Adsorption and desorption kinetics of tetrakis(dimethylamino)titanium and dimethylamine on TiN surfaces

Applied Surface Science 137 Ž1999. 113–124

Adsorption and desorption kinetics of tetrakis ždimethylamino/titanium and dimethylamine on TiN surfaces L.A. Okada, S.M. George

)

UniÕersity of Colorado, Department of Chemistry and Biochemistry, Boulder, CO 80309-0215, USA Received 29 December 1997; accepted 5 August 1998

Abstract Titanium nitride ŽTiN. can be deposited using the organometallic precursor tetrakisŽdimethylamino.titanium ŽTiwNŽCH 3 . 2 x 4 . ŽTDMAT.. Deviations from conformal TiN film growth have been observed in trench structures using TDMAT. This nonconformal deposition may be associated with readsorption and site blocking by the dimethylamine ŽHNŽCH 3 . 2 . ŽDMA. reaction product. To understand the deviations from conformal TiN deposition in trench structures, the adsorption and desorption kinetics for TDMAT and DMA were measured on a sputter-deposited TiN surface using laser induced thermal desorption ŽLITD. techniques. The LITD measurements revealed that DMA has a higher sticking coefficient than TDMAT. The sticking coefficients for both TDMAT and DMA were also dependent on surface coverage. The initial sticking coefficient for TDMAT is S0 s 0.23 with a coverage-dependence approximated by SŽ u . s 0.25 expŽy4.7 u . where u is the normalized surface coverage. The initial sticking coefficient for DMA is S0 s 0.70 and the coverage-dependence is approximated by SŽ u . s 0.86 expŽy3.7 u .. The observed desorption kinetics of DMA following TDMAT and DMA exposures on TiN were also coverage-dependent. The isothermal desorption measurements could be fit using a simple first-order desorption rate expression, k d s nd expwyEdŽ u .rRT x where EŽ u . is the coverage-dependent desorption energy, EdŽ u . s E0 y E1u . Assuming a desorption preexponential of nd s 1 = 10 13 sy1 , the observed isothermal desorption measurements for DMA desorption following both TDMAT and DMA exposures could be fit using E0 s 29.1 kcal moly1 and E1 s 8.2 kcal moly1. These measured adsorption and desorption kinetics are consistent with DMA readsorption as a major contributor to nonconformal TiN growth in trench structures using TDMAT. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Adsorption; Desorption; TiN surfaces

1. Introduction Titanium nitride ŽTiN. is an excellent diffusion barrier material because of its low bulk resistivity, good chemical and thermal stability, impermeability )

Corresponding author.

to Si diffusion, and excellent adhesion to silicon and silicon dioxide films w1–3x. Although TiN can be deposited using physical vapor deposition ŽPVD. techniques, these methods are reaching their limits as aspect ratios increase in silicon microelectronic devices. Collimation of sputtering deposition is a viable alternative for the near future; however, limited bot-

0169-4332r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 3 7 5 - 4

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L.A. Okada, S.M. Georger Applied Surface Science 137 (1999) 113–124

tom and sidewall coverage will hinder its application in sub-0.25 mm device structures w4x. Because of their ability to deposit in high aspect ratio structures w5,6x and excellent resulting film properties w5–7x, chemical vapor deposition ŽCVD. techniques have become increasingly popular. TiN CVD can be performed using the TiCl 4 q NH 3 system w8–16x, but suffers from high thermal budgets and Cl incorporation issues w11–14x. Consequently, many organometallic precursors are currently being examined for TiN thin film growth w5 – 7,17 – 30 x. Tetrakis Ž dimethylamino . titanium wTiŽN CH 34 2 .4 x ŽTDMAT. w6,17–21x, TDMATq NH 3 w5,7,22–28x, and tetrakisŽdiethylamino.titanium wTiŽN CH 2 CH 34 2 .4 x ŽTDEAT. q NH 3 w18,29,30x are the most commonly studied organometallic systems for TiN deposition. In particular, TDMAT has been actively investigated and many studies have demonstrated that a 1:1:1 titanium–carbon–nitride film, TiCN, is deposited by TDMAT thermal decomposition w6,17–21x. These TiCN films are suitable as barrier materials in IC devices w21x and they have excellent conformality and reasonable film resistivities w21x. Although TiCN films may be applicable for devices, much lower carbon concentrations and lower resistivities can be obtained using the binary system, TDMATq NH 3 . However, the TDMATq NH 3 deposition system suffers from nonuniform step coverage. These deviations from conformal deposition are attributed to efficient gas phase reactions between TDMAT and NH 3 w5,22,24,26,31x. An efficient gas phase transamination reaction between TDMAT and NH 3 is believed to produce a reactive intermediate that is responsible for the low carbon concentrations and high quality TiN growth. The transamination reaction happens readily in the gas phase at temperatures as low as 300 K w22,24,25x and the intermediate is believed to have a polymeric structure w31x. This polymeric reactive intermediate may have a high sticking coefficient and low surface mobility that may explain the nonconformal TiN deposition. Another explanation for the deviation from conformal growth is readsorption of the reaction product, either dimethylamine radical or dimethylamine ŽDMA. ŽHNwCH 3 x 2 ., on the TiN surface w32x. Readsorption of the reaction products could inhibit TiN growth by blocking the reactive surface sites. Read-

sorption effects would be minimized on flat surfaces if carrier gases flow past the surface and remove the reaction products. Product readsorption would be most problematic in low conductance high aspect ratio trench structures. This product readsorption process has been termed ‘byproduct inhibition’ w32,33x. Byproduct inhibition has been used successfully to model nonconformal deposition in trench structures for several systems w32,33x. Either gas phase reactions or product readsorption may explain the nonconformal TiN step coverage observed in trench structures. Forcing the transamination reaction to occur only on the surface may produce good TiN film quality with enhanced conformality w24x. The reaction of surface-adsorbed TDMAT with NH 3 was studied on a wide variety of surfaces at T s 300–500 K w5,24x. However, no transamination was observed under UHV conditions and the surface-adsorbed TDMAT was believed to have insufficient thermal motion to accommodate facile transamination w24x. However, the partial decomposition of TDMAT at higher temperatures or higher pressures may overcome this difficulty. To provide kinetic parameters to assess the product readsorption mechanism, this study examined the adsorption and desorption kinetics of TDMAT and DMA on sputter-deposited TiN surfaces. These experiments were conducted using laser induced thermal desorption ŽLITD. techniques. LITD measurements of TDMAT and DMA uptake onto TiN surfaces at various constant pressures determined the sticking coefficients. LITD measurements of TDMAT and DMA removal during isothermal desorption at various constant temperatures yielded the desorption kinetics. The measured sticking coefficients and desorption kinetics for TDMAT and DMA help to evaluate the product readsorption mechanism as an explanation for nonconformal TiN deposition in high aspect ratio trench structures.

2. Experimental The laser induced thermal desorption ŽLITD. adsorption and desorption measurements were performed in an ultrahigh vacuum ŽUHV. apparatus that has been described in detail in previous publications w34x. Briefly, the UHV chamber is equipped with an

L.A. Okada, S.M. Georger Applied Surface Science 137 (1999) 113–124

Extrel C-50 quadrupole mass spectrometer that was used to detect the desorption products from LITD experiments. The vacuum chamber also contains an electron gun and cylindrical mirror analyzer for Auger electron spectroscopy ŽAES.. AES studies were conducted to check surface cleanliness. The UHV chamber was pumped with a 300 l sy1 ion pump and a titanium sublimation pump with liquid nitrogen cooled cryopanels. Typical operating chamber pressures were ; 2 = 10y9 Torr. The UHV chamber was recently modified to incorporate an internal high pressure cell ŽIHPC.. The IHPC allows high pressure experiments to be performed without compromising the UHV conditions necessary for surface sensitive experiments. The IHPC design was modified from an earlier IHPC design constructed in our group w35x. Compared with our original IHPC design w35x, the IHPC utilized in this experiment was simplified by elimination of the He thermal switch. Rapid surface cooling to temperatures below room temperature was not required for these current high pressure experiments. Consequently, the He thermal switch was replaced with a simple Cu rod. Liquid nitrogen cooled cryotraps were employed to evacuate the IHPC to ; 1 mTorr. After the pressure in the IHPC was reduced, a Balzers 50 l sy1 turbomolecular pump was utilized to further lower the pressure in the IHPC. Once pressures of ; 5 = 10y7 Torr were achieved, the IHPC was opened to the main chamber. The UHV chamber pressure rapidly rose to ; 5 = 10y8 Torr after opening the IHPC, but decreased to ; 5 = 10y9 Torr within a few minutes. The TiN surfaces used in these LITD studies were ; 1 mm thick TiN films that were sputter-deposited onto SiŽ100. wafers in a N2 ambient. Typical TiN film resistivities were ; 200 mV cm. These TiN metallized silicon wafers were then cut into 1.0 = 1.1 cm samples. Holes 0.030 in. in size were drilled in the top of the sample to accommodate a thermocouple junction. A WrRe thermocouple was attached to the sample using a thermally conductive ceramic glue ŽAremco 516.. This sample mounting scheme is similar to our previously described designs for mounting silicon wafers w34x. The cleaning of the sputter-deposited TiN surface was attempted with a wide variety of methods. Upon

115

exposure to air, TiN films typically incorporate C and O impurities w21x. Removal of these C and O impurities was not achieved by a simple anneal to 1000 K in UHV. Earlier studies concluded that N2 sputtering was effective in cleaning TiN single-crystal surfaces w36,37x. Unfortunately, N2 sputtering did not offer significant improvements to the cleanliness of the sputter-deposited TiN surface in our vacuum apparatus. TiN exposure to a N2 plasma and an NH 3 plasma were also attempted and found to be inefficient at removing the C and O surface impurities. The lack of success with annealing, sputtering and plasma cleaning prompted us to try to bury the C and O impurities with a CVD grown TiN film. The thermal decomposition of TDMAT was chosen for TiN deposition. This procedure was expected to decrease O and increase C based on earlier results w17–19,21x. The growth of TiN films was attempted at T s 3508C and P s 0.002 Torr for 100 min. Although a continuous flow was not possible in the IHPC, the TDMAT partial pressure is comparable to previous CVD growth studies w17–19,21x. These ˚ growth conditions should have deposited a ; 200 A thick TiN film w17x. However, the C and O impurity levels measured by AES after this CVD growth showed no significant improvements. These results indicate either that the deposited TiN film was rapidly contaminated or that negligible TiN deposition was obtained using these growth conditions. Since only minimal improvements were observed using the various cleaning procedures, the sputter-deposited TiN films were utilized with finite C and O impurity levels. The TiN surface composition measured by AES was  Ti q N4 Ž387 q 379 eV. s 60 " 2%, O Ž503 eV. s 17 " 2%, C Ž271 eV. s 20 " 2% and Si Ž92 eV. s 3 " 2% using the following AES equation: % X s Ž I xrS x . r  Ž I TiqN rSTiqN . q Ž IC rSC . q Ž IO rSO . q Ž ISirSSi . q . . . 4 where I x is the intensity of the differentiated AES peak and S x is the sensitivity factor. The sensitivity factors are SC s 0.2, SO s 0.51, and SSi s 0.37 w38x. The sensitivity factors of the Ti and N features are an average of S Ti s 0.45 and S N s 0.35, i.e., S TiqN s 0.4. This averaging was necessary because the Ti ŽLMM. and NŽKLL. lines overlap w39,40x. This

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L.A. Okada, S.M. Georger Applied Surface Science 137 (1999) 113–124

freeze, pump, thaw cycles. Anhydrous NH 3 and dimethylamine ŽDMA. were obtained from Matheson and Aldrich, respectively, and used without further purification. Low pressure TDMAT and DMA exposures were performed by backfilling the UHV chamber. Higher pressure TDMAT and DMA exposures utilized the IHPC. A TiN surface temperature of ; 310 K was used during the sample exposures.

Table 1 TDMAT mass cracking pattern

Fig. 1. Mass spectrum of TDMAT showing the molecular parent ion at m r es 224 amu and the electron impact ionization cracking pattern. The intensities were scaled relative to the peak at m r es 44 amu.

equation assumes that the Ti:N ratio is 1:1. The surface compositions measured by AES were constant over the duration of all the experiments. Laser induced thermal desorption ŽLITD. experiments utilized a Q-switched, TEM 00 ruby laser w34x. ˚ the TiN absorbs At a laser wavelength of 6943 A, strongly and has an optical penetration depth of 200 ˚ w41x. The laser pulse widths were ; 100–130 ns A and typical laser energies prior to entering the vacuum chamber were ; 2.9 mJ per pulse. The beam was focused by a 1.0 m focal length lens and impinged on the TiN surface at an angle of 558. This angle of incidence produces an elliptical desorption area on the TiN surface with approximate dimensions of 400 mm = 700 mm. The laser beam was translated on the TiN surface using turning mirrors mounted on piezoelectric-driven translation stages. The desorbing surface species were detected on their initial pass through the ionizer of the Extrel C-50 mass spectrometer. The TDMAT precursor was obtained from Schumacher, in Carlsbad, California. Several freeze, pump, thaw cycles were performed to remove the N2 overpressure and any volatile precursor impurities. No additional purification was performed after the

Mass

Intensity

Mass

Intensity

Mass

Intensity

12 13 14 15 16 18 26 27 28 29 30 31 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 56 58 59 60 61 62 63 64 65 66 67 68 72

1.2 2.9 7.3 37.4 2.7 16.6 2.5 9.3 52.5 5.1 1.1 0.7 2.2 4.4 7.4 10.6 54.2 16.5 100 32.2 7.6 0.8 5.6 3.4 0.7 1.2 0.8 0.9 5.5 1.6 0.9 1.4 4.1 6.8 1.6 0.9 1.2 2.2 0.6 0.7

73 74 75 76 77 78 79 80 81 87 87 89 90 91 92 93 94 101 102 103 104 105 106 107 114 115 116 117 118 119 120 121 122 128 129 130 131 132 133 134

1.8 4.5 10.5 12.2 7.4 3.7 1.8 1.4 0.6 1.9 1.9 10.8 20.7 13 7.7 2.4 0.5 2 5.4 10.6 14.4 6.7 2.8 1.4 0.5 2.2 4.8 14.5 11.9 12.8 4.2 3.6 0.7 0.8 4.1 6 18.8 20.6 19.2 10.8

135 136 137 143 144 145 146 147 148 149 149 158 159 160 161 162 163 164 165 166 167 168 170 171 172 173 174 175 176 177 178 179 180 181 182 183 222 223 224 225 226

5.6 2.5 0.9 2.5 1.4 5.8 6.4 14.4 4.1 2.6 2.6 3.1 1.9 10.4 4.2 25 6.8 3.2 5.2 12.9 1.6 1 1.8 1.4 9.3 5.1 15.2 10.4 21.7 13 22.5 21.4 10.1 15 2.3 1 4.6 7.4 39.7 9.6 3.7

L.A. Okada, S.M. Georger Applied Surface Science 137 (1999) 113–124

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3. Results 3.1. TDMAT cracking pattern and LITD yields Fig. 1 displays the mass spectrum of TDMAT. This cracking pattern was obtained using a VG 7070 EQHF mass spectrometer with an ionizer energy of 70 eV. The spectrum was scanned from 1000 amu to 10 amu. All peaks with intensities0 0.5% relative to mass 44 are displayed in Fig. 1. The molecular parent ion is clearly observed at mre s 224 amu. The 44 amu fragment is associated with NŽCH 3 .q 2 that originates from the ligands on TDMAT. The mre s 44 amu peak may also contain some contributions from impurities resulting from TDMAT synthesis and decomposition products. Decomposition products may result from the special handling requirements necessary to obtain this spectrum which included heat sealing the TDMAT in a

Fig. 3. Mass 44 LITD signals versus time during exposures of Ža. TDMAT and Žb. DMA to the TiN surface at pressures of 2.7= 10y7 Torr and 2.0=10y7 Torr, respectively.

Fig. 2. Mass 44 LITD signals versus time during exposures of Ža. TDMAT and Žb. DMA to the TiN surface at pressures of 4.0= 10y8 Torr and 2.5=10y8 Torr, respectively.

capillary tube. Table 1 lists the masses and corresponding intensities that were 0 0.5% relative to the 100% peak for mre s 44 amu. LITD signals versus mass following saturation TDMAT exposure on the sputter-deposited TiN surface are very similar to the observed TDMAT gas phase mass spectrum displayed in Fig. 1. LITD signals at the various masses below mre s 60 amu are in agreement with Fig. 1. Because the quadrupole mass spectrometer was tuned for the low mass range, the LITD signals at masses above mre s 60 amu are small relative to the noise levels. However, the larger masses in the TDMAT cracking pattern could be observed by the mass spectrometer up to mre s 120 amu by backfilling the UHV chamber with TDMAT. The LITD adsorption and desorption measurements were performed at mre s 44 amu because this LITD signal was the most intense following TDMAT saturation exposure. The main cracking

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L.A. Okada, S.M. Georger Applied Surface Science 137 (1999) 113–124

these adsorption experiments, the TiN surface was initially annealed to 700 K immediately prior to adsorption at 310 K. The sticking coefficients can be obtained from these measurements of TDMAT and DMA uptake. To observe the saturation behavior, TDMAT and DMA adsorption experiments were also conducted at higher pressures. Fig. 3a shows TDMAT adsorption at a TDMAT pressure of 2.7 = 10y7 Torr at 310 K. Fig. 3b displays DMA adsorption at a DMA pressure of 2.0 = 10y7 Torr at 310 K. At these higher pressures, Fig. 3 shows that the adsorption saturates after long exposure times. The determination of sticking coefficients from the adsorption results in Figs. 2 and 3 requires a coverage calibration of the mass 44 LITD signals. To calibrate the LITD signals, the TDMAT and DMA

Fig. 4. Sticking coefficient for Ža. TDMAT and Žb. DMA on TiN surface versus coverage at 310 K. The solid lines show the fits to the measurements.

fragment of DMA is also at mre s 44 amu; consequently, the mre s 44 amu LITD signal may arise from the desorption of either TDMAT or DMA or even NŽCH 3 . 2 radical. The appearance of large mre s 45 amu LITD signals argues against the laser desorption of the NŽCH 3 . 2 radical. The presence of the LITD signals around mre s 57 amu also indicates that DMA is not the only LITD product following TDMAT exposures on the TiN surface. 3.2. Adsorption kinetics The adsorption of TDMAT and DMA on the TiN surface was measured using LITD techniques w42– 44x. Fig. 2a shows TDMAT adsorption measured by mass 44 LITD signals at a TDMAT pressure of 4.0 = 10y8 Torr at 310 K. Fig. 2b displays DMA adsorption measured by mass 44 LITD signals at a DMA pressure of 2.5 = 10y8 Torr at 310 K. For

Fig. 5. Mass 44 LITD signal versus temperature from TiN surface exposed to a saturation coverage of Ža. TDMAT and Žb. DMA. Mass 17 LITD signal versus temperature from TiN surface exposed to Žc. NH 3 .

L.A. Okada, S.M. Georger Applied Surface Science 137 (1999) 113–124

coverages on TiN after the saturation exposures in Fig. 3 were assumed to correspond to approximately one monolayer, i.e., A 0 s 5 = 10 14 molecules cmy2 . The coverage of TDMAT or DMA after a saturation exposure was also represented by a normalized surface coverage of u s 1.0. Using this calibration, the sticking coefficients for TDMAT and DMA on TiN versus coverage could be determined from the adsorption data shown in Figs. 2 and 3. Fig. 4a and b show the sticking coefficients for TDMAT and DMA on TiN versus coverage. The initial sticking coefficient for TDMAT is S0 s 0.23. The initial sticking coefficient for DMA is S0 s 0.70. The fits to the sticking coefficients versus coverage are shown by the solid lines. For TDMAT, the solid line represents SŽ u . s 0.25 expŽy4.7 u .. For DMA, the solid line represents SŽ u . s 0.86 expŽy3.7 u .. The C and O impurities on the TiN surface may act to reduce these sticking coefficients slightly relative to sticking coefficients on a clean TiN surface.

119

Fig. 7. Coverage measured by mass 44 LITD signal versus time during isothermal desorption after saturation TDMAT exposure at 310 K. The solid lines show the fit to the coverage-dependent desorption activation energy.

3.3. Desorption kinetics Fig. 5a and b display the thermal stability of DMA following TDMAT and DMA exposures, respectively. These LITD measurements were performed after saturation exposures. The TDMAT and DMA LITD measurements were conducted at mass 44. Similar thermal stability results were obtained after TDMAT exposures by monitoring the LITD

Fig. 6. Normalized LITD signals from Fig. 5 for mass 44 following TDMAT and DMA exposures and mass 17 after NH 3 exposure.

signal at mre s 57 amu. Fig. 5c shows the thermal stability of NH 3 . The NH 3 LITD measurements were recorded using mass 17. There is remarkable

Fig. 8. Coverage measured by mass 44 LITD signal versus time during isothermal desorption after saturation DMA exposure at 310 K. The solid lines show the fit to the coverage-dependent desorption activation energy.

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L.A. Okada, S.M. Georger Applied Surface Science 137 (1999) 113–124

similarity in the thermal stability of these adsorbed species on TiN. To demonstrate this similarity, the normalized LITD signals for DMA following TDMAT and DMA exposures and NH 3 are shown in Fig. 6. The isothermal desorption of TDMAT or its reaction products were measured by monitoring mass 44 LITD signals versus time at various temperatures after saturation TDMAT exposures at 310 K. LITD results from some representative temperatures are shown in Fig. 7. Similar isothermal desorption results were recorded following TDMAT exposures by monitoring the LITD signal at mre s 57 amu. The isothermal desorption of DMA was also measured by monitoring mass 44 LITD signals versus time at various temperatures after saturation DMA exposures at 310 K. Some representative LITD measurements are displayed in Fig. 8.

3.4. Readsorption DMA is the main product of the thermal decomposition of TDMAT. Adsorption of DMA on the TiN surface may block surface sites and inhibit TDMAT adsorption. DMA adsorption was a noticeable problem after cleaning a TiN surface previously exposed to TDMAT. Fig. 9 shows DMA adsorption and desorption versus surface temperature. During these LITD measurements, the background pressure in the UHV chamber was only P s 7 = 10y9 Torr. As expected from Fig. 5b, the DMA coverage is insignificant in Fig. 9 at TiN surface temperatures ) 450 K. When the temperature decreases below 300–350 K, the DMA coverage increases rapidly because of DMA adsorption. Subsequent increases in the temperature easily remove the DMA coverage. However, Fig. 9 illustrates the persistence of DMA readsorption at low temperatures at fairly low vacuum pressures. Although the DMA residence times on TiN are less at higher temperatures, much larger DMA partial pressures will be encountered during TiN growth with TDMAT.

4. Discussion 4.1. Adsorption kinetics

Fig. 9. Mass 44 amu LITD signal and temperature versus time showing adsorption and desorption of DMA on TiN surface at a low total background pressure in the UHV chamber of 7=10y9 Torr.

Understanding the adsorption and desorption kinetics of TDMAT and DMA on TiN may provide insights about TiN growth using TDMAT thermal decomposition. The sticking coefficients of TDMAT and DMA were determined at 310 K using LITD techniques. Initial sticking coefficients determined from Fig. 2 were S0 s 0.25 for TDMAT and S0 s 0.70 for DMA. Fig. 4 also reveals that the sticking coefficients for both TDMAT and DMA were coverage dependent. Although the trends are similar, the coverage dependence of the TDMAT sticking coefficient is somewhat more severe. The larger sticking coefficient for DMA will affect TiN growth in high aspect ratio structures. Decomposition of TDMAT on the TiN surface produces DMA, surface carbon and DMA radicals. The DMA radicals are believed to have a high sticking coefficient that enables them to readsorb on the TiN surface and inhibit the TiN film growth using

L.A. Okada, S.M. Georger Applied Surface Science 137 (1999) 113–124

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TDMATq NH 3 w32x. The large sticking coefficient for DMA indicates that the reaction inhibition from DMA readsorption must also be considered in models of reaction product inhibition. Because DMA is the main reaction product, DMA readsorption may have a larger effect than DMA radical readsorption. Fig. 4 shows that the measured sticking coefficients for TDMAT and DMA are coverage dependent. A sticking coefficient that decreases as Ž1 y u . is expected from a simple Langmuir model w45x. The sticking coefficients in Fig. 4 decrease more than the simple Ž1 y u . coverage dependence. This behavior suggests additional adsorbate–adsorption interactions w45x.

stabilities are virtually indistinguishable. This behavior also argues that TDMAT decomposition on TiN surfaces produces primarily DMA reaction products. The thermal stability of NH 3 on TiN is shown for comparison in Figs. 5 and 6. NH 3 displays a stability that is in close correspondence to DMA from either TDMAT or DMA exposures. This similarity argues that DMA is chemisorbed to TiN though the nitrogen lone pair electrons. NH 3 and DMA are Lewis bases and both would be expected to bind to the electropositive Ti 3q surface sites by electron donation from their nitrogen lone pair electrons.

4.2. Desorption kinetics

DMA has a higher sticking coefficient on TiN than TDMAT. The coverage-dependent desorption activation energies of DMA following TDMAT and DMA exposures on TiN are also equivalent. These adsorption and desorption results suggest that DMA may act as a reaction product inhibitor during TiN film growth using TDMAT decomposition. This byproduct inhibition may cause nonconformal TiN growth in high aspect ratio structures. A competitive adsorption model can be utilized to estimate the effect of DMA on TDMAT adsorption on TiN. This model assumes that TDMAT and DMA both compete for the same surface sites. The TDMAT and DMA coverages can then be determined assuming that: all sites are equivalent; each site can hold only one species; and there are no interactions between adjacent adsorbates. For simplicity, TDMAT is also assumed to occupy only one site even though TDMAT decomposes to yield DMA. These assumptions lead to the following coverage equations that are identical to the Langmuir adsorption isotherm equations for competitive nondissociative adsorption w45x:

Figs. 7 and 8 display the isothermal desorption of DMA from TiN surfaces exposed to TDMAT and DMA, respectively. The desorption rates do not follow simple first-order desorption kinetics. The coverages versus time could be modeled using a first-order rate expression with a coverage-dependent desorption activation energy, Ed Ž u ., i.e., yd urdt s k d u where k d s nd expwyEd Ž u .rRT x. A typical desorption preexponential of nd s 10 13 sy1 was assumed for both the TDMAT and DMA results. The coverage-dependent desorption activation energy was assumed to have a form Ed Ž u . s E0 y E1 u . For both TDMAT and DMA, E0 s 29.1 kcal moly1 and E1 s 8.2 kcal moly1 . Fits to the desorption data using these parameters are shown as the solid lines in Figs. 7 and 8. The identical coverage-dependent desorption energies following TDMAT and DMA exposures strongly suggest that DMA product removal is the rate-limiting step during TDMAT decomposition. If another process was rate-limiting, the TDMAT and DMA desorption kinetics would not be expected to be similar. The identical desorption activation energies for TDMAT and DMA also argue that DMA is the primary TDMAT reaction product. We have no evidence for DMA radical formation at the temperatures of these isothermal desorption experiments. The identical desorption kinetics for TDMAT and DMA is also consistent with the thermal stabilities of DMA following TDMAT and DMA exposures on TiN shown in Figs. 5 and 6. The DMA thermal

4.3. CompetitiÕe adsorption of TDMAT and DMA

uTs uDs

K T PT 1 q K D PD q K T PT K D PD 1 q K D PD q K T PT

,

Ž 1.

.

Ž 2.

In these equations, TDMAT and DMA are represented by the subscripts ‘T’ and ‘D’, respectively. PT and P D are the TDMAT and DMA pressures, respectively. K T and K D are the equilibrium con-

L.A. Okada, S.M. Georger Applied Surface Science 137 (1999) 113–124

122

stants for TDMAT and DMA where K T s k a,Trk d,T and K D s k a,D rk d,D . k a and k d are the rate constants for adsorption and desorption, respectively. The desorption kinetics for DMA from either TDMAT or DMA exposure on TiN are identical, i.e., k d,T s k d,D . Consequently, the differences between u T and u D will arise from differences between the adsorption rates for TDMAT and DMA. The desorption rate is rd s k d A1 where A1 is the occupied site coverage Žsites cmy2 . and k d is the desorption rate constant Žmolecules sy1 sitey1 .. The adsorption rate is ra s k a PSŽ u .Ž A 0 y A1 . where P is the pressure Žatm., SŽ u . is the coverage-dependent sticking coefficient Žunitless. and A 0 is the total site coverage Žsites cmy2 .. The adsorption rate constant per site Žmolecules sy1 sitey1 atmy1 . is k a s - Õ )r4 gTA 0 where - Õ ) is the average gas velocity Žcm sy1 ., and g is the gas constant Žatm Lrmolecule K.. A 0 is necessary for the product k a P to yield the gas flux per surface site. Fig. 4 shows that the ratio between the sticking coefficients is SŽ u . D rSŽ u . T f 3.0. In addition, TDMAT and DMA have molecular masses of 224.2 and 45.1 amu, respectively. These mass differences lead to a velocity ratio of - Õ D )r-Õ T )s Ž224.2r45.1. f 2.2. As a result, the ratio between the adsorption rates is k a,D rk a,T s w- Õ D ) SŽ u . D P D xrw- Õ T ) SŽ u . T PT x f 6.5P D rPT . The TDMAT and DMA coverages can be calculated at a given temperature and various TDMAT and DMA partial pressures. With the simplification that k d,T s k d,D s k d , Eqs. Ž1. and Ž2. can be rewritten as:

uTs

PT kd

q

k a ,T

uDs

K a ,D

ž / k a ,T

k a ,D

q

K a ,T

ž / k a ,D

Ž 3.

.

Ž 4.

PD q PT

PD kd

,

PT q PD

For a typical growth temperature of 650 K and TDMAT pressure of PT s 200 mTorrs 2.6 = 10y4 atm, u T and u D can be calculated using Eqs. Ž3. and Ž4.. Given the high reactive sticking coefficient for TDMAT on TiN, the pressure of the DMA reaction product, P D , should be comparable to PT , i.e., PD

f PT . In addition, k d s nd expwyEd Ž u .rRT x as described in Section 4.2. For simplicity, k d was evaluated at Ed Ž u . s 29.1 kcal moly1 . Using the growth conditions described above with P T s PD s 2.6 = 10y4 atm, the TDMAT coverage determined from Eq. Ž3. is u T s 0.13. Likewise, the DMA coverage derived from Eq. Ž4. is u D s 0.86. Given that the desorption rates are equal, the =6.7 faster adsorption rate for DMA leads to a much higher DMA coverage on TiN during typical growth conditions with P T s PD . At 650 K and P T s 2.6 = 10y4 atm, the TDMAT coverage will be u T s 0.50 at much lower DMA pressures of PD s 3.7 = 10y5 atm. Even lower DMA pressures of PD s 2.7 = 10y6 atm are required for a TDMAT coverage of u T s 0.9 with P T s 2.6 = 10y4 atm. These calculations indicate that DMA is easily readsorbed and competes effectively for reaction sites on the TiN surface. The predicted TDMAT and DMA coverages are consistent with significant DMA reaction product inhibition. Because of lower conductance, the DMA product pressures encountered in a trench structure will be larger than DMA pressures outside the trench. Consequently, TiN growth inhibition by DMA product readsorption will be more pronounced in the high aspect ratio trench structures. 4.4. Surface transamination Numerous studies have reported the growth of high quality, low C containing TiN films using TDMATq NH 3 w5,7,26,31x. Unfortunately, the efficient gas phase transamination reaction that produces the intermediate responsible for the growth also results in TiN films with poor step coverage w5,7,26,31x. Recent studies suggest that the surface controlled transamination of TDMAT may be a viable alternative to produce high quality TiN film growth with enhanced film conformality. The low pressure surface transamination reaction has been studied on a variety of surfaces at moderate temperatures of 300–500 K w5,24x. However, no reaction was observed at these temperatures. Studies of coadsorbed TDMAT and NH 3 at low temperatures also showed no reaction upon surface heating w24x. NH 3 desorption prior to thermal activation of the TDMAT bonds is the most likely explanation for

L.A. Okada, S.M. Georger Applied Surface Science 137 (1999) 113–124

no facile transamination surface reaction w5x. Higher pressures or higher temperatures may be necessary for the surface mediated transamination reaction. We utilized higher pressures of 1 = 10y3 Torr to investigate the surface transamination reaction of TDMAT and NH 3 in an atomic layer processing ŽALP. scheme w46–50x. This method employs selflimiting surface chemistry where each precursor is exposed to the surface in a sequential fashion w46x. Unfortunately, we observed no reaction at 300 K using this approach. However, monitoring the surface controlled reaction was difficult because of DMA readsorption at these low temperatures. The thermal stability curves for DMA following TDMAT and DMA exposures and NH 3 displayed in Fig. 4a,b and c, respectively, suggest that the surface-controlled transamination reaction will be very difficult. The surface stabilities of DMA and NH 3 are very similar. Fig. 5 shows that both species decrease rapidly between 350–450 K. There is no self-limiting surface reaction at 300 K. At higher temperatures where the reaction may be facilitated, the coverages of NH 3 and DMA from either TDMAT or DMA exposures decrease rapidly. Consequently, the NH 3 and DMA species needed for self-limiting surface chemistry may not be stable on TiN at the higher required reaction temperatures.

5. Conclusions Nonconformal deposition of TiN is observed in trench structures using the organometallic precursor tetrakisŽdimethylamino.titanium ŽTDMAT.. To determine whether this nonconformal deposition may result from the readsorption of the dimethylamine ŽDMA. reaction product, the adsorption and desorption kinetics of TDMAT and DMA were examined on a sputter-deposited TiN surface using laser induced thermal desorption ŽLITD. techniques. The initial sticking coefficient determined by LITD measurements of coverage versus time during TDMAT exposure was S0 s 0.23 and the coverage-dependent sticking coefficient was approximated by SŽ u . s 0.25expŽy4.7 u .. The LITD measurements for DMA yielded an initial DMA sticking coefficient of S0 s 0.70 and a coverage-dependent sticking coefficient approximated by SŽ u . s 0.86expŽy3.7 u ..

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The desorption kinetics for DMA determined by isothermal LITD measurements following either TDMAT or DMA exposures to the TiN surface were equivalent. Using a first-order desorption rate expression with a coverage-dependent desorption activation energy, the DMA isothermal desorption measurements following TDMAT and DMA exposures were equivalent and could be fit using Ed Ž u . s E0 y E1Ž u . where E0 s 29.1 kcal moly1 and E1 s 8.1 kcal moly1 . The identical desorption kinetics suggest that DMA desorption is rate-limiting for TDMAT decomposition and indicate that the competitive adsorption of TDMAT and DMA on TiN will be determined by the TDMAT and DMA adsorption kinetics. Using a competitive adsorption model, the TDMAT and DMA coverages on TiN were calculated using the measured adsorption and desorption kinetics and typical reaction conditions. The DMA surface coverage is predicted to be significant and site-blocking by the DMA reaction product can explain nonconformal TiN deposition. Additional simulations are required to test the reaction product inhibition model in high aspect ratio structures. However, this study illustrates that the DMA reaction products may act to block surface sites, prevent TDMAT reactant adsorption and inhibit TiN thin film growth. Reaction product inhibition should be considered when evaluating molecular precursors as candidates for thin film deposition.

Acknowledgements This work was supported by the Air Force Office of Scientific Research and the Colorado Advanced Materials Institute ŽCAMI.. Equipment used in this research was provided by the Office of Naval Research. The authors thank Dr. Ofer Sneh of Lucent Technologies–Bell Laboratories, Optoelectronics Center in Breinigsville, PA for providing the sputter-deposited TiN wafers. The authors also thank Dr. Robert Barkley in the Department of Chemistry and Biochemistry at the University of Colorado for measuring the gas phase TDMAT mass spectrum and Dr. Shawn Riahi of Schumacher, in Carlsbad, CA for preparation of the TDMAT samples for mass spectrometric analysis.

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