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On the surface analysis of copper oxides: the difficulty in detecting Cu3O2
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Vacuum 77 (2004) 27–35 www.elsevier.com/locate/vacuum
On the surface analysis of copper oxides: the difficulty in detecting Cu3O2 D.E. Mencera, M.A. Hossainb, R. Schennachb, T. Gradyc, H. McWhinneyc, J.A.G. Gomesb, M. Kesmezb, J.R. Pargad, T.L. Barre, D.L. Cockeb, a Department of Chemistry, Wilkes University, Wilkes-Barre, PA 18776, USA Gill Chair of Chemistry and Chemical Engineering, Lamar University, Beaumont, P.O. Box 10022, TX 77710, USA c Department of Chemistry, Prairie View A&M University, Prairie View, TX 77446, USA d Institute Technology of Saltillo, Department of Metallurgy and Materials Science, V. Carranza 2400, Saltillo Coah. Mexico, C.P. 25000 e Department of Materials, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA b
Abstract The existence of Cu3O2, a gross defect structure of Cu2O, has been documented experimentally since the early 1960s. However, discussions of the oxidation of copper often neglect the importance of this phase; in fact, it is often omitted entirely from such discussions. This results from the difficulty in determining the chemical state during sputter depth profiling and relying on techniques that have difficulty providing chemical state information. The occurrence of sputter reduction during the depth profiling of copper oxide layers has been demonstrated with XPS depth profiles on a series of copper samples oxidized, for varying lengths of time, in air at a temperature of either 423 or 523 K. Under these conditions, a thin layer of CuO/Cu(OH)2 terminates the oxide layers. Beneath this layer, the presence of Cu3O2 is expected on the samples prepared at 423 K. However, immediately upon the beginning of sputtering, only Cu1+ is detected in the oxide layers. A zone of constant Cu:O ratio of (approximately 1.5) is found throughout most of the oxide layer even though Cu2+ is not detected. On the samples prepared at 523 K, the presence of CuO is anticipated. However, Cu2+ is not detected after sputtering is initiated and a region of constant Cu:O ratio of ca. 1.5 is detected. The inherent difficulties involved in investigating oxide layer growth and vertical oxide layer structure using sputter depth profiling are discussed in light of this experimental data. r 2004 Elsevier Ltd. All rights reserved. Keywords: Sputter reduction; XPS; Copper; Cu3O2; Oxidation
Corresponding author. Tel.: +1-409-880-1862; fax: +1-409-880-8374.
E-mail address: [email protected] (D.L. Cocke). 0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.068
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1. Introduction The presence of Cu3O2 during the growth of oxide layers on copper has been documented since the 1960s. Previous experiments [1–7] focused on the spectroscopic and structural properties of these oxide films through the use of UV–Vis–NIR Diffuse Reflectance [1], photoluminescence [2], and studies employing X-ray diffraction (XRD) [2,3] that indicate that Cu3O2 is a gross defect structure of Cu2O. There is a very small difference in the XRD of these two phases that results from a 1% expansion in the lattice of Cu3O2 when compared to Cu2O [2,3]. IR Reflectance measurements [4] support this crystallographic data. In addition, Cu3O2 exhibits an optical band gap greater than that found in Cu2O [5]. Excitation spectra [2] indicate the presence of oxygen vacancies, which are also consistent with the interpretation of the luminescence data, mentioned previously. Cocke et al. [8] performed electrochemical study of copper oxides using Linear Sweep Voltammetry (LSV) and suggested that the formation of Cu3O2 is linked to the three zones important to the oxide growth: the metal/oxide interface, the bulk zone where Cu3O2 is located and the oxide/fluid interface. Sun [9] did VLEED numerical quantification of the Cu3O2 bondforming kinetics on the Cu (0 0 1) surface and revealed that annealing disrupts bond formation in Cu3O2. In an attempt to investigate the formation of CuO at the surface of growing oxide layers on copper at 473 and 573 K, Lenglett et al. [6] employed the electron microprobe, XRD, spectroscopy (UV–Vis–NIR), and X-ray Photoelectron Spectroscopy (XPS). That study confirmed the presence of the Cu3O2 at both oxidation temperatures, and CuO was only present in significant quantities at the terminal surface of the oxides at these temperatures. Additional linear sweep voltammetry experiments by these researchers [7] have also supported the presence of this phase. These authors also point out that the magnetic properties of copper oxide films described by O’Keefe and Stone [10] can also be explained by the formation of Cu3O2. Schennach and Gupper [11] used Raman Spectroscopy to study copper
oxidation in air and found the formation of Cu2O between 343 and 403 K, formation of Cu3O2 between 403 and 493 K, formation of CuO and decrease of Cu3O2 between 493 and 523 K and formation of CuO at temperatures higher than 533 K. In fact, the growth of stable layers of Cu3O2 at temperatures of 423 K has been reported [5,12,13]. This paper addresses several issues. The first is the presence of discrepancies between the results of depth profiling surface analysis studies and the other work described above. This seems to be the result of the phenomenon of sputter reduction and results in apparent inability of XPS and depth profiling to detect Cu3O2. Other issues include the effect of underlying substrate on the oxide growth and the decomposition of oxide due to the low activity of oxygen at the metal/ oxide interface.
2. Experimental Copper discs (1 mm thick, by 1 cm diameter) were cut from oxygen free high conductivity (OFHC, minimum purity 99.95%) copper rod. The discs were then wet polished in alternating directions with 220, 320, 400, 600 and 1500 grit silicon carbide sandpaper. The polished discs were rinsed with water (Aldrich 99.5+%, A.C.S., reagent), isopropyl alcohol (LabChem Inc. 70% v/v Lot No 3126-6), and water again, followed by daubing dry on laboratory wipes. The copper samples were never touched with bare hands and oxidation was accomplished by placing the samples on a preheated iron block in a laboratory oven at 423 or 523 K in air for various lengths of time. XPS data was collected using a Perkin Elmer PHI 5600 ci X-ray Photoelectron Spectrometer. System background pressure was approximately 109 mbar range. Mg Ka (1253.6 eV) radiation generated using a standard dual anode source operating at 15 kV and an emission current of 23 mA was used in all experiments. The signal from adventitious carbon (284.6 eV) was used for calibrating the XPS data. XPS data were collected in the sequence C 1s, O 1s, O KVV, Cu 2p, and Cu LMM. XPS depth profiling was carried out with He and Ne (for thicker oxide layers) ion
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Table 1 Sputtering conditions for copper discs heated in air Temperature (K) Ambient/Pristine 423 523
Oxidation time (min)
Gas
Sputtering interval (min)
Total sputtering time (min)
30 300 05 60
He He He He Ne
0.1 0.2 0.3 1.0 1.5
0.1 1.6 3.9 11.0 13.5
sputtering. A He or Ne gas pressure of (10–15) 103 mP (0.01–0.015 mbar) at 25 mA (emission) and an acceleration voltage of 4 kV were employed. The analyzed spot size and the sputtered area were 0.4 mm2 and ca. 1 mm2, respectively. Sputtering conditions are given in Table 1. Surface concentrations were determined with Perkin Elmer software using peak heights and relative sensitivity factors to give surface concentrations in atomic percent. Routine deconvolutions were performed when necessary using the Perkin Elmer software. Since all the samples were prepared and handled in air prior to thermal oxidation, each was covered by native oxide formed on the freshly polished metal surface. Since this native oxide layer could influence oxide film growth, the native oxide layer was analyzed as a reference to the thermally oxidized samples.
3. Results and discussion The copper oxide films grown in this study are terminated with a thin layer of CuO/Cu(OH)2. Fig. 1a shows the changes observed in the Cu 2p region during the depth profile of a copper oxide film grown at 423 K for 30 min. Curve i shows the unsputtered surface and the spectra are indicative of CuO/Cu(OH)2. This is evidenced by the presence of the telltale shake-up lines (940.0 and 943.2 eV) on the Cu 2p peaks and the hydroxide peak (530.4 eV) in the O 1s region, and is in agreement with the work of Barr [14] and others [15]. The relative size of these shake-up peaks relative to the 2p peaks, along with the peak position of the Cu 2p3/2 (933.3 eV) [16], indicates that the upper surface of the oxide layer is
composed primarily of Cu2+ species. After the first cycle of He sputtering, the Cu2+ is no longer detected (see Fig. 1a curve ii). This finding is inconsistent with the prior studies [1–7,10,12,13] that find Cu3O2 in copper oxide layers grown under these conditions. The removal of the Cu(OH)2 along with the adventitious carbon is expected. However, no indication of Cu2+ remains (the absence of the shake-up features mentioned above and of a peak at the binding energy corresponding to Cu2+ in the 2p region). However, the presence of Cu1+ is confirmed by the position of the X-ray induced LMM Auger (336.4 eV KE, after the 1st sputtering) [17,18], which allows one to differentiate between Cu1+ and Cu0 which exhibit essentially the same Cu 2p peak positions. In Fig. 1a curve iii the copper 2p region is shown after the 6th sputtering cycle. This is the first cycle during which the characteristic features of zero valent copper are observed in the LMM Auger region. The metallic copper is detected through the remainder of the Cu1+ oxide and no Cu2+ is detected. Sputtering was continued until the O 1s signal disappeared at which time only copper metal was detected. Fig. 1b shows the Cu 2p region for the depth profiles of a copper oxide film grown at 423 K for 5 h. Again the unsputtered surface is composed of CuO/Cu(OH)2 (curve i). However, after the first He sputtering cycle (see curve ii), the oxide layer again contains only Cu1+. In Fig. 1b curve iii the copper 2p region is shown after the 7th sputtering cycle. This is the first cycle during which the characteristic features of zero valent copper are observed in the LMM Auger region. During the depth profiles of the samples prepared at 423 K, a region of constant Cu:O ratio (1.5)
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Fig. 1. Cu 2p region during the depth profile of a copper oxide film grown at (a) 423 K for 30 min and (b) 423 K for 5 h. In each set of spectra the curves are normalized and offset for ease of comparison and the spectra are (i) the unsputtered surface, (ii) after the first sputtering cycle, (iii) first cycle in which metallic copper is observed.
corresponding to the formula of Cu3O2 is found. Fig. 2 shows the Cu:O ratio of the two samples as a function of sputtering time (depth). From these plots it is clear that there is a region of essentially constant stoichiometric oxide phase corresponding to a formula of Cu3O2 after the first two sputtering cycles. The region of constant Cu:O ratio of about (1.7) extends for a greater depth for the sample oxidized for 5 h (curve b) than the one grown for only 30 min (curve a), since this is a thicker oxide layer. It is also worth mentioning that the appearance of Cu0, as indicated by the change in
the peak position and peak profile shape of the Cu LMM, does not occur until the end of this region of constant composition. Fig. 3 shows the changes observed in the Cu 2p region during the depth profile of a copper oxide film grown at 523 K for (a) 5 min (b) for 1 h. These oxide films are also terminated by a layer composed of Cu2+ species. This is again demonstrated by the presence of, and relative size of, the shake-up lines on the Cu 2p peaks. This outer layer is composed of CuO/Cu(OH)2 as indicated by the hydroxide peak (529.4 eV BE) in the O 1s region.
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10.00
30 min
8.00
5 hr
Cu/O
6.00
4.00
2.00
0.00 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Sputter Time (min)
Fig. 2. Cu:O ratios during the depth profiles on the copper oxide layers prepared at 423 K (a) 30 min growth time and (b) 5 h growth time.
Immediately upon Ne ion sputtering, the Cu2+ is no longer detected, as shown in curves ii in Fig. 3a and b. This is demonstrated clearly by the disappearance of the Cu2+ features described above and the appearance of Cu1+ again confirmed by the position of the X-ray induced LMM Auger (335.7 eV KE). For the remainder of the depth profile, no Cu2+ species are detected. In curves iii in Fig. 3a and b, the copper 2p region is shown after the 8th and 5th sputtering cycles, respectively. This is the first cycle during which the characteristic features of zero valent copper are observed in the LMM Auger region. Fig. 4 shows the Cu:O ratio of the two samples as a function of sputtering time (depth). The region of constant Cu:O ratio of about (1.8) extends for a greater depth for the sample oxidized for 1 h (curve b) than the one grown for only 5 min (curve a), since this is a thicker oxide layer. A constant Cu:O ratio of about (1.5) is expected for Cu3O2. This exact Cu:O ratio of 1.5 is not observed due to the phenomenon of sputter reduction. Several models of the formation of this phase during the growth of copper oxide films have been proposed. Most agree that the first step in oxide
growth is the formation of a chemisorbed oxygen layer followed by the creation of a non-stoichiometric precusor oxide [19]. Nucleation and growth of Cu2O islands that coalesce into a contiguous layer and thicken [20–23] is then followed by the formation of CuO at the gas-oxide interface [19,24–27]. Once this thin CuO surface layer has formed, the Cu2O is converted into a defect structure by the incorporation of Cu2+ and oxide ion (O2) vacancies to create Cu3O2. The presence of this phase has been well established [1–7,10,12,13] and yet it is not detected using surface analysis techniques. An earlier depth profiling study [28] of the oxide layers formed on copper indicated that CuO did not appear to form in air at 573 K (the conditions expected based on the Cu/O phase diagram and the studies discussed above). In this study, Bubert et al. [28] considered sputter reduction as a possible explanation, but instead used Rutherford Backscattering (RBS) and chemical analysis to argue that Cu2O was indeed the major species grown on copper in air at 573 K. In a follow-up study, Bubert et al [29] used depth profiling XPS, RBS, and glow discharge optical emission spectroscopy (GDOES) to study copper oxide film growth
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Fig. 3. Cu 2p region during the depth profile of a copper oxide film grown at (a) 523 K for 5 min and (b) 523 K for 1 h. In each set of spectra the curves are normalized and offset for ease of comparison and the spectra are (i) the unsputtered surface, (ii) after the first sputtering cycle, (iii) first cycle in which metallic copper is observed.
on copper and copper films on silicon. However, the factor analysis treatment of the XPS data for the samples grown on copper seems to rely on spectra in which mixtures of CuO and Cu2O are found. The XPS and RBS data shown for the samples grown on copper metal (at both 473 and 573 K in air and 10% O2 in Ar) seem to clearly indicate the presence of layered oxide structures terminated in CuO/Cu(OH)2. On the other hand, the samples grown on the thin layers (70 nm) of copper on silicon at 473 and 673 K clearly show that CuO is the primary species detected in the oxide layers. This seems to indicate that the
oxidation process of copper is modified when isolated from an underlying layer of bulk copper. This is consistent with earlier work by Mencer et al. [30] that demonstrated titanium films deposited on copper- and gold-coated copper displayed differences in the chemical species present during both vacuum annealing and low pressure oxidation. The presence of an intervening layer of gold made the titanium more active toward oxidation. The authors attributed the results in terms of the role of oxygen absorbing into the bulk of the underlying copper, and the role of gold as a barrier to the diffusion of oxygen
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10.00 523 K, 5 min
523 K, 1 hr
8.00 Ne+ ion sputtering
b
Cu/O
6.00
He+ ion sputtering 4.00
a
2.00
0.00 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Sputter Time (min)
Fig. 4. Cu:O ratios during the depth profiles on the copper oxide layers prepared at 523 K (a) 5 min growth time and (b) 1 h growth time.
into the copper. Other studies have also demonstrated the facile nature of copper oxides on the surface of copper metal [18]. Sputter reduction is quite common in oxides [26,30–35]. Therefore, one must question whether the copper to oxygen ratio is from the thermal oxidation process or from the altered oxide caused by the preferential sputtering of oxygen. This question is perhaps best discussed in context of the reverse chemical reactions model proposed by Vasilyev and coworkers [36–40]. This model takes into account the chemical activity of the elements in question during the ion induced altered layer formation. Normally during sputtering, the impacting ions energy is greater than the energy of the chemical bonds present before alteration. The model predicts substantial destruction of chemical bonds and the formation of decomposition species that may remain or react further with each other. This results in a new surface composition that reflects the stability of species under conditions of existing oxygen activity (since oxygen is expected to be lost) and the redox potential in the layer. The species formed are expected to be those with the preferred free energies of formation. In the case of copper the
following species are available: CuO ! Cu3 O2 þ O ";
(1)
Cu3 O2 ! Cu2 O þ O ";
(2)
Cu2 O ! Cux O þ O " :
(3)
Under mild ion impact the first reaction could be considered to be dominant. The ease of oxidation state changes is well known in copper oxides [18]. The oxide ion is not stable except in the positive charge field of cations. The preferential sputtering of oxygen atoms is expected to release electrons that reduce available copper ions that further destabilize the O2 ions which eventually sets off a chemical cascade that destroys the oxide and drives the system to the next stable oxide with lower copper valencies. The Cu:O ratio of the altered layer appears to indicate Cu3O2. However, this compound requires Cu2+ to Cu1+ at a ratio of 1 to 2. This inconsistency could indicate that CuxO is the main component of the altered layer and to achieve the Cu3O2 phase much milder sputter conditions would be required. The phenomenon of sputter reduction [26,30–35] is well known and provides a very likely explanation
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for the disparity between the results obtained using surface analytical techniques and techniques which do not employ ion sputtering.
and the Texas Air Research Center under Grant Nos. 129LUB001A and 2001LUB2001A. This material is based in part upon work supported by the Texas Advanced Technology Program under Grant No. 003581-0013-1999.
4. Conclusion In order to determine the presence of the compound Cu3O2 an effort was made to combine sputter depth profiling with XPS. No matter how the oxides were prepared similar depth profile results were obtained. Using the X-ray induced Cu LMM Auger peak position (and line shape), the Cu 2p3/2 peak position, and the shake-up peak for Cu2+ it is possible to differentiate Cu2+ and Cu1+ in copper oxide films. The outer surface of the oxide films are composed of CuO/Cu(OH)2. However, once sputtering is initiated it is no longer possible to determine the presence of Cu2+. After removal of this layer by He or Ne ion sputtering, Cu3O2 appears to be the oxide in the ion impact altered layer, which is explained by the reverse chemical model of sputtering. The problem lies with the use of sputtering to uncover the Cu3O2 layer. Reduction of Cu2+ to Cu1+ in the ion beam provides a clear explanation for the absence of Cu2+. As long as sputter reduction is recognized, the XPS results are not inconsistent with earlier spectroscopic, structural, optical, and magnetic studies. It is important to exercise great caution when using depth profiling to determine the complex structure of thermally oxidized layers on copper or other metals. Cu3O2 is an important component in the low temperature copper oxidation of copper. This oxide phase should be included in models of low temperature copper oxidation and its presence should not be neglected in the interpretation of copper oxidation data.
Acknowledgements This paper has been supported in part by the Robert A. Welch Foundation (Houston, TX) under Grant No. V-1103, the Gulf Coast Hazardous Substance Research Center under Grant No. EPA-118LUB3653, the Texas Hazardous Waste Research Center under Grant No. 0669LUB0745,
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Vacuum 77 (2004) 27–35 www.elsevier.com/locate/vacuum
On the surface analysis of copper oxides: the difficulty in detecting Cu3O2 D.E. Mencera, M.A. Hossainb, R. Schennachb, T. Gradyc, H. McWhinneyc, J.A.G. Gomesb, M. Kesmezb, J.R. Pargad, T.L. Barre, D.L. Cockeb, a Department of Chemistry, Wilkes University, Wilkes-Barre, PA 18776, USA Gill Chair of Chemistry and Chemical Engineering, Lamar University, Beaumont, P.O. Box 10022, TX 77710, USA c Department of Chemistry, Prairie View A&M University, Prairie View, TX 77446, USA d Institute Technology of Saltillo, Department of Metallurgy and Materials Science, V. Carranza 2400, Saltillo Coah. Mexico, C.P. 25000 e Department of Materials, University of Wisconsin-Milwaukee, Milwaukee, WI 53201, USA b
Abstract The existence of Cu3O2, a gross defect structure of Cu2O, has been documented experimentally since the early 1960s. However, discussions of the oxidation of copper often neglect the importance of this phase; in fact, it is often omitted entirely from such discussions. This results from the difficulty in determining the chemical state during sputter depth profiling and relying on techniques that have difficulty providing chemical state information. The occurrence of sputter reduction during the depth profiling of copper oxide layers has been demonstrated with XPS depth profiles on a series of copper samples oxidized, for varying lengths of time, in air at a temperature of either 423 or 523 K. Under these conditions, a thin layer of CuO/Cu(OH)2 terminates the oxide layers. Beneath this layer, the presence of Cu3O2 is expected on the samples prepared at 423 K. However, immediately upon the beginning of sputtering, only Cu1+ is detected in the oxide layers. A zone of constant Cu:O ratio of (approximately 1.5) is found throughout most of the oxide layer even though Cu2+ is not detected. On the samples prepared at 523 K, the presence of CuO is anticipated. However, Cu2+ is not detected after sputtering is initiated and a region of constant Cu:O ratio of ca. 1.5 is detected. The inherent difficulties involved in investigating oxide layer growth and vertical oxide layer structure using sputter depth profiling are discussed in light of this experimental data. r 2004 Elsevier Ltd. All rights reserved. Keywords: Sputter reduction; XPS; Copper; Cu3O2; Oxidation
Corresponding author. Tel.: +1-409-880-1862; fax: +1-409-880-8374.
E-mail address: [email protected] (D.L. Cocke). 0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.068
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1. Introduction The presence of Cu3O2 during the growth of oxide layers on copper has been documented since the 1960s. Previous experiments [1–7] focused on the spectroscopic and structural properties of these oxide films through the use of UV–Vis–NIR Diffuse Reflectance [1], photoluminescence [2], and studies employing X-ray diffraction (XRD) [2,3] that indicate that Cu3O2 is a gross defect structure of Cu2O. There is a very small difference in the XRD of these two phases that results from a 1% expansion in the lattice of Cu3O2 when compared to Cu2O [2,3]. IR Reflectance measurements [4] support this crystallographic data. In addition, Cu3O2 exhibits an optical band gap greater than that found in Cu2O [5]. Excitation spectra [2] indicate the presence of oxygen vacancies, which are also consistent with the interpretation of the luminescence data, mentioned previously. Cocke et al. [8] performed electrochemical study of copper oxides using Linear Sweep Voltammetry (LSV) and suggested that the formation of Cu3O2 is linked to the three zones important to the oxide growth: the metal/oxide interface, the bulk zone where Cu3O2 is located and the oxide/fluid interface. Sun [9] did VLEED numerical quantification of the Cu3O2 bondforming kinetics on the Cu (0 0 1) surface and revealed that annealing disrupts bond formation in Cu3O2. In an attempt to investigate the formation of CuO at the surface of growing oxide layers on copper at 473 and 573 K, Lenglett et al. [6] employed the electron microprobe, XRD, spectroscopy (UV–Vis–NIR), and X-ray Photoelectron Spectroscopy (XPS). That study confirmed the presence of the Cu3O2 at both oxidation temperatures, and CuO was only present in significant quantities at the terminal surface of the oxides at these temperatures. Additional linear sweep voltammetry experiments by these researchers [7] have also supported the presence of this phase. These authors also point out that the magnetic properties of copper oxide films described by O’Keefe and Stone [10] can also be explained by the formation of Cu3O2. Schennach and Gupper [11] used Raman Spectroscopy to study copper
oxidation in air and found the formation of Cu2O between 343 and 403 K, formation of Cu3O2 between 403 and 493 K, formation of CuO and decrease of Cu3O2 between 493 and 523 K and formation of CuO at temperatures higher than 533 K. In fact, the growth of stable layers of Cu3O2 at temperatures of 423 K has been reported [5,12,13]. This paper addresses several issues. The first is the presence of discrepancies between the results of depth profiling surface analysis studies and the other work described above. This seems to be the result of the phenomenon of sputter reduction and results in apparent inability of XPS and depth profiling to detect Cu3O2. Other issues include the effect of underlying substrate on the oxide growth and the decomposition of oxide due to the low activity of oxygen at the metal/ oxide interface.
2. Experimental Copper discs (1 mm thick, by 1 cm diameter) were cut from oxygen free high conductivity (OFHC, minimum purity 99.95%) copper rod. The discs were then wet polished in alternating directions with 220, 320, 400, 600 and 1500 grit silicon carbide sandpaper. The polished discs were rinsed with water (Aldrich 99.5+%, A.C.S., reagent), isopropyl alcohol (LabChem Inc. 70% v/v Lot No 3126-6), and water again, followed by daubing dry on laboratory wipes. The copper samples were never touched with bare hands and oxidation was accomplished by placing the samples on a preheated iron block in a laboratory oven at 423 or 523 K in air for various lengths of time. XPS data was collected using a Perkin Elmer PHI 5600 ci X-ray Photoelectron Spectrometer. System background pressure was approximately 109 mbar range. Mg Ka (1253.6 eV) radiation generated using a standard dual anode source operating at 15 kV and an emission current of 23 mA was used in all experiments. The signal from adventitious carbon (284.6 eV) was used for calibrating the XPS data. XPS data were collected in the sequence C 1s, O 1s, O KVV, Cu 2p, and Cu LMM. XPS depth profiling was carried out with He and Ne (for thicker oxide layers) ion
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Table 1 Sputtering conditions for copper discs heated in air Temperature (K) Ambient/Pristine 423 523
Oxidation time (min)
Gas
Sputtering interval (min)
Total sputtering time (min)
30 300 05 60
He He He He Ne
0.1 0.2 0.3 1.0 1.5
0.1 1.6 3.9 11.0 13.5
sputtering. A He or Ne gas pressure of (10–15) 103 mP (0.01–0.015 mbar) at 25 mA (emission) and an acceleration voltage of 4 kV were employed. The analyzed spot size and the sputtered area were 0.4 mm2 and ca. 1 mm2, respectively. Sputtering conditions are given in Table 1. Surface concentrations were determined with Perkin Elmer software using peak heights and relative sensitivity factors to give surface concentrations in atomic percent. Routine deconvolutions were performed when necessary using the Perkin Elmer software. Since all the samples were prepared and handled in air prior to thermal oxidation, each was covered by native oxide formed on the freshly polished metal surface. Since this native oxide layer could influence oxide film growth, the native oxide layer was analyzed as a reference to the thermally oxidized samples.
3. Results and discussion The copper oxide films grown in this study are terminated with a thin layer of CuO/Cu(OH)2. Fig. 1a shows the changes observed in the Cu 2p region during the depth profile of a copper oxide film grown at 423 K for 30 min. Curve i shows the unsputtered surface and the spectra are indicative of CuO/Cu(OH)2. This is evidenced by the presence of the telltale shake-up lines (940.0 and 943.2 eV) on the Cu 2p peaks and the hydroxide peak (530.4 eV) in the O 1s region, and is in agreement with the work of Barr [14] and others [15]. The relative size of these shake-up peaks relative to the 2p peaks, along with the peak position of the Cu 2p3/2 (933.3 eV) [16], indicates that the upper surface of the oxide layer is
composed primarily of Cu2+ species. After the first cycle of He sputtering, the Cu2+ is no longer detected (see Fig. 1a curve ii). This finding is inconsistent with the prior studies [1–7,10,12,13] that find Cu3O2 in copper oxide layers grown under these conditions. The removal of the Cu(OH)2 along with the adventitious carbon is expected. However, no indication of Cu2+ remains (the absence of the shake-up features mentioned above and of a peak at the binding energy corresponding to Cu2+ in the 2p region). However, the presence of Cu1+ is confirmed by the position of the X-ray induced LMM Auger (336.4 eV KE, after the 1st sputtering) [17,18], which allows one to differentiate between Cu1+ and Cu0 which exhibit essentially the same Cu 2p peak positions. In Fig. 1a curve iii the copper 2p region is shown after the 6th sputtering cycle. This is the first cycle during which the characteristic features of zero valent copper are observed in the LMM Auger region. The metallic copper is detected through the remainder of the Cu1+ oxide and no Cu2+ is detected. Sputtering was continued until the O 1s signal disappeared at which time only copper metal was detected. Fig. 1b shows the Cu 2p region for the depth profiles of a copper oxide film grown at 423 K for 5 h. Again the unsputtered surface is composed of CuO/Cu(OH)2 (curve i). However, after the first He sputtering cycle (see curve ii), the oxide layer again contains only Cu1+. In Fig. 1b curve iii the copper 2p region is shown after the 7th sputtering cycle. This is the first cycle during which the characteristic features of zero valent copper are observed in the LMM Auger region. During the depth profiles of the samples prepared at 423 K, a region of constant Cu:O ratio (1.5)
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Fig. 1. Cu 2p region during the depth profile of a copper oxide film grown at (a) 423 K for 30 min and (b) 423 K for 5 h. In each set of spectra the curves are normalized and offset for ease of comparison and the spectra are (i) the unsputtered surface, (ii) after the first sputtering cycle, (iii) first cycle in which metallic copper is observed.
corresponding to the formula of Cu3O2 is found. Fig. 2 shows the Cu:O ratio of the two samples as a function of sputtering time (depth). From these plots it is clear that there is a region of essentially constant stoichiometric oxide phase corresponding to a formula of Cu3O2 after the first two sputtering cycles. The region of constant Cu:O ratio of about (1.7) extends for a greater depth for the sample oxidized for 5 h (curve b) than the one grown for only 30 min (curve a), since this is a thicker oxide layer. It is also worth mentioning that the appearance of Cu0, as indicated by the change in
the peak position and peak profile shape of the Cu LMM, does not occur until the end of this region of constant composition. Fig. 3 shows the changes observed in the Cu 2p region during the depth profile of a copper oxide film grown at 523 K for (a) 5 min (b) for 1 h. These oxide films are also terminated by a layer composed of Cu2+ species. This is again demonstrated by the presence of, and relative size of, the shake-up lines on the Cu 2p peaks. This outer layer is composed of CuO/Cu(OH)2 as indicated by the hydroxide peak (529.4 eV BE) in the O 1s region.
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10.00
30 min
8.00
5 hr
Cu/O
6.00
4.00
2.00
0.00 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Sputter Time (min)
Fig. 2. Cu:O ratios during the depth profiles on the copper oxide layers prepared at 423 K (a) 30 min growth time and (b) 5 h growth time.
Immediately upon Ne ion sputtering, the Cu2+ is no longer detected, as shown in curves ii in Fig. 3a and b. This is demonstrated clearly by the disappearance of the Cu2+ features described above and the appearance of Cu1+ again confirmed by the position of the X-ray induced LMM Auger (335.7 eV KE). For the remainder of the depth profile, no Cu2+ species are detected. In curves iii in Fig. 3a and b, the copper 2p region is shown after the 8th and 5th sputtering cycles, respectively. This is the first cycle during which the characteristic features of zero valent copper are observed in the LMM Auger region. Fig. 4 shows the Cu:O ratio of the two samples as a function of sputtering time (depth). The region of constant Cu:O ratio of about (1.8) extends for a greater depth for the sample oxidized for 1 h (curve b) than the one grown for only 5 min (curve a), since this is a thicker oxide layer. A constant Cu:O ratio of about (1.5) is expected for Cu3O2. This exact Cu:O ratio of 1.5 is not observed due to the phenomenon of sputter reduction. Several models of the formation of this phase during the growth of copper oxide films have been proposed. Most agree that the first step in oxide
growth is the formation of a chemisorbed oxygen layer followed by the creation of a non-stoichiometric precusor oxide [19]. Nucleation and growth of Cu2O islands that coalesce into a contiguous layer and thicken [20–23] is then followed by the formation of CuO at the gas-oxide interface [19,24–27]. Once this thin CuO surface layer has formed, the Cu2O is converted into a defect structure by the incorporation of Cu2+ and oxide ion (O2) vacancies to create Cu3O2. The presence of this phase has been well established [1–7,10,12,13] and yet it is not detected using surface analysis techniques. An earlier depth profiling study [28] of the oxide layers formed on copper indicated that CuO did not appear to form in air at 573 K (the conditions expected based on the Cu/O phase diagram and the studies discussed above). In this study, Bubert et al. [28] considered sputter reduction as a possible explanation, but instead used Rutherford Backscattering (RBS) and chemical analysis to argue that Cu2O was indeed the major species grown on copper in air at 573 K. In a follow-up study, Bubert et al [29] used depth profiling XPS, RBS, and glow discharge optical emission spectroscopy (GDOES) to study copper oxide film growth
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Fig. 3. Cu 2p region during the depth profile of a copper oxide film grown at (a) 523 K for 5 min and (b) 523 K for 1 h. In each set of spectra the curves are normalized and offset for ease of comparison and the spectra are (i) the unsputtered surface, (ii) after the first sputtering cycle, (iii) first cycle in which metallic copper is observed.
on copper and copper films on silicon. However, the factor analysis treatment of the XPS data for the samples grown on copper seems to rely on spectra in which mixtures of CuO and Cu2O are found. The XPS and RBS data shown for the samples grown on copper metal (at both 473 and 573 K in air and 10% O2 in Ar) seem to clearly indicate the presence of layered oxide structures terminated in CuO/Cu(OH)2. On the other hand, the samples grown on the thin layers (70 nm) of copper on silicon at 473 and 673 K clearly show that CuO is the primary species detected in the oxide layers. This seems to indicate that the
oxidation process of copper is modified when isolated from an underlying layer of bulk copper. This is consistent with earlier work by Mencer et al. [30] that demonstrated titanium films deposited on copper- and gold-coated copper displayed differences in the chemical species present during both vacuum annealing and low pressure oxidation. The presence of an intervening layer of gold made the titanium more active toward oxidation. The authors attributed the results in terms of the role of oxygen absorbing into the bulk of the underlying copper, and the role of gold as a barrier to the diffusion of oxygen
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10.00 523 K, 5 min
523 K, 1 hr
8.00 Ne+ ion sputtering
b
Cu/O
6.00
He+ ion sputtering 4.00
a
2.00
0.00 0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Sputter Time (min)
Fig. 4. Cu:O ratios during the depth profiles on the copper oxide layers prepared at 523 K (a) 5 min growth time and (b) 1 h growth time.
into the copper. Other studies have also demonstrated the facile nature of copper oxides on the surface of copper metal [18]. Sputter reduction is quite common in oxides [26,30–35]. Therefore, one must question whether the copper to oxygen ratio is from the thermal oxidation process or from the altered oxide caused by the preferential sputtering of oxygen. This question is perhaps best discussed in context of the reverse chemical reactions model proposed by Vasilyev and coworkers [36–40]. This model takes into account the chemical activity of the elements in question during the ion induced altered layer formation. Normally during sputtering, the impacting ions energy is greater than the energy of the chemical bonds present before alteration. The model predicts substantial destruction of chemical bonds and the formation of decomposition species that may remain or react further with each other. This results in a new surface composition that reflects the stability of species under conditions of existing oxygen activity (since oxygen is expected to be lost) and the redox potential in the layer. The species formed are expected to be those with the preferred free energies of formation. In the case of copper the
following species are available: CuO ! Cu3 O2 þ O ";
(1)
Cu3 O2 ! Cu2 O þ O ";
(2)
Cu2 O ! Cux O þ O " :
(3)
Under mild ion impact the first reaction could be considered to be dominant. The ease of oxidation state changes is well known in copper oxides [18]. The oxide ion is not stable except in the positive charge field of cations. The preferential sputtering of oxygen atoms is expected to release electrons that reduce available copper ions that further destabilize the O2 ions which eventually sets off a chemical cascade that destroys the oxide and drives the system to the next stable oxide with lower copper valencies. The Cu:O ratio of the altered layer appears to indicate Cu3O2. However, this compound requires Cu2+ to Cu1+ at a ratio of 1 to 2. This inconsistency could indicate that CuxO is the main component of the altered layer and to achieve the Cu3O2 phase much milder sputter conditions would be required. The phenomenon of sputter reduction [26,30–35] is well known and provides a very likely explanation
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for the disparity between the results obtained using surface analytical techniques and techniques which do not employ ion sputtering.
and the Texas Air Research Center under Grant Nos. 129LUB001A and 2001LUB2001A. This material is based in part upon work supported by the Texas Advanced Technology Program under Grant No. 003581-0013-1999.
4. Conclusion In order to determine the presence of the compound Cu3O2 an effort was made to combine sputter depth profiling with XPS. No matter how the oxides were prepared similar depth profile results were obtained. Using the X-ray induced Cu LMM Auger peak position (and line shape), the Cu 2p3/2 peak position, and the shake-up peak for Cu2+ it is possible to differentiate Cu2+ and Cu1+ in copper oxide films. The outer surface of the oxide films are composed of CuO/Cu(OH)2. However, once sputtering is initiated it is no longer possible to determine the presence of Cu2+. After removal of this layer by He or Ne ion sputtering, Cu3O2 appears to be the oxide in the ion impact altered layer, which is explained by the reverse chemical model of sputtering. The problem lies with the use of sputtering to uncover the Cu3O2 layer. Reduction of Cu2+ to Cu1+ in the ion beam provides a clear explanation for the absence of Cu2+. As long as sputter reduction is recognized, the XPS results are not inconsistent with earlier spectroscopic, structural, optical, and magnetic studies. It is important to exercise great caution when using depth profiling to determine the complex structure of thermally oxidized layers on copper or other metals. Cu3O2 is an important component in the low temperature copper oxidation of copper. This oxide phase should be included in models of low temperature copper oxidation and its presence should not be neglected in the interpretation of copper oxidation data.
Acknowledgements This paper has been supported in part by the Robert A. Welch Foundation (Houston, TX) under Grant No. V-1103, the Gulf Coast Hazardous Substance Research Center under Grant No. EPA-118LUB3653, the Texas Hazardous Waste Research Center under Grant No. 0669LUB0745,
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