PECVD Silicon Nitride Films Research Articles

Novel fabrication of fixed suspended silicon nitride structure for MEMS devices with dry etching

A method of fabricating suspended LPCVD and PECVD silicon nitride structure is demonstrated for a wide range of MEMS (Micro-Electro-Mechanical Systems) applications. Low stress LPCVD and PECVD silicon nitride film of 1 μm thickness were selected for the structure separately. Optical Lithographic parameters, viz, photoresist (PR) thickness, PR variety and baking parameters were optimized to obtain the PR suitable for selective etching of silicon and silicon nitride. Parameters of the dry etching process were also optimized to achieve anisotropic etching of silicon nitride and isotropic etching of Si to release the silicon nitride beam. The silicon nitride structures, thus released, were characterized using Scanning Electron Microscope (SEM) and Laser Doppler Vibrometer (LDV). Finite Element Method (FEM) analysis was carried out using COMSOL, to compare with the experimental modes of vibration investigated using the Laser Doppler Vibrometer (LDV). Thus we demonstrate that the first mode at 30 kHz was indeed the optimum match.

Temperature accelerated discharging processes through the bulk of PECVD silicon nitride films for MEMS capacitive switches

The effect of temperature on discharging processes of PECVD silicon nitride films has been investigated in Metal-Insulator-Metal (MIM) capacitors and RF MEMS capacitive switches with the aid of Kelvin Probe method. The surface potential decay of MIM capacitors during discharge has been monitored using a Single-point Kelvin probe system at various temperatures (300K–400K) while the shift of bias at minimum up-state capacitance during discharge has been employed in MEMS capacitive switches in order to determine the discharge current through the bulk of the film at different temperatures. The results indicate that the discharging process through the bulk of silicon nitride films is thermally activated and the corresponding value of the discharging current increases with temperature, due to the increase of the film conductivity. Moreover, the value of minimum up-state capacitance of the switches is found to be significantly affected by temperature, by the mechanical properties of the moving armature as well as by the variance of charge density distribution. Finally, different thermally activated mechanisms have been observed in MIM capacitors and MEMS capacitive switches, due to different charging processes on these devices.

Induced charging phenomena on SiNx dielectric films used in RF MEMS capacitive switches

Contact-less charging process has been found to constitute a compensation mechanism to dielectric charging of MEMS capacitive switches. The present paper aims to provide a better knowledge of induced charging mechanisms that appear in HF PECVD silicon nitride films. The characteristics of this process as well as the degree of compensation are investigated for different stressing field intensities of both polarities in MEMS capacitive switches. The experimental results indicate that the degree of compensation in HF PECVD SiNx films seems to increase with the intensity of the applied electric field and it is also affected by the polarity of the stressing bias. A new theoretical model that describes the build-up of induced charging is also proposed. Taking into account that the electric field during contact-less charging is quite low, the model assumes that induced charging arises from charge displacement inside the film through hopping conduction processes.

Efficient passivation of solar cells by silicon nitride

In this study we investigated the deposition parameters of a PECVD silicon nitride (SiN) film leading to an efficient passivation of multicrystalline silicon solar cells. The parameters investigated were the temperature, the power and the refractive index through the gas process flow rate ratio SiH4/NH3.Using a symmetrical structure SiN/Si/SiN and Quasi-Steady-State Photo-Conductance (QSSPC) characterization we found that 380 °C and 4500 W are the optimal temperature and optimal power, respectively. The optimal refractive index was determined using a method which encompasses optical and electrical properties of SiN films deposited on multicrystalline silicon solar cells. This method, based on the calculation of the short circuit current densities, revealed that the optimal refractive index is 1.9.

A study of deposition conditions on charging properties of PECVD silicon nitride films for MEMS capacitive switches

The present paper aims to provide a better insight to the electrical characteristics of silicon nitride films that have been deposited with PECVD method under different conditions. The effect of film thickness, substrate temperature and the frequency that produces the plasma in PECVD method on the dielectric charging phenomenon in silicon nitride films has been investigated with the aid of thermally stimulated depolarization currents (TSDC) and Kelvin Probe (KP) techniques. The results indicate that the decrease of film thickness and substrate temperature as well as the deposition at high frequency plasma (13.56MHz) seems to produce silicon nitride films that are less prone to dielectric charging and thus better candidates for MEMS capacitive switches.

TSV 웨이퍼 공정용 Si3N4후막 스트레스에 대한 공정특성 분석

Department of Mechanical and Industrial Engineering, University of Toronto, Toronto M5S 2J7, Canada (Received December 30, 2013; Revised January 23, 2014; Accepted February 11, 2014)Abstract: With the recent advent of through silicon via (TSV) technology, wafer level-TSV interconnection become feasible in high volume manufacturing. To increase the manufacturing productivity, it is required to develop equipment for backside passivation layer deposition for TSV wafer bonding process with high deposition rate and low film stress. In this research, we investigated the relationship between process parameters and the induced wafer stress of PECVD silicon nitride film on 300 mm wafers employing statistical and artificial intelligence modeling. We found that the film stress increases with increased RF power, but the pressure has inversely proportional to the stress. It is also observed that no significant stress change is observed when the gas flow rate is low.Keywords: Through silicon via, Modeling, PECVD, TSV backside passivation, RSM

Nanoindentation fracture and fatigue characterization of PECVD silicon nitride films subjected to rapid thermal annealing

In this work, the fracture and fatigue behaviors of PECVD silicon nitride films deposited on silicon substrates after rapid thermal annealing were characterized. Specimens were fabricated using different fabrication parameters and post-deposition annealing temperatures and duration. Indentation methods were primarily used for qualitatively examining the effect of process conditions on the resultant mechanical properties. The experimental results indicated that the residual stress, fracture toughness, interfacial strength, and fatigue crack propagation, strongly depend on the processing conditions, such as deposition temperatures, chamber pressures, temperature, and duration of annealing. Specifically, preliminary results indicated that the specimen deposited at a lower temperature and a lower pressure exhibited much less residual tensile stress and better interface strength. Meanwhile, the study of rapid thermal annealing duration on the film stress demonstrated that the evolution of the residual stress was mainly controlled by the intrinsic stress generation dynamics and stress relaxation. It was also found that rapid thermal annealing could enhance the interfacial strength but the high tensile stress generated could actually reduce the equivalent toughness and lead to structural reliability concerns. In sum, the characterization results should provide useful information for correlating mechanical reliability and processing parameters for future structural design optimization and for improving the structural integrity of PECVD silicon nitride films for MEMS and IC fabrication.

과량의 N2에 희석된 SiH4 가스를 이용한 150℃ 저온 PECVD 실리콘 질화물 박막의 물성이 TFT 소자 특성에 미치는 영향

과량의 N2에 희석된 SiH4 가스를 이용한 150℃ 저온 PECVD 실리콘 질화물 박막의 물성이 TFT 소자 특성에 미치는 영향

Temperature effects on the bulk discharge current of dielectric films of MEMS capacitive switches

Kelvin probe method has been directly applied to capacitive MEMS switches in order to investigate temperature activated mechanisms in PECVD Silicon Nitride (SiNx) films. The bulk discharge current of MEMS capacitive switches has been determined for different charging and discharging temperatures, in the range of 300–400K. The increase of discharging temperature leads to an increase of the magnitude of the bulk discharge current and the relaxation time of the discharging process is found to be thermally activated. Finally, it is shown that the increase of charging temperature assists trapping at centers characterized by time constants even longer than the time window of observation, i.e. 104s.

Wet Etching Studies of PECVD Silicon Nitride Films in Doped TMAH Solutions

It has been reported that the etch rate of exposed aluminum lines and pads on MEMS chips can be significantly reduced by dissolving an appropriate amount of silicon (or silicic acid, water glass) and ammonium persulfate (AP) in TMAH solution. However the etch rates of the PECVD silicon nitride films，which is usually underlying aluminum lines and pad, were rarely reported in previous literatures. In this paper, silicon nitride films of high compressive stress, low compressive stress, micro-stress, low tensile stress and high tensile stress are prepared by adjusting the flow ratio of SiH4 to NH3 and plasma power. Then the films are etched in four kinds of previously reported doped TMAH solutions. The experimental results show that silicon nitride films depositing at low flow ratio of SiH4 to NH3 and lager RF power were etched more slowly in doped TMAH solution than that of depositing at high flow ratio of SiH4 to NH3 and low RF power.

Effect of Deposition Gas Ratio, RF Power, and Substrate Temperature on the Charging/Discharging Processes in PECVD Silicon Nitride Films for Electrostatic NEMS/MEMS Reliability Using Atomic Force Microscopy

The dependence of the electrical properties of silicon nitride, which is a commonly used dielectric in nano- and micro-electromechanical systems (NEMS and MEMS), on the deposition conditions used to prepare it and, consequently, on material stoichiometry has not been fully understood. In this paper, the influence of plasma-enhanced chemical vapor deposition conditions on the dielectric charging of films is investigated. The work targets mainly the dielectric-charging phenomenon which constitutes a major failure mechanism in electrostatically driven NEMS/MEMS devices and particularly in capacitive MEMS switches. The charging/discharging processes are studied using two nanoscale characterization techniques: Kelvin probe force microscopy (KPFM) and, for the first time, force-distance curve (FDC) measurements. KPFM is used to investigate dielectric charging at the level of a single asperity, while FDC is employed to measure the multiphysics coupling between the charging phenomenon and tribological issues, mainly meniscus force. The electrical properties of the films obtained from both techniques show a very good correlation. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy material characterization techniques are also used to determine the compositions and chemical bonds, respectively, of the films. An attempt to correlate between the chemical and electrical properties of films is made.

Kelvin probe force microscopy-based characterization techniques applied for electrostatic MEMS/NEMS devices and bare dielectric films to investigate the dielectric and substrate charging phenomena

In this study, two different characterization techniques based on Kelvin probe force microscopy (KPFM) have been used to investigate the dielectric and substrate charging in electrostatic micro- and nano-electromechanical systems (MEMS and NEMS). The first technique (KPFM-MEMS) has been employed to study the discharging process on a microscopic scale in a charged MEMS dielectric film. This has been performed by monitoring the surface potential decay with time of charged PECVD silicon nitride films implemented in electrostatic capacitive MEMS switches. The second methodology, KPFM-thin films (KPFM-TF), has been applied to investigate the charging/discharging processes in bare SiNx films as well as the substrate charging phenomenon. It makes use of the atomic force microscope tip to simulate charge injection through a single asperity, and then measure the induced surface potential. The influence of the SiNx film thickness and deposition conditions has been studied. Moreover, the impact of bias amplitude and bias polarity applied during charge injection has been explored. Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy material characterization techniques have been used to determine the chemical bonds and compositions, respectively, of the SiNx films being investigated. The nanoscale KPFM results obtained in this study reveal an accurate understanding of both the dielectric charging and the substrate charging that take place in electrostatic MEMS/NEMS devices.

Nanoindentation Characterization of PECVD Silicon Nitride on Silicon Subjected to Mechanical Fatigue Loading

ABSTRACTIn this work, the mechanical properties of PECVD silicon nitride deposited on silicon substrates by two different processing conditions were investigated. Indentation method was primary used for qualitatively examining the effect of process conditions to the achieved mechanical properties. The experimental results indicated that the residual stress, fracture toughness and interfacial strength, as well as the fatigue crack propagation were strongly depended on the processing conditions such as deposition temperatures and chamber pressures. Preliminary results indicated that the specimen deposited at a lower temperature and a lower pressure exhibited a much less residual tensile stress and a better interface strength. On the other hand, it was found that RTA could enhance the interfacial strength but the generated high tensile strength could actually reduce the equivalent toughness and leads to structural reliability concerns. In summary, the characterization results should be possible to provide useful information for correlating the mechanical reliability with the processing parameters for future structural design optimization and for improving the structural integrity of PECVD silicon nitride films for MEMS and IC fabrication.

A systematic reliability investigation of the dielectric charging process in electrostatically actuated MEMS based on Kelvin probe force microscopy

This paper presents a comprehensive investigation for the dielectric charging problem in electrostatically actuated microelectromechanical system (MEMS) devices. The approach is based on Kelvin probe force microscopy (KPFM) and targets, in this specific paper, thin PECVD silicon nitride films for electrostatic capacitive RF MEMS switches. KPFM has been employed in order to mimic the potential induced at the dielectric surface due to charge injection through asperities. The effect of dielectric thickness has been investigated through depositing SiNx films with different thicknesses. Then, in order to simulate the different scenarios of dielectric charging in real MEMS switches, SiNx films have been deposited over thermally grown oxide, evaporated gold and electroplated gold layers. Also, the effect of the deposition conditions has been investigated through depositing dielectric films using low and high frequency PECVD methods. The investigation reveals that thin dielectric films have larger relaxation times compared to thick ones when the same injection bias is applied, independently of the substrate nature. For the same SiNx film thickness, the decay time constant is found to be smaller in dielectric films deposited over metallic layers compared to the ones deposited over silicon substrates. Finally, the material stoichiometry is found to affect the surface potential distribution as well as the relaxation time constant.

Effect of Humidity on Dielectric Charging Process in Electrostatic Capacitive RF MEMS Switches Based on Kelvin Probe Force Microscopy Surface Potential Measurements

AbstractIn this article we investigate the effect of relative humidity on dielectric charging/discharging processes in electrostatically actuated MEMS devices. The assessment procedure is based on surface potential measurements using Kelvin Probe Force Microscopy (KPFM) and it targets in this specific work PECVD silicon nitride films in view of application in electrostatic capacitive RF MEMS switches. Charges have been injected through the AFM tip and the induced surface potential has been measured under different relative humidity levels. The impact of the charge injection duration and the bias level as well as bias polarity applied during the charge injection step, have been explored. Finally, the effect of the dielectric film thickness under different relative humidity levels has been assessed through depositing SiN films with different thicknesses.

A study of stacked PECVD silicon nitride films used for surface micromachined membranes

Silicon nitride stacked films containing three layers differing in mechanical–chemical properties are synthesized using plasma enhanced chemical vapor deposition method from monosilane (SiH 4) and ammonia (NH 3) mixture. The composition is analyzed using X-ray Photoelectron Spectroscopy and stress is measured using a substrate bending method. The ability to obtain stacked films with the custom tensile stress in the overall structure was demonstrated by the series of experiments. The tensile stress in the top and bottom films was obtained between 200 and 300 MPa whereas the stress in the middle film could be adjusted from compressive 60 MPa to tensile 300 MPa. Since the appropriate stress value is important in achieving required mechanical properties of the membranes, the results obtained are discussed in the context of surface micromachined membrane structures.

EDITORIAL

EDITORIAL

Use of a neural network to characterize the charge density of PECVD-silicon nitride films

Silicon nitride (SiN) films were deposited using a plasma-enhanced chemical vapor deposition system (PECVD). The charge density of the SiN films was modeled using a generalized regression neural network (GRNN). The PECVD process was characterized by means of a face-centered Box Wilson experiment. The prediction performance of the GRNN model was optimized using a genetic algorithm (GA). The GA-GRNN model significantly improved the GRNN prediction performance by more than 55%. The optimized GA-GRNN model was used to investigate the effects of various parameters on the charge density. A higher charge density was obtained at higher temperatures (i.e. a lower H concentration). Increasing the pressure increased the charge density at all temperatures levels with a much stronger impact at a lower H concentration. The effects of the SiH4 and N2 (or NH3) flow rates on the charge density were similar in that a higher charge density was achieved at a lower Si−N ratio (N-rich films). A considerable increase in the charge density with a radio frequency power at a lower NH3 flow rate was attributed to the generation of more Si−H than N−H bonds.

The application of polyimide/silicon nitride dual passivation to Al xGa 1− xN/GaN high electron mobility transistors

PECVD silicon nitride passivation is quite frequently done at the end of AlGaN/GaN HEMT fabrication before substrate back-side lapping. However, the PECVD silicon nitride process is likely to produce pinholes in the passivation film. A very thick PECVD silicon nitride film may produce mechanical stress on the underlying device. Polyimide passivation has also been known to be effective for AlGaN/GaN HEMT and it can also serve as a stress buffer. However, polyimide can take up water while PECVD silicon nitride is a good diffusion barrier for water, etc. Thus it is expected that a dual PECVD silicon nitride/polyimide passivation will be a better choice than just a single layer of PECVD silicon nitride or polyimide. In this paper, we will demonstrate the application of a dual PECVD silicon nitride/polyimide passivation to AlGaN/GaN HEMT process.

Indentation Characterization of Fracture Toughness and Interfacial Strength of PECVD Nitrides after Rapid Thermal Annealing

AbstractThis work presents the result of mechanical characterization of the fracture toughness and interfacial strength of PECVD silicon nitride films deposited on silicon subjected to rapid thermal annealing (RTA) processing between 200 and 800 °C. Both micro- and nano-indentation techniques are employed to perform the experiments. In conjunction with the model proposed by Marshall and Lawn for data reduction, the fracture toughness of un-heat treated nitride is obtained as 2.2 MPa√m based on a series of Vickers micro-indentation tests and this value is essentially unchanged if the RTA temperatures are below 400°C. Further increase in RTA temperature would significantly enhance the fracture toughness. On the other hand, using nanoindentation testing in conjunction with the model proposed by Marshall and Evans, the interfacial strength between the nitride and silicon is determined as 17.2 J/m2 for un-heat treatment nitrides and it could also be significantly enhanced by RTA processes with temperatures exceeding 400°C. These results should be useful for related MEMS or IC structure fabrication for the concerns of maintaining the structural integrity and improve fabrication performance in related applications.