Patent Number: 048067695
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the preferred embodiments of the present invention, an example of a prior art system is described with reference to the drawings, for comparison. FIG. 1 is a block diagram of an ion implantation system to which the present invention can be applied. In FIG. 1, the ion implantation system includes an outer cabinet 4, an inner cabinet 41, and a first power supply 48 connected between the outer cabinet 4 and the inner cabinet 41. The first power supply 48 provides typically a maximum voltage of 160 KV to 175 KV. In the inner cabinet 41 are provided an ion source 43, and an ion extraction electrode 44. A second power supply 45 is provided between the ion source 43 and the ion extraction electrode 44 and provides typically a voltage of approximately 25 KV to 40 KV, to cause the emission of ion beams from the ion source 43. The emitted ion beams are introduced into an analysis magnet tube 46 installed in the inner cabinet 41. When a magnetic flux B is directed thereto as shown in the drawing, light mass ions i.sub.l and heavy mass ions i.sub.h, both contained in the ion beams, are impinged on an inner wall of the analysis magnet wall 46, as shown in the drawing, and are absorbed thereat. As a result, ions i having a predetermined mass are emitted from the analysis magnet tube 46, and a speed thereof is accelerated at an accelerator 42. A low potential side of the power supply 45, the analysis magnet tube 46, and an entrance side of the accelerator 42 are commonly connected to the inner cabinet 41. The voltage of the accelerator 42 is provided by the voltage of the power supply 48, the accelerator 42 accelerating the input ions i by the electric field provided by the above voltage. The accelerated ions are introduced into an exposure chamber 5. In the implantation process, atmosphere in an inner space of the exposure chamber 5 is pumped out to produce a vacuum therein. A target mechanism is placed in the inner space prior to this pumping. The operation for ion implantation of semiconductor wafers is described below. FIG. 2 is a sectional view of the target mechanism 3 of the prior art. The target mechanism 3 includes a support shaft 32 of aluminum (Al) or stainless steel and a target disk 31 of Al mounted thereon. A plurality of semiconductor wafers 2 to be implanted are mounted on a surface of the circumference of the target disk 31. The target mechanism 3 is placed in the exposure chamber 5, which is brought to a vacuum condition, in the direction as shown by the broken line arrow in FIG. 1, and is rotated around a rotation center line 0--0'. Ion implantation of the semiconductor wafers 2 mounted on the target disk 31 is achieved sequential exposure to the ion beams ION. The target mechanism 3 can be moved in a vertical direction V--V' in the exposure chamber 5, ensuring an ion implantation of the whole surface of the semiconductor wafers 2. After completion of the ion implantation, the target mechanism 3 is extracted from the exposure chamber 5. At this time the target mechanism 3 is rotated by 9020 , as shown by a broken line in FIG. 1, to bring the wafer mounting surface of the target disk 31 to the horizontal plane. The target disk 31 is then removed from the support shaft 32 by releasing holding claws 33. A new target disk 31 on which untreated semiconductor wafers 2 are already mounted is then mounted on the support shaft 32. The target mechanism 3 with the new target disk 31 is inserted into the exposure chamber 5, and the ion implantation is carried out for the untreated semiconductor wafers 2. The ion implanted semiconductor wafers are detached from the target disk 31 removed from the support shaft 32 outside the exposure chamber 5, and thereafter, untreated semiconductor wafers are mounted on the removed target disk 31. The above disk exchange operation can be applied in the present invention. The problems of the prior art are now discussed in detail. Assuming an implantation energy of 80 KeV and a high ion implantation dosage of approximately 1.times.10.sup.16 cm.sup.-2, the beam current should be 10 mA, and the beam power should be 800 W. Also, assuming a permanent implantation as for a theoretical analysis is carried out under the above condition, the temperature of the semiconductor wafer may rise to approximately 180.degree. C., giving a temperature difference of approximately 17.degree. C. between the wafer and the target disk, and a temperature difference of approximately 160.degree. C. between the target disk and the support shaft, etc. The above temperature rise depends on the beam power. In practice, the implantation time is approximately 1 to 30 minutes and given a diameter of the target disk of 70 cm, the actual temperature of the wafer may rise by approximately 120.degree. C. to 130.degree. C. during an usual implantation time. This temperature rise will damage resists on the semiconductor wafers, because the temperature tolerance of the resists is approximately 100.degree. C. In addition, ion implantation at such high temperatures has an adverse affect on the quality of the wafers, because of the difficulty of lattice recovery of the wafer after implantation. An embodiment of a target mechanism 1 of the present invention will be described with reference to FIGS. 3a to 3d. The ion implantation system of this embodiment is similar to that in FIG. 1, except for the target mechanism 1. The target mechanism 1 includes a target disk 11 of Al, a support 12 of Al or stainless steel, and a thermal contact means such as a silicon (Si) rubber sheet 13 inserted between the target disk 11 and the support 12. Semiconductor wafers 2 to be implanted are mounted on a surface of the target disk 11 opposite to the surface of the target disk 11 in contact with the Si rubber sheet 13. In FIG. 3a, the structure of the support 12 includes a first shaft 12-1, a base 12-2, and a second shaft 12-4. A cavity 12-3 is provided in the base 12-2, and a hole 12-5 communicating with the cavity 12-3 is formed in the second shaft 12-4. The support 12 can be rotated in a direction A with respect to a center axis 0--0'. The structure of the target disk 11 comprises a disk 11-1 having a center hole through which the first shaft 12-1 is fitted, and claws 11-3 detachably holding the target disk 11 to the base 12-2 of the support 12. A thermal transportation means 11-2 is provided in the disk 11-1. The lower surface of the disk 11-1 and the top surface of the base 12-2 facing the lower surface of the disk 11-1 are precision-machined. However, from a microscopic view point, these surfaces are uneven, and thus, if brought into face-to-face contact, the contact therebetween may be considered to be a point contact. This point contact would limit the thermal transfer from the target disk 11 to the support 12, causing a temperature rise of the target disk 11, and accordingly, a temperature rise of the semiconductor wafers 2 during the ion implantation. The Si rubber sheet 13, as shown in FIG. 3b, is inserted between the lower surface of the disk 11-1 and the top surface of the base 12-2, and functions as a contact means to cause a perfect face-to-face contact between those surfaces. As a result, the above prior art temperature difference between the target disk and the support of a theoretical value of approximately 160.degree. C. can be reduced to a theoretical value of approximately 40.degree. C. The Si rubber sheet 13 has a thermal transportation coefficient greater than 20 mW/cm.sup.2..degree. C. FIG. 3c is a plan view of the disk 11-1 having the wafers 2 mounted thereon, taken along a line H.sub.1 --H.sub.1 ' in FIG. 3a. Eight wafers 2 mounted around the circumference of the disk 11-1 are rotated together with the support 12 in the direction A during the implantation. FIG. 3d is a plan view of the disk 11-1 and the thermal transportation means 11-2 provided therein, taken along a line H.sub.2 --H.sub.2 ' in FIG. 3a. The thermal transportation means 11-2 includes eight heat pipes 11-2-1 to 11-2-8 provided radially with respect to the rotation center. The heat pipes 11-2-1 to 11-2-8 are installed independently from each other, and each heat pipe is provided with a cavity extending along the longitudinal direction and webs or slots on an inner wall thereof. A cooling medium, such as water, a hydrocarbon fluoride gas e.g., "Freon" (trade name) or ethanol, is inserted in the cavity. The principle of the heat pipe is widely known. When the ion beam exposure causes a rise in the temperature of the wafers 2, the cooling medium at the circumferential edges of the heat pipes beneath the wafers 2 is also heated. Accordingly, a thermal convection is produced between the high temperature portions, i.e., the circumferential edge of the heat pipes, and low temperature portions, i.e., a portion adjacent to the rotation center, resulting in a high speed flow of the cooling medium between the high temperature portions and the low temperature portion through the webs or the slots in accordance with a capillary action, and accordingly, a high speed transporting of thermal energy from the high temperature portions to the low temperature portion. Thus, the thermal transportation means 11-2 contributes to a lowering of the temperature of the wafers 2. Further, a cooling medium, for example, water, is circulated through the cavity 12-3 of the base 12-2, which accelerates the cooling of the wafers 2. Namely, the thermal energy at the wafer 2 is distributed by the thermal transportation means 11-2, and the distributed thermal energy is forcibly transferred to the base 12-2 force-cooled from the inner wall of the cavity 12-3 in the base 12-2. In addition, during the ion implantation, the target disk 11-1 is rotated at a high speed, for example, 1000 rpm, and therefore the cooling medium at the center of the target disk 11-1 is forcibly moved to the circumference thereof, where the temperature is high, accelerating the thermal transportation. According to the embodiment, the ion implantation process is carried out under the following conditions: Beam power: 800 W PA1 Implantation time: approximately 15 minutes PA1 Size of Si rubber sheet: 1000 cm.sup.2 PA1 Cooling medium: Freon gas This results in the temperature at the wafers being approximately 80.degree. C. This temperature satisfies the requirements necessary for the protection of the resist on the wafer and of the wafer quality. The thermal contact means 13 should have the characteristics of good contactability with metal, high thermal conductivity, and stability in vacuum conditions. Accordingly, the thermal contact means 13 can be a polyfloraethylene film, e.g., "Teflon" (trade name), an RTV (Room Temperature Vulcanization) Si rubber, an Indium (In) film, etc., instead of the Si rubber sheet mentioned above. Freon gas is vaporized at a temperature higher than approximately 50.degree. C., and this vaporization will accelerate the thermal convection in the heat pipe. Preferably, Freon gas is used as the cooling medium in the heat pipe, rather than water having a boiling point of 100.degree. C.. Ethanol, which vaporizes at approximately 80.degree. C., is also preferable to water as the cooling medium. As shown by a broken-line circle in FIG. 3d, the heat pipes can be commonly connected at the center portion of the mechanism, where the temperature is low. After ion implantation, the heated (to approximately 80.degree. C.) target mechanism 1 is taken out of the exposure chamber 5, and the target disk 11 having the ion implanted wafers 2 mounted thereon is removed from the support 12. During the target disk exchange operation, preferably, the target mechanism 1 is forcibly water-cooled while outside exposure chamber 5 by supplying water to the hole 12-5 of the support 12. Another embodiment of the thermal transportation means will be described with reference to FIG. 4. FIG. 4 is a plan view of a second variety of heat pipes 11-2-1`to 11-2-8', taken along the line H.sub.2 -H.sub.2 ' in FIG. 1 and corresponding to FIG. 3d. In FIG. 4, a circumferential end of each heat pipe, above which the wafer to be ion-implanted is mounted, is bent to form an L-shape, to increase an inner space to which the cooling medium is inserted and from which the heat is removed. The heat pipes 11-2-1' to 11-2-8' can be replaced by boxes drilled in the mechanism per se. Still another embodiment of a target mechanism of the present invention will be described with reference to FIG. 5. FIG. 5 is a sectional view of the target mechanism and corresponds to FIG. 3a. In FIG. 5, an Si rubber sheet 13' as the thermal contact means is inserted not only between the lower surface of the target disk 11-1 and the upper surface of the base 12-3 but also between the first shaft 12-1 and an inner wall of the target disk 11-1, thereby increasing the thermal contact area. Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in this specification, except as defined in the appended claims.