Technique for isocentric ion beam scanning

A technique for isocentric ion beam scanning is disclosed. In one particular exemplary embodiment, the technique may be realized by an apparatus for isocentric ion beam scanning. The apparatus may comprise an end station having a mechanism for holding and translating a wafer. The apparatus may also comprise a deflector that tilts an ion beam to a predetermined angle and directs the ion beam into the end station. The wafer may be translated with respect to the ion beam for isocentric scanning at least a portion of a surface of the wafer, and wherein the ion beam is maintained at the predetermined angle during isocentric scanning.

FIELD OF THE DISCLOSURE

The present disclosure relates generally to semiconductor manufacturing and, more particularly, to a technique for isocentric ion beam scanning.

BACKGROUND OF THE DISCLOSURE

Ion implantation is a process of depositing chemical species into a substrate by direct bombardment of the substrate with energized ions. In semiconductor manufacturing, ion implanters are used primarily for doping processes that alter the type and level of conductivity of target materials. A precise doping profile in an integrated circuit (IC) substrate and its thin-film structure is often crucial for proper IC performance. To achieve a desired doping profile, one or more ion species may be implanted in different doses, at different energy levels, and sometimes at different incident angles.

For a uniform distribution of dopant ions, an ion beam is typically scanned across the surface of a target wafer.FIG. 1ashows a typical beam path10formed by scanning an ion beam spot102across a wafer104. The ion beam spot102may be scanned back and forth in the X direction while the wafer104is translated in the Y direction, thereby forming the zigzag pattern of the beam path10.FIG. 1bshows another typical beam path11formed by scanning the ion beam spot102across the wafer104. Here, the ion beam spot102is scanned along straight lines in the X direction and makes square turns at the end of a sweep.

As the semiconductor industry is producing devices with smaller and smaller feature sizes, ion beams with lower energies are required for wafer implantation. Compared with high- or medium-energy ion beams, low-energy ion beams present some unique challenges. For example, a low-energy ion beam usually has a large beam spot, which can cause problems for beam utilization and uniformity tuning. In addition, the shape or current density of a low-energy ion beam can change substantially as it propagates down a beam line. As a result, if different portions of a wafer meet the low-energy ion beam at different positions along the beam line, different dopant profiles may be created for the different portions of the wafer. This problem is illustrated inFIG. 2, wherein an ion beam202is propagating along the Z direction. If a wafer204meets the ion beam202at a first position Z=Z1, the wafer204will see a beam spot206. If the wafer204meets the ion beam202at a second position Z=Z2, for example, when the ion beam202is scanned across the surface of the wafer204, the wafer204will see a beam spot208that may be quite different from the beam spot206. Therefore, the portion of the wafer204implanted with the ion beam at beam spot208may have a different dopant profile from the portion implanted with the ion beam at beam spot206. The problem of beam-line variation is not unique to low-energy ion beams but can also be observed in some high-current ion beams.

As a countermeasure to the beam-line variation problem, a concept known as “isocentric scanning” has been proposed. Isocentric scanning generally involves moving a wafer with respect to an ion beam in such a way that the point in space where the ion beam strikes the wafer surface remains the same no matter which part of the wafer surface meets the ion beam. It should be noted that isocentric scanning does not have to cover the entire surface of a wafer but may cover only a portion of the wafer surface.

FIG. 3illustrates isocentric scanning of a wafer304. An ion beam302may propagate along the Z direction, and the wafer304may be translated within the X-Y plane to perform the isocentric scanning. Effectively, the wafer304is moved in such a way that a beam spot306(i.e., a cross section of the ion beam302) having the same shape and current density distribution will strike different portions of the surface of the wafer304at a same point (X0, Y0, Z0) in space.

FIG. 3only shows the simplest example of isocentric scanning wherein the ion beam302has a normal incidence on the wafer304. These days, however, it is often necessary to implant a wafer at one or more different incident angles other than the normal incident angle, which technique is known as “angled ion implantation.” Sometimes, it may also be desirable to implant different portions of a wafer at different incident angles. In existing ion implantation systems, angled ion implantation is typically accommodated by maintaining an ion beam along a fixed reference direction and tilting a target wafer with respect to the ion beam. To achieve isocentric scanning on a tilted wafer, the wafer has to be moved in a complex pattern with respect to the ion beam.

FIG. 4illustrates a complex movement of a tilted wafer404for isocentric scanning by an ion beam402. The ion beam402propagates along the Z direction. For isocentric scanning, the wafer404is moved in such a way that the same beam spot (or ion beam cross section)406strikes the surface of the wafer404. Since the wafer404is tilted with respect to the ion beam402, the wafer404is moved not only in the X-Y plane, but also in the Z direction. For example, at position A, the center of the tilted wafer404may be at Z=ZA. At position B, the center of the tilted wafer404may be at Z=ZB(ZB≠ZA). The movement in the Z direction has to precisely coordinated with the movement in the X-Y plane in order to meet the isocentric scanning requirement.

To precisely control the tilting and the three-dimensional (3-D) translation of a wafer for isocentric scanning, existing ion implantation systems have to be equipped with a fairly complex end station to hold, tilt, and move the wafer.FIG. 5illustrates a typical setup for isocentric scanning in existing ion implantation systems. An end station508may be a process chamber or similar component that houses a mechanism (not shown) for holding a wafer504and for controlling wafer movements. An ion beam502may propagate along a reference direction50and may be directed into the end station508via an aperture510. With respect to the end station508, the angle of the ion beam502is relatively fixed. Isocentric scanning is achieved by tilting and moving the wafer504, all inside the end station508. For example, the wafer504may be tilted by an angle θ so that the normal direction52of the wafer504is at an angle θ with respect to the reference direction50. The mechanism in the end station508is capable of moving the wafer504in all three dimensions based on the tilt angle θ and the isocentric scanning requirement. Such capabilities necessitate a sophisticated design of the end station508that is often expensive to build and maintain.

In view of the foregoing, it would be desirable to provide a technique for isocentric ion beam scanning which overcomes the above-described inadequacies and shortcomings.

SUMMARY OF THE DISCLOSURE

A technique for isocentric ion beam scanning is disclosed. In one particular exemplary embodiment, the technique may be realized by an apparatus for isocentric ion beam scanning. The apparatus may comprise an end station having a mechanism for holding and translating a wafer. The apparatus may also comprise a deflector that tilts an ion beam to a predetermined angle and directs the ion beam into the end station. The wafer may be translated with respect to the ion beam for isocentric scanning at least a portion of a surface of the wafer, and wherein the ion beam is maintained at the predetermined angle during isocentric scanning.

In accordance with other aspects of this particular exemplary embodiment, the deflector may tilt the ion beam with an electrostatic field, a magnetic field, or an electromagnetic field.

In accordance with further aspects of this particular exemplary embodiment, the wafer may be translated in a two-dimensional movement during isocentric scanning.

In accordance with additional aspects of this particular exemplary embodiment, the end station may further comprise a plasma flood gun that can be repositioned based on the predetermined angle of the ion beam.

In accordance with another aspect of this particular exemplary embodiment, the end station may further comprise an aperture that can be repositioned to admit the ion beam into the end station based on the predetermined angle. The aperture may block one or more particles in the ion beam that are not tilted by the deflector.

In another particular exemplary embodiment, the technique may be realized as a method for isocentric ion beam scanning. The method may comprise tilting an ion beam to a predetermined angle. The method may also comprise directing the ion beam into an end station holding a wafer. The method may further comprise translating the wafer with respect to the ion beam for isocentric scanning at least a portion of a surface of the wafer, wherein the ion beam is maintained at the predetermined angle during isocentric scanning.

In accordance with other aspects of this particular exemplary embodiment, the deflector may tilt the ion beam with an electrostatic field, a magnetic field, or an electromagnetic field.

In accordance with further aspects of this particular exemplary embodiment, the wafer may be translated in a two-dimensional movement during isocentric scanning.

In accordance with additional aspects of this particular exemplary embodiment, the method may further comprise preventing charge buildup on the wafer surface with a plasma flood gun that can be repositioned based on the predetermined angle of the ion beam.

In accordance with another aspect of this particular exemplary embodiment, the method may further comprise repositioning an aperture to admit the ion beam into the end station based on the predetermined angle of the ion beam.

In accordance with yet another aspect of this particular exemplary embodiment, the method may further comprise blocking one or more particles in the ion beam that are not tilted by the deflector.

In yet another particular exemplary embodiment, the technique may be realized as at least one signal embodied in at least one carrier wave for transmitting a computer program of instructions configured to be readable by at least one processor for instructing the at least one processor to execute a computer process for performing the method as recited above.

In still another particular exemplary embodiment, the technique may be realized as at least one processor readable carrier for storing a computer program of instructions configured to be readable by at least one processor for instructing the at least one processor to execute a computer process for performing the method as recited above.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the present disclosure provide a new approach for isocentric ion beam scanning wherein an ion beam is tilted to a predetermined angle before the ion beam is directed into an end station for isocentric scanning of a target wafer. With the ion beam maintained at the predetermined angle, there may be no need to tilt the target wafer itself. As a result, isocentric scanning of the target wafer may involve a two-dimensional (2-D), rather than a three-dimensional (3-D), translation of the target wafer. Since it is no longer necessary to tilt the target wafer or to coordinate its 3-D movements, the end station may have a simpler and therefore less expensive design.

FIG. 6illustrates an exemplary method for isocentric ion beam scanning in accordance with an embodiment of the present disclosure. In an ion implantation system, an end station608may hold a wafer604. The normal direction of the wafer604may be aligned with the Z direction.

First, an ion beam602may be generated and an incident angle (e.g., θ′) may be determined for an angled ion implantation on the wafer604or a portion thereof.

Next, the ion beam602may be tilted to the predetermined angle θ′ with respect to the Z direction before the ion beam602is directed into the end station608via an aperture610. Tilting of the ion beam602may be achieved in a number of ways. Typically, the ion beam602may be tilted with an electrostatic, magnetic, or electromagnetic field. The tilt angle may also be achieved, for example, by adjusting the relative position of the end station608with respect to a reference direction60of the ion beam602. Meanwhile, the wafer604may maintain its position inside the end station608without tilting.

Then, with the ion beam602stationary (i.e., at the angle θ′), the wafer604may be translated in the X-Y plane, without any movement in the Z direction, so that the portion(s) of the wafer604to be implanted will meet the same beam spot606. Thus, an isocentric scanning of the wafer604may be achieved for different incident angles without tilting the wafer604or moving it in a complex 3-D pattern.

FIG. 7shows an exemplary system700for isocentric ion beam scanning in accordance with an embodiment of the present disclosure. The system700may comprise an end station708and a deflector device712.

The deflector device712may employ an electrostatic field, a magnetic field, or a combination thereof to change the propagation direction of an ion beam702. The deflector device712may cause the ion beam702to be tilted at an angle θ″ with respect to a reference direction (e.g., the Z direction).

The end station708may comprise a wafer handling mechanism706that can hold a wafer704and control its movements (e.g., translation or rotation within the X-Y plane). The end station708may also comprise a sliding aperture710that can be repositioned to admit the ion beam702into the end station708. The position of the sliding aperture710may depend on the tilt angle θ″ of the ion beam702. Particles in the ion beam702that have not been deflected to the angle θ″ may be blocked by a sidewall of the aperture710or a sidewall of the end station708. The end station708may further comprise a plasma flood gun (PFG)714that prevents charge buildup on the wafer704.

In operation, the deflector device712may be adjusted to tilt the ion beam702to a predetermined angle (e.g., θ″). Then, the sliding aperture710(or a relative position between the deflector712and the end station708) may be adjusted to allow the properly tilted ion beam702to pass therethrough and enter the end station708. Then, the wafer handling mechanism706may be activated to move the wafer704with respect to the ion beam702to effectuate isocentric scanning.

According to embodiments of the present disclosure, during an isocentric scanning of a wafer as described herein, the resulting beam path on the wafer is not limited to any particular pattern. When performing an isocentric scanning with a spot beam, for example, the beam path may have a zig-zag pattern as illustrated inFIG. 1aor a raster pattern as illustrated inFIG. 1b.

When performing an isocentric scanning with a ribbon beam, only one-dimensional translation of the wafer may be needed if the ribbon beam is at least as wide as the wafer. One example is shown inFIG. 8.FIG. 8shows an exemplary system800for isocentric ion beam scanning in accordance with an embodiment of the present disclosure. The system800may be similar to the system700shown inFIG. 7, except that the system800employs a ribbon beam802for isocentric scanning of the wafer704. The ribbon beam802may be deflected by the deflector712to a tilt angle (e.g., θ′″) before being directed into the end station708via the sliding aperture710. The ribbon beam802may be made no narrower than the diameter of the wafer704. Since the ribbon beam802is wide enough to cover the width of the wafer704in the X direction, there may be no need to translate the wafer704in the X direction. Instead, an isocentric scanning of the ribbon beam802on the wafer704may only involve translation of the wafer704in the Y direction.

At this point it should be noted that the technique for isocentric ion beam scanning in accordance with the present disclosure as described above typically involves the processing of input data and the generation of output data to some extent. This input data processing and output data generation may be implemented in hardware or software. For example, specific electronic components may be employed in an ion implanter or similar or related circuitry for implementing the functions associated with isocentric ion beam scanning in accordance with the present disclosure as described above. Alternatively, one or more processors operating in accordance with stored instructions may implement the functions associated with isocentric ion beam scanning in accordance with the present disclosure as described above. If such is the case, it is within the scope of the present disclosure that such instructions may be stored on one or more processor readable carriers (e.g., a magnetic disk), or transmitted to one or more processors via one or more signals.