Patent Number: 048779628
Section: description

In the drawings, the same reference numerals indicate the same or corresponding parts. DESCRIPTION OF THE PREFERRED EMBODIMENT Hereinbelow, an example of ion implantation performed in accordance with the method of the present invention will be described while referring to FIGS. 3 and 4 of the accompanying drawings. FIG. 3 is a schematic view of a silicon substrate in the form of a silicon wafer 7 which is mounted on a rotatable base 5 of an electrostatic scanning ion implantation apparatus like the one illustrated in FIG. 1. FIG. 4 is a front view of the wafer 7. The wafer 7 is made of (100) Si and has a flat 7a lying in a (110) crystal plane. The base 5 is able to tilt the wafer 7 with respect to incident ion beams 6, and it is able to rotate the wafer 7 in its own plane. The angle of tilt of the wafer 7 is indicated by T, and the angle of rotation of the wafer flat 7a with respect to a horizontal plane 8 is indicated by R. In this example, the direction of the ion beam 6 is such that when the wafer flat 7a is horizontal, the (110) crystal planes of the wafer 7 are aligned with the ion beam 6, and therefore the angle of rotation R is measured with respect to the horizontal plane 8. In order to perform ion implantation in accordance with the present invention, the base 5 is tilted by a tilt angle T of about 7.degree. and is rotated with respect to the horizontal plane 8 by a rotational angle R of 15.degree. to 75.degree.. Most preferably, the angle of rotation R is about 45.degree.. When the wafer 7 is secured in this first position, indicated by P1, the wafer 7 is irradiated with a dose of ions equal to approximately 1/4 of the total dose with which it is to be irradiated. In this position, the (110) crystal planes of the wafer 7 are not aligned with the ion beam 6, so planar channeling is largely prevented. After performing ion implantation in this first position, the wafer 7 is then rotated by the base 5 in the plane of the wafer 7 by 90.degree. in the counterclockwise direction in FIG. 4 to a second position P2. At this second position, the wafer 7 is again irradiated with approximately 1/4 of the total dose of ions. Next, the wafer 7 is again rotated counterclockwise by 90.degree. in its own plane to a third position P3 and irradiated with approximately 1/4 of the total dose, after which it is rotated counterclockwise by 90.degree. to a fourth position P4 and irradiated with the remaining approximately 1/4 of the total dose. After irradiation at the fourth position, the total dose of ions has been implanted, and ion implantation of the wafer 7 is complete. The (100) crystal plane of silicon has four-fold symmetry, so a wafer 7 having a flat 7a lying in a (110) crystal plane has &lt;110&gt; crystal axes which are parallel to and perpendicular to the flat 7a. If the angle between the flat 7a and a horizontal plane 8 is 0.degree. for such a wafer 7, during ion implantation, planar channeling occurs along the (110) planes, as was the case with the wafer 7 of FIG. 2. However, in the method of the present invention, as a wafer 7 is rotated in its own plane by an intitial angle of 15.degree. to 75.degree. away from a position in which the (110) crystal planes would be aligned with an incident ion beam, the (110) crystal planes are no longer aligned with an incident ion beam and little planar channeling takes place. Furthermore, as the wafer is rotated by 90.degree. at a time to four different positions and ion implantation is performed with the same dose of ions at each position, planar channeling is yet further decreased. Therefore, the uniformity of the depth of implantation of ions in the surface of a wafer is greatly increased. In the above-described example, the initial angle of rotation R of the wafer 7 was 15.degree.-75.degree., but as the wafer 7 is rotated four times by 90.degree. at a time to four different positions, the same effects can be obtained if the initial angle of rotation is (R+90.degree.), (R+180.degree.), or (R+270.degree.).