Filip Geenen

Monte Carlo simulations of atomic layer deposition on 3D large surface area structures: Required precursor exposure for pillar- versus hole-type structures

Due to its excellent conformality, atomic layer deposition (ALD) has become a key method for coating and functionalizing three dimensional (3D) large surface area structures such as anodized alumina (AAO), silicon pillars, nanowires, and carbon nanotubes. Large surface area substrates often consist of arrays of quasi-one-dimensional holes (into which the precursor gas needs to penetrate, e.g., for AAO), or “forests” of pillars (where the precursor gas can reach the surface through the empty 3D space surrounding the pillars). Using a full 3D Monte Carlo model, the authors compared deposition onto an infinite array of holes versus an infinite array of pillars. As expected, the authors observed that the required exposure to conformally coat an array of holes is determined by the height to width ratio of the individual holes, and is independent of their spacing in the array. For the pillars, the required exposure increases with decreasing center-to-center distance and converges in the limit to the exposure of...

Anisotropic thermal expansion of Ni, Pd and Pt germanides and silicides

Silicon or germanium-based transistors are nowadays used in direct contact with silicide or germanide crystalline alloys for semiconductor device applications. Since these compounds are formed at elevated temperatures, accurate knowledge of the thermal expansion of both substrate and the contact is important to address temperature depending effects such as thermal stress. Here we report the linear coefficients of thermal expansion of Ni-, Pd- and Pt-based mono-germanides, mono-silicides and di-metal-silicides as determined by powder-based x-ray diffraction between 300 and 1225 K. The investigated mono-metallic compounds, all sharing the MnP crystal structure, as well as Pd2Si and Pt2Si exhibit anisotropic expansion. By consequence, this anisotropic behaviour should be taken into account for evaluating the crystal units cell at elevated temperatures.

Controlling the formation and stability of ultra-thin nickel silicides - An alloying strategy for preventing agglomeration

The electrical contact of the source and drain regions in state-of-the-art CMOS transistors is nowadays facilitated through NiSi, which is often alloyed with Pt in order to avoid morphological agglomeration of the silicide film. However, the solid-state reaction between as-deposited Ni and the Si substrate exhibits a peculiar change for as-deposited Ni films thinner than a critical thickness of tc = 5 nm. Whereas thicker films form polycrystalline NiSi upon annealing above 450 ° C, thinner films form epitaxial NiSi2 films that exhibit a high resistance toward agglomeration. For industrial applications, it is therefore of utmost importance to assess the critical thickness with high certainty and find novel methodologies to either increase or decrease its value, depending on the aimed silicide formation. This paper investigates Ni films between 0 and 15 nm initial thickness by use of “thickness gradients,” which provide semi-continuous information on silicide formation and stability as a function of as-deposited layer thickness. The alloying of these Ni layers with 10% Al, Co, Ge, Pd, or Pt renders a significant change in the phase sequence as a function of thickness and dependent on the alloying element. The addition of these ternary impurities therefore changes the critical thickness tc. The results are discussed in the framework of classical nucleation theory.The electrical contact of the source and drain regions in state-of-the-art CMOS transistors is nowadays facilitated through NiSi, which is often alloyed with Pt in order to avoid morphological agglomeration of the silicide film. However, the solid-state reaction between as-deposited Ni and the Si substrate exhibits a peculiar change for as-deposited Ni films thinner than a critical thickness of tc = 5 nm. Whereas thicker films form polycrystalline NiSi upon annealing above 450 ° C, thinner films form epitaxial NiSi2 films that exhibit a high resistance toward agglomeration. For industrial applications, it is therefore of utmost importance to assess the critical thickness with high certainty and find novel methodologies to either increase or decrease its value, depending on the aimed silicide formation. This paper investigates Ni films between 0 and 15 nm initial thickness by use of “thickness gradients,” which provide semi-continuous information on silicide formation and stability as a function of as...

Formation of ultrathin, stable and epitaxial silicides for semiconductor contacts

The current field-effect transistors use nickel-based silicides for contacting the source and drain regions. The thickness of this silicide is trending to ever-thinner values. The influence of this thickness decrease on the formation of nickel silicides was systematically investigated between 0 and 15 nm. Annealing thickness gradients enabled us to distinguish two regimes: films above a critical thickness tend to agglomerate at high temperature, while ultra-thin films below a critical thickness are morphologically extremely stable. The critical thickness for pure Ni films is about 5nm. Through alloying of the initial Ni layer, we could influence this critical thickness to higher (Al, Co) and lower (Ge, Pd, Pt) values. Synchrotron based pole figure measurements provided texture information that helps to understand the observed shift in critical thickness and the morphological stability of the ultra-thin silicide layers.

Influence of alloying elements on the phase formation of ultrathin Ni (<10nm) on Si(001) substrates

The influence of Ni thickness on the formation of Nickel suicides was systematically investigated between 0 and 15nm. Annealing thickness gradients distinguishes Alms that agglomerate (>5nm) and films that are morphologically stable (<;5nm). Alloying the initial Ni layer influences this critical thickness to higher (Al, Co) and lower (Ge, Pd, Pt) values. Pole figures and in situ XRD provides information to understand this observed shift in critical thickness.