Diffusion Furnace for Doping, Diffusion and Activation of Dopants
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A semiconductor is a material, that is conductive at special conditions,
unlike metals which are permanent conductors and isolators which are non-conductive.
Typical semiconductors are silicon and germanium (4th group of the PSE). Most common compound semiconductors
are III/V semiconductors consisting of elements of the 3rd and 5th group of the periodic
system of the elements PSE, like Galliumarsenide GaAs or Indiumphosphide InP. II/VI compound semiconductors
consist of elements of the 2nd and 6th group of the PSE like Cadmiumsulfide CdS.
In order to adjust the conductivity of these semiconductors, small amounts of dopants are
injected in the bulk material, which have usually one electron more or less compared to the main material.
This way, excessive electrons are available and the result is a negative n-conductivity
or electrons are missing. So called holes lead to a positive p-conductivity.
Silicon is the most used semiconductor material and for n-doping quite ofter Phosphorous is used.
P-doping is done with Boron usually.
The semiconductor material is doped by ion bombardment in an ion implanter. The penetration depth of dopants depends on the acceleration in the ion implanter. After implantation, an activation or annealing is required. For this purpose, diffusion furnaces are used.
The dopand is inserted in the silicon by diffusion in a diffusion furnace. Doping can be achieved by solid source doping (doping-wafers), by liquid source doping as e.g. TMB (Trimethoxyboran, Trimethylborate, (CH3O)3B ) or TMP (Trimethoxyphosphine, Trimethylphosphite, (CH3O)3P) or Phosphorus Oxychloride POCl3 and by gaseous doping like Boran BH3, Phosphane PH3. POCl3, TMB and TMP have gained significant acceptance. Benefits are the ease of handling a liquid source, less health hazards and improved purity levels. The liquid is supplied in a bubbler. Nitrogen passes at a well defined temperature through the liquid and is transporting the dopant. Typical doping temperatures are 800 - 900°C. Anneal steps allow the activation and diffusion of dopants in the silicon.
After the doping in the ion implanter, the dopants are partially not yet electrically active. By a temperature step, the activation can be done. By prolonged heat treatment of the semiconductor wafers at an elevated temperature, the dopants diffuse in the material and create a greater depth distribution. In a diffusion furnace the doping profile can be adjusted.
Both, for the doping in the furnace as well as for activation and diffusion of the dopants in state of the art production of integrated circuits, vertical furnaces are used. They are available in various models and sizes, for research and development (JTEKT VF1000), as well as for mass production (JTEKT VF5100/VF5300 or JTEKT VF5700/VF5900). JTEKT Thermo Systems (previously Koyo Thermo Systems) vertical furnaces are equipped with cost saving LGO heaters.
The doping is either done by the deposition of a doping glass and following diffusion in a conveyor furnace or in a tube furnace, using Phosporousoxychloride POCl3. The doping methode, using doping glass is simple and can be done in a continuous process in a conveyor diffusion furnace. However this methode requires two process steps more compared to the POCl3-doping process, because the doping glass has to be deposited and removed. In case that the POCl3-doping is used, mainly horizontal diffusion furnaces are selected for cost reasons and because of the low demands to this process.
