Ion implantation apparatus

Abstract: An ion implantation apparatus includes an ion source (1) comprising an arc chamber (2) generating ions and a drawing electrode (3) drawing ions from the arc chamber (2), a mass separator (25) transporting only ions necessary for implantation in the drawn ions to a material to be implanted by ions, an ion implantation chamber (5) in which the material to be implanted by ions are arranged and controlling means (10) for automatically controlling the distance between the arc chamber (2) and the drawing electrode (3) by quantization in accordance with a theoretical calculation using conception of normalization perveance at a beam drawing part in accordance with setting of the kind of ions to be implanted, an accelerating voltage and an amount of current, and current density distribution.

Description: The present invention relates to an ion implantation apparatus as claimed in the preamble of claim 1. Such kind of ion implantation apparatus is already known from: Nuclear Instruments and Methods in Physics Research, Vol. B 37/38, 1989, pages 612 - 615. Said known ion implantation apparatus comprising: an ion source comprising an arc chamber generating ions and a drawing electrode drawing ions from said arc chamber; a mass separator transporting only such ions of the drawn ions which are necessary for implantation to a material into which ions shall be implanted; an ion implantation chamber in which said material is arranged; and a controlling means for controlling the amount of current and the current density distribution of the ion beam provided by said ion source; wherein said controlling means is arranged to adjust said amount of current and the current density distribution by controlling the distance d between said arc chamber and said drawing electrode. Figure 4 is a diagram showing another example of a conventional ion implantation apparatus disclosed in, for example Japanese Patent Laid Open Gazette No. 62-241247. In figure 15, an ion beam 22 is applied to an ion implantation chamber 21 and corrected by an ion beam correcting lens 27. Thereafter, the corrected ion beam 22 passes through an ion beam measuring means 33 which functions as a sensor of a controller 32 for controlling the ion beam correcting lens 27. Then, the ion beam 22 passes through a Faraday cage 34 and is implanted in a disk 24 arranged on a rotary disc 23. This rotary disc 23 is connected to an ion beam ampere meter 26. Then, operation thereof will be described in detail hereinafter. The ion beam 22 is applied from the left side of figure 15 to the ion implantation chamber 21 and then implanted in the wafer 24 arranged on the rotary disc 23. The disc 23 serves as a bottom of the Faraday cage 34 and an amount of ion beam applied to the Faraday cage 34 can be measured by the ion beam ampere meter 26 connected to the disc 23. The ion beam 22 passes through the ion beam correcting lens 27 and the ion beam measuring means 33 just before it reaches the Faraday gage 34. Figure 5 is a view showing the ion beam measuring means 33 seen from the side of the wafer 24. Referring to figure 16, a probe 29 formed of a high melting point metal is driven by a motor 40 so as to be rotated by an appropriate angle and vertically crosses the ion beam 22, whereby a current flows through the ampere meter 41. A potentiometer and a rotary encoder are attached on the motor 40 and then a signal obtained by the ampere meter 41 is monitored by an oscilloscope with appropriate synchronization, with the result that waveform shown in figure 6 is obtained. Thus, a vertical length, a position, and an approximate configuration of the ion beam can be obtained from the waveform and positions a and b in figure 6. The computer 32 receives this information and controls the ion beam correcting lens 27, whereby the ion beam 22 is corrected. However, since the ion beam correcting lens 27 is inserted in front of the rotary disc 23 in the above conventional ion implantation apparatus, size of the apparatus is increased and a structure thereof becomes complicated, with the result that production cost is increased. In addition, when the beam correcting lens 27 is inserted, a beam transporting distance is increased, so that the number of ions which collide with neutral particles and scatter during its transportation is increased. As a result, transporting efficiency of the beam is reduced. In order to prevent it, the correcting lens has to be dispensed with so that the transporting distance of the beam may be short, particularly in a large current type ion implantation apparatus which makes great account of the transporting efficiency of the beam, like a predeposition type ion implantation apparatus (disclosed in, for example "Electron and Ion Beam Handbook") which is the main current in the present. However, if there is no ion beam correcting lens, it is difficult to appropriately control a current and current density of the ion beam. Ion implantation apparatuses not requiring an ion correcting lense, but which can anyhow appropriately control the current and current density of an ion beam are known from Patent Abstracts of Japan, Vol. 13, No. 150 (E-742) (3498) (Abstract for JP-A-63 310549) and from Patent Abstracts of Japan, Vol. 12, No. 115 (E-599) (2962) (Abstract for JP-A-62 243231). The appartus described in the former reference comprises a current measuring means and shims which are rotated by a shim driving control in a way to adjust the measured ion current to a specified value. In the apparatus of the latter reference, a certain ion beam current is set and then the ion beam current density is measured at a plurality of points. When a certain current density is desired when operating the apparatus, the corresponding ion beam current is set. It is the object of the invention to provide an ion implantation apparatus capable of controlling the amount of current and current density distribution of an ion beam with a simple structure having no ion beam correcting lense. The ion implantation apparatus of the present invention is defined by the teaching of appended claim 1. In this apparatus, the controlling means is arranged to adjust the distance between the arc chamber and the drawing electrode. According to the invention, this control is done in a feedback manner by performing a measurement with an ion beam measuring means and by adjusting the distance in a way to keep said measuring signal at values not exceeding a set value. When feedback control using the ion current density distribution measuring apparatus is performed so that a maximum amount of implantation current which does not exceed a set maximum current density may be obtained, it is possible to control the ion beam with high productivity and yield (low electrostatic breakdown coefficient). Figure 1 is a view showing a structure of an ion implantation apparatus of the present invention; Figure 2 is a view showing the relation between a distance d between an arc chamber and a beam drawing electrode, a current and a current density distribution of a beam; Figure 3 is a view showing an accumulated beam current distribution in the disc scanning direction when d is i through iv; Figure 4 is a view showing an example of an ion implantation apparatus comprising a conventional ion beam correcting lens; Figure 5 is a view showing ion beam measuring means of the apparatus in figure 4; and Figure 6 is a view showing a current density distribution detected by the ion beam measuring means in figure 5 and monitored by an oscilloscope. Embodiments of the present invention will be described in detail in reference to the drawings hereinafter. Figure 1 is a view showing a structure of an ion implantation apparatus of the present invention. In figure 1, an ion source 1 comprises an arc chamber 2 generating ions and a drawing electrode 3 drawing ions from the arc chamber 2. A mass separator 25 takes out an ion beam 11b having required ions from an ion beam 11a drawn by a drawing electrode 3 and then the ion beam 11b is applied to an ion implantation chamber 5. A rotary disc 6 is provided in the ion implantation chamber 5 and a wafer 28 is put on the rotary disc 6. An ion beam measuring means 7 is provided at the lower part of the rotary disc 6. A controlling means 10 controls a drawing electrode driving means 8 in accordance with a data obtained by the ion beam measuring means 7. Then, operation thereof will be described in detail hereinafter. Ions generated from the arc chamber 2 in the ion source 1 are drawn by the beam drawing electrode 3 and then radiated as an ion beam 11a. The ion beam lla passes through a mass separator 25 and then an ion beam llb having required ions is only applied to the ion implantation chamber 5. The ions applied to the ion implantation chamber 5 are implanted in the wafer 28 put on the rotary disc 6 in the ion implantation chamber 5. The ion beam measuring means 7 for measuring an amount of current and a current density distribution of the ion beam is provided at the lower part of the rotary disc 6 corresponding to the position to which the ion beam 11b is applied. Data obtained by the ion beam measuring means 7 is received and processed by the controlling means 10, whereby the drawing electrode driving means 8 is controlled. As a result, the distance between the arc chamber 2 and the drawing electrode 3 is adjusted and the ion beams 11a and 11b are corrected. It is considered that the ion beam can be measured with highest precision when, for example a method of scanning an ion beam detector of a multihole Faraday type is used as the ion beam measuring means 7. However, the same type as that of the ion beam measuring means 23 in a conventional apparatus shown in figure 4 may be used. Referring to figure 1, the ion beam is drawn out of the arc chamber 2 by the beam drawing electrode 3. Since the ion beam consists of positive electric charges, when especially an ion density is high, the ion beam scatters by repulsive force of ions. This phenomenon is generally called "divergence caused by a space charge of an ion". First, control (a primary control) is performed in disregard of the divergence of beam caused by the space charge. A distance D between a beam drawing electrode 3, which provides a minimum beam divergence angle when the divergence of beam caused by the space charge is not regarded, and an arc chamber 2 is found by a theory and a numeral expression described by Junzo Ishikawa, in "Ion Source Engineering", pp.178-186 published by Ionics Kabushiki Kaisha as follows. D = 2 1/2 SC ε0 9I (2αV 3 ) 1/4 where S is an area of a drawing hole, C is a constant which is 0.6 when the drawing hole is a circle and 0.68 when it is a slit, ε 0 is a dielectric constant of a vacuum, α is an effective specific charge, I is a drawing current and V is a drawing voltage. The above effective specific charge α is expressed as follows. α -1/2 I = Σ i 1/2 m i Z i e I i where m i , Z i and I i are mass, ionization number and an amount of current of each ion comprised in I respectively and e is a prime charge. S and C in the above equation are constants decided depending on each apparatus and V and I are set as implantation conditions. If α in accordance with I is obtained by an experiment or the like, controlling means 10 calculates a value of D by several arithmetic operation and the position of the beam drawing electrode 3 is adjusted by driving means 8, whereby the beam drawing operation when the divergence caused by the space charge can be disregarded can be implemented with the best convergence. The above is the primary control. In addition, α can be easily found by an experiment using a mass separator 25. The drawn ion beam passes through the mass separator 25, whereby a beam having required ions is only separated. Thereafter, it is applied to the wafer 28 on the rotary disc 6 arranged in the ion implantation chamber 5. The ion beam measuring means 7 measures an amount of current and a current density distribution of the beam radiated to the wafer 28. At this time, if divergence of the beam caused by the space charge can be disregarded, the measured amount of current shows a maximum value. However, when an effect of the space charge can not be disregarded and the amount of current is not maximum or when there is considerable variation in the current density even if the amount of current is maximum and this variation should be corrected, the following more precise control, is performed as follows. Figure 2 is a graph showing d (the distance between the arc chamber and the drawing electrode) dependency of an amount of current and a current density distribution of an ion beam which are measured by the ion beam measuring means 7. The horizontal axis in figure 2 shows that d is increased at equal intervals in the order of i, ii, iii and iv. Figure 3 shows a current density distribution of an accumulated beam in the disc scanning direction at each position of i through iv. As can be seen from figures 2 and 3, variation of d has the same effect to the beam as that of a convergent lens. Therefore, when the value of d is varied in the iv direction and then the controlling means 10 performs feedback control, the amount of current measured by the ion beam measuring means 7 may be maximum. In addition, when the current density distribution is made uniform, the value of d is varied in the i direction and the controlling means 10 performs feedback control while monitoring the current density distribution of the ion beam measuring means 7. As a monitoring parameter at this time, for example a value of a maximum current density or maximum current density / average current density is set and then a maximum amount of current which does not exceed that set value can be obtained by the controlling means 10. As described above, since the ion beam can be adjusted without the ion beam correcting lens, the beam transporting distance is not increased. Therefore, the amount of current and the current density distribution of the ion beam can be automatically controlled, while the beam transporting efficiency is not reduced. It is needless to say that the above control can be applied to a serial control in which continuous control is implemented in real time or to batch process in which an ion beam is adjusted at appropriate intervals of time. Although the above control is performed by monitoring the ion beam after mass separation, the beam before the mass separation may be monitored. At this time, especially when the current density distribution is controlled, it is necessary to previously find correlation of the current density distributions of the ion beam before and after the mass separation by calculation or the like.

Claims: An ion implantation apparatus comprising an ion source (1) comprising an arc chamber (2) generating ions and a drawing electrode (3) drawing ions from said arc chamber (2); a mass separator (25) transporting only such ions of the drawn ions which are necessary for implantation to a material into which ions shall be implanted; an ion implantation chamber (5) in which said material is arranged; and a controlling means for controlling the amount of current and the current density distribution of the ion beam provided by said ion source; wherein said controlling means (10) is arranged to adjust said amount of current and the current density distribution by controlling the distance (d) between said arc chamber (2) and said drawing electrode (3), characterized by an ion beam measuring means (7) outputting to said control means a measuring signal in dependence of the amount of current and the current density distribution of said ion beam, in which apparatus said control means is arranged to control the distance (d) in a way to keep the measuring signals at values not exceeding a set value. An ion implantation apparatus in accordance with claim 1, wherein said set value is a maximum current density. An ion implantation apparatus in accordance with claim 1, wherein said set value is the value maximum current density/average current density.

Inventors: FUJISHITA NAOMITSU, KATOH SUSUMU, NAITOU MASAO, NAKANISHI TETSUYA, NOGUCHI KAZUHIKO, SASAKI SHIGEO, SHONO KAZUHIRO, TANJO MASAYASU, FUJISHITA, NAOMITSU, KATOH, SUSUMU, NAITOU, MASAO, NAKANISHI, TETSUYA, NOGUCHI, KAZUHIKO, SASAKI, SHIGEO, SHONO, KAZUHIRO, TANJO, MASAYASU, FUJISHITA, NAOMITSU, C/O MITSUBISHI DENKI K. K., KATOH, SUSUMU, C/O MITSUBISHI DENKI K. K., NAITOU, MASAO, C/O NISSHIN DENKI KABUSHIKI KAISHA, NAKANISHI, TETSUYA, C/O MITSUBISHI DENKI K. K., NOGUCHI, KAZUHIKO, C/O MITSUBISHI DENKI K. K., SASAKI, SHIGEO, C/O MITSUBISHI DENKI K. K., SHONO, KAZUHIRO, C/O MITSUBISHI DENKI K. K., TANJO, MASAYASU, C/O NISSHIN DENKI K. K.
Applicants: MITSUBISHI ELECTRIC CORP, NISSHIN DENKI KABUSHIKI KAISHA, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1990
Main classification: H01J37
IPC codes: H01J37