Page 139 - Semiconductor Manufacturing Handbook

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ION IMPLANTATION AND RAPID THERMAL PROCESSING

10.12 WAFER PROCESSING

10.4.2 Implant Angle Control
The angle between the incident ion beam and the surface of the wafer is an important parameter in
determining the final distribution of dopant in the wafer. The primary factors related to the implant
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angle which determine this distribution are channeling in the crystalline silicon and shadowing of
areas of the wafer by the edges of various masks.
Channeling occurs as implanted ions migrate along and between the crystallographic planes and
rows of the silicon lattice. Significant variations in the depth a given ion will travel can occur depend-
ing on whether or not it is aligned with a particular channel. A critical angle can be defined to
describe how close to a particular channel an incident ion needs to be before it can travel in that chan-
nel. The critical angle is a function of the energy (E) of the incident ion and, for both axial and pla-
nar channeling, is proportional to
1
E

Shadowing can occur in implants that have masks of heights that are large compared to some crit-
ical device dimensions, such as channel lengths. Whenever such an implant is performed at angles
other than zero degree, either by design or due to some variation, parts of the wafer surface will be
shadowed and not receive any implant dose.
Control over the implant angle is achieved via control over the angle at which the wafer is posi-
tioned during implantation, both in multi-wafer and single-wafer architectures. Sources of variation
in the implant angle during an implant, or from run-to-run, occur as a result of a number of possible
factors, including
• Variation in beam tuning causing the incident ion beam to not be parallel to the axis of the beam-
line. Up to several degrees of variation as a result of beam steering have been reported for various
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high-current architectures. Typical variations for medium-current and high-energy architectures
tend to be significantly lower, of the order of a few tenths of a degree.
• Variation in wafer crystal-cut error as a result of the bulk wafer manufacturing process. These vari-
ations typically do not exceed 0.5°, and can be controlled from batch to batch to better than 0.1°.
• Variation in wafer positioning as a result of scanning architecture. Some multi-wafer spinning disk
geometries introduce a systematic variation in the incident angle across the wafer as they scan and
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rotate in front of the beam. This across-wafer variation can be of the order of 1°, and can be mit-
igated by reductions in the wafer pad angle offset (that is used to improve cooling) to be no more
than a few tenths of a degree.
Further mitigations for variations in the implant angle may be achieved through the use of “quad-
mode” segmented implants in which the wafer is deliberately rotated by 90° during each of four
equal dose segments of the overall implant. These rotations of the wafer serve to average out any
angle variation that might exist in one or more planes and have been shown to effectively reduce the
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device sensitivity to these variations. Newer developments in angle control for all types of tools
involve active correction for certain types of detectable angle variation prior to implant. Automated
changes to beam tuning are capable of correcting beam steering variations. Repositioning of the
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wafer at a tilt angle to compensate for other known sources of variation is also now possible. As
devices continue to shrink and generally become more sensitive to variations in the implant angle,
this area of implanter control will remain a fertile one for new developments.

10.4.3 Contamination
Several types of contaminations are of critical concern during the implantation process. Surface con-
taminants, both elemental and particulate, can result from the sputtering processes occurring as the
ion beam strikes surfaces in close proximity to the wafer. Deposition and resputtering of material
(both dopants and bulk beamline structural materials) over time puts the in-process wafers at some

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