Page 185 - Sami Franssila Introduction to Microfabrication

164 Introduction to Microfabrication

Implant dose is monitored during implantation by the annealing. Its dislocation densities can be a million/cm ,
2
Faraday cup current measurement. This is the basis for orders of magnitude more than in bulk silicon. Implan-
the high degree of doping control in implantation as tation time poses another limitation: the required doses
compared to diffusion, which has no, whatsoever, in situ are two orders of magnitude higher than those in com-
17
monitoring method. mon usage. A low dose SIMOX with 4 × 10 /cm 2
implantation helps to minimize both the aforementioned
problems. There are further limitations that are inherent
15.4.2 Safety aspects
to the implant process: with 200 keV maximum energy,
Ion implanters pose a number of safety issues that have the implant depth is fairly shallow and, therefore, the
to be tackled. The obvious one is the high voltage device silicon thickness is rather limited. The thickness
that is present inside the machines. The second issue of buried is also limited by the implant process.
is X-rays that are produced as ions decelerate. Lead
radiation protection is routinely used around the parts
where X-rays are generated. If hydrogen is implanted, as 15.6 EXERCISES
in the Smart-cut process (to be presented in Chapter 17),
nuclear reactions are possible at fairly low energies of 1. What will be the implant time for a 200 mm
150 keV and gamma rays are then generated. diameter wafer, when arsenic ions are implanted
15
2
Implant gases AsH 3 , PH 3 and BF 3 are extremely with doses of 10 /cm and implant current of
toxic. Toxic gas detectors are placed inside the system 100 µA? 11 + 49 +
to sniff for leaks. Operation and maintenance of an 2. What is the range of 20 keV B and BF 2 ions?
implanter can, therefore, be carried out by highly trained 3. How thick a silicon dioxide layer will be formed
staff only. More discussions on safety issues can be inside the silicon when the implant dose is 2 ×
18
2
found in connection with cleanrooms, in Chapter 35. 10 /cm in SIMOX?
4. What is the range of 100 keV germanium implanta-
tion?
15.5 SIMOX: SOI BY ION IMPLANTATION 5S. How thick an oxide layer is needed to mask boron
implantation? Present your results as a function of
In SIMOX technology, a SOI structure is realized in two
boron energy.
main steps. The first step is oxygen implantation into a
6S. Check by simulator the range of 100 keV phospho-
silicon wafer and the second step is a high-temperature rus ions and compare it with the simple estimate
anneal during which the implanted oxygen atoms form discussed in the text.
an oxide layer inside the silicon (Table 15.2). This oxide 7. At what energy is electronic and nuclear stopping
is known as buried oxide (BOX). The top silicon layer, equal for phosphorus?
known as the device layer, becomes insulated from the
bottom layer, known as the handle.
SIMOX material exhibits inherent defect problems: REFERENCES AND RELATED READINGS
the device silicon layer is damaged by the implanta-
tion process and it cannot be fully recovered during Chanson, E. et al Ion beams in silicon processing and charac-
terization, J. Appl. Phys., 81 (1997), 6513–6561.
Cheung, N.: Plasma immersion ion implantation for semicon-
Table 15.2 SIMOX process ductor processing, Mater. Chem. Phy., 46 (1996), 132.
Current, M.: Ion implantation for silicon device manufacturing:
Implant conditions a vacuum perspective, J. Vac. Sci. Technol., A14 (1996),
1115.
18
Oxygen dose 2 × 10 /cm 2 Izumi, K.: History of SIMOX material, MRS Bull., 23(12)
Oxygen energy 150–200 keV Special issue on Silicon-on-insulator technology (1998), 20.
◦
Wafer temperature 550–650 C
LeCoeur, F. et al: Ion implantation by plasma immersion:
interest, limitations and perspectives, Surf. Coat. Technol.,
Anneal conditions
125 (2000), 71.
White, N.R.: Moore’s law: implications for ion implant
Temperature 1300–1350 C th
◦
Time 4–6 h equipment – an equipment designer’s perspective, Proc. 11
Intl. Conference on Ion Implantation Technology Austin
Atmosphere Ar + 0.5% oxygen
(1996), p. 355.