ELECTROSTATIC (LANGMUIR) PROBE MEASUREMENTS IN RF DRIVEN He, N2, BCl3, AND BCl3/N2 PLASMAS

A Langmuir probe diagnostic system was developed to measure the electron density distribution in BCl3/N2 mixtures to determine if an average electron energy increase might be partially responsible for the previously observed etch rate enhancement of GaAs with nitrogen addition. The system was validated in both helium and nitrogen plasmas. Probe measurements showed that as the nitrogen concentration in BCl3/N2 plasmas increased, the average electron energy actually decreased (5.52 eV @ 0% N2, 4.14 eV @ 30% N2, and 3.69 eV @ 60% N2). However, an increase in negative ion density was observed with nitrogen addition reaching a maximum at 40% N2. Although negative ions play no role in etching, this trend seems to correlate with the observed etch rate enhancement. A plausible explanation for both the increase in etch species and the increase in negative ion density is an increase in dissociation due to energy transfer from N2 metastables

Computational Intelligence Techniques for OES Data Analysis

Semiconductor manufacturers are forced by market demand to continually deliver lower cost and faster devices. This results in complex industrial processes that, with continuous evolution, aim to improve quality and reduce costs. Plasma etching processes have been identified as a critical part of the production of semiconductor devices. It is therefore important to have good control over plasma etching but this is a challenging task due to the complex physics involved. Optical Emission Spectroscopy (OES) measurements can be collected non-intrusively during wafer processing and are being used more and more in semiconductor manufacturing as they provide real time plasma chemical information. However, the use of OES measurements is challenging due to its complexity, high dimension and the presence of many redundant variables. The development of advanced analysis algorithms for virtual metrology, anomaly detection and variables selection is fundamental in order to effectively use OES measurements in a production process. This thesis focuses on computational intelligence techniques for OES data analysis in semiconductor manufacturing presenting both theoretical results and industrial application studies. To begin with, a spectrum alignment algorithm is developed to align OES measurements from different sensors. Then supervised variables selection algorithms are developed. These are defined as improved versions of the LASSO estimator with the view to selecting a more stable set of variables and better prediction performance in virtual metrology applications. After this, the focus of the thesis moves to the unsupervised variables selection problem. The Forward Selection Component Analysis (FSCA) algorithm is improved with the introduction of computationally efficient implementations and different refinement procedures. Nonlinear extensions of FSCA are also proposed. Finally, the fundamental topic of anomaly detection is investigated and an unsupervised variables selection algorithm tailored to anomaly detection is developed. In addition, it is shown how OES data can be effectively used for semi-supervised anomaly detection in a semiconductor manufacturing process. The developed algorithms open up opportunities for the effective use of OES data for advanced process control. All the developed methodologies require minimal user intervention and provide easy to interpret models. This makes them practical for engineers to use during production for process monitoring and for in-line detection and diagnosis of process issues, thereby resulting in an overall improvement in production performance

Neutral Beam Etching

The aim of this research is to better understand the behaviour of pulsed discharges and electron dynamics for the purpose of tailoring the plasma properties for neutral beam etching (NBE) applications. A capacitively coupled plasma formed in a research system was used for a study of pulsed tailoring in an electropositive plasma. A combination of high time resolved optical diagnostics, plasma imaging and optical emission spectroscopy, and hairpin probe measurements were used to study the electron density and the energy distribution function during the ignition phase of a repetitively pulsed plasma. Two different waveforms were used to modulate the envelope of the input RF-voltages in order to control the ignition phase, by changing the increase rate of the electron density and evolution of the electron energy distribution function (EEDF). The results of this study indicate that the increase rate of the electron density and the EEDF, during operation, can be influenced and even controlled to some extent by pulse tailoring. Electron densities of the order of 1016 m-3 were obtained, and EEDFs of a highly non-Maxwellian nature were characterised during the ignition phase. Also, the ignition timescales were controlled by applying pulse tailoring from a few microseconds (typically 2 μs) to a few tens of microseconds (80 μs) for the different input waveforms. An inductively coupled plasma in an industrial plasma etching tool was used to study pulse tailoring in electropositive and electronegative discharges. The same environment was used to create a source to from energetic negative ions which could then be extracted and neutralised. Similar diagnostic techniques, as those used in the research source, in addition to RF-probes were used to characterise the inductive source. Optical emission spectroscopy and electron density measurements showed that the plasmas, almost instantaneously, ignite in the H-mode. The EEDFs were characterised by a Maxwellian distribution with an electron temperature ranging between 1.2 up to 1.6 eV, and electron densities of the order 1018 m-3 were measured, depending on the operating conditions. This source was also used for preliminary NBE studies. Neutralisation efficiencies ranging between 70% and 95% were measured, and etch rates of 25 and 30 nm/min were found. Finally, a novel technique was developed to monitor in-situ and in real time the plasma-induced damage on thin films, in particular low-k dielectrics. This technique uses wafer tiles to simultaneously measure the plasma properties and plasma-induced damage on the thin films. An analytical model, based on the ion flux probe model, was used to to extract the plasma parameters and mimic the existence of thin on the probe surface, by a resistor and a capacitor connected in parallel. The model managed to extract exact values of a dummy load, while a reasonable fit could not be achieved for thins silicon dioxide films. Initial results showed that, even for a well characterised material of a known permittivity and thickness, the model could not extract absolute resistivity and capacitance. Plasmas surface interactions were deduced to be the main factor for this result, which were not included in the model used in this study. However, plasma-induced damage/changes on low-k thins films by different plasma chemistries, such as argon and hydrogen, were qualitatively measured in real-time. The wafer probe was used to monitor the etch rate of an SF6 plasma for low-k thin films in realtime and in-situ. This technique could also be used to measure the neutralisation efficiency and possibly the etch rates of our NBE source in real time

RATIONAL DESIGN OF NON-DAMAGING CAPACITIVELY COUPLED PLASMA ETCHING AND PHOTORESIST STRIPPING PROCESSES FOR ULTRALOW K DIELECTRIC MATERIALS

Resistance-capacitance delay, crosstalk, and power dissipation associated with the increasing capacitance of interconnect structures limits the performance of high-speed microelectronics and leads to the demand for porous ultralow dielectric constant (ULK) material introduction. Process integration of ULK dielectrics requires plasma etching of dielectric material, stripping of the post-etching photoresist (PR) mask, and surface cleaning of plasma-etching-related residues, without damaging the dielectric. Dual frequency capacitively coupled plasma (CCP) reactor are becoming the standard for etching of ULK materials. In this work, we evaluated ULK-compatible PR stripping using both remote plasma and in situ ashing processes coordinated with CCP fluorocarbon (FC)-based ULK etching. Remote H2 plasma enabled a high PR ashing rate while introducing little ULK damage at an elevated substrate temperature (275 °C), and was the best for our remote plasma ashing processes. In situ ashing, with the advantage of no need for an additional dedicated reactor, is preferable to the remote plasma ashing for industry, and we studied in detail CO2 in situ ashing process. The ULK damage introduced during CO2 in situ ashing increased with atomic oxygen density as a function of chamber pressure. To compare the performance of different ashing processes for PR stripping from ULK material, we introduced an ashing efficiency (AE) parameter which is defined as the thickness of PR removed over the thickness of ULK simultaneously damaged, and can be considered a process figure of merit. A high AE can be obtained under low pressure operation, which suppresses ULK damage with minimal atomic oxygen while combining with a RF bias to enhance the PR ashing rate. The preceding ULK etching process using 10% C4F8/Ar plasma deposits FC coating on ULK feature sidewalls. For H2-based remote plasma at high temperature, most of FC coating was removed rapidly and its impact on ULK ashing damage was minor. For CO2 in situ ashing, FC coating remained on ULK sidewalls and provided effective protection of ULK. FC protection was essential for the success of the CO2 in situ ashing process. A strong decrease of ULK post-ashing damage with increasing FC coverage was found, which may be due to surface protection by FC surface coverage along with pore-sealing by the FC material

Virtual metrology for plasma etch processes.

Plasma processes can present dicult control challenges due to time-varying dynamics and a lack of relevant and/or regular measurements. Virtual metrology (VM) is the use of mathematical models with accessible measurements from an operating process to estimate variables of interest. This thesis addresses the challenge of virtual metrology for plasma processes, with a particular focus on semiconductor plasma etch. Introductory material covering the essentials of plasma physics, plasma etching, plasma measurement techniques, and black-box modelling techniques is rst presented for readers not familiar with these subjects. A comprehensive literature review is then completed to detail the state of the art in modelling and VM research for plasma etch processes. To demonstrate the versatility of VM, a temperature monitoring system utilising a state-space model and Luenberger observer is designed for the variable specic impulse magnetoplasma rocket (VASIMR) engine, a plasma-based space propulsion system. The temperature monitoring system uses optical emission spectroscopy (OES) measurements from the VASIMR engine plasma to correct temperature estimates in the presence of modelling error and inaccurate initial conditions. Temperature estimates within 2% of the real values are achieved using this scheme. An extensive examination of the implementation of a wafer-to-wafer VM scheme to estimate plasma etch rate for an industrial plasma etch process is presented. The VM models estimate etch rate using measurements from the processing tool and a plasma impedance monitor (PIM). A selection of modelling techniques are considered for VM modelling, and Gaussian process regression (GPR) is applied for the rst time for VM of plasma etch rate. Models with global and local scope are compared, and modelling schemes that attempt to cater for the etch process dynamics are proposed. GPR-based windowed models produce the most accurate estimates, achieving mean absolute percentage errors (MAPEs) of approximately 1:15%. The consistency of the results presented suggests that this level of accuracy represents the best accuracy achievable for the plasma etch system at the current frequency of metrology. Finally, a real-time VM and model predictive control (MPC) scheme for control of plasma electron density in an industrial etch chamber is designed and tested. The VM scheme uses PIM measurements to estimate electron density in real time. A predictive functional control (PFC) scheme is implemented to cater for a time delay in the VM system. The controller achieves time constants of less than one second, no overshoot, and excellent disturbance rejection properties. The PFC scheme is further expanded by adapting the internal model in the controller in real time in response to changes in the process operating point

The use of 3D surface topography analysis techniques to analyse and predict the alteration of endosseous titanium dental implants generated during the surgical insertion

Dental implants have emerged as the preferred choice for tooth replacement, reaching about one million implantations per year. Among factors affecting dental implant success, surface topography has been recognised as an important parameter since it is known to affect the cellular response and, ultimately, osseointegration. Although it has generally been overlooked, the topography integrity may be jeopardised due to the stresses to which implant surfaces are subjected during surgical insertion. The study of this phenomenon is of great importance because the surface topography alteration may lead to a cellular response that differs from the ones analysed in in vitro assays, as well as to titanium particle release, which has been related to increased bone resorption, peri-implantitits and implant failure. The main goal of the present dissertation was to analyse and predict the topographic alteration of endosseous dental implants generated during the surgical insertion. In light of the lack of standardisation regarding dental implant topographic characterisation, the effect of the data acquisition and processing variables on the 3D topographical parameters was first evaluated. Then, based on the results obtained, a generalized topographical characterization strategy for endosseous dental implant was established. Aimed at elucidating the effect of the insertion forces on the topography of implants, commercial dental implants were inserted into cow rib bones using standard surgical procedures. 3D topographical characterisation techniques were employed to quantify the modification and to approximate the material loss. Post-inserted implants showed wear and plastic deformation of the most prominent peaks, and released titanium particles were observed in the bone site underlying the implants. The obtained results suggest that the surface modification is a complex phenomenon conditioned by the combination of surface treatment, implant macrogeometry and surgical procedure. Additionally, an in vitro study was conducted to investigate whether the topographic alteration generated during dental implant insertion affects the cell response. Within the limits of the study, the experimental data rejected the hypothesis that osteoblast attachment and proliferation could be significantly affected by the topographical modification generated during dental implant insertion. Finally, the correlation between the surface modification and certain physically meaningful properties of surface topography was successfully established through the newly developed product of 3D topographical parameters named Si. This novel parameter is presented as a useful tool to predict whether a surface is more prone to suffer alterations during insertion, and may therefore contribute to foster advancements in the design of better performing endosseous implants.Los implantes dentales se han convertido en la opción preferida para la sustitución de piezas dentales, llegando a la colocación de alrededor de un millón implantes dentales por año. Entre los factores que afectan al éxito del implante dental, la topografía de la superfície ha sido reconocida como un parámetro importante ya que se sabe que afecta la respuesta biológica, y en última instancia, la osteointegración. Aunque en general se ha pasado por alto, la integridad de la topografía puede verse comprometida debido a los esfuerzos a los que las superfícies de los implantes son sometidos durante la inserción quirúrgica. El estudio de este fenómeno es de gran importancia ya que la alteración de la topografía superficial puede conducir a una respuesta celular que difiere de la analizada en ensayos in vitro, así como a la liberación de partículas de titanio, que se ha relacionado con el aumento de resorción ósea, periimplantitits y fracaso del implante. El objetivo principal de la presente tesis fue analizar y predecir la alteración topográfica de los implantes dentales endoóseos generado durante la inserción quirúrgica. A la luz de la falta de estandarización en cuanto a la caractetrización topográfica de implantes dentales, en primer lugar se analizó el efecto de los parámetros de adquisición y procesado de datos en los parámetros topográficos 3D y se estableció una estrategia para la caracterización topográfica de implantes dentales endoóseos. Con el objetivo de dilucidar el efecto de las fuerzas de inserción en la topografía de los implantes, se insertaron implantes dentales comerciales en hueso de costilla de bovino mediante procedimientos quirúrgicos estándar, y se emplearon técnicas de caracterización topográfica 3D para cuantificar la modificación y aproximar la pérdida de material. Los implantes post-insertados mostraron desgaste y deformación plástica de los picos más prominentes, y se observaron partículas de titanio en el hueso subyacente a los implantes. Los resultados obtenidos sugieren que la modificación de la superfície es un fenómeno complejo condicionado por la combinación de tratamiento de superfície, macrogeometría del implante y procedimiento quirúrgico. Adicionalmente se llevó a cabo un estudio in vitro para investigar si la alteración topográfica generada durante la inserción del implante dental afecta a la respuesta celular. Dentro de las limitaciones del estudio, los datos experimentales rechazaron la hipótesis de que la adhesión y proliferación de los osteoblastos podrían verse afectadas de manera significativa por la modificación topográfifica generada durante la inserción. Por último, la relación entre la modificación superficial y algunas propiedades topográficas se estableció con éxito a través del nuevo producto de parámetros topográficos 3D denominado Si. Este nuevo parámetro se presenta como una herramienta útil para predecir si una superfície es más propensa a sufrir alteraciones durante la inserción, y por lo tanto, puede contribuir a fomentar avances en el diseño de implantes endoóseos de mejor rendimiento.Hortz inplanteak hortzak ordezkatzeko lehen aukera bilakatu dira, eta urtean milioi bat inguru inplante ezartzera iritsi dira. Hortz inplante arrakastatsu bat lortzeko erabakigarriak diren faktoreen artean, gainazalaren topografia parametro garrantzitsutzat jotzen da, jakina baita erantzun biologikoan eta, finean, osteointegrazioan eragina duela. Orokorrean aintzat hartu ez den arren, gainazalaren topografiaren osotasuna konprometitua egon daiteke intsertzio kirurgikoan sortzen diren esfortzuak direla eta. Fenomeno horren azterketa garrantzi handikoa da, topografiaren alterazioaren ondorioz erantzun zelularra in vitro saiakuntzetan aztertutakoaren ezberdina izan eta titanio partikulak askatu daitezkeelako, zeina hezurraren erresortzio, periinplantitis eta inplantearen porrotarekin lotu den. Tesi doktoral honen helburu nagusia, intsertzio kirurgikoan sorturiko hortz inplanteen gainazalaren topografiaren alterazioa aztertu eta aurreikusteko teknikak garatzea izan da. Hortz inplanteen karakterizazio topografikorako estandarizaziorik ez dagoela eta, lehenik eta behin datuak eskuratzen eta prozesatzen parte hartzen duten parametroek 3Dko parametro topografikoen balioetan duten eraginaren analisi bat egin da, eta, lorturiko emaitzetan oinarrituz, hortz inplanteen topografia karakterizatzeko estrategia bat garatu da. Intserzio kirurgikoaren esfortzuek hortz inplanteen gainazalean duten efektua aztertzeko helburuarekin, hortz inplante komertzialak behien saiheski hezurrean txertatu ziren prozedura kirurgiko estandarrak jarraituz, eta 3Dko karakterizazio teknikak erabili ziren modifikazioa kuantifikatu eta material galera kalkulatzeko. Txertatu ondoren, hortz inplanteek tontor garaienen desgastea eta deformazio plastikoa erakutsi zuten, eta titanio partikulak aurkitu ziren inplantea inguratzen duen hezurrean. Lorturiko emaitzek iradoki zuten hortz inplanteen modifikazioa fenomeno konplexua dela, gainazalaren tratamendu, inplantearen makro-geometri eta prozesu kirurgikoarekiko duen menpekotasunaren ondorioz. Are gehiago, in vitro analisi bat burutu zen ea intsertzioan zehar sorturiko alterazio topografikoek erantzun zelularrean efekturik duen ikertzeko. Ikerketaren mugak kontuan izanik, datu esperimentalek baztertu egin zuten modifikazio topografikoak osteoblastoen ugalketan eta adhesioan efektua dutelako hipotesia. Azkenik, gainazalaren modifikazioaren eta propietate topografikoen arteko korrelazioa arrakastarekin ezarri zen, garatutako 3Dko parametro topografikoen produktu berri baten bitartez. Si izena eman zaion parametro berri horri esker, gainazal batek alterazioak jasateko duen joera aurreikusi daiteke, eta beraz, hortz inplante funtzionalagoen diseinuan aurrerapenak sustatzeko erreminta egokitzat aurkezten da