Thin Film by Chemical Vapour Deposition

The present invention relates to chemical vapour deposition of hydridospherosiloxanes to generate films of SiO₂ at low temperatures on substrates that cannot withstand high temperatures.

Prior Art

SiO₂ film deposition has traditionally been accomplished primarily through chemical vapour deposition using TEOS at temperatures of about 650⁰C through the oxidation of silane which yield poor quality of films. The knowledge about synthesis, properties and uses of hydridospherosiloxanes exited since 1980. However, synthesis of the right grade of hydridospherosiloxanes to generate SiO₂ thin films at low temperatures was not known. As a result it was difficult to deposit thin films of SiO₂ on substrates such as GaAs and aluminum which cannot withstand high temperatures.

Summary of the invention

A biphasic reaction medium is prepared, having a first solvent phase and a second solvent phase. The first solvent phase can contain an aliphatic or aromatic hydrocarbon solvent like pentane, hexane, heptane, benzene, toluene xylene. Halogenated solvent such as carbon tetrachloride may also be considered in this context. The second solvent phase is a concentrated solution of a metal salt in a polar organic solvent like alcohols and ethers such as methanol, ethanol, isopropanol, glycerol, diethyl ether, diglyme. The metal salts used are from chloride, iron sulphate and calcium chloride. A silicon represented by the formula HSiX, where X is a group which is, hydrolyzable in the solvent of said first solvent phase, is added to the biphasic reaction medium. X can be CI or OCH. The commonly used compound is trichlorosilane, HSiCl₃. The first solvent phase is separated from the said second solvent phase and then the separated first solvent phase is treated with a metal salt, preferably sodium or potassium carbonate or calcium carbonate. Slow evaporation of the first solvent phase solvent is employed to isolate a mixture of crystals of (HSiO_3/2)₈ and (HSiO_3/2)₁₀. The crystal mixture is washed with a hydrocarbon solvent and crystals of (HSiO_3/2)₈ are isolated.

The figure illustrates the CVD process used for the formatio of SiO₂ films. Pure hydridospherosiloxane is heated in a chamber at a constant temperature which is maintained through the use of a silicon oil bath. Oxygen is fed to the chamber from a conventional source of oxygen. The vapour so generated passes into a deposition chamber. A plurality of substrates are suspended within the deposition chamber to receive the vapour. The vapour form the film. Temperature and pressure inside the deposition chamber are regulated strictly, as these parameters determine the deposition rate which is also determined by the flow rate of oxygen and the precursor temperature. The deposition rate can vary from about 5.0 nm/min to about 15.0 nm/min. The precursor (hydridospherosiloxane) temperature, oxygen flow rate and the total substrate chamber pressure are 80⁰C, 10 sccm and 10 torr respectively. The deposited SiO₂ film has a refractive index of 1.434 at a wavelength of 32.8 nm which corresponds to a packing density of 0.95. After high temperature annealing in argon, the refractive index of the film reaches a value of 1.459 which corresponds to a packing density of 0.998. The temperature of the precursor is maintained around 100⁰C and that of the substrate is kept at a temperature lower than 500⁰C.

Example:

FeCl₃ (anhydrous, 50 g) was taken in a 1 L round bottomed flask and concentrated HCL (20 ml) was added followed by CH₃OH (40 ml). Hexane (Mixture of isomers, 350 ml) and toluene (50 ml) were then added and the biphasic mixture was stirred by heavy magnetic stirrer. A solution of HSiCl₃ (20 ml, 0.2 mol) in hexane (150 ml) was added drop-wise over a period of 9 hours. After an additional 30 minutes of stirring, the upper hexane layer which had a slight yellow color was transferred to another round-bottomed flask along with some suspended yellow solid, K₂Co₃ (14 g) and CaCl₂ (10 g) were added to the flask and the contents stirred overnight. The mixture was filtered and the volume of the colorless filtrate was reduced by evaporation until it was ca.20 ml. The crystals that were pooled and the volume reduced by evaporation until it was <10 ml. A second crop of crystals was collected and washed with hexane (Ca. 10 ml). The total yield of crystals was 2.88 g. the solid non-crystalline soluble residue weighted 4.9 g.

The crystals were analyzed by ‘H NMR and shown to be a mixture of HT₈ and HT₁₀ (ca.3.5:1). Yield 27.2% based on HSiCl₃.

Pure HT₈ was isolated (1.85, 17.5% yield based on HSiCl₃) by washing off the HT₁₀ along with some HT₈ using hexane (ca. 45 ml).

The Patent has 16 claims. The major claims are given below.

The low temperature generation of films of SiO₂, on substrates that cannot withstand temperatures greater than about 500⁰C, comprising the step of chemical vapour deposition of hybridospherosiloxane, wherein said hydridospherosiloxane is produced by the step of :

preparing a biphasic reaction medium having a first solvent phase and a second solvent phase, said second solvent phase comprising a concentrated solution of a metal salt in a polar organic solvent and water ;
adding to the biphasic reaction medium of step a), a silicon compound represented by the formula : HSiX₃ - where X is group which is hydrolyzable in the solvent of said first solvent phase ;
separating said first solvent phase from said second solvent phase;
treating said separated first solvent phase with a metal salt containing carbonate ions, for a time period which is just sufficient to remove by-products;
isolating a mixture of crystals of (HSiO_3/2)₈ and (HSiO_3/2)₁₀ by evaporation of said first solvent phase solvent ; and
washing said mixture of step e) with a hydrocarbon solvent and isolating crystals of (HSiO_3/2)_8.

The low temperature SiO₂ film generation method of claim 1, wherein said first solvent phase contains a hydrocarbon solvent.

The low temperature SiO2 film generation method of claim 1, wherein the solvent of said first solvent phase is an aliphatic or aromatic hydrocarbon.

The low temperature SiO₂ film generation method of claim 1, wherein the solvent of said second solvent phase is methanol or ethanol.