The production of power semiconductor devices strongly relies on the ability to finely tune the electrical properties of semiconductor materials through the process of doping. When doping silicon for the production of semiconductor devices, the level of silicon doping required depends on the specific electronic component which must be produced. This ability to fine tune doped semiconductor materials allows the creation of a wide range of semiconductor devices for use in endless applications.
Typical criteria to be considered when doping silicon include the following:
- Type of Doping (n- or p-type doping)
- Specific Resistance in Ω cm as a measure of the level of the doping required
- Material or element used for doping: for n-type - P, As, Sb; and for p-type - B, Al, Ga
In cases where n-type doping is required, doping silicon rods by a process known as Neutron Transmutation Doping (NTD) is an effective solution.
Doping a silicon rod using the the neutron transmutation doping process
92% of all silicon atoms are made up of 14 protons and 14 neutrons (14Si28 ). However, 3% of silicon atomic nuclei contain 16 neutrons (14Si30). Through the capture of a thermal neutron, this silicon isotope can be changed into 14Si31, which is radioactive, and by converting a neutron into a proton and an electron (β-decay), the silicon isotope can be converted to phosphorus 15P31. The half-life of 14Si31 is 157.3 min.
Since the converted silicon isotope 14Si30 is very evenly distributed in the crystal, the resulting dopant atoms 15P31 are also distributed very evenly.
During the NTD process, a silicon rod with very little doping is put into an atom reactor where thermal neutrons are released until the required amount of doping is achieved. The silicon rod is then left in the reactor for a few weeks in order to rid the silicon of all the radioactive material.
The advantage of using the NTD method is that even at low concentration ("high ohmic" silicon), very homogenous doping can be achieved. The main disadvantage is that only n-type doping can be performed using this method, and the higher the level of doping the more expensive is the doping process due to greater and thus longer lasting radioactivity.
The fluctuations of the specific electrical resistance from the middle to the edge of the wafer are higher in the conventionally doped wafers than in wafers doped using the NTD method (figures 1 and 2). This is mostly important for large surface components.
Figure 1. Radial Resistance in a conventional doped silicon disk
Figure 2. Radial resistance in a silicon disk doped using NTD process
Processing neutron transmutation doped silicon rods
After the neutron transmutation doping precess, the silicon rods are rounded to the desired diameter. The usual diameters are 100 mm(4"), 125mm (5"), 150mm (6"), 200 mm (8"), 300mm (12").
The silicon rod is flattened by grinding. The flats are used for coarse adjustment of the wafer in the process. The crystal orientation is coded depending on the number and position of the flats. Flats are an alternative to notches.
The rods are cut into slices, called disks or wafers. This can be done either with an annular saw or a wire saw. The main goal is to have uniform wafer thickness and minimum waste.
Figure 3. Wire saw for cutting silicon rods
Structuring the neutron transmutation doped silicon disks
The disk edges can be rounded off by grinding in order to avoid edge breakage and to make sure that the wafer does not break as well. The edges of any given wafer are often custom formed to meet a particular customer's need.
Figure 4. Rounding off the edges of the silicon wafers
Surface treatment of transmutation doped wafers
Crystal imperfections occur on the wafer surface due to the use of the saw. These imperfections must be removed prior to the production of most components. This is done by one or two sided etching. Lapping or polishing must be used for fine structures.
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