SERDAR AKSU: RESEARCH: PROJECT DESCRIPTION
The Role of Complexing Agents in the Chemical Mechanical Planarization (CMP) of Copper Thin Films
A new era in chip manufacturing is about to start with use of copper as the interconnect metal. Copper, which has been regarded as one of the “poisons” in the microelectronics industry, now makes its way to the “veins” of the integrated circuits (ICs). Modern ICs contain tens to hundreds of millions of logic devices to achieve complex functions. Interconnects which are embedded in interlayer dielectric material (ILD) are the wire connections to supply electrical signals to these devices. By replacing Al alloys (Al-Cu-Si alloys), which have been used for over thirty years, with copper as the interconnect metal, it is possible to manufacture faster, more reliable and cheaper chips.
Transition from aluminum to copper has not been easy. First appropriate diffusion barriers for copper were needed to be developed to prevent copper diffusing to SiO2 and then subsequently to Si devices. Copper has several deep levels in Si, which would act as combination centers. Therefore copper is known as one of “life-time killers” for silicon devices. The second problem with copper is that conventional dry etching does not work for copper patterning. Since copper does not form compounds with adequate volatility at the normal operating temperatures, severe mass transport limitations are imposed on the removal of etching products. In order to overcome these problems copper interconnects are fabricated using a new process, which is called as the dual damascene method. Using dual damascene method it is possible to form multilevel copper interconnects in planar fashion. Chemical mechanical planarization (CMP) of the copper thin films is an integral part of the dual damascene method and is one of the most important fabrication steps in manufacture of copper lines. The CMP process is a polishing process in which surface roughness of a film on a wafer is removed by smoothing and plaining aided by both chemical and mechanical forces using chemically active slurries abrasive slurry.
Future optimization of CMP for copper patterning will require a deeper understanding of the interactions between the slurry and copper film during polishing. In copper chemical mechanical planarization process (Cu CMP), electrochemical and mechanical mechanisms responsible for planarization and material removal from the wafer surface are not well understood due mainly to the proprietary nature of the slurry development research in the microelectronics industry and even at some universities. Slurries are generally formulated empirically at present. From the open literature it is apparent that the main chemical additives of typical Cu CMP slurry are oxidizing agents, complexing agents. Sometimes organic corrosion inhibitors are also included.
The present research project aims primarily at delineation of the role of complexing agents in the chemical mechanical planarization of copper. Although complexing agents are one of the vital chemical additives in preparation of commercial copper slurries, there have been only very limited amount of work examining their behavior relevant to Cu CMP process. The role of complexing agents during Cu CMP process is still unclear. One of the main goals of the current project is to develop a fundamental basis for the characteristic behavior of copper CMP slurries containing complexing agents. The results of the project are expected to provide an in-depth understanding for the formulation of superior Cu CMP slurries. A range of organic complexing agents with carboxylate and ammonia groups will be examined. Glycine, ethylenediamine (EN) and ethylenediaminetetraacetic acid (EDTA) are the complexing agents under investigation. Glycine has been chosen as the model reagent to be used in experiments. Thus far electrochemical dissolution and polishing of copper in glycine and water solutions without glycine were examined. The electrochemical processes involved in the oxidative dissolution of copper were investigated by potentiodynamic polarization studies and correlated with the potential-pH diagram for the copper-water and copper-water-glycine systems. In-situ electrochemical experiments such as polarization experiments and open circuit potential measurements were conducted during polishing.
Results so far indicate
that experimentally measured polarization curves are in good agreement with the
predictions of the potential-pH diagrams. While no significant changes in
the in-situ polarization are present when copper exhibits active behavior,
mechanical components significantly affect the in-situ polarization when copper
shows active-passive behavior. Experimental results also revealed
information about the possible planarization mechanism during polishing.
One of the major findings or the projects is that glycine (or complexing agents
in general) controls the thickness of the passivation layer formed on
copper. While the investigation is still continuing, the results points
out that use of complexing agents in the CMP slurry play an important role in
enhancing the polishing efficiency between the protruding and recessed regions
of a copper thin film.
Interested? Would like to hear more? If so, here is the link to my publications.
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You can reach me at: serdar@socrates.berkeley.edu |