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Title:Comparison of high power impulse magnetron sputtering and modulated pulsed power sputtering for interconnect metallization
Author(s):Meng, Liang
Director of Research:Ruzic, David N.
Doctoral Committee Chair(s):Ruzic, David N.
Doctoral Committee Member(s):Stubbins, James F.; Dolan, Thomas; Li, Xiuling
Department / Program:Nuclear, Plasma, & Rad Engr
Discipline:Nuclear Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):High Power Impulse Magnetron Sputtering (HiPIMS)
High Power Pulsed Magnetron Sputtering (HPPMS)
Modulated Pulsed Power (MPP)
Interconnect Metallization
Copper Seed
Ionized Physical Vapor Deposition (IPVD)
Plasma diagnostics
Plasma Transport
Ionization Fraction
Abstract:Ionized physical vapor deposition is used to deposit the barrier and seed layers in the state of the art interlevel metallization process. As the critical dimension keeps shrinking, it has become increasingly difficult to use the current techniques to form thin, continuous and stable barrier/seed layers, and more likely to form void during the following process of electroplating. An enhanced metal ionization is believed to be critical. Ions in the deposition flux can increase the nucleation density and adhesion of Cu to Ta due to their surface penetration, and create less overhang due to the high directionality of ion flux. In this work, high power impulse magnetron sputtering (HiPIMS) and its derivative, modulated pulsed power (MPP) magnetron sputtering, with their claimed high ionization capability, are proposed for the application of barrier/seed layer deposition. Their plasma properties and metal ionization fractions are characterized using various pulsing and discharge parameters. Depositions on patterned wafers are performed to evaluate their potential for interconnect metallization application. Time- and spatially-resolved plasma diagnostics are further performed to investigate the physical mechanisms involved in the pulsed plasma generation, evolution, and plasma transport. Specially designed experiments and plasma modeling are used to further understand some key features during HiPIMS, such as the self-sputtering process. Fundamental studies of HiPIMS discharge are conducted in a planar magnetron. Very high peak current up to 750 A can be achieved. Triple Langmuir probe (TLP) is adopted to measure the electron density ne and electron temperature Te. High electron densities (ne) during the pulse to about 5×1017 m-3 are measured on the substrate and reach 3×1018 m-3 later after the pulse ends. Cu ionization fractions (IF) are measured on the substrate level using a gridded energy analyzer (GEA) combined with a quartz crystal microbalance (QCM). Up to 60% has been achieved using a 200 Gauss magnetic field configuration, much higher than the DC magnetron sputtering. It basically increases with higher charging voltage and longer pulse length due to higher plasma densities. However, lower ion extraction efficiency at stronger B field, however, leads to lower Cu ionization in spite of a higher plasma density. HiPIMS has been shown to have some complicated and distinctive features. Their possible effects on the application, as well as the underlying physics are investigated. Plasma expansion is observed with a high plasma density peak moving from the target to the substrate. It has varied speed and preferred orientation. Different parameters such as the charging voltage, pulse duration, and magnetic field strength are found to affect the plasma transport. A large plasma potential drop is observed in the presheath and extends into the bulk plasma region during HiPIMS discharge. It not only affects the plasma expansion but also determines the ion extraction efficiency, which is critical for the interconnect metallization application. A direct evidence of the self-sputtering effect is provided by measuring the incident fluxes to the cathode through a hole in the target. Plasma is initially ignited only in a long strip in the race track where the B field is strong and drifts toward the weak-B region. High fraction of Cu+ flux is determined. To provide more insights into the development of Cu ion and Ar ion species, a time-dependent model is built to describe the ionization region where plasma is confined by magnetic field. The important processes such as plasma-target interactions, electron collision ionizations, and gas rarefaction are incorporated in the model. The test results of the model show the capability to predict the temporal development of the electron density, the degrees of ionization for Cu and Ar, and the ratio of Cu+ ions to Ar+ ions. Magnetic field configurations are modified specifically for the HIPIMS. The race track pattern is varied to optimize the target utilization and the downstream plasma uniformity. A closed path for electrons to drift along is found essential in the design. The configuration of wider race track generates a higher pulse current, and extends the intense plasma coverage on the substrate. A spiral-shaped magnetic field configuration is able to generate high pulse current, achieve a downstream plasma with superior uniformity, and yield a better target utilization even without the assistance of magnet rotation. Modulated pulsed power (MPP) magnetron sputtering is a new derivative of the HiPIMS that may allow unprecedented user control over the growth process. It has some distinctive features, such as flexible control over the discharge voltage and current waveforms. In this study, a thorough characterization of the MPP discharge using two different models (Solo and Cyprium) is performed in the Galaxy planar magnetron to better understand this pulsing technique. The effects of various pulsing and discharge parameters, as well as the magnetic field, are studied. For the test of deposition on patterned wafers, a hollow cathode magnetron is chosen. All three types of power supplies, DC, MPP and HiPIMS are first subject to the plasma characterization, both to study the discharge mechanisms on HCM and to develop potentially good recipes with high Cu ionization fractions. Both MPP and HiPIMS increase the Cu ionization fraction in the deposition flux (up to 25% and up to 30% respectively) as compared with the normal DC magnetron sputtering (as below 20%). Ultimately, Cu is deposited on patterned wafers with trenches of different widths as narrow as 70 nm. The conformality of the Cu film on the trench will be compared using cross-section scanning electron microscopy (SEM). Reduced overhang is achieved using MPP Solo as compared with DC sputtering. More significant improvements have been seen using the MPP Cyprium model. HiPIMS also shows slightly better conformality and may be further improved with appropriate substrate biasing. The potential of applying HiPIMS and MPP for barrier/seed layer deposition will be further discussed.
Issue Date:2013-08-22
Rights Information:Copyright 2013 Liang Meng
Date Available in IDEALS:2013-08-22
Date Deposited:2013-08

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