room temperature [32], however the increase in solderability lifetime was not as dramatic as that observed for SAM coated samples. The main reason for the significant reduction in oxidation of the SAM coated samples stored in the freezer is likely to be the increased order within the ODT SAM at low temperature. This can be inferred from studies of the photo-oxidation of SAMs on gold and silver, where longer chain length has been shown to dramatically decrease the rate of photo-oxidation of the thiol head groups to sulfonates [46,47]. In these studies, the SAM was thought to provide a barrier to the penetration of the oxidising species to the S atoms and for chain lengths greater than 10 C atoms, a substantial reduction in the rate of photo-oxidation was observed, which was attributed to greater ordering of the chains through the maximisation of the van der Waals forces at room temperature. For SAMs on copper, several studies have investigated the effects of chain length on oxidation and corrosion protection. Jennings et al. [15] used impedance and capacitance measurements to investigate the rate of oxidation and corrosion of copper substrates as a function of the monolayer chain length and observed that for samples stored at room temperature in air, there was a significant slowing of the oxidation with increasing chain length. However, of particular note from their work was the observation that SAMs prepared from long chain molecules that included an ether linkage in the chain, did not display the same resistance behaviour as straight chain thiol molecules of the same length: rather, the coating resistance performance mirrored that of a shorter chain molecule with a similar bulk material melting point. The deduction from this was that the interactions that determine the melting point of a material play a significant role in determining the order and penetrability of the monolayer. This is in agreement with other work by Cooper and Leggett
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[48,49] that demonstrated that for SAMs composed of molecules of the same methylene chain length, much slower rates of photo-oxidation were observed for carboxylic acid terminated molecules compared to methyl terminated molecules. This was attributed to the relatively strong hydrogen bonding interaction at the tail group that increased the order in the underlying carbon chains thereby providing a more substantial barrier layer. In the present study, it is proposed that by reducing the temperature of the ODT SAM on Cu in a refrigerator or freezer, that the chains in the SAM can become more ordered due to the reduction in thermal energy capable of disrupting the van der Waals forces between them. With a more densely packed SAM, the penetration of oxygen/water vapour from the air to the Cu interface is restricted, such that there is a limited quantity of oxygen present at the interface and the rate of oxidation is therefore reduced. At present, insufficient data is available to determine accurately the rates of oxidation as a function of temperature, however by cooling in a freezer, a step change in the rate of oxidation appears to take place, that is, the oxidation process is almost completely terminated. This may indicate that a more significant change in the structure of the layer occurs at low temperatures, possibly through a twodimensional liquid to solid ‘‘crystallisation’’ or by ‘‘freezing’’ of the monolayer chain positions, similar to the glass transition seen in polymers and suggested previously by Cooper and Leggett [48]. A number of experimental and theoretical investigations of the structure of SAMs on gold surfaces have been carried out [e.g. 50–53] that have revealed a variety of phase transitions which are dependent on the chain length and temperature. These studies have identified alterations to the orientation and conformation of the chains over similar temperature ranges to those of interest in this work. Such structural changes could significantly influence the ability of a SAM to prevent the diffusion of oxygen/water vapour and subsequently lead to a sharp variation with temperature of the substrate oxidation rate, as observed here.
5. Conclusion This work has identified a method for depositing SAMs of octadecanethiol onto copper surfaces leading
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to freshly prepared materials with no detectable oxygen inclusion. This has been achieved using simple aqueous solution based etching treatments to remove the native copper oxide and through the use of a modified thiol solution that included a quantity of acetic acid to act as a final in situ etchant. The ability of these monolayers to prevent the oxidation of the underlying copper during storage in air at different temperatures was investigated and at low temperatures the rate of oxidation of the samples was found to decrease dramatically. Storage of test pieces in a freezer at 30 8C prevented any detectable oxidation of the copper even after 10 weeks. This was attributed to the greater ordering of the methylene chains at low temperature through the limited disruption of the van der Waals interactions resulting in a monolayer that presented a more substantial barrier to oxygen/water vapour penetration. The results indicate that the early stages of oxygen incorporation into the samples involve oxidation of the copper substrate in preference to the S head group and are consistent with a model proposed by Laibinis and Whitesides [14] that involves the SAM remaining on top of a copper surface that progressively oxidises. Further work needs to be conducted to measure the rate of oxidation in detail to provide a clearer understanding of the thermal stability of SAMs that may further widen the range of application of these useful materials.
Acknowledgments The authors thank Dr I. Mathieson for assistance in collecting the XPS data and the Engineering and Physical Sciences Research Council for financial support under grant no. GR/RO4157. The technical support of Shipley Europe Limited (now Rohm and Haas Electronic Materials Europe Limited), Celestica Limited and Nortel Networks is also gratefully acknowledged.
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