Copper oxides formation by a low pressure RF oxygen plasma

MATERIALS SCIENCE & EWCIWEERIMG ELSEVIER

B

Materials Science and Engineering B41 (1996) 206-216

Copper oxides formation N. Bellakhal”, “Laboratoire

d’Analyse bURA

Spectroscopique 230-CORM,

by a low pressure RF oxygen plasma

K. Draou”, B.G. Chitronb, J.L. BrisseP*

et de Traitement de Surface des Mathiaux (Equipe LEICA), Universitk de Romz, UFR F-76821, Mom Saint Aigitan Cedex, Frmzce Unioemitd de Roueiz, UFR de;v Sciences, F- 76821, Mont Sniizt Aignarz Cedes, Frarzce Received

6 February

des Sciences,

1996

Abstract Copper foils are exposed at low pressure (200 Pa) to an inductively coupled oxygen plasma which is also examined spectroscopically to identify the major reactive species. The metal surface is oxidized and the resulting oxides are identified by optical methods: FTIR, photoluminescence and UV-Visible-NIR diffuse reflectance spectroscopies. The following oxides were characterized: a precursor oxide Cu,O of mixed valency character, copper {I) oxides (CqO, CqOJ and copper (II) oxide CuO. The nature of the oxidized layer and its thickness depend on the time of exposure to the plasma and on the working conditions. Keywords:

CuO;

Cu,O;

Cu,O;

Cu,O,;

FTIR;

Oxygen

plasma;

Photoluminescence

1. Introduction Plasma processing has become an important branch of material science, mainly due to applications in the surface modification of substrates for IC technology. The control of the physical-chemical plasma properties used in extractive metallurgical plasma field is based on the thermodynamic properties of the plasma gas. The gas dischargesat low pressure(i.e., the “cold” plasmas) and especially the RF dischargesunder specific geometry conditions of the tubular reactor are widely used for surface modifications. The required processings (e.g., etching, deposition, nitriding) in such dischargesgenerally result from poorly understood physical and chemical processes occurring in the gas phase and at the gas/solid interface; they also depend on the energy transfer from the plasma to the solid [1,2]. The chemical reactions between the reactive gas and the material (e.g. oxidation, nitriding, carburation, etc.) lead to an evolution of the material composition. Low pressure glow-discharges at 13.56 MHz are common in plasma surface technology, such as.RF sputtering, plasma deposition and plasma etching. These processesare now emerging as important tools for the synthesis of new materials, and the development of new electronic * Corresponding

author.

devices. Industrial material processing often requires oxygen plasmas for their ability to clean, oxidize, and etch samples without heating them to high temperatures. Applications include dry etching of photoresist, polymer and biological materials, formation of oxide films and ashing for pretreatment of speciesin chemical analysis. The oxygen system is particularly suitable for basic studies because of the large data base relevant to atomic and molecular processesinvolving oxygen [3]. Several models were proposed to interpret the elementary processesin oxygen discharges [4-61: all agree that the molecular dissociation from electronic impact of levels A3C,t and B3Z; constitutes the dominant path [7]. This dissociation mainly yields first oxygen atoms of the fundamental O(3P) state and metastable O(‘D) and 0(‘S) states. Wicknamayaka [S] indicates how the state of the created atomic oxygen depends on the injected RF power (e.g., for a 1000 W discharge, O(‘D) constitutes 95% of the oxygen atom population). In addition, this low pressure plasma contains charged species: Stoffeles [9] shows that the major negative speciesare 0 -, 0” - and O3-, 0 - being predominant. Shibata [lo] indicated that the major ions are O*+ and O- with number densities of about 10IGmm3 at 50 Pa, four orders of magnitude lower than the atomic density. 0921-5107/96/$15.00 0 1996 - Elsevier Science S.A. All rights reserved PIISO921-5107(96)01674-l

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RF Generator

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Baratron h ulator

Matchmg network m

Faradaycage

Evacuation

voltmeter Primq

1 Gas alimentation

pump

Fig.

1.

Experimental setup

The study of the chemical properties of a non-equilibrium plasma has been undertaken a decade ago by one of us in the case of a corona discharge [ll] and is now developed in an inductively coupled oxygen plasma in order to investigate the collective properties of the electronically and vibrationally excited molecules and atoms towards solid surfaces. We selected copper which yields several oxides as a convenient indicator to evidence these properties and to precise the influence of the discharge parameters (e.g., the induced power, the gas pressure, the flow rate and the distance between the treated sample and the first inductive coil). The choice of copper was also governed by its universal use in the electrotechnical industries and by the fact that oxides often form in relation with electric discharges and breakdowns. The well-known oxidizing properties of dioxygen are relevant to particles in the fundamental state. As previously mentioned, a noticeable part of the gaseous species in a plasma is raised to some electronically or vibrationally excited state, which induces deep mod& cations of the electron repartition and hence of the chemical reactivity, To go on with our investigation of the enhanced chemical properties of the activated gas, we focused on the oxidizing character of particular plasmas and we selected the oxidation of metallic copper by an oxygen plasma as an illustrative example.

During the past few years semiconductor oxides have been extensively investigated for their unique physical and chemical properties in connection with microelectronics, photochemistry, heterogeneous catalysis, production of superconductivity materials and so on. The copper oxides aroused an increased interest due to their use as catalysts in the methanol synthesis or as components in solar cells [12,13]. They are usually prepared by thermal oxidation of copper [14], electrodeposition or reactive sputtering for Cu,O [15,16], sputtering or thermal chemical vaporation deposition (CVD) for CuO [17]. In fact the copper oxides are not limited to the well known species Cu,O and CuO, and other oxidized forms were recently identified. Cu30, [18] is shown to have the same crystalline structure as the stoichiometric oxide Cu,O, with one copper vacancy per unit cell and could not be characterized by its UV-Vis reflectance spectra although the photoluminescence spectra were found to be different for the two copper (I) oxides (Cu,O and Cu,O,) [18]. Our investigation of the oxidizing properties of an oxygen RF plasma led us to identify the nature of the oxygen species present in the discharge by emission spectroscopy. The particules inductively created in this non-equilibrium luminescent electric discharge react at the copper surface and form an oxidized layer. After treatment the yielded copper oxides were characterized

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Table 1 Chemical compositions of copper A foils treated by oxygen plasma Impurity @pm)

ETP copper (For copper A, according to French nomenclature)

0

Ag

As

Pb

Fe

Ni

Sb

S

100





$10

$5

<5

by optical methods, like FTIR, UV-Vis-NIR diffuse reflectance and photoluminescence spectroscopies. The film thickness was measured by interferometry. This paper is devoted to the relevant results.

2. Experimental The plasma device (Fig. 1) consists of a Pyrex tube reactor 0.1 m in diameter, 0.3 m in length. The driving frequency is transferred from a 13.56 MHz generator to the gas by means of an impedance matching network and a seven-turn coil. The sample copper foil is positioned normally to the gas flow on a stainless steel quenching head. This quenching head is water-cooled so that its temperature remains fairly constant during the experiments and never exceeds 308 K. Due to the tight contact the quenching head and the sample temperatures can be regarded as the same. The low pressure (i.e., 200 Pa) is sustained by a primary pump. The input gas pressure is measured by a MKS pressure transducer type 122 A in the O-lo4 Pa

800

700 Wavelenth

600

(nm)

Fig. 2. Typical emission spectrum of the oxygen plasma recorded at d= 0.1 m from the 1st coil (injected power, 300 W; gas pressure, 200 Pa).

Te $5

range; the gauge is linked to a PID regulator which controls the pressure and allows the billsticking of the imposed values. The oxygen flowrate is controlled by a mass flowmeter so that the discharge is continuous and the plasma is stable. For the present investigations the oxygen flow rate is fixed at 0.25 N 1 min-’ (4.17 lo-” N m3 s - ‘) in standard conditions. The RF imput power may be adjusted in the range 100-2200 W. This work was performed at 300 and 200 W, unless otherwise specified. The metal samples used for our study are ETP-grade copper (i.e., copper A grade according to the french nomenclature), which originally contains 100 ppm of oxygen in the form of Cu,O. The chemical composition of this industrial starting material is reported in Table 1. The samples (0.3 mm thick; 110 mm2 area) were polished with various grinding Sic papers (400, 800 for 5 min and 1200 grade for 10 mm), then washed in absolute ethanol and nitrogen dried before being exposed to the plasma. The W-visible emission spectroscopy of the plasma is performed using a 1.0 m Ebert-FastiC monochromator with a 1200 grooves mm-’ grating. Using 50 pm slit widths, the spectral resolution is 0.3 nm and the field of view in the piasma 0.3 x 5 mm2. In the 300-900 nm spectral range, the sensitivity of the optical set-up is calibrated with a tungsten strip lamp. The detector is a UVP photomultiplier. The UV-Vis-NIR diffuse spectra of the treated metal are recorded on a Perkin Elmer Lambda 9 spectrophotometer equipped with an integrating sphere (BaSO,) and the luminescence spectra with a JobinYvon 3C spectrophotometer (analyzed range: 300-1200 nm). The specular reflectance infrared spectra are recorded on a Nicolet FTIR 710 spectrometer (analyzed range: 5000-225 cm- ‘; angle of incidence 80”; RS SO).

3. Characterization 900

Se

of the copper oxides

Previous studies on the thermal oxidation of copper films at low temperature evidenced the formation of several oxides which were characterized by their reflectance spectra in the W, visible and IR range and by their photoluminescence spectra [l&19].

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E (cm-‘)

Ionization limit

10983t

3P

8863 1 8663 1 86627 86625

I

844.650 nm 777.196 nm 777.418 nm 777.540 nm 7679: j-

w vv

7376s5-. \

5s

J

3P Ground state

OJ Fig. 3. An energy diagram of atomic oxygen.

3.1. Pwczasor oxide Cu,O p > 4)

As attested by the pertinent literature the thermal oxidation process of copper begins with the growth of a precursor oxide CqO (x > 4) which has the same crystdllographic structure as Cu,O but presents different

XPS and UV spectra [20]. The XPS and Auger analyses of the precursor oxide evidence the mixed valency character of copper in the oxidation states (O-I) which results from the scattering of interstitial Cu(0) in the Cu(1) oxide phase and corresponds to a diffuse interface approximately lU- 15 nm thick: the oxygen and

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copper concentration profiles vary continuously from the surface to the bulk copper PO]. In addition, the

Fig. 4. Variation of the density of the atomic oxygen for given pressue (P = 1040 W, d= 0.1 m).

200

500

Wavelength

1000 (nm)

1500

Fig. 5. W-Vis-NIR diffuse reflectance spectra of copper oxidized in oxygen plasma (injected power, 300 W, pressure, 200 Pa) during 5 min at different distances between the copper target and the first inductive coil. (a) 0.05 m, (b) 0.1 m, (c) 0.2 m.

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oxide Cu,O is electrochemically reduced at a higher potential than Cu,O: its electrochemical reduction curve is only characterized by one minor peak near - 0.55 VjSCE which is attributed to the reduction of &(I) in the oxide phase [14]. The W-Vis-NIR reflectance spectrum of the precursor oxide is close to that of metallic copper except an absorption band in the 360380 nm range and the shift of the maximum absorption from 285 to 300 nm [20].

In the oxidation state (I), copper is found under two forms of cuprous oxides: $0 and CU~O,. The UVVis-NIR spectrum of the stoichiometric oxide C&O has been extensively studied theoretically and experimentally for decades [21,22]. The optical spectrum of non-stoichiometric cuprous oxide (polycrystalline samples, thin or thick films grown on copper at high or low temperature) exhibits intense bands between 450 and 650 nm. The relevant Tolstoi’s model was developed by Machefert to explain the influence of the electronic defects on the optical spectrum of cuprous oxide [23,24]. The calculation with the oft-quoted experimental gap value of 2.2 eV leads to an absorption band at 510 and 670 nm for copper and oxygen vacancies respectively. The non-stoichiometry absorption may be correlated with the analysis of excitation photoluminescence spectra [25,26]. It has long been recognized that the three peaks in the luminescence spectrum are related to different activator centers. The long-wave luminescence at 930 mn is attributed to copper vacancies and the two other emission bands centered at 820 and 720 nm are attributed to oxygen vacancies. Czanderna prepared a composition CUO,,~~ (or Cu,O,) by low temperature oxidation (393-433 K) of polycrystalline copper [27]. This composition is a gross defect structure of Cu,O and corresponds to one copper vacancy per unit cell, on the average. The most significant aspect of the X-ray and electron diffraction measurements is that CuO,,,, yields the same diffraction lines as those of Cu,O and no extra line. A deviation from stoichiometry of this magnitude has not been reported previously and hence Cu,O, is probably a unique metastable phase [19]. The absorption spectrum of Cu,O, is characterized by the lack of an absorption band in the range 450-550 nm and by a very strong unexplained absorption band below the band gap 1141. The electrochemical properties of Cu,O, and C&O were studied [ZS]: these oxides cannot be distinguished from the potentials of their cathodic reduction peaks, i.e. Cu,O, and Cu,O are reduced together at the same potential. Lefez [19] confirms the differences between the optical properties of Cu,OZ and Cu,O which are respectively characterized by luminescence emission bands at 760 nm (intense) and 720 or

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820 nm. The IR reflectance spectrum of the Cu,O/Cu system consists of two bands around 650 cm-’ (LO mode) and 610 cm-’ (TO mode) [18]. 3.3. Copper (II) oxide (CuO) The IR reflectance spectrum of the CuO/Cu system consists of many bands around 605,530,470 cm-’ (TO mode) and 620, 580, 550 cm-’ (LO mode) [18]. The optical spectrum of the cupric oxide is characterized by an abrupt absorption rise in the range 800-900 nm [14]. No other salient feature can be observed up to 200 nm. The UV-Vis-NIR spectrum of the 3d9 Jahr-Teller effect in an octahedral environment is well known and consists of three transitions: 0,, 2B,, -+ 2A1,, 02, 2B1,-t ‘B,,, O,, ?Big-+2Eg in the range 600-1300 nm (e.g., for MgO:Cu* + , two bands are observed at about 9500 and 11230 cm - ’ [29]). The broad absorption band observed for CuO is due to the complex magnetic structure and to a large overlapping between the d-d transitions and the charge transfer bands. The electrochemical reduction curve of thin CuO films formed after oxygen plasma treatment was also studied [28]. All the voltammograms present two peaks (i.e.. one at - 0.70 VjSCE and a shoulder at - 0.80 VSCE) which can be assigned respectively to the first and second reduction step of CuO. The two peaks for CuO were not resolved when high scan rates were used or thick films investigated.

4. Results and discussion

We first report here on the oxygen plasma emission spectroscopy analysis since it provides us with information on gaseous species involved in the modification of the metal surface. We also deal with the analysis of the different copper oxides formed after the plasma treatments of the foils. Since the treatments depend strongly on the working parameters of the plasma, we varied the time of exposure, the position of the sample (i.e., the distance d between the sample and the first inductive coil) and the plasma power.

I

-I

700

600

800 Wavelenth (nm)

900

Fig. 6. Emission photoluminescence spectra (A,,, = 530 nm) of copper oxidized in oxygen plasma (injected power, 300 W, pressure, 200 Pa; d = 0.1 m). (a) 10 min, (b) 15 min.

coil (HT). Numerous spectra are recorded at different operating conditions: the injected power and the pressure respectively vary in the 140-1040 W and 120-250 Pa ranges. All the spectra exhibit only atomic lines. See for instance Fig. 2 which shows the spectrum recorded at 300 W and 200 Pa. Four lines are present, the wavelengths of which are 777.196, 777.418, 777.540 and 844.650 nm. Fig. 3 shows that the three former lines are respectively emitted from the *P (86 631 cm - ‘), jP (86 627 cm-‘) and 5P (86 625 cm-‘)-these levels will

4.1. Oxygetlz phmn nnalysis Optical emission spectroscopy is extensively used for in situ diagnostics of weakly ionized plasmas in material processing [303. Optical emission spectra allow to identify the emitting species and to follow qualitative changes in the plasma properties as a function of the external parameters such as the gas nature, RF power and pressure. This spectroscopy affords direct information only on the excited species. Our analysis and measurements are mainly focused in a region located at 0.1 m downstream from the first

200

500

1000 Wavelength (nm)

l! 00

Fig. 7. UV-Vis-NIR diffuse reflectance spectra of copper oxidized in oxygen plasma (injected power, 300 W; pressure, 200 Pa; d = 0.1 m) during 20 min.

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Fig. 8. UV-Vis-NIR diffuse reflectance spectra of copper oxidized in oxygen plasma (injected power, 200 W; pressure, 200 Pa; d= 0.1 m). (a) 10 min, (b) 15 min, (c) 20 min, (d) 25 min, (e) 30 min, (f) 35 min, (g) 40 min.

be gathered in latter from the The intensity optically thin density of the relation:

this paper under the label ‘P-and the 3P (88 631 cm-‘) [31-331. I,, of an atomic line emitted by an medium is linked to the N, number upper state of the transition by the

where A,, is the Einsten probability of the transition, CT nm, its wavenumber, and L the thickness of the medium. Fig. 4 displays the variations of the 0 (‘P) and 0 (‘P) excited state number density as a function of the reactor pressure at 1 mm over the copper surface. Similar profiles have been observed in the free flow. The injected power is 1040 W. This plot exhibits a steep evolution of the atomic number density around 210 Pa and a slight decrease over this value. This experimental result is not surprising if we consider previous works devoted to HF low pressure oxygen plasmas. Indeed Capitelli [6] has shown how the dissociation degree depends on the electrical reduced field: at low pressure (E/N greater than lO-‘O V m’), the plasma is highly dissociated, which leads to a strong lowering of the vibrational temperature (TV = 500 K). Under this critical value, the plasma is weakly dissociated and the vibrational temperature is around 5000 K.

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The 5P number density is the sum of the three 5P(3), jp(z), and jp(l) sub-level number densities. From the measurement of the spectrally resolved 777.196, 777.418 and 777.540 nm line intensities, we have checked with a very good accuracy that these number densities are in the ratio to their statistical weights. Let us assume a Boltzmann equilibrium between the 3P and the 5P levels at T,,, = lo4 K which is a plausible value for the electron temperature in this kind of plasma: then the O(3P)/O(5P) number density ratio would be equal to 0.45. Within our pressure range, the actual value deduced from the line intensities varies between 0.57 and 0.89 in the boundary layer. A similar order of magnitude is obtained in the free flow. This departure cannot be ascribed to the experimental uncertainties. As a matter of fact, these levels are populated through different reactions [34-361: the 3P level is excited from the atomic ground state by direct electron impact and the 5P level is created from the electron dissociation of molecular oxygen from its repulsive levels. Therefore the departure from equilibrium between levels might be expected. In contrast with N,, O2 is a weak light emitter because, for most of its excited states, transitions to ground state are strongly forbidden [37]. The B3C; + X3C; Schumann Runge transition dominates the spectrum of molecular oxygen 1371. Discrete bands of this allowed transition span the region 535-175 nm; an even stronger dissociation continuum spans the region 175-130 nm. Most other transitions which have been observed under high resolution are forbidden by electric dipole selection rules. In contrast with 02, the b4C,-+ a4111, and A2n[, +X211, systems of 02 are electric dipole allowed. In our pressure and excitation conditions, neither 0, nor 02 molecular band has been detected within the investigated spectral range. So we have no direct measurement of the molecular and atomic number density in the test reactor. The spectroscopic signals are few. Consequently they only lead to qualitative conclusions about the state of the plasma. Nevertheless, it is possible to estimate the number density of its major species from extrapolating other works [6,10] to our experimental situation which is defined by the transit time through the reactor (C= 44 ms), the static pressure (200 Pa), the excitation frequency (13.56 MHz) and the electrical injected power (1040 W). The so-obtained E/.N ratio is 1.2 x 10m20 V m2, and the dissociation degree is around 0.10. These values lead to: [O,] g 2.6 x 1O22mB3, [0] g 2.9 x 1021 m -3, [O,t] z 4 x 10IG m-’ and [e-] ~5 x lOI m-‘. Emission spectroscopy analysis of the plasma allows us to state only the occurrence of oxygen atoms in our discharge. As recently suggested [38], the efficiency of the oxygen plasma treatments seems to be related to the oxygen atoms present: the oxidation of organic liquids by non-equilibrium oxygen plasmas involves [39] both

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atoms 0 (“P) and excited molecules OZ (‘Ag). In addition, the influence of the oxygen atoms was emphasized in the plasma treatment of YBaCuO thin films [40] and yzo, 1411. 4.2. Copper oxides jbmntion

4.2.1. Precursor oxide We paid special attention to the first oxidation state of copper obtained at the beginning of the oxidation process. The copper samples were thus exposed to the plasma for 5 min (injected power: 300 W; pressure: 200 Pa) at three different distances from the first inductive coil (i.e., d = 0.05, 0.1 and 0.2 m) leading in each case to the formation of a thin oxide layer. The diffuse reflectance spectra of these films were recorded in the range 200-1000 nm. The typical spectrum (Fig. 5) illustrates the absorption band in the range 350-380 nm which is specific of the precursor oxide Cu,O. Evidence is thus given of the occurrence of this oxide for various working conditions, and in particular for different values of the plasma energy density PjiV (where P refers to the injected power and N to the oxygen mole number in the reactional volume which varies with d) The quantitative information of the IR reflectance spectra could not be obtained because the electric signals relevant to films thinner than 0.1 pm were lost in the background noise.

R

4.22. Cu(I) oxides As evidenced by the UV-Vis-NIR spectra of the treated samples, the oxide initially formed Cu,O is affected by a longer exposure to the plasma. The relevant reaction is attributed to the effect of the inductively created particles on the precursor oxide to yield copper (I) oxides (Cu,O and Cu,O,). The copper (I) oxides are also prepared by plasma treatment of the metal samples. Emission photoluminescence spectra were realized with an excitation wavelength of 530 nm. Fig. 6 shows the photoluminescence spectra of copper samples treated for 10 (Fig. 6a) and 15 min (Fig. 6b) by oxygen plasma (injected power 300 W; pressure 200 Pa; d = 0.1 m). Two luminescence emission bands are evidenced: an intense band at 760 nm (Cu,O,) and a second at 820 nm (Cu?O), which indicate that the thin film formed is constituted by an intimate mixture of Cu,O, and Cu,O. 4.2.3. Cu(II) oxide. A copper sample treated by the oxygen plasma for 20 min (injected power: 300 W; pressure 200 Pa; d = 0.2 m) was examined by W-Vis-diffuse reflectance spectroscopy (Fig. 7) and the absorption bands identified; the bands in the range 700-800 nm which are attributed to CuO, and the bands between 400 and 550 nm which are attributed to the remaining non stoichiometric cuprous oxide.

700

500 600 Wavenumber (cm-l)

400

Fig. 9. Infrared reflectance spectra of copper oxidized in oxygen plasma (injected power, 200 W; pressure, 200 Pa; d = 0.1 m), at 80” of normal. {a) 10 min, (b) 15 tin, (c) 20 min, (d) 25 min, (e) 30 min, (f) 35 min, (g) 40 min.

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Table 2 Influence of the treatment time on the thickness of the oxides layer (injected power, 200 W; pressure, 200 Pa; d= 0.1 m) Treatment time (min) Thickness (pm)

15 0.15

20 0.22

25 0.35

In addition, the treatment of a Cu(1) oxides film induces further oxidation and yields CuO. For example exposure to the plasma for an extra 5 min of the previously examined 15 min treated sample, leads to a new oxide layer identified as CuO, in place of the starting Cu(I) oxides. The photoluminescence spectrum performed on the relevant sample confirms the evolution of the surface since only Cu,O could then be detected. The photoluminescence spectroscopy reveals that the formation of CuO can be obtained by oxidation of Cu,02 as in the case of a thermal treatment [lS].

5. Influence of the working conditions

For a given distance between the copper target and the first coil (e.g., d = 0.1 m) and a given net transmitted power (e.g., P = 200 W corresponding to the energy density 1.28 x lo7 W mol- ‘), the oxidation kinetics of copper samples and the nature of the oxide formed depend only on the time of exposure. The spectrophotometric investigation in the WVis-NIR range of the plasma oxidation process confirms that the initially formed oxide is the precursor oxide Cu,O. Longer exposure to the activated gaseous species leads to Cu(1) oxides (Cu,O, Cu,O,) and uiti-

30 0.50

35 0.77

40 0.98

mately to CuO. This result is backed up by complementary optical techniques such as diffuse and IR reflectance spectroscopy and photoluminescence. 5.1. Diffhe re~ectnnce spectroscopy

The spectrum of a sample treated for 10 min presents the absorption band at 350 nm specific of the precursor oxide Cu,O. The band intensity decreases for longer treatments (15-40 min) and the optical spectrum exhibits an intense absorption band in the range 420-580 nm due to the occurrence of the non-stoichiometric cuprous oxide Cu,O. For exposures longer than 25 min the absorption band at 700 nm proper to CuO increases drastically (Fig. 8). 5.2. Injizred rej7ectancespectra

The IR reflectance spectra (Fig. 9) of a sample treated for at least 15 min were recorded under an incidence angle of 80”. They present an intense band at 650 cm-’ which characterizes the longitudinal opticai vibrations (LO) of both Cu,O and Cu30,. The band intensity decreases as the treatment duration increases, i.e. when the thickness of the oxides layer increases. For layers thicker than 0.25 11111,corresponding to treatments longer than 25 min in the given conditions, a new band is observed at 610 cm - l. This band is attributed to the transverse optical vibrations (TO) of Cu,O. CuO is identified by a series of low intensity bands at 605, 530 and 470 cm-l (TO) present on the spectra of samples treated for at least 30 min. The thickness of the oxide fYm was measured by interferometry methods and the relevant results are gathered in Table 2. They evidence that the film thickness is a linear function of the exposure time to the plasma for given treatment conditions after an induction period close to 20 min. One can guess that the growth of the film may be related to the formation of Cu(1) and Cu(I1) oxides while the induction period may be an argument in the favour of the formation of the precursor oxide. 5.3. Photoluminescencespectroscopy

1

700

,

800 Wavelenth (nm)

900

Fig. 10. Emission photoluminescence spectra (A,.., = 530 nm) of copper oxidized in oxygen plasma (injected power, 200 W; pressure, 200 Pa; d=O.l m), (a) 15 min, (b) 20 min.

The copper (I) oxides (i.e., Cu,O and Cu,OJ are the only copper oxides that can be identified by photoluminescence spectroscopy. Fig. 10 shows the recorded spectra of samples treated for 15 and 20 min by the oxygen plasma. The spectrum

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UV-Vis-NIR

cu,o

320-380 400-580 700-800

(nm)

FTIR (cm-‘)

Photoluminescence (nm)

650 W,, 610 W, 650 W,, 610 W, 620, 580, 550, 510 W, 605, 530, 470 W,

720, 820 740, 760 -

of the 15 min treated sample exhibits two emission bands: an intense luminescence emission at 760 nm and a second one at 820 nm due respectively to the presence of Cu,O, and Cu,O in the oxides am. For the 20 min treated sample, the band relevant to Cu,O, disappears and only the band at 820 nm can be observed (t&O). The evolution of the 760 nm emission band shows that Cu,O, is initially present in the oxides layer and is later oxidized into CuO. In other words, the photoluminescence spectra evidence that the formation of CuO results from the oxidation of both oxides Cu,O and cu,o,.

6. Conclusions A low pressure oxygen plasma treatment is proposed as a new method to prepare metal oxides and we tested the technique in the case of copper. Thin oxide films were formed at the metal surface. They were identified by means of various spectrophotometric methods (i.e., UV-Vis, FTIR reflectance, and photoluminescence spectroscopies) and the results are summarized in Table 3. Our results agree with those of Lenglet and Machefert [19,20,24] relevant to the thermal oxidation of the metal, since we also evidence the formation of a

precursor oxide Cu,O, as well as the Cu(1) and Cu(I1) oxides. We varied the exposure of the samples to the plasma in given working conditions (the treatment time and the plasma density P/N). We got successively the precursor oxide, the two Cu(1) oxides (Cu,O and Cu,O,) and ultimately the superior oxide CuO (Table 4). The formation of the precursor oxide was evidenced as the major species for short treatments and the relevant layer thickness remains inferior to 0.1 llrn. The formation of Cu,O can thus be considered as the &rst step in the oxidation process at the surface and is related with treatment conditions involving a limited flux of the activated species, i.e. short exposure, long distance d from the first coil or low plasma density (Table. 4). The conditions for the formation of the precursor oxide are now more precise: this presents an obvious interest since this oxide is involved in the adhesion of polymers on copper surfaces [42]. The oxygen plasma oxidation mechanism is complex as already mentioned [43] and remains to be detailed, since we have only identified the oxides formed, and the gaseous species present in the plasma and responsible for the metal oxidation. Only oxygen atoms were detected by their emission lines: i.e., the triplet state O(5P) at 777.4 nm and the singlet state O(3P) at 844.6 nm.

Table 4 Formation of copper oxides for different working conditions d (ml

P/N (W mol-‘) P=200

w

0.05

P=300

w

2.56 x IO7

t<5 min Cu,O 5 < t < 20 min Cu(1) t > 20 min Cu(I), Cu(II)a

3.61 x 10’

1< 5 min Cu,O 5 15 min Q(I), Cu(I1) :

1.28 x IO’

t< 10 min CU,~O lOit<25min &(I) t&25 min G(I), Cu(II)a 1

1.80 x 10’

t $5 min Cu,O 5 -c t i 20 min &(I) : t > 20 min G(l), Cu(I1)

0.64 x 10’

r<15min Cu,O 15, 35 min &(I), Cu(I1)

0.90 x lo7

t< 10 min Cu,O lO 30 min Cu(I), Cu(I1)

0.1

0.2

a Cu(II) is major.

215

of the copper oxides

Copper oxides cu,o C&O, cue

et al. / Materiais

IV. Bellakhal

216

et al. i Materials

Science and Engineering

341 (1996)

206-216

Acknowledgements

PO1J.M. Machefert, M. Lenglet, D. Blavette, A. Menand and A.D’

The authors thank Pr. M. Lenglet for his valuable advice and help in recording the spectra.

WI [22] ~231

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