Silicon oxide vapour

Thin oxide films may be prepared by substrate oxidation or by vapour deposition onto a suitable substrate. An example of the fomrer method is the preparation of silicon oxide thin-films by oxidation of a silicon wafer. In general, however, the thickness and stoichiometry of a film prepared by this method are difficult to control. 

Fig. 13.20. Optical heterodyne force microscopy (OHFM) and its application to a copper strip of width 500 nm, thickness 350 nm, on a silicon substrate, with subsequent chemical vapour deposition (CVD) of a silicon oxide layer followed by polishing and evaporation of a chromium layer of uniform thickness 100 nm and flatness better than 10 nm (a) amplitude (b) phase 2.5 [im x 2.5 m. Ultrasonic vibration at fi = 4.190 MHz was applied to the cantilever light of wavelength 830 nm was chopped at fo = 4.193 MHz and focused through the tip to a spot of diameter 2 im with incident mean power 0.5 mW the cantilever resonant frequency was 38 kHz. The non-linear tip-sample interaction generates vibrations of the cantilever at the difference frequency f2 — f = 3 kHz (Tomoda et al. 2003).

At the turn of the 18th century many scientists believed that silica, or silica earth, contained an unknown chemical element and tried to isolate it in a free state. H. Davy attempted to decompose silica with an electric current—the method by which a number of alkali metals had already been prepared—but without success. The scientist s attempt to prepare free silicon by passing metallic potassium vapour over red-hot silicon oxide also failed. In 1811 L.J. Gay Lussac and L. Thenard applied themselves to the problem. They observed a vigorous reaction between silicon tetra-fluoride and metallic potassium a reddish brown compound was formed in the reaction. The scientists could not reveal the nature of the product most likely, it was contaminated amorphous silicon. 

This process allows the deposition of silicon dioxide on silicon. Silicon oxidation is made at high temperature (typically between 800 and 1100 °C) in an oven. It is performed with the reaction of an oxidizing gas (generally O2) on the silicon surface. It can be made more effective by the use of water in the form of vapour. This process is particular because the material for the deposition (atoms of Si) is taken directly from the substrate. For instance when a thickness of 1 //m is grown on the Si substrate, the thickness of the substrate is decreased of a few hundreds of nm. 

The MIS structure is an example of a semiconductor structure of high importance for sensor apphcations. This structure consists of three layers arranged as a stack. The sequence is metal (M), insulator (I) and semiconductor (S). Usually, the set-up starts with a substrate of a semiconductor material such as P-type silicon. This is covered first by a thin silicon oxide (Si02) layer formed by oxidation in an oxygen-containing atmosphere. Next, a thin metallic layer is applied by vapour deposition. Instead of MIS, often the abbreviation MOS is used, since the insulating layer (I) is often formed by an oxide (0). Thus, a field effect transistor (FET) in a MOS structure is called a MOSFET . 

Hozumi A, Takai O. (1998) Preparation of silicon oxide films having a water-repellent layer by multi-step microwave plasma-enhanced chemical vapour deposition. Thin Solid Films 334(1-2) 54-59. 

Z.Q. Cao, X. Zhang, Size-dependent creep behaviour of plasma-enhanced chemical vapour deposited silicon oxide films. J. Phys. D-Appl. Phys. 39(23), 5054—5063 (2006)... 

Increased Si content at the surface of steel leads to a beneficial effect improving the anti-oxidation behaviour. The surface protection is achieved by several mechanisms that take place at the same time, including the formation of silicon oxide films, increased Cr diffusion from the bulk to the surface, formation of phases such as y-Fe, Cr203, Si02, Si FCy, passivation of the surface by oxidation and surface diffusion. As mentioned in references [1-4], silicon seems to retard breakaway in the presence of water vapour in the environment and may facilitate Cr rediffusion from the bulk which would help repassivation observed after breakaway. As a consequence the level of the Cr reservoir possibly may be kept lower than for Si-free steels. At least, a continuous silica layer is not the reason for the improved oxidation behaviour. The positive effect of silicon seems to stabilise at values above 0.5% Si. A possible reason for the influence of silicon seems to be that silicon enhances the diffusion of Cr in the metal matrix. 

The reaction of magnesium oxide witli silicon produces a veiy small vapour pressure of magnesium... 

Heating with the following solids, their fusions, or vapours (a) oxides, peroxides, hydroxides, nitrates, nitrites, sulphides, cyanides, hexacyano-ferrate(III), and hexacyanoferrate(II) of the alkali and alkaline-earth metals (except oxides and hydroxides of calcium and strontium) (b) molten lead, silver, copper, zinc, bismuth, tin, or gold, or mixtures which form these metals upon reduction (c) phosphorus, arsenic, antimony, or silicon, or mixtures which form these elements upon reduction, particularly phosphates, arsenates,... 

The production of germanium and silicon is carried out by the reduction of the oxides, SiCh and GeC>2. There is a considerable difference in the stabilities of the oxides, and GeC>2 can be reduced by hydrogen at temperatures around 1100K, but SiC>2 must be reduced by carbon at temperatures around 2300 K. The elements only reach a vapour pressure of 10-6 atmos at 1500 K and 1750 K respectively, and so the reduction of GeC>2 does not lead to any significant vaporization, but this is not so in the production of silicon. The gaseous composition of a 1 2 mixture of Si(>2 and carbon at 2000 K is as follows ... 

More modern diffusion pumps are operated with a range of synthetic oils having room-temperature vapour pressures of between 10 and 10 Torr. Although many of the oils previously used in oil diffusion pumps were not particularly stable to chemical attack at the normal working temperature of the diffusion pump, the oils available today (based on a variety of materials such as naphthalene, poly(phenyl ether), or silicones) are generally stable to oxidation at their normal working temperatures and many are particularly suitable when contact with more aggressive materials cannot be ruled out. 

Nitric acid and nitrogen oxides are driven off together with water vapour and pass from the tower through the top to a Bleacher (3) filled with disks or cylinders where they pass in counter-current to the condensed acid which returns from the S-bend condenser (4) of high-silicon iron to flow through (5) to storage. 

---