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Differences Between Chemical and Physical Deposition
The fundamental difference between chemical and physical thin film deposition techniques rests in how the atoms or molecules which comprise the film are delivered to the substrate. Chemical deposition techniques rely on a fluid precursor which reacts chemically with the substrate. Because the thin film material is conducted through a fluid, chemical deposition is conformal, approaching the substrate without preference to a particular direction. Physical deposition techniques rely on mechanical or electromechanical means to deposit the thin film on the substrate. Particles deposited are brought to the substrate by taking advantage of temperature or pressure differentials or by physically separating atoms from a target which will later condense. Physical thin film deposition techniques are directional in nature since particles will follow a straight path from the target to the substrate.
Chemical Vapor Deposition
Chemical vapor deposition, or CVD, is a chemical thin film deposition technique used in the manufacture of semiconductors and synthetic diamonds. In CVD the fluid precursor is a gaseous form of the element deposited. The gas is typically a halide or hydride, although organometallic gases are used for particular applications. The precursor gas is moved into a chamber with the substrate at low pressure. A chemical reaction between the substrate and the precursor occurs, increasing the thickness of the thin film. The reaction is allowed to persist until the film has reached the desired thickness.
Sputtering
Sputtering is a kind of physical thin film deposition technique where atoms from a target material are broken off and allowed to come to rest on the substrate. In thin film deposition, sputtering uses plasmas of a noble gas such as argon to knock atoms from the target. Noble gas use ensures that no undesired chemical reactions occur. Sputtering rapidly achieves desired thickness levels, making it a fast and efficient technique for thin film deposition.
Molecular Beam Epitaxy
Molecular beam epitaxy, or MBE, combines elements of chemical and physical thin film deposition techniques, allowing it to combine the advantages of both. Target materials deposited are heated until they convert directly from solid to gaseous form. The gaseous elements are then allowed to react chemically with the substrate to grow the thin film. Although MBE is a slow technique, it achieves high levels of purity and allows for epitaxial film growth which is desirable for sensitive devices such as quantum wells or dots. The development of MBE has allowed for these devices to become integrated into everyday devices such as light emitting diodes, or LEDs.