1. Field of the Invention
The present invention relates to a method for operating an ion source for use with an ion beam irradiation apparatus such as an ion implantation system, and the ion beam irradiation apparatus for practicing the operation method, and more particularly to means for leading out an ion beam containing indium ions from an ion source stably.
2. Description of the Related Art
In the fields of semiconductor manufacture, indium ion has drawn a great deal of attention.
In the publication of Japanese Patent Unexamined Publication No. Hei.3-13576 (JP-A-3-13576), a method for leading out an ion beam containing indium ions from the ion source has been disclosed in which after a heating furnace (carbon container) containing indium iodide is preheated up to temperatures from 100xc2x0 C. to 200xc2x0 C., the heating furnace is further heated to temperatures from 300xc2x0 C. to 500xc2x0 C. to produce a vapor from indium iodide, and this vapor is introduced through a vapor conduit tube into a plasma production vessel (discharge chamber) to ionize the vapor by arc discharge.
With the above method, because the melting point of indium iodide is 210xc2x0 C., indium iodide is completely molten within the heating furnace to become a liquid state, if the heating furnace is operated in the temperature range from 300xc2x0 C. to 500xc2x0 C. Consequently, liquefied indium iodide is deposited in mucus on the heating furnace, the plasma production vessel and the vapor conduit tube connecting both of them to contaminate their inside, or clog the vapor conduit tube, so that the ion beam can not be led out stably.
Owing to the contamination, the replacement or cleaning of the heating furnace, the plasma production vessel and the vapor conduit tube is required frequently, for example, every time the operation of ion source is stopped, which is impractical.
Thus, it is an object of this invention to provide means for leading out an ion beam containing indium ions from an ion source stably, while preventing the contamination of the heating furnace, the vapor conduit tube and the plasma production vessel.
According to this invention, there is provided a method for operating an ion source comprising a heating furnace for heating a solid material to produce a vapor, and a plasma production vessel for producing a plasma by ionizing the vapor supplied through a vapor conduit tube from the heating furnace, wherein the solid material is indium fluoride, and an ion beam containing indium ions is led out by producing a plasma within the plasma production vessel, while the temperature of the heating furnace is kept in a range from 450xc2x0 C. to 1170xc2x0 C., and below a temperature of the plasma production vessel.
Namely, supposing that when an ion beam is led out, the temperature of the heating furnace is T1[xc2x0 C.], and the temperature of the plasma production vessel is T2[xc2x0 C.], the following relation holds to lead out the ion beam.
[Numerical Expression 1]
450xe2x89xa6T1 less than 1170 and T1 less than T2 (unit: xc2x0 C.) 
According to the experiments, an ion beam containing indium ions could be led out by elevating the temperature T1 of the heating furnace to 450xc2x0 C. or higher to produce a vapor of indium fluoride. When the temperature T1 is below 450xc2x0 C., the vapor of indium fluoride hardly occurs, whereby few indium ions can not be led out, which is impractical.
And since the melting point of indium fluoride is 1170xc2x0 C., and the temperature T1 of the heating furnace is kept below this melting point in case of leading out the ion beam, the vapor is produced but indium fluoride is not molten. Accordingly, it is possible to prevent liquefied indium fluoride from sticking to the heating furnace, the plasma production vessel, and the vapor conduit tube connecting them, thereby causing the contamination of them or clogging of the vapor conduit tube.
Further, since the temperature T1 of the heating furnace is kept below the temperature T2 of the plasma production vessel when leading out the ion beam, in other words, the temperature T2 of the plasma production vessel is higher than the temperature T1 of the heating furnace, the vapor conduit tube connecting the heating furnace and the plasma production vessel gives rise to a temperature gradient in which the temperature rises towards the plasma production vessel. Owing to this temperature gradient, the vapor produced within the heating furnace can be prevented from recoagulating in the vapor conduit tube or plasma production vessel, thereby it can be prevented from contaminating the plasma production vessel or vapor conduit tube or clogging the vapor conduit tube. This effect can be obtained without too great difference between the temperatures T1 and T2. For example, this temperature difference may be as small as about 10xc2x0 C. to 20xc2x0 C.
As a result, an ion beam containing indium ions can be led out stably from the ion source, while preventing the contamination of the heating furnace, the vapor conduit tube and the plasma production vessel.
In the publication JP-A-3-13576, there was a description that indium fluoride had the problem, and was unsuitable to produce indium ions. However, according to this invention, an ion beam containing indium ions can be led out stably, while preventing the contamination of the plasma production vessel, as previously described. Accordingly, the indium ion beam can be utilized stably.
Note that a control unit for keeping the temperature T1 of the heating furnace when leading out the ion beam from the ion source in accordance with the Numerical Expression 1 may be provided. With such control unit, the ion source can be operated with labor-saving and automatically.
The ion beam is preferably led out by keeping the temperature T1 of the heating furnace in the range from 450xc2x0 C. to 1170xc2x0 C. after the heating furnace and the plasma production vessel are preheated in a vacuum atmosphere at temperatures from 250xc2x0 C. to 450xc2x0 C. The vacuum atmosphere is used to prevent the reaction with oxygen or water content in the residual gases. This preheating may be performed for the heating furnace and the plasma production vessel at the same time, or separately. Indium fluoride may be put within the heating furnace at the time of preheating, or after preheating.
By preheating at temperatures of 250xc2x0 C. or greater, the water content can be removed almost completely in short time from inside the heating furnace and the plasma production vessel, and further indium fluoride, if any. At the temperatures below 250xc2x0 C., it takes too much time to effect the preheating and it is difficult to remove the water content completely. As a result of this preheating, it is possible to avoid production of hydrofluoric acid or hydrogen fluoride due to reaction between indium fluoride and the water content. Therefore, the contamination of the plasma production vessel or the vacuum vessel containing the ion source can be prevented more securely. Though hydrofluoric acid or hydrogen fluoride is a strong acid substance that is unfavorable for a vacuum pump or the human body, the production of hydrofluoric acid or hydrogen fluoride can be prevented.
And by preheating at temperatures below 450xc2x0 C., evaporation of indium fluoride at the time of preheating can be prevented. If indium fluoride is evaporated at the time of preheating, there is the possibility that the plasma production vessel or the vacuum vessel may be contaminated. However, in the present invention, contamination of the plasma production vessel or the vacuum vessel can be prevented.
Halides of indium include sulfur bromide, indium chloride, and indium iodide, besides indium fluoride, but because they have lower melting points than indium fluoride, the temperature of the heating furnace immediately rises above those melting points to obtain the vapor amount required to lead out an ion beam. Also, owing to preheating at temperatures as previously described, a considerable molecular weight has already evaporated. As a result, the plasma production vessel or the vacuum vessel is contaminated by bromine, chlorine or iodine. The vapor conduit tube may be clogged. On the contrary, among indium halides, indium fluoride has the highest melting point, and does not cause the problem as above described. Accordingly, indium fluoride is most preferable. In this way, the inventors have found that indium fluoride is optimal to lead out indium ions.
Indium fluoride is hydrous or anhydrous, and it is preferable to use hydrous indium fluoride. This is more beneficial to prevent the contamination of the heating furnace or plasma production vessel, or the production of strong acid substance.
Also, indium fluoride maybe preheated within the heating furnace at the same time of preheating the heating furnace. In this way, the water content can be removed more completely to be more beneficial to prevent the contamination. And if the preheating is performed at the same time, the operation is simplified. Alternately, indium fluoride which has been heated at temperatures from 250xc2x0 C. to 450xc2x0 C. in other place to remove the water content beforehand may be brought into the heating furnace while keeping it away from the water content. In this way, the starting time of the ion source can be shortened.
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