Amorphous Silicon Nitride Thin Films Research Articles

Suppression of crack formation in wafer-scale amorphous SiNx films by residual hydrogen-ligands manipulation

Plasma-Enhanced Chemical Vapor Deposition (PECVD) of amorphous silicon nitride (SiNx) thin films is a critical procedure in microelectronics serving as a surface passivation layer and dielectric barrier. However, intrinsic film stress continuously builds up along with PECVD growth, leading to film cracking. How to achieve crack-free PECVD amorphous SiNx film within a large thickness range remains a critical unresolved challenge in semiconductor industry. In this study, we revealed that high residual NH ligands from the NH3 precursor could induce excessive tensile strain at the SiNx/Si wafer interface and consequently aggravate SiNx film crack formation. With a heating pretreatment on the wafer, residual H ligands were effectively reduced to achieve homogenous chemical composition in SiNx film. As a result, the crack number declined ∼42% and the remaining crack length was substantially shorter in contrast to the original SiNx film. This work demonstrates the crucial role of residual ligands on internal strain regulation and points out a pathway to achieve crack-free PECVD SiNx films in industrial manufacturing.

Effect of dehydrogenation on optical constants of silicon nitride thin films

This study presents annealing experiments conducted on near-stoichiometric hydrogenated amorphous silicon nitride (a-SiNx:H) thin films deposited via Plasma Enhanced Chemical Vapor Deposition. The heat treatment was carried out under an inert atmosphere within a temperature range of 400–1000 °C, with increments of 100 °C. After each annealing step, a-SiNx:H films were subjected to characterization using visible and FTIR spectroscopies, evaluating their optical and structural properties, as well as hydrogen concentration. The infrared analysis reveals a significant reduction in Si–H bond concentration within the temperature range of 400–700 °C, becoming virtually undetectable at higher annealing temperatures. Conversely, the N–H bond concentration primarily decreases at elevated temperatures, persisting at 23% of its initial value at 1000 °C. Optical constants were determined from transmittance spectra using a dispersion model that combines two unbounded Tauc-Lorentz oscillators. The refractive index experiences an increase up to 600 °C due to material densification, followed by a decrease at higher temperatures attributed to optical gap widening. Dispersion curves of the extinction coefficient reveal a wide sub-gap absorption band, which vanishes after a 900 °C annealing step, rendering the material suitable for waveguide applications.

Low-temperature fabrication of silicon nitride thin films from a SiH4+N2 gas mixture by controlling SiNx nanoparticle growth in multi-hollow remote plasma chemical vapor deposition

High-quality amorphous silicon nitride (SiNx) thin films were fabricated by the controlled growth of nanoparticles during SiH4+N2 multi-hollow remote plasma chemical vapor deposition (CVD) at low substrate temperature 100 °C. Measurements from quartz crystal microbalances showed that a higher amount of nanoparticle incorporation in the SiNx film corresponded to a higher ratio of N/Si in the film, implying that the nanoparticles were nitrided in the plasma phase. We controlled the size of the nanoparticles by tuning the gas flow ratio of N2/SiH4 and the total gas flow rate. Transmission electron microscopy and energy-dispersive X-ray spectroscopy showed that smaller nanoparticles in the plasma led to a higher ratio of N/Si in the film and a lower hydrogen content. We attribute these results to the low heat capacity and large specific surface area of the nanoparticles, which enabled active chemical reactions on their surface in the plasma.

Structural evolution and mechanical properties of nitrogen doped hydrogenated amorphous silicon thin films

Nitrogen doped hydrogenated amorphous silicon thin films, also recorded as silicon rich hydrogenated amorphous silicon nitride thin films, were deposited by plasma enhanced chemical vapor deposition. The structural evolution and mechanical properties of the films with different nitrogen contents were studied by Fourier transform infrared spectroscopy, Raman scattering spectroscopy, and the density and stress measurement system, respectively. The results showed that with the increase in ammonia gas flow rate from 0.5 SCCM to 20 SCCM, the tensile stress and the density of the films decreased from 600 MPa to 280 MPa and from 2.31 g/cm3 to 2.08 g/cm3, respectively. The hydrogen bonding configurations, hydrogen content, and structural ordering evolution were investigated to reveal the relationship between the structural and mechanical properties of the films. A qualitative model was proposed to explain the role of nitrogen and hydrogen atoms during the film growth.

Effects of various substrate materials on structural and optical properties of amorphous silicon nitride thin films deposited by plasma-enhanced chemical vapor deposition

The plasma-enhanced chemical vapor deposition (PECVD) technique is well suited for fabricating optical filters with continuously variable refractive index profiles; however, it is not clear how the optical and structural properties of thin films differ when deposited on different substrates. Herein, silicon nitride films were deposited on silicon, fused silica, and glass substrates by PECVD, using silane and ammonia, to investigate the effects of the substrate used on the optical properties and structures of the films. All of the deposited films were amorphous. Further, the types and amounts of Si-centered tetrahedral Si–SivN4-v bonds formed were based upon the substrates used; Si–N4 bonds with higher elemental nitrogen content were formed on Si substrates, which lead to obtaining higher refractive indices, and the Si–SiN3 bonds were mainly formed on glass and fused silica substrates. The refractive indices of the films formed on the different substrates had a maximum difference of 0.05 (at 550 nm), the refractive index of SiNx films formed on silicon substrates was 1.83, and the refractive indices of films formed on glass were very close to those formed on fused silica. The deposition rates of these SiNx films are similar, and the extinction coefficients of all the films were lower than 10−4.

The use of MACE technique on amorphous silicon-rich silicon nitride thin films for the formation of spherical silica nanoparticles

We present a method that employs the metal-assisted chemical etching technique and a subsequent NH3 plasma treatment for obtaining silica nanoparticles (SNPs) uniformly distributed on an amorphous silicon-rich silicon nitride (SRN) film. These particles ranging from 50 to 300 nm are formed directly from the SRN films, and their density is related to the flow rate of the precursor gas (SiCl4). We determined that the SNPs are a SiOx compound (x > 1) with amorphous structure. Furthermore, the chemical composition of the bulk of the remaining SRN film after the etching in a HF/H2O2-based solution and plasma treatment maintain the properties of a pristine one if the initial thickness of the film is large enough (≥ 1 µm). This method enables the formation of silica nanoparticles directly on a silicon-rich silicon nitride film that could have potential optoelectronic applications, implicit in the optical measurements.

Fabrication of amorphous silicon nitride thin films by radio-frequency sputtering assisted by an inductively coupled plasma

We have investigated the preparation of amorphous silicon nitride (a-Si1-xNx:H) thin films by reactive RF sputtering assisted by inductively coupled plasma (ICP) with argon (Ar), hydrogen (H2) and nitrogen (N2). The ICP assist system gives the high density ICP mode plasma near the substrate with relatively low plasma potential and electron temperature by applying >50W to the ICP antenna coil. The E04 band gap is successfully controlled from 2.2eV to 3.3eV by changing the RF power to the antenna coil. The ICP mode plasma significantly enhances the formation of Si-N bonds in a-Si1-xNx:H films, due to the effective generation of N radicals near the substrate. The ICP mode plasma also gives the high deposition rate for wide band gap a-Si1-xNx:H films. A small N2 gas ratio (<1%) is enough to produce wide band gap a-Si1-xNx:H films, therefore, a sufficient Ar gas ratio can be maintained for the Si sputtering. This method gives a simple and effective process to produce wide band gap a-Si1-xNx:H films with a relatively high deposition rate.

Defect states in amorphous SiNx:H compounds using thermally stimulated exo-electron emission

Defect states of hydrogenated amorphous silicon nitride (a-SiNx:H) thin films were measured using the thermally stimulated exoelectron emission spectroscopic method. The defect states of films with nitrogen to silicon composition ratios (x) of 0.92, 1.21, and 1.44 were found to be located 1.66, 1.77, and 1.82eV below the conduction band minimum, respectively.We confirmed that the hydrogen concentration decreased as x was increased via an elastic recoil detection analysis experiment, while as the hydrogen concentration increased, the defect concentration decreased. From these results, we concluded that the defect centers could be related to dangling bonds, the so-called K centers, in silicon nitride. Furthermore, as the concentration of SiH was increased with decreasing x via the infrared spectroscopy, the defect density was reduced through the defect passivation.

Mechanical and electrical properties of RF magnetron sputter deposited amorphous silicon-rich silicon nitride thin films

Amorphous silicon nitride thin films in a thickness range of 40 to 500nm are deposited onto (100) silicon wafers using radio frequency magnetron sputter deposition. Analysis of variance techniques are used to determine which deposition parameter has a significant impact on the film properties. The biaxial stress of the layers is found to be compressive independent of the plasma chamber pressure levels and to increase with increasing plasma power. The chemical composition of the films is silicon-rich, resulting in an index of refraction (IOR) of 2.55 independent of deposition conditions. Both IOR and X-ray photoelectron spectroscopy measurements indicate a nitrogen to silicon ratio in the range of 0.71–0.85. The etch rates for HF wet chemical etching and for CF4:O2 reactive ion etching are found to be much higher compared to direct current sputter deposited silicon nitride films with only a weak dependency on the deposition conditions. Temperature dependent leakage current measurements using Au/Cr/SiNx/Si structures between 25 and 300°C show two dominating leakage current mechanisms: ohmic conduction dominates at low applied electric field values below 0.1 MV/cm and Poole–Frenkel type conduction above 0.3MV/cm. The extracted electrical parameters such as the activation energy or the barrier height are found to be nearly unaffected by the deposition parameters.

Thermal activation and temperature dependent PL and CL of Tb doped amorphous AlN and SiN thin films

AbstractThe effect of terbium (Tb) doping on the photoluminescence (PL) and cathodoluminescence (CL) spectra of amorphous aluminum nitride (a ‐AlN) and amorphous silicon nitride (a ‐SiN) thin films has been investigated for different temperature conditions. The samples were prepared by RF dual magnetron reactive sputtering technique with a Tb concentration of about ∼1 at% An enhancement of the light emission is obtained after thermal annealing treatments following the activation of luminescent centers. Furthermore, the Tb related integrated light emission intensity is reported exhibiting a continuous increase with the samples temperature well below thermal quenching for both materials. This behavior suggests a phonon assisted energy migration mechanism which contributes to the effective energy transfer from the matrix to the Tb ions. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Mechanical and electrical properties of DC magnetron sputter deposited amorphous silicon nitride thin films

Amorphous silicon nitride SiNx thin films in a thickness range of 40 to 500nm were deposited onto (100) silicon wafers using DC magnetron sputtering. The biaxial stress of the films was found to be tuneable in the range of −1300MPa (compressive) to almost 0 by varying the plasma power density and the back pressure in the deposition chamber. The films were close to stoichiometric composition with x ranging from 1.27 to 1.34. The refractive index n decreases from 2.10 to 1.97 with increasing back pressure, indicating compositional changes in the thin films. This finding, however, was not confirmed by Fourier-Transform Infrared Spectroscopy measurements. On the other hand, lower wet chemical etch rates revealed a larger robustness of layers deposited at conditions where the mean kinetic energy of the sputtered particles is higher. Temperature dependent leakage current measurements using Au/Cr/SiNx/Si MIS (metal–insulator–semiconductor) structures between 25 and 300°C have shown that ohmic and Poole–Frenkel conduction mechanisms dominate the leakage current behaviour at electrical fields ranging up to 0.5MV/cm. The extracted physical parameters such as the corresponding activation energies were found to be mildly affected by the deposition parameters.

Highly Conformal Amorphous W–Si–N Thin Films by Plasma-Enhanced Atomic Layer Deposition as a Diffusion Barrier for Cu Metallization

Ternary and amorphous tungsten silicon nitride (W–Si–N) thin films were grown by atomic layer deposition (ALD) using a sequential supply of a new fluorine-free, silylamide-based W metallorganic precursor, bis(tert-butylimido)bis(bis(trimethylsilylamido))tungsten(VI) [W(NtBu)2{N(SiMe3)2}2], and H2 plasma at a substrate temperature of 300 °C. Here, W(NtBu)2{N(SiMe3)2}2 was prepared through a metathesis reaction of W(NtBu)2Cl2(py)2 (py = pyridine) with 2 equiv of LiN(SiMe3)2 [Li(btsa)]. The newly proposed ALD system exhibited typical ALD characteristics, such as self-limited film growth and linear dependency of the film growth on the number of ALD cycles, and showed a high growth rate of 0.072 nm/cycle on a thermally grown SiO2 substrate with a nearly zero incubation cycle. Such ideal ALD growth characteristics enabled excellent step coverage of ALD-grown W–Si–N film, ∼100%, onto nanotrenches with a width of 25 nm and an aspect ratio ∼4.5. Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy analysis confirmed that the incorporated Si and W were mostly bonded to N, as in Si–N and W–N chemical bonds. The film kept its amorphous nature until annealing at 800 °C, and crystallization happened at local areas after annealing at a very high temperature of 900 °C. An ultrathin (only ∼4 nm thick) ALD-grown W–Si–N film effectively prevented diffusion of Cu into Si after annealing at a temperature up to 600 °C.

Observation of “fast” and “slow” decay processes in oxygen-doped hydrogenated amorphous silicon nitride thin films

Oxygen-doped amorphous silicon nitride thin films were fabricated in a plasma enhanced chemical vapor deposition system by using H2 diluted gases under a substrate temperature of 250 °C. An intense photoluminescence was achieved under 325 nm laser excitation and the luminescence is reduced to about 70% of the initial emission intensity after 1hr laser irradiation, which is improved compared with that deposited at low substrate temperature. Time-resolved photoluminescence behaviors were characterized and two recombination processes were observed, one is a “fast” nanosecond PL decay component which was explained in terms of the relaxation process to the localized O-Si-N related states, and the other is a “slow” microsecond radiative recombination component via the localized states associated with O-Si-N bands.

Effect of nitrogen mole fraction on hydrogenated amorphous silicon nitride deposited by DC magnetron sputtering: transition between metallic and reactive sputtering

AbstractThe hydrogenated amorphous silicon nitride thin films are deposited by DC magnetron sputtering in argon, molecular hydrogen and nitrogen plasma mixture. The films are deposited at 150 °C and at 130 W sputtering power with wide range of nitrogen mole fraction. The plasma is characterized by the target voltage measurement. The samples are characterized by the optical transmission measurements and the physicochemical structure is studied by the FTIR absorption spectroscopy. When the nitrogen mole fraction increases from 0.075 to 0.24 the target voltage decrease from 413 to 325 V and increases suddenly to 450 V when nitrogen mole fraction increases to 0.69. The decrease of target voltage may be due to the transition from metallic to reactive sputtering process and its increase can be explained by the implantation of reactive ions in the target. Sputtering rate and refractive index decrease respectively from 8.33 to 1.73 Å/sec and from 2.52 to 1.68 with nitrogen mole fraction. The value of refractive index at critical nitrogen mole fraction is about 1.8. When the nitrogen mole fraction increases the frequency related to Si‐H band, on FTIR spectrum, shift from 2097 cm‐1 to reach 2209 cm‐1 at the critical nitrogen mole fraction. Thereafter it varies sharply to 2190 cm‐1 and remains constant. A sudden change is observed on the sputtering rate, on the intensity of the band attributed to the stretching vibration of N‐H and Si‐H bonds. From these results we believe that the stoichiometric composition is reached at the critical nitrogen mole fraction and beyond this the deposited films are rich of nitrogen and hydrogen. This is consistent with the explanation given of the variation of the target voltage. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of Introducing Free Gaseous Radicals of Trichlorosilane and Ammonia Precursors on Growth and Characteristics of LPCVD a-SiNx Ultra Thin Films

Preparation and characteristics of amorphous silicon nitride (a-SiNx) thin films deposited by low pressure chemical vapor deposition (LPCVD) are investigated. Free gaseous radicals of trichlorosilane (TCS) and ammonia (NH3) are produced by passing each of the precursor gases separately over Pt-Ir/Al2O3 catalyst at the temperature of 600 C. Kinetics studies of the LPCVD are carried out in different total pressures, NH3/TCS flow rate ratios and temperatures. Surface topography, chemical concentrations, growth rate and thickness are studied by Ellipsometry, x-ray photo-electron spectroscopy (XPS), atomic force microscopy (AFM) and auger depth profiling (ADP). Analysis of experiments indicates that at the temperatures between 730 C and 830 C, the growth rate of thin films follows an Arrhenius behavior with activation energy of 166.3 KJ.mol-1. The measured hydrogen contamination in a-SiNx ultra thin films is 1.05 at% which is 17 times lower than the corresponding contamination in the films produced by (PECVD) and 3.4 times lower than the contamination in the LPCVD thin films with silane (SiH4) or dichlorosilane (DCS) and Ammonia. The surface topography of the prepared films is smooth and uniform and the thickness varies between 23 and 101 nanometers.

Optical and Structural Properties of Ammonia-Free Amorphous Silicon Nitride Thin Films for Photovoltaic Applications

ABSTRACT Hydrogenated amorphous silicon nitride (a-SiNx:H) thin films have been deposited through the green chemistry route using silane (SiH4) and nitrogen (N2) as process gases with SiH4 flow being variable and N2 flow being constant without the use of pollutant and corrosive ammonia (NH3) by the plasma-enhanced chemical vapor deposition technique at 13.56 MHz. Fourier transform infrared spectroscopy analysis shows various possible vibrational modes of Si-H, Si-N, and N-H bonds present in the film. Raman spectroscopy is performed on these samples to calculate volume fractions corresponding to amorphous phases present in the a-SiNx:H films. The refractive index (η) values are calculated using Swanepoel's method, which are in the range of 2.89 to 3.17. The thickness of the deposited films has been evaluated using transmission spectra. Absorption coefficient and band gap (E g) values are obtained from optical absorption studies. An increase in the E g and a decrease in the η value have been observed for the samples grown with decreasing SiH4 flow.

Raman and FTIR Studies on PECVD Grown Ammonia-Free Amorphous Silicon Nitride Thin Films for Solar Cell Applications

Ammonia- (NH3-) free, hydrogenated amorphous silicon nitride (a-SiNx:H) thin films have been deposited using silane (SiH4) and nitrogen (N2) as source gases by plasma-enhanced chemical vapour deposition (PECVD). During the experiment, SiH4 flow rate has been kept constant at 5 sccm, whereas N2 flow rate has been varied from 2000 to 1600 sccm. The effect of nitrogen flow on SiNx:H films has been verified using Raman analysis studies. Fourier transform Infrared spectroscopy analysis has been carried out to identify all the possible modes of vibrations such as Si–N, Si–H, and N–H present in the films, and the effect of nitrogen flow on these parameters is correlated. The refractive index of the above-mentioned films has been calculated using UV-VIS spectroscopy measurements by Swanepoel’s method.

Amorphous silicon nitride thin films implanted with cerium ions for cathodoluminescent light source

Cerium ions were implanted into amorphous silicon nitride thin films, and the cathodoluminescence (CL) of the thin films was investigated. The Ce ions were distributed within a depth of around 20nm from the surface of the amorphous silicon nitride, and they formed three kinds of cations in the thin films. By implanting the Ce ions, the CL of the films was distributed around a wavelength of 405nm. The CL increase of more than 2.2 times compared to silicon nitride thin films was observed at 25-kV electron beam excitation. This increase was significantly enhanced by adopting a high-temperature annealing process after the cerium-ion implantation.

A comparison of the mechanical stability of silicon nitride films deposited with various techniques

A comparison of mechanical properties of amorphous silicon nitride thin films deposited with various techniques used for microelectronic applications was conducted. Nitride films with thicknesses less than 80nm were deposited on (001) oriented silicon wafers by using various methods: low pressure chemical vapor deposition (LPCVD), rapid thermal CVD (RTCVD), atomic layer deposition (ALD) and plasma enhanced CVD (PECVD). The wafer curvature method was used to show that the as-deposited LPCVD, RTCVD and ALD films exhibited tensile residual stresses that decreased with silicon richness. In contrast, the stress of the PECVD as-deposited layers ranged from tensile to ultra-compressive, depending on the exposure to high plasma power and ion bombardment during growth. After high temperature annealing, the LPCVD, RTCVD and ALD nitride stresses were almost unchanged, indicating that these films/substrate systems have significant thermal mechanical stability. In contrast, it was observed that, regardless of the initial stress, the annealed PECVD films developed tensile stress after high temperature treatment, with the same dependence of stress on refractive index as was found with the other deposition techniques. The Young's moduli, measured by performing nano-indentation on 200nm thick nitride layers, were found for most samples to be correlated with film density.

Optical and Structural Features of Silicon-Rich Hydrogenated Amorphous Silicon Nitride Thin Films

A systematic study of fabrication and effect of post-deposition processing on the optical and structural features of silicon-rich hydrogenated amorphous silicon nitride thin films deposited by Hg-sensitized Photo-Chemical Vapour Deposition technique is presented. Both deposition parameters and post-deposition thermal treatment resulted into substantial change in the refractive index associated with the densification of the film. Our studies reveal that the presence of hydrogen and its out-diffusion upon thermal treatment play a crucial role in the overall structural evolution, specially the stabilization of individual phases such as Si and Si3N4. We further report the room-temperature photoluminescence from as-deposited films, which is due to formation of silicon nanostructures in crystalline and amorphous forms. These studies are of great interest from the prospective of commercially viable Si-based technology.