Status of Low-Dose Implantation for VLSI
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Bipolar technologies have recently been reviewed by Hill and Hunt5 and will not be discussed here. Manufacturing practices are as critical as understanding phenomena or applications to devices. Dose accuracy is discussed in the context of very low doses. Cost effectiveness is a key consideration for the future. Both continuous improvement of the existing machines and revolutionary changes are needed for cost-effective manufacturing.
Physical Phenomena There is a need to improve understanding of implantation depth profiles (particularly at low energies), damage, and annealing phenomena. These effects are critically important for controlling shallow impurity profiles used in modern devices. An emerging picture includes the role of channeling, a linkage between stably displaced disorder (and the generation
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Equipment Control: Dose, Species Energy Contamination Cost
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Phenomena Profiles Channeling Damage Diffusion
Figure 1. Relationship of vertically integrated technologies and key elements reviewed in this paper.
of dislocations) and, in turn, the role of dislocations in dopant diffusion.
Profiles Ideal random-depth distributions of implanted dopant impurities are obtained by implanting into preamorphized silicon.6 These random-depth distributions are described in the simplest terms by Gaussian (two moments, range and straggle) and in better detail by Pearson-4 (four moments, add skew and kurtosis) mathematical models. Ideal random profiles are not obtained in crystalline material. The profiles always show some channeling character, even when the incident direction of the implanting ions is selected to be in the least favorable direction for channeling. Channeling is the capture and steering of ions between rows and planes of atoms in a crystalline target. Figure 2 shows recent boron depth profiles for a systematic sequence of rotation angles of the implantation direction at a fixed tilt angle with respect to the surface normal of the crystal. These and other similar data have been fit to a dual Pearson-4 mathematical model.7 A great deal of data of this kind now exists. Yu8 concluded that a tilt of 7-10° and rotation zones corresponding to dense atom directions minimize channeling. It is common practice to try to limit channeling in the tail of the profile by implanting through an amorphous film such as a screen oxide. This presumably scatters the incoming beam into a larger range of incoming angles, spreading (some of) the beam outside the critical angle for channeling. However, under certain conditions, use of amorphous overlayers results in even less random profiles (see Figure 3). The implants in Figure 3 were all carried out at "optimum" tilt and rotation. At the higher implant energies, the broadening of the angular spread of the beam by the capping oxide increases the spreading of the beam, which increases the fraction of the ions that find channeling axes and planes rather than diverting the ions from the channels.9 "Ideal" channeled data, where ions p
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