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Comparative PEEM and AES study of surface morphology and composition of Cu films on Si and 3C–SiC substrates
Surface Science 547 (2003) 193–200 www.elsevier.com/locate/susc
Comparative PEEM and AES study of surface morphology and composition of Cu films on Si and 3C–SiC substrates Z. An, M. Hirai, M. Kusaka, M. Iwami
*
Research Laboratory for Surface Science, Faculty of Science, Okayama University, Okayama 700-8530, Japan Received 2 April 2003; accepted for publication 1 October 2003
Abstract We have conducted photoemission electron microscope (PEEM) and Auger electron spectroscopy (AES) studies on the Cu(30 nm)/3C–SiC(1 0 0) and Cu(30 nm)/Si(1 0 0) samples annealed successively up to 850 C. With PEEM, lateral diffusion of Cu atoms on the 3C–SiC substrate was observed at 400 C while no lateral diffusion was seen for the Cu/ Si(1 0 0) samples up to 850 C. The 30 nm Cu thin film on 3C–SiC began to agglomerate at 550 C, similar to the case for the Cu/Si(1 0 0) system. No further spread of the lateral diffusion region was found in subsequent annealing up to 850 C for Cu/3C–SiC while separated regular-sized dot structures were found at 850 C for Cu/Si(1 0 0). AES studies of Cu/Si(1 0 0) system showed partial interface reaction during Cu deposition onto the Si(1 0 0) substrate and oxidation of the resultant Cu3 Si to form SiO2 on the specimen surface at room temperature in air. Surface segregation of Si and C was observed after annealing at 300 C for Cu/Si(1 0 0) and at 850 C for the Cu/3C–SiC system. We have successfully elucidated the observed phenomena by combining PEEM and AES considering diffusion of the constituent elements and/or reaction at interfaces. 2003 Elsevier B.V. All rights reserved. Keywords: Metal–semiconductor interfaces; Silicon carbide; Silicon; Copper; Metallic films; Surface structure, morphology, roughness, and topography; Electron microscopy; Auger electron spectroscopy
1. Introduction Copper is an attractive interconnect metal due to its low resistivity and high electromigration resistance compared with aluminium. The interface reaction in the Cu (film)/Si (substrate) system has been found to be extremely easy and rapid [1–5]. * Corresponding author. Tel.: +81-86-251-7897; fax: +81-86251-7903. E-mail addresses: an@film.rlss.okayama-u.ac.jp (Z. An), [email protected] (M. Iwami).
Cu3 Si, the Si-richest of several Cu–Si compounds according to the Cu–Si phase diagram [6], was found to be the only product present in the Cu/Si system annealed at 175–850 C [1–5]. On the other hand, the reaction and the formation of either Cu3 Si or graphite at the interface between the Cu film and the 3C–SiC substrate were found only at annealing temperatures above 850 C [1]. The difference in reactivity between the Cu/Si system and the Cu/3C–SiC system should be embodied in their surface morphological and chemical characteristics when thermally treated.
0039-6028/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2003.10.002
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Heat treatment is an important and indispensable link in the fabrication of semiconductor devices, because it gives rise to interface diffusion and reaction that significantly improve the device contacts by the formation of metal silicides and/or carbides/graphite in metal/Si and SiC systems [7,8]. On the other hand, it results in a degradation of electron devices and limitation of the microminiaturization of electron devices due to possible agglomeration and lateral diffusion of the metal film. Photoemission electron microscopy (PEEM) is an interesting tool for the study of surface morphological and/or chemical changes caused by thermal treatments, oxidation, and other surface reactions. In this study, we have imaged morphological and chemical changes of surfaces of air exposed Cu/Si(1 0 0) and Cu/3C–SiC specimens by PEEM in ultra high vacuum (UHV: 108 Pa) caused by successive thermal treatment from room temperature (RT) to 850 C. In addition, we have also conducted surface composition analyses for air exposed Cu/Si(1 0 0) and Cu/3C–SiC specimens annealed in UHV by Auger electron spectroscopy (AES). The AES samples were annealed for 5 min. No significant difference in the AES spectra between annealing times of 5 and 10 min was observed in a comparison experiment. So the AES results are comparable with the PEEM experiments in which the annealing time was 10 min.
2. Experimental Si(1 0 0) and 3C–SiC(1 0 0) (1 lm thick, epitaxially grown on Si(1 0 0), 4 off, HOYA, Inc.) were used as substrates. The substrates were cleaned as follows: They were rinsed in ethanol bath with ultrasonic agitation, put in an HNO3 solvent to remove metal ions, and etched in a diluted HF solution to remove a native oxide layer. The cleaned Si and SiC substrates were degassed at 630 C for more than 3 h and then flash-heated at 900 C in a high vacuum chamber with a base pressure of 106 Pa by passing an alternating current through the substrates. Cu depositions were carried out by the evaporation of Cu from a W basket in the high vacuum chamber at RT. 30 and 20 nm thick Cu square dot films for PEEM
experiments were prepared by evaporation through a wire mesh (640 lm · 640 lm) that almost touched the substrates. This process resulted in the formation of 600 lm · 600 lm square dot films for the PEEM experiments. On the other hand, the 30 nm Cu films for AES experiments were prepared without using the wire mesh. The nominal Cu film thickness was monitored by a quartz microbalance. All specimens were then air exposed for 36 h. The subsequent heat-treatments of the Cu/Si(1 0 0) and Cu/3C–SiC(1 0 0) samples were made in situ in the PEEM and AES chambers in UHV(108 Pa) from RT to 850 C. The sample temperature was measured with a W5%Re vs. W26%Re thermocouple in the PEEM experiments, and monitored by an optical pyrometer in the AES analyses. PEEM imaging was conducted using the ELMITEC PEEM III system [9]. Both a high-pressure Hg arc lamp (Eph 5 eV) and 40 eV photons from synchrotron radiation were used as light sources for PEEM. The PEEM system is installed at the beamline BL-5 in HiSOR, Hiroshima University, Japan. All the PEEM images in this paper were taken during an acquisition time of 16 s after a sample was annealed and cooled below 100 C. The contrast in the PEEM images taken using the Hg lamp as a light source (Eph ¼ 5 eV) are attributed to differences in local work function and surface topography. The contrast of images taken with 40 eV photons from synchrotron radiation as a light source is primarily determined by the difference in photo-excitation cross-section of bound electrons and the mean free paths of photo-electrons due to valence electron excitation and of secondary electrons.
3. Results and discussion 3.1. PEEM characterization 3.1.1. Cu(film)/3C–SiC Fig. 1 shows the PEEM images of the boundary between Cu and SiC for the Cu(30 nm)/3C–SiC sample at RT (a) and after the successive heat treatments at 200 C (b), 400 C (c), 550 C (d), and 700 C (e)–(g) for 10 min, respectively. The
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Fig. 1. PEEM images of the Cu film boundary of an air exposed Cu (30 nm)/3C–SiC sample at (a) RT and annealed for 10 min at (b) 200 C, (c) 400 C, (d) 550 C, and (e)–(g) 700 C, respectively, in UHV. An acquisition time of these images is 16 s. The FOV of each image is 130 lm. A Hg lamp (Eph ¼ 5 eV) was used as light source for (a)–(f). In (g), 40 eV photons from synchrotron radiation were used, where we expected to enhance either element selectivity or surface sensitivity. Lateral diffusion of Cu can be seen in (b)–(e). Agglomeration of Cu occurred by heating above 550 C. The almost uniform image in (g) indicates Si and/or C segregation on top of specimen.
dark spot in the middle of the each image is a damaged area in the micro-channel plate of the PEEM system and not a surface structure. The images in Fig. 1 were taken at a field of view (FOV) of 130 lm. All images in Fig. 1, except the image (f) which is from another region, were recorded at almost the same location. Photons from a Hg lamp (Eph ¼ 5 eV) and 40 eV photons from synchrotron radiation were used as light sources
for Fig. 1(a)–(g), respectively. By using 40 eV photons, we intended to have either element selective or surface sensitive information due to either more than one order of magnitude enhancement in photo-excitation cross-section for Cu 3d electrons or shorter mean free path of valence band photo-electrons. In Fig. 1(a), the Cu film at RT appears rather dark compared with the later images. This is
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attributed to adsorption of molecules containing C and O atom and oxidation of the Cu surface during exposure to air for 36 h before loading it into the PEEM chamber. After annealing at 200 C for 10 min, one can see a partial brightening in the Cu region, i.e., in part of square, in Fig. 1(b). By heating at 400 C for 10 min, the brighter Cu region spreads over the whole Cu area as can be seen in Fig. 1(c). The brightening of the sample surface is considered to be due to desorption of adsorbed gases which are expected primarily to contain carbon and oxygen as will be discussed later. Comparing Fig. 1(b) and (c), considerable lateral diffusion of Cu on the substrate can be observed at 400 C. The lateral diffusion region is indicated by arrows in Fig. 1(c), and the lateral diffusion length is estimated to be 30 lm. But no further spread of the lateral diffusion region was found upon subsequent annealing processes at 550 C (Fig. 1(d)) and 700 C (Fig. 1(e)). Some small dark spots can be seen in the diffusion region near the edge of the Cu film as shown by arrows in Fig. 1(d), which indicates the start of partial agglomeration of the Cu film at 550 C annealing. After annealing at 700 C, Cu agglomeration expanded from the edge to the center of the Cu film as shown by arrows in Fig. 1(f). This should be attributed to the weaker interface bonding and the thinner film thickness at the edge than those at the center. In addition, if there had not been the interface (vertical) diffusion between Cu film and SiC substrate, the original Cu film should become brighter with increasing annealing temperature from 400 to 700 C as a result of further degassing and deoxidation. Contrary to this consideration, the gradual darkening of the Cu film in Fig. 1(e) and (f) compared with that in Fig. 1(c) indicates that SiC appeared onto the surface after annealing at 700 C and its amount on the surface increased with increasing annealing temperature, which will be confirmed by the AES study below. One can clearly distinguish the agglomerated structures in Fig. 1(e) (Hg lamp), but can not see them clearly in Fig. 1(g) (Eph ¼ 40 eV). The difference will be attributed to the fact that the agglomerated surface is covered to some extent by Si and/or C which had diffused to the surface, al-
though possible effects of the wide energy distribution of photoelectrons and secondary electrons upon excitation with 40 eV photons obscure differences in local work function and surface topography. 3.1.2. Cu(film)/Si(1 0 0) Fig. 2(a)–(c) show images of the boundary between Cu and Si of the Cu(30 nm)/Si(1 0 0) sample at RT (a) and after successive heat treatments at 400 C (b), and 700 C (c) for 10 min, respectively. The images (FOV ¼ 130 lm) in Fig. 2 were taken at the same location using a Hg lamp as a light source. The Cu-covered region is brighter due to lower work function than the Si region in Fig. 2(a), where a thin SiO2 layer is considered to be formed on both Si- and Cu-covered regions, as will be discussed later. During the annealing from RT to 400 C (Fig. 2(b)), the image in the Cu film region gradually brightened, which is considered to be due to desorption of adsorbed gases. But, in the subsequent annealing process, the brightness of the image in the Cu film region dramatically decreased in intensity, resulting in a dark image at 700 C (Fig. 2(c)). Darkening of the image at 700 C cannot be attributed to an interface reaction as this should have already been completed in the initial annealing stage. The darkening could be due to segregation and chemical reaction of Si atoms near surface region, e.g., Cu2 O + Si fi SiO2 + Cu3 Si, thereby forming SiO2 with higher work function on the surface, as the free energy of formation of Cu2 O ()11.6 kcal/g atom) is less than that of SiO2 ()70 kcal/g atom) [5]. At 850 C (Fig. 2(d)), the image in Cu-deposited region at first brightened again, which is considered to be due to deoxidation of Sioxide in the surface region. In Fig. 2(d), Cu film agglomeration was observed with agglomerated dot structures with a size of about 600 nm. When the same surface was imaged using 40 eV photons, one can hardly distinguish the dot structures as shown in Fig. 2(e), in which there is little difference in brightness between the Si substrate and the initially Cu covered region. This observation can be explained by considerations similar to those in Section 3.1.1 for Fig. 1(g). With the Hg lamp irradiation contrast appeared due to differences in local work function and sur-
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Fig. 2. PEEM images of the Cu film boundary of an air exposed Cu(30 nm)/Si(1 0 0) sample: (a) at RT, (b) and (c) annealed for 10 min at 400 and 700 C, respectively, in UHV. The FOV is 130 lm. The PEEM images (d) and (e) are from an air exposed Cu(20 nm)/ Si(1 0 0) sample annealed for 10 min at 850 C. The FOV of (d) and (e) is 140 lm. An acquisition time of these images is 16 s. A Hg lamp was used as light source, except for (e), where 40 eV photons from synchrotron radiation were used.
face topography. On the other hand, for the 40 eV photon case, no contrast was visible due to possible Si segregation onto the agglomerated structure. However, in the latter case, possible effects of worse resolution could not be excluded due to the wide energy distribution of the photo-electrons. In Fig. 2, no lateral diffusion that was observed in the Cu(30 nm)/3C–SiC sample can be seen for the Cu/Si(1 0 0) samples. The difference between Cu/3C–SiC and Cu/Si(1 0 0) indicates the effect of interface reaction on the bonding strength at the interface. As strong chemical bonding at the interface of the Cu/Si(1 0 0) samples has formed during the initial low temperature annealing stage, the lateral diffusion of Cu atoms is considered to be impossible even at 850 C. 3.2. AES characterization 3.2.1. Cu(30 nm)/3C–SiC Fig. 3 shows the AES spectra of the Cu(30 nm)/ 3C–SiC sample at RT (a), and after successive
annealing at 300 C (b), 400 C (c), 550 C (d), 700 C (e), and 850 C (f) for 5 min, respectively. The spectrum of a 3C–SiC substrate degassed and flashed in UHV is also shown in Fig. 3(g) for comparison. At RT, in Fig. 3(a), Cu MVV, C KLL, and O KLL signals can be seen at around 65, 280, and 529 eV, respectively. After annealing at 300 C (Fig. 3(b)), the C and O signals decreased rapidly and the signal of Cu MVV significantly increased due to the desorption of molecules including C and O atoms. After 400 C annealing (in Fig. 3(c)), no significant change could be seen compared with Fig. 3(b). Therefore, C and O atom-containing molecules are expected to be mainly adsorbed physically on the Cu/3C–SiC sample surface before heating. The oxidation of Cu film at RT in air for 36 h was not remarkable. This can explain the rapid brightening of the PEEM images in Fig. 1(a)–(c). At 550 C (in Fig. 3(d)), the slight increase of the C KLL signal and the decrease of Cu MVV signal is attributed to agglomeration of the Cu film revealing partially the substrate. Therefore
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AES spectra (a)
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Fig. 3. AES spectra of a Cu(30 nm)/3C–SiC sample exposed to air for 36 h: at RT (a), annealed for 5 min at 300 C (b), 400 C (c), 550 C (d), 700 C (e), and 850 C (f), respectively, in UHV. (g) is from a 3C–SiC substrate degassed and flash heated in UHV for comparison.
the Cu region in Fig. 1(d) appears somewhat darker compared with Fig. 1(c). The C KLL signals significantly increased upon further annealing at 700 and 850 C as can be seen in Fig. 3(e) and (f). However, no increase of the Si LVV signal proportional to C KLL signal could be seen. The peak to peak (P–P) height ratio of the C KLL to the Si LVV signal is about 4 in Fig. 3(e) and (f) which is two times larger than the P–P height ratio of the 3C–SiC substrate (Fig. 3(g)). The increases of C KLL and Si LVV signals can be attributed to the strong agglomeration of the Cu film as seen in Fig. 1(e) or (f). The reason for the larger C KLL/Si LVV P–P height ratio at 850 C than that of 3C– SiC is considered to be due to increased C concentration on the agglomerated surface caused by the interface reaction between Cu and 3C–SiC to form Cu3 Si at 850 C [1]. In addition, one can see a double peak characteristic [5] of the Si LVV in Cu3 Si at around 92 eV in Fig. 3(f). The increased C concentration on the agglomerated Cu surface may be responsible for the brightness of the Cudeposited part in Fig. 1(g). Otherwise it may be brighter than the substrate part.
3.2.2. Cu(30 nm)/Si(1 0 0) AES spectra of the Cu(30 nm)/Si(1 0 0) sample are shown in Fig. 4: at RT (a) and annealed at 350 C (b), 450 C (c), 550 C (d), 700 C (e), and 850 C (f) for 5 min, respectively, in UHV. The spectrum of a Si(1 0 0) substrate degassed and flashheated in UHV is also shown in Fig. 4(g) for comparison. One can hardly see any change in the intensity of the C KLL and O KLL signals upon annealing from RT to 550 C as shown in Fig. 4(a)–(d). This fact indicates stable chemical adsorption of O and C atom-containing adsorbates on the sample surface. If there were no interdiffusion and/or reaction between Cu and Si during the deposition of Cu at RT, no difference in the spectral shape should be seen at RT between Fig. 3(a) and Fig. 4(a), because the two samples have the same Cu film thickness and the same exposure time to air. The spectral characteristics in the electron energy range of 63–80 eV of Fig. 4(a)–(d) should be attributed mainly to the Si LVV signal of SiO2 formed due to the rapid oxidation of Cu3 Si (Cu3 Si + O2 fi SiO2 + Cu or Cu2 O) at RT [1,5,10]. This can explain the darker PEEM image of the AES spectra
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Fig. 4. AES spectra of the Cu(30 nm)/Si(1 0 0) sample exposed to air for 36 h: at RT (a), annealed for 5 min at 300 C (b), 450 C (c), 550 C (d), 700 C (e), and 850 C (f), respectively, in UHV (g), is that for a Si(1 0 0) substrate degassed and flash heated in UHV for comparison.
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of the Cu/Si(1 0 0) system are illustrated in Fig. 5. In the process of heat-treatment from RT to 850 C, the film or the agglomerated structure surface had been covered at RT to some extent by the adsorbed molecules including C and O atom and the oxides (SiO2 and/or Cu2 O), at 400 C by the adsorbed molecules, oxides and segregated Si, at 700 C by SiO2 and segregated Si, and at 850 C by segregated Si, respectively. Therefore, the AES intensity of the Cu MVV signal hardly changes in Fig. 4(a)–(f). Considering that the Cu MVV signal can be seen in spite of attenuation by adsorbed molecules, oxides, and/or segregated Si on top of the Cu, this top layer is estimated to have a . thickness of around 5 A
4. Summary Fig. 5. Schematic illustration for the Cu/Si(1 0 0) specimen exposed to air: at RT (a), annealed in UHV at 400 C (b), 700 C (c), and 850 C (d).
Cu(30 nm)/Si(1 0 0) sample Fig. 2(a) than that of the Cu(30 nm)/3C–SiC sample at RT. One can see the characteristic peak of the Si LVV in Si at around 92 eV in Fig. 4(b)–(f). However, if there were no Si segregation to the surface, especially before the agglomeration of the film occurs, in Fig. 4(b) and (c), the peak at around 92 eV (if appearance) should have the double peak characteristic of the Si LVV in Cu3 Si. A gradual increase in intensity of the Si LVV signal can be seen from Fig. 4(b)–(d) due to the successive annealing from RT to 550 C. This is attributed to Si segregation to the surface due to the prolonged annealing, which was also reported [5] and discussed [11] elsewhere. Therefore the darkening of the PEEM images in Fig. 2 during further annealing should be due to the segregation of Si atoms to the surface and the chemical reaction, Cu2 O + Si fi SiO2 + Cu3 Si, in the near surface region, resulting in a SiO2 layer on top. The dramatic increase of the Si LVV signal in Fig. 4(e) and especially in Fig. 4(f) indicates that the Si substrate had been partially exposed due to the increasing agglomeration in the Cu covered region and Si segregation onto the agglomerated structure had possibly increased. The phenomena in the surface and interface region
The changes of the surface morphology and composition of air-exposed Cu films on 3C– SiC(1 0 0) and Si(1 0 0) substrates upon heating in UHV were investigated by PEEM and AES. The following can be concluded. (1) Lateral diffusion occurred at 400 C in the Cu/ 3C–SiC system. No further spread of the lateral diffusion region could be seen upon subsequent annealing above 550 C. No lateral diffusion could be seen in the Cu/Si(1 0 0) system up to 850 C. At 850 C, separated regular dot structures (600 nm) were formed for Cu(20 nm)/Si(1 0 0). (2) Agglomeration of the Cu film on the 3C–SiC substrate and on the Si(1 0 0) substrate was found to begin upon annealing at 550 C. The agglomeration was observed to start at the edge and to spread to the central region of the Cu/3C–SiC sample, but to develop simultaneously in whole region covered by the Cu film for Cu/Si(1 0 0) with increasing annealing temperature. (3) Significant surface oxidation of the Cu/Si(1 0 0) system at RT in air is due to partial interface reaction during Cu deposition. Surface segregation of Si and C was observed after annealing at 300 C for Cu/Si(1 0 0) and at 850 C for the Cu/3C–SiC system.
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Acknowledgements The authors would like to express their thanks to the members of HiSOR, Hiroshima University, Japan for their support in the PEEM experiment. References [1] Z. An, A. Ohi, M. Hirai, M. Kusaka, M. Iwami, Surf. Sci. 493 (2001) 182–187. [2] C.S. Liu, L.J. Chen, J. Appl. Phys. 74 (1993) 5501. [3] S.Q. Hong, C.M. Comrie, S.W. Russell, J.W. Mayer, J. Appl. Phys. 70 (1991) 3655.
[4] M. Onishi, H. Miura, Trans. Jpn. Inst. Met. 18 (1977) 107. [5] A. Cros, M.O. Aboelfotoh, K.N. Tu, J. Appl. Phys. 67 (1990) 3328. [6] T.B. Massalski, Binary Alloy Phase Diagrams, vol. 1, America Society for Metals, Metals Park, OH, 1986, p. 961. [7] B.Y. Tsaur, C.K. Chen, C.H. Anderson Jr., J. Appl. Phys. 57 (1985) 1890. [8] J. Park, K. Landry, J. Perepezko, Mater. Sci. Eng. A 259 (1999) 279. [9] E. Bauer, J. Electron. Spectrosc. Relat. Phenom. 114–116 (2001) 975. [10] J.M.E. Harper, A. Charai, L. Stolt, F.M. dÕHeurle, P.M. Fryer, Appl. Phys. Lett. 56 (1990) 2519. [11] T.C. Frank, J.L. Falconer, Appl. Surf. Sci. 14 (1982) 359.
Comparative PEEM and AES study of surface morphology and composition of Cu films on Si and 3C–SiC substrates Z. An, M. Hirai, M. Kusaka, M. Iwami
*
Research Laboratory for Surface Science, Faculty of Science, Okayama University, Okayama 700-8530, Japan Received 2 April 2003; accepted for publication 1 October 2003
Abstract We have conducted photoemission electron microscope (PEEM) and Auger electron spectroscopy (AES) studies on the Cu(30 nm)/3C–SiC(1 0 0) and Cu(30 nm)/Si(1 0 0) samples annealed successively up to 850 C. With PEEM, lateral diffusion of Cu atoms on the 3C–SiC substrate was observed at 400 C while no lateral diffusion was seen for the Cu/ Si(1 0 0) samples up to 850 C. The 30 nm Cu thin film on 3C–SiC began to agglomerate at 550 C, similar to the case for the Cu/Si(1 0 0) system. No further spread of the lateral diffusion region was found in subsequent annealing up to 850 C for Cu/3C–SiC while separated regular-sized dot structures were found at 850 C for Cu/Si(1 0 0). AES studies of Cu/Si(1 0 0) system showed partial interface reaction during Cu deposition onto the Si(1 0 0) substrate and oxidation of the resultant Cu3 Si to form SiO2 on the specimen surface at room temperature in air. Surface segregation of Si and C was observed after annealing at 300 C for Cu/Si(1 0 0) and at 850 C for the Cu/3C–SiC system. We have successfully elucidated the observed phenomena by combining PEEM and AES considering diffusion of the constituent elements and/or reaction at interfaces. 2003 Elsevier B.V. All rights reserved. Keywords: Metal–semiconductor interfaces; Silicon carbide; Silicon; Copper; Metallic films; Surface structure, morphology, roughness, and topography; Electron microscopy; Auger electron spectroscopy
1. Introduction Copper is an attractive interconnect metal due to its low resistivity and high electromigration resistance compared with aluminium. The interface reaction in the Cu (film)/Si (substrate) system has been found to be extremely easy and rapid [1–5]. * Corresponding author. Tel.: +81-86-251-7897; fax: +81-86251-7903. E-mail addresses: an@film.rlss.okayama-u.ac.jp (Z. An), [email protected] (M. Iwami).
Cu3 Si, the Si-richest of several Cu–Si compounds according to the Cu–Si phase diagram [6], was found to be the only product present in the Cu/Si system annealed at 175–850 C [1–5]. On the other hand, the reaction and the formation of either Cu3 Si or graphite at the interface between the Cu film and the 3C–SiC substrate were found only at annealing temperatures above 850 C [1]. The difference in reactivity between the Cu/Si system and the Cu/3C–SiC system should be embodied in their surface morphological and chemical characteristics when thermally treated.
0039-6028/$ - see front matter 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2003.10.002
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Z. An et al. / Surface Science 547 (2003) 193–200
Heat treatment is an important and indispensable link in the fabrication of semiconductor devices, because it gives rise to interface diffusion and reaction that significantly improve the device contacts by the formation of metal silicides and/or carbides/graphite in metal/Si and SiC systems [7,8]. On the other hand, it results in a degradation of electron devices and limitation of the microminiaturization of electron devices due to possible agglomeration and lateral diffusion of the metal film. Photoemission electron microscopy (PEEM) is an interesting tool for the study of surface morphological and/or chemical changes caused by thermal treatments, oxidation, and other surface reactions. In this study, we have imaged morphological and chemical changes of surfaces of air exposed Cu/Si(1 0 0) and Cu/3C–SiC specimens by PEEM in ultra high vacuum (UHV: 108 Pa) caused by successive thermal treatment from room temperature (RT) to 850 C. In addition, we have also conducted surface composition analyses for air exposed Cu/Si(1 0 0) and Cu/3C–SiC specimens annealed in UHV by Auger electron spectroscopy (AES). The AES samples were annealed for 5 min. No significant difference in the AES spectra between annealing times of 5 and 10 min was observed in a comparison experiment. So the AES results are comparable with the PEEM experiments in which the annealing time was 10 min.
2. Experimental Si(1 0 0) and 3C–SiC(1 0 0) (1 lm thick, epitaxially grown on Si(1 0 0), 4 off, HOYA, Inc.) were used as substrates. The substrates were cleaned as follows: They were rinsed in ethanol bath with ultrasonic agitation, put in an HNO3 solvent to remove metal ions, and etched in a diluted HF solution to remove a native oxide layer. The cleaned Si and SiC substrates were degassed at 630 C for more than 3 h and then flash-heated at 900 C in a high vacuum chamber with a base pressure of 106 Pa by passing an alternating current through the substrates. Cu depositions were carried out by the evaporation of Cu from a W basket in the high vacuum chamber at RT. 30 and 20 nm thick Cu square dot films for PEEM
experiments were prepared by evaporation through a wire mesh (640 lm · 640 lm) that almost touched the substrates. This process resulted in the formation of 600 lm · 600 lm square dot films for the PEEM experiments. On the other hand, the 30 nm Cu films for AES experiments were prepared without using the wire mesh. The nominal Cu film thickness was monitored by a quartz microbalance. All specimens were then air exposed for 36 h. The subsequent heat-treatments of the Cu/Si(1 0 0) and Cu/3C–SiC(1 0 0) samples were made in situ in the PEEM and AES chambers in UHV(108 Pa) from RT to 850 C. The sample temperature was measured with a W5%Re vs. W26%Re thermocouple in the PEEM experiments, and monitored by an optical pyrometer in the AES analyses. PEEM imaging was conducted using the ELMITEC PEEM III system [9]. Both a high-pressure Hg arc lamp (Eph 5 eV) and 40 eV photons from synchrotron radiation were used as light sources for PEEM. The PEEM system is installed at the beamline BL-5 in HiSOR, Hiroshima University, Japan. All the PEEM images in this paper were taken during an acquisition time of 16 s after a sample was annealed and cooled below 100 C. The contrast in the PEEM images taken using the Hg lamp as a light source (Eph ¼ 5 eV) are attributed to differences in local work function and surface topography. The contrast of images taken with 40 eV photons from synchrotron radiation as a light source is primarily determined by the difference in photo-excitation cross-section of bound electrons and the mean free paths of photo-electrons due to valence electron excitation and of secondary electrons.
3. Results and discussion 3.1. PEEM characterization 3.1.1. Cu(film)/3C–SiC Fig. 1 shows the PEEM images of the boundary between Cu and SiC for the Cu(30 nm)/3C–SiC sample at RT (a) and after the successive heat treatments at 200 C (b), 400 C (c), 550 C (d), and 700 C (e)–(g) for 10 min, respectively. The
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Fig. 1. PEEM images of the Cu film boundary of an air exposed Cu (30 nm)/3C–SiC sample at (a) RT and annealed for 10 min at (b) 200 C, (c) 400 C, (d) 550 C, and (e)–(g) 700 C, respectively, in UHV. An acquisition time of these images is 16 s. The FOV of each image is 130 lm. A Hg lamp (Eph ¼ 5 eV) was used as light source for (a)–(f). In (g), 40 eV photons from synchrotron radiation were used, where we expected to enhance either element selectivity or surface sensitivity. Lateral diffusion of Cu can be seen in (b)–(e). Agglomeration of Cu occurred by heating above 550 C. The almost uniform image in (g) indicates Si and/or C segregation on top of specimen.
dark spot in the middle of the each image is a damaged area in the micro-channel plate of the PEEM system and not a surface structure. The images in Fig. 1 were taken at a field of view (FOV) of 130 lm. All images in Fig. 1, except the image (f) which is from another region, were recorded at almost the same location. Photons from a Hg lamp (Eph ¼ 5 eV) and 40 eV photons from synchrotron radiation were used as light sources
for Fig. 1(a)–(g), respectively. By using 40 eV photons, we intended to have either element selective or surface sensitive information due to either more than one order of magnitude enhancement in photo-excitation cross-section for Cu 3d electrons or shorter mean free path of valence band photo-electrons. In Fig. 1(a), the Cu film at RT appears rather dark compared with the later images. This is
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attributed to adsorption of molecules containing C and O atom and oxidation of the Cu surface during exposure to air for 36 h before loading it into the PEEM chamber. After annealing at 200 C for 10 min, one can see a partial brightening in the Cu region, i.e., in part of square, in Fig. 1(b). By heating at 400 C for 10 min, the brighter Cu region spreads over the whole Cu area as can be seen in Fig. 1(c). The brightening of the sample surface is considered to be due to desorption of adsorbed gases which are expected primarily to contain carbon and oxygen as will be discussed later. Comparing Fig. 1(b) and (c), considerable lateral diffusion of Cu on the substrate can be observed at 400 C. The lateral diffusion region is indicated by arrows in Fig. 1(c), and the lateral diffusion length is estimated to be 30 lm. But no further spread of the lateral diffusion region was found upon subsequent annealing processes at 550 C (Fig. 1(d)) and 700 C (Fig. 1(e)). Some small dark spots can be seen in the diffusion region near the edge of the Cu film as shown by arrows in Fig. 1(d), which indicates the start of partial agglomeration of the Cu film at 550 C annealing. After annealing at 700 C, Cu agglomeration expanded from the edge to the center of the Cu film as shown by arrows in Fig. 1(f). This should be attributed to the weaker interface bonding and the thinner film thickness at the edge than those at the center. In addition, if there had not been the interface (vertical) diffusion between Cu film and SiC substrate, the original Cu film should become brighter with increasing annealing temperature from 400 to 700 C as a result of further degassing and deoxidation. Contrary to this consideration, the gradual darkening of the Cu film in Fig. 1(e) and (f) compared with that in Fig. 1(c) indicates that SiC appeared onto the surface after annealing at 700 C and its amount on the surface increased with increasing annealing temperature, which will be confirmed by the AES study below. One can clearly distinguish the agglomerated structures in Fig. 1(e) (Hg lamp), but can not see them clearly in Fig. 1(g) (Eph ¼ 40 eV). The difference will be attributed to the fact that the agglomerated surface is covered to some extent by Si and/or C which had diffused to the surface, al-
though possible effects of the wide energy distribution of photoelectrons and secondary electrons upon excitation with 40 eV photons obscure differences in local work function and surface topography. 3.1.2. Cu(film)/Si(1 0 0) Fig. 2(a)–(c) show images of the boundary between Cu and Si of the Cu(30 nm)/Si(1 0 0) sample at RT (a) and after successive heat treatments at 400 C (b), and 700 C (c) for 10 min, respectively. The images (FOV ¼ 130 lm) in Fig. 2 were taken at the same location using a Hg lamp as a light source. The Cu-covered region is brighter due to lower work function than the Si region in Fig. 2(a), where a thin SiO2 layer is considered to be formed on both Si- and Cu-covered regions, as will be discussed later. During the annealing from RT to 400 C (Fig. 2(b)), the image in the Cu film region gradually brightened, which is considered to be due to desorption of adsorbed gases. But, in the subsequent annealing process, the brightness of the image in the Cu film region dramatically decreased in intensity, resulting in a dark image at 700 C (Fig. 2(c)). Darkening of the image at 700 C cannot be attributed to an interface reaction as this should have already been completed in the initial annealing stage. The darkening could be due to segregation and chemical reaction of Si atoms near surface region, e.g., Cu2 O + Si fi SiO2 + Cu3 Si, thereby forming SiO2 with higher work function on the surface, as the free energy of formation of Cu2 O ()11.6 kcal/g atom) is less than that of SiO2 ()70 kcal/g atom) [5]. At 850 C (Fig. 2(d)), the image in Cu-deposited region at first brightened again, which is considered to be due to deoxidation of Sioxide in the surface region. In Fig. 2(d), Cu film agglomeration was observed with agglomerated dot structures with a size of about 600 nm. When the same surface was imaged using 40 eV photons, one can hardly distinguish the dot structures as shown in Fig. 2(e), in which there is little difference in brightness between the Si substrate and the initially Cu covered region. This observation can be explained by considerations similar to those in Section 3.1.1 for Fig. 1(g). With the Hg lamp irradiation contrast appeared due to differences in local work function and sur-
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Fig. 2. PEEM images of the Cu film boundary of an air exposed Cu(30 nm)/Si(1 0 0) sample: (a) at RT, (b) and (c) annealed for 10 min at 400 and 700 C, respectively, in UHV. The FOV is 130 lm. The PEEM images (d) and (e) are from an air exposed Cu(20 nm)/ Si(1 0 0) sample annealed for 10 min at 850 C. The FOV of (d) and (e) is 140 lm. An acquisition time of these images is 16 s. A Hg lamp was used as light source, except for (e), where 40 eV photons from synchrotron radiation were used.
face topography. On the other hand, for the 40 eV photon case, no contrast was visible due to possible Si segregation onto the agglomerated structure. However, in the latter case, possible effects of worse resolution could not be excluded due to the wide energy distribution of the photo-electrons. In Fig. 2, no lateral diffusion that was observed in the Cu(30 nm)/3C–SiC sample can be seen for the Cu/Si(1 0 0) samples. The difference between Cu/3C–SiC and Cu/Si(1 0 0) indicates the effect of interface reaction on the bonding strength at the interface. As strong chemical bonding at the interface of the Cu/Si(1 0 0) samples has formed during the initial low temperature annealing stage, the lateral diffusion of Cu atoms is considered to be impossible even at 850 C. 3.2. AES characterization 3.2.1. Cu(30 nm)/3C–SiC Fig. 3 shows the AES spectra of the Cu(30 nm)/ 3C–SiC sample at RT (a), and after successive
annealing at 300 C (b), 400 C (c), 550 C (d), 700 C (e), and 850 C (f) for 5 min, respectively. The spectrum of a 3C–SiC substrate degassed and flashed in UHV is also shown in Fig. 3(g) for comparison. At RT, in Fig. 3(a), Cu MVV, C KLL, and O KLL signals can be seen at around 65, 280, and 529 eV, respectively. After annealing at 300 C (Fig. 3(b)), the C and O signals decreased rapidly and the signal of Cu MVV significantly increased due to the desorption of molecules including C and O atoms. After 400 C annealing (in Fig. 3(c)), no significant change could be seen compared with Fig. 3(b). Therefore, C and O atom-containing molecules are expected to be mainly adsorbed physically on the Cu/3C–SiC sample surface before heating. The oxidation of Cu film at RT in air for 36 h was not remarkable. This can explain the rapid brightening of the PEEM images in Fig. 1(a)–(c). At 550 C (in Fig. 3(d)), the slight increase of the C KLL signal and the decrease of Cu MVV signal is attributed to agglomeration of the Cu film revealing partially the substrate. Therefore
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AES spectra (a)
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Fig. 3. AES spectra of a Cu(30 nm)/3C–SiC sample exposed to air for 36 h: at RT (a), annealed for 5 min at 300 C (b), 400 C (c), 550 C (d), 700 C (e), and 850 C (f), respectively, in UHV. (g) is from a 3C–SiC substrate degassed and flash heated in UHV for comparison.
the Cu region in Fig. 1(d) appears somewhat darker compared with Fig. 1(c). The C KLL signals significantly increased upon further annealing at 700 and 850 C as can be seen in Fig. 3(e) and (f). However, no increase of the Si LVV signal proportional to C KLL signal could be seen. The peak to peak (P–P) height ratio of the C KLL to the Si LVV signal is about 4 in Fig. 3(e) and (f) which is two times larger than the P–P height ratio of the 3C–SiC substrate (Fig. 3(g)). The increases of C KLL and Si LVV signals can be attributed to the strong agglomeration of the Cu film as seen in Fig. 1(e) or (f). The reason for the larger C KLL/Si LVV P–P height ratio at 850 C than that of 3C– SiC is considered to be due to increased C concentration on the agglomerated surface caused by the interface reaction between Cu and 3C–SiC to form Cu3 Si at 850 C [1]. In addition, one can see a double peak characteristic [5] of the Si LVV in Cu3 Si at around 92 eV in Fig. 3(f). The increased C concentration on the agglomerated Cu surface may be responsible for the brightness of the Cudeposited part in Fig. 1(g). Otherwise it may be brighter than the substrate part.
3.2.2. Cu(30 nm)/Si(1 0 0) AES spectra of the Cu(30 nm)/Si(1 0 0) sample are shown in Fig. 4: at RT (a) and annealed at 350 C (b), 450 C (c), 550 C (d), 700 C (e), and 850 C (f) for 5 min, respectively, in UHV. The spectrum of a Si(1 0 0) substrate degassed and flashheated in UHV is also shown in Fig. 4(g) for comparison. One can hardly see any change in the intensity of the C KLL and O KLL signals upon annealing from RT to 550 C as shown in Fig. 4(a)–(d). This fact indicates stable chemical adsorption of O and C atom-containing adsorbates on the sample surface. If there were no interdiffusion and/or reaction between Cu and Si during the deposition of Cu at RT, no difference in the spectral shape should be seen at RT between Fig. 3(a) and Fig. 4(a), because the two samples have the same Cu film thickness and the same exposure time to air. The spectral characteristics in the electron energy range of 63–80 eV of Fig. 4(a)–(d) should be attributed mainly to the Si LVV signal of SiO2 formed due to the rapid oxidation of Cu3 Si (Cu3 Si + O2 fi SiO2 + Cu or Cu2 O) at RT [1,5,10]. This can explain the darker PEEM image of the AES spectra
(a ) (b ) (c ) (d )
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Fig. 4. AES spectra of the Cu(30 nm)/Si(1 0 0) sample exposed to air for 36 h: at RT (a), annealed for 5 min at 300 C (b), 450 C (c), 550 C (d), 700 C (e), and 850 C (f), respectively, in UHV (g), is that for a Si(1 0 0) substrate degassed and flash heated in UHV for comparison.
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of the Cu/Si(1 0 0) system are illustrated in Fig. 5. In the process of heat-treatment from RT to 850 C, the film or the agglomerated structure surface had been covered at RT to some extent by the adsorbed molecules including C and O atom and the oxides (SiO2 and/or Cu2 O), at 400 C by the adsorbed molecules, oxides and segregated Si, at 700 C by SiO2 and segregated Si, and at 850 C by segregated Si, respectively. Therefore, the AES intensity of the Cu MVV signal hardly changes in Fig. 4(a)–(f). Considering that the Cu MVV signal can be seen in spite of attenuation by adsorbed molecules, oxides, and/or segregated Si on top of the Cu, this top layer is estimated to have a . thickness of around 5 A
4. Summary Fig. 5. Schematic illustration for the Cu/Si(1 0 0) specimen exposed to air: at RT (a), annealed in UHV at 400 C (b), 700 C (c), and 850 C (d).
Cu(30 nm)/Si(1 0 0) sample Fig. 2(a) than that of the Cu(30 nm)/3C–SiC sample at RT. One can see the characteristic peak of the Si LVV in Si at around 92 eV in Fig. 4(b)–(f). However, if there were no Si segregation to the surface, especially before the agglomeration of the film occurs, in Fig. 4(b) and (c), the peak at around 92 eV (if appearance) should have the double peak characteristic of the Si LVV in Cu3 Si. A gradual increase in intensity of the Si LVV signal can be seen from Fig. 4(b)–(d) due to the successive annealing from RT to 550 C. This is attributed to Si segregation to the surface due to the prolonged annealing, which was also reported [5] and discussed [11] elsewhere. Therefore the darkening of the PEEM images in Fig. 2 during further annealing should be due to the segregation of Si atoms to the surface and the chemical reaction, Cu2 O + Si fi SiO2 + Cu3 Si, in the near surface region, resulting in a SiO2 layer on top. The dramatic increase of the Si LVV signal in Fig. 4(e) and especially in Fig. 4(f) indicates that the Si substrate had been partially exposed due to the increasing agglomeration in the Cu covered region and Si segregation onto the agglomerated structure had possibly increased. The phenomena in the surface and interface region
The changes of the surface morphology and composition of air-exposed Cu films on 3C– SiC(1 0 0) and Si(1 0 0) substrates upon heating in UHV were investigated by PEEM and AES. The following can be concluded. (1) Lateral diffusion occurred at 400 C in the Cu/ 3C–SiC system. No further spread of the lateral diffusion region could be seen upon subsequent annealing above 550 C. No lateral diffusion could be seen in the Cu/Si(1 0 0) system up to 850 C. At 850 C, separated regular dot structures (600 nm) were formed for Cu(20 nm)/Si(1 0 0). (2) Agglomeration of the Cu film on the 3C–SiC substrate and on the Si(1 0 0) substrate was found to begin upon annealing at 550 C. The agglomeration was observed to start at the edge and to spread to the central region of the Cu/3C–SiC sample, but to develop simultaneously in whole region covered by the Cu film for Cu/Si(1 0 0) with increasing annealing temperature. (3) Significant surface oxidation of the Cu/Si(1 0 0) system at RT in air is due to partial interface reaction during Cu deposition. Surface segregation of Si and C was observed after annealing at 300 C for Cu/Si(1 0 0) and at 850 C for the Cu/3C–SiC system.
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Acknowledgements The authors would like to express their thanks to the members of HiSOR, Hiroshima University, Japan for their support in the PEEM experiment. References [1] Z. An, A. Ohi, M. Hirai, M. Kusaka, M. Iwami, Surf. Sci. 493 (2001) 182–187. [2] C.S. Liu, L.J. Chen, J. Appl. Phys. 74 (1993) 5501. [3] S.Q. Hong, C.M. Comrie, S.W. Russell, J.W. Mayer, J. Appl. Phys. 70 (1991) 3655.
[4] M. Onishi, H. Miura, Trans. Jpn. Inst. Met. 18 (1977) 107. [5] A. Cros, M.O. Aboelfotoh, K.N. Tu, J. Appl. Phys. 67 (1990) 3328. [6] T.B. Massalski, Binary Alloy Phase Diagrams, vol. 1, America Society for Metals, Metals Park, OH, 1986, p. 961. [7] B.Y. Tsaur, C.K. Chen, C.H. Anderson Jr., J. Appl. Phys. 57 (1985) 1890. [8] J. Park, K. Landry, J. Perepezko, Mater. Sci. Eng. A 259 (1999) 279. [9] E. Bauer, J. Electron. Spectrosc. Relat. Phenom. 114–116 (2001) 975. [10] J.M.E. Harper, A. Charai, L. Stolt, F.M. dÕHeurle, P.M. Fryer, Appl. Phys. Lett. 56 (1990) 2519. [11] T.C. Frank, J.L. Falconer, Appl. Surf. Sci. 14 (1982) 359.