In-situ XPS Investigation of the Surface Chemistry of a Cu(I) Beta-Diketonate Precursor and the ALD of Cu2O

This poster was presented in the Materials for Advanced Metallization (MAM) 2014 Conference in Chemnitz, Germany. Abstract: Atomic Layer Deposition (ALD) has emerged as an ubiquitous method for the deposition of conformal and homogeneous ultra-thin films on complex topographies and large substrates in microelectronics. Electrochemical deposition (ECD) is the first choice for the deposition of copper (Cu) into the trenches and vias of the interconnect system for ULSI circuits. The ECD of Cu necessitates an electrically conductive seed layer for filling the interconnect structures. ALD is now considered as a solution for conformal deposition of Cu seed layers on very high aspect ratio (AR) structures also for technology nodes below 20 nm, since physical vapor deposition is not applicable for structures with high AR. Cu seed layer deposition by the reduction of Cu2O, which has been deposited from the Cu(I) β-diketonate precursor [(nBu3P)2Cu(acac)], has been successfully carried out on different substrates like Ta, TaN, SiO2, and Ru [1, 2]. However, still many questions are unanswered regarding the underlying surface chemistry of the precursor on many substrates, leading to different growth modes during ALD. In this work, the surface chemistry of [(nBu3P)2Cu(acac)] on SiO2 substrate is investigated by in-situ X-ray photoelectron spectroscopy (XPS), reporting vital information about the oxidation state and the atomic concentration after chemisorption on the substrates kept at different temperatures. The aim of the investigation is to understand the stepwise change in the precursor oxidation state with increasing substrate temperature and to identify the temperature limit for the thermal ALD with this Cu precursor on SiO2. For the experiments, the Cu precursor was evaporated on SiO2 substrates kept at temperatures between 22 °C and 300 °C. The measured C/Cu and P/Cu concentration indicated that most of the nBu3P ligands were released either in the gas phase or during adsorption (Fig. 1a). No disproportionation was observed for the Cu precursor in the temperature range between 22 °C and 145 °C. Similarly, in this temperature range the Auger parameter calculated from Cu 2p3/2 and Cu L3VV spectra was found to be 360.0±0.2 eV, comparable to Cu(I) oxidation state [3]. However, disproportionation of the Cu precursor was observed above 200 °C, since C/Cu concentration ratio decreased and substantial metallic Cu was present on the substrate. Hence, 145 °C is the temperature limit for the ALD of Cu2O from this precursor, as the precursor must not alter its chemical state after chemisorption on the substrate. 500 ALD cycles with the probed Cu precursor and wet O2 as co reactant were carried out on SiO2 at 145 °C. After ALD, in situ XPS analysis confirmed the presence of Cu2O on the substrate. Ex-situ spectroscopic ellipsometry indicated an average film thickness of 2.5 nm of Cu2O deposited with a growth per cycle of 0.05 Å/cycle, comparable to previous experiments. References: [1] T. Waechtler, S. Oswald, N. Roth, A. Jakob, H. Lang, R. Ecke, S. E. Schulz, T. Gessner, A. Moskvinova, S. Schulze, M. Hietschold, J. Electrochem. Soc., 156 (6), H453 (2009). [2] T. Waechtler, S. -F. Ding, L. Hofmann, R. Mothes, Q. Xie, S. Oswald, C. Detavernier, S. E. Schulz, X. -P. Qu, H. Lang, T. Gessner, Microelectron. Eng., 88, 684 (2011). [3] J. P. Espinós, J. Morales, A. Barranco, A. Caballero, J. P. Holgado, A. R. González Elipe, J. Phys. Chem. B, 106, 6921 (2002)

Optical Properties of Sputtered Tantalum Nitride Films Determined by Spectroscopic Ellipsometry

Tantalum and tantalum nitride thin films are routinely applied as diffusion barriers in state-of-the-art metallization systems of microelectronic devices. In this work, such films were prepared by reactive magnetron sputtering on silicon and oxidized silicon substrates and studied by spectroscopic ellipsometry in the spectral range from 190 nm to 2.55 μm. The complex refractive index for thick films (75 to 380 nm) was modeled using a Lorentz-Drude approach. These models were applied to film stacks of 20 nm TaN / 20 nm Ta on unoxidized and thermally oxidized Si. With free oscillator parameters, accurate values of the film thicknesses were obtained according to cross-sectional scanning electron microscope (SEM) measurements. At the same time, a strong variation of the optical properties with film thickness and substrate was observed

High Power GaN/AlGaN/GaN HEMTs Grown by Plasma-Assisted MBE Operating at 2 to 25 GHz

Heterostructures of the materials system GaN/AlGaN/GaN were grown by molecular beam epitaxy on 6H-SiC substrates and high electron mobility transistors (HEMTs) were fabricated. For devices with large gate periphery an air bridge technology was developed for the drain contacts of the finger structure. The devices showed DC drain currents of more than 1 A/mm and values of the transconductance between 120 and 140 mS/mm. A power added efficiency of 41 % was measured on devices with a gate length of 1 µm at 2 GHz and 45 V drain bias. Power values of 8 W/mm were obtained. Devices with submicron gates exhibited power values of 6.1 W/mm (7 GHz) and 3.16 W/mm (25 GHz) respectively. The rf dispersion of the drain current is very low, although the devices were not passivated.Heterostrukturen im Materialsystem GaN/AlGaN/GaN wurden mittels Molekularstrahlepitaxie auf 6H-SiC-Substraten gewachsen und High-Electron-Mobility-Transistoren (HEMTs) daraus hergestellt. Für Bauelemente mit großer Gateperipherie wurde eine Air-Bridge-Technik entwickelt, um die Drainkontakte der Fingerstruktur zu verbinden. Die Bauelemente zeigten Drainströme von mehr als 1 A/mm und Steilheiten zwischen 120 und 140 mS/mm. An Transistoren mit Gatelängen von 1 µm konnten Leistungswirkungsgrade (Power Added Efficiency) von 41 % (bei 2 GHz und 45 V Drain-Source-Spannung) sowie eine Leistung von 8 W/mm erzielt werden. Bauelemente mit Gatelängen im Submikrometerbereich zeigten Leistungswerte von 6,1 W/mm (7 GHz) bzw. 3,16 W/mm (25 GHz). Die Drainstromdispersion ist sehr gering, obwohl die Bauelemente nicht passiviert wurden

In-situ XPS Investigation of the Surface Chemistry of a Cu(I) Beta-Diketonate Precursor and the ALD of Cu2O

This poster was presented in the Materials for Advanced Metallization (MAM) 2014 Conference in Chemnitz, Germany. Abstract: Atomic Layer Deposition (ALD) has emerged as an ubiquitous method for the deposition of conformal and homogeneous ultra-thin films on complex topographies and large substrates in microelectronics. Electrochemical deposition (ECD) is the first choice for the deposition of copper (Cu) into the trenches and vias of the interconnect system for ULSI circuits. The ECD of Cu necessitates an electrically conductive seed layer for filling the interconnect structures. ALD is now considered as a solution for conformal deposition of Cu seed layers on very high aspect ratio (AR) structures also for technology nodes below 20 nm, since physical vapor deposition is not applicable for structures with high AR. Cu seed layer deposition by the reduction of Cu2O, which has been deposited from the Cu(I) β-diketonate precursor [(nBu3P)2Cu(acac)], has been successfully carried out on different substrates like Ta, TaN, SiO2, and Ru [1, 2]. However, still many questions are unanswered regarding the underlying surface chemistry of the precursor on many substrates, leading to different growth modes during ALD. In this work, the surface chemistry of [(nBu3P)2Cu(acac)] on SiO2 substrate is investigated by in-situ X-ray photoelectron spectroscopy (XPS), reporting vital information about the oxidation state and the atomic concentration after chemisorption on the substrates kept at different temperatures. The aim of the investigation is to understand the stepwise change in the precursor oxidation state with increasing substrate temperature and to identify the temperature limit for the thermal ALD with this Cu precursor on SiO2. For the experiments, the Cu precursor was evaporated on SiO2 substrates kept at temperatures between 22 °C and 300 °C. The measured C/Cu and P/Cu concentration indicated that most of the nBu3P ligands were released either in the gas phase or during adsorption (Fig. 1a). No disproportionation was observed for the Cu precursor in the temperature range between 22 °C and 145 °C. Similarly, in this temperature range the Auger parameter calculated from Cu 2p3/2 and Cu L3VV spectra was found to be 360.0±0.2 eV, comparable to Cu(I) oxidation state [3]. However, disproportionation of the Cu precursor was observed above 200 °C, since C/Cu concentration ratio decreased and substantial metallic Cu was present on the substrate. Hence, 145 °C is the temperature limit for the ALD of Cu2O from this precursor, as the precursor must not alter its chemical state after chemisorption on the substrate. 500 ALD cycles with the probed Cu precursor and wet O2 as co reactant were carried out on SiO2 at 145 °C. After ALD, in situ XPS analysis confirmed the presence of Cu2O on the substrate. Ex-situ spectroscopic ellipsometry indicated an average film thickness of 2.5 nm of Cu2O deposited with a growth per cycle of 0.05 Å/cycle, comparable to previous experiments. References: [1] T. Waechtler, S. Oswald, N. Roth, A. Jakob, H. Lang, R. Ecke, S. E. Schulz, T. Gessner, A. Moskvinova, S. Schulze, M. Hietschold, J. Electrochem. Soc., 156 (6), H453 (2009). [2] T. Waechtler, S. -F. Ding, L. Hofmann, R. Mothes, Q. Xie, S. Oswald, C. Detavernier, S. E. Schulz, X. -P. Qu, H. Lang, T. Gessner, Microelectron. Eng., 88, 684 (2011). [3] J. P. Espinós, J. Morales, A. Barranco, A. Caballero, J. P. Holgado, A. R. González Elipe, J. Phys. Chem. B, 106, 6921 (2002)

New Precursors for CVD Copper Metallization

A novel CVD copper process is described using two new copper CVD precursors, KI3 and KI5, for the fabrication of IC or TSV (Through Silicon Via) copper interconnects. The highly conformal CVD copper can provide seed layers for subsequent copper electroplating or can be used to directly fabricate the interconnect in one step. These new precursors are thermally stable yet chemically reactive under CVD conditions, growing copper films of exceptionally high purity at high growth rates. Their thermal stability can allow for elevated evaporation temperatures to generate the high precursor vapor pressures needed for deep penetration into high aspect ratio TSV vias. Using formic acid vapor as a reducing gas with KI5, copper films of > 99.99 atomic % purity were grown at 250°C on titanium nitride at a growth rate of > 1500 Å/min. Using tantalum nitride coated TSV type wafers, ~ 1700 Å of highly conformal copper was grown at 225°C into 32 μm × 5 μm trenches with good adhesion. With ruthenium barriers we were able to grow copper at 125°C at a rate of 20 Å/min to give a continuous ~ 300 Å copper film. In this respect, rapid low temperature CVD copper growth offers an alternative to the long cycle times associated with copper ALD which can contribute to copper agglomeration occurring. © 2008 Elsevier B.V

ALD of Copper and Copper Oxide Thin Films For Applications in Metallization Systems of ULSI Devices

As a possible alternative for growing seed layers required for electrochemical Cu deposition of metallization systems in ULSI circuits, the atomic layer deposition (ALD) of Cu is under consideration. To avoid drawbacks related to plasma-enhanced ALD (PEALD), thermal growth of Cu has been proposed by two-step processes forming copper oxide films by ALD which are subsequently reduced. This talk, given at the 8th International Conference on Atomic Layer Deposition (ALD 2008), held in Bruges, Belgium from 29 June to 2 July 2008, summarizes the results of thermal ALD experiments from [(nBu3P)2Cu(acac)] precursor and wet O2. The precursor is of particular interest as it is a liquid at room temperature and thus easier to handle than frequently utilized solids such as Cu(acac)2, Cu(hfac)2 or Cu(thd)2. Furthermore the substance is non-fluorinated, which helps avoiding a major source of adhesion issues repeatedly observed in Cu CVD. As result of the ALD experiments, we obtained composites of metallic and oxidized Cu on Ta and TaN, which was determined by angle-resolved XPS analyses. While smooth, adherent films were grown on TaN in an ALD window up to about 130°C, cluster-formation due to self-decomposition of the precursor was observed on Ta. We also recognized a considerable dependency of the growth on the degree of nitridation of the TaN. In contrast, smooth films could be grown up to 130°C on SiO2and Ru, although in the latter case the ALD window only extends to about 120°C. To apply the ALD films as seed layers in subsequent electroplating processes, several reduction processes are under investigation. Thermal and plasma-assisted hydrogen treatments are studied, as well as thermal treatments in vapors of isopropanol, formic acid, and aldehydes. So far these attempts were most promising using formic acid at temperatures between 100 and 120°C, also offering the benefit of avoiding agglomeration of the very thin ALD films on Ta and TaN. In this respect, the process sequence shows potential for depositing ultra-thin, smooth Cu films at temperatures below 150°C

High Power GaN/AlGaN/GaN HEMTs Grown by Plasma-Assisted MBE Operating at 2 to 25 GHz

Heterostructures of the materials system GaN/AlGaN/GaN were grown by molecular beam epitaxy on 6H-SiC substrates and high electron mobility transistors (HEMTs) were fabricated. For devices with large gate periphery an air bridge technology was developed for the drain contacts of the finger structure. The devices showed DC drain currents of more than 1 A/mm and values of the transconductance between 120 and 140 mS/mm. A power added efficiency of 41 % was measured on devices with a gate length of 1 µm at 2 GHz and 45 V drain bias. Power values of 8 W/mm were obtained. Devices with submicron gates exhibited power values of 6.1 W/mm (7 GHz) and 3.16 W/mm (25 GHz) respectively. The rf dispersion of the drain current is very low, although the devices were not passivated.Heterostrukturen im Materialsystem GaN/AlGaN/GaN wurden mittels Molekularstrahlepitaxie auf 6H-SiC-Substraten gewachsen und High-Electron-Mobility-Transistoren (HEMTs) daraus hergestellt. Für Bauelemente mit großer Gateperipherie wurde eine Air-Bridge-Technik entwickelt, um die Drainkontakte der Fingerstruktur zu verbinden. Die Bauelemente zeigten Drainströme von mehr als 1 A/mm und Steilheiten zwischen 120 und 140 mS/mm. An Transistoren mit Gatelängen von 1 µm konnten Leistungswirkungsgrade (Power Added Efficiency) von 41 % (bei 2 GHz und 45 V Drain-Source-Spannung) sowie eine Leistung von 8 W/mm erzielt werden. Bauelemente mit Gatelängen im Submikrometerbereich zeigten Leistungswerte von 6,1 W/mm (7 GHz) bzw. 3,16 W/mm (25 GHz). Die Drainstromdispersion ist sehr gering, obwohl die Bauelemente nicht passiviert wurden

New Precursors for CVD Copper Metallization

A novel CVD copper process is described using two new copper CVD precursors, KI3 and KI5, for the fabrication of IC or TSV (Through Silicon Via) copper interconnects. The highly conformal CVD copper can provide seed layers for subsequent copper electroplating or can be used to directly fabricate the interconnect in one step. These new precursors are thermally stable yet chemically reactive under CVD conditions, growing copper films of exceptionally high purity at high growth rates. Their thermal stability can allow for elevated evaporation temperatures to generate the high precursor vapor pressures needed for deep penetration into high aspect ratio TSV vias. Using formic acid vapor as a reducing gas with KI5, copper films of > 99.99 atomic % purity were grown at 250°C on titanium nitride at a growth rate of > 1500 Å/min. Using tantalum nitride coated TSV type wafers, ~ 1700 Å of highly conformal copper was grown at 225°C into 32 μm × 5 μm trenches with good adhesion. With ruthenium barriers we were able to grow copper at 125°C at a rate of 20 Å/min to give a continuous ~ 300 Å copper film. In this respect, rapid low temperature CVD copper growth offers an alternative to the long cycle times associated with copper ALD which can contribute to copper agglomeration occurring. © 2008 Elsevier B.V