In PVD thermal evaporation, low tension is used for evaporation of the material for thin film deposition and high tension is used for plasma creation for cleaning purposes. I hope this will help.

What is the difference between HT and LT and what are their characters ?

HT means high tension or high voltage, where in a small current is used with a very high voltage(for creating plasma). LT means low tension where in a low voltage is used with very high current(for heating the resistive boats).

In a typical thermal evaporation unit: you have two kinds of power supplies.

1) HT power supply - This means it will provide a high volta in (KV), DC voltae, which is useful for plasma glow cleaning, and to a small extent DC sputtering, if the KV is provided upto 4 to5 Kilo volts.

    Basicall it is a step up transformer, which buils the high KV, and then a rectifier, which converts the stepped up ac voltage in to DC voltage.

2) LT power supply.  

     This is providing low voltage but high current. Because for evaporation of any metal you need a high current (20 to 70 Amps), to melt the evaporation material kept on the boat.  So you need a LT power supply. 

     It basically consists for a step down transformer, which produces a low voltage and a high current at the secondary, and you you evaporation electrodes where you connect the tungsten filament are connected to this output.

Prof. K. Sreenivas