FIELD OF THE INVENTION The invention is related to the field of gettering pumps, in particular, to the problem of outgasing of hydrogren from semiconductor substrate wafers during ion implantation processes.

BACKGROUND OF THE INVENTION Statement of the Problem In an ion implanter, an ion beam of dopant atoms is generated by an ion source and analyzed by a mass analyzer magnet to achieve the proper atom and charge state for implanting. The ion beam is then either scanned by an applied electronic or magnetic field or shaped by a magnetic field to a ribbon before striking the wafer.

Prior to ion implantation, a semiconductor wafer substrate is coated with a hydrogen- containing"resist"film, which has holes in it where the ion beam will be allowed to strike the wafer. The resist stops the ion beam from entering the wafer in the areas that are not to be doped with the ions. Unfortunately, the energetic ion beam tends to crack the hydrocarbon resist film and causes the release, or"outgassing", of hydrogen,

which degrades the vacuum in the ion implanter. Hydrogen atoms migrate up the ion beam path in the direction opposite of the ion beam, and collisions between the ions and the hydrogen atoms change the charge state of some of the ionized dopant atoms.

The change in charge state interferes with direction control by the applied field, causing the dopant atoms to follow the wrong path. This results in the wafer being exposed non-uniformly. It also interferes with the dosemetry, which controls the dose by measuring the ion current.

The hydrogen is hard to pump out of the system because it tends to migrate upstream from the wafer substrate up the beam path rather than toward the back of the chamber holding the wafer substrate, where pumps tend to be located. For example, it is known in the art of integrated circuit fabrication, in particular, ion implantation, to use titanium-based gettering pumps located in the substrate processing chamber to remove contaminants caused by outgassing. These gettering pumps are typically non- evaporative gettering (NEG) pumps.. In ion implantation systems of the prior art, a gettering pump cartridge is located in the ion implantation chamber and its operation depends on migration of contaminants, such as hydrogen, into an opening of the cartridge. Turbomechanical, cryogenic and roughing pumps are conventionally used in ion implantation systems to create and to maintain a high vacuum, but these pumps do not pump hydrogen efficiently.

Generally, pumps used in systems of the prior art are located downstream of the substrate wafer being processed. Hydrogen atoms cause serious problems, however, upstream of the substrate wafer in an ion implantation process. The hydrogen atoms released by outgassing from the resist layer tend to migrate upstream into the ion beam path being directed at the substrate. If the ion beam hits the hydrogen after it has passed through the ion-beam angle-correcting elements and changes charge state as a result, it may still implant into the wafer to the correct depth.

The effects are more serious if the change in charge state occurs further up in the ion optical elements since the ions take the wrong path and hit the wafer and an incorrect angle and with an incorrect energy. This can cause nonuniformities in dosage from one part of the wafer to another and an incorrect depth of implantation of the dopant

into the semiconductor substrate. If the charge state of the ion is changed before it exits the analyzing magnet, the dose of dopant atoms is disturbed because the analyzing magnet monitors the number of dopant atoms by detecting ion current.

SUMMARY OF THE INVENTION The invention helps solve some of the problems mentioned above by providing a system and the method in which a gettering pump surrounds the ion beam path just ahead of the place where it impinges on an integrated circuit wafer substrate.

And ion implanter system in accordance with the invention includes the improvement of a pumping system for reducing the effects of wafer outgassing on beam purity.

Preferably, the pumping system comprises a titanium sublimation pump.

A first basic embodiment of a pumping system in accordance with the invention includes a gettering chamber located between ion-beam measuring and control elements and the implantation chamber of an ion implanter. An ion beam passes through the gettering chamber on its way downstream from the ion-beam measuring and control elements into the implantation chamber. A sorption surface is located in the gettering chamber, and a solid film of gettering metal is disposed on the sorption surface. The pumping system further includes a gettering-metal source for forming the solid film of gettering metal.

It is a feature of a system and method in accordance with the invention that the gettering-metal source is separated from the semiconductor substrate wafer in such way that there is no line-of-sight connection between the gettering-metal source and the substrate. In this way, gettering-metal atoms are inhibited from impinging on the semiconductor substrate. Typically, the gettering chamber has an ion-beam inlet and and ion-beam outlet. The ion-beam inlet through which the ion beam enters the pumping system typically is proximate the ion optical element, and the ion-beam outlet typically is proximate or connected to the inlet of the implantation chamber.

Preferably, an ion implanter system having a pumping system in accordance with the invention comprises a pump housing located between the ion-beam measuring and control elements and the implantation chamber. The gettering

chamber is located in the pump housing. Further, a film-formation chamber is located in the pump housing. The film-formation chamber is fluidically separated from the gettering chamber so that gettering-metal atoms in vaporous or other material states are inhibited from migrating from the film-formation chamber to the gettering chamber. The sorption surface, however, on which the solid film of gettering metal is disposed, is movable between the film-formation chamber and gettering chamber. In a preferred embodiment in accordance with the invention, a gettering element separates the film-formation chamber from the gettering chamber, and the sorption surface is located on the gettering element. Preferably, a pumping system in accordance with the invention comprises a plurality of gettering elements that are contiguous to each other and form a seal between the film-formation chamber and gettering chamber. A series of cooled baffles may serve to separate the film- formation chamber from the gettering chamber, and it provides a large surface area for sorption of gettering. Preferably the gettering element is shaped so that the sorption surface is sequentially movable between the film-formation chamber and gettering chamber. Typically, the gettering element has a cylindrical shape and is rotatable on its axis. Preferably, cylindrical gettering elements have teeth-like longitudinal ridges and grooves that increase surface area for sorption and that fit together to improve sealing between the film-formation chamber and a gettering chamber.

Preferably the gettering-metal source is located in the film formation chamber.

Typically, the gettering-metal source is a metal filament that is heated to sublimate gettering metal from the source, or a sputtering target from which gettering-metal is sputtered. As a sorption surface moves through the film-formation chamber, appropriate energy is applied to the gettering-metal source to form a fresh solid film of gettering metal on the sorption surface. As a sorption surface with the fresh solid film of gettering metal moves into the gettering chamber, the gettering metal reacts with reactive hydrogen atoms that have migrated upstream from the substrate in the implantation chamber into the gettering chamber.

Typically, the gettering-metal source comprises a metal selected from the group consisting of titanium, zirconium and tantalum. Similarly, the solid film of gettering metal typically comprises a metal selected from the group consisting of titanium, zirconium and tantalum. Preferably, the gettering-metal source and the solid film comprised titanium because titanium is very reactive with hydrogen and, therefore, is very effective in"gettering"hydrogen.

In a second basic embodiment of a method and a system in accordance with the invention, a gettering element located upstream from the semiconductor substrate between the semiconductor substrate and the ion-beam measuring and controlling elements. There is no separate gettering chamber. A film-formation chamber is contiguous to the gettering element, and the interior of the film-formation chamber is fluidically separated from the implantation chamber, the ion-beam path and the semiconductor substrate. A sorption surface is located on the gettering element and is movable between the interior and the exterior of the film-formation chamber. A solid film of gettering metal is disposed on the sorption surface; and a gettering-metal source in the interior of the film-formation chamber is used to deposit a fresh solid film of gettering metal on the movable sorption surface.

A system and method in accordance with the invention are particularly useful in ion implanter system and process for removing hydrogen contamination and reducing its effects on the ion beam purity. It is clear, however, that the invention is also useful in other fabrication processes of the integrated circuit art, as well as in other industrial areas in which high purity is important. Therefore, a pumping system and a method in accordance with the invention are useful in a generalized high- vacuum processing system comprising a critical process step conducted at a process- step location and having a source of contaminants that migrate along a path to the process-step location and interfere with the critical process step. A generalized pumping system for reducing the effects of contamination in such a system comprises: a gettering chamber located in the path between the source of contaminants and the process-step location; a sorption surface located in the gettering chamber; a solid film

of gettering metal disposed on said sorption surface; and a gettering-metal source for forming said solid film.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 depicts in generalized schematic form an ion implantanter system including the improvement of a gettering pump for reducing the effects of wafer outgassing on the ion beam purity; FIG. 2 depicts in more detail the pumping system of FIG. 1; FIG. 3 depicts a further embodiment of an ion implanter system 300 and having a pumping system 310 in which pump housing 340 contains evacuated gettering chamber 334 and evacuated film-formation chamber 342; FIG. 4 depicts the pumping system of FIG. 3 in greater detail; FIGS. 5-6 depict in schematic form an embodiment in accordance with the invention in which a pumping system 510 has a cylindrical pump housing 540 with ion inlet 542 and ion outlet 543; FIG. 6 depicts pump housing 540 encloses evacuated gettering chamber 534, titanium-filaments (gettering-metal sources) 538 located in evacuated film-formation chamber 542, and cylindrical rotatable gettering elements 544 separating gettering chamber 534 and film-formation chamber 542.

FIG. 7 shows an alternative embodiment of a pumping system 710 in accordance with the invention in which rotatably movable gettering elements 744 have relatively smooth sorption surfaces; and FIG. 8 depicts in schematic form a second basic embodiment of a pumping system in accordance with the invention in which a film-formation chamber isolates a gettering-metal source from the ion beam path in the gettering element is located close to the semiconductor substrate.

DETAILED DESCRIPTION OF THE INVENTION The system and the method of the invention are described herein with reference to Figures 1-8. It should be understood that the systems and devices

depicted in Figures 1-8 are not intended to represent an actual particular embodiment of the invention. Rather, Figures 1-8 are used merely to assist understanding the invention, the scope of which is set out in the claims below. It is contemplated that a pumping system and a method in accordance with the invention are useful in reducing the harmful effects of hydrogen gas caused by outgassing in an ion implantation system. It is understood, however, that a gettering pumping system and method in accordance with the invention may be used in other applications of the integrated circuit and related arts to reduce harmful effects of gas-borne contaminants.

FIG. 1 depicts in generalized schematic form an ion implanter system 100 including the improvement of a pumping system 110 for reducing the effects of wafer outgassing on the ion beam purity. Ion implanter system 100 includes ion beam source 112, which produces a stream of ionized dopant atoms, analyzing magnet 114, accelerator tube 116, in which ion stream 116 is accelerated before entering upon deflection plates 118. Analyzing magnet unit 114 detects the charged ions in ion beam 120, thereby measuring the dose of ions being implanted. Angle correcting element 122 corrects the direction of the ion beam 124. In a method in accordance with the invention, ion beam 124 passes through pumping system 110 before entering implantation chamber 126 and impinging on semiconductor substrate 128 to implant dopant atoms therein. In FIG. 1, the dashed line extending from angle correcting element 122 to substrate 128 represents errant path 129 of a dopant atom of which the charge state has been altered by collision with contaminant hydrogen. Pumping system 110 includes gettering-metal source 130, in particular, a titanium filament 130 for sublimating titanium atoms. Ion implanter system 100 also typically includes one or several turbomechanical,"roughing"and cryogenic pumps, which are not represented in FIG. 1. Analyzing magnet 114, accelerator tube 116, deflection plates 118, and angle correcting element 122 are ion-beam measuring and control elements known in the art. The exact configuration of devices for performing the functions of ion-beam measuring and control elements may vary between embodiments of the invention.

FIG. 2 represents in more detail pumping system 110. In operation, an ion beam follows ion beam path 232 and passes through evacuated gettering chamber 234 before impinging on semiconductor substrate 128 in implantation chamber 126. A sorption surface 236 is located in gettering chamber 234 and a solid film 238 of gettering metal is disposed on sorption surface 236. Hydrogen atoms released by outgassing of a resist layer on substrate 128 tend to migrate upstream, as indicated by arrows 239. In a system and method in accordance with the invention, the highly reactive hydrogen atoms react with the gettering metal of solid film 238 at sorption surface 236. The hydrogen atoms are thereby removed from the ion beam path 132 and deleterious effects of the hydrogen atoms on the ion beam purity are reduced. In FIG. 2, gettering chamber 234 is enclosed in pump housing 240, and it has an ion- beam inlet 242 proximate to ion optical element 118 (see FIG. 1), and an ion-beam outlet 243 proximate to implantation chamber 126. Gettering-metal source 230, preferably a titanium filament, is located in gettering chamber 234 and serves in the formation of solid film 238 of gettering metal on sorption surface 236. As illustrated in FIG. 2, a basic feature of a system and method in accordance with the invention is that a gettering-metal source 238 is located in such that it is not in line-of-sight contact with semiconductor substrate 128. This reduces the likelihood of gettering- metal atoms impinging on semiconductor substrate 128 instead of sorption surface 236.

FIG. 3 depicts a further embodiment of an ion implanter system 300 and having a pumping system 310 in which pump housing 340 contains evacuated gettering chamber 334 and evacuated film-formation chamber 342. Pump housing 340 also includes gettering elements 344, which preferably are rotatable cylinders, each having a sorption surface 336 on which a solid film 338 of gettering metal is disposed. Thus, as cylindrical gettering element 344 rotates on its axis, any particular point on sorption surface 336 moves sequentially from film-formation chamber 342 into gettering chamber 334 and back again, in a continuous cycle. When a particular point on sorption surface 336 is in film-formation chamber 342, gettering-metal atoms deposited on that part of the sorption surface form a fresh solid film 338 of gettering

metal. As gettering element 344 continues to rotate on its axis, fresh un-reacted atoms of gettering metal rotate into gettering chamber 334. A freshly deposited solid film 338 is exposed to contaminants in gettering chamber 334 and react with them at sorption surface 336. In particular, titanium gettering atoms and solid film 338 react with hydrogen atoms from outgassing, and remove the atoms from ion beam path 232.

Gettering-metal source 350 is located in film-formation chamber 342. Preferably, a plurality of gettering-metal sources 350 are located in film-formation chamber 342.

In a method in accordance with the invention, energy is applied to gettering-metal source 350 so that gettering-metal atoms move from gettering-metal source 350 onto sorption surface 336 to form solid film 338. Preferably, gettering-metal source 350 is a titanium filament that is heated so that titanium atoms sublimate from the filament and condense on sorption surface 336. In an alternative embodiment, the gettering- metal source is a sputter target from which gettering-metal atoms are sputtered to form solid film 338 on sorption surface 336. Gettering chamber 334 and film- formation chamber 342 are"fluidically separated"from each other. This means that gaseous or particulate gettering metal cannot migrate from film-formation chamber 342 into gettering chamber 334. Thus, when gettering metal atoms sublimate from a gettering-metal source, such as a titanium filament 338, then the vaporous gettering metal condenses on sorption surface 336 and a does not migrate into gettering chamber 334, where it would interfere with the ion beam and the implantation process. Similarly, particulate gettering metal sputtered from a gettering-metal target deposits on sorption surface 336 in film-formation chamber 342 and does not pass through into gettering chamber 334.

FIG. 4 depicts pump housing 340 and other components of pumping system 310 in greater detail. Preferably, cylindrical gettering elements 344 have teeth-like longitudinal grooves 352 in sorption surface 336. Grooves 352 increase the total exposed surface area of sorption surface 336, thereby increasing the capacity and velocity of gettering pump system 310. Furthermore, grooves 352 mesh together as gettering elements 344 rotate, thereby enhancing the seal between film-formation chamber 342 and gettering chamber 334. End guards 354 provided a seal between

pump housing wall 356 and portion 358 of gettering element 344 proximate to wall 356. Titanium filament 350 in film-formation chamber 342 is connected to the exterior through conventional vacuum seals 360. During operation, and the ion beam enters from upstream through inlet for 42 along ion-beam path to 32 passing through gettering chamber 334 and exits throughout less for 43 and downstream direction into an implantation chamber forwardly semiconductor substrate.

FIG. 5 depicts in schematic form and embodiment in accordance with the invention in which a pumping system 510 has a cylindrical pump housing 540 with ion inlet 542 and ion outlet 543. An ion beam passes through pumping system 510 along the ion beam path 532. FIG. 6 is a cross-sectional view along the cylindrical axis of pump housing 540. In FIG. 6, pump housing 540 encloses evacuated gettering chamber 534, titanium-filaments (gettering-metal sources) 550 located in evacuated film-formation chamber 542, and cylindrical rotatable gettering elements 544 separating gettering chamber 534 and film-formation chamber 542. Gettering elements 544 have teeth-like grooves 352 that enhance the separation of gettering chamber 534 from film-formation chamber 542. During operation, atoms of titanium gettering-metal are deposited in solid titanium film 538 on sorption surfaces 536 by known sublimation techniques. Slow, synchronized rotation of gettering elements 544 continuously exposes freshly deposited titanium in gettering chamber 534, where it reacts to remove contaminants, especially hydrogen, from ion beam path 532.

FIG. 7 shows an alternative embodiment of a pumping system 710 in accordance with the invention in which rotatably movable gettering elements 744 have relatively smooth sorption surfaces 736. Since gettering elements 744 do not have intermeshing teeth as any embodiments of FIGS. 3-6, conventional vacuum sealing means 762 are applied to enhance the separation provided by gettering elements 744 between gettering chamber 734 and film-formation chamber 742.

FIG. 8 depicts in schematic form a second basic embodiment of a pumping system in accordance with the invention. Pumping system 810 includes evacuated implantation chamber 826 containing semiconductor substrate 828. An ion beam following ion beam path 832 enters implantation chamber 826 through implanter inlet

825 to impinge on semiconductor substrate 828. At least one and preferably a plurality of gettering elements 844, each having a sorption surface 836, are located upstream of semiconductor substrate 828 relative to the downstream direction of ion beam path 832. Preferably, gettering elements 844 are located in implantation chamber 826 between semiconductor substrate 828 and implanter inlet 825. Pumping system 810 further comprises a film-formation chamber 842 contiguous to each of gettering elements 844. Each film-formation chamber 842 has an interior 870 and the exterior 871. Each film-formation chamber 842, that is, the interior 870 of the film- formation chamber, is fluidically separated from implantation chamber 826 and semiconductor substrate 828. That means that vaporous atoms or small particles of gettering metal cannot migrate from the interior 870 to the exterior 871, where they could interfere with the ion beam 832 or or semiconductor substrate 828. A gettering- metal source 830 is located within film-formation chamber 842. Gettering-metal source 830 is, for example, a filament for sublimating gettering metal or a sputtering target. Using techniques known in the art, gettering-metal atoms from gettering-metal source 830 are deposited on sorption surface 836 to form a fresh solid film 838 of gettering metal. Gettering-metal source 830 and solid film 838 typically comprise a gettering-metal selected from the group of titanium, zirconium and tantalum.

Preferably, they comprise titanium. As depicted in FIG. 8, the gettering element 844 preferably is a rotatable cylinder. The sorption surface 836 is movable between the interior 870 and the exterior 871 of film-formation chamber 842. As each gettering element 844 is slowly rotated, a freshly deposited solid film 838 of gettering metal on sorption surface 836 is rotated into the space 880 between semiconductor substrate 828 and implanter inlet 825, where the reactive gettering metal reacts with contaminants, especially with hydrogen atoms resulting from outgassing. As in other embodiments of system in a method in accordance with the invention, a system and method described with reference to FIG. 8 provides a freshly deposited solid film of gettering metal in the ion-beam path upstream from the semiconductor substrate, between the semiconductor substrate and ion-beam measuring and control elements.

During operation, a gettering-metal source is continuously used to form a fresh solid

film of gettering metal, but the gettering-metal source is isolated from the ion-beam path and the semiconductor substrate so that gettering-metal atoms cannot migrate in and interfere with the ion implantation process.

There have been described an improved system and an improved method of ion implantation in semiconductor substrates. The importance of certain features of the invention has been described; for example, the importance of locating a gettering chamber upstream of the implantation chamber, between the ion optical element and the semiconductor substrate. Also, the importance of isolating the formation of the solid film of gettering metal from the semiconductor substrate and the ion beam path has been described. It is obvious that a gettering pumping system in accordance with the present invention is useful to reduce the presence and the deleterious effects of contaminants in other types of high-vacuum industrial processes in which contaminants tend to migrate from a source of contamination to another location in the processing system where the contaminants interfere with a critical process step.

As in the embodiments described above, an essential feature of a pumping system in accordance with the invention is locating a gettering element along a process path between the source of the contamination and the critical process step to be protected, while isolating a gettering-metal source from the ion-beam path. It is, therefore, understood that material compositions, process step sequences, operating conditions and apparati may vary from the particular embodiments described herein without departing from the essential elements of the invention, which are claimed below.