The main raw materials in fiber optic manufacturing are SiCl and GeCl, both requiring a purity of 99.9999% or higher. Microdisk impurities in the raw materials will cause significant losses in the manufactured fiber. Therefore, SiC and GeCl need to be purified to achieve the required purity before being used as raw materials in the preform process.
Here, we will discuss the purification process using SiC as an example; a similar process applies to GeCl. In the epitaxial stage of SiCl, the most significant impurity in the raw material is trichlorosilane (SiHC), with a content of approximately 7000 ppm; there are also impurities such as C-H bond compounds (hydrocarbons, especially chlorohydrins), with a content of approximately 300 ppm; silanols are present at approximately 40 ppm; and iron impurities are present at approximately 200 ppb.
If trichlorosilane remains in the manufactured fiber, it will cause significant losses in the 0.9–2.5 μm wavelength range. SiHClO and other various impurities in SiCl can be removed using conventional distillation methods. This section describes the use of photochlorination combined with distillation to remove trifluorosilanes and other impurities from SiCl, achieving a high℃of SiCl purification.
The raw material, SiC, enters the photochlorinator through a filter. The principle of photochlorination is to utilize the strong reactivity of the Si-H bond with halogen elements, converting the Si-H bond into a Si-Cl bond. In the photochlorinator, gas is introduced, and under ultraviolet irradiation with a wavelength of 240-400 nm, atoms are generated, resulting in the following reaction: SiHCl₄ + Cl₂ → SiCl + HCl.
The SiCl and HCl produced in the photochlorinator enter a scrubber, where gas-liquid extraction causes HCl or residual Cl₄ to be carried away by N₂ and discharged from the top of the scrubber. SiCl then flows back from the bottom of the scrubber to the distillation column for further purification.
In the distillation column, in addition to the continued removal of residual HCl and Ch₂ from the top, SiCl and other impurities can be separated by their different boiling points under the heating of the bottom electrically heated boiling flask. SiC has the lowest boiling point (57℃), making it easily separated from impurities. In particular, hydroxyl hydrogens and similar hydroxyl impurities in silanols are easily removed from SiCl by fractional distillation in environments with low HCl content. The scrubber before the distillation column removes a large amount of HCl, creating the necessary conditions.
In this system, in addition to SiCl purification (impurity removal), passivation also occurs, for example, converting harmful silanol impurities into harmless siloxane impurities. The latter, remaining in the optical fiber, will not cause light absorption in the operating band.
The SiCl separated from distillation column I enters distillation column II for further fractional purification. The system combining distillation columns I and II can also remove non-volatile C-Cl impurities. These impurities are produced by multi-H bond impurities during photochlorination, as compounds containing multiple C-H bonds can only be partially photo-oxidized, leaving C-C bond compounds. Simultaneously, iron-containing impurities (including soluble iron and volatile iron) are also removed.
Coolers (5℃) are located at the top of both the distillation column and the scrubber to recover the SiCl feedstock and improve the recovery rate. The system achieves a SiCl recovery rate of 99%, while a GeCl recovery rate of 99.99% is required. The latter is expensive and corrosive, making it unsuitable for discharge. SiCl discharged from distillation column II is cooled by a heat exchanger and then stored for later use.
An infrared spectrometer is used to determine the purity of SiCl. The impurity content in SiCl purified by this system is as follows:
C-H group impurities <20 ppm, optimally up to 5 ppm
<20 ppm, optimally up to 5 ppm
<20 ppb, optimally up to 2 ppb
