Boron implantation

Step 6. The doping concentration of the "typ substrate under the gate oxide is adjusted by another boron implantation. Boron passes through the thin gate oxide. This provides the threshold voltage adjustment for the final device. 

Boron implant with laser anneal. Boron atoms are accelerated into the backside of the CCD, replacing about 1 of 10,000 silicon atoms with a boron atom. The boron atoms create a net negative charge that push photoelectrons to the front surface. However, the boron implant creates defects in the lattice structure, so a laser is used to melt a thin layer (100 nm) of the silicon. As the silicon resolidihes, the crystal structure returns with some boron atoms in place of silicon atoms. This works well, except for blue/UV photons whose penetration depth is shorter than the depth of the boron implant. Variations in implant depth cause spatial QE variations, which can be seen in narrow bandpass, blue/UV, flat fields. This process is used by E2V, MIT/LL and Samoff. 

MBE growth of very thin layer of boron and silicon. The problems associated with boron implant and laser anneal can be overcome by growing a very thin (5 nm) layer of silicon with boron atoms on the backside of the thinned CCD (1% boron, 99% silicon). The growth is applied by molecular beam epitaxy (MBE) machines. This process was developed by JPL and MIT/LL. 

Figure 13. Damage and concentration profiles for single and multiple ion beam (boron) implantation into silicon.

Fig. 5. Depth profile of deuterium in boron-implanted Si after deuteration at 150°C for 30 min. After Johnson (1985). 

Fig. 16. Infrared absorption spectra for boron-implanted silicon after passivation in either monatomic hydrogen or deuterium. The specimens were passivated at 150°C for 1 h (per surface), and the spectral resolution is 4 cm-. From Johnson (1985). 

Fig. 18. Angle-lapped section of n-type Si that has been boron-implanted and then partly hydrogenated to form a 0.23 /u.m-thick layer of n-type, H-neutralized material. The n-type regions are preferentially stained.

Another effect of lattice contraction of a boron-implanted layer might be to cause reorientation of the B—H complexes in the layer. Calculations (Denteneer et al., 1989) and measurements (Stavola et al., 1988) show that the B—H complex can readily reorient itself in response to an applied stress at room temperature. Thus, reorientation might occur in a boron-implanted layer and vitiate the analysis of the channeling experiment. In a (100) sample, however, all the (111) directions lie at the same angle to the stress axis if relaxation in the plane of the sample is isotropic, and all orientations of the B—H complex are energetically equivalent. This would not be true in (111) material. 

In the semiconductor industry, SIMS has been particularly useful for the depth profiling of dopants that are present in silicon in very low concentrations. As an example, a SIMS depth profile for boron implanted into silicon is shown in Figure 27. One of the significant features is that we can detect about 101S boron atoms/cm3 in a silicon matrix of 5 x 1022 atoms/cm3. This illustrates an ability to detect 20 ppb. Also, the method spans 5 orders of magnitude in boron concentration. No other technique can span such a large range accurately. 

Figure 27 SIMS profile of boron implanted into silicon.

Figure 36 Comparison of normal incidence profiling in vacuum to 60° incident profiling with oxygen flooding. Profiles of boron implanted in silicon. (From Ref. 122.)...

Generally speaking most of the shallow impurity levels which we shall encounter are based on substitution by an impurity atom for one of the host atoms. An atom must also occupy an interstitial site to be a shallow impurity. In fact, interstitial lithium in silicon has been reported to act as a shallow donor level. All of the impurities associated with shallow impurity levels are not always located at the substitutional sites, but a part of the impurities are at interstitial sites. Indeed, about 90% of group-VA elements and boron implanted into Si almost certainly take up substitutional sites i.e., they replace atoms of the host lattice, but the remaining atoms of 10% are at interstitial sites. About 30% of the implanted atoms of group-IIIA elements except boron are located at either a substitutional site or an interstitial site, and the other 40% atoms exist at unspecified sites in Si [3]. The location of the impurity atoms in the semiconductors substitutional, interstitial, or other site, is a matter of considerable concern to us, because the electric property depends on whether they are at the substitutional, interstitial, or other sites. The number of possible impurity configurations is doubled when we consider even substitutional impurities in a compound semiconductor such as ZnO and gallium arsenide instead of an elemental semiconductor such as Si [4],... 

F. N. Schwettmann, R. J. Dexter, and D. F. Cole, Etch rate characterization of boron-implanted thermally grown Si02, J. Electrochem. Soc. 120, 1566, 1973. 

When we have a system with more than one species, as in the case of boron implantation in silicon, it is also important to identify the final location of the implanted atom, critical in this particular example, since the activation of the dopant will depend on this configuration. The different possibilities for a stable configuration of boron in a silicon lattice, together with their occurrence probability are shown in Table 1. It must be emphasized with high probability that a boron atom has to occupy a substitutional site, called B. ... 

The author would like to thank Dr. James R. Ehrsteln of the National Bureau of Standards for providing Figures 6a and 6b and Dr. Marek Pawllk of the GEC Research Laboratories for supplying the boron implant profiled in Figure 8. 

Implantation. Ziegler and co-workers (1,14,24,25) introduced NDP by determining the range and shape of boron implantation distributions in intrinsic and doped silicon wafers. With the resultant profiles, they were able to calculate diffusion coefficients for boron in crystalline, amorphous, and arsenic-doped silicon. Since little experimental data existed for the case of boron to judge the validity of the current range theories, the shape of the boron profiles from NDP were of great interest. NDP and other techniques have since been able to show that a Pearson IV model rather than a gaussian profile is required to describe accurately the Implant distribution (21,26-28). 

In subsequent experiments, Biersack, et al. (29) used the boron (n,alpha) reaction to show the effect of pre- and post-irradiation damage on boron implantation profiles. By post-irradiating a boron implant in silicon with 200 keV H2, a migration of the boron to the Induced damage sites was observed. In the same paper, diffusion and trapping of lithium ions in niobium were reported. Using the lithium (n,alpha) reaction, they mapped irradiation Induced crystal defects through a depth of several micrometers with respect to several sample treatment conditions. 

Interfacial Profiling. Neutron depth profiling is well suited for measurements across interfacial boundaries. Kvitek et al. ( ) and others (16,17,21,30) have studied profiles of boron implanted and diffused across the interfacial region of Si/Si02. Other NDP experiments (33,34) have been described for interfaces of silicon, silicon dioxide or metal on metal, where diffusion distributions and segregation coefficients were studied. 

Fig. 14.10. Threshold voltages of n-channel (VTn) transistors and p-channel (Vj-p) as a function of boron implantation doses (Ohzone et al. 1980)...

Most impurity and defect states in SiC can be considered as deep levels. Both capacitance and admittance spectroscopy provide data on these deep levels which can act as donor or acceptor traps. Bulk 6H-SiC contains intrinsic defects which are thermally stable, up to 1700 °C. In epitaxial films of 6H-SiC a deep acceptor level is seen in boron-implanted samples but not when other impurities are implanted. Other centres, acting as electron traps, are also seen in p-n junction and Schottky barrier structures. Irradiation of 6H-SiC produces 6 deep levels, reducing to 2 after annealing. Only limited studies have been carried out on the 3C-SiC polytype, in the form of epitaxial films on silicon substrates. No levels were seen in thick films but electron traps were seen in thin n-type films and a hole trap (structural defect) was found to be a mobility killer. Neutron irradiation produces defects most of which can be removed by annealing. Two levels were found in Al-implanted 4H-SiC. 

Figure 16.6 shows the process sequence executed in p-well formation. The lithographic masking step (involving mask 2) is the same as that for the u-well implant mask. It defines the area of the inverter to be implanted to form the p-wells for the nMOS transistors. A boron implantation is performed, followed by resist stripping and cleaning. Annealing is done in basically the same manner as described above for the u-well. 

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