Journal of the Korean Physical Society, Vol. 53, No. 3, September 2008, pp. 14681474
Plasma Diagnostics during Plasma-Enhanced Chemical-Vapor Deposition of Low-Dielectric-Constant SiOC(-H) Films from TES/O2 Precursors R.
Navamathavan, An Soo Jung, Chang Young Kim and Chi Kyu Choi
Nano-Thin Film Materials Laboratory, Department of Physics, Cheju National University, Jeju 690-756
Lee
Heon Ju
Department of Mechanical, Energy and Production Engineering, Cheju National University, Jeju 690-756
(Received 12 December 2007, in nal form 2 March 2008) Low-dielectric-constant SiOC(-H) lms were deposited on p -type Si (100) substrates by using plasma-enhanced chemical-vapor deposition (PECVD) from triethoxysilane (TES; C6 H16 O3 Si) and oxygen gas as precursors. Factors a ecting the carbon and hydrogen incorporation into the SiO2 lms during plasma-enhanced chemical vapor deposition were studied for di erent experimental parameters by using an in-situ residual gas analyzer (RGA), optical emission spectroscopy (OES) and Langmuir probe measurements. Detailed information of the relative radical densities, the electron density (Ne ) and the electron temperature (Te ) of the bulk plasma was obtained in order to better understand the deposition process of the low- SiOC(-H) thin lms. The silane-to-oxygen (precursor ow rate) ratio in the process gas mixture and the radio-frequency (rf) power determine the chemical nature of the substrate surface and the incorporation rates of Si-O-C and Si-O-Si into the lm. We interpret in detail the evolutions of plasma discharge parameters in terms of the variations in the composition of the plasma. PACS numbers: 77.55.+f, 77.22.-d, 81.15.Gh, 78.55.Mb Keywords: Low-k materials, SiOC(-H) lms, TES, PECVD, RGA, OES, FTIR
I. INTRODUCTION
Materials with a low dielectric (low-) constant are required as interlayer dielectrics for the on-chip interconnection of ultra-large scale integration (ULSI) devices to provide high speed, low dynamic power dissipation and low cross-talk noise. The requirements include high thermal and mechanical stability, good adhesion to other interconnect materials, resistance to processing chemicals, low moisture absorption and low cost [1{4]. Therefore, in recent years, there have been widespread e orts for researchers to develop low- materials that can simultaneously satisfy all of these requirements. One of the most promising low-k candidates is carbon-doped SiO2 (SiOC(-H)). The chemical structure of SiOC(-H) lm is basically composed of a silicon-oxygen backbone with methyl (CH3 ) incorporation. The methyl groups have lower polarization and can reduce the lm density considerably, hence decreasing the dielectric constant. Though dielectric constant is a primary factor to rank the performance of a low- interlayer dielectric (ILD), thermal and mechanical stabilities are also necessary for a successful integration into multilayer structures [5{8]. How E-mail:
[email protected]
ever, lowering the dielectric constant generally causes detrimental e ects on thermo-mechanical properties such as the modulus, the hardness, the cohesion/adhesion strength and the thermal expansion coecient. Low- dielectrics have to meet stringent requirements in materials properties in order to be successfully integrated. Use of appropriate starting materials with a lower value to develop porous SiOC(-H) thin lms will have a de nite advantage in interconnect technology. The processes of plasma deposition and the plasma characteristics are found to be important to fabricate good-quality low- lms at high rates [9, 10]. If the physical mechanisms during the plasma processes are to be understood, it is essential to analyze the various species present inside the plasma. In this present study, we investigated the plasma characteristics during the deposition of SiOC(-H) thin lms by using plasma enhanced chemical vapor deposition with triethoxysilane (TES) and oxygen as precursors. We analyzed the information on the reactive discharges, the plasma composition and the plasma parameters by using a residual gas analyzer (RGA), optical emission spectroscopy (OES) and a Langmuir probe, respectively, which provided the physical mechanisms and a better understanding of the formation of low- SiOC(-H) lms.
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Fig. 1. Schematic view of the TCP-CVD system. II. EXPERIMENTS
SiOC(-H) thin lms were deposited on p -Si(100) substrates by using a mixture of TES (C6 H16 O3 Si) and oxygen gases as precursors in a transformer-coupled plasma chemical-vapor deposition (TCP-CVD) system (Figure 1) at room temperature. The plasma was generated using an rf power supply with a frequency of 13.56 MHz between the two electrodes. Before depositing the SiOC(H) thin lms, we investigated the in-situ bulk plasma characteristics by using RGA, OES and a Langmuir probe for the di erent experimental parameters. We evacuated the chamber to a pressure of less than 10 6 Torr and we maintained the working pressure at 100 mTorr. The total ow rate of the precursors was maintained at 50 sccm. Two di erent experimental conditions were used: (i) The ow rate ratio of [TES/(O2 +TES)] 100 % was varied from 20 to 100 % at a constant rf power of 700 W and (ii) the rf power was varied from 100 to 800 W at a constant ow rate ratio of 80 %. The TES precursor was a colorless transparent liquid with a purity of 98 %, a density of 0.875 g/cm3 and a boiling point of 131.5 C. In order to prevent the recondensation of the TES precursor, we heated the bubbler bath and the gas delivery lines and kept them at a constant temperature of 40 C. A simple experiment for determining the natural abundance can be performed by using an RGA. The in-situ RGA spectrum was recorded by means of a RGA (Stanford Research Systems) quadrupole mass analyzer and was analyzed by using a standard software for an ultrahigh vacuum. OES was performed through a quartz window by using an optical ber and a monochromator (Triax series 320) equipped with a photomultiplier
tube. The plasma parameters, such as the electron density (Ne ) and the electron temperature (Te ), were measured by using a Wise probe 2000 (P and A Solution) technique, by inserting one electrode into the plasma, with a constant or time-varying electric potential between the electrode and the surrounding chamber. The refractive index of the deposited lm was measured by using an ellipsometer at a wavelength of 632.8 nm. In order to determine the dielectric constant, we measured the capacitance-voltage (C-V) characteristics for the metalinsulator-semiconductor (MIS) structure at 1 MHz by using a semiconductor parameter analyzer (HP4280A).
III. RESULTS AND DISCUSSION
The in-situ RGA partial-pressure measurement in the mass range of 0 { 50 amu during the fabrication of the low- SiOC(-H) thin lms in a vacuum chamber is shown in Figures 2(a), 2(b) and 2(c). This is a simple experiment, which allows us to measure the absolute value of the partial pressure in the reactive gas during the process. Direct quantitative measurement of the isotopic abundance can be obtained during the experimental work and can be compared with that of the literature. Figures 2(a) and 2(b) show the RGA data for a mixture of Ar+TES+O2 gas for di erent ow rate ratios with plasma o and on conditions, respectively. In both gures, we can observe similar data, except for a slight variation in the intensities. We can clearly see a signal around 26 { 29 amu corresponding to the TES precursor for the plasma o condition, which is not observed in the plasma on state, as shown in the Figures
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Fig. 2. RGA partial pressure measurement in the mass range of 0 { 50 atomic mass units (amu) for the Ar+TES+O2 mixtures: (a) plasma o and (b) plasma on with di erent ow rate ratios and (c) with di erent rf powers.
2(a) and 2(b). We observed an important consumption of molecular oxygen at higher ow rates in the reactive gas. We attribute this consumption to the destruction of O2 molecules by electronic collision to give atomic
species [11]. Figure 2(c) shows the RGA data of the Ar+TES+O2 bulk plasma during the deposition of low SiOC(-H) thin lms with di erent rf powers. These data look similar for all the rf power, but a slight inten-
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Fig. 3. Optical emission spectra of Ar+TES+O2 bulk plasma (a) with di erent precursor ow rate ratios and (b) with di erent rf powers.
sity variation is noted around 26 { 29 amu, which might be due to the TES precursor. These peaks are observed to disappear at higher rf powers of 700 and 800 W, which indicates that the precursor molecules fully dissociate at the higher rf powers. We observed a drastic drop in the intensities shown in Figure 2(a) compared to those shown in Figure 2(b). These di erences are attributed to the reactivity of the radicals in the precursor molecules. During the plasma discharge, numerous reactions take place to produce reactive ions and radicals. The main mechanisms are electron impact with precursor molecule to form highly reactive species at low pressure. Some other reactions also occur, for example, associative ionization of Ar atoms [12]. Figures 3(a) and 3(b) show the typical optical emission spectra of an Ar+TES+O2 bulk plasma with di erent
ow rate ratios and rf powers, respectively. Figure 3(a) shows the OES spectra for di erent ow rate ratios vary from 20 to 100 % at a constant rf power of 700 W. The in uence of the precursor ow on the radical intensities in the plasma was studied. When the ow rate ratio was increased, the emission intensities corresponding to each species increased. Figure 3(b) shows the OES spectra of the Ar+TES+O2 bulk plasma during the deposition of SiOC(-H) thin lms for di erent rf powers at a constant
ow rate ratio of 80 %. During the plasma polymerization, many reactive species are dissociated into small species, forming a SiOC(-H) lm on the Si substrate.
From the OES data as shown in Figures 3(a) and 3(b), the detected species are identi ed as SiH+ (399.3 nm), C3 (405 nm), SiO (426 nm), CH (431.3 nm), SiH2 (552.7 nm), H2 (602 nm), H (656.5 nm), Ar (725 { 825 nm) and O (844 nm) [13{15]. As shown, there is no discharge for an rf power of 300 W; the intensities of the species gradually increased with the applied rf power. When the rf power became 700 W, the plasma color turned full bright; thus, a complete discharge had occurred. This is an effect of the increasing electron density in the chamber due to inelastic collisions of electrons with TES molecules. The changes in the relative intensities of the detected species, such as SiH+ , C3 , SiO, CH, H , H2 , Ar and O, demonstrate the complete dissociation of the precursor by the plasma power. As the rf power was increased, the emission intensity of each species increased rapidly, which shows the complete dissociation of the precursors in the bulk plasma. Figures 4(a) and 4(b) show the normalized emission intensity (Ix /IAr ) of each species as a function of the
ow rate ratio and the rf power, respectively, where Ix is the emission intensity of species and IAr is the intensity of argon at 750 nm. We are interested in the evolutions of the changes in the normalized intensities of the CH, C3 , SiH2 , H and SiO species as functions of the ow rate ratio and the rf power. The behaviors of the normalized lines of the species for the ow rate ratio are di erent from those for the rf power. As shown in Figure 4(a), all
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Fig. 4. Normalized emission line intensities of the species SiH, C3 , SiO, CH, SiH2 , H , H2 and O for di erent (a) ow rate ratios and (b) rf powers.
Fig. 5. Electron temperature (Te ) and electron density (Ne ) of the Ar+TES+O2 bulk plasma as functions of the (a) precursor ow rate ratio (%) and the (b) rf power.
the normalized species (SiH+ , C3 , SiO, SiH2 , CH, H , H2 and O) are decreased slightly as the ow rate ratio is increased. Some of the radicals, for example, SiO and C3 , are not observed for the lower ow rate ratios (from 20 to 60 %). In the case of the rf power, as shown in Figure 4(b), the normalized intensities of some species (C3 , SiO, CH, SiH2 and H ) are slightly increased and those of other species (SiH, H2 and O) are found to be decreased with increasing rf power. This result means that the ethyl groups in the TES precursor are decomposed by the release of H radicals. The dissociation of SiO, Si and O can take place due to break up of Si-O network chains by the application of rf power. Because the thermal stability of the Si-O-Si bond in organosilicon compounds is lower than that of the Si-O bond in SiO2 , the breaking of SiO chains is easier [9,16]. Although the intensities of all emitted species were found to increase with increasing
ow rate ratio and rf power, as presented in Figures 3(a) and 3(b), the normalized intensities of some species (C3 , SiO, CH, SiH2 and H ) decreased with increasing of ow rate ratio [Figure 4(a)]. Thus, the radical intensities in the bulk plasma were observed to be strongly in uenced by the ow rate ratio and the rf power. Figure 5 shows the electron density and the electron temperature of the Ar+TES+O2 bulk plasma as func-
tions of the ow rate ratio and the rf power, respectively, measured by means of a Wise probe method. The electron density increased and the electron temperature decreased monotonously with increasing ow rate ratio and rf power, respectively. As the ow rate ratio (from 20 to 100 %) and the rf power (from 100 to 900 W) were increased, the corresponding electron density and electron temperature (Te ) were found to vary from 1.52 109 to 6 1010 cm 3 and from 6.9 108 to 4.2 1010 cm 3 and from 3.4 to 1.7 eV and from 3.5 to 1.4 eV, respectively. This result clearly demonstrates that a higher
ow rate ratio and plasma power will induce an abundant electron density in the chamber, which enhances the generation of more reactive radicals. It is likely that the applied rf power in the PECVD chamber will exhibit a rapid transition between electrostatic (E) and electromagnetic (H) modes of the plasma discharge following the application of an rf power of around 700 W [see Figure 5(b)]. Mode transition is determined by a sudden and huge change in the luminance. The H mode is characterized by a much higher luminance and plasma density, so this transition may be connected with the dynamics of the electron density and with the metastable density of the atoms and the direct ionization of the atoms by the electron impact. At the H mode, many ionic and atomic
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Fig. 6. Correlations of the deposition rate of the SiOC(-H) lms with the ow rate ratio and the rf power.
spectra are observed compared to the E mode. During the generation of the plasma, free electrons gain kinetic energy from the applied rf power and lose it in collisions with neutral molecules and atoms. As the rf power is increased beyond 700 W, which results in a full discharge of bulk plasma in the chamber, the internal energy of the gas molecules becomes high and the molecules can then be excited, dissociated and ionized [17]. Thus, a complete dissociation of the precursor take place, leading to a greater abundance of reactive radicals and ions in the chamber. Figure 6 shows the relation of the deposition rate of the SiOC(-H) to the ow rate ratio and the rf power. As expected, the deposition rate of the SiOC(-H) lm was observed to be slower (30 nm/min) for a lower ow rate and rf power. The deposition rate of the SiOC(-H) lm remains almost constant for rf powers from 300 to 600 W and for ow rate ratios from 20 to 40 %. Above these range, the deposition rate increased rapidly with increasing rf power and ow rate ratio. This rapid increase in the deposition rate is attributed to the full plasma discharge (increased electron density), as evidenced by the OES and Langmuir probe data (see Figures 4 and 5). The deposition rate is increased by about 3 times for the lms deposited at 800 W (90 nm/min) compared to those deposited at 400 W (30 nm/min). The same trend was also observed for the case of the ow rate ratio; the deposition rate increased from 20 nm/min (for a ow rate of 20 %) to 75 nm/min (for a ow rate of 100 %). Normally, the deposition rate of the SiOC(-H) lms depends on the plasma parameters, which is clearly evidenced from Figure 5; as the rf power increases, the electron density increases rapidly, resulting in a higher deposition rate. Figures 7(a) and 7(b) show the dielectric constant and the refractive index of SiOC(-H) lms as functions of the
ow rate ratio and the rf power, respectively. For lower
ow rate ratios and rf powers, the dielectric constant and the refractive index were found to be higher. Figure 7(a)
Fig. 7. Dielectric constant and refractive index of SiOC(H) thin lms as functions of (a) the ow rate ratio and (b) the rf power.
shows the dielectric constant and the refractive index for SiOC(-H) lms with the di erent ow rate ratios from 50 to 100 % (the data for ow rate ratios from 20 to 40 % are not shown here, for which the values of and the refractive index are the same as those for 50 %). As can be seen from the Figure 7(b), a similar trend was observed for the case of the rf power. The dielectric constant lies between 3.6 and 3.7, very close to the value of SiO2 -like lms, thereafter, yielding a drastic reduction in the value upon incorporation of carbon into the lm. This carbon can be attached to the Si-O-Si network chain, resulting in the formation of a more cross-linked structure in the lm. The drastic reduction in the refractive index of the SiOC(-H) lm with increasing of rf power is also strong evidence for more abundant presences of carbon and of a Si-O-Si cross-linking network in the lm. These low values of refractive index are likely to be related to a higher fraction of voids in the SiOC(-H) lms [13]. As the
ow rate ratio and the rf power were raised, SiOC(-H) was formed with increasing carbon content. Therefore, a very drastic reduction in the dielectric constant value was obtained for the SiOC(-H) lms deposited at higher precursor ow rate ratios and higher rf powers.
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Journal of the Korean Physical Society, Vol. 53, No. 3, September 2008 IV. CONCLUSIONS
REFERENCES
The detailed plasma diagnostics during the fabrication of low- SiOC(-H) thin lms on p -Si(100) substrates by using PECVD with a mixture of TES and oxygen gases was studied. The detailed plasma parameters were analyzed by using RGA, OES and Langmuir probe methods. When the ow rate ratio (from 20 to 100 %) and rf power (from 100 to 900 W) were increased, the corresponding electron density (Ne ) and electron temperature (Te ) were found to vary from 1.52 109 to 6 1010 cm 3 and from 6.9 108 to 4.2 1010 cm 3 and from 3.4 to 1.7 eV and from 3.5 to 1.4 eV, respectively. The deposition rate was increased by about 3 times for the lms deposited at 800 W (90 nm/min) compared to the lms deposited at 400 W (30 nm/min). The dielectric constant and the refractive index of the SiOC(-H) lms were found to decrease with increasing precursor
ow rate ratio and applied rf power. Complete discharge of the precursor molecules resulted in a high-quality lm and, hence, high-quality structural and electrical properties. An appropriate choice of precursor and experimental parameters can result in a high-quality SiOC(-H) lm with a lower dielectric constant.
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ACKNOWLEDGMENTS
This work was supported by a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea Government Ministry of Science and Technology (MOST), (No. R01-2007-000-10181-0). The authors involved in this study were supported by a grant from the 2nd Stage BK-21 Project.
