Diamondoid-containing materials in microelectronics

Details Diamondoid-containing materials in microelectronics Diamondoid-containing materials in microelectronics

2002-01-14

2004-08-31

Kunemund, Robert (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Forming from vapor or gaseous state

C117S086000, C117S104000, C117S929000, C423S446000

Reexamination Certificate

active

06783589

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
Embodiments of the present invention are directed toward novel uses of both lower and higher diamondoid-containing materials in the field of microelectronics. These embodiments include, but are not limited to, the use of such materials as heat sinks in microelectronics packaging, passivation films for integrated circuit devices (ICs), low-k dielectric layers in multilevel interconnects, thermally conductive films, including adhesive films, thermoelectric cooling devices, and field emission cathodes.
2. State of the Art
Carbon-containing materials offer a variety of potential uses in microelectronics. As an element, carbon displays a variety of different structures, some crystalline, some amorphous, and some having regions of both, but each form having a distinct and potentially useful set of properties.
A review of carbon's structure-property relationships has been presented by S. Prawer in a chapter titled “The Wonderful World of Carbon,” in
Physics of Novel Materials
(World Scientific, Singapore, 1999), pp. 205-234. Prawer suggests the two most important parameters that may be used to predict the properties of a carbon-containing material are, first, the ratio of sp
2
to sp
3
bonding in a material, and second, microstructure, including the crystallite size of the material, i.e. the size of its individual grains.
Elemental carbon has the electronic structure
1
s
2
2
s
2
2
p
2
, where the outer shell
2
s
and
2
p
electrons have the ability to hybridize according to two different schemes. The so-called sp
3
hybridization comprises four identical &sgr; bonds arranged in a tetrahedral manner. The so-called sp
2
-hybridization comprises three trigonal (as well as planar) &sgr; bonds with an unhybridized p electron occupying a &pgr; orbital in a bond oriented perpendicular to the plane of the &sgr; bonds. At the “extremes” of crystalline morphology are diamond and graphite. In diamond, the carbon atoms are tetrahedrally bonded with sp
3
-hybridization. Graphite comprises planar “sheets” of sp
2
-hybridized atoms, where the sheets interact weakly through perpendicularly oriented &pgr; bonds. Carbon exists in other morphologies as well, including amorphous forms called “diamond-like carbon,” and the highly symmetrical spherical and rod-shaped structures called “fullerenes” and “nanotubes,” respectively.
Diamond is an exceptional material because it scores highest (or lowest, depending on one's point of view) in a number of different categories of properties. Not only is it the hardest material known, but it has the highest thermal conductivity of any material at room temperature. It displays superb optical transparency from the infrared through the ultraviolet, has the highest refractive index of any clear material, and is an excellent electrical insulator because of its very wide bandgap. It also displays high electrical breakdown strength, and very high electron and hole mobilities. If diamond as a microelectronics material has a flaw, it would be that while diamond may be effectively doped with boron to make a p-type semiconductor, efforts to implant diamond with electron-donating elements such as phosphorus, to fabricate an n-type semiconductor, have thus far been unsuccessful.
Attempts to synthesize diamond films using chemical vapor deposition (CVD) techniques date back to about the early 1980's. An outcome of these efforts was the appearance of new forms of carbon largely amorphous in nature, yet containing a high degree of sp
3
-hybridized bonds, and thus displaying many of the characteristics of diamond. To describe such films the term “diamond-like carbon” (DLC) was coined, although this term has no precise definition in the literature. In “The Wonderful World of Carbon,” Prawer teaches that since most diamond-like materials display a mixture of bonding types, the proportion of carbon atoms which are four-fold coordinated (or sp
3
-hybridized) is a measure of the “diamond-like” content of the material. Unhybridized p electrons associated with sp
2
-hybridization form &pgr; bonds in these materials, where the &pgr;
0
bonded electrons are predominantly delocalized. This gives rise to the enhanced electrical conductivity of materials with sp
2
bonding, such as graphite. In contrast, sp
3
-hybridization results in the extremely hard, electrically insulating and transparent characteristics of diamond. The hydrogen content of a diamond-like material will be directly related to the type of bonding it has. In diamond-like materials the bandgap gets larger as the hydrogen content increases, and hardness often decreases. Not surprisingly, the loss of hydrogen from a diamond-like carbon film results in an increase in electrical activity and the loss of other diamond-like properties as well.
Nonetheless, it is generally accepted that the term “diamond-like carbon” may be used to describe two different classes of amorphous carbon films, one denoted as “a:C—H,” because hydrogen acts to terminate dangling bonds on the surface of the film, and a second hydrogen-free version given the name “ta-C” because a majority of the carbon atoms are tetrahedrally coordinated with sp
3
-hybridization. The remaining carbons of ta-C are surface atoms that are substantially sp
2
-hybridized. In a:C—H, dangling bonds can relax to the sp
2
(graphitic) configuration. The role hydrogen plays in a:C—H is to prevent unterminated carbon atoms from relaxing to the graphite structure. The greater the sp
3
content the more “diamond-like” the material is in its properties such as thermal conductivity and electrical resistance.
In his review article, Prawer states that tetrahedral amorphous carbon (ta-C) is a random network showing short-range ordering that is limited to one or two nearest neighbors, and no long-range ordering. There may be present random carbon networks that may comprise 3, 4, 5, and 6-membered carbon rings. Typically, the maximum sp
3
content of a ta-C film is about 80 to 90 percent. Those carbon atoms that are sp
2
bonded tend to group into small clusters that prevent the formation of dangling bonds. The properties of ta-C depend primarily on the fraction of atoms having the sp
3
, or diamond-like configuration. Unlike CVD diamond, there is no hydrogen in ta-C to passivate the surface and to prevent graphite-like structures from forming. The fact that graphite regions do not appear to form is attributed to the existence of isolated sp
2
bonding pairs and to compressive stresses that build up within the bulk of the material.
The microstructure of a diamond and/or diamond-like material further determines its properties, to some degree because the microstructure influences the type of bonding content. As discussed in “Microstructure and grain boundaries of ultrananocrystalline diamond films” by D. M. Gruen, in
Properties, Growth and Applications of Diamond
, edited by M. H. Nazaré and A. J. Neves (Inspec, London, 2001), pp. 307-312, recently efforts have been made to synthesize diamond having crystallite sizes in the “nano” range rather than the “micro” range, with the result that grain boundary chemistries may differ dramatically from those observed in the bulk. Nanocrystalline diamond films have grain sizes in the three to five nanometer range, and it has been reported that nearly 10 percent of the carbon atoms in a nanocrystalline diamond film reside in grain boundaries.
In Gruen's chapter, the nanocrystalline diamond grain boundary is reported to be a high-energy, high angle twist grain boundary, where the carbon atoms are largely &pgr;-bonded. There may also be sp
2
bonded dimers, and chain segments with sp
3
-hybridized dangling bonds. Nanocrystalline diamond is apparently electrically conductive, and it appears that the grain boundaries are responsible for the electrical conductivity. The author states that a nanocrystalline material is essentially a new type of diamond film whose properties are largely determined by the bonding of the carbons within grain boundaries.
Another allotrope of carbon known

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