ION SOURCE

Abstract: An ion source equipped with an ion beam exit slit (7) for extracting ions from plasma generated in feed gas introduced into a discharge chamber (5), and with a gas inlet or inlets (110) for introducing the feed gas into the discharge chamber (5) in the proximity of the ion beam exit slit (7). Ion extraction can be made stably without any deposit on the ion beam exit slit (7) even when a boron halide is used as the feed gas. The effect of the ion source can be further enhanced by adding oxygen, hydrogen or gas of an oxygen-containing compound to the feed gas, and by using a microwave.

Description: This invention relates to an ion source for an ion implanter, an ion milling machine or the like as described in the first part of claim 1, and more particularly to an ion source suitable for obtaining a stable ion beam for an extended period of time, and further particularly to a microwave ion source suitable for obtaining a B⁺ ion beam. An ion source of this type has been described in Ep-A- 0.101.867 Figure 1 is a schematic illustration of the structure of a conventional microwave ion source. The microwave ion source consists of rectangular waveguides 2a and 2b as the waveguide for propagating a microwave, discharge electrodes 4 constituting a ridged waveguide structure, a discharge chamber 5 made of boron nitride and disposed between the ridged waveguides, and extraction electrodes 8a, 8b and 8c which extract the ion beam 21. An axial magnetic field generated by the excitation of a coil 13 is applied to the discharge chamber 5, and a feed gas for discharge is introduced into the discharge chamber 5 through a gas introduction pipe 6. Figure 2 shows in detail the discharge chamber and portions near the chamber. Figure 3 is a sectional view of the discharge chamber 5 and portions near the chamber, and is useful for explaining the discharge chamber 5, a gas inlet 10 and an ion beam exit slit 7. In the drawings, reference numeral 1 represents a microwave generator; 3 is a microwave introducing flange; 5a is a lining of the discharge chamber 5; 7a is a portion near the ion beam exit slit; 11 is a dielectric filler; and 12 is an insulator. PH₃ (phosphine), AsH₃ (arsine) or the like as a hydride is used as the feed gas in order to obtain the beam of P⁺ (phosphorus) ion, As⁺ (arsenic) ion beam or the like that is used for the ion implantation for a semicondcutor in the ion source shown in Figure 1. In this case, the P⁺ or As⁺ ion beam can be extracted stably for an extended period of time. If a BF₃ gas is introduced in order to obtain B⁺ ion beam necessary for the ion implantation for a semiconductor, however, two problems develop, which have made it difficult so far to obtain stably a high current ion beam for an extended period of time: (1) deposit at the ion beam extraction opening portion (i.e., ion beam exit slit 7); (2) deposit inside the discharge chamber 5. If the deposit of the item (1) occurs, the opening area is reduced so that the extracted beam current drops. In a conventional ion source, such as, for example, the ion source described in the already cited EP-A-0 101 867, the gas inlet 10 is positioned near the center of the discharge chamber 5. Therefore, if plasma is generated by a halide gas such as BF₃, boron nitride (BN) constituting the lining 5a of the discharge chamber 5, particularly its portion near the gas inlet 10, is etched, and an etching product (most of which is BN) precipitates at the other portions particularly at the ion beam exit slit 7. In consequence, the slit width is recuced, and the current drops eventually. According to an experiment which uses the BF₃ gas as the feed gas, the area of the opening portion is reduced almost by half in the course of the operation of the ion source for about four hours. When the deposit is hit by the plasma or ion beam, it is peeled off from the ion beam exit slit 7 and flies sometimes into the space to which an electric field for extracting the ions is applied. The peeled matter strikes the electrode 8b and generates secondary electron emission, which in turn, generates an abnormal discharge between the electrode 8a to which a high positive voltage is applied and the electrode 8b to which a high negative voltage is applied. This results in unstability of the ion source. The deposit is often peeled off in the case of (2), too, and the plasma state becomes unstable, and, in addition, an abnormal discharge develops between the electrodes in the same way as in the case of (1), and reduces the stability of the ion source. If the plasma is generated in a conventional ion source with a halide gas such as BF₃, the extracted ion beam current decreases and the stability of the ion source drops, as described above. It is believed that the reason why the deposit occurs when the BF₃ or BCℓ₃ gas is used is that since the fluorine or chlorine atom generated by microwave discharge is extremely active chemically, it corrodes and dissociates boron nitride (BN) that constitutes the lining 5a of the discharge chamber 5. As a matter of fact, when the deposit is physically analyzed, it is determined to be boron nitride. In order to prevent the occurrence of such a deposit, it is effective to constitute the discharge chamber in a thermally isolated structure, and to raise its operation temperature so as to thermally dissociate or evaporate the deposit. However, the temperature can not be raised beyond a certain limit due to the structural liminations of the discharge chamber (generally, the approximate upper limit is 800 to 900°C), and it has been difficult in practice in the past to restrict the quantity of deposit to a level which presents no practical problems. As a prior art reference disclosing the state of art in this field, mention can be made of Japanese Patent Laid-Open No. 132754/1981. The object of the present invention is obtained with an ion source as defined in claim 1. The dependent claims set out particular embodiments of the invention. SUMMARY OF THE INVENTION The present invention is directed to provide an ion source, particularly a microwave ion source, which can eliminate the problems with the prior art described above, which does not reduce the width of an ion beam ext slit even when a halide gas such as BF₃ or BCl₃ is used as a feed gas to be introduced into a discharge chamber, and makes it possible to stably extract an intended ion beam for an extended period of time. To accomplish this object, the ion source in accordance with the present invention is equipped with an gas inlet located within a determined distance from an ion beam exit slit for extracting ions from the plasma of the feed gas introduced into a discharge chamber. The effect of the present invention can be further enhanced by using, as the feed gas to be introduced into the discharge chamber, a mixed gas prepared by mixing at least one gas selected from the above-mentioned boron halide gases with at least one gas, which reacts with boron to form a boron compound, such as oxygen, hydrogen or oxygen-containing compound gas. If d is the distance between the gas inlet and the end surface of the discharge chamber on which the ion exit slit is disposed, d should be up to 2d' and preferably, is up to d', which is the distance between the wall of the protion of the discharge chamber at which the gas inlet is disposed and the wall opposing the former, and if the gas inlet is positioned closer to the ion beam exit slit,a better result can be obtained. In the structure described above in which the gas inlet is disposed in the proximity of the ion beam exit slit, the gas pressure is elevated in the proximity of the ion beam exit slit, and the portion of the exit slit is etched in the same way as the walls inside the discharge chamber, thereby making it possible to reduce the deposit onto the exit slit portion. The quantity of the gas which reacts with boron to form a boron compound and which is to be mixed with the halide gas is at least 0.1% of the halide gas in a pressure ratio, and the effect of the invention increases remarkably as reactive gas quantity increases. If the gas quantity is below 0.1%, the effect of the invention can not be observed. If the quantity of the mixing gas such as oxygen increases, the quantity of the intended ions contained in the extracted ion beam decreases in proportion to the mixing quantity, so that the upper limit of the mixing gas is determined in accordance with the object of use of the ion source. When the mixing gas is hydrogen, its mixing quantity is the same as the oxygen described above. Examples of the oxygen-containing compounds described already include CO, CO₂, NO, N₂O, SO₂ and H₂O. The mixing gas may be any one of the above-mentioned gases of hydrogen, oxygen and various compounds containing oxygen, but two or more of these gases may be mixed and be used as the mixing gas. Addition of the mixing gas such as oxygen to the gas to be introduced into the discharge chamber provides naturally the effect of the invention in combination with disposition of the gas inlet in the proximity of the ion beam exit slit, but when it is used alone, addition of the mixing gas stabilizes ion beam extraction. Microwaves in a magnetic field are generally used as plasma generation means, and can also be used for the ion source of the present invention. Besides the application to the ion source plasma, the field of utilization of the plasma generated by microwave discharge in the magnetic field includes etching of Si by plasma. It is generally known that the etching rate drops due to the mixture of oxygen depending on the microwave plasma etching. It is known that when the B⁺ beam is extracted by introducing BF₃ into the microwave ion source, relatively great quantities of ions of oxygen-containing compounds such as BO⁺, BOF⁺ or the like are detected as being generated if H₂O as the residual gas inside a vacuum chamber is great. It can be understood from above that if O2 is positively mixed with the feed gas for the microwave ion source, deposition can be prevented because etching of BN as the material of the discharge chamber can be restricted, and at the same time the dissociated BN molecules change to BO⁺, BOF⁺ , and the like. Even if deposition occurs, it escapes in the form of BO⁺, BOF⁺ or the like, so that a remarkable reduction of the deposition rate can be expected. As an ion source for obtaining B⁺ ion beam of a mA class, an ion source utilizing low voltage arc discharge with a hot filmaent is known. BF₃ is also used for such an ion source. Since the hot filament is rapidly corroded by oxygen, operation of the ion source for an extended period can not be expected even if oxygen (O₂) is introduced in order to obtain a stable B⁺ beam. In this sense, introduction of O₂ is a method which can be characterizingly used for a microwave ion source not containing a hot filament. In the description above, oxygen is used as a typical example, but the effect of reduction of deposition can be obtained with those gases which react with boron and which readily form a boron compound, such as H₂ gas. Heretofore known technique or knowledge in the field of ion sources can be used for the ion source of the present invention for those matters which are not particularly described in this specification. Figure 1 is a schematic illustration showing the structure of a conventional microwave ion source; Figure 2 is a perspective view showing a discharge chamber and portions nearby; Figure 3 is a sectional view showing the discharge chamber and portions nearby; Figure 4 is a sectional view showing the discharge chamber and portions nearby in a microwave ion source in accordance with one embodiment of the present invention; Figure 5 is a sectional view taken along line A - A of Figure 4; Figure 6 is a schematic illustration showing the overall structure of the microwave ion source in accordance with one embodiment of the present invention; Figure 7 is a diagram showing the relation between the position of gas inlets and the deposition rate at an ion beam exit slit; Figures 8a and 8b are sectional and plan views showing the discharge chamber and portions nearby in the microwave ion source in accordance with another embodiment of the present invention, respectively; Figure 9 is a sectional view showing the discharge chamber and portions nearby in the microwave ion source in accordance with still another embodiment of the present invention; and Figures 10 and 11 are schematic illustrations showing the overall structure of the microwave ion source in accordance with other embodiments of the present invention, respectively. Embodiment 1: Figures 4 and 5 are sectional views, each showing the structure of a discharge chamber and portions nearby in a microwave ion source in accordance with a first embodiment of the present invention, and Figure 5 is a sectional view taken along line A - A in Figure 4. This embodiment uses microwave discharge in a magnetic field as means for generating plasma. The microwave is introduced into the discharge chamber 5 from the left side of Figure 4 through the space between the discharge electrodes 4. The magnetic field is applied in a direction crossing an electric field by the microwave, and the interaction of these fields generate the plasma of the feed gas inside the discharge chamber 5. Ion beam 21 is extracted from an ion beam exit slit 7 which is disposed on one end surface of the discharge chamber. The feed gas is introduced into the discharge chamber 5 through a plurality of gas inlets 110 (five inlets are shown in Figure 5) that are disposed in the proximity of the ion beam exit slit. As described earlier, the distance d between these gas inlets 110 and the end surface 120 of the discharge chamber, on which the exit slit 7 is disposed, should be smaller than 2d', and is preferably smaller than d', which is the distance between the wall surface, on which the gas inlets are disposed, and the wall surface of the discharge chamber 5 facing the former. When a halide gas such as BF₃ is used as the feed gas in this embodiment, the deposit to the ion beam exit slit 7 can be reduced because the inner walls of the discharge chamber 5 close to the gas inlets 110 and the surface exposed to the plasma at the ion beam exit slit 7 are also etched. The diameter of the gas inlet 110 is 2 mm, the length of the ion beam exit slit 7 is substantially the same as that of the discharge chamber, the thickness of the slit 7 is about 1 mm, and the discharge chamber 5 is 8 mm wide, 20 mm deep and 46 mm long. The same material is used for each portion as in the prior art device. That is, the discharge electrode 4 is made of stainless steel, the dielectric filler 11 is made of BN, the lining 5a of the discharge chmaber is made of BN, and carbon is used for the portion 7a in the proximity of the ion beam exit slit 7. Figure 6 is a schematic illustration showing the overall structure of the microwave ion source of this embodiment. Next, the disposition of the gas inlets in this embodiment will be examined in detail. Figure 7 is a diagram showing the relation between the position of the gas inlets 110 and the deposition quantity of the deposit at the ion beam exit slit. The abscissa represents d/d' ratio (R) wherein d' is the distance between the gas inlet and the inner wall of the discharge chamber facing the gas inlet and d is the distance between the end surface of the discharge cahmber on which the ion beam exit slit is disposed and the gas inlet, and the ordinate represents the deposition rate (mm/hr). The ion beam exit slit and the discharge chamber have the same structure as described already. The microwave power to be charged into the plasma is 800 W, the intensity of magnetic field is 5 x 10⁻²T (500 Gs) and the feed gas quantity expressed in terms of pressure is 1x10⁻¹ Pa, and these values are kept constant. A negative deposition rate indicates that the ion beam exit slit has been etched. As can be seen clearly from Figure 7, undesirable deposition occurs greatly in the region where R > 2. Within the range of 0 ≦ R ≦ 2, the effect of the present invention can be observed, and the smaller the value R, the smaller the deposition quantity. Particularly in the region where R ≦ 1, the operating life enough for the practical use as an ion source for ion implanter can be obtained. In other words, (implant current) x (implant time) = 40 mA·hr can be achieved. In accordance with the prior art, the operating life is at most 10 mA·hr. According to an experiment, life has been found to be more than 120 mA·hr. (the ion source can be used for more than 30 hrs at 4 mA) when R = 0.25. Embodiment 2: Figures 8a and 8b are sectional and plan views showing a discharge chamber and portions nearby in a microwave ion source in accordance with a second embodiment of the present invention. The feed gas is introduced into the discharge chamber in the same way as in the first embodiment shown in Figure 4. In this embodiment, the ion beam exit slit is divided into a plurality of units in a thermally isolated state so that the temperature of the portions of the ion beam exit slit 7, which are exposed to plasma, can be further raised. The ion beam exit slit 7 in this embodiment consists of three members 7a, 7b and 7c, whereby 7a is made of stainless steel, 7b is made of an electrically conductive composite material of electrically conductive BN and Ti, and 7c is made of carbon. By employing this split structure, the ion beam exit slit 7 is thermally isolated and consists of a plurality of units as shown in Figure 8b. This split structure raises the temperature of BN by thermally isolating the slit 7, and makes the slit 7 more readily etchable when the discharge gas passes therethrough. This embodiment can further enhance the etching effect of the ion beam exit slit 7 by the feed gas, and can eliminate the deposit on the ion beam exit slit 7. Embodiment 3: Figure 9 is a sectional view showing the structure of the discharge chamber of the microwave ion source and portions nearby in accordance with a third embodiment of the present invention. The feed gas is introduced into the discharge chamber through the gas inlet 110 disposed close to the ion beam exit slit 7 and through a gas inlet 110a which is additionally disposed on the surface at the depth of the discharge chamber 5 (on the surface on the left hand of Figure 9). The gas introduction pipes 6 and 6a communicating with these two gas inlets 110 and 110a have separate gas flow rate control valves 9a and 9b, respectively, and can independently control the gas flow rate. This embodiment can uniformly reduce the deposit inside the discharge chamber 5 by adjusting the respective gas flow rates to match the state of deposit at the ion beam exit slit 7 and on the deep surface of the discharge chamber 5. Embodiment 4: This embodiment is the same as the ion source shown in the first embodiment except that leak valves 14 and 14' are fitted to the gas introduction pipe 6 as shown in Figure 10, and a gas mixture, boron trifluoride (BF₃) and oxygen (O₂), is introduced. When the mixing quantity of O₂ is changed to 5%, 10% and 20%, respectively, the proportion of B⁺ contained in the extracted ion beam tends to decrease somewhat in proportion to the concentration of O₂, but no deposit is seen occurring in any case inside the discharge chamber 5 and at the portion of the ion beam exit slit 7. As a result, a B⁺ beam of at least 60 keV and at least 4 mA can be stably obtained for more than four hours by mass-separating the ion beam extracted from the ion source with a sector magnet mass separator. The effect of reducing the deposit can be seen occurring from the 0.1% O₂ concentration with respect to the pressure of the BF₃, and can also be seen increasing with an increasing O₂ concentration. Besides the generation of various atom and molecule ions, large quantities of chemically active, neutral atoms and molecules are generated in the plasma. Their quantities and component ratios vary in a complicated manner with microwave power, temperature, gas pressure, and so forth. For this reason, chemical analysis in further detail will be necessary in order to clarify the reason why discharge free from the deposit can be maintained by introducing O₂. A similar effect can be obtained when boron trichloride (BCℓ₃) is used in place of BF₃. Embodiment 5: The microwave ion source in accordance with a fifth embodiment of the invention is the same as the ion source of the fourth embodiment except that BF₃ and O₂ are introduced into the discharge chamber 5 through separate gas introduction pipes 6 and 6', as shown in Figure 11. A stable B⁺ beam free from deposition can be obtained in this embodiment in the same way as in the fourth embodiment shown in Figure 10. Reference numerals 15 and 15' in Figure 11 represent the leak valves, respectively. Embodiment 6: The fourth and fifth embodiments of the invention shown in Figures 10 and 11, respectively, mix the gases with the two needle valves. However, if a gas tank storing therein a mixed gas is used, the mixed gas can be introduced into the discharge chamber through one needle valve as shown in Figure 6, and the same effect as those of the fourth and fifth embodiments can be obtained. Embodiment 7: When an oxygen-containing gas such as CO₂ is used as the gas to be mixed with BF₃ or BCℓ₃ in place of oxygen used in the fourth, fifth and sixth embodiments, the same effect as when oxygen is introduced can be obtained. A stable B⁺ beam can be obtained similarly when two or more kinds of oxygen-containing gases are mixed. Embodiment 8: When about 10% H₂ is mixed in place of O₂ with BF₃ in the fourth, fifth and sixth embodiments of the invention, a stable B⁺ beam can be extracted without causing any deposition. It is known that among boron compounds, hydrogen compounds such as B₂H₆ are relatively stable. It is therefore believed that when H₂ is mixed, the reactive hydrogen radical particles react with the deposit or the like, thereby exhibiting the effect of reducing deposition. As described above, even when a halide such as BF₃ or BCℓ₃ is used as the feed gas, the present invention can prevent the deposition on the ion beam exit slit, and hence can extract ions stably for an extended period of time. For example, a B⁺ ion beam of at least 4 mA, which has not so far been accomplished by the prior art ion source, can now be obtained for 4 or more hours stably. In view of the fact that an ion implant current of a semiconductor ion implanter on a production line remains at about 2 mA at present, the present invention makes it possible for the first time to carry out high current B⁺ ion implantation at a practical level, and provides a great effect in practice. Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practised otherwise than as specifically described herein.

Claims: An ion source including a microwave generator (1), a discharge chamber (5) of a rectangular parallelepiped shape adapted to have a plasma therein generated in a feed gas introduced therein, a gas inlet (110) for introducing said feed gas into said discharge chamber (5) located on a side wail surface of said discharge chamber (5), and an ion beam exit slit (7) disposed on an end surface (120) of said discharge chamber for extracting ions (21) from said plasma generated in the feed gas introduced into said discharge chamber (5), characterized in that the distance d between the end surface (120) of said discharge chamber (5) and said gas inlet (110), and the distance d' between the side wall surface of the portion of said discharge chamber (5) on which said gas inlet (110) is disposed and a wall surface facing the former, satisfy the relation d ≦ d' and the gas inlet (110) is located closer to the ion beam exit slit (7) than the center of the discharge chamber (5) to the exit slit (7) such that the gas pressure is elevated in the proximity of the ion beam exit slit (7) so that the deposit on the exit slit portion is reduced. An ion source according to claim 1, characterized in that the position at which said gas inlet (110) is located, is nearer to said end surface (120) than to the center of said discharge chamber (5). An ion source according to one of the claim 1, characterized in that said ion source further includes a magnetic field generator (13) and that said distance d and said distance d' satisfy the relation d ≦ d'. An ion source according to one of the precedent claims, characterized in that another gas inlet (110a) is disposed on a side wall surface of said discharge chamber (5) closer to an end surface of the discharge chamber (5) facing the end surface (120) than to said end surface (120). An ion source according to claim 4, characterized by a gas introduction pipe (6) communicating with said gas inlet (110) and another introduction pipe (6a) communicating with said gas inlet (110a), and in which a gas flow rate control valve (9b, 9a) is disposed in each of said gas introduction pipes (6, 6a) wherein feed gases of BF₃ and O₂ are separately introduced into said discharge chamber (5) through said gas introduction pipes (6, 6a). An ion source according to one of the claims 1 to 5, characterized in that said ion beam exit slit (7) is formed by a plurality of thermally isolated members (7a, 7b, 7c), and in that the ion beam exit slit (7) consists of a plurality of ion beam exit slits which are situated in the member (7b). An ion source according to claim 6, characterized in that electrically conductive composite material of electrically conductive BN and Ti is used for at least a part of the member (7b). Use of a feed gas in an ion source according to one of the claims 1 to 7, characterized in that said feed gas is a mixed gas prepared by mixing at least one halide gas selected from boron halide gases with at least one gas which reacts with boron and forms a boron compound. Use of a feed gas in an ion source according to claim 8, characterized in that said boron halide gas is at least one member selected from the group consisting of boron trifluoride and boron trichloride, and said gas to be mixed with said halide gas is at least one member selected from the group consisting of oxygen gas, hydrogen gas, and gas of an oxygen-containing compound and their mixtures. Use of a feed gas in an ion source according to claim 9, characterized in that said gas of an oxygen-containing compound is CO, CO₂, NO, N₂O, SO₂ or H₂O. Use of a feed gas in an ion source according to claims 9 or 10, characterized in that the quantity of said gas to be mixed with said halide gas is at least 0.1% of said halide gas in terms of pressure ratio. Ion source according to one of the claims 1 to 7, further characterized in that said feed gas is a mixed gas prepared by mixing at least a halide gas selected from the group consisting of boron trifluoride and boron trichloride with at least one gas selected from the group consisting of oxygen gas, hydrogen gas, and gas of an oxygen-containing compound and their mixtures.

Inventors: KOIKE, HIDEMI, NINOMIYA, KEN, OKADA, OSAMI, OZASA, SUSUMU, SAKUDO, NORIYUKI, TOKIGUCHI, KATSUMI
Applicants: HITACHI, LTD.
Application year: 1985
Main classification: H01J27
IPC codes: H01J27, H01J37