Optimizing the Selection and Supply of Hf Precursor Candidates for Gate Oxide

MATERIALS & GASES Optimizing the selection and supply of Hf precursor candidates for gate oxide A. Soulet, L. Duquesne, G. Jursich & R. Inman, American Air Liquide, Chicago research Center, IL, USA, A. Misra, Air Liquide Electronics U.S. LP, Dallas, TX, USA, N. Blasco, C. Lachaud, Y. Marot, R. Prunier & M. Vautier, Air Liquide Centre de Recherches Claude Delorme, Jouy-en-Josas, France, & S. Anderson, P. Clancy & M. Havlicek, Air Liquide – Balazs™ Analytical Services, CA, USA ABSTRACT Selecting a precursor for gate oxide deposition requires extensive characterization of targeted molecules. Critical analytical parameters for MOCVD and ALD precursors, like vapor pressure, thermal stability, and metallic contamination are presented and results discussed herein for the most promising Hf precursors currently under investigation: Tetrakis(ethylme thylamino)hafnium (TEMAH), tetrakis (dimethylamino)hafnium (TDMAH), tetrakis(diethylaminohafnium) (TDEAH), tetrakis(tertbutoxy)hafnium (HTB) and hafnium tetrachloride(HfCl4). In this article, we show how these Figure 1. Hf precursor candidates for MOCVD and ALD. characterizations help in preventing identified in the literature and compared HfCl 4 is the most prominent precursors’ degradation, identifying in terms of deposition results [2–4], there member of the hafnium halide family gas-phase species and defining is surprisingly very little comparative – it is the most commonly used process windows and distribution information on their thermal properties halogenated hafnium precursor. It is a parameters. and purity requirements. In this work, moisture reactive white powder, with these characteristics are assessed for a set a high sublimation point of 320°C of commonly considered Hf precursors. at atmospheric pressure and a vapor Introduction Trace-metal contamination in the pressure of 1 torr at 190°C. HfCl4 Today there is considerable interest in precursor can be best examined by ICP- allows carbon-free deposition, but its the development and optimization MS analysis, provided robust analytical use in deposition processes results in of CVD films particularly in the protocols are followed. Thermal behavior corrosive by-products, such as HCl, search and evaluation of precursors of these metal-organic precursors is best which may affect both the film quality for metal organic chemical vapor assessed by the use of upgraded thermal and CVD chamber integrity, which deposition (MOCVD) and atomic layer gravimetric analysis (TGA) and differential can potentially limit its use. HfCl4 deposition (ALD) of hafnium oxide scanning calorimetry (DSC). The is, however, a strategic compound films. Hafnium based oxides are being upgrades include special inert atmosphere for chemical suppliers, as it is often considered as promising candidates for handling designed for air- and moisture- the starting material to synthesize high dielectric constant gate oxide for sensitive precursors, ability to work other metal-organic precursors. Its the 65nm node and beyond as well as both under vacuum and at atmospheric purity specifications are therefore of for next-generation DRAM capacitors. pressure and coupling with real-time significant importance. For optimal handling, distribution exhaust analysis techniques (FTIR, QMS) Commonly available hafnium and processing, the most important for gas-phase stability studies. At typical complexes with amino ligands mainly precursor selection criteria include distribution temperatures, the stability of consist of tetrakis(dimethylamido)hafnium physical state, vapor pressure, thermal these precursors can also be well assessed (TDMAH), tetrakis(ethylmethylamido) stability at reasonable vaporization by NMR measurements. Overall, five hafnium (TEMAH) and tetrakis(diethy temperatures [1] and purity. The optimal hafnium precursors are considered in the lamido)hafnium (TDEAH). Successful following sections. candidate needs to be reactive enough ALD and CVD of HfO2 and HfN from to enable non-reversible monolayer such hafnium amides has been previously ALD, highest growth rate at reasonable Hafnium precursor choice reported [5–7]. process temperatures, and yet stable – promising candidates Hafnium complexes with alkoxy ligands enough to enable proper handling, shelf- The most frequently investigated precursors notably consist of tetrakis(t-butoxy) life and contamination-free delivery to for hafnium oxide film deposition typically hafnium or Hf(OtBu)4 (HTB). Other the process reactor. Even though some fall into the alkylamino, alkoxy or halide possible alkoxy precursors are homoleptic of the promising precursors have been ligand families. and have bidendate ligands such as TH 74 SEMICONDUCTOR FABTECH – 27 EDITION WWW . FABTECH . ORG MATERIALS & GASES tetrakis(methoxymethylpropanoxy) be (a) hydrolysis from trace moisture pressure gauge is kept at 150°C. The hafnium (Hf(mmp)4), tetrakis(acetyl contamination, (b) thermal pyrolysis, pressure gauge is calibrated at an elevated acetonate)hafnium, Hf(acac)4, and more and (c) oligomerization. Whereas partial temperature of 150°C. recently tetra(2,2,7-trimethyl-3,5- oligomerization of product will not Between measurements, a turbo pump octanedionate)hafnium (Hf(tod)4) [8]. The change its observed vapor pressure very helps achieve vacuum below 1 millitorr, use of bidentate ligands leads to a higher- much, degradation by trace hydrolysis which also determines the detection order complexation between the metal or thermal decomposition will produce limit of the measurement. Vapor pressure and ligands that provides a shielding effect relatively high-volatility by-products curves are plotted and the data is fitted around the metal. It should lead to less that will significantly influence the to the Clausius–Clapeyron equation in reactivity to moisture. In addition, there observed vapor pressure. Therefore order to estimate its heat of vaporization, are heteroleptic alkoxy precursors that vapor pressure measurements require which is another important thermal incorporate mono- and bi-dentate ligands, special attention. The description of a property of the precursor. such as bis (t-butoxy)bis(methoxymethylp vapor pressure measurement setup at Air Vapor pressure measurements ropanoxy) hafnium (Hf(OtBu)2(mmp)2 Liquide’s R&D centers is given below. further called HTBMMP) which have Samples are prepared and loaded into also been reported [9]. Successful ALD Vapor pressure setup description precursor vessel in a glove box under and CVD of Hafnium oxides and silicates, A stainless steel precursor vessel is attached a nitrogen atmosphere. The nitrogen for instance from HTB, have also been to a temperature-controlled manifold remaining in the vessel is removed by reported [2,4,10]. equipped with a capacitance manometer freeze-pump-thaw cycles on the sample Precursor candidates that are both that is also temperature controlled. The vessel. These cycles are repeated until liquid and very volatile are most precursor vessel can be cooled to as reproducible vapor pressures are observed. preferred, for easier delivery to the low as 77K using a liquid nitrogen bath Measurements are considered reliable process chamber either by bubbling or heated with a resistive band heater. only after the pressure in the manifold or by direct liquid injection. Solid Combination of both heating and cooling is stable over time, as there may be some precursors – such as those with is necessary in order to induce freeze- passivation of the manifold upon addition bidentate ligands, would generally pump-thaw cycles thereby discriminating of vapors of a new precursor. At each be transformed into liquid form by between true precursor vapor pressure temperature, measurements are repeated dissolution into a high-purity solvent and artificially high vapor pressure from to ensure that results are reproducible, media in order to help control a precise decomposition by-products that may indicating that decomposition by- and regular flow of precursor, but potentially be present. products are not influencing vapor this introduces additional risk of the The precursor vessel is maintained pressure measurements. solvent interfering with the process at a lower temperature than all other In Figure 3, measured values of deposition chemistry particularly in portions of the manifold. This is done TDEAH vapor pressure are given as a CVD processes using oxidizing gases to ensure that precursor vapors cannot function of temperature along with the with hydrocarbon solvents. Optimal condense anywhere in the system other Clausius–Clapeyron plot indicating the selection of a precursor and how it is than the precursor vessel. The manifold heat of vaporization. used in high-volume semiconductor is maintained at 120°C, while the Table 1 lists a comparison of vapor manufacturing requires detailed thermal property evaluation, which is not well documented today for many precursors. Thermal properties of selected Hf precursors Vapor pressure measurement One of the most critical thermal parameters of a precursor is its vapor pressure as a function of temperature. The latter needs to be known to optimize the delivery of precursor to the deposition chamber. Sufficient precursor vapor density is needed to allow adequate deposition rates and yet, care is needed not to overheat the precursor beyond its self- decomposition temperature. Thus, from knowledge of the thermal decomposition temperature of the precursor and its vapor pressure, one can assess its maximum permissible vapor density for the process. The vapor pressure

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