Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Patent
1997-09-08
1999-08-03
Nguyen, Kiet T.
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
250398, H01J 37317
Patent
active
059328827
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to ion implanters for implanting ions into substrates, such as semiconductor wafers, in electronic device fabrication, and in particular to ion implanters capable of processing wafers on a commercial scale with relatively low implant energies.
DESCRIPTION OF THE PRIOR ART
Ion implantation techniques are commonly used as one of the processes employed in the manufacture of integrated circuits, to modify the electrical transport properties in predefined regions of a semiconductor material by doping these regions with a predetermined concentration of impurity atoms. The technique generally involves generating a beam of a preselected specie of ions and directing the beam towards a target substrate. The depth of the ion implant depends, inter alia, on the energy of the ion beam at the substrate. As the density of devices on a single wafer increases and the lateral dimensions of individual devices decrease for ultra-large scale integrated circuits (ULSI), the ability of an ion implanter to form shallow junctions using low energy ions, e.g. of about 2 keV to 10 keV, becomes increasingly important. At the same time, in commercial ion implantation, it is also important to be able to process an individual wafer in as short a time as possible and this requires the ion beam current to be as large as possible. Unfortunately, the requirement of a low energy beam tends to conflict with the requirement of a high current beam, since it is extremely difficult to transport an ion beam at low energy and high current due to space charge effects.
A known scheme to avoid the problem of beam expansion and loss of beam current is to transport the ion beam at high energy and then decelerate the beam to the desired low energy just before the beam impacts the substrate. For example, Applied Physics Letters 53 (18) Oct. 31, 1988, pages 1741 to 1743, S. N. Hong et al describes a conventional ion implanter which has been modified in order to study implant depth profiles by incorporating into the implant chamber a decelerating lens system in which a fixed target substrate is held. A retarding power supply is connected between the retarding lens and the beam extraction power supply such that the final energy of the ions just prior to impacting the target is determined solely by the retarding potential generated by the retarding power supply. Ions are extracted from the ion source at an energy of 35 keV and are passed through an analyser magnet which analyses the ions transported in the beam according to their mass. The mass analysed beam is then passed to an X-Y scanner which deflects the beam from the path between the magnet and the scanner along another path directed at the target. The retarding lens and target, which are biased at 34 kV, decelerate the beam from 35 keV to the fixed implant energy of 1 keV. Thus, ions are transported at high energy along the path between the ion source and the implant chamber to minimize beam expansion due to space charge effects and consequential loss of current. The energy of the ion beam is then only reduced just in front of the target, before impact, so that the beam travels a very short distance at low energy, again to minimize beam expansion.
One problem with the method of transporting the ion beam at relatively high energy and then decelerating the ion beam very close to the target is that, along the passage of the high energy beam, a proportion of the beam ions are neutralized through charge exchange processes with residual gas atoms and become high energy neutrals which, if directed at the target, will pass through the retarding lens without being slowed. These high energy neutrals penetrate more deeply into the substrate than the low energy ions, which is particularly undesirable when forming shallow junctions. The effect of these high energy neutrals on the implant depth can be seen as a high energy tail in the depth profile as measured by secondary ion mass spectroscopy (SIMS).
Review of Scientific Instruments 65(8) August 1994, pages 2680 to
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S.N. Hong et al. "Formation of . . . a modified Ion Implanter", Appl. Phys. Lett. 53 (18), Oct. 31 1968 pp. 1741-1743.
A.H. Al-Bayati et al., "Performance of mass . . . materials research", Rev.Sci Instrum. 65(8), Aug. 1994 pp. 2680-2692.
Daniel F. Downey, "Low energy . . . and processing," Nuclear Instruments & Method in Physics Research B74 (1993 pp. 160-169.
Adibi Babak
England Jonathan Gerald
Taylor Mitchell C.
Applied Materials Inc.
Nguyen Kiet T.
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