Patent Number: 
Section: claims

1. A system for implanting a substrate, comprising:a substrate holder disposed within a process chamber of the system and being electrically grounded;a driven electrode disposed within the process chamber and coupled to a power source, the power source configured to supply voltage to the driven electrode as an unbalanced voltage pulse train, wherein a negative peak voltage during a negative voltage pulse period of the unbalanced voltage pulse train is the same as or higher than a positive peak voltage during a positive voltage pulse period of the unbalanced pulse train; anda movable mask configured to move between a first position proximate the substrate holder, and a second position proximate the driven electrode. 2. The system of claim 1, wherein the unbalanced voltage pulse train comprises a square wave pulse train. 3. The system of claim 1, wherein the positive peak voltage is less than about 2 kV, and an absolute value of the negative peak voltage is greater than about 2 kV. 4. The system of claim 1, wherein the system is configured to couple the movable mask to ground under a first operating condition, and to couple the movable mask to the power source under a second operating condition. 5. The system of claim 4, wherein the movable mask is coupled to ground when in the first position, and coupled to the power source when in the second position. 6. The system of claim 1, wherein the negative voltage pulse period is about 10-100 μs, and wherein the positive voltage pulse period is about 10-50 μs. 7. The system of claim 1, wherein the movable mask comprises a frame having a central aperture configured to accommodate a mask plate. 8. The system of claim 1, wherein the movable mask comprises a shaft slidably disposed with respect to a linear feedthrough of the process chamber. 9. A method of implanting a substrate in a plasma chamber of an ion implantation system, comprising;coupling a substrate holder disposed in the plasma chamber to ground;providing from a power supply an unbalanced voltage pulse train to an electrode disposed in the plasma chamber; andpositioning a mask during a first exposure proximate the substrate, wherein the mask is coupled to ground. 10. The method of claim 9, further comprising positioning the mask during a second exposure proximate the driven electrode. 11. The method of claim 10, further comprising electrically coupling the mask to the unbalanced voltage pulse train during the second exposure. 12. The method of claim 10, wherein during the first exposure the mask is disposed within a plasma sheath region created by a positive voltage pulse period of the unbalanced voltage pulse train, the method further comprising:positioning during the first exposure portions of the substrate on the substrate holder proximate the mask to receive a patterned implant; andpositioning portions of the substrate on the substrate holder during the second exposure to receive a blanket implant. 13. The method of claim 9, comprising providing the unbalanced voltage pulse train as a series of square wave pulses in which a positive peak voltage is the same as or less than an absolute value of a negative peak voltage of each of the square wave pulses. 14. The method of claim 9, further comprising providing a negative voltage pulse period of the unbalanced voltage pulse train of about 10-100 μs. 15. The method of claim 9, further comprising providing a positive voltage pulse period of the unbalanced voltage pulse train of about 10-50 μs. 16. The method of claim 13, wherein the positive peak voltage is less than about 2 kV and an absolute value of the negative peak voltage is greater than about 2 kV. 17. A system for implanting a substrate, comprising:a process chamber having an electrode electrically coupled to a power source and a substrate holder electrically coupled to ground; anda movable mask, wherein the movable mask is configured to move between a first position proximate the substrate holder and a second position proximate the electrode;wherein the power source is configured to supply voltage to the electrode as an unbalanced voltage pulse train, wherein an absolute value of a negative peak voltage of the unbalanced voltage pulse train is equivalent to or higher than a positive peak voltage of the unbalanced voltage pulse train, andwherein the system is configured to couple the movable mask to ground when in the first position, and to couple the movable mask to the power source when in the second position. 18. The system of claim 17, wherein a duration of a negative voltage pulse period is about 10-100 μs, and wherein a duration of a positive voltage pulse period is about 10-50 μs. 19. The system of claim 17, wherein the positive peak voltage is less than about 2 kV, and an absolute value of the negative peak voltage is greater than about 2 kV. 20. The system of claim 17, the process chamber further comprising first and second portals coupled to respective first and second load lock chambers, wherein gas conductance through the first and second portals is configured to provide continuous plasma operation when the first and second portals are open. 21. The system of claim 20, wherein the system is configured to continuously transport substrates from the first load lock chamber into the process chamber and therefrom to the second load lock chamber while a plasma is maintained in the process chamber.