Recently, the art of processing materials in a selective manner has garnered significant attention. Especially in the past few years, the topic of making atomic scale processing techniques, such as atomic layer deposition (ALD) and atomic layer etching (ALE), more selective has become a highly active field of research in both academia and industry.1–4 This is primarily due to the ever increasing complexity in fabricating devices consisting of various layers of nanoscale materials and 3D features. This has led to the need for expanding the current portfolio of deposition and etching techniques with novel approaches in selective processing. In this post, I will discuss about the concept of topographically selective processing and how it is currently being explored as a means for expanding the toolbox of selective materials processing on 3D substrates. I will start off by introducing the categories of selective deposition as defined for planar substrates which will then make it easy to translate those concepts to the 3rd dimension.
Selective processing on 2D substrates
In a perspective article published last month in Chemistry of Materials, Mackus and co-workers1 discuss the current status of the field of area-selective ALD and new approaches to improve process selectivity during film growth. Although area-selective deposition is currently a hot topic, it is but one out of several flavors in the menu of selective deposition. Different categories of selectivity during film growth have been defined in the literature as early as 1990 in an article published by Jan Otto Carlsson.5 These categories were termed as area-, phase-, microstructure-, or chemical composition-selective deposition on a pre-patterned planar substrate composed of different surface materials. They are shown in the figure below taken from that early publication.5
Schematic overview of the various flavors of selective deposition on a patterned planar substrate composed of different surface materials, A and B, as outlined by Carlsson in 1990.5
In area-selective deposition using gas-phase reactants on a patterned planar substrate composed of, e.g., two different surface materials, film growth occurs only on the surface area of one substrate material and not on the other. The selectivity arises from a difference in the interfacial reactions between the different surface material areas of the patterned substrate and the gas phase reactants that the substrate is exposed to.5 For example, an area-selective ALD process for In2O3:H was demonstrated by Mameli et al.6 two years ago on a patterned planar substrate consisting of OH-terminated and H-terminated Si surface areas. The In2O3:H film grew only on the OH-terminated Si surface areas and not on the H-terminated surface areas, as shown in the figure below.6
Photo of a wafer (planar substrate) on which the logo of the Eindhoven University of Technology (TU/e) has been prepared by area-selective ALD of In2O3:H. Prior to commencing deposition, a pattern of the logo was initially made on the planar substrate using a micro-plasma printer that led to the formation of OH-terminated surface regions on an H-terminated Si wafer surface.6
In phase-selective deposition, films with different material phases (e.g., crystalline/amorphous) are grown simultaneously and selectively on the different surface areas of the different materials that form a patterned substrate.5 Similar to phase-selective deposition, films with different microstructures (e.g., void-rich/deficient, large/small grain size, etc.) or different chemical compositions (e.g., impurity rich/deficient, over/under stoichiometric, etc.) may be deposited simultaneously on the different surface areas of the different materials that form a patterned substrate. This ultimately leads to microstructure- or chemical composition-selective deposition.5 For instance, a case for microstructure-selective deposition may be inferred from recent work reported in the literature where ALD of TiO2 was performed on a patterned substrate composed of Al2O3 and SiO2 surface regions, as shown in the figure below.7 The authors reported on the growth of TiO2 films on both regions of the patterned substrate with the simultaneous and selective formation of large grains on Al2O3 and small grains on SiO2 surface regions.7
(a) Schematic process flow outlining the formation of a patterned planar substrate composed of different surface materials – Al2O3 and SiO2 – followed by selective deposition of TiO2 in terms of simultaneous growth of large and small grains on SiO2 and Al2O3 surface regions, respectively. (b) Optical image of as-deposited TiO2 film where the contrast difference corresponds to the difference in grain size formed on the different surface regions of the patterned substrate. (c) Magnified AFM image of as-deposited TiO2 film taken at the boundary of the patterned substrate showing the presence of a sharp interface between the regions of large and small TiO2 grains formed simultaneously by selective deposition.7 Such simultaneous growth of films with different properties (e.g., grain size) on the different surface regions of a patterned planar substrate enables material property-selective deposition.
Since the categories of phase-, microstructure-, and chemical composition-selective deposition have been defined based on the inherent differences in material properties that transpire after growth on a patterned substrate, one could argue that these cases are simply different examples of one common selective deposition process. As a result, the aforementioned cases of simultaneous film growth with different material properties can be collectively represented by a single term, namely material property-selective deposition. This is illustrated schematically in the figure below and compared to the case of area-selective deposition. The process of area-selective deposition has come to be well-understood as the occurrence of film growth on one surface with respect to another, where the difference between the two starting surfaces are characterized by the corresponding differences in their material properties. In this regard, material property-selective deposition can be intuitively understood as the occurrence of film growth on two different starting surfaces, each with different material properties, which induce differences in the properties of films growing concurrently on those surfaces.
Schematic illustration of selective materials deposition and etching processes on a planar substrate. (a) Area-selective deposition of films on different surface regions of a patterned planar substrate composed of different surface materials. (b) Material property-selective deposition of a film simultaneously on the different surfaces of the patterned planar substrate, where film regions with different colors denote material having different properties, e.g., phase, microstructure, chemical-composition, etc. Note that films with different material properties are shown over here to have the same thickness and growth rate, which may or may not be the case in reality. (c) Material property-selective deposition followed by selective etching of films having different material properties on the different surfaces of the patterned planar substrate. The selective etching can be performed by additional processing (e.g., wet- or dry-etch treatment) either post-deposition or during deposition (e.g., combining selective deposition and selective etch steps in a sequential process cycle).
In principle, films obtained on a patterned planar substrate by such material property-selective deposition processes have the potential to undergo further processing in a selective context. For instance, these films can be subjected to an appropriate post-deposition processing step, such as a wet- or dry-etch treatment. This additional processing may very well be carried out during the deposition itself, e.g., in a cyclic process that sequentially combines selective deposition and selective etch steps. These approaches could in principle, selectively etch films from one region of the patterned substrate while leaving films grown on the other regions intact (or vice-versa) based on the inherent differences in film properties at those regions. This is also illustrated schematically in the figure above. Such methods could potentially enable films to be obtained only at the desired areas of a patterned planar substrate if, for instance, area-selective bottom-up deposition processes for obtaining the films selectively at those desired areas do not exist. For example, in the figure above the final feature layouts formed in schematic (a) by area-selective deposition resemble the respective layouts obtained in schematic (c). This could be achieved by conducting a selective etch treatment in an additional processing step after or during property-selective deposition depicted by the layout in schematic (b).
Topographically selective processing on 3D substrates
With regard to the cases discussed above for patterned planar substrates having different surface materials, selective deposition can also be carried out in an analogous fashion on 3D substrates having different surface orientations. An example of such a process was recently demonstrated by Bent and co-workers8 in 2016. They performed area-selective ALD of Pt on 3D trench nanostructures where the Pt film was reported to grow only on the vertically oriented sidewall surfaces of the trench nanostructures and not on the horizontally oriented (i.e., planar) top and bottom surface regions of the trenches.8 From a conceptual point of view, one may interpret this as a case of anisotropic deposition. It could very well be considered as the ideal or most extreme case of non-conformal film growth on a 3D substrate. Such an approach for growing films in an anisotropic and area-selective manner on 3D substrates having different geometries or surface orientations was termed as topographically selective deposition.8 Conversely speaking, any materials processing technique carried out on a 3D substrate, be it deposition or etching, could be considered to be topographically selective if there is an anisotropic characteristic associated with it. In this regard, typical ion-driven etch processes, such as reactive ion etching (RIE) or directional plasma ALE, performed on 3D substrates could be considered as topographically selective etch processes. By the same reasoning, one could also expect the cases of material property-selective deposition (discussed earlier for a planar substrate) to bring about topographically selective deposition when performed on a 3D substrate. Going back to the case of area-selective ALD of Pt on 3D trench nanostructures, the anisotropic film growth was achieved by selectively treating the planar surface regions of the 3D trench nanostructures using anisotropic ion implantation of fluorocarbon species. This led to the creation of an ultrathin hydrophobic surface layer only at the planar trench regions which inhibited growth of Pt films at those regions, thereby yielding topographically selective deposition at the vertical sidewalls.8
Cross-sectional TEM image of a three-dimensional (3D) trench-shaped nanostructure where Pt films have been grown by area-selective deposition only on the vertical sidewalls. Directional ion implantation of fluorocarbon species was carried out prior to commencing Pt deposition forming hydrophobic surface layers only at the planar top and bottom regions of the 3D trenches, which inhibited film growth at those regions. Such anisotropic or non-conformal growth of films on 3D substrates has been termed in the literature as topographically selective deposition.8
George and co-workers9 recently demonstrated a topographically selective deposition process on 3D trench nanostructures that was complementary to the results reported by Bent and co-workers.8 The former reported area-selective deposition on the planar top and bottom surfaces of 3D trench nanostructures using electron-enhanced ALD of BN.9 Electron-enhanced ALD itself is an emerging concept whereby deposition is carried out using sequential and self-limiting exposures of electrons and a precursor.9 Dangling bonds have been reported to form at surfaces undergoing electron exposure through the process of electron stimulated desorption, after which the dangling bonds facilitate precursor adsorption and hence, film growth.9,10 For a 3D trench, the dangling bonds should, in principle, form only at the planar top and bottom trench surfaces aligned perpendicular to the directional electron flux, and not on the vertical sidewalls aligned parallel to the same electron flux. This strategy of selective dangling bond formation was reported to enable topographically selective deposition of BN on the 3D trench nanostructures in terms of area-selective film growth on the planar trench surfaces.9 This case of anisotropic deposition reported by George and co-workers9 using ALD is a typical outcome of the widely-used continuous, flux-dependent and line-of-sight based deposition processes (e.g., physical vapour deposition, PVD), which offer no precise growth control. The sequential and self-limiting process of ALD, which offers atomic scale growth control, has been historically adopted to prevent such anisotropic or non-conformal film deposition. However, as mentioned before, the increased complexity in manufacturing future devices consisting of various multi-material layers across planar and 3D layouts calls for atomic scale processing techniques that are also selective in nature.
The aforementioned cases of topographically selective deposition on 3D substrates are schematically illustrated in the figure below (in analogy to the earlier schematics for planar substrates). The 3D substrates consist of trench-shaped nanostructures where the surface planes of vertical sidewalls are perpendicularly aligned to the corresponding planes of the planar top and bottom trench regions. Schematic (a) illustrates area-selective deposition on either the vertical sidewalls or the planar top and bottom regions of the 3D trenches. Schematic (b) illustrates simultaneous material-property selective deposition of films with different properties on the differently oriented surfaces of 3D trench nanostructures.
Schematic illustration of topographically selective materials deposition and etching processes on a three-dimensional (3D) substrate composed of trench-shaped nanostructures. (a) Area-selective deposition of films on the differently oriented surface regions (planar top and bottom regions, vertical sidewalls) of the 3D trench-shaped substrate. (b) Material property-selective deposition of a film simultaneously on the differently oriented surfaces of the 3D trenches, where film regions with different colors denote material having different properties, e.g., phase, microstructure, chemical-composition, etc. Note that films with different material properties are shown over here to have the same thickness and growth rate, which may or may not be the case in reality. (c) Material property-selective deposition followed by selective etching of films having different material properties on the differently oriented surfaces of the 3D trenches. The selective etching can be performed by additional processing (e.g., wet- or dry-etch treatment) either post-deposition or during deposition (e.g., combining selective deposition and selective etch steps in a sequential process cycle). Note that the tapered corners of the films in (b) and (c) serve to indicate an approximate example of how the properties of films at those corners might evolve during growth under the influence of energetic and directionally impinging species.
Similar to the case discussed previously for patterned planar substrates, films obtained on 3D trench nanostructures in such a material property-selective manner also have the potential to undergo additional processing in a selective context. Exposing these films to an appropriate post-deposition processing step, e.g., wet- or dry-etch treatment, could in principle, lead to topographically selective film etching. Likewise, the additional processing could also be performed during deposition, e.g., in a cyclic process that sequentially combines selective deposition and selective etch steps. For instance, film regions could be selectively etched from the planar surfaces of the 3D trenches while those at the vertical sidewalls are left intact (or vice-versa), based on inherent differences in the properties of films formed at those differently oriented regions by material property-selective deposition. This is illustrated schematically in the figure above. Analogous to the case for planar substrates, such methods have the potential to yield films only at the desired surface orientations of 3D substrates if, for instance, area-selective deposition processes for obtaining the films only at those desired surfaces do not exist. This is illustrated in the figure above. It shows that the final feature layouts formed in schematic (c) upon conducting a topographically selective etch treatment on the layout obtained after or during property-selective deposition in schematic (b) resemble the respective layouts obtained in schematic (a) by area-selective deposition.
An example of material-property selective deposition on 3D substrates can be inferred from the work conducted by Agarwal and co-workers for plasma ALD of SiCxNy on 3D trench nanostructures.11 Film deposition was followed by a wet-etch treatment, which resulted in the selective removal of SiCxNy film regions on the vertical trench sidewalls while those on the planar top and bottom trench surfaces remained behind. The results are shown in the images below.11 The post wet-etch image is similar to one of the layouts illustrated in schematic (c) above. The observed results were attributed to the anisotropic nature of (mild) ion bombardment during plasma exposure (on a grounded substrate) which was speculated to densify film regions selectively at the planar trench surfaces.11 It consequently made the film regions growing at the planar trench surfaces comparatively more wet-etch resistant than those formed on the vertical sidewalls. Based on these observations, one could infer that topographically selective deposition of SiCxNy films was attained on the 3D trench nanostructures in terms of microstructure-selective growth of low density films at the sidewalls and high density films at the planar top and bottom trench surfaces.
Cross-sectional TEM images of (left) as-deposited and (right) post wet-etch (in dilute hydrofluoric acid) SiCxNy films grown on 3D trench nanostructures by plasma ALD consisting of a methylamine plasma exposure step on a grounded substrate.11 Directional impingement of ions during plasma exposure was reported to form wet-etch resistant material only at the planar top and bottom regions of the 3D trenches, not at the vertical sidewalls. Conducting a wet-etch treatment after this material property-selective deposition on a 3D substrate led to selective etching of film regions at the vertical sidewalls while those at the planar top and bottom trench surfaces remained behind. Such selective deposition and etching of films on 3D substrates enables topographically selective processing.
Based on the experiments conducted during my doctoral research, I was able to demonstrate a topographically selective deposition process on 3D trench nanostructures12 that basically flipped the results observed by Agarwal and co-workers.11 As part of my research, I carried out plasma ALD of SiNx on 3D trench nanostructures by implementing RF substrate biasing during the nitrogen plasma exposure step.12 This enhanced the energy of the directional ions impinging on the substrate (which I independently verified by measuring ion flux-energy distribution functions using a retarding field energy analyzer13) such that the planar top and bottom regions of the trench were bombarded with much more energy than the vertical sidewalls. It culminated in the degradation of SiNx films only at those planar trench regions which were selectively removed by a post-deposition wet-etch treatment, while the films formed at the vertical sidewalls remained intact. The results are shown in the images below.12 Based on these observations, a topographically selective deposition process for SiNx on 3D trench nanostructures was reported in terms of microstructure-selective growth of high density films at the vertical sidewalls and low density films at the planar surfaces of the trenches. Note that when plasma ALD of SiNx was performed without RF substrate biasing, dense and wet-etch resistant films were obtained at all regions of the 3D trench nanostructures.14 This eliminated the process selectivity enabled in the previous case by directional and energetic ions during plasma ALD with RF substrate biasing.
Cross-sectional TEM images of (left) as-deposited and (right) post wet-etch (in dilute hydrofluoric acid) SiNx films grown on 3D trench nanostructures by plasma ALD consisting of a nitrogen plasma exposure step on an RF biased substrate.12 Directional and energetic ion bombardment during plasma exposure degraded material properties only at the planar top and bottom regions of the 3D trenches, not at the vertical sidewalls. Conducting a wet-etch treatment after this material property-selective deposition on a 3D substrate led to selective etching of film regions at the planar top and bottom surfaces of the trenches while those at the vertical sidewalls remained behind. Such selective deposition and etching of films on 3D substrates enables topographically selective processing.
In other instances of plasma ALD with RF substrate biasing, I was able to demonstrate topographically selective deposition of TiO2 and HfO2 films on 3D trench nanostructures in terms of phase-selective film growth.12 For both these cases, an example of which is shown below, polycrystalline and amorphous films were obtained simultaneously on the planar and vertical surfaces, respectively, of the 3D trench nanostructures.12 This was again due to the anisotropic nature of ions that delivered much more energy to the planar trench surfaces compared to the vertical sidewalls when RF substrate biasing was implemented during plasma ALD.12,13
Cross-sectional TEM image of TiOx film grown on 3D trench nanostructures by plasma ALD consisting of an oxygen plasma exposure step on an RF biased substrate.12 Directional and energetic ion bombardment during plasma exposure induced growth of polycrystalline material at the planar top and bottom trench regions while amorphous material formed at the vertical sidewalls of the trenches. Such selective growth of films with different properties on the differently aligned surfaces of 3D substrates leads to topographically selective deposition.
The examples discussed above clearly elucidate how using and controlling the impingement of directional species before or during film growth by ALD on 3D substrates are key to performing topographically selective processing. Besides these and many other examples reported in academia, several examples have also been reported by industrial sources that can be considered to lie under the umbrella of topographically selective processing on 3D substrates, as shown in the images below.
Cross-sectional TEM images of films obtained either on (a, b) planar surface orientations or (c, d) vertical surface orientations of 3D trench-shaped nanostructures after topographically selective processing.4,15,16,17 The films in (a) and (c) consist of SiNx , while those in (b) consist of TiOx . The films in (d) denote HfO2 and Al2O3.
A report from ASM just a few months ago shows how films were obtained by topographically selective processing only at planar regions of 3D trench nanostructures and not its sidewalls.15 Another example from ASM showed the complete reverse whereby films were obtained only at vertical sidewalls after topographically selective processing.16 Similar examples were also reported by Tokyo Electron.4,17 Films obtained in such a manner at specific surface orientations have been reported to serve purposes such as horizontal or vertical etch stop layers during device fabrication.4,15 These examples highlight how the industry is currently looking at new ways for selective materials processing in the 3rd dimension as 3D features become the norm in both device fabrication and final device architectures. To quote Robert Clark and co-workers,4 “Ultimately integration engineers would like to have a variety of processes to choose from including selective, non-selective, isotropic, and anisotropic processes for both functional and sacrificial materials, and all with atomic level thickness and variability control, representing an area ripe for new innovations.” On this accord, topographically selective processing on 3D substrates is well poised to address upcoming challenges in transferring next-generation device technologies from the lab to the fab.
(1) Mackus, A. J. M.; Merkx, M. J. M.; Kessels, W. M. M. From the Bottom-Up: Toward Area-Selective Atomic Layer Deposition with High Selectivity. Chem. Mater. 2019, 31 (1), 2–12.
(2) Faraz, T.; Roozeboom, F.; Knoops, H. C. M.; Kessels, W. M. M. Atomic Layer Etching: What Can We Learn from Atomic Layer Deposition? ECS J. Solid State Sci. Technol. 2015, 4 (6), N5023–N5032.
(3) Kanarik, K. J.; Tan, S.; Gottscho, R. A. Atomic Layer Etching: Rethinking the Art of Etch. J. Phys. Chem. Lett. 2018, 9 (16), 4814–4821.
(4) Clark, R.; Tapily, K.; Yu, K.; Hakamata, T.; Consiglio, S.; Meara, D. O.; Wajda, C.; Smith, J.; Leusink, G. Perspective : New Process Technologies Required for Future Devices and Scaling. APL Mater. 2018, 6 (5), 058203.
(5) Carlsson, J. O. Selective Vapor-Phase Deposition on Patterned Substrates. Crit. Rev. Solid State Mater. Sci. 1990, 16 (3), 161–212.
(6) Mameli, A.; Kuang, Y.; Aghaee, M.; Ande, C. K.; Karasulu, B.; Creatore, M.; Mackus, A. J. M.; Kessels, W. M. M.; Roozeboom, F. Area-Selective Atomic Layer Deposition of In2O3:H Using a μ-Plasma Printer for Local Area Activation. Chem. Mater. 2017, 29 (3), 921–925.
(7) Cho, C. J.; Kang, J. Y.; Lee, W. C.; Baek, S. H.; Kim, J. S.; Hwang, C. S.; Kim, S. K. Interface Engineering for Extremely Large Grains in Explosively Crystallized TiO2 Films Grown by Low-Temperature Atomic Layer Deposition. Chem. Mater. 2017, 29 (5), 2046–2054.
(8) Kim, W.-H.; Minaye Hashemi, F. S.; Mackus, A. J. M.; Singh, J.; Kim, Y.; Bobb-Semple, D.; Fan, Y.; Kaufman-Osborn, T.; Godet, L.; Bent, S. F. A Process for Topographically Selective Deposition on 3D Nanostructures by Ion Implantation. ACS Nano 2016, 10 (4), 4451−4458.
(9) Sprenger, J. K.; Cavanagh, A. S.; Sun, H.; Roshko, A.; Blanchard, P.; George, S. M. Topographical Selectivity with BN Electron-Enhanced ALD. Presented at the 65th AVS International Symposium & Exhibition; Long Beach, California, 2018.
(10) Sprenger, J. K.; Sun, H.; Cavanagh, A. S.; Roshko, A.; Blanchard, P. T.; George, S. M. Electron-Enhanced Atomic Layer Deposition of Boron Nitride Thin Films at Room Temperature and 100 °C. J. Phys. Chem. C 2018, 122 (17), 9455–9464.
(11) Ovanesyan, R. A.; Leick, N.; Kelchner, K. M.; Hausmann, D. M.; Agarwal, S. Atomic Layer Deposition of SiCxNy Using Si2Cl6 and CH3NH2 Plasma. 2017, 29 (15), 6269–6278.
(12) Faraz, T.; Knoops, H. C. M.; Verheijen, M. A.; van Helvoirt, C. A. A.; Karwal, S.; Sharma, A.; Beladiya, V.; Szeghalmi, A.; Hausmann, D. M.; Henri, J.; Creatore, M.; Kessels, W. M. M. Tuning Material Properties of Oxides and Nitrides by Substrate Biasing during Plasma-Enhanced Atomic Layer Deposition on Planar and 3D Substrate Topographies. ACS Appl. Mater. Interfaces 2018, 10 (15), 13158–13180.
(13) Faraz, T.; Arts, K.; Karwal, S.; Knoops, H. C. M.; Kessels, W. M. M. Energetic Ions during Plasma-Enhanced Atomic Layer Deposition and Their Role in Tailoring Material Properties. Plasma Sources Sci. Technol. 2018, DOI: 10.1088/1361-6595/aaf2c7
(14) Faraz, T.; Van Drunen, M.; Knoops, H. C. M.; Mallikarjunan, A.; Buchanan, I.; Hausmann, D. M.; Henri, J.; Kessels, W. M. M. Atomic Layer Deposition of Wet-Etch Resistant Silicon Nitride Using Di(Sec-Butylamino)Silane and N2 plasma on Planar and 3D Substrate Topographies. ACS Appl. Mater. Interfaces 2017, 9 (2), 1858-1869.
(15) ASM Analyst and Investor Report. Presented at Semicon West; 2018.
(16) ASM Internal Data, Courtesy of Ivo Raaijmakers; 2018.
(17) Iwashita, S.; Suzuki, A.; Shindo, T.; Kikuchi, T.; Matsudo, T.; Morita, Y.; Moriya, T.; Uedono, A. Capacitively Coupled DC/RF Discharges for PEALD Process of Titanium Dioxide Films. Presented at the 65th AVS International Symposium & Exhibition; Long Beach, California, 2018.