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A study on structural and electrical properties of low dielectric constant SiOC(–H) thin films deposited via PECVD
Journal of Physics and Chemistry of Solids 73 (2012) 641–645
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
A study on structural and electrical properties of low dielectric constant SiOC(–H) thin films deposited via PECVD R. Navamathavan a,n, R. Nirmala b, Chang Young Kim c, Cheul-Ro Lee a, Chi Kyu Choi c a Semiconductor Materials Process Laboratory, School of Advanced Materials Engineering, Engineering College, Research Center for Advanced Materials Development, Chonbuk National University, Chonju 664-14, South Korea b Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju 561-756, South Korea c Nano Thin Film Materials Laboratory, Department of Physics, Cheju National University, Ara 1 Dong, Jeju 690–756, South Korea
a r t i c l e i n f o
abstract
Article history: Received 17 February 2010 Received in revised form 2 January 2012 Accepted 2 January 2012 Available online 12 January 2012
Low-dielectric constant SiOC(–H) films were deposited on p-type Si(100) substrates using plasma enhanced chemical vapor deposition (PECVD) at different radio frequency (rf) powers. The structural characteristics of the SiOC(–H) films were characterized using Fourier transform infrared spectroscopy (FTIR) in the absorbance mode. The bonding configurations of the SiOC(–H) films remained unchanged upon annealing, showing their good thermal stability. Electrical characteristics of the SiOC(–H) thin films with Al/SiOC(–H)/p-Si(100)/Al metal-insulator-semiconductor (MIS) structures were analyzed using capacitance–voltage (C–V) and conductance–voltage (G/o–V) at different frequencies. The conductance and the capacitance measurements were used to extract the interface state density in the MIS structures. From the experimental data and the subsequent quasi-static C–V analysis, the energy distribution of interface state density was obtained. The interface state density of the as-deposited and 400 1C annealed MIS structures increased with increasing rf powers, whereas the fixed charge density decreased with increasing rf powers. The interface state densities and their electrical properties of the SiOC(–H) films strongly affected by the radio frequency power. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Thin film B. Plasma deposition C. Infrared spectroscopy D. Electrical properties D. Dielectric properties
1. Introduction The microelectronic industry depends on continuous improvement of device speed and functionality. As integrated circuit dimensions continue to shrink, highly packed multilevel interconnections with low-resistance metal and low-dielectric constant materials have attracted much attention as a method for increased ultra-large scale integrated (ULSI) circuits operating speed. To address this issue, low-resistivity Cu metallization and low-dielectric constant materials are being used to replace the conventional Al/SiO2 interconnect structure [1–3]. Carbon-doped silicon oxides (SiOCH) thin films deposited using plasma enhanced chemical vapor deposition (PECVD) are currently being used in interconnect applications [4–7]. Optimizing the relationship between film properties and precursor structure will enable development of materials suitable for integration and use in multiple interlayer dielectrics (ILD). Therefore, in addition to a low dielectric constant, the ILD must satisfy a large number of diverse requirements in order to be successfully integrated. However, in order to satisfy the process compatibility and ensure the desired electrical performance and the device reliability, the low-k materials need to meet a demanding list of electric, chemical, mechanical and
n
Corresponding author. Tel.: þ82 63 270 2304; fax: þ82 63 270 2305. E-mail address: [email protected] (R. Navamathavan).
0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2012.01.002
thermal requirements. One of this integration challenges with new ultra low-k generation materials is their electrical properties and reliability issues [8–10]. The electric property at the interface between a dielectric/ semiconductor [SiOC(–H)/p-Si(100)] structure is known to influence the device characteristics and its stability under an electric field. The performance of a metal-insulator-semiconductor (MIS) structure depends on various factors, such as the presence of the localized interface states existing at the dielectric/semiconductor interface, interface preparation process, metal to semiconductor barrier height, interfacial insulator layer formation and series resistance. Therefore, more detailed investigation on these interface state of the SiOC(–H)/p-Si(100) structure should be required. In this present study, we investigated the electrical properties of MIS, Al/SiOC(–H)/p-Si(100)/Al structure and we report the results of a systematic analyses on the frequency dependence of the electrical properties of MIS structure. To do this, the electrical properties of MIS structure were carried out in the frequency range of 1 kHz to 5 MHz at room temperature.
2. Experimental SiOC(–H) thin films were deposited on p-Si(100) substrates using a mixture of MTMS (C4H12O3Si) and oxygen gases as precursors in a PECVD system at room temperature. The plasma
R. Navamathavan et al. / Journal of Physics and Chemistry of Solids 73 (2012) 641–645
As-deposited
Si-O-C
Si-O-Si
Si-CH3
Si-CH3
Absorbance (arb. units)
was generated using radio frequency (rf) power supply with a frequency of 13.56 MHz between the two electrodes. Before depositing the SiOC(–H) films, the p-Si(100) substrates were degreased with 10% HF solution for 30 s and then rinsed in deionized water to remove the native oxides on the substrate. And then the wafer was loaded into the chamber, which was evacuated to a pressure of less than 10 6 Torr and the working pressure was kept at 110 mTorr. The SiOC(–H) films were deposited at room temperature and the rf power was varied from 600 to 900 W. The total flow rate of the precursors were maintained at 40 sccm and the flow rate ratio of [MTMS/(O2 þMTMS)] 100¼ 90%. The MTMS precursor is a colorless transparent liquid with a boiling point of 101 1C at standard atmospheric pressure. In order to prevent the recondensation of MTMS precursor, the bubble bath and all the gas delivery lines were heated and kept at a constant temperature of 40 1C. After depositing SiOC(–H) films, 250, 500 and 750 mm circular patterns of high purity Al metal (99.999%) with a thickness of 300 nm was thermally evaporated using a shadow mask. To form ohmic contacts, we evaporated the Al metal with a thickness of 200 nm onto the whole back surface of the p-Si(100) wafer. After deposition, the films were annealed at 400 1C for 30 min in an Ar ambient. The thickness of the deposited film was measured by the field-emission scanning electron microscopy (FESEM, JSM6700 F). The capacitance–voltage (C–V) and conductance–voltage (G/o–V)characteristics were measured at 1 MHz and in the frequency range of 1 kHz to 5 MHz at room temperature for the MIS (Al/SiOC(–H)/p-Si(1 0 0)/Al) structure using semiconductor device analyzer (Agilent B1500A) and a C–V meter (4280 A, HP/ Agilent Technologies), respectively.
900 W
-OH
-CHx
800 W
700 W
600 W 1000
Si-O-Si
1500
2000 2500 3000 Wavenumber (nm)
Si-O-C
Si-CH3
3500
4000
Annealed at 400°C
Si-CH3
Absorbance (arb. units)
642
900 W
-CHx
-OH
800 W
700 W
3. Results and discussion Fig. 1(a) and (b) shows the FTIR spectra of as-deposited and annealed SiOC(–H) films for different rf powers. The SiOC(–H) films showed absorption bands due to Si–O–Si (around 1055 cm 1), Si–O–C (around 1105 cm 1) and Si–CH3 (around 950 and 1250 cm 1), in addition to those of the Si substrate. The bonding mode at 1100 cm 1 is due to Si–O–C asymmetric stretching modes in an open link and Si–C cage link [11]. The intensity of the Si–O–Si bonding mode increased simultaneously with increasing rf power. The CHx terminating groups give rise to stretch the bands in the region 2800–3000 cm 1, and the bands around 850 and 1250 cm 1 are attributed to the Si–CH3 groups. The broad peak between 3500 and 3800 cm 1 corresponds to a stretching of the –OH related and to the physisorbed moisture on the surface in several modes. Since the deposition of the SiOC(–H) film depends on the precursor, in the case of the MTMS precursor, as the rf power increased from 600 to 900 W, the corresponding peak of Si–O–Si increased, as shown in Fig. 1(a). Fig. 1(b) shows the FTIR spectra of SiOC(–H) films deposited at different rf powers after annealed at 400 1C. The absorption bands were observed to be steeper and well separated for the annealed samples, as shown in Fig. 1(b). Almost there was no change in the peak positions before and after annealing the films when comparing Fig. 1(a) and (b). This result showed that the thermal stability of the SiOC(–H) films upon annealing process. However, the relative intensity of the Si–O–Si peak increased as shown in the dotted lines in Fig. 1. The distinguishable separation of peak positions in the range of 950–1250 cm 1 for the Si–O–Si and the Si–O–C bonding structures was consistently observed for all samples after annealing. Additionally, when the annealing temperature increased, the –OH related bond (around 3500–3800 cm 1) decreased. This can also account for the observed decrease in the dielectric constant in the film. The peak structure of the prominent and clearly separated
600 W 1000
1500
2000 2500 3000 Wavenumber (nm)
3500
4000
Fig. 1. FTIR spectra of SiOC(–H) films deposited at different rf powers for (a) asdeposited and (b) annealed at 400 1C.
Si–O–Si and Si–O–C bonds indicated that the caged Si–C bonds exist in the SiOC(–H) films and an evidence of enhanced porosity in the film [12]. A way to reduce the dielectric constant of a material is to reduce its weight density by increasing the free volume in the Si–O network [13]. After annealing, numerous Si–O–Si network were appeared to be generated due to the rearrangement of the bonding structures in the SiOC(–H) films, as shown in Fig. 1(b). Fig. 2(a) and (b) shows the voltage dependence of the measured C–V and G/o–V characteristics of the as-deposited Al/SiOC(–H)/p-Si(100)/Al structures with different rf powers at various frequencies ranging from 1 kHz to 5 MHz. The capacitance decreased with increasing rf power for as-deposited samples, however, the capacitance values are higher for the lower frequency measurements as shown in Fig. 2(a). The flat band voltage shifted more towards negative region of the C–V curve for the film with rf power of 800 and 900 W, we did not obtain the accumulation region due to the limited bias voltage range (only from 25 to 25 V) of the instrument. In the accumulation region, for a given applied voltage, the value of capacitance increased with decreasing frequency due to the time dependent response of the interface states. Additionally, appreciable frequency dependent dispersion was observed at the depletion region. Also, the frequency dependent G/o–V curves showed peaks in the
R. Navamathavan et al. / Journal of Physics and Chemistry of Solids 73 (2012) 641–645
80
35 5MHz 100kHz 10kHz 1kHz
30 25
As-deposited 900 W
20
40
1MHz 50kHz 5kHz
As-deposited 900 W
20 0
15 -20
30
-15
-20
60
-10
25
-15
-10 800 W
40
800 W
20
20
15 50
-20
-15
-10
G/ω (pF)
Capacitance (pF)
5MHz 100kHz 10kHz 1kHz
60
1MHz 50kHz 5kHz
643
-5
40 700 W
30
0 60
-20
-15
-10
-5
40
700 W
20
20
0
125
-15
-10
-5
50
100
25
0 -20
-10
0
-5
600 W
200
600 W
75
-10
-15
300
100
10
-20
-10
Voltage (V)
0
10
Voltage (V)
Fig. 2. The measured (a) capacitance (C) and (b) conductance (G/o) characteristics versus applied voltage at various frequency ranging from 1 kHz to 5 MHz for the as-deposited Al/SiOC(–H)/p-Si(100)/Al structures with different rf powers.
5MHz 100kHz 10kHz 1kHz
30 25
1MHz 50kHz 5kHz
60 Annealed at 400°C 900 W
20
-10
-15
-5 800 W
-20
30
-10
-15
-5 800 W
20
20
10
16 -20
-15
-5
-10
0
5 700 W
30
G/ω (pF)
Capacitance (pF)
900 W
0 -20
24
0 40
-20
30
-15
-5
-10
0
5 700 W
20 10
20
0 -15
-10
-5
0
80
5
150
600 W
100
-15
-10
-5
0
5 600 W
0
5
50
40 0 -15
Annealed at 400°C
20
10
120
1MHz 50kHz 5kHz
40
15
40
5MHz 100kHz 10kHz 1kHz
0 -10
-5 Voltage (V)
0
5
-15
-10
-5 Voltage (V)
Fig. 3. The measured (a) capacitance (C) and (b) conductance (G/o) characteristics versus applied voltage at various frequency ranging from 1 kHz to 5 MHz for the Al/SiOC(–H)/p-Si(100)/Al structures annealed at 400 1C with different rf powers.
depletion region as shown in Fig. 2(b), which was attributed to the particular distribution of surface states between the dielectric/semiconductor interface. However, in the inversion region no significant frequency dispersion was observed at this frequency
range. Fig. 3(a) and (b) shows the voltage dependence of the measured C–V and G/o–V characteristics of the Al/SiOC(–H)/pSi(100)/Al structures prepared with different rf powers after annealed at 400 1C for various frequencies ranging from 1 kHz
644
R. Navamathavan et al. / Journal of Physics and Chemistry of Solids 73 (2012) 641–645
to 5 MHz. The similar trends were observed for the samples annealed at 400 1C, however, the values of capacitance and conductance were reduced, as shown in Fig. 3(a) and (b). From these results, we infer that under the applied voltage the interface states were responsible for the observed frequency dispersion in C–V and G/o–V curves. Therefore, depending on the relaxation time of the interface states and the frequency of the alternating current (ac) signal, there may be a capacitance due to interface states [14]. Fig. 4(a) shows the interface state density of the as-deposited and 400 1C annealed Al/SiOC(–H)/p-Si(100)/Al, MIS structures were calculated using conductance and capacitance method as functions of rf powers. In both methods, the interface state density decreased with increasing rf power. The value of NSS for the as-deposited and annealed at 400 1C Al/SiOC(–H)/p-Si(100)/Al structure prepared with an rf power of 600 W (7.8 1012 eV 1 cm 2 and 3.9 1012 eV 1 cm 2) were considerably higher than with an rf power of 900 W (2.4 1012 eV 1 cm 2 and 2.1 1012 eV 1 cm 2) by the conductance method. The same trend was the case for the capacitance method. However, the extracted NSS values from capacitance method were found to be two orders lower than that of the conductance method. The value of NSS for the as-deposited and annealed at 400 1C Al/SiOC(–H)/p-Si(100)/Al structure prepared with an rf power of 600 W (5.05 1010 eV 1 cm 2 and 1.01 1010 eV 1 cm 2) was considerably higher than with an rf power of 900 W (4.03 1010 eV 1 cm 2 and 0.16 1010 eV 1 cm 2) by the
5 Conductance Method As-deposited Annealed at 400°C
6
4
Capacitance Method As-deposited Annealed at 400°C
3 2
4
Nss (× 1010 eV-1cm-2)
Nss (× 1012 eV-1cm-2)
8
1 2 600
700 800 rf power (W)
900
0
capacitance method. It can be explained the interface state density (NSS) can follow the ac signal and yield an excess capacitance and conductance, which depends on the relaxation time of NSS and frequency of the applied ac signal [14,15]. In particular, at the depletion region for a given applied voltage the value of capacitance increases with decreasing frequency due to the time dependent response of interface states. In the inversion region no appreciable frequency dispersion is evident in this frequency. Also, the capacitance and conductance curves at low frequencies give a peak in the depletion region. Such behavior of the capacitance and conductance peaks are attributed to particular distribution of surface states between SiOC(–H)/p-Si(100) interface. Depending on the relaxation time of the interface states and the frequency of the ac signal, there may be a capacitance due to interface states in excess to depletion layer. This excess capacitance can reduce the interface state as observed in the capacitance method. Therefore, the estimated the NSS value was differed for the capacitance and conductance methods. From these results, we conclude that the Si–O bonds can be converted to Si–C by addition of the –CH3 groups owing to compensation of positive and negative charges at the interface. As a result, the interface states decreased with increasing rf power [16]. However, the value of NSS was further reduced for the samples annealed at 400 1C, which was due to the structural rearrangement by the annealing. This result is in good agreement with the structural data as shown in Fig. 1(b). Fig. 4(b) shows the flat band voltage and the fixed charge of the as-deposited and 400 1C annealed Al/SiOC(–H)/p-Si(100)/Al MIS structure as a function of rf powers. As the rf power increase, the flat band voltage shifted more towards the negative direction. This result means that the positive charge carrier due to excessive chemical bonds, such as –CH3 or Si–CH3 and Si–H in the SiOC(–H) films increased with increasing rf power. The fixed charges monotonically increased with increasing rf power. As shown in Fig. 2(a), the C–V curves shifted towards the negative region, which confirmed the enhanced fixed charges in the SiOC(–H) film with increasing rf power. However, for the annealed samples, the value of VFB slightly reduced, and hence the value of Nf also decreased. This result is attributed to the rearrangement of excessive oxygen and the –CH3 groups concentration with annealing. Fig. 5 shows the plot of dielectric constant of the as-deposited and the annealed samples as a function of rf powers. As expected, the calculated dielectric constant values of SiOC(–H) films decreased with increasing rf power. From a combination of the structural and electrical behaviors shown in Figs. 1, 3 and 4, it is
20 0
Annelaed at 400°C
-20
10
Fixed charge As-deposited Annealed at 400°C
-30 5 -40
Dielectric constant
15
Flat band voltage As-deposited
As-deposited Annealed at 400°C
3.4 Nf (× 1011 cm-2)
VFB (V)
-10
3.6
3.2 3.0 2.8 2.6 2.4
0 600
700 800 rf power (W)
900
Fig. 4. (a) Surface state density of the as-deposited and the annealed samples calculated using conductance and capacitance method as a function of rf powers, and (b) Flat band voltage and fixed charge density of the as-deposited and the annealed samples as a function of rf powers.
2.2 600
650
700
750
800
850
900
rf power (W) Fig. 5. Dielectric constant of the as-deposited and the annealed SiOC(–H) films as a function different rf powers.
R. Navamathavan et al. / Journal of Physics and Chemistry of Solids 73 (2012) 641–645
evident that the incorporation of more carbon atoms due to rearrangement of bonding configurations within SiOC(–H) film can reduce the dielectric constant. This is attributed to the lower polarizability of Si–CH3 bonds, which gives rise to the lower dielectric constant. Thus, a very drastic reduction in the dielectric constant value was obtained as the rf power was increased. Therefore, it is concluded that the bonding rearrangement due to annealing and the influence of rf powers in the SiOC(–H) films is crucial for obtaining a low-dielectric constant.
4. Conclusions In this study, low-dielectric constant SiOC(–H) thin films were deposited on a p-Si(100) substrates using PECVD with a mixture of MTMS and oxygen gases for different rf powers ranging from 600 to 900 W. The C–V and G/o–V characteristics for Al/SiOC(–H)/ p-Si(100)/Al MIS structure were carried out as a function of frequency. From the electrical characteristics, the interface state density and the fixed charges were calculated for the MIS structure. The value NSS of the as-deposited Al/SiOC(–H)/ p-Si(100)/Al structure prepared with an rf power of 600 W (7.8 1012 eV 1 cm 2) was considerably higher than with an rf power of 900 W (3.9 1012 eV 1 cm 2) by the conductance method. The value VFB was shifted more towards negative region of the C–V curve and Nf was found to be increased with increasing rf powers because of more carbon atoms incorporated into the SiOC(–H) film. The interface state density was observed to be lower for the annealed MIS structures than that of the as-deposited MIS structure. From a combination of the structural
645
and the electrical properties, it was confirmed that the incorporation of more carbon atoms due to rearrangement of bonding configurations within SiOC(–H) film reduces the dielectric constant.
References [1] M. Morgen, E.T. Ryan, J.H. Zhao, C. Hu, T. Cho, P.S. Ho, Annu. Rev. Mater. Sci. 30 (2000) 645. [2] K. Maex, M.R. Baklanov, D. Shamiryan, F. Lacopi, S.H. Brongersma, Z.S. Yanovitskaya, J. Appl. Phys. 93 (2003) 8793. [3] W. Volksen, R.D. Miller, G. Dubois, Chem. Rev. 110 (2010) 56. [4] M. Mikhail, R. Baklanov, K. Maex, Phil. Trans. R. Soc. A 364 (2006) 201. [5] K. Endo, K. Shinoda, T. Tatsumi, J. Appl. Phys. 86 (1999) 2739. [6] R. Navamathavan, C.Y. Kim, A.S. Jung, C.K. Choi, H.J. Lee, J. Korean Phys. Soc. 53 (2008) 351. [7] I.S. Bae, S.J. Cho, W.S. Choi, H.J. Cho, B. Hong, H.D. Jeong, J.H. Boo, Prog. Org. Coatings 61 (2008) 245. [8] C.Y. Kim, A.S. Jung, R. Navamathavan, C.K. Choi, K.M. Lee, J. Korean Phys. Soc. 53 (2008) 371. [9] M. Aimadeddine, V. Arnal, A. Farcy, C. Guedj, T. Chevolleau, N. Posseme, T. David, M. Assous, O. Louveau, F. Volpi, J. Torres, Microelectron. Eng. 82 (2005) 341. [10] C.M. Garner, G. Kloster, G. Atwood, L. Mosley, A.C. Palanduz, Microelectron. Reliab. 45 (2005) 919. [11] A. Grill, V. Patel, Appl. Phys. Lett. 79 (2001) 803. [12] A. Grill, D.A. Neumayer, J. Appl. Phys. 94 (2003) 6697. [13] A. Grill, V. Patel, K.P. Rodbell, E. Huang, M.R. Baklanov, K.P. Mogilnikov, M. Toney, H.C. Kim, J. Appl. Phys. 94 (2003) 3427. [14] B. Tataroglu, S. Altindal, A. Tataroglu, Microelectron. Eng. 83 (2006) 2021. [15] K. Martens, B.D. Jaeger, R. Bonzom, J.V. Steenbergen, M. Meuris, G. Groesneken, H. Maes, IEEE Electron. Device Lett. 27 (2006) 405. [16] A.S. Zakirov, R. Navamathavan, Y.J. Jang, A.S. Jung, C.K. Choi, K.M. Lee, J. Korean Phys. Soc. 50 (2007) 1809.
Contents lists available at SciVerse ScienceDirect
Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
A study on structural and electrical properties of low dielectric constant SiOC(–H) thin films deposited via PECVD R. Navamathavan a,n, R. Nirmala b, Chang Young Kim c, Cheul-Ro Lee a, Chi Kyu Choi c a Semiconductor Materials Process Laboratory, School of Advanced Materials Engineering, Engineering College, Research Center for Advanced Materials Development, Chonbuk National University, Chonju 664-14, South Korea b Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju 561-756, South Korea c Nano Thin Film Materials Laboratory, Department of Physics, Cheju National University, Ara 1 Dong, Jeju 690–756, South Korea
a r t i c l e i n f o
abstract
Article history: Received 17 February 2010 Received in revised form 2 January 2012 Accepted 2 January 2012 Available online 12 January 2012
Low-dielectric constant SiOC(–H) films were deposited on p-type Si(100) substrates using plasma enhanced chemical vapor deposition (PECVD) at different radio frequency (rf) powers. The structural characteristics of the SiOC(–H) films were characterized using Fourier transform infrared spectroscopy (FTIR) in the absorbance mode. The bonding configurations of the SiOC(–H) films remained unchanged upon annealing, showing their good thermal stability. Electrical characteristics of the SiOC(–H) thin films with Al/SiOC(–H)/p-Si(100)/Al metal-insulator-semiconductor (MIS) structures were analyzed using capacitance–voltage (C–V) and conductance–voltage (G/o–V) at different frequencies. The conductance and the capacitance measurements were used to extract the interface state density in the MIS structures. From the experimental data and the subsequent quasi-static C–V analysis, the energy distribution of interface state density was obtained. The interface state density of the as-deposited and 400 1C annealed MIS structures increased with increasing rf powers, whereas the fixed charge density decreased with increasing rf powers. The interface state densities and their electrical properties of the SiOC(–H) films strongly affected by the radio frequency power. & 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Thin film B. Plasma deposition C. Infrared spectroscopy D. Electrical properties D. Dielectric properties
1. Introduction The microelectronic industry depends on continuous improvement of device speed and functionality. As integrated circuit dimensions continue to shrink, highly packed multilevel interconnections with low-resistance metal and low-dielectric constant materials have attracted much attention as a method for increased ultra-large scale integrated (ULSI) circuits operating speed. To address this issue, low-resistivity Cu metallization and low-dielectric constant materials are being used to replace the conventional Al/SiO2 interconnect structure [1–3]. Carbon-doped silicon oxides (SiOCH) thin films deposited using plasma enhanced chemical vapor deposition (PECVD) are currently being used in interconnect applications [4–7]. Optimizing the relationship between film properties and precursor structure will enable development of materials suitable for integration and use in multiple interlayer dielectrics (ILD). Therefore, in addition to a low dielectric constant, the ILD must satisfy a large number of diverse requirements in order to be successfully integrated. However, in order to satisfy the process compatibility and ensure the desired electrical performance and the device reliability, the low-k materials need to meet a demanding list of electric, chemical, mechanical and
n
Corresponding author. Tel.: þ82 63 270 2304; fax: þ82 63 270 2305. E-mail address: [email protected] (R. Navamathavan).
0022-3697/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2012.01.002
thermal requirements. One of this integration challenges with new ultra low-k generation materials is their electrical properties and reliability issues [8–10]. The electric property at the interface between a dielectric/ semiconductor [SiOC(–H)/p-Si(100)] structure is known to influence the device characteristics and its stability under an electric field. The performance of a metal-insulator-semiconductor (MIS) structure depends on various factors, such as the presence of the localized interface states existing at the dielectric/semiconductor interface, interface preparation process, metal to semiconductor barrier height, interfacial insulator layer formation and series resistance. Therefore, more detailed investigation on these interface state of the SiOC(–H)/p-Si(100) structure should be required. In this present study, we investigated the electrical properties of MIS, Al/SiOC(–H)/p-Si(100)/Al structure and we report the results of a systematic analyses on the frequency dependence of the electrical properties of MIS structure. To do this, the electrical properties of MIS structure were carried out in the frequency range of 1 kHz to 5 MHz at room temperature.
2. Experimental SiOC(–H) thin films were deposited on p-Si(100) substrates using a mixture of MTMS (C4H12O3Si) and oxygen gases as precursors in a PECVD system at room temperature. The plasma
R. Navamathavan et al. / Journal of Physics and Chemistry of Solids 73 (2012) 641–645
As-deposited
Si-O-C
Si-O-Si
Si-CH3
Si-CH3
Absorbance (arb. units)
was generated using radio frequency (rf) power supply with a frequency of 13.56 MHz between the two electrodes. Before depositing the SiOC(–H) films, the p-Si(100) substrates were degreased with 10% HF solution for 30 s and then rinsed in deionized water to remove the native oxides on the substrate. And then the wafer was loaded into the chamber, which was evacuated to a pressure of less than 10 6 Torr and the working pressure was kept at 110 mTorr. The SiOC(–H) films were deposited at room temperature and the rf power was varied from 600 to 900 W. The total flow rate of the precursors were maintained at 40 sccm and the flow rate ratio of [MTMS/(O2 þMTMS)] 100¼ 90%. The MTMS precursor is a colorless transparent liquid with a boiling point of 101 1C at standard atmospheric pressure. In order to prevent the recondensation of MTMS precursor, the bubble bath and all the gas delivery lines were heated and kept at a constant temperature of 40 1C. After depositing SiOC(–H) films, 250, 500 and 750 mm circular patterns of high purity Al metal (99.999%) with a thickness of 300 nm was thermally evaporated using a shadow mask. To form ohmic contacts, we evaporated the Al metal with a thickness of 200 nm onto the whole back surface of the p-Si(100) wafer. After deposition, the films were annealed at 400 1C for 30 min in an Ar ambient. The thickness of the deposited film was measured by the field-emission scanning electron microscopy (FESEM, JSM6700 F). The capacitance–voltage (C–V) and conductance–voltage (G/o–V)characteristics were measured at 1 MHz and in the frequency range of 1 kHz to 5 MHz at room temperature for the MIS (Al/SiOC(–H)/p-Si(1 0 0)/Al) structure using semiconductor device analyzer (Agilent B1500A) and a C–V meter (4280 A, HP/ Agilent Technologies), respectively.
900 W
-OH
-CHx
800 W
700 W
600 W 1000
Si-O-Si
1500
2000 2500 3000 Wavenumber (nm)
Si-O-C
Si-CH3
3500
4000
Annealed at 400°C
Si-CH3
Absorbance (arb. units)
642
900 W
-CHx
-OH
800 W
700 W
3. Results and discussion Fig. 1(a) and (b) shows the FTIR spectra of as-deposited and annealed SiOC(–H) films for different rf powers. The SiOC(–H) films showed absorption bands due to Si–O–Si (around 1055 cm 1), Si–O–C (around 1105 cm 1) and Si–CH3 (around 950 and 1250 cm 1), in addition to those of the Si substrate. The bonding mode at 1100 cm 1 is due to Si–O–C asymmetric stretching modes in an open link and Si–C cage link [11]. The intensity of the Si–O–Si bonding mode increased simultaneously with increasing rf power. The CHx terminating groups give rise to stretch the bands in the region 2800–3000 cm 1, and the bands around 850 and 1250 cm 1 are attributed to the Si–CH3 groups. The broad peak between 3500 and 3800 cm 1 corresponds to a stretching of the –OH related and to the physisorbed moisture on the surface in several modes. Since the deposition of the SiOC(–H) film depends on the precursor, in the case of the MTMS precursor, as the rf power increased from 600 to 900 W, the corresponding peak of Si–O–Si increased, as shown in Fig. 1(a). Fig. 1(b) shows the FTIR spectra of SiOC(–H) films deposited at different rf powers after annealed at 400 1C. The absorption bands were observed to be steeper and well separated for the annealed samples, as shown in Fig. 1(b). Almost there was no change in the peak positions before and after annealing the films when comparing Fig. 1(a) and (b). This result showed that the thermal stability of the SiOC(–H) films upon annealing process. However, the relative intensity of the Si–O–Si peak increased as shown in the dotted lines in Fig. 1. The distinguishable separation of peak positions in the range of 950–1250 cm 1 for the Si–O–Si and the Si–O–C bonding structures was consistently observed for all samples after annealing. Additionally, when the annealing temperature increased, the –OH related bond (around 3500–3800 cm 1) decreased. This can also account for the observed decrease in the dielectric constant in the film. The peak structure of the prominent and clearly separated
600 W 1000
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2000 2500 3000 Wavenumber (nm)
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Fig. 1. FTIR spectra of SiOC(–H) films deposited at different rf powers for (a) asdeposited and (b) annealed at 400 1C.
Si–O–Si and Si–O–C bonds indicated that the caged Si–C bonds exist in the SiOC(–H) films and an evidence of enhanced porosity in the film [12]. A way to reduce the dielectric constant of a material is to reduce its weight density by increasing the free volume in the Si–O network [13]. After annealing, numerous Si–O–Si network were appeared to be generated due to the rearrangement of the bonding structures in the SiOC(–H) films, as shown in Fig. 1(b). Fig. 2(a) and (b) shows the voltage dependence of the measured C–V and G/o–V characteristics of the as-deposited Al/SiOC(–H)/p-Si(100)/Al structures with different rf powers at various frequencies ranging from 1 kHz to 5 MHz. The capacitance decreased with increasing rf power for as-deposited samples, however, the capacitance values are higher for the lower frequency measurements as shown in Fig. 2(a). The flat band voltage shifted more towards negative region of the C–V curve for the film with rf power of 800 and 900 W, we did not obtain the accumulation region due to the limited bias voltage range (only from 25 to 25 V) of the instrument. In the accumulation region, for a given applied voltage, the value of capacitance increased with decreasing frequency due to the time dependent response of the interface states. Additionally, appreciable frequency dependent dispersion was observed at the depletion region. Also, the frequency dependent G/o–V curves showed peaks in the
R. Navamathavan et al. / Journal of Physics and Chemistry of Solids 73 (2012) 641–645
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Fig. 3. The measured (a) capacitance (C) and (b) conductance (G/o) characteristics versus applied voltage at various frequency ranging from 1 kHz to 5 MHz for the Al/SiOC(–H)/p-Si(100)/Al structures annealed at 400 1C with different rf powers.
depletion region as shown in Fig. 2(b), which was attributed to the particular distribution of surface states between the dielectric/semiconductor interface. However, in the inversion region no significant frequency dispersion was observed at this frequency
range. Fig. 3(a) and (b) shows the voltage dependence of the measured C–V and G/o–V characteristics of the Al/SiOC(–H)/pSi(100)/Al structures prepared with different rf powers after annealed at 400 1C for various frequencies ranging from 1 kHz
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to 5 MHz. The similar trends were observed for the samples annealed at 400 1C, however, the values of capacitance and conductance were reduced, as shown in Fig. 3(a) and (b). From these results, we infer that under the applied voltage the interface states were responsible for the observed frequency dispersion in C–V and G/o–V curves. Therefore, depending on the relaxation time of the interface states and the frequency of the alternating current (ac) signal, there may be a capacitance due to interface states [14]. Fig. 4(a) shows the interface state density of the as-deposited and 400 1C annealed Al/SiOC(–H)/p-Si(100)/Al, MIS structures were calculated using conductance and capacitance method as functions of rf powers. In both methods, the interface state density decreased with increasing rf power. The value of NSS for the as-deposited and annealed at 400 1C Al/SiOC(–H)/p-Si(100)/Al structure prepared with an rf power of 600 W (7.8 1012 eV 1 cm 2 and 3.9 1012 eV 1 cm 2) were considerably higher than with an rf power of 900 W (2.4 1012 eV 1 cm 2 and 2.1 1012 eV 1 cm 2) by the conductance method. The same trend was the case for the capacitance method. However, the extracted NSS values from capacitance method were found to be two orders lower than that of the conductance method. The value of NSS for the as-deposited and annealed at 400 1C Al/SiOC(–H)/p-Si(100)/Al structure prepared with an rf power of 600 W (5.05 1010 eV 1 cm 2 and 1.01 1010 eV 1 cm 2) was considerably higher than with an rf power of 900 W (4.03 1010 eV 1 cm 2 and 0.16 1010 eV 1 cm 2) by the
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capacitance method. It can be explained the interface state density (NSS) can follow the ac signal and yield an excess capacitance and conductance, which depends on the relaxation time of NSS and frequency of the applied ac signal [14,15]. In particular, at the depletion region for a given applied voltage the value of capacitance increases with decreasing frequency due to the time dependent response of interface states. In the inversion region no appreciable frequency dispersion is evident in this frequency. Also, the capacitance and conductance curves at low frequencies give a peak in the depletion region. Such behavior of the capacitance and conductance peaks are attributed to particular distribution of surface states between SiOC(–H)/p-Si(100) interface. Depending on the relaxation time of the interface states and the frequency of the ac signal, there may be a capacitance due to interface states in excess to depletion layer. This excess capacitance can reduce the interface state as observed in the capacitance method. Therefore, the estimated the NSS value was differed for the capacitance and conductance methods. From these results, we conclude that the Si–O bonds can be converted to Si–C by addition of the –CH3 groups owing to compensation of positive and negative charges at the interface. As a result, the interface states decreased with increasing rf power [16]. However, the value of NSS was further reduced for the samples annealed at 400 1C, which was due to the structural rearrangement by the annealing. This result is in good agreement with the structural data as shown in Fig. 1(b). Fig. 4(b) shows the flat band voltage and the fixed charge of the as-deposited and 400 1C annealed Al/SiOC(–H)/p-Si(100)/Al MIS structure as a function of rf powers. As the rf power increase, the flat band voltage shifted more towards the negative direction. This result means that the positive charge carrier due to excessive chemical bonds, such as –CH3 or Si–CH3 and Si–H in the SiOC(–H) films increased with increasing rf power. The fixed charges monotonically increased with increasing rf power. As shown in Fig. 2(a), the C–V curves shifted towards the negative region, which confirmed the enhanced fixed charges in the SiOC(–H) film with increasing rf power. However, for the annealed samples, the value of VFB slightly reduced, and hence the value of Nf also decreased. This result is attributed to the rearrangement of excessive oxygen and the –CH3 groups concentration with annealing. Fig. 5 shows the plot of dielectric constant of the as-deposited and the annealed samples as a function of rf powers. As expected, the calculated dielectric constant values of SiOC(–H) films decreased with increasing rf power. From a combination of the structural and electrical behaviors shown in Figs. 1, 3 and 4, it is
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Fig. 4. (a) Surface state density of the as-deposited and the annealed samples calculated using conductance and capacitance method as a function of rf powers, and (b) Flat band voltage and fixed charge density of the as-deposited and the annealed samples as a function of rf powers.
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evident that the incorporation of more carbon atoms due to rearrangement of bonding configurations within SiOC(–H) film can reduce the dielectric constant. This is attributed to the lower polarizability of Si–CH3 bonds, which gives rise to the lower dielectric constant. Thus, a very drastic reduction in the dielectric constant value was obtained as the rf power was increased. Therefore, it is concluded that the bonding rearrangement due to annealing and the influence of rf powers in the SiOC(–H) films is crucial for obtaining a low-dielectric constant.
4. Conclusions In this study, low-dielectric constant SiOC(–H) thin films were deposited on a p-Si(100) substrates using PECVD with a mixture of MTMS and oxygen gases for different rf powers ranging from 600 to 900 W. The C–V and G/o–V characteristics for Al/SiOC(–H)/ p-Si(100)/Al MIS structure were carried out as a function of frequency. From the electrical characteristics, the interface state density and the fixed charges were calculated for the MIS structure. The value NSS of the as-deposited Al/SiOC(–H)/ p-Si(100)/Al structure prepared with an rf power of 600 W (7.8 1012 eV 1 cm 2) was considerably higher than with an rf power of 900 W (3.9 1012 eV 1 cm 2) by the conductance method. The value VFB was shifted more towards negative region of the C–V curve and Nf was found to be increased with increasing rf powers because of more carbon atoms incorporated into the SiOC(–H) film. The interface state density was observed to be lower for the annealed MIS structures than that of the as-deposited MIS structure. From a combination of the structural
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and the electrical properties, it was confirmed that the incorporation of more carbon atoms due to rearrangement of bonding configurations within SiOC(–H) film reduces the dielectric constant.
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