Vapor Deposited Silicon Nitride Films Research Articles

Selectively patterned Mg-doped GaN by SiNx-driven hydrogen injection

We demonstrate a method to achieve selectively patterned Mg-doped GaN layers using hydrogen drive-in through plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiNx) films. Activated Mg-doped GaN layers were selectively deactivated by patterned PECVD SiNx films with low-temperature annealing and showed high-resistive behavior. Spatially resolved photoluminescence measurements were used to optically verify the deactivation of Mg acceptors and showed distinct features corresponding to activated and deactivated Mg in GaN. The method suggested here provides a simple and effective method to achieve patterned Mg-doped GaN regions without thermal and plasma damage, which could cause degradation of device performance. The proposed method could provide a way to achieve future high-performance GaN lateral and vertical devices that rely on laterally patterned doping.

Improved PECVD SixNy film as a mask layer for deep wet etching of the silicon

Although plasma enhanced chemical vapor deposition (PECVD) silicon nitride (SixNy) films have been extensively investigated by many researchers, requirements of film properties vary from device to device. For some applications utilizing SixNy film as the mask Layer for deep wet etching of the silicon, it is very desirable to obtain a high quality film. In this study, SixNy films were deposited on silicon substrates by PECVD technique from the mixtures of NH3 and 5% SiH4 diluted in Ar. The deposition temperature and RF power were fixed at 400 °C and 20 W, respectively. By adjusting the SiH4/NH3 flow ratio, SixNy films of different compositions were deposited on silicon wafers. The stoichiometry, residual stress, etch rate in 1:50 HF, BHF solution and 40% KOH solution of deposited SixNy films were measured. The experimental results show that the optimum SiH4/NH3 flow ratio at which deposited SixNy films can perfectly protect the polysilicon resistors on the front side of wafers during KOH etching is between 1.63 and 2.24 under the given temperature and RF power. Polysilicon resistors protected by the SixNy films can withstand 6 h 40% KOH double-side etching at 80 °C. At the range of SiH4/NH3 flow ratios, the Si/N atom ratio of films ranges from 0.645 to 0.702, which slightly deviate the ideal stoichiometric ratio of LPCVD Si3N4 film. In addition, the silicon nitride films with the best protection effect are not the films of minimum etch rate in KOH solution.

Gettering of interstitial iron in silicon by plasma-enhanced chemical vapour deposited silicon nitride films

It is known that the interstitial iron concentration in silicon is reduced after annealing silicon wafers coated with plasma-enhanced chemical vapour deposited (PECVD) silicon nitride films. The underlying mechanism for the significant iron reduction has remained unclear and is investigated in this work. Secondary ion mass spectrometry (SIMS) depth profiling of iron is performed on annealed iron-contaminated single-crystalline silicon wafers passivated with PECVD silicon nitride films. SIMS measurements reveal a high concentration of iron uniformly distributed in the annealed silicon nitride films. This accumulation of iron in the silicon nitride film matches the interstitial iron loss in the silicon bulk. This finding conclusively shows that the interstitial iron is gettered by the silicon nitride films during annealing over a wide temperature range from 250 °C to 900 °C, via a segregation gettering effect. Further experimental evidence is presented to support this finding. Deep-level transient spectroscopy analysis shows that no new electrically active defects are formed in the silicon bulk after annealing iron-containing silicon with silicon nitride films, confirming that the interstitial iron loss is not due to a change in the chemical structure of iron related defects in the silicon bulk. In addition, once the annealed silicon nitride films are removed, subsequent high temperature processes do not result in any reappearance of iron. Finally, the experimentally measured iron decay kinetics are shown to agree with a model of iron diffusion to the surface gettering sites, indicating a diffusion-limited iron gettering process for temperatures below 700 °C. The gettering process is found to become reaction-limited at higher temperatures.

A generic "micro-Stoney" method for the measurement of internal stress and elastic modulus of ultrathin films.

Accurate measurement of the mechanical properties of ultra-thin films with thicknesses typically below 100 nm is a challenging issue with an interest in many fields involving coating technologies, microelectronics, and MEMS. A bilayer curvature based method is developed for the simultaneous determination of the elastic mismatch strain and Young's modulus of ultra-thin films. The idea is to deposit the film or coating on very thin cantilevers in order to amplify the curvature compared to a traditional "Stoney" wafer curvature test, hence the terminology "micro-Stoney." The data reduction is based on the comparison of the curvatures obtained for different supporting layer thicknesses. The elastic mismatch strain and Young's modulus are obtained from curvature measurements of cantilevers before and after the film deposition. The data reduction scheme relies on both analytical and finite element calculations, depending on the magnitude of the curvature. The experimental validation has been performed on ultra-thin low pressure chemical vapor deposited silicon nitride films with thickness ranging between 54 and 133 nm deposited on silicon cantilevers. The technique is sensitive to the cantilever geometry, in particular, to the thickness ratio and width/thickness ratio. Therefore, the precision in the determination of the latter quantities determines the accuracy on the extracted elastic mismatch strain and elastic modulus. The method can be potentially applied to films as thin as a few nanometers.

Effect of hyperthermal annealing on LPCVD silicon nitride

The objective of this paper is to understand the effects of 1100°C annealing on film thickness, refractive index and especially residual stress of low-pressure chemical vapor deposition (LPCVD) silicon nitride films. The annealing effect on Young's modulus of silicon nitride films is also discussed. For these purposes, a number of 1100°C furnace annealing processes in N2 atmosphere were carried out. With the increase of annealing time, film thickness decreases exponentially and correspondingly the refractive index increases. Both film thickness and refractive index reach a stable value after several times annealing. Due to the film densification and viscous flow, residual stress of Si–Si3N4 system increases in the first 10min annealing treatment and then decreases in the following annealing processes. Based on the Maxwell viscoelastic model, an improved model which considers film densification and viscous flow simultaneously is built to explain the effect of annealing process on residual stress.

Hydrogen passivation of interstitial iron in boron-doped multicrystalline silicon during annealing

Effective hydrogenation of interstitial iron in boron-doped multicrystalline silicon wafers is reported. The multicrystalline silicon wafers were annealed with plasma-enhanced chemical vapour deposited silicon nitride films, at temperatures of 400 °C – 900 °C and for times from minutes to hours. At low temperatures where a combined effect of hydrogenation and precipitation of dissolved Fe is expected, results show that the hydrogenation process dominates the effect of precipitation. The concentrations of dissolved interstitial iron reduce by more than 90% after a 30-min anneal at temperatures between 600 and 900 °C. The most effective reduction occurs at 700 °C, where 99% of the initial dissolved iron is hydrogenated after 30 min. The results show that the observed reductions in interstitial Fe concentrations are not caused by the internal gettering of Fe at structural defects or by an enhanced diffusivity of Fe due to the presence of hydrogen. The hydrogenation process is conjectured to be the pairing of positively charged iron with negatively charged hydrogen, forming less recombination active Fe-H complexes in silicon.

Determination of the suitable refractive index of solar cells silicon nitride

In silicon solar cells studies, the optimal refractive index of plasma- enhanced chemical vapor deposited silicon nitride films is usually determined by an electrical characterization. This technique is done by minority carrier lifetime or surface recombination velocity measurement. We developed in this study a method which encompasses electrical and optical film properties. This method is based on the calculation of the short circuit current densities of multicrystalline silicon solar cells. The optimal refractive index is determined by the maximum short circuit current density.Films with the following refractive indices were studied: 1.9, 2.0, 2.1 and 2.4. The thicknesses are those of an optimal anti-reflection coating (one-quarter wavelength).The optical characterization of these films deposited on multicrystalline silicon wafers and on corning glass gave a minimal weighted reflection for refractive index of 2.0 and a maximum transmission for refractive index of 1.9, respectively.The QSSPC characterization revealed that the film refractive index of 1.9 performed the best passivation quality. Internal quantum efficiencies of simulated multicrystalline silicon solar cells coated with these films were determined by PC1d program simulation.Short-circuit current densities calculated using these experimental and simulated data revealed that the optimal refractive index is 1.9.

Thermal annealing effect on ultraviolet-light-induced leakage current in low-pressure chemical vapor deposited silicon nitride films

We report the effects of isothermal annealing on the current component, the paramagnetic K0 centers, and charge accumulation, induced by exposing silicon nitride films and silicon nitride–silicon dioxide double-layer films to 4.9-eV ultraviolet (UV) illumination. The UV-induced current component decayed as a result of the isothermal annealing at temperatures ranging from 27°C to 240°C, and was induced once again by UV exposure following the annealing. The density of the current component showed a close correlation with the density of the K0 centers. Based on detailed analysis, we show that electron–hole pair generation in the bulk of the silicon nitride film is the possible source of the UV-induced current component.

Modeling stress development and hydrogen diffusion in plasma enhanced chemical vapor deposition silicon nitride films submitted to thermal cycles

We conducted isochronal stress hysteresis measurements coupled with thermal desorption spectroscopy on silicon nitride thin films obtained by performing plasma enhanced chemical vapor deposition on (001) silicon wafers. Above the deposition temperature, we observed irreversible stress build-up in parallel to substantial hydrogen effusion out of the films. We confirmed that the hydrogen dissociation and stress build-up can be modeled with similar kinetic equations. The hydrogen dissociation and stress development activation energies as well as the hydrogen diffusion coefficients were determined by fitting the experimental data with solutions to the kinetics and Fickian diffusion equations obtained with the finite difference method. A first order correlation was found between the hydrogen diffusion coefficients calculated between 400 and 800 °C and the silicon nitride film density.

Hollow Waveguides With Low Intrinsic Photoluminescence Fabricated With PECVD Silicon Nitride and Silicon Dioxide Films

A new type of integrated hollow core anti-resonant reflecting optical waveguide (ARROW) with low intrinsic photoluminescence (PL) fabricated with plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiN) and silicon dioxide (SiO2) films is demonstrated. The waveguide is fabricated by surrounding the hollow core with alternating layers of SiN and SiO2. The thicknesses and indices of the layers are designed to meet an anti-resonance condition. Compared to earlier ARROW structures made by depositing high-temperature (HT) SiN (250 °C), the PL background decreases by a factor of ~ 10 when low-temperature (LT) SiN (100 °C ) films are used. Therefore, LT SiN ARROWs are attractive platforms for sensitive fluorescence and Raman spectroscopy measurements of biomolecules.

Nanotribology-based novel characterization techniques for the dielectric charging failure mechanism in electrostatically actuated NEMS/MEMS devices using force–distance curve measurements

The work presents a comprehensive package of novel nanoscale characterization techniques to study dielectric charging in electrostatic nano- and microelectromechanical systems (NEMS and MEMS). The proposed assessment methodologies are based on the force–distance curve (FDC) measurements performed using an atomic force microscope (AFM) to measure, for the first time, the induced surface potential and adhesive force over charged dielectric films. They were employed to study plasma enhanced chemical vapor deposition (PECVD) silicon nitride films for application in electrostatic capacitive RF MEMS switches. Three different techniques were introduced including the application of FDC measurements to study charging in bare SiN x films, metal–insulator–metal (MIM) capacitors, and MEMS switches. The results from the three methods were correlated and compared with the published data from other characterization techniques, mainly charge/discharge current transient (C/DCT) and Kelvin probe force microscopy (KPFM). The unique advantages of the proposed FDC-based characterization techniques are twofold. First, they can measure the multiphysics coupling between the dielectric charging phenomenon and tribological issues at the interface between the switch bridge and the dielectric surface. Second, the FDC-based techniques can measure larger levels of induced surface potential over charged dielectric films which results from the high electric field normally used to actuate MEMS switches. Based on the proposed FDC techniques, the influence of several parameters on dielectric charging/discharging processes was investigated: the dielectric film thickness, deposition conditions, substrate, and electrical stress conditions.

Investigation of silicon nitride charging

The TSDC spectra of MIM capacitors with different SiN film thicknesses were analyzed assuming a minimum number of three charging mechanisms, which are attributed to dipolar and space charge polarization. Two relatively narrow contributions appear in the low temperature region and a broad contribution appears in high temperature region. All these contributions exhibit a relatively small dependence on the film thickness. The paper investigates the charging mechanisms of plasma enhanced chemical vapor deposition (PECVD) silicon nitride films, which are used in MEMS capacitive switches. The assessment has been carried out in metal-insulator-metal (MIM) capacitors with symmetric metal contacts and film thicknesses from 100nm to 500nm, with the aid of thermally stimulated depolarization currents (TSDC) method. Various polarization fields with intensities from 20kV/cm to 1MV/cm and of both field polarities were applied and the stored charge measured in the external circuit was calculated. The decomposition of TSDC spectra revealed the contribution of discrete polarization mechanisms, attributed to dipolar and space charge polarization. In low field intensities the TSDC spectra were attributed to domination of dipolar polarization and in high fields to space charge polarization.

Mechanical property characterization of sputtered and plasma enhanced chemical deposition (PECVD) silicon nitride films after rapid thermal annealing

In this paper, the mechanical and fracture properties of silicon nitride films subjected to rapid thermal annealing (RTA) have been systemically tested. The residual stress, Young's modulus, hardness, fracture toughness, and interfacial strength of both sputtered and plasma-enhanced chemical vapor deposition (PECVD) silicon nitride films deposited on silicon wafers were measured and compared. The results indicated that the Young's modulus and hardness of both types of silicon nitride films significantly increased when the RTA temperature increased. Furthermore, RTA processes could also alter the state of residual stress. The initial residual compressive stress of sputtered silicon nitride film was gradually relieved, and the film became tensile after the RTA process. For PECVD silicon nitride, the tensile residual stress reached its peak after a 600 °C RTA, then dropped after further increases in RTA temperature, due to stress relaxation. The tendency of the equivalent fracture toughness was to exhibit a strong correlation with that shown in the residual stress of silicon nitride. By considering the effect of residual stress, the real fracture toughness of both types of silicon nitride films were slightly enhanced by using RTA processes. Finally, experimental results indicated that the interfacial strength of PECVD silicon nitride could also be significantly improved by RTA processes at 600–800 °C. On the other hand, the initial interfacial strength of the sputtered silicon nitride was sufficiently strong, and the RTA processes only resulted in minor improvements. The characterization flow could be applied to other brittle films, and these specific test results should be useful for improving the structural integrity and process optimization of related MEMS and IC applications.

On the influence of environment gases, relative humidity and gas purification ondielectric charging/discharging processes in electrostatically driven MEMS/NEMSdevices

In this paper, we investigate the impact of environment gases and relative humidity ondielectric charging phenomenon in electrostatically actuated micro- and nano-electromechanicalsystems (MEMS and NEMS). The research is based on surface potential measurementsusing Kelvin probe force microscopy (KPFM). Plasma-enhanced chemical vapor deposition(PECVD) silicon nitride films were investigated in view of applications in electrostaticcapacitive RF MEMS switches. Charges were injected through the atomic forcemicroscope (AFM) tip, and the induced surface potential was measured using KPFM.Experiments have been performed in air and in nitrogen environments, both underdifferent relative humidity levels ranging from 0.02% to 40%. The impact of oxygengas and hydrocarbon contaminants has been studied for the first time by usingdifferent gas purifiers in both air and nitrogen lines. Voltage pulses with differentbias amplitudes have been applied during the charge injection step under allinvestigated environmental conditions in order to investigate the effect of biasamplitude. The investigation reveals a deeper understanding of charging anddischarging processes and could further lead to improved operating environmentconditions in order to minimize the dielectric charging. Finally, the nanoscaleKPFM results obtained in this study show a good correlation with the devicelevel measurements for capacitive MEMS switches reported in the literature.

Correlation Between Composition and Stress for High Density Plasma CVD Silicon Nitride Films

Intrinsic film stress is an important issue for the fabrication, performance, and application of a device. In the present study, a correlation between stress in high density plasma chemical vapor deposited silicon nitride films and their composition has been established. This has led to the low temperature deposition of low stressed films. The films exhibit a relatively negligible amount of chemically nonbonded hydrogen atoms. The density of the films calculated from the results of Rutherford backscattering spectroscopy and nuclear reaction analysis techniques is high, around , indicating a dense network. The films exhibit a relatively high coefficient of thermal expansion of around , indicating a fairly short-range order in the film. Furthermore, a reversible thermally induced stress, i.e., a negligible stress hysteresis upon thermal cycling between room temperature and , has been observed in the film. From the stress response to the thermal cycling experiments, contributions from the thermal and athermal components to the net room temperature stress have been deconvoluted.

The Effect of Silicon Nitride Stoichiometry on Charging Mechanisms in RF-MEMS Capacitive Switches

This paper discusses the mechanisms responsible for charging of plasma enhanced chemical vapor deposition (PECVD) silicon nitride films used in the fabrication of RF microelectromechanical (MEMS) switches. Nitride films deposited at different temperatures are characterized in order to better understand the effect of deposition conditions on material stoichiometry and stress. Both RF MEMS switches and metal-semiconductor-metal capacitors with PECVD silicon nitride as the dielectric layer were fabricated and their charging mechanisms were examined. Measurements indicate that charging arises from the formation of a defect band where charge transport occurs through a Poole-Frenkel-like effect. The calculated activation energy exhibits direct relation to material stoichiometry, and therefore to the nitride bandgap. Finally, it is viewed that lower temperature nitride is less prone to dielectric charging.

Analysis of Stress in Plasma Enhanced Chemical Vapor Deposition Silicon Nitride Film Irradiated with Ultraviolet Light

Plasma enhanced chemical vapor deposition (PE-CVD) silicon nitride (SiN) film was annealed with ultraviolet (UV) light for producing strained silicon. Stress increase of 700 MPa in 30-nm-thick SiN film was obtained after annealing time of 5 min using a low pressure mercury (Hg) lamp with irradiation power of 22 mW/cm2. The stress increase was found to be generated by not only the UV annealing but also thermal annealing. The behavior of stress increase can be described by a simple experimental formula including the UV and thermal annealing effects. And the stress increase in the thinner SiN film was calculated. The most useful method for obtaining larger stress increase was proposed.

Modeling the stress enhancement of plasma enhanced chemical vapor deposited silicon nitride films by UV post treatment – impact of the film density

Plasma Enhanced Chemical Vapour Deposited (PECVD) tensile nitride liners have been introduced in the 90 nm CMOS technology node to generate mechanical strain within the transistor silicon channel. With strain, because of the silicon piezoresistance effect, the electron mobility is enhanced within the silicon channel, so the nMOS transistor performance. Since that time, significant work has been carried out to develop silicon nitride films with enhanced tensile stress values to increase the gain provided by these process induced stressors along the introduction of the new technologies. For the 45 nm node, an additional UV post treatment has been introduced to achieve highly tensile residual stress. This study determines the relevant as deposited PECVD nitride film properties that modulate the final stress achievable after UV cure. Thanks to a simple stress model, it is demonstrated that as deposited nitride films must present a nitrogen rich composition and an intrinsic low density to become highly tensile after post UV treatment. With these design rules, nitride films with final stress of 1.6 GPa have been synthesized.

Methods of producing plasma enhanced chemical vapor deposition silicon nitride thin films with high compressive and tensile stress

Various methods of generating high stress in thin plasma enhanced chemical vapor deposition (PECVD) silicon nitride (SiN) films are reported. Besides the mainstream variation of plasma power and other process parameters, novel techniques such as creation of high density layers in multilayer PECVD structures or exposure of SiN films to ultraviolet radiation are shown to increase intrinsic film stress. Thin PECVD SiN films have been analyzed by a variety of analytical techniques including Fourier transform infrared spectroscopy, x-ray reflectivity (XRR), time of flight secondary ion mass spectrometry, and transmission electron microscopy to collect data on bonding, density, chemical composition, and film thickness. The level of bonded hydrogen as well as film density has been found to correlate with film stress. Creation of multilayer structures and high density layers help to build up more stress compared to a standard single layer film deposition. Both the density and number of layers in a film, characterized by XRR, affect the stress. Higher density layers affect diffusion profiles and show impurity oscillations corresponding to a multilayer film structure. Ultraviolet cure allows the film to achieve higher level of tensile stress at relatively low temperatures (400–500°C), comparable to the result of film high temperature annealing.

Improved Silicon Nitride Surfaces for Next-Generation Microarrays

This work reports how the use of a standard integrated circuit (IC) fabrication process can improve the potential of silicon nitride layers as substrates for microarray technology. It has been shown that chemical mechanical polishing (CMP) substantially improves the fluorescent intensity of positive control gene and test gene microarray spots on both low-pressure chemical vapor deposition (LPCVD) and plasma-enhanced chemical vapor deposition (PECVD) silicon nitride films, while maintaining a low fluorescent background. This results in the improved discrimination of low expressing genes. The results for the PECVD silicon nitride, which has been previously reported as unsuitable for microarray spotting, are particularly significant for future devices that hope to incorporate microelectronic control and analysis circuitry, due to the film's use as a final passivating layer.