Effects of Solvent on the Formation of the MUA ... - Springer Link

Electron. Mater. Lett., Vol. 10, No. 3 (2014), pp. 671-678 DOI: 10.1007/s13391-013-3339-0

Effects of Solvent on the Formation of the MUA Monolayer on Si and Its Diffusion Barrier Properties for Cu Metallization Mohammad Arifur Rahman, Jung Suk Han, Kyunghoon Jeong, Ho-seok Nam, and Jaegab Lee* School of Advanced Material Engineering, Kookmin University, Seoul 136-702, Korea (received date: 2 December 2013 / accepted date: 14 December 2013 / published date: 10 May 2014) We investigated the effects of solvents, such as ethanol and isooctane, on self-assembly of the mercaptoundecanoic acid (MUA) monolayer on Si and its diffusion barrier properties for Cu metallization. The use of isooctane as a solvent produced MUA self-assembled monolayers (SAMs) (~1.3 nm thick) on Si. These acted as an effective diffusion barrier against Cu diffusion up to 200°C. In contrast, the MUA SAMs produced by ethanol allowed the diffusion of Cu to a MUA-Si interface at 200°C, stimulating the out-diffusion of Si into Cu and thus resulting in the degraded diffusion barrier properties. This was possibly due to the partial formation of interplane hydrogen bonding between the terminal groups of the bound acid and free thiol groups. This provided less dense thiol surface groups, thus leading to poor adhesion of Cu to MUA SAMs. The fabricated Cu/isooctane-assisted MUA source/drain electrode a-Si:H thin film transistors with a channel length of 10 µm exhibited an excellent electron mobility of 0.74 cm2/V-s, threshold voltage of −0.51 V, I /I ratio of 3.25 × 106, specific contact resistance of 4.24 Ω-cm2 after annealing at 200°C. on

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Keywords: SAMs, MUA, thiol surface group, diffusion barrier, TFTs

1. INTRODUCTION Molecular self-assembly offers a versatile method for achieving a desired surface functionality[1] and topography. Structural flexibility of self-assembled monolayers is used in a variety of applications, such as coating, electronic devices, chemical and bio-sensors. Most of these applications require good control over the structure and stability of the monolayers.[2] Experimental conditions, such as solvent, temperature, concentration, immersion time, and substrate cleanliness significantly affect the formation and structure of alkanethiol self-assembled monolayers (SAMs).[3-5] Solvent choice is an important factor determining the structural quality and formation kinetics of SAMs.[6-8] Changes to the chemical nature of the solvent impacts a number of parameters, such as steric constrains, polarity, viscosity, mobility and solubility for a given SAM molecule.[9] The formation and structure of alkanethiol SAMs on Au surfaces have been extensively studied using various surface characterization techniques.[2,5,10,11] However, materials, Si is the most important of various materials in micro ~ nano electronics and mechatronics. Thus, MUA SAMs on Si are of special interest to integrate Si micro ~ nanodevices, especially with the use of the organic monolayer as an interface modifier or diffusion barrier for Cu metallization. Cu metallization has been recently employed as the gate *Corresponding author: [email protected] ©KIM and Springer

and source/drain (S/D) electrodes of the development of aSi:H TFT-LCDs in mass production due to their superior material properties[12,13] and the development of the Cu wetetching processes to address the difficulty of dry etching of Cu.[14] In addition, the typical Cu interconnection and S/D electrodes for a-Si:H TFTs and Hf-In-Zn-O (HIZO) thin film transistors consist of Cu/Ti (or Mo) bilayer structures,[15,16] where a Ti (or Mo) thin film serves as the diffusion barrier/ adhesion layers. However, the formation of a bilayer structure requires complex process steps, including photolithography, sputter deposition and wet-etching of a bilayer interconnect, thus resulting in a high cost fabrication process. The facile fabrication of Cu/MUA SAMs/Si structures leads to a simple process, and thus achieves low cost production. In this study, the effect of solvent on SAMs formed on aSi:H as an interfacial barrier for Cu source/drain electrode was studied. We selected a mercaptoundecanoic acid (MUA) monolayer consisting of -SH surface group/carbon chain (11)/-COOH head groups and used either ethanol or isooctane as a solvent for SAMs formation. The SAMs were used as the diffusion barrier properties of the Cu/Si interfaces through the formation of the Cu-S and Si-O bond with the terminal and head groups of the SAMs, respectively. Therefore, this study focused on the investigation of the solvent effects of the MUA SAMs formation and its diffusion barrier properties of Cu/SAMs/Si structures. Finally, we fabricated Cu/SAMs S/D a-Si:H TFTs and measured their electrical characteristics.

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2. EXPERIMENTAL PROCEDURE The Si (100) (p-type, resistivity = 1 - 30 Ω-cm) substrate was cleaned using a piranha solution (H2SO4 : H2O2 = 1 : 1) for 10 min at 110°C and then dipped in 0.3% HF solution for 10 s. The Si substrate was rinsed thoroughly with deionized (DI) water and dried in an N2 flow. The SAMS were formed on a pre-cleaned silicon substrate by dipping the substrate in a solution of MUA in either isooctane or ethanol at a concentration of 0.1 mM for 1 h. Then, the sample was rinsed with DI water and dried with a flow of N2. The measured water contact angle of MUA was ~67° and the thickness of the MUA monolayer formed on Si was measured to be approximately 1.3 nm by ellipsometry. After this, an approximately 100 nm-thick copper film was formed at a deposition rate of 0.3 nm/s by evaporation in a vacuum of 2 × 10−6 torr. This resulted in Cu/MUA layer/Si structures. For comparison, the same thickness of Cu was deposited on Si(001) substrate to investigate the interface reaction between Cu and Si.[17] The Cu/SAMS/Si(001) and Cu/Si(001) samples annealed in a vacuum at 100°C - 300°C for 1 h were analyzed by x-ray diffraction (XRD) and electrically measured by a four-point probe. For the Cu/SAMs bilayer S/D electrode a-Si:H thin film transistors, n+ a-Si (50 nm)/a-Si:H (200 nm)/Si3N4/Mo gate electrode/glass substrates were patterned to define n+ a-Si:H S/D by photolithography and then cleaned in dilute HF solution, followed by the selective coating of MUA on the S/ D regions by dipping them in an isooctane assisted MUA solution. The patterning of Cu on the SAMs coated S/D regions was made by the lift-off technique. Transmission line model (TLM) patterns were used to characterize the specific contact resistance[18] of the Cu/ MUA/n+ Si (n-type, resistivity = 0.005 Ω-cm) structures. A semiconductor parameter analyzer (HP4155C) was used to measure the electrical characteristics. In addition, the binding energy of the MUA was examined using x-ray photoelectron spectroscopy (XPS) (PHI 5000 VersaProbe (Ulvac-PHI), Al Ka (1486.6 eV).

3. RESULTS AND DISCUSSION Evidence of the formation of a MUA monolayer on Si was provided using XPS. Figure 1 shows the spectra of the elements in both ethanol and isooctane assisted MUA SAMs at a take-off angle of 90°. In Figure 1(a) and 1(b), the C1s spectra reveal the characteristic signals for hydrocarbon (C-C) at 284.4 eV, C-COOH at 286 eV and oxidized carbon (C-O) at 288 eV in both MUA SAMs.[19] In addition, Fig. 1(c) and 1(d) show the oxidized sulfur species [S(2p) binding energy ~168 eV)] of MUA SAMs that could be oxidized sulfur -SOxH.[20] Wang et al.[21] reported the presence of a significant amount of oxidized sulfur species SO32−, in the NH2

terminated SAMs of alkanethiolates prepared with ethanol. The -SOxH group arises from the surface containing -SH groups, and it has stronger affinity to Cu than -SH group. Both SAMs do not show any bound and unbound thiol peaks at ~162 eV and ~164 eV in Fig. 1(c) and 1 (d).[21] The spectra of O (532 eV) in Fig. 1(e) and Fig. 1(f) correspond to -COOH groups.[12] Furthermore, the spectra of Si in Fig. 1(g) and Fig. 1(h) correspond to Si (99.4 eV) and SiO2 (~103 eV).[22,23] 3.1 Diffusion barrier properties We annealed the samples in a vacuum at 100°C - 300°C to evaluate the thermal stability of Cu/MUA/Si structures, as well as the diffusion barrier properties of the MUA SAMs. The variation of the resistivity of the Cu/MUA/Si structures with the annealing temperature is shown in Fig. 2(a). For comparison, the Cu/Si structure was annealed to investigate the resistivity variation with the anneal temperature of 100°C -300°C. This revealed that the resistivity of Cu/Si structure abruptly increased at 200°C, after a slight increase of the resistivity at 100°C. In contrast, the resistivity of Cu/ MUA/Si decreased upon annealing at 100°C. However, further increase of the temperature up to 200°C showed different temperature dependence of resistivity with the solvents, such as ethanol and isooctane, i.e. 200°C annealing of Cu/isooctane-assisted MUA/Si structure decreased the resistivity to 3.0 µΩ-cm, indicating that the SAMs acted as an effective diffusion barrier up to 200°C for Cu metallization. However, the resistivity of Cu/ethanol-assisted MUA/Si structure likely increased the resistivity to 11 µohm-cm as the temperature increased to 200°C. SEM images of the annealed Cu films on either isooctane or ethanol MUA surfaces at 200°C are presented in Fig. 2(b) and Fig. 2(c), respectively. Voids can be seen in the annealed Cu films on ethanol-MUA, compared to the continuous Cu films on isooctane-MUA. The formation of voids can be a result of poor adhesion of Cu to SAMs, and thus reflects the non-uniformly distributed thiol-surface groups, since the Cuthiol group’s bonding is sufficiently strong to hold Cu atoms on the MUA monolayer. Figure 3(a) and Fig. 3(b) show the XRD peaks of both Cu/ ethanol-MUA/Si and Cu/isooctane- MUA/Si annealed at 100°C - 300°C, respectively. The as-deposited Cu films on both ethanol- and isooctane-MUA monolayers showed (200) preferred orientation. However, annealing the samples at a high temperature (≥100°C) had different influences on the evolution of Cu texture, depending on the solvent, i.e., (200) texture of Cu films on ethanol-MUA/Si did not change upon annealing with a significant reduction in the peak intensity of the annealed Cu at 200°C and the accompanied formation of Cu3Si, an indicator of barrier failure. In contrast, the texture of Cu films on isooctane-MUA has been evolved into (111) from (200), which started at 100°C and

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Fig. 1. XPS spectra of the element, such as C, O, S, Si obtained at take-off angles of 90°, respectively.

continued up to 200°C with the increase of the intensity of (111). The evolution of (111) texture is a result of surface energy minimization. Further increase of anneal temperature up to 300°C is likely to deplete the Cu on the surface and to lead to the formation of Cu silicide. This is consistent with the diffusion barrier properties of Cu films, as shown in Fig. 2.

Figure 4 shows the variation of thickness of MUA SAMs with dipping time into either ethanol or isooctane containing MUA. Similar dependence of MUA thickness on dipping time can be seen with both solvents. The thickness of both ethanol and isooctane assisted MUA SAMs increased until 10 min and then reached almost plateau value with dipping time, which probably correspond to full coverage of the


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Fig. 2. The variation of resistivity (a) Cu/MUA/Si structures with annealing temperature, (b) Scanning Electron Microscopic (SEM) images of 200°C annealed MUA surface prepared by (b) ethanol (c) isooctane solvent.

Fig. 3. XRD peaks of as-annealed (a) Cu/ethanol-MUA/Si (b) (a) Cu/isooctane-MUA/Si structures at 100°C - 300°C.

Fig. 4. Variation of thickness of MUA SAMs with substrate dipping time into (a) ethanol containing MUA (b) isooctane containing MUA.


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Fig. 5. Cross-sectional images and corresponding EDX spectrum depth profiles of (a) as deposited with ethanol and isooctane assisted MUA SAMs on Cu/MUA/Si structure (b, c) as-annealed (200°C) with ethanol and isooctane assisted MUA SAMs on Cu/MUA/Si structure.

Fig. 6. Adhesion test (a) Bare Cu/Si (b) Ethanol assisted MUA SAMs (c) Isooctane assisted MUA SAMs.

substrate surface. Figure 5 shows the cross-sectional TEM images and the corresponding EDX spectrum depth profiles of the asdeposited Cu/MUA/Si structure and the as-annealed sample at 200°C. Two distinct regions can be seen of the as-deposited Cu/ethanol MUA/Si structure (Fig. 5(a)). In addition, the corresponding EDX depth profile shows the shift of the Cu profile to the left by ~1.5 nm, which is approximately MUA’s thickness. This provides evidence for the presence of a MUA interlayer between Cu and Si. In addition, annealing the sample at 200°C allowed for the in-diffusion of Cu to the MUA-Si interface, as shown in the EDX depth profile (inset of Fig. (b)). It was noted that Si was out-diffused into Cu from the Si-substrate, forming Cu3Si, as confirmed by fast Fourier Transformation (FFT) patterns in high resolution images. This was consistent with the XRD patterns of the

annealed Cu/ethanol-MUA/Si at 200°C (Fig. 3). It was reported that for the Cu/Si system, the formation of the interface Cu-Si bonds occurs at low temperature and weakens the Si-Si bonds, resulting in the facile breaking of the Si bonds for the release of Si from the Si substrate.[24] Therefore, we believe that the diffused Cu to the MUA-Si interface stimulated the release of Si, and the released Si was outdiffused and reacted with Cu to form Cu3Si. In contrast to the annealed Cu/ethanol MUA/Si at 200°C, the annealed Cu/ isooctane MUA/Si at 200°C shows the clear Cu-Si interface with negligible amount of Si out-diffused into Cu, indicating that the isooctane MUA can act as an effective diffusion barrier for Cu metallization up to approximately 200°C. The difference in diffusion barrier properties between ethanoland isooctane-MUA monolayers can be explained by the different densities of the thiol surface group of MUA


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Fig. 7. Illustration of the structures of (a) MUA SAMs formed on Si:H in ethanol (b) MUA SAMs formed on a-Si:H in isooctane solvent.

monolayers formed on Si using different solvents. The proposed mechanism of MUA SAMs formation with ethanol and isooctane solvents is illustrated in Fig. 7. Due to the presence of highly polar ethanol solvent (dipole moment: 1.69 D), interplane hydrogen bonds forms in the surface between the terminal groups of unbound thiolate of one MUA (HS-(CH2)10-COOH) molecule and unbound carboxylic acid of another MUA (HS-(CH2)10-COOH) molecule in SAMs (Fig. 7(a)) when it was prepared in ethanol.[18] This

interplane hydrogen bonds led to form another hydrogen bond in the bottom between the unbound thiols and bound acids, besides the SAMs forming by Si and oxygen. The side ways interplane hydrogen bonding interrupted the organized self assembly of MUA and thus forming a less dense or less packed MUA SAMs. In contrast, this type of interplane hydrogen bond was not likely to be formed when a non polar solvent isooctane (dipole moment: 0 D) was used (Fig. 7(b)), resulting in highly densely packed MUA SAMs. The Scotch tape test of the as-deposited Cu/Si and Cu/ MUA/Si structure revealed that the adhesion properties of the Cu/MUA/Si were enhanced through the strong chemical interaction of both the interfacial Cu and Si surface with the MUA layer. However, the adhesion properties were slightly weaker with ethanol assisted MUA compared to isooctane assisted MUA layer presented in Fig. 6. This indicates that ethanol assisted MUA self-assembled layers less effectively act as interfacial adhesion enhancers in S/D electrodes compared to isooctane assisted MUA layers for a-Si:H TFTs. Since the isooctane assisted MUA SAMs exhibited relatively better performance as a diffusion barrier of Cu metallization compared to ethanol assisted MUA SAMs, we thus fabricated Cu/isooctane-assisted MUA S/D electrode aSi:H TFTs with a channel length of 10 µm. Figure 8 shows

Fig. 8. Transfer and output characteristics of (a) the as-deposited Cu/MUA S/D a-Si:H TFTs and (b) the as-annealed TFT at 200°C, respectively.


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the transfer and output characteristics of the as-formed Cu/ SAMs S/D a-Si:H TFT and the as-annealed TFT at 200°C, respectively. The threshold voltage VT, subthreshold slope S, on/off current ratio ION /IOFF, and electron mobility at saturation µn, obtained from the curve of the as-annealed TFT at a drain voltage of Vd = 15 V were −0.24 V, 2.60 V/decade, 1.4 × 106, 0.3 cm2/V-s, respectively. In contrast, the annealed TFTs show −0.51 V of threshold voltage, 1.01 V/decade of subthreshold and 3.25 × 106 of Ion /Ioff, 0.74 cm2/V-s of mobility. In addition, the contact resistance Rc was calculated from the output characteristics shown in Fig. 10 using the following equation.[25,26] L RT = RCH + 2Rc = -------------------------------------- + 2Rc WµCox ( VG – VT )

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ACKNOWLEDGEMENTS This work was supported by the Technology Innovation Program (Industrial Strategic Technology Development Program, 10035430, Development of reliable fine-pitch metallization technologies) funded by the Ministry of Knowledge Economy (MKE, Korea). This research was also supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (2013K1A4A3055679).

REFERENCES

(1)

where W is the channel width, L is the channel length, µ is the linear mobility on the contact region, while Cox and VT are the gate oxide capacitance per unit and threshold voltage, respectively. The specific contact resistance, ρc, calculated from the as-annealed TFT was 4.24 Ω-cm2 and the contact resistance, Rc, was 1.0 × 105 Ω, about one order of magnitude lower than the channel resistance (3 × 106 Ω). The as-deposited TFTs show a much higher specific contact resistance than as-annealed TFT, which leads to current crowding effects and lower mobility. For the electrical properties of Cu/SAMs S/D a-Si:H TFT mentioned above, it is clear that as-annealed TFT exhibits better performances when isooctane assisted MUA SAMs work as a diffusion barrier. However, ethanol assisted MUA SAMs behaved as degraded barrier layers with the production of Cu silicide during annealing. Therefore, isooctane assisted SAMs will be the better choice for Cu/SAMs S/D a-Si:H TFT fabrication.

4. CONCLUSIONS We demonstrated the solvent effects on the formation of the MUA monolayer on Si and its diffusion barrier properties for Cu metallization. Ethanol assisted MUA SAMs in Cu/ MUA/Si structures allowed the Cu atoms to diffuse to the MUA-Si interface and at the same time the outdiffusion of Si to Cu, which led to the formation of Cu silicide at 200C. In contrast, isooctane assisted MUA SAMs exhibit relatively good performance as an interface diffusion barrier, as a result of uniformly distributed thiol surface functional groups that are able to hold Cu atoms tightly. The less organized ethanol assisted MUA SAMs is attributed to the formation of sideways interplane hydrogen bonds. The as-fabricated Cu/ SAMs S/D electrode a-Si:H TFTs had a mobility of 0.30 cm2/V-s and a specific contact resistance of 23.8 Ω-cm2, while the as-annealed TFT at 200°C exhibited a higher mobility (0.74 cm2/V-s) and lower contact resistance (4.24 Ω-cm2).

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