Allotropes of Carbon - Jeff Kopel Jan 98

Excerpts from the Proposal to Sematech for Study of Candidate Materials for Apertures Used at the End Station of Ion Implanters 

Jeff Kopel, Glemco, Inc.

Conventions

It is important to define the terms for carbon based materials at the outset, as they are often used differently in different situations  For the purpose of this paper, the following conventions will apply.  Please note that some of the materials defined are not included in the proposed study.  I have defined them because they have been used in implanters.

 Carbon will refer only to the element, without distinguishing structure. 

Amorphous carbon will refer to carbon that has no discernable crystalline structure. 

Vitreous carbon will refer to amorphous carbon artifacts produced by a specific process.

Graphite will refer to specialty graphite, polycrystalline carbon with greater than 40% crystalline structure, but no single crystal larger than a hundred microns in length (arbitrary choice). 

Pyrolytic graphite will refer to HOPG (highly ordered pyrolytic graphite) that is produced by CVD (Chemical Vapor Deposition) and is highly anisotropic. 

Vitreous impregnated graphite (also called vitreous graphite) will refer to graphite that has been impregnated with a PVC (polyvinyl chloride) based resin and then pyrolized. 

-3 (suffix) graphite will refer to any POCO graphite that is impregnated with polymeric furfural alcohol and pyrolized.  The -3 suffix is a trade name of Poco Graphite, Inc. 

Glassy Carbon Coated Graphite will refer to any graphite that has been coated with a PVC based resin and then pyrolized.  Glassy Carbon Coating^® is a registered trademark of Graphite Die Mold. 

FABMATE^® will refer to POCO DFP-2 graphite that is CVD coated with amorphous carbon. FABMATE^® is a registered trademark of Poco Graphite, Inc. 

In addition, a brief description of the manufacturing process and material characteristics for these various forms of carbon will help in understanding their characteristics. 

Vitreous carbon is produced by mixing a resin, usually furan, with a pore forming agent, casting it to a near net shape and then pyrolizing it to drive out the volatile content.  It has no discernable grain structure, no measurable crystallinity, and is isotropic.  It is typically around 30% porous, with a very fine, closed pore structure.  It is very brittle and difficult to machine.  The thermal conductivity is relatively low, in the order of 5 W/mK (Watts per meter Kelvin).  The purity of the final part depends on the purity of the starting materials. 

Graphite is produced by grinding a coke into a fine powder, mixing it with a binder material, molding it into a block or cylinder, baking it to remove volatile content, and then firing it to reach the desired degree of graphitization.  Graphites have a wide range of properties.  For ion implanter applications, the grain size will be 1 micron to 150 microns.  The graphitic crystallites will range from 300 angstroms to 100 microns, largely depending on graphitization temperature.  Properties can be isotropic or have up to 15% anisotropy. Porosity will be from 15%-30%.  The pore size can be from 0.2 to 10 microns.  The porosity will typically be 70%-90% open.  It is somewhat brittle, but machines easily.  The thermal conductivity is relatively high, in the order of 60-200 W/mK.  It can be subjected to a high temperature halogen gas purification to remove metallic contamination remaining from the starting materials. 

Pyrolytic graphite is produced by cracking methane at a very high temperature and allowing the carbon to be deposited on a substrate or mandrel.  The resulting material has no grain structure and virtually no porosity.  It  has extremely large crystallographic planes and can approach single crystal structure.  It is highly anisotropic, with some properties varying 200:1 when measured in the plane of deposition (A-B direction) and through the plane (C direction).  Machining is tricky, but not difficult once the proper techniques are applied.  Thermal conductivity is around 190 W/mK in the A-B direction, but only 1 W/mK in the C direction.  The purity of the final part depends on the purity of the starting materials. 

Vitreous impregnated graphite is produced by impregnating any graphite with a PVC based resin and firing.  In some cases, this process will be repeated to increase density.  The impregnation penetrates about 0.25" and does not affect any of the material properties except porosity.  There is a slight filling of the pore structure that will increase the density by 0.01 to 0.05 Gr/CC (grams per cubic centimeter).  The intention of this process is to coat the inside of the pore structure with amorphous carbon to help bind the graphite grains into the main body of the part, thereby reducing particle release during the erosion caused by the ion beam.  It is important to note that the vitreous impregnation process has been sold to several companies, and can be applied to any graphite.  The purity of the final part depends on the purity of the graphite and the purity of the PVC resin used for impregnation. 

-3 graphite is material produced for many years by Poco Graphite, Inc.  It is very similar to the vitreous impregnated graphite except that a polymeric furfural alcohol is used instead of a PVC resin.  There is some question whether the -3 coats the inside of the pore or simply leaves a lump of carbon bonded inside the pore.  There is no empirical method to determine the form of either the -3 or VI impregnant in the graphite.  Unlike the vitreous impregnation, the -3 has a penetration of around 2".  It is the sole process of Poco Graphite and is used only on POCO manufactured graphites.  Historically, POCO has recommended DFP-3-2 for ion implanter parts.  DFP-1 is a 5 micron, 1.77 density, 0.8 micron pore size graphite.  The -3 suffix indicates the carbon impregnation and the -2 suffix indicates it has been subjected to a high temperature halogen gas purification to remove metallic contamination remaining from the starting materials. 

Glassy Carbon Coating^® is produced by a process in which PVC resin is repeatedly brushed on the surface of the graphite and pyrolyzed.  This completely fills the porosity near the surface of the part.  The coating will typically penetrate around 30 microns and build up another 10 microns on the surface.  The coating does not significantly affect any physical properties of the graphite except permeability.  The purity of the final part depends on the purity of the graphite and the purity of the PVC resin used for coating.

The following table is a summary of the three basic carbon materials considered for ion implanters.  The graphite column is not significantly changed by any of the impregnants or coatings discussed earlier in this paper. 

Characteristics of Carbon Based Materials Used in Ion Implanters

Notes:

1. Theoretically - in reality it is turbostratic with disclinations and other structural anomalies

2. Gr/CC - theoretic density is 2.26, apparent density includes pore structure

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