Electroless copper on refractory and noble metal substrates with an ultra-thin plasma-assisted atomic layer deposited palladium layer

Electrochimica Acta 51 (2006) 2400–2406

Electroless copper on refractory and noble metal substrates with an ultra-thin plasma-assisted atomic layer deposited palladium layer Young-Soon Kim a , Hyung-Il Kim a , Joong-Hee Cho a , Hyung-Kee Seo a , M.A. Dar a , Hyung-Shik Shin a , Gregory A. Ten Eyck b , Toh-Ming Lu b , Jay J. Senkevich c,∗ a

Thin Film Technology Lab, School of Chemical Engineering, Chonbuk National University, Jeonju 561756, Republic of Korea b Department of Physics, Rensselaer Polytechnic Institute, Troy, NY 12180, USA c Brewer Science Inc., Rolla, MO 65401, USA Received 30 May 2005; received in revised form 6 July 2005; accepted 13 July 2005 Available online 18 August 2005

Abstract Electroless Cu was investigated on refractory metal, W and TaNX , and Ir noble metal substrates with a plasma-assisted atomic layer deposited palladium layer for the potential back-end-of-the-line (BEOL) metallization of advanced integrated devices. The sodium and potassium-free Cu electroless bath consisted of: ethylenediamine tetraacetic acid (EDTA) as a chelating agent, glyoxylic acid as a reducing agent, and additional chemicals such as polyethylene glycol, 2,2 -dipyridine and RE-610 as surfactant, stabilizer and wetting agent respectively. The growth and chemical characterization of the Cu films was carried out with a field emission scanning electron microscope (FE-SEM), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectrometry (RBS). Group VIII metals such as Pt, Pd, etc., are stable in the electroless bath and catalytic towards the oxidation of glyoxylic acid and therefore work well for the electroless deposition of Cu. From RBS analysis, the amount of carbon and oxygen in Cu films were less than 1–3%. The Cu films were electroless deposited at 45–50 ◦ C on patterned tantalum nitride with plasma-assisted atomic layer deposited (PA-ALD) Pd as a catalytic layer. Electroless Cu trench fill was successful with ultrasonic vibration, RE-610, and lowering the temperature to 45–50 ◦ C on TaNX with the PA-ALD Pd catalytic layer. © 2005 Elsevier Ltd. All rights reserved. Keywords: Copper electroless deposition; Diffusion barrier; XPS; RBS

1. Introduction Copper has replaced the conventional aluminum-base wiring in present ultra-large scale integrated (ULSI) logic devices as of result of its low resistivity (1.67 ␮
cm), high melting point (1085 ◦ C), and electromigration resistance [1–2]. Cu sputtering, chemical vapor deposition (CVD), and electrochemical deposition (ECD), have all been investigated for barrier seed deposition and trench fill. Continuous improvement of bath chemistries and tools may extend ECD Cu to feature sizes found in 45–22 nm nodes; however, high aspect ratio features (thermal and through-wafer vias), found ∗ Corresponding author. Tel.: +1 573 364 0444x1375; fax: +1 573 364 0650. E-mail address: [email protected] (J.J. Senkevich).

0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.07.018

in three-dimensional devices, necessitate the development of alternative Cu filling methods [3]. Cu readily diffuses into silicon dioxide and other carbondoped silicate (C:SiOX ) low dielectric constant materials. It also weakly interacts with these materials along with polymeric-based chemical vapor deposited dielectrics that could be used to pore seal porous interlayer dielectrics [4]. As a result, copper needs a diffusion barrier layer exhibiting good adhesion. Further, the diffusion barrier layer needs to be an electrochemically stable, i.e. dissolution should not take place. Additionally, if an electroless deposition method is employed for metallization of the dielectric or barrier layer then the surface should be catalytically active towards the oxidation of the organic reducing agent. In the current work that organic is glyoxylic acid. The current barrier layer of choice is TaNX for Cu metallization, a logical step from TiN,

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which has been used for Al metallization. The TaNX /Ta/Cu sputtered stack is currently used, where Ta provides adhesion between Cu and TaNX and Cu is used as a seed layer to maintain uniform current over the 300 mm wafer during electrochemical Cu trench fills under galvanostatic control [5,6]. Electroless copper plating, a technology already in-use for electronic packaging, has been proposed to fill copper on-chip interconnects [7,8]. In recent years, copper electroless deposition (ELD) has emerged as an efficient way to fill nanofeatures [9,10]. It is a selective process and therefore current does not need to be uniform over large area wafers or surfaces since a reducing agent is added in the solution as a source of electrons [11–13]. The later benefit will enable the barrier layer to be thinned and without the need for a seed layer; however, the barrier layer should be catalytic. It is possible that electroless plating processes will offer significant advantages for future wiring structures in nano-scale materials fabrication of the metal nanowire for biosensor and micro-electromechanical system (MEMS) and three-dimensional integrated devices. Cu ELD requires a catalytic substrate to oxidize the organic reducing agent. The metals, which are thermodynamically stable in both acid and base are candidates for autocatalytic reduction in aqueous media. Such metals are Pt, Pd, Ir and Cu. Various deposition techniques such as sputtering, CVD, and atomic layer deposition (ALD) [14] have been proposed for the preparation of a thin stable metal for the catalytic seed layer. With the scaling of interconnects it is becoming more difficult to deposit a ‘seed’ layer or catalyst conformally on trench sidewalls and high aspect ratio features. Palladium has been recently deposited on the copper barrier via atomic layer deposition and it is also an appropriate catalyst for the electroless deposition of copper. In a previous study, Ten Eyck et al. examined a palladium layer by plasma-assisted atomic layer deposition (PA-ALD) for a broad range of applications on Ir, W and Si substrates [14]. This PA-ALD Pd catalyst layer had good coverage. In addition Kim et al. demonstrated copper electroless deposition on TaNX with an ultrathin Pd catalyst by PA-ALD [15]. This study investigates sodium and potassium-free electroless Cu deposition with a PA-ALD Pd layer on various refractory metal and noble metal substrates. Moreover, we have also tried Cu trench fill on 3–1 aspect ratio 130 nm trenches with ELD by decreasing the rate of deposition by lowering the plating bath temperature, using ultrasonic vibration, and with a wetting agent.

2. Experimental We used PA-ALD Pd films with a thickness of 2–4 nm on TaNX , Ir, and W substrates. Specific information of PA-ALD Pd can be obtained from the literature [14,15]. We also used bare W and Pt substrates. For the Cu electroless deposition, ethylenediamine tetraacetic acid (EDTA) was

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Table 1 Composition of the Cu electroless Components

Concentration

CuSO4 ·5H2 O (g/l) C10 H16 N2 O8 (EDTA) (g/l) Glyoxylic acid monohydrate (g/l) Tetramethyammonium hydroxide to achieve a pH Polyethylene glycol (␮l) 2,2 -Dipyridine (mg) RE-610 (␮l) Temperature (◦ C)

7.615 10.258 7 11.8, 12.5 5 4 0, 10 45–60

used as a chelating agent, glyoxylic acid as a reducing agent, and additional chemicals such as polyethylene glycol, 2,2 -dipyridine and RE-610 as surfactant, stabilizer, and wetting agent respectively. The pH of bath was adjusted with tetramethyammonium hydroxide (TMAH) at room temperature. The solution temperature was maintained at 45–60 ◦ C. The detailed bath composition is presented in Table 1. The surface chemical bonding for each sample was analyzed by using X-ray photoelectron spectroscopy (XPS). The samples were loaded into the XPS chamber with a chamber base pressure of (1–2) × 10−9 Torr and a spectrum collection pressure of (3–4) × 10−9 Torr. The X-ray Mg K␣ source (PHI model 04-151) used in this experiment has a primary energy 1253.6 eV, and a double pass cylindrical mirror analyzer (PHI Model 15-255G) with a pass energy of 50 eV used for high-resolution scans. The thickness of Pd and Cu were characterized by RBS on the 4.0 MeV Dynamitron accelerator at the Ion Beam Laboratory in the Department of Physics at the University at Albany. Measurements were made with 2.0 MeV alpha particles on the 30◦ beam line. The RBS-determined areal density was converted into an equivalent thickness by dividing by the bulk atomic density of the metal. Spectra were collected with a 10 mm2 beam spot, 2–4 ␮C of charge, and with 3 nA of current [4,16]. The microstructures of samples were examined by a field emission scanning electron microscope (FESEM, JEOL JSM-6330F).

3. Results and discussion Prior to immersing the barrier substrates in the bath, Pd catalyst films were deposited by plasma-assisted atomic layer deposition on TaNX , Ir, and W. From RBS measurements, the ˚ on TaNX , 14.5 A ˚ on Ir, thicknesses of the Pd films were 30 A ˚ on W for 100 cycles. Ten Eyck et al. reported that and 27 A the thickness differences of the Pd films were a result of different initiation times for the various substrates [14]. In all cases, there was a hydrogen plasma pre-clean to remove the native oxide on TaNX and W. The hydrogen plasma has a difficult time removing the adventitious carbon from noble metal substrates. This may be the explanation for the thick-

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Fig. 1. XPS spectra of the Cu surface deposited for 2 min by electroless plating on blanket films; TaNX , Ir and W at a solution temperature of 60 ◦ C and a pH of 12.5.

ness difference between Ir and the refractory metals from the previous work with Pd ALD on Ir [17]. The Cu films were deposited for 2 min by electroless plating on blanket barriers; Pd/TaNX , Pd/Ir, and Pd/W at a solution temperature of 60 ◦ C and a pH of 12.5 without RE-610. Fig. 1 shows the XPS spectra of Cu on the aforementioned substrates. The peak positions of each sample were referenced to adventitious carbon at 284.75 eV. As can be seen, in a typical wide scan spectra of Fig. 1(a) all the standard photoelectron lines of Cu are present; Cu 3p, Cu 2p, O 1s, and adventitious carbon C 1s. The peak, located at around 335 eV, is characterized as the Cu LMM Auger line. The oxidation states of copper ions can be differentiated by their binding energies. Cu(II), as with other paramagnetic compounds [18], the intensity of its shake-up satellite may approach that of the main peak. The absence of the shake-up satellite is usually characteristic of elemental (Cu) or diamagnetic Cu(I). That is Cu2+ can be differentiated from Cu+ or Cu0 based on their binding energies. Cu2+ has a satellite peak due to its open-shell 3d9 configuration, while Cu+ /Cu0 does not show a satellite peak due to its closed-shell configuration (3d10 ) [18]. Fig. 1(b) exhibits a 20.2 eV doublet separation from the Cu 2p spectral line characterized by a binding energy of

934.5 ± 0.2 eV (2p3/2 ) and 954.7 ± 0.2 eV (2p1/2 ) both with satellite shake-up peaks. The Cu 2p3/2 and 2p1/2 shake-up satellite intensities approach that of the main line with satellite intensity at around 943.6 ± 0.2 and 963.3 ± 0.2 eV. The binding energy at 934.5 ± 0.2 eV corresponds to Cu2+ and is observed for all samples. It was confirmed that spectral line of Cu 2p with satellite shake-up peaks is due to CuO on the Cu surface. There is no indication of incorporation of any of the electroless reagents at the surface of the films, specifically amines at ∼400 eV and sulfates at ∼168 eV. The spectral lines of O 1s were only curve fitted using Shirley type base line with mixed Gaussian (30%) and Lorentzian (70%) profiles. The best fitting was achieved with two components at the binding energies of 530.8 and 532.2 eV. The O 1s peak position at 530.8 eV that corresponds to copper oxide and is consistent with the Cu 2p and Auger peaks [18]. The additional O 1s peak at 532.2 eV is associated with hydroxyl groups from moisture. Fig. 1(d) shows the spectral line of C 1s, which is due C H and C O. There is a clear C C or C H peak present, which is likely due to adventitious carbon; however, RBS is needed to determine this unless sputtering is undertaken with XPS. The fitting of the spectra revealed the presence of a C O peak. The C O

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Fig. 3. RBS spectrum of Cu film (Cu/W/Si) deposited for 15 min on W/Si at a solution temperature of 60 ◦ C and a pH of 11.8.

Fig. 2. (a) RBS spectra and (b) depth profiling of the copper film deposited for 2 min on Pt at a solution temperature of 60 ◦ C and a pH of 12.5.

area with the W substrate is slightly emphasized relative to the TaNX and Ir substrates. The peak associated with OH bonding is also emphasized for the W sample. This might be an indication of a rougher film absorbing more water vapor from the ambient conditions. RBS analysis can determine the film thickness to an accuracy of ±8% with ultra-thin films defined by spectra that do not exhibit a flattop [19]. Fig. 2(a) shows the RBS spectra of Pt with and without electroless deposited Cu. These films were deposited for 2 min at 60 ◦ C and a pH of 12.5 without RE-610. Simulating the spectra in Fig. 2(a) with XRUMP [20], the thickness of the platinum substrate was 57.5 nm and the thickness of ELD Cu on Pt is 140 nm. The lip around 1.5 MeV is due to the Cu layer on top of the Pt. The Pt peak shifts towards lower energies due to the presence of the Cu thin film on top of it. Depth profiling of the copper film is shown in Fig. 2(b). There is an apparent intermixing region between Cu and the Pt substrate of ∼25 nm. This alloying layer results in beneficial adhesion; however, that it exists might not be beneficial with using Pt as a catalytic layer. It is not clear from the work here but it may be that Pt is etched in the alkaline electroless

solution and thus the reason for the alloying layer. Ultrathin noble metal layers that can activate barrier layers are attractive to electroless processes but only if they are not etched in the electroless solution and do not diffuse into the copper trench at elevated temperature anneals, e.g. >400 ◦ C. Diffusion can be a problem for cubic or hexagonal close packed structures that have lower melting points like group VIII noble metals. Cu was deposited directly on tungsten via ELD. Fig. 3 shows the RBS spectrum of the Cu film deposited for 15 min by electroless plating on W/Si at a solution temperature of 60 ◦ C and at pH of 11.8. Simulating Fig. 3 with XRUMP results in a Cu film thickness of 180 nm. Previously, we reported that an incubation time of 3–5 min is needed to dissolute tungsten oxide surface films to tungsten ions [21]. The deposition rate of copper was ∼11.2 nm/min. Furthermore, the carbon content in Cu ELD film was less than 3% with the use of a tungsten/carbon substrate via RBS analysis. Fig. 4 shows the RBS spectra of copper films deposited on W/C and the bare W/C substrate. This Cu film was deposited for 10 min at 60 ◦ C and a pH of 11.8. As can be seen in Fig. 4(a), the thickness of tungsten on the carbon substrate is 48 nm. After Cu ELD, the thicknesses of the W substrate and Cu are 8.5 and 114 nm. The tungsten substrate is etched in a solution of copper ELD from 48 to 8.5 nm during electroless deposition. From Fig. 4(b), the amount of oxygen in the Cu films is less than 1%. It is also evident from Fig. 4(c) that only 2.5 nm of W exists below the Cu/W alloying layer in Table 2 Table 2 Simulation layers for RBS analysis of ELD Cu on the W/C substrate Sublayers

Thickness (nm)

Composition

1 2 3 4 5 6 7

3 5 55 65 2.5 5 10000

W:O = 1:2 (surface oxide) Cu:O = 2:l (surface oxide) Cu Cu:W = 1:0.09 W C:O = 1:2 (oxygen on substrate) C

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Fig. 4. RBS spectra of Cu deposited for 10 min on tungsten deposited on carbon (Cu/W/C) and without Cu (W/C) at a solution temperature of 60 ◦ C and a pH of 11.8: (a) wide view of the spectra; (b) zoomed on the oxygen region.

(fifth of sublayer). The modeling of Fig. 4(b) and (c) with the parameters in Table 2 shows that there is a significant copper layer that contains 5–8% tungsten. In the present study, metallization of tungsten and copper by ELD results in an alloy of copper–tungsten with a thickness of 65 nm. Table 3 shows depth profiling of the Cu film on tungsten, if there is no etching during the deposition of copper (open circles in Fig. 4(c)). The two peaks at 1.65–1.8 eV are representative of W. The lower energy peak is what is left of the W substrate peak after W was etched in the ELD solution. The higher energy peak is the alloyed W in the Cu ELD film. The alloying is predominately at the bottom of the Cu film but Table 3 Simulation of ELD Cu on W/C assuming no alloying between Cu and W Sublayers

Thickness (nm)

Composition

1 2 3 4 5 6

0.3 5 111 48 5 10000

W:O = 1:2 (surface oxide) Cu:O = 2:l (surface oxide) Cu W C:O = 1:2 (oxygen on substrate) C

very evident. The depletion at 1.45 eV is attributed to this alloying. This etching of W in the alkaline bath has limited the use of W as a substrate for electroless deposition of Cu in applications where W should be thin; however, the alloying layer does result in excellent adhesion between Cu and W. The Pourbaix diagram provides useful information about the phase diagram [22]. According to the Pourbaix diagram, the WOX species easily forms anionic WO4 2− in the electroless bath and W is not stable under highly alkaline conditions. It is not clear whether or not WNX will also be etched in these Cu electroless solutions. The electroless deposition of Cu has emerged as a convenient method to fill nano features, but reference to a conventional potential-pH equilibrium diagram is advised to evaluate the stability of the substrate metal or metal nitride. We used a patterned substrate with a sputtered TaNX layer to look at the trench fill capabilities of the ELD Cu solution. PA-ALD Pd was deposited under the same conditions as the ˚ Pd film exhibited good surface blanket TaNX layers. A 30 A ˚ on coverage and had a RMS surface roughness of ∼0.9 A the blanket TaNX substrate. We also reported previously a

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At higher deposition temperatures the trench fill is poor and the film surface is rough. Ultrasonic vibration and RE-610 likewise have a smoothing effect on the film. 4. Conclusions

Fig. 5. FE-SEM image of Cu on patterned TaNX (3–1 aspect ratio 130 nm) deposited by electroless plating at 50 ◦ C at a pH of 12.5 for (a) 2 min and (b) 5 min.

deposition rate of 25 nm/min at 65 ◦ C and 15 nm/min at 60 ◦ C with 5 ␮l of RE-610 [15]. Fig. 5 shows cross-sectional FESEM images of Cu films on the Pd/TaNX substrates deposited by electroless plating at pH of 12.5, deposited at 50 ◦ C with RE-610 for (a) 2 min and (b) 5 min. From Fig. 5(a) it can be seen that very fine copper coalesced to give uniform coverage. The deposition rate for the film deposited in Fig. 5(b) was 10 nm/min. The deposition rate for the electroless films is highly dependent on the solution bath temperature. Fig. 6 shows a cross-sectional FE-SEM image of Cu trench fill of the same 3–1 aspect ratio 130 nm trench on the PA-ALD Pd on sputtered 10 nm TaNX barrier layer. The electroless deposition was undertaken at pH of 12.5 at 45 ◦ C for 30 min. The deposition was initiated with a one second ultrasonic vibration. The RE-610 wetting agent, which is a phosphate ester was also added to the solution. We suggest that trench fill can be improved via electroless deposition by lowering the plating temperature, initiating the reaction with ultrasonic vibration, and using a wetting agent such as RE-610.

Fig. 6. FE-SEM images of Cu on patterned TaNX (3–1 aspect ratio 130 nm) deposited for 30 min by electroless plating at 45 ◦ C at a pH of 12.5. Additionally, ultrasonically vibrated for 1 s at the beginning of the deposition and the wetting agent RE-610 was present.

In the present study, copper was deposited on various refractory and noble metals using via an electroless solution. For the copper electroless process, copper sulfate, ethylenediamine tetraacetic acid (EDTA), and glyoxylic acid were used. Additional chemicals such as polyethylene glycol, 2,2 dipyridine and RE-610 as surfactant, stabilizer and wetting agent were also used respectively. Electroless depositions were undertaken on plasma-assisted atomic layer deposited Pd on tantalum nitride (TaNX ), W and Ir at 45–60 ◦ C. For Cu interconnects where electroless deposition is employed, a diffusion barrier that is catalytic itself or interacts favorably with a catalytic layer such as Pt, Pd, Au, Ru, etc., is needed. The electroless Cu films deposited on PA-ALD Pd on sputtered TaNX did not delaminate during deposition. The trench fill of electroless Cu on PA-ALD Pd deposited on TaNX in 3–1 aspect ratio 130 nm trenches has been demonstrated via FE-SEM cross-sections. We suggest that ultrasonic vibration, a wetting agent, and lowering the temperature to 45 ◦ C facilitates the trench fill. What is more, hydrogen evolution is sequestered with a low temperature bath, ultrasonic vibration and use of the wetting agent RE-610. Acknowledgments This work was supported by the Korea Science and Education Foundation (KOSEF) and Korea Ministry of Science and Technology (KMOST). We also thank Intel for providing the patterned substrates.

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