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The Challenge of Improving
Electrical Insulation A group of researchers at IBM recognized that progress toward more densely integrated circuits would be impeded by the material properties of the current industry standard for insulation, silicon dioxide. There are a number of materials with dielectric constants (k)—a measure of the ease with which electrons pass through a material—lower than that of silicon dioxide. The ideal insulator, in terms of its impermeability to electron flow and fields, is a vacuum. Air (e.g., air pockets in foam) is an excellent alternative: by creating pores filled with air in a material, its dielectric constant can be decreased. Thus, the researchers sought to figure out how to control the process of creating pores in a material (controlled porosity), with the goal of developing a method of providing new insulating materials to successive generations of IC devices. Research too Uncertain
for Company Funding At the time, company
resources addressing electrical interconnection problems were focused
on a research effort to substitute copper for aluminum in the fabrication
of chip wires. IBM management believed that substituting copper for aluminum
in the fabrication of wires might circumvent the need for improved insulators,
copper being a much better conductor than aluminum. Some believed that
the substitution of copper would allow for increases in signal speed sufficient
to eliminate the need for the increased
levels of integration that would make a new type of insulator necessary.
These factors combined
to make the company unwilling to proceed with the research on controlled
porosity without assistance. The researchers submitted their proposal
in ATP’s 1992 General Competition and received an award. The ATP
award reduced the risk of the research to a level that IBM management
was willing to provide the necessary internal support to pursue the research. The ATP awarded IBM
$1.8 million for a three-year, $5.8 million project to research and develop
alternative methods for producing insulating foams using organic polymers.
These foams were to make use of the insulation quality of air by creating
tiny, nanometer-scale air pockets in a polymeric structure. IBM researchers
aimed to develop organic polymer nanofoams with dielectric constants (k)
as low as 2.0, almost twice as good as that of silicon dioxide, with its
dielectric constant of 3.9. The Project Team Researchers Pursue
Three Parallel Research Efforts The researchers pursued
three different approaches to the development of low-k dielectric foams
for insulation of microelectronics devices. One approach investigated
closed-cell molecular foams. These foams incorporated a cage-like molecular
structure inside a polymer, forming tiny pores in which the polymer would
encapsulate air or other gaseous molecules. Given the lack of data on
this technology, however, there was concern as to whether molecular foams
with dielectric constants of less than two could be achieved while still
meeting the other requirements for microelectronic applications. Researchers
anticipated this approach might need to be combined with one of the other
two processes. The second approach
to constructing polymer foams built on a method called induced phase separation,
which had been developed by Sandia National Laboratories. The researchers
sought to determine whether this process could produce extremely thin,
heat-resistant foam structures suitable for microelectronic applications.
The third approach investigated the block copolymer method, considered the most promising by IBM. This approach combined two different polymers with different chemical and physical properties to create a structure that would provide the desired performance as an insulator while withstanding the heat employed in the process of fabricating aluminum circuitry. Achievements The researchers carried
out extensive experimentation with the second approach, which used induced
phase separation. This technique developed extremely good electrical insulating
foams, some of which had dielectric constants lower than two. The pores
in these foams, however, were too large. It was not possible to use them
in an insulating layer thin enough for an integrated circuit of the required
density. Because the size of the air pockets could not be sufficiently
reduced, the team chose not to pursue this approach further. Using the block copolymer method, IBM and its colleagues generated foams with the desired thermal characteristics, as well as foams with the desired electrical insulation, but could not produce foam that had both qualities at the desired levels. The researchers did develop significantly better microelectronic insulators, but these insulators would only be usable in the future if copper wiring were to be substituted for aluminum, so that fabrication temperatures could be kept below 400° C. Post-Project Developments Since the end of the
ATP project, IBM has made considerable progress in the development of
improved block copolymer foams, but much work remains in the development
of a viable process for incorporating these materials into the fabrication
of microprocessors. Indeed, low dielectric constant insulators (k <2.5),
including polymer foams, have yet to be fully developed or incorporated
into integrated circuits. Separately, however, IBM’s first series
of integrated circuits with copper wiring (CMOS 7S) have been developed,
lowering IC processing temperatures and increasing the possibility of
lower-k polymer dielectrics being used in future circuits. In the meantime, IBM also undertook R&D on nonporous, lower-k dielectric materials that may serve in place of silicon dioxide. For instance, IBM researchers initiated a study of “toughened organosilicates,” which have dielectric constants in the range of 2.6–3.0. ATP-supported Research
Stimulates Industry-Wide R&D Efforts As a result, many
U.S. corporations, including Lucent Technologies, Texas Instruments, Motorola,
Dow Corning, BF Goodrich, Allied Signal, Dow, Dow Chemical, and Du Pont,
as well as IBM, are now reviving research into low-k dielectric materials.
A number of key strategic alliances (Allied Signal–Nanopore, IBM–Siemens–Toshiba,
and IBM–Apple–Motorola) have been formed to pursue R&D in
advanced devices and materials. Low-k dielectric materials research has
also been stimulated through industry consortia such as SEMATECH.(1)
Consequently, U.S. corporations remain major global players in the development
of new low-k dielectric materials for on-chip applications. The results of IBM’s earlier research were substantial enough to attract the interest of a major materials supplier, Dow Chemical Company. Dow formed a joint venture involving IBM’s Almaden Research Center and Yorktown Heights facilities, applied to the ATP, and, in October 1998, was awarded ATP funding. The new project aims to identify and develop polymers to produce nanofoams with a dielectric constant as low as 1.5, and integrate them in common IC fabrication. If the Dow-IBM partnership proves successful, it could help establish a U.S. supplier base for high-performance insulating materials. ATP support in developing new ways to produce insulators for new generations of high-performance microelectronics has helped to secure an important technical option for the industry. It represents one of the efforts underway to sustain progress in integrated circuit technology and increase U.S. producers’ share of the global microelectronics market.
____________________ Return to Table of Contents or go to next section. Date created: April
2002 |
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