# Patent

## Title
Systems and methods for insitu lens cleaning in immersion lithography

## Abstract

An immersion lithography apparatus is provided that includes an energy source, a projection optical system, a stage, a showerhead including an immersion liquid supply device and an immersion liquid discharge device that produces a flow of liquid within an exposure zone, and a cleaning device that cleans a portion of the projection optical system having been contacted with the immersion liquid by means of a cleaning gas. In an embodiment, the cleaning device includes a gas supply device and a gas discharge device that produce a flow of cleaning gas into the exposure zone. In embodiments, the apparatus includes a stage that includes a dose sensor and/or an ultra-violet light source. A method for insitu cleaning of a final lens element within an immersion lithography system having an immersion fluid showerhead that provides immersion fluid to an exposure zone of the immersion lithography system is also provided.

## Background
BACKGROUND
1. Field of the Invention
The present invention relates to immersion lithography, more particularly, to providing methods for insitu lens cleaning in immersion lithography systems.
2. Related Art
Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays (e.g., liquid crystal displays), semiconductor wafers, circuit boards, various integrated circuits, print heads, macro/nano-fluidic substrates, and the like. During lithography, a substrate, which is disposed on a substrate stage, is exposed to an image projected onto the surface of the substrate by exposure optics located within a lithography apparatus.
The projected image produces changes in the characteristics of a layer, for example, photoresist, deposited on the surface of the substrate. These changes correspond to the features projected onto the substrate during exposure. Subsequent to exposure, the layer can be etched or otherwise processed to produce a patterned layer. The pattern corresponds to those features projected onto the substrate during exposure. The patterned layer is then used to remove or further process exposed portions of underlying structural layers within the substrate, such as conductive, semiconductive, or insulative layers. This process is repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the substrate.
In the field of immersion lithography the exposure operation is conducted with an immersion liquid, which typically has been water, between the last lens element of the projection optics and the substrate. The surface of the final lens element is manually cleaned periodically using solvents. The residues that are removed are generally from processing associated with resist leaching from the ultra-violet exposure of photoresist on substrates being processed. This approach to lens cleaning is time consuming, as the lens must typically be removed. Moreover, the approach is not well suited for new immersion lithography exposure systems. New immersion lithography systems use high refractive index immersion fluids. The high refractive index immersion fluids are organic and breakdown under ultraviolet exposure to produce carbonaceous deposits, which are not easily manually cleaned from the surface of lens.
What are needed are methods for insitu cleaning of lens used in immersion lithography systems operated with high refractive index immersion fluids.
SUMMARY
The present invention is directed to immersion lithography apparatus that includes an energy source, a projection optical system, a stage that moves a substrate on which the substrate is place, a showerhead including an immersion liquid supply device and an immersion liquid discharge device that produces a flow of liquid between the projection optical system and the substrate and a cleaning device that cleans a portion of the projection optical system having been contacted with the immersion liquid by means of a cleaning gas. In an embodiment, the cleaning device includes a gas supply device and a gas discharge device. The gas supply device produces a flow of cleaning gas between the final lens element of the projection optical system and the stage. The final lens element comes in contact with the immersion fluid, and may also be referred to as the wet lens element. In embodiments, the apparatus includes a wafer stage that includes a dose sensor to determine when cleaning of a final lens element is needed. The wafer stage can also include an ultra-violet light source or one or more mirrors used to reflect ultra-violet light onto the surface of a final lens element that is being cleaned. In an embodiment, the energy source of the system provides the ultra-violet light for cleaning.
The invention also provides a method for insitu cleaning of a final lens element within an immersion lithography system having an immersion fluid showerhead that provides immersion fluid to an exposure zone of the immersion lithography system. The method includes determining that the final lens element requires cleaning. Once this determination is made, the immersion fluid is drained from the showerhead and exposure zone. A cleaning gas is then passed through the showerhead across the final lens element surface exposed to the immersion fluid. Alternatively the gas can be passed through means other than the showerhead. While the final lens element is exposed to the cleaning gas, the final lens element is also exposed to ultra-violet light. In the presence of the ultra-violet light, the cleaning gas reacts with contaminants on the surface of the wet lens to remove the contaminants/debris from the lens. The debris that is vaporized by the reaction is swept away by the flowing cleaning gas, thereby cleaning the final lens element.
The invention provides an efficient system and method for insitu cleaning of final lens element that does not damage the final lens element, nor does it require the painstaking task of lens removal and repositioning.



BRIEF DESCRIPTION OF THE FIGURES
The present invention is described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. The drawing in which an element first appears is indicated by the left-most digit in the corresponding reference number.
FIG. 1 is a block diagram of an immersion lithography system.
FIG. 2 is a block diagram of an immersion lithography system, according to an embodiment of the invention.
FIG. 3 is a block diagram of an immersion lithography system with an ultra-violet light source positioned for final lens element cleaning, according to an embodiment of the invention.
FIG. 4 is a block diagram of an immersion lithography system with a mirror positioned to reflect ultra-violet light for final lens element cleaning, according to an embodiment of the invention.
FIG. 5 is a flowchart of a method for compensating for a heating effect of exposure energy in an immersion fluid across an exposure zone of an immersion lithography system using pre-calibrated aberration information, according to an embodiment of the present invention.



DETAILED DESCRIPTION
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
In immersion lithography systems, liquid is injected into the space between a projection optical system exit window and a substrate surface that is being worked. FIG. 1 is a block diagram of immersion lithography system 100. System 100 includes energy source 110, pattern generator 120, and projection optical system 130. Energy source 110 produces exposure doses of energy that are used to pattern a substrate, such as substrate 150, that is being worked. Pattern generator 120 generates the pattern to be produced on substrate 150.
Projection optical system 130 includes optical element 132, optical element 134 and optical element 136, which is also referred to as an exit POS element or a final lens element. Projection optical system 130 conditions the energy provided by energy source 110 to properly focus it onto substrate 150 to produce the desired pattern.
Substrate 150 is secured to the surface of wafer stage 160. Wafer stage 160 is mechanically coupled to a lithography machine and moves to adjust the position of the substrate underneath the projection optics depending on the area of the substrate being patterned. As individuals skilled in the art will know, projection optical systems may have a wide range of optical elements, both in number and type. Projection optical system 130 provides a simplified example for illustration purposes and is not intended to limit the scope of the invention.
Immersion liquid 140 fills the space between substrate 150 and final lens element 136, referred to as the exposure zone. An immersion lithography showerhead (not shown) provides for the flow of immersion liquid 140 across the area of the substrate being worked. Over time deposits resulting from the ultra-violet exposure process and impurities in the immersion liquid form on the surface of final lens element 136. These deposits impair the performance of the immersion lithography apparatus and need to be periodically cleaned.
Cleaning the lens surface of lens element 136 proposes significant challenges. For example, the cleaning process should not damage the lens. Unfortunately, many cleaning solutions are corrosive to the materials used in lens elements and will roughen the lens surface. Also, since the lens element is embedded within the tool, the cleaning temperature must not become too high. Given these limitations, significant care is taken to keep the lens clean, such as adjusting the distance between the last lens element and keeping a high flow rate of immersion fluid to keep debris from settling on the lens surface. Additionally, purification of the immersion fluid occurs to keep ultra-violet transmission high. While these steps can reduce debris and ultra-violet activated deposition, they do not eliminate it. Thus, often the lens element must be accessed or removed for cleaning, which is also undesirable because of the time it takes to remove and replace the lens, and the increased probability of damage to the lens during the removal process.
In a recent study, hydrogen peroxide and ozone were tested to see if the combination could oxidize organic contaminants. The study concluded that the oxidant was too weak to remove the contamination. The study also noted that different kinds of surfactant including ionic and anionic were also tried, but with no clear improvement. See Chang, et. al., Development of Cleaning Process for Immersion Lithography, Proc. of SPIE, Vol. 6154 (2006) (“Chang Study”).
U.S. patent application Ser. No. 11/217,464, entitled Liquid Immersion Optical Tool Method for Cleaning Liquid Immersion Optical Tool, and Method for Manufacturing Semiconductor Device, filed by Higashiki on Sep. 2, 2005 (“Higashiki Patent Application”) describes an apparatus for cleaning a final lens element using a cleaning solution. The cleaning solution is passed across the final lens element to clean debris from the lens. Fulictioni water such as ozone water, ionized water, carbonated water, or peroxide water is used as the cleaning solution. Alternatively, acid is used as a cleaning solution. In addition, cleaning may be carried out using water vapor. Higashiki Patent Application at ¶39. An ultrasound wave, a water jet or a cavitation jet can be used to generate cavities in the cleaning solution to provide for a scrubbing action as the solution is passed through the area to be cleaned. Specialized nozzles are used. Id. at ¶40-41. The cleaning solution is evacuated once the cleaning process has ended through a rinse process. Id. at ¶48.
In contrast to the above system described in the Higashiki Patent Application and the results of the Chang Study, the present invention addresses cleaning of a final lens element, such as lens 136, by using an ultra-violet activated oxygen based gaseous cleaning process to periodically remove absorbed deposits on the wet lens surface of lens 136. Rather than use a cleaning solution, as is the case in the Higashiki Patent Application, the present invention uses an oxygen-based gas that is activated by ultra-violet light to provide the cleaning action.
FIG. 2 provides a diagram of immersion lithography apparatus 200, according to an embodiment of the invention. As in the case of immersion lithography apparatus 100, immersion lithography apparatus 200 includes an energy source 110, pattern generator 120, and projection optical system 130. Energy source 110 produces exposure doses of energy that are used to pattern a substrate, such as substrate 150 that is being worked. Pattern generator 120 generates the pattern to be produced on substrate 150.
Immersion lithography apparatus 200 also includes an immersion liquid supply device 230 and an immersion liquid discharge device 240 that produces a flow of liquid 140 between the final lens element 136 and substrate 150. Immersion liquid 140 flows through supply tube 210, across the area between final lens element 136 and substrate 150, referred to as the exposure zone, and exits through discharge tube 220 into immersion liquid discharge device 240. Immersion liquid supply device 230, immersion liquid discharge device 240, supply tube 210 and discharge tube 220 are intended to depict the primary elements of a showerhead. The invention is not limited to use with a showerhead, as described, but rather individuals skilled in the art will be able to able the invention to other types of showerhead designs based on the teachings herein. In an embodiment, either or both of immersion liquid supply device 230 and immersion liquid discharge device 240 include a vent or other means for evacuating the immersion liquid entirely in preparation for when the cleaning process takes place.
Immersion lithography apparatus 200 also includes a cleaning device that cleans a portion of the projection optical system. The cleaning device includes gas supply device 250 and gas discharge device 260. The gas supply device 250 produces a flow of cleaning gas between the final lens element 136 and stage 160. In the present embodiment, gas supply device 250 is a vent upon which a gas supply tube (not shown) can be affixed. Likewise gas discharge device 260 is a vent upon which a gas discharge tube (not shown) can be affixed to remove cleaning gas. Gas supply device 250 and gas discharge device 260 can include any type of device or apparatus that can produce a flow of gas. The cleaning gas can include air, oxygen enhanced air or oxygenated noble gases, for example.
Substrate 150 is secured to the surface of wafer stage 205. Wafer stage 205 is mechanically coupled to a lithography machine and moves to adjust the position of the substrate underneath the projection optics depending on the area of the substrate being patterned. In other embodiments, wafer stage 205 can be replaced by wafer stage 160 or wafer stage 405, as discussed below.
Additionally, in an embodiment wafer stage 205 includes dose sensor 270 and ultra-violet light source 280. Dose sensor 270 can be positioned under final lens element 136 to determine whether cleaning of the lens is needed. Dose sensor 270 senses the received energy transmitted through projection optics 130. When the sensed energy is below a certain threshold, or uniformity across the exposure zone is degraded below an acceptable level, a lens cleaning indication is provided. The threshold varies by the type of energy source 110, the energy dose, the projection optical system 130 and the type of immersion fluid being used.
Ultra-violet light source 280 generates ultra-violet light that is used to illuminate portions of final lens element 136 during the cleaning process. When the cleaning process has begun, wafer stage 160 is positioned such that ultra-violet light source 280 is positioned underneath the area being cleaned. Note that wafer stage 160 includes an opening with a clear surface that permits the ultra-violet light to impinge upon final lens element 136. In an embodiment, energy source 110 can provide the ultra-violet light needed to support cleaning.
Referring to FIG. 4, wafer stage 405 can also be used in embodiments of immersion lithography apparatus 200 as an alternative to wafer stage 205. When wafer stage 405 is used, an ultra-violet light source 420 transmits ultra-violet light to mirror 410, which is mounted on the bottom of wafer stage 405. Mirror 410 directs the ultra-violet light to impinge on the optical element being cleaned, typically final lens element 126.
FIG. 5 provides a flowchart of method 500 for insitu cleaning of a final lens element within an immersion lithography system, according to an embodiment of the invention. Method 500 begins in step 510. In step 510, an indication that a final lens element requires cleaning is determined. As was discussed above, carbonaceous deposits build up on the surface of final lens elements. In one embodiment, a dose sensor, such as dose sensor 270, can be used to determine whether a final lens element, such as final lens element 136 requires cleaning. Alternatively, based on the type of immersion fluid and the fluid handler cleaning a determination can be made whether it is time to clean the final lens element.
In step 520 immersion fluid is drained. For example, immersion fluid 140 can be drained from immersion liquid supply device 230 and immersion liquid discharge device 240. In an embodiment, immersion liquid discharge device includes a vent or other means for evacuating the immersion liquid entirely in preparation for the cleaning process.
In step 530 a cleaning gas is passed across the final lens element. In embodiments, the cleaning gas can include air, oxygen enhanced air, or oxygenated noble gases. Cleaning gas supply device 250 forces the cleaning gas into the area between final lens element 135 and wafer stage 205, for example. Cleaning gas discharge device 240 receives gas being forced across final lens element 135. In an embodiment, gas discharge device 240 can include a vacuum to extract the cleaning gas. External gas pumps and vacuums can be attached to gas supply device 230 and gas discharge device 240. In an embodiment, gas supply device 230 and, gas discharge devices 240 are vents that are coupled to external supply and discharge apparatus. Prior to exposing the final lens element to ultra-violet light during the cleaning process, the final lens element should be dried. In an embodiment, step 530 occurs sufficiently long enough to ensure that the surface of the final lens element is dried. In an alternative embodiment a separate drying gas is passed across the final lens element following step 520 and before the introduction of the cleaning gas in step 530. The drying gas is passed across the final lens element for a sufficiently long enough period to time to ensure that the surface of the final lens element to be cleaned is dried.
In step 540 the final lens element that has been dried is exposed to ultra-violet light in the presence of the cleaning gas that is being passed across the final lens element. The duration of steps 530 and 540 can range from a few seconds to hours based on the thickness of debris that has accumulated on the final lens element. The action of the ultra-violet light on the oxygen in the cleaning gas produces ozone, which removes the organic/carbonaceous deposits formed a final lens element surface. During step 540 the debris is vaporized by the ozone or other cleaning gas to produce CO2 and water vapor. The ultra-violet light can either be generated by energy source 110, by ultra-violet light source 280 or by ultra-violet light source 420.
In step 550 debris, which was vaporized in the previous step, is flushed away by the flowing cleaning gas. In step 560, method 500 ends. Following the cleaning process, immersion liquid is reintroduced into immersion liquid supply device 230 and immersion liquid discharge device 240.
Conclusion
Exemplary embodiments of the present invention have been presented. The invention is not limited to these examples. These examples are presented herein for purposes of illustration, and not limitation. Alternatives (including equivalents, extensions, variations, deviations, etc., of those described herein) will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. Such alternatives fall within the scope and spirit of the invention.

## Claims

1. An immersion lithography apparatus, comprising:
a projection optical system;
a stage configured to move a substrate on which the substrate is placed;
a showerhead including an immersion liquid supply device and an immersion liquid discharge device configured to produce a flow of liquid between the projection optical system and the substrate; and
a cleaning device configured to clean a portion of the projection optical system having been contacted with the immersion liquid by means of a cleaning gas to remove solid deposits therefrom, the cleaning device comprising:
a gas supply and discharge device configured to produce a flow of cleaning gas to a space between the projection optical system and the stage when the portion of the projection optical system having been contacted with the immersion liquid is dried,
wherein the gas supply and discharge device is affixed to the showerhead and includes a gas supply vent located on the liquid immersion supply device and a gas discharge vent located on the liquid immersion discharge device, the gas supply vent and gas discharge vent being arranged on opposite sides of said portion of the projection system such that, in use, said cleaning gas flows across the portion of the projection optical system from the gas supply vent to the gas discharge vent,

wherein the gas supply and discharge device is configured to produce a flow of gas to dry the portion of the projection optical system having been contacted with the immersion liquid, and wherein, in use, said gas flows across the portion of the projection optical system from the gas supply vent to the gas discharge vent to dry the portion of the projection optical system having been contacted with the immersion liquid, and
wherein the cleaning device comprises an energy source configured to expose a projection optical system element to ultra-violet light while cleaning gas flows between the proiection optical system and the stage so as to remove solid deposits formed on the projection optical system element, wherein after completing exposure of the projection optical system element to the ultraviolet light, the cleaning device is configured to supply the cleaning gas to flush away removed solid deposits from said space toward the gas discharge vent.

2. The immersion lithography apparatus of claim 1, wherein the energy source is configured to illuminate a portion of the projection optical system that has been exposed to immersion liquid during a cleaning process.

3. The immersion lithography apparatus of claim 2, wherein the stage includes a region that permits light from the energy source to be transmitted to a surface to be cleaned.

4. The immersion lithography apparatus of claim 2, wherein the stage includes a mirror that reflects light from the energy source through a region on the stage that permits light from the ultra-violet light source to be transmitted to a surface to be cleaned.

5. The immersion lithography apparatus of claim 1, wherein the cleaning gas includes air, oxygen enhanced air, or oxygenated noble gases.

6. The immersion lithography apparatus of claim 1 further comprising a dose sensor configured to sense the energy dose level transmitted through a final lens element to determine whether cleaning of a final lens element is necessary.

7. The immersion lithography apparatus of claim 6, wherein the dose sensor is located on the stage.

8. The immersion lithography apparatus of claim 1, wherein the solid deposits comprise organic deposits, carbonaceous deposits, or both.

9. A method for in situ cleaning of a final lens element within an immersion lithography system having an immersion fluid showerhead configured to provide immersion fluid to an exposure zone of the immersion lithography system, comprising:
(a) draining the immersion fluid from the showerhead and exposure zone;
(b) passing a cleaning gas through the showerhead across the final lens element surface exposed to the immersion fluid, the cleaning gas being supplied by a gas supply and discharge device configured to flow gas to a space between the final lens element and a stage, wherein the gas supply and discharge device is affixed to the showerhead and includes a gas supply vent located on the shower head and a gas discharge vent located on the showerhead, the gas supply vent and gas discharge vent being arranged on opposite sides of said final lens element such that, in use, said cleaning gas flows across the final lens element from the gas supply vent to the gas discharge vent;
(c) after the final lens element has been dried, exposing the final lens element to ultra-violet light in the presence of the cleaning gas to remove solid deposits from the final lens element; and
(d) after completing exposure of the final lens element to ultraviolet light in (c), flushing the removed deposits away from the final lens element toward the gas discharge vent using the flow of cleaning gas,
wherein between (a) and (b), the method comprises passing a drying gas through the showerhead across the final lens element surface exposed to the immersion fluid, said drying gas flowing across the portion of the projection optical system from the gas supply vent to the gas discharge vent to dry the portion of the projection optical system having been contacted with the immersion liquid.

10. The method of claim 9, wherein the cleaning gas comprises air, oxygen enhanced air, or oxygenated noble gases.

11. The method of claim 9, wherein the duration of steps (b) and (c) range from a few seconds to hours based on the thickness of deposits accumulated on the final lens element.

12. The method of claim 9, wherein exposing the final lens element to ultra-violet light includes transmitting the ultra-violet light through the immersion lithography system.

13. The method of claim 9, wherein exposing the final lens element to ultra-violet light includes transmitting the ultra-violet light from beneath the final lens element.

14. The method of claim 9, wherein the solid deposits comprise organic deposits, carbonaceous deposits, or both.

15. The method of claim 9, comprising sensing an energy dose level transmitted through the final lens element to determine whether cleaning of the final lens element is necessary.