Supervisory control of LPCVD silicon nitride

This paper presents a scheme for deposition time and temperature control of low pressure chemical vapor deposition silicon nitride off product wafers. A Kalman filter based estimation scheme is presented for deposition time control. Deposition temperature control is treated in additional detail, including the impact of sampling the furnace load. Furthermore, stability metrics are also derived capturing the allowable modeling error for ensuring closed-loop stability. This is important for enabling the same model to be used across multiple tools and processes. A two-step iterative scheme is presented for implementing a "batch as you go" controller. Finally, the controller is applied in high-volume production.     

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.     

Inverse model-based real-time control for temperature uniformity ofRTCVD

A reduced-order model describing a rapid thermal chemical vapor deposition (RTCVD) process is utilized for real-time model based control for temperature uniformity across the wafer. Feedback is based on temperature measurements at selected points on the wafer surface. The feedback controller is designed using the internal model control (IMC) structure, especially modified to handle systems described by ordinary differential and algebraic equations. The IMC controller is obtained using optimal control theory on singular arcs extended for multi-input systems. Its performance is also compared with one based on the Hirschorn inverse of the model. The proposed scheme is tested with extensive simulations where the full-order model is used to emulate the process. Several cases of significant uncertainty, including model parameter errors, process disturbances, actuator errors, and measurement noise are used to test the robustness of the controller to real life situations. Both controllers succeed in achieving temperature uniformity well within the desirable bounds, even in cases where several sources of uncertainty are simultaneously present with measurement noise     

Optimization of low temperature silicon nitride processes for improvement of device performance

This paper gives some insights in the applications where PECVD nitrides can be introduced to replace the LPCVD layers and how the process parameters need to be varied to obtain the desired properties. Film properties like stress, hydrogen content, wet etch rate and deposition rate are reported. The nitrides are optimized for specific applications and examples on the influence of nitride properties on device performance are given. It is important to investigate that the advantage of the high film integrity of nitride layers used in the past is not lost due to the strong demand for developing new process schemes with low thermal budget layers. We show that PECVD films are a valid alternative for LPCVD and that the majority of the film properties satisfy the criteria to use PECVD films as contact-etch-stop layers, silicidation blocking films and spacer materials.     

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.<>     

Model-based uniformity control for epitaxial silicon deposition

Semiconductor fabrication requires tight specification limits on thickness and resistivity for epitaxially deposited silicon films. Our testbed system for integrated, model-based, run-to-run control of epi films incorporates a Centura tool with an epi deposition chamber, an in-line epi film thickness measurement tool, and off-line thickness and resistivity measurement systems. Automated single-input-single-output, run-to-run control of epi thickness has been successfully demonstrated. An advanced multi-objective controller is described, which seeks to provide simultaneous epi thickness control on a run-to-run basis using the in-line sensor, as well as combined thickness and resistivity uniformity control on a lot-to-lot basis using off-line thickness and resistivity sensors. Control strategies are introduced for performing combined run-to-run and lot-to-lot control, based on the availability of measurements. Techniques are proposed for dealing with multiple site measurements of multiple film characteristics taken at different sampling rates, as well as the use of time-based inputs and rate-based models. These concepts are widely applicable for semiconductor fabrication processes.     

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     

Silicon nitride for the improvement of silicon inversion layer solar cells

It is demonstrated that CVD Si-nitride films as transparent dielectric satisfy all requirements for achieving MIS/IL solar cells with high efficiency and long term stability. Deposited on silicon by the SiH₄/NH₃ reaction at temperatures lower than usual (between 600° and 650°C) fixed positive interface charge densities Q_N/q up to 7 × 10¹² cm^?2 with excellent stability have been obtained. Utilizing the Si-nitride charge storage effect, the highest known Q_N/q values (> 10¹³cm^?2) combined with low values of N_it have been achieved. The charge distribution is discussed and an energy band diagram modified according to new analytical results is presented. MIS/IL solar cells with AM1 efficiencies of 15% (active area) and high UV sensitivity have been obtained.     

一种自适应实时事务并发控制新方法

传统的基于锁的并发控制机制重点考虑数据的一致性和事务的并发度,不能很好地满足实时数据库系统对时态一致性的要求.结合锁,多版本和有效性确认等并发控制机制,提出了一种并发控制的新方法——自适应多版本实时事务并发控制方法:AMVCC.此并发控制方法可根据具体情况自适应地采用不同的并发控制机制,能够有效提高事务的并发度和截止时间内完成事务的数量.通过可行性分析知:此方法在性能上优于传统基于锁的并发控制方法.     

Low temperature polycrystalline silicon thin film transistor withsilicon nitride ion stopper

The authors have fabricated a new low temperature polycrystalline silicon (poly-Si) thin film transistor (TFT) with silicon nitride (SiN _x) ion-stopper and laser annealed poly-Si. The fabricated poly-Si TFT using SiN_x as the ion-stopper as well as the gate insulator exhibited a field effect mobility of 110 cm²/Vs, subthreshold voltage of 5.5 V, subthreshold slope of 0.48 V/dec., and on/off current ratio of ~10⁶. Low off-state leakage current of 2.4×10^-2 A/μm at the drain voltage of 5 V and the gate voltage of -5 V was achieved     

Room temperature deposition of silicon nitride films using very low frequency (50Hz) plasma CVD

Silicon nitride films have been deposited by low frequency 50Hz plasma CVD using a nitrogen and silane mixture at room temperature. To deposit high quality silicon nitride, the silane fraction in the nitrogen and silane mixture has to be less than 5 %. The refractive index, breakdown field strength and resistivity of the obtained silicon nitride film were 2.0, 1.2x10⁷ V/cm and 6x10¹⁵ Ωcm, respectively. Mechanism of the deposition of high quality silicon nitride is discussed on the basis of the experimentally observed light emission spectrum from the plasma and of the electron energy distribution function in the plasma theoretically calculated by the Boltzmann equation method.     

Dynamically adaptive real-time disparity estimation hardware using iterative refinement

The computational complexity of disparity estimation algorithms and the need of large size and bandwidth for the external and internal memory make the real-time processing of disparity estimation challenging, especially for High Resolution (HR) images. This paper proposes a hardware-oriented adaptive window size disparity estimation (AWDE) algorithm and its real-time reconfigurable hardware implementation that targets HR video with high quality disparity results. Moreover, an enhanced version of the AWDE implementation that uses iterative refinement (AWDE-IR) is presented. The AWDE and AWDE-IR algorithms dynamically adapt the window size considering the local texture of the image to increase the disparity estimation quality. The proposed reconfigurable hardware architectures of the AWDE and AWDE-IR algorithms enable handling 60 frames per second on a Virtex-5 FPGA at a 1024×768 XGA video resolution for a 128 pixel disparity range.     

New applications of low temperature PECVD silicon nitride films for microelectronic device fabrication

Silicon nitride has been widely used in microelectronic device fabrication processes for encapsulation, surface passivation and isolation. In this paper we report new applications of plasma-enhanced chemical vapor deposition (PECVD) silicon nitride films that can be deposited at a temperature lower than the soft bake temperature of normal photoresists. Lift-off of the silicon nitride film was carried out using standard positive photoresist. GaAs MESFETs and InP MISFETs with self-aligned gates were successfully fabricated using this lift-off process of low temperature PECVD silicon nitride.     

Effects of deposition temperature on the oxidation resistance andelectrical characteristics of silicon nitride

A study was made of the effects of deposition temperature on the oxidation resistance and electrical characteristics of silicon nitride. It was found that silicon nitride below a certain limit thickness has no oxidation resistance. This threshold falls as the deposition temperature is lowered. 3-nm-thick silicon nitride deposited at 600°C has sufficient oxidation resistance For wet oxidation at 850°C, while 5 nm film deposited at 750°C has no oxidation resistance. The electrical characteristics also improve as the deposition temperature is lowered. 6-nm-thick silicon nitride deposited at 600°C shows a TDDB lifetime that is about two orders longer than that of 6-nm-thick silicon nitride deposited at 700°C. It was also found that the silicon nitride transition layer which is deposited at the initial stage of deposition influences the oxidation resistance and electrical characteristics of thin silicon nitride. It was concluded that lowering the deposition temperature reduces the influence of the transition layer and improves the oxidation resistance and electrical characteristics of thin silicon nitride     

Stable InP MIS device using silicon nitride/anodic oxidedouble-layer dielectric

MIS devices are fabricated on InP using a double-layer dielectric consisting of anodic oxide and PECVD silicon nitride. Two different sets of experiments are conducted using tartaric acid and oxalic acid based AGW electrolyte solutions respectively for growing the anodic oxide layer. Devices having the oxalic acid grown anodic oxide layer exhibit capacitance-voltage (C-V) characteristics close to the ideal and are stable for 7200 s under applied bias conditions. The stability of MIS devices is evaluated by determining the accumulation layer charge density versus time     

Memory characteristics of MOSFET with silicon nanoclusters formed using a pulse-type gas-feeding technique in the LPCVD system

The digital gas-feeding method was used in this study, with Si₂H₆ as the source gas, in a low-pressure chemical-vapor deposition system, to grow Si nanoclusters with high densities and uniform sizes. The densities of the Si nanoclusters rose to 7 × 10¹¹ cm⁻², and their sizes slightly changed at about 7 nm based on the frequency of gas-pulse feeding in the digital process. MOSFETs containing Si nanoclusters as a floating gate in the gate stack were fabricated, and the various nonvolatile-memory characteristics of MOSFET were investigated. The total threshold voltage shift of 3.7 V was achieved, and the program/erase times were found to be 5 μs/50 ms when the program/erase voltages were +18/−20 V, respectively. The charge-storage memory window was extrapolated over 1 year to be 1.5 V in the retention measurements of the fabricated Si nanocluster floating-gate memory device.     

Measuring temperature of silicon monocrystals using spectral pyrometry

Spectrums of the thermal radiation of silicon monocrystals that are heated by a continuous laser beam (wavelength of 1.064 μm) are recorded within the wavelength range λ = 200–2500 nm. Silicon temperatures are determined within the interval T = 900–1700 K using the spectral pyrometry. The processing of a sequence of spectrums recorded with the frequency 100–1000 Hz allows the evolution of the crystal temperature to be restored during laser heating in the case when heating rates are sufficiently small. Peculiarities of different spectral intervals are discussed as applied to the problem of measuring the silicon temperature. During the laser heating of silicon, the temperature of a surface layer is shown to be heterogeneous with respect to depth, which is manifested in differences between average values calculated using thermal radiation spectrums and the surface temperature.     

Model-based adaptive process control for surface finish improvement in traverse grinding

The paper presents a process controller aimed at improving the surface quality generated by traverse grinding, avoiding the surface defects caused by vibrations onset. The innovation provided by the proposed controller consists in suppressing vibration occurrence by means of a model-based and self-learning approach: a monitoring layer classifies occurring problems and a control logic exploits these indications to select the proper mitigation actions. Since wheel-regenerative chatter represents one of the most important problems during traverse grinding in terms of achievable productivity and finishing quality, the main control variable is the wheel velocity. This variable is tuned exploiting an adaptive Speed tuning Map computed by the controller using a heuristic approach and learning methodology. The control can manage also the other sources of vibration by means of proper identification and mitigation strategies. Experimental tests are carried out on a roll grinder to validate the control system. Good performances are achieved after some training tests to allow controller learning.     

Real-time fetal heart monitoring in biomagnetic measurements using adaptive real-time ICA

Electrophysiological signals of the developing fetal brain and heart can be investigated by fetal magnetoencephalography (fMEG). During such investigations, the fetal heart activity and that of the mother should be monitored continuously to provide an important indication of current well-being. Due to physical constraints of an fMEG system, it is not possible to use clinically established heart monitors for this purpose. Considering this constraint, we developed a real-time heart monitoring system for biomagnetic measurements and showed its reliability and applicability in research and for clinical examinations. The developed system consists of real-time access to fMEG data, an algorithm based on Independent Component Analysis (ICA), and a graphical user interface (GUI). The algorithm extracts the current fetal and maternal heart signal from a noisy and artifact-contaminated data stream in real-time and is able to adapt automatically to continuously varying environmental parameters. This algorithm has been named Adaptive Real-time ICA (ARICA) and is applicable to real-time artifact removal as well as to related blind signal separation problems.     

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.