J. Phys. Chem. 1992,96,9855-9861
information no longer exists. Hence, there is no longer an uncertainty in the values of the C-H bond energies associated with these former disparities.
Acknowledgment. Support for this research was provided by the National Science Foundation, Chemistry Division, under Grant CHE9102038. We thank Jh.Irene R. Slagle for her advice, help, and assistance. J.T.N. thanks the Neste Oy Foundation for a fellowship. This collaboration was made passible in part through NATO Research Grant 0518/87, and this contribution is also gratefully acknowledged. RWby NO. CH,, 2229-07-4; C2H5,2025-56-1; i-C3H7,2025-55-0; t-C4H9, 1605-73-8; HBr, 10035-10-6; Br, 10097-32-2; C2H6, 74-84-0; C3H8, 74-98-6; m-C4Hlo, 106-97-8; i-C4Hlo, 75-28-5; H2, 1333-74-0; s-CIH~,2348-55-2.
9855
(18) (a) Halocarbon Wax is a product of the Halocarbon Wax Corp., Hackensack, NJ. (b) Teflon, 852-200. (19) Talukdar, R. K.; Vaghjiani, G.L.; Ravishankara, A. R. J. Chem. Phys. 1992, 96, 8194. (20) Donovan, R. J.; Husain, D. Chem. Rev. 1970, 70,489. (21) H e m n , J. T.; Huie, R. E. J. Phys. Chem. 1969, 73, 3327. , (22) Walker, R. W. Reaction Kinetics Vol. 1; Specialiet Paiodicd Reports; The Chemical Society: London, 1975; p 161. (23) Washida, N.; Bayes, K. D. J. Phys. Chem. 1980,81, 1309. (24) Michael, J. V.; Keil, D. G.; Klemm, R.B. Int. J . Chem. Kinet. 1983, 15, 705. (25) Atkinson, R. Int. 1. Chem. K i m . 1987, 19, 799. (26) Fettis, G. C.; Knox, J. H.; Trotman-Dickenson, A. F. J. Chem. Soc. 1960,4177. (27) Brouard, M.; Lightfoot, P. D.; Pilling, M. J. J. Phys. Chem. 1986, 90, 445. (28) Chen, Y.; Rauk, A.; Tschuikow-Roux, E. J . Phys. Chem. 1990,94, 2775. (29) Chen, Y.; Rauk, A.; Tschuikow-Row, E. J. Phys. Chem. 1990,94, 6250; Erratum J. Phys. Chem. 1992, 96, 6854. (30) Recalculated Arrhenius parameters with error limits for Br R H reactions b a d on experiments in ref 26 which were used in the current thermochemical calculations were reported by: Fettis, G. C.; Knox, J. H.In Progress in Reaction Kinetics; Porter, G., Ed.; Pergamon: New York, 1964; Chapter 1. (31) Donaldson, D. J.; Leone, S.R. J. Phys. Chem. 1986,90,936. (32) Richards, P. D.; Ryther, R. J.; Weitz, E. J . Phys. Chem. 1990, 91, 3663. (33) Kistiakowski, G. B.; Van Artsdalen, E. R. J. Chem. Phys. 1944,12, 469. (34) Coomber, J. W.; Whittle, E. Trans. Faraday Soc. 1966,62, 2643. (35) King, K. D.; Golden, D. M.; Benson, S.W. Trans. Faraday Soc. 1970, 66, 2794. (36) Islam, T. S. A.; Benson, S.W. Int. J. Chem. Kinet. 1984,16,995. (37) S.W.; Kondo, 0.;Marahall, R. M . Int. J. Chem. Kinet. 1H7, 19, 829. (38) Hanning-Lee, M. A.; Green, N. J. B.; Pilling, M. J.; Robertson, S . H. J. Phys. Chem. Submitted for publication. (39) Pacey, P. D.; Wimalesena, J. H. J. Phys. Chem. 1984, 88, 5657. (40)Pannar, S.S.;Benson, S. W. J. Am. Chem. Soc. 1989, 111, 57. (41) Atkinson, R.; Baulch, D. L.; Cox, R. A,; Hampaon, R. F.; Ken, J. A.; Troe. J. J . Phys. Chem. Ref. Data 1989, 18, 881. (42) Pittam, D. A,; Pilcher, G. J. Chem. Soc., Faraday Trans. I 1972,68, 2224. (43) Chase, M. W.; Davies, C. A.; Downey, J. R.; F ~ r i pD. , J.; McDonald, R. A.; Syraned, A. N. J. Phys. Chem. Ref. Data 1985.14, Suppl. 1 (JANAF Thermochemical Tables, 3rd 4.). (44) Burcat, A. In Combustion Chemistry; Gardiner, W. C., Ed.; Springer-Verlag: New York, 1984; Chapter 8.
+
References and Notes (1) Golden, D. M.; Benson, S.W. Chem. Rev. 1969, 69, 125. (2) O”ea1, H. E.; Benson, S.W. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 2, Chapter 17. (3) McMillen, D. F.; Golden, D. M. Annu. Reu. Phys. Chem. 1982, 33, 493. (4) Tsang, W. Int. J . Chem. Kinet. 1978, 10, 821. ( 5 ) Tsang, W. J. Am. Chem. Soc. 1985, 107, 2872. (6) Russell, J. J.; Seetula, J. A.; Timonen, R. S.;Gutman, D.; Nava, D. F. J. Am. Chem. Soc. 1988.110, 3084. (7) Russell, J. J.; Seetula, J. A.; Gutman, D. J. Am. Chem.Soc. 1988,110. 3092. ( 8 ) Seetula, J. A.; Gutman, D. J. Phys. Chem. 1990, 94,7529. (9) Gutman, D. Acc. Chem. Res. 1990, 23, 375. (10) Seetula, J. A.; Russell, J. J.; Gutman, D. J. Am. Chem. Soc. 1990, 112, 1347. (1 1) Seetula, J. A.; Feng, Y.; Gutman, D.; Seakins, P. W.; Pilling, M. J. J. Phys. Chem. 1991. 95, 1658. (12) Niiranen, J. T.; Gutman, D.; Krasnoperov, L. N. J . Phys. Chem. 1992, 96, 5881. (13) Sectula, J. A.; Gutman, D. J . Phys. Chem. 1992, 96, 5401. (14) Rate constants for the sec-C,Hg + HBr reaction were obtained recently and have been published.’ The kinetics of this reaction was not restudied in the current investigation. (15) Seakins, P. W.; Pilling, M. J. J . Phys. Chem. 1991, 95, 9874. (16) Nicovich, J. M.; van Dijk, C. A.; Kruetter, K. D.; Wine, P. H. J . Phys. Chem. 1991. 95,9890. (17) Slagle, I. R.; Gutman, D. J . Am. Chem. SOC.1985, 207, 5342.
Laser Studies of the Reactivity of NH(X%)
with the Surface of Sllicon Nitride
Ellen R. Fisher, Pauline Ho, William G. Breiland, and Richard J. Buss* Sandia National Laboratories, Albuquerque, New Mexico 87185-5800 (Received: June 12, 1992)
The reactivity of NH(X3Z-) with the surface of both a silicon nitride film and a depositing hydrogenated silicon nitride film has been measured to be essentially zero with an upper limit of 0.1 for substrate temperatures of 3OU-700 K. The reactivity was directly determined using spatially resolved laser-induced fluorescence of NH in a plasma-generated molecular beam incident on the surface. The NH adsorbs and then desorbs from the surface with a spatial distribution consistent with a cosine angular distribution. No dependence of reactivity on rotational state of the NH was observed.
I. htroduction Understanding the interactions of molecules with surfaces is important for the development of many thin-film materials processing technologies, including chemical vapor deposition (CVD), plasma-enhanced CVD, and plasma etching. These processes are often controlled by chemical reactions that occur when gas-phase molecules collide with a substrate. Unfortunately, very little information is available for gas-surface reactions within these plasma environments, particularly those involving radicals.l Thus, 0022-3654/92/2096-9855~03.00/0
direct measurements of radical reactivities are needed to build a database and develop an understanding of such reactions. The NH radical is present in NH3 and NH3/SiH, plasmas’ used for plasma deposition and etching in the semiconductor industry. There is considerable interest in the silicon nitride products deposited from these plasmas at low substrate temperatures since silicon nitride films are used as ate dielectrics and barrier coatings in microelectronicdevices‘sfand for protective Specifically, plasma-depcwited amorphous SiN,:H Q 1992 American Chemical Society
9856 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 DYE LASER BEAM
, \ VACUUM CHAMBER \\
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\
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Figure 1. Experimental schematic. (a) Top view of apparatus. (b) Detail of interaction region and collection optics. Specular scattering of molecular beam is illustrated.
films are promising materials in photovoltaic conversion or as high band gap materials in a-Si:H/a-SiN,:H superlattices. Further, Selwyn and Lin have shown that N H is one of the central intermediates in the decomposition of NH3 on Pt and Fe surfaces8 Despite this considerable interest, very little work has been done to characterize the fundamental interactions between NH, species and silicon or silicon nitride surfaces. In addition, the NH radical has been observed in comets9 and is an important intermediate in a number of combustion processes,lovllparticularly in the production of NO, species. The present work applies the IRIS (imaging of radicals interacting with surfaces) technique12to the NH radical produced both from a pure NH3 plasma incident on a thermally grown silicon nitride film and from an NH3/SiH4 plasma which deposits a silicon nitride film on the surface. This technique allows us to measure the steady-state reactivity of a radical species during deposition of a thin film. The IRIS technique is relatively new and has been used previously to study the reactivity of SiH radicals with the surface of an a-Si:H film12and S i 0 molecules with the surface of a silicon oxide film.13
JI. Experimental Section A. General. The method used for studying the reactivity of a radical with the surface of a depositing film has been discussed in detail previ~usly.~Z~~ Figure 1 is a schematic of the experimental apparatus, showing the experimental geometry of the IRIS apparatus. A brief description is given here, concentrating on those aspects specific to the present study. The method uses spatially resolved laser-induced fluorescence (LIF) spectroscopyto detect both molecules in a collimated, near-effusive molecular beam and those coming from a surface. The selectivity of the LIF technique is used to monitor one species in a molecular beam made from a plasma which is the source of the radicals. The beam thus contains virtually all the species present in the plasma and continuously exposes the surface to the plasma environment. Comparison of the LIF signals observed with the surface in and out
Fisher et al. of the path of the molecular beam yields the fraction of the incident molecules (of the species being detected) that react at the surface. A new feature of the experimental apparatus is a 3-mm-thick copper block mounted on the main chamber side of the differential wall in the path of the molecular beam. The copper block was continuously cooled with liquid nitrogen, forming a "cold shield" for the molecular beam. The cold shield was installed because the spatial distribution of N H in the molecular beam appeared to contain contributionsfrom N H molecules effusing around the standard slit configurations. Even with the cold shield in place, but not cooled, the N H signal appeared to contain additional components. The beam profile had the expected spatial distribution only when the cold shield was completely cooled to liquid nitrogen temperatures, which was monitored during the experiment via a chromel/alumel thermocouple attached to the copper block. For most of the reactivity studies, a pure NH3 plasma was used as the source of the radical-containing molecular beam. The plasma chamber is a cylindrical Pyrex tube 50 mm in diameter and -20 cm long. Glass wool was inserted into the rear of the plasma tube to confine the plasma in the front half of the plasma tube. An NH3 flow rate of 10 sccm and 40 W of inductively coupled rf power were used. The pressure in the plasma tube was -30 mTorr when the plasma was on. In addition to the pure ammonia plasma, the reactivity of N H produced in an NH3/SiH4 plasma was also investigated. For these studies, an NH3 flow rate of 10 sccm was maintained, while the SiH4 flow rate was varied from 5 to 10 sccm. For most of the experiments, a relatively narrow molecular beam, defined by three slits, was used. The first slit was 0.9 mm wide and was mounted on the differential wall, 2.54 cm from the 1.O-cmdiameter hole in the front of the plasma tube. The second and third slits were each 1.0 mm wide and were located on either side of the liquid nitrogen cold shield, 2.81 and 3.1 1 cm in front of the plasma tube, respectively. The height of the slits was adjusted to give a beam with a rectangular cross section 1.0 cm high at the substrate to enhance detection of the scattered molecules. The laser beam intersected the center of the molecular beam 4.2 cm in front of the plasma tube. Obtaining the fluorescence excitation spectrum did not require spatial resolution of the fluorescence, so a wider molecular beam was used to improve signal levels. The two slits on the cold shield were replaced with a single 2.1-mm-wide slit mounted directly on the differential wall in the main chamber, which gave a beam that was 1.5 cm wide at the laser. The substrate was placed parallel to the laser beam at a 43.3' angle relative to the molecular beam (Figure 1). The samples used in these experiments were 1- X 2-cm pieces of silicon with 158 A of thermally grown nitride. The sample was held by pieces of tantalum foil and was mounted such that it could be rotated precisely in and out of the molecular beam. The laser-surface distance was varied by translating the substrate along the axis of the molecular beam. The substrate temperature could be varied by resistive heating, using a current regulated power supply and a chromel/alumel thermocouple attached to the back of the sample with a high-temperature ceramic glue. On removing the substrate after an experiment, the thin film of deposited a-SiN:H (from the NH3/SiH4plasma) was observed as a rectangular spot with the dimensions of the molecular beam. Tunable laser light around 335 nm (-0.7 mJ/pulse) was produced using an excimer-pumped (XeCl, 100-150 mJ, 15 ns, 25 Hz) dye laser system with p-terphenyl. Light in the 3203 3 0 . ~ 1range was produced using sulforhadamine640 in the dye laser and frequency doubling the output. Initially; the LIF signal was spectrally dispersed through a 0.75-m monochromator (1200 grooves/mm grating) and detected with a doubly intensified diode array. For the fluorescence excitation spectrum and the reactivity experiments, the light was imaged directly onto the diode array (Figure 1). The fluorescence was collected with a single 200mm-focal length quartz (50-mm diameter) lens. The optics d e magnified the image so that the 25-mm-long diode array viewed a region 34.3 mm long, making it possible to spatially resolve both the incident and scattered beams simultaneously. For the
The Journal of Physical Chemistry, Vol. 96, No. 24, I992 9857
Reactivity of NH(X3Z-) with Silicon Nitride
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fluorescence excitation spectrum, the laser wavelength was scanned at 1.6 X lo-' nm s-l. The sync-out pretrigger from the excimer laser was c ~ ~ e ~tot ae delayed-pulse d generator which was then used to gate the detector. The detector was gated on after the pulse to avoid interferencefrom scattered laser light. Because the radiative lifetime of NH is 440 f 15 ns,14a detector gate width of 1 ps was used to collect the fluorescence. Signals for reactivity experiments were generally accumulated on the array for 25 laser pulses between readings and integrated for 100 samples before being stored in the computer. Signals were alternately measured with the surface in and out of the molecular beam path to account for possible long-term changes in the plasma or laser tuning and alignment. Minor background signals resulting from continuous emission from the plasma and pixel noise in the diode array were also measured and subtracted from the NH signals. The data reported are the average of several sets of data taken with and without the surface in place. B. Numerical Simulation. A numerical simulation which accounts for the experimental geometry is needed to relate the signal intensities observed to a molecular reactivity. The simulation procedure, described previously,'2 is based on the known slit, laser beam, and substrate configurations. The numerical result is defmed as a scattering coefficient, S,which is the ratio of the peak height of the signal from the scattered NH molecules to the peak height of the signal from the NH in the incident molecular beam. Use of the ratio of the scattered and beam signals to determine reactivity eliminates the effects of laser power saturation.I2 One difference in the calculations for the present experiment is that the numerical simulation now includes effects from Doppler broadening, which was not important in our previous experiments but is noticeable with the lighter NH molecule. This effect arises in our reactivity experiments because we essentially measure the number density of the N H molecules in the incident beam, NB, and the number density of the NH molecules coming off the surface, Ns, at various positions in front of the substrate. In optimizing the N H signal, the dye laser is tuned to the peak of an NH rotational line, maximizing the signal in the incident molecular beam and thus maximizing the detectability of NB.N H molecules coming from the substrate have velocity components along the laser which differ from N H molecules in the molecular beam, and thus they experience a Doppler shift in absorption frequency. Since the molecules coming off the substrate are not all scattered specularly, the Doppler effect causes the detectability of N B and Ns to differ. This effect is included in the calculation because the line width of the laser, although broad (-0.5 cm-I), is narrow enough that the effect of molecular velocity is detectable. The magnitude of the Doppler
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Excitation Wavelength (nm)
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Figure 3. Experimental fluorescence excitation spectrum of NH in the molecular beam. The lines labeled [l], [3], and [ 5 ] are the Q I ( J= l), RI(J = 3), and RI(J = 5 ) lines used in this study. Below the experimental spectrum is a plot of line positions calculated from the spectral
data of Brazier et al." effect is thus dependent on the velocity distribution of the N H in the incident beam and scattered from the surface. The effect of the Doppler shift on the simulation is shown in Figure 2, which compares the calculated scattering signal if the laser is tuned directly to the peak of the rotational line at 334.954 nm with that for tuning off the peak by f0.003 nm. With the laser tuned directly to the peak of the rotational line, the ratio of scattered signal to incident signal is 0.461. When the laser is shifted off the line in either direction, the calculated value of S changes to either 0.602 or 0.376, depending on the direction of the shift. In addition, the location of the peak of the scattered N H distribution migrates with respect to the beam as the laser wavelength changes. These effects on scattering ratio and peak position were observed experimentally because the dye laser exhibited minor wavelength drift over long time periods. We were able to exploit this effect to extract an estimate of the velocity of the N H molecules in the beam as discussed below in section IIIC.
III. Results A. Spectroscopy. The spectral selectivity of the LIF technique was used to identify N H and study it separately from the other species in the molecular beam. Indeed, the spectrally dispersed spectrum of the LIF of NH in the molecular beam contained only the expected emission at the excitation wavelength. Figure 3 shows an experimental fluorescence excitation spectrum for NH in the molecular beam. Underneath the experimental spectrum, the figure depicts the calculated line positions from a numerical simulation of the well-known spectroscopy of the A311-X3Z (0,O) band of NH,IS-" specifically the spectral constants from Brazier et al.17 The agreement between the calculated line positions and experimental spectra shows that the fluorescing species is unquestionably NH. The IRIS method for measuring radical reactivities is not sensitive to saturation effects because the signal from desorbing molecules is normalized to the incident molecular beam signal. Saturation effects are addressed, however, because the laser power, pulse duration, and spot size used yield an irradiance well above that required to saturate the transitions in NH in the collision-free environment of the molecular beam. A measurement of the fluorescence signal as a function of pulse energy remained slightly sublinear in power from 0.1 to 1.1 mJ/pulse. This type of behavior is well-known in saturation spectroscopy18and was observed in our SiH reactivity measurements.'* In addition to the triplet ground state, the spectroscopy of the alA state of N H is also known. We attempted to probe the reactivity of NH in this electronically excited state by examining the clIl-alA transition in the 320-330-nm range. No LIF signal was observed at any of the previously assigned spectral lines for this transition. As a further check that the failure to observe
9858 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
Fisher et al.
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from the surface. The small solid curve is the LIF signal from NH molecules coming from a room tempcrature surface for a lasersurface distance of 2 mm, i.e., the difference between the "surface in" and "beam" only signals. The laser was tuned to the R,(3) rotational line of NH (334.954 nm). The x-axis zero is choscn as the location where the laser beam intersects the center of the molecular beam. fluorescence was not caused by any experimental errors associated with using the doubled dye laser system, the laser was tuned to spectral lines from the A311-X3Z- transition located at -333.5 nm. LIF from triplet N H was satisfactorily observed in this spectral region. B. SprW kpdence. Figure 4 shows the spatially resolved LIF signal of N H in the molecular beam. For most of the reactivity experiments, the laser was tuned to 334.954 nm, which excites primarily the R,(3) line of N H but also excites the RQ32(1) line at approximately one-third the intensity. This overlap was accounted for in all numerical simulations of the data. The N H signal observed when a room temperature surface is placed in the path of the molecular beam 2.0 mm from the laser beam is also shown in Figure 4. This curve corresponds to the N H in the molecular beam plus any N H emanating from the surface. The difference between these two curves is also shown in Figure 4, corresponding to the N H coming from the surface only. From this figure, it is clear that a significant amount of N H is coming from the surface. Note that these experiments measure the spatial distribution of state-specific N H coming from the surface but do not distinguish between N H molecules that adsorb/desorb from the surface without reaction and N H molecules produced by the surface reaction of some other species in the molecular beam such as NH2 or NH3 or by ion bombardment. Figure 5 shows the signals from the incident molecular beam and from desorbed N H molecules obtained using laser-surface distances of 2.0,3.0,4.0, and 5.0 mm. The experimental curves show the increase in peak width, the shift in peak-maximum position, and decrease in peak height with inmasing lasef-surface distance expected for molecules desorbing with a m i n e angular distribution. Also shown are calculated curves assuming cosine desorption of 100%of the N H in the molecular beam. The data are cumistent with the m i n e distribution as well as the assumption of 100%desorption from the surface. Other considerations included in the calculated curves of Figure 5 a "the temperature of the N H molecules and are discugped further in the next section. NH reactivity measurements were also made using the q Z 1 ( 1 ) (335.392 nm) and Rl(5) (334.183 nm) spectral lines. Results for these two rotational states are essentially identical to those shown in Figures 4 and 5 with respect to the general shape and lasersurface distance dependence. This suggests there is little or no differemx between the reactivity of different rotational states. The reactivity measurements obtained using specific rotational lines
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Figure 5. LIF signal for NH in the molecular beam only and for NH coming from a 300 K surface at laser-surface distancm of 2, 3,4, and 5 nun with the laser tuned to the R1(3) rotational line. Dashed linea are the numerical simulation of the experiment assuming the NH radicals desorb from the surface with a cosine spatial distribution and 100% probability. The x-axis zero is the location where the laser beam intersects the center of the molecular beam. can thus be reasonably extrapolated to NH molecules as a whole. C. Temperature Depe". To obtain a quantitative value for the reactivity of the N H molecules, the temperature of the molecules incident on the substrate and emanating from the surface must be known. The reactivity of N H with the substrate is defied as one minus the ratio of the flux of N H molecules emanating from the surface to the flux of incoming NH molecules. Our LIF measurements, however, are sensitive to the number density of NH molecules in a specific set of rotational states. The experimental signals are thus affected not only by the reaction probability but also by the relative temperatures of the scattered and incident molecules. The effect of temperature is twofold. First, since the LIF signal is proportional to the number density of the gas, the signal level (for a constant flux) is inversely praportional to the average molecular velocity and depends, therefore, on the translational temperature of the gas. Second, since the laser excites a specific set of N H rotational states, which change in population with temperature, the signal level is also related to rotational temperature. Thus, quantitative interpretation require? knowledge of the internal temperature of both the scattered and incident molecules. We can estimate the translational and rotational temperatures of the incident NH m o l d e s in several ways. One way to estjmate the rotational temperature is to analyze the J-state dependence of the reactivity as a function of substrate temperature. If the beam temperature and the substrate temperature are the same, the population of a specific rotational state in the incident beam is the same as that for the molecules coming from the surface. The rotational temperature of the molecules no longer affects the reactivity measurement, and therefore, all rotational states exhibit the same value for S (defined in section IIB). If the beam and substrate temperatures differ, then the populations of some J states will increase and some will decrease upon thermal equilibration with the subtrate. This results in an apparent change in reactivity for different rotational states. Figure 6 shows S for three different rotational states of NH taken at three different surface temperatures, 300,433, and 668 K. Also shown in Figure 6 is a numerical simulation of S, accounting for the relative J-state population changes with substrate temperature. The dashed lines in Figure 6 are a least-squares fit of the calculated ratio to the nine experimental data points. (The adjustable parameter is the plasma temperature, and all points are fit simultaneously.) Assuming that the N H molecules equilibrate at the surface temperature, the intersection point of the calculated curves for all rotational lines should give the tem-
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9859
Reactivity of NH(X3Z-) with Silicon Nitride
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Substrate Temperature ( K ) Figure 6. Substrate temperature dependence of scattering coefficient, S, for the vzl(l) (solid triangles), R1(3)(solid circles), and R1(5)(solid squares) rotational lines of NH. Vertical lines represent uncertainties in the experimental measurements. The three dashed lines represent the simulated substrate temperature dependence with a plasma temperature (beam rotational temperature) of 370 K.
perature of the beam. Our Figure 6 fit estimates the plasma temperature to be 370 f 20 K with 100 f 10%of the molecules emanating from the surface. Although rotational temperatures can often be obtained from excitation spectra, we believe that the other methods discussed in this paper are more reliable for determining the temperature of the molecular beam in this particular experiment. Laser power variations and the spatial and temporal effects of saturation mentioned in ref 12 result in partially saturated line intensities, making it difficult to fit the experimental spectrum in Figure 3 with a simulated spectrum. A passable fit was obtained, however, by assuming a linear power dependence. An uncongested section of the spectrum between 337 and 339 nm was chosen to minimize complications due to overlapping lines. This yielded a rough estimate of 350 K for the rotational temperature, which is consistent with the other temperature estimations. An attempt to fit the same section of the experimental spectrum assuming a completely saturated spectrum gave a much poorer fit and a temperature of 460 K. To estimate the translational temperature of the NH in the beam, we examined the effect of the Doppler shift on the data. As shown in Figure 2,slight fluctuations in the laser wavelength resulted in variations in the shape and intensity of different data sets, The value of S was correlated with the distance between the beam position and the peak in the scattered NH as expected if the observed variation was Doppler induced. Figure 7 demonstrates this effect for one set of experiments (J = 5 with a substrate temperature of 300 K). Each point represents a separate measurement for which the scattered ratio is graphed as a function of the distance from the beam to the peak of the scattered NH signal. The magnitude of the Doppler effect is related to the translational temperature of the NH in the molecular beam as is illustrated by the calculated lines in Figure 7. These lines show the predicted slope for three different NH translational temperatures and are least-squares fits to the data. An NH translational temperature of 390 40 K was extracted from the data by taking the best fit to each of the nine data sets (three rotational states at three substrate temperatures). This value is in good agreement with the rotational temperature derived from the independent analysis of the rotational state/substrate temperature dependence of the NH scattering described above. The orifice in the plasma chamber wall from which the beam is generated is large, 1 cm, comparable to the mean free path of species in the plasma. Thus, the rotational and translational distributions in the beam need not be identical nor even Maxwell-Boltzmann. Our data suggest, however, that the plasma is
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I
500 600 700 Substrate Temperature (KI Figure 8. Comparison of NH scattering coefficient, S, from NH formed in a pure NH, plasma (solid triangles) and NH formed in a 2:l NH3/ SiH4 plasma (solid squares) as a function of substrate temperature. Vertical linea reprcaent the uncertainty in the experimental measurements for the NH,/SiH4 plasma. The laser was tuned to the R1(3)rotational line of NH (334.954 nm).
approximately 380 K and that the NH is near effusive without substantial rotational cooling or velocity increase. Knowledge of the substrate temperature and the temperature of the incident NH molecules allows us to calculate the simulated curves of Figure 5 . As noted.above, these calculations assume a cosine distribution and 100% adsorption/desorption of the NH from the surface. The good agreement between the experimental data and simulated curves indicates that the NH radicals adsorb with near-unit probability and desorb with a cosine distribution. It is, therefore, reasonable to assume that the rotational and translational degrees of freedom of the scattered molecules arc in thermal equilibrium with the surface. D. N H @ i i Plasma. Figure 8 compares S as a function of substrate temperature for NH radicals produced in a 2:l NH3/SiH4plasma with that from NH radicals produced in a pure NH3 plasma. It is clear from Figure 8 that the reactivity of the NH molecules produced in the silane plasma environment is the same as that with the pure NH3 plasma within our experimental uncertainty. In addition, the substrate temperature dependence of the N H molecules is essentially the same with both plasmas. FTIR analysis of the film deposited by the NH3/SiH4 plasma both in the plasma tube and on the substrate (at a slower rate) shows that the depositing film is a silicon nitride. The major
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The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
absorption occurring at 900 cm-l was identified as the Si-N stretch, and minor peaks observed at 3330 and 2150 cm-' showed both N-H and Si-H functionalities.
IV. Discussion The kinetics of film growth in a plasma depend on the detailed interactions of the plasma-generated species with the film surface. In order to understand how the film chemistry, morphology, and electronic properties are related to plasma variables, it is necessary to know how the individual plasma species interact at the surface. The technique used here allows the study of one specific molecule interacting at a surface while the surface is being bombarded by the full range of plasma species. The surface of the deposited f h is difficult to characterize precisely because the continuous bombardment of the surface with charged particles, reactive neutral molecules, metastables, and high-energy photons results in a steady-state surface composition that is difficult to determine with current analytical techniques. The substrates used here were silicon coated with thermally grown silicon nitride. As expected, no film is deposited during exposure of this surface to a molecular beam generated from a pure NH3 plasma. We think it plausible that at steady state the surface is predominantly terminated with NH, groups, although unpaired electrons and physisorbed molecules should also be present. Although the chemistry of NH at surfaces is unknown, it is energetically possible for NH to insert into an NH bond (AH -45 kcal/molIg) and very favorable to add to an unpaired electron (e.g., NH SiH3 HNSiH3, AH = -83 kcal/m01'~). Other possible reaction paths include an NH molecule physisorbing and migrating to a reactive site or remaining on the surface until a gas-phase radical collides with it. We estimate, on the basis of the plasma pressure and distance to the substrate, that the collision frequency at any site on the surface is about 15O/s. Our observation that essentially all the NH molecules desorb without reaction indicates that none of these suggested reactions have significant rates. When silane is added to the plasma and a film of a-SiN,:H is being deposited, the steady-state surface coverage is expected to include SiH, groups. Insertion of NH into an X3SiH bond to give X3SiNH2 is energetically very favorable (AH -110 kcal/mollg). Again, we observe that NH is unreactive with this surface, indicating that either the SiH, groups are in very low steady-state concentrations or the insertion reaction has a very low cross section regardless of the energetics. The details of the surface condition during deposition, however, are not known. Our measurements demonstrate that NH interacting even with the surface of depositing silicon nitride during plasma exposure physisorbs and equilibrates with the surface but then desorbs without detectable reaction. This is true over a wide range of surface temperatures and for different rotational states. It is possible, however, that NH formed under different conditions and plasma exposure may exhibit a different reactivity than that observed here. Failure to observe significant reactivity of NH radicals on the surface of a depositing hydrogenated silicon nitride film suggests that NH is not an important species in the deposition process. This result is in accord with the results of Smith et al., who examined the mechanism of SiN,H, deposition from NH3/Si% plasmas?0q2' They suggest that the triaminosilane radical, Si(NH&, is the key deposition precursor and that the NH, radicals can be eliminated as deposition precursors due to the morphology of the film deposited.2o Smith and -workers have also examined the deposition of SiN,H, from N2/SiH4 plasmas.22In contrast to the NH3/SiH4 plasma, they do not observe any gas-phase Si-N species and therefore postulate that the deposition process occurs by direct surface reaction between high sticking coefficient species such as SiH, radicals and N atoms. Again, this suggests that NH is not a likely deposition precursor candidate. NH is the third molecule to be examined with the IRIS technique. Previous IRIS experiments demonstrated that SiH radicals react with the surface of a depositing amorphous hydrogenated silicon film with 294 3% probability.I2 In contrast, the reactivity
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Fisher et al. of S i 0 with the surface of a depositing silicon oxide film was found to be essentially zero over a substantial range of substrate temperature~.'~ The high reactivity found for SiH can be understood considering the coordinately unsaturated nature of the SiH species. The extremely low reactivity for SiO, however, was somewhat surprising since S i 0 is also coordinately unsaturated and has a reactive Si multiple bond. Although NH radicals are much less studied than their isoelectronic counterparts,0 atoms and CH2 radicals, the gas-phase. reactions of NH with a number of small molecules have recently received more a t t e n t i ~ n . ' ~ -Although ~~ the analogy between gas-phase and gas-urface reactions is not strict, these studies show that while the singlet NH molecules are highly reactive, their triplet counterparts are not nearly as reactive with such reaction partners as NO, NH3, N1, CO, and 02?4b3C27 This reactivity trend is quite similar to that of the isoelectronic O('P)/O('D) and CH2 triplet/singlet pairs. In these cases, the singlet species are found to be highly reactive and to insert readily into various bonds whereas the triplet species are much less reactive. Unfortunately, we were unable to detect LIF of the singlet NH in this plasma system and thus could not perform the interesting comparison of reactivity. Failure to observe LIF of singlet NH molecules is somewhat surprising as emissions due to the NH alA and the NH c'II species have been observed previously in both NH3 and NH3/SiH4glow discharges: and both states were observed in emission from our plasma. As is well-hown, however, the observation of emission from the excited triplet and singlet states of NH does not relate directly to the concentration of the lower electronic states in the plasma environment. If the NH singlet is indeed highly reactive with the plasma chamber walls and gas-phase species in the plasma, its steady-state concentrationin the plasma may remain low.
V. Summary The reactivity of NH radicals with the surface of silicon nitride and a depositing hydrogenated silicon nitride film has been measured to be essentially zero with an upper limit of 10% using a combination of spatially resolved LIF and molecular beam methods. The NH is observed to adsorb, equilibrate energetically with the surface, and then desorb with a spatial distribution consistent with a cosine angular distribution. This result is in contrast to previous surface reactivity results for S M but is similar to results for SiO. Although NH might be expected to be a highly reactive species, it appears that NH is not likely to be a major deposition precursor for silicon nitride films. Acknowledgment. This work was performed at Sandia National Laboratories and was supported by the US.Department of Energy under Contract DE-AC-0476DP00789. We also thank Pamela Ward for her technical assistance. Registry No. Imidogen, 13774-92-0; silicon nitride, 12033-89-5; hydrogen, 1333-74-0.
References and Notes (1) We follow HerzbergV example and use the term "radical" to refer to transient intermediate species, rather than limiting the term to only those species with an unpaired electron. ( 2 ) Herzberg, G. The Spectra and Structures of Simple Free Radicals; Cornell University Press: Ithaca, NY, 1971. (3) Ho, P.;Buss, R.J.; Lochman, R. E. J . Mater. Res. 1989, I ,873-881. (4) Kapoor, V. J.; Stein, H. J. Silicon Nitride Thin Insulating Films; Symposium Proceedings; The Electrochemical Society: Pennington, NH, 1983; Vol. 83-8. ( 5 ) Belyi, V. I.; Vasilyeva, L. L.; Ginovker, A. S.;Gritsenko, V. A.; Repinsky, s. M.;Sinitsa, S.p.; Smirnova, T. p.; Edelman. F.L. Silicon Nitride in Electronics; Materials Science Monographs; Elsevier: Amsterdam, 1988; Vol. 34. (6) Galasso, F.S.;Veltri, R.D.; Croft, W. J. Am. Ceram. Soc. Bull. 1978, 57, 453. (7) Gebhardt, J. J.; Tanzilli, R. A.; Harris, T. A. J. Electrochem. Soc. 1976, 123, 1578. (8) Selwyn, G. S.;Lin, M. C. Chem. Phys. 1982,67, 213-220. (9) Strobel, D. F . Reu. Geophys. Space Phys. 1975, 13, 372. (10) Roose, T. R.;Hanson, R. K.; Kruger, C. H. Eighteenth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1981; pp 853-863.
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J. Phys. Chem. 1992, 96,9861-9865 (11) Miller, J. A.; Branch, M. C.; McLean, W. J.; Chandler, D. W.; Smooke, M.D.; Kee, R. J. Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; pp 673-684. Bian. J.; Vandooren, J.; Van Tiggelen, P. J. Twenty-First Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1986; pp 953-963. (12) Ho, P.; Breiland, W. G.; Buss, R. J. J. Chem. Phys. 1989, 91, 2627-2634. (13) Buss, R. J.; Ho, P.; Weber, M.E. Plasma Chem. Plasma Process., in press. (14) Fairchild, P. W.; Smith, G. P.; Crosley, D. R.; Jeffries, J. B. Chem. Phys. Lett. 1984, 107, 181-186. (15) Veseth, L. J. Phys. B 1972,5,229-241. Ni, T.; Yu,S.;Ma, X . ; Kong, F. Chem. Phys. Lett. 1986, 126, 413-416. (16) Huber, K. P.; Herzberg, G. Molecular Spectra and Molecular Structure, IV. Constants of Diatomic Molecules; Van Nostrand Reinhold: New York, 1979. (17) Brazier, C. R.; Ram, R. S.; Bernath, P. F. J. Mol. Spectrosc. 1986, 120, 381-402. (18) He, L.-W.; Burkhardt, C. E.;Ciocca, M.;Leventhal, J. J. Phys. Rev.
Lett. 1991, 67, 2131-2134 and references therein. (19) Melius, C. F.; Ho, P. J . Phys. Chem. 1991, 95, 1410-1419. (20) Smith, D. L.; Alimonda, A. S.;Chen, C.-C.; Ready, S.E.; Wacker, B. J. Electrochem. Soc. 1990, 137, 614-623. (21) Smith, D. L. Mater. Res. Soc. Symp. Proc. 1990, 165, 69-78. (22) Smith, D. L.; Alimonda, A. S.; von Reissig, F. J. J. Vac.Sei. Technol. B 1990,8, 551-557. (23) Adams, J. S.;Pasternack, L. J . Phys. Chem. 1991,95, 2975-2982. Kenner, R. D.; Phannenberg, S.;Heinrich, P.; Stuhl, F. J. Phys. Chem. 1991, 95,65856593, (24) (a) Hack, W.; Wilms, A. Z . Phys. Chem. 1989, 161, 107-121. (b) Hack, W.; Wilms, A. J. Phys. Chem. 1989.93, 3540-3546. (c) Hack, W.; Rathmann, K. J. Phys. Chem. 1990, 94, 3636-3639. (25) (a) Cox, J. W.; Nelson, H. H.; McDonald, J. R. Chem. Phys. 1985, 96, 175-182. (b) Harrison, J. A.; Whyte, A. R.; Phillips, L. F. Chem. Phys. Lett. 1986, 129, 346-352. (26) Hack, W.; Rathmann, K. J. Phys. Chem. 1992, 96, 47-52. (27) Yamasaki, K.; Okada, S.;Koshi, M.;Matsui, H. J. Chem. Phys. 1991, 95, 5087-5096.
Pyrolysis of Styrene. Kinetics and Mechanism of the Equilibrium Styrene c)Benzene Acetylene
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M. A. Grela, V. T. Amorebieta, and A. J. Colussi* Department of Chemistry, University of Mar del Plata, 7600 Mar del Plata. Argentina (Received: June 16, 1992; In Final Form: August 3, 1992)
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The thermal unimolecular decomposition of styrene into benzene and acetylene, C6H&H=CH2 CsH6 + HCCH ( l ) , was investigated in a low pressure (=lo mTorr) flow reactor by on-line mass spectrometry between 1180 and 1350 K. Measured rates can be calculated, via RRKM extrapolation, from the expression log (k,,,s-') = 14.38 - 17 076/T, which was derived by detailed balance from high-pressure ( 4 0 Torr) low-temperature (878-973 K) kinetic data for the reverse reaction. This value of E l , = 77.9 kcal/mol allows for the generation of vinylidene, H z W , the carbene isomer of acetylene, as a primary product of the title reaction. A non-radical process involving the rate-determining extrusion of HzC=C: from a [4.1.0]7-methylene cyclohepta-2,4-diene intermediate in equilibrium with styrene is consistent with kinetic and thermochemical considerations.
Introduction The kinetics and mechanism of the thermal isomerization and decomposition of the multitude of CsH8isomers is a fascinating subject.' Benzene and acetylene are ultimately formed via nonradical processes through a maze of polycyclic intermediates. Thus for example, the low-temperature pyrolysis of cubane, the most exergonic isomer, leads to highly excited cyclooctatetraene with an activation energy of 43 kcal/moL2 This value, some 40 kcal/mol smaller than the dissociation energy of a normal C-C bond or about 25% of the cubane strain, militates against a biradical intermediate preserving the structure of the original molecular cage. The dissociations of hot cyclooctatetraene and styrene photoexcited at 193 nm also adhere to this pattern: a common bicyclic [4.2.0] isomer lies at the gateway of their exit channek3 We have recently investigated the mechanism of the thermal dimerization of acetylene below lo00 K,reaching the conclusion that occurs by prior isomerization of HCCH into HzC=C:, followed by fast addition of this species to a second HCCH m ~ l e c u l e . ~A- ~similar scheme was thought to apply to the association of acetylene with benzene, which proceeds at similar rates: Since styrene is the most stable isomer of the CsHs manifold, Le., the final product of the HCCH + CsH6 reaction at high temperatures, we decided to confirm the nonradical nature of these processes and perhaps the participation of vinylidene by examining whether the rates of styrene one-step dissociation into benzene and acetylene and its reverse satisfy detailed balance.g910 In this paper we report results showing that (1) the kinetics of acetylene decay in c,&/HccH mixtures can be quantitatively accounted for by known bimolecular reactions, i.e., without in0022-3654/92/2096-9861%03.00/0
voking free monoradical chains, and (2) the rates of reaction -1 thus derived can be used, in conjunction with independent thermodynamio data, to obtain a high-pressure Arrhenius expression for reaction 1 that accurately reproduces present measurements on the pyrolysis of styrene. The fact that reaction 1 demonstrably occurs in one chemical step definitely rules out freeradical chain mechanisms for the title equilibrium. We found that the activation energy of reaction 1 is compatible with the formation of vinylidene, a condition that also applies to the reported kinetic and thermodynamic data for the thermal elimination of acetylene from barrelene, the [2.2.2] bicyclic isomer of styrene," and vinylacetylene.@ On this basis we discard concerted processes involving closed-shell HCCH. Finally, we show that thermochemical considerations also exclude biradical intermediates in reaction 1.
Experimental Section A steady flow of styrene vapor (2 X lOI4-2 X l O I 5 molecules cm-' s-l) circulated through a heated, fused silica reactor (volume = 90 cm') at pressures below 10 mTorr. Mixtures of reactant and products effused into the differential chamber of a modulated beam mass spectrometer (Extrel) for on-lineanalysis. Under such conditions, secondary reactions are effectively s u p p r d and labile intermediate species, such as free radicals, can be consistently detected. Further details of this technique can be found in previous publications from this laboratory.'' The dynamic parameters of the reactor used were: kc,M= 0.214(T/M)'/2 s-l (escape rate constant of a species of mass M , in daltons) and uM= 4530( T/M)1/2S-I (gas-wall collision frequency); they were routinely checked by measuring the rates of ethylbenzene decomposition, which are known with considerably certainty.I4 Temperatures 0 1992 American Chemical Society
