1.1149/1.2727414, The Electrochemical Society A New Liquid Precursor for Pure Ruthenium Depositions J. Gatineau, C. Dussarrat Air Liquide Laboratories, Wadai 28, Tsukuba city, Ibaraki Prefecture, 3-4247, JAPAN High purity ruthenium films were deposited using a new metal organic precursor, dubbed CHORUS. This liquid precursor is very volatile and is delivered without the addition of solvents to the reaction system. CHORUS enables to deposit ruthenium films from temperature as low as 25 C using hydrogen as a reducing co-reactant in Chemical Vapor Deposition (CVD) mode. This new process is characterized by the high purity of the obtained ruthenium films, the non-oxidation of the sub-layer during the process, the good adherence to many kinds of substrates and the absence of incubation time. Introduction Ruthenium is of potential interest for next generation devices in applications such as metal electrode in FeRAM and DRAM memory devices and seed layers, which acts as a glue layer for copper. Ruthenium has a suitable work function for metal electrode and, compared to other noble metals, it has a high melting point and will thus easily support the stringent conditions encountered during the process, its oxide is conductive, meaning that potential oxygen diffusion from adjacent materials or processes will not affect its properties of the device, and it can be easily etched by dry technique. It can also be used as a seed layer for copper electroplating, if the process allows deposition of ruthenium films without oxidizing the metal nitride barrier sub-layer (usually TaN). Numerous works has been conducted in order to develop appropriate ruthenium precursors, whether for ruthenium and/or ruthenium oxide depositions. A quick look at the existing literature enables us to see that many families of precursors were assessed: β- diketonate, inorganic, carbonyl, olefin, cyclodienyl among the most used types of ligands (1-5). Regardless of the type of family they belong to, many kinds of precursors are reported to give depositions of good quality, but some interface oxidation and adhesion issues, as well as long incubation times are faced in these processes (1). Another main drawback of the ruthenium precursors is their relatively low vapor pressure (.25 Torr at 85 C for Ru(EtCp) 2 ) (6). Moreover, the impurity content, in the film or at its interface with the sub-layer, of carbon and oxygen, coming from the precursor itself and also from the reactant gas O 2, can be relatively high (7, 8). Finally, issues with the poor adhesion and the non-uniformity of the films even on flat surfaces are faced (9). This work aims to present a new precursor that solves many of these problems. It is a highly volatile liquid precursor, named CHORUS, which is very reactive in thermal processing. Ruthenium depositions are obtained without incubation time in Low Pressure CVD (LPCVD) mode using hydrogen as the only co-reactant. This precursor enables us to deposit metallic films with very low oxygen content in the film as well as at the interface with the sub-layer. 33
Experimental For the characterization of the material, we estimated the freezing point of the molecule by soaking a glass ampoule of CHORUS in a bath whose temperature was controlled. We also performed NMR (JEOL, JNR-ECA) and isothermal curve of vaporization analyses, using a TG-DTA apparatus at atmospheric pressure under nitrogen environment (Seiko Instruments). From these data, we concluded that the vapor pressure of the precursor was high enough to allow the delivery of its vapors to the reaction chamber by using the bubbling method. Low concentration hydrogen (less than 2%) was used as a reducing agent. Nitrogen or helium was used as carrier and dilution gas. Films were deposited in a cold-wall type furnace equipped with a 4 inch heating chuck. Downstream to the pump, a butterfly valve enabled to set the pressure in the reaction furnace in the range of.1-1 Torr. The ruthenium films were mainly deposited on silicon oxide wafers. Some tests were also performed onto tantalum nitride substrates. The films were analyzed by an Auger Electron Spectrometer (Perking Elmer, PHI 65), allowing in depth profile monitoring. The thickness of the films was measured by an EDX instrument (EDX-HS, Shimadzu). Results and discussion CHORUS is a yellow liquid which was designed and synthesized by Air Liquide. Its melting point is less than -5 C. An ampoule of the precursor was soaked for a few hours in a low melting point liquid whose temperature was set at -5 C and even though its viscosity appeared higher, it did not solidify. For comparison purposes, the same test was performed with water, which froze in less than three minutes. Isothermal curve of vaporization analyses of CHORUS were performed and precious information were deduced. First of all, when the precursor is heated at a constant temperature of C, as presented in Figure 1, it vaporizes entirely, without leaving residues, at a rate of.14 mg/min. Due to apparatus constraints, the precursor had to be exposed to air before the analysis. The absence of residues indicates that no reactions occur between air and the precursor for a short period of time. Mass loss (%) 9 7 5 3 2 1 5 15 2 25 Time (mn) Figure 1. Isothermal curve of vaporization of CHORUS at C, under nitrogen atmosphere (flow rate of nitrogen: 2 ml/min, initial weight: 31 mg). 34
Ruthenium films were obtained from temperature as low as 25 C in CVD mode using hydrogen as a co-reactant. The inner pressure of the reactor was set at.5 Torr. While the bubbler of CHORUS was set at a temperature of 1 C, a deposition rate of 1.5-2 nm/min was obtained in the transport-limited state, as measured by EDX. The ruthenium films deposited on silicon oxide and tantalum nitride substrates showed good adhesion, as verified by Scotch tape peeling tests. The purity of the obtained films was assessed by Auger spectrometry analyses. As shown in Figure 2, oxygen is not present in the film. Analyses by SIMS are now being performed to verify the results and data will be published in a future paper. During the reaction-rate limited regime, the deposition rate increases with the temperature. We used these data to estimate the energy of activation of the CHORUS process on silicon oxide substrates. The deposition rates expressed in nm/min as a function of the inverse of the temperature are shown in Figure 3. Applying the Arrhenius equation to the trend line leads to an activation energy of.44 ev, which is relatively low for a ruthenium precursor, proving the reactivity of this precursor. As a comparison, the activation energy of Ru(EtCp) 2 on Si substrates in CVD mode is.95 ev, and values as high as 1.4 ev on SiO 2 /Si wafers have been reported (6,8). O Si Ru Atomic concentration (%) 2 25 5 75 125 Assessed thickness (nm) Figure 2. Auger in-depth profile of a ruthenium film obtained from CHORUS (35 C.25 Torr 15 min). Deposition rate (nm/min) 1 1 y = 26643e -5.91x R 2 =.9997 1.65 1.75 1.85 1.95 /T [1/K] Figure 3. Assessment of the activation energy of ruthenium films deposited from CHORUS in CVD mode onto SiO 2 substrates. Ruthenium processes usually face major problems with formation delays. Such phenomenon, referred to as incubation time, can reach tens of minutes in CVD mode or hundreds of cycles in ALD mode (1, 1). Films were deposited in CVD mode with CHORUS on two types of substrates that were selected as representative of the kind of 35
layers on which ruthenium films may be deposited on. For transistor applications, metaloxide materials may be used as gate dielectric and ruthenium may be deposited on their top as a metal gate. Silicon oxide was chosen as a representative of this application. For back-end of line applications, a barrier layer and a seed layer for copper ECD are expected to be used between the low dielectric material and copper. Ruthenium is an attractive material for the seed layer material whereas metal-nitride films will be used as barrier material. A substrate of tantalum nitride was chosen to represent this application. The ruthenium films were deposited at 35 C, the inner pressure in the reaction system being set to.5 Torr. The films were deposited simultaneously on both substrates for experiments performed during 5, 1, 2 and 3 minutes. A complementary test was performed on silicon oxide alone during minutes. As presented in Figure 4, the process exhibits a constant deposition rate, with the trend line crossing the x-axis close to the origin, on both silicon oxide and tantalum nitride substrates. It means that the process using CHORUS as a ruthenium precursor enables to deposit films with negligible incubation time whatever the substrate. Estimated thickness (nm) 2 y = 1.5x + 1.13 R 2 =.998 SiO2 TaN y = 1.52x -.82 R 2 =.997 2 Deposition time (mn) Figure 4. Thickness evolution vs. deposition time on SiO 2 and TaN substrates (CVD 35 C -.5 Torr). Conclusion Ruthenium films were successfully deposited at low pressure in CVD mode with a new metal organic precursor, dubbed CHORUS. This molecule evaporates at C without leaving particles, even after exposure to air. The use of this liquid and volatile precursor, when reacted with hydrogen, enables to obtain highly pure ruthenium films from 25 C in a cold wall type reaction furnace. Deposition rates close to 2 nm/min were obtained while the material was kept at 1 C. The absence of incubation time was confirmed on both silicon oxide and tantalum nitride sub-layers. The good adhesion of the ruthenium films was confirmed by Scotch tape tests. The characteristics of the material as well as the properties of the deposited films mean that CHORUS is a promising MOCVD precursor for ruthenium depositions. References 1. T. Shibutami, K. Kawano, N. Oshima, S. Yokoyama, H. Funakubo, J. Electrochem. Soc., 6, 9 (199). 2. M. L. Green, M. E. Gross, L. E. Papa, K. J. Schnoes, and D. Brasen, J. Electrochem. Soc., 132, 11 (1985). 36
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