Method and apparatus for polarizing electromagnetic radiation

According to one aspect of the invention, a method and apparatus for polarizing electromagnetic radiation is provided. The electromagnetic radiation may be divided into first and second portions, substantially all of the first portion may be linearly polarized in a first direction and substantially all of the second portion may be linearly polarized in a second direction, the first direction being substantially orthogonal to the second direction. The linear polarization of at least one of the first and second portions may be changed such that substantially all of both of the first and second portions are linearly polarized in a third direction. At least one of the first and second portions may be redirected such that substantially all of both the first and second portions are propagating in a fourth direction.

BACKGROUND OF THE INVENTION

1). Field of the Invention

Embodiments of this invention relate to a method and apparatus for polarizing electromagnetic radiation, particularly for use in semiconductor substrate processing.

2). Discussion of Related Art

Integrated circuits are formed on semiconductor wafers. The wafers are then sawed (or “singulated” or “diced”) into microelectronic dice, also known as semiconductor chips, with each chip carrying a respective integrated circuit. Each semiconductor chip is then mounted to a package, or carrier, substrate. Often the packages are then mounted to a motherboard, which may then be installed into a computing system.

Numerous steps may be involved in the creation of the integrated circuits, such as the formation and etching of various semiconductor, insulator, and conductive layers. Before the various layers may be etched, a layer of light-sensitive photoresist is formed on the substrate to protect the portions of the substrate that are not to be etched.

Machines referred to as photolithography steppers are used to expose the desired pattern in the photoresist layer. In order to achieve the desired pattern, light is directed through a reticle, or “mask,” and focused onto the substrate. Typically, the light sources used in the steppers emit light that is randomly polarized, which leads to a lack of precision in the exposure, as well as an increase in the size of the features that can be exposed.

Recently, attempts have been made to linearly polarize the light using a polarizing beam splitter which only captures 50 percent of the light that passes through it. However, because of the light used often has very small wavelengths, and is thus expensive to create, such a method is not cost effective as a large portion of the light is wasted.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, various aspects of the present invention will be described, and various details set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced with only some or all of the aspects of the present invention, and the present invention may be practiced without the specific details. In other instances, well-known features are admitted or simplified in order not to obscure the present invention.

It should be understood thatFIGS. 1 through 9Bare merely illustrative and may not be drawn to scale.

FIG. 1toFIG. 9Billustrate a method and apparatus for polarizing electromagnetic radiation, according to one aspect of the present invention. The electromagnetic radiation may be divided into first and second portions, substantially all of the first portion may be linearly polarized in a first direction and substantially all of the second portion may be linearly polarized in a second direction, the first direction being substantially orthogonal to the second direction. The linear polarization of at least one of the first and second portions may be changed such that substantially all of both of the first and second portions are linearly polarized in a third direction. At least one of the first and second portions may be redirected such that substantially all of both the first and second portions are propagating in a fourth direction.

According to another aspect of the present invention, a beam of electromagnetic radiation, having an axis, may be directed through a substantially uniform magnetic field. The electromagnetic radiation may be substantially linearly polarized in a first direction, and flux lines of the magnetic field may extend in a second direction. The first direction may be substantially orthogonal to the second direction. A first line extending from the axis of the beam and a first portion of the beam may be substantially perpendicular to the first direction. A second portion of the beam may be propagated through a first material, within the magnetic field, having a first Verdet value. The second portion of the beam may be linearly polarized in a third direction after propagating through the first material. A second line extending from the axis of the beam and the second portion of the beam may be substantially perpendicular to the third direction.

FIG. 1illustrates a semiconductor processing apparatus, or a photolithographic stepper10, according to an embodiment of the present invention. The stepper10may include a frame12, a substrate transport subsystem14, an exposure subsystem16, and a computer control console18. The substrate transport subsystem14may be attached to and located at a lower portion of the frame12and may include a substrate support20and a substrate track22. The substrate support20may be sized to support semiconductor substrates, such as wafers with diameters of, for example, 200 or 300 mm. Although not illustrated in detail, the substrate support20may include various actuators and motors to move the substrate support20in an X/Y coordinate system which may be substantially perpendicular to the sheet, or page, on whichFIG. 1is shown. The substrate track22may include various components to place a semiconductor substrate onto the substrate support20and remove the semiconductor substrate therefrom.

The exposure subsystem16may be connected to the frame12and suspended substantially over the substrate support20. The exposure subsystem16may include an electromagnetic radiation source24, a polarization subsystem26, a collector28, a reticle30, and imaging optics32.

The electromagnetic radiation source24may be a visible light source, such as a laser source, and be connected to the frame12. In one embodiment, the electromagnetic radiation source24may be a deep ultraviolet (DUV) light source capable of emitting ultraviolet light having wavelengths of, for example, 248 nanometers, 193 nanometers, and/or 157 nanometers (nm).

FIGS. 2–4illustrate the polarization subsystem26. The polarization subsystem26may include a linear polarization subsystem34and an annular polarization subsystem36. As illustrated inFIGS. 2 and 4, the linear polarization subsystem34may include a polarizing beam splitter38, a wave-plate40, and a reflecting system42. The polarizing beam splitter38may include a central axis44which extends through a central portion of the wave-plate40, and through a central portion of the annular polarization subsystem36as illustrated inFIG. 2. The wave-plate40may be a half wave-plate as is commonly understood in the art.

The reflecting system42may include a first reflective device46, a second reflective device48, and a third reflective device50. The reflective devices46,48, and50may be mirrors. The first reflective device46, as illustrated inFIGS. 2 and 3, may be substantially circular and be positioned such that a reflective surface thereof faces toward the central axis44of the polarizing beam splitter38at an angle of approximately 45 degrees. The second reflective device48may be substantially be conic in shape with a circular outer edge and have a reflective surface with an angled cross-section, as illustrated inFIG. 3. The second reflective device48may be positioned a distance from the first reflective device42in a direction that is substantially parallel to the central axis44of the polarizing beam splitter38. The third reflective device50may be substantially circular in shape with a circular opening52at a central portion thereof. The third reflective device50may be positioned such that the central axis44of the polarizing beam splitter38passes through a central portion of the opening52and a reflective surface thereof faces partially toward the second reflective device48at an angle of approximately 45 degrees to the central axis44.

Referring again toFIG. 2, the annular polarization subsystem36may include a magnetic field generator54and an annular polarization device56. The magnetic field generator54may include, for example, two Helmholtz coils58, as is commonly understood in the art. The Helmholtz coils58may include numerous coils of electrically conductive wire within and be positioned a distance apart with the central axis44perpendicular thereto and passing through a central portion thereof. The annular polarization device56may be cylindrically shaped and positioned such that the central axis44of the polarizing beam splitter38passes through a central portion thereof. Although not illustrated in detail, it should be understood that the Helmholtz coils58may have a central axis that is congruent with the central axis44of the polarizing beam splitter38. Although not illustrated, the magnetic field generator54may also include a power supply that is electrically connected to the coils of wire within the Helmholtz coils58.

FIG. 4illustrates the annular polarization device56in greater detail. The annular polarization device56may be made of a material with a high Verdet constant, such as terbium gallium garnet (TGG), which has a Verdet constant of 0.12 min/Oe.cm at 1064 nm. The high Verdet constant material may be shaped to have a gradually increasing thickness around a central axis60of the annular polarization device56. The central axis60of the annular polarization device56may be congruent with the central axis44of the polarizing beam splitter38.

At a first portion62of the annular polarization device56, extending radially from the central axis60, the high Verdet constant material may have a first thickness64which may be negligible. At a second portion66of the annular polarization device56extending radially from the central axis60, the high Verdet constant material may have a second thickness68which may be greater than the first thickness64. At a third portion70of the annular polarization device56, the high Verdet constant material may have a third thickness72which may be greater than the second thickness68. Thus, as illustrated inFIG. 4, the upper surface of the high Verdet constant material “spirals” upwards around the central axis60.

As one skilled in the art will appreciate, each portion of the annular polarization device56may be understood to have a particular “Verdet value” which may be based on the particular Verdet constant of the particular material and the thickness of the material at each particular portion. If either the thickness of the high Verdet constant material is increased, or the material itself is changed to increase the Verdet constant of the material, the amount of Faraday rotation experienced by electromagnetic radiation as it propagates through each particular portion of the material will increase as the Verdet value increases.

Although not illustrated for clarity, it should be understood that all of the components of the polarizing subsystem26may be connected to the frame12.

Referring again toFIG. 1, the collector28, the reticle30, and the imaging optics32may be connected to the frame12and positioned under the polarization subsystem26. The collector28may be in the form of a large lens, as is commonly understood in the art. The reticle30may be positioned below the collector28and may be in the form of “mask,” as is commonly understood in the art, and may include a plurality of openings therein. The imaging optics32may be positioned below the reticle30and, although not illustrated in detail, may include a plurality of lenses of varying shapes and sizes.

The computer control console18may be in the form a computer having memory for storing a set of instructions and a processor connected to the memory for executing the instructions, as is commonly understood in the art. The computer control console18may be electrically connected to both the substrate transport subsystem14and the exposure subsystem16, as well as all of the various components thereof, and may control and coordinate the various operations of the stepper10.

In use, a semiconductor substrate73, such as a wafer having a diameter of, for example, 200 or 300 mm, may be placed on the substrate support20by the substrate track22. The substrate73may have a plurality of integrated circuits, divided amongst multiple microelectronic dice, formed thereon and a layer of photoresist deposited over the dice. The power supply of the magnetic field generator54may then be activated to supply power to the Helmholtz coils58, resulting in a substantially uniform magnetic field, indicated by arrows75inFIG. 2, being generated between the Helmholtz coils58. The magnet field may have a strength of between, for example, 0.1 and 1.0 Tesla.

The electromagnetic radiation source24may be activated to emit electromagnetic radiation, such as DUV light, toward the polarization subsystem26. Referring now toFIGS. 2 and 3, the electromagnetic radiation74may enter the polarizing beam splitter38on a side opposite the wave-plate40and in a direction that is substantially parallel with the central axis44of the polarizing beam splitter38. Although not illustrated in detail, as the electromagnetic radiation74enters the polarizing beam splitter38, the electromagnetic radiation74may be randomly, or arbitrarily, polarized.

As the electromagnetic radiation74propagates through the polarizing beam splitter38along the central axis44, the electromagnetic radiation74is split, or divided, into a first portion76and a second portion78. The first portion76of the electromagnetic radiation74may pass through the polarizing beam splitter38along the central axis44and into the wave-plate40. The second portion78of the electromagnetic radiation74may be reflected, or redirected, in a first direction80which is substantially perpendicular to the central axis44toward the first reflective device46.

It should be understood that horizontal symbols82may indicate that the electromagnetic radiation74at that point may be linearly polarized in a direction that is substantially parallel to the arrows shown on the horizontal symbols82and/or the page on which the horizontal symbols82appear. Vertical symbols84may indicate that the electromagnetic radiation74at that point is linearly polarized in a direction that is perpendicular to the sheet on which the vertical symbols84appear. Therefore, electromagnetic radiation associated with a horizontal symbol82may have a linear polarization in a direction that is orthogonal, or perpendicular, to electromagnetic radiation that is associated with a vertical symbol84. As one skilled in the art will appreciate, the directions indicated by symbols82and84may specifically refer to the direction of linear polarization of the E-field of the electromagnetic radiation.

Thus, as illustrated inFIG. 3, the first portion76of the electromagnetic radiation74may become linearly polarized in a direction as indicated by the horizontal symbols82as it passes through the polarizing beam splitter38. Likewise, the second portion78of the electromagnetic radiation74may become linearly polarized in a direction as indicated by the vertical symbols84as it is redirected by the polarizing beam splitter38.

As the first portion76of the electromagnetic radiation74passes through the wave-plate40, the first portion76may become linearly polarized in a direction as indicated by the vertical symbols84. Thus, after the first portion76has passed through the wave-plate40both the first76and second78portion of the electromagnetic radiation74may be linearly polarized in a direction as indicated by the vertical symbols84.

The first portion76of the electromagnetic radiation74may continue to propagate along the central axis44of the polarizing beam splitter38and pass through the opening52of the third reflective device50.

The second portion78of the electromagnetic radiation74may be reflected by the first reflective device46toward the second reflective device48in a direction that is substantially parallel to the central axis44of the polarizing beam splitter38. The second portion78may then be reflected by the second reflective device48toward the third reflective device50.

Still referring toFIG. 3, because of the conic shape of the reflective surface of the second reflective device48, substantially all of the second portion78of the electromagnetic radiation74may be reflected toward the reflective surface of the third reflective device50. In particular, substantially none of the second portion78of the electromagnetic radiation74may be reflected by the second reflective device48into the opening52of the third reflective device50. The second portion78of the electromagnetic radiation74may be reflected by the third reflective device50in a direction substantially parallel to the central axis44of the polarizing beam splitter38away from the polarizing beam splitter38and, as illustrated inFIG. 2, toward the annular polarization subsystem36.

As the first76and second78portions of the electromagnetic radiation propagate away from the linear polarization subsystem34, the first76and second78portions may jointly form a beam of electromagnetic radiation.FIG. 5illustrates the beam86of electromagnetic radiation as it propagates toward the annular polarization subsystem36. The beam86of electromagnetic radiation may include the first portion76and the second portion78of electromagnetic radiation74from the linear polarization subsystem34. All of the electromagnetic radiation of the beam86may linearly polarized in substantially the same direction as indicated by the horizontal symbols82.

It should be understood that because of the change of the field of view betweenFIG. 3andFIG. 5, the vertical symbols84ofFIG. 3and the horizontal symbols82ofFIG. 5may indicate a linear polarization in the same direction.

Referring toFIG. 5, the beam86, when viewed in cross-section perpendicular to the central axis44of the polarizing beam splitter38may be substantially circular with the central axis44of the polarizing beam splitter38being congruent with a central axis thereof. A line88extending from the central axis44to a first portion90of the beam86, which extends from the axis44to an outer edge of the beam86, may be perpendicular to the direction of linear polarization as indicated by the horizontal symbols82. Although not illustrated, it should be understood that the same may be true for a line drawn between a portion of the beam on a side of the central axis44directly opposing the first portion90. However, lines92and96extending from the central axis44to second94and third98portions of the beam86, not directly opposite the first portion90, may not be perpendicular to the direction of linear polarization as indicated by the horizontal symbols82inFIG. 5.

Referring again toFIG. 2, the beam86may continue to propagate into the magnetic field generated by the magnetic field generator54and into the annular polarization device56. Although not illustrated in detail, it should be understood that the flux lines of the magnetic field may extend in the direction indicated by arrows75, and thus may be orthogonal to the direction of linear polarization as indicated inFIG. 5.

Referring now to bothFIGS. 4 and 5, the annular polarization device56may be positioned so that the first portion90of the beam86of electromagnetic radiation passes through the first portion62of the annular polarization device56. The second portion94of the beam86of electromagnetic radiation may propagate through the second portion66of the annular polarization device. The third portion98of the beam86of electromagnetic radiation may propagate through the third portion70of the annular polarization device. Due to the high Verdet constant of the material of the annular polarization device56, as well as the magnetic field generated by the magnetic field generator54, as the electromagnetic radiation propagates through the high Verdet constant material, the polarization of the electromagnetic radiation may be rotated a particular amount depending upon the thickness of the high Verdet constant material and the value of the Verdet constant of the particular material used. Thus, after the beam86has propagated through the annular polarization device56, the electromagnetic radiation of the beam86may no longer be linearly polarized as illustrated inFIG. 5.

FIG. 6illustrates the beam86of electromagnetic radiation after having propagated through the annular polarization device56. The beam86may now be “annularly” polarized as illustrated inFIG. 6. Both the first76and second78portions of electromagnetic radiation may now be polarized such that a line drawn between the central axis44and each respective portion, or section, of either the first76or second78portions will be perpendicular to the linear polarization of each respective section. Thus, as illustrated inFIG. 6, the line88between the central axis44and the first portion90of the beam may remain perpendicular to the direction of linear polarization as indicated by the horizontal symbol821. Likewise, the line92drawn between the central axis44and the second portion94of the beam86may now be perpendicular to the direction of linear polarization as indicated by the horizontal symbol822which indicates the direction of linear polarization for the second section94. The same may be true for the third section98, as the line96from the central axis44to the third portion98of the beam86may be perpendicular to the direction of linear polarization indicated by the horizontal symbol823for the third section98.

Referring toFIGS. 1 and 2, the beam of electromagnetic radiation may then propagate from the annular polarization subsystem36and into the collector28. The collector28may focus the electromagnetic radiation through the reticle30and into the imaging optics32. The imaging optics32may further focus the electromagnetic radiation before the electromagnetic radiation is directed onto the semiconductor substrate73, where the electromagnetic radiation exposes the layer of photoresist, as is commonly understood in the art. It should be understood that the electromagnetic radiation may retain the annular polarization, as illustrated inFIG. 6, as the photoresist is exposed.

The wafer support20may move the semiconductor substrate73in the X/Y coordinate system so that individual sections of the semiconductor substrate73, which may correspond with one or more of the dice, may be exposed one at a time, as is common understood in the art. When the entire photoresist layer has been exposed, the substrate track22may remove the semiconductor substrate73from the substrate support22, and replace it with a second semiconductor substrate to be exposed as described above.

One advantage is that the electromagnetic radiation, as it exposes the photoresist layer, may be annularly polarized, as illustrated inFIG. 6. Therefore, size of the features exposed on the semiconductor substrate may be minimized and the accuracy with which the features are exposed can be increased. Another advantage is that the electromagnetic radiation may be linearly polarized, and annularly polarized, without a substantial portion of the electromagnetic radiation being lost, as all of the electromagnetic radiation is directed onto the substrate. Therefore, an increased amount of electromagnetic radiation may be directed onto the semiconductor wafer, increasing the exposure rate of the stepper. A further advantage is that the polarization of the electromagnetic radiation may be controlled regardless of the polarization of the electromagnetic radiation being emitted by the electromagnetic radiation source.

FIG. 7Aillustrates a linear polarization subsystem100according to another embodiment of the present invention. The linear polarization subsystem100may include a polarizing beam splitter102, a wave-plate104, a reflecting system106, including a first108and a second110reflective device, and a defractive optical element112. The polarizing beam splitter102and the wave-plate104may be similar to the polarizing beam splitter38and the wave-plate40illustrated inFIGS. 2 and 3. Referring again toFIG. 7A, the polarizing beam splitter102may have a central axis114extending through a central portion thereof. The first reflective device may be positioned a distance away from the polarizing beam splitter102in a direction that is substantially perpendicular to the central axis114so that a reflective surface thereof faces the central axis114at an angle. The defractive optical element112may be positioned along the central axis114of the polarizing beam splitter102. The second reflective device110may be positioned on a side of the defractive optical element112opposite the polarizing beam splitter102.

In use, the linear polarization subsystem110may be used within the stepper10illustrated inFIG. 1in place of the linear polarization subsystem34illustrated inFIGS. 2 and 3. Electromagnetic radiation116may be directed into the polarizing beam splitter102in a direction substantially parallel to the central axis114toward the defractive optical element112. The electromagnetic radiation may be split, or divided, into a first portion118and a second portion120by the polarizing beam splitter102. The first portion118may propagate through the polarizing beam splitter102in a direction substantially parallel to the central axis114and may be linearly polarized in a direction as indicated by the horizontal symbols82. The second portion120of the electromagnetic radiation116may be redirected toward the first reflective device108and have a linear polarization in a direction as indicated by the vertical symbols84as it propagates from the polarizing beam splitter102.

As the first portion118of the electromagnetic radiation116passes through the wave-plate104, the linear polarization of the first portion118may be changed such that the first portion118is linearly polarized in a direction as indicated by the vertical symbols84. The first portion118of the electromagnetic radiation116may then propagate to the defractive optical element112where it may be defracted away from the defractive optical element112in a direction that is substantially perpendicular to the defractive optical element112.

The second portion120of the electromagnetic radiation116may be reflected by the first reflective device108, through the electromagnetic radiation116propagating from the defractive optical element112, to the second reflective device110. The second portion120may be reflected by the second reflective device110toward the defractive optical element112and be defracted away from the defractive optical element112in a direction substantially parallel to the first portion118.

FIG. 7Billustrates a linear polarization subsystem122according to a further embodiment of the present invention. The linear polarization subsystem122may include a polarizing beam splitter124, a wave-plate126, a reflecting system128, including a first130and a second132reflecting device, and a volume hologram134. The polarizing beam splitter124, the wave-plate126, and the first reflective device130may be similar to the polarizing beam splitter38, the wave-plate40, and the first reflective device46illustrated inFIGS. 2 and 3. Referring again toFIG. 7B, the second reflective device132may be positioned a distance from the first reflective device130in a direction that is substantially parallel to a central axis136of the polarizing beam splitter124with a reflective surface of the second reflective device132facing the central axis136at an angle of approximately 45 degrees. The hologram134may be positioned along the central axis136of the polarizing beam splitter124on a side of the wave-plate126opposite the polarizing beam splitter124.

In use, electromagnetic radiation138may be directed into the polarizing beam splitter124toward the hologram134in a direction that is substantially parallel to the central axis136of the polarizing beam splitter124. The polarizing beam splitter124may split, or divide, the electromagnetic radiation138into a first portion140and a second portion142. The first portion140of the electromagnetic radiation138may propagate through the polarizing beam splitter124in a direction that is substantially parallel to the central axis136and may be linearly polarized in a direction as indicated by the horizontal symbols82. The first portion140may then pass through the wave-plate126where the linear polarization of the first portion140may be changed to a linear polarization in a direction as indicated by the vertical symbols84. The first portion140of the electromagnetic radiation138may then propagate into the hologram134in a direction that is substantially parallel to the central axis136of the polarizing beam splitter124.

The second portion142of the electromagnetic radiation138may be redirected by the polarizing beam splitter124toward the first reflective device130in a direction that is substantially perpendicular to the central axis136with a linear polarization in a direction as indicated by the vertical symbols84. The second portion of142of the electromagnetic radiation138may then be reflected by the first reflective device130toward the second reflective device132in a direction that is substantially parallel to the central axis136of the polarizing beam splitter124. The second portion142may then be reflected by the second reflective device toward the hologram134in a direction that is substantially perpendicular to the central axis136of the polarizing beam splitter124. As is commonly understood in the art, the first140and second142portions of electromagnetic radiation138may be co-linearly diffracted by the hologram134and redirected away from the central axis136of the polarizing beam splitter124at an angle of approximately 45 degrees from the central axis136.

FIG. 8illustrates an annular polarization device144according to another embodiment of the present invention. The annular polarization device144may be made of similar materials and work according to the same principles as the annular polarization device56illustrated inFIG. 4. Referring again toFIG. 8, the annular polarization device144, however, may include a high Verdet constant material with a gradually increasing thickness approximately half way around a central axis146thereof. The high Verdet constant material may be shaped such that a first portion148and a second portion150, at an opposing side of the central axis146, have approximately the same thickness. As will be appreciated by one skilled in the art, the high Verdet constant material and the varying thicknesses of the high Verdet constant material may be selected such that a linearly polarized beam of electromagnetic radiation, similar to that illustrated inFIG. 5, may be propagated through the annular polarization device144such that when the beam of electromagnetic radiation leaves the annular polarization device144, the beam may have an annular polarization similar to that illustrated inFIG. 6.

Other embodiments of the annular polarization device may be glass or crystal structures, such as garnet, doped with materials, such as rare-earth ions, to vary the Verdet constant in the material used in the annular polarization device so that the thickness of the material need not vary as much as the example illustrated yet still have the varying Verdet value as described above. Other materials may be used to as the material of the annular polarization device such as an engineered polymer, a doped glass, and yttrium iron garnet. Additionally, the magnetic field in which the annular polarization device is placed may not be uniform, but may be varied in order to change the Verdet value of particular portions of the device. The magnetic field may be produced by different types of magnetic field generators, such as a samarium cobalt magnet.

FIGS. 9A and 9Billustrate a polarization subsystem152according to another embodiment of the present invention. The polarization subsystem152may include a polarizing beam splitter154having a central axis156, a wave-plate158, and a reflecting system160. The reflecting system160may include a first reflective device162, a second reflective device164, and a third reflective device166. The polarizing beam splitter154, the wave-plate158, the first reflective device162, the second reflective device164, and the third reflective device166may be radially symmetric about the central axis156of the polarizing beam splitter154.

The first reflective device162may be annularly shaped and positioned at a distance from the polarizing beam splitter154in a direction that is substantially perpendicular to the central axis156of the polarizing beam splitter154. The first reflective device162may have a reflective surface that faces toward the wave-plate158in a direction that is approximately 45 degrees to the central axis156. The second reflective device164may be similar to the first reflective device, however, the second reflective device164may be positioned a distance away from the first reflective device162in a direction that is substantially parallel to the central axis156of the polarizing beam splitter154and have a reflective surface that faces toward the wave-plate158in a direction that is approximately 45 degrees to the central axis156. The third reflective166device may be annularly shaped and positioned within the second reflective device164and lie directly between the second reflective device164and the central axis156. The third reflective device166may have a reflective surface which faces away from the wave-plate158and the central axis156at an angle that is approximately 45 degrees to the central axis156.

In use, electromagnetic radiation168may be directed into a side of the polarizing beam splitter154opposite the wave-plate158in a direction that is substantially parallel to the central axis156. As the electromagnetic radiation168passes through the polarizing beam splitter154, the electromagnetic radiation may be split, or divided, into a first portion170and a second portion172.

As illustrated inFIG. 9B, when the first portion170of the electromagnetic radiation168leaves the polarizing beam splitter154in a direction that is substantially parallel to the central axis156, the first portion170may be linearly polarized in a direction as indicated by the horizontal symbols82. As illustrated inFIG. 9B, the horizontal symbols82indicate that the first portion170of electromagnetic radiation168, as it leaves the polarizing beam splitter154, may be linearly polarized in a direction that is parallel to radial lines extending from the central axis156.

Although only one cross-section of the polarization subsystem152is illustrated, it should be understood that the linear polarization of the electromagnetic radiation168may appear identical in any cross-section of the polarization subsystem152taken at the central axis156of the polarizing beam splitter154, similarly to the cross-section illustrated inFIG. 9B.

The first portion170of the electromagnetic radiation168may then pass through the wave-plate158. The first portion170of the electromagnetic radiation168may then be linearly polarized in a direction as indicated by the vertical symbols84. In other words, as illustrated inFIG. 9B, after the first portion170passes through the wave-plate158, the first portion170is linearly polarized in a direction that is perpendicular to lines extending radially from the central axis156of the polarizing beam splitter154.

The second portion172of the electromagnetic radiation168may be redirected radially away from the central axis156and be linearly polarized in a direction as indicated by the vertical symbols84. In other words, as illustrated inFIG. 9B, after the second portion172of the electromagnetic radiation168propagates radially from the polarizing beam splitter154, the second portion172may be linearly polarized in a direction that is perpendicular to radial lines extending from the central axis156of the polarizing beam splitter154. The second portion172may be reflected by the first reflective device162toward the second reflective device164and reflected by the second reflective device164toward the third reflective device166. The third reflective device166may reflect the second portion172of the electromagnetic radiation168away from the wave-plate158in a direction that is substantially parallel to the central axis156.

Thus, as both the first170and second172portions of the electromagnetic radiation168propagate from the polarization subsystem152, both the first170and second172portions are linearly polarized in a direction that is perpendicular to lines extending radially from the central axis156of the polarizing beam splitter154. Therefore, a beam of electromagnetic radiation propagating from the polarization subsystem152may have an annular polarization similar to that illustrated inFIG. 6.