Review—Silicon Nitride and Silicon Nitride-Rich Thin Film Technologies: Trends in Deposition Techniques and Related Applications

This article provides an overview of the state-of-the-art chemistry and processing technologies for silicon nitride and silicon nitride- rich ﬁlms, i.e., silicon nitride with C inclusion, both in hydrogenated (SiN x :H and SiN x :H(C)) and non-hydrogenated (SiN x and SiN x (C)) forms. The emphasis is on emerging trends and innovations in these SiN x material system technologies, with focus on Si and N source chemistries and thin ﬁlm growth processes, including their primary effects on resulting ﬁlm properties. It also illustrates that SiN x and its SiN x (C) derivative are the focus of an ever-growing research and manufacturing interest and that their potential usages are expanding into new technological areas. non-hydrogenated

Silicon nitride and carbide thin films, primarily in the form of silicon nitride (SiN x ), silicon carbide (SiC y ), and silicon carbo-nitride (SiN x C y ), where 0 < x < 1.33 and 0 < y < 1, are experiencing a burgeoning of research interest across multiple application sectors. The appeal of these Si-based coatings is attributed to their highly desirable combination of physical, mechanical, electrical, and optoelectronic properties making them prime candidates for applications in the automotive, aerospace, computer chip, solar, light-emitting, and medical industries. [1][2][3][4][5][6] In consideration of the intense current interest in SiN x and SiN x C y , and the expectation that their applications will continue to witness further expansion and extensive diversification, we present an overview on the latest trends and developments in hydrogenated and non-hydrogenated silicon nitride and silicon carbonitride deposition techniques and associated post-deposition processing technologies. Given the fast-moving nature of SiN x and SiN x C y technological advances, the intent is to present an survey of work published within the last five years for silicon nitride and silicon nitride-rich films, i.e., silicon nitride with C inclusion, both in hydrogenated (SiN x :H and SiN x :H(C)) and non-hydrogenated (SiN x and SiN x (C)) forms. Prior years reports will be discussed only in the context of providing appropriate background and support for the more contemporary results summarized herein. This article is not intended to be a comprehensive review, but instead is meant to provide the reader with a focused analysis of research directions particularly where, in the authors' experience, they are reflective of potential commercial relevance.
The silicon nitride literature presents a complex picture of its properties: mechanical, thermal, electrical, tribological, etc. In the best cases, the properties are associated with well-defined chemical compositions and morphologies. However, silicon nitride is frequently described by the process by which it is formed, and reported properties are for compositions that are not fully defined. Accordingly, this review is centered on silicon nitride deposition and the processes and selected properties associated with specific modifications in deposition techniques. While a detailed analysis of film physical, chemical, electrical and optical properties over the range of deposition technologies and conditions is not presented herein, salient properties are summarized in tabular form. Analyses of thin film properties as function of most researched deposition techniques, and an evaluation of resulting applications as they pertain to film properties, will be presented in a subsequent review.
For instance, SiN x , SiC y , and SiN x C y 1-4 are employed as hard protective coatings under challenging thermal, environmental, and chemical conditions due to their high hardness (potentially in ex-cess of 40GPa), effective oxidation resistance, elevated temperature and thermal shock resistance as well as chemical stability, and attractive mechanical, tribological and dielectric properties. [5][6][7][8][9] In particular, bonding in SiN x , SiC y , and SiN x C y exhibits substantially greater covalent character than in silicon dioxide (SiO 2 ). SiN x , SiC y , and SiN x C y can therefore provide an intrinsically greater resistance to diffusive migration than SiO 2 , a feature that is of utmost impact at nanoscale dimensions.
Ultrathin films of SiN x and SiC y are utilized in a broad spectrum of applications in integrated circuitry (IC) technologies, particularly in the microprocessor unit (MPU), system-on-a-chip (SoC), flash memory, and the vertical stacking of electronic devices in what is commonly referred to as three-dimensional (3D) integrated systems. 10 For one, SiN x is widely adopted as diffusion barrier for silicon dioxide (SiO 2 ) dielectric with the introduction of copper metallization structures. Additionally, SiC y , SiN x C y , and SiC y O z are applied as diffusion barriers in combination with low dielectric constant (κ) material replacements to SiO 2 . [11][12][13] Similarly, SiN x and SiN x C y are used as capping layers and etch stops for copper interconnects either individually or, more recently, in combination with selective cobalt capping. 14 The dominant low interlevel dielectric (ILD) film for the bottom few metal layers of the MPU is SiOCH, and for flash memory architectures it is SiOC. 10 Other IC applications incorporate SiN x as a dielectric such as metal-insulator-metal capacitors and thin film transistors (TFTs) due to its high dielectric constant which enables the deposition of thinner films while preserving higher breakdown voltage and lower leakage current. 15,16 In an analogous manner, SiN x and SiC y thin films are successfully incorporated into active optical and optoelectronic devices due to their wide bandgap (2.3 eV for SiC y and 5.1 eV for SiN x ), and elevated electrical breakdown voltage, including panel displays, lighting, and light-emitting devices. 4,[17][18][19] In this respect, both types of Si-based coatings are employed as permeation barriers and encapsulation layers in light-emitting devices (LEDs), and organic LEDs (OLEDs), [20][21][22][23] as well as in the fabrication of various planar optical systems and optical waveguides. 24 Additionally, SiN x C y and SiN x coatings are used or suggested as passivation layers in flexible electroluminescent devices. 25,26 It should also be noted that amorphous hydrogenated SiC x O z thin films are the subject of intense exploration as potential candidates for optoelectronic devices, due to their appealing photoluminescence characteristics. These include both white emission as well as emission in the blue at the highly desirable 1540 nm optical wavelength, when doped with erbium (Er). [27][28][29][30] Nitrogen-rich silicon and SiN x films also serve as host matrix for Si nanocrystals for use in optoelectronic device applications. 31,32 Other applications include the use of hydrogenated amorphous siliconnitrogen (a-SiN x :H) films as an insulating layer in thin-film transistors P692 ECS Journal of Solid State Science and Technology, 6 (10) P691-P714 (2017) (TFT) for liquid-crystal display (LCD) and other emerging display systems. 15,16,33,34 The application of SiN x , SiC y , and SiN x C y also extends into the green energy field, 15,16 primarily in solar cell applications. For example, microcrystalline and amorphous SiC y coatings are employed as window layers in thin film solar cells. 35 And much like the case of the hard coatings and computer chip industries, SiN x and SiC y thin films are applied as passivation layers in silicon solar cells. 36,37 Other applications include the use of Si-rich SiN x as host matrix for Si nanocrystals and Si nanoscale inclusions (Si-ni) light emitters for solar cell applications. 31,38 Likewise, SiN x is witnessing extensive use in biotechnology and medical fields, especially in medical devices due to its high chemical stability, enhanced wear endurance, improved fracture toughness, and, unlike its carbide analog, elevated thermal shock resistance, and good biocompatibility. 39 The resulting functions include usages in medical devices similar to the applications described above for IC systems.
In addition, SiN x can be employed as a protective coating in in vivo and in vitro environments, including, for example, viewing windows for medical devices, insulating membranes for cell electroporation, as well as in biosensors for a variety of health-related applications. 40 In vivo studies demonstrate that SiN x can be considered biostable, although differences in bio-dissolution rates have been observed in films prepared by different deposition techniques. 32,[41][42][43] The SiN x materials are non-irritating. and are considered non-cytotoxic. 44 Furthermore, they are not substrates for bacterial colonization and do not inhibit osteogenesis. 45 It is also worth noting that the mechanical, optical, and electronic characteristics of SiN x and SiC y can be tightly controlled and systematically customized as a function of carbon (C) and nitrogen (N) concentrations. 46,47 This feature makes the SiN x C y phase a prime candidate for applications which require micro-modulation of the SiN x C y system to enable adjustable properties, such as those requiring tunable optical bandgaps and refractive indicies. In particular, SiN x C y films were grown with a tunable bandgap in the range of 2.3eV to 5.0eV, depending on their C and N content. 48,49 However, in most cases, the incorporation of other elements, particularly hydrogen, is not accounted for. It is more proper to designate amorphous hydrogenated silicon nitride as a-SiN x :H. Not only does the amount of hydrogen incorporation affect physical, optical, and dielectric properties (in accordance with the Lorentz-Lorenz relationship) 50 in what is commonly referred to as SiN x :H, the nature of the Si-H versus N-H bonding also plays a significant role in tailoring the resulting film characteristics. [46][47][48][49] Another influencing factor is the Pauling relative electronegativity of the Si, N, and H elements (namely, Si:1.90; N:3.04; H:2.20). Si-N and N-H bonds have relatively high dipole moments, while Si-H bonds have relatively low dipole moments. Thus, even if the atom % of film compositions prepared with different precursors are identical, the resulting films may have different atom bonding arrangements and the dielectric properties of the resulting films will vary.

Fundamental Properties of Silicon Nitride
A review of the state of the art in silicon nitride processing technologies must begin with a summary of the fundamental properties of known phases of silicon nitride. Unfortunately, the last fully comprehensive review of silicon nitride both in monolithic (bulk) and thin film forms predates current film deposition technologies. 51 At present, there is no up-to-date compendium of single reference data that completely describes the properties of silicon nitride in all of its' forms and compositions. More recent articles have examined specific aspects of silicon nitride technologies. 52,53 In particular, the report by Riley 52 provides a historical review of the progress of the silicon nitride ceramic system, including its evolution into a variety of high grade ceramic materials. Corresponding properties such as crystal structure, lattice diffusion and defect chemistry, oxidation, production and general properties were assessed in detail. The work also surveyed the development of microstructure-properties functionality relationships. Also, the review by Hampshire 53 presented a survey of silicon nitride ceramics structure, processing, and properties, including microstructural development, sialons, and applications. Newer review articles of silicon nitride thin film deposition techniques are discussed in Overview of silicon nitride formation and deposition techniques section.
Silicon nitride with the nominal stoichiometry Si 3 N 4 (c-Si 3 N 4 ) occurs in three crystalline forms: α, β, and γ, while amorphous SiN x (a-SiN x ) exists in various forms that display a range of physical, chemical, electrical, and mechanical properties. Additionally, the literature generally refers to crystalline and amorphous silicon nitride with different ratios of silicon to nitrogen as silicon nitride (namely, a-SiN x , or c-SiN x with 0 < x < 1.33), although a few reports described significantly higher N/Si ratio. 25 Table I presents nominal properties for silicon nitride. The data compilation should be considered as a guide by the reader for bulk or crystalline (c-SiN x ), polycrystalline (pc-SiN x ), amorphous nonhydrogenated (a-SiN x ) and hydrogenated (a-SiN x :H) thin films. The data is intended to represent salient, but not absolute, properties of the various forms of SiN x as determined at temperatures in the range of 20 • -100 • C, except for self-evident thermal properties. In this context, the data should provide a baseline in the review and analysis of the properties of SiN x films as reported in the published work.

Overview of Silicon Nitride Formation and Deposition Techniques
In general, silicon nitride monolithic (bulk) and film structures can be prepared by: (1) solid phase synthesis (e.g., nitriding of Si, hot isostatic pressing of silicon nitride particles); (2) liquid phase synthesis (porous composite infiltration with thermal conversion or spin-on deposition with thermal conversion from liquid or polymeric precursors); and (3) vapor phase synthesis in primarily thin and ultrathin films.
Silicon nitride monolithic (bulk) growth methodologies.-Solid phase synthesis of silicon nitride is usually associated with structural ceramic synthesis and the huge literature in this area is often associated with aerospace and military applications, 53,54 but recently the technique has been extended to semiconductor substrates. 55 Liquid phase synthesis, although it straddles application areas, is the least studied of the processes and less is known about the resulting film or bulk properties. Liquid phase synthesis, including spin-on and sol-gel techniques, is usually associated with linear (thermoplastic) polymers or crosslinked (thermoset) resins with alternating silicon and nitrogen atoms in their backbone, and are broadly denoted polysilazanes and aminosilanes. [56][57][58][59][60] Early work in this area was directed toward thermolytic or pyrolytic conversion of polysilazanes directly into shaped or structural silicon nitride ceramics. 61 The successful production of structural ceramics from liquid phase synthesis has not been achieved to date due to issues associated with ceramic yield, by-product diffusion, phase composition, density and chemical composition.
In terms of chemical composition one of the two major classes of polysilazanes, organopolysilazanes (structure 1), has organic carbon substitutions on the backbone leading in general to silicon carbonitrides. 62 Inorganic polysilazanes, alternately termed perhydridopolysilazanes (structure 2), lead more directly to silicon nitride structures, but the polymers themselves are apt to have stability issues that lead to variability in performance.  Limited commercial success of liquid phase synthesis has been the infiltration of porous ceramics followed by pyrolytic conversion to produce densified ceramic-matrix composites (CMCs), 63 as binders in metal-matrix composites (MMCs), 64 and as spin-on film for photolithography with materials supplied initially by Kion Corp and transitioned to Clariant and E Merck Corp. 65 Similarly, commercial acceptance was achieved in fabrication of dielectric layers by spin-on deposition techniques for semiconductor devices in a process in which silicon nitride initially formed from perhydridopolysilazane was ultimately converted to silicon dioxide 66,67 with materials supplied by Tonen Corp, but this approach has largely been abandoned. 68 Silicon nitride thin film vapor processing technologies.-Silicon nitride thin film vapor processing technologies include physical vapor deposition (PVD), primarily sputtering; chemical vapor deposition (CVD) in its various forms, including thermal, hot wire (HW-CVD), plasma-enhanced (PE-CVD), and remote plasma-enhanced; and atomic layer deposition (ALD), also in thermal, plasma-assisted (PA-ALD), plasma-enhanced (PE-ALD), and remote plasma forms. In this context, Takeyama, 25 King,11,12 and Meng, 13 present valuable historical reviews of silicon nitride PVD, CVD, and ALD deposition techniques.
PVD, which in principle involves the transport but not the formation of silicon nitride is an area of continued exploration since film composition and properties are influenced by sputtering methods as well as transport, and deposition environments. 69 There is emerging interest and reports of alternatives to ALD for ultra-thin films for both SiN x and SiN x C y by self-limiting processes associated with self-assembled monolayer (SAM) deposition 70 and molecular layer deposition (MLD). 68,71,72 Vapor phase synthesis is associated with the semiconductor, medical, aerospace, energy, and automotive sectors. As such, the present article will predominantly review the latest research work in vapor phase synthesis. In this respect, Tables II and III present details of very recent vapor phase deposition techniques of SiN x thin films, along with a synopsis of intended applications. 15,16,[20][21][22][23][24][25][31][32][33][34][36][37][38] More specifically, Table II summarizes PVD and CVD work, while  Table III focuses exclusively on atomic layer deposition ALD.
It is worth noting that, historically, CVD in its various forms, including thermal, HW-CVD, PA-CVD, PE-CVD, and remote plasmaenhanced, had been the method of choice for growing SiN x thin films, followed by PVD, mainly magnetron sputtering. More recently, however, ALD (including thermal, PA-ALD, and PE-ALD) appears to be receiving the most attention due to the need for extremely thin SiN x films with increasingly tight control of composition and properties.
In what follows, an overview is first presented of silicon nitride source chemistries. In this context, Table IV outlines the Si-and Nbond dissociation energies for selected SiN x source chemistries 102,103 while Table V focuses on relevant properties of many of the recently studied CVD and ALD SiN x source precursors. 104 Subsequently, the various CVD, PVD, and ALD processes are summarized, respectively, in Appendices A, B, and C and discussed in more detail in the relevant sections.

Overview of Silicon Nitride Source Chemistries
It should be noted that in the case of CVD and ALD processes, the differences in formation and behavior of SiN x thin films can be attributed, in part, to fundamental thermodynamic and chemical properties of precursors. In this context, Table IV lists bond dissociation energies for selected SiN x source chemistries, not only those that are most commonly used, specifically, silane (SiH 4 ) and ammonia (NH 3 ). The table is intended to provide the reader with a baseline comparison of bond dissociation energies of selected organic and inorganic Si and N source chemistries with the most commonly used precursors, namely, silane (SiH 4 ) and ammonia (NH 3 ). Table V The large bond dissociation energy of N 2 and relatively high bond dissociation energy of SiH 4 are consistent with the fact that energetic environments, typically either thermal or plasma, are necessary for thin film formation. The resulting films tend to be rich in H and the H typically resides on the Si atoms, with post deposition thermal treatment commonly applied to reduce H content. 36,38,41,42 Since the N atom is trivalent and the silicon atom is tetravalent, H content in the film has minimal effect on the mobility of films: conformality is difficult to achieve and reflow is not observed in post-deposition process windows. 34,85,91 In contrast, the lower dissociation energies associated with NH 3 and SiI 4 allows deposition at lower temperatures. 105,106 Additionally, chemical pathways associated with silicon diimide and diiodosilylene formation from SiI 4 are enabled for deposition. In this case, H mostly resides on the N atom on films formed from silicon halides and NH 3 with the important consequence that the mobility of H substituted N atoms are constrained in two dimensions rather than three. Conformality is therefore expected to be easier to achieve. This advantage is offset by the fact that the chemical deposition pathway leads to gas phase depletion reactions independent of the substrate. 73 The discussion above is partly simplified, particularly in the case of CVD. Higher energy environments associated with CVD often induce gas phase depletion reactions initiated by unimolecular decomposition of a precursor and/or direct reaction of precursors in the gas phase preceding or concomitant with deposition. 107 ALD more often proceeds by direct reaction with surfaces or by a dissociative adsorption on surfaces (in rate limiting steps) and, as a result, associated reactions, by definition, proceed sequentially. 82,90

Chemical Vapor Deposition (CVD)
Appendix A presents a synopsis of SiN x thermal, plasmaenhanced, hot-wire, and remote plasma-enhanced CVD parameters and post processing treatments (where applicable). Table II and Appendix A show the following common trends in recent CVD work: CVD SiN x films are employed as barrier/protective layer, etch stop, and higher dielectric constant (κ) dielectric in IC microprocessor and memory (primarily metal-insulator-metal capacitor structures) devices; 75 transparent and, moisture permeation barriers, and dielectric layer in solar cells 32,37 and organic light-emitting devices (OLEDs). 22,23 More unique applications include SiN x overlays on cylindrical fused silica optical fibers 76 and host material for Si nanocrystals as active light emitters, 38 both for use in photoluminescence (PL) and optoelectronics devices. One common attribute in all these applications is the inherent ability of CVD to provide conformal SiN x coatings in complex topographical structures, such as high aspect ratio channels, vias, and trenches in IC applications. Silane (SiH 4 ) and ammonia (NH 3 ) are the predominant source chemistries for, respectively, silicon and nitrogen. Under PE-CVD conditions, high quality SiN x films with minimal hydrogen content can be obtained in the temperature range of 300-400 • C. This temperature range is also needed to yield higher density films with low porosity and surface roughness, since it allows longer surface diffusion length of adsorbed radicals in comparison to when lower substrate temperatures are applied. 31 Significantly lower substrate temperature (<100 • C) was used in the case of polymer flexible and polyethyleneterephthalate substrates 20-23,79 to minimize thermal budget induced dam-age to the fragile plastics, resulting in a-SiN x :H films with significant hydrogen content (>5%). 79 The inclusion of increasingly higher hydrogen content (up to 30%) with gradual decrease in processing temperatures was supported by other work. 25 CVD SiN x thin film formation appears to involve the typical CVD reaction pathways: 108,109 (1) transport of gaseous SiH 4 and NH 3 precursor species to the substrate surface, in the case of thermal CVD, or the occurrence of gas phase reactions, in the case of PE-CVD or PA-CVD, followed by transport of the resulting gaseous reactants (e.g., SiH x where x < 4 and NH y where y < 3) to the surface; (2) adsorption of the Si-bearing and Nbearing species to substrate surface; (3) surface diffusion with potential desorption of some reactant groups; (4) surface reaction with film nucleation in island growth or step growth mode; and (5) emission of resulting volatile reaction products. 110 As mentioned above, higher substrate temperature allows longer surface diffusion lengths, leading to extended surface reaction times, and resulting in improved step coverage and reduced hydrogen incorporation. Alternatively, the pre-adsorption reactions in the case of PA-CVD or PE-CVD could produce more active reactant species, leading to higher surface reaction rates at shorter surface diffusion lengths, potentially producing less contaminated SiN x films but with poorer step coverage and lower etch resistance. 107 Although a significant body of research in thermal CVD SiN x can be found in the literature prior to 2010, 73,105,106,[111][112][113][114][115] there are very few recent reports (within the last five years) on the topic, due most likely to the high thermal budget required for dissociation and reaction of the Si and N chemistries, except for P698 ECS Journal of Solid State Science and Technology, 6 (10) P691-P714 (2017) noteworthy research on the thermal CVD reaction of dichlorosilane (SiH 2 Cl 2 ) and nitrogen (N 2 ). This reaction required a substrate temperature of 750 • C, which is prohibitively high for most electronic and solar applications. 77 A significant report 34 included the use of pulsed RF generated by modulating a continuous 200Hz low-frequency wave signal generator with 50% duty cycle in the PE-CVD reaction of SiH 4 and NH 3 at 150 • C. The process yielded film densification (over 20% increase in film density) and smoothing (a decrease in average surface grain size standard deviation from 0.2nm 2 to 0.04nm 2 ), with the SiN x films exhibiting smoother surface morphology and lower void density. A common rule of thumb 32,38,75 in the PE-CVD and PA-CVD work appears to be that lower R = NH 3 /SiH 4 flow ratios (R < 1) lead to Si-rich films (Si/N ratio > 1.1), while higher R (R > 1) produces N-rich films (N/Si ratio > 1.4), with inclusions of Si nanostructures or nanoscale intrusions at even higher R values. Also, lower substrate temperatures tend to yield a-SiN x :H films, with thermal annealing required to reduce H content and lead to film crystallization, while higher processing temperatures produce c-SiN x films with reduced hydrogen content.
Comparable results were obtained in the case of PE-CVD SiN x from a N 2 +SiH 4 mixture, with Si-rich films being formed at higher N 2 flows. 75 Studies of post-deposition thermal annealing effects were also reported under different annealing modes, gases, and durations, with 21 or without vacuum break between the deposition and annealing steps: 36,38,41,42 In the case of in situ annealing, 21 SiN x multilayered permeation barrier stacks deposited on PET substrates <100 • C were transported to a PE-CVD chamber between every two successive 50-nm thick SiN x layers and exposed to a 13.56 MHz RF Ar plasma at varying power density, working pressure, and treatment duration. The work led to the identification of an optimized Ar plasma treatment recipe for the formation of improved permeation barriers, with the finding being attributed to the role of Ar plasma in rearranging Si and N atoms at the SiN x layers interfaces, thus causing a densification of the interfacial regions, and an ensuing reduction in the permeability of the SiN x multilayered stacks. In the case of annealing in a conventional oven with vacuum break, one study 41,42 performed annealing of Sirich SiN x films in a conventional oven in N 2 flow, with thermal treatment at 1100 • C leading to the formation of Si nanocrystals (Si-NCs) within the SiN x host matrix. The Si-NCs acted to significantly enhance the SiN x films photoluminescence properties due to quantum confinement effects. In another study, 36 SiN x samples were annealed in a conventional oven at 830 • C to improve hydrogen passivation and ensure reduced leakage current in the resulting metal-insulator-semiconductor (MIS) structures. Alternatively, another approach 38 implemented rapid thermal annealing (RTA) with vacuum break in pure Ar, Ar with 20% O 2 , and Ar with 50% O 2 of Si nanoscale intrusions (Si-ni) embedded in Si-rich SiN x films. It was found that only at temperatures above 950 • C did the thermal treatment have any effect on the optical properties of the Si nanoscale intrusions, although the effect was less significant than the NH 3 to SiH 4 ratio during the deposition step.

Physical Vapor Deposition (PVD)
Appendix B presents a synopsis of SiN x sputtering parameters and post processing treatments (where applicable). A review of Table  II and Appendix B shows the following common trends in recent sputtering work: Most common applications for sputtered SiN x films consist primarily of a barrier/passivation coating and etch stop in microelectromechanical systems (MEMS); 74,116 high refractive index material for solar cells, 25 through-Si vias for three-dimensional (3D) semiconductor devices, electroluminescent devices, and display devices; 25,31 and high κ dielectric layer in stacked high-dielectric constant (κ) structures for non-volatile memory (NVM) devices. 75 A unique application is as a host material for Si nanocrystals as active light emitters for uses in PL and optoelectronic devices. Given that sputtering tends to be a line of sight technique, the application of sputtering techniques is primarily limited to topographies that are less aggressive with more relaxed design rules and smaller aspect ratios than their CVD and ALD counterparts. DC or RF magnetron sputtering were the deposition techniques of choice for PVD, although the deposition rates for RF magnetron sputtered SiN x were significantly lower than their DC counterparts. 74,116 Furthermore, the DC magnetron sputtered SiN x films exhibited superior chemical and physical properties than their RF magnetron sputtered analogs, while displaying equivalent electrical characteristics in MEMS devices.

P700
ECS Journal of Solid State Science and Technology, 6 (10) P691-P714 (2017) These findings support the conclusion that DC magnetron sputtered films are more suitable than their RF equivalents for most SiN x applications, except in cases where substrates are mechanically or chemically fragile, thus requiring reduced impact energy to minimize ion and radical Induced damage. In DC magnetron sputter work, 116 it was shown that the N 2 plasma back pressure played a key role in modulating the N/Si ratio in the resulting SiN x films, with higher back pressures leading to increased N content in the films. Alternatively, in another report, the N/Si ratio in SiN x films was controlled by employing RF Magnetron sputtering to produce Si-rich films, and PE-CVD to yield N-rich films. 74 A pertinent report 25 compared the properties of SiN x films grown by RF magnetron sputtering and PE-CVD at low temperature. The resulting findings indicated that sputtering was more appropriate than PE-CVD in yielding higher quality SiN x films with enhanced density, with the lower density in the PE-CVD coatings being attributed to the inclusion of hydrogen (due to the lower processing temperature employed). 25 Another report of note 31 focused on the formation of Si nanocrystals (Si-NC) in multilayered structures consisting of alternating Si-rich SiN x (SRN) and Si 3 N 4 ultrathin films. In this case, the N/Si ratio in the SRN layers was regulated by co-sputtering from Si (DC magnetron sputtering) and Si 3 N 4 (RF magnetron sputtering) targets. The Si content in the resulting SRN films was modulated by adjusting the deposition rates from the targets through control of the power applied to the targets, with the application of higher DC power to the Si target leading to an increase in Si concentration. Alternatively, stoichiometric Si 3 N 4 ultrathin films were achieved by RF magnetron sputtering from the stoichiometric target. After the formation of 25 alternating layers consisting of 5 nm thick SRN and 1nm thick Si 3 N 4 , the structures were capped with a 10 nm-thick Si 3 N 4 protective coating and annealed above 900 • C in N 2 environment to form Si-NCs in the Si-rich layers. The annealing step led to improved PL performance, which was attributed to improved crystallization and enhanced nitride passivation in the Si-rich layers.

Atomic Layer Deposition (ALD)
Appendix C presents a synopsis of SiN x thermal, plasma-assisted, and plasma-enhanced ALD parameters and post processing treatments (where applicable). A simple first-order observation is that under similar conditions, regardless of technique, growth per cycle (GPC) is significantly greater with precursors containing multiple silicon atoms. For example, neopentasilane has a higher growth rate than silane. Similarly, hexachlorodisilane has a higher growth rate than dichlorosilane. 113 Table III and Appendix C reveal the following additional trends in recent ALD work: The most common applications for ALD SiN x films are barrier/protective layer, etch stop, passivation layer, spacer material, and high dielectric constant (κ) dielectric in emerging nanoscale IC microprocessor and memory devices 100 and, to a lesser extent, transparent barrier, anti-reflective coating, antimoisture permeation layer, and dielectric layer in solar cell and OLED systems. 88 Other applications include host matrix for ruthenium nanocrystals as seed/barrier layer for copper metallization in IC structures; 97,98 and hydrofluoric acid etch stop layer and electrically insulating spacer in MEMS and medical devices. 80,81 One common attribute in all these applications is the inherent ability of ALD to provide stringent atomic level control and excellent conformality 55 for SiN x coatings in challenging geometries where CVD begins to show its deficiencies and shortcomings. These geometries include highly complex topographical structures, such as extremely high aspect ratio or exceedingly narrow channels, vias, and trenches in IC applications.
In contrast to recent CVD work, very few ALD reports used SiH 4 and other perhydridosilanes, such as neopentasilane ((SiH 3 ) 4 Si), as Si source chemistry. 11,85 Instead, the Si sources employed in the most recent ALD investigations could be organized into two categories: (a) inorganic Si sources, including hydridosilanes, such as monochlorosilane (SiH 3 Cl), 93 dichlorosilane (SiH 2 Cl 2 ), 93 and diiodosilane (SiH 2 I 2 ); 101 and halosilanes, such as hexachlorodisilane (Si 2 Cl 6 ), 82,88,117 Octachlorotrisilane (Si 3 Cl 8 ), 87 and tetraiodosilane (SiI 4 ); 86,100,101 and (b) organic Si sources, primarily amidosilanes, such as BT-BAS (SiN 2 C 8 H 22 ). 92,93 For nitrogen, the majority of the work described the use of NH 3 or N 2 . One report suggested the additional use of hydrazine (N 2 H 4 ), 101 which is quite undesirable given its elevated toxicity and high instability, while another proposed the utilization of t-butylhydrazine (C 4 H 12 N 2 ). 100 Although no dissociation energy is available in the literature for t-butylhydrazine, it is estimated to be significantly lower than N 2 based on reports in the literature on the dissociation energy of N 2 H 2 , 103 thus making t-butylhydrazine more conducive for ALD growth of SiN x at lower temperatures than N 2 . ALD SiN x work 86,87,100,117 was carried out in three different modes: thermal (no plasma), PA-ALD (where the plasma was generated in the reactor directly above the substrate), and PE-ALD (where the plasma was generated remotely and transported into the reactor). For thermal ALD, the majority of the work focused on the reaction of halosilanes and N-bearing reactants, including: (a) SiI 4 and NH 3 or C 4 H 12 N 2 in the temperature range of 175-250 • C; 100 (b) Si halides containing Br and/or I (e.g., SiI 4 , SiBr 4 , SiBr 4-x I x (x = 1-3), or Si y X 2y+2, where y > 2, and X is one or more Br or I) and a N-containing reactant, such as NH 3 , in the temperature range of 350-600 • C; 86 (c) Si 3 Cl 8 and NH 3 in the temperature range of 310-500 • C; 87 (d) Si 2 Cl 6 and NH 3 in the temperature range of 515-573 • C. 117 Only two of the reports 87,117 presented compositional analysis results for the SiN x films. The findings indicated that lower deposition temperatures produced substoichiometric films that oxidized upon exposure to air. Higher processing temperatures generated films that were closer to a N/Si ratio of ∼1.3, and led to a reduction but not complete elimination of oxidation upon removal from the ALD reactor. These results suggest that thermal ALD might require prohibitively higher deposition temperatures (well above 573 • C) to yield stoichiometric films with effective resistance to oxidation. PA-ALD SiN x is the subject of relatively few reports, 86,88 and the work focused on the reaction of halosilanes and N-bearing reactants, primarily: (a) Si halides containing Br and/or I (See section Overview of silicon nitride source chemistries) and a N-containing reactant, such as NH 3 , in the temperature range of 350-600 • C; 86 and (b) Si 2 Cl 6 and NH 3 in the temperature range 350-450 • C. 88 The apparent lack of interest in PA-ALD SiN x could be attributed, at least in part, to concerns about the potential adverse effects of plasma generation directly above the substrate, including the potential inclusion of contaminants in the films. Furthermore, compositional analysis 88 showed that the PA-ALD films were N-rich (N/Si ratio ∼1.71) with significant H incorporation (e.g., as high as 23% at 400 • C). Infrared studies 88 supported the preferential reaction of Si 2 Cl 6 with surface NH 2 clusters, instead of NH groups, with the latter being incorporated in the SiN x films due to their reduced reactivity with Si 2 Cl 6 . Interestingly, the infrared analysis demonstrated that the inclusion of H in the films was primarily in the form of NH species. This is consistent with earlier reports of Atmospheric Pressure CVD (APCVD) silicon nitride generated from iodosilanes. 105 in PE-ALD SiN x , with primary focus on the effects of low thermal budget on PE-ALD process characteristics and resulting film composition and chemical and electrical properties. One such report investigated low temperature PE-ALD growth from bis(t-butylamino)silane (BTBAS) and a N 2 +Ar plasma at 150 • C. 91 Subsequent structural and chemical characterization of the SiN x layers indicated the absence of open pores larger than 0.3nm in diameter, with films as thin as 10 nm displaying good barrier properties. A second investigation examined low temperature PE-ALD (<300 • C) 85 of SiN x films from neopentasilane (NPS) as source chemistry using trisilylamine (TSA) as comparative baseline. The study determined that both precursors exhibited similar N 2 plasma saturation behavior, with NPS displaying higher growth rates. The films were Si rich (Si/N ratio ∼1.13) with minimal O and C contaminants. A third study 94 analyzed the thermal dependence of PE-ALD SiN x films grown from trisilylamine (TSA) and NH 3 in the temperature range 250-350 • C. All the films were nearly stoichiometric (N/Si ratio increased from 1.32 at 250 • C to 1.35 at 350 • C). Alternatively, hydrogen content decreased from ∼13% to ∼8% with the rise in thermal budget. In terms of higher temperature growth, processing temperatures of 400 • C and 500 • C were used in the PE-ALD of SiN x from NH 3 and, respectively, monochlorosilane (MCS) and dichlorosilane (DCS). 93 The work demonstrated that the resulting SiN x spacer (grown at 400 • C) and gate encapsulation (deposited at 500 • C) were crucial components in successful high-κ metal gate applications. Similar findings were presented 100 for PE-ALD SiN x from Si precursors containing an iodine ligand (such as HSiI 3 , H 2 SiI 2 , or H 3 SiI) and a N-containing plasma. The resulting N/Si ratio ranged from 0.5 to 2.0. The findings are consistent with prior work that employed tetraiodosilane and titanium tetraiodide to generate Ti-Si-N diffusion barriers for copper metallization at low temperatures. 73 In terms of the PE-ALD adsorption and decomposition mechanisms for inorganic sources, one relevant report 82 analyzed the reactivity of β-Si 3 N 4 surface sites with Si 2 Cl 6 (using SiH 4 as comparative baseline) during the PE-ALD Si 2 Cl 6 substrate exposure step by combining ab initio density functional theory calculations with actual PE-ALD SiN x film deposition. The analysis examined three types of substrate surface sites: (a) hydrogen passivated N and Si sites (NH/SiH); (b) NH and SiNH 2 sites formed during the NH 3 exposure step (NH/SiNH 2 ); and (c) under-coordinated bare Si=N sites. It was determined that the bare Si=N sites were more energetically favorable than their NH/SiH and NH/SiNH 2 counterparts to react with the Si or Cl atoms from the source precursors. It was also concluded that the reaction energy was lower for Si 2 Cl 6 than SiH 4 . These findings led to the identification of a 3 step PE-ALD process to attain the most energetically favorable surface sites during the Si source PE-ALD substrate exposure step. 82 Another investigation 11 also explored the role of a N 2 plasma pre-treatment prior to the SiH 4 exposure step on Si substrates and found that atomic N and N + are the central reactant species that adsorb to the Si surface to form Si-N. The latter then act as reactive adsorption spots for SiH 4 at N dangling bond sites, generating adsorbed SiH x and NH x species. Further adsorption is excluded until subsequent exposure to the N 2 plasma, leading to a repetition of the previous N species adsorption cycle. Reiterating the alternating N 2 plasma/SiH 4 exposure steps results in the growth of a complete Si-N layer followed by the formation of a continuous SiN x film. This alter-nating exposure process was applied to produce SiN x :H films with enhanced conformality and improved moisture barrier behavior. In terms of the PE-ALD adsorption and decomposition mechanisms for organic sources, a similar theoretical and experimental study 82 of the energies of adsorption and decomposition of bis(dimethylaminomethylsilyl)trimethylsilylamine (DTDN2-H2, C 9 H 29 N 3 Si 3 ) during DTDN2-H2 exposure step on the growing PE-ALD SiN x film surface. The bare Si=N sites (as formed by the N 2 plasma) were found to be the most energetically favorable for the adsorption and reaction of DTDN2-H2. The study also showed that the N/Si ratio in the films increased from 0.98 to 0.99 as the substrate temperature was raised from 300 • C to 400 • C, with oxygen content ∼7.5% due to oxidation upon exposure to air. Further increase in substrate temperature to 500 • C caused higher C incorporation, as well as a significant increase in O content. Alternatively, another study 90 combined first-principles density functional theory with experimentation to examine the effects of N-bearing plasmas (N 2 , H 2 , N 2 -H 2 , NH 3 ) on the mechanisms of adsorption of bis(t-butylamino)silane (BTBAS, SiN 2 C 8 H 31 ) on β-Si 3 N 4 (0001) surfaces with various surface terminations.
The study concluded that the use of H 2 , N 2 -H 2 , and NH 3 plasmas caused termination of reactive β-Si 3 N 4 (0001) surface sites with H and NH x species, thus inhibiting precursor adsorption and film formation. The study also determined that the application of a N 2 plasma did regenerate reactive surface sites terminated with H and NH 2 groups. Interestingly, a complementary investigation 92 for PE-ALD SiN x from BTBAS pointed to the existence of so-called "redeposition effects" resulting from the dissociation of reaction species in the plasma and the redeposition of fragments of such species on the surface of the growing SiN x film. It was found that this effect is driven primarily by the plasma gas residence time, with a shorter residence time leading to a reduction in re-deposition effects and yielding films of higher purity and improved quality. A report of note 96 incorporated ab initio techniques into theoretical models to examine the effects of PE-ALD reaction mechanisms on precursor adsorption and decomposition pathways for a variety of inorganic and organic Si precursors, including SiH 4 , SiH 2 Cl 2 , SiH 2 (CH 3 ) 2 , (Si 3 N 4 ) 4 (NH 3 ) 12 , and SiN 2 C 8 H 22 . The techniques employed realistic cluster models of aminecovered surfaces to derive the configurations and energies of chemisorption and reaction of these Si sources via functional groups removal. These calculations were combined with density functional theory derivations that determined that the initial precursor physisorption phase was essential toward SiN x film formation, which led to accurate predictions regarding the reactivity of a collection of amino-silane precursors. The theoretical derivations also provided correct projections on H retention in the PE-ALD SiN x films. A recent report used density functional theory to model the dissociative chemisorption of silicon nitride precursors (mono(alkylamino)silanes) on silicon dioxide to determine the effect of different aminoalkyl ligands. 118 Adsorption energies, driven primarily by hydrogen bonding did not vary significantly with size of aminoalkyl ligands, however, a large variation in the reaction energy barriers was observed with ligand size due to transition state interactions and steric effects. The ALD window for suitable thin film growth was found to be widest for diisopropylaminosilane (DIPAS) and dipropylaminosilane (DPAS) precursors (∼100 • C-500 • C). Another report 97,98 of note developed a PE-ALD SiN x process as part of forming RuSiN x films as diffusion barriers for copper (Cu) interconnects for IC applications. The process employed tris(isopropylamino)silane (TIPAS) and NH 3 for SiN x . RuSiN x films with varying Ru/SiN x ratios were formed by controlling the number of PE-ALD SiN x formation cycles while maintaining that for PE-ALD Ru constant. The resulting RuSiN x ternary phase consisted of an amorphous SiN x host matrix con-

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ECS Journal of Solid State Science and Technology, 6 (10) P691-P714 (2017) taining Ru nanoclusters of ∼3 nm in diameter, and exhibited stable diffusion barrier performance against Cu diffusion up to 650 • C. Studies of pre-, during, and post-deposition plasma treatment effects were also reported under different treatment modes, gases, and durations, with and without vacuum break between the deposition and treatment steps. Some of the key findings are reported below: In the case of pre-deposition plasma treatment, substrate exposure to Ar plasma prior to SiN x thermal ALD was shown to yield significantly lower SiN x wet etch rates (WERs) compared to the case of no plasma treatment. 86 Alternatively, the opposite effect was observed in the case of a post-deposition H 2 plasma treatment, 81 which was attributed to the role of H 2 plasma in removing contaminants from the film or causing its densification. A similar finding was reported in the post-deposition treatment in a H 2 plasma for 3 hours at 350 • C, which was ascribed to the efficacy of the H 2 plasma at residual C removal from the films. Ar plasma was also observed to improve surface adsorption and activation pathways for PE-ALD SiN x from DIPAS and NH 3 . 84 In this work, successful SiN x low temperature chemisorption (325 • C) was achieved through the application of an additional Ar plasma treatment step after the Si precursor purge step, but prior to the NH 3 exposure cycle. However, when the intermediate number of Ar plasma treatments was increased from 1 to 3, the resulting films exhibited a rise in C and O contamination from, respectively, 0 to 10% and ∼15 to 30%. The films were Si rich with Si/N ratio over 2. Alternatively, in PE-ALD of SiN x from BTBAS and N 2 , 92 it was shown that a constant increase in N 2 plasma exposure time led to a continuous decrease in C content at lower deposition temperatures (<500 • C). For example, for films grown at 200 • C, C content decreased from ∼15% at 1s N 2 plasma exposure time to ∼8% at 15s N 2 plasma exposure time. Films formed at N 2 plasma exposure times below 15s showed high affinity to O, while those deposited at 15s exhibited good resistance to oxidation. However, the composition of SiN x films grown at 500 • C was nearly stoichiometric (Si 3 N 4 ), with minimal O and C contents, irrespective of N 2 plasma exposure time.

Summary and Commentary
The authors have presented an overview of the most recent published work (last five years or so) for SiN x and SiN x -rich films, primarily SiN x with C inclusion, SiN x (C). This survey highlights major emerging developments in the SiN x material system technologies, with focus on Si and N source chemistries and thin film deposition processes and their effects on resulting properties. It also demonstrates that SiN x is the subject of an ever-growing interest and that its use is expanding into new technological areas.
From a chemistry perspective, and while generalizations are difficult when such a wide range of SiN x applications is considered, certain trends are observed. Traditionally, SiN x deposition, particularly in IC devices, utilized Si-and N-rich precursors such as SiH 4 and N 2 in relatively high-energy environments (>700 • C or the use of plasma activation to achieve deposition at lower substrate temperatures). More recently, there has been a continual push toward lower energy deposition processes (reduced processing temperature) driven by the need to minimize thermal budget induced damage to thermally fragile substrates, such as low dielectric constant (κ) materials in IC devices and polymer materials in OLED applications. This need has engendered the utilization of precursors that possess "pre-assembled" Si-N bonds as exemplified by "single-source" precursors such as trisilylamine and, in order to achieve deposition in even lower energy environments, precursors that contain C, such as bis(tbutylamino)silane. Concomitant with this trend, there was recognition that desirable dielectric or passivation properties could be achieved despite the adventitious incorporation of C into films. Functional or performance properties, independent of a simple compositional definition of SiN x , expanded interest in reproducible film formation with controlled and reproducible C inclusion. Silicon carbonitride films became widely accepted and deposition techniques and conditions were explored. As dimension constraints became tighter and ALD techniques became generally accepted, the differences in gas-phase and substrate reactivity with Si-N, Si-C, C-N and Si-halide bonds have been exploited and furthered the evolution and introduction of new precursors.
Another independent trend that is readily recognized is the acceptance of silicon halides as precursors. Historically, while silicon halides react with ammonia and other amines at low temperature, thus making them attractive for lower energy deposition processes, the low gravimetric percentage of silicon in precursors and the troublesome ammonium salt byproducts would have eliminated them from consideration. The acceptance of hexachlorodisilane as a SiN x precursor in full-scale manufacturing is leading to consideration of other silicon halides such as tetraiodosilane in near-term full-scale manufacturing. Inherent in the development of SiN x films is the evolution of SiN x as an encompassing descriptor of the technology to Si-N rich films, such as SiN x with C inclusion, SiN x (C), as well as the precursors and the deposition techniques used to achieve these films.
From a processing perspective, CVD in its various forms, including thermal, hot wire (HW-CVD), plasma-enhanced (PE-CVD), and remote plasma-enhanced CVD, had been historically the method of choice for growing SiN x thin films, followed by physical vapor deposition (PVD), primarily magnetron sputtering. More recently, however, ALD has been receiving the most attention due to the need for extremely thin SiN x films with tight control in composition and properties. This trend is attributed to the inherent ability of ALD to provide strict atomic level control and excellent conformality for SiN x thin films in aggressive geometries where CVD begins to suffer from poor step coverage. These geometries include highly complex topographical structures, such as extremely high aspect ratio or exceedingly narrow channels, vias, and trenches in IC applications.
Another trend that has contributed to the growth of interest in Si-N rich films is the area of heterodevices. Until now, the primary driver for thin film materials has been the manufacture of IC devices. Despite the enormous scholarly literature in virtually every area of thin film technology for microelectronics, there has been little opportunity for new techniques to achieve adoption in full-scale IC manufacturing. Heterodevices, particularly those associated with life sciences, have different sets of material requirements and can be successful at lower manufacturing scale. Heterodevices open a wide range of new commercialization opportunities for silicon nitrogen-rich materials. The field of Si-N rich films will continue to evolve with new film requirements, new techniques such as Molecular Layer Deposition (MLD) and Self-Assembled Monolayers (SAMs) and associated new Si-and N-precursors.