Determining beam alignment in ion implantation using Rutherford Back Scattering

An ion implanter comprising:

• a process chamber having a chamber wall;
• an ion beam generator to produce a beam of ions for implantation, said beam having a nominal beam direction in said chamber;
• a single wafer holder in said chamber to carry for ion implantation therein a single planar crystalline wafer having a crystal axis nominally normal to the plan of the wafer, said wafer holder having a normal direction which is nominally normal to the plane of a wafer on said holder;
• a tilt mechanism for adjusting the tilt angle of said normal direction of the holder relative to said nominal beam direction about an axis perpendicular to said normal direction and to said nominal beam direction;
• a twist mechanism for rotating the holder, while holding a wafer thereon in the ion beam, to selected rotational positions about an axis nominally parallel to said normal direction;
• a scattered ion receiver mounted on said chamber wall to receive beam ions back scattered from a wafer on the holder;
• and
• a scattered ion current detector for determining the current of said back scattered beam ions received by said receiver.

A method of calibrating the direction of an ion beam relative to a single wafer holder in an implant chamber of an ion implanter, comprising:

• mounting on the holder a single crystalline material so that the crystal lattice of the crystalline material has a known nominal orientation relative to the holder;
• directing the ion beam at the single crystalline material on the holder;
• effecting relative movement between the direction of the ion beam and the orientation of the holder while the crystalline material on the holder is continuously in the ion beam;
• monitoring the intensity of beam ions back scattered from the crystalline material during said relative movement to identify, from a corresponding minimum in said monitored intensity, at least one selected orientation of the crystal lattice of the crystalline material relative to the ion beam;
• and
• using the relative orientation of the ion beam and the holder when said selected crystal lattice orientation is identified and the known nominal orientation of the crystal lattice to the holder to provide a calibration point for the direction of the ion beam relative to the wafer holder.

A method of calibrating the wafer holder in the process chamber of an ion implanter of the type having a process chamber,

• an ion beam generator to produce a beam of ions for implantation, said beam having a predetermined direction in said chamber,
• a single wafer holder in said chamber having a normal direction which is normal to the plane of a single wafer on the holder,
• a tilt mechanism for adjusting the tilt angle of said normal direction relative to said beam direction about a tilt axis perpendicular to said beam direction and to said normal direction, and
• a twist mechanism for rotating the holder, while holding a wafer thereon in the ion beam, about a twist axis which is perpendicular to said tilt axis and nominally parallel to said normal direction;
• the method comprising the steps of:
• a) mounting on the holder a crystalline material so as to have a predetermined crystal axis which is normally aligned with said normal direction;
• b) tilting the holder to a predetermined tilt angle;
• c) rotating the holder about the twist axis between at least selected ranges of twist angles, with the wafer thereon continuously in the ion beam;
• d) monitoring the intensity of beam ions back scattered from the crystalline material as the holder is rotated to identify the twist angles corresponding to minima in said monitored intensity;
• and
• e) using the values of said identified twist angles with said predetermined tilt angle to calculate the values of tilt angle and twist angle required to compensate for any deviation between the twist axis and said normal direction and to direct said normal direction parallel to said beam direction.