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Thermal decomposition of 1,1-dimethylhydrazine on Si(100)-2 × 1
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surface s c i e n c e ELSEVIER
Applied Surface Science 120 (1997) 299-305
Thermal decomposition of 1,1-dimethylhydrazine on Si(100)-2 × 1 J.L. Armstrong, Y.-M. Sun, J.M. White * Department of Chemistry and Biochemistry and Center for Materials Chemistry, Universi~' of Texas at Austin, Austin TX 78712, USA
Received 21 January 1997; accepted 1 June 1997
Abstract
The surface reaction of 1,1-dimethylhydrazine (DMH) with Si(100) has been studied with temperature programmed desorption spectroscopy (TPD), temperature programmed static secondary ion mass spectrometry (TPSSIMS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Adsorption of DMH on Si(100) at 170 K followed by annealing to 1100 K results in significant decomposition to form surface carbide and nitride. TPD results show that the only gas phase desorption products are hydrogen and dimethylamine. Furthermore, decomposition occurs over a broad temperature range; XPS and TPSIMS results indicate C-N bond cleavage beginning at 400 K and by 600 K, all the C-N bonds have dissociated. We propose a molecular level mechanism that involves partial decomposition upon adsorption followed by extensive bond cleavage to form surface carbide and nitride. © 1997 Elsevier Science B.V.
1. Introduction
Silicon nitride and oxynitride have many properties that are useful in the construction of electronic devices, including a dielectric constant slightly higher than for silicon oxide; a diffusion barrier that resists thermal migration of dopants and impurities; and lattice match better than that of SiO 2 with bulk silicon, which relieves strain [1-3]. As devices become more complicated, the need to produce and process materials at lower temperatures, where damage can be minimized, takes on greater importance. The ongoing search for nitrogen bearing precursors, for making quality silicon nitride (or oxynitride) films, has yielded the potential nitriding agents NH 3, N 2 H 4 , NO, N O 2 and N20 [4-9].
* Corresponding author.
NH 3 [10-13] and N2H 4 [14-16] are among the most studied nitrogen containing molecules on silicon surfaces. Yates and co-workers published a study comparing the reactions of ammonia on Si(100) and S i ( l l l ) [13]. They concluded, and it is generally accepted, that ammonia bonds dissociatively on Si(100) and, at high temperatures ( ~ 600 K), recombines (as much as 70% of the monolayer) as NH 3. Hydrazine has been postulated to bind with both nitrogen atoms down, across the dimer bond, and decompose to form silicon nitride [14]. Ammonia and hydrazine have been the molecules of choice in the past, but each has disadvantages as nitrogen sources: ammonia is inefficient at low temperatures, and hydrazine, an explosive, is potentially hazardous. The reactivity of hydrazine may be decreased by substituting alkyl groups on the nitrogen atoms; however, very little is known about the reactions of alkyl amines or alkylated hydrazine derivatives with
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 7 ) 0 0 2 3 6 - 5
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J.L. Armstrong et al. /Applied Surface Science 120 (1997) 299-305
silicon. Recently, data has appeared on the reaction of 1,1-dimethylhydrazine (DMH) with Si(111)-7 x 7 at high pressures, with DMH cited as a possible nitridant of silicon [17]. Lin et al. have also looked at the reaction of methylhydrazine with Si(lll)-7 X 7 [15]. Alkylated hydrazine derivatives, such as DMH, are also being studied as possible nitrogen sources for producing metal nitrides (such as TiN) through chemical vapor deposition [27]. Consequently, it is valuable to study the reaction of alkylated nitrogen precursors on Si(100)-2 X 1. We have studied the reaction of DMH on Si(100) over a wide temperature range (150-1100 K). XPS, TPD, and SSIMS were used to observe the thermal decomposition of DMH. We found, from TPD and XPS, that DMH binds strongly to the silicon surface. A bonding model consistent with the data, and similar to what has been postulated for hydrazine, will be presented.
2. Experimental The experimental apparatus has been described previously [18,19]. Experiments were performed in an ultra high vacuum (UHV) chamber pumped by a turbo molecular pump and a titanium sublimation pump to a base pressure of 1 X 10 -9 Torr. The chamber is equipped with a quadrupole mass spectrometer fitted with a Bessel box, a double pass cylindrical mirror analyzer, an X-ray source, and an argon ion gun. In this configuration, the chamber is capable of temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and temperature programmed SSIMS (TPSSIMS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Two rectangular pieces (1.2 X 1.0 cm) of silicon were cut from p-type Si(100) with a resistivity of 20 cm. A strip (1.8 × 0.9 cm) of thin (0.127 mm) Ta foil was placed between the two silicon pieces and the resulting unit strapped together with two small pieces of Ta wire. The Ta foil was spot welded to two Ta posts in contact with a LN 2 reservoir. A chromel-alumel thermocouple was mounted to one side of the 'sandwich' assembly by a Ta clip. This method of sample mounting allowed cooling to 150 K and heating to 1500 K with good uniformity
(temperature gradient < 5 K across the length of the sample surface measured with an optical pyrometer). The native oxide was removed by thermal annealing to 1100 K for 1 min followed by slow cooling to base temperature ( < 2 K/s). Residual contaminants were cleaned by several sputter-anneal cycles until AES showed there was no carbon, oxygen, or nitrogen on the surface. During the cleaning procedure, instrumentation filaments were turned off to avoid activation of background species, particularly hydrogen, towards adsorption on the freshly cleaned surface. 1,1-dimethylhydrazine (DMH) was obtained from Aldrich Chemical and purified through several freeze-pump-thaw cycles. The dosing assembly consisted of a 10 ~ m aperture fitted between two VCR fittings with a 0.25 in. diameter stainless steel tube protruding into the vacuum chamber. The silicon sample could be dosed by positioning it within 2 cm of the dosing assembly. In all experiments, unless otherwise noted, DMH was dosed to saturation at a temperature of 170 K, and exposures are indicated in Langmuir units (1 L = 1 X 10 -6 T o r t s).
3. Results 3.1. TPD DMH on Si(lO0) First, consider the TPD results for DMH adsorbed onto Si(100) at 170 K. Fig. 1 shows the TPD spectra we observe for DMH dosed on a freshly prepared surface. The sample was dosed with ~ 5 L of DMH, sufficient to saturate the surface with DMH. The upper panel shows molecular hydrogen ( m / e = 2) desorption with a peak temperature of 780 K, consistent with H 2 recombinative desorption from the monohydride state on a Si(100) surface [20,21]. Mass 44 and mass 45 traces (also in the upper panel) show small responses near 620 K, which we assign to dimethylamine desorption. The lower panel shows desorption for parent DMH (mass 60 and fragment 59). The peak desorption temperature is near that known for multilayer DMH [22,23]. The peak desorption temperature for dimethylamine (620 K) is near that for recombinative desorption of ammonia on Si(100) [13]. To help explain this result, we performed a hydrogen co-adsorption experiment in which the surface was first dosed with
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J.L. Armstrong et al. /Applied Surface Science 120 (1997) 299-305
this is presented graphically in Fig. 3 which plots the Auger peak-to-peak ratio, normalized and corrected for sensitivity factors. Fig. 3 also shows the corresponding H 2 TPD with cycle number; while the peak temperature remains constant, the total peak area decreases by about 12% per cycle.
D M H / S i ( 1 0 0 ) ~
rn/ ¢-
~
620 K
45 rn/e I
. --. I
I
3.2. X P S anneal series I
I
e-
I
I
200
400
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~
600 800 Temperature (K)
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1000
Fig. I. Temperature programmed desorption for the saturation dose of DMH on Si(100). Dosing temperaturewas 150 K and the heating rate was 3 K/s. approximately 0.2 ML of hydrogen, followed by a saturation dose of DMH (5 L exposure, Fig. 2). Owing to the fact that fewer adsorption sites are available, we measure as much as 632% increase in total peak area for dimethylamine desorption, although the total hydrogen peak area remains relatively the same (to within 5%). This dramatic increase suggests that the presence of hydrogen (or other contaminants) may produce different chemistries in the adsorbed layer. In any event, the hydrogen needed to yield dimethylamine on the clean surface (upper panel Fig. 1) probably comes from adsorption of background water following cool-down from the cleaning procedure. Finally, the extent to which the surface is passivated by decomposition products is of some importance. Fig. 3 shows a series of experiments in which the sample was dosed, annealed, then re-dosed without cleaning. We tracked the increase in carbon and nitrogen on the surface by monitoring the peak intensities in the Auger spectrum (Fig. 3(a)). With increasing cycles, the carbon and nitrogen grow in monotonically, with the nitrogen (398 eV) concentration increasing more rapidly than carbon (273 eV);
An XPS anneal series is presented in Fig. 4 for N(ls) and C(ls) photoelectrons. The N(ls) series is plotted in Fig. 4(a), with DMH dosed to saturation at 150 K. Beginning at 150 K and increasing the temperature to 1100 K, we note several interesting trends. First, at 150 K the N(ls) spectrum peaks at a binding energy of 400.8 eV, consistent with N(ls) binding energies for alkylated hydrazine derivatives previously studied on Si(lll), [15] as well as DMH on GaAs [22] and Pt(111) [23]. Upon heating to 300 K, there is a significant shift to lower binding energy (399.5 eV), accompanied by a clear decrease in peak
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. I
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.
.
. I
.
:
.... I
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.
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Fig. 2. Hydrogenco-dosed temperatureprogrammeddesorption. The initial H2 coverage was approximately0.2 ML with 5 L DMH dose. Heatingrate was 3 K/s.
302
J.L. Armstrong et al. / Applied Surface Science 120 (1997) 299-305
273
387
Cycle 60 50 40
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~ 20
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(a)
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i
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i
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i
3
Cyclenumber
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400
600 800 Temperature(K)
1000
Fig. 3. Post anneal/dose experiments; each cycle consisted of dosing 5 L DMH followed by annealing to 1100 K. (a) Auger spectra for Si (90 eV), C (273 eV), and N (390 eV); (b) TPD following H 2 desorption with a ramp rate of 3 K/s.
T
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1100V L " ~
~
~ooor, 900
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280
285
290
Fig. 4. XPS spectra at the following temperatures: 150, 300, 400, 600, 700, 900, 1000, and 1100 K. (a) N(ls), (b) C(ls).
J.L Armstrong et al. / Applied Surface Science 120 (1997) 299-305
area. This agrees with the desorption shown in TPD between 180 and 300 K. The initial shift to 399.5 eV is assigned to cleavage of N - N bonds producing NH x and N(CH3) x fragments [14,24-26]; furthermore, the peak remains at 399.5 eV up to 600 K, then narrows and shifts to even lower binding energy. After annealing to 1100 K, the N(ls) peak settles at 397.0 eV binding energy, which we assign to the formation of silicon nitride. We are unable to distinguish between amine (NH 2) and dimethylamido (N(CH 3)2 ) fragments directly from the N(1 s) chemical shifts, since it has been shown that the presence of alkyl substituents have little effect on the N(ls) binding energy compared to the effect of hydrogen bonded to nitrogen [26]. The 150 K C(ls) spectra (Fig. 4(b)) show a peak located at 286.0 eV, which is attributed to the methyl substituents in DMH. Annealing to 400 K results in two very distinct carbon states, one at 284.1 eV and the other at 285.8 eV. Similar results have been observed for methylhydrazine on Si(111), [15] and the existence of two states strongly suggests C - N bonds are breaking. With increasing temperature, the contribution from the 285.8 eV state and the 284.1 state remain up to 600 K, and near 700 K the 284.1 eV state becomes dominant. Upon heating to 900 K, the 285.8 eV state disappears and the 284.1 eV state shifts closer to 283 eV. Finally, after annealing to 1100 K, the 283 eV peak remains, which we assign to carbon in silicon carbide. The C(ls) spectra provides solid evidence of C - N bond cleavage, occurring at temperatures as low as 400 K.
3.3. Temperature programmed SSIMS results TPD allows us to observe products released into the gas phase, but we are unable to make direct measurements of the products that remain on the surface. While XPS yields some information on the chemical states for the surface fragments, it does so only at set temperatures and not contiguously over the entire temperature range. With temperature programmed SSIMS, on the other hand, surface products can be monitored throughout the entire anneal. Fig. 5 shows five m/e signals followed by SSIMS as a function of temperature. The DMH for this experiment was dosed to 1 monolayer. The left panel shows dimethylamido fragments (mass 43 and 44) and the fight panel Sill ÷, CH~, and DMH ÷ for the
303
5000
4000
3000
2000
I000
o
200
400
600
800 200 Temperature (K)
600
1000
Fig. 5. Temperature programmed SSIMS; left panel dimethylamido surface fragment (masses 44 and 43) and right panel Sill + ( m / e = 29), CH~ ( m / e = 15), and DMH + ( m / e = 6 0 ) .
parent molecule (mass 29, 16, and 60). For the dimethylamido, we observe a decreasing signal, which tracks the decrease for the parent DMH. This agrees with the XPS anneal series data, where we observe the transfer of methyl groups to surface sites beginning as low as 400 K. The rapid decrease in SSIMS signal beginning near 400 K is consistent with decomposition on the surface. A very sharp rise in Sill ÷ SSIMS signal begins near 500 K and tracks the decrease in signal for CH 7; hence, we are able to detect decomposing methyl groups. Beginning near 720 K, the Sill ÷ signal decays rapidly, which corresponds to the leading edge of hydrogen desorption observed in TPD.
4. Discussion When modeling the adsorption of any molecule on Si(100) we must consider the surface dimer that exists on the reconstructed (2 X 1) surface. Ammonia, for example, binds dissociatively on Si(100), with the N - H bond cleaving, placing an N H : group across from an adsorbed hydrogen atom, each on a silicon atom of a dimer pair. However, our data suggests that DMH does not initially dissociate C - N ,
304
J.L Armstrong et aL / Applied Surface Science 120 (1997) 299-305
cH3CH3
/H
i
Fig. 6. Adsorptionmodelfor DMH on Si(100). N-N, or N - H bonds. We propose that DMH, upon adsorption, bridge bonds as a Lewis base across the dimer bond. A similar model has been proposed for hydrazine (see Fig. 6) [14]. The data support the bridge bonded model in several ways. First, TPSSIMS shows no surface hydrogen near the adsorption temperature, but we do observe parent and dimethylamido fragments. Though, the mass 29 signal is above baseline (Fig. 6), the intensity is consistent with the 29Si isotope, based on comparison between the DMH covered Si(100) surface and the signal for clean Si(100). Second, XPS shifts suggest an intact the molecule, whereas, at higher temperatures, XPS clearly shows that the molecule decomposes. Finally, TPD data affirm a strong DMH/Si(100) interaction: there is negligible signal for molecular desorption at temperatures above the physisorbed state. We rule out a single point attachment since such an adsorption model - i.e., cleavage of N - H bonds, forming Si-N - should manifest a higher intensity in the mass 29 signal (TPSSIMS) due to surface hydrogen, as well as shifts in the nitrogen XPS. Recombinative desorption (similar to NH 3 on Si(100)) would be possible; however, we observe no molecular desorption (other than physisorbed multilayer desorption near 160 K). We do observe a small, but detectable, signal for mass 45 (dimethylamine). This desorption probably results from dimethylamido groups (after N - N bond cleavage) scavenging surface hydrogen, which comes from the water contamination alluded to earlier. When the surface is heated to room temperature (300 K), there is a dramatic change in the XPS binding energies for C(ls) and N(ls), indicating C - N bond cleavage. The shift in the C(ls) binding energy, concurrent with a shift to lower binding energy for N(ls), observed near 400 K is similar to that observed by Lin et al. for methylhydrazine [15].
We take this as evidence that DMH decomposes through two major steps, one by cleavage of N - N bonds up to a temperature of 300-400 K, and another by cleavage of C - N bonds, at 400 K and higher, indicated by the shift to lower binding energy for C(ls). These conclusions are further supported by SSIMS. In TPSSIMS we observe a moderate signal for the parent mass of DMH upon adsorption. With increasing temperatures, the SSIMS signal for mass 60 decreases dramatically and the signal approaches baseline at 250 K, corresponding to the desorption seen in TPD for DMH near 180 K. The initial increase in mass 44 signal, which we attribute to dimethylamine in TPSSIMS, also reflects possible N - N bond cleavage occurring before or near 350 K; the mass 44 signal (and the cracking fragment, mass 43) peaks near 350 K and begins to decay, in agreement with our XPS data, where we concluded that C - N bonds cleave at temperatures as low as 400 K. As methyl groups transfer to the surface, the TPSSIMS signal for this species would necessarily begin to decay. Mass 15, CH~-, decreases slightly but remains above baseline until 500-550 K, then drops steadily; we observe a simultaneous increase in mass 29 intensity (Sill+), indicating that methyl groups remain on the surface where they either desorb or decompose. The decrease in TPSSIMS CH~- coincides with molecular hydrogen desorption observed in the TPD data, as pointed out earlier, and suggests that methyl groups decompose through cleavage of C - H bonds, forming CH x(a) (x = 0, 1, 2) and H(a). Since dehydrogenation occurs near the hydrogen desorption temperature for Si-H on Si(100), H 2 appears in the gas phase immediately after H transfers from CH x to Si. After decomposition, XPS and AES verify the presence of both carbon and nitrogen on the surface with binding energies that agree with carbide and nitride formation. After annealing to 1100 K, we were able to re-dose the surface and track the decrease in H 2 desorption. Approximately 12% of the surface reactive sites is removed in forming the nitride and carbide. After about 5 or 6 dosing/anneal cycles, an inert surface composed of Si3N 4 and SiC is formed. The XPS peak area includes a significant amount of surface carbon and nitrogen, but our post anneal/dosing cycles do not account for all the
J.L. Armstrong et al. / Applied Surface Science 120 (1997) 299-305
decomposition. We propose that at the extreme of the anneal, some carbon or nitrogen diffuses into the bulk, which explains the remaining activity of Si(100) (post annealed) towards DMH adsorption.
5. Conclusions We have shown that the reaction of DMH with silicon yields a number of surface decomposition products. DMH strongly adsorbs onto Si(100) at 150 K, with a proposed bonding scheme analogous to the model for hydrazine adsorption, in which the two nitrogen atoms bond to two silicon atoms of a dimer pair. Near 300-400 K, C - N bonds begin to break, forming surface methyl species. As the temperature increases, hydrogen leaves the surface via surface mediated dehydrogenation, immediately followed by desorption. Finally, annealing to 900 K is sufficient to produce a mixture of surface nitride and carbide.
Acknowledgements This work is supported by the Motorola Partnership in Research and by the National Science Foundation through its office of Science and Technology infrastructure under grant No. CHE8920120. Special thanks goes to Arvind Kamath and to Dr. R.L. Hance for insightful discussions.
References [1] F.H.P.M. Habraken, A.E.T. Kuiper, Mater. Sci. Eng. R 12 (1994) 123. [2] J.T. Milek, Silicon Nitride for Microelectronic Applications, IFI, Plenum, New York, 1971-1972.
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[3] S.M. Sze, Physics of Semiconductor Devices, 2nd ed., Wiley/Interscience, New York, 1981. [4] A. Namiki, S. Suzuke, H. Kato, T. Nakamura, T. Suzaki, Surf. Sci. 283 (1993) 9. [5] P. Avouris, F. Bozso, R.J. Hamers, J. Vac. Sci. Technol. B 5 (1987) 1387. [6] J. Stober, B. Eisenhut, G. Rangelov, Th. Fauster, Surf. Sci. 321 (1994) 111. [7] M. Bhat, A. Kamath, D.L. Kwong, Y.-M. Sun, J.M. White, Appl. Phys. Lett. 65 (1994) 1314. [8] G. Rangelov, J. Stober, B. Eisenhut, Th. Fauster, Phys. Rev. B 44 (1954) 1316. [9] E.G. Keim, L. Wolterbeek, A. Van Silfhout, Surf. Sci. 180 (1987) 57. [10] C.U.S. Larsson, A.S. Flodstrt~m, Surf. Sci 241 (1991) 353. [11] P. Avouris, F. Bozso, R.J. Hamers, J. Vac. Sci. Technol. B 5 (1987) 1387. [12] J. Stober, B. Eisenhut, G. Rangelov, T. Fauster, Surf. Sci. 302 (1994) 111. [13] P.J. Chen, M.L. Colaianni, J.T. Yates, Surf. Sci. 274 (1992) L605. [14] Y. Bu, M.C. Lin, Surf. Sci. 311 (1994) 385. [15] Y. Bu, D.W. Shin, M.L. Lin, Surf. Sci. 276 (1992) 184. [16] E.A. Slaughter, J.L. Gland, J. Vac. Sci. Technol. A 10 (1992) 184. [17] Y. Takami, Y. Egashira, I. Honma, H. Komiyama, Appl. Phys. Lett. 66 (1995) 1527. [18] X.-L. Zhou, C.R. Flores, J.M. White, Appl. Surf. Sci. 62 (1992) 223. [19] J.R. Creighton, J.M. White, Appl. Surf. Sci. 35 (1988) 5700. [20] M.P. D'Evelyn, Y.L. Yang, L.F. Sutcu, J. Chem. Phys. 96 (1992) 852. [21] M.C. Flowers, N.B.H. Jonathan, Y. Liu, A. Morris, J. Chem. Phys. 99 (1993) 7038. [22] Y.-M. Sun, D.W. Sloan, M. McEllistrem, A.L. Schwaner, J.M. White, J. Vac. Sci. Technol. A 13 (3) (1995) 1455. [23] A.L. Schwaner, M. Kovar, D.J. Alberas, J.M. White, J. Vac. Sci. Technol. A. 13 (3) (1995) 1368. [24] G. Dufour, F. Rouchet, H. Roulet, F. Sirotti, Surf. Sci. 304 (1994) 33. [25] J.L. Bischoff, F. Lutz, D. Bolmont, L. Kubler, Surf. Sci. 251/252 (1991) 170. [26] Y. Bu, M.C. Lin, Surf. Sci. 301 (1994) 118. [27] Y.-M. Sun, unpublished work.
surface s c i e n c e ELSEVIER
Applied Surface Science 120 (1997) 299-305
Thermal decomposition of 1,1-dimethylhydrazine on Si(100)-2 × 1 J.L. Armstrong, Y.-M. Sun, J.M. White * Department of Chemistry and Biochemistry and Center for Materials Chemistry, Universi~' of Texas at Austin, Austin TX 78712, USA
Received 21 January 1997; accepted 1 June 1997
Abstract
The surface reaction of 1,1-dimethylhydrazine (DMH) with Si(100) has been studied with temperature programmed desorption spectroscopy (TPD), temperature programmed static secondary ion mass spectrometry (TPSSIMS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). Adsorption of DMH on Si(100) at 170 K followed by annealing to 1100 K results in significant decomposition to form surface carbide and nitride. TPD results show that the only gas phase desorption products are hydrogen and dimethylamine. Furthermore, decomposition occurs over a broad temperature range; XPS and TPSIMS results indicate C-N bond cleavage beginning at 400 K and by 600 K, all the C-N bonds have dissociated. We propose a molecular level mechanism that involves partial decomposition upon adsorption followed by extensive bond cleavage to form surface carbide and nitride. © 1997 Elsevier Science B.V.
1. Introduction
Silicon nitride and oxynitride have many properties that are useful in the construction of electronic devices, including a dielectric constant slightly higher than for silicon oxide; a diffusion barrier that resists thermal migration of dopants and impurities; and lattice match better than that of SiO 2 with bulk silicon, which relieves strain [1-3]. As devices become more complicated, the need to produce and process materials at lower temperatures, where damage can be minimized, takes on greater importance. The ongoing search for nitrogen bearing precursors, for making quality silicon nitride (or oxynitride) films, has yielded the potential nitriding agents NH 3, N 2 H 4 , NO, N O 2 and N20 [4-9].
* Corresponding author.
NH 3 [10-13] and N2H 4 [14-16] are among the most studied nitrogen containing molecules on silicon surfaces. Yates and co-workers published a study comparing the reactions of ammonia on Si(100) and S i ( l l l ) [13]. They concluded, and it is generally accepted, that ammonia bonds dissociatively on Si(100) and, at high temperatures ( ~ 600 K), recombines (as much as 70% of the monolayer) as NH 3. Hydrazine has been postulated to bind with both nitrogen atoms down, across the dimer bond, and decompose to form silicon nitride [14]. Ammonia and hydrazine have been the molecules of choice in the past, but each has disadvantages as nitrogen sources: ammonia is inefficient at low temperatures, and hydrazine, an explosive, is potentially hazardous. The reactivity of hydrazine may be decreased by substituting alkyl groups on the nitrogen atoms; however, very little is known about the reactions of alkyl amines or alkylated hydrazine derivatives with
0169-4332/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 9 - 4 3 3 2 ( 9 7 ) 0 0 2 3 6 - 5
300
J.L. Armstrong et al. /Applied Surface Science 120 (1997) 299-305
silicon. Recently, data has appeared on the reaction of 1,1-dimethylhydrazine (DMH) with Si(111)-7 x 7 at high pressures, with DMH cited as a possible nitridant of silicon [17]. Lin et al. have also looked at the reaction of methylhydrazine with Si(lll)-7 X 7 [15]. Alkylated hydrazine derivatives, such as DMH, are also being studied as possible nitrogen sources for producing metal nitrides (such as TiN) through chemical vapor deposition [27]. Consequently, it is valuable to study the reaction of alkylated nitrogen precursors on Si(100)-2 X 1. We have studied the reaction of DMH on Si(100) over a wide temperature range (150-1100 K). XPS, TPD, and SSIMS were used to observe the thermal decomposition of DMH. We found, from TPD and XPS, that DMH binds strongly to the silicon surface. A bonding model consistent with the data, and similar to what has been postulated for hydrazine, will be presented.
2. Experimental The experimental apparatus has been described previously [18,19]. Experiments were performed in an ultra high vacuum (UHV) chamber pumped by a turbo molecular pump and a titanium sublimation pump to a base pressure of 1 X 10 -9 Torr. The chamber is equipped with a quadrupole mass spectrometer fitted with a Bessel box, a double pass cylindrical mirror analyzer, an X-ray source, and an argon ion gun. In this configuration, the chamber is capable of temperature programmed desorption (TPD), static secondary ion mass spectrometry (SSIMS) and temperature programmed SSIMS (TPSSIMS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Two rectangular pieces (1.2 X 1.0 cm) of silicon were cut from p-type Si(100) with a resistivity of 20 cm. A strip (1.8 × 0.9 cm) of thin (0.127 mm) Ta foil was placed between the two silicon pieces and the resulting unit strapped together with two small pieces of Ta wire. The Ta foil was spot welded to two Ta posts in contact with a LN 2 reservoir. A chromel-alumel thermocouple was mounted to one side of the 'sandwich' assembly by a Ta clip. This method of sample mounting allowed cooling to 150 K and heating to 1500 K with good uniformity
(temperature gradient < 5 K across the length of the sample surface measured with an optical pyrometer). The native oxide was removed by thermal annealing to 1100 K for 1 min followed by slow cooling to base temperature ( < 2 K/s). Residual contaminants were cleaned by several sputter-anneal cycles until AES showed there was no carbon, oxygen, or nitrogen on the surface. During the cleaning procedure, instrumentation filaments were turned off to avoid activation of background species, particularly hydrogen, towards adsorption on the freshly cleaned surface. 1,1-dimethylhydrazine (DMH) was obtained from Aldrich Chemical and purified through several freeze-pump-thaw cycles. The dosing assembly consisted of a 10 ~ m aperture fitted between two VCR fittings with a 0.25 in. diameter stainless steel tube protruding into the vacuum chamber. The silicon sample could be dosed by positioning it within 2 cm of the dosing assembly. In all experiments, unless otherwise noted, DMH was dosed to saturation at a temperature of 170 K, and exposures are indicated in Langmuir units (1 L = 1 X 10 -6 T o r t s).
3. Results 3.1. TPD DMH on Si(lO0) First, consider the TPD results for DMH adsorbed onto Si(100) at 170 K. Fig. 1 shows the TPD spectra we observe for DMH dosed on a freshly prepared surface. The sample was dosed with ~ 5 L of DMH, sufficient to saturate the surface with DMH. The upper panel shows molecular hydrogen ( m / e = 2) desorption with a peak temperature of 780 K, consistent with H 2 recombinative desorption from the monohydride state on a Si(100) surface [20,21]. Mass 44 and mass 45 traces (also in the upper panel) show small responses near 620 K, which we assign to dimethylamine desorption. The lower panel shows desorption for parent DMH (mass 60 and fragment 59). The peak desorption temperature is near that known for multilayer DMH [22,23]. The peak desorption temperature for dimethylamine (620 K) is near that for recombinative desorption of ammonia on Si(100) [13]. To help explain this result, we performed a hydrogen co-adsorption experiment in which the surface was first dosed with
301
J.L. Armstrong et al. /Applied Surface Science 120 (1997) 299-305
this is presented graphically in Fig. 3 which plots the Auger peak-to-peak ratio, normalized and corrected for sensitivity factors. Fig. 3 also shows the corresponding H 2 TPD with cycle number; while the peak temperature remains constant, the total peak area decreases by about 12% per cycle.
D M H / S i ( 1 0 0 ) ~
rn/ ¢-
~
620 K
45 rn/e I
. --. I
I
3.2. X P S anneal series I
I
e-
I
I
200
400
I
~
600 800 Temperature (K)
I
1000
Fig. I. Temperature programmed desorption for the saturation dose of DMH on Si(100). Dosing temperaturewas 150 K and the heating rate was 3 K/s. approximately 0.2 ML of hydrogen, followed by a saturation dose of DMH (5 L exposure, Fig. 2). Owing to the fact that fewer adsorption sites are available, we measure as much as 632% increase in total peak area for dimethylamine desorption, although the total hydrogen peak area remains relatively the same (to within 5%). This dramatic increase suggests that the presence of hydrogen (or other contaminants) may produce different chemistries in the adsorbed layer. In any event, the hydrogen needed to yield dimethylamine on the clean surface (upper panel Fig. 1) probably comes from adsorption of background water following cool-down from the cleaning procedure. Finally, the extent to which the surface is passivated by decomposition products is of some importance. Fig. 3 shows a series of experiments in which the sample was dosed, annealed, then re-dosed without cleaning. We tracked the increase in carbon and nitrogen on the surface by monitoring the peak intensities in the Auger spectrum (Fig. 3(a)). With increasing cycles, the carbon and nitrogen grow in monotonically, with the nitrogen (398 eV) concentration increasing more rapidly than carbon (273 eV);
An XPS anneal series is presented in Fig. 4 for N(ls) and C(ls) photoelectrons. The N(ls) series is plotted in Fig. 4(a), with DMH dosed to saturation at 150 K. Beginning at 150 K and increasing the temperature to 1100 K, we note several interesting trends. First, at 150 K the N(ls) spectrum peaks at a binding energy of 400.8 eV, consistent with N(ls) binding energies for alkylated hydrazine derivatives previously studied on Si(lll), [15] as well as DMH on GaAs [22] and Pt(111) [23]. Upon heating to 300 K, there is a significant shift to lower binding energy (399.5 eV), accompanied by a clear decrease in peak
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Fig. 2. Hydrogenco-dosed temperatureprogrammeddesorption. The initial H2 coverage was approximately0.2 ML with 5 L DMH dose. Heatingrate was 3 K/s.
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J.L. Armstrong et al. / Applied Surface Science 120 (1997) 299-305
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Fig. 3. Post anneal/dose experiments; each cycle consisted of dosing 5 L DMH followed by annealing to 1100 K. (a) Auger spectra for Si (90 eV), C (273 eV), and N (390 eV); (b) TPD following H 2 desorption with a ramp rate of 3 K/s.
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Fig. 4. XPS spectra at the following temperatures: 150, 300, 400, 600, 700, 900, 1000, and 1100 K. (a) N(ls), (b) C(ls).
J.L Armstrong et al. / Applied Surface Science 120 (1997) 299-305
area. This agrees with the desorption shown in TPD between 180 and 300 K. The initial shift to 399.5 eV is assigned to cleavage of N - N bonds producing NH x and N(CH3) x fragments [14,24-26]; furthermore, the peak remains at 399.5 eV up to 600 K, then narrows and shifts to even lower binding energy. After annealing to 1100 K, the N(ls) peak settles at 397.0 eV binding energy, which we assign to the formation of silicon nitride. We are unable to distinguish between amine (NH 2) and dimethylamido (N(CH 3)2 ) fragments directly from the N(1 s) chemical shifts, since it has been shown that the presence of alkyl substituents have little effect on the N(ls) binding energy compared to the effect of hydrogen bonded to nitrogen [26]. The 150 K C(ls) spectra (Fig. 4(b)) show a peak located at 286.0 eV, which is attributed to the methyl substituents in DMH. Annealing to 400 K results in two very distinct carbon states, one at 284.1 eV and the other at 285.8 eV. Similar results have been observed for methylhydrazine on Si(111), [15] and the existence of two states strongly suggests C - N bonds are breaking. With increasing temperature, the contribution from the 285.8 eV state and the 284.1 state remain up to 600 K, and near 700 K the 284.1 eV state becomes dominant. Upon heating to 900 K, the 285.8 eV state disappears and the 284.1 eV state shifts closer to 283 eV. Finally, after annealing to 1100 K, the 283 eV peak remains, which we assign to carbon in silicon carbide. The C(ls) spectra provides solid evidence of C - N bond cleavage, occurring at temperatures as low as 400 K.
3.3. Temperature programmed SSIMS results TPD allows us to observe products released into the gas phase, but we are unable to make direct measurements of the products that remain on the surface. While XPS yields some information on the chemical states for the surface fragments, it does so only at set temperatures and not contiguously over the entire temperature range. With temperature programmed SSIMS, on the other hand, surface products can be monitored throughout the entire anneal. Fig. 5 shows five m/e signals followed by SSIMS as a function of temperature. The DMH for this experiment was dosed to 1 monolayer. The left panel shows dimethylamido fragments (mass 43 and 44) and the fight panel Sill ÷, CH~, and DMH ÷ for the
303
5000
4000
3000
2000
I000
o
200
400
600
800 200 Temperature (K)
600
1000
Fig. 5. Temperature programmed SSIMS; left panel dimethylamido surface fragment (masses 44 and 43) and right panel Sill + ( m / e = 29), CH~ ( m / e = 15), and DMH + ( m / e = 6 0 ) .
parent molecule (mass 29, 16, and 60). For the dimethylamido, we observe a decreasing signal, which tracks the decrease for the parent DMH. This agrees with the XPS anneal series data, where we observe the transfer of methyl groups to surface sites beginning as low as 400 K. The rapid decrease in SSIMS signal beginning near 400 K is consistent with decomposition on the surface. A very sharp rise in Sill ÷ SSIMS signal begins near 500 K and tracks the decrease in signal for CH 7; hence, we are able to detect decomposing methyl groups. Beginning near 720 K, the Sill ÷ signal decays rapidly, which corresponds to the leading edge of hydrogen desorption observed in TPD.
4. Discussion When modeling the adsorption of any molecule on Si(100) we must consider the surface dimer that exists on the reconstructed (2 X 1) surface. Ammonia, for example, binds dissociatively on Si(100), with the N - H bond cleaving, placing an N H : group across from an adsorbed hydrogen atom, each on a silicon atom of a dimer pair. However, our data suggests that DMH does not initially dissociate C - N ,
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J.L Armstrong et aL / Applied Surface Science 120 (1997) 299-305
cH3CH3
/H
i
Fig. 6. Adsorptionmodelfor DMH on Si(100). N-N, or N - H bonds. We propose that DMH, upon adsorption, bridge bonds as a Lewis base across the dimer bond. A similar model has been proposed for hydrazine (see Fig. 6) [14]. The data support the bridge bonded model in several ways. First, TPSSIMS shows no surface hydrogen near the adsorption temperature, but we do observe parent and dimethylamido fragments. Though, the mass 29 signal is above baseline (Fig. 6), the intensity is consistent with the 29Si isotope, based on comparison between the DMH covered Si(100) surface and the signal for clean Si(100). Second, XPS shifts suggest an intact the molecule, whereas, at higher temperatures, XPS clearly shows that the molecule decomposes. Finally, TPD data affirm a strong DMH/Si(100) interaction: there is negligible signal for molecular desorption at temperatures above the physisorbed state. We rule out a single point attachment since such an adsorption model - i.e., cleavage of N - H bonds, forming Si-N - should manifest a higher intensity in the mass 29 signal (TPSSIMS) due to surface hydrogen, as well as shifts in the nitrogen XPS. Recombinative desorption (similar to NH 3 on Si(100)) would be possible; however, we observe no molecular desorption (other than physisorbed multilayer desorption near 160 K). We do observe a small, but detectable, signal for mass 45 (dimethylamine). This desorption probably results from dimethylamido groups (after N - N bond cleavage) scavenging surface hydrogen, which comes from the water contamination alluded to earlier. When the surface is heated to room temperature (300 K), there is a dramatic change in the XPS binding energies for C(ls) and N(ls), indicating C - N bond cleavage. The shift in the C(ls) binding energy, concurrent with a shift to lower binding energy for N(ls), observed near 400 K is similar to that observed by Lin et al. for methylhydrazine [15].
We take this as evidence that DMH decomposes through two major steps, one by cleavage of N - N bonds up to a temperature of 300-400 K, and another by cleavage of C - N bonds, at 400 K and higher, indicated by the shift to lower binding energy for C(ls). These conclusions are further supported by SSIMS. In TPSSIMS we observe a moderate signal for the parent mass of DMH upon adsorption. With increasing temperatures, the SSIMS signal for mass 60 decreases dramatically and the signal approaches baseline at 250 K, corresponding to the desorption seen in TPD for DMH near 180 K. The initial increase in mass 44 signal, which we attribute to dimethylamine in TPSSIMS, also reflects possible N - N bond cleavage occurring before or near 350 K; the mass 44 signal (and the cracking fragment, mass 43) peaks near 350 K and begins to decay, in agreement with our XPS data, where we concluded that C - N bonds cleave at temperatures as low as 400 K. As methyl groups transfer to the surface, the TPSSIMS signal for this species would necessarily begin to decay. Mass 15, CH~-, decreases slightly but remains above baseline until 500-550 K, then drops steadily; we observe a simultaneous increase in mass 29 intensity (Sill+), indicating that methyl groups remain on the surface where they either desorb or decompose. The decrease in TPSSIMS CH~- coincides with molecular hydrogen desorption observed in the TPD data, as pointed out earlier, and suggests that methyl groups decompose through cleavage of C - H bonds, forming CH x(a) (x = 0, 1, 2) and H(a). Since dehydrogenation occurs near the hydrogen desorption temperature for Si-H on Si(100), H 2 appears in the gas phase immediately after H transfers from CH x to Si. After decomposition, XPS and AES verify the presence of both carbon and nitrogen on the surface with binding energies that agree with carbide and nitride formation. After annealing to 1100 K, we were able to re-dose the surface and track the decrease in H 2 desorption. Approximately 12% of the surface reactive sites is removed in forming the nitride and carbide. After about 5 or 6 dosing/anneal cycles, an inert surface composed of Si3N 4 and SiC is formed. The XPS peak area includes a significant amount of surface carbon and nitrogen, but our post anneal/dosing cycles do not account for all the
J.L. Armstrong et al. / Applied Surface Science 120 (1997) 299-305
decomposition. We propose that at the extreme of the anneal, some carbon or nitrogen diffuses into the bulk, which explains the remaining activity of Si(100) (post annealed) towards DMH adsorption.
5. Conclusions We have shown that the reaction of DMH with silicon yields a number of surface decomposition products. DMH strongly adsorbs onto Si(100) at 150 K, with a proposed bonding scheme analogous to the model for hydrazine adsorption, in which the two nitrogen atoms bond to two silicon atoms of a dimer pair. Near 300-400 K, C - N bonds begin to break, forming surface methyl species. As the temperature increases, hydrogen leaves the surface via surface mediated dehydrogenation, immediately followed by desorption. Finally, annealing to 900 K is sufficient to produce a mixture of surface nitride and carbide.
Acknowledgements This work is supported by the Motorola Partnership in Research and by the National Science Foundation through its office of Science and Technology infrastructure under grant No. CHE8920120. Special thanks goes to Arvind Kamath and to Dr. R.L. Hance for insightful discussions.
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