Supercritical fluid chemical deposition of Cu in Ru and TiN-lined deep nanotrenches using a new Cu(I) amidinate precursor

Microelectronic Engineering 137 (2015) 32–36

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Supercritical fluid chemical deposition of Cu in Ru and TiN-lined deep nanotrenches using a new Cu(I) amidinate precursor Md Rasadujjaman a,b,⇑, Mitsuhiro Watanabe a, Hiroshi Sudoh c, Hideaki Machida c, Eiichi Kondoh a a

Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8511, Japan Department of Physics, Dhaka University of Engineering & Technology, Gazipur 1700, Bangladesh c Gas-Phase Growth Ltd., 2-24-16 Naka, Koganei, Tokyo 184-0012, Japan b

a r t i c l e

i n f o

Article history: Received 17 June 2014 Received in revised form 21 November 2014 Accepted 26 November 2014 Available online 4 December 2014 Keywords: Supercritical carbon dioxide Cu(I) amidinate Copper Thin films Adhesion strength Nanotrenches

a b s t r a c t We report supercritical fluid chemical deposition (SFCD) of Cu films in Ru- and TiN-lined deep nanotrenches using a new non-fluorinated Cu(I) amidinate precursor. The Cu(I) amidinate precursor dispersed well in an acetone/CO2 solution with molecular hydrogen as the reducing agent at very low temperatures. High-purity Cu films were grown on both Ru and TiN substrates at a lower deposition temperature (140 °C) than typical temperatures reported for CuII(hfac)2 and CuII(dibm)2 precursors. The temperature dependence of the growth rate was studied and the growth rates were determined to be 10–14 nm/min and 6–13 nm/min for Ru and TiN substrates, respectively, in a temperature range of 140–240 °C. On the Ru surface, Cu nucleated densely, forming smooth, strongly adherent films. At 140 °C, the excellent filling capability of Cu in Ru- or TiN-lined nanotrenches was demonstrated. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction In microelectronic interconnections, aluminium has been replaced by copper because of its low resistivity and superior low resistance, which prevents electromigration [1]. For this application, deep nanotrenches or nanoholes in insulators are filled with copper [2]. Much attention has been focused on the deposition of copper in increasingly narrow trenches [3,4]. However, a precise method for depositing uniform, conformal Cu films and filling deep nanotrenches is required. Various deposition techniques are currently used for thin-film growth in microelectronics applications. Physical vapor deposition (PVD) methods such as sputtering and evaporation are the most common, and chemical vapor deposition (CVD) is also widely used. Thin metal film deposition in supercritical fluids, especially in supercritical carbon dioxide (scCO2), has attracted extensive attention in recent years [5–10]. ScCO2 has zero surface tension, excellent diffusivity, and high density, making scCO2 fluid a promising transport medium for delivering metal–organic precursors to surfaces. The high solubility and diffusivity of the scCO2 fluid allow the rapid fabrication of conformal films and nanotrench filling ⇑ Corresponding author at: Interdisciplinary Graduate School of Medicine and Engineering, University of Yamanashi, 4-3-11 Takeda, Kofu, Yamanashi 400-8511, Japan. Tel./fax: +81 55 220 8472. E-mail address: [email protected] (Md. Rasadujjaman). http://dx.doi.org/10.1016/j.mee.2014.11.021 0167-9317/Ó 2014 Elsevier B.V. All rights reserved.

[11–13]. This deposition process is called supercritical fluid chemical deposition (SFCD). SFCD can be used to replace current deposition technologies such as PVD and CVD [14]. SFCD is highly effective because of its excellent step coverage and the conformability of the films on high-aspect-ratio features [15], which will satisfy the future requirements of microelectronic metallization. However, a lack of suitable precursors has hindered the use of SFCD for Cu metallization and other applications. To the best of our knowledge, there are no precursors with satisfactory properties, including low deposition temperature, and oxygen- and halogenfree ligands, that form conformal, high-aspect-ratio Cu films. Relatively air-stable Cu(I) precursors were studied as oxygen-free compounds for atomic layer deposition and CVD to prepare pure Cu films [16,17]. This method has already been used with several organic Cu(I) b-diketonates, such as [Cu(I)(hfac)L], [Cu(I)(COD)], [Cu(I)(VTMS)], and [Cu(I)(hfac)(2-methyl-1-hexane-3-yne)], and Cu(II) b-diketonates, such as Cu(hfac)2 and Cu(dibm)2, as precursors for Cu film deposition (hfac = hexafluoroacetylacetonate, COD = cyclooctadiene, VTMS = vinyltrimethylsilane, dibm = diisobutylmethanate) [12,18]. These precursors have one or more shortcomings. The reactivity of the precursors is low and they require a high decomposition temperature. Decomposition of the precursors leads to the incorporation of carbon and oxygen impurities into the films, thus degrading their quality. The fluorine in the [Cu(I)(hfac)L], [Cu(I)(COD)], [Cu(I)(hfac)(2-methyl-1-hexane3-yne)], and Cu(hfac)2 precursors causes fluorine contamination

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2. Experimental details The deposition of Cu films was performed in a batch reactor system. Fig. 2 shows a schematic of the apparatus used in this work. In this system, substrate and precursor are enclosed in a pressureresistant stainless steel reactor (2.1 cm3). [Cu(DIPPA)]2 was used as the precursor for the Cu source and Ru- or TiN-coated SiO2/Si test wafers were used as substrates. The size of the substrates was approximately 0.7 cm2. Prior to deposition, wafers were cleaned with acetone in an ultrasonic bath. A known amount of the [Cu(DIPPA)]2 precursor (3.34  10 5 mol), H2 (1 MPa), and the substrate were placed in the reactor, usually prior to pumping in CO2 (13 MPa). The reactor was heated with a heating mantle to the operating temperature. The reactor temperature was measured with a thermocouple inserted in a small hole drilled in the reactor wall. Initially, a supercritical CO2 (scCO2) environment was used to deposit the films; however, the precursor did not dissolve well. An auxiliary solvent was selected in preliminary experiments. Fig. 3 shows the solubility of [Cu(DIPPA)]2 in 1 mL of various common organic solvents. The best solubility was in acetone, and the solubility in hexane and ethanol was lower than that of acetone. Next, Cu thin films were deposited at different deposition temperatures for 15 min using different auxiliary solvents to dissolve a fixed

CH3 CH2 H3C

CH3 HC

N

N

Cu

Cu

CH CH3

H3C H3C

CH3 HC

H3C

N

N CH2

CH

Pump

Air Reactor Back pressure

Cooling unit

Regulator

regulator Valve Precursor & substrate Mantle heater

CO2 cylinder

H2 cylinder Fig. 2. Schematic illustration of scCO2 fluid chemical deposition apparatus.

250

Solubility (mg/ml)

of the films, which reduces the adhesion of Cu to the substrate. In addition, the reactivity of the precursors with H2 is low because of the Cu–O bonds, which require a high decomposition temperature, leading to rough Cu films contaminated with carbon and oxygen [19]. We report the SFCD deposition of Cu films using a new Cu precursor, the diisopropylpropion-amidinate (DIPPA) Cu(I) dimer, [Cu(DIPPA)]2. The chemical structure of this compound is shown in Fig. 1. Its equilibrium vapor pressure is as high as 0.1 Torr at 120 °C (sublimation) and the melting point is 123 °C. It has six pendant alkyl groups that can adjust the properties of the precursors. The advantages of this Cu(I) amidinate precursor are that it contains no halogens and it is thermally stable. Owing to the weak Cu–N bond, it is also sufficiently reactive with molecular H2 to deposit pure Cu films at a relatively low decomposition temperature (120 °C), which is lower than any previous precursor. The lower-temperature deposition provides smoother film morphology because less agglomeration occurs. Unlike Cu halides, the byproduct vapors do not corrode substrates or deposition equipment or cause poor film adhesion.

200

150

100

50

0

Acetone

Hexane

Ethanol

Fig. 3. Solubility of [Cu(DIPPA)]2 in common organic solvents.

amount of [Cu(DIPPA)]2 in an scCO2–H2 mixer with a batch reactor. Uniform, shiny Cu films were obtained by using acetone as an auxiliary solvent. However, hexane and ethanol produced rough, dirty Cu films. The film thickness was determined by using a surface profilometer (DekTak) and a Hitachi scanning electron microscope (SEM). For X-ray diffraction measurements, an X-ray diffractometer (XRD-6000, Shimadzu) operating with Cu Ka radiation was used. The depth composition profile of the deposited films was evaluated using an Auger electron spectrometer (AES; JAMP7810, JEOL). The surface morphology of the deposited films was observed by SEM at an accelerating voltage of 15 kV. Cross sections and surface maps of the films were acquired by an energy dispersive X-ray spectrometer (EDS) equipped with a field-emission SEM (FE-SEM; JSM6500, JEOL) at an acceleration voltage of 15 kV. The adhesion strength of the films was evaluated with a microscratch tester (CSR-2000, Rhesca) following JIS R3255 specifications, where the load applied to the horizontally vibrating stylus was increased continuously with the traveling displacement of the stylus. The critical adhesion strength was defined as the applied load at which delamination occurred. The delamination point was determined from a micrograph [20]. 3. Results and discussion

CH3

CH3 Fig. 1. Chemical structure of [Cu(DIPPA)]2.

3.1. Deposition of Cu and nanotrench filling on Ru- and TiN-lined substrates 3.1.1. Deposition characteristics The main characteristic of SFCD Cu deposition from the [Cu(DIPPA)]2 precursor is a strong dependency of the growth

M. Rasadujjaman et al. / Microelectronic Engineering 137 (2015) 32–36

4.0

-cm)

Ru substrate TiN substrate

3.5

(

mechanism on the substrate material. Cu films were deposited in a scCO2 environment on Ru- and TiN-coated substrates, and the Cu started to grow at around 140 °C. This is much lower than typical SFCD temperatures for Cu(hfac)2 and Cu(dibm)2 precursors. For instance, Kondoh et al. and Blackburn et al. reported that Cu deposition started at higher temperatures (above 180 °C) from the Cu(hfac)2 precursor during SFCD [18,21]. Matsubara et al. achieved Cu deposition from the Cu(dibm)2 precursor at a higher temperature of 200 °C [5]. Fig. 4 shows the Arrhenius plot for SFCD of Cu films on Ru and TiN substrates at deposition temperatures of 140–240 °C. The concentration of the [Cu(DIPPA)]2 precursor was fixed at 3.34  10 5. The growth rate for the Cu films was determined by the film thickness and divided by the deposition time. At temperatures below 140 °C, no Cu deposition occurred. The growth rates of the Cu film were determined at 10–14 nm/min for Ru and 6–13 nm/min for TiN. The Arrhenius plot shows the two slopes between 140–160 °C and 160–240 °C. The growth rate for the Ru substrate increased from 10 nm/min at 140 °C to 13 nm/min for deposition at 160 °C and the activation energy was roughly estimated as 0.22 eV. The growth rate became almost saturated at 160 °C, indicating that the films grew by a mass transport-limited mechanism with an activation energy of 0.012 eV in the temperature range of 160–240 °C. However, the growth rate for the TiN substrate increased from about 6 nm/min at 140 °C to 9 nm/min at 160 °C with an estimated energy of 0.23 eV, and then increased from 160 °C, suggesting the transition of the growth rate between the reaction-limited and mass transport-limited mechanisms. The estimated activation energy was 0.094 eV in the temperature range of 160–240 °C. Interestingly, this value is almost half that reported for Cu(dibm)2 and Cu(hfac)2 [18,22,23], indicating that the [Cu(DIPPA)]2 precursor dissociated a low temperature. For forming interconnects, film continuity and electrical conductivity are very important. The Cu films were shiny, metallic, and electrically conductive. The film resistivity of the Cu films was investigated by four-point probe measurements. Fig. 5 shows the resistivity of the Cu films deposited on Ru and TiN substrates at different deposition temperatures. The lowest resistivities were 1.93 and 2.07 lX cm for Ru and TiN, respectively, at 140 °C, which is the lowest ever reported for the Cu(I) amidinate precursor [16] and close to the resistivity of pure bulk Cu (1.67 lX cm), confirming that the Cu films were good quality. The Cu film resistivity was not affected by the substrates. At lower temperatures, SFCD allows denser nucleation and smoother surface morphology on metallic Ru and TiN surfaces, leading to lower resistivity. The change in

3.5

Resistivity,

34

2.5

2.0

1.5 120

140

160

180

200

220

240

260

Temperature (ºC) Fig. 5. Resistivity of Cu films on Ru and TiN substrates as a function of deposition temperature.

Cu Ru TaN

100 nm

(a) Cu

Ru

Ta

N

Si

C

F

O

Interface

2.8

Ln(deposition rate) (nm/min)

Ea = 0.012 eV

Ru substrate TiN subsrate

2.6

2.4

(b)

Ea = 0.22 eV

Ea = 0.094 eV

2.2

2.0 Ea = 0.23 eV

1.8 1.9

Fig. 6. (a) Cross-sectional SEM image and (b) EDS maps of the Cu film deposited at 240 °C on the Ru substrate.

2.0

2.1

2.2

2.3

2.4

2.5

1000/T (1/K) Fig. 4. Arrhenius plot of Cu deposition from scCO2 fluid on Ru and TiN substrates.

the resistivity of the deposited films with substrate temperature was negligible. The higher resistivity observed at 240 °C was probably caused by poor connectivity between grains. EDS was used to examine the impurities in the films. Fig. 6(a) and (b) shows the SEM image and EDS maps of Cu films deposited at 240 °C on Ru. EDS revealed that the film exhibits almost no impurity elements such as C, O, and F. However, there is a clear evidence of diffusion between Cu and Ru. Fig. 7(a) and (b) shows typical Auger depth profiles of the Cu films deposited on TiN at deposition temperatures of 140 and 240 °C, respectively. Very small amounts of impurities, such as C,

35

Intensity (arb. unit)

Intensity (arb. unit)

Cu Ti N O C F Si

Cu(200)

(a)

Cu(111)

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240 °C 140 °C

0

5

10

15

20

25

30

40

45

Etching cycle

Cu Ti N O C F Si

10

15

60

120

Adhesion strength (mN)

Intensity (arb. unit)

5

55

Fig. 8. X-ray diffraction pattern of Cu films deposited at different deposition temperatures.

(b)

0

50

2 (degree)

20

25

100

80

60

40

30

Etching cycle Fig. 7. AES depth profiles of Cu films deposited at (a) 140 and (b) 240 °C on the TiN substrate.

O, and F, were detected on the surfaces of each sample. Furthermore, after argon-ion etching to remove the top layers, all evidence of C, O, and F disappeared, confirming that the quality of the Cu films was good. The film texture is technologically important because the reliability of the ultra-large-scale-integration interconnect improves as the ratio of the (1 1 1) to the (2 0 0) plane intensity increases. The X-ray diffraction studies of the Cu films are shown in Fig. 8. The (1 1 1)/(2 0 0) intensity ratio was higher at 240 °C (3.27) than the ratio at 140 °C (2.29), indicating preferential growth of (1 1 1) planes parallel to the substrate, which is typical for vapor deposition or SFCD Cu thin films. This is important for Cu interconnect applications because (1 1 1)-textured interconnects have higher electromigration resistance [24]. The adhesion properties of the deposited Cu films on the Ru substrate were investigated by using the microscratch tester. SFCD Cu films generally show poor adhesion [12,25]; typical Cu adhesion to SiO2 is less than 10 mN [20]. However, on Ru, no delamination of the films occurred, even at a stylus force of 1000 mN. Because Ru is a more noble metal element, and consequently has good resistance to oxidation and adheres strongly to Cu [16]. In addition, Cu has better wettability on a Ru surface than on a SiO2 surface [26] and the Ru(0 0 2) plane has a lower lattice mismatch, which suggests that the interface energy should be lower, enhancing the adhesion of Cu. In addition, Cu diffusion into the Ru, which was confirmed by the EDS maps, resulted in a further increase in the adhesion

20 120

140

160

180

200

220

240

260

Temperature (°C) Fig. 9. Adhesion of Cu films deposited on the TiN substrate at different deposition temperatures.

strength. The use of our non-fluorinated [Cu(DIPPA)]2 precursor dramatically improves the adhesion strength of the Cu film to more than 1000 mN, which meets the adhesion strength required by microelectronic interconnects [27]. Fig. 9 shows the adhesion strength of Cu films on the TiN substrate as a function of temperature. The adhesion strength decreased from 112 to 29 mN as the temperature increased from 140 to 240 °C. This confirms that lower temperature deposition enhanced the adhesion strength and interfacial bonding between the Cu and TiN layer. However, the TiN surface was oxidized as the deposition temperature increased, creating a surface Ti–O layer that is much less electrically conductive. Therefore, the decomposition of the precursor was retarded, which reduced the number of nucleation sites producing a larger grain size. Therefore, Cu atoms adhered weakly to the TiN substrate. 3.1.2. Filling of Ru-lined nanotrenches with Cu One challenge in supercritical fluid metallization is to fill nanotrenches with Cu at low deposition temperatures. Fig. 7 shows cross-sectional SEM images of a nanotrench on a Ru-lined substrate filled at different temperatures. At lower temperatures (140 and 160 °C) [Fig. 10(a) and (b)] the 140 and 180 nm trenches were completely filled with Cu. This temperature for Cu filling was

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140 nm

180 nm

140 nm

100 nm

100 nm

180 nm

100 nm

(a) 140 °C

100 nm

(b) 160 °C

140 nm

180 nm

100 nm

100 nm

(c) 240 °C

Fig. 10. Cross-sectional SEM images of Cu films deposited in Ru-lined nanotrenches at (a) 140 (b) 160 and (c) 240 °C. The nominal dimension is defined as the trench width at the opening. The magnification is the same in all micrographs.

(c) (a)

125 nm

150 nm

250 nm

Cu SFCD. The [Cu(DIPPA)]2 precursor also filled Ru- and TiN-lined nanotrenches well at a very low temperature (140 °C). In conclusion, the SFCD method confirms the utility of our [Cu(DIPPA)]2 precursor for depositing Cu films with excellent adhesion strength that are suitable for Cu interconnections in microelectronics. Acknowledgments This research was supported by the Green Energy Conversion Science and Technology program, University of Yamanashi, Japan. References

100 nm

100 nm

100 nm

Fig. 11. Cross-sectional SEM images of Cu films deposited in TiN-lined nanotrenches using the [Cu(DIPPA)]2 precursor at 140 °C. The nominal dimension is defined as the trench width at the opening. The magnification is the same in all micrographs.

much lower than the optimum temperatures for Cu(II) precursors; practically no Cu deposition occurred from Cu(II) precursors at 140 °C. However, when the temperature was increased to 240 °C [Fig. 10(c)] the speed of the Cu diffusion increased leading to grain worsening and agglomeration [28]. 3.1.3. Filling of TiN-lined nanotrenches with Cu Fig. 11 shows the cross-sectional SEM images of nanotrenches filled with Cu at a deposition temperature of 140 °C on a TiN-lined substrate. The 125, 150, and 250 nm trenches were completely filled with Cu. The filling temperature was lower than the Cu(II) precursors reported by Kondoh et al., who achieved the filling of SFCD Cu in nanotrenches at a higher deposition temperature of 250 °C [14]. 4. Conclusions Cu thin films from a non-fluorinated new Cu(I) amidinate precursor deposited from scCO2 fluids were characterized. The [Cu(DIPPA)]2 dimer was especially attractive owing to its low melting point and high surface reactivity at lower temperatures. Good quality films with very low levels of impurities were deposited when H2 was added as reductant, which enabled low-temperature

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