US20060232753A1 - Liquid immersion lithography system with tilted liquid flow - Google Patents

Abstract

A liquid immersion lithography system including a projection optical system for directing electromagnetic radiation onto a substrate, and a showerhead for delivering liquid flow between the projection optical system and the substrate. The showerhead includes an injection nozzle and a retrieval nozzle located at different heights. The liquid flow is tilted relative to the substrate. A direction from the injection nozzle to the retrieval nozzle is tilted at approximately 1 to 2 degrees relative to the substrate.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The present invention is related to liquid immersion lithography, and more particularly, to efficient recirculation of liquid in immersion lithography systems.
• 2. Description of the Related Art
• An integrated circuit (“IC”) integrates a large number of electronic circuit elements, including transistors. The circuit elements are manufactured and interconnected on a semiconductor substrate, e.g., on a single crystalline silicon wafer. During manufacturing, the wafers undergo cycles of film deposition and lithography, in addition to other processing. Film deposition is the process of depositing a layer of material, e.g., insulating or metallic, over the entire substrate; lithography is the process of patterning the deposited layer. The first step in lithography involves coating the wafer with photoresist that is sensitive to particular radiation, typically ultra-violet light. During the next step—exposure—the substrate is exposed to a radiation pattern stored on a mask, also called a reticle. Radiation locally changes the physical or chemical properties of the photoresist, and the exposed (or unexposed) areas are selectively dissolved during a developing step that leaves behind a pattern of photoresist. The patterned photoresist provides a pattern for a subsequent etching step. The etching step removes undesired areas of the deposited layer, leaving behind structures associated with circuit elements, such as wires, resistors and transistors, and the like.
• Highly integrated circuits require small circuit elements. Since the radiation pattern shapes the circuit elements, the smallest feature size depends on the resolution achieved in the lithography exposure step, or the resolution of the projection device used to project the radiation pattern onto the substrate. According to the Raleigh criterion, this resolution is proportional to the wavelength λ of the projected light and to an adjustment factor k₁, and inversely proportional to the sine function of the marginal, or capture, angle θ of the projection optics, where:
  Resolution=k ₁*λ/sin(θ)
• The resolution can be decreased, i.e., improved, in one of three ways. First, the wavelength λ of the projected light can be decreased. A shorter wavelength, however, may require new photoresist and a number of changes in the projection device, such as using a different light source and light filters, and special lenses for the projection optics. Second, the resolution can be decreased by decreasing the adjustment factor k₁. Decreasing k₁ may also require the use of different photoresist and high precision tools. Third, the marginal angle θ can be increased by increasing the size of the projection optics. The effect of this increase, however, is limited by the sine function above.
• One way to reduce the wavelength λ of the projected light is through the use of immersion lithography, where a liquid is injected between the projection optics and the wafer, taking advantage of the higher refractive index of the liquid compared to air (and, therefore, resulting in a smaller effective wavelength λ).
• One of the persistent problems in immersion lithography relates to ensuring purity and lack of contamination of the immersion liquid. The immersion liquid is generally recirculated, using some form of an injection system to inject the liquid into the volume between the projection optics and the substrate, and extracted using some form of a extraction, or suction system to extract the liquid from the exposure area back into recirculation. However, the liquid can get contaminated, for example, due to pickup of particles from the air, or due to pickup of material from the photo resist that is being exposed. Normally, filtering systems are in place to remove the contaminants. However, not all of the liquid that is injected into the exposure area can actually be recirculated. This is due to the surface tension that exists between the liquid and the substrate surface. Although most of the liquid can be extracted, using the suction pressure of the extraction/recirculation system, some droplets of liquid remain on the surface of the way of the substrate, together with their contaminants. Increasing the suction pressure generally does not help past a certain point, since this will increase the recirculation speed, but will not address the problems caused by the surface tension of the liquid.
• Even with the current designs, many liquid injection and extraction systems utilize a fairly complex showerhead design to deliver and extract the immersion liquid. However, even in such complex designs, the problem of the surface tension of the liquid is not entirely solved.
• Accordingly, what is needed is an approach that ensures that all of the injected liquid is collected by the extraction system in an immersion lithography tool.
SUMMARY OF THE INVENTION
• The present invention is directed to liquid immersion lithography system with tilted liquid flow that substantially obviates one or more of the problems and disadvantages of the related art.
• There is provided a liquid immersion lithography system including a projection optical system for directing electromagnetic radiation onto a substrate, and a showerhead for delivering liquid flow between the projection optical system and the substrate. The showerhead includes an injection nozzle and a retrieval nozzle located at different heights. The liquid flow is tilted relative to the substrate. A direction from the injection nozzle to the retrieval nozzle is tilted at approximately 1 to 2 degrees relative to the substrate.
• In another aspect, a liquid immersion lithography system includes a projection optical system for exposing a substrate, an injection nozzle and a retrieval nozzle for delivering tilted liquid flow between the projection optical system and the substrate. The liquid flow is tilted at approximately 1 to 2 degrees relative to the substrate.
• In another aspect, an exposure system includes, in order of light propagation, an illumination source, a condenser lens, a mask (or contrast device) and projection optics. A liquid delivery system provides liquid to an exposure area below the projection optics. The exposure system also includes means for providing tilted liquid flow of the liquid.
• In another aspect, an exposure system includes, in order of light propagation, an illumination source, a condenser lens, a mask and projection optics. A liquid delivery system provides liquid to an exposure area of a substrate. The substrate is tilted relative to a horizontal.
• Additional features and advantages of the invention will be set forth in the description that follows. Yet further features and advantages will be apparent to a person skilled in the art based on the description set forth herein or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
• It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
BRIEF DESCRIPTION OF THE DRAWINGS/FIGS.
• The accompanying drawings, which are included to provide a further understanding of the exemplary embodiments of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. In the drawings:
• FIG. 1 illustrates one embodiment of the invention that uses a tilted showerhead for injection and extraction of the immersion liquid.
• FIG. 2 illustrates a close-up view of the tilted showerhead arrangement of FIG. 1.
• FIG. 3 is another illustration of the exposure area of a liquid immersion lithography system with a liquid in the exposure area.
• FIG. 4 illustrates a meniscus region A of FIG. 3.
• FIG. 5 illustrates a meniscus region B of FIG. 3.
• FIG. 6 illustrates a three-dimensional isometric view of the embodiment illustrated in FIGS. 1-5.
• FIG. 7 shows an exemplary photolithographic system that uses the tilted showerhead.
DETAILED DESCRIPTION OF THE INVENTION
• Reference will now be made in detail to the embodiments of the present invention, examples of which are illustrated in the accompanying drawings.
• The inventors have discovered a rather unexpected phenomenon:
• when the liquid flow in an immersion lithography system is tilted, even by a relatively small angle, as little as one or two degrees, the tilt, and the corresponding effect of gravity on the liquid flow, is sufficient to overcome the residual surface tension forces acting on the liquid. Accordingly, with such a tilted arrangement, the pooling of the immersion liquid in certain portions of the exposure area can be avoided, reducing the possibility of contamination.
• FIG. 7 shows an exemplary photolithographic system that uses the tilted showerhead. As shown in FIG. 7, the lithographic system 700 (shown in side view) includes a light source (illumination source) 710, such as a laser or a lamp, illumination optics 712 (such as a condenser lens), and a reticle (i.e., a, mask, or a contrast device) 714, which is usually mounted on a reticle stage (not shown). Note that the reticle 714 can be a plate with an exposure pattern on it, or a spatial light modulator array, such as used in maskless lithography. Light from the reticle 714 is imaged onto a wafer 718 using projection optics 716. The wafer 718 is mounted on a wafer stage 720.
• FIG. 1 illustrates one embodiment of the invention that uses a tilted showerhead for injection and extraction of the immersion liquid. As shown in FIG. 1, the projection optics 716 is mounted above a substrate, such as the semiconductor wafer 718. One portion of a showerhead is used for liquid injection, and a second portion of the showerhead is used for liquid extraction.
• FIG. 1 thus illustrates a cross-sectional view (at the top) and a plan view (at the bottom) of one embodiment of the immersion lithography system with tilted liquid flow arrangement. A lower portion of the projection optical system 716 is located above the wafer 718 (note that the last element of the projection optical system 716 can be a prism, or a lens, or a glass window). Also shown in FIG. 1 is a immersion head, or shower head 110, which is shown in cross section. Liquid flow enters the exposure area through an injection nozzle 106, and exits through a retrieval, or suction, nozzle 108. Note the different heights of the injection nozzle 106 and the retrieval nozzle 108 in the exposure area illustrated in more detail in FIG. 2, which provides for a height differential, and therefore for a tilted liquid flow.
• The dimension of the gap between the lowest element of the projection optics 716 (which can be either a lens or a prism) and the surface of the substrate 718 is usually on the order of approximately one millimeter (typically ranging between about 0.5 millimeters and 2 millimeters). Thus, it is possible to leave one of the nozzles (either injection or extraction) in its original place and correspondingly raise or lower the other nozzle, so as to create a tilt.
• The relative arrangement of the two nozzles is such that there is a natural gravity-induced liquid flow from the injection showerhead to the extraction showerhead. As noted above, the tilt can be relatively small, even as little as one or two degrees. Note that the important aspect is the relative arrangement of the injection and extraction nozzles, such that there is a relative tilt of the liquid flow compared to the plane of the wafer 102 (which is normally horizontal).
• FIG. 2 illustrates a close up view of the tilted nozzle arrangement. Note in particular the bottom surface of the showerhead, labeled X in FIG. 2, which is tilted at approximately one or two degrees as shown in the figure. Note also that the suction nozzle 108 is located below a bottom surface 202 of the projection optical system 716 by a distance t (see left hand side of the figure).
• FIG. 3 is another illustration of the exposure area of a liquid immersion lithography system, similar to FIG. 2, but showing a liquid 302 in the exposure area, as would be the case during actual operation of the device. Note also a withdrawal pressure p_w, and two regions A and B, which include two meniscus regions, discussed further below.
• FIG. 4 illustrates a meniscus region A from FIG. 3. Note a height “h,” which refers to a gap height, and the shape of the meniscus, which is outward.
• The pressure in the liquid adjacent to the meniscus (p_m) will be reduced due to the effect of surface tension. The exact pressure at this location will depend on the detailed shape of the meniscus and include effects related to the contact angles. However, an order of magnitude estimate of this pressure depression is given by: $p_m=p_{\mathrm{amb}}-p_w-\frac{4\sigma }{h}$
• where p_amb is the ambient pressure, p_w is the withdrawal pressure, σ is the surface tension and h is the gap height (see FIG. 4). (See generally J. Fay, Introduction to Fluid Mechanics, MIT Press, Cambridge, Mass. (1994), which is incorporated herein by reference.
• FIG. 5 illustrates a region B from FIG. 3 with a gap height “H” between the showerhead 110 and the wafer 718. Note that the meniscus is inward shaped, with the pressure in the liquid given by: $p_M=p_{\mathrm{amb}}-p_w-\frac{4\sigma }{H}$
• Because H>h, p_M>p_m and liquid will start flowing from the side that has the bigger gap.
• FIG. 6 is another illustration of the embodiment illustrated in FIGS. 1-5, in this case, a three-dimensional isometric view. Shown in FIG. 6 is the wafer 718 positioned below the projection system 716 (only a portion of which is shown). The showerhead 110 is visible in the figure, with the liquid 302 flowing under the projection optical system 716.
• In another embodiment, it is possible to accomplish the tilting effect by tilting the wafer surface. Normally, in conventional systems, the wafer 718 is kept perfectly horizontal (or, as horizontal as practical), to ensure good image quality. However, it is possible to tilt the wafer 718 (and, correspondingly, the rest of the exposure optics, such that the wafer surface is tilted by approximately one or two degrees, so as to encourage liquid flow in the direction from the injection port to the extraction port. Such an approach is more complicated to implement than the embodiment described above, since tilting the entire lithographic tool may be undesirable or mechanically problematic. However, such a tilting of the entire lithographic tool will accomplish the same purpose—creation of a preferred direction of liquid flow even in the absence of suction pressure for extraction.
• As yet a third embodiment, the tilting effect can be simulated using forced air flow. In other words, even if the injection and extraction ports are level with each other, and the substrate is also oriented perfectly horizontally, an air pressure gradient in the direction from the injection port to the extraction port will also achieve a similar effect—that is, overcoming the surface tension forces that otherwise impede liquid flow.
CONCLUSION
• While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention.

Claims (8)

1. A liquid immersion lithography system, comprising:

a projection optical system to direct electromagnetic radiation onto a substrate; and

a showerhead to deliver liquid flow between the projection optical system and the substrate,

wherein the showerhead includes an injection nozzle and a retrieval nozzle located at different heights.

2. The liquid immersion lithography system of claim 1, wherein the liquid flow is tilted relative to the substrate.

3. The liquid immersion lithography system of claim 1, wherein a direction from the injection nozzle to the retrieval nozzle is tilted at approximately 1 to 2 degrees relative to the substrate.

4. A liquid immersion lithography system comprising:

a projection optical system to expose a substrate; and

an injection nozzle and a retrieval nozzle to deliver tilted liquid flow between the projection optical system and the substrate.

5. The liquid immersion lithography system of claim 4, wherein the tilted liquid flow is tilted at approximately 1 to 2 degrees relative to the substrate.

6. An exposure system comprising:

in order of light propagation, an illumination source, a condenser lens, a contrast device and projection optics;

a liquid delivery system to provide liquid to an exposure area below the projection optics; and

means for providing tilted liquid flow of the liquid.

7. An exposure system comprising:

in order of light propagation, an illumination source, a condenser lens, a contrast device and projection optics;

a liquid delivery system to provide liquid to an exposure area of a substrate; and

means for tilting the substrate relative to a horizontal.

8. The system of claim 6, wherein the means for tilting also tilts the projection optics.

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