Low-dose ion implantation

For some doped layers, such as a low-dose ion implant for a MOS transistor, this procedure does not reveal the entire doping profile.10 In this case, a MOS structure is examined rather than a Schottky diode. Typically, a bare silicon wafer is oxidized, and then aluminum dots are sputtered on to form many MOS structures. When a MOS device is examined, Equations (8) and (9) have to be supplemented by... 

Paine, B.M., Hurvitz, N.N., Sperious, V.S. Strain in GaAs by low-dose ion implantation. 

During ion implantation, each ion produces a region of disorder around the ion track. As the implantation dose increases, the disorder increases until all the atoms have been displaced and an amorphous layer is produced over a depth Rp. The buildup and saturation of disorder are shown in Fig. 10.1 for 40 keV phosphorus ions incident on Si. In this example, about 4 x 1014 phosphorus ions cnT2 are required to form an amorphous layer. Except for low doses or implantation with light ions, we can anticipate that an amorphous layer is formed during the implantation process. This assumes that no recovery of lattice order occurs around the ion track. 

Fig. 9. Schematic view of the development of the concentration profile of ions implanted from low (L), medium (M), and high (H) doses. The projected...

Figure 20. Ion implantation defect production models for low- and high-dose B implants into crystalline and preamorphized Si. Csoi is the solid solubility...

In Table 5 the few data available for the formation and destruction of compounds between the implanted ion and the host are specified. The initial percentage is listed at which the compound is formed under low-dose conditions together with the dose necessary to decompose = 90% of the initial compound. 

Table 5. Decomposition of compounds formed during ion implantation. Low-dose formation yield and dose necessary for 90% decomposition...

Fig. 1.3. A schematic representation of the disorder produced in room-temperature implantations of heavy ions at energies of 10-100 keV. At low doses, the highly disordered regions around the tracks of the ions are spatially separated from each other. The volume of the disordered region is determined primarily by the stopping point of the ion and the range of the displaced lattice atoms (dashed arrows). At high doses, the disordered regions can overlap to form an amorphous layer...

Gas bubble formation and blistering effects have been widely observed in high-dose implantations of inert-gas ions. Backscattering measurements of depth distributions often show very low concentrations of implanted species in the nearsurface region. This indicates that the inert-gas atoms can escape from the material even without sputtering. In these cases, the simple model described in the previous sections does not apply. 

Thermal Anneal. Ion-implanted wafers should be annealed at the lowest temperature possible in order not to diffuse the impurities further into the wafer. At low implanted doses (1011 - 1(H2 cm-2) the electrical activation process can occur at temperatures as low as 260 degrees C in silicon whereas for higher doses (1013 - 1011 cm 2) 570 degrees C may be required. For higher doses (1015 cm"2) higher temperatures around 900 degrees C will be required for electrical activation (26). Most anneals are for 30 minutes in a nonoxidizing ambient. Reference 26 may be consulted for more details of the thermal anneal. 

Ion implantation has also been used to increase the barrier height of metal-semiconductor Schottky-barrier diodes (46). This is accomplished by implanting low energy ions of opposite conductivity type into the semiconductor surface. The implanted ions change the field and potential in the surface region and reduce the diode current. Figure 16 shows the variation of current density versus forward voltage for various values of ion implantation dose (58). 

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