Cu metallizations

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applied surface science ELSEVIER

Applied Surface Science 91 (1995) 277-284

LPCVD Re xSiyN z diffusion barriers in S i / S i O 2 / C u metallizations A.-M. Dutron a,,, E. Blanquet a, C. Bernard a A. Bachli b, R. Madar c a INPG, ENSEEG, LTPCM, BP 75, 38402 Saint-Martin-d'H~res, France b California Institute of Technology, Pasadena, CA 91125, USA c 1NPG, ENSPG, LMGP, BP 46, 38402 Saint-Martin-d'l~res, France

Received 20 March 1995; accepted for publication 12 April 1995

Abstract

Low pressure chemical vapor deposition RexSiyN z thin films are investigated as diffusion barriers between Cu overlayers and oxidized silicon substrates. Gaseous precursors are silane, in situ fabricated rhenium chloride, ammonia, hydrogen and argon. Thermodynamic simulation of the R e - S i - N system is combined to the experimental study. The as-deposited RexSiyN z films are found to be amorphous or nanocrystalline. The (Si)/SiO2/RexSi vNz(200 nm)/Cu(100 nm) metallizations are tested up to 1273 K by a 1 min RTP annealing in vacuum. The barrier performance is characterized with SEM, RBS and AES, for different R e x S i y N z film composition and annealing temperature. R e x S i y N z films properties are compared with M e - S i - N (where Me = Ta, W, Mo, Ti) films obtained by physical deposition methods.

1. I n t r o d u c t i o n

The decreasing dimensions of electronic devices clearly intensify the need for thermal stability of adjoining layers. Aluminum is the metal of choice for metallization in silicon integrated circuits in LSI (large scale integration) because of its low contact resistance and ease o f processing. But in sub 0.25 /xm geometries, aluminum suffers from junction spiking and electromigration. That is the reason why copper could be an attractive alternative owing to its lower resistivity and higher electromigration resistance [1]. However one major drawback o f copper in metallization overlayers is its rapid diffusion into silicon or SiO 2 at low temperature [2,3]. One way to * Corresponding author.

improve the stability of conventional metallizations is to interpose a diffusion barrier between them. In this case, the requirements on this layer's properties are that it yields acceptable contact resistance, and is stable on temperature treatments. Polycrystalline barrriers such as Ta [4-7], W [8,9], TiW [10], Cr [11], TiN [12-14], TaN [5], W N [15,16], have been studied for use in copper metallizations. Because these barriers contain grain boundaries (which may act as fast diffusion paths for metals) and because their properties (such as resistiw ity) depend on impurity content, for example, TiN is "stuffed" with oxygen which blocks the grain boundaries diffusion paths [17], they provide inadequate protection and show modest performances [18]. That is the reason why more recently another class of barriers is investigated: " a m o r p h o u s " barriers.

0169-4332//95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(95)00131-X

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278

Nicolet and coworkers showed that reactive deposition N 2 or NH 3 by sputtering could produce materials such as W - N [19-21], T a - S i - N [6,22,23], W - S i - N and M o - S i - N [24] with a structure that appeared to be amorphous by XRD analysis. In this paper we survey ternary R e - S i - N alloys, produced by LPCVD. A combined thermodynamic and experimental study of LPCVD RexSiyN z films is presented, based on thermochemical modelling of the CVD process [25]. The used thermodynamic approach and analyses of films morphologies will be presented thoroughly elsewhere [26]. The gaseous precursors were: in situ fabricated rhenium chloride, ammonia and silane diluted in hydrogen and argon. In situ metal chlorination was used to fabricate the rhenium precursor as already reported for tungsten, tantalum and iron [27-29]. The experimental set up was designed according to an a priori thermodynamic calculation of the R e - S i N ternary phase diagram. A 100 nm thick copper overlayer was sputtered on the R e - S i - N barrier film and the /Si02/RexSiyNz(200 nm)/Cu(100 rim) systems were tested up to 1273 K for 1 rain by RTA (rapid thermal annealing) in vacuum (5 X 10 -4 Torr = 6.65 X 10 -2 Pa). As-deposited and annealed RexSiyN z films properties and ( S i > / S i O 2 / RexSiyNz/Cu performances were evaluated by XRD, RBS, NRA, SEM and resistivity measurements. Finally, LPCVD RexSiyN z layers perform-

ances as a diffusion barrier are compared with M e Si-N (Ta, W, Mo, Ti) obtained by physical methods

2. Thermodynamic simulation A thermodynamic simulation was performed with MELANGE software [30] to provide the ternary phase diagram, optimum experimental conditions for chlorination and deposition [31]. The thermodynamic simulation is based on the minimization of the Gibbs free energy of the total R e - S i - H - C 1 - N - A r system. No ternary phase has been reported in the literature, therefore, we firstly assumed that no ternary phase does exist. Similarly, we did not consider a thermodynamic description of any amorphous phase, even though an amorphous film could be expected to be deposited. We assumed that the amorphous material can be simulated as a mixture of crystalline compounds, for the same ternary composition. The experimental chlorination temperature, the deposition temperature and the total pressure were fixed to 823 K, 1073 K and 1.197 × 103 Pa (9 Torr), respectively. The gaseous precursor ReC15(g) is expected to be fabricated and transported [32]. The four experimental conditions labeled A, B, C, D, and the corresponding predicted phases as represented in the ternary phase diagram R e - S i - N of Fig. 1 are listed in Table 1.

Table 1 The selected deposition conditions and corresponding calculated phases Conditions Gases flow rates (sccm/min) Ar Sill, NH 3 Corresponding calculated phases

A

B

C

D

340 550 50

240 650 10

243 650 7

245 650 5

Re + Si3N 4 + IN2 ]

Re + ResSi ~ + Si3N 4

ResSi 3 7- Si3N 4

ResSi 3 + Si3N 4 + ReSi 2

Constant: Cl 2 ~ 5 Sccm/min; H 2 = 90 s c c m / m i n . Chlorination temperature: 823K. Depositiontemperature: 1073 K.

A.-M. Dutron et aL/Applied Surface Science 91 (1995) 277-284 Re+Re5Si3+Si3N4

Rf

ResSi3 ~ ResSi3+ReSiz+Si3NNN~/ N4 ~ "~ ~_

279

nm)/Cu(100 nm) was annealed in vacuum for 1 min between 873 and 1273 K by RTA in vacuum (6.65 X 10 -z Pa).

Re+Si3N4+[N 2] 4. Results and discussion 4.1. RexSiyN z films

3. Experimental procedure

Mirror-like RexSi,,N z films were deposited on a SiO2/Si substrate and were observed in SEM. Cross-section micrographs indicated fine morphology, plane interface SiO2/barrier film, and film thickness of about 2000 A for a deposition time of 7 min. On the surface, no grain boundaries were really noticeable, even at high magnification. Annealing

Ternary R e - S i - N films were deposited in a vertical cold wall low pressure reactor which has been described in detail elsewhere [33]. The rhenium chloride gaseous precursor was processed by in situ chlorination in the top section of the reactor. Rhenium pellets were set up in a quartz tube and heated by a lamp furnace at 923 K, whereas chlorine passes through and forms rhenium chloride ReC15(g). Prior to deposition, the chlorination chamber was regenerated by heating rhenium pellets at 923 K in a hydrogen reducing atmosphere during 30 min. This procedure was carried out to remove some metal oxides, oxychlorides which may be present on the surface of the rhenium pellets. Films were deposited on thermally oxidized (100> oriented silicon (100 nm S i t z thickness). For each deposition, the substrate was cleaned with dilute HC1, rinsed with ultra-pure water and dried with inert gas. Then the substrate was loaded in the reactor and set up on a graphite susceptor heated by RF induction. Deposition process was carried out at 1073 K under a total pressure of 1.197 X 103 Pa (9 Torr) for a deposition time of 7 min. According to the thermodynamic calculation, the different gas flow rates were fixed, and only ammonia flow rate was varied from 5 to 50 c c / m i n maintaining a total flow rate of 1 f / m i n . All gas flows, chlorination and deposition temperatures are reported in Table 1. After the deposition of amorphous RexSiyN z, a 100 nm thick copper layer was RF sputtered. The resulting s t r u c t u r e (Si)/SiO2/RexSiyNz(200

Fig. 2. (a) Surface scanningelectron micrographof a ReSiN film that was elaborated (condition B) at 1073 K after annealing at 1273 K. (b) Surface micrographof a ReSiN film elaborated in A condition after annealing at 1273 K.

Si

Si3N4

I/2N

2

Fig. I. Ternary phase diagram Re-Si-N calculated at 1273K with the indicated calculated experimental conditions A, B, C, D.

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leads to grain growth and at quite high magnification ( X 4 0 0 0 0 ) some grains can be distinguished (Fig. 2a). Some observations have been made for the B, C, D conditions. Only for the A condition, some holes appeared on the surface after annealing at 1073 K indicating that some nitrogen may have been liberated (Fig. 2b). X-ray diffraction on as-deposited layers and after annealing from 873 to 1173 K, indicated a nanocrystalline or amorphous structure whatever experimental deposition conditions (Fig. 3a). No defined diffraction peak was observed. After annealing at 1273 K, for each experimental point B, C, D, the film has now crystallized: three diffraction peaks could be attributed to Re and in some cases, two peaks to rhenium oxide ReO3(s) (Fig. 3b). The oxygen is likely to originate from annealing and crystallization. The composition has been confirmed by RBS analysis which gives an average composition of

Re

Si

SiaN 4

1/2N

2

O ¢ A O A' B' C' D' : experimental compositions ea A" : composition after annealing of dot A' • * • • A B C D : simulated experimental conditions

Fig. 4. Plotted experimental compositionsas determined by RBS in the ternary phase diagram.

Re0.26Si0.34N0.40 corresponding probably to a mixa

5 t~

b

O v

s

O

20

30

40

50

60

70

80

2 Them Fig. 3. X-ray diffraction pattern of ReSiN film (C condition) (a) a~s-depositedand (b) annealing at 1273 K, some peaks corresponding to Re are indicated by vertical bars (ASTM 5-702).

ture of about 1 / 4 Re + 3 / 4 amorphous "Si3N4" with an excess of Si for each of the B, C, D conditions. Oxygen and chlorine impurity content are each about 3%-5%. Oxygen contamination was also found in the case of low pressure chemical vapor deposition of ReSi2 [32]. RBS spectrum on the layer elaborated in the A condition (dot A~ on Fig. 4) can be analyzed as resulting from a mixture of Re and SiaN 4 with an excess of N 2. After annealing at 1073 K, RBS analysis showed a decrease of the nitrogen content and gave a similar composition (dot ~ ' ) to the one obtained for the three other conditions (dot B', C', D'). This could be due to the fact that the film was too rich in gaseous nitrogen which was liberated during the annealing in vacuum making some holes as observed by SEM until the film reached its final composition On a straight line starting from the N 2 angle of the ternary phase diagram. RBS analysis showed that all the as-deposited layers have nearly the same composition, located in an "amorphous zone" inside the [Re + ResSi 3 -I- Si3N 4 ] domain near the Re + Si3N 4 equilibrium tie-line (Fig. 4). The obtained composition really differs from the compositions predicted by thermodynamic simulation.

A.-M. Dutron et al. /Applied Surface Science 91 (1995) 277-284

Fig. 5. Cross-section of a R e - S i - N film deposited over a patterned SiO 2 substrate.

281

system and those annealed at 873 and 1073 K spectra are quite similar and revealed only two Cu peaks (Fig. 7a and Fig. 7b). The spectrum obtained after annealing at 1273 K showed Re peaks (Fig. 7c), and looks like those taken o n Reo.26Sio.34No.40 films after crystallization. RBS analysis revealed that for the as-deposited sample according to the shape of the Cu and Re peaks, no diffusion has occurred. After 873 and 1073 K annealing, the RBS spectra are difficult to interpret since the Cu layer is inhomogeneous in thickness, and not continuous as represented in the SEM pictures. For this last sample, the Cu islands were etched with HNO 3 to allow an Auger electron spectroscopy analysis after a sputtering time of 5 min just on the barrier layer to check if Cu has diffused into

These discrepancies can be explained by several factors: • the formation of nanocrystalline or amorphous material while all the data were determined for crystallized materials; • the equilibrium is not reached; • the uncertainties on some thermodynamic data. Some R e - S i - N films were deposited on patterned substrates and were observed by SEM. On Fig. 5, a cross-section micrograph indicates a very good step coverage for the R e - S i - N layer on SiO 2 steps and confirms that CVD process is very attractive for submicronic devices. 4.2. RexSiyN z as diffusion barrier after metallization Re0.26Si0.34No.40 films were covered by 100 nm thick copper film deposited by sputtering and were tested in temperature under vacuum. After 873 K, and" more after 1073 K annealing of the Si/SiO2/Reo.26Sio.34No.4o//Cu systems, the barrier surface is no longer covered continuously by Cu as confirmed by SEM pictures (Fig. 6a and Fig. 6b). After 1073 K annealing, we observed the formation of Cu islands on the surface. The annealing at 1273 K leads to the evaporation of Cu and the copper islands disappeared. XRD spectra of S i / / S i O 2 / / R e o . 2 6 S i o . 3 4 N o . 4 o / / C u were taken on the as-deposited system and after annealing at 873, 1073 and 1273 K. The as-deposited

Fig. 6. (a) Surface, and (b) cross-section scanning electron micrographs of Si/SiO 2 / R e S i N / C u systems after annealing at (a) 873 K, and (b) 1073 K.

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A.-M. Dutron et al. /Applied Surface Science 91 (1995) 277-284

the layer; But as shown in Fig. 8a and Fig. 8b, no Cu seemed to be present in the film. Finally, in the sample annealed at 1273 K, Cu is not detected by RBS confirming that all the copper is evaporated and that no Cu has diffused into the barrier or substrate.

=. 5. Comparison to physically deposited M e - S i - N layers Amorphous diffusion barriers M e - S i - N (Me = Ta, W, Mo) have been elaborated by reactive sputtering in NH 3 first by Nicolet et al. in the 1990's. More recently, Iijima and co-workers deposited by PVD

b Z

a 0

190

380 570 Energy (eV)

760

950

Fig. 8. Auger electron spectra on Si/ReSiN/Cu after annealing at 1073 K, (a) on the layer, and (b) after etching and erosion. /

°~

similar T i - S i - N material [34]. I f we consider that the " a m o r p h i z a f i o n " phenomenon in these M e - S i N systems results from the difference between M e - N or S i - N bonds then different situations m a y occur since for instance T a - N in its crystalline form presents stoichiometric compounds ( f l - T a 2 N , e-TaN, 6-TAN) [35], whereas no R e - N compound is reported. The resistivity o f the as-deposited Re0.26 Si0.34 N0.no films was measured by the four point probes technique at room temperature. The average value is obtained of p = 15 × 10 3 / x ~ - cm which is higher than for sputtered M e - S i - N films as given in Table 2.

b

O~ ca

~

C

1 20 '

30

40

*peakduetothesubs~rate **non-identifiedpeak

50

60 2 Theta

70

80

Fig. 7. XRD diffraction pattern of Si/SiO 2/ReSiN/Cu systems (a) as-deposited, and after annealing at different temperatures: (b) 1073 K, and (c) 1273 K.

Table 2 Resistivity values for different Me-Si-N films Me-Si-N Resistivity p (/xfl. cm) Reo.26Sio.34No.4o[This work]

W0,24Si0.38N0.38[24] Ta0.36Si0.]4N0.50 [6] TilSi0.6Nl.6 [25] TiN

15 X 103 1800 625 500 50-100

A.-M. Dutron et al. /Applied Surface Science 91 (1995) 277-284

283

Table 3 Properties of the reported best barrier performances Me-Si-N Tao,36Sio.laNo.so [6,20-24]

Wo.24Sic~.38N0.38

Crystallization Crystallizedphases temperature

Barrier stable during annealing(min/°C)

Notes

Ta2N, TasSi 3, Ta4.sSi

30/900 in vacuum(4 × 10-7 Torr) annealingin a classical furnace

900°C

W

30/900 in vacuum(4 × 10-7 Torr) annealingin a classical furnace

Cu inducesTa-Si-N premature crystallizationat 900°C. Once crystallizationhas been achieved, the barrier fails. Formation of Cu3Si compound Cu inducesW-Si-N premature crystallizationat 850°C. No compoundformationwith Cu. The majorityof copper vaporized during vacuumannealingat 950°C leavingbehind Cu droplets.

800°C

Mo3Si and Mo5Si 3

30/800 in vacuum(4 X 10-7 Tort) classical furnace

1100°C

[24]

Mo-Si-N [24]

TiiSi0.rN1.6 [34]

> 600°C

-

Reo.26Sio.34No.4o [This work]

> 900°C

Re

The characteristics and optimal performances of the physically deposited barriers and LPCVD ReSiN are summarized in Table 3. The M e - S i - N films were annealed to determine their crystallization temperature and the Si or S i O 2 / b a r r i e r / C u systems were annealed to test their efficiency as diffusion barriers. Their annealing procedure was usually 30 min in a classical furnace in vacuum. As it is reported these amorphous barriers have many similar properties. The nitrogen concentration is one of the key points: for some presented cases when the layers contain a excess of nitrogen, after annealing in vacuum the N 2 loss induces pinholes in films and accelerates the barrier failure. However, in the reported case of T a - S i - N [6,22-24], no significant N 2 loss is observed and failure occurs with crystallization. These different behaviors could be related to the existence and the formation of stable refractory metal nitride compounds.

30/600 classical furnace 1/1000 RTA in vacuum (5 X 10-4 Torr)

No premature crystallizationwith Cu. The barrier does not react with Cu at 850°C Barrier fails in the junction after 30 min annealingin N2 : H2 = 0.75 : 1 No premature crystallizationwith Cu. Higher N content layers liberate nitrogen. No compoundformation with Cu after annealingup to at least 900°(2,

6. Conclusion We have investigated R e - S i - N films as a diffusion barrier for Cu metallization. Experimental study of ReSiN films elaborated by LPCVD, combined with an a priori thermodynamic calculation, was carried out. Reo.26Sio.34No.40 composition was obtained under all experimental conditions. It was found to be amorphous or nanocrystalline as-deposited and after annealing up to 1173 K. The crystallization occurs around 1273 K with the appearance of metallic Re and probably silicon nitride. Si/SiO2/Re0.26Si0.34N0.40/Cu systems seemed to be stable until at least 1073 K. Moreover these ternary films can be deposited on patterned substrates with very good step coverage. Compared with reactive sputtered M e - S i - N films (Me -- Ta, W, Mo, Ti), R e - S i - N behaves quite similarly to W - S i - N and appears to be Very stable with temperature. Although its resistivity is quite high

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A.-M. Dutron et al. / Applied Surface Science 91 (1995) 277-284

c o m p a r e d to T i - S i - N , these results showed that LPCVD Re-Si-N c o u l d b e a p r o m i s i n g b a r r i e r as thin layers ( 3 0 n m ) . H o w e v e r , a b e t t e r c o m p r o m i s e h a s to b e f o u n d b e t w e e n its b a r r i e r p e r f o r m a n c e , thickness and resistivity. The present high resistivity ( 1 5 0 0 /x12. c m ) p r e c l u d e s t h e a p p l i c a t i o n as b a r r i e r layer.

Acknowledgements T h e a u t h o r s w i s h to t h a n k V. G h e t t a , J. G a r d e n , J.C. O b e r l i n a n d J. T o r t e s for t h e i r h e l p in f i l m s a n a l y s e s a n d fruitful d i s c u s s i o n s .

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