Properties of low-k SiCOH films prepared by plasma-enhanced chemical vapor deposition using trimethylsilane

Properties of low-k SiCOH films prepared by plasma-enhanced chemical vapor deposition using trimethylsilane

Microelectronics Journal 33 (2002) 971–974 www.elsevier.com/locate/mejo Properties of low-k SiCOH films prepared by plasma-enhanced chemical vapor de...

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Microelectronics Journal 33 (2002) 971–974 www.elsevier.com/locate/mejo

Properties of low-k SiCOH films prepared by plasma-enhanced chemical vapor deposition using trimethylsilane B. Narayanan*, R. Kumar, P.D. Foo Department of Deep Sub-micron Integrated Circuit, Institute of Microelectronics, 11 Science Park Road, Science Park II, Singapore, 117685 Received 2 May 2002; revised 6 June 2002; accepted 13 June 2002

Abstract The properties of low-k SiCOH film deposited by plasma-enhanced chemical vapor deposition using trimethylsilane are reported here. The deposition process was performed at different temperatures from 200 to 400 8C. The influence of deposition temperature on the films were characterized using Fourier transform infrared spectroscopy (FTIR) to understand its impact on the studied properties. The films were annealed at ,450 8C in an inert ambient after deposition in all the cases. The deposition rate decreases with increase in deposition temperature. The refractive index of the films increases as a function of deposition temperature. From FTIR spectra, OH-related bonds were not detected in films even when deposited at 200 8C. The Si – CH3 bonds were detected in all the films and decreased monotonically from 200 to 400 8C. All deposition conditions studied resulted in films with dielectric constant less than 3, the lowest being ,2.7 when deposited at 200 8C. All films exhibited good thermal stability. q 2002 Published by Elsevier Science Ltd. Keywords: Low-k; Plasma-enhanced chemical vapor deposition; Trimethylsilane; SiCOH; RC delay

1. Introduction The new generation integrated circuits, with shrinking device dimensions will, employ a combination of low dielectric constant material films and copper metallization for multilevel interconnections [1]. The low dielectric constant materials are required to reduce the RC time constant signal delay. Low-k dielectric materials should have a high thermal stability to withstand temperatures of 400 – 450 8C, which are encountered in a typical back end of line IC processing flow. Equally important are the mechanical stability and chemical compatibility during various fabrication processes including plasma etching, resist-stripping, wetchemical cleaning and chemical mechanical polishing. To solve the above problems several organic and inorganic materials have been proposed [2,3]. Organic materials have more integration issues than inorganic materials, such as thermal instability, poor adhesion, and low resistance to O2 plasma. On the other hand, films made from inorganic materials have better thermal and mechanical stability and film adhesion during process integration though they have higher dielectric * Corresponding author. Tel.: þ 65-6770-5721; fax: þ 65-6773-1914. E-mail address: [email protected] (B. Narayanan). 0026-2692/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S 0 0 2 6 - 2 6 9 2 ( 0 2 ) 0 0 0 7 5 - 7

constant. Accordingly, films having both organic and inorganic components are very promising as low-k dielectric materials with stable properties. One such candidate material is carbon doped silicon oxide, which consists of Si – CH3 groups as well as Si – O groups. Carbon’s lower polarizability, in Si – CH3 bonding helps reduce the dielectric constant. Additionally, the presence of space-occupying alkyl groups in these materials produce ‘free-volumes’ in the film and decrease the dielectric constant [4]. Replacement of SiO2 as the interlayer dielectric (ILD) with a carbon doped oxide has attracted considerable attention due to the ease of processing using a conventional plasma-enhanced chemical vapor deposition (PECVD) approach as well as their superior thermal and mechanical stability over the organic materials [4]. This work focuses on the properties of carbon doped oxide film (SiCOH) deposited by PECVD using trimethylsilane as the precursor. Films deposited at different temperatures were characterized for the refractive index and dielectric constant. The Fourier transform infrared spectroscopy (FTIR) was used to analyze the bonding states in the material of the film. Films with dielectric constant of , 2.7 with good thermal stability were achieved at a deposition temperature of 200 8C.

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2. Experiment The SiCOH films were prepared on 8 in. p-type (100) silicon substrates in a radio frequency (13.56 MHz) PECVD reactor (Applied Materials, DXZ) using a gas mixture of trimethylsilane (3MS) and oxygen. The deposition temperatures employed ranged between 200 and 400 8C and other process parameters including gas flow rates and pressure were kept same for the different growth runs. The refractive index of the films was measured using Opti-probe (Therma-wave) system. FTIR analysis was carried out using a BioRad, 2200ME and dielectric constant was measured using SSM mercury probe CV system (SSM 495). All the films were characterized after annealing at , 450 8C unless otherwise indicated. It is found that the deposition temperatures have significant effects on the properties of the films.

3. Results and discussion 3.1. Temperature dependency of the deposition Fig. 1 shows the deposition rate of low-k SiCOH films as function of deposition temperature. It was found that the deposition rate reduced as the deposition temperature increased. This type of behavior is different from a typical thermally activated process encountered in the formation of films such as silicon oxide or nitride. Kim et al. [5] have suggested that the apparently higher growth rates at lower temperatures could be associated with the presence of voids within the films due to Si –CH3 bonds. Accordingly, the density of the films deposited at lower temperatures will be lower which can reflect on the refractive index values (lower the density, lower the refractive index). Fig. 2 shows the refractive index for the lower temperature deposited films. The activation energy of the thermally ‘inactivated’ [5] process is calculated using Arrhenius plot as shown in Fig. 3. The data points fit very well to an equation RðTÞ ¼ 283 expð20:13 eV=kTÞ A=min

Fig. 1. The dependence of deposition rate on the deposition temperature.

Fig. 2. Refractive indexes of SiCOH films as a function of deposition temperature.

3.2. FTIR spectra The information on the vibration states as studied using FTIR is shown in Fig. 4(a) and (b) for films deposited at different temperatures. The strong peaks at 1036 and 1100 cm21 are derived from Si – O asymmetric and Si – O –Si stretch bonds, respectively [5]. The small peak seen at 1268 cm21 is assigned to the Si –CH3 bond. It is clear that the Si – CH3 bond related vibration are prominently seen in the films deposited at lower temperatures and its presence decreases as deposition temperature increases. This shift means that the Si – O – Si framework is loosened by substitution of Si – CH3 for Si –O [6]. The Si – O stretch peak at 1036 cm21 contains a shoulder, indicating that the open chain and caged Si – O structural configuration coexist in this bond [4,7]. The deposition at different temperatures ranging from 200 to 400 8C, keeping other parameters identical caused a reduction of C and H concentration as observed from the FTIR spectra. Similarly Si – CH3 peak reduced, as temperature is increased. Meanwhile the film deposited at 2008C had the strongest Si – CH3 and C – H peak compared with films deposited at temperatures above 250 8C. The dielectric constant of all these, as deposited films were between 2.8 and 3.0. Post-annealing of these films had little or no impact on the dielectric constant except for the film deposited at 200 8C. For the film deposited at 200 8C, the dielectric

Fig. 3. Arrhenius plot for deposition vs temperature.

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film in Fig. 2, due to the decrease in the voids in the films. Also the polarization of Si – CH3 is smaller than Si – O; it is suggested that substitution of Si – CH3 for Si –O decreases the refractive index [6]. To get a lower dielectric constant in SiCOH film, one way is to increase the voids with in an acceptable range, evenly distributed. Our microscopic inspection did not prove the presence of voids. It could be due to the dimensions of voids less than several tens of nanometers. The relative concentration of carbon incorporated in SiCOH films were estimated from the peak intensity relation of Si – CH3 to Si –O from the following equation Relative carbon content ð%Þ ¼ {AC =ðAO þ AC Þ} £ 100 where AO and AC are the peak intensities of Si –O stretching vibration mode at 1036 cm21 and Si – CH3 stretching vibration mode at 1268 cm21, respectively. Fig. 5 shows the relative carbon content incorporated into the film as a function of deposition temperature. Carbon incorporation into the SiO2 in the form of Si –CH3 bonding, create nonporous network in the SiCOH film. 3.3. Annealing effect and dielectric constant

Fig. 4. (a) and (b) FTIR spectra of the film deposited at different temperature.

constant reduced to 2.7 from 3.0 after a post-anneal at , 450 8C. Water was not observed in as deposited films as observed from FTIR spectra. From above results, the different competing reactions might attribute to the formation of Si –CH3 and C – H related bonds. Si – CH3 bonds and C –H related bonds decreases as temperature increases. Consequently the density of the film must be increasing as shown from the refractive index of the

Fig. 5. Relative carbon content incorporated in the film as function of deposition temperature.

Post-annealing was carried out to investigate the thermal stability of the films. The films deposited at 300 –400 8C did not have any change in the studied properties after annealing at a temperature of , 450 8C. Meanwhile film deposited at 200 8C showed a significant reduction in the dielectric constant, from a value of 3– 2.7 after post-annealing. The relative carbon content remains unchanged after postannealing. This suggests that the Si – CH3 bond is thermally stable at , 450 8C. Fig. 6 presents the dielectric constants as function of deposition temperature. From Fig. 6 it can be observed that the dielectric constant follows the same trend as the refractive index in Fig. 2. The percentage of dielectric constant decrease for the films deposited between 300 and 400 8C is less than 5%, but the decrease in dielectric constant for the films deposited at 200 8C is more than 15% after annealing (Fig. 7). The decrease of the dielectric

Fig. 6. Dielectric constant as a function of temperature.

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level of carbon content coming from the strong peak of Si – CH3 bond. The more prominent shoulder at Si –O stretch peak indicates the existence of Si – O caged network structure. As OH was not observed in the film deposited at 200 8C, this will be a better and stable SiCOH low-k film with dielectric constant 2.7 after post-annealing.

Acknowledgments

Fig. 7. Variation of dielectric constant and thickness before and after annealing for film deposited at 2008C.

constant for film deposited at 200 8C after post-annealing could be due to the Si – O network rearrangement [4].

4. Conclusion The characteristics of SiCOH low-k film deposited at different temperatures were investigated. The growth rate of the film followed an exponential function with temperature. The film deposited at 200 8C had the lower dielectric constant after post-annealing. The films have acceptable

The authors would like to thank the PECVD group for the film deposition and measurements and Dr Bala for his inputs and comments.

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