Papers

AVS1997 Session PS+TF+MS-ThM: Ionized PVD

Thursday, October 23, 1997 8:20 AM in Room A7/8

Thursday Morning

Start	Invited?	Item
8:20 AM		PS+TF+MS-ThM-1 The Application of I-PVD to Very High Aspect Ratios: Limits and Opportunities S.M. Rossnagel (IBM T.J. Watson Research Center) Ionized magnetron sputter deposition, known as I-PVD or commercially as IMD, was developed as a means to controllably deposit sputtered atoms into high aspect ratio (AR) features. I-PVD is intrinsically more flexible and efficient than collimated sputtering, which is a subtractive filtering process. I-PVD has been successfully applied to high AR (AR = 7 experimentally, AR = 10 theoretically) contact metalization, which is predominantly a directional-only process. Conformal liners require an interplay between isotropic and directional deposition and resputtering, and appear to be limited to a maximum AR of about 7 theoretically and 5 (so far) experimentally. Filling requires high directionality and low resputtering and seems limited in practical tools (<75% ionization) to AR = 2 at room temperature. Higher temperatures facilitate surface reflow which allows AR = 4 (at least) but lead to pattern density issues, which are very limiting in high pitch applications. Several fundamental relations have emerged which can be used to characterize I-PVD. (1) At high AR, bottom step coverage equals relative ionization. (2) A magnetron-to-rf power ratio of 1:1 equals roughly 50% ionization in many systems. (3) Resputtering is deleterious for filling at <0.75 micron feature width. (4) Reflow without directionality fails at small feature size or high deposition rate. (5) High pitch, high AR features require significant directionality; reflow is too anemic. This paper will attempt to set out the intrinsic advantages and limitations of I-PVD and/or reflow deposition schemes for high AR, densely or irregularly packed semiconductor features.
8:40 AM		PS+TF+MS-ThM-2 Plasma Physics of Ionized Physical Vapor Deposition J. Hopwood (Northeastern University) Experiments and models which elucidate the plasma physics of ionized physical vapor deposition (I-PVD) systems are described. The method of I-PVD seeks to deposit thin films from ions, rather than neutral atoms. The ions are generated using conventional PVD sources in conjunction with a high electron density plasma source. Over 80% of an aluminum metal vapor flux may be ionized. In this work, a conventional DC magnetron is coupled with an inductively coupled plasma to generate aluminum ions. Initially, a zero-dimensional model is constructed to investigate the role of both electron impact and Penning ionization in the creation of aluminum ions. The results of the model are compared with measured ionization fractions and found to agree. When the electron density exceeds ~10¹¹ cm^-3, the dominant ionization mechanism shifts from Penning to electron impact. It is also observed from the model that high metal density quenches the electron temperature and limits the maximum ionization. Langmuir probe measurements are used to confirm the modeled quenching mechanism. Measured aluminum neutral and ion density along the central axis of the reactor between the magnetron target and the wafer position shows a monotonically increasing ionization fraction. The aluminum atom density decays exponentially, while the aluminum ion density remains relatively constant. Thermalization of the sputtered metal flux is shown to be critical to I-PVD. A 2-D diffusion model is also presented to describe the axial evolution of ionization. Finally, the radial distribution of aluminum ions and neutrals is discussed. These experiments were performed using a Varian Quantum^TM magnetron with a 30 cm diam. Al target. The 50 cm inductively coupled plasma uses an internal Faraday shield and an external coil. Initial uniformity of the system is <8% on 200 mm wafers. The radial uniformity of the ion fraction will be discussed in terms of measurements and a 2-D model.
9:20 AM		PS+TF+MS-ThM-4 High Aspect Ratio Deposition of Copper and Aluminum in an ECR Ionized PVD Reactor W.M. Holber, L. Bourget, X. Zhang, X. Chen (Applied Science and Technology, Inc.); J. Urbahn (Applied Science and Technology, Inc. (Presently at Eaton Corp.)); S. Jin (Applied Science and Technology, Inc. (Presently at Intel Corp.)); T. Yao, K. Ngan, Z. Xu, S. Ramaswami (Applied Materials, Inc.) One of the key technology requirements for advanced interconnect schemes is that of depositing metals such as aluminum and copper into high aspect ratio features. We have focused the work described here on the use of an Electron-Cyclotron-Resonance (ECR) ionized PVD reactor which was designed to explore the low-pressure range of operation (0.5-5 mTorr), where the collisionality between the metals ions and atoms and the background argon gas is low, and where the plasma density and electron temperature can be quite high - up to 3 x 10¹² cm^-3 and 10 eV, respectively. In this reactor, aluminum or copper atoms sputtered from a planar target pass through the dense plasma column, where there is a high probability of ionization. The operating parameters which were varied include background argon pressure, sputter target voltage, microwave power, target to substrate distance, and substrate bias technique and bias voltage. Deposition samples consisted of 0.30 - 0.75 micron-sized lines or vias, with a depth of 1.2 microns. In all cases the temperature of the sample during deposition was kept quite low, typically 100-200 °C. The parameter having the most effect on fill was substrate bias, and the resultant resputtering ratio for the depositing material. Fills having aspect ratios of 3.0 or greater have been demonstrated.
9:40 AM		PS+TF+MS-ThM-5 Comparison of Modeling and Experimental Results for Copper and Aluminum Deposition in an ECR Ionized PVD Reactor X. Chen, X. Zhang, W.M. Holber, L. Bourget (Applied Sci. & Tech., Inc.); J. Urbahn (Applied Sci. & Tech., Inc. (Presently at Eaton Corp.)); S. Jin (Applied Sci. & Tech., Inc. (Presently at Intel Corp.)); K. Shadman (Applied Sci. & Tech., Inc. (Presently at M.I.T.)); T. Yao, K. Ngan, Z. Xu, S. Ramaswami (Applied Materials, Inc.) Most proposed advanced interconnect schemes require the deposition of aluminum or copper into high aspect ratio features. One potential means of achieving this is through the use of ionized PVD, in which an ionized flux of metal is used to fill features. The characteristics of the depositing metal flux which effect the fill capability include fraction ionized, angular dispersion at the substrate, and degree of resputtering from the substrate. In the work described here, an Electron-Cyclotron-Resonance (ECR) ionized PVD reactor is used to explore fill capability in the lower-pressure (0.5-5 mTorr) range of operation. SEMs of fills from the cases in which there is no resputtering of the deposited metal from the substrate can be used to derive information as to the angular spread of the depositing species. Optical emission measurements and deposition rates are used to estimate the ionization fraction of the depositing flux, and are then compared to the numbers obtained by calculating the ionization path length of the sputtered metal atoms in the plasma. It is shown that aspect ratios of 2.0-3.0 are the most aggressive that can be filled with non-biased directional deposition using this nearly fully-ionized source.
10:00 AM		PS+TF+MS-ThM-6 Modeling Ionized Metal Physical Vapor Deposition In Inductively Coupled Plasma Tools M. Li, D.B. Graves (University of California, Berkeley) Ionized physical vapor deposition (IPVD) is being explored as means to fill high aspect ratio features and to form barrier or adhesion layers with good bottom coverage. While promising results have been obtained to date, the complex couplings between the metal atoms sputtered from the target and the high density plasma, the roles of the immersed rf coils, the gas flow and temperature pattern in the tool, and related issues are as yet not fully understood. For example, the variables that determine spatial uniformity of film deposition and the ratio of neutral metal to ionized metal flux at the substrate are not well understood at present. We report results from a fluid model and a fluid-kinetic hybrid model of IPVD for aluminum/argon and titanium nitride/argon plasmas. The model geometry is two-dimensional, axisymmetric, with sputtered metal atom flux from the target taken as a boundary condition. Rf inductive power from an immersed coil sustains the plasma. Capacitive coupling from the coils and/or from the substrate is included in the model. The major issues we have explored with the model include the effects of tool geometry, immersed coil position, sputter ed metal atom flux and flux profile from the target, applied power and gas pressure. A key result of the model is the demonstration that much of the metal sputtered from the target is ionized relatively near the target and returns to the target in the form of metal ions. Only metal neutrals that are able to diffuse past the peak in the plasma potential are able to impact the substrate. The majority of these metal species are ionized under most of the conditions we have examined. Model predictions have been compared to the available experimental measurements, including the effects of the metal atoms on the plasma density and electron energy distribution.
10:20 AM		PS+TF+MS-ThM-7 An Investigation of an Ar/Cu Plasma for Ionized Sputtering* W. Wang, J.E. Foster, J.H. Booske, A.E. Wendt, N. Hershkowitz (University of Wisconsin, Madison) As semiconductor device sizes approach the sub-quarter micron regime, conventional physical vapor deposition (PVD) techniques such as sputtering become insufficient to fill deep trenches with a high aspect ratio for interconnect and contact applications. Ionized sputtering of metals (Al, Cu) has been suggested for this purpose. In ionized sputtering, a high density argon plasma is produced between the sputtering source and substrate. Sputtered metal vapor becomes ionized when it travels through the high density argon plasma region. By applying a negative bias (0, -100V) to the substrate, metal ions are attracted and deposited into trenches of high aspect ratio with good directionality. It has been demonstrated that trenches with an aspect ratio of 4:1 can be filled with Cu by this technique.¹ Recently a prototype inductively coupled rf ionized sputtering system was built at the Engineering Research Center (ERC) for Plasma-aided Manufacturing of University of Wisconsin-Madison. In this presentation, we report Ar/Cu plasma properties such as electron temperature(T_e), plasma density (n_e), electron energy distribution function (EEDF) and optical emission spectra as a function of sputtering power during ionized sputtering of Cu at different argon pressures. A monotonic decrease in plasma density with increasing sputtering power is confirmed. However, no change of electron temperature as indicated by Langmuir probe I-V characterization is observed at different sputtering powers. Preliminary results indicate a slight modification to the EEDF as the sputtering power increases. The drop in plasma density with the sputtering power may be related to the loss of energetic electrons in the Ar/Cu plasma. The intensity ratio of the Cu⁺ ion line at 213 nm to the Cu neutral line at 216 nm is used to monitor the behavior of the ionization fraction at a variety of rf powers and Ar pressures. The intensity measurements indicate that the Cu⁺/Cu ratio increases with increasing Ar pressure which is consistent with global modeling predictions. Addition of a multi-cusp magnetic confinement field is observed to significantly enhance Ar plasma density and appears to thereby enhance the Cu ionization efficiency. *Acknowledgement This work was supported by National Science Foundation (NSF) under grant No. EEC-8721545. ¹1. W.M. Holber, J.S. Logan, H.J. Grabarz, J.T. Yeh, and J.B.O. Caughman, J. Vac. Sci. Technol.A, Vol.11, No.6, Nov/Dec(1993).
10:40 AM		PS+TF+MS-ThM-8 Investigation of Ionized PVD With Various Plasma Sources D.B. Hayden, D.R. Juliano, D.N. Ruzic (University of Illinois, Urbana) Ionized physical vapor deposition (IPVD) has been used to enhance the sputtering of metal into trenches and vias. A magnetron¹ coupled with an added inductively coupled plasma (ICP) coil has been investigated with different working gases and targets. This has increased the ionization of the sputtered metals, which has allowed the metal ions to be normally accelerated across the plasma sheath to a biased substrate. The ionization fraction of the sputtered metal flux to the bottom of a trench has reached upwards of 85%, which is promising for filling higher aspect ratios. However, there has been difficulty in achieving uniformity due to shadowing and the increased distance from the target to substrate to accommodate the ICP coil. For this reason other secondary plasma sources have been investigated, which can possibly improve the uniformity while still ionizing an adequate percentage of metal neutrals. Plasma sources include coil designs at wider radii which are not in the line-of-sight from the target to substrate, as well as external plasma sources such as remote radio-frequency and helicon configurations. ¹Donated by Materials Research Corporation, a wholly-owned subsidiary of Sony Corporation of America.
11:00 AM		PS+TF+MS-ThM-9 Ionized PVD Deposition of Ti and TiN Liners J.C. Forster, P. Gopalraja, Y. Tanaka, T. Tanimoto, R. Hofmann, Z. Xu (Applied Materials) Ionized physical vapor deposition has received increased attention recently as the microelectronics industry heads towards sub-0.35µ back end of line manufacturing. The Vectra IMP (Ionized Metal Plasma) is a commercially available, production worthy ionized PVD source for the deposition of Ti and TiN barrier and adhesion layers. In this source an RF inductively coupled plasma is used to ionize metal sputtered from a magnetron source. The influence of various process parameters, such as target power, inductive coil power, pressure, and wafer bias, on deposition rate, uniformity and bottom coverage have been determined, and are discussed in the context of physical processes occurring in the plasma. Uniformities of 2% 1σ over an 8$B!I(B diameter are possible, with bottom coverages of greater than 50% in a 0.35x1.2µm contact. Simple scalings for both uniformity and bottom coverage with the DC sputter power, the RF inductive power, the pressure and the wafer bias have been found that are consistent with physical processes occurring in the plasma. A comparison between experimental results with some models of the ionized PVD process shows that proper treatment of the collisional slowing down of sputtered neutrals is essential to properly predict process performance.
11:20 AM		PS+TF+MS-ThM-10 Properties of TiN Films Deposited by Ion Metal Plasma (IMP) Y. Tanaka, E. Kim, J.C. Forster, Z. Xu (Applied Materials) Ionized metal plasma (IMP) deposition uses inductively coupled plasma to ionize DC sputtered materials and to give the ions independent directionality and energy control. The IMP deposition technique allows controllability of the TiN microstructure, which is sensitive to ion energy flux and temperature. Operating the source at relatively high RF inductive power (>2kW) and substrate bias creates a high energy ion flux that bombards the substrate, densifying the TiN film. With constant RF power (3kW), the TiN film grain size increases from 200Å to over 700Å as bias voltage increases from 0 to -60V. Further increase in bias voltage causes the grain features to disappear. Film resistivity decreases with growing grain size, to as low as 50µΩ-cm. Increasing the bias energy, hence increasing bombardment energy, beyond a certain threshold, results in increased resistivity which correlates with weaker film texture, especially <200>. Interactions between plasma distribution and TiN resistivity uniformity have been studied. Due to the different recombination rates of argon and nitrogen, the plasma density uniformity will depend on the relative ratio between Ar and N₂, and the RF power. Provided that the inductive coil is parallel to the process cavity sidewall, an Ar rich plasma has its peak at the center of the coil loop. On the other hand, an N₂ rich plasma has a density peak closer to the edge. The IMP process parameters can be controlled to provide uniform TiN resistivity across a wafer.
11:40 AM		PS+TF+MS-ThM-11 Film Properties of Ti/TiN Bilayers Deposited Sequentially by Ionized Physical Vapor Deposition F. Cerio, J. Drewery, E. Huang, G. Reynolds (Varian Ginzton Research Center) Ionized physical vapor deposition (iPVD) has received much attention as a method for depositing material at the bottom and on the sidewalls of the high aspect ratio features proposed for sub-0.25 micron integrated circuits. In the following paper, we describe the film properties of Ti/TiN bilayers deposited sequentially using the iPVD technique. The experimental configuration consisted of a conventional planar magnetron in combination with an inductively coupled RF plasma. TiN was reactively sputtered from a titanium target which remained non-nitrided throughout the deposition, a process commonly referred to as operating in the non-nitrided mode (NNM). These films were analyzed by cross-sectional SEM and TEM, automated 4-point sheet resistance probe, X-ray diffraction, X-ray fluorescence, stress gauge, Rutherford backscattering, reflectance mapping, and secondary ion mass spectrometry. Highly oriented <111> TiN was observed on <002> oriented Ti underlayers. At aspect ratios of 4:1 with vertical sidewalls, bottom coverage approaching 100% was obtained. Varying process parameters did not change the bulk resistivity significantly, and values as low as 23 µΩ-cm were measured for the NNM TiN films. Mechanical stress was strongly influenced by temperature, similar to what has been observed for both conventionally sputtered and collimated Ti/TiN. Below 200°C, the films were highly compressive, but values below 2 GPa were obtained at 400°C. RF plasma power and pressure were also found to affect stress. Reflectivity was easily controlled in the range 15-23% at 440 nm; by adjusting process parameters, the reflectivity could be matched to that of collimated TiN, which is typically 20% at this wavelength. Deposition rates of 900 Å/min were measured, corresponding to a specific deposition rate of 1.8 Å/kW-s.