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Electroless copper deposition on silicon with titanium seed layer
Materials Chemistry and Physics 98 (2006) 95–102
Electroless copper deposition on silicon with titanium seed layer Rajendra K. Aithal, S. Yenamandra, R.A. Gunasekaran ∗ , P. Coane, K. Varahramyan Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA 71272, USA Received 12 July 2005; accepted 28 August 2005
Abstract The characteristics of the diffusion barrier layer, that prevents copper migration into the silicon substructure, are critical to the successful use of copper as a conductor in integrated circuits (ICs). This paper describes an electroless deposition of Cu on silicon with titanium (Ti) seed layer, which also serves as adhesion promoting layer and barrier layer to Cu diffusion. The deposition rate and surface morphology are studied as a function of different plating parameters such as the concentration of complexing and reducing agent, temperature, deposition time, pH and additive concentration. All the electroless deposits with thickness up to 100 nm were found to adhere well to the substrates. Surface morphology of the deposited films studied using scanning electron microscope (SEM) and atomic force microscopy (AFM) showed that the roughness and grain size tend to increase with increasing temperature and pH with an optimum being reached at 50 ◦ C and pH 12.5. X-ray diffraction (XRD) analysis shows that the peak intensity ratio I(1 1 1)/I(2 0 0) of plated copper increased after thermal annealing at 300 ◦ C for 1 h in N2 ambient and without any copper diffusion into Ti/SiO2 interface. The electrical resistivity of copper films as determined by four-probe measurement showed that the resistivity decreased with increasing annealing time and a low resistivity of 2.9 -cm was obtained after annealing at 300 ◦ C in N2 ambient for 90 min. © 2005 Elsevier B.V. All rights reserved. Keywords: Electroless copper deposition; Thin films; Annealing; Copper interconnects
1. Introduction Currently, the device interconnect technology is being greatly improved to produce the next-generation high-density and highspeed integrated circuits (ICs) [1]. In the design and fabrication of ICs, resistance and capacitance play an important role in signal propagation delays in the form of RC time delay within the devices and interconnects [1]. Aluminum and its alloys are commonly used for metallization in vias and interconnects in integrated circuits [2]. As the devices become faster and more complex, the interconnection lines must be made very narrow and reliable. The narrowing of the conductor causes an increase in the resistance–capacitance (RC) delay, which in turn controls the speed of the device [1]. The need for low RC values will require the use of alternative materials, which offer lower resistance, such as copper for the circuit interconnect technology. Copper has an advantage over aluminum because of its lower electrical conductivity. It has about two-thirds the electrical resistance of aluminum and
∗
Corresponding author. Tel.: +1 318 257 5115; fax: +1 318 257 5104. E-mail address: [email protected] (R.A. Gunasekaran).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.08.079
high resistance to electromigration and stress induced voiding [3–6]. Several different deposition methods such as physical vapor deposition (PVD), chemical vapor deposition (CVD), electroplating and electroless plating are used for copper metallization and interconnection applications [7–10]. The high costs associated with PVD and CVD processes make them unattractive methods of choice for metallization and in the large-volume production line [11,12]. Electroless deposition has emerged as the method of choice for metallization and interconnection in the recent years due to its low cost, fast deposition, good filling capability, good uniformity and low processing temperatures [13,14]. However, the use of electroless copper in interconnects possess some challenges [15]. Copper diffuses rapidly in silicon, resulting in the deterioration of devices even at lower temperatures. The adhesion of copper to dielectrics is also poor [16,17]. In order to prevent copper from diffusing into silicon and to improve its adhesion to dielectrics, barrier layers such as cobalt, chromium, nickel, palladium, tantalum, tantalum silicon nitride, titanium, titanium nitride, silicon nitride, tungsten nitride, tungsten silicon nitride are commonly used [18–22].
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The diffusion barrier layer prevents the migration of copper into the silicon substrate and enables the use of copper in the ICs. While much research has been done on the electroless copper plating using metal nitride barrier layers such as tantalum nitride (TaN), tungsten nitride (WN) and titanium nitride (TiN) [23–25], little effort has been applied towards investigating direct electroless copper plating on metal barrier layers. In this work titanium (Ti) was used both as a seed layer and barrier layer because of its low electrical resistivity and high thermal stability. Titanium has been recognized as an excellent barrier material for preventing copper penetration at annealing temperatures of 400 ◦ C and provides good adhesion of copper to dielectrics [26,27]. In the present work, an attempt was made to use single metal layer as seed, adhesion and barrier layer. 2. Experimental In this work, silicon substrates with Ti seed layer were used for electroless copper deposition. A 50 nm thick layer of Ti was sputter deposited on (1 0 0) silicon substrate. Prior to electroless plating, substrates were cleaned with acetone and isopropyl alcohol to remove any organic impurities present on the surface and then rinsed with DI water. The activation of the seed layer was performed by immersing the substrate in a acidic colloidal solution comprising of 2.5 g of tin chloride (SnCl2 ·2H2 O), 0.05 g of palladium chloride (PdCl2 ) and 15 ml of hydrochloric acid (HCl) in 30 ml of DI water for 1–3 min followed by immersion in an acceleration solution consisting of 1:1 H2 O and HCl for 1–2 min to remove excess tin from the activated surfaces and expose the catalytic Pd. The electroless copper bath is composed of 0.01 M copper sulfate (CuSO4 ·5H2 O), which serves as the source of Cu ions, 0.025 M ethylenediaminetetraacetic acid (EDTA) as complexing agent, 2–3 ml l−1 formaldehyde (HCHO) as a reducing agent and sodium hydroxide (NaOH) as a pH regulator. The deposition temperature (Td ) and pH were varied in the range of 40–60 ◦ C and 12.0–13.0, respectively, with deposition time (t) kept constant at 10 min. Small amounts of surfactants such as 2,2 -dipyridyl and Triton X-100 were also added to the bath to ensure complete wetting of the substrate surface and enhance film uniformity by retarding the formation of hydrogen gas and decreasing the surface tension of the plating solution. Bubbling of the bath with N2 was also done to expel hydrogen formed during plating process. The copper deposited substrates were post-annealed in nitrogen at 300 ◦ C to reduce any defects formed in the deposits and also to reduce the resistivity of the as-plated copper films. The crystal structure and the orientation of the copper films deposited by the electroless method before and after annealing were analyzed using Siemens D5000 X-ray diffractometer. The radiation used ˚ The patterns were recorded at a tube was Cu K␣ with a wavelength of 1.5418 A. voltage of 40 kV and a tube current of 30 mA, applying a fixed scan rate of 2◦ per minute in the angular range of 20–100◦ . The intensity and 2θ of the peaks obtained for the experimental samples were compared with the standard values of Joint Committee Powder Diffraction Standards (JCPDS). The surface morphology of the copper deposit was determined using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The thickness of the deposit was determined by using a roughness-step tester (RST). The electrical resistivity of the plated copper film was measured by using the Van der Pauw method [28]. All the samples were square shaped with dimensions approx. 1.5 cm × 1.5 cm. The resistivity of the sputtered Ti layer was measured before the electroless plating of the samples. The surface morphology, electrical resistivity and crystallography measurements were made on as-plated copper and annealed copper films. The annealing was carried out at 300 ◦ C in nitrogen (N2 ) ambient for 15, 30 45, 60 and 90 min.
Fig. 1. Deposition rate of copper as a function of complexing agent (EDTA) concentration ([Cu2+ ] = 0.01 M, pH 12.5, Td = 50 ◦ C).
concentration of 0.01 M, temperature at 50 ◦ C and a pH of 12.5. As seen from Fig. 1, the thickness of the electrolessly deposited copper initially increases with an increase in EDTA concentration, reaching a maximum value at 0.036 M and decreases with further increase in the EDTA concentration. At EDTA concentrations less than 0.024 M the Cu2+ is not completely complexed, leading to the precipitation of copper. For higher concentrations of EDTA, the Cu2+ ions are complexed too strongly, making it hard for the reducing agent (HCHO) to reduce the copper ions and deposit metallic copper on the catalytic surface. Because of the instability of the copper deposition rate at certain ratios, EDTA concentration less than 0.024 M and greater than 0.036 M was not used for subsequent experiments. Also the bath is not stable at lower and higher EDTA concentrations. Similarly, experiments were performed to test the effect of CuSO4 concentration on the thickness of the deposits. Fig. 2 shows the copper deposit thickness as a function of varying CuSO4 concentration with fixed EDTA concentration (0.0275 M) in the bath. At any fixed EDTA concentration, the deposition rate slightly increased with the increase in CuSO4 concentration. Higher concentrations of Cu2+ ions in the bath require higher concentrations of complexing agent (EDTA),
3. Results and discussion Fig. 1 shows the electroless deposition of copper as a function of complexing agent (EDTA) concentration with a fixed CuSO4
Fig. 2. Deposition rate of copper as a function of CuSO4 concentration ([EDTA] = 0.0275 M, pH 12.5, Td = 50 ◦ C).
R.K. Aithal et al. / Materials Chemistry and Physics 98 (2006) 95–102
Fig. 3. Deposition rate of copper as a function of formaldehyde concentration (pH 12.5, Td = 50 ◦ C).
thereby affecting the deposition rate. Fig. 3 shows the copper deposit thickness as a function of HCHO concentration. With increasing HCHO concentration, the rate of deposition increases linearly. The number of electrons needed for reduction increases with the increase in the reducing agent concentration, thereby increasing the rate at which metallic copper is deposited on the titanium surface. To understand the influence of different bath conditions on the deposited copper, parameters such as pH and temperature were also varied. Fig. 4 shows the deposition of copper as a function of pH at three different deposition temperatures (Td ). As the pH increases from 12.0 to 13.0, the rate of deposition increases with increasing temperature and a thickness of 91 nm was observed for a deposition time (t) of 10 min for a pH of 13.0 and temperature of 60 ◦ C. At lower pH, the rate of transfer of electrons is low and as a consequence, the rate of nucleation is also low. Once the surface is coated with Cu, the subsequent layers are deposited at a faster rate than the initial layers. This explains the linear rise in the curves at temperatures 50 and 60 ◦ C for pH > 12.5. At Td = 50 ◦ C, the rate of deposition is nearly linear as the pH value is increased. This shows that at this temperature (50 ◦ C) and with the increase in pH, there seems to be an equal influence of both OH− and HCHO on the rate of deposition.
Fig. 4. Thickness of electroless copper deposits as a function of pH and temperature (Td ).
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However, at 60 ◦ C, as the pH increases, the OH− ions tend to have a dominant effect on the rate of deposition as compared to HCHO resulting in the observed steep increase in the deposition rate at pH > 12.5. Fig. 5 shows the copper deposit thickness as a function of dipyridyl concentration at two different Triton X concentrations. With the concentration of Triton X-100 fixed at 0.05 ml l−1 , the rate of deposition increases with increasing 2,2 -dipyridyl concentration up to a maximum of 300 mg l−1 , beyond which there was a drastic reduction in the rate, with no deposition observed for 500 mg l−1 of dipyridyl concentration. Similar effect was also observed at 0.1 ml l−1 Triton X-100 concentration. For certain concentrations of the two additives there was an accelerating effect and for others there was decelerating effect. A steep drop in deposition rate may be due to the increased adsorption of 2,2 -dipyridyl on the copper surface thereby interfering with the reduction process of Cu2+ on the surface. It was reported earlier by Kouda et al. [29], that the adsorption of 2,2 -dipyridyl interfered with the reduction of cupric complex ions. It should be noted that the result obtained in the present work is contrary to what was observed by Oita et al. [30]. In their study, the deposition rate decreased with increase in 2,2 -dipyridyl concentration up to 300 mg l−1 (Triton X-100 was not used) and was constant thereafter for any increase in the concentration [30]. However, in this work there was an initial increase in the deposition rate followed by a rapid decrease with increasing dipyridyl concentration. This particular behavior is not well understood and needs to be investigated. Smooth and pin-hole free films were obtained for 0.1 ml l−1 Triton X-100. SEM images showing the surface morphology of electroless plated copper films on Ti/SiO2 /Si deposited at different temperatures are shown in Fig. 6 (a–c). At 40 ◦ C (Fig. 6a), the films deposited were rough and non-uniform while at 50 ◦ C (Fig. 6b) the films were smooth and continuous with the presence of very few blisters. The films deposited at 60 ◦ C (Fig. 6c) were not continuous and showed more number of blisters, which are formed as a result of the release of hydrogen. This release of hydrogen also inhibits the deposition of Cu, eventually leading to
Fig. 5. Deposition rate of copper as a function of 2,2 -dipyridyl concentration at fixed Triton X-100 concentrations of (a) 0.05 ml l−1 and (b) 0.1 ml l−1 .
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Fig. 6. SEM images of electroless copper plated films on Ti/SiO2 /Si without stirring and N2 bubbling at (a) 40 ◦ C, (b) 50 ◦ C and (c) 60 ◦ C.
Fig. 7. SEM images of electroless copper plated films with N2 bubbling and stirring on Ti/SiO2 /Si at 50 ◦ C.
Fig. 8. SEM images of electroless copper plated films after thermal annealing at 300 ◦ C for 1 h.
the dome shaped features on the plated film. The size of the blisters increased with the increase in film thickness resulting in poor adherence of the film to the substrate and subsequent oxidation of the copper when exposed to ambient. Fixing the temperature of the plating solution at 50 ◦ C and bubbling the solution with nitrogen along with continuous stirring, reduced the blister formation due to hydrogen gas release to a maximum extent as shown in Fig. 7. Small amount of defects and pin-holes still existed due to entrapped hydrogen and this was effectively removed by thermal annealing the sample (shown in Fig. 7) in N2 ambient at 300 ◦ C (Fig. 8). AFM images of the electrolessly deposited copper at a pH of 12.5 and deposition time (t) of 10 min are shown in Fig. 9. It can be observed from the images that the copper film deposited at 60 ◦ C is non-uniform and rough while the films deposited
at 50 ◦ C and 40 ◦ C are uniform and smooth. The average surface roughness of electroless copper films deposited for 10 min as a function of pH and temperature are plotted in Fig. 10. A minimum roughness of 4.2 nm is observed for a copper thickness of approx. 40 nm and a maximum roughness of 10 nm is observed for a thickness of 91 nm. The adhesion of the plated copper films was tested using the scotch tape test. All the electroless deposits up to a thickness of 100 nm adhered well to the substrate. Fig. 11 shows the grain size of electrolessly deposited copper films at 50 ◦ C and at different pH levels. The deposition time (t) was kept constant at 10 min. Fig. 11a shows the surface morphology of deposited copper at pH of 12.0. Since the deposition at a pH of 12.0 is slow, the grains grow small and sharp in size. Their distribution is not even over the entire surface, resulting
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Fig. 9. AFM images of the electroless deposited copper films at different temperatures: (a) 40 ◦ C, (b) 50 ◦ C and (c) 60 ◦ C.
in a rough surface. The size of the grains increases as the pH increases leading to a complete coverage of the entire surface. Fig. 11b shows the surface of the copper film deposited at pH 12.25. The size of the grains are bigger than that deposited at pH 12.0 (Fig. 11a). At this point the film growth is in the final
stages of nucleation. As the pH increases to 12.5 (Fig. 11c) the film growth becomes the dominant factor and nucleation does not affect the grain size as much. At pH 12.75 and 13.0, the films become smoother with minimum number of grains on the surface (Fig. 11d). At higher pH values, the surface is affected
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R.K. Aithal et al. / Materials Chemistry and Physics 98 (2006) 95–102 Table 1 Peak intensity ratio I(1 1 1)/I(2 0 0) and FWHM of (1 1 1) peak of electrolessly deposited copper films
Fig. 10. Surface roughness of electroless copper films deposited as a function of temperature and pH.
XRD parameters
As-deposited
Annealed (15 min)
Annealed (60 min)
I(1 1 1)/I(2 0 0) FWHM (1 1 1)
2.95 0.209◦
4.57 0.203◦
5.81 0.190◦
by blisters formed from hydrogen entrapment which results in a rougher surface (Fig. 11e). Fig. 12a and b shows the X-ray diffraction (XRD) pattern of the Ti seed layer and electrolessly deposited copper on Ti/SiO2 /Si substrate. The peaks corresponding to the crystalline copper and copper oxide (CuO) were observed in the XRD patterns. Table 1 shows the peak intensity ratio of I(1 1 1)/I(2 0 0) and the full width at half maximum (FWHM) for the (1 1 1) peak of the plated copper. A peak intensity ratio I(1 1 1)/I(2 0 0)
Fig. 11. Grain size as a function of pH: (a) 12.0, (b) 12.25, (c) 12.5, (d) 12.75 and (e) 13.0 (Td = 50 ◦ C).
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Fig. 12. XRD patterns of (a) Ti sputtered seed layer on Si, (b) electrolessly deposited copper on Tiseed /SiO2 /Si and (c) electrolessly deposited copper films on Tiseed /SiO2 /Si substrate annealed at 300 ◦ C for 60 min.
of 2.95 and the full width at half maximum (FWHM) of 0.21◦ was obtained for the as-plated copper. Comparison of the main diffraction peaks (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of the plated copper peaks and the standard JCPDS card for powdered copper (Table 2) did not lead to the observation of a specific orientation in the as-plated copper films on Ti/SiO2 /Si, indicating randomly oriented copper grains. After thermal annealing at 300 ◦ C for 1 h in N2 ambient, the peak intensity ratio I(1 1 1)/I(2 0 0) of plated copper increased from 2.95 to 5.81 and the FWHM reduced from 0.209 to 0.19◦ (Fig. 12c) indicating the improved crystallinity and stress release in the film. Stress-induced voiding and elecTable 2 Joint Committee Powder Diffraction Standards (JCPDS) of copper 2θ
Intensity
(h k l)
43.3◦ 50.4◦ 74.1◦ 89.9◦ 95.1◦
100 46 20 17 5
(1 1 1) (2 0 0) (2 2 0) (3 1 1) (2 2 2)
tromigration strongly relates to the crystallographic texture in metal interconnects [31]. It has been proven that (1 1 1) texture significantly inhibits stress-induced voiding in aluminum interconnects. The (1 1 1)-textured copper has higher resistance to electromigration because highly textured microstructures suppress grain boundary and interfacial diffusion of metal atoms [31,32]. A lower oxidation rate was observed in the (1 1 1)-textured copper layer which implies that a highly textured (1 1 1) copper is favorable for the interconnect technology [33]. The electrical resistivity of the deposited copper films was determined by Van der Pauw method [28]. The resistivity of the sputtered Ti (50 nm) seed layer was found to be 68 -cm, which is, however, high compared to the value of bulk titanium (40 -cm). This may be due to the presence of impurities or defects in the sputtered Ti film. The electrical resistivity of the as-plated copper was found to be 6.23 -cm, which is significantly higher than that of bulk copper, which is 1.67 -cm. The presence of defects, hydrogen entrapment in the films and film discontinuities are the major factors contributing to the observed increase in the resistivity of the plated films. Fig. 13
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References
Fig. 13. Resistivity of copper film as a function of annealing time at 300 ◦ C.
shows the electrical resistivity of annealed copper films as a function of annealing time. The electrical resistivity of copper films decreased with increasing annealing time and a minimum resistivity value of 2.9 -cm was obtained after annealing at 300 ◦ C in N2 ambient for 90 min. The resistivity did not decrease further for annealing times greater than 90 min. The decrease in resistivity resulting from the increased annealing time is attributed to the reduction in defects and the number of grain boundaries in the film. 4. Conclusion Uniform copper films with good surface morphology were obtained on Ti/SiO2 /Si substrates by electroless copper plating. The optimization of activation procedure, plating solution and plating parameters such as stirring and nitrogen bubbling, led to the inhibition of blister formation due to hydrogen entrapment. The roughness of the plated film increased with film thickness. The average roughness obtained was ∼10 nm for a thickness of ∼0.1 m. A unique effect in the deposition thickness was observed with the variation in additive concentrations. XRD analysis showed that the peak intensity ratio I(1 1 1)/I(2 0 0) of plated copper increased and the FWHM reduced as the annealing times increased. The electrical resistivity of copper films decreased with increasing annealing time and a low of 2.9 cm was obtained after annealing at 300 ◦ C in N2 ambient for 90 min.
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Electroless copper deposition on silicon with titanium seed layer Rajendra K. Aithal, S. Yenamandra, R.A. Gunasekaran ∗ , P. Coane, K. Varahramyan Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA 71272, USA Received 12 July 2005; accepted 28 August 2005
Abstract The characteristics of the diffusion barrier layer, that prevents copper migration into the silicon substructure, are critical to the successful use of copper as a conductor in integrated circuits (ICs). This paper describes an electroless deposition of Cu on silicon with titanium (Ti) seed layer, which also serves as adhesion promoting layer and barrier layer to Cu diffusion. The deposition rate and surface morphology are studied as a function of different plating parameters such as the concentration of complexing and reducing agent, temperature, deposition time, pH and additive concentration. All the electroless deposits with thickness up to 100 nm were found to adhere well to the substrates. Surface morphology of the deposited films studied using scanning electron microscope (SEM) and atomic force microscopy (AFM) showed that the roughness and grain size tend to increase with increasing temperature and pH with an optimum being reached at 50 ◦ C and pH 12.5. X-ray diffraction (XRD) analysis shows that the peak intensity ratio I(1 1 1)/I(2 0 0) of plated copper increased after thermal annealing at 300 ◦ C for 1 h in N2 ambient and without any copper diffusion into Ti/SiO2 interface. The electrical resistivity of copper films as determined by four-probe measurement showed that the resistivity decreased with increasing annealing time and a low resistivity of 2.9 -cm was obtained after annealing at 300 ◦ C in N2 ambient for 90 min. © 2005 Elsevier B.V. All rights reserved. Keywords: Electroless copper deposition; Thin films; Annealing; Copper interconnects
1. Introduction Currently, the device interconnect technology is being greatly improved to produce the next-generation high-density and highspeed integrated circuits (ICs) [1]. In the design and fabrication of ICs, resistance and capacitance play an important role in signal propagation delays in the form of RC time delay within the devices and interconnects [1]. Aluminum and its alloys are commonly used for metallization in vias and interconnects in integrated circuits [2]. As the devices become faster and more complex, the interconnection lines must be made very narrow and reliable. The narrowing of the conductor causes an increase in the resistance–capacitance (RC) delay, which in turn controls the speed of the device [1]. The need for low RC values will require the use of alternative materials, which offer lower resistance, such as copper for the circuit interconnect technology. Copper has an advantage over aluminum because of its lower electrical conductivity. It has about two-thirds the electrical resistance of aluminum and
∗
Corresponding author. Tel.: +1 318 257 5115; fax: +1 318 257 5104. E-mail address: [email protected] (R.A. Gunasekaran).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.08.079
high resistance to electromigration and stress induced voiding [3–6]. Several different deposition methods such as physical vapor deposition (PVD), chemical vapor deposition (CVD), electroplating and electroless plating are used for copper metallization and interconnection applications [7–10]. The high costs associated with PVD and CVD processes make them unattractive methods of choice for metallization and in the large-volume production line [11,12]. Electroless deposition has emerged as the method of choice for metallization and interconnection in the recent years due to its low cost, fast deposition, good filling capability, good uniformity and low processing temperatures [13,14]. However, the use of electroless copper in interconnects possess some challenges [15]. Copper diffuses rapidly in silicon, resulting in the deterioration of devices even at lower temperatures. The adhesion of copper to dielectrics is also poor [16,17]. In order to prevent copper from diffusing into silicon and to improve its adhesion to dielectrics, barrier layers such as cobalt, chromium, nickel, palladium, tantalum, tantalum silicon nitride, titanium, titanium nitride, silicon nitride, tungsten nitride, tungsten silicon nitride are commonly used [18–22].
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The diffusion barrier layer prevents the migration of copper into the silicon substrate and enables the use of copper in the ICs. While much research has been done on the electroless copper plating using metal nitride barrier layers such as tantalum nitride (TaN), tungsten nitride (WN) and titanium nitride (TiN) [23–25], little effort has been applied towards investigating direct electroless copper plating on metal barrier layers. In this work titanium (Ti) was used both as a seed layer and barrier layer because of its low electrical resistivity and high thermal stability. Titanium has been recognized as an excellent barrier material for preventing copper penetration at annealing temperatures of 400 ◦ C and provides good adhesion of copper to dielectrics [26,27]. In the present work, an attempt was made to use single metal layer as seed, adhesion and barrier layer. 2. Experimental In this work, silicon substrates with Ti seed layer were used for electroless copper deposition. A 50 nm thick layer of Ti was sputter deposited on (1 0 0) silicon substrate. Prior to electroless plating, substrates were cleaned with acetone and isopropyl alcohol to remove any organic impurities present on the surface and then rinsed with DI water. The activation of the seed layer was performed by immersing the substrate in a acidic colloidal solution comprising of 2.5 g of tin chloride (SnCl2 ·2H2 O), 0.05 g of palladium chloride (PdCl2 ) and 15 ml of hydrochloric acid (HCl) in 30 ml of DI water for 1–3 min followed by immersion in an acceleration solution consisting of 1:1 H2 O and HCl for 1–2 min to remove excess tin from the activated surfaces and expose the catalytic Pd. The electroless copper bath is composed of 0.01 M copper sulfate (CuSO4 ·5H2 O), which serves as the source of Cu ions, 0.025 M ethylenediaminetetraacetic acid (EDTA) as complexing agent, 2–3 ml l−1 formaldehyde (HCHO) as a reducing agent and sodium hydroxide (NaOH) as a pH regulator. The deposition temperature (Td ) and pH were varied in the range of 40–60 ◦ C and 12.0–13.0, respectively, with deposition time (t) kept constant at 10 min. Small amounts of surfactants such as 2,2 -dipyridyl and Triton X-100 were also added to the bath to ensure complete wetting of the substrate surface and enhance film uniformity by retarding the formation of hydrogen gas and decreasing the surface tension of the plating solution. Bubbling of the bath with N2 was also done to expel hydrogen formed during plating process. The copper deposited substrates were post-annealed in nitrogen at 300 ◦ C to reduce any defects formed in the deposits and also to reduce the resistivity of the as-plated copper films. The crystal structure and the orientation of the copper films deposited by the electroless method before and after annealing were analyzed using Siemens D5000 X-ray diffractometer. The radiation used ˚ The patterns were recorded at a tube was Cu K␣ with a wavelength of 1.5418 A. voltage of 40 kV and a tube current of 30 mA, applying a fixed scan rate of 2◦ per minute in the angular range of 20–100◦ . The intensity and 2θ of the peaks obtained for the experimental samples were compared with the standard values of Joint Committee Powder Diffraction Standards (JCPDS). The surface morphology of the copper deposit was determined using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The thickness of the deposit was determined by using a roughness-step tester (RST). The electrical resistivity of the plated copper film was measured by using the Van der Pauw method [28]. All the samples were square shaped with dimensions approx. 1.5 cm × 1.5 cm. The resistivity of the sputtered Ti layer was measured before the electroless plating of the samples. The surface morphology, electrical resistivity and crystallography measurements were made on as-plated copper and annealed copper films. The annealing was carried out at 300 ◦ C in nitrogen (N2 ) ambient for 15, 30 45, 60 and 90 min.
Fig. 1. Deposition rate of copper as a function of complexing agent (EDTA) concentration ([Cu2+ ] = 0.01 M, pH 12.5, Td = 50 ◦ C).
concentration of 0.01 M, temperature at 50 ◦ C and a pH of 12.5. As seen from Fig. 1, the thickness of the electrolessly deposited copper initially increases with an increase in EDTA concentration, reaching a maximum value at 0.036 M and decreases with further increase in the EDTA concentration. At EDTA concentrations less than 0.024 M the Cu2+ is not completely complexed, leading to the precipitation of copper. For higher concentrations of EDTA, the Cu2+ ions are complexed too strongly, making it hard for the reducing agent (HCHO) to reduce the copper ions and deposit metallic copper on the catalytic surface. Because of the instability of the copper deposition rate at certain ratios, EDTA concentration less than 0.024 M and greater than 0.036 M was not used for subsequent experiments. Also the bath is not stable at lower and higher EDTA concentrations. Similarly, experiments were performed to test the effect of CuSO4 concentration on the thickness of the deposits. Fig. 2 shows the copper deposit thickness as a function of varying CuSO4 concentration with fixed EDTA concentration (0.0275 M) in the bath. At any fixed EDTA concentration, the deposition rate slightly increased with the increase in CuSO4 concentration. Higher concentrations of Cu2+ ions in the bath require higher concentrations of complexing agent (EDTA),
3. Results and discussion Fig. 1 shows the electroless deposition of copper as a function of complexing agent (EDTA) concentration with a fixed CuSO4
Fig. 2. Deposition rate of copper as a function of CuSO4 concentration ([EDTA] = 0.0275 M, pH 12.5, Td = 50 ◦ C).
R.K. Aithal et al. / Materials Chemistry and Physics 98 (2006) 95–102
Fig. 3. Deposition rate of copper as a function of formaldehyde concentration (pH 12.5, Td = 50 ◦ C).
thereby affecting the deposition rate. Fig. 3 shows the copper deposit thickness as a function of HCHO concentration. With increasing HCHO concentration, the rate of deposition increases linearly. The number of electrons needed for reduction increases with the increase in the reducing agent concentration, thereby increasing the rate at which metallic copper is deposited on the titanium surface. To understand the influence of different bath conditions on the deposited copper, parameters such as pH and temperature were also varied. Fig. 4 shows the deposition of copper as a function of pH at three different deposition temperatures (Td ). As the pH increases from 12.0 to 13.0, the rate of deposition increases with increasing temperature and a thickness of 91 nm was observed for a deposition time (t) of 10 min for a pH of 13.0 and temperature of 60 ◦ C. At lower pH, the rate of transfer of electrons is low and as a consequence, the rate of nucleation is also low. Once the surface is coated with Cu, the subsequent layers are deposited at a faster rate than the initial layers. This explains the linear rise in the curves at temperatures 50 and 60 ◦ C for pH > 12.5. At Td = 50 ◦ C, the rate of deposition is nearly linear as the pH value is increased. This shows that at this temperature (50 ◦ C) and with the increase in pH, there seems to be an equal influence of both OH− and HCHO on the rate of deposition.
Fig. 4. Thickness of electroless copper deposits as a function of pH and temperature (Td ).
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However, at 60 ◦ C, as the pH increases, the OH− ions tend to have a dominant effect on the rate of deposition as compared to HCHO resulting in the observed steep increase in the deposition rate at pH > 12.5. Fig. 5 shows the copper deposit thickness as a function of dipyridyl concentration at two different Triton X concentrations. With the concentration of Triton X-100 fixed at 0.05 ml l−1 , the rate of deposition increases with increasing 2,2 -dipyridyl concentration up to a maximum of 300 mg l−1 , beyond which there was a drastic reduction in the rate, with no deposition observed for 500 mg l−1 of dipyridyl concentration. Similar effect was also observed at 0.1 ml l−1 Triton X-100 concentration. For certain concentrations of the two additives there was an accelerating effect and for others there was decelerating effect. A steep drop in deposition rate may be due to the increased adsorption of 2,2 -dipyridyl on the copper surface thereby interfering with the reduction process of Cu2+ on the surface. It was reported earlier by Kouda et al. [29], that the adsorption of 2,2 -dipyridyl interfered with the reduction of cupric complex ions. It should be noted that the result obtained in the present work is contrary to what was observed by Oita et al. [30]. In their study, the deposition rate decreased with increase in 2,2 -dipyridyl concentration up to 300 mg l−1 (Triton X-100 was not used) and was constant thereafter for any increase in the concentration [30]. However, in this work there was an initial increase in the deposition rate followed by a rapid decrease with increasing dipyridyl concentration. This particular behavior is not well understood and needs to be investigated. Smooth and pin-hole free films were obtained for 0.1 ml l−1 Triton X-100. SEM images showing the surface morphology of electroless plated copper films on Ti/SiO2 /Si deposited at different temperatures are shown in Fig. 6 (a–c). At 40 ◦ C (Fig. 6a), the films deposited were rough and non-uniform while at 50 ◦ C (Fig. 6b) the films were smooth and continuous with the presence of very few blisters. The films deposited at 60 ◦ C (Fig. 6c) were not continuous and showed more number of blisters, which are formed as a result of the release of hydrogen. This release of hydrogen also inhibits the deposition of Cu, eventually leading to
Fig. 5. Deposition rate of copper as a function of 2,2 -dipyridyl concentration at fixed Triton X-100 concentrations of (a) 0.05 ml l−1 and (b) 0.1 ml l−1 .
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Fig. 6. SEM images of electroless copper plated films on Ti/SiO2 /Si without stirring and N2 bubbling at (a) 40 ◦ C, (b) 50 ◦ C and (c) 60 ◦ C.
Fig. 7. SEM images of electroless copper plated films with N2 bubbling and stirring on Ti/SiO2 /Si at 50 ◦ C.
Fig. 8. SEM images of electroless copper plated films after thermal annealing at 300 ◦ C for 1 h.
the dome shaped features on the plated film. The size of the blisters increased with the increase in film thickness resulting in poor adherence of the film to the substrate and subsequent oxidation of the copper when exposed to ambient. Fixing the temperature of the plating solution at 50 ◦ C and bubbling the solution with nitrogen along with continuous stirring, reduced the blister formation due to hydrogen gas release to a maximum extent as shown in Fig. 7. Small amount of defects and pin-holes still existed due to entrapped hydrogen and this was effectively removed by thermal annealing the sample (shown in Fig. 7) in N2 ambient at 300 ◦ C (Fig. 8). AFM images of the electrolessly deposited copper at a pH of 12.5 and deposition time (t) of 10 min are shown in Fig. 9. It can be observed from the images that the copper film deposited at 60 ◦ C is non-uniform and rough while the films deposited
at 50 ◦ C and 40 ◦ C are uniform and smooth. The average surface roughness of electroless copper films deposited for 10 min as a function of pH and temperature are plotted in Fig. 10. A minimum roughness of 4.2 nm is observed for a copper thickness of approx. 40 nm and a maximum roughness of 10 nm is observed for a thickness of 91 nm. The adhesion of the plated copper films was tested using the scotch tape test. All the electroless deposits up to a thickness of 100 nm adhered well to the substrate. Fig. 11 shows the grain size of electrolessly deposited copper films at 50 ◦ C and at different pH levels. The deposition time (t) was kept constant at 10 min. Fig. 11a shows the surface morphology of deposited copper at pH of 12.0. Since the deposition at a pH of 12.0 is slow, the grains grow small and sharp in size. Their distribution is not even over the entire surface, resulting
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Fig. 9. AFM images of the electroless deposited copper films at different temperatures: (a) 40 ◦ C, (b) 50 ◦ C and (c) 60 ◦ C.
in a rough surface. The size of the grains increases as the pH increases leading to a complete coverage of the entire surface. Fig. 11b shows the surface of the copper film deposited at pH 12.25. The size of the grains are bigger than that deposited at pH 12.0 (Fig. 11a). At this point the film growth is in the final
stages of nucleation. As the pH increases to 12.5 (Fig. 11c) the film growth becomes the dominant factor and nucleation does not affect the grain size as much. At pH 12.75 and 13.0, the films become smoother with minimum number of grains on the surface (Fig. 11d). At higher pH values, the surface is affected
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R.K. Aithal et al. / Materials Chemistry and Physics 98 (2006) 95–102 Table 1 Peak intensity ratio I(1 1 1)/I(2 0 0) and FWHM of (1 1 1) peak of electrolessly deposited copper films
Fig. 10. Surface roughness of electroless copper films deposited as a function of temperature and pH.
XRD parameters
As-deposited
Annealed (15 min)
Annealed (60 min)
I(1 1 1)/I(2 0 0) FWHM (1 1 1)
2.95 0.209◦
4.57 0.203◦
5.81 0.190◦
by blisters formed from hydrogen entrapment which results in a rougher surface (Fig. 11e). Fig. 12a and b shows the X-ray diffraction (XRD) pattern of the Ti seed layer and electrolessly deposited copper on Ti/SiO2 /Si substrate. The peaks corresponding to the crystalline copper and copper oxide (CuO) were observed in the XRD patterns. Table 1 shows the peak intensity ratio of I(1 1 1)/I(2 0 0) and the full width at half maximum (FWHM) for the (1 1 1) peak of the plated copper. A peak intensity ratio I(1 1 1)/I(2 0 0)
Fig. 11. Grain size as a function of pH: (a) 12.0, (b) 12.25, (c) 12.5, (d) 12.75 and (e) 13.0 (Td = 50 ◦ C).
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Fig. 12. XRD patterns of (a) Ti sputtered seed layer on Si, (b) electrolessly deposited copper on Tiseed /SiO2 /Si and (c) electrolessly deposited copper films on Tiseed /SiO2 /Si substrate annealed at 300 ◦ C for 60 min.
of 2.95 and the full width at half maximum (FWHM) of 0.21◦ was obtained for the as-plated copper. Comparison of the main diffraction peaks (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of the plated copper peaks and the standard JCPDS card for powdered copper (Table 2) did not lead to the observation of a specific orientation in the as-plated copper films on Ti/SiO2 /Si, indicating randomly oriented copper grains. After thermal annealing at 300 ◦ C for 1 h in N2 ambient, the peak intensity ratio I(1 1 1)/I(2 0 0) of plated copper increased from 2.95 to 5.81 and the FWHM reduced from 0.209 to 0.19◦ (Fig. 12c) indicating the improved crystallinity and stress release in the film. Stress-induced voiding and elecTable 2 Joint Committee Powder Diffraction Standards (JCPDS) of copper 2θ
Intensity
(h k l)
43.3◦ 50.4◦ 74.1◦ 89.9◦ 95.1◦
100 46 20 17 5
(1 1 1) (2 0 0) (2 2 0) (3 1 1) (2 2 2)
tromigration strongly relates to the crystallographic texture in metal interconnects [31]. It has been proven that (1 1 1) texture significantly inhibits stress-induced voiding in aluminum interconnects. The (1 1 1)-textured copper has higher resistance to electromigration because highly textured microstructures suppress grain boundary and interfacial diffusion of metal atoms [31,32]. A lower oxidation rate was observed in the (1 1 1)-textured copper layer which implies that a highly textured (1 1 1) copper is favorable for the interconnect technology [33]. The electrical resistivity of the deposited copper films was determined by Van der Pauw method [28]. The resistivity of the sputtered Ti (50 nm) seed layer was found to be 68 -cm, which is, however, high compared to the value of bulk titanium (40 -cm). This may be due to the presence of impurities or defects in the sputtered Ti film. The electrical resistivity of the as-plated copper was found to be 6.23 -cm, which is significantly higher than that of bulk copper, which is 1.67 -cm. The presence of defects, hydrogen entrapment in the films and film discontinuities are the major factors contributing to the observed increase in the resistivity of the plated films. Fig. 13
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References
Fig. 13. Resistivity of copper film as a function of annealing time at 300 ◦ C.
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