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ION IMPLANTATION AND RAPID THERMAL PROCESSING

10.10 WAFER PROCESSING

of up to 400 mm. The wafer is typically held on an electrostatic chuck that may be gas cooled to
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maintain adequate wafer temperature. The wafer can usually be tilted at angles of up to 60° and
rotated through a full 360° either while on the chuck or prior to being placed there. When operating
at maximum throughput, a typical single-wafer architecture can process up to 300 wafers per hour.

10.4 KEY PROCESS AND MANUFACTURING ISSUES

There are several critical process parameters, in addition to the implant species, which describe and
differentiate various ion implantation processes. These parameters include implant energy, dose, and
angle. In addition, demands in the area of wafer contamination and charge control each present
unique process control challenges that are also dependent on the beamline and process chamber
architecture.

10.4.1 Dose Uniformity and Repeatability

The delivery of a uniform dose of ions across the entire surface of the wafer is one of the key met-
rics in any ion implant tool. How this delivered dose is controlled during the implantation process
varies depending on the particular architecture of the implant tool and the range of the dose being
delivered.
Both multi-wafer and single-wafer implant tools typically control the speed with which the
wafers are scanned across the ion beam in one direction based on a feedback signal that is related to
the flux of atoms and ions arriving at the wafer. In most cases, this signal comes from a Faraday cup
sampling the electrical charge, arriving during a known period of time when the beam is not incident
on the wafers. Changes in the flux of atoms and ions incident on the wafer (as a result of changes in
the ion source output, or beamline transport, for example) lead to changes in the wafer scan speed to
maintain constant flux over the entire surface of the wafer.
In multi-wafer tools with a spinning process disk and a fixed position ion beam, the ion flux can
be measured either in real time during each revolution of the process disk through a slot of known
dimension in the disk itself and/or during the period of time when the disk has scanned completely
out of the path of the beam. 5
In single-wafer tools with a scanned ion beam, the ion flux can be measured at the end points of
each scan of the beam. In single-wafer tools with a fixed ribbon ion beam, the ion flux can be esti-
mated from the edges of the ribbon beam, which are not incident on the wafer, or can be measured
during the period when the wafer has scanned completely out of the path of the beam. 18
How the delivered dose is measured in the wafer following implantation depends on the species,
energy, and range of dose being delivered. Common measurement techniques include modulated
reflectance (commonly known as Therma-wave) and four-point probe sheet resistance. 19
Therma-wave is an optical technique that measures damage to the crystal structure of the silicon fol-
lowing implantation and can be used to infer a delivered ion dose, if the sensitivity of the damage mea-
surement at the particular energy and dose range is known. Therma-wave measurements have the
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highest sensitivity at energies above a few tens of keV and for doses in the range of 10 to 10 . The
sensitivity in this range can be as high as approaching unity (i.e., fractional changes in Therma-wave
units of measurement correspond to the same fractional changes in the ion dose). Below these high-
sensitivity energies and/or above these high-sensitivity doses, Therma-wave sensitivity is usually no
better than approximately 0.1 and is generally not used. Therma-wave measurements have the advan-
tage of being performed on wafers as they are implanted, and require no interim annealing or other
postimplant processing step. Therma-wave measurements can also be made for implant species that are
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not electrically active (common examples of nonelectrically active implant species are Ge and N ).
Sheet resistance measurements with a four-point probe are contact electrical measurements
requiring the presence of a p-n junction. Because of this, they can only be performed for implant
species that can be electrically activated and then only following such activation (typically via a ther-
mal annealing process as described in the next section).

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