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. 2026 May 11;11(20):29943–29949. doi: 10.1021/acsomega.6c01006

Low-Temperature Plasma-Enhanced Atomic Layer Deposition of Cu Films Using Liquid Copper Precursor Bis(1-ethylmethylamino-2-propoxy)copper, Cu(emap)2

Akihiro Nishida †,‡,*, Atsushi Yamashita †, Masaki Enzu †, Atsushi Sakurai †, Yasutaka Matsuo ‡
PMCID: PMC13216949  PMID: 42222780

Abstract

Cu atomic layer deposition (ALD) is necessary to deposit thin Cu seed layers with good coverage, high film continuity, and low resistivity onto a high-aspect-ratio substrate. In addition, the Cu metal should be deposited at as low a temperature as possible, ideally ≤100 °C, to minimize the surface roughness, promote facile nucleation, and form continuous films even at thicknesses of a few nanometers. However, Cu ALD requires a temperature greater than 80 °C when conventional Cu ALD precursors are used because they exhibit low volatility and high melting points. In the present study, we investigated Cu aminoalkoxide precursors and found an excellent candidate, bis­(1-ethylmethylamino-2-propoxy)­copper, Cu­(emap)2, which exhibits both high volatility and a low melting point. We also demonstrated PE-ALD of a thin Cu metal film using Cu­(emap)2 and H2 as a coreactant and prepared shiny metallic Cu films at 40 °C. The growth rate of Cu on Ru and SiO2 substrates was 0.22 and 0.41 Å/cycle, respectively, and the resistivity of the films on the SiO2 substrate was measured to be 4.1 μΩ·cm. No carbon, nitrogen, or oxygen contaminants were detected by X-ray photoelectron spectroscopic analysis of the resultant Cu metal films. Regarding film morphology, we verified by field-emission scanning electron microscopy that continuous films less than 10 nm thick were deposited on planar Ru and Co substrates. Therefore, we concluded that, compared with conventional Cu ALD precursors, Cu­(emap)2 is better suited for Cu ALD manufacturing processes. The present results indicate that their application to TH-ALD could, in the future, enable Cu ALD on high-aspect-ratio substrates.

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Introduction

Cu is an attractive compound for use in semiconductor devices because of its high electrical conductivity, high thermal conductivity, and comparatively good electromigration resistance. , It has been used in various industrial applications, including as an interconnect material in semiconductor devices. , Three-dimensional integration technology using interposers has been especially important for recent device scaling. In particular, glass via interposers fabricated using through-glass via (TGV) technology are attracting attention because of their simple fabrication process and lower manufacturing costs compared with those of Si interposers. Physical vapor deposition (PVD) has been one of the most popular methods to deposit thin Cu seed layers for microelectronic devices. However, depositing a conformal and continuous film on high-aspect-ratio substrates for scaled semiconductor devices is difficult. Cu atomic layer deposition (ALD) is necessary to deposit thin Cu layers with good coverage, high film continuity, and low resistivity (ρ).

The ALD of Cu metal films using various Cu precursors has long been a topic of study. However, these processes have film agglomeration issues, which makes them unsatisfactory for Cu ALD. − Cu is easily agglomerated by thermal energy during the ALD process; thus, the deposition temperature should be lower than 100 °C to avoid film agglomeration. , However, almost all reported Cu ALD processes are carried out at temperatures greater than 100 °C because of the thermal properties and reactivity of Cu precursors.

As examples, researchers have carried out Cu ALD using a highly reactive reducing agent such as ZnEt2, trimethylaluminum (TMA), borans, or hydrazine as a coreactant to achieve a low-temperature process (∼100 °C). − However, these processes have several critical shortcomings, such as Zn or Al contamination in films prepared using ZnEt2 or TMA, respectively. Using hydrazine as a coreactant introduces handling concerns from a safety perspective. As an alternative approach, plasma-enhanced ALD (PE-ALD) using H2 has been reported. Researchers have used PE-ALD to carry out Cu ALD at temperatures less than 100 °C. − However, the volatility of these precursors is insufficient for ALD to be conducted at a temperature less than 100 °C. In fact, the vaporization temperatures of these precursors are higher than the corresponding process temperatures, as shown in Table . For example, the ALD process temperature of Cu­(acac)2 (acac = acetylacetonate) is 85–140 °C and its vaporization temperature is 138 °C; similarly, the process temperature of Cu­(thd)2 (thd = 2,2,6,6-tetramethylheptane-3,5-dionate) is 60–180 °C and its sublimation temperature is 127.5 °C, and the process temperature of Cu­( i Pr-amd)2 ( i Pr-amd = N,N′-diisopropylacetamidinate) is 50–100 °C, whereas its vaporization temperature is 90 °C. These properties indicate that the risk of chamber contamination and particle generation in the ALD chamber increases because of precursor condensation. In addition, the melting points of these Cu precursors are higher than their volatilization temperatures, potentially leading to clogging of the precursor supply lines during low-temperature processes. Based on the above issues, we conclude that conventional precursors are not suitable for Cu ALD, especially industrial Cu ALD processes.

1. Comparison of ALD Precursors for Cu Films .

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a

The clogging risk of Cu­(emap)2 is extremely low compared with that of conventional precursors because of its low melting point and higher volatility.

Therefore, to develop Cu precursors with greater volatility and a lower melting point, we investigated Cu precursors with novel structures. Cu precursors with amidinate and aminoalkoxide structures are considering promising because they lead to pure Cu films via ALD at comparatively low deposition temperatures. The melting points and vapor pressures of amidinate precursors with various substituents have been reported. However, none of the reported compounds exhibit the preferred properties. Regarding aminoalkoxide, Cu­(dmap)2 (dmap = dimethylamino-2-propoxide) is a well-known Cu ALD precursor because of its high reactivity and can be used to generate pure Cu films. ,,, However, the melting point of Cu­(dmap)2 is 140 °C, which makes low-temperature ALD difficult because of clogging and poor volatility. CTA-1, which has an aminoalkoxide structure, although its detailed structure has not been reported, exhibits better volatility and has enabled low-temperature ALD at 30 °C. However, CTA-1 is a solid precursor; thus, clogging issues persist.

In the present study, we focused on aminoalkoxides and investigated the structures that exhibit both low melting points and high vapor pressures. We found an excellent candidate, bis­(1-ethylmethylamino-2-propoxy)­copper (Cu­(emap)2) (Table ), which exhibits both high volatility and a low melting point. As shown in Table , the improved vapor pressure of the present precursor results in a smaller temperature difference between the vaporization temperature and the deposition temperature during low-temperature ALD, thereby reducing the risk of precursor condensation and line clogging. In addition, for other reported precursors, the melting points are higher than their vaporization temperatures, meaning that they sublimate. Consequently, if condensation occurs, solid particles may form and deposit on internal components such as gas lines. Although the gas lines are heated, heat transfer to solid particles is inefficient under reduced pressure, which increases the risk of clogging. In contrast, the precursor used in this study has a melting point below its vaporization temperature; therefore, even if condensation occurs, the precursor becomes liquid, making it more likely to re-evaporate upon line heating. Therefore, we investigated the low-temperature ALD behavior and film characterization with the aim of using Cu­(emap)2 to attain a Cu film with a thickness less than 10 nm.

Experimental Section

Volatility and Thermal Stability Testing

Thermogravimetric analysis (TGA) of Cu­(emap)2 was carried out at 10 Torr in an Ar-filled glovebox using a Rigaku ThermoPlus2 TG8120. The melting point and thermal decomposition temperature were measured by differential scanning calorimetry (DSC; Bruker AXS DSC 3100).

ALD Testing of Cu­(emap)2 with H2 Plasma

Cu films were deposited using a commercial ALD reactor (CN1 Atomic Premium) with H2 as a coreactant. Thermal oxide SiO2 blanket wafers were purchased from Sumco. Ru substrates were prepared with a Cr adhesion layer on the thermal oxide SiO2 substrates at room temperature using a magnetron sputtering system (Ulvac QAM4). Co substrates also were prepared on the Ru/SiO2 substrates using a magnetron sputtering system. The sputtering targets were 2 in. diameter disks of Ru (99.99%) and Co (99.99%). The thicknesses of the SiO2, Ru, and Co substrates were ∼100, ∼20, and ∼15 nm, respectively. Prior to deposition, the SiO2, Ru, and Co substrates were cut into 25 mm × 25 mm squares and precleaned with H2 plasma for 10 min in the ALD reactor to remove the surface oxide layer and organic contamination. Before the ALD process, the wafer was heated for 30 min to stabilize its temperature.

The Cu­(emap)2 was synthesized by the method described in the literature and was provided by Adeka (product name: Adeka Orcera CTA-5). The Cu­(emap)2 precursor was added to a stainless steel canister and heated at 65 °C. The precursor was supplied by vapor drawing using Ar carrier gas flowing at 50 sccm. The chamber pressure was maintained at ∼0.80 Torr by controlling the Ar process gas. The deposition mode was PE-ALD, and the plasma type was direct plasma. The radio-frequency (RF) power was 50 W. The ALD cycles were performed in the following sequence: Cu­(emap)2 (supply time: 5–30 s)/Ar purge (15 s)/H2 plasma (60 s)/Ar purge (15 s).

Characterization of Deposited Cu Films

The extent of Cu deposition was measured by X-ray fluorescence (XRF; Rigaku ZSX Primus IVi). The stoichiometry and elemental bonding states in the films were investigated by X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific K-Alpha). The film morphology and thickness were evaluated by field-emission scanning electron microscopy (FE-SEM; Hitachi High-Tech S-4800) and atomic force microscopy (AFM; Bruker Multimode 8), respectively. The crystallinity and the film density were characterized by X-ray diffraction (XRD; Rigaku Ultima IV). The electrical resistivity was measured via measurement of the sheet resistivity using a four-point probe (low-resistivity meter; Nittoseiko Analytech Loresta-GX II MCP-T710). To further clarify the layered structure, we conducted cross-sectional observations by scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (STEM-EDS) analysis using instruments (JEM-ARM200F, Jeol and HD-2000, Hitachi High-Tech) operating at 200 kV. The cross-sectional specimens were prepared by focused ion beam (FIB, FB-2000).

Results and Discussion

Volatility and Thermal Stability of Cu­(emap)2 Precursors

Figure a shows the TGA curves of Cu­(emap)2 and conventional precursor Cu­(dmap)2. The TGA curve of Cu­(emap)2 is clean, showing a single step without decomposition or residue formation at 10 Torr. The 50% volatile temperature (TG1/2) at 10 Torr was 103 °C. Compared with Cu­(dmap)2, Cu­(emap)2 exhibited greater volatility. The TG1/2 of Cu­(dmap)2 was 115 °C.

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(a) TGA curves of Cu­(emap)2 and Cu­(dmap)2 at 10 Torr. (b) DSC thermograms of Cu­(emap)2 and Cu­(dmap)2.

Figure b shows the DSC thermograms of Cu­(emap)2 and Cu­(dmap)2. Cu­(emap)2 is a solid precursor at room temperature. When Cu­(emap)2 was heated, an endothermic peak was observed. The onset of the endothermic peak was observed at 38 °C, indicating that the melting point of Cu­(emap)2 is 38 °C. A thermal decomposition peak was observed at temperatures greater than 195 °C in the thermogram of Cu­(emap)2. Therefore, Cu­(emap)2 maintains a liquid state from 38 to 195 °C. By contrast, the melting point of Cu­(dmap)2 is 140 °C. A thermal decomposition peak was observed at temperatures greater than 198 °C in the thermogram of Cu­(dmap)2. Therefore, Cu­(dmap)2 is a solid until 140 °C. Cu­(dmap)2 is difficult to use at temperatures less than 100 °C because of clogging issues related to its high melting point and low volatility.

Figure S1 shows the DSC cyclic scans for the measurement of the melting point of Cu­(emap)2; the results show that Cu­(emap)2 melts near room temperature. The temperature range was −20 to 90 °C. The onset of the endothermic peak was observed at 30.6 °C during the first heating. This result indicates that the melting point of Cu­(emap)2 is 30.6 °C. After the melting process, Cu­(emap)2 maintained its liquid state by supercooling; thus, even if it is cooled at −20 °C, the melted Cu­(emap)2 remained in the liquid state for several days after this measurement. Cu­(emap)2 has an ethylmethylamino structure; thus, its molecular symmetry is lower than that of Cu­(dmap)2, which has a dimethylamino structure. The aforementioned results show that the melting point of Cu­(emap)2 is substantially lower than that of Cu­(dmap)2 and that Cu­(emap)2 does not solidify immediately even if cooled. , We conclude that Cu­(emap)2 can behave almost as a liquid precursor at room temperature.

Low-Temperature PE-ALD Testing of the Cu­(emap)2 Precursors

We evaluated the optimal dosing time to study its effect on the growth rate using Cu­(emap)2 with H2 as a coreactant. The Cu­(emap)2 dosing time was varied while the substrate temperature was maintained at 40 °C. Figure a shows the thickness per cycle as functions of the Cu­(emap)2 dose. The film growth rate increased when the Cu­(emap)2 dosing time was increased from 5 to 10 s. However, the Cu­(emap)2 growth rate saturated when the Cu­(emap)2 dosing time was longer than 10 s. The Cu­(emap)2 half-reaction reached a self-limited state.

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ALD behavior of the Cu­(emap)2 when H2 plasma is used as a coreactant. (a) Growth rate as a function of the Cu­(emap)2 supply time at a substrate temperature of 40 °C on a Ru substrate. (b) Growth rate as a function of the H2 plasma supply time at a substrate temperature of 40 °C on a Ru substrate. (c) Film thickness as a function of the number of ALD cycles at 40 °C on Ru and SiO2 substrates. (d) Temperature dependence of the growth rate of the Cu­(emap)2 on Ru and SiO2 substrates.

The H2 plasma dosing time was also optimized. Figure b shows the dependence of the film growth rate at 40 °C on the H2 plasma dosing time. A constant growth rate was observed when the dosing time was 60 s or longer. Variations in the growth rate at shorter dosing times were confirmed to be due to incomplete reaction of the metal precursor. Because carbon contamination originating from the ligand was a concern in this range of short dosing times, a dosing time of at least 60 s was considered necessary.

Figure c shows the film thickness as a function of the number of ALD cycles. The film thickness, which was measured by FE-SEM and XRF, increases linearly with increasing number of ALD cycles at 40 °C on Ru and SiO2 substrates. This result indicates that the film grew at a constant growth rate; that is, the Cu­(emap)2 exhibited excellent thickness control characteristics. The linear dependence and saturation behavior were verified, and we therefore concluded that the Cu­(emap)2 exhibited ALD-type growth behavior. The difference in Cu film growth rates on Ru and SiO2 substrates was considered to originate from the variation in the initial growth behavior of the Cu film. On the Ru substrate, a nucleation delay of approximately 10 cycles was observed, whereas no nucleation delay was detected on the SiO2 surface. Cu exhibits poor adhesion to the SiO2 substrate and therefore readily undergoes aggregation during the PE-ALD process. Cu showed better adhesion to the Ru substrate than to the SiO2 substrate; thus, a comparatively smooth Cu surface was generated and a lower growth rate was observed on the Ru substrate. The Cu film morphology data will be discussed in detail later in the paper.

Figure d shows the temperature dependence of the growth rate in the Cu­(emap)2 + H2 PE-ALD test process. A constant film growth rate was observed when the deposition temperature was increased from 40 to 100 °C. When the deposition temperature was further increased to 125 °C, the Cu film growth rate sharply increased. This increase in growth rate was attributed to the onset of H2-assisted CVD reactions and to the aggregation of Cu layer in this temperature range. It is well-known that deposited Cu readily undergoes aggregation at temperatures above 100 °C. The sharp increase in growth per cycle (GPC) is considered to result from the aggregation of Cu, which increases the surface area of the deposited Cu film and thereby enhances the amount of Cu precursor adsorbed on the surface, together with the CVD-like reactions described above. Therefore, we estimated the ALD window of the Cu­(emap)2 to be the temperature from 40 to 100 °C. The GPC on the SiO2 substrate is higher than that on the Ru substrate as a whole.

Characterization of Cu Films Deposited Using Cu­(emap)2 with H2 Plasma as a Coreactant

The compositions of the films and the concentrations of impurities in the films were measured by XPS analysis. Figure shows the XPS depth profiles (Figure a) and high-resolution spectra of Cu 2p (Figure b) and C 1s (Figure c) of the as-deposited Cu films with a thickness of 22 nm on Ru substrates when H2 plasma was used as a coreactant. A high-purity Cu film (>99 atom % Cu) was observed after Ar+-ion bombardment for 1 min to eliminate the surface oxidation layer resulting from air exposure. Impurities were not detected in the film by XPS. The Cu 2p spectrum shows a single Cu–Cu peak at 932.8 eV. No Cu–O peak was observed at ∼933.5 eV. This Cu–Cu binding energy well matches the value reported for Cu metal, indicating that Cu­(emap)2 leads to a pure Cu film even when the deposition temperature is 40 °C. The C 1s spectrum shows that carbon contamination was not observed even when a low deposition temperature was used. These results indicate that Cu­(emap)2, which has aminoalkoxide ligands, exhibits excellent reactivity with H2 plasma at 40 °C.

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(a) XPS depth profiles and high-resolution (b) Cu 2p and (c) C 1s spectra of as-deposited Cu films grown on a Ru substrate at 40 °C, as characterized by XPS.

Figure shows the XRD patterns of Cu films with a thickness of 22 nm on a Ru substrate and 41 nm on a SiO2 substrate when deposited at 40 °C. Sharp peaks such as strong Cu(111) and weak Cu(200) and Cu(220) peaks are observed. Because of the high reactivity of the aminoalkoxide structure, crystalline Cu films can be deposited using the Cu­(emap)2 precursor and H2 plasma as a coreactant at a deposition temperature of 40 °C, without requiring a postannealing process.

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XRD patterns of Cu films with thicknesses of 22 and 41 nm deposited onto Ru and SiO2 substrates using the Cu­(emap)2 precursor and H2 plasma as a coreactant at deposition temperatures of 40 °C.

Figure a shows FE-SEM images. Cu films were deposited via 1000 PE-ALD cycles. The film deposited at 40 °C exhibited a continuous Cu layer with a thickness ∼22 nm on a Ru substrate. Figure b shows the corresponding AFM images of Cu films deposited onto Ru and SiO2 substrates. The rms roughness value of the Cu film on a Ru substrate was 1.69 nm. A continuous Cu layer was also observed for the film on a SiO2 substrate. The thickness and the rms roughness were ∼41 and 3.91 nm, respectively. The GPC of Cu films on Ru and SiO2 substrates was 0.22 and 0.41 Å/cycle, respectively. The resistivity of the Cu film on a SiO2 substrate was 4.7 μΩ·cm, which is similar to that of a sputtered Cu film (5–6 μΩ·cm). This thickness and rms roughness difference were caused by differences in the film growth behavior. Cu growth is strongly dependent on the substrate material. The Cu grew on the SiO2 substrate as isolated islands, whereas Ru exhibited greater wettability than SiO2, resulting in a comparatively continuous Cu film. Therefore, not only is low-temperature ALD important, but also the metal underlayer plays a critical role is attaining a thin continuous Cu film.

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Film morphology characterization. (a) FE-SEM images of Cu films with thicknesses of 22 and 41 nm deposited onto Ru and SiO2 substrates using the Cu­(emap)2 precursor and H2 plasma as a coreactant at a deposition temperature of 40 °C. (b) Corresponding AFM images of the Cu films deposited onto Ru and SiO2 substrates.

To deposit Cu films less than 10 nm thick, we reduced the number of ALD cycles from 1000 to 300. Figure shows FE-SEM images of the top-view surface and cross-sectional morphologies of Cu thin films grown on Ru, Co, and SiO2 substrates using 300 cycles of Cu PE-ALD. Similar to the Cu films grown for 1000 PE-ALD cycles on a SiO2 substrate, those grown for 300 cycles consist of isolated islands. It has been reported that pronounced island-like growth occurs on nonwetting substrates such as SiO2, and the present result is in good agreement with these previous findings. By contrast, a continuous Cu layer was observed on the Ru and Co substrates owing to their excellent wettability toward Cu. Previous studies have reported that strong Cu–Ru substrate interactions on Ru substrates effectively suppress three-dimensional Cu growth at the surface, which is consistent with the absence of isolated island growth in the corresponding top-view images. The thicknesses of the Cu films on the Ru and Co substrates were estimated to be 8.6 and 8.4 nm, respectively. The resistivity of Cu deposited on Ru and Co substrates was calculated by treating the Ru or Co substrate and the Cu film as parallel resistive components, based on their individually measured sheet resistances. The calculated resistivity of the Cu films on the Ru and Co substrates was 19 and 17 μΩ·cm, respectively. The higher resistivity compared with that of thicker films is attributed to resistivity increasing with decreasing Cu film thickness, especially for films less than 20 nm thick. In addition, the easy oxidation of the very thin Cu films (∼8 nm) during sample transfer from the ALD chamber to the resistivity meter likely affected the results.

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Comparison of the Cu growth on Ru, Co, and SiO2 substrates, as observed in the top-view surface images and cross-sectional FE-SEM images of the Cu films prepared using the Cu­(emap)2 precursor and H2 plasma as a coreactant at a deposition temperature of 40 °C.

Figure a shows the STEM images of a Cu film on a Co substrate, where the Cu film is 8.4 nm thick and was deposited from Cu­(emap)2 with H2 plasma as a coreactant at 40 °C. The Cu film is clearly visible on the Co layer. The corresponding EDS mapping data in Figure b indicate that the Cu layer was continuously deposited onto the Co layer. These observations are consistent with the reasonable resistivity value of the Cu film. Therefore, improving the volatility and melting point of the Cu ALD precursor expanded the ALD process range, enabling stable low-temperature ALD at 40 °C. As a result, a continuous Cu film with a thickness less than 10 nm was attained.

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(a) STEM images of Cu films and (b) the corresponding EDS mapping data for the layer structure of the Cu film on a Co substrate.

Summary and Conclusions

We evaluated a newly developed Cu­(emap)2 precursor that has a low melting point of 30.6 °C and exhibits excellent volatility compared with conventional precursors. We demonstrated PE-ALD of Cu films using H2 as a coreactant. The ALD window was estimated to range from 40 to 100 °C, and the estimated growth rate on a Ru substrate was 0.22 Å/cycle at 40 °C. XPS analysis showed that a high-purity Cu film (>99%) was deposited. In particular, continuous Cu films less than 10 nm thick were deposited onto metal substrates such as Ru and Co at 40 °C. These results indicate that Cu­(emap)2 solves Cu ALD problems and expands the process range for future semiconductor device manufacturing. In addition, although plasma-assisted ALD was adopted in this initial study, thermal ALD will be crucial for achieving conformal Cu deposition in high-aspect-ratio structures. The favorable low-temperature properties of Cu­(emap)2 suggest strong potential for the development of low-temperature thermal Cu ALD in future studies. We consider Cu­(emap)2 to be a potential replacement for current Cu film deposition methods (e.g., sputtering) for high-aspect-ratio substrates and low-heat-resistance underlayers. We conclude that the proposed Cu­(emap)2 precursor is an excellent candidate Cu ALD precursor for use at temperatures less than 100 °C.

Supplementary Material

ao6c01006_si_001.pdf (337.7KB, pdf)

Acknowledgments

This work was partly supported by JST-Research and Development Program for Next-generation Edge AI Semiconductors Grant Number JPMJES2522, “Crossover Alliance to Create the Future with People, Intelligence and Materials” and “Advanced Research Infrastructure for Materials and Nanotechnology Japan (ARIM),” Grant Nos. JPMXP1222HK0072 and JPMXP1223HK0013, from the Ministry of Education, Culture, Sports, Science and Technology (MEXT).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c01006.

  • DSC cyclic scans for the measurement of the melting point of Cu­(emap)2 (PDF)

The authors declare no competing financial interest.

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