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Characteristics of low-κ SiOC films deposited via atomic layer deposition
Thin Solid Films 645 (2018) 334–339
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Characteristics of low-κ SiOC films deposited via atomic layer deposition a
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Jaemin Lee , Woochool Jang , Hyunjung Kim , Seokyoon Shin , Youngkyun Kweon , ⁎ Kunyoung Leeb, Hyeongtag Jeona,b, a b
Department of Nano-scale Semiconductor Engineering, Hanyang University, Seoul 133-791, South Korea Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Octamethylcyclotetrasiloxane Atomic layer deposition Low-dielectric constant Silicon oxycarbide Thin film
The deposition of SiOC thin films via remote plasma atomic layer deposition was investigated. Octamethylcyclotetrasiloxane (OMCTS) and O2, Ar, H2 plasmas were respectively used as a precursor and reactants during the deposition process at 400 °C. Plasma and deposition temperatures had a significant effect on the physical and electrical characteristics of the films. When Ar and H2 plasma was respectively used during the deposition process, films exhibited low dielectric constants while incorporating carbon; however, O2 plasma yielded carbon free SiO2 films. Low dielectric constants resulted in low film densities and the presence of carbon within the films. When Ar and H2 plasma was used as the reactant gas, pores within the films with loose structures and SieC bonds served to lower the dielectric constant. As a result, Ar and H2 plasma conditions exhibited low dielectric constants of 2.7 and 3.1 at 100 °C, respectively. Meanwhile, the presence of carbon and low film densities caused leakage paths within the films. X-ray photoelectron spectroscopy supported analyses demonstrating the bonding characteristics of Si, C, O components.
1. Introduction There has been a continuous requirement or impetus to reduce pattern sizes and increase their integration within semiconductor devices. As the gate widths of integrated circuits are reduced to sub10 nm, many issues begin to arise. One issue is a resistive-capacitive (RC) delay due to the non-negligible parasitic capacitance of dielectrics. This device performance deteriorating RC delay should be reduced by changing materials. In particular, silicon dioxide (SiO2) and silicon nitride (SiN) should be replaced with a low-k material. In the case of SiO2, which is generally employed as a gap-fill oxide in trench isolation, as an interlayer dielectric (ILD), and as a capping layer within semiconductor devices, SiO2 should be substituted with a low dielectric constant material for superior device performance [1]. On the other hand, SiN must be replaced with a low-k material due to its relatively high dielectric constant. SiN has been used as a gate spacer due to its high electrical stability, good etching characteristics, barrier properties against wet chemicals, and dopant diffusion properties [2]. However, SiN possesses a high dielectric constant of 7, causing RC delays and degrading device performance. Therefore, substituting a SiN spacer with a low-k material should not only satisfy having a dielectric constant lower than 7 but also yield excellent film stabilities and etch properties. A number of candidates are under development for materials such ⁎
as SiOC, SiC, SiOF, and SiCN, with various deposition methods for low-k materials [3]. Among these films, SiOC is considered to be the most promising. SiOC possesses the lowest dielectric constant among the candidates and also exhibits good electrical stability, thermal stability, and etch selectivity against SiO2 [4,5,6,7]. Carbon content within the SiOC films is responsible for lowering the dielectric constant by two factors: porosity and polarization. With regard to porosity, methyl (eCHx) groups are easily desorbed, rendering pores within the films during deposition due to weak bond strengths to silicon. Pores exhibiting a dielectric constant of 1 caused the dielectric constant of the film to decrease [8]. Secondly, the carbon content altered the polarization of the films. Polarization, made up of electronic, dipolar, and ionic components, could hold charges within the film, increasing the dielectric constant. Among the three polarization types, the ionic contribution dominated within the SiOC film [9,10]. When SieO bonds were replaced with SieC bonds, the ionic polarization decreased due to the electronegativity difference of SieC being smaller than that of SieO. As a result, a high concentration of carbon was needed within the SiOC film to generate a low-k thin film. However, immoderate alkyl groups could degrade the electrical and mechanical stability of the films. Therefore, carbon content and porosity values should be optimized to yield a low-k without deteriorating the film quality [11,12]. As devices scale down in size, micro-loading effects on pattern densities should be avoided during the deposition process. Also, the
Corresponding author. E-mail address: [email protected] (H. Jeon).
https://doi.org/10.1016/j.tsf.2017.10.045 Received 17 January 2017; Received in revised form 18 October 2017; Accepted 20 October 2017 Available online 25 October 2017 0040-6090/ © 2017 Elsevier B.V. All rights reserved.
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source and drain regions within metal-oxide-semiconductor field-effect transistor have become significant in terms of the thermal diffusion of dopants caused by shallow junction depths. Additionally, high aspect ratio trenches are used in devices which require high step coverage during the film deposition process. These new requirements demand a low temperature deposition process. Atomic layer deposition (ALD) is considered to be a potential solution to these challenges. ALD processes feature a self-limiting reaction, yielding conformal, high density, and pin hole free films regardless of an uneven pattern density. Also, plasma ALD processes can deposit films at low temperatures. (< 400 °C) Therefore, the development of a low-k film via plasma ALD processes is essential for low-k dielectric materials of future devices. In this study, we investigate the physical properties, electrical stability, and etch properties of SiOC dielectric films. Films were prepared via remote plasma atomic layer deposition (RPALD) using an octamethylcyclotetrasiloxane (OMCTS) precursor with O2, Ar, and H2 plasmas. Remote plasmas can reduce plasma-induced damage to the film surface. The OMCTS precursor possesses a SieO backbone with two methyl groups bonded to each silicon [13]. Therefore, we expect the deposited SiOC films to possess a carbon component with a low dielectric constant.
O2 plasma, the ALD process window was shown to be 250 to 300 °C. In the case of Ar plasma, steady film growth rates appeared at temperatures ranging from 100 to 400 °C. With regard to H2 plasma, there was no ALD process window and film growth rates gradually decreased with an increase in substrate temperature. Nevertheless, films deposited via H2 plasma exhibited constant growth rates through an increase in ALD process cycles, as shown in Fig. 1(b). Therefore, H2 plasma conditions took advantage of control over film thickness by fixing the substrate temperature and other plasma conditions. 2.2. Auger electron spectroscopy (AES) analysis Fig. 2 shows the AES analysis results of films deposited by O2, Ar, and H2 plasma at 100 °C. As shown in Fig. 2(a), the carbon concentration was a negligible quantity within the film produced via O2 plasma. It could be assumed that the composition yielded extremely reactive O2 plasma radicals causing the methyl groups to detach from the SieO backbone bonds. Also, the dielectric constant of these film was 4.0, which is close to the dielectric constant of SiO2 films measured via C-V analysis. Therefore, we could conclude that O2 plasma was not suitable for depositing SiOC films in this study. On the other hand, Fig. 2(b) and (c) showed the AES results of films deposited via Ar and H2, respectively. The AES results both showed that the chemical composition of the films contained a sizable amount of carbon contents and these carbon concentration changed as a function of deposition temperature [14]. Table 1 shows the concentration of carbon and oxygen within a film produced via Ar and H2 plasma. In SiOC films, carbon content plays an important role with regard to reducing the dielectric constant. In both, the carbon concentrations decreased gradually as the substrate temperature increased. The decrease in carbon appeared to be related to the thermal desorption of methyl groups from the SieO main bonds. It was reasonable to assume that higher deposition temperatures stimulated the breaking of SieC bonds due to their relatively weak bond strengths as compared to SieO. In the case of Ar plasma conditions, the oxygen concentration increased with substrate temperature as the carbon concentration decreased. This was the result of Ar plasma radicals breaking precursors in the absence of a chemical reaction. On the other hand, the oxygen content within films deposited via H2 plasma decreased with an increase in deposition temperature. This decline was due to the reaction of H2 plasma with oxygen in the OMCTS precursor. It was assumed that H2 plasma removed oxygen by producing H2O as a by-product.
1.1. Experimental Prior to SiOC film deposition, 2 in. p-type silicon (100) substrates were cleaned with a dilute hydrofluoric acid solution (HF:dionized water = 1:100) for 2 min to remove the native oxide layer. The substrates were then loaded into the ALD chamber and O2, Ar, and H2 remote plasmas were used to deposit the SiOC film at a temperature between 100 °C and 400 °C. The plasma gas flow rate was 50 sccm and Ar was used as the carrier gas. An OMCTS {[(CH3)2SiO]4} canister was heated to 40 °C to yield a suitable vapor pressure for the ALD process (2 Torr at 40 °C). The SiOC RPALD recipe consisted of four steps: (1) OMCTS precursor injection into the chamber, (2) Ar purging to remove residues and by-products, (3) O2, Ar, and H2 plasma, (4) Ar purging to remove residues and by-products. O2, Ar, and H2 plasmas were generated via inductively coupled plasma discharging with a radio frequency (RF) of 13.56 MHz. The plasma power of the Ar, O2, and H2 plasma was 30, 30, and 100 W, respectively. 1.2. Characterizations Metal-insulator-semiconductor (MIS) structures were fabricated to measure the leakage current and breakdown voltage of the SiOC thin film. 70 nm thick Au layers were deposited as the top electrode via electron beam evaporation. After fabrication of the MIS structures, annealing was performed under ambient N2 at 400 °C for 30 min to remove any moisture or defects produced during deposition. Various analyses were performed to investigate the influence of substrate temperature and the type of plasma gas used. Film thickness and refractive index measurements were performed with a spectroscopic ellipsometer (SE). X-ray reflectometry (XRR) was also used to measure film thickness and density values. The chemical composition was measured via Auger electron spectroscopy (AES). The bonding characteristics were investigated by X-ray photoelectron spectroscopy (XPS) with an Mg Kα Xray source and C 1s bonding was used as a charge reference for XPS spectra.
2.3. Refractive index (RI) and dielectric constant Fig. 3 shows the refractive index values of films deposited via Ar and H2 plasma. Refractive index values are known to be proportional to film density. Film density is another primary factor related to dielectric constant values in addition to carbon content. Pores and voids produced during the deposition process have a dielectric constant of 1 and can lower the overall dielectric constant of films. In the case of both Ar and H2 plasma conditions, the refractive index of films increased as deposition temperatures increased from 100 to 300 °C. In the case of Ar plasma conditions, the refractive index of films increased from 1.23 to 1.31 in the as-deposited state. Refractive index values increased slightly after annealing at 400 °C under ambient N2 for 30 min. Films generated by H2 plasma conditions resulted in a similar tendency to that of the Ar plasma condition. The refractive index of films generated by H2 plasma increased from 1.31 to 1.4 as temperatures increased [15]. Annealed films also exhibited higher refractive index values than the as-deposited films. The increase in refractive index after annealing could be attributed to the dissociation of carbon groups and moisture. Moisture generated during deposition should be removed because H2O has a high dielectric constant of 80 and can increase the k-value of the entire film [16,17]. Dissociation caused film thicknesses to shrink and densify. Through SE measurements, the thickness shrinkage of films was verified
2. Results and discussion 2.1. ALD process window and physical properties Fig. 1(a) presents the film thickness values of the deposited films at substrate temperatures of 100 to 400 °C with various plasma conditions. The ALD process window signifies the ideal deposition conditions for stable thin film growth. Accordingly, ALD should be performed at a substrate temperature within the ALD window region. With regard to 335
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Fig. 1. (a) Film growth rates for the 400 cycle RPALD of SiOC films at 100–400 °C under various plasma conditions. (b) Film thickness values over an increase in ALD process cycles under various plasma conditions.
Fig. 2. AES depth profiles of SiOC films deposited at 100 °C via (a) 30 W O2 plasma, (b) 30 W Ar plasma, and (c) 100 W H2 plasma.
the film density values were fairly low due to their exhibiting considerable porosity. Porosity could be the reason for the low dielectric constants and poor mechanical strength when lying down [8]. Fig. 4 presents dielectric constants of Ar and H2 plasma conditions measured from C-V curves. Au-gated MIS capacitors of SiOC (100–150 Å) dielectric films were fabricated for C-V measurements. All films made from Ar and H2 plasma exhibited lower dielectric constant values (2.7–3.4) than that of general SiO2 films (3.9) and O2 plasma conditions (4.0) in this study. With regard to Ar and H2 plasma conditions, dielectric constants of films decreased with deposition temperature. Dielectric constant trends were influenced by the carbon content of films and film densities as mentioned above. At lower temperatures, films generated under Ar and H2 plasma conditions possessed higher carbon compositions and lower film densities than those at high temperatures. High carbon content bonded to Si could decrease the influence of ionic polarization, which is proportional to the dielectric
Table 1 Carbon and oxygen atomic concentrations of SiOC films deposited via Ar and H2 plasmas at a temperature of 100, 200, and 300 °C.
Ar plasma H2 plasma
Carbon Oxygen Carbon Oxygen
100 °C
200 °C
300 °C
11% 55% 20% 50%
9% 57% 18% 47%
6% 58% 15% 45%
Deposition condition: OMCTS injection (1s), Ar Purge (10s), plasma (5s), Ar purge (10s). Plasma power: Ar plasma (30 W), H2 plasma (100 W).
to be 3–7%. XRR results revealed the specific densities of films. Films deposited at 100 °C via Ar and H2 plasma yielded film densities of 1.40 and 1.60 g/cm3, respectively in the as-deposited state. After annealing, film densities increased to 1.52 and 1.65 g/cm3. It was well known that
Fig. 3. Refractive index values of SiOC films deposited via (a) 30 W Ar plasma and (b) 100 W H2 plasma at various deposition temperatures in the as-deposited and annealed states at 400 °C for 30 min under ambient N2.
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XPS analyses of films made from Ar plasma at 100, 200, and 300 °C were also conducted to investigate the influence of deposition temperature with regard to bonding states. Fig. 6(a) and (b) represent the peak area ratios of C 1s and Si 2p spectra, respectively. According to the XPS results, carbon related peaks such as C-Si, C-O, Si-C1O3, and SiC2O2 peaks decreased with an increase in deposition temperature [22,23]. These trends coincided with the results of AES analysis. According to the XPS results, the quantity of SieC bonds was proportional to the carbon content within the film. 2.5. Electrical stability Fig. 7 presents the leakage current density of the films. The films were annealed at 400 °C under ambient N2 for 30 min to remove moisture and defects. Au-gated MIS capacitors of SiOC dielectric films were prepared for I-V measurements. The current density at 1 MV/cm was chosen in the I-V curve to obtain the leakage current. Under Ar plasma conditions, the leakage current densities at 1 MV/cm for the 100, 200, and 300 °C samples were 9.07 × 10− 6 A/cm2, 6.8 × 10− 6 A/cm2, and 8.18 × 10− 7 A/cm2, respectively. In the case of the H2 plasma condition, leakage current densities of the 100, 200, 300 °C samples were 1.13 × 10− 7 A/cm2, 4.56 × 10− 8 A/cm2, and 1.53 × 10− 8 A/cm2, respectively. The films deposited at lower substrate temperatures exhibited poor electrical stability because additional carbon content at lower deposition temperatures more easily decomposed the remaining pores. The pores rendered a larger effective surface area in addition to a greater quantity of unbound bonds within the film, potentially creating a high density of moisture and defects, thus inducing leakage paths. The improved electrical properties of films from H2 plasma as opposed to Ar plasma may be attributed to a higher film densities and passivation effects [24]. The higher film density of SiOC thin films tended to reduce the existence of defects and H2O within the pores. Additionally, H2 plasma exhibited passivation effects, increasing the contact angle of water on the surface, yielding a hydrophobic surface [25]. Fig. 7 also shows the dielectric breakdown induced by permanent conducting paths through the dielectric material. Similar to the leakage current density trend, higher deposition temperatures generated dielectric breakdown at higher electric fields. The dielectric breakdown voltage of films was 2.1, 3.2, and 4.5 MV/cm when the films were deposited via Ar plasma at 100, 200, and 300 °C. From H2 plasma, dielectric breakdown occurred at 7.25, 8.86, and 9.21 MV/cm at deposition temperatures of 100, 200, and 300 °C. This tendency was presumably due to the influence of SieO bonds possessing a higher thermochemical energy than SieC bonds [13,19]. Films deposited at higher temperatures with both plasmas yielded more SieO bonds than SieC bonds. The SieO bonds required higher electric fields to induce dielectric breakdown.
Fig. 4. Dielectric constant values of SiOC films deposited via Ar and H2 plasmas at 100, 200, and 300 °C measured after annealing at 400 °C.
constant in SiOC films. Ionic polarization could be lowered when Si is bonded to C as opposed to O due to the difference in electronegativity of SieC being smaller than that of SieO [10]. Thus, an increase of SieC and decrease of SieO in films at lower deposition temperatures played a crucial role with regard to reducing the dielectric constant [17,18,19]. Film density is another important factor having an effect on the dielectric constant. Film densities can be decreased by desorbing methyl groups on the remaining pores within the film during deposition. Empty spaces in the film tend to be covered at higher deposition temperatures with shrinking film thicknesses [8]. Thus, films deposited at lower temperatures can maintain a larger quantity of pores. In our study, the carbon content was 6% at 300 °C and increased to 11% at 100 °C under Ar plasma conditions. Additional carbon content was incorporated within the films deposited via H2 plasma. The carbon concentration within the films was 15% at 300 °C and 20% at 100 °C. With regard to film density, refractive index and XRR results demonstrated the generation of lower film densities with a decrease in deposition temperatures. As a result, the dielectric constants of films produced by Ar and H2 plasma followed the ideal trends mentioned above. Under Ar plasma conditions, the dielectric constant of films yielded the lowest value of 2.7 at 100 °C. Films deposited by H2 plasma at 100 °C exhibited the lowest dielectric constant of 3.1. H2 plasma conditions yielded films with additional carbon content and relatively high film densities, rendering higher dielectric constants than that of Ar plasma conditions. These results implied that film density was a primary factor as opposed to polarization between the Ar and H2 plasma deposition conditions in terms of the resultant dielectric constant value.
2.4. X-ray photoelectron spectroscopy (XPS) analysis 3. Conclusions XPS was used to investigate the bonding characteristics of SiOC films. Fig. 5(a) presents the C 1s spectra deconvoluted into three different moieties. SiOC films deposited via O2, Ar, and H2 plasma at 100 °C were used in the XPS analysis. The deconvoluted peaks were assigned as follows: CeO (286.2 eV), CeC (284.6 eV), and CeSi (283.4 eV) [20]. The CeSi bond was significantly related to the dielectric constant of film because this bond lowered the ionic polarization. Films with O2 plasma featured no C-Si peaks. On the other hand, films deposited via Ar and H2 plasma exhibited C-Si peaks and films under H2 plasma conditions showed a higher ratio of C-Si peaks. Fig. 5(b) shows the Si 2p spectra of a film deposited via O2, Ar, and H2 plasmas. The peak could be deconvoluted to Si-O4 (103.4 eV), Si-O3C1 (102.2 eV), and Si-O2C2 (101.1 eV) peaks [21]. Films deposited from O2 plasma featured no Si-O3C1 and Si-O2C2 peaks because the films did not have any carbon. Meanwhile, films deposited from Ar plasma revealed a quantity of these peaks and films deposited from H2 plasma had more carbon related peaks.
In this study, the influence of plasma and deposition temperatures with regard to the properties of SiOC low-k films was investigated using OMCTS precursors and ALD processes. Contrary to our expectations, O2 plasma did not preserve carbon content within the films, which was essential toward reducing dielectric constants. On the other hand, the carbon component of the films still survived under Ar and H2 plasma conditions. Films deposited from H2 plasma had a greater carbon composition while the density of films made from Ar plasma were lower than that of H2 plasma. These two factors (carbon content and low film densities) had the effect of lowering the dielectric constant of the films. As a result, Ar and H2 plasma exhibited dielectric constant values of 3.1 and 2.7, respectively. XPS analysis demonstrated the bonding characteristics, which explained the results of the electrical and etching properties. SieC bonds in SiOC decreased the dielectric constant due to a low ionic polarization while the carbon content acted as a defect that induced leakage currents. Consequently, low substrate conditions with 337
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Fig. 5. XPS analysis of the (a) C 1s peak and (b) Si 2p peak of films deposited via O2, Ar, and H2 plasmas at 100 °C.
Fig. 6. Peak area ratio of (a) CeO and CeSi peaks from C 1s spectra and (b) Si-C1O3 and Si-C2O2 peaks from Si 2p spectra. The films used in the XPS analysis were deposited via Ar plasma at 100, 200, and 300 °C.
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Fig. 7. Leakage current densities of 10 nm thick SiOC films deposited via (a) Ar plasma and (b) H2 plasma at 100, 200, and 300 °C.
a high carbon content yielded low dielectric constant values but also exhibited high leakage current densities that degraded film quality.
[13]
Acknowledgements [14]
This study was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (Grant No. NRF-2015R1A2A1A10052324).
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Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Characteristics of low-κ SiOC films deposited via atomic layer deposition a
b
a
b
T
a
Jaemin Lee , Woochool Jang , Hyunjung Kim , Seokyoon Shin , Youngkyun Kweon , ⁎ Kunyoung Leeb, Hyeongtag Jeona,b, a b
Department of Nano-scale Semiconductor Engineering, Hanyang University, Seoul 133-791, South Korea Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Octamethylcyclotetrasiloxane Atomic layer deposition Low-dielectric constant Silicon oxycarbide Thin film
The deposition of SiOC thin films via remote plasma atomic layer deposition was investigated. Octamethylcyclotetrasiloxane (OMCTS) and O2, Ar, H2 plasmas were respectively used as a precursor and reactants during the deposition process at 400 °C. Plasma and deposition temperatures had a significant effect on the physical and electrical characteristics of the films. When Ar and H2 plasma was respectively used during the deposition process, films exhibited low dielectric constants while incorporating carbon; however, O2 plasma yielded carbon free SiO2 films. Low dielectric constants resulted in low film densities and the presence of carbon within the films. When Ar and H2 plasma was used as the reactant gas, pores within the films with loose structures and SieC bonds served to lower the dielectric constant. As a result, Ar and H2 plasma conditions exhibited low dielectric constants of 2.7 and 3.1 at 100 °C, respectively. Meanwhile, the presence of carbon and low film densities caused leakage paths within the films. X-ray photoelectron spectroscopy supported analyses demonstrating the bonding characteristics of Si, C, O components.
1. Introduction There has been a continuous requirement or impetus to reduce pattern sizes and increase their integration within semiconductor devices. As the gate widths of integrated circuits are reduced to sub10 nm, many issues begin to arise. One issue is a resistive-capacitive (RC) delay due to the non-negligible parasitic capacitance of dielectrics. This device performance deteriorating RC delay should be reduced by changing materials. In particular, silicon dioxide (SiO2) and silicon nitride (SiN) should be replaced with a low-k material. In the case of SiO2, which is generally employed as a gap-fill oxide in trench isolation, as an interlayer dielectric (ILD), and as a capping layer within semiconductor devices, SiO2 should be substituted with a low dielectric constant material for superior device performance [1]. On the other hand, SiN must be replaced with a low-k material due to its relatively high dielectric constant. SiN has been used as a gate spacer due to its high electrical stability, good etching characteristics, barrier properties against wet chemicals, and dopant diffusion properties [2]. However, SiN possesses a high dielectric constant of 7, causing RC delays and degrading device performance. Therefore, substituting a SiN spacer with a low-k material should not only satisfy having a dielectric constant lower than 7 but also yield excellent film stabilities and etch properties. A number of candidates are under development for materials such ⁎
as SiOC, SiC, SiOF, and SiCN, with various deposition methods for low-k materials [3]. Among these films, SiOC is considered to be the most promising. SiOC possesses the lowest dielectric constant among the candidates and also exhibits good electrical stability, thermal stability, and etch selectivity against SiO2 [4,5,6,7]. Carbon content within the SiOC films is responsible for lowering the dielectric constant by two factors: porosity and polarization. With regard to porosity, methyl (eCHx) groups are easily desorbed, rendering pores within the films during deposition due to weak bond strengths to silicon. Pores exhibiting a dielectric constant of 1 caused the dielectric constant of the film to decrease [8]. Secondly, the carbon content altered the polarization of the films. Polarization, made up of electronic, dipolar, and ionic components, could hold charges within the film, increasing the dielectric constant. Among the three polarization types, the ionic contribution dominated within the SiOC film [9,10]. When SieO bonds were replaced with SieC bonds, the ionic polarization decreased due to the electronegativity difference of SieC being smaller than that of SieO. As a result, a high concentration of carbon was needed within the SiOC film to generate a low-k thin film. However, immoderate alkyl groups could degrade the electrical and mechanical stability of the films. Therefore, carbon content and porosity values should be optimized to yield a low-k without deteriorating the film quality [11,12]. As devices scale down in size, micro-loading effects on pattern densities should be avoided during the deposition process. Also, the
Corresponding author. E-mail address: [email protected] (H. Jeon).
https://doi.org/10.1016/j.tsf.2017.10.045 Received 17 January 2017; Received in revised form 18 October 2017; Accepted 20 October 2017 Available online 25 October 2017 0040-6090/ © 2017 Elsevier B.V. All rights reserved.
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source and drain regions within metal-oxide-semiconductor field-effect transistor have become significant in terms of the thermal diffusion of dopants caused by shallow junction depths. Additionally, high aspect ratio trenches are used in devices which require high step coverage during the film deposition process. These new requirements demand a low temperature deposition process. Atomic layer deposition (ALD) is considered to be a potential solution to these challenges. ALD processes feature a self-limiting reaction, yielding conformal, high density, and pin hole free films regardless of an uneven pattern density. Also, plasma ALD processes can deposit films at low temperatures. (< 400 °C) Therefore, the development of a low-k film via plasma ALD processes is essential for low-k dielectric materials of future devices. In this study, we investigate the physical properties, electrical stability, and etch properties of SiOC dielectric films. Films were prepared via remote plasma atomic layer deposition (RPALD) using an octamethylcyclotetrasiloxane (OMCTS) precursor with O2, Ar, and H2 plasmas. Remote plasmas can reduce plasma-induced damage to the film surface. The OMCTS precursor possesses a SieO backbone with two methyl groups bonded to each silicon [13]. Therefore, we expect the deposited SiOC films to possess a carbon component with a low dielectric constant.
O2 plasma, the ALD process window was shown to be 250 to 300 °C. In the case of Ar plasma, steady film growth rates appeared at temperatures ranging from 100 to 400 °C. With regard to H2 plasma, there was no ALD process window and film growth rates gradually decreased with an increase in substrate temperature. Nevertheless, films deposited via H2 plasma exhibited constant growth rates through an increase in ALD process cycles, as shown in Fig. 1(b). Therefore, H2 plasma conditions took advantage of control over film thickness by fixing the substrate temperature and other plasma conditions. 2.2. Auger electron spectroscopy (AES) analysis Fig. 2 shows the AES analysis results of films deposited by O2, Ar, and H2 plasma at 100 °C. As shown in Fig. 2(a), the carbon concentration was a negligible quantity within the film produced via O2 plasma. It could be assumed that the composition yielded extremely reactive O2 plasma radicals causing the methyl groups to detach from the SieO backbone bonds. Also, the dielectric constant of these film was 4.0, which is close to the dielectric constant of SiO2 films measured via C-V analysis. Therefore, we could conclude that O2 plasma was not suitable for depositing SiOC films in this study. On the other hand, Fig. 2(b) and (c) showed the AES results of films deposited via Ar and H2, respectively. The AES results both showed that the chemical composition of the films contained a sizable amount of carbon contents and these carbon concentration changed as a function of deposition temperature [14]. Table 1 shows the concentration of carbon and oxygen within a film produced via Ar and H2 plasma. In SiOC films, carbon content plays an important role with regard to reducing the dielectric constant. In both, the carbon concentrations decreased gradually as the substrate temperature increased. The decrease in carbon appeared to be related to the thermal desorption of methyl groups from the SieO main bonds. It was reasonable to assume that higher deposition temperatures stimulated the breaking of SieC bonds due to their relatively weak bond strengths as compared to SieO. In the case of Ar plasma conditions, the oxygen concentration increased with substrate temperature as the carbon concentration decreased. This was the result of Ar plasma radicals breaking precursors in the absence of a chemical reaction. On the other hand, the oxygen content within films deposited via H2 plasma decreased with an increase in deposition temperature. This decline was due to the reaction of H2 plasma with oxygen in the OMCTS precursor. It was assumed that H2 plasma removed oxygen by producing H2O as a by-product.
1.1. Experimental Prior to SiOC film deposition, 2 in. p-type silicon (100) substrates were cleaned with a dilute hydrofluoric acid solution (HF:dionized water = 1:100) for 2 min to remove the native oxide layer. The substrates were then loaded into the ALD chamber and O2, Ar, and H2 remote plasmas were used to deposit the SiOC film at a temperature between 100 °C and 400 °C. The plasma gas flow rate was 50 sccm and Ar was used as the carrier gas. An OMCTS {[(CH3)2SiO]4} canister was heated to 40 °C to yield a suitable vapor pressure for the ALD process (2 Torr at 40 °C). The SiOC RPALD recipe consisted of four steps: (1) OMCTS precursor injection into the chamber, (2) Ar purging to remove residues and by-products, (3) O2, Ar, and H2 plasma, (4) Ar purging to remove residues and by-products. O2, Ar, and H2 plasmas were generated via inductively coupled plasma discharging with a radio frequency (RF) of 13.56 MHz. The plasma power of the Ar, O2, and H2 plasma was 30, 30, and 100 W, respectively. 1.2. Characterizations Metal-insulator-semiconductor (MIS) structures were fabricated to measure the leakage current and breakdown voltage of the SiOC thin film. 70 nm thick Au layers were deposited as the top electrode via electron beam evaporation. After fabrication of the MIS structures, annealing was performed under ambient N2 at 400 °C for 30 min to remove any moisture or defects produced during deposition. Various analyses were performed to investigate the influence of substrate temperature and the type of plasma gas used. Film thickness and refractive index measurements were performed with a spectroscopic ellipsometer (SE). X-ray reflectometry (XRR) was also used to measure film thickness and density values. The chemical composition was measured via Auger electron spectroscopy (AES). The bonding characteristics were investigated by X-ray photoelectron spectroscopy (XPS) with an Mg Kα Xray source and C 1s bonding was used as a charge reference for XPS spectra.
2.3. Refractive index (RI) and dielectric constant Fig. 3 shows the refractive index values of films deposited via Ar and H2 plasma. Refractive index values are known to be proportional to film density. Film density is another primary factor related to dielectric constant values in addition to carbon content. Pores and voids produced during the deposition process have a dielectric constant of 1 and can lower the overall dielectric constant of films. In the case of both Ar and H2 plasma conditions, the refractive index of films increased as deposition temperatures increased from 100 to 300 °C. In the case of Ar plasma conditions, the refractive index of films increased from 1.23 to 1.31 in the as-deposited state. Refractive index values increased slightly after annealing at 400 °C under ambient N2 for 30 min. Films generated by H2 plasma conditions resulted in a similar tendency to that of the Ar plasma condition. The refractive index of films generated by H2 plasma increased from 1.31 to 1.4 as temperatures increased [15]. Annealed films also exhibited higher refractive index values than the as-deposited films. The increase in refractive index after annealing could be attributed to the dissociation of carbon groups and moisture. Moisture generated during deposition should be removed because H2O has a high dielectric constant of 80 and can increase the k-value of the entire film [16,17]. Dissociation caused film thicknesses to shrink and densify. Through SE measurements, the thickness shrinkage of films was verified
2. Results and discussion 2.1. ALD process window and physical properties Fig. 1(a) presents the film thickness values of the deposited films at substrate temperatures of 100 to 400 °C with various plasma conditions. The ALD process window signifies the ideal deposition conditions for stable thin film growth. Accordingly, ALD should be performed at a substrate temperature within the ALD window region. With regard to 335
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Fig. 1. (a) Film growth rates for the 400 cycle RPALD of SiOC films at 100–400 °C under various plasma conditions. (b) Film thickness values over an increase in ALD process cycles under various plasma conditions.
Fig. 2. AES depth profiles of SiOC films deposited at 100 °C via (a) 30 W O2 plasma, (b) 30 W Ar plasma, and (c) 100 W H2 plasma.
the film density values were fairly low due to their exhibiting considerable porosity. Porosity could be the reason for the low dielectric constants and poor mechanical strength when lying down [8]. Fig. 4 presents dielectric constants of Ar and H2 plasma conditions measured from C-V curves. Au-gated MIS capacitors of SiOC (100–150 Å) dielectric films were fabricated for C-V measurements. All films made from Ar and H2 plasma exhibited lower dielectric constant values (2.7–3.4) than that of general SiO2 films (3.9) and O2 plasma conditions (4.0) in this study. With regard to Ar and H2 plasma conditions, dielectric constants of films decreased with deposition temperature. Dielectric constant trends were influenced by the carbon content of films and film densities as mentioned above. At lower temperatures, films generated under Ar and H2 plasma conditions possessed higher carbon compositions and lower film densities than those at high temperatures. High carbon content bonded to Si could decrease the influence of ionic polarization, which is proportional to the dielectric
Table 1 Carbon and oxygen atomic concentrations of SiOC films deposited via Ar and H2 plasmas at a temperature of 100, 200, and 300 °C.
Ar plasma H2 plasma
Carbon Oxygen Carbon Oxygen
100 °C
200 °C
300 °C
11% 55% 20% 50%
9% 57% 18% 47%
6% 58% 15% 45%
Deposition condition: OMCTS injection (1s), Ar Purge (10s), plasma (5s), Ar purge (10s). Plasma power: Ar plasma (30 W), H2 plasma (100 W).
to be 3–7%. XRR results revealed the specific densities of films. Films deposited at 100 °C via Ar and H2 plasma yielded film densities of 1.40 and 1.60 g/cm3, respectively in the as-deposited state. After annealing, film densities increased to 1.52 and 1.65 g/cm3. It was well known that
Fig. 3. Refractive index values of SiOC films deposited via (a) 30 W Ar plasma and (b) 100 W H2 plasma at various deposition temperatures in the as-deposited and annealed states at 400 °C for 30 min under ambient N2.
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XPS analyses of films made from Ar plasma at 100, 200, and 300 °C were also conducted to investigate the influence of deposition temperature with regard to bonding states. Fig. 6(a) and (b) represent the peak area ratios of C 1s and Si 2p spectra, respectively. According to the XPS results, carbon related peaks such as C-Si, C-O, Si-C1O3, and SiC2O2 peaks decreased with an increase in deposition temperature [22,23]. These trends coincided with the results of AES analysis. According to the XPS results, the quantity of SieC bonds was proportional to the carbon content within the film. 2.5. Electrical stability Fig. 7 presents the leakage current density of the films. The films were annealed at 400 °C under ambient N2 for 30 min to remove moisture and defects. Au-gated MIS capacitors of SiOC dielectric films were prepared for I-V measurements. The current density at 1 MV/cm was chosen in the I-V curve to obtain the leakage current. Under Ar plasma conditions, the leakage current densities at 1 MV/cm for the 100, 200, and 300 °C samples were 9.07 × 10− 6 A/cm2, 6.8 × 10− 6 A/cm2, and 8.18 × 10− 7 A/cm2, respectively. In the case of the H2 plasma condition, leakage current densities of the 100, 200, 300 °C samples were 1.13 × 10− 7 A/cm2, 4.56 × 10− 8 A/cm2, and 1.53 × 10− 8 A/cm2, respectively. The films deposited at lower substrate temperatures exhibited poor electrical stability because additional carbon content at lower deposition temperatures more easily decomposed the remaining pores. The pores rendered a larger effective surface area in addition to a greater quantity of unbound bonds within the film, potentially creating a high density of moisture and defects, thus inducing leakage paths. The improved electrical properties of films from H2 plasma as opposed to Ar plasma may be attributed to a higher film densities and passivation effects [24]. The higher film density of SiOC thin films tended to reduce the existence of defects and H2O within the pores. Additionally, H2 plasma exhibited passivation effects, increasing the contact angle of water on the surface, yielding a hydrophobic surface [25]. Fig. 7 also shows the dielectric breakdown induced by permanent conducting paths through the dielectric material. Similar to the leakage current density trend, higher deposition temperatures generated dielectric breakdown at higher electric fields. The dielectric breakdown voltage of films was 2.1, 3.2, and 4.5 MV/cm when the films were deposited via Ar plasma at 100, 200, and 300 °C. From H2 plasma, dielectric breakdown occurred at 7.25, 8.86, and 9.21 MV/cm at deposition temperatures of 100, 200, and 300 °C. This tendency was presumably due to the influence of SieO bonds possessing a higher thermochemical energy than SieC bonds [13,19]. Films deposited at higher temperatures with both plasmas yielded more SieO bonds than SieC bonds. The SieO bonds required higher electric fields to induce dielectric breakdown.
Fig. 4. Dielectric constant values of SiOC films deposited via Ar and H2 plasmas at 100, 200, and 300 °C measured after annealing at 400 °C.
constant in SiOC films. Ionic polarization could be lowered when Si is bonded to C as opposed to O due to the difference in electronegativity of SieC being smaller than that of SieO [10]. Thus, an increase of SieC and decrease of SieO in films at lower deposition temperatures played a crucial role with regard to reducing the dielectric constant [17,18,19]. Film density is another important factor having an effect on the dielectric constant. Film densities can be decreased by desorbing methyl groups on the remaining pores within the film during deposition. Empty spaces in the film tend to be covered at higher deposition temperatures with shrinking film thicknesses [8]. Thus, films deposited at lower temperatures can maintain a larger quantity of pores. In our study, the carbon content was 6% at 300 °C and increased to 11% at 100 °C under Ar plasma conditions. Additional carbon content was incorporated within the films deposited via H2 plasma. The carbon concentration within the films was 15% at 300 °C and 20% at 100 °C. With regard to film density, refractive index and XRR results demonstrated the generation of lower film densities with a decrease in deposition temperatures. As a result, the dielectric constants of films produced by Ar and H2 plasma followed the ideal trends mentioned above. Under Ar plasma conditions, the dielectric constant of films yielded the lowest value of 2.7 at 100 °C. Films deposited by H2 plasma at 100 °C exhibited the lowest dielectric constant of 3.1. H2 plasma conditions yielded films with additional carbon content and relatively high film densities, rendering higher dielectric constants than that of Ar plasma conditions. These results implied that film density was a primary factor as opposed to polarization between the Ar and H2 plasma deposition conditions in terms of the resultant dielectric constant value.
2.4. X-ray photoelectron spectroscopy (XPS) analysis 3. Conclusions XPS was used to investigate the bonding characteristics of SiOC films. Fig. 5(a) presents the C 1s spectra deconvoluted into three different moieties. SiOC films deposited via O2, Ar, and H2 plasma at 100 °C were used in the XPS analysis. The deconvoluted peaks were assigned as follows: CeO (286.2 eV), CeC (284.6 eV), and CeSi (283.4 eV) [20]. The CeSi bond was significantly related to the dielectric constant of film because this bond lowered the ionic polarization. Films with O2 plasma featured no C-Si peaks. On the other hand, films deposited via Ar and H2 plasma exhibited C-Si peaks and films under H2 plasma conditions showed a higher ratio of C-Si peaks. Fig. 5(b) shows the Si 2p spectra of a film deposited via O2, Ar, and H2 plasmas. The peak could be deconvoluted to Si-O4 (103.4 eV), Si-O3C1 (102.2 eV), and Si-O2C2 (101.1 eV) peaks [21]. Films deposited from O2 plasma featured no Si-O3C1 and Si-O2C2 peaks because the films did not have any carbon. Meanwhile, films deposited from Ar plasma revealed a quantity of these peaks and films deposited from H2 plasma had more carbon related peaks.
In this study, the influence of plasma and deposition temperatures with regard to the properties of SiOC low-k films was investigated using OMCTS precursors and ALD processes. Contrary to our expectations, O2 plasma did not preserve carbon content within the films, which was essential toward reducing dielectric constants. On the other hand, the carbon component of the films still survived under Ar and H2 plasma conditions. Films deposited from H2 plasma had a greater carbon composition while the density of films made from Ar plasma were lower than that of H2 plasma. These two factors (carbon content and low film densities) had the effect of lowering the dielectric constant of the films. As a result, Ar and H2 plasma exhibited dielectric constant values of 3.1 and 2.7, respectively. XPS analysis demonstrated the bonding characteristics, which explained the results of the electrical and etching properties. SieC bonds in SiOC decreased the dielectric constant due to a low ionic polarization while the carbon content acted as a defect that induced leakage currents. Consequently, low substrate conditions with 337
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Fig. 5. XPS analysis of the (a) C 1s peak and (b) Si 2p peak of films deposited via O2, Ar, and H2 plasmas at 100 °C.
Fig. 6. Peak area ratio of (a) CeO and CeSi peaks from C 1s spectra and (b) Si-C1O3 and Si-C2O2 peaks from Si 2p spectra. The films used in the XPS analysis were deposited via Ar plasma at 100, 200, and 300 °C.
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Fig. 7. Leakage current densities of 10 nm thick SiOC films deposited via (a) Ar plasma and (b) H2 plasma at 100, 200, and 300 °C.
a high carbon content yielded low dielectric constant values but also exhibited high leakage current densities that degraded film quality.
[13]
Acknowledgements [14]
This study was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (Grant No. NRF-2015R1A2A1A10052324).
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