Ion implantation

Ion implantation is referred to a base material a doping method for introducing impurities (in the form of ions). In this manner, the material properties can be (in most cases, the electrical properties ) of the base fabric change. The method is used inter alia, in the semiconductor technology. Appropriate systems for ion implantation are known as ion implanters.

Principle

The basic method is the bombardment of solids in a high vacuum with accelerated ions. It can be divided into the following steps:

• Generating ions in an ion source
• Extraction of the ions by an electrostatic field
• Separation of ions according to mass in a mass separator
• Accelerating the ions
• Deflection by means of electric fields
• Implantation into the sample

The most important parameters for the characterization of ion implantation, the acceleration energy, which can range from 500 eV to 6 MeV, and the implantation dose is in the range of 1011 to 1018 cm -2. They determine the range of the ions in the solid state and the doping concentration.

During implantation arise depending on the mass of the implanted ions and the implantation dose of radiation damage in the crystal lattice of the semiconductor. Therefore, the substrate must be annealed according to one implantation step. This is done by a high temperature process in which the impurities are incorporated into the lattice, and so electrically activated, and the lattice structure is restored. The annealing process can be realized by a furnace or rapid thermal annealing process.

Range of ions

In the ion implantation, the range of the ions plays a crucial role. An important theory to describe the range of ions in amorphous solids was erected in 1963 by Jens Lindhard, Scharff Morten and Hans E. Schiott, commonly known as LSS theory. Describes the deceleration of the ions by the electrons of the braking medium, the electron gas is a kind of viscous medium (electronic braking ).

The LSS theory describes the dopant concentration in amorphous solids with good accuracy. Also for monocrystalline or polycrystalline solids it can be applied. It may function may cause large deviations. For example, it may be a larger dopant concentration in greater depth. The reason for this lies in the so-called lattice guiding effect which can occur in crystalline solids.

The lattice guiding effect (English: channeling ) is an undesired effect on the doping of monocrystalline silicon wafers ( wafers ). Depending on the crystal structure in the disk, there is the possibility that ions and therefore able to penetrate almost unabated due to the uniform crystal structure through the spaces between the atoms undesirable deep into the substrate. The effect disturbs the exact process management, since it can only be described very hard on statistical correlations, but this goes very well with scattered ions. The guide grid can be prevented by inclining the substrate surface by about 7 ° and 22 ° rotated with respect to the <100 > directions, and / or these are coated prior to implantation with a thin screen oxide.

Application

With the ion implantation can change a variety of material properties on the application area. In the semiconductor art, the ion implantation is used, inter alia for the introduction of foreign atoms for doping the semiconductor crystal, thereby the change in the electrical conductivity and the charge carrier mobility is the main goal. In this region the ion implantation has become the most important process and this largely replaced diffusion processes. Typical dopants are aluminum, antimony, arsenic, boron, gallium, germanium, indium, carbon, phosphorus, nitrogen or oxygen, which is used for example, in the SIMOX technique (Separation by Implanted Oxygen).

However, there are also numerous applications outside of microelectronics. They are aimed primarily at changing the color, hardness, optical properties, etchability, liability, gas diffusion and composition of a material.

Pros and Cons

The ion implantation as compared to other methods such as diffusion, a number of advantages, but also some disadvantages, which are summarized briefly below.

Advantages:

• Short process times.
• High homogeneity and reproducibility.
• Possibility of implantation by already deposited thin layers.
• So-called " buried layer " is generated below the surface (such as the SIMOX technique).
• The main process takes place at room temperature ( relatively low thermal load only when annealing ).

Cons:

• Generation of radiation damage in the crystal lattice.
• Implantation is limited to near-surface layers.
• It can no profiles are produced with sharp boundaries implantation.
• There may be differences between the actual and the theoretical profile due to additional effects.