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organosilicon plasmas and thin SiOxCyHz films deposited in a helicon reactor
Thin Solid Films 359 (2000) 188±196 www.elsevier.com/locate/tsf
A comparative study of oxygen/organosilicon plasmas and thin SiOxCyHz ®lms deposited in a helicon reactor K. Aumaille*, C. ValleÂe, A. Granier, A. Goullet, F. Gaboriau, G. Turban Laboratoire des Plasmas et des Couches Minces - Institut des MateÂriaux Jean Rouxel de Nantes, Universite de Nantes - CNRS, 2 rue de la HoussinieÁre, BP 32229, 44322 Nantes Cedex 3, France. Received 21 June 1999; accepted in ®nal form 18 October 1999
Abstract Thin SiOxCyHz ®lms have been prepared by plasma enhanced chemical vapor deposition (PECVD) on silicon substrates at low pressure (2 mTorr) and 300 W rf power, using tetraethoxysilane (TEOS) or hexamethyldisiloxane (HMDSO) as a monomer and oxygen as a reactive gas. The plasma composition, the structure and properties of the deposited ®lms are studied as a function of the organosilicon fraction (Xorg). Optical emission spectroscopy is carried out in order to identify the species in the plasma. The layers are characterized by in situ spectroscopic ellipsometry and by several ex situ diagnostics including infra-red spectroscopy, X-ray photoelectron spectroscopy, gravimetry and chemical etching. At low values of Xorg, the structure and properties of the ®lms and optical emission spectra are very similar whatever the organosilicon precursor. At high values of Xorg, the structure and properties of the deposited ®lms and emitting species signi®cantly depend on the organosilicon precursor. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Organosilicon ®lms; Plasma processing and deposition; Silicon oxide
1. Introduction The use of silicon dioxide thin ®lms in many applications as optics [1], barrier ®lms for food packaging [2,3], interlayer dielectrics [4,5] and corrosion protection layers [6,7], explains the increasing interest for these ®lms. In addition to chemical vapor deposition, several other processes are employed for the deposition of silicon dioxide ®lms. Among these processes, plasma enhanced chemical vapor deposition (PECVD) has become one of the most important thin ®lm deposition processes because of the possibility of preparing good quality coatings at low substrate temperature. The most utilized organosilicon precursors are tetraethoxysilane (TEOS; SiZ(OZ(C2H5))4) and hexamethyldisiloxane (HMDSO; OZ(SiZ(CH3)3)2), pure or mixed with oxygen (O2) or nitrous oxide (N2O). O2 and N2O are added to organosilicon vapor in order to oxidize organic groups and to deposit near stoichiometric dioxide ®lms [8]. Both TEOS and HMDSO are non-toxic, nonexplosive, and much safer than silane. HMDSO has the further advantage of a higher room temperature vapor pressure (48 Torr) than TEOS (4 Torr) which allows easier use.
According to the organosilicon fraction in the O2/organosilicon mixture, either inorganic SiO2-like ®lms or organic SiOxCyHz-like ®lms are deposited. In this work, we have studied both O2/TEOS and O2/ HMDSO discharges, using optical emission spectroscopy (OES) to detect the excited species created in the discharge, and spectroscopic ellipsometry (SE), Fourier transform infra-red spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), gravimetry and chemical etching (p-etch) analyses to determine the ®lm composition and structure. The aim of this paper is to compare the structure and properties of the deposited ®lms obtained with different silicon precursors, TEOS and HMDSO, in a rf helicon reactor and to correlate the analysis of the plasma with the one of ®lms. After a brief description of the helicon reactor and the diagnostics in section 2, the results obtained from different diagnostics in O2/TEOS and O2/HMDSO plasmas will be presented in section 3 and discussed in section 4.
2. Experiment 2.1. Plasma reactor
* Corresponding author. Tel.: 1332-40-373-9654; fax: 1332-40-373959. E-mail address: [email protected] (K. Aumaille)
A detailed description of the helicon plasma diffusion reactor can be found in references [9,10]. The experimental
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0088 3-4
K. Aumaille et al. / Thin Solid Films 359 (2000) 188±196
set-up is shown in Fig. 1. In brief, the main components are a Pyrex tube serving as a plasma source and a stainless steel diffusion chamber where a p-type (100) silicon substrate with a 10±30 V cm resistivity is positioned. During the deposition process, the silicon substrate was neither heated nor cooled and left at the ¯oating potential. The plasma was generated in the source via an external helicon antenna. The oxygen gas was introduced from the top of the helicon source whereas the organosilicon precursor (TEOS or HMDSO) was injected into the diffusion chamber via a dispersal ring placed 8 cm above the substrate. The organosilicon vapor ¯ow rate was controlled in the 0±20 sccm range using a MKS 500C mass ¯ow. To achieve good operating conditions, liquid TEOS was heated up to 408C whereas HMDSO was cooled down to 108C. Argon was used as an actinometer. The deposition conditions were: a ®xed total ¯ow rate (oxygen1organosilicon) of 16 sccm which corresponds to a pressure of 2 mTorr before plasma ignition, a 300 W rf power and a magnetic ®eld equal to 60 Gauss at the center of the helicon source. In this study, the thickness of the ®lms deposited from O2/TEOS plasmas was about 1 and 0.5 mm when O2/HMDSO plasmas were used. 2.2. Diagnostics In order to characterize the oxygen/organosilicon plasmas and deposited ®lms, several diagnostics were performed. The light emitted by the discharge was sampled through a quartz window. The optical emission spectrum of the plasma was detected by a Jobin±Yvon monochromator (HR460) equipped with two gratings (2400 and 1200 g/ mm gratings were used for the 180±420 and 420±850 nm spectral range, respectively), and a photomultiplier (R928 Hamamatsu). Emission spectra were recorded in the spectral range 180±850 nm with a resolution of 0.3 nm. The ellipsometer used for refractive index and ®lm thickness measurements was a spectroscopic phase modulated UV-Visible (1.5±5 eV) ellipsometer from ISA Jobin Yvon (UVISEL). The ellipsometer was mounted on the diffusion chamber, allowing in situ measurements [20].
Fig. 1. Experimental set-up of the deposition helicon reactor.
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The infra-red absorption spectra of the deposited ®lms were recorded using a Nicolet 20 SXC spectrometer equipped with a HgCdTe (MCT) detector. The resolution was 4 cm 21 and 300 scans were recorded and averaged. A Leybold±Heraeus surface analysis system allows ex situ X-ray photoelectron spectroscopy (XPS) measurements. XPS measurements were made using a MgKa Xray source (1253.6 eV). The analyzer resolution was approximately 0.5 eV using a pass energy of 76 eV. As XPS analyses can not be performed in situ, the behavior of samples exposed to air has been studied. The experiments have shown that the surface contamination can lead to erroneous conclusions. In the case of inorganic SiO2-like ®lm, the as-deposited ®lm was submitted to a 40 eV argon ions bombardment for a few min duration to eliminate the surface contamination by carbon. In contrast, in the case of organic SiOxCyHz-like ®lm, the as-deposited ®lm was analyzed without any argon ions bombardment to avoid any ®lm modi®cation by Ar 1 ions (preferential sputtering of C) [11]. The ®lm density was measured by gravimetry. This method was previously shown to be reliable as compared to X-ray re¯ectivity. All the samples were weighed before and after the deposition using a micro-balance (Mettler AT20) with an accuracy of 10 mg. The density of the deposited ®lms was then determined from the weight increase to ®lm thickness ratio. Chemical etching (p-etch) was carried out to evaluate the quality of the deposited ®lms. For this purpose, the ®lms were etched in a 3:2:60 HF(50%)/HFNO3(67.5%)/ H2O(100%) solution. 3. Results 3.1. Optical emission spectroscopy Semi quantitative OES analysis was performed with 5% argon added to the O2/organosilicon mixture. When the organosilicon precursor was added to oxygen, the argon line intensity decreased. As previously demonstrated by Langmuir probe measurements [12], this decrease is due to a decrease of both the plasma density and electron temperature. Thus, the intensities of all the emission lines were divided by the 750 nm argon line intensity in order to partly avoid the variations of electron temperature and density. The emission systems observed in oxygen/organosilicon plasmas have been described in detail in several publications [12,13]. In brief, the main results are as follows: (1), at high dilution of the organosilicon in oxygen, the main excited species responsible for the optical emission spectrum in the 180±850 nm spectral range are O, CO, OH and H, but weaker emissions from CO 1, CO21 were also detected; (2), as the organosilicon fraction (Xorg) increased, new emissions from excited CH, C2 and H2 were identi®ed for both organosilicons whereas the emission from oxygen
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atoms decreased; (3), in pure organosilicon plasmas, the emission from oxygen atoms completely disappeared. CO* and OH* emissions were still observed in TEOS containing plasmas whereas they vanished in pure HMDSO plasmas [13]; (4), the emission of Si containing species (Si, SiO, SiH) was detected in O2/HMDSO and HMDSO plasmas whereas it was absent in O2/TEOS and TEOS plasmas. The normalized intensities of O* (844 nm), CO* (266 nm), OH* (306 nm), Hg (431 nm), H2* (752 nm), CH* (434 nm), SiO* (248 nm) and Si (288 nm) are plotted versus Xorg in Figs. 2a,b and 3a,b. Several facts are immediately apparent from these ®gures: (1), the values of the normalized intensities are very similar whatever the organosilicon precursor used. The intensities clearly depend on Xorg; (2), in both O2/TEOS and O2/HMDSO plasmas, the actinometric signal IO/IAr ®rst increases with Xorg, reaches a maximum around 10% and then decreases quickly. The actinometric signal IO/IAr is proportional to [O], since the latter can be written under the simple form I O =I Ar A T e [O]/[Ar] where A(Te) is the oxygen to argon excitation coef®cient ratio, which is nearly independent of the electron tempera-
Fig. 3. (a) Normalized emission intensities: (X), CO (266 nm); (W), OH (306 nm); (A), Hg (431 nm) as a function of the TEOS fraction. (b) Normalized emission intensities: (X), CO (266 nm); (W), OH (306 nm); (A), Hg (431 nm) as a function of the HMDSO fraction.
Fig. 2. (a) Normalized emission intensities: (X), O (844 nm); (W), CH (434 nm); (A), H2 (752 nm) as a function of the TEOS fraction. (b) Normalized emission intensities: (X), O (844 nm); (W), CH (434 nm); (A), H2 (752 nm); (S), Si (288 nm); (B), SiO (252 nm) as a function of the HMDSO fraction.
ture Te [9]. The increase in IO/IAr corresponds to a decrease in the recombination of oxygen atoms on the reactor walls due to the competition between O and new radicals formed in the plasma (H, OH, etc.). The decrease in IO/IAr when organosilicon precursor is further added to oxygen is attributed to the consumption of O by oxidation reactions occurring at the surface of the growing ®lm or in the gas phase [9]; (3), the emissions of CO, OH and H follow the same trends. Their normalized intensities ®rst increase with Xorg, go through a maximum and further decrease. The maximum is reached at X org 20% for OH and H, against 40% for CO. It is worth noting that the variations of all normalized CO systems and all hydrogen Balmer lines intensities are the same as a function of Xorg. As previously mentioned, one of the most striking differences between pure TEOS plasmas and pure HMDSO plasmas is the absence of CO* and OH* emissions in pure HMDSO plasmas; (4), H2 lines are detected from X org 20%, and above this threshold their intensities increase linearly with Xorg; (5), the intensities of C-containing radicals (CH) increase rapidly with Xorg. CH*
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emission is observed when emission of O* tends to disappear; (6), as previously reported, the emissions of Si and SiO are only detected in HMDSO-containing plasmas from X org 20 and 50%, respectively. 3.2. Spectroscopic ellipsometry The in¯uence of the organosilicon fraction on the deposition rate and the refractive index of the deposited ®lms at 1.96 eV (i.e., 633 nm) is reported in Figs. 4 and 5. It is clear that a simple law between the deposition rate and the organosilicon fraction does not exist (Fig. 4). At low values of Xorg, low deposition rates of about 6±10 nm/min are obtained for both organosilicons. Upon adding the organosilicon precursor to oxygen, the deposition rate increases and saturates at 25 nm/min for the ®lms deposited in pure TEOS plasmas and 50 nm/min for the ®lms deposited in HMDSO plasmas. Thus, at high values of Xorg, the deposition rate is around two times larger in HMDSO-derived ®lms than in TEOS-derived ®lms. This result is in agreement with what was found by Latreche [22] and Sawada [23]. Fig. 5 shows the variations of the in situ refractive index of the ®lms obtained from TEOS and HMDSO as a function of the organosilicon fraction. At a low Xorg, the ®lms are transparent in the 1.5±5 eV energy range, and their refractive index at 1.96 eV is very close to the one of a thermal oxide (n 1:46). The decrease of the refractive index as Xorg is increased, indicates the presence of defects such as microporosity. The Bruggeman effective medium approximation (BEMA) has been used to evaluate the fraction of voids, assuming that the ®lm is a homogeneous mixture of amorphous SiO2 and voids [20]. The values of void fraction increase with Xorg, reaching 6% for the ®lms deposited in 20% TEOS/80% O2 plasmas, against 2% for the ®lms elaborated by 20% HMDSO/80% O2 plasmas. As Xorg increases above 30%, both the extinction coef®cient and refractive index increase. The extinction coef®cient in the spectral
Fig. 4. Variations of the deposition rate (Vd) as a function of the organosilicon fraction: (X), TEOS; (W), HMDSO.
Fig. 5. Variations of the refractive indices (in situ measurements) measured at 1.96 eV versus the organosilicon fraction: (X), TEOS; (W), HMDSO.
range of 1.5±5 eV determined for the ®lms deposited at X org 100% is plotted in Fig. 6. As can be seen, the HMDSO-derived ®lms are more absorbent than TEOSderived ®lms in the UV. Moreover, at high values of Xorg, the organic ®lms derived from HMDSO have a refractive index higher than the organic ®lms derived from TEOS. 3.3. Infra-red absorption spectroscopy FTIR absorption spectroscopy was performed to determine the nature of the bonding groups in the ®lms. Typical infra-red spectra in the 400±4000 cm 21 spectral range of the ®lms deposited at different organosilicon fractions are shown in Fig. 7a,b. When X org , 33%, the infra-red spectra of the ®lms prepared from TEOS and HMDSO are very similar. Both spectra exhibit the three characteristic peaks of SiO2 near 450, 800 and 1070 cm 21 and absorption bands at 3450 and
Fig. 6. The extinction coef®cient of the ®lms deposited from pure organosilicon plasmas as a function of the wavelength: (continuous line), TEOS; (broken line), HMDSO.
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Fig. 7. (a) Normalized FTIR absorbance spectra for different TEOS fractions: XTEOS 10, 40, 50, 100%. (b) Normalized FTIR absorbance spectra for different HMDSO fractions: XHMDSO 10, 33, 50, 100%.
3650 cm 21 assigned to OZH vibration of associated SiOH species and isolated SiOH groups, respectively [24]. It is worth noting that the integrated normalized absorbance of SiOSi (between 950 and 1300 cm 21) stretching peak is higher in the case of HMDSO-derived ®lms. The presence of water near 1650 cm 21 is very dif®cult to detect. When 33% # Xorg # 100%, the infra-red spectra of TEOS and HMDSO-derived ®lms exhibit the presence of two new common bands. As can be seen in Fig. 7a,b, SiZH absorption peak appears at the same position (2250 cm 21), whatever the organosilicon used. This peak is clearly visible from X org 33% in the case of ®lms deposited in O2/
HMDSO mixtures, whereas it appears from X org 50% in the case of ®lms deposited in O2/TEOS mixtures. CZH stretches are positioned at around 2900, 2930 and 2960 cm 21 for both organosilicons, but are observed from X org 33% for HMDSO-derived ®lms and from X org 40% for TEOS-derived ®lms. In addition to the above mentioned peaks, new speci®c absorption structures appear: SiZ(CH3)x (810, 840, 890, 1250 cm 21) and SiZ(CH2)xZSi (1360, 1400 cm 21) for HMDSO-derived ®lms [15], CvO (1700 cm 21) and CHx (1380, 1460 cm 21) for TEOS-derived ®lms [12]. When the HMDSO fraction increases, the SiZO (1050 cm 21) peak position is down-
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shifted while its intensity strongly decreases. This evolution goes along with a strong decrease in the hydroxyl band intensity (2800±3800 cm 21). Finally, no additional structures are evidenced in the spectra recorded at X org 100%. However peak intensity variations are exhibited: (1), a decrease in CvO (1700 cm 21), an increase in SiH (2250 cm 21) and CHx (2900± 2960 cm 21) for TEOS-deposited ®lms; (2), an increase in SiZ(CH3)x (1250 cm 21) and SiH (2150 cm 21), a decrease in OH (3450 cm 21) and SiOH (3650 cm 21) combined with a disappearance of CHx (2930 cm 21) peak for HMDSO deposited ®lms. Moreover, when the HMDSO fraction increases, the frequency of the SiZH absorption peak is down-shifted to 2150 cm 21. 3.4. X-ray photoelectron spectroscopy The dif®culty of X-ray photoelectron spectroscopy measurements results from the surface contamination containing carbonZoxygen bonds which can lead to erroneous conclusions. In order to obtain quantitative elemental and chemical state information, XPS analyses were carried out on a silica (SiO2) standard to determine exact oxygen, silicon and carbon binding energy and sensitivity factors. Then, all the XPS spectra were corrected of the charging effect using the O 1s peak position of 532.7 eV as a reference. This procedure is often employed [11,17]. The O/Si and C/Si content ratios measured by means of XPS analyses are plotted versus Xorg in Fig. 8a,b. For X org , 33%, the two elements detected are silicon and oxygen. Carbon is below the detection limit of XPS analyses. Whatever the organosilicon, the O/Si content is close to two. The value of Si 2p binding energy (103.2 eV) is around 0.2 eV lower than the one of a thermal silicon oxide (103.4 eV), and corresponds to Si surrounded by four oxygen atoms denoted Si(ZO)4 as in silica [17]. The O 1s and Si 2p peaks are symmetrical and positioned at about 533 and 103 eV, respectively. Thus, the energy difference between O 1s and Si 2p is around 430 eV, which is consistent with stoichiometric SiO2 ®lms [18]. By increasing the organosilicon fraction, the carbon content in the ®lms increases very rapidly, whereas the oxygen and silicon contents decrease (Table 1). In the case of TEOS-derived ®lms, the increase in Xorg does not affect the shape and position of the Si 2p peak. As can be seen in Fig. 9a, the Si 2p peak is symmetrical and is always maximum at ~103 eV which is characteristic of the Si(ZO)4 environment. In contrast, in the case of HMDSO-derived ®lms, the Si 2p spectrum is not symmetrical due to the presence of SiZO, SiZC and SiZH bonds, as shown in Fig. 9b. The binding energy of Si 2p in HMDSO-derived ®lms is less than that determined for TEOS-derived ®lms. Moreover, examination of the experimental Si 2p width before the silicon peak decomposition, indicates that the Si 2p full width half-maximum (FWHM) is higher in the case of HMDSO-deposited ®lms. This con®rms that there is
Fig. 8. (a) O/Si determined from XPS spectra versus Xorg: (X), TEOS; (W), HMDSO. (b) C/Si determined from XPS spectra versus Xorg: (X), TEOS; (W), HMDSO.
more than one chemical state present within this peak. The decomposition of the carbon C 1s peak, for the ®lms deposited in O2/TEOS gas mixtures, has shown the presence of three peaks positioned at ~285 (CZC, CZH), , 286 (CZO) and ,288 eV (CvO, OZCZO). In the HMDSOderived ®lms, C 1s peaks at 286 and 288 eV are not observed. The values of C/Si and O/Si content ratios significantly differ according to the organosilicon. The C/Si ratio is higher in TEOS-derived ®lms than in HMDSO-derived ®lms. The O/Si ratio is always larger than two in TEOSderived ®lms and less than two in HMDSO-derived ®lms.
Table 1 O, Si and C content determined from XPS spectra for two different organosilicon fractions: Xorg 10% and 100%, respectively.
O (%) Si (%) C (%)
Xorg 10% TEOS
HMDSO
Xorg 100% TEOS
HMDSO
67 33 0
68 32 0
34 13 53
34 24 42
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carbon-related species such as CvO and CHx. This suggests that silicon and oxygen atoms are replaced by light carbon and hydrogen atoms. This decrease is also noticed for ®lms obtained from O2/HMDSO plasmas. Similarly, the exchange of SiZO bonds by SiZC, SiZH and CZH bonds contributes to a decrease in the ®lm density. 4. Discussion 4.1. High organosilicon dilution (X org , 33%)
Fig. 9. (a) The XPS Si 2p spectrum of the ®lm deposited from pure TEOS plasmas. (b) The XPS Si 2p spectrum of the ®lm deposited from pure HMDSO plasmas.
3.5. Gravimetry and X-ray re¯ectivity The density of the ®lms as measured by gravimetry and XRR is plotted as a function of the organosilicon fraction in Fig. 10. The density ranges between 2.4 and 1.3 g/cm 3 depending on the nature of the deposited ®lm (silicon oxide ®lm and organosilicon ®lm). Independently of the nature of the organosilicon precursor, the ®lm density decreases as Xorg increases. Films deposited in oxygen-rich mixtures have a density very close to that of the thermal oxide (2.2 g/cm 3). The density is slightly higher in the case of HMDSO; this is consistent with p-etch measurements since the lowest petch rate is obtained at low HMDSO fraction (5%) and is equal to two times the p-etch rate of the thermal oxide, against four times for a ®lm deposited with 5% TEOS. Then, when Xorg increases, the ®lm density decreases and approaches a constant value around 1.4 g/cm 3 in the case of HMDSO-derived ®lms, and around 1.6 g/cm 3 in the case of TEOS-derived ®lms. The abrupt transition which occurs between 30 and 50% TEOS coincides with the decrease in the integrated area of the SiO peak and the appearance of
At low values of Xorg, optical emission spectra are very similar whatever the organosilicon precursor used. In addition to O lines, the optical emission spectra are dominated by the strong emissions of excited CO, OH and H which are indicative of the formation of etching products during the deposition such as CO, CO2 and H2O. All the analyses exhibit that the ®lms prepared from TEOS and HMDSO are both SiO2-like ®lms with properties very close to those of a thermal oxide. The only signi®cant difference lies in the density of the ®lms, which is slightly higher in the case of HMDSOderived ®lms. This difference is likely to be related to a higher SiZO bond concentration. The integrated normalized absorbance of SiOSi (A(SiOSi) between 950 and 1300 cm 21) stretching peak has been calculated. It is shown that A(SiOSi)HMDSO . A(SiOSi)TEOS. These results are consistent with p-etch measurements which have shown that the chemical etch-rate of the ®lms deposited in 95% O2/ 5% HMDSO plasmas is two times faster than for the thermal oxide, against four times for the ®lms deposited in 95% O2/ 5% TEOS plasmas. 4.2. Low organosilicon dilution (X org $ 33%) At high values of Xorg, the plasma and ®lm compositions signi®cantly depend on the organosilicon used. The most striking difference between pure TEOS plasmas and pure HMDSO plasmas is the absence of Si*, SiO* and SiH*
Fig. 10. Density of the ®lms as a function of the organosilicon fraction: (X), TEOS; (W), HMDSO.
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emissions in pure TEOS plasmas. In the case of TEOS, four strong SiZO bonds must be broken in order to obtain an isolated excited Si atom, whereas in the case of HMDSO, its formation only requires the breaking of one SiZO bond and three SiZC bonds which are easier to break than SiZO. This observation is consistent with the optical emission spectra produced by controlled electron impact on TEOS and HMDSO molecules [14]. A further difference between pure TEOS plasmas and pure HMDSO plasmas is the absence of CO* emissions in pure HMDSO plasmas, whereas CO* systems are easily observed in pure TEOS plasmas. These discrepancies are again connected to the nature of the organosilicon precursor, since the CO bond is only existing in the TEOS molecule and the emission of CO* systems is only identi®ed in pure TEOS plasmas. It is worth noting that the appearance of the CH emission in the plasma coincides with the incorporation of carbon in the ®lms observed in the XPS and FTIR spectra, as demonstrated by Lamendola et al. [21]. The analysis of organic SiOxCyHz-like ®lms by spectroscopic ellipsometry has shown that: (1), the deposition rate is around two times larger in HMDSO-derived ®lms than in TEOS-derived ®lms, which can be attributed to either a lower sticking coef®cient of fragments stemming from the dissociation of TEOS molecules, or a lower number silicon atoms in the TEOS monomer. In fact, HMDSO is a molecule containing two silicon atoms, against one in TEOS molecule; (2), n increases with Xorg, and the HMDSOderived ®lms have a refractive index higher than those of TEOS-derived ®lms. The enhancement of carbon content in the ®lms with Xorg could be a cause of the increasing index. Nevertheless, other effects can compete with carbon incorporation such as ®lm density. For the ®lms elaborated by O2/ HMDSO plasmas, the increase of the carbon content in the ®lms is not the main reason for refractive index increase, since XPS analyses have shown that: (C)HMDSO , (C)TEOS and as shown in Fig. 5, we have nHMDSO . nTEOS. For HMDSO fractions greater than 33%, FTIR spectra reveal a peak at around 2250 cm 21 assigned to SiH, and the increase of the refractive index corresponds to the appearance of SiH groups. In Fig. 11, the area of the SiH normalized absorption band (integrated between 2060 and 2285 cm 21) is plotted as a function of the refractive index; the linear correlation supports the idea that the n increase is strongly linked to the density of SiZH bonds. The infra-red analysis of all organic ®lms has evidenced that the structure of the deposited ®lms signi®cantly differs according to the organosilicon. The most important results obtained by means of Fourier transform infra-red spectroscopy are: (1), the absorption peaks of SiZCHx are completely absent in the case of TEOS-deposited ®lms, whereas they are easily observed in the case of HMDSO-deposited ®lms. This result can be related to the presence of SiZC bonds in the HMDSO molecule; (2), the presence of ethoxy groups in the TEOS molecule induces three peaks at 2900, 2930 and 2960 cm 21 due to CH3 and CH2 stretching vibra-
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Fig. 11. The area of the SiH normalized absorption band (integrated between 2060 and 2285 cm 21) as a function of the refractive index of HMDSO-derived ®lms.
tions, whereas the methyl groups in the HMDSO monomer are characterized by two peaks at 2900 and 2960 cm 21 which can be assigned to the symmetric and asymmetric stretching vibration of a CH3 group [15,16]; (3), the OH absorption is higher in the case of TEOS-derived ®lms than in the case of HMDSO-derived ®lms. This clearly demonstrates that pure HMDSO plasmas provide excellent barrier ®lms. Del®no et al. [19] have shown that the formation of SiZH bonds prevents water incorporation in the ®lms. On the other hand, it was noted from XPS results that: (1), TEOS-derived ®lms have C/Si content ratios higher than those of HMDSO-derived ®lms. This discrepancy between C/Si content ratios can be due to the composition of the monomers. Indeed, C/Si is equal to eight in the TEOS molecule, against three in the HMDSO molecule; (2), in the case of organic ®lms derived from TEOS plasmas, the O/Si ratio is larger than two, whereas in the case of organic ®lms derived from HMDSO plasmas, the O/Si ratio is less than two. This suggests that in the case of TEOS-derived ®lms, oxygenZsilicon bonds (ZOZSiZ) are replaced by oxygenZhydrogen (ZOZHZ) and oxygenZcarbon (ZOZCZ) bonds. Similarly, in the case of HMDSOderived ®lms, the substitution of siliconZoxygen (ZSiZOZ) bonds by siliconZhydrogen (ZSiZHZ) and siliconZcarbon (ZSiZCZ) bonds can explain the decrease of the O/Si ratio; (3), the ®tting of the silicon Si 2p peak has revealed the presence of SiZO, SiZC and SiZH groups with HMDSO, which is in good agreement with FTIR analysis of these ®lms. 5. Conclusion Thin SiOxCyHz ®lms have been deposited from oxygen/ organosilicon (TEOS and HMDSO) plasmas in an helicon
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reactor. The composition of the plasma and the composition and structure of the deposited ®lms have been studied as a function of the organosilicon fraction (Xorg). For low values of Xorg, it has been shown that whatever the organosilicon used, the structure of the deposited ®lms and optical emission spectra are very similar. All the analyses exhibit that the ®lms prepared from TEOS and HMDSO are both SiO2like ®lms with properties very close to those of a thermal oxide. As Xorg increases, the structure of the ®lms changes from dense and transparent inorganic SiO2±like ®lms to less dense and nontransparent organic SiOxCyHz-like ®lms. The carbon content, refractive index and deposition rates of the deposited ®lms increase with the organosilicon fraction in the mixture. For high values of Xorg, the composition of the plasma, the composition and structure of the organic ®lms differ according to the organosilicon used. Indeed, the analysis of the plasma has evidenced that the signi®cantly emitting species depend on the organosilicon precursor. Strong similitaries between the structure of the organosilicon monomer and the structure of the organic ®lms have been observed. This suggests that TEOS and HMDSO molecules are certainly fragmented in heavy precursors by electron impact or oxygen atoms. In order to identify the nature of the fragments responsible for the ®lm deposition, the use of other in situ diagnostics such as mass spectrometry or infra-red absorption spectroscopy will be necessary.
Acknowledgements The authors would like to thank Dr. A. van der Lee, C. MarlieÁre and J. Durand (Laboratoire des MateÂriaux et ProceÂdeÂs Membranaires de Montpellier) for the analysis of the ®lms by X-ray re¯ectivity. We also would like to thank Dr. C. Cardinaud and V. Fernandez (Laboratoire des Plasmas et des Couches Minces de Nantes) for their help in the analysis of XPS spectra.
References [1] P. Tien, G. Smolinsky, R. Martin, J. Appl. Opt. 11 (1972) 637. [2] N. Inagaki, S. Kondo, M. Hirata, H. Urushibata, J. Appl. Polym. Sci. 30 (1989) 3385. [3] P. Fayet, C. Holland, B. Jaccoud, A. Roulin, Proc. 38th SVC Conf., 1995, pp. 15±17. [4] T. Templer, L. Vallier, R. Madar, J.C. Oberlin, R.A.B. Devine, Thin Solid Films 241 (1994) 251. [5] C. Falcony, A. Ortiz, S. Lopez, J.C. Alonso, S. Muhl, Thin Solid Films 199 (1991) 269. [6] J.M. Okura, White, Appl. Surf. Sci. 29 (1987) 223. [7] H.P. Schreiber, M.R. Wertheimer, A.M. Wrobel, Thin Solids Films 72 (1980) 487. [8] N. Selamoglu, J.A. Mucha, D.E. Ibboston, D.L. Flamm, J. Vac. Sci. Technol. B 7 (1989) 1345. [9] A. Granier, F. Nicolazo, C. ValleÂe, A. Goullet, G. Turban, B. Grolleau, Plasma Sources Sci. Technol. 6 (1997) 147. [10] R.W. Boswell, Phys. Lett. 33A (1970) 457. [11] F. Fracassi, R. d'Agostino, P. Favia, J. Electrochem. Soc. 139 (1992) 2636. [12] F. Nicolazo, A. Goullet, A. Granier, C. ValleÂe, G. Turban, B. Grolleau, Surf. Coat. Technol. 98 (1998) 1578. [13] A. Granier, L. Mage, F. Nicolazo, et al., Proc. 23rd Int. Conf. on Phenomena in Ionized Gases 3 (1997) 74±75. [14] P. Kurunczi, A. Koharian, K. Becker, K. Martus, Plasma Phys. 36 (1996) 723. [15] C. Rau, W. Kulisch, Thin Solid Films 249 (1994) 28. [16] M.G.M. van der Vis, R.J.M. Konings, A. Oskam, T.L. Snacck, J. Mol. Struct. 274 (1992) 47. [17] M.R. Alexander, R.D. Short, F.R. Jones, M. Stollenwerk, J. Zabold, W. Michaeli, J. Mater. Sci. 31 (1996) 1879. [18] C. Bourreau, Y. Catherine, P. Garcia, Plasma Chem. Plasma Processes 10 (1990) 247. [19] M. Del®no, W. Tsai, G. Reynolds, M.E. Day, Appl. Phys. Lett. 63 (1993) 3426. [20] C. ValleÂe, A. Goullet, F. Nicolazo, A. Granier, G. Turban, J. NonCryst. Solids 216 (1997) 48. [21] R. Lamendola, R. d'Agostino, P. Favia, F. Fracassi, Plasma Processing of Polymers, in: R. d'Agostino, P. Favia, F. Fracassi (Eds.), Series E, Vol 346, Kluwer Academic Publishers, The Netherlands, 1997, pp. 321±333. [22] Latreche, M., PhD Thesis, Grenoble 1 (1995). [23] Y. Sawada, S. Ogawa, M. Koyama, J. Phys. D: Appl. Phys. 28 (1995) 1667. [24] W.A. Pliskin, J. Vac. Sci. Technol. 14 (1977) 1064.
A comparative study of oxygen/organosilicon plasmas and thin SiOxCyHz ®lms deposited in a helicon reactor K. Aumaille*, C. ValleÂe, A. Granier, A. Goullet, F. Gaboriau, G. Turban Laboratoire des Plasmas et des Couches Minces - Institut des MateÂriaux Jean Rouxel de Nantes, Universite de Nantes - CNRS, 2 rue de la HoussinieÁre, BP 32229, 44322 Nantes Cedex 3, France. Received 21 June 1999; accepted in ®nal form 18 October 1999
Abstract Thin SiOxCyHz ®lms have been prepared by plasma enhanced chemical vapor deposition (PECVD) on silicon substrates at low pressure (2 mTorr) and 300 W rf power, using tetraethoxysilane (TEOS) or hexamethyldisiloxane (HMDSO) as a monomer and oxygen as a reactive gas. The plasma composition, the structure and properties of the deposited ®lms are studied as a function of the organosilicon fraction (Xorg). Optical emission spectroscopy is carried out in order to identify the species in the plasma. The layers are characterized by in situ spectroscopic ellipsometry and by several ex situ diagnostics including infra-red spectroscopy, X-ray photoelectron spectroscopy, gravimetry and chemical etching. At low values of Xorg, the structure and properties of the ®lms and optical emission spectra are very similar whatever the organosilicon precursor. At high values of Xorg, the structure and properties of the deposited ®lms and emitting species signi®cantly depend on the organosilicon precursor. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Organosilicon ®lms; Plasma processing and deposition; Silicon oxide
1. Introduction The use of silicon dioxide thin ®lms in many applications as optics [1], barrier ®lms for food packaging [2,3], interlayer dielectrics [4,5] and corrosion protection layers [6,7], explains the increasing interest for these ®lms. In addition to chemical vapor deposition, several other processes are employed for the deposition of silicon dioxide ®lms. Among these processes, plasma enhanced chemical vapor deposition (PECVD) has become one of the most important thin ®lm deposition processes because of the possibility of preparing good quality coatings at low substrate temperature. The most utilized organosilicon precursors are tetraethoxysilane (TEOS; SiZ(OZ(C2H5))4) and hexamethyldisiloxane (HMDSO; OZ(SiZ(CH3)3)2), pure or mixed with oxygen (O2) or nitrous oxide (N2O). O2 and N2O are added to organosilicon vapor in order to oxidize organic groups and to deposit near stoichiometric dioxide ®lms [8]. Both TEOS and HMDSO are non-toxic, nonexplosive, and much safer than silane. HMDSO has the further advantage of a higher room temperature vapor pressure (48 Torr) than TEOS (4 Torr) which allows easier use.
According to the organosilicon fraction in the O2/organosilicon mixture, either inorganic SiO2-like ®lms or organic SiOxCyHz-like ®lms are deposited. In this work, we have studied both O2/TEOS and O2/ HMDSO discharges, using optical emission spectroscopy (OES) to detect the excited species created in the discharge, and spectroscopic ellipsometry (SE), Fourier transform infra-red spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), gravimetry and chemical etching (p-etch) analyses to determine the ®lm composition and structure. The aim of this paper is to compare the structure and properties of the deposited ®lms obtained with different silicon precursors, TEOS and HMDSO, in a rf helicon reactor and to correlate the analysis of the plasma with the one of ®lms. After a brief description of the helicon reactor and the diagnostics in section 2, the results obtained from different diagnostics in O2/TEOS and O2/HMDSO plasmas will be presented in section 3 and discussed in section 4.
2. Experiment 2.1. Plasma reactor
* Corresponding author. Tel.: 1332-40-373-9654; fax: 1332-40-373959. E-mail address: [email protected] (K. Aumaille)
A detailed description of the helicon plasma diffusion reactor can be found in references [9,10]. The experimental
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0088 3-4
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set-up is shown in Fig. 1. In brief, the main components are a Pyrex tube serving as a plasma source and a stainless steel diffusion chamber where a p-type (100) silicon substrate with a 10±30 V cm resistivity is positioned. During the deposition process, the silicon substrate was neither heated nor cooled and left at the ¯oating potential. The plasma was generated in the source via an external helicon antenna. The oxygen gas was introduced from the top of the helicon source whereas the organosilicon precursor (TEOS or HMDSO) was injected into the diffusion chamber via a dispersal ring placed 8 cm above the substrate. The organosilicon vapor ¯ow rate was controlled in the 0±20 sccm range using a MKS 500C mass ¯ow. To achieve good operating conditions, liquid TEOS was heated up to 408C whereas HMDSO was cooled down to 108C. Argon was used as an actinometer. The deposition conditions were: a ®xed total ¯ow rate (oxygen1organosilicon) of 16 sccm which corresponds to a pressure of 2 mTorr before plasma ignition, a 300 W rf power and a magnetic ®eld equal to 60 Gauss at the center of the helicon source. In this study, the thickness of the ®lms deposited from O2/TEOS plasmas was about 1 and 0.5 mm when O2/HMDSO plasmas were used. 2.2. Diagnostics In order to characterize the oxygen/organosilicon plasmas and deposited ®lms, several diagnostics were performed. The light emitted by the discharge was sampled through a quartz window. The optical emission spectrum of the plasma was detected by a Jobin±Yvon monochromator (HR460) equipped with two gratings (2400 and 1200 g/ mm gratings were used for the 180±420 and 420±850 nm spectral range, respectively), and a photomultiplier (R928 Hamamatsu). Emission spectra were recorded in the spectral range 180±850 nm with a resolution of 0.3 nm. The ellipsometer used for refractive index and ®lm thickness measurements was a spectroscopic phase modulated UV-Visible (1.5±5 eV) ellipsometer from ISA Jobin Yvon (UVISEL). The ellipsometer was mounted on the diffusion chamber, allowing in situ measurements [20].
Fig. 1. Experimental set-up of the deposition helicon reactor.
189
The infra-red absorption spectra of the deposited ®lms were recorded using a Nicolet 20 SXC spectrometer equipped with a HgCdTe (MCT) detector. The resolution was 4 cm 21 and 300 scans were recorded and averaged. A Leybold±Heraeus surface analysis system allows ex situ X-ray photoelectron spectroscopy (XPS) measurements. XPS measurements were made using a MgKa Xray source (1253.6 eV). The analyzer resolution was approximately 0.5 eV using a pass energy of 76 eV. As XPS analyses can not be performed in situ, the behavior of samples exposed to air has been studied. The experiments have shown that the surface contamination can lead to erroneous conclusions. In the case of inorganic SiO2-like ®lm, the as-deposited ®lm was submitted to a 40 eV argon ions bombardment for a few min duration to eliminate the surface contamination by carbon. In contrast, in the case of organic SiOxCyHz-like ®lm, the as-deposited ®lm was analyzed without any argon ions bombardment to avoid any ®lm modi®cation by Ar 1 ions (preferential sputtering of C) [11]. The ®lm density was measured by gravimetry. This method was previously shown to be reliable as compared to X-ray re¯ectivity. All the samples were weighed before and after the deposition using a micro-balance (Mettler AT20) with an accuracy of 10 mg. The density of the deposited ®lms was then determined from the weight increase to ®lm thickness ratio. Chemical etching (p-etch) was carried out to evaluate the quality of the deposited ®lms. For this purpose, the ®lms were etched in a 3:2:60 HF(50%)/HFNO3(67.5%)/ H2O(100%) solution. 3. Results 3.1. Optical emission spectroscopy Semi quantitative OES analysis was performed with 5% argon added to the O2/organosilicon mixture. When the organosilicon precursor was added to oxygen, the argon line intensity decreased. As previously demonstrated by Langmuir probe measurements [12], this decrease is due to a decrease of both the plasma density and electron temperature. Thus, the intensities of all the emission lines were divided by the 750 nm argon line intensity in order to partly avoid the variations of electron temperature and density. The emission systems observed in oxygen/organosilicon plasmas have been described in detail in several publications [12,13]. In brief, the main results are as follows: (1), at high dilution of the organosilicon in oxygen, the main excited species responsible for the optical emission spectrum in the 180±850 nm spectral range are O, CO, OH and H, but weaker emissions from CO 1, CO21 were also detected; (2), as the organosilicon fraction (Xorg) increased, new emissions from excited CH, C2 and H2 were identi®ed for both organosilicons whereas the emission from oxygen
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atoms decreased; (3), in pure organosilicon plasmas, the emission from oxygen atoms completely disappeared. CO* and OH* emissions were still observed in TEOS containing plasmas whereas they vanished in pure HMDSO plasmas [13]; (4), the emission of Si containing species (Si, SiO, SiH) was detected in O2/HMDSO and HMDSO plasmas whereas it was absent in O2/TEOS and TEOS plasmas. The normalized intensities of O* (844 nm), CO* (266 nm), OH* (306 nm), Hg (431 nm), H2* (752 nm), CH* (434 nm), SiO* (248 nm) and Si (288 nm) are plotted versus Xorg in Figs. 2a,b and 3a,b. Several facts are immediately apparent from these ®gures: (1), the values of the normalized intensities are very similar whatever the organosilicon precursor used. The intensities clearly depend on Xorg; (2), in both O2/TEOS and O2/HMDSO plasmas, the actinometric signal IO/IAr ®rst increases with Xorg, reaches a maximum around 10% and then decreases quickly. The actinometric signal IO/IAr is proportional to [O], since the latter can be written under the simple form I O =I Ar A T e [O]/[Ar] where A(Te) is the oxygen to argon excitation coef®cient ratio, which is nearly independent of the electron tempera-
Fig. 3. (a) Normalized emission intensities: (X), CO (266 nm); (W), OH (306 nm); (A), Hg (431 nm) as a function of the TEOS fraction. (b) Normalized emission intensities: (X), CO (266 nm); (W), OH (306 nm); (A), Hg (431 nm) as a function of the HMDSO fraction.
Fig. 2. (a) Normalized emission intensities: (X), O (844 nm); (W), CH (434 nm); (A), H2 (752 nm) as a function of the TEOS fraction. (b) Normalized emission intensities: (X), O (844 nm); (W), CH (434 nm); (A), H2 (752 nm); (S), Si (288 nm); (B), SiO (252 nm) as a function of the HMDSO fraction.
ture Te [9]. The increase in IO/IAr corresponds to a decrease in the recombination of oxygen atoms on the reactor walls due to the competition between O and new radicals formed in the plasma (H, OH, etc.). The decrease in IO/IAr when organosilicon precursor is further added to oxygen is attributed to the consumption of O by oxidation reactions occurring at the surface of the growing ®lm or in the gas phase [9]; (3), the emissions of CO, OH and H follow the same trends. Their normalized intensities ®rst increase with Xorg, go through a maximum and further decrease. The maximum is reached at X org 20% for OH and H, against 40% for CO. It is worth noting that the variations of all normalized CO systems and all hydrogen Balmer lines intensities are the same as a function of Xorg. As previously mentioned, one of the most striking differences between pure TEOS plasmas and pure HMDSO plasmas is the absence of CO* and OH* emissions in pure HMDSO plasmas; (4), H2 lines are detected from X org 20%, and above this threshold their intensities increase linearly with Xorg; (5), the intensities of C-containing radicals (CH) increase rapidly with Xorg. CH*
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emission is observed when emission of O* tends to disappear; (6), as previously reported, the emissions of Si and SiO are only detected in HMDSO-containing plasmas from X org 20 and 50%, respectively. 3.2. Spectroscopic ellipsometry The in¯uence of the organosilicon fraction on the deposition rate and the refractive index of the deposited ®lms at 1.96 eV (i.e., 633 nm) is reported in Figs. 4 and 5. It is clear that a simple law between the deposition rate and the organosilicon fraction does not exist (Fig. 4). At low values of Xorg, low deposition rates of about 6±10 nm/min are obtained for both organosilicons. Upon adding the organosilicon precursor to oxygen, the deposition rate increases and saturates at 25 nm/min for the ®lms deposited in pure TEOS plasmas and 50 nm/min for the ®lms deposited in HMDSO plasmas. Thus, at high values of Xorg, the deposition rate is around two times larger in HMDSO-derived ®lms than in TEOS-derived ®lms. This result is in agreement with what was found by Latreche [22] and Sawada [23]. Fig. 5 shows the variations of the in situ refractive index of the ®lms obtained from TEOS and HMDSO as a function of the organosilicon fraction. At a low Xorg, the ®lms are transparent in the 1.5±5 eV energy range, and their refractive index at 1.96 eV is very close to the one of a thermal oxide (n 1:46). The decrease of the refractive index as Xorg is increased, indicates the presence of defects such as microporosity. The Bruggeman effective medium approximation (BEMA) has been used to evaluate the fraction of voids, assuming that the ®lm is a homogeneous mixture of amorphous SiO2 and voids [20]. The values of void fraction increase with Xorg, reaching 6% for the ®lms deposited in 20% TEOS/80% O2 plasmas, against 2% for the ®lms elaborated by 20% HMDSO/80% O2 plasmas. As Xorg increases above 30%, both the extinction coef®cient and refractive index increase. The extinction coef®cient in the spectral
Fig. 4. Variations of the deposition rate (Vd) as a function of the organosilicon fraction: (X), TEOS; (W), HMDSO.
Fig. 5. Variations of the refractive indices (in situ measurements) measured at 1.96 eV versus the organosilicon fraction: (X), TEOS; (W), HMDSO.
range of 1.5±5 eV determined for the ®lms deposited at X org 100% is plotted in Fig. 6. As can be seen, the HMDSO-derived ®lms are more absorbent than TEOSderived ®lms in the UV. Moreover, at high values of Xorg, the organic ®lms derived from HMDSO have a refractive index higher than the organic ®lms derived from TEOS. 3.3. Infra-red absorption spectroscopy FTIR absorption spectroscopy was performed to determine the nature of the bonding groups in the ®lms. Typical infra-red spectra in the 400±4000 cm 21 spectral range of the ®lms deposited at different organosilicon fractions are shown in Fig. 7a,b. When X org , 33%, the infra-red spectra of the ®lms prepared from TEOS and HMDSO are very similar. Both spectra exhibit the three characteristic peaks of SiO2 near 450, 800 and 1070 cm 21 and absorption bands at 3450 and
Fig. 6. The extinction coef®cient of the ®lms deposited from pure organosilicon plasmas as a function of the wavelength: (continuous line), TEOS; (broken line), HMDSO.
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Fig. 7. (a) Normalized FTIR absorbance spectra for different TEOS fractions: XTEOS 10, 40, 50, 100%. (b) Normalized FTIR absorbance spectra for different HMDSO fractions: XHMDSO 10, 33, 50, 100%.
3650 cm 21 assigned to OZH vibration of associated SiOH species and isolated SiOH groups, respectively [24]. It is worth noting that the integrated normalized absorbance of SiOSi (between 950 and 1300 cm 21) stretching peak is higher in the case of HMDSO-derived ®lms. The presence of water near 1650 cm 21 is very dif®cult to detect. When 33% # Xorg # 100%, the infra-red spectra of TEOS and HMDSO-derived ®lms exhibit the presence of two new common bands. As can be seen in Fig. 7a,b, SiZH absorption peak appears at the same position (2250 cm 21), whatever the organosilicon used. This peak is clearly visible from X org 33% in the case of ®lms deposited in O2/
HMDSO mixtures, whereas it appears from X org 50% in the case of ®lms deposited in O2/TEOS mixtures. CZH stretches are positioned at around 2900, 2930 and 2960 cm 21 for both organosilicons, but are observed from X org 33% for HMDSO-derived ®lms and from X org 40% for TEOS-derived ®lms. In addition to the above mentioned peaks, new speci®c absorption structures appear: SiZ(CH3)x (810, 840, 890, 1250 cm 21) and SiZ(CH2)xZSi (1360, 1400 cm 21) for HMDSO-derived ®lms [15], CvO (1700 cm 21) and CHx (1380, 1460 cm 21) for TEOS-derived ®lms [12]. When the HMDSO fraction increases, the SiZO (1050 cm 21) peak position is down-
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shifted while its intensity strongly decreases. This evolution goes along with a strong decrease in the hydroxyl band intensity (2800±3800 cm 21). Finally, no additional structures are evidenced in the spectra recorded at X org 100%. However peak intensity variations are exhibited: (1), a decrease in CvO (1700 cm 21), an increase in SiH (2250 cm 21) and CHx (2900± 2960 cm 21) for TEOS-deposited ®lms; (2), an increase in SiZ(CH3)x (1250 cm 21) and SiH (2150 cm 21), a decrease in OH (3450 cm 21) and SiOH (3650 cm 21) combined with a disappearance of CHx (2930 cm 21) peak for HMDSO deposited ®lms. Moreover, when the HMDSO fraction increases, the frequency of the SiZH absorption peak is down-shifted to 2150 cm 21. 3.4. X-ray photoelectron spectroscopy The dif®culty of X-ray photoelectron spectroscopy measurements results from the surface contamination containing carbonZoxygen bonds which can lead to erroneous conclusions. In order to obtain quantitative elemental and chemical state information, XPS analyses were carried out on a silica (SiO2) standard to determine exact oxygen, silicon and carbon binding energy and sensitivity factors. Then, all the XPS spectra were corrected of the charging effect using the O 1s peak position of 532.7 eV as a reference. This procedure is often employed [11,17]. The O/Si and C/Si content ratios measured by means of XPS analyses are plotted versus Xorg in Fig. 8a,b. For X org , 33%, the two elements detected are silicon and oxygen. Carbon is below the detection limit of XPS analyses. Whatever the organosilicon, the O/Si content is close to two. The value of Si 2p binding energy (103.2 eV) is around 0.2 eV lower than the one of a thermal silicon oxide (103.4 eV), and corresponds to Si surrounded by four oxygen atoms denoted Si(ZO)4 as in silica [17]. The O 1s and Si 2p peaks are symmetrical and positioned at about 533 and 103 eV, respectively. Thus, the energy difference between O 1s and Si 2p is around 430 eV, which is consistent with stoichiometric SiO2 ®lms [18]. By increasing the organosilicon fraction, the carbon content in the ®lms increases very rapidly, whereas the oxygen and silicon contents decrease (Table 1). In the case of TEOS-derived ®lms, the increase in Xorg does not affect the shape and position of the Si 2p peak. As can be seen in Fig. 9a, the Si 2p peak is symmetrical and is always maximum at ~103 eV which is characteristic of the Si(ZO)4 environment. In contrast, in the case of HMDSO-derived ®lms, the Si 2p spectrum is not symmetrical due to the presence of SiZO, SiZC and SiZH bonds, as shown in Fig. 9b. The binding energy of Si 2p in HMDSO-derived ®lms is less than that determined for TEOS-derived ®lms. Moreover, examination of the experimental Si 2p width before the silicon peak decomposition, indicates that the Si 2p full width half-maximum (FWHM) is higher in the case of HMDSO-deposited ®lms. This con®rms that there is
Fig. 8. (a) O/Si determined from XPS spectra versus Xorg: (X), TEOS; (W), HMDSO. (b) C/Si determined from XPS spectra versus Xorg: (X), TEOS; (W), HMDSO.
more than one chemical state present within this peak. The decomposition of the carbon C 1s peak, for the ®lms deposited in O2/TEOS gas mixtures, has shown the presence of three peaks positioned at ~285 (CZC, CZH), , 286 (CZO) and ,288 eV (CvO, OZCZO). In the HMDSOderived ®lms, C 1s peaks at 286 and 288 eV are not observed. The values of C/Si and O/Si content ratios significantly differ according to the organosilicon. The C/Si ratio is higher in TEOS-derived ®lms than in HMDSO-derived ®lms. The O/Si ratio is always larger than two in TEOSderived ®lms and less than two in HMDSO-derived ®lms.
Table 1 O, Si and C content determined from XPS spectra for two different organosilicon fractions: Xorg 10% and 100%, respectively.
O (%) Si (%) C (%)
Xorg 10% TEOS
HMDSO
Xorg 100% TEOS
HMDSO
67 33 0
68 32 0
34 13 53
34 24 42
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carbon-related species such as CvO and CHx. This suggests that silicon and oxygen atoms are replaced by light carbon and hydrogen atoms. This decrease is also noticed for ®lms obtained from O2/HMDSO plasmas. Similarly, the exchange of SiZO bonds by SiZC, SiZH and CZH bonds contributes to a decrease in the ®lm density. 4. Discussion 4.1. High organosilicon dilution (X org , 33%)
Fig. 9. (a) The XPS Si 2p spectrum of the ®lm deposited from pure TEOS plasmas. (b) The XPS Si 2p spectrum of the ®lm deposited from pure HMDSO plasmas.
3.5. Gravimetry and X-ray re¯ectivity The density of the ®lms as measured by gravimetry and XRR is plotted as a function of the organosilicon fraction in Fig. 10. The density ranges between 2.4 and 1.3 g/cm 3 depending on the nature of the deposited ®lm (silicon oxide ®lm and organosilicon ®lm). Independently of the nature of the organosilicon precursor, the ®lm density decreases as Xorg increases. Films deposited in oxygen-rich mixtures have a density very close to that of the thermal oxide (2.2 g/cm 3). The density is slightly higher in the case of HMDSO; this is consistent with p-etch measurements since the lowest petch rate is obtained at low HMDSO fraction (5%) and is equal to two times the p-etch rate of the thermal oxide, against four times for a ®lm deposited with 5% TEOS. Then, when Xorg increases, the ®lm density decreases and approaches a constant value around 1.4 g/cm 3 in the case of HMDSO-derived ®lms, and around 1.6 g/cm 3 in the case of TEOS-derived ®lms. The abrupt transition which occurs between 30 and 50% TEOS coincides with the decrease in the integrated area of the SiO peak and the appearance of
At low values of Xorg, optical emission spectra are very similar whatever the organosilicon precursor used. In addition to O lines, the optical emission spectra are dominated by the strong emissions of excited CO, OH and H which are indicative of the formation of etching products during the deposition such as CO, CO2 and H2O. All the analyses exhibit that the ®lms prepared from TEOS and HMDSO are both SiO2-like ®lms with properties very close to those of a thermal oxide. The only signi®cant difference lies in the density of the ®lms, which is slightly higher in the case of HMDSOderived ®lms. This difference is likely to be related to a higher SiZO bond concentration. The integrated normalized absorbance of SiOSi (A(SiOSi) between 950 and 1300 cm 21) stretching peak has been calculated. It is shown that A(SiOSi)HMDSO . A(SiOSi)TEOS. These results are consistent with p-etch measurements which have shown that the chemical etch-rate of the ®lms deposited in 95% O2/ 5% HMDSO plasmas is two times faster than for the thermal oxide, against four times for the ®lms deposited in 95% O2/ 5% TEOS plasmas. 4.2. Low organosilicon dilution (X org $ 33%) At high values of Xorg, the plasma and ®lm compositions signi®cantly depend on the organosilicon used. The most striking difference between pure TEOS plasmas and pure HMDSO plasmas is the absence of Si*, SiO* and SiH*
Fig. 10. Density of the ®lms as a function of the organosilicon fraction: (X), TEOS; (W), HMDSO.
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emissions in pure TEOS plasmas. In the case of TEOS, four strong SiZO bonds must be broken in order to obtain an isolated excited Si atom, whereas in the case of HMDSO, its formation only requires the breaking of one SiZO bond and three SiZC bonds which are easier to break than SiZO. This observation is consistent with the optical emission spectra produced by controlled electron impact on TEOS and HMDSO molecules [14]. A further difference between pure TEOS plasmas and pure HMDSO plasmas is the absence of CO* emissions in pure HMDSO plasmas, whereas CO* systems are easily observed in pure TEOS plasmas. These discrepancies are again connected to the nature of the organosilicon precursor, since the CO bond is only existing in the TEOS molecule and the emission of CO* systems is only identi®ed in pure TEOS plasmas. It is worth noting that the appearance of the CH emission in the plasma coincides with the incorporation of carbon in the ®lms observed in the XPS and FTIR spectra, as demonstrated by Lamendola et al. [21]. The analysis of organic SiOxCyHz-like ®lms by spectroscopic ellipsometry has shown that: (1), the deposition rate is around two times larger in HMDSO-derived ®lms than in TEOS-derived ®lms, which can be attributed to either a lower sticking coef®cient of fragments stemming from the dissociation of TEOS molecules, or a lower number silicon atoms in the TEOS monomer. In fact, HMDSO is a molecule containing two silicon atoms, against one in TEOS molecule; (2), n increases with Xorg, and the HMDSOderived ®lms have a refractive index higher than those of TEOS-derived ®lms. The enhancement of carbon content in the ®lms with Xorg could be a cause of the increasing index. Nevertheless, other effects can compete with carbon incorporation such as ®lm density. For the ®lms elaborated by O2/ HMDSO plasmas, the increase of the carbon content in the ®lms is not the main reason for refractive index increase, since XPS analyses have shown that: (C)HMDSO , (C)TEOS and as shown in Fig. 5, we have nHMDSO . nTEOS. For HMDSO fractions greater than 33%, FTIR spectra reveal a peak at around 2250 cm 21 assigned to SiH, and the increase of the refractive index corresponds to the appearance of SiH groups. In Fig. 11, the area of the SiH normalized absorption band (integrated between 2060 and 2285 cm 21) is plotted as a function of the refractive index; the linear correlation supports the idea that the n increase is strongly linked to the density of SiZH bonds. The infra-red analysis of all organic ®lms has evidenced that the structure of the deposited ®lms signi®cantly differs according to the organosilicon. The most important results obtained by means of Fourier transform infra-red spectroscopy are: (1), the absorption peaks of SiZCHx are completely absent in the case of TEOS-deposited ®lms, whereas they are easily observed in the case of HMDSO-deposited ®lms. This result can be related to the presence of SiZC bonds in the HMDSO molecule; (2), the presence of ethoxy groups in the TEOS molecule induces three peaks at 2900, 2930 and 2960 cm 21 due to CH3 and CH2 stretching vibra-
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Fig. 11. The area of the SiH normalized absorption band (integrated between 2060 and 2285 cm 21) as a function of the refractive index of HMDSO-derived ®lms.
tions, whereas the methyl groups in the HMDSO monomer are characterized by two peaks at 2900 and 2960 cm 21 which can be assigned to the symmetric and asymmetric stretching vibration of a CH3 group [15,16]; (3), the OH absorption is higher in the case of TEOS-derived ®lms than in the case of HMDSO-derived ®lms. This clearly demonstrates that pure HMDSO plasmas provide excellent barrier ®lms. Del®no et al. [19] have shown that the formation of SiZH bonds prevents water incorporation in the ®lms. On the other hand, it was noted from XPS results that: (1), TEOS-derived ®lms have C/Si content ratios higher than those of HMDSO-derived ®lms. This discrepancy between C/Si content ratios can be due to the composition of the monomers. Indeed, C/Si is equal to eight in the TEOS molecule, against three in the HMDSO molecule; (2), in the case of organic ®lms derived from TEOS plasmas, the O/Si ratio is larger than two, whereas in the case of organic ®lms derived from HMDSO plasmas, the O/Si ratio is less than two. This suggests that in the case of TEOS-derived ®lms, oxygenZsilicon bonds (ZOZSiZ) are replaced by oxygenZhydrogen (ZOZHZ) and oxygenZcarbon (ZOZCZ) bonds. Similarly, in the case of HMDSOderived ®lms, the substitution of siliconZoxygen (ZSiZOZ) bonds by siliconZhydrogen (ZSiZHZ) and siliconZcarbon (ZSiZCZ) bonds can explain the decrease of the O/Si ratio; (3), the ®tting of the silicon Si 2p peak has revealed the presence of SiZO, SiZC and SiZH groups with HMDSO, which is in good agreement with FTIR analysis of these ®lms. 5. Conclusion Thin SiOxCyHz ®lms have been deposited from oxygen/ organosilicon (TEOS and HMDSO) plasmas in an helicon
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reactor. The composition of the plasma and the composition and structure of the deposited ®lms have been studied as a function of the organosilicon fraction (Xorg). For low values of Xorg, it has been shown that whatever the organosilicon used, the structure of the deposited ®lms and optical emission spectra are very similar. All the analyses exhibit that the ®lms prepared from TEOS and HMDSO are both SiO2like ®lms with properties very close to those of a thermal oxide. As Xorg increases, the structure of the ®lms changes from dense and transparent inorganic SiO2±like ®lms to less dense and nontransparent organic SiOxCyHz-like ®lms. The carbon content, refractive index and deposition rates of the deposited ®lms increase with the organosilicon fraction in the mixture. For high values of Xorg, the composition of the plasma, the composition and structure of the organic ®lms differ according to the organosilicon used. Indeed, the analysis of the plasma has evidenced that the signi®cantly emitting species depend on the organosilicon precursor. Strong similitaries between the structure of the organosilicon monomer and the structure of the organic ®lms have been observed. This suggests that TEOS and HMDSO molecules are certainly fragmented in heavy precursors by electron impact or oxygen atoms. In order to identify the nature of the fragments responsible for the ®lm deposition, the use of other in situ diagnostics such as mass spectrometry or infra-red absorption spectroscopy will be necessary.
Acknowledgements The authors would like to thank Dr. A. van der Lee, C. MarlieÁre and J. Durand (Laboratoire des MateÂriaux et ProceÂdeÂs Membranaires de Montpellier) for the analysis of the ®lms by X-ray re¯ectivity. We also would like to thank Dr. C. Cardinaud and V. Fernandez (Laboratoire des Plasmas et des Couches Minces de Nantes) for their help in the analysis of XPS spectra.
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