1. Field of the Invention
The present invention relates to an ion implantation method and an ion implantation apparatus capable of forming a non-uniform distribution of a dose amount within a substrate.
2. Description of the Related Art
Ion implantation process is one of the manufacturing processes of a semiconductor substrate. In the ion implantation process, ion implantation is sometimes performed such that a distribution of an ion implantation amount (also referred to as a dose amount) that is implanted within a substrate (for example, a wafer or a glass substrate) is non-uniform.
For example, in the manufacturing process of the semiconductor substrate, there is a problem that characteristics of a semiconductor device that is manufactured on a one substrate become non-uniform within the substrate.
To compensate for such a non-uniform distribution of the characteristics of the semiconductor device, conventionally, a method is used by which a dose amount implanted within the substrate in the ion implantation process is non-uniformly distributed within the substrate.
A concrete method is disclosed in Patent Document 1. Patent Document 1 discloses an ion implantation apparatus that realizes implantation of ions into the substrate by reciprocally driving the substrate in a Y direction, and scanning a spot-like ion beam in an X direction that is orthogonal to the Y direction by virtue of an electric field or a magnetic field. Such an ion implantation apparatus is called a hybrid scanning system. With such an ion implantation apparatus, a non-uniform dose amount distribution can be formed within the substrate by changing a scanning speed of the ion beam depending on a position of the ion beam on the substrate.
Moreover, in the manufacturing process of the semiconductor substrate, to improve a utilization efficiency of the substrate, a technique has been used from the past in which a semiconductor device having different characteristics is manufactured in different regions on a single substrate. An example of such a technique is disclosed in Patent Document 2.
In Patent Document 2, ion implantation into the substrate is performed using the ion implantation apparatus of the hybrid scanning system similar to Patent Document 1. First, to form distribution of two different dose amounts on either side of a center portion of the substrate, ion implantation into the substrate is performed by changing either the scanning speed of the ion beam or a driving speed of the substrate to a different value when the ion beam crosses the center portion of the substrate. Subsequently, the substrate is rotated by 90 degrees and the ion implantation is performed by changing either the scanning speed of the ion beam or the driving speed of the substrate to a different value when the ion beam again crosses the center portion of the substrate. In this manner, four regions having different dose amount distributions are formed in the substrate.    Patent Document 1: Japanese Patent Application Laid-open No. 2010-118235 (FIGS. 3 to 10 and FIGS. 12 to 18)    Patent Document 2: Japanese Patent Application Laid-open No. 2003-132835 (FIGS. 1 to 10, Paragraphs 0062 to 0064, and 0096).
As described in Paragraphs 0062 to 0064 and 0096 of Patent Document 2, changing the scanning speed of the ion beam or the driving speed of the substrate to a desired value takes time; however, short. If the ion beam is irradiated on the substrate while the speed is being changed to the desired speed, a region of an undesired dose amount distribution called a transition region is formed in the substrate.
FIG. 11 depicts states of the transition region. (A) depicts an intended dose amount distribution to be formed beneath the surface of the substrate. In (A), a concentric dose amount distribution is explained as an example. It is an objective to form a region of a dose amount D2 at a central region and a region of a dose amount D1 at an outer circumferential region. (B) depicts a state of the dose amount distribution when the substrate is cut along a line A-A shown in (A). An axis of abscissa shown in each of the graphs (B) to (E) of FIG. 11 indicates a position on the line A-A. The line A-A shown in FIG. 11 at (A) and (F) passes through a center of the substrate and divides the substrate into two portions.
In this example, for the sake of simplicity, a current density of the ion beam and the driving speed of the substrate are always assumed to be constant in the ion implantation apparatus of the hybrid scanning system. In this case, the dose amount implanted into the substrate is inversely proportional to the scanning speed of the ion beam. Therefore, to obtain the dose amount distribution shown in (B), it is necessary to change the scanning speed of the ion beam in the manner shown in (C).
However, because some time is required for changing the scanning speed, the scanning speed of the ion beam is actually changed in the manner shown in (D). As a result, the dose amount distribution shown in (E) is formed on the line A-A. In the dose amount distribution finally formed within the substrate, a transition region R is formed in a region other than the regions of the dose amounts D1 and D2 shown in (F).
A larger transition region leads to insufficient compensation of the characteristics distribution of the semiconductor device. Thus, it is desirable that the transition region be as small as possible. In Patent Document 2, a technique of reducing a size of the ion beam is proposed for making the transition region smaller. Concretely, to change the scanning speed of the ion beam scanned in the X direction at the center portion of the substrate, Wx that is a size of the ion beam in the X direction, is reduced. When the size of the ion beam is reduced, a beam current reduces by an equivalent amount. Therefore, if the ion implantation process to the substrate is performed using such an ion beam, a longer time is required for achieving a desired dose amount distribution. To solve this problem, a method is proposed in which Wy that is a size of the ion beam in the Y direction, which is orthogonal to the X direction, is increased and reduction of the beam current of the ion beam is restrained by performing the ion implantation process using the ion beam that is roughly elliptical.
However, even if an elliptical ion beam proposed in Patent Document 2 is used for forming a circular dose amount distribution on the substrate as mentioned in Patent Document 1 or shown in FIG. 11, the time required to perform the ion implantation process cannot be shortened while reducing the transition region and restraining reduction in the beam current.
In the circular dose amount distribution mentioned in Patent Document 1 or shown in FIG. 11, the dose amount distribution changes in the Y direction when the ion beam is scanned on the substrate in the X direction. As a result, because it is necessary to make the transition region smaller even in the Y direction, it is not sufficient to use only the elliptical ion beam as proposed in Patent Document 2.