Characterization of self-assembled monolayers for CuCu bonding technology

Accepted Manuscript Characterization of self-assembled monolayers for Cu-Cu bonding technology

M. Lykova, E. Langer, K. Hinrichs, I. Panchenko, J. Meyer, U. Künzelmann, M.J. Wolf, K.D. Lang PII: DOI: Reference:

S0167-9317(18)30470-2 doi:10.1016/j.mee.2018.09.008 MEE 10864

To appear in:

Microelectronic Engineering

Received date: Revised date: Accepted date:

8 May 2018 5 September 2018 30 September 2018

Please cite this article as: M. Lykova, E. Langer, K. Hinrichs, I. Panchenko, J. Meyer, U. Künzelmann, M.J. Wolf, K.D. Lang , Characterization of self-assembled monolayers for Cu-Cu bonding technology. Mee (2018), doi:10.1016/j.mee.2018.09.008

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ACCEPTED MANUSCRIPT CHARACT ERIZAT ION OF SELF-ASSEMBLED MONOLAYERS FOR CU-CU BONDING T ECHNOLOGY

M. Lykovaa,* , E. Langerb, K. Hinrichsc, I. Panchenkoa,d, J. Meyera, U. Künzelmannb, M. J. Wolfd and K.-D. Lange a

Institute of Electronic Packaging Technology (IAVT), TU Dresden, 01062 Dresden, Germany Institute of Semiconductors and Microsystems (IHM), TU Dresden, 01062 Dresden, Germany c Leibniz-Institut für Analytische Wissenschaften – ISAS – e. V., Schwarzschildstr. 8, 12489 Berlin, Germany d All Silicon System Integration Dresden (IZM-ASSID), Fraunhofer Institute for Reliability and Microintegration (IZM), Ringstr. 12, 01468 Dresden, Germany e Fraunhofer Institute for Reliability and Microintegration (IZM), Gustav-Meyer-Allee 25, 13355 Berlin, Germany

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ACCEPTED MANUSCRIPT Cu-Cu bonding process is intensively investigated nowadays because of the possibility of obtaining fine -pitch interconnects in 3D stacks for efficient electronic and sensing applications. These interconnects possess high shear strength, as well as exce llent electrical and thermal conductivity compared to commonly used solder joints. The bottleneck of Cu -Cu bonding technology is the rapid oxidation of Cu and adsorption of a contamination layer upon exposure to air. This effect can be strongly reduced by us ing self-assembled monolayers (SAMs) as temporary protective coatings on Cu substrates. The results of X-ray photoelectron spectroscopy (XPS) yielded a drastic decrease in o xidation rate of both sputtered and electroplated Cu surfaces during storage of passivated samples at -40 °C for 7 days. The results of infrared spectroscopic ellipsometry (IRSE) showed that after 10 days of air exposure at room temperature the long -chain 1-hexadecanethiol (C16) SAMs appear to still remain on the Cu surface and to have much slower oxidatio n rate in comparison to the short-chain 1-hexanethiol (C6) SAMs. Therefore, we can make a conclusion that the application of long -chain alkanethiols or storage of passivated samples at low temperature can provide a long-term oxidation protection for Cu surfaces stored in air.

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Keywords — self-assembled monolayer, XPS, infrared spectroscopic ellipsometry, IRSE, Cu-Cu bonding, Cu passivation

ACCEPTED MANUSCRIPT 1.

INT RODUCT ION

SAMs are organic assemblies that start to form spontaneously on the solid surface due to exposure to an active surfactant. SAM molecules of alkanethiols (Fig. 1) consist of S head group, CH2 alkane chain and CH3 terminal group [1]. The affinity of the S head group to certain solid surfaces makes it possible for a closely packed monolayer to form. The presence of CH 2 alkane chain in the surfactant molecule is responsible for lateral interactions (van der Waals forces) between the neighbor molecules. CH3 terminal group has major impact on the modification of surface characteristics , i.e. its wetting properties.

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-H - terminal group - alkane chain

- S head group - substrate

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Fig. 1. Schematic diagramms: (a) 1-Hexanethiol with a tilt angle ; (b) thiolates chemisorbed and arranged on the substrate.

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SAMs have various applications in microelectronics industry. The organic monomolecular layer was used as a diffusion barrier between porous low-k films and metals [2], [3]. Another broad application for SAM is surface patterning in the lithography process [4], [5]. In this study we want to focus on a further SAM application, namely, on the oxidation protection for Cu interconnects [6], [7]. As Cu oxidizes within seconds of air exposure, it remains a challenge to use this metal as an interconnect material in 3D systems widely in industry. Usually, a certain period of time passes between the electroplating of Cu microbumps and further bonding procedures. During this time Cu should be protected from oxidation. SAMs with a thickness of few nanometers can be used for this purpose. Generally, they are removed from the Cu surface via thermal annealing before the chips are pressed together in a bonding chamber with an inert gas [8]. This guaranties that pure Cu surfaces are exposed to thermocompression bonding and a high interconnect quality is achieved. Nevertheless, SAMs tend to degrade and oxidize upon several hours of air exposure. Therefore, the prolongation of its effectiveness remains an important goal. D. F. Lim et. al. reported that SAM does not degrade on Cu after 3 days of storage in a fridge at 4 °C [6]. The inspection of monolayer quality was carried out by surface wettability analysis (contact angle technique). Si wafer with 100 nm of sputtered Cu on top was used as a substrate. Furthermore, D. Hutt and C. Liu conducted X-ray photoelectron spectroscopy (XPS) analysis of the passivated Cu coupons after storage in a freezer at -30 °C for 10 weeks [9]. No essential evidences of oxidation process were found after this storage time. These results suggest that storage of passivated samples at low temperatures has a high potential for a long -term oxidation protection of Cu surface without the need of using inert gas atmosphere. Cu surface characteristics influence SAM quality along with storage conditions. SAMs are usually passivated on the substrates with low roughness to achieve high ordering degree (e.g. on sputtered metal). Nevertheless, Cu microbumps are usually fabricated by electrochemical deposition (ECD) which causes higher roughness , higher grain size and lower amount of grain boundaries . Hence, the investigation of SAM performance on the ECD Cu surface is an essential point of this study. One more way to prolong SAM performance is to choose assemblies with longer alkane chains. The aging of SAMs with different alkane chain lengths on Au surfaces in air conditions has been widely investigated by infrared spectroscopy [10], [11]. Nevertheless, there is still a lack of knowledge about behavior of these films on Cu surfaces. Infrared spectroscopic ellipsometry (IRSE) is a non-destructive analytical method which gives information about molecular orientation of thin films and thus can monitor the change in their chemical state. Therefore, the aims of the study are to investigate the influence of Cu deposition method, chain length of alkanethiols and storage conditions on SAM protective capability.

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2.

M AT ERIALS AND MET HODS

2.1 Sample description and preparation

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Two types of samples were used for the surface characterization: the first type is a 13 × 13 mm2 Si die (750 µm thickness) with Ti (150 nm), Cu seed (150 nm) and ECD Cu (2 µm) on top; the second type is a 13 × 13 mm2 Si die (750 µm) with Ti (150 nm) and sputtered Cu (2 µm) on top, later referred to as physically vapor deposited (PVD) Cu. The arithmetical mean roughness value (Ra ) of ECD Cu was approximately 13 nm, the roughness of PVD Cu was approximately 2 nm. The surface roughness values were measured by a confocal microscope µSurf (NanoFocus) using a 320-S lens. All samples were etched with dilute sulfuric acid H2 SO4 10 % and rinsed with deionized (DI) water before the other procedures. To perform passivation with SAMs, the etching and passivation procedures were carried out in a glove box system (Braun, Labstar) with an Ar atmosphere (O2 < 0.5 ppm). After etching the samples were rinsed with ethanol 99.9 % and immersed into the 1 mMol solution of 1-hexanethiol (C6, Sigma-Aldrich, ≥ 95 %) or 1-hexadecanethiol (C16, Sigma-Aldrich, ≥ 95 %) in ethanol for 2 h. During this time a monolayer is forming on the Cu surface. After the passivation the samples were taken out of the solution and rinsed with ethanol to remove excess SAM precursors .

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2.2 XPS Characterization of the achieved monolayer quality and storage tests followed the passivation procedure. In the first experiment samples with PVD and ECD Cu with and without C6 passivation were stored at -40 °C for 1 week. SAM quality was analyzed by XPS right after the passivation and after the storage tests. Analysis of the chemical state of the Cu surface was carried out by means of XPS system PHI ESCA 5700 (Physical Electronics). A source with a power of 250 W and non-monochromatic Al-Kα radiation was used. Fig. 2 shows atomic concentrations of chemical components (O, Cu, S and C) on the ECD and PVD Cu surfaces. X-axis corresponds to the time and energy of Ar+ ion bombardment or Ar+ ion sputtering of the Cu surface. After each sputtering step a certain fraction of the Cu surface, including the coating films on the top, is removed. This is conducted in order to compare , how fast O, C or S compounds are sputtered away from the metal substrate. The first sputtering step has a low Ar+ ion energy of 500 eV (ion current 100 nA), because it is used to remove the organic contamination layer that is adsorbed within seconds by each metal surface upon exposure to air. The next sputtering steps have higher bombardment energy of 2 keV (ion current of about 1µA), because they are actually responsible for sputtering of oxide layer away and cleaning of the Cu surface. 2.3 IRSE

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To investigate SAMs with different chain lengths, samples with PVD Cu were passivated with C6 and C16 and stored at room temperature for different durations in air. After the storage tests each sample surface was characterized using IRSE technique. IRSE is a thin film sensitive method that can measure changes in polarization and amplitude of the incident infrared light caused by the interaction with the sample surface. The reflected light typically is elliptically polarized and can be described by the amplitude ratio tan(Ψ) and phase shift

ACCEPTED MANUSCRIPT difference Δ of p- and s-polarized components. Light in the mid infrared spectral range can be absorbed due to excitation of molecular vibrations of the probed SAM. The ellipsometer was connected to the IR spectroscopic system Bruker 55. The incidence angle was set to 80°. Further details on the ellipsometric method can be found in [12]. 3.

RESULT S AND DISCUSSION samples with PVD Cu and ECD Cu for comparison

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3.1 Influence of storage conditions on SAM quality on

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Fig. 2. XPS-analysis of PVD and ECD Cu-surfaces with C6 (SAM) and without (no SAM) passivation after preparation at room temperature and after 7 days storage at -40 °C. T he investigated chemical element is given in the lower right of each chart.

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The results of the quantification of high-energy resolution core level XPS measurements for PVD and ECD Cu surfaces can be seen on Fig. 2. Both sample types with SAM pass ivation, stored at -40 °C for a week, show a very low O content after 120 s bombardment with 500 eV Ar+ ions. This is the evidence that after removing of the organic contamination layer a clean Cu surface without any oxide is seen. The samples without SAM that were not stored at low temperature, were still exposed to air for maximum 2 h due to transportation. Although it is a comparably short period, the samples indicate almost the same level of O contamination as samples stored in air for a week. This suggests that the rate of Cu oxidation is very fast in the first few hours, whereby the oxide thickness remains almost unchanged in the next 3 weeks , approximately 2-3 nm [13]. In contrary, samples with SAM passivation without storage in air show nearly no O content after 120 s of sputtering with 500 eV which confirms an excellent protection of the monolayer. For Cu 2p spectra the atomic concentrations are the highest for samples with SAM passivation because of the absence of oxide and contamination layer after the first sputtering step . The C 1s spectra are shown to prove that after the 120 s of a cleaning step there is no more organic contamination on the Cu surface. Comparison of the C 1s and the O 1s data shows that for passivated substrates all O is removed with the organic contaminations (and the SAM) in the first sputtering step, while for not -passivated Cu the O is still present and hence directly bound to the Cu.

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Fig. 3. S2p spectra of ECD (a) and PVD Cu (b) after storage at -40 °C for 1 week

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Fig. 3 shows two peaks: a low intensity peak at 168.2 ± 0.3 eV which can be assigned to sulfonates and a high intensity peak at 162.9 ± 0.3 eV which can be assigned to thiolates [14]. Thiolates are the products of chemisorption between S head group and Cu atoms. Sulfonates form upon oxidation of the S head group due to SAM degradation. A slight intensity of the sulfonate peaks suggests that a small amount of defects is detected in SAMs. A relatively high intens ity of the thiolate peaks means that not degraded SAMs were present on the Cu surface after 1 week of storage at low temperatures. This suggests that storage of Cu with SAM at low temperature almost termin ates oxidation process. This can be caused by fewer disruptions in van der Waals forces due to thermal movement of the atoms in molecules . This, in turn, leads to higher ordering of the monolayer and, hence, better protective capabilities. 3.2 Influence of the chain length of SAM passivation on its degradation rate There are four characteristic absorption bands of the alkanethiol spectrum on Cu: νs (CH2 ), νa(CH2 ), νs (CH3 ) and νa(CH3 ). ABSORP TION BANDS IN CM-1 OF CH2 AND CH3 STRETCHING BANDS C6, C16 ON CU SURFACE AND OF P URE CU SURFACE SP ECTRA

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2964 2964

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Table 1 shows frequencies of band maxima of CH2 and CH3 related stretching bands. The results show that the s ample without SAM and with C6 has absorption bands at almost the same frequencies whereby C16 absorption bands are found at lower wavenumbers. This can be explained by the various disordering levels of the adsorbed organic contamination layer on sample without SAM (No SAM), samples with short-chain thiols (C6) and long-chain thiols (C16). The higher is the ordering, the better is the interaction between the molecules. The characteristic CH 2 stretching bands tend to change their peak positions towards higher frequencies with lower ordering of the layer. Therefore, short-chain thiols have been proven to have similar ordering to the liquid phase, whereby long-chain thiols tend to have a crystalline structure with a higher ordering [15]. Peak intensities for the bands ν s (CH3 ) in Fig. 4 are important to differentiate between samples with SAM and without it and to measure the packaging density and surface coverage of SAMs. Due to the terminal CH 3 group, samples with SAM have obviously more of ordered CH3 molecules which can be proven by higher intensities for these peaks. The stronger resonance of ν s (CH3 ) for the longer alkane chains also shows a higher packaging density of the CH 3 -groups due to the higher ordering of the SAM compared to the shorter alkane chains.

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Fig. 4. Measured IR spectra of C6, C16 monolayers on PVD Cu without storage in air and the spectra of clean PVD Cu without and with storage in air for 2 weeks

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tan(Ψ) xhours /tan(Ψ) 17hours

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Fig. 5 shows spectra for samples with and without passivation after 113-115 h and 186-187 h of storage at room temperature in air, respectively. To emphasize the changes during storage, these spectra were divided by the spectra of samples stored at room temperature in air for 17 hours.

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Fig. 5. IR spectra of samples with and without SAM passivation after different storage durations in air: (a) without SAM after 115 h; (b) with C6 after 114 h; (c) with C16 after 113 h; (d) without SAM after 187 h; (e) with C6 after 188 h; (f) with C16 after 186 h.

The increase of CH2 band intensity with the storage time and minor changes of CH 2 bands for non-passivated samples are detected. This can be explained by the fact that Cu surface without SAM protection quickly build up a contamination layer directly after preparation and no significant changes happened during the characterization experiments. In contrary, the modified band signatures of the passivated surface indicate a change in a chemical state. The increase of band amplitudes of νs (CH2 ) and νa(CH2 ) vibration modes can be explained by an increase of the average tilt angle α of the SAM (Fig.3). Due to the surface selection rule in infrared spectroscopy, for ultrathin films on metallic surfaces only those components can adsorb infrared radiation, for which the transition dipole moment is perpendicular to the substrate. This mea ns that only Z-component of the projection of transition dipole moment has to be taken into account [16]. Monolayer tilting is most likely due to the oxidation of thiolate head group (S) and hence formation of sulfonates [10]. Therefore the spectra suggest that the oxidation rate of SAM with shorter chain lengths (C6) is higher. This may be explained by lower ordering of SAMs and more defects (pinholes), as well as lower density of SAM molecules and, in turn, low coverage.

ACCEPTED MANUSCRIPT Another evidence of better protective functions of C16 is the peak position which arises still at the frequencies of 2849 and 2918 cm-1 even after 188 h. These peak positions suggest that C16 remains present on the Cu surface even after 188 h of storage in air and has not been fully oxidized. 4.

CONCLUSIONS

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Surface chemical state of PVD and ECD Cu with and without SAM with and without storage at low temperatures was analyzed. ECD and PVD Cu can be protected from oxidation by alkanethiol-SAMs. Almost no oxidation of Cu surface with SAMpassivation was detected after storage at -40 °C in air for 1 week. The results of the investigation show definite protective abilities of SAM against oxidation of Cu surfaces. This can be explained by the better ordering of such monolayers due to strengthened van der Waals forces between the alkane chains at low temperature. SAMs with longer chain lengths are more effective than the shorter ones. The protective properties of alkanethiol SAMs increase with increasing chain length and decreasing temperature. This can be interpreted as a direct dependence of the prote ctive effect and the ordering of the SAM, which also increases with longer chains and lower temperatures. The protection of Cu surfaces by alkanethiol SAMs in combination with low-temperature storage offers an effective and reliable opportunity for temporary protection against oxidation, without the need of an inert atmosphere or a permanent protective layer.

ACCEPTED MANUSCRIPT A CKNOWLEDGEMENT S

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This work was founded by Graduate Academy of TU Dresden (Scholarship Program for the Promotion of Early -Career Female Scientists of TU Dresden (without Faculty of Medicine). The samples were manufactured by the Fraunhofer Institute for Reliability and Microintegration (IZM), Dresden Branch of the Institute (ASSID), in cooperation with Fraunhofer IZM, Berlin Branch of the Institute. We thank I. Engler from Leibniz-Institut für Analytische Wissenschaften for laboratory assistance. Financial support by the Ministerium für Innovation, Wissenschaft und Forschung des Landes Nordrhein -Westfalen, the Regierende Bürgermeister von Berlin - Senatskanzlei Wissenschaft und Forschung, and the Bundesministerium für Bildung und Forschung is gratefully acknowledged, as well as the European Union through the EFRE program EFRE 1.8/13. The authors would like to gratefully acknowledge the staff of the Institute of Electronic Packaging Technology (IAVT) and Institute of Semiconductors and Microsystems (IHM) of TU Dresden for active support and fruitful discussions.

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ACCEPTED MANUSCRIPT Highlights

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Oxidation of the Cu surface can be prevented using self-assembled monolayers No oxide on the Cu surface after 1 week storage of passivated samples at -40 °C in air Long-term oxidation protection for both electroplated, and sputtered Cu surfaces Oxidation rate of Cu decreases with increasing the chain length of the monolayer

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