Diffusion Furnace
	Koyo Thermo Systems Co., Ltd. is represented in Europe by Crystec Technology Trading GmbH
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Diffusion Furnace

Diffusion and Doping

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 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.

Furnaces

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, in the past horizontal diffusion furnaces have been selected in most cases for cost reasons and because of the low demands to this process.

For higher demands to the homogeneity of the doping profile or to the automation level, vertical furnaces are available. The smallest version of a Koyo Thermo Systems vertical furnace can be very well used especially for the use in research and development of solar cells. The furnace model VF1000 is designed as a mini-batch furnace, has a manual loading and is therefore very flexible regarding sample sizes. This vertical furnace is equipped with a cost saving LGO heating element. The process performance equals the big production versions of vertical furnaces for IC production. The price of this unit is similar to the price of a horizontal tube in a horizontal furnace model 206.

For mass production, Koyo Thermo Systems developed a special vertical furnace, which gives better process results compared to a horizontal one, but still does not increase much the production costs for solar cells. This twin furnace loads 3-4 boats in one vertical tube and has therefore a capacity of 600-800 solar cells. This is the same capacity that you get on a horizontal furnace. Automation is easier to do on a vertical furnace.

Crystec Technology Trading GmbH will be pleased to further discuss details with you.
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