5
Silicon oxynitride from microwave plasma: fabrication and characterization S. BLAIN, J. E. KLEMBERG-SAPIEHA, AND M. R. WERTHEIMER' Groupe des couches minces (GCM) et Dkpartement de genie physique, ~ c o l e Polytechnique, C.P. 6079, Succursale XA", Montre'al (Que'bec),Canada H3C 3A7 AND S. C. GUJRATHI Laboratoire de physique nucle'aire, Dkpartement de physique, Universitk de Montrkal, C.P. 6128, Succursale "A'', Montreal (Que'bec), Canada H3C 3J7 Received August 11, 1988 Plasma silicon nitride (P-SiN), oxynitride (P-SiON), and silicon dioxide (P-SiO,) films have been prepared from SiH4- NH3-N,O mixtures in a large volume microwave plasma (LMPR, 2.45 GHz) apparatus at T, = 280°C. Film compositions, determined by X-ray photoelectron spectroscopy and nuclear elastic recoil detection analysis, reveal about 15 at.% hydrogen in P-SiN, <2% in P-SiO,, and intermediate values in P-SiON. Various physicochemical and electrical properties (density, refractive index, intrinsic stress, permittivity, and conductivity) vary systematically with film composition, 0 / ( 0 + N), determined from the above analyses. The present microwave plasma enhanced chemical vapour deposition (PECVD) films compare favorably with the best PECVD and low pressure chemical vapour deposition (LPCVD) materials reported in the literature. Nous avons prepare du nitrure (P-SiN), de l'oxynitrure (P-SiON) et de l'oxyde de silicium (P-SiO,) en couches minces partir de melanges gazeux SiH4-NH3-N,O dans un appareil a plasma microonde (LMPR, 2,45 GHz), sur des substrats chauffks a T, = 280°C. Les analyses chimiques des couches, dCterminCes par ESCA et ERDA, revblent environ 15% atomique d'hy- drogbne dans le P-SIN, <2% dans le P-SiO, et des valeurs intermediaires dans le P-SiON. Nous avons mesure plusieurs propriCtes physico-chimiques et 61ectriques, dont la densite, l'indice de refraction, les contraintes intrinsbques, la permittivite et la conductivite electrique. Toutes ces propriCtCs varient de fagon systematique avec la composition, 0 / ( 0 + N), determinee l'aide des analyses dkja mentionnees. Les prksentes couches pr6parCes par PECVD microonde se comparent favorablement avec les meilleurs materiaux PECVD et m&meLPCVD dCcrits dans la litterature. Can. I. Phys. 67, 190 (1989) 1. Introduction Thin dielectric layers of silicon compounds are of great importance in numerous Si and GaAs based microelectronics applications, and in macroelectronic applications such as amor- phous silicon-based flat panel displays, high temperature capac- itor dielectrics, etc. Silicon dioxide (SiO,), nitride (Si,N,), and oxynitrides can be prepared by a variety of techniques, includ- ing low pressure chemical vapour deposition (LPCVD) and plasma enhanced chemical vapour deposition (PECVD). In the latter case, these materials are often designated P-SiO,, P-SIN, and P-SiON, respectively, to account for their distinct character and properties. Whereas the literature on P-SIN and P-SiO, is extensive, until very recently P-SiON has received less atten- tion. Since the former two materials have limitations in certain application areas, P-SiON might be more useful than either if it can combine dielectric properties such as high relative per- mittivity and low losses, low permeability to moisture and ion transport, and low internal stresses. Promising results have been published in the past few years regarding silicon oxynitride films prepared by various LPCVD (1, 2) and PECVD techniques (3 -12). We recently reported (13) our work on the fabrication and characterization of P-SiN films prepared by large volume microwave plasma (LMPR) reaction of binary (SiH,, NH,) or ternary gas mixtures (the third gas being Ar or N,). That research has now been extended to include P-SiO, and P-SiON, the object of the present report. An important advantage of PECVD over LPCVD is the much lower deposition tempera- ture, T,, that can be used (typically 300°C versus over 800"C, 'Author to whom correspondence may be addressed. respectively). An additional advantage of microwave (mw) plasma over the radio (rf) or lower excitation frequencies used in conventional PECVD is the following: Wertheimer et al. (14) have shown that at mw frequencies, plasma deposition of thin films occurs more rapidly than under lower frequency excita- tion, under otherwise identical fabrication conditions. They attribute this to differences in the electron energy distribution function (EEDF), which in the mw case has a much higher population of very energetic electrons. In the case of P-SiN, for example, Tessier et al. (13) found deposition rates exceed- ing 100 A s- ', about an order of magnitude higher than most values reported in the literature. The purpose of the present work, then, is to report on high- rate deposition of P-SiON and P-SiO, films with the same microwave plasma apparatus used by Tessier et al. (13) and Wertheimer et al. (14), and to describe the structures and prop- erties of the resulting films. The optimized materials are then destined for use in a variety of device-oriented applications, for example, as high temperature capacitor dielectrics (15), encap- sulation for VLSI microelectronics, etc. After describing the deposition and characterization techniques in Sect. 2, the chem- ical composition of the samples is discussed in Sect. 3.1. Sec- tion 3.2 presents measured physicochemical and electrical properties, namely density, refractive index, mechanical stress, permittivity, and conductivity, all as functions of film compo- sition. Finally, we discuss these results in the light of compa- rable data from the literature. 2. Experimental The large volume microwave (LMPR, 2.45 GHz) plasma apparatus used previously to produce P-SiN, organosilicone, Pnnled In Canadailmprlm6 au Canada Can. J. Phys. Downloaded from www.nrcresearchpress.com by Santa Cruz (UCSC) on 11/25/14 For personal use only.

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Silicon oxynitride from microwave plasma: fabrication and characterization

S. BLAIN, J. E. KLEMBERG-SAPIEHA, AND M. R. WERTHEIMER' Groupe des couches minces (GCM) et Dkpartement de genie physique, ~ c o l e Polytechnique, C.P. 6079, Succursale XA",

Montre'al (Que'bec), Canada H3C 3A7

AND

S. C. GUJRATHI Laboratoire de physique nucle'aire, Dkpartement de physique, Universitk de Montrkal, C.P. 6128, Succursale "A'', Montreal

(Que'bec), Canada H3C 3J7

Received August 1 1, 1988

Plasma silicon nitride (P-SiN), oxynitride (P-SiON), and silicon dioxide (P-SiO,) films have been prepared from SiH4- NH3-N,O mixtures in a large volume microwave plasma (LMPR, 2.45 GHz) apparatus at T, = 280°C. Film compositions, determined by X-ray photoelectron spectroscopy and nuclear elastic recoil detection analysis, reveal about 15 at.% hydrogen in P-SiN, <2% in P-SiO,, and intermediate values in P-SiON. Various physicochemical and electrical properties (density, refractive index, intrinsic stress, permittivity, and conductivity) vary systematically with film composition, 0 / ( 0 + N), determined from the above analyses. The present microwave plasma enhanced chemical vapour deposition (PECVD) films compare favorably with the best PECVD and low pressure chemical vapour deposition (LPCVD) materials reported in the literature.

Nous avons prepare du nitrure (P-SiN), de l'oxynitrure (P-SiON) et de l'oxyde de silicium (P-SiO,) en couches minces partir de melanges gazeux SiH4-NH3-N,O dans un appareil a plasma microonde (LMPR, 2,45 GHz), sur des substrats chauffks a T, = 280°C. Les analyses chimiques des couches, dCterminCes par ESCA et ERDA, revblent environ 15% atomique d'hy- drogbne dans le P-SIN, <2% dans le P-SiO, et des valeurs intermediaires dans le P-SiON. Nous avons mesure plusieurs propriCtes physico-chimiques et 61ectriques, dont la densite, l'indice de refraction, les contraintes intrinsbques, la permittivite et la conductivite electrique. Toutes ces propriCtCs varient de fagon systematique avec la composition, 0 / ( 0 + N), determinee

l'aide des analyses dkja mentionnees. Les prksentes couches pr6parCes par PECVD microonde se comparent favorablement avec les meilleurs materiaux PECVD et m&me LPCVD dCcrits dans la litterature.

Can. I. Phys. 67, 190 (1989)

1. Introduction Thin dielectric layers of silicon compounds are of great

importance in numerous Si and GaAs based microelectronics applications, and in macroelectronic applications such as amor- phous silicon-based flat panel displays, high temperature capac- itor dielectrics, etc. Silicon dioxide (SiO,), nitride (Si,N,), and oxynitrides can be prepared by a variety of techniques, includ- ing low pressure chemical vapour deposition (LPCVD) and plasma enhanced chemical vapour deposition (PECVD). In the latter case, these materials are often designated P-SiO,, P-SIN, and P-SiON, respectively, to account for their distinct character and properties. Whereas the literature on P-SIN and P-SiO, is extensive, until very recently P-SiON has received less atten- tion. Since the former two materials have limitations in certain application areas, P-SiON might be more useful than either if it can combine dielectric properties such as high relative per- mittivity and low losses, low permeability to moisture and ion transport, and low internal stresses. Promising results have been published in the past few years regarding silicon oxynitride films prepared by various LPCVD (1, 2) and PECVD techniques (3 -12).

We recently reported (13) our work on the fabrication and characterization of P-SiN films prepared by large volume microwave plasma (LMPR) reaction of binary (SiH,, NH,) or ternary gas mixtures (the third gas being Ar or N,). That research has now been extended to include P-SiO, and P-SiON, the object of the present report. An important advantage of PECVD over LPCVD is the much lower deposition tempera- ture, T,, that can be used (typically 300°C versus over 800"C,

'Author to whom correspondence may be addressed.

respectively). An additional advantage of microwave (mw) plasma over the radio (rf) or lower excitation frequencies used in conventional PECVD is the following: Wertheimer et al. (14) have shown that at mw frequencies, plasma deposition of thin films occurs more rapidly than under lower frequency excita- tion, under otherwise identical fabrication conditions. They attribute this to differences in the electron energy distribution function (EEDF), which in the mw case has a much higher population of very energetic electrons. In the case of P-SiN, for example, Tessier et al. (13) found deposition rates exceed- ing 100 A s- ', about an order of magnitude higher than most values reported in the literature.

The purpose of the present work, then, is to report on high- rate deposition of P-SiON and P-SiO, films with the same microwave plasma apparatus used by Tessier et al. (13) and Wertheimer et al. (14), and to describe the structures and prop- erties of the resulting films. The optimized materials are then destined for use in a variety of device-oriented applications, for example, as high temperature capacitor dielectrics (15), encap- sulation for VLSI microelectronics, etc. After describing the deposition and characterization techniques in Sect. 2, the chem- ical composition of the samples is discussed in Sect. 3.1. Sec- tion 3.2 presents measured physicochemical and electrical properties, namely density, refractive index, mechanical stress, permittivity, and conductivity, all as functions of film compo- sition. Finally, we discuss these results in the light of compa- rable data from the literature.

2. Experimental The large volume microwave (LMPR, 2.45 GHz) plasma

apparatus used previously to produce P-SiN, organosilicone,

Pnnled In Canadailmprlm6 au Canada

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BLAIN ET AL. 191

and fluorocarbon films has been described elsewhere (14, 16). In the present depositions of P-SiON films, SiH, was the source of Si, while NH, and N,O were the source gases for N and 0 , respectively. Microwave power was kept constant at P = 100 W and substrate temperature was held at Ts = 280°C, while the total reactor pressure was fixed at p = 100 mTorr (13.3 Pa). The respective partial flow rates of SiH,, NH,, and N20 ranged from 15 to 2 5 , 0 to 50, and 0 to 50 sccm; total flow rate was maintained at 60-80 sccm (standard cubic centimetres per minute).

Chemical compositions of the film samples were determined using X-ray photoelectron spectroscopy (XPS or ESCA) and nuclear elastic recoil detection analysis (ERDA), the latter using the tandem van de Graaf accelerator at the UniversitC de Mont- rCal (12, 17). This method is particularly important, as it yields total hydrogen content and permits nondestructive depth pro- filing of all the component elements. Hydrogen is believed to be of great importance, as it contributes to degradation of chem- ical stability and thereby of dielectric and other physical prop- erties (18). The ESCA measurements were performed using a VG ESCALAB MKII instrument, equipped with MgKa and AlKa X-ray sources. The atomic composition of samples was calculated from N (Is), 0 (Is), and Si (2p) line intensities, using previously determined sensitivity factors. To avoid errors from surface contamination, resulting from exposure to ambient air, the samples were argon ion etched before analysis. This was done with relatively low energy (0.5 - 1.0 keV) Arf ions to minimize surface damage, preferential sputtering, and bonding changes. This experimental approach was verified using ther- mally grown SiO, and LPCVD Si,N, (n = 2.01), and SiOJ, films with accurately known elemental compositions from other sources. ERDA and ESCA (the latter combined with Ar ion erosion for depth profiling) revealed the compositions of these sample materials to be virtually the same at the cleaned surface as in the bulk; furthermore, we found results from the two tech- niques to be in very good agreement.

Most of the films were deposited on polished single crystal Si wafers, and film thickness (and hence also deposition rate) was measured by both profilometry (Sloan Dektak) and ellip- sometry (Gaertner Scientific, model L 117)' for maximum accuracy. The film density was calculated from thickness and gravimetric measurements, the latter obtained with an elec- tronic microbalance (Sartorius, model 2600). The refractive index n was also obtained from ellipsometry at A = 6328 A.

Mechanical stress in the films was measured from the cur- vature induced by the deposit in a very thin (0.005 cm) rectan- gular (1 X 0.1 cm) beam of polished, vitreous silica, according to the method of P i n ~ h . ~ The radius of curvature was measured using two independent optical methods, namely:

(i) interferometry in monochromatic light (Newton's ring method) based on the air wedge formed between the curved beam and an optically flat substrate;

(ii) height difference measurements (z, to k 0 . 5 pm) using micrometer stages (x,y, and z) coupled with an optical micro- scope having high magnification (1000 X ) and small depth of field. From the concave (or convex) radius of curvature R , the tensile (or compressive) stress S can then be calculated from ref. 19:

'We are grateful to Professor A. Filion, Collkge Militaire Royal de St-Jean, for the use of this instrument.

3H. Pinch. David Sarnoff Research Center, Princeton, NJ. Private communication.

N,O FLOW (sccrn) FIG. 1. Sample composition of P-SiON as a function of nitrous oxide

flow rate, FN2, . FSiH4 = 15 sccm; (FNH3 + FNZO) = 45 sccm; for other fabrication conditions, see text. 0, silicon; + , nitrogen; 0 , oxy- gen; A , hydrogen.

where Ys and us are Young's modulus and Poisson's ratio (7 X 10" dyn cmP2 or 7 X 10" Pa, and 0.17 for vitreous SiO,, respectively), and 1, and 1, respectively are the substrate and film thicknesses. It turns out (20) that a contribution to S result- ing from different thermal expansion coefficients between film and substrate is negligible in the present circumstances. Film thicknesses studied in this work ranged between 0.3 and 1 pm.

Finally, for electrical characterizations, the dielectric films were deposited on Al-coated glass slides, following which a thin (- 1000 A) top electrode was evaporated. Complex relative permittivity K* was measured at room temperature over the fre- quency range lo2 - lo5 Hz using a computer-interfaced LCR meter (Hewlett Packard, model HP 4274 A). Direct current I- V measurements were performed, also at ambient temperature, using a computer-controlled voltage source (Keithley, model 230) and electrometer (Keithley, model 619). For further details regarding the electrical measurements, the object of separate investigations, see ref. 15.

3. Results

3.1 Deposition rates, film uniformity, and composition Wertheimer and co-workers (13, 14) have discussed

microwave plasma deposition rates of P-SiN, and of other thin film materials. Typically, they report order-of-magnitude higher deposition rates at 2.45 GHz than at 13.56 MHz radio frequency, when the experiments are carried out in the same plasma reactor system (for example, the one used in the present work) under otherwise identical conditions (14). Similar observations have also been made in the present case for P-SiO, and P-SiON $eposition, namely that deposition rates can easily exceed 100 A s- ' (0.6 p m min- ') for sufficiently high reagent gas flow rates. Under such conditions, however, good film quality and reproducibility require quite precise tuning of other fabrication parameters. This is much less critical when fabrication conditions are chosen as listed in Sect. 2, which qevertheless permit deposition rates in the range up to 20 or 30 A s-I. This is still substantially higher than typical literature data reported for the same film materials by LPCVD (1) ( S 1

s - ') and PECVD at audio or radio frequency (3,5,7,9) (1 -

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192 CAN. 1. PHYS. VOL. 67. 1989

N,O FLOW (sccm)

FIG. 2. Composition of P-Si02 samples, as a function of nitrous oxide flow rate, FNZO . (FSIH4 + FNZO) = 80 sccm; for other fabri- cation conditions, see text. 0, a, silicon; 0 , + , oxygen; A, hydrogen; + , nitrogen. The open and filled symbols pertain to ERDA and ESCA, respectively.

16 A s-'). Regarding film uniformity, the present LMPR reactor, having a 15 cm diameter heatable rotating substrate holder, routinely permits us to obtain 2 2 % film thickness uniformity on a 4 in. (10 cm) diameter Si wafer by mw PECVD.

Figure 1 presents the silicon, oxygen, nitrogen, and hydrogen concentrations, determined by combined ESCA and ERDA analyses, of a typical series of samples prepared by varying the N,O flow rate (F, ,, abscissa) in the feed gas mixture. We note that all compoiitions, ranging from nearly stoichiometric P-SIN (Si N,.,:H) when FN2, = 0 to nearly stoichiometric P- SiO, (Si0,:H) at the other extreme, can be obtained simply by varying the N,O/NH, flow proportion. The intermediate "metastable" oxynitrides are, in fact, very stable materials. From Fig. 1 it appears that the N,O partial flow rate is the most important variable in controlling sample composition. We note that the total 0 and N concentrations, respectively, increase and decrease almost linearly with increasing N20 (and decreasing NH,) flow rate, giving rise to the observed smooth transition from P-SiN to P-SiO,. The H concentration is also seen to decrease almost linearly from its maximum value (-15 at.%) in P-SIN, while the total Si content remains almost constant over the entire FNZO range. These results are in general agreement with those reported by others (see, for example, refs. 5, 7 ,9 , and lo), with the exception that the present films have substantially lower H contents than do P-SiON films prepared in audio- or radio-frequency plasmas (5, 7, 9, 10, 12). This latter issue deserves some additional comment; the Grenoble workers, who use a 50 kHz power source for their PECVD preparations (8, 12), regularly perform ERDA analyses of their samples at the Universite de Montrkal. This permits direct comparison of features in their and our own sample materials. In Tessier et al. (13), we have already pointed out the substantially higher H content (by a factor of about 2) of their 50 kHz P-SiN compared with our 2.45 GHz materials. Comparing the present data and those in ref. 12, it would appear that a similar factor also applies to P-SiO, and P-SiON. We speculate that this is due to differing fragmentations of the feed gas molecules in mw and rf discharges, on account of the already mentioned differences in their respective EEDFs (14).

FIG. 3. Composite plot of data from the present work and of certain results from the literature, having as common abscissa the film com- position parameter 0 / ( 0 + N), see text. (a) Relative permittivity K'

measured at 1 kHz; the dashed curve represents data of Remmerie et al. (2); (6) refractive index n; a, present data; e, data of Kuiper et al. (1); (c) dc electrical conductivity, log,, u; (4 intrinsic mechanical stress S; positive and negative values refer to tensile and compressive stress, respectively. (1 dyne = 10 pN)

Figure 2 presents additional detailed results for P-SiO, produced from mw plasma reaction of SiH, and N,O alone. Except at low N,O flow rates (FNZO 6 5 5 sccm), we note that

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BLAIN ET AL. 193

near-stoichiometric P-SiO, is readily produced, with very little hydrogen and negligible nitrogen concentrations. This figure also reveals the excellent agreement obtained between independent ESCA and ERDA measurements on the same samples, a fact that also applies to the P-SiON samples of Fig. 1.

3.2 Sample properties In view of the nearly constant Si content, and the relatively

minor variations in H concentration of P-SiON sample materials discussed in Sect. 3.1 (see Fig. l), it is logical to discuss any systematic variations of their physicochemical properties in terms of their relative 0 and N concentrations. As numerous other workers (1,2,5,6) have done, we choose for this purpose the parameter 0 / ( 0 + N), deduced from analyses such as those presented in Sect. 3.1. Figure 3 is a composite plot with 0 / ( 0 + N) as the common abscissa, in which parts (a) - (d) respec- tively illustrate variations of the following properties with sam- ple composition: Fig. 3a shows the relative permittivity K ' ,

measured at 1 kHz (15); the dashed curve, shown for compar- ison, represents similar measurements on LPCVD films of Remmerie et al. (2). Values of K ' are seen to decrease roughly linearly with 0 / ( 0 + N) from 7, the accepted literature value for Si3N4, to about 4, that for SiO,. Figure 3b shows the refrac- tive indices of our films (square symbols) compared with those of Kuiper et al. (I) for LPCVD films (filled circles). The dashed curve represents a calculation of the refractive index according to the Bmggeman formula (21)

where subscripts 0 and N refer to SiO, and Si3N4, respectively, V are their volume fractions in the oxynitride, and <n> is the effective refractive index of the medium. According to the Bmggeman model, SiON films are taken to be mixtures of pure Si,N4 and SiO, components having n values of 2.00 and 1.46, respectively. Whereas Kuiper's data are in good agreement with the model, our measurements deviate at low 0 / ( 0 + N) values; this will be discussed further in Sect. 4.

Figure 3c presents the variation of dc conductivity a , meas- ured at 300 K and E = 0.5 MV cm-', as a function of 0 / ( 0 + N). Samples are found to be ohmic up to about 0.5 MV cm- '; beyond this field value the I-V characteristics are com- plex and cannot readily be attributed to any simple conduction mechanism(s), as pointed out by Rabiller et al. (15). From Fig. 3c we note that a decreases from P-SIN ( a = 2 x 10-I4 S cm- ') to P-SiO, ( a = 10-16 S cm- ') by more than two orders of magnitude. This behaviour resembles that reported by Rem- merie et al. (2), but the present a values are some orders of magnitude smaller.

Finally, Fig. 3d shows the results of internal stress measure- ments, S, versus 0/(0 + N), positive and negative S values representing tensile and compressive stresses, respectively. Par- ticularly noteworthy is the fact that tensile stresses in nitrogen- rich P-SiON films and P-SIN become compressive for 0 / ( 0 + N) 0.4. Tensile stresses are undesirable, as they tend to cause cracking in the film upon relaxation (22).

Beside the data in Fig. 3, we have performed other types of measurements on these films, for example, infrared spectros- copy and density determinations. The density p drops smoothly from its value for P-SiN (p = 2.9 g cmP3) to that for P-SiO, (p = 2.2 g cmP3). In the following section we discuss further the results presented in Figs. 1-3.

4. Discussion Several authors (5,7,8) have proposed that P-SiON consists

of two separate phases, Si0,:H and SiN,:H, rather than being a single homogenous alloy with random bonding. This view is strongly supported by infrared absorption spectra of the mate- rials, which show that all the oxygen is incorporated in the films as S i U S i groups (5,8). The permittivity and refractive index measurements of Fig. 3 also lend credence to a two-phase model. For the case of LPCVD films, which tend to have higher density and lower hydrogen content than their PECVD coun- terparts, the measurements of Kuiper et al. (1) agree very well with the Bmggeman mixture,.formula over the entire range of 0 / ( 0 + N). Our own PECVD data (Fig. 3b) fall below the Bmggeman curve for P-SiN and N-rich P-SiON (0 S ( 0 / ( 0 + N) < 0.5), but are in good agreement with it for 0.5 (0 / (0 + N) S 1. This, we feel, can be well explained on the basis of density differences between LPCVD and PECVD films, as follows. Let us assume validity of the Lorentz-Lorenz rela- tionship (23)

where Il is the molar refraction, M is the molecular weight, and other symbols have already been defined. Taking p = 2.9 g cm-3 for P-SiN (13), p = 3.4 g cm-3 and n = 2.00 for stoichiometric, crystalline Si3N4 (and also for LPCVD material (l)), [3] yields n = 1.8 for P-SiN, somewhat below our meas- ured value (see Fig. 3b, for 0/(0 + N) = 0). Since our best P-SIN is very nearly stoichiometric (Si/N = 0.75 (13)), the assumption in the above calculation that ( I I IM) = constant is presumably valid; the several at. % of chemically bound hydro- gen in the P-SiN may account for the difference between cal- culated and measured n values. Now, regarding the other com- positional extreme, 0 / ( 0 + N) = 1, we pointed out in Sect. 3 that the present P-SiO, is practically stoichiometric, very low in hydrogen and nitrogen (see Fig. 2), and that its measured density is close to that for crystalline SiO,, p = 2.2 g cmP3. Based on the above arguments, then, the agreement between our measured n values and the Bmggeman curve for 0.5 G 0 / ( 0 + N) S 1 is not surprising. In contrast, Schoenholtz and Hess (10) found no satisfactory correlation between their n val- ues and the Bmggeman approximation, a fact they attribute to a high content of bonded hydrogen in their 13.56 MHz PECVD films. Therefore, the above-noted satisfactory agreement for our mw PECVD films further supports the view, expressed in Sect. 3.1, that their hydrogen content is significantly lower than that of most comparable rf materials.

Comparing Figs. 3a and 3b, we notice that Maxwell's rela- tion (23)

[4] K ' = K', = n2

where K ' and K ' ~ are the static and optical permittivities, respec- tively, is invalid for P-SiON over the.entire composition range. The reason for this is the strong infrared dispersion displayed by all these materials (5,8,10); the possible existence of two separate phases, discussed above, could provide additional dis- persion in the form of an interfacial (or Maxwell-Wagner) polarizability (23).

Finally, we discuss the mechanical stresses, illustrated in Fig. 3d. Several authors (9,20) have recently published very detailed accounts of stress measurements in P-SiN, P-SiO,, and P-SiON films produced under various fabrication conditions. Kanicki et al. (20) have, in addition, examined films produced in

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194 CAN. J. PHYS. VOL. 67, 1989

commercial LPCVD, photo-CVD, and microwave ECR reac- tors. They found all their LPCVD and PECVD nitride films to exhibit tensile stress, with a maximum value (S = 10" dyn cm-,) occuning for stoichiometric material. This tensile stress, according to Claassen (9), derives from hydrogen desorption and cross-linking reactions (formation of Si-N bonds) that continue for some time at elevated T , when plasma deposition has ceased. Compressive stress, on the other hand, derives from film expansion due to ion bombardment, particularly when T , is not high enough to anneal out implantation damage. Now, the intensity and mean energy of ion bombardment are strongly affected by most of the common fabrication variables used in capacitively coupled PECVD reactors (power, pressure, elec- trode distance, excitation frequency, feed gas composition), and to this Claassen attributes the observed strong dependence of S upon the deposition parameters (9). Although we, like the other authors, find tensile stress in P-SiN and N-rich P-SiON (see Fig. 3 4 , its value S = 1.2 x lo9 dyn cmP2 is substantially lower than typical values (4 < S < 12 x lo9 dyn cm-,) reported for P-SiN prepared in high radio-frequency discharges (9, 10, 20). The reason for this is not clear to us at present. Like Claassen (9) and Schoenholtz and Hess (lo), we find that addition of N,O strongly decreases the intrinsic stress. The lat- ter authors even report the same phenomenon of "zero stress" (i.e., the cross-over point between tensile and compressive stress regimes) illustrated in Fig. 3d; in our case, this occurs for about 20% N,O in the feed gas mixture, which differs from their value of about 60%. This, once again, may be a symptom of different EEDFs in 2.45 GHz and 13.56 MHz plasmas. The zero stress P-SiON having 0 / ( 0 + N) = 0.4 may prove to be a very attractive material for a wide variety of electronic and other applications.

5. Conclusions A large vodume microwave plasma (LMPR) apparatus allows

rapid (-30 A s-') deposition of high quality P-SiN, P-SiON, and P-SiO, films at T , = 280°C, with * 2% uniformity on 10 cm diameter Si wafers. Compositions over the entire range can readily be achieved by varying the NH,/N,O flow rate ratio, while keeping the total feed gas flow rate constant. By com- bining ESCA and ERDA measurements, we have been able to accurately determine absolute film compositions, including total hydrogen content, which appears to be substantially lower in the mw PECVD materials than in their rf counterparts. Var- ious physicochemical and electrical properties (density, refrac- tive index, intrinsic stress, permittivity, and dc conductivity) vary systematically with film composition 0 / ( 0 + N), as determined from the above analyses. The refractive index, n, varies according to the Bruggeman mixture formula, lending support to a two-phase model for P-SiON films proposed by several authors. Since n is easily measured by ellipsometry, this provides a convenient means for determining film composition. Finally, intrinsic stresses in our mw PECVD films have been found to be relatively low, and a zero-stress regime was found for film compositions near 0 / ( 0 + N) = 0.4.

In summary, on the basis of the above characterization results, the present P-SiON films compare favorably with the best rf PECVD materials and even with LPCVD materials

reported in the literature. The fact that deposition is more rapid in microwave plasma constitutes an important economic advantage.

Acknowledgements The authors are grateful to Mr P. Rabiller for valuable dis-

cussions and for electrical measurements. This work has been funded in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) and by the Fonds pour la formation de chercheurs et aide 2 la recherche (FCAR) of QuCbec.

1. A. E. T. KUIPER, S. W. K m , F. H. P. M. HABRAKEN, AND Y. TAMMINGA. J. Vac. Sci. Technol. B: 1, 62 (1983).

2. J. REMMERIE, H. E. MAES, R. DE KEERSMAEKER, F. H. P. M. HABRAKEN, J. OUDE-ELFERINK, AND W. VAN DER WEG. In Silicon nitride and silicon dioxide thin insulating films. The Electrochem- ical Society, Pennington, NJ. 87-18, 50 (1987).

3. V. S. NGUYEN, S. BURTON, AND P. PAN. J. Electrochem. SOC. 131, 2348 (1984).

4. V. S. NGUYEN, W. A. LANFORD, AND A. L. RIEGER. J. Electro- chem. Soc. 133, 970 (1986).

5. W. A. P. CLAASSEN, H. A. J. TH.v.D.PoL, A. H. GOEMANS, AND

A. E. T. KUIPER. J. Electrochem. Soc. 133, 1458 (1986). 6. A. HASHIMOTO, M. KOBAYASHI, T. KAMUOH, H. TAKANO, AND

M. SAKUTA. J. Electrochem. Soc. 133, 1464 (1986). 7. W. G. M. VAN DEN HOEK. Mater. Res. Soc. Symp. Proc. 68,335

(1986). 8. Y. CROS, D. JOUSSE, J. LIU, AND R. C. ROSTAING. J. Nan-Cryst.

Solids, 90, 287 (1987). 9. W. A. P. CLAASSEN. Plasma Chem. Plasma Process. 7, 109

(1987). 10. J. E. SCHOENHOLTZ AND D. W. HESS. Thin Solid Films, 148,285

(1987). 11. J. VUILLOD. J. Vac. Sci. Technol. A: 5, 1675 (1987). 12. J. C. ROSTALNG, Y. CROS, S. C. GUJRATHI, AND S. POULAIN.

Proc. 12th Int. Conf. on amorphous and liquid Semiconductors, Prague (Aug. 1987). J. Non-Cryst. Solids, 97-98, 1051 (1987).

13. Y. TESSIER, J. E. KLEMBERG-SAPIEHA, S. POULIN-DANDURAND, M. R. WERTHEIMER, AND S. C. GUJRATHI. Can. J. Phys. 65,859 (1987).

14. M. R. WERTHEIMER, M. MOISAN, J. E. KLEMBERG-SAPIEHA, AND

R. CLAUDE. Pure Appl. Chem. 60, 815 (1988). 15. P. RABILLER, S. BLAIN, J. E. KLEMBERG-SAPIEHA, M. R. WERTH-

EIMER, AND A. YELON. Conf. Rec. IEEE Int. Symp. Electr. Insul. Boston, MA (June 1988). IEEE Doc. 88-CH2594-0-DEI. p. 149.

16. M. R. WERTHEIMER, J. E. KLEMBERG-SAPIEHA, AND H. P. SCHREIBER. Thin Solid Films, 115, 109 (1984).

17. R. GROLEAU, S. C. GUJRATHI, AND J. P. MARTIN. Nucl. Instrum. Methods Phys. Res. 218, 1 1 (1983).

18. D. L. FLAMM, D. E. IBBOTSON, C. P. CHANG, AND J. A. MUCHA. Proc. IUPAC Int. Symp. on plasma chemistry, ISPC 8, Tokyo, Japan (Aug. 1987). p. 1124.

19. G. G. STONEY. Proc. R. Soc. London A: 82, 172 (1909). 20. J. KANICKI, P. WAGNER, J. KARASINSKI, AND J. ANGILELLO. C0nf.

Rec. IEEE Int. Symp. Electr. Insul., Boston, MA (June 1988). IEEE DOC. 88-CH2594-0-DEI. p. 153.

21. D. A. G. BRUGGEMAN. Ann. Phys. Leipzig, 24, 636 (1935). 22. A. K. SINHA, H. J. LEVINSTEIN, T. E. SMITH, G. QUINTANA, AND

S. E. HASZKO. J. Electrochem. Soc. 125, 601 (1978). 23. A. R. VON HIPPEL. Dielectrics and waves. MIT Press, Cambridge,

MA. 1966. p. 228.

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