Patent Number: 
Section: description

Referring now to FIG. 1 of the drawings, an ion beam implanter is shown generally at 10. The ion implanter includes an ion source 12, for producing a generally positively charged ion beam 14 that is extracted therefrom by known means, for example, an extraction electrode. A mass analysis magnet 16 mass analyzes the extracted ion beam 14 and outputs a mass analyzed ion beam 18 which includes only those ions having a charge-to-mass ratio that falls within a prescribed range. The mass analyzed ion beam 18 passes through a resolving aperture 20 and is implanted into wafers W situated upon pedestals situated about the periphery of a rotating support or disk 22. The rotating disk in the disclosed embodiment is made of aluminum, although it may be coated with a layer of silicon. In the case of an aluminum rotating disk 22, the disk would be more electrically conductive than the wafers situated thereon. In the case of a silicon-coated disk 22, the disk would generally be less electrically conductive than the wafers situated thereon (dependent upon whether or not a patterned insulating surface such as a photoresist is applied to the wafers). Generally, the invention acknowledges that the electrical conductivity of the wafers and the portions of the disk that surround them are different. This difference in electrical conductivity may be used to determine whether or not the wafer charge accumulation is adversely affecting the beam passing thereover. The disk 22 is vertically translated along an axis Y by means of a motor 24 and leadscrew 26. The disk 22 is rotated by means of motor 28, in a direction indicted by arrow 29, about an axis that passes through disk center 31 perpendicularly to the plane of the disk. The wafers W are positioned about the periphery of the disk 22 at locations that are substantially equidistant from the disk center. The full surface area of the wafers W are implanted as they rotate in a circular path (in the xe2x80x9cX scanxe2x80x9d direction) and are vertically translated (xe2x80x9cin the Y scan directionxe2x80x9d) before the fixed position ion beam 18. Ion dosage received by the wafers W is determined by rotational velocity and the vertical translational velocity of the spinning disk 22, both of which are determined by the motor control 30. Charge neutralization system 33 is provided for neutralizing the positive charge that would otherwise accumulate on the wafers as they are implanted by the generally positively charged ion beam 18. U.S. Pat. No. 5,959,305, which discloses a known type of charge neutralization system, is hereby incorporated by reference as if fully set forth herein. The present invention is embodied as an in-process charge monitor and control system 32. The system 32 includes means to measure the amount of charge accumulation on the wafers W that can cause the ion beam to change shape as the disk rotates, causing the beam to successively pass from the wafers to the intermediate portions of the conductive aluminum disk surface. In response to these measurements, the operation of the charge neutralization system 33 may be adjusted or tuned, as further explained below. Alternatively, the output of system 32 may be used as and input to a dose control system 35 to control the rotation and translation of the spinning disk 22 to insure a uniform implant dose across the entire surface of the wafers W being implanted. The dose control system 35 includes known elements such as a Faraday cage 34 providing an output signal 36. The output 36 from the Faraday cage 34 and an output 41 from a pressure monitor disposed within the implantation chamber, such as an ion gauge 43, are input to the control circuitry 50. The circuitry 50 uses these inputs to determine an appropriate X-scan and Y-scan speed of the wafer in front of the ion beam 18, as is known in the art. Specifically, the Faraday cage 34 is mounted behind the spinning disk 22 and is used to measure the ion beam current that passes through slot 62 in the disk. The length of the slot 62 is at least as long as the diameter of the wafers being implanted (e.g., 200 mm or 300 mm) so that the slot will receive ion beam current throughout the entire range of the Y-scan of the wafers (see also FIG. 3). The dose control circuitry 50 outputs control signal 52 to motor control 30 based on the outputs of the Faraday cage 34 and the ion gauge 43. Motor control 30 in turn outputs rotational control signal 54 to motor 28 and vertical translational signal 56 to motor 24, in order to maintain a uniform implantation across the surface of the wafers being implanted. In this manner, the outputs of Faraday cage 34 and the ion gauge 43 are used by the control circuitry 50 to thereby determine the dose of ions implanted into the wafers. The control circuitry also includes memory 58 and a user console or interface 60. The use of the output of the Faraday cage 34 and ion gauge 43 to control rotational and translation movement of the wafers W in front of the ion beam 18 is known. However, using only these mechanisms may result in non-uniform wafer implants because the ion beam current measurement provided by Faraday cage 34 does not take into account changes or disturbances to the ion beam profile as it passes from portions of the conductive aluminum disk surface intermediate wafers, to the insulative charged surface of a particular wafer being implanted. For example, the ion beam may xe2x80x9cblow-upxe2x80x9d, or become less controllably focused, if it is exposed to a sufficiently positive charge accumulation over the wafer being implanted. As such, a non-uniform wafer implant may be obtained. FIG. 2 shows one example of such a non-uniform implant, commonly referred to as a xe2x80x9cbull""s-eyexe2x80x9d pattern of non-uniform ion implantation. As shown in FIG. 2, the areas of the implanted wafer marked with xe2x80x9c+xe2x80x9d indicate areas of overdose (low sheet resistivity), and the areas marked with xe2x80x9cxe2x88x92xe2x80x9d indicate areas of underdose (high sheet resistivity). FIG. 2 resulted from implanting a 200-mm wafer with boron (B) ions at an energy level of 2 kilo-electron-volts (keV). As such, the present invention provides an additional ion beam measurement mechanism that takes into account changes or disturbances in ion beam profile, in order to improve dose uniformity across the surface of the wafer. Referring back to FIGS. 1 and 3, the in-process charge monitor and control system 32 includes electrical charge pick-ups or monitors 38 and 40 for outputting signals 42 and 44, respectively, and a comparator 46 for comparing the signals 42 and 44. Apertures 64 and 66 are provided in the disk 22 to receive portions of the ion beam current when it passes thereover. As shown in FIG. 1, portions of the ion beam are shown in phantom as reference numerals 18a and 18b as indicative of the portions of the beam that will pass through aperture 64 and 66, respectively, when the disk 22 rotates from the position shown in FIG. 1. Aperture 64 and aperture 66 are located the same distance d from disk center 31. As the disk 22 rotates, a first portion 18a of the ion beam current passes through aperture 64 and is measured by charge pick-up or monitor 40, which produces output signal 44. As the disk 22 continues to rotate, a second portion 18b of the ion beam current passes through aperture 66 and is measured by charge pick-up or monitor 38, which produces output signal 42. Aperture 64 is selected at a location where the ion beam is unaffected by the charge accumulation on the wafer, and aperture 66 is selected at a location where the ion beam is affected by the charge accumulation on the wafer. In other words, aperture 66 is located closer to a wafer than is aperture 64. Alternatively, the first and second apertures may each be located equidistant from a wafer but surrounded by portions of the disk having different electrical conductivity characteristics. For example, aperture 64 may be provided in a portion of the disk that is aluminum, and aperture 66 may be provided in a portion that is silicon coated. In either case, comparator 46 compares the output signals of charge monitors 38 and 40 to determine the effect, if any, that the charged insulative surfaces of the wafers have on the ion beam profile. For example, in the disclosed embodiment of FIG. 3, if the comparator 46 detects no measurable difference in the first and second portions of the beam current, it can be determined that there is no adverse effect causing beam xe2x80x9cblow-upxe2x80x9d. The negligible comparator output 48 indicates that the charge neutralization system 33 of the ion implanter is operating to effectively neutralize any charge accumulation on the wafers and permit a uniformly dosed implant. However, if the comparator 46 detects a measurable difference in the first and second portions of the beam current, it can be determined that there is an adverse effect causing beam xe2x80x9cblow-upxe2x80x9d. For example, if the beam is xe2x80x9cblown-upxe2x80x9d, the peak ion beam current measured at aperture 66 would be less than that measured at aperture 64. Alternatively, one can measure the time distribution of the beam as it passes apertures 64 and 66. If the beam is detected for a longer period of time at aperture 66, it indicates a beam xe2x80x9cblow-upxe2x80x9d condition. In either case, the measurable comparator output 48 indicates that the charge neutralization system 33 of the ion implanter is not operating to effectively neutralize any charge accumulation on the wafers and permit a uniformly dosed implant. As such, the operation of the charge control system (33) may be adjusted or tuned, using comparator output 48, to provide a greater supply of low energy electrons for neutralizing this excess wafer charge accumulation. Alternatively, the output 48 of comparator 46 may be used instead to adjust the dose control circuitry 50. (As shown in FIG. 1, comparator output 48 is shown in phantom as an alternative input to dose control circuitry 50.) For example, the bull""s-eye pattern of FIG. 2 may be correlated to the output 48 of comparator 46. As such, the dosage control circuitry 50 may be programmed to adjust the X-scan and Y-scan speeds of the disk in real time to correct for the anticipated dosage errors. In effect, the dosage control circuitry 50 uses comparator output 48, in addition to the outputs of the ion gauge 43 and the Faraday cage 34, to modify its output control signal 52 to motor control 30. However, it is anticipated that the invention may be more directly implemented as a means to tune the operation of the charge neutralization system 33, as described above. Accordingly, a preferred embodiment of an in-process charge monitor and control system has been described. With the foregoing description in mind, however, it is understood that this description is made only by way of example, that the invention is not limited to the particular embodiments described herein, and that various rearrangements, modifications, and substitutions may be implemented with respect to the foregoing description without departing from the scope of the invention as defined by the following claims and their equivalents.