A Turgambaeva

N Prud'homme

V Krisyuk

C Vahlas

Decomposition Schemes of Copper(I) N N ′ -Diisopropylacetamidinate During Chemical Vapor Deposition of Copper

Keywords: Copper(I) Amidinate, Mass Spectrometry, MOCVD, Thermal Decomposition Mechanism, Gas Phase Composition, Copper Film

Copper(I) N N ′ -diisopropylacetamidinate [Cu(amd)] 2 (amd = CH(CH 3 2 NC(CH 3 NCH(CH 3 2 , an oxygen and halogen-free compound, was previously tested as precursor for pure copper CVD and ALD films. The present work deals with the investigation of the composition and of the reactivity of the gas phase during the CVD process. The work was performed by mass spectrometry as a function of temperature in two different, though complementary environments: (A) in a miniature, low pressure hot wall CVD reactor, (B) in a cold wall reactor operating at subatmospheric pressure. (A) revealed that the onset of thermal decomposition is 140 C and 130 C in vacuum and in the presence of hydrogen, respectively; maximal decomposition degree is reached at temperature higher than 200 C. The protonated ligand H(amd) is the main gaseous decomposition by-product; propene CH 2 CHCH 3 , acetonitryle CH 3 C≡N and iminopropane CH 3 C(CH 3 NH are also observed in vacuum. Heterogeneous decomposition mechanism both in vacuum and hydrogen presence is discussed.

INTRODUCTION

Cu-containing intermetallic alloy films present attractive properties for manufacturing interconnects in microelectronic circuits, 1 for low friction and corrosion and wear resistant coatings, or in catalysis. [2][3] Cu alloys can be approximants of ternary Al-Cu-TM (transition metal) quasicrystalls. 4 Thermal behaviour in the gas phase of metal amidinates and guanidinates are carefully investigated in view of the potential use of these oxygen free compounds for metallization by ALD and CVD. [5][6] A range of new copper(I) quanidinates and amidinates has been synthesized and the condensed and gaseous decomposition products of some of them were characterized. [7][8] Investigation of such thermal decomposition products and of the corresponding mechanisms provide insight into the reactivity and thermal stability and thus contributes to the optimization of the deposition process and to the design of novel, improved precursors. Possible decomposition mechanisms of single molecules were also theoretically investigated by * Author to whom correspondence should be addressed. using density functional theory (DFT) calculations. It was proposed that thermolysis of copper(I) guanidinates proceeds through a -hydrogen elimination route, 9-10 whereas decomposition of copper(I) amidinates should proceed through another mechanism which still remains unclear. 11 The aim of the present work is to investigate the composition and the reactivity of the gas phase during CVD of copper from copper(I) N N ′ -diisopropylacetamidinate [Cu(amd)] 2 (amd = CH(CH 3 2 NC(CH 3 NCH(CH 3 2 . This compound was identified in previous reports as a promising CVD and ALD precursor. [5][6] The work was performed by mass spectrometry as a function of temperature in two different, though complementary environments: in a miniature, low pressure hot wall CVD reactor directly linked with a time-of-flight mass spectrometer (TOF MS) A, and in a cold wall reactor operating at subatmospheric pressure, connected with a quadrupole mass spectrometer via a capillary tube (B).

In case (A) mass spectrometry allowed in situ, i.e., without any sampling device, tracking the changes of gas phase composition during the programmed heating of the precursor. The obtained information concerned the temperature range of, and the composition of gas phase during precursor vaporization. Also, the temperature ranges of the onset and of the maximum rate of precursor decomposition, the nature of gaseous products during precursor thermolysis and the influence on these parameters of the addition in the input gas of a reactive gas (hydrogen). The feature of this configuration is that it provides information in conditions which are offset those prevailing in a CVD reactor, namely pressure is below 10 -5 Torr.

A simple sampling system was used in case (B): sampling from the deposition zone was realized without using any skimmer to separate in the mass spectrometer the signals from the precursor and from the carrier/dilution gas. It allowed obtaining reproducible results relatively rapidly and contributes to the determination of a precursor decomposition mechanism. In a complementary way, the use of setup (B) allows tracking the evolution of characteristic peaks in mass spectra before, during and after the CVD process. Information obtained in case (B) can be linked with the characteristics of the deposited films. This configuration provides global information which does not allow straightforward access to the elementary mechanisms which control the deposition process. Combination of the two techniques provides compatible and complementary results. This approach has been successfully illustrated in the case of MOCVD of Cu from CpCuPEt 3 . 12 13

EXPERIMENTAL DETAILS

[Cu(amd)] 2 was provided by NanoMePS (France) and was purified by vacuum sublimation at 100 C before use.

Experimental setup (A): The CVD system consisting of the evaporator and the temperature-controlled cell; i.e., the <<reactor>> serves as the input system built into TOF MS. Ionization of the gas phase was performed by electrons with energy ca. 70 eV. Reaction mixture entered directly into the TOF MS ion source through the 0.2 mm effusive orifice of the reactor. The system is detailed in Ref. [14]. with only minor modifications with respect to the adopted configuration. Procedure was the following. During the experiment, the evaporation temperature of the precursor was maintained unchanged at 95 C, while the "reactor" was heated from 105 C to 300 C with the heating rate 5 deg/min. Full range mass spectra were recorded each 10 deg during the reactor heating; experiments were performed with and without hydrogen in the input gas.

Experimental setup (B): A stagnant flow, vertical, tubular, warm walls CVD reactor was used, described in details in Ref. [15]. During deposition experiments, the gas phase containing precursor vapors, hydrogen as a gas reactant, nitrogen as a carrier gas and decomposition by-products was continuously sampled by using a heated glass lined capillary tubing of internal diameter 0.8 mm. One end of the capillary was positioned at 5 mm above the substrates. The other end was connected with a microvalve which allowed regulation of the gas flow. Analysis was performed by quadrupole mass spectrometer with ionization energy EI, ca. 70 eV. The precursor vapours were delivered to the reactor chamber using a stainless-steel container maintained at 95 C. The deposition pressure was 10 Torr and the temperature of the substrates was 280 C. Temperatures at inlet and reactor walls were maintained at 110 C and 95 C respectively, during the deposition experiments. The total flow rate of the input gas was 325 standard cubic centimeters per minute (sccm) and was composed of 50 sccm of nitrogen as carrier gas though the precursor container, 175 sccm of nitrogen as dilution gas and 100 sccm of hydrogen as reducing gas. Deposition was run from 240 to 25 min, depending on the operating conditions. The copper films were deposited on polished steel substrates, (111) silicon wafers and thermally oxidized silicon substrates (140 nm SiO 2 .

In this paper we report only results on gas phase monitoring in the real CVD reactor. Detailed description of CVD experiments and copper films properties have already been published. 6

RESULTS AND DISCUSSION

Mass Spectrum and Vaporization Stability

In mass spectrum of the precursor peak with the highest mass-to-charge ratio (m/z) corresponds to [ 63 Cu 2 (amd) 2 ] + at m/z 408, indicating that the compound is dimeric in the gas phase as it is in the solid state. 16 This peak was selected for monitoring of the precursor behaviour. Other copper-containing peaks with relative intensity above 5% with regard to the main peak are During thermal cycling of the evaporator up to 100 C no changes of the mass spectrum of the precursor were observed, illustrating its good vaporisation stability of [Cu(amd)] 2 .

Thermal Decomposition Pathways

The investigation of the evolution of the intensity of the ion peaks in a mass spectrum as a function of temperature provides useful information on the nature of the gaseous products resulting from the thermal decomposition of the precursor on the surface. It is recalled that the intensity of an ion peak is proportional to the concentration; i.e., the partial pressure of the corresponding species in the gas phase. Hence, upon reaching the decomposition onset temperature the intensity of the ion peak corresponding to the precursor decreases while those of ion peaks corresponding to the products increase.

Figures 1 and2 present the evolution, in vacuum and in the presence of hydrogen, respectively, of the intensity of various ion peaks, derived from full mass range spectra, as a function of the "reactor" temperature in setup (A). Low pressure conditions (10 -5 Torr) in the experimental setup (A) were favourable to heterogeneous precursor decomposition without any reactions in the gas phase. Decomposition of [Cu(amd)] 2 starts at 140 ± 10 C in vacuum and at 130 ± 10 C in the presence of hydrogen. Above 200 C the intensity of Cu-containing peaks is close to background value indicating that there is no intact precursor molecules in the gas phase. In both cases, the intensity of the peak corresponding to the protonated ligand H(amd) increase with increasing temperature and for this reason this species is considered as a gaseous product of thermal decomposition. Ion peaks at m/z 42, 41, 57 were attributed to propene CH 2 CHCH 3 , to acetonitryle CH 3 C≡N and to iminopropane CH 3 C(CH 3 NH, respectively. In vacuum (Fig. 1), these species behave similarly to H(amd) and they are considered as gaseous products as well. In the presence of hydrogen (Fig. 2) such a growth of the intensity of these three peaks is not observed indicating that they are not related to gaseous products of the decomposition of [Cu(amd)] 2 . It is concluded that decomposition of [Cu(amd)] 2 in vacuum and in the presence of hydrogen proceeds by two different ways. Based on the above information, the following pathways were proposed for the thermal decomposition in vacuum of the adsorbed [Cu(amd)] 2 on the growing surface. In a first step the protonated ligand H(amd) is desorbed from, and a copper atom is formed on the surface, due to the migration of a proton from the neighbour ligand and to the cleavage of copper-ligand bond, as is illustrated in reaction ( 1

+ CHCN + CH 2 = CHCH 3 gas (2)
This decomposition scheme is based on an intramolecular mechanism of protonated ligand formation, which is typical for the decomposition of other metallorganic compounds e.g., for metal -diketonates. [17][18] In the presence of hydrogen, formation of H(amd) occurs mainly due to the reduction of copper(I) complex by hydrogen according to reaction (3): Cu amd 2 + H 2 → 2Cu surf + 2H amd gas (3) Suggested decomposition mechanisms presume that deposition in both cases proceeds in a way which is favourable for the formation of pure Cu films. This was confirmed for films processed in the cold wall CVD reactor as is detailed in Ref. [6]. Investigation of pure ligand decomposition at the same conditions would be desirable to complete the study like we usually do 17 but it is unstable under storage and can not be separated easily. Also DFT calculations could provide some support for our interpretations. The authors have planned investigations with the aim to make progress towards these directions.

GAS PHASE MONITORING DURING CVD EXPERIMENTS

Experimental setup (B) allowed monitoring the gas phase during deposition in the CVD reactor operating in technologically compatible conditions. Figure 3 presents results at one of the typical CVD experiment with monitoring the gas phase. It shows the time dependence of the intensity main peaks which are representative of the behaviour of the gas phase. It is observed that, after the initiation of precursor feeding the concentration of the protonated ligand is considerably increased in comparison with a background value and remains at approximately constant level during 25 min of deposition, before the precursor feeding is shut off. At that point its concentration sharply decreases. Such behavior is typical for a deposition product. It is in agreement with the results obtained in the setup (A) indicating that H(amd) is the main gaseous product of the precursor decomposition in hydrogen. Thus, relatively simple surface reaction (3) occurs under CVD from [Cu(amd)] 2 and hydrogen, assuming pure copper metal as the only solid product that was truly confirmed under investigation of films properties. 6

CONCLUSION

Decomposition ways of copper(I) N N ′ -diisopropylacetamidinate vapor on the hot surface of the growing film were suggested basing on the gas phase composition. Decomposition mechanisms of the precursor and formation of pure copper films were confirmed in real CVD reactor equipped with mass spectrometric system to monitor the gas phase composition over the substrate.

  [ 63 Cu 2 (amd) 2 -CH 3 ] + (m/z 393, most intense), [ 63 Cu(amd) 2 -CH 4 ] + (m/z 329) and [ 63 Cu(amd)-CH 3 ] + (m/z 189).

Fig. 1 .

 1 Fig. 1. Intensities of ion peaks of mass spectrum characterising gas phase versus temperature, during an experiment in vacuum performed in setup (A).

Fig. 2 .

 2 Fig.2. Intensities of ion peaks of mass spectrum versus temperature, characterising gas phase upon experiment in hydrogen presence performed in the setup (A).

Fig. 3 .

 3 Fig. 3. Time dependence of gas phase composition during CVD of Cu. 1-H(amd) (m/z 142), 2-[Cu(amd)] 2 (m/z 408).

  ): Cu amd 2 → Cu amd -H surf +H amd gas +Cu surf (1) The surface intermediate {Cu(amd) -H} which is formed during the previous step is further decomposed forming copper on the surface and releasing propene, acetonitryle and iminopropane in gas phase according to reaction (2): Cu amd -H surf → Cu surf + HN = C CH 3 2

Acknowledgments: This work was supported by the EC under contract MC-IIF-39728 and by the French Agence Nationale de la Recherche (ANR) under contract no. NT05-3_41834.