LPCVD silicon nitride uniformity improvement using adaptive real-time temperature control

An effective approach to improve silicon nitride thickness uniformity has been demonstrated on a batch LPCVD furnace platform. Implementation of adaptive real-time temperature control provides accurate, real-time estimation of substrate temperature profiles that enables model-based optimization of process temperature. Optimization of a 200-nm silicon nitride deposition yielded long-term, overall nitride thickness uniformity of 0.79% 1/spl sigma/ over a seven-week period, compared to 1.24% for an equivalent PID-tuned process. Three sequential silicon nitride deposition iterations were implemented in the process recipe to enable increased temperature ramp rates for more efficient optimization of within-wafer uniformity. The optimized process requalified quickly after major and minor equipment maintenance, and is suitable for use in a manufacturing environment. The ART-optimized temperature ramp intervals used in this study are comparable to temperature deltas often used to offset dichlorosilane depletion effects encountered in some large-batch vertical furnace depositions. SIMS depth profiling of ART-optimized silicon nitride does reveal small oxygen and chlorine peaks, indicating slight interface formation between deposition steps.     

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 $\mathrm{Si}–{\mathrm{Si}}_3{\mathrm{N}}_4$ system increases in the first 10 min 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.     

Analysis of LPCVD process conditions for the deposition of low stress silicon nitride. Part I: preliminary LPCVD experiments

A wide range of process conditions were investigated to optimize conditions for the deposition of low stress silicon nitride films by low-pressure chemical vapor deposition. Experiments carried out in a standard, multi-wafer batch system generated films with an index of refraction ranging from about 2.04 to 2.82 and residual stress ranging from about 700 MPa tensile to –90 MPa compressive. The relationship between residual stress and index of refraction was characterized and results compared to those presented in the technical literature. Increase in the index of refraction beyond about 2.3 by means of increasing the gas flow did not reduce the residual stress appreciably but had a significant detrimental impact on the thickness uniformity and deposition rate. In contrast to results reported by other researchers, uniformity was not observed to increase with increasing DCS/NH₃ ratio in this study. Efforts to minimize thickness non-uniformity by suppressing deposition rate at the gas inlet region of the deposition system while increasing deposition rate at the rear were not successful. While increasing the temperature at the exhaust end of the system was intended to improve thickness non-uniformity, significant thickness and index of refraction uniformity was not realized. The reduction in deposition rate and corresponding increase in index of refraction at the exhaust end of the system indicated a variation in gas species from inlet to exhaust of the system. These experimental results revealed that the index of refraction decreased while the deposition rate decreased with increasing partial pressure of DCS. This suggests that the inhomogeneity observed within the repeatability runs is due to ammonia depletion along the length of the load.     

Real-time multivariable control of PECVD silicon nitride filmproperties

This paper reports on the application of quadrupole mass spectrometry (QMS) sensing to real-time multivariable control of film properties in a plasma-enhanced CVD silicon nitride process. Process variables believed to be most important to film deposition are defined (i.e., disilane pressure, triaminosilane pressure, and dc bias voltage) and their responses to system inputs are modeled experimentally. Then, a real-time controller uses this information to manipulate the process variables and hence film performance in real time during film deposition. The relationships between gas concentrations and film performance are shown explicitly where the controller was used to drive the concentrations to constant setpoints. Also, an experiment investigating the effects of an out-of-calibration mass flow controller demonstrates the compensating ability of the real-time controller. The results indicate that in situ sensor-based control using quadrupole mass spectrometry can significantly assist in optimizing film properties, reducing drift during a run, reducing run-to-run drift, creating a better understanding of the process, and making the system tolerant to disturbances     

Properties of thin LPCVD silicon oxynitride films

Thin, uniform silicon oxynitride films with films thicknesses of ≤ 10 nm were successfully deposited by low pressure chemical vapor deposition (LPCVD). The reactant gases were SiH₂Cl₂, N₂O, and NH₃. The compositional uniformity of these films as a function of depth was good. The structure of these oxynitride films was found to be dominated by the mixed matrix of Si, N, and 0, rather than a physical mixture of SiO₂ and Si₃N₄ clusters. N-H bonding was observed and the total amount of hydrogen in the as-deposited film was on the order of 5 x l0²⁰/cm³. No H-OH or Si-OH bonds were detected. Excellent dielectric breakdown distributions were found for oxynitride films with equivalent oxide film thicknesses as low as 7.5 nm. The conduction of Si-N-0 films depended on film composition. A small capacitor-voltage (C-V) window (< 0.1 V) was observed for the Si-N-O/Si structures. The midgap surface state density was on the order of 5 x 10¹⁰/cm² /eV. Either trapping of holes or the generation of positive states were found after high field stressing of the oxynitride films.     

Selective silicon epitaxial growth by LPCVD using silane

Chlorine chemistry can have adverse effects on LPCVD systems and components. Results demonstrating the growth of selective silicon epitaxial layers in excess of 0.7 mu m thickness showing excellent uniformity and selectivity without the use of HCl are presented.<>     

Electrical measurements on ion-implanted LPCVD polycrystalline silicon films

The electrical conduction properties of ion implanted polycrystalline silicon films have been studied. The polysilicon films were deposited by pyrolysis of silane at 647°C in LPCVD system onto oxide-coated silicon wafers to a thickness of 0.6 μm. Dopants were itroducd by implanting with boron or phosphorus ions, accelerated to 145 keV; doses ranged from 1 × 10¹² cm^?2 to 1 × 10¹⁵ cm^?2. Film resistivities spanning 8 orders of magnitude were obtained using this doping range. Current-voltage characteristics of polysilicon resistors were measured at temperatures ranging from 24 to 140°C. The associated barrier heights and activation energies were derived. The grain-boundary trapping states density was estimated to be 5 × 10¹² cm^?2. We found that both dopant atom segregation and carrier trapping at the grain boundaries play important roles in polysilicon electrical conduction properties. However, within the dose range studies, the dopant atom segragation is most detrimental to the film conductivity for doses < 1 × 10¹³ cm^?2; as the dose is increased, carrier trapping effects become more pronounced for doses up to 5 × 10¹⁴ cm^?2. For doses ? 5 × 10¹⁴ cm^?2, conduction due to carriers tunneling through the potential barriers at grain boundaries has to be considered.     

High-performance thin-film transistors in low-temperature crystallized LPCVD amorphous silicon films

Thin-film transistors (TFT's) were fabricated in low-temperature (550°C) crystallized amorphous LPCVD silicon films. The performance of these devices was found to depend upon the deposition temperature. Low threshold voltages and effective mobilities as high as 32 cm²/V.s are reported for devices fabricated in 150-nm-thick films with maximum processing temperature of 860°C. The performance of these devices is shown to be far superior to devices fabricated in as-deposited polycrystalline silicon films.     

Trapping levels in silicon nitride

The trapping levels in silicon nitride have been investigated using the thermally stimulated current technique. Traps at between 0.50 and 0.90eV below the conduction band of the nitride were found, as well as a distribution of traps at and near the silicon-silicon nitride interface about 0.10eV below the conduction band of the silicon.     

Stress control of silicon nitride films deposited by plasma enhanced chemical vapor deposition

Stress controllable silicon nitride(Si Nx) films deposited by plasma enhanced chemical vapor deposition(PECVD) are reported. Low stress Si Nx films were deposited in both high frequency(HF) mode and dual frequency(HF/LF) mode. By optimizing process parameters, stress free(-0.27 MPa) Si Nx films were obtained with the deposition rate of 45.5 nm/min and the refractive index of 2.06. Furthermore, at HF/LF mode, the stress is significantly influenced by LF ratio and LF power, and can be controlled to be 10 MPa with the LF ratio of 17% and LF power of 150 W. However, LF power has a little effect on the deposition rate due to the interaction between HF power and LF power. The deposited Si Nx films have good mechanical and optical properties, low deposition temperature and controllable stress, and can be widely used in integrated circuit(IC), micro-electro-mechanical systems(MEMS) and bio-MEMS.     

Silicon nitride processing for control of optical and electronic properties of silicon solar cells

Thin films of SiN are well suited as antireflection (AR) coatings for Si solar cells because their optical properties, such as refractive index and absorption coefficient, can be tailored during deposition to match those of Si solar cells. The SiN layers, particularly those deposited by a plasma-enhanced chemical vapor deposition (PECVD) process, can serve other functions in Si solar-cell fabrication. They can be excellent buffer layers through which the front metal contact can be fired. The PECVD nitridation also introduces H into the Si surface, which diffuses deep into the solar cell and passivates residual impurities and defects during metal-contact firing. The optimization of SiN properties and processing conditions may have conflicting demands based on its multifunctional role. To fully exploit these multiple functions, the SiN processing sequence must be optimized based on the properties of the nitride, the diffusion behavior of H, and the interactions of metal with the SiN/Si composite substrate.     

Residual stress in silicon nitride films

The mechanical stress caused by Si₃N₄ films on (111) oriented Si wafers was studied as a function of the Si₃N₄ film thickness, deposition rate, deposition temperature and film composition. The Si₃N₄ films were prepared by the reaction of gaseous SiH₄ and NH₃ in the temperature range 700–1000°C. The curvature of the Si substrates caused by the Si₃N₄. films is related to the film stress; the substrate curvature was measured by an optical interference technique. The measured Si₃N₄. film stress was found to be highly tensile with a magnitude of about 10¹⁰ dynes/cm². For the thickness range of 2000–5000Å, there was no change in the measured stress. The total film stress was observed to decrease for decreasing deposition rate and increasing deposition temperature. A large change in film stress was observed for films containing excess Si; the stress decreased with increasing Si content. Based on published values for the thermal expansion coefficients for Si and Si₃N₄, a published value for Young’s Modulus for Si₃N₄, and the measured total stress values, a consistent argument is developed in which the total stress consists of a compressive component due to thermal expansion coefficient mismatch and a larger tensile intrinsic stress component. Both the thermal and intrinsic stress components vary with film deposition temperature in directions which decrease the total room temperature stress for higher deposition temperatures.     

A bibliography on silicon nitride films

A bibliography of ca. 600 references on the preparation, properties and applications of silicon nitride (Si₃N₄) films has been collected from the literature.The bibliography comprises a list of papers appearing in Chemical Abstracts, journals, conference proceedings and patent abstracts, covering the period from 1966 (when the first article appeared describing an application of silicon nitride films in the solid state technology) through to 1978.     

Double-layered silicon nitride antireflection coatings for multicrystalline silicon solar cells

Silicon nitride coating possesses both optical antireflection and electrical passivation effects for crystalline silicon solar cells. In this work, we employed a double-layered silicon nitride coating consisting of a top layer with a lower refractive index and a bottom layer (contacting the silicon wafer) with a higher refractive index for multicrystalline silicon solar cells. Double-layered silicon nitride coating provides a lower optical reflection and better surface passivation than those of single-layered silicon nitride. Details for optimizing the double-layered silicon nitride coating are presented. In order to get statistical conclusions, we fabricated a large number of multicrystalline silicon solar cells using the production line for both the double-layered and single-layered cell types. It was statistically demonstrated that the double-layered silicon nitride coating provided a consistent enhancement in the photovoltaic performance of multicrystalline silicon solar cells over those of the single-layered silicon nitride coating.     

氮化硅的ECCP刻蚀特性研究

本文对氮化硅的增强电容耦合等离子刻蚀进行研究,为氮化硅刻蚀工艺的优化提供参考。针对SF_6+O_2气体体系,通过设计实验考察了功率、压强、气体比、氦气等对刻蚀速率和均一性的影响,并对结果进行机理分析和讨论。实验结果表明:功率越大,刻蚀速率越大,与源极射频电力相比,偏置射频电力对刻蚀速率的影响更为显著;压强增大,刻蚀速率增大,但压强增大到一定程度后,刻蚀速率基本不变,刻蚀均匀性随着压强增大而变差;在保证SF_6/O_2总流量保持不变下,O_2的比例增大,刻蚀速率先增大后减小,刻蚀均匀性逐步变好;He的添加可以改善刻蚀均匀性,但He的添加量过多时,会造成刻蚀速率降低。     

Supervisory control of remote manipulation

The relatively short distances to be spanned in using the manipulators presently available permit real-time control systems, frequently employing direct linkages. The advent of space flight necessitates the design of manipulation systems that can perform complicated tasks, on the moon and beyond, upon command from earth. Such a design must compensate for the communications time delay due to the distances involved and also for a difference in environment not directly observable by the human operator. The answer to the problem seems to lie in a computer-controlled remote unit, capable of making limited decisions of its own but supervised from home base.     

Supervisory control of hybrid systems

In this paper, the supervisory control of hybrid systems is introduced and discussed at length. Such control systems typically arise in the computer control of continuous processes, for example, in manufacturing and chemical processes, in transportation systems, and in communication networks. A functional architecture of hybrid control systems consisting of a continuous plant, a discrete-event controller, and an interface is used to introduce and describe analysis and synthesis concepts and approaches. Our approach highlights the interaction between the continuous and discrete dynamics, which is the cornerstone of any hybrid system study. Discrete abstractions are used to approximate the continuous plant. Properties of the discrete abstractions to be appropriate representations of the continuous plant are presented, and important concepts such as determinism and controllability are discussed. Supervisory control design methodologies are presented to satisfy control specifications described by formal languages. Several examples are used throughout the paper to illustrate our approach     

Photoemission spectroscopy of heterojunctions of amorphoushydrogenated silicon with silicon oxide and nitride

A study of the growth and electronic structure of a-Si:H/a-SiN_x:H and a-Si:H/a-SiO_x:H heterojunctions by photoemission spectroscopy is discussed. The interfaces in a-Si:H/a-SiO_x are atomically abrupt, except for the SiO_x on the Si interface which is graded over ~3 Å due to plasma oxidation. The offset energies between the a-Si:H valence band and those of a-SiN_x:H and a-SiO_x:H are 1.2 and 4.0 eV, respectively. Extra H(~2×10¹⁵) is incorporated in the Si on the SiN_x interface region. The hole wave functions in a-Si:H are localized on a scale of 1-2 interatomic distances     

Arguments for electron conduction in silicon nitride

The carrier type conducted in the nitride under + VGhas been interpreted as an electron by some and as a hole by others. Under - VGhole conduction is generally accepted. Our I-V data on the Si3N4dielectric agree with those reported in the literature but we interpret the nitride conduction as electron flow for both + VGand - VG. New arguments to support electron conduction are derived from three types of Si3N4:SiO2dual dielectric silicon-gate transistors.