BACKGROUND OF THE INVENTION

The present invention relates to an ion implanter for implanting ions of a predetermined sort into a sample such as semiconductor wafer.

In ion implanters, it has heretofore been common practice that various ion beams extracted from an ion source are once separated according to masses by means of a magnetic mass separator so as to select only a specified sort of ions, whereupon the selected ions are implanted into a semiconductor wafer. Figure 1 shows the fundamental setup of a prior-art ion implanter. Referring to the figure, an ion beam 3 emergent from an ion source 1 is separated into ion beams 3', 3", 3"' etc. of individual ion sorts by a static magnetic field B which a magnetic mass separator 2 establishes. A semiconductor wafer 5 is mounted on the surface of a sample holder 4. The holder is mounted on the surface of a rotating disk. Owing to a combination of the rotational motion and a radial motion of the disk, the semiconductor wafer 5 is uniformly irradiated with the ion beam 3" consisting only of predetermined ions. In this case, the mass resolution of the magnetic mass separator 2 is proportional to the beam radius r_m in the magnetic sector. In order to improve the mass resolution, the radius r has to be increased. Therefore, the magnetic mass separator 2 and the whole ion implanter becomeslarge in size. Moreover, the transmissivity of the mass separator 2 for the ion beam is usually several tens %, and the beam loss is inevitable. Accordingly, the effective utilization factor of the ion implanting current compared to the ion beam 3 extracted from the ion source 1 is ordinarily 30% or so.

In recent years, an ion implant equipment which changes the surface property of a sample by implanting ions into the surface has been applied to various fields. In this case, the purity of the ion beam is not so important, and it is only required to perform the ion implantation into a large surface of the sample at high speed by increasing the ion implant current. Hitherto, there has not been an ion implanter which can satisfy such requirement.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide an ion implanter which realizes ion implantation at higher current level.

In order to accomplish the object, according to the present invention, an ion implanter is constructed of an ion source, a sample hclder for supporting a sample to be implanted with ions, and scanning means for scanning an ion beam extracted from said ion source, the sample being located at a position where it can avoid irradiation with neutral particles which propagate rectilinearly from said ion source. An example of the scanning means is constructed of an air-core coil which is disposed between said ion source and the sample, and an A.C. power supply which provides said air-core coil with alternatinq current. Another example of the scanning means is constructed of a permanent magnet which is disposed between said ion source and the sample, and an A.C. power supply which is connected to said ion source in order to minutely change the ion acceleration energy A.C.-wise.

Owing to such characterizing features of the present invention, it has become possible to implant ions into a sample with a high current ion beam. As a result, an ion implanter capable of high-speed ion implantation has been provided.

BRIEF DESCRIPTION OF THE DRAWINGS

• Figure 1 is a view showing the fundamental setup of a prior-art ion implanter,
• Figure 2 is a view showing the fundamental setup of an ion implanter according to the present invention, and
• Figure 3 is a view showing the setup of an ion implanter according to another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 2 shows the fundamental setup of an ion implanter according to the present invention. A semiconductor wafer 5 placed on a sample holder (not shown) is located in the direction in which an ion beam 3 from an ion source 1 propagates rectilinearly. Further, an air-core coil 6 for deflecting the ion beam 3 is located in substantially the middle position between the ion source 1 and the semiconductor wafer 5, and alternating current is supplied to the air-core coil 6 using an A.C. power supply 7 so as to oscillate the ion beam 3 in the-lateral direction in the sheet of the drawing as indicated by arrows. On the other hand, the semiconductor wafer 5 is mechanically moved perpendicularly to the sheet of the drawing. Thus, the semiconductor wafer 5 is uniformly irradiated with the ion beam 3. In this case, the semiconductor wafer 5 is moved sideways so that one end part A thereof may not be irradiated with the ion beam 3 at the time of a magnetic field B = 0. This is intended to prevent fast neutral particles emergent from the ion source 1 from becoming implanted into the semiconductor wafer 5. Neutral particle implantation results in dose non-uniformity over the semiconductor wafer 5.

According to the present invention, unlike the prior-art ion implanter shown in Figure 1, the mass separator 2 is not included, and the distance between the ion source 1 and the semiconductor wafer 5 is shortened. Therefore, loss of the ion beam 3 becomes extremely low, and the ion beam 3 extracted from the ion source 1 can be effectively utilized as it is.

In the ion implanter according to the present invention, mass separation is not performed. In this regard, there may be employed an ion source arrangement in which a solid material corresponding to the sort of implant ions is heated and vaporized and then introduced into the ion source 1, and the vapor is used as a working gas for the ion source 1, whereby the purity of the ion beam to be extracted is enhanced. As the ion source arrangement, it is possible to use, for example, a coaxial microwave ion source as disclosed in Japanese Patent Application No. 55-152140.

Experiments were carried out using the setup shown in Figure 2, and the results will now be described. In this case, the ion source 1 used was a microwave ion source producing the high current ion beam 3, and the semiconductor wafer 5 was a Si wafer. The experiments were conducted on two kinds of deflection fields; one established by the air-core coil 6 alone, and the other established by the air-core coil 6 in which a magnetic material circuit was put. As a result, a P implant current of 50-100 mA being several times greater than in the prior art was obtained at the position of the semiconductor wafer 5. By shifting one end A of the semiconductor wafer 5 so as not to be irradiated with the ion beam 3 at the magnetic field B=O, the mixing of the neutral particles could be prevented, and the dose uniformity within the 4" Si wafer could be suppressed to the order of several % with the standard deviation (1 α). This is equivalent to the uniformity in the prior art ion implanter. In addition, since the ion implant current increased, the ion implant time shortened and higher wafer throughput was realized. Further, when the junction characteristics of the wafer subjected to the ion implantation were investigated, an I (current) - V (voltage) curve equivalent to that attained by the prior art ion implanter was obtained, and no problem was involved in the performance of a semiconductor device.

Now, Figure 3 shows the setup of an ion implanter according to another aspect of the present invention. In this embodiment, a static magnetic field is imposed by using a permanent magnet 8 instead of the air-core coil 6 shown in Figure 2. The ion beam 3 is deflected in the static magnetic field generated by the permanent magnet 8, depending upon the voltage of a D.C. power supply (V_DC) 12 for extracting the ion beam 3 from the ion source 1. At this time, the setup is so arranged that the ion beam 3 impinges upon the center of the semiconductor wafer 5. Of course, one end part A of the semiconductor wafer 5 has its position shifted so as not to be irradiated with neutral particles which propagate rectilinearly from the ion source 1. Subsequently, an A.C. voltage from an A.C. power supply (VAC) 11 is superposed on the D.C. voltage from the D.C. power supply (V_DC) 12, whereby the ion beam 3 is laterally oscillated on the semiconductor wafer 5 as indicated by arrows. Since the extraction voltage of the ion beam 3 fluctuates due to the application of the A.C. voltage, the extraction characteristics of the ion beam 3 change. In an experiment, however, the A.C. voltage V_AC was approximately 1 - 2 kV , when the D.C. acceleration-voltage V_DC was 30 kV. The voltage fluctuation to this degree scarcely changes the extraction characteristics (beam current and shape). In the figure, numeral 13 designates a plasma chamber, numeral ₁0 a negative electrode and numeral 9 a ground electrode, and these constitute the ion source 1. In this embodiment, the ion beam 3 is scanned with the center on the semiconductor wafer 5. Since, in the embodiment of Figure 2, the ion beam 3 is scanned with the center on the rectilinear propagation part of the beam of the ion source 1, it needs to be scanned greatly. In contrast, in the embodiment shown in Figure 3, the amplitude of the oscillations can be made small, and the utilization factor of the beam current is improved.

As set forth above, according to the present invention, ion implantation with a high current ion beam is realized without degrading the uniformity of the ion implantation, and the effect is remarkable in practical use.