Expanding the toolbox of atomic scale processing

Tahsin Faraz
Abstract & Cover

As we enter an era of atomic scale device dimensions, it has become imperative to  utilize deposition and etching techniques that allow for processing materials at the  atomic level. Furthermore, next-generation devices consist of various material layers  across both planar and three-dimensional (3D) layouts which has led to an additional  need for processing materials in a selective manner. As a result, it is now vitally  important to retain proper control over the thickness and properties of materials grown  or removed during fabrication of nanoscale devices with 3D geometries. Plasmaenhanced atomic layer deposition (ALD) has obtained a prominent position in  synthesizing ultra-thin films of functional materials with atomic scale precision. Uniform  and conformal film deposition even on challenging 3D substrate topographies can be  attained by virtue of the sequential and self-limiting precursor and plasma exposure  steps of plasma ALD. Highly reactive plasma radicals are generated during the plasma  step and the contribution of these electrically neutral species toward film growth is a  well-known feature of plasma ALD. However, the ions generated during plasma  exposure can also play a significant role in the deposition process which has been  relatively less explored. Furthermore, the challenges related to current plasma based  dry-etching processes provide a window of opportunity for being potentially tackled by  the etch counterpart of ALD, i.e., atomic layer etching (ALE). This dissertation  investigates plasma-enhanced atomic scale processing of functional materials and the  role of ions during these processes on planar and 3D substrate topographies, relevant  for next-generation device technologies.  In the first part of this work, a new ALD process for SiNx was developed using a  novel organosilane precursor (DSBAS) and N2 plasma. Dense and wet-etch resistant SiNx  films that can be synthesized at low temperatures serve as spacers or encapsulation  layers for protecting sensitive device components; e.g., gate stacks in 3D transistors or  magnetic tunnel junctions in emerging magnetoresistive memories. SiNx films with a  high density and low impurity content were obtained at low substrate temperatures on  planar substrates using the process developed in this work. Deposition were also  performed on high aspect ratio 3D trench nanostructures to investigate SiNx film  conformality and wet-etch resistance. Sources limiting conformality on 3D substrates  were attributed to factors occurring in the N2 plasma step. Identification of factors  associated with plasma processing conditions is a prerequisite for addressing the  challenge of growing conformal SiNx on 3D substrates. Yet, very low wet-etch rates  were observed at different regions throughout the trenches, confirming high quality SiNx could be grown at low substrate temperature on 3D substrates using the developed  process.  Next, the effects substrate biasing during plasma ALD on the properties of  materials (oxides and nitrides of Ti, Hf, and Si) grown on planar and 3D substrate  topographies were investigated. A commercial 200-mm remote plasma ALD system  equipped with RF substrate biasing was used to control the ion energy during the  plasma exposure step. This technique was demonstrated to significantly enhance the  versatility of plasma ALD processes by providing additional knobs for controlling a wide  range of material properties, appropriate for numerous applications. Substrate biasing  during plasma ALD increased the refractive index and mass density of TiOx and HfOx and  enabled control over their crystalline properties. Plasma ALD of these oxides with  substrate biasing formed crystalline films at a low temperature which would otherwise  yield amorphous films without biasing. Substrate biasing drastically reduced the  resistivity of conductive TiNx and HfNx films. Furthermore, biasing enabled the residual  stress of these materials to be altered from tensile to compressive. The properties of  SiOx were slightly improved whereas those of SiNx were degraded as a function of  substrate biasing. Plasma ALD on 3D trench nanostructures with biasing induced  differing film properties at different regions of the 3D substrate which demonstrated  the potential of this technique in enabling new approaches for topographically selective  deposition.  Ion energy characteristics on grounded and biased substrates during plasma  exposure were also measured to investigate their role in tailoring material properties.  Insights from such measurements are essential toward understanding how a given  plasma ALD process at different operating conditions can be influenced by energetic  ions. Ion flux-energy distribution functions (IFEDFs) were measured using a retarding  field energy analyzer for reactive plasmas typically used in plasma ALD (O2, H2, N2)  without and with RF biasing. The properties of materials (TiOx, HfNx, SiNx) grown using  these plasmas were analyzed as a function of the ion energy and flux parameters  derived from IFEDFs. These results have provided more insight on the relation between  energetic ions and the ensuing material properties, e.g., by providing energy maps of  material properties in terms of the ion energy dose during plasma ALD. They  demonstrate how the measurement and control of ion energy characteristics during  plasma ALD provide a platform for synthesizing ultra-thin films with the desired  properties.  In the final part of this work, past research efforts on ALE were reviewed and the  key defining characteristics of ALE identified. These include cyclic step-wise processing,  self-limiting surface chemistry, repeated removal of atomic layers (not necessarily a full monolayer) of the material, and the presence or absence of directional species that lead  to anisotropic or isotropic ALE processes, respectively. Subsequently, further parallels  were drawn with the more mature and mainstream technology of ALD from which  lessons and concepts were extracted that can be beneficial for advancing the field of  ALE.  To conclude, this dissertation elucidates important aspects associated with plasmaenhanced atomic scale processes that provide deeper insight on the fundamental and  technological opportunities afforded by these techniques, relevant for future 3D device  architectures. It serves to exemplify how the properties of functional materials can be  tailored by accurate control and optimization of plasma based processing conditions  

Source of Information
Harm Knoops
Eindhoven University of Technology
(Eindhoven, Netherlands)
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