Thermal reactions of fluorine and nitrogen ... - ACS Publications

J . Phys. Chem. 1988, 92, 2827-2834

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proper counting of the corresponding vibrational densities of states. When counting the vibrational densities of states, it has been shown that truncation is necessary due to dissociation thresholds for the fragments themselves.8 The discrepancy between the calculated fit and the experimental results can possibly represent the onset of a competing decay pathway or some interplay between isomerization and decay processes such as is seen at the lower ion internal energies for 2,4-hexadiyne. In fact, Baer et al.' invoked a competition between two C4H4+formation channels which switch above an ion internal energy -3 eV. It could be possible that, at the higher energies used in this study, this lower energy channel may still be contributing to the energy dependence of the C4H3+/C4H4+branching ratio.

Concluding Remarks The results of the statistical computation indicate that, but for an entropic barrier for the formation of C4H4+(and perhaps for the formation of cyclic C3H3+),the multiphoton dissociation of 2,4-hexadiyne is largely statistical. In particular, this means that the primary products, in the experiments under consideration, are primarily the low-energy isomers. A second point is that this provides supporting evidence for the parent ions having a relatively well defined energy. The reason is that the fragmentation pattern for parent ions with a wide energy distribution is quite different. The difference is not only in details such as the branching ratio

2827

but also in the general appearance of the At well-defined energies we find that only a few reaction paths will contribute at any particular energy. In contrast, for 2,4-hexadiyne ions with a wide energy distribution, many reaction paths contribute and channels with quite different thresholds can be simultaneously important. Specifically, and as seen in the experimental results,I2 there is extensive fragmentation to the C6H5+product ion, particularly so at lower energies. This is just as in benzene.5*8Hence, we conclude that the higher energy isomers indicated by singlephoton ionization mass spectrometry2 are possibly due to isomerization and/or competition between radiative vs nonradiative d e ~ a y . ~ The , ' ~ statistical breakdown computations do not allow for radiative decay and predict negligible survival of parent ions, hence the conclusion that such parent ions which are observed are due to fluorescence. The statistical computations also provide additional support for a narrow energy distribution of the initially produced parent ions.

Acknowledgment. M.A.E. acknowledges the financial support of the National Science Foundation. The Fritz Haber Research Center is supported by the Minerva Society. Registry No. 2,4-Hexadiyne, 2809-69-0. (12) Gobeli, D. A. El-Sayed, M. A. J . Phys. Chem. 1985,89, 1722. (13) Allen, M.; Maier, J. P.; Marthaler, 0.;Kloster-Jensen, E. Chem. Phys. 1978, 29, 33 1 .

Thermal Reactions of F, and NF, with Silicon(ll0) Studied by Laser Ionization Mass Spectrometry D. W. Squire,? J. A. Dagata,+D. S. Y. Hsu,C. S. Dulcey,f and M. C. Lin* Chemical Kinetics Group, Code 6105, Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5000 (Received: February 2, 1987; In Final Form: November 9, 1987)

The techniques of laser and electron ionization mass spectrometry have been employed to study the thermal etching of Si(ll0) by F, and NF3 at substrate temperatures between 300 and 1200 K. By two-photon resonance-enhanced ionization of SiFz via the BIB2 state, the apparent activation energy for gaseous silicon difluoride production was found to be 8.9 f 0.3 and 22.1 1.7 kcal/mol for F, and NF,, respectively. SiF was not detected. An extensive search for SiF, during etching by F2 at 1000 K, by means of resonance ionization from 320 to 325 and from 416 to 510 nm, also showed no signs of the species. Both SiF and SiF, are thermochemically unimportant etch products under the conditions employed. In F2 etching, SiF4 and total silicon fluoride CSiF,+ signals as measured by electron ionization rose rapidly at lower temperatures and stabilized between 700 and 900 K before rising again. No such behavior was observed for SiF, production from F2 or for the products formed in NF, etching. Apparent activation energies for total silicon fluoride and S1F4production are similar. For F,, they were found to be about 9 kcal/mol in the low-temperature region, and for NF, both were measured to be about 21 kcal/mol. A proposed reaction mechanism explaining these and related results is discussed.

Introduction Plasma etching is one of the most important methods of commercial semiconductor processing. Research into etching mechanisms has employed ion-assisted etching or reactive ion etching as models for plasma Confusing and seemingly contradictory results from such model systems as well as the growing interest in laser-assisted etching have led to increased study of the thermal (unassisted) etching of silicon and related ~ubstrates.~~~-~~ Etching by means of surface halogenation to form volatile products has been studied by using techniques such as chemiluminescence, modulated beam mass spectrometry, laser-induced fluorescence (LIF), and etch-depth analysis, in combination with standard surface analytic techniques.2*s12SiF, and SiF, have been alternately proposed as the principal surface species (the former usually considered to be in the form of (SiF,),,), and SiF, SiF2, 'NRC/NRL Cooperative Research Associate. f Geo-Centers, Inc, 4710 Auth Place, Suitland, MD 20746.

0022-3654/88/2092-2827$01.50/0

SiF,, SiF4, and Si2F6as major gas-phase products under varying conditions of ion bombardment, reagent, substrate doping, etch temperature, and detection t e ~ h n i q u e . ~ '-13 ~ ~ ~ ~Few - ~ *of' the (1) Mogab, C. J.; Levinstein, H.J. J . Vac. Sei. Technol. 1980, 17, 721. (2) Winters, H.F.; Coburn, J. W.; Chuang, T. J. J . VUC.Sci. Technol., B 1983, I , 469. (3) Winters, H.F.; Houle, F. A. J . Appl. Phys. 1983, 54, 1218. (4) Lee, Y. H.;Chen, M . M . J . Vac. Sci. Technol., B 1986, 4, 468. (5) Matsumi, Y.; Toyoda, S.; Hayashi, T.; Miyamura, M.; Yoshikawa, H.; Komiya, S. J . Appl. Phys. 1986, 60, 4102. (6) Houle, F. A. J . Appl. Phys. 1986, 60, 3018. (7) Vasile, M. J.; Stevie, F. A. J . Appl. Phys. 1982, 53, 3799. (8) Roop, B.; Joyce, S.; Schuitz, J. C.; Shinn, N. D.; Steinfeld, J. I. Appl. Phys. Left. 1985, 46, 1187. (9) Mitchell, M. J.; Suto, M.; Lee, L. C.; Chuang, T. J. J . VUC.Sei. Technol. B 1987, 5, 1444. (IO) Donnelly, V. M.; Flamm, D. L. J . Appl. Phys. 1980, 51, 5273. (11) Ibbotson, D. E.; Flamm, D. L.; Mucha, J. A.; Donnelly, V. M. Appl. Phys. Left. 1984, 44, 1129. (12) McFeely, F. R.; Morar, J. F.; Himpsel, F. J. Sutf Sei. 1986, 165, 277. (13) Flamm, D. L.; Donnelly, V. M.; Mucha, J. A. J . Appl. Phys. 1981, 52, 3633.

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surface through a 1.6-mm-diameter stainless steel tube. Reaction products desorbing normal to the surface passed through a 4-mm skimmer into the ionization region of a quadrupole mass spectrometer (50 mm from the surface). After ionization by 70-eV electrons or by a laser focussed through a 0.25-m focal length quartz lens, ionic products were mass filtered and detected with the signal subsequently amplified, digitized, and stored under computer control. Fluorine and NF, were the fluorinating agents used in these experiments. The fluorine (Air Products), 10% in helium, was used as supplied. The NF, (PCR) was used without dilution. In addition, SiF4 (Matheson) was used undiluted to establish the cracking pattern of SiF4 under the experimental conditions employed, and N2F4(Stauffer), distilled trap-to-trap and diluted to 10% in helium, was employed as a source of NF2. A needle valve in the inlet manifold was used to control the reagent gas flow into the reaction chamber. The gas flow settings maintained a constant ambient pressure in the reaction chamber. Reagents encountered only Viton, Teflon, and stainless steel between their source vessel Figure 1. Schematic of the apparatus used in these experiments. See text for details and discussion. gnd the silicon surface. Bayard-Alpert ionization gauges calibrated for air but uncorrected for the speclfic reactant gas monitored chamber pressure. techniques employed in etching studies can be used in situ during the etch process, and a post-mortem analysis of the etched surface The gas flow was set to produce similar ionization gauge readings without regard to either the reactant or its dilution. Background may not reflect the conditions and composition existing during the etching process. Some in situ techniques, such as chemilupressures in the untrapped reaction chamber and the liquid niminescence or LIF, require that the experiment be performed at trogen trapped detector chamber were 3.0 X lo-’ and 7.0 X lo-* Torr, respectively, rising to 2.0 X lo4 and 4.0 X lo-’ Torr during pressures where secondary gas-phase reactions can occur, to maximize the signal (for example, chemiluminescence studies the experiment. monitoring SiF, by means of its reaction with a fluorinating species The silicon( 110) single crystal (Virginia Semiconductor) was p-doped (BN doping) to a resistivity of 20-30 Q cm. As shown to form SiF, require relatively high pressures). In electron ionization mass spectrometric (EI/MS) techniques, dissociation of in Figure 1, the 2.5-mm thick wafer was cut into 10 X 10 mm squares with 1-mm grooves put into opposing edges. Tungsten the ionizing molecule often occurs. Since the fluorosilanes tend wires (0.5 mm) were placed into the grooves and then spot welded to lose one or more fluorine atoms upon ionization, identification of nascent ionized species is a recurrent problem in etching studies. onto tantalum-10% tungsten electrodes, providing support and Multiphoton ionization mass spectrometry (MPI/MS) is an resistively heating the silicon. A chromel-alumel thermocouple, spot-welded to a tantalum spring clamp inserted into a 1-mm hole extremely powerful, highly sensitive technique for exploring redrilled into an open edge of the silicon, directly measured substrate actions involving gas-phase radicals.I4-l6 Radicals tend to have low ionization energies because of their unpaired electrons. The temperature. In the ionization region of the quadrupole mass spectrometer (Extrel), either electrons or photons ionized gaseous resultant closed-shell ion generally has no low-lying excited species. Electron ionization (EI) was carried out with 70-eV electronic states. Hence, radical species show little or no fragmentation under laser ionization conditions, and the combination electrons at emission currents of 70 FA. Laser ionization was performed with either the fundamental or doubled output of an of wavelength selection and mass analysis permits precise species excimer-pumped dye laser (Lambda Physik). Laser dyes (Exciton) identification. used were rhodamine 640 (doubled using a KDP crystal), stilbene In the experiments reported in this article, this technique is 420, and coumarins 440, 460, 480, and 500 (all undoubled). applied to the thermal etching of silicon(ll0) single crystals by Typically, laser powers were 600 MJ per laser shot doubled and F2 and NF,. Because of their differing bond dissociation energies, 1-2 mJ per laser shot undoubled. the contrast between F2and NF, etching provides insight into the Ion counting was handled somewhat differently for laser and effect of reagent bond strength on etch rates and products. Due electron ionization. In both cases, a Channeltron electron multo the similarity of the NF2-F and FXe-F bond strengths, the tiplier (Galileo) detected the ions. In laser ionization, the amplified contrasting results from NF, and XeF2 (which has been commonly used as a model etchant) are particularly i n t e r e ~ t i n g . ~ ~ ” ion signal was measured by a boxcar integrator (Stanford Research) with the boxcar output digitized and computer averaged. Moreover, the known M P I / M S spectroscopy of SiF and SiF, permits these products to be directly and unambiguously moniIn electron ionization, an ORTEC pulse-counting system was used with its output digitized and computer averaged. The use of a tored.I5 Coupling these data with results on SiF, production pulse-counting system provided continuous wave detection (no determined in our ancillary experiments using EI/MS, one can downtime) under E1 conditions. Electron ionization therefore had generate a consistent view of the mechanism of fluorine etching a 10’ duty cycle advantage over laser ionization and thus an of silicon. absolute sensitivity advantage under the experimental conditions Experimental Section employed. Both systems were operated a t 10 H z with typically 400 samples per data point. The apparatus used in these studies is a modification of that used previously for SiF and SiF2 ~pectroscopy’~ and is similar to Results the instrument employed for chemical vapor deposition (CVD) studies.14 The apparatus is shown schematically in Figure 1. SiF, MPIIMS Results. SiF2 was detected desorbing from the Briefly, reagent molecules were directed onto a heated silicon Si surface by using the technique of multiphoton ionization mass spectrometry (MPI/MS). Ionization was achieyd by two-photon excitation (peak wavelength 320.9 nm) to the BIB2 state of SiFz (14) Squire, D. W.; Dulcey, C. S.;Lin, M. C. J . Vac. Sci. Technol., B followed by a third (ionizing) photon, as demonstrated earlier in 1985, 3, 1513. this 1ab0ratory.l~ ( 1 5 ) Dagata, J. A.; Squire, D. W.; Dulcey, C. S.; Hsu, D. S.Y.; Lin, M. C. Chem. Phys. Lett. 1987, 134, 1 5 1 . Figure 2 shows the mlz 66 (SiF2) ion signal at (a) 1000 and (16) Lin, M. C.; Smders, W. A. Advances in Multiphoton Processes and (b) 300 K for F, impinging on the single-crystal silicon as the laser Spectroscopy; Lin, S . H., Ed.; World Scientific: Singapore, 1986; p 333. was scanned between 320 and 322 nm. The ambient pressure in (17) Dagata, J. A.; Squire, D. W.; Dulcey, C.S.;Hsu, D. S . Y.; Lin, M. the reaction chamber was 1.0 X lo-’ Torr, giving a F, partial C. J . Vac. Sei. Technol. B 1987, 5, 1495.

Thermal Reactions of F2 and NF3 with Silicon( 110)

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The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2829

1

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320.9 nm

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Figure 4. Laser mass spectra ( m / z 20-1 10) of gas-phase etch products

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MASS

-i 320

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F2 on Si (110)

b) 300 K

321

322

WAVELENGTH (nm)

of F2 attack on silicon at (a) 1000 and (b) 310 K. The wavelength is 320.9 nm (the peak of the 0band of the BIB2two-photon transition, and the chamber pressure is 1.0 X Torr (F, pressure was 1.0 X lo6 Torr).

Figure 2. Mass 66 (SiF2) ion signal as a function of laser wavelength

between 320 and 322 nm for F2 attacking silicon at (a) 1000 K and (b) Torr (F210% in 300 K. Ambient chamber pressure was 1.0 X helium). Excitation is a two-photon resonance through the BIB2state of SiF2 with a third photon ionizing.

1 20

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bl 310 K

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MASS

Figure 5. Laser mass spectra ( m / z 20-1 10) of gas-phase products of NF, etching of silicon at (a) 900 and (b) 310 K. The wavelength is 320.9 nm (the peak of the 0-0 band of the BIB2 two-photon transition), and the chamber pressure is 1.0 X Torr. 320

321

322

WAVELENGTH (nm)

Figure 3. SiF2ion signal as a function of laser wavelength between 320

and 322 nm for NF, etching silicon at (a) 900 and (b) 310 K. Ambient chamber pressure was 1.0 X lo4 Torr. Excitation is a two-photon resonance through the BIB2 state. pressure of 1 X IOd Torr. Figure 3 shows a similar plot of SiF2+ signal from 320 to 322 nm at (a) 900 and (b) 310 K for NF3 striking the crystal. However, the ambient chamber pressure in this case was 1.0 X IO4 Torr, a reagent pressure 2 orders of magnitude higher in order to achieve a peak signal intensity half as large as in the F2case (albeit at a temperature 100 K lower). Observed peaks correspond to identifications of Gole et a1.'* and Dagata et al.Is This assignment classifies the peaks to the red of 321.1 nm as a vibrational sequence in the v2 bending mode while peaks to the blue of 321.1 nm are considered ,to be rotational structure associated with the 0-0 band of the B1B2state. The relative intensity of these rotational bands in the Fz-generated spectrum, as contrasted to the NF3-generated one, is striking. Specifically, the intensity of the lower rotational bands near 321 nm indicates that SiF2 generated from F2 is rotationally colder than that generated from NF,. Subsequent studies of SiF, production _kinetics were carried out a t the peak of the 0-0 transition of the BIB2 transition at 320.9 nm. The laser-generated mass spectrum (for m/z 2C-110) at this (18) Gole, J. L.; Hauge, Spectrosc. 1972, 43, 441.

R. H.; Margrave, J. L.; Hastie, J. W. J . Mol.

PRESSURE

(lo-'

TORR)

Figure 6. F2 pressure dependence of the SiF, ion signal at an ionizing

wavelength of 320.9 nm (ambient pressure; fluorine is an order of magnitude lower). These data were taken at a surface temperature of 750 K, but at other temperature the pressure is equally linear. NF3 also produces a linear pressure dependence of the SiF2signal. wavelength for Fz attack on silicon is shown in Figure 4, for substrate temperatures of (a) 1000 and (b) 310 K. The ambient chamber pressure was 1.0 X Torr (F2 partial pressure 1.0 X Torr). Figure 5 shows similar laser mass spectra for NF3-generated products at surface temperatures of (a) 900 and (b) 310 K. The ambient pressure in the reaction chamber was 7.5 X Torr. In both figures, the entire mass spectrum is obtained for the SiFz resonant condition, and all peaks other than mlz 66 can be attributed to nonresonant photoionization of ambient background.

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Squire et al.

NF3 on Si (1 10) 320.9 nm


E,=8.9 +0.3

kcal/mol

l I

300

l

l

1

600

1

1

1

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900

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300

1200

Figure 6 shows the linear pressure dependence of the SiF, ion signal at a substrate temperature of 750 K. SiF2derived from NF3 also demonstrates a linear pressure dependence, but the far smaller ion signals strength leads to much larger scatter in the data. Under E1 conditions, ionic intensities also demonstrate linear Torr), indicating that no pressure dependences (below 5 X secondary gas-phase reactions are taking place. Figure 7 shows the laser-generated m / z 66 ion (SiF2+)signal dependence on substrate temperature for F2 striking the Si( 110) substrate. The different symbols employed indicate separate experimental runs. Data were superimposable whether recorded as substrate temperature increased or recorded as it decreased. Torr (1.1 X Torr of The ambient pressure was 1.1 X F2). Data from different days were normalized at 900 K (to correct for variations in laser power and mass spectrometer sensitivity). These data were least-squares fitted to an Arrhenius form with an apparent activation energy E,[SiF,] = 8.9 f 0.3 kcal/mol. Figure 8 is similar to Figure 7, showing the SiF, ion signal as a function of substrate temperature, but for NF, striking the silicon Torr. These data were fitted at an ambient pressure of 7.5 X to an apparent Arrhenius activation energy E,[SiF2] = 22.1 f 1.7 kcal/mol. To establish the origin of the etching F atoms (NF,, NF2, and/or NF), we carried out an experiment by employing N2F4 as a source of the NF, radical. No SiF, ion signal could be seen at substrate temperatures up to 1000 K. Search for SiF and SiF3. On the basis of the spectroscopic results of Dulcey and HudgensIg and Dagata et al.,15 an extensive search was made for desorbed SiF radicals during these thermal etching processes. The ionizing laser was set at 437.6 nm, the wavelength resonant at both the one- and two-photon levels (with the A2Z+and the C”zZ+ states of SiF, to have maximum detection sensitivity to SiF desorbing from the surface.Ig SiF could not be seen during etching by F2 or NF, except at extremely high substrate temperatures (1 100 K), where only sporadic ion signals were observed. Accordingly, SiF does not appear to be a significant gaseous product in the thermal etching of silicon by fluorinating species. (19) Dulceq. C

S , Hudgens, J W. Chem Phys Lett 1985, 118, 444

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TEMPERATURE (K)

TEMPERATURE ( K )

Figure 7. Laser-generated ion signal at mass 66 versus substrate temperature for F2 impinging on silicon. The wavelength was 320.9 nm; ambient chamber pressure was 1.1 X Torr (10% fluorine). Data from different experimental runs (the different symbols) were normalized at 900 K. A least-squares fit to an Arrhenius form gave an apparent activation energy of 8.9 f 0.3 kcal/mol.

I

Figure 8. Laser-generated SiF2ion signal versus surface temperature for NF3 striking silicon. The wavelength was 320.9 nm; ambient chamber Torr. Data from different runs (the different pressure was 7.5 X symbols) were normalized at 900 K. A least-squaresfit to an Arrhenius form gave an apparent activation energy of 22.1 f 1.7 kcal/mol.

An extensive search was also carried out in an attempt to detect gas-phase SiF3 radical products from fluorine etching. To our knowledge, the only spectroscopy of SiF, (other than chemiluminescence from etching processes) is two chemiluminescence studies by Wang et aL20and, subsequently, Conner et both observing emission in the 210-270-nm region. On the basis of the results of Duignan et aLZ2on the resonance-enhanced MPI spectrum of CF3 and the similarity between silicon and carbon halide spectroscopy, SiF3 is expected to possess a two-photon resonant state with a one-photon wavelength lying somewhere b_etwee_n420 and 530 nm. This transition, reassigned as the A2Al-X2A, resonance by Conner et al., offered the best possibility for the detection of the SiF3 radical. With F2 at an ambient Torr and a substrate temperature chamber pressure of 1.0 X of 1000 K, an exhaustive search was undertaken for the SiF, radical. Using five laser dyes covering the wavelength ranges from 320 to 325 and from 416 to 510 nm, we could not detect SiF, desorbing from the surface. Assuming that a resonant state exists for SiF, in this wavelength region, one can conclude that SiF3 is not produced as a significant gas-phase product of surface reactions in F2 thermal etching of silicon. Further work to establish the spectroscopy of SiF, using a reactor source is planned. Electron Ionization Results. Brief ancillary experiments were carried out to detect stable products such as SiF4 with electron ionization. The mass spectrometer used in the present experiments, however, had an ion transmission optimized for the laser detection of the smaller silicon fluorides. When SiF4 was introduced into this apparatus, the S i p (mass 47), SiF2+(mass 6 6 ) , and SiF4+ (mass 104) signal intensities were found to be 27, 3, and 275, respectively, of the SiF3+ (mass 85) signal at an electron energy of 70 eV, rather than the literature values of 3.8, 0.7, and 1 . 6 % ~ ~ ~ It should be stressed that this transmission bias presented no difficulty when analyzing the variation of a single mass with substrate temperature or gas pressure but makes comparisons (20) Wang, J. L.-F.; Krishnan, C. N.; Margrave, J. L. J . Mol. Spectrosc.

-.

1973. -, 48. 346. - ~

(21) Conner, C. P.; Stewart, G. W.; Lindsay, D. M.; Gole, J. L. J . Am. Chem. SOC.1977, 99, 2540. (22) Duignan, M. T.; Hudgens, J. W.; Wyatt, J. R. J . Phys. Chem. 1982, 86, 4156. (23) Dibeler, V . H.; Mohler, F. L. J . Res. Natl. Bur. Stand. ( U S . ) 1948, IO, 25.

Thermal Reactions of Fz and N F 3 with Silicon( 110)

The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2831 0

NF3 on Si (110) SiF,' S1M

4


L

Figure 9. Total SiF,' signal CSiF,' (W) shown versus silicon substrate temperature for fluorine (10% in helium; ambient reaction chamber pressure 2.0 X 10" Torr). Also shown are the raw ion signals of SiF+ (m/z 47, X), SiF2' ( m / z 66, D), SiF3+( m / z 8 5 , 0 ) , and SiF4' ( m / z 104, A). These data were taken by using 70-eV electron ionization; no correction for fragmentationduring E1 detection has been made. Note that loss of at least one fluorine is typical during fluorosilane electron ionization. A least-squares fit of ZSiF,' versus T to an Arrhenius form gives an apparent activation energy E,[CSiF,+] of ca. 8 kcal/mol for T < 500

K. between different masses problematic. Electron ionization leads to significant fragmentation following ionization. Silicon fluorides tend to lose a fluorine atom quite easily under electron ionization condition^,^*^ in contrast to their behavior under MPI conditions. The major ion product from SiF,, as indicated by the cracking pattern given above, is SiF3+ at electron energies of 70 eV. SiF+, the principal product of SiFz electron ionization, can also arise from SiF4 fragmentation but is a smaller peak than SiF3+. Since no masses above SiF, could be observed in these experiments and no evidence for the production of gas-phase SiF3could be found with laser ionization, the SiF3+ ion signal is derived solely from SiF4desorbing from the surface. With the assumption that the fragmentation of SiF, under electron ionization conditions is independent of substrate temperature, the SiF3+ion signal provides a direct monitor of SiF, production under all conditions. This assumption means that SiF, product yield and its activation energy are based solely on the SiF3+ data, thereby avoiding any possible contamination effects. Contaminants such as COFz (m/z 66) have been observed in other work.3 In laser ionization, the effect of such contamination, if it is present a t all, is totally unimportant because of wavelength selectivity. Using 70-eV electrons for ionization, other investigators have found COFz to have a m / z 47:m/z 66 ratio of 10:3.s At substrate temperatures where SiF4 yield (as measured by m / z 85 signal) is small, however, we find virtually no m/z 66, an indication that carbonaceous contamination is negligible in the present experiments. The total silicon ion signal CSiF,+ (the uncorrected sum of all SiF,+ ions (x = 1-4)) is used in the present report to approximate the total silicon removal rate, and the apparent Arrhenius activation energy Ea[CSiF,+] derived from CSiF,+ as a function of surface temperature is compared to the results of Donnelly et a1.,11,13.24 who obtained an activation energy for the total etch rate by measuring etch depth as a function of exposure time (see following and Discussion). Because of the mass spectrometer transmission, both CSiF,+ and Ea[CSiF,+] are subject to some uncorrected systematic error. Figure 9 displays the electron ionization (EI) signal for F, etching silicon( 110). The total silicon fluoride ion signal CSiF,' (24) Mucha, J. A.; Donnelly, V. M.; Flamm, D. L.; Webb, L. M. J . Phys. Chem. 1981, 85, 3529.

.

4

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/1

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'

T (K)

'

l

'

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1000

Figure 10. Total SiF,+ signal CSiF,' (m) plotted versus silicon substrate temperature for NF3 (ambient reaction chamber pressure 2.0 X 10" Torr). Also shown are the raw ion signals of SiF' ( m / z 47, X), SiF2+ ( m / z 66, a), SiF3+( m / z 85, 0),and SiF4+( m / z 104, A). These data were taken by using 70-eV electron ionization (no correction for frag mentation during E1 detection has been made). ZSiF,+ has an apparent Arrhenius activation energy E,[xSiF,+] of 21.7 1.3 kcal/mol.

*

(x = 1-4) is plotted versus substrate temperature for an ambient chamber pressure of 2.0 X 10" Torr (fluorine 10% in helium). The individual ion signals for SiF+ ( m / z 47), SiFz+ (m/z 6 6 ) , SiF3+ ( m / z 85), and SiF4+ (m/z 104) are also shown (SiF4+ is sporadic and too small to be meaningful). The "break" in CSiF,+ signal at substrate temperatures between 700 and 900 K is a consistent feature of F2etching, although the onset, duration, and severity of the break vary slightly with pressure and sample changes. CSiF,+ is least-squares fitted to a simple Arrhenius form in the temperature region below 500 K, generating a lowtemperature Ea[CSiF,+] of ca. 8 kcal/mol. An activation energy evaluated over the entire temperature range would not be meaningful, but the low-temperature data can be evaluated for comparison purposes. The choice of 500 K as the upper limit in the analysis minimized the influence of the break on E,[XSiF,']. Figure 10 shows comparable results with NF3 as an etchant at an ambient chamber pressure of 2.0 X 10" Torr. CSiF,+ (x = 1-4), SiF+ ( m / z 47), SiFz+ ( m / z 6 6 ) , SiF3+(m/z 85), and SiF4+(m/z 104) raw ion signals are plotted versus silicon substrate temperature. A least-squares fit to an Arrhenius form of the CSiF,+ data between 400 and 1200 K gives E,[CSiF,+] = 21.7 f 1.3 kcal/mol (errors are given as lo). Note that SiF+ is the dominant ion observed until the surface is at 1000 K. SiFz, the parent radical of SiF+, appears to be the major gaseous etch product in the temperature region where the overall etch rate as measured by CSiF,+ is quite small, as was also found at surface temperatures below 450 K for fluorine etching (Figure 9). Note that Figures 9 and 10 are qualitative measurements and that only the SiF3+(monitoring SiF,) ion signal is analyzed quantitatively. In Figures 9 and 10 the SiF4+ signal is very small, since most SiF, fragments upon ionization, and the uncertainty in this signal renders any interpretation meaningless. In a pure SiF, sample, the SiF4+signal was 2% of the SiF3+ion signal, illustrating the difficulty inherent in measuring SiF4+accurately when signals are small. In addition, the rise in SiFz+and SiF+ when compared to SiF3+at temperatures greater than 1000 K is taken as signifying a shift from SiF, to SiF, production. The low-temperature E1 signals at m / z 47 and 66 are attributed to SiF2desorbing from the surface, since COF, has been ruled out on the grounds of the m / z 47:m/z 66 ratio at low temperatures. This is in agreement with the results of Donnelly, Flamm, et al.,'1913924 who have observed the low-temperature production of SiF2in their experiments. The fact that our MPI results did not show significant SiFz at low temperatures is due to the lower duty cycle of the MPI

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TABLE I: Apparent Activation Energy (E,) for the Etching of Silicon ( I = A exD(-E./RT)"

r

~~

product/ method xSiF,*/ED

xSiF,*/EI SiF4/EI SiF,/CL SiF2/LI

E,, kcal/mol Fz NF3

T, K 220-400 -3.6' 270-360 8.7 f 1.8d 337-47 3 450-630 5.7c 21.7 f 1.3 7.5 f 0.9 300-1200 -8' 300-470 20.0 f 1.2 5.6 f 0.u 300-1200 -lo, 300-470 220-400 2.2 f 0.3* 8.0 f O.ld 337-460 8.9 f 0.3 22.1 f 1.7 6.7 f 0.5' 300-1200

F atom 2.1 f O.lb

XeF,

X X X

0 X

X

v)

I) I)

X

0

"E1 = electron ionization, ED = etch depth, CL = chemiluminescence, LI = laser ionization. bReference 13. cReference 11. dReference 24. 'These values are only approximate due to the presence of breaks that limit the number of data points available for the least-squares analysis. fReference 17. Note: xSiF,+ has no contribution from SiF2+due to contamination by Xe2+. Downloaded by UNIV OF NEBRASKA-LINCOLN on August 31, 2015 | http://pubs.acs.org Publication Date: May 1, 1988 | doi: 10.1021/j100321a027

Discussion The most significant results of the present study are the direct observation by MPI/MS of the gaseous SiF, radical and direct elimination of SiF and, most probably, SiF, as gas-phase products in the thermal etching of silicon. These findings, free from secondary reactions (as in chemiluminescence studies) or fragmentation effects (as in EI/MS), establish the thermal gas-phase products in silicon etching, separate surface from gas-phase reactions, and permit the evaluation of proposed etching mechanisms. Although SiF, cannot be fully eliminated as a gaseous product, due to lack of detailed knowledge on its MPI spectroscopy, it is quite likely that the radical is not produced in the thermal etching of silicon because of the expected high strength of the =Si-SiF3 bond. Additionally, since the gas-phase decomposition of the SiF3-SiF, molecule predominantly produces SiF, and SiF, rather than two SiF, radicals,25 a similar type of disproportionation reaction may be expected to be dominant on the surface, viz. SiF3

F

I

l

or, schematically SiF,(a)

-

F(a)

+ SiF2(g)

where a = adsorbed and g = gaseous. Table I summarizes the results of F, and NF, etching as well as those of XeF2,l7 F, and F2 by Donnelly, Flamm, and coworkers.11*13,24 Shown are the apparent activation energies (E,) found for various etch products, evaluated by using the expression (25) Bains, S. K.; Noble, P. N.; Walsh, R. J. Chem. SOC.,Faraday Trans. 2 1986, 82, 837.

B

X

I)

Y

1

technique compared to E1 in this particular application. The laser produces a 10-ns pulse at 10 H z for a duty cycle of but electron ionization counts total ionic signal at a given mass for a full 0.1 s for an effective duty cycle of 1. Additionally, the cross-sectional area of the ionization region is much smaller for MPI than for EI. For these reasons E1 was more capable than MPI a t detecting SiF, under these conditions. When SiF, intensity, derived from the SiF3+ion signal, is plotted versus surface temperature, Figure 11 results. SiF3+ion signal is shown for Fz and NF,. The apparent activation energies E,[SiF,] derived from these results by least-squares fitting to an Arrhenius form are ca. 10 (F, below 500 K) and 20.0 f 1.2 kcal/mol (NF,). If the experimental SiF, cracking pattern and the SiF3+intensity are used to subtract "SiF," from all fragments in Figures 9 and 10, the remaining total ion signals have Arrhenius activation energies (F,, E,[SiF2] = 9.5 f 1.0 kcal/mol; NF3, E,[SiF2] = 20.1 f 2.2 kcal/mol) in reasonable agreement with those determined for SiF2 by using MPI.

a

x%xx:

.-u* -

"

"

300

"

"

600

I

1200

900

TEMPERATURE ( K )

Figure 11. SiF4 (derived from SiF3+) ion signal as a function of surface impinging on a silicon surface at temperatures for F2 ( X ) and N F 3 (0) 2.0 X 10" Torr (F, pressure was 2.0 X lo-' Torr) by employing 70-eV electron ionization mass spectrometric detection. A least-squares fit to an Arrhenius form gives an apparent activation energy of ca. 10 kcal/mol ( T 500 K) for the production of SiF4 from F2 and 20.0 f 1.2 kcal/mol from NF3. TABLE II: Bond Dissociation Energies of Etchants

etchant

bond strength, kcal/mol

ref

etchant

bond strength, kcal/mol

ref

F-F FXe-F Xe-F

37.8 59 4.0

21 28 21

F,N-F FN-F N-F

58.6 70.4 82

29 29 27

I = A exp(-E,/RT). Table I1 shows the gas-phase bond strengths of relevant etchants. The results of the present investigation using Fz and NF3 as etchants reveal an apparent correlation between observed activation energies for SiF, and SiF4 production and the bond dissociation energies D(F-F) and D(F,N-F). The lack of reactivity observed for NzF4 (NF,) is also consistent with this bond-energy correlation. This finding, together with the apparent low activation energies for etch rate and SiF, chemiluminescence in the F-atom etching reaction determined by Flamm et al.,13 suggests the generation of F atoms on the surface as the ratelimiting step in the F2 and NF3 etching reactions. This step must involve the initial dissociative adsorption of etchant molecules on the Si surface.2s26 This bond-energy correlation should be invoked cautiously, because factors other than bond energy can influence the dissociative adsorption of etchant molecules. This point is vividly illustrated by the vast difference between the results for NF, and XeF,,' 1,17 which have approximately the same strength for their first F bond (Table 11). In the XeF, system not only is the apparent activation energy for SiFzproduction considerably lower, but t h e overall etch rate as measured by Ibbotson et al." exhibits a negative temperature dependence in the range 270-360 K. The latter observation was attributed to the low-temperature ( T < 360 K) formation of a XeF, overlayer, which in turn increases the etch rate." This enhanced adsorption and overlayer formation is consistent with the high polarizability of XeF,," in contrast with the more tightly bound, less polarizable NF, molecule, for which the surface must be heated to 1000 K before any measurable etching can be detected. The results summarized in Table I can be rationalized on the basis of the following simplified general reaction scheme:

(26) Winters, H. F. J . App/. Phys. 1978, 49, 5165

The Journal of Physical Chemistry, Vol. 92, No. 10, 1988 2833

Thermal Reactions of F2 and N F 3 with Silicon(ll0) XF(a)


nF(a)

-

+ F(a)

- X(a)

+ Si(s) SiF,(a) n = 1-3 F(a) + SiF3(a) SiF,(a)

(2) (3)

(4)

SiF,(a)

SiF,(g)

(5)

SiF2(a)

SiF2(g)

(6)

where X = F, NF2, FXe, and s = solid. The surface fluorosilyl layer SiF,(a) (n = 1-3) may have a polymeric structure, such as =SiF-(SiF,),-SiF,, with a terminal -SiF3 group (SiF3(a)) and the anchoring =SiF- group (SiF(a)) attaching the chain to the bulk silicon. (This simple model for SiF,(a) omits cross-linkage and chain branching, which are also expected to occur.) The relative importance of these steps will be explored as a function of surface temperature. In the present experiments, reactions 5 and 6 are the only reactions giving rise to gaseous products, SiF, and SiF,, respectively. SiF and SiF, radicals do not desorb as discussed previously. Figures 9 and 10 suggest that SiF, formation dominates only in the presence of adequate F(a) concentration via reaction 2. Below 400 K for F2 etching and below 1000 K for NF, etching, SiF2desorption occurs faster than SiF, formation. The very low CSiF,' signal when mass 47 is the major observable species (under E1 conditions) and the very different surface temperature ranges over which SiF, desorption predominates over SiF, production for F2 and NF3 indicate dissociative adsorption is limiting the reaction. The low concentration of F(a) a t low temperatures restricts the surface fluorination reactions 3 and 4. Low-temperature fluorination of the surface is so slow that SiF2 desorbs via reaction 6 before further fluorination to SiF4 can occur (surface fluorine starvation). This is consistent with the observation of Flamm et a l l 3 that the overall etch rate and SiF, chemiluminescence intensity in F-atom etching have the same small temperature dependence (the activation energy is 2.1 f 0.3 kcal/mol). In this case there is no adsorption barrier and therefore no fluorine starvation on the surface. For F-atom etching, the rate-limiting steps are most likely reactions 3 and 4, the production of SiFz(a) and SiF4(a). In the case of Fz etching, as surface temperature increases, F2 dissociation becomes faster, resulting in a faster etch rate and a rapid growth in SiF4production (Figure 9). As the F(a) concentration increases and ceases to limit further reaction, the sequential fluorination reactions 3 and 4 dominate over thermal desorption of SiF,. The percentage of total silicon fluoride appearing as SiF4 continues to rise, as does CSiF,' itself, until 700 K. Between 700 and 900 K, the percentage of SiF, in CSiF,+ reaches its maximum, while the absolute SiF4 signal and the CSiF,+ intensity stabilize (the break seen in Figures 9 and 1 l), and the SiF, signal begins to rise (Figure 7). Above 900 K, CSiF,' and SiF4 rise again, but the SiF2rises more rapidly and the percentage of SiF4 in CSiF,' decreases. The key to understanding this break is the rise in SiF2 signal. As shown in reaction 6, SiF2 originates from the thermal desorption of SiFz(a), while SiF4is produced by the further sequential fluorination of the surface fluorosilyl species. A rise in SiF, indicates that the thermal cleavage of Si-Si bonds, for example (SiF,)n(s)

-

-

mSiFz(g)

+ (SW,ds)

(7)

is becoming competitive with F-atom attack on such bonds: F(a)

+ (SiF,),(s)

SiF,(a)

+ SiF2(a) + (SiFz)n-z(s)

(8)

The thermal dissociation process leaves highly reactive dangling silicon bonds that compete for available F atoms. The opening of this reaction channel stops the increase in the SiF, signal and the total silicon fluoride signal CSiF,+. As the surface temperature continues to rise, the SiF4 and CSiF,+ signals start increasing again, but with activation energies altered due to the competing SiF2desorption. Above 1050 K, the SiF4signal starts to stabilize for a second time as the thermal desorption of SiFz becomes faster than F-atom addition to SiF2(a). The Debye temperature of silicon is known to be 625 K,2' which is consistent

with a high degree of lattice excitation above 700 K. For both F2 and NF, the SiF2 signals observed fit a simple Arrhenius form within the scatter of the data. The Arrhenius activation energies found for gaseous SiF2production in F2etching and in XeF2 etching" are in reasonable agreement with those determined by chemiluminescence.' 1~13*24These results confirm that the chemiluminescence is a direct indicator of SiF, concent r a t i ~ n . ~ , " - " *Assuming ~~ that the nature of SiF2(a) is identical for all etchants, the variation in Ea[SiF2] must be due to differences in the dissociative adsorption of the etchants. In NF3 etching, dissociative adsorption is slow until 900 K. As a result, the buildup of the fluorosilyl layer SiF,(a) (n = 1-3) occurs only above that temperature. This buildup leads to the rise in SiF4 as a fraction of CSiF,' signal at high temperatures (see Figure 10). The observed apparent activation energies for formation of both products (SiF, and CSiF,') are about the same, i.e., 22.1 f 1.7 versus 20.0 f 1.2 kcal/mol. In the related experiments using XeF2 as an etchant,I7 different behavior is observed. Even at room temperature SiF4 is the dominant etch product. Dissociative adsorption of XeF2 occurs so readily that no restriction on the sequential fluorination occurs when the SiF2 channel opens, and thus no break is apparent in the SiF4signal. An interesting aspect of the etching data summarized in Table I is that for each etchant, including the F atom, the apparent activation energies for SiF2 production agree closely with those found for SiF, production and the overall etch rate. This point suggests that the production of SiF2 and that of SiF, proceed concurrently, following the breakup of the surface fluorosilyl layer, either by decomposition (reaction 7) or by F-atom attack (reaction 8). The different activation energies for different etchants arise largely from the varying activation energies for surface F-atom production (Le., dissociative adsorption). For example, the activation energies for SiF2 and SiF, production are low in atomic fluorine etching and high in NF, etching. At no time in these experiments could any mass larger than SiF4 be observed. The Si2F6postulated by Houle6 in XeF, etching was not observed with use of either F2 or NF, as an etchant, possibly due to the low-mass bias of the present mass spectrometer. The present results were obtained by using a moderately p-doped (1 10) single crystal. In the literature, a dependence of product distribution on doping has been Experiments on the nature of doping and crystal orientation effects on product distribution are planned. Finally, it should be noted that etch rates cannot be derived from this data. N o flow rates were actually measured, only the ambient chamber pressure at some distance from the surface. Thus the pressure at the surface is somewhat higher than measured. Further, rates measured at these pressures may not be applicable at higher pressures where the flow is nonmolecular (and vice versa). If the SiF, to SiF, ratio is dependent on the fluorine-atom surface concentration, then a higher fluorinating gas pressure may change both the rate and the relative ratio of SiF2 and SiF, but not the energetics or mechanism presented here. Conclusions We have employed MPI and EI/MS to examine the thermal etching of silicon( 110) single crystal by Fz and NF, at surface temperatures between 300 and 1200 K. The SiF2 radical has been unambiguously detected by using MPI. SiF, production was found to have activation energies of 8.9 f 0.3 and 22.1 f 1.7 kcal/mol for F2 and NF,, respectively. For F2, these results for SiF2 production are in reasonable agreement with previous work.11.13,24 Apparent activation energies for total silicon fluoride and SiF, production have been measured by using electron ionization. For (27) Kerr, J. A. Handbook of Chemistry and Physics, 67th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1986. (28) Johnson, G . K.; Malm, J. G.; Hubbard, W. N. J . Chem. Thermodyn. 1972, 4, 879. (29) Karapet'yants, M. Kh.; Karapet'yants, M. L. Thermodynamic Constants of Inorganic and Organic Compounds; Ann Arbor Humphrey: Ann Arbor, MI, 1970.

2834

J . Phys. Chem. 1988, 92, 2834-2841


F2 etching, they are ca. 8 kcal/mol for total silicon fluoride and ca. 10 kcal/mol for SiF4 at low temperatures. For NF3 etching, the same products had activation energies of 21.7 f 1.3 and 20.0 f 1.2 kcal/mol. These data support a mechanism where the controlling step in the thermal etch rate is dissociative adsorption of the etchant. Under low adsorbed F-atom conditions (fluorine starvation) desorption of SiF2 occurs more readily than further reaction to SiF4. Attack by F atoms on surface silicon bonds is considered the principal fluorination reaction. As surface F-atom concentration rises, further reaction to SiF, becomes competitive with and then dominates over SiF, desorption. At higher temperatures, the thermal cleavage of Si-Si bonds leads to increased SiF2production. At extremely high temperatures SiFz is expected to desorb before further reaction to SiF, can occur. SiF was not detected as a gas-phase product in thermal silicon etching. Its absence is likely the result of its high reactivity toward F atoms and its high desorption energy. Our preliminary search for SiF, by MPI/MS also failed to detect its presence. The

desorption of SiF, is believed to be thermodynamically unfavorable. This work continues our exploration of semiconductor surface chemistry using MPI/MS techniques.'4J5 The capability of direct, unambiguous observation of gas-phase radical products offers the best possibility to differentiate these radical species from their stable parent molecules. The extension of this etching research to n-doped silicon and other substrates (GaAs, etc.) and etchants is clearly indicated. Acknowledgment. We thank D. R. Anderson of the NRL ceramics shop for aid in the machining of the silicon surfaces and the Chemistry Department of the Catholic University of America for the loan of the photon-counting system. We gratefully acknowledge partial support from the Office of Naval Research. D.W.S. and J.A.D. thank the National Research Council for their Cooperative Research Associateships. Registry No. Si, 7440-21-3; F2, 7782-41-4; NF3, 7783-54-2.

Paramagnetically Enhanced Nuclear Relaxation In Lyotropic Lamellar Phases. Application to Double-Tailed Surfactantst C. Chachaty* CEAIIRDI, Dgpartement de Physico- Chimie, DPCIBP 121, CEN de Saclay, 91 191 Gif-sur- Yvette cedex, France

and J.-P. Korb Laboratoire de Physique de la MatiPre Condensge, Groupe de Recherche no. 38 du CNRS, Ecole Polytechnique. 91 128 Palaiseau cedex, France (Received: July 10, 1987; In Final Form: November 1 1 , 1987)

Lamellar phases from double-tailed sodium alkyl phosphate/water systems have been investigated by phosphorus and carbon-13 relaxations enhanced by paramagnetic divalent ions exchanging rapidly among the surfactant polar heads at the interface with water layers. So that both structural and dynamical information can be extracted from these experiments,the paramagnetic probes were chosen such that their electron spin-lattice relaxation times are significantly larger (MnZ+,VO2+) or smaller (Ni2+)than the time scales of the surfactant molecular motions generally found in the 10 ps-3 ns range. The paramagnetic relaxation of surfactant nuclei results from intra- and intermolecular contributions ( J . Phys. Chem. 1987, 91, 1255). The latter, corresponding to dipolar interactions in a two-dimensional fluid, has been the subject of a recent theoretical work. The sum of these two contributions has been computed as an average over all accessible surfactant conformers resulting from the trans-gauche isomerizations of alkyl chains, taking into account the sterical constraints and fitted to the experimental relaxation rates at several magnetic field strengths. This procedure allows determinations of the populations of the main conformers, of the reorientation correlation times, and of the lateral diffusion coefficients of the surfactant, which are sometimes difficult to obtain by other NMR methods.

Introduction The conformation of surfactant molecules in lyotropic liquid crystals is generally obtained from the deuteron quadrupolar splittings of alkyl chains. In the case of surfactants with single normal alkyl chains, the all-trans conformer predominates, and the deuteron quadrupolar splittings decrease continuously from the polar head to the methyl extremity (see, for instance, ref 1). This behavior is a consequence of packing constraints of surfactant molecules in the liquid crystalline phases.2 The assignment of quadrupolar splittings is less obvious in the case of double-tailed surfactants where the intra- and intermolecular constraints result in an enhancement of the gauche conformers in some points of the alkyl chains. Recent studies on lyotropic lamellar phases from sodium dialkyl phosphates3-' prompted us to use the paramagnetically enhanced carbon-1 3 longitudinal relaxation as a subsidiary method of conformational study. This method, which could 'Presented at the 8th Specialized Ampere Colloque, Lisbon, 17-21 September, 1987.

0022-3654/8S/2092-2834$01.50/0

be in some cases an alternative to the difficult and expensive deuterium labeling, has been proposed by McConnell et aL8s9for determining the mean depth of a carbon atom in a phospholipid bilayer, the paramagnetic relaxation being induced by a nitroxide probe attached to a lipid guest molecule. The relevant theory ~~

~

(1) Charvolin, J.; Hendryks, Y. In Nuclear Magnetic Resonance in Liquid Crystals; Emsley, J. W., Ed.; Reidel: Dordrecht, Netherlands, 1985; p 449. (2) Gruen, D. W. R. J . Phys. Chem. 1985, 89, 146; 1985, 89, 153. (3) Chachaty, C.; Quaegebeur, J. P. J. Phys. Chem. 1983, 87, 4341. (4) Quaegebeur, J.-P.; Perly, B.; Chachaty, C. In Surfactants in Solution;

Mittal, K. L., Bothorel, P., Eds.; Plenum: New York, 1987; Vol. 4, p 449. (5) Faure, A.; Lovera, J.; Gregoire, P.; Chachaty, C. J . Chim. Phys. Phys.-Chim. B i d . 1985, 82, 779. (6) Faure, A. Dr. Sci. Thesis, Nancy, 1987. CEA Report R-5387, 1987. (7) Caniparoli, J. P.; Faure, A.; Tistchenko, A.-M.; Chachaty, C., submitted for publication in J . Phys. Chem. (8) Kornberg, R. D.; McConnell, H. M. Proc. Natl. Acad. Sci. U . S . A . 1971, 68, 2564. (9) Brdlet, P.; McConnell, H. M. Proc. Natl. Acad. Sci. U.S.A.1975, 72. 1421.

0 1988 American Chemical Society

Thermal reactions of fluorine and nitrogen ... - ACS Publications

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