Thin Solid Films 497 (2006) 35 – 41 www.elsevier.com/locate/tsf
Remote nitrogen microwave plasma chemical vapor deposition from a tetramethyldisilazane precursor. 2. Properties of deposited silicon carbonitride films I. Blaszczyk-Lezak a, A.M. Wrobel a,*, D.M. Bielinski b a
Centre of Molecular and Macromolecular Studies, Polish Academy of Sciences, Sienkiewicza 112, PL-90-363 Lodz, Poland b Institute of Polymers, Faculty of Chemistry, Technical University of Lodz, 90-924 Lodz, Poland Received 11 February 2005; received in revised form 15 September 2005; accepted 19 September 2005 Available online 16 November 2005
Abstract The physical, optical, and mechanical properties of silicon carbonitride (Si:C:N) films produced by the remote nitrogen plasma chemical vapor deposition (RP-CVD) from tetramethyldisilazane have been investigated in relation to their chemical composition and structure. The films deposited at different substrate temperature (30 – 400 -C) were characterized in terms of their density, refractive index, hardness, elastic modulus, and friction coefficient. The correlations between the film compositional parameters, expressed by the atomic concentration ratios N / Si, C / Si, and N / C, as well as structural parameters described by the relative integrated intensities of the infrared absorption bands from the Si – N, Si – C, and SiMe units (controlled by substrate temperature) were investigated. On the basis of the results of these studies, reasonable structure – property relationships have been determined. D 2005 Elsevier B.V. All rights reserved. Keywords: Silicon carbonitride film; Density; Refractive index; Mechanical properties
1. Introduction Among variety of silicon-based coatings silicon carbonitride (Si:C:N) films, owing to their superior useful properties, attract recently great interest. This material exhibits high hardness [1 – 13], low friction coefficient [7,9], strong adhesion to the substrate [8], wide optical bandgap [1,14 –16], good field emission characteristics [17 –19], low electrical conductivity [15,16], good moisture barrier properties [20], and outstanding resistance to oxidation at high temperatures [21]. In view of the unique properties Si:C:N films are very promising coatings for a wide range of technological applications. Moreover, silicon carbonitride is considered as a serious rival for another superhard materials, such as, e.g., cubic boron nitride [22] or carbon nitride [23]. The present report deals with the Si:C:N films formed by remote nitrogen plasma chemical vapor deposition (RP-CVD)
* Corresponding author. E-mail address:
[email protected] (A.M. Wrobel). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.09.191
from a 1,1,3,3-tetramethyldisilazane, (Me2HSi)2NH, (TMDSN) precursor. The films produced at various deposition (or substrate) temperatures were characterized in terms of their properties important for technological applications. In particular, density, refractive index, hardness, elastic modulus, and friction coefficient of the films have been examined. It is worth to mention that the results of our earlier study [24 – 26] on the Si:C:N films formation by remote nitrogen – hydrogen plasma CVD from TMDSN showed that the use of this gas mixture for plasma generation substantially deteriorates their mechanical properties due to incorporation of the hydronitrene plasma species to the film structure. In view of this finding we use in the current study pure nitrogen as an upstream gas. The aim of the present work was to determine the relationships between the chemical structure (controlled by the substrate temperature) and resulting properties of the Si:C:N films. For this purpose, we used the film compositional and structural parameters reported in the first part of this work [27]. Since the properties of silicon carbonitride films reported in the literature are mostly related to the parameters of the deposition process, the structure – property relationships pre-
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I. Blaszczyk-Lezak et al. / Thin Solid Films 497 (2006) 35 – 41 2.8 2.0
DENSITY (g cm-3)
1.8 2.4 1.6
2.2
2.0
REFRACTIVE INDEX
2.6
pyramidal diamond indenter. Measurements were carried out at increasing load from 0.05 to 4.2 mN with the constant rate of 0.45 mN s 1. Hardness and elastic modulus were evaluated from the indentation data according to the method developed by Oliver and Pharr [29] using the software provided by the manufacturer of the instrument. To reduce the effect of the
(a)
1.4
2.0 0
100
200
300
400
Fig. 1. Density (>) and refractive index (q) of Si:C:N film as a function of substrate temperature.
sented in this paper seem to be an important aspect for enhancing our knowledge of these useful thin-film materials.
2.6 1.8
2.4
2.2 1.6 2.0
2. Experimental
1.8 0.3
0.5
0.6
1.4
0.7
AES ATOMIC RATIO N/Si
2.1. Film deposition
(b) 2.8
SUBSTRATE TEMPERATURE (oC) 400 200 100 30 2.0
DENSITY (g cm-3)
2.6 1.8 2.4
2.2
1.6
REFRACTIVE INDEX
The microwave RP-CVD system and the procedure used for the formation of Si:C:N films have been presented and described in detail elsewhere [24,28]. The films were deposited using the following parameters: a microwave (2.45 GHz) power input P = 120 W, pressure p = 45 Pa, upstream flow rate of nitrogen-gas F(N2) = 100 sccm, feeding rate of the CVD reactor with the TMDSN precursor F(TMDSN) = 3.1 T 0.9 mg min 1 = 0.52 T 0.15 sccm. The distance between the plasma edge and the precursor inlet in the CVD reactor was 35 cm. Films were deposited on a Fisher microscope cover glass plates (45 50 0.2 mm) and on p-type c-Si wafers (3 cm 3 cm 0.4 mm) using the substrate temperature T S = 30– 400 -C.
0.4
REFRACTIVE INDEX
SUBSTRATE TEMPERATURE (oC)
DENSITY (g cm-3)
1.8
2.8
SUBSTRATE TEMPERATURE (oC) 100 300 400
2.0 1.4 1.8 0.2
0.6
1.0
1.4
1.8
AES ATOMIC RATIO C/Si
2.2. Film characterization 2.8
SUBSTRATE TEMPERATURE (oC) 100 300 400 2.0
DENSITY (g cm-3)
2.6 1.8
2.4
2.2
1.6
REFRACTIVE INDEX
Thickness and refractive index of the films deposited on c-Si wafers were measured ellipsometrically using a Nippon Infrared Industrial Co. EL-101D ellipsometer equipped with a 632.8 nm He – Ne laser. For each film sample the average thickness and refractive index values were calculated from at least five ellipsometric measurements. The thickness of the film samples used in the present study for the properties characterization was in the range 0.8 –1.5 Am. The Auger electron spectroscopy (AES) compositional and infrared (IR) structural parameters used for the present study have been determined in the first part of this work [27] by means an AES ULVAC AQM 808 system and a FTIR-Infinity ATI Matson spectrophotometer. Nanoindentation measurements were performed at ambient temperature (21 – 22 -C) for the films deposited on the p-type cSi(100) substrate using the Nano Test 600 (Micro Materials Ltd.) instrument, equipped with a Berkovitch type trigonal
(c)
2.0 1.4 1.8 0.0
0.5
1.0
1.5
2.0
2.5
AES ATOMIC RATIO N/C
Fig. 2. Density (>) and refractive index (q) of Si:C:N film as a function of the AES atomic concentration ratios: N / Si (a), (C / Si) (b), and N / C (c), controlled by substrate temperature.
I. Blaszczyk-Lezak et al. / Thin Solid Films 497 (2006) 35 – 41 SUBSTRATE TEMPERATURE (oC) 100 350 400
(a) 2.8
2.0
DENSITY (g cm-3)
1.8
2.4
2.2 1.6
REFRACTIVE INDEX
2.6
2.0 1.8 25
30
35
40
1.4 50
45
RELATIVE INTENSITY OF Si-N IR BAND (%) SUBSTRATE TEMPERATURE (oC)
(b)
100
2.2
250
200
350
2.1 1.8 2.0 1.6 1.9
1.8 8
9
10
11
12
REFRACTIVE INDEX
DENSITY (g cm-3)
2.0
1.4 13
RELATIVE INTENSITY OF Si-C IR BAND (%) SUBSTRATE TEMPERATURE (oC) 400 350 200 100 30
(c) 2.8
2.0
DENSITY (g cm-3)
1.8
2.4
2.2
1.6
REFRACTIVE INDEX
2.6
37
equipped with a friction attachment. Measurements were carried out for the films deposited on c-Si substrate at the normal load of 8 mN and the sliding speed of 100 nm s 1 over the distance of 100 Am, under ambient conditions (temperature of 21 – 22 -C and relative humidity of 44%). Topography of the scanned surface was registered along with the friction force to eliminate influence of possible artifacts on friction. 3. Results and discussion 3.1. Density and refractive index Film density, an important property strongly sensitive to crosslinking, was calculated from independently determined mass and thickness values reported previously [27], which were respectively measured gravimetrically and ellipsometrically. Fig. 1 shows the density (q) and refractive index (n) of the Si:C:N film as a function of substrate temperature. As can be seen from the plots in this figure q and n increases markedly with increasing T S reaching at T S = 400 -C high values q = 2.65 g cm 3 and n = 2.01. For comparison, the density and refractive index values reported for Si:C:N films produced by the direct plasma CVD from hexamethyldisilazane – nitrogen mixture at T S = 250– 350 -C are q = 2.15– 2.49 g cm 3 and n = 1.85– 2.02 [5]. The trends observed for q and n curves in Fig. 1 are due to thermally induced crosslinking reactions resulting in the formation of the Si– C and Si –N networks, which give rise to the density and refractive index. However, the most informative are direct relations between the examined properties and the compositional and structural parameters of Si:C:N film. This is demonstrated in Fig. 2, which presents the density and refractive index as a function of the AES atomic concentration ratios N / Si (a), C / Si (b), and N / C (c), controlled by substrate temperature. The increase of q and n values noted with increasing ratios N / Si (Fig. 2a) and N / C (Fig. 2c) can be ascribed to rising content of Si –N and C – N crosslinks [27], whereas the drop of these values with increasing ratio C / Si
16
2.0
250
1
2
3
4
5
6
RELATIVE INTENSITY OF SiMe IR BAND (%)
Fig. 3. Density (>) and refractive index (q) of Si:C:N film as a function of the relative integrated intensities of the IR absorption bands from the Si – N (a), Si – C (b), and SiMe (c) units, controlled by substrate temperature.
substrate material, the analyzed data were collected from the penetration depth mostly not exceeding 20% of the film thickness. Friction force of the stainless steel ball with the radius of 5 Am sliding over the film surface was measured using the same instrument as for nanoindentation (Nano Test 600), this time
HARDNESS (GPa)
0
200
12 10
150 8 6
100
4 50
2 0 100
200
300
SUBSTRATE TEMPERATURE
ELASTIC MODULUS (GPa)
14 1.4 1.8
400 (oC)
Fig. 4. Hardness (>) and elastic modulus (q) of Si:C:N film deposited on c-Si substrate as a function of substrate temperature.
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I. Blaszczyk-Lezak et al. / Thin Solid Films 497 (2006) 35 – 41 SUBSTRATE TEMPERATURE (oC) 16
100
300
400 200
HARDNESS (GPa)
12
150
10 100
8 6
50 4 2 0.3
0.4
0.5
0.6
ELASTIC MODULUS (GPa)
14
0 0.8
0.7
AES ATOMIC RATIO N/Si
(b) 16
SUBSTRATE TEMPERATURE (oC) 400 300 100
3.2. Hardness and elastic modulus
200
150
10 100
8 6
50
2 0.2
0.4
0.6
0.8
1.0
1.2
0 1.6
1.4
AES ATOMIC RATIO C/Si
(c) 16
SUBSTRATE TEMPERATURE (oC) 100 300 400
12
150
10 100
8 6
50
140
o
350 C
120
8
100 80
6 60 40
4
20 2
0 6
8
10
12
14
16
RELATIVE INTENSITY OF Si-C IR BAND (%) SUBSTRATE TEMPERATURE (oC)
(b) 16
400
350
200
100
250
14 200
0.5
1.0
1.5
2.0
2.5
0 3.0
AES ATOMIC RATIO N/C
Fig. 5. Hardness (>) and elastic modulus (q) of Si:C:N film deposited on c-Si substrate as a function of the AES atomic concentration ratios: N / Si (a), (C / Si) (b), and N / C (c), controlled by substrate temperature.
12 150
10 8
100
6 50 4
(Fig. 2b) is presumably due to the increase of the content of organic moiety and lowering crosslink density in the film, which take place with decreasing T S. The decrease of the density with rising ratio C / Si is also reported for Si:C:N films produced by the direct plasma CVD from hexamethyldisilazane (HMDSN) [5].
2 0.5
ELASTIC MODULUS (GPa)
4
ELASTIC MODULUS (GPa)
HARDNESS (GPa)
10
200
14
2 0.0
SUBSTRATE TEMPERATURE (oC) 100 200 300
(a)
ELASTIC MODULUS (GPa)
4
Fig. 4 illustrates the substrate temperature dependencies of the Si:C:N film hardness (H) and elastic modulus (E). The
HARDNESS (GPa)
HARDNESS (GPa)
12
ELASTIC MODULUS (GPa)
14
Fig. 3 shows q and n values as a function of the relative integrated intensities of the IR bands from the Si– N (a), Si– C (b), and SiMe (c) units. These relationships show that q and n values increase with rising intensities of the Si– N (Fig. 3a) and Si– C (Fig. 3b) bands, and on the other hand, they decrease with increasing intensity of the SiMe band (Fig. 3c). It is noteworthy that the trend of the plots in Fig. 3c is similar to that of the curves in Fig. 2b and proves that q and n values are strongly affected by the content of organic moiety in the film. From the data in Fig. 3 can be inferred that the increasing contents of the Si –N and Si– C bonds in the film network give rise to the density and refractive index, whereas the opposite effect is observed for the content of non-converted SiMe groups. It is worth to mention that the density and refractive index plots of similar trends as those in Fig. 3b one can obtain using the q, n, and Si– C IR band intensity data reported for Si:C:N films produced by the direct plasma CVD from HMDSN [5].
HARDNESS (GPa)
(a)
0 1.5
2.5
3.5
RELATIVE INTENSITY OF SiMe IR BAND (%)
Fig. 6. Hardness (>) and elastic modulus (q) of Si:C:N film on c-Si substrate as a function of the relative integrated intensities of the IR absorption band from the Si – C (a) and SiMe (b) units, controlled by substrate temperature.
I. Blaszczyk-Lezak et al. / Thin Solid Films 497 (2006) 35 – 41 0.07
FRICTION COEFFICIENT
0.06 0.05 0.04 0.03 0.02 0.01
0
100
200
300
400
SUBSTRATE TEMPERATURE (°°C)
Fig. 7. Friction coefficient (against stainless steel) of Si:C:N film deposited on c-Si substrate as a function of substrate temperature.
increase of these important mechanical parameters to the values H = 14.3 GPa and E = 210.5 GPa with rising T S to 400 -C (Fig. 4) is due to mentioned earlier enhanced crosslinking
(a)
and resulting densification of the film, as revealed by the density plot in Fig. 1. The compositional plots of hardness and elastic modulus are presented in Fig. 5, which shows H and E values in a function of the AES atomic ratios N / Si (a), C / Si (b), and N / C (c) controlled by substrate temperature. The values of H and E increase sharply with rising ratios N / Si (Fig. 5a) and N / C (Fig. 5c) due to crosslinking via the formation of the Si – N and C – N networks, respectively [27], and resulting hardening and stiffening of the film. The marked drop of these values observed with increasing ratio C / Si (Fig. 5b) is ascribed to rising content of organic groups accompanied by reduced crosslinking in the film, as mentioned earlier. The effect of the structural parameters, represented by the relative integrated intensity of the IR bands from the Si– C and SiMe units, on the H and E values are shown in Fig. 6. From the presented plots can be inferred that H and E increase with rising content of the Si– C bonds (Fig. 6a) and they drop with increasing content of non-converted SiMe
SUBSTRATE TEMPERATURE (°C) 100
300 350
400
(a)
0.030
0.025
0.020
0.015
0.010
SUBSTRATE TEMPERATURE (°C) 30 100
0.07
FRICTION COEFFICIENT
FRICTION COEFFICIENT
39
0.3
(b)
0.4 0.5 0.6 0.7 AES ATOMIC RATIO N/Si
400
300
100
400
0.06 0.05 0.04 0.03 0.02 0.01 20
25
30
35
40
45
50
RELATIVE INTENSITY OF Si-N IR BAND (%)
SUBSTRATE TEMPERATURE (°C) 0.07
200
(b)
30
0.07
FRICTION COEFFICIENT
FRICTION COEFFICIENT
30° 0.06 0.05 0.04 0.03 0.02 0.01 0.0
0.06 0.05 0.04 100° 0.03
200° 400°
0.02
350° 250°
0.01 0.5
1.0
1.5
2.0
AES ATOMIC RATIO C/Si
Fig. 8. Friction coefficient of Si:C:N film deposited on c-Si substrate as a function of the AES atomic concentration ratios: N / Si (a) and (C / Si) (b), controlled by substrate temperature.
7
8
9
10
11
12
13
RELATIVE INTENSITY OF Si-C IR BAND (%)
Fig. 9. Friction coefficient of Si:C:N film deposited on c-Si substrate at various temperatures as a function of the relative integrated intensities of the IR absorption bands from the Si – N (a) and Si – C (b) bonds.
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I. Blaszczyk-Lezak et al. / Thin Solid Films 497 (2006) 35 – 41
groups (Fig. 6b). It is noteworthy that the latter plot reveals the same trend as that in Fig. 5b. 3.3. Friction coefficient Fig. 7 shows the substrate temperature dependence of the friction coefficient (l) of the Si:C:N film, against stainless steel. Friction coefficient decreases to a very small values l = 0.013 and 0.012 with increasing T S to 250 and 300 -C, respectively. For comparison, the friction coefficient determined for the c-Si substrate was l = 0.072 and for our silicon carbide Si:C films deposited on c-Si at T S = 300 -C by remote hydrogen plasma CVD from (dimethylsilyl)(trimethylsilyl)methane we found l = 0.042 [30]. The compositional dependencies of the friction coefficient presented in Fig. 8 show the l value in a function of the AES atomic ratios N / Si (a) and C / Si (b) controlled by substrate temperature. In view of the plots in Fig. 8 the friction coefficient reaches a minimum value l = 0.012 at N / Si = 0.43 (Fig. 8a) and C / Si = 0.77 (Fig. 8b). The plot of similar trend as that in Fig. 8b we also reported for Si:C films produced by remote hydrogen plasma CVD from a (dimethylsilyl)(trimethylsilyl)methane precursor [30]; a minimum value of l = 0.042 appeared at C / Si = 1.18 (T S = 300-C). Fig. 9 shows the friction coefficient in a function of the relative integrated intensities of the IR absorption bands from the Si– N (Fig. 9a) and Si –C (Fig. 9b) bonds. These plots account for the decrease of l value with increasing content of both Si– N and Si –C bonds in the film network structure. Referring to the curve in Fig. 9b the appearance of a minimum value of l = 0.013 can be distinguished at 10.6% relative intensity of the Si– C band. The discussed results indicate that the friction coefficient is very sensitive to the chemical composition and structure of the investigated Si:C:N films and reveals very low values for high contents of inorganic Si– C and Si– N bonds.
film (caused by the drop of substrate temperature). The increase in the content of the Si– C bonds gives rise to the q, n, H, and E values. The same effect is observed for the Si –N bonds with respect to q and n. Friction coefficient reaches a minimum value l = 0.01 for the optimum film composition determined by the atomic concentration ratios N / Si = 0.4 and C / Si = 0.8– 0.9. The l value was found to decrease with increasing content of the Si– N and Si– C bonds in the film network. In view of relatively high hardness (H = 14.3 GPa), high refractive index (n = 2.0), and low friction coefficient (l = 0.02, against stainless steel) found for Si:C:N film deposited at T S = 400 -C, this material may be useful coating for practical use. Acknowledgements This work has been financed by Polish Ministry of Scientific Research and Information Technology in a frame of the research project No. 3T08C00728. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
4. Summary The presented results reveal distinct relationships existing between the properties of investigated Si:C:N films and their chemical composition and structure (controlled by the substrate temperature). The compositional parameters expressed by AES atomic concentration ratios N / Si, C / Si, and N / C, as well as the structural parameters represented by relative integrated intensity of IR absorption bands from the Si– N, Si –C bonds and SiMe groups, reflecting the crosslinking of silicon carbonitride network, were found to strongly influence the film properties characterized by: density, refractive index, hardness, elastic modulus, and friction coefficient. The values of q, n, H, and E increase with increasing atomic ratios N / Si and N / C, whereas they drop with rising ratio C / Si. The latter trend is consistent with the structural dependencies, which imply decrease of the q, n, H, and E values with rising content of unconverted SiMe units in the
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