Patent Number: 
Section: description

The present invention teaches a novel low-energy ion implant method involving the separation of the charged ion beam from the uncharged neutralized particles. FIG. 2 is a diagram of the current invention. The diagram of the ion beam implant system includes the ion source 105, the mass analyzer magnet assembly 125, beamline chamber 140, post analysis deceleration electrode assembly 135, plasma shower 145, and target chamber 150 for implanting a target wafer 120 with an ion beam 110. Under normal operation, the ion beam is mass-selected and decelerated by the decel electrode assembly 135, and is transported to the target wafer 120. The plasma shower 145 helps to reduce the space charge of the decelerated ion beam 110 and increase the beam transportation efficiency from the decel electrode assembly 135 to the wafer 120. As the ion beam 110 travels through the resolving chamber 140 some charged particles may be neutralized through the process of charge exchange with residual gas in the beamline. The deceleration voltage will not decelerate these neutralized particles because they do not carry any charge. The speed and direction of the neutral particles are not affected by the electric field. When these neutral particles with higher energy reach the target wafer 120 together with the decelerated ion beam, they will cause energy contamination with deeper implant profile. Separating the neutral particle beam and the ion beam to prevent the neutral beam from reaching the wafer is the most effective way to eliminate the energy contamination. In this invention, the beam is steered downward (FIG. 3a) or upward (FIG. 3b) in decel-mode by displacing one or several of the decel electrodes off the beam line symmetric axis on the dispersive plane defined by the mass analyzer magnet The non-symmetric electric field bends the ion beam with an off-axis angle as a function of the decel electrode displacements and the decel electrode voltages. After passing through the decel electrode assembly 135, the path of the neutralized particles and the charged particles are therefore separated during deceleration and become two separate beams 110-1 and 110-2. The neutralized particle beam 110-1 travels along a straight line while the charged ion beam 110-2 is travels along a path with a slightly downward (or upward) angle, in a range of three to fifteen degrees, such that the beam is directed at the target wafer 120. Note that the angle can be different depending on a particular system configuration. A beam stopper 155 is employed in 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 tilted with a small slant angle relative to the vertical axis such that the wafer normal is parallel to the incident ion beam 110-2. The wafer is also moved downward (or upward) from the normal implant position as shown in FIG. 2 to a new position as shown in FIG. 3a (or FIG. 3b) to accept the steered ion beam. The invention discloses an ion implantation method that requires the use of a target chamber for containing a target for implantation and an ion source chamber that includes an ion source with a mass analyzer for generating an ion beam with specific mass at original energy. The ion source chamber further includes beam deceleration optics for decelerating the ion beam from the original energy to the desired final energy. The beam deceleration optics further includes an ion beam steering means for generating an electrostatic field. The electrostatic field is applied to steer the ions to the targeted ion-beam direction that is slightly different from the original ion beam direction. The targeted ion-beam direction has a small downward (or upward) angle, in a range from three to fifteen degrees, while the neutralized beam particles are unaffected by the deceleration and steering means and travel in the original beam direction. The target chamber containing the target for implantation is tilted backward (or forward), as shown in FIG. 3a and 3b, at a small angle in a range from three to fifteen degrees toward the ion-source chamber whereby the target for implantation may be perpendicular to the ion beam. A beam stopper is provided in the neutralized beam path to prevent the neutralized beam from reaching the implant target in the target chamber. The energy contamination from high-energy neutral particles is therefore eliminated regardless how many neutral particles are created from ion beam interaction with the residual gas molecules. Low energy contamination of less than 0.1% can be achieved even low vacuum environment exists in the beamline. In a specific 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 as low as 200 eV with a beam energy contamination of less than 0.1%. 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 increased 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 25 cm, the beam height would be at least 10 cm. After the neutralized beam is separated from the decelerated ion beam, a beam stopper can be applied in the neutralized beam path to prevent the neutrals with higher energy from reaching the wafer and therefore minimize energy contamination. For an ion source with a narrow extraction aperture, the aspect ratio of an ion beam usually decreases when the beam travels from the ion source/extraction region to the deceleration and steering region because the space charge blow-up is more severe in the dispersive plane than in the non-dispersive plane defined by the analyzer magnet To obtain an ion beam with aspect ratio larger than 4 in the deceleration and steering region, the aspect ratio of the ion source extraction aperture should be several times larger than 4. We consider that the aspect ratio of the ion source extraction aperture is at least equal to 20 to provide high aspect ratio beams in the region of deceleration and steering for successful separation of the decelerated and steered ion beam and the neutralized beam. According to FIGS. 2 and 3, this invention discloses a method for performing an ion implantation. The method includes steps of a) providing a target chamber for containing a target for implantation and an ion source chamber including an ion source for generating an ion beam; b) providing a beam deceleration optics that includes a beam deceleration means in the ion source chamber for decelerating the ion beam for producing a low energy ion beam; c) providing a beam steering means to the beam deceleration optics to separate neutralized particles out of the ion beam by keeping the neutralized particles propagating in a neutralized-particle direction slightly different from a steered targeted ion-beam direction; and d) employing the ion-beam deceleration optics for transmitting the ion beam along the targeted ion-beam direction to the target for implantation and for blocking the neutralized particles from reaching the target for implantation. In a preferred embodiment, the method further includes a step of e) providing an analyzer magnet to the ion source chamber for mass filtering. In a preferred embodiment, the step of employing the beam deceleration means further includes a step of providing a deceleration electric-field means for generating a deceleration electric-field for decelerating the ion beam for producing a low energy ion beam. In a preferred embodiment, the step of employing the ion beam steering means for generating an electrostatic field for keeping the neutralized particle to transmit along a trajectory different than the ion beam carrying electric charges comprising a step of steering the ion beam to transmit in a targeted ion-beam direction slightly different from the neutralized-particle direction. In a preferred embodiment, the step of employing an ion-beam deceleration optics further includes a step of employing a neutralized beam blocking means for blocking the neutralized particle from reaching the target of implantation in the target chamber. In a preferred embodiment, the step of providing an ion source in an ion source chamber is a step of providing an ion source for generating a positive charged ion beam. And, the step of employing the beam deceleration means includes the step of employing a deceleration electric-field means for generating a negative electric-field for decelerating the ion beam for producing a low energy ion beam. In a preferred embodiment, the step of employing the ion beam steering means comprising a step of steering the ion beam carrying electric charges to transmit in the targeted ion-beam direction at a small deflected angle. In a preferred embodiment, the step of employing the ion beam steering means to steer the ion beam carrying electric charges to transmit in the targeted ion-beam direction comprising a step of steering the ion beam at a small deflected angle in a range of three to fifteen degrees relative to the horizontal axis. In a preferred embodiment, the step of providing the ion source in the ion source chamber comprising a step of providing the ion source chamber and the target chamber with a vacuum in the range of 10xe2x88x925 Torr. And, the step of employing the ion beam deceleration means comprising a step of decelerating the ion beam to an energy level as low as about 200 eV with an energy contamination of less than about 0.1%. In essence, this invention discloses a method for generating an implantation ion beam from an ion source projecting a plurality of ions. The method includes steps of a) employing a beam deceleration means for decelerating the ions projected from the ion source; b) employing a beam steering means for generating an electrostatic field for separating a plurality of neutralized particles from the ion ions by keeping the neutralized particles propagating in a neutralized-particle direction slightly different from a targeted ion-beam direction of the ions. In a preferred embodiment, the method further includes a step c) arranging a wafer implant position corresponding to the targeted ion-beam direction for accepting the ions projected thereto. In a preferred embodiment, the step of employing a means for transmitting the ions to a target of implantation comprising a step of employing a means for blocking the neutralized particles from reaching the target of implantation. In a preferred embodiment, the step of separating the neutralized particles from the ions comprising a step of providing a charged particle deflection means for deflecting the trajectory of the ions at a small angle from the trajectory of the neutralized particles. In a preferred embodiment, the method further comprising a step of configuring the ion beam deceleration means for decelerating and processing the ions into an ion beam having a large beam-height to beam-width ratio. In another preferred embodiment, the method further comprising a step of providing a beam block for blocking the neutralized particles propagating in the neutralized-particle direction. In a preferred embodiment, the method further includes a step of projecting the ions in forming the implantation ion beam with high beam current and low and a ratio of a beam height to a beam width equal or larger than 20. In another preferred embodiment, the step of forming the implantation ion beam having a ratio of a beam height to a height to a beam width equal or larger than 20 comprising a step of providing an extraction aperture for the ion source with an aspect ratio equal or larger than 20. In another preferred embodiment, the step of configuring the ion beam deceleration means for decelerating and processing the ions into an ion beam having a large beam-height to beam-width ratio comprising a step of processing the ions into an ion beam having a beam-height to beam-width ratio equal or greater than 4. And, the step of processing the ions into an ion beam having a beam-height to beam-width ratio equal or greater than 4 comprising a step of providing an aperture of a deceleration and steering optics having a beam-height to beam-width ratio equal or greater than 4. In a preferred embodiment, the step of providing a charged particle deflection means for deflecting the trajectory of the ions at a small angle from the trajectory of the neutralized particles comprising a step of deflecting the trajectory of the ions at an angle in the range of three to fifteen degrees. Therefore, the present invention provides a new low energy implant method used to form shallow p-type and n-type junctions in semiconductor devices. Specifically, a new ion beam deceleration method is disclosed for decelerating a charged ion beam and for separating a neutralized beam from the ion beam. The neutral beam is composed of neutral particles propagating at energies higher than the desired energy. The neutral beam is separated and stopped by a neutral-particle-stopping block so that it is unable to reach the target wafer. The problem of energy contamination in very low energy implants using decel-mode is thus resolved using this invention. 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.