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Metal Nitrides

Refractory metal nitride films are widely used in applications requiring materials with good conductivity and resistant to high temperatures and oxidizing ambients. These films are used for electrodes in devices such as capacitors and transistors, and as an adhesion / barrier layer for interconnects in semiconductor chips. The properties that make them useful are low resistivity (<100uohm-cm), high melting points (>1200C), oxidation resistance, and limited reactivity with  either oxides or metals such as Al and Cu.

The most widely used metal nitride film is TiN, followed by TaN and MoN. These films have been deposited at IMI either by reactive physical vapor deposition (PVD) , where a metal target is sputtered in an Ar-N2 mixture, or by atomic layer deopistion (ALD), where metal containing precursors react with NH3 cycle-by-cycle, to build up the film one atomic layer at a time.

Choosing a deposition depends on the specific application. PVD can be done at relatively low temperatures and allows for the tuning of the properties based on the substrate temperature (Ar/N2 ratio, deposition pressure, etc.). ALD  requires much higher deposition temperatures (above 400C), and  is used when the films have to be deposited on nanoscale high aspect ratio structures.

Each application determines the important properties of the metal nitride studied. IMI performs exhaustive characterization for every film in order to tune the films to specific applications. Crystal orientation and texture is determined by X-Ray Diffraction (XRD). The exact stoichiometry is determined by X-ray Fluorescence (XRF) and X-ray Photoelectron Spectroscopy (XPS). The density  of the films is measured by X-ray reflectivity (XRR).

In addition to the sheet resistance of the film, the Work Function provides an important electrical parameter for many device applications. This function affects the barrier height to oxides and semiconductors, which, in turn, controls the  threshold voltages for transistors and leakage in  MOS and MIM capacitors. The barrier height can be affected by other phenomena at the interface, such as surface states and dipoles. IMI’ s engineers use internal photoemission (IPE) to measure the actual barrier height.

The following examples illustrate how IMI optimizes metal nitride films for different semiconductor technology applications.

DRAM

Nitride films are used as the bottom and top electrodes in capacitors with high-k oxides as the main dielectric. The high aspect ratio of the capacitors require nitride films to be deposited by ALD. The dielectric constant of the oxide, which is critical for achieving high capacitance, can be impacted by the underlying electrode.

The  crystal texture of the bottom electrode can enhance the formation of oxide phases that have a high dielectric constant. In order to reduce leakage, the  electrode material must have a high barrier height relative to the oxide. Therefore, IPE measurements are critical in optimizing the deposition and processing of the  nitride films.

Another factor contributing to leakage is oxygen vacancies in the dielectric, which can be created by the nitride reducing the oxides (both during  deposition and post deposition anneals). IMI models simulate the measured I-V curves to infer the defect density and distribution in the dielectric layer. This allows the electrode material  and process to be optimized.

Logic Transistors

Advanced MOS transistors, including FinFets, use high-k oxides such as HfO2 in conjunction with metal gates. These metal gates consist of a thin layer of metal nitride (usually TiN around 10nm thick), followed by a metal with higher conductivity such as W  or Al.

The performance of the transistor is determined by the  Effective Work function of the nitride layer. Leakage and reliability are affected by interaction with the metal oxide. Controlling the stoichiometry of the TiN, including the level of O incorporated in it, greatly influences the  Effective Gate Oxide Thickness (EOT). In a gate-last sequence, currently used  logic processes, the gate stack (including the TiN layer) has to be deposited in a trench with high aspect ratio, making ALD the preferred deposition method. IMI’s expertise in ALD, combined with the ability to simulate the material properties, interactions, and defect generation, enable us to help customers optimize their gate stacks.

Interconnects

State-of-the-art semiconductor processes use Cu based interconnects deposited into trenches and etched into a dielectric like SiO2. Cu ions can easily diffuse through the SiO2  causing shorts and transistor failures. Encapsulating the Cu lines with barrier materials prevents this phenomenon.

The three sides of the trench have to be lined with a very  thin metal nitride layer  blocking the diffusion of Cu. The metal nitride has to possess low resistivity to allow Cu to form a continuous film with the right crystal orientation, and to not react with the Cu during subsequent process steps. The preferred metal nitride is TaN, which is typically deposited using PVD; however, as line widths shrink PVD is being replaced by ALD.

As the above examples illustrate, the deposition and properties of nitride films require fine-tuning for  different applications, taking into consideration not only the overall process integration constraints, but also the impact on the circuits’ electrical properties.