Zone refining is a technique used to purify material with sharp melting points from 99.95% to 99.9999+% and above. It is being widely used in the semiconductor industry for refining of germanium and silicon to purity levels of 9N and above (i.e. >=99.9999999%) also for organic compounds having sharp melting points. An automatic zone melting unit has been indigenously designed and developed with indigenous available components.
Zone refining is a technique used to purify material with sharp melting points from 99.95% to 99.9999+% and above. It is being widely used in the semiconductor industry for refining of germanium and silicon to purity levels of 9N and above (i.e. >=99.9999999%) also for organic compounds having sharp melting points.
The zone refining technique involves melting a narrow zone of long ingot of the material to be processed. This molten zone is then slowly moved by moving the heater gradually along the length of the ingot in one direction. This process is repeated multiple times till the required purity level is reached. In this process difference between solubility of impurities in the molten and solid phases is used to concentrate impurities at the end section of the ingots.
Zone melting is a family of techniques, variation of which are applied in semiconductor industry viz. Zone refining for purification of semiconductor elements, zone leveling for uniform addition of dopants to the semiconductors and Bridgman's or float zone technique for crystal growth. It is being widely used in the semiconductor industry for refining of germanium and silicon to purity levels of 9N and above (i.e. >=99.9999999%).
The zone refining technique involves melting a narrow zone of long ingot of the material to be processed. This molten zone is then slowly moved by moving the heater gradually along the length of the ingot in one direction. This process is repeated multiple times till the required purity level is reached. In this process difference between solubility of impurities in the molten and solid phases is used to concentrate impurities at the end section of the ingots.
An automatic zone refining unit has been indigenously designed and developed using indigenous available components. The unit is capable of variable forward and reverse traverse. The unit is mounted with a digital man- machine interface (MMI) for easy indication/ operation of parameters such as temperature, speeds, count, etc. Both resistance heaters and induction heating coils can be mounted with little customization depending on the requirement of the process.
With use of induction heating the time required for achieving the melting point is faster as compared to a resistance heater. Another advantage is minimized heat loss as compared to the conventional resistance heater which involves heat transfer from the heating coil to the actual charge through various barriers like ambient air, the container wall and inert gas used or vacuum through convection. An induction heater heats only the charge / material or the container carrying the charge. A wide range of temperature (RT to 2800°C) not achievable by conventional resistance heater (RT to 800°C) is achievable
The machine is capable of handling 678 cm3 (maximum) (calculated considering a tube of 6.5cm diameter with 60 cm working length, and suitable materials container usually a semicircular boat) of material in a single batch. For e.g. purification of 5 kg (approx) of germanium (99.99%) having density of 7.435 gm/cm3 in a single batch will yield 3.5 kg of pure material (99.9999%) in 15 passes. The duration of one batch may vary depending on number of passes, speed selected (depends on cross section of charge) and desired purity level. The end portions of the processed material where the concentration of impurity builds up while purifying the central portion can be further reprocessed.
Specifications The zone refining unit is fully fabricated with SS304 grade stainless steel and houses the following components
| Sr. No. | Major Item Description |
|---|---|
| 1 | Screw chuck arrangement for tube mounting |
| 2 | Traverse using acme screw |
| 3 | Variable speed stepper motor with controller. |
| 4 | PLC unit with app. 8 digital input and 6 digital outputs |
| 5 | Proximity switches and mechanical limit switches |
| 6 | Digital temperature controller with PID with the thermocouple |
| 7 | Optical pyrometer |
| 8 | Induction heater with solid state power supply |
| 9 | Related protective switchgear including relays, MCBs, auxiliary contactors, push buttons. |
Infrastructural facilities/equipment for a typical zone refining setup
| Sr. No. | Item Description |
|---|---|
| 1 | A laboratory with 15 ft. x 20 ft with air curtains at entry gates is sufficient for a typical zone refining setup both under inert gas or vacuum with three phase power supply (32A) |
| 2 | A gas distribution setup if the refining is to be carried out under inert gas |
| 3 | A Diffusion vacuum pump if the refining is to be carried out under vacuum |
| 4 | A class 100 clean bench for material handling. |
| 5 | Facility for washing glassware with exhaust (for chemical cleaning of glassware) |
| 6 | Water distillation unit for de-ionized water for cleaning. |
