Although CVD has been in use in various ways since the late 1800’s, the process did not begin to mature until the 1980’s when small electronics started commercializing. In the 1980’s, nearly every industry needing to create electrically conductive films or coatings, especially in layering processes, recognized CVD as the best choice. It has been found to be extremely effective in industries utilizing silicon as a substrate, such as semiconducting and solar cells, where controls are needed to deposit specific elements at specific temperatures. This process is not without its problems, however. These include metallic contamination; flaking of deposits from earlier depositions off reactor walls; oxidation due to release of chlorides or hydrogen; and decomposition of various parts of the substrate.
During the creation of polysilicon, silicon dioxide materializes forming a membrane on the silicon, protecting it from oxidation. This protective layer creates the perfect surface for deposition of electrically conductive chemicals such as HCDS. During formation of the membrane, voids are often created as a result of metallic impurities, such as titanium, being deposited on the substrate along with the processing gases. HCDS has been found to effectively fill the voids and create an even distribution of deposition on both top and bottom layers due to its higher purity.
Over the past few decades, HCDS has been utilized as a precursor in CVD layering of silicon wafers. This is due to HCDS requiring a low temperature environment and its ability to assimilate well with SiO2 and promote film growth in areas of the substrate that decompose during the heating and deposition process. HCDS is successful in developing polycrystalline, monocrystalline, and amorphous thin films, depending on the nature of the initial feedstock. HCDS also has etching advantages and capabilities many chemicals lack, requiring mixing with other chemicals or gases that may result in lower electrical selectivity. Electrical selectivity and high purity are extremely important when it comes to fabricating microelectronic components.
The Siemens and CVD processes described above form the bases of today’s fabrication of integrated circuits, microprocessors, plasma screens, LED displays, and other electronic related components. This fabrication relies on stable electric selectivity within the silicon substrate upon which the resulting microchips are produced. Oxidation is a limiting factor in electrical conductivity and this is where HCDS fills the gap. Specific chemical properties of HCDS allow it to create a diffusion barrier film that prevents oxidation of Cu or other materials deposited on the substrate to enhance electrical performance and reliability in electronic devices. One of the key factors in determining the electrical conductivity of the substrate is the grade of the silicon feedstock (i.e. metallurgic grade or electronic grade). Metallurgic grade (MG) has many metallic particles within it, rendering it a low purity, thus decreasing the electrical selectivity. Just as purified HCDS has, electronic grade (EG) silicon goes through multiple refinements and is either high or ultrahigh purity.
Micromachining of integrated circuits began in the 1960’s and by the 1980’s was utilized mainly for selective etching to increase the modulus of the substrate. In the technology boom of the 1990’s, microelectromechanical systems (MEMS) came into the picture as a result of an influx of research funding by the government in order to find novel ways of etching patterns on the substrate to increase deposition and conductivity. MEMS components of are fabricated at the nano scale, between 0.001 and 0.1 mm and are developed in the same manner as their larger counterparts (e.g. polysilicon production followed by CVD). Current MEMS applications include inkjet printers, portable device microphones, DLP displays, and piezoelectrics.
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