Unique Materials Formed by Atomic Deposition Processes

Amorphous materials are those that have no detectable crystal structure. Materials can be naturally amorphous, such as the nonmetallic glasses. Some normally crystalline materials can be formed in the solid amorphous condition by rapidly cooling ("quenching") from the liquid phase (Ref 78).

Amorphous film materials can be formed by:

• Deposition of a natural "glassy" material such as a glass composition

• Deposition at low temperatures where the adatoms do not have enough mobility to form a crystalline structure

• Ion bombardment of high-modulus materials (Ref 79)

• Deposition of materials some of whose bonds are partially saturated by hydrogen (e.g., a-Si:H, a-C:H, and a-B:H, where "a" stands for amorphous)

• Deposition of complex metal alloys

Hydrogen seems to play a very unusual role in the growth of some materials. In the case of depositing silicon from the silane (SiH4) precursor gas in CVD, the incomplete decomposition of the precursor results in the deposition of a-Si, which can contain 10 to 15 at.% H. The hydrogen prevents Si-Si bonding, thus causing the film material to be amorphous in much the same way as "glass formers" do in forming glasses by melting. The a-Si seems to deposit much like a polymer film, giving very good surface replication and low void density in the early stages of film growth.

The unique applications of amorphous materials arise because of:

• Absence of grain boundaries—no grain boundary diffusion

• Low void/pinhole content

• Considerable compositional latitude

• Unique optical properties

• Unique electronic properties

• Ease of fabrication

Some semiconductor materials grow with some of their bonds unsaturated ("dangling"). This can lead to unacceptable electron trapping. It has been demonstrated that the unsaturated bonds in a-Si can be passivated by hydrogen doping, thus raising the electron mobility in the material. Hydrogen ion bombardment is now used to treat polycrystalline silicon photovoltaic materials to improve photoconversion efficiency.

Metastable or labile phases are unstable phases of materials that are easily changed if energy is available for mass transport processes to occur. Deposition processes allow the development of metastable forms of the material. Metastable crystal structures can be formed by rapid quenching of high-temperature phases of the deposited material, or they can be stabilized by residual stresses or impurities in the film. For example, diamond is a metastable phase of carbon that is formed naturally in a high-pressure and high-temperature environment and changes to graphitic carbon on heating. However, diamond films can be deposited using the proper deposition techniques. Metastable film compositions can also be formed under deposition conditions that do not allow precipitation of material when it is above the solubility limit of the system. For example, concurrent low-energy ion bombardment using "dopant ions" allows doping of semiconductor films to a level greater than that obtainable by diffusion-doping techniques (Ref 80).

Diamond and Diamond-like Carbon (DLC) Films. Recently great progress has been made in the deposition of diamond and diamond-like carbon (DLC) coatings for industrial applications. Natural diamond, with its high hardness, low coefficient of friction, high thermal conductivity (1.5 times that of silver), high packing density, good visible and infrared transparency, and chemical inertness, has long provided a goal for the thin film deposition community.

Diamond is a carbon material with a specific crystallographic structure (diamond structure) and specific chemical bonding (sp3 bonding). DLC is an amorphous carbon material with mostly sp3 bonding that exhibits many of the desirable properties of crystalline diamond. DLC material is sometimes called "amorphous diamond," but that term is an oxymoron and should be avoided.

The property of the carbon sp3 bonding that allows the deposition of both diamond and DLC coatings is its relative chemical inertness to hydrogen reduction. The sp3 bonds formed during deposition are stable against hydrogen etching. Any sp2 (graphite) bond formed, however, is susceptible to hydrogen etching.

Polycrystalline diamond films can be formed if the deposition temperature is high enough (>600 °C, or 1100 °F) to allow atomic rearrangement during deposition. DLC films are formed at lower temperatures (room temperature and even below) where the atoms cannot arrange themselves into the diamond structure, giving an amorphous material. The DLC films have varying amounts of sp2 bonding and included hydrogen that affect their properties. The sp3-bonded material can be deposited by a number of techniques, all of which involve "activating" both a hydrocarbon species, to allow carbon deposition, and hydrogen, to provide the etchant species.

Polycrystalline diamond films are most often deposited by the hot filament CVD technique, the combustion flame technique, or PECVD using a microwave plasma. In all cases, the diamond film that is formed is polycrystalline and has a rough surface. This is due to the method of film nucleation on the substrate surface and the nature of the film growth. This rough surface gives a high coefficient of friction, and a great deal of work is being done to try to improve this surface morphology for wear and friction applications. Other properties can approach those of natural diamond.

DLC films are made primarily using PECVD or ion beam techniques with low substrate temperatures (Ref 81). The DLC films are smooth with properties approaching those of natural diamond, except that thermal conductivity is much lower. DLC films can also be deposited by ion bombardment processes that do not involve hydrogen. These films are sometimes called "i-C" films (Ref 79). DLC films are being used as coatings on optical products such as eyeglasses, sunglasses and IR optics, and as wear-resistant coatings on storage media and cutting surfaces.

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