CMP后清洁

1.INTRODUCTION

With the rapid shrinking ofthe device dimensions and the strict requirements ofachieving extremely smooth surfaces, there is an increasing need for global sur-face planarization of various thin films in the front- and back-end processesusing chemical mechanical planarization (CMP) [1]. During CMP, the padmaterial and the wafer surface are in intimate contact in the presence of slurry[2]. The slurry particles remain on the wafer surface at the end of the CMP pro-cess, and if not removed, they can cause various types of defects (scratches, cor-rosion spots, etc.) in the subsequent processing steps, which affects thefunctionalization ofthe integrated circuit (IC). These defects, in some instances.may also arise from the pad and the diamond disc conditioner. The other com-mon forms of contaminants include organic residues and metallic impurities.The residues typically originate from slurry additives or the pad material andrequire subsequent processing such as plasma ashing or an oxidant (e.g..peroxide)-based chemical cleaning [3]. The metallic impurities that are lefton the surface in concentrations of 1011.1012atoms/cm' emanate either from abrasion caused to the metal lines or from metal ions present in the slurryTable 4.1 summarizes different types of contaminants and their effects duringprocessing of devices (4]. Particulate contamination in the form of abrasive par-ticles, precipitates, or fragments of films and pad can enhance local surfaceroughness, impact the photolithography process by blocking the UV light, or cause shorting when the particles are conductive. Metallic impurities can affectelectrical characteristics when they are highly mobile or cause dissolution ofsilicon in the case of more noble metal ions. Organic residues influence the wet-tability of the films and reduce the adhesion of deposited films.

In an effort to significantly lower the defect density on various films prior tothe next processing step, the demand for an effective and efficient post-CMPcleaning process has been continuously rising. As a result, there is a tremendousinterest towards improving the fundamental understanding of the interfacialphenomena and the underlying physical and mechanical mechanisms that drivethe cleaning process with the ultimate objective of developing innovativeapproaches with newer chemistries and advanced tools that can effectivelyachieve desired cleaning with minimum damage. In this chapter, a review ofbatch- and single-wafer brush and megasonic cleaning processes with emphasison the effect of different process variables on particle removal is provided. Thefirst few sections briefly describe the forces that drive particle adhesion andremoval in a post-CMP cleaning process. The last part of the chapter focuseson the typical chemical formulations and the role of additives in post-CMPcleaning of silicon dioxide, tungsten, and copper films.

2 FORCES ON PARTICULATE CONTAMINANTSIN A POST-CMP CLEANING PROCESS

A particle on a polished surface immersed in a liquid and subjected to an exter-nal force such as that in a brush cleaning or megasonic cleaning experiencesseveral types of forces acting on it. Some of these forces and their dependencyon the particle dimension are shown in Table 4.2 (5]. As can be seen from thistable, the dominant forces are van der Waals adhesion forces and double-layerinteractions (electrostatic forces of interaction) due to their higher magnitudecompared to other forces for the same size of the particle.

2.1 van der Waals Forces

The van der Waals forces are one of the most important forces in the adhesion ofparticles to any substrate. These forces originate due to the instantaneous dipoleinteractions between molecules formed due to the positions of the electronssurrounding the nuclei. There are three types of dipole interactions, namely,permanent dipole-permanent dipole (Keesom interactions), permanentdipole-induced dipole (Debye interactions), and induced dipole-induced dipoleinteractions (London forces) which collectively contribute to van der Waalsforces (6]. The van der Waals forces depend on the composition of the interacting particle, substrate and the medium, the distance of separation between them.and the surface roughness and geometry of the particle and the substrate (71.