Crystec Technology Trading GmbH

Doping of Semiconductors

A semiconductor is a material that becomes conductive under certain conditions, whereas metals are conductive at all times and insulators do not conduct electric current. Typical semiconductors are silicon and germanium (group 14 of the periodic table). Most compound semiconductors belong to the III/V group, consisting of an element from group 13 combined with an element from group 15, such as gallium arsenide (GaAs) or indium phosphide (InP). II/VI compound semiconductors consist of an element from group 12 and one from group 16, e.g. cadmium sulfide (CdS). To control the conductivity of the semiconductor, small amounts of dopants are introduced that have either one more or one fewer valence electron than the host material. In this way either excess electrons are introduced and the material becomes n-type, or electrons are missing and holes lead to p-type conduction. Silicon is the most commonly used semiconductor. It is typically doped with phosphorus to create n-type material or with boron to make it p-type.

Doping in the Ion Implanter

The semiconductor material is doped by bombarding it with ions in an ion implanter. The penetration depth of the dopants depends on the acceleration used in the implanter. After implantation, an activation or annealing step is required. Diffusion furnaces are used for this purpose.

Doping in the Furnace

Doping can be achieved using solid dopant sources (doping wafers), liquid dopants such as TMB (trimethoxyborane, trimethylborate, (CH₃O)₃B) or TMP (trimethoxyphosphite, trimethylphosphite, (CH₃O)₃P), or phosphorus oxychloride POCl₃, and by dopant gases such as diborane (B₂H₆) / borane derivatives or phosphine (PH₃). POCl₃, TMB and TMP are widely used. The advantages of these liquid dopants are their easy handling, the lower health risk for operators, and the high purity of these products. They are supplied as a liquid dopant in a bubbler. Nitrogen is passed through the liquid at a defined temperature and becomes enriched with the dopant. Typical doping temperatures are in the 800-900°C range. Annealing steps can activate dopants and allow them to diffuse into the silicon.

Activation and Diffusion of Dopants

After ion implantation the dopants are often not yet electrically active. An annealing step at elevated temperature can activate them. During prolonged anneals at elevated temperature the dopants diffuse in the material and produce a deeper distribution. The diffusion furnace is used to set the dopant profile.

For both furnace-based doping and for activation and diffusion of dopants, vertical furnaces are used today in the manufacture of integrated circuits. They are available in various sizes and designs for research and development (VF1000) as well as for mass production (VF5100/VF5300 or VF5700/VF5900). JTEKT (formerly Koyo Thermo Systems) vertical furnaces are equipped with a cost-saving LGO heating element.

Doping is either performed by applying a dopant glass, e.g. using a screen-printing process and subsequent diffusion in a conveyor furnace or in tube furnaces with phosphorus oxychloride POCl₃. Doping with dopant glass is simple and can be done continuously in conveyor furnaces. However, it requires two additional process steps compared to POCl₃ doping because the dopant layer must be applied and later removed. If POCl₃ doping is used, mainly horizontal diffusion furnaces are used for cost reasons and due to the modest requirements of this process.