Copper–Titanium Thin Film Interaction

Microelectronic Engineering 76 (2004) 153–159 www.elsevier.com/locate/mee

Copper–titanium thin ﬁlm interaction

L. Castoldi a, G. Visalli a, S. Morin a, P. Ferrari a, S. Alberici b, G. Ottaviani c,d,*, F. Corni c,d, R. Tonini c,d, C. Nobili c,d, M. Bersani e

a ST Microelectronis, Cornaredo, Milano, Italy b Central R&D, Physics and Material Characterization Laboratory, ST Microelectronis, Via Olivetti 2, I-20041 Agrate Brianza, Milano, Italy c Dipartimento di Fisica, via Campi 213/a 41100 Modena, Italy d MASEM, Materiali Avanzati per Sistemi, lElettroMeccanici, via Vivaldi 70, 41100 Modena, Italy e ITC-irst, via Sommarive 18, 38050 Povo, Trento, Italy

Available online 12 August 2004

Abstract

Interaction between 5 lm thick copper and 50 nm thin titanium films was investigated as a function of annealing temperature and time using MeV 4He+ Rutherford backscattering, X-ray diffraction and dynamic Secondary Ion Mass Spectrometry. Samples were made by depositing 10 nm of titanium on a PECVD silicon oxynitride, followed by 50 nm of titanium nitride and 50 nm of titanium in the said order. In the same system 100 nm of copper were subsequently sputtered; finally 5 lm of copper were grown by electroplating. This complex structure was chosen in order to investigate the possibility of using copper interconnects also in power devices. To investigate the composition and growth of Ti–Cu compound on the buried interface, it was necessary to develop a special procedure. The results of the investiga- tion show the formation of a laterally non-uniform layer of TiCu4, which is presumably preceded by the formation of CuTi. The growth of the compound is kinetically controlled by means of a diffusion coefficient having 1.7 eV activation energy and a 5 · 102 cm2/s pre-exponential factor. The formation of a titanium–copper compound ensures a reliable and low resistance electrical contact especially at the vias. The Ti/TiN/Ti acts efficiently as a sacrificial and inert diffusion barrier. No copper was detected on the silicon oxynitride surface even after a 20-min 500 C heat treatment. 2004 Published by Elsevier B.V.

PACS: 68.35.Fx; 6630.Xj; 68.55.Nq Keywords: Copper-titanium compounds; Power devices; Electrical contact; Diﬀusion barrier; Interconnect; Vias

1. Introduction

* Corresponding author. Tel.: +39 039 6035168; fax: +39 039 6035530. Copper metallization is considered as a E-mail address: [email protected] substitute for aluminum-based interconnects for (G. Ottaviani). 0.1–0.15 lm advanced devices [1]. For these

0167-9317/$ - see front matter 2004 Published by Elsevier B.V. doi:10.1016/j.mee.2004.07.043 154 L. Castoldi et al. / Microelectronic Engineering 76 (2004) 153–159 applications, a few hundredths nanometer thick and each piece was annealed (at the desired tem- films, generally in contact with low-k oxides, are perature–time couples) in a 106 mbar vacuum considered. To prevent copper diffusion, barriers furnace. are necessary; polycrystalline tantalum and amor- During the heat treatments Ti and Cu react; in phous tantalum nitride films are currently used [2]. order to observe the buried reacted film, the unre- The stability of such barriers against copper diffu- acted copper was selectively removed by a wet sion is very good, since, according to the equilib- chemical etching at room temperature with a mix- rium phase diagrams, no Ta–Cu compounds are ture of inorganic acids (phosphoric acid and acetic expected, and amorphous TaN inhibits grain acid) and hydrogen peroxide, diluted in water boundaries diffusion. (H2O2:H3PO4:CH3COOH:H2O = 1:1:1:20). Due to the advantage offered by copper in terms The surface morphology of the copper film was of resistance, electromigration properties, and unveiled by means of an atomic force microscope integrability in the process, its use has been consid- (AFM) and of a focused ion beam (FIB). Heat ered also for power devices. For these applications, treatments induce grain growth but no correlation mostly due to the necessity to deliver high power, was found with the Ti–Cu interaction process. Nu- the properties required to the interconnection clear techniques, Rutherford backscattering scheme, and as a consequence also the structures spectrometry (RBS) and elastic recoil detection and materials, are different than those necessary (ERD) techniques [3] with a 2–2.5 MeV 4He+ for memory or microprocessor applications. Actu- beam, were used to investigate the Cu–Ti interac- ally, for power devices the metal films are thicker, tions and the hydrogen redistribution. The experi- around a few microns for copper, the insulator is mental apparatus allows to acquire two spectra at generally PECVD deposited silicon oxynitride, two different scattering angles at the same time; and above all, the contact resistance in the vias 160 and 120 for RBS and 15 and 160 for should be as low as possible, as well as reproduci- ERD. In this way a double check can be per- ble. For the last requirement a metallization formed. Titanium and copper interact forming a scheme ensuring reproducibility through metallur- compound, to establish the nature of which RBS gical interaction is desirable. and X-ray diffraction have been used. Secondary The purpose of this paper is to investigate the ion mass spectrometry (SIMS) has been performed properties of a multilayer metallic structure in con- to monitor the diffusion of copper trough the Ti/ tact with PECVD deposited silicon oxynitride film. TiN/Ti layer [4,5]. The estimated sensitivity of The multilayer being considered consists of cop- such technique is 1017 atoms/cm3 of copper in tita- per, titanium and titanium nitride films. nium. The situation in the TiN layer is different, as the copper signal, mostly the 63 amu signal, inter- feres with the TiN signal and the sensitivity is not 2. Experimental better than a few atomic per cent. The results showed that, within the sensitivity of the tech- Samples have been prepared by depositing 1000 nique, there is no copper contamination at the nm of PECVD silicon oxynitride on p-type silicon Ti-oxynitride interface as far as all the annealed wafer, followed by 10 nm of titanium, 50 nm of conditions considered. titanium nitride and 50 nm of titanium. Subse- quently, 100 nm of copper were sputter-deposited, and finally 5 lm of copper were grown by electro- 3. Results plating in the same system. Before each heat treatment the copper films were kept at room Fig. 1 shows RBS spectra made with 2.2 MeV temperature. The resistivity decreased from 2.3 to 4He+ and the detector at the scattering angle of 1.8 lX-cm in 10 hrs, reaching its steady state value. 120 with respect to the incoming ion beam ob- To study in detail the Ti–Cu interaction processes tained from various samples after chemical etching 10 · 10 mm2 pieces where cut from a single wafer of copper. The positions of Ti and Cu on the sur- L. Castoldi et al. / Microelectronic Engineering 76 (2004) 153–159 155

Energy (MeV) Energy (MeV) 1.6 1.7 1.8 0.6 0.8 1.0 1.2 1.4 1.6 35 60

as-deposited 500C 20 min 30 350 C 10 min 400 C 5 min 400 C 5 min 50 25 40 20 30 15

20 NormalizedYield

10 Normalized Yield

5 10 Ti Cu N O Si Ti Cu 0 0 400 420 440 460 480 500 100 150 200 250 300 350 400 450 Channel Channel

Fig. 1. 2.2 MeV 4He+ RBS spectra taken on as-deposited and Fig. 2. 2 MeV 4He+ RBS spectra taken in two samples post-annealing samples. The unreacted copper film has been annealed at two different temperatures, after unreacted copper removed by chemical etching. The spectra show only the copper etched. The signals from Ti and Cu are consistent with a layer and titanium portions; the position of the elements, if on the having a composition of Cu4Ti. surface, is shown. face are indicated. The signal between channels senting measurable thickness of the compound. 400 and 460 is due to Ti atoms, whereas above Fig. 2 shows the spectrum obtained from the sam- channel 460 the signal comes from Cu. The signal ple annealed at 500 C 20 min with 2 MeV 4He+. from silicon oxynitride is not shown. The area un- The spectrum obtained from the sample annealed der the Ti peak is proportional to the total amount at 400 C 5 min is plotted for comparison. The of titanium atoms in the sample for unit area. In position of the various elements, if located on the the as-deposited sample this quantity is 5.3 · 1017 surface, is indicated. On the surface, a slight oxy- atoms/cm2; within a small atomic percentage, the gen contamination of about 2 · 1016 atoms/cm2 is same value was found in all samples, independ- present. The simulation of the spectrum taken at ently of the heat treatments undergone and of 500 C is quite complex due to the multilayer the quantity of compound formed. No copper is structure of the sample and to the fact that the present in the spectrum of the as-deposited sample, edges are not sharp, indicating a laterally non-uni- indicating that chemical etching is extremely effi- form sample, as the AFM pictures have also cient in removing metallic copper. The spectra of shown. Further complication in the analysis is the samples annealed at 350 C for 10 min and due to the presence of hydrogen. at 400 C for 5 min, show 5 · 1015 and 22 · 1015 By combining the results from RBS and ERD copper atoms/cm2 respectively. The amount of spectra (not shown here), the structure of the sam- copper not etched and likely bound to titanium ples, both as-deposited and post 500 C 20 min atoms increases with the annealing temperature annealing, is outlined in Fig. 3. The thickness of and/or time until a saturation value is reached. the various layers was obtained by assuming the This fact, together with the constancy of titanium following densities: 1, 0.57, 0.8 · 1023 atoms/cm3 content, gives the experimental evidence that only for TiN, Ti and TiCu4 respectively. The thickness unreacted copper is removed, and that the selectiv- and composition of the films in the as-deposited ity of chemical etching among copper, titanium sample are those expected from the deposition and copper–titanium compound is rather good. parameters. At 500 C, about 80 nm of TiCu4 is The atomic composition of the Ti–Cu com- formed and 20 nm attributed to TiCu compound. pound formed has been inferred from spectra pre- A small quantity of Cu is present in the TiN layer. 156 L. Castoldi et al. / Microelectronic Engineering 76 (2004) 153–159

as-deposited

>1000 15 40 40 10

Si2.3N2.8O1H0.9 Ti TiN1.1H0.04 Ti1H0.01

TiH0.25 500 oC 20 min

>1000 50 10 45 20 50 30

Ti Cu H Si2.3N2.8O1H0.9 Si2.3N2.8O1H0.5 Ti1.2N1H0.3Cu0.07 Ti1Cu4H0.1 1 4 0.28

TiH0.15 Ti1.H0.2Cu0.8

Fig. 3. Schematic diagram showing the composition of the as-deposited and 500 C 20 min annealed samples. The thickness in the 23 3 ﬁgures is expressed in nm evaluated by assuming as densities 1, 0.57, 0.8 · 10 atoms/cm values for TiN, Ti and TiCu4 materials respectively. The diagrams have been aligned with the oxynitride/titanium interface.

To confirm the presence of TiCu4 compounds, By measuring the quantity of Cu after anneal- X-ray diffraction measurements were performed. ing at different times and temperatures and after Only the spectra of two representative samples chemical etching, it is possible to study the kinetics are shown in Fig. 4. The 350 C annealed sample of the compound formation. Fig. 5 shows the shows, similarly to the as-deposited samples, Ti amount of residual Cu as a function of square root and TiN peaks. The 500 C treatment induces of time after chemical etching. The points follow a the formation of a TiCu4 compound [6]; a peak straight line before saturation occurs, thus indicat- corresponding to the TiCu [7] phase is also pre- ing that the process of growth is controlled by dif- sent. The observation of only one peak is generally fusion. The S slope of the various lines is not sufficient to obtain a certain identification of a proportional to the D diffusion coefficient through compound. However, they supported the presence the relation of TiCu. After 500 C annealing, (002) and (101) S2 ¼ 4D: titanium peaks disappeared, and a narrowing of (111) titanium nitride peaks and an increase in The S slope expressed in copper atoms/cm2 min1/2 intensity of (100) titanium peaks were observed. is converted into a thickness of Cu4Ti, assuming The presence of the (100) titanium peak can be for the compound the density of 7.89 g/cm3. justified by assuming that the (100) peak can be Fig. 6 shows the diffusion coefficient as a func- attributed to the titanium film between the tita- tion of temperature in an Arrhenius plot; the nium nitride and the oxynitride, whereas the points can be fitted by a straight line indicating a (002) and (101) peaks belong to the titanium film process with 1.7 eV activation energy and a between the titanium nitride and copper films. 5 · 102 cm2/s pre-exponential factor. Only the outermost titanium film is converted into copper–titanium compounds. Heat treatments im- prove only the crystal quality of the (100) titanium 4. Discussion and conclusion film. The different texture of the two titanium films is due to the different substrates – oxynitride and The Cu-Ti binary phase diagram [8] implies six titanium nitride – where the films start growing. different compounds, Cu4Ti (orthorhombic) [6], L. Castoldi et al. / Microelectronic Engineering 76 (2004) 153–159 157

350C, 10' 4500 Ti (002)

4000

3500

3000

2500

I (a.u.) 2000 TiN (111) 1500 Ti (101) 1000 Ti (100)

500

0 30 32 34 36 38 40 42 44 46 48 50 (a) 2-theta (degrees)

500C, 20' 5000 Cu4Ti (211) 4500

4000

3500

3000

2500 I (a.u.) 2000 Ti (100) Cu Kb line for 1500 TiN (111) Cu4Ti CuTi (102) 1000

500

0 30 32 34 36 38 40 42 44 46 48 50 Kb (b) 2-theta (degrees)

Fig. 4. X-ray diﬀraction spectra of 350 and 500 C annealed samples. The 350 C data are substantially similar to the as-deposited ones.

Cu2Ti, Cu3Ti2,Cu2Ti3, CuTi (tetragonal) [7] and position is the closest to eutectic composition. CuTi2. At low temperatures, around 600 C, the According to the empirical rules [10] adopted to phase diagram shows a finite solid solubility for predict the first phase formation, CuTi is expected titanium in copper but no solubility for copper in to be the first phase to be formed. We have only a titanium. This means that copper can diffuse weak evidence of CuTi formation. Due to the pres- through titanium only via grain boundaries or ence of an excess of copper with respect to tita- through compound formation. The eutectic points, nium, the compound at equilibrium will be with the lowest temperature occurring with 27 Cu4Ti. Liotard et al. [11] investigated Cu–Ti thin at.% of Ti, close as a composition to Cu3Ti. Cu3Ti film interaction and found that the first phase to has been reported as a metastable phase [9]. CuTi be formed is actually CuTi, followed by the is the congruently melting compound which com- appearance of a second phase which they attribute 158 L. Castoldi et al. / Microelectronic Engineering 76 (2004) 153–159

10 Table 1 o Comparison of the activation energy and pre-exponential factor 500 C o )

2 450 C for the diffusion coefficient published and obtained in this paper 8 2 Source Q (eV) Do (cm /s) 425 oC This study (350–450 C); thin films 1.7 5 · 102 atoms/cm Liotard et al. (350–450 C); thin filmsa 1.82 7.73 · 102 17 6 Gershinskii (400–600 C); thin filmsa 1.78 1.78 · 102 400 oC Schatt et al. (750–850 C); bulka 0.96 7.16 · 106 4 a All these authors refer to a compound having Ti2Cu7 atomic composition. 350 oC 2 ported in literature [11–13]. A reasonable consist- Reacted Copper (10 ency is shown, indicating that, though hydrogen 0 0 5 10 15 20 originally plays a role in the oxynitride film, it does Heat Treatment Time 1/2 (minutes1/2) not significantly affect its kinetics. Ti/TiN/Ti acts as a mixture of sacrificial and in- Fig. 5. Quantity of reacted copper plotted as a function of the square root of the annealing time at various temperatures. The ert barrier against copper diffusion. The time nec- straight lines are fitting curves on the first points and are used to essary to fully transform the outermost titanium evaluate the diffusion coefficient for the growth of the Ti–Cu layer – the sacrificial barrier – is around half an compound. hour at 500 C. Once Ti has been consumed, copper could diffuse through the polycrystalline TiN layer – the inert barrier – through grain boundary Temperature ( oC) diffusion. According to the data reported in litera- 550 500 450 400 350 10-12 ture, this requires much higher temperatures [14] than those considered here. If diffusion occurs, the innermost titanium layer will act at this time

/sec) -13 2 10 as a further sacrificial barrier. SIMS data do not show any copper at the oxynitride interface even after 500 C 20 min annealing. 10-14 In conclusion, we investigated the growth kinetics of TiCu4 compound. The process is diffusion- controlled with a diffusion coefficient of 1.7 eV 10-15 E =1.7eV t activation energy and a 5 · 102 cm2/s pre-expo- -2 2 Diffusion Coefficient (cm Do = 5x10 cm /sec nential factor. The presence of a TiN layer en-

-16 hances the barrier properties of the structure. 10 1.2 1.3 1.4 1.5 1.6 1.7 The hydrogen contained in the oxynitride layer -1 1000/T (K ) does not affect the growth kinetics of the com- Fig. 6. Arrhenius plot of the diffusion coefficient as a function pound. The inner titanium layer does not react with of the inverse of annealing temperature. copper even after a 500 C heat treatment. The formation of Ti–Cu compound ensures an intimate electrical contact between the various metallic films. to the metastable phase Cu3Ti. Their RBS data show a layer with Ti0.88Cu3.2 composition, very close to Cu4Ti. Since they started with thin films having a Ti/Cu composition close to 1/3, it is rea- References sonable to assume that they have a mixture of two [1] R.Rosenberg, D.C.Edelstein, C.-K.Hu, K.P.Rodbell, compounds, Cu4Ti and presumably CuTi. Annu. Rev. Mater. Sci. 30 (2000) 229. In Table 1 the parameters for the growth of [2] A.E.Kaloyeros, E.Eisenbraun, Annu. Rev. Mater. Sci. 30 Cu4Ti compound are compared with the data re- (2000) 363. L. Castoldi et al. / Microelectronic Engineering 76 (2004) 153–159 159

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