Patent Number: 
Section: description

FIG. 1 is a functional block diagram for showing the ion implant system 100 of this invention. The deceleration optics described below can decelerate an ion beam from high energy, e.g. 5 keV, to energy as low as 0.2 keV, and at the same time disperse the decelerated ion beam in an angular-spread-out beam according to the ion particle energy range. The angular-spread-out characteristic of the ion beam provides a convenient method for selectively blocking out the beam in a certain energy range by employing a simple mechanical means known as a beam stop. Referring to FIG. 1, the ion beam implant system 100 includes an ion source associated with ion-beam formation electrodes 105, the mass analyzer magnet 125, post analysis deceleration electrodes 135, and target chamber 150 for implanting a target wafer 120 with an ion beam 110. Under normal operation (no ion beam deceleration), the ion beam 110, mass-filtered by the mass analyzer magnet 125, is transported through the decel electrodes 135 and reaches the wafer. In this situation, there is no voltage difference between the entrance electrode and exit electrode of the decel electrode assembly so that neither deceleration nor acceleration occurs for the ion beam. There is also no non-symmetric field applied in the region of the decel electrodes so that the ion beam is not steered away from the beamline symmetric axis. Under the operation of ion beam deceleration, after the ion beam 110 passes through the magnetic analyzer 125, a deceleration voltage 130 is applied to decelerate the ion beam 110 as shown in FIG. 1. When the ion beam 110 is a positively charged ion beam, a negative voltage 130 is applied. As the ion beam 110 travels through the ion beam system 100, some charged particles may be neutralized. The deceleration voltage will not decelerate the neutralized particles because they do not carry a net charge. The energy and direction of such particles are not affected by the electric field. After passing through the deceleration optics 135 the path of the neutral particles and the charged particles are therefore separated during deceleration and become two separate beams 110-1 and 110-2. The neutral particle beam 110-1 travels along a straight line while the charged ion beam 110-2 becomes spread out by employing a special deceleration optics as will be discussed below. The charged ion beam becomes an angularly spread-out beam and travels along a path with a slightly downward angle, e.g., a six-degree downward angle, to reach the target wafer 120. Note that the charged ion-beam is spread out over an angular range depending on the energy of the ion particles as will be discussed below. A beam stop 155 is employed on the path of the neutralized particle beam 110-1 to block the neutralized beam 110-1 from reaching the target wafer 120. The target wafer 120 is placed with a small slant angle, e.g., a six-degree angle relative to a vertical direction of the perpendicularly facing charged ion beam 110-2. By putting a beam stop 155 after the deceleration optics, but in the original beam path 110-1, the neutral particles are blocked and hence removed. By making the steering angle sufficiently large (at least 3 degrees) the problem of energy contamination associated with the neutral fraction in charged ion beams can be overcome. In this way, the problem of energy contamination in decel-mode operation can be resolved. Referring to FIG. 2, the angular spread of the ion beam generated by the deceleration optics provides a steering function that is specifically configured as an energy filter. For a given configuration of the deceleration optics, the individual ions in the beam will be deflected downward at a large (small) angle for ions having a relatively low (high) energy. Suppose that the steering angle is xcex8O for ions with initial energy EO decelerated to a final energy EF. The ion beam is typically composed of ions with a range of energies from EOxe2x88x92dE1 to EO+dE2, where dE1 and dE2 represent the lower and upper ion beam energy increment limits, respectively. Referring to FIG. 2, ions with energy much higher than EO will be deflected by a small deflection angle xcex8 less than  less than xcex8O and thus will be blocked by the upper part of the beam stop as shown in FIG. 2. Ions with higher energy, but close to EO, will be partially blocked. Ions with energy smaller than EO will be blocked by the lower part of the beam stop although energy contamination is not as serious a problem for ions with energy much less than EO. The problems caused by energy contamination can be significantly resolved with a beam stop 155 as that shown in FIGS. 1 and 2. Even during high voltage glitch conditions, which may cause the original ion beam to have a large energy range, the implant profile will not be adversely affected by energy contamination. The opening of the beam stop also defines the targeted ion-beam direction when the deceleration optics decelerates and steers the ion beam through the opening to the target wafer. FIGS. 3 and 4 show the schematic diagrams of the deceleration optics 135 and the electrical voltage arrangement of the electrodes employed in the deceleration optics 135 of the present invention. The deceleration optics consists of three electrodes A, B, and C. The voltages of the ion source, the extraction suppression electrode, and the source terminal are shown in FIG. 3 as VS, VE, and VT, respectively, where VS and VT are referenced to ground while VE is referenced on the source terminal. Electrode A is at a potential VA and is equal to the ion source termination potential VT(VA=VT). The deceleration suppression electrode B is at a potential VB that is more negative than VA(VB less than VA). Electrode C is at a potential VC that is equal to the potential of the processed wafers, and is more positive than VA(VA less than VC). The original ion energy EO is equal to q(VSxe2x88x92VT)=q(VSxe2x88x92VA), and the decelerated ion energy EF is equal to q(VSxe2x88x92VA)xe2x88x92q(VCxe2x88x92VA)=q(VSxe2x88x92VC), where q is the charge of an ion in the beam and is usually positive. In most ion implanters, it is preferable for the processed wafers to be connected to ground (VC=0) or nearly so. In this configuration, the ion source power supply is floated or referenced on the source terminal potential which itself is floated or referenced on the ground potential. The resulting energies are, EO=q(VSxe2x88x92VT), EF=qVs, where, VC=0, VB less than 0, and in decel-mode, VT less than 0. Also, the extraction power supply, VE less than 0, is referenced on the source terminal and VB less than VA=VT less than 0 is referenced on the ground potential. Regardless of the configuration, VB is more negative than VA and VC(VB less than VA less than VC), so that Electrode B can suppress both the upstream and downstream electrons. Electrode B also provides focusing while the beam is being decelerated and steered. From the electrode cross-section diagram in FIG. 4, it can be seen that Electrode B and Electrode C can be displaced transversely off the centerline of electrode A. Both the electric field between Electrode A and B and the field between Electrode B and C steer the ion beam downward. Electrodes B and C are controlled by a manipulator and can move transversely to steer the ion beam with the correct angle so that the ion beam can reach the wafer position. The steering angle is a function of the original and final energies of the ion beam and the electric field distribution in the deceleration region. For different original and final energies of the ion beam, the parameters affecting the electric field distribution, including the suppression voltage VB and the transverse positions of Electrodes B and C, have to change to keep the steering angle unchanged so that the ion beam can reach the same wafer position. Because the suppression voltage VB is primarily used to focus the ion beam, its value is usually changed to give the proper focusing while the transverse positions of Electrodes B and C are changed to give the proper steering. The original beam is required to have small beam width for separating the decelerated and steered ion beam with the neutralized beam in a position not far from the deceleration region to significantly reduce energy contamination. Assume that the steering angle is xcex8O, the beam width is w for both the neutralized beam and decelerated ion beam, and the travel distance for completely separating the neutralized beam and the steered ion beam is L. The steering angle xcex8O should be maintained small, usually from three degrees to fifteen degrees, to minimize corresponding wafer position change and possible beam current loss. The travel distance L should be short to maximize beam current delivery to the wafer when space charge blow-up occurs for low energy and high current beam. Since the relation among these parameters is approximately w=L tanxcex8O, the beam width is required to be small, too. For instance, when xcex8O is equal to 6 degrees and L equal 30 cm, w will become 3.2 cm Considering that large beam cross section is required to minimize space charge blow-up for low energy and high current beam, the beam height should be large when the beam width is limited to be small. In other words, an ion beam with large aspect ratio (or large height-to-width ratio) is required in the deceleration and steering region for successfully separating the decelerated and steered ion beam from the neutralized beam, and transporting the production worthy low energy beam currents. An aspect ratio of 4 is considered to be the minimum requirement for separation of a low energy and high current ion beam from the corresponding neutralized beam. Since the beam width is usually larger than 2.5 cm, the beam height has to be at least 10 cm. After the neutralized beam is separated from the decelerated ion beam, a beam stop can be applied in the neutralized beam path to prevent the neutrals with higher energy from reaching the wafer and therefore minimize energy contamination. FIG. 5 shows a three-dimensional perspective view of the mechanical design of the deceleration electrode assembly. The apertures of the three electrodes are narrow and tall because they are designed to decelerate narrow and tall beams, or high aspect ratio beams as discussed above. Electrode B has a larger width than Electrode A and C to prevent ion beams from striking on Electrode B, generating large secondary electron emissions, and thereby overloading the suppression power supply. Another reason is to provide a better focusing field distribution. When the width of Electrode B is smaller than that of Electrode C, the transverse field components at the edge of Electrode C is high, which may inappropriate deflection of the beam. The deceleration optics of the present invention provides an apparatus to decelerate ion beams and at the same time steer these decelerated beams off the path of the original ion beams. In this way, the decelerated ion beam is steered in the target direction and the neutralized beam travels in the direction of the original ion beam. By blocking the neutralized beams with a beam stop, the energy contamination resulting from deceleration can be eliminated. The invention thus discloses an ion implantation apparatus, which includes a target chamber for containing a target for implantation and an ion source chamber includes an ion source for generating an ion beam. The ion source chamber further includes beam deceleration optics for decelerating the ion beam to produce a low energy ion beam. The deceleration optics further includes an ion beam steering means for generating an electrostatic field for separating neutralized particles by steering the charged particles to transmit in a targeted charged-particle direction that is slightly different from the neutral beam direction. The ion-beam deceleration optics further includes electrodes for generating a spread-out ion beam over an angular range along the beam line of the ion beam. The angular spread is determined by the energy of each ion in the ion beam and is used for more accurately controlling the energy of the ions for implantation and for blocking the neutralized particles and ions above a maximum implant energy from reaching the target for implantation. In a preferred embodiment, the ion-beam deceleration optics includes a first, second, and third electrode arranged along the direction of the ion beam for generating a filtering electric field wherein the second electrode is provided with a more negative voltage than the first electrode, and the third electrode is provided with a more positive voltage than the first electrode. In a preferred embodiment, the first electrode is provided with a voltage that is the same as the ion source terminal voltage and the third electrode is provided with a voltage that is the same as a wafer voltage. In another preferred embodiment, the third voltage is provided with a wafer voltage connected to a ground voltage. The ion-beam deceleration optics further includes a neutral beam blocking means for blocking the neutralized particles from reaching the target of implantation in the target chamber. The beam deceleration optics further includes a high energy beam blocking means for blocking ions of the ion beam having an energy higher than a maximum implant energy by placing the high energy beam blocking means at a pre-designated angular position along the beam line corresponding to an angular range for blocking ions of the ion beam having an energy higher than the maximum implant energy. The ion source generates a positively charged ion beam and the beam deceleration optics includes the electrodes for generating an energy filtering electric-field for decelerating and filtering the ion beam by producing a spreading-out ion beam over an angular range along the primary beam direction. The steered ion beam transmits in the targeted ion-beam direction having a small vertically deflected angle, e.g. six degrees, relative to a horizontal axis as shown in FIGS. 1 and 2. And, the target chamber containing the target for implantation leans at a small angle, e.g. six-degrees, relative to a vertical axis perpendicular to the horizontal axis whereby the target for implantation is perpendicular to the incident angle of the ion beam. In another preferred embodiment, the ion source chamber is provided with a vacuum in the range of 10xe2x88x925 torr and the ion beam may be decelerated to an energy level of 200 eV or less with a beam energy contamination of about 0.1%. In summary, an ion source apparatus for generating and directing an ion beam is disclosed in this invention. The ion source apparatus includes a beam deceleration optics used for decelerating the ion beam. The beam deceleration optics further includes a plurality of electrodes for generating an electric field used for spreading out the ion beam over an angular range according to energy of each ion of the ion beam for more accurately directing an ion beam with desired low energy to a target wafer. According to above descriptions, this invention further discloses a method for generating an implantation ion beam. The method includes the steps of (a) providing an ion source for generating an ion beam; (b) employing an analyzer magnet for steering the ion beam through a curved beam-trajectory to a targeted ion-beam direction; (c) applying the ion beam steering means for coordinating with the beam deceleration means for generating an electromagnetic field for separating a neutralized particle by steering a neutralized particle to transmit in a neutralized-particle direction slightly different from the targeted ion-beam direction; and (d)employing a beam deceleration optics for decelerating and filtering the ion beam for producing a spreading out beam over an angular range along a beam line of said ion beam according to an energy of ions of the ion beam and employing a high energy ion blocking means for blocking out ions having an energy higher than a maximum implant energy. Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.