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|3D Integration: 300mm MOCVD Copper Seed Layer Deposition For High Aspect Ratios TSV|
|Keywords: TSV, MOCVD Copper, High Aspect Ratio|
|3D integration approaches significantly increase device performance and functionality, with “Through Silicon Via” (TSV) technology forming a lead element. To achieve these improvements, the design has evolved. Today, TSV Mid-process interconnect feature sizes are more challenging for metallization, with 10:1 to 15:1 aspect ratios appearing in designs. TSV Last High density with 10:1 aspect ratio with a TSV diameter inferior or equal to 1 µm. These devices with narrow, high aspect ratio vias require highly conformal thin metal depositions, sometimes with thermal budget restrictions in force, typically 200°C. An MOCVD TiN film was developed that offers a high coverage film with robust liner/barrier properties with an ability up to 200°C. To complete the requirements for all 3D schemes, a new deposition method was required to obtain a continuous, conformal Cu seed layer after barrier deposition to initiate conformal filling by electroplating. In this context, we developed an ultra-thin copper film deposited by MOCVD at 175°C, using a fluorine-free organometallic precursor to give a universal MOCVD barrier/seed solution for high aspect ratio TSV with or without temperature constraints. To give a full MOCVD metallization solution at low temperature provides a solution for all 3D integration schemes and overcomes the step coverage limitations of I-PVD approaches used with lower aspect ratio designs. Furthermore, MOCVD enables to use similar material as those used for PVD, which do not implies more development for integration. It’s also well known that this deposition technique offers a faster deposition rate than ALD for instance, which is more compatible with an industrial solution. The TiN barrier and the copper seed layer films were deposited on 300mm wafers using an SPTS Sigma fxP systemTM. The equipement includes two C3M reactors dedicated for each film. All TiN/Cu stacks studied were deposited without a vacuum break. In each case, precursor is introduced into the reactor with or without co- reactants using a dual showerhead system to prevent any upstream preliminary reactions. The MOCVD TiN film deposited using TDEAT as the precursor, offers a high conformal coverage even at 10µmx200µm, interesting barrier properties evaluated by TOF-SIMS along the sidewalls post annealing TSV . Since a strong reactivity is well known between Ti and F to form Ti-F and Ti-N-F bonds, and to avoid generating non-volatile compounds linked to potential adhesion issues from subsequent reactions between the TiN and the copper film  , a fluorine-free Cu precursor was chosen. Argon was used as carrier gas and hydrogen to play two roles: pretreating the TiN surface to promote the increase in the nucleation density sites and as a co-reactant to help ligand dissociation. This study was performed on 300mm blanket and TSV wafers. In the first part, we studied surface preparation which has been shown to be key parameter to successful the nucleation and subsequent growth of Cu films. During the early stages of the deposition sequence and onto the coalescence of the film, we investigated the impact of co- reagents, their order of introduction, and also the influence of an additional plasma pretreatment (NH3/H2). In the second part, we focused on high aspect ratio structures to confirm molecular transport and chemical reactions down the structure to the TSV bottom in order to obtain a continuous coverage of Cu required for subsequent plate up steps. The copper film conformality was measured by SEM (Hitachi 5500) cross section along the TSV sidewalls. Continuity was confirmed by via fill using Cu electroplating and observed by FIB (HELIOS) on 10x100µm TSV structures, depositing less than 100nm of MOCVD copper. The overburden post- plating, is around 2µm whilst compared with using an i-PVD seed layer, 1,5µm Cu seed thickness is required to ensure good plating of a 10x80µm TSV and the overburden is around 5µm. Reducing the copper seed layer thickness is another advantage for this MOCVD copper deposition approach, for further planarization processes, and also decreasing significantly film constraints and cost. MOCVD copper film microstructure was analyzed by X-ray diffraction (XRD X’pert Pro), we observed a preferential orientation in (111) direction, which is the targeted texture to prevent from electromigration . Compositional analysis was done by X-ray photo- electron spectroscopy (XPS), which revealed at 175°C we obtain a pure copper film, no oxygen, no carbon was detected inside the copper film. At low temperature, inside the reactor, this fluorine-free precursor is dissociated without any ligand fragmentation. Roughness was characterized by Atomic Force Microscopy (AFM Fastscan). The stress of the MOCVD copper film and the full stack barrier/seed was evaluated by measurement on blanket wafers (FRT Microprofilometer). Adhesion was checked by a scribe test post anneal (400°C, 5min under N2H2 flow). An interface TiN/Cu analysis by XPS and TEM (FEI Titan Themis), was performed to investigate the good adhesion observed between the deposited layers. We defined a protocol to manage constraints evaluation inside the TSV and the propagation inside the silicon by XRD measurement. In parallel, the gaseous phase of the selected precursor was studied. We aim to determine its thermodynamics parameters (vapor pressure), decomposition mechanisms using a Hertz-Knusden’s cell only or coupled to a mass spectrometer.|
|Sabrina FADLOUN, Senior Process Engineer / PhD student
Annecy le Vieux, FRANCE