A Study of Tungsten-Titanium Barriers in Silver Metallization
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A Study of Tungsten-Titanium Barriers in Silver Metallization S. Bhagat1, N. D. Theodore1,2, and T. L. Alford1 1 School of Materials, Arizona State University, Tempe, AZ, 85281 2 Freescale Semiconductor Inc., Tempe, AZ, 85284 ABSTRACT This work investigated the viability of tungsten-titanium barrier layers for silver metallization. Reactive sputtered W-Ti was deposited on a Si wafer followed by an Ag thin film over layer. These samples were then annealed in vacuum at temperatures up to 700 oC. Characterization of these samples included using x-ray diffractometry, Rutherford backscattering spectrometry, scanning transmission microscopy, secondary ion mass spectroscopy, transmission electron microscopy, and four point probe analysis. The results indicated that the metal/diffusion barrier stack was stable up to 600 oC. Silicon started moving into the tungsten-titanium film at temperatures above 600 oC. Movement of Si resulted in local Si voiding. These results showed the promise of W-Ti as an effective barrier layer for silver metallization for process temperatures below 600 oC.
INTRODUCTION Aluminum, copper, and their alloys are the most commonly used interconnect materials for ultra-large-scale integration applications. But aluminum has problems such as poor electromigration resistance, RC delays, electrical leakage, and power consumption. Similarly, though copper has better conductivity than aluminum, at high temperatures Cu diffuses into silicon causing device degradation. These problems have lead to the investigation of other suitable interconnect materials, with possible candidates being silver, tungsten, gold, and their alloys. Silver has higher conductivity and better electromigration resistance than aluminum [1]. However, problems associated with silver include poor adhesion to dielectric layers, and diffusion into silicon substrates at temperatures higher than 400 oC [2, 3]. Introduction of a barrier layer between the silicon substrate and the silver thin film can reduce the above stated problems and enhance performance. Tungsten-titanium barrier layers have been investigated extensively for Al and Cu interconnects [4]. Limited research work has been done towards investigation of W-Ti as a barrier layer for silver metallization [5, 6]. This study focuses on the thermal stability of metal/diffusion barrier stack exposed to annealing in vacuum at high temperatures. The results support the potential use of W-Ti for barrier layers for silver metallization.
EXPERIMENTAL DETAILS Silicon wafers were cleaned in acetone and methanol followed by a hydrofluoric acid dip to etch the native oxide layer. A thin W-Ti barrier layer of 170 nm thickness was deposited by
reactive direct-current sputtering of a standard W-Ti target at a power of 100 W using a 5.08 cm magnetron gun. Argon was used as the plasma gas. The base pressure was 2.7x10-5 Pa and the working pressure of Ar was 0.133 Pa. This deposition was followed by deposition of 200 nm of silver in a CHA electron- beam evaporator. The base pressure was
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