Plasma CVD of Nano-particle Composite Porous SiOCH Films

Masaharu Shiratani1, Shinya Iwashita1, Hiroshi Miyata1, Kazunori Koga1, Hidefumi Matsuzaki1, Morito Akiyama2

1Department of Electronics, Kyushu University, 744 Motooka, Fukuoka 819-0395, Japan

2Advanced Industrial Science and Technology, 807-1 Shuku-machi, Tosu, Saga 841-0052, Japan Email: siratani@ed.kyushu-u.ac.jp

Abstract: We have proposed plasma CVD for synthesizing nano-particle composite porous SiOCH films using pulse RF discharges with amplitude modulation (AM). The AM discharge have two definite advantages; one is increasing the deposition rate of films due to rapid transport of nano-particles, and the other is avoiding agglomeration of nano-particles. We have succeeded in depositing films of a dielectric constant of 1.4-2.9, a porosity of 56-3.5%, a high thermal stability up to 673 K, and a Young’s modulus above 10 GPa. Such high mechanical strength is realized probably because these films have grained structure composed of small nano-particles of 3.5 nm in size. Substrate temperature is a key parameter that determines the porosity and dielectric constant of the films. AM discharge is a promising method for depositing nano-particle composite porous SiOCH films having a low dielectric constant and high mechanical strength. Keywords: plasma CVD, nano-particle, porous low-k film, RF discharge

1. Introduction

As microelectronic circuits become smaller, intermetal insulators in the circuits need to have lower dielectric constant k for preventing signal delays and cross-talks. In addition to the dielectric constant, sufficient mechanical strength is a key to successful integration of the low-k materials [1-4]. Introducing pores in low-k films is an effective method for decreasing the dielectric constant, although the mechanical strength markedly decreases when the porosity of the low-k film increases [5].

For synthesizing ultraporous low-k films having sufficient mechanical strength, we have proposed a novel one-step method [6]. For this method, nano-particles as nano-building blocks and radicals as adhesives are produced in reactive discharge plasmas; both are transported to a substrate and nano-particles are eventually codeposited there together with radicals. Control of the size of nano-particles as well as that of their transport rate from their generation region to a substrate is important in realizing the tailored structure of the films.

We have succeeded in controlling the size of nano-particles by pulse radio frequency (RF) discharge [6, 7] and realized their rapid transport towards a substrate by pulse RF discharge with the amplitude modulation (AM) of discharge voltage [8, 9]. On the basis of the obtained results, we have deposited nano-particle composite porous SiOCH films by pulse RF discharges without and with AM, and have evaluated the porosity, dielectric constant, Young’s modulus, RMS roughness, and thermal stability of the films. In this paper, we describe the experimental results and discuss criterion for increasing the deposition

rate of films without agglomerating nano-particles during their transport towards a substrate. 2. Experimental

Experiments were carried out using a capacitively coupled RF discharge reactor, as shown in Fig. 1 [6-9]. Two grounded electrodes of 60 mm in diameter were placed at a distance of 40 mm in a reactor of 260 mm in inner diameter and 230 mm in height. A powered disc electrode of 20 mm in diameter and 1 mm in thickness was set at 9 mm above the lower grounded electrode. The flow rates of Si(CH3)2(OCH3)2 and Ar were 0.2 and 40 sccm, respectively. The total gas pressure was 133 Pa. The temperature of the reactor wall was kept at 358-368 K to avoid the liquefaction of Si(CH3)2(OCH3)2 on the wall surface. To dissociate Si(CH3)2(OCH3)2 and generate nano-particles, we sustained discharges by applying a 770-780 peak-to-peak voltage of 13.56 MHz to the

Fig. 1. Experimental setup.

powered electrode for a discharging period Ton of 0.3 s having an intermission period of 1.0 or 10 s. The self-bias voltage was -288 V. The corresponding discharge power was 75 W. For an AM discharge, the discharge voltage was modulated, as shown in Fig. 2. The peak-to-peak voltage VAM during the modulation and the modulation period Δt were set to be 1090 V and 50 ms, respectively.

Spatiotemporal evolution of the size and density of nano-particles was measured by a two-dimensional laser light scattering method [10] combined with a simple method for deducing their size and density.

For deposition experiments, Si substrates of 5x10 mm2 were placed on the lower grounded electrode. The substrate temperature Ts was set to be in the range of 368-403 K. The mass of the porous SiOCH films was measured with a precision balance. Their cross-sectional and surface images were obtained by scanning electron microscopy (SEM). The root-mean-square (RMS) roughness of the films was measured with an atomic force microscope (AFM). The dielectric constant k was deduced from the capacitance of a metal/insulator/metal (MIM) structure having Al electrodes measured at 100 kHz. The atomic compositions and chemical bonds of the films were analyzed by energy-dispersive X-ray spectroscopy (EDS) and Fourier transform infrared spectroscopy (FT-IR), respectively. 3. Results and Discussion

Nano-particles grow via CVD processes on their surface during a discharging period, and they agglomerate each other after turning off discharges. For synthesizing nano-particle composite porous SiOCH films, such agglomeration should be avoided to keep the size of nano-particles close to the value just after turning off discharges. The deposition rate (DR) of nano-particle composite porous SiOCH films can be expressed as , (1)

where, M and ρ are the mass and mass density, n and v are the number density and velocity of nano-particles, β and s are the effective surface reaction probability and effective sticking one, respectively [11]. The deposition rate is proportional to vp and np. When the particle dynamics is dominated by thermal agglomeration, the decrease in number density after turning of discharges is obtained as

, (2)

where, K is the agglomeration coefficient [7, 12]. The characteristic time of the nano-particle transport towards a substrate is defined as . (3) The criterion for increasing the deposition rate of films without agglomerating nano-particles during their transport towards a substrate is , (4) where Δ is the distance between the powered electrode and a substrate. Using Eq. (4), the criterion is given by , (5) . (6) Rapid transport of nano-particle towards a substrate is a promising method for increasing the deposition rate of films without agglomerating nano-particles during their transport. Up to now we have realized such rapid transport of nano-particles towards a substrate by pulse RF discharges with AM: their velocity is more than 60 cm/s, which is at least 7 times as high as that without AM [8, 9]. The size of nano-particles deposited to a substrate with AM is 3.5 nm, which is close to the size of 2.5 nm obtained immediately after turning off the discharge, whereas that obtained without AM is 6.5 nm. The deposition rate

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discharge (a) without and (b) with AM. Vpp =770-780 V, VAM = 1090 V, Ton =0.3 s, and Δt = 50 ms.

obtained with AM and the intermission period = 1 s is 0.65 nm/s, which is 3 times as high as that without AM and intermission period = 10 s. Figures 3 and 4 show typical surface SEM images and AFM ones of the nano-particle composite porous SiOCH films deposited by pulse RF discharges without and with AM for Ts = 368 K. The dielectric constant and porosity of the films obtained (a) without AM are 1.3 and 81.1 %, and those (b) with AM are 1.4 and 60 %, respectively.

With increasing the substrate temperature from 368 to 393 K, the porosity of the films obtained without AM decreases from 81.1 to 37.6 % and that with AM decreases from 60 to 21.4 %. Such decrease in the porosity leads to an increase in their dielectric constant from 1.3 to 2.3 for the films obtained without AM, and from 1.4 to 2.6 for those with AM, respectively. Atomic compositions of the films weakly depend on the substrate temperature, indicating that thedielectric constant of the films is mainly determined by their porosity [12]. Figure 5 shows the substrate temperature dependence of the RMS roughness of the films obtained without and with AM. Without AM the RMS roughness decreases from 309 to 37 nm with increasing substrate temperature

from 368 to 393 K. With AM the RMS roughness decreases from 99 to 18 nm with increasing substrate temperature from 368 to 374 K, and it weakly depends on substrate temperature in the range of 374-403 K [12]. Figure 6 shows dielectric constant of nano-particle composite porous SiOCH films deposited by pulse RF discharges with AM after annealing at 273-673 K in vacuum for 1 hour. The films exhibit high thermal stability up to 673 K. The leak current and breakdown electric field are less than 1 nA/cm2 and more than 1 MV/cm, both of which meet the requirements for interlayer dielectric materials of ULSI. It should be noted that the films have a nearly constant Young’s modulus of 20 GPa irrelevant to their porosity of 3.5-56 %. The relationship between Young’s modulus and porosity is different from that in [13] expressed as the equation as follows, , (7)

Fig. 4. AFM images of films obtained (a) without and (b)

with AM. Ts 368 K, Ar 40 sccm, Si(CH3)2(OCH3)2 0.2 sccm, 1.0 Torr, Vpp = 770 V, VAM = 1090 V, 75 W, Ton = 0.3 s, Δt = 50 ms, intermission period (a) 10 s and (b) 1.0 s.

2)1(/ pEE b !=

where E, Eb, and p are Young’s modulus of porous films, that of the bulk and porosity, respectively. The high mechanical strength of our films is realized probably because the films have grained structure composed of nano-particles of 3.5 nm in size. 4. Conclusions

We have proposed a novel one-step method for synthesizing nano-particle composite porous SiOCH films by pulse RF discharge with AM. The following conclusions are obtained in this study.

1. The deposition rate obtained with AM is 0.65 nm/s, which is at least 3 times as high as that without AM. Deposition of nano-particle composite porous SiOCH films by pulse RF discharge with AM is a

promising method for increasing the deposition rate with suppressing agglomeration.

2. The porosity of the films increases from 3.5 to 60 % with decreasing substrate temperature from 403 to 368 K, leading to a reduction in their dielectric constant from 2.9 to 1.4. Substrate temperature is a key parameter that determines the porosity and dielectric constant of the films. Young’s modulus of the films is above 20 GPa irrelevant to their porosity.

3. The RMS roughness decreases from 99 to 18 nm with increasing substrate temperature from 368 to 374 K and it weakly depends on substrate temperature in the range of 374 – 403 K.

4. Nano-particle composite porous SiOCH films exhibit high thermal stability up to 673 K.

Acknowledgement

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(2006). [5] J. P. Simon, et al., J. Appl. Crystallogr. 40, s363

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Fig. 5. Substrate temperature dependence of RMS

roughness of films obtained without and with A M . T s 3 6 8 - 4 0 3 K , A r 4 0 s c c m , Si( CH 3 ) 2 (OCH 3 ) 2 0 .2 sccm, 1 .0 To r r, Vpp = 770 V, VAM = 1090 V, 75 W, Ton = 0.3 s, Δt = 50 ms, intermission period (a) 10 s and (b) 1.0 s.

Fig. 6. Dielectric constant of films obtained with AM

after annealing at 273-673 K in vacuum for 1 hour.