Abstract:
A method and system for in-situ cleaning of a reticle. Ionized gas is directed onto the reticle by a delivery device. The ionized gas neutralizes electro-static attraction between the reticle and particulate contaminants on the reticle and thereby dislodges them from the reticle. The ionized gas and particulate contaminants are then removed from the reticle by a vacuuming pump and transported from the system by a contaminant collector. As a result of the in-situ cleaning method and system, the overall risk of chip defects due to reticle particulate contaminants is reduced, and chip and wafer yield is improved.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to lithography systems. More particularly, this invention relates to cleaning a reticle during use in a lithography system.  
           [0003]    2. Related Art  
           [0004]    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, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer.  
           [0005]    While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art.  
           [0006]    During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by an exposure system located within a lithography system. The exposure system includes a reticle (also called a mask) for projecting an image onto the wafer. The exposure system also includes an illumination system, a projection optics system, and a wafer alignment stage.  
           [0007]    Particulate contamination on the reticle can be imaged on each pattern on the wafer. As the particle is not part of the desired pattern, the reticle image produced contains a defect (the image of the particle). In many cases, this defect can cause the functional failure of every pattern on every wafer printed with the contaminated reticle. As shorter and shorter wavelengths are employed to improve the lithography process and improve the capability to print finer features, the system becomes more sensitive to smaller particles. These are more difficult (or currently impossible) to detect on the reticle prior to its installation in the lithography tool. In addition, it is more difficult (if not impossible) to maintain the reticle environment clean enough to prevent the deposition of such small particles on the reticle.  
           [0008]    While exposure optics are used in the case of photolithography, a different type of exposure apparatus can be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each can require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.  
           [0009]    The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the image features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those image features projected onto the wafer during exposure. This patterned layer is then used to remove or further process exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive, or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface, or in various layers, of the wafer.  
           [0010]    Step-and-scan technology works in conjunction with a projection optics system that has a narrow, typically rectangular imaging slot called the exposure field. Rather than expose the entire wafer at one time, individual fields are scanned onto the wafer one at a time. This is done by moving the wafer and reticle simultaneously such that the imaging slot is moved across the field during the scan. The wafer stage must then be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the quality of the image projected onto the wafer is maximized. While using a step-and-scan technique generally assists in improving overall image quality, image distortions generally occur in such systems due to imperfections within the projection optics system, illumination system, and the particular reticle being used. An exemplary step and-scan lithography system is the Microscan II, manufactured by Silicon Valley Group, Inc., San Jose, Calif.  
           [0011]    The illumination system of a lithographic system includes a light source. Excimer lasers are one such light source and operate at several characteristic wavelengths ranging from vacuum ultraviolet light to greater than 400 nanometers (nm) depending on the gas mixture used. By shortening the wavelength of the light, the resolution of the projection system is improved. Thus, in a lithography system, it is desirable to utilize a light source with wavelengths within the vacuum ultraviolet range, i.e., below 200 nm.  
           [0012]    As shorter wavelength light sources are used in lithography, organic contamination in the exposure area of the lithography system becomes a greater problem. It is well known that organic contaminates have high optical absorption coefficients at shorter wavelengths, particularly at 157 nm and below. A 1 nm film of organic contaminant belonging to the alkane group will drop the optical transmission at 157 nm by 1%. Further, an acetone residue left on the surface of a calcium fluoride optical element reduces the transmission by 4% at 157 nm. (See, T. M. Bloomstein et al., Optical Materials and Coatings at 157 nm, 3676 S.P.I.E. Proceedings 342-9 (1999); which is incorporated herein by reference). Optical intensity is an important issue as the number of optical elements increases in a lithography system. It is for this reason that organic contamination can be detrimental to optical elements in  157  nm and below lithography systems.  
           [0013]    Sources of organic contamination within a lithography system include out-gassed products from polymer materials and solvents used for degreasing tool parts, for example. Extremely low levels of organic contamination are critical for the exposure path in the lithography system, and an active purge system and strict material selection are required for those areas of the system associated with this path.  
           [0014]    Therefore, what is needed is a technique of removing particle contamination of the mask over an extended period of time.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention makes practical maintaining near zero particle contamination of the mask over an extended period of time by repetitive cleaning of the mask during the actual exposure process. The repeated cleaning shortens the effective amount of time that the mask is exposed to contamination, making realistic levels of environmental control (Class 1 to Class 10) consistent with a near zero particulate requirement.  
           [0016]    The present invention utilizes a cleaning system in which the reticle is passed underneath a delivery device using a step and scan method. In one embodiment, the delivery device remains stationary. The delivery device transports a gas, which becomes ionized before being directed onto the mask surface of the reticle. The ionized gas neutralizes electro-static attraction between the mask and particulates, thereby “blowing off” the particulates. The ionized gas and particulates are then transported away from the mask surface of the reticle by a contaminant collector. A positive or negative charge can be applied to the contaminant collector to better promote collection of particulate contamination from the mask.  
           [0017]    Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 0   
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0018]    The accompanying drawings, which are incorporated herein and form part of the specification, illustrate the present invention and together with the description further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.  
         [0019]    [0019]FIG. 1 is a diagram of an example embodiment of the cleaning apparatus of the present invention.  
         [0020]    [0020]FIG. 2 is a block diagram of the cleaning apparatus of the present invention.  
         [0021]    [0021]FIG. 3 is a schematic diagram of a delivery device contained in the system of the present invention.  
         [0022]    [0022]FIG. 4 is a cross-sectional view of an example embodiment of the present invention.  
         [0023]    [0023]FIG. 5 is a perspective view of an example embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    An example embodiment of the present invention is now described with reference to the figures where like reference numbers indicate identical or functionally similar elements. The left most digit of each reference number corresponds to the figure in which the reference number is first used. While specific configurations and arrangements are discussed, it should be understood that this is done for illustrative purposes only. A person skilled in the relevant art will recognize that other configurations and arrangements can be used without departing from the spirit and scope of the invention. It will be apparent to a person skilled in the relevant art that this invention can also be employed in a variety of other applications.  
         [0025]    [0025]FIG. 1 is a diagram of an example embodiment of the cleaning apparatus of the present invention. Reticle  110  is translated within an imaging station (not shown), where in-situ cleaning occurs. An exposure field for lithography is shown at  105 . The exposure field  105 , which is produced by the illumination system, is stationary. As described above, to expose portions of a wafer (not shown), the reticle  110  is translated in two dimensions to step and scan across the exposure field  105 , as would be apparent to a person skilled in the relevant art. According to the present invention, a cleaning apparatus  145  is positioned in close proximity to the mask surface of the reticle  110 . By the term “close proximity”, the inventors mean that those skilled in the art can, without undue experimentation, determine just how close the cleaning apparatus  145  is to the reticle  110  based on empirical data and other variables, such as the volume of gas flow, amount of contamination, size of the contaminants, physical characteristics of the cleaning apparatus  145 , and the like. The cleaning apparatus  145  comprises a delivery device  130  and a contaminant collector  115 .  
         [0026]    As the reticle  110  is stepped and scanned to expose the wafer (not shown), the reticle  110  passes repeatedly underneath the cleaning apparatus  145 . The exposure field  105  and the cleaning apparatus  145  remain stationary. The delivery device  130  directs the ionized gas  140  onto the mask surface of the reticle  110 . The ionized gas  140  reduces electro-static attraction between the mask (i.e., top) surface of the reticle  110  and particulate contamination lodged thereon (not shown). The ionized gas  140  is used to dislodge particulate contamination from the mask surface of the reticle  110 . As a result, particulate contamination of the reticle  110  is reduced to an acceptable level.  
         [0027]    The gas is preferably nitrogen that is ionized to produce a stream of N 2  anions and electrons. Other gases can be used. “Off-the-shelf” devices that produce such an ion stream are commercially available (e.g., NRD Inc., Grand Island, N.Y.,—Nuclecel™, model 2021CR).  
         [0028]    Alternatively, the ionized gas  140  can be produced by bombarding or otherwise exposing a gas with alpha particles produced by a radioactive isotope. The radioactive isotope can be located within the cleaning apparatus  145 , or the gas can be exposed to the radioactive isotope upstream (e.g., closer to the gas source (not shown)). For example, Polonium with an atomic weight of  210  produces alpha particles. The radioactive isotope Americium also produces alpha particles. In yet another embodiment, the ionized gas  140  can be produced by electrostatically charging a gas, using techniques that would become apparent to a person skilled in the relevant art.  
         [0029]    The delivery device  130  carries the ionized gas via an internal cavity. The delivery device  130  contains delivery ducts  135 . The delivery ducts  135  are holes, slots, slits, or jets used to direct the ionized gas  140  onto the mask surface of the reticle  110 . The size, shape, number and location of the holes, slots, slits, or jets used to deliver the ionized gas  140  will be determined by implementation requirements.  
         [0030]    The contaminant collector  115  vacuums the ionized gas  140  from the mask surface of the reticle  110  after the ionized gas  140  is directed onto the mask surface of the reticle  110  to dislodge particulate contaminants from the mask surface of the reticle  110 . The contaminant collector  115  contains vacuum ducts  140 . The vacuum ducts  155  are holes, slots, slits, or jets used to remove particulate contaminants  150  and the ionized gas  140  from the reticle  110 . The size, shape, number and location of the holes, slots, slits, or jets used to vacuum the particulate contaminants  150  and the ionized gas  140  will be determined by implementation requirements.  
         [0031]    Particulate contaminants  150  and ionized gas  140  are vacuumed through the vacuum ducts  155  and are removed from the mask surface of the reticle  110  by the contaminant collector  115 . The vacuum ducts  155  can be positively or negatively electrically biased to provide additional attraction of contaminants dislodged from the mask surface of the reticle. The size, shape, number and location of the vacuum ducts  155  will be determined by implementation requirements. In a further embodiment, the position of the delivery device  130  and the contaminant collector  115  can be reversed.  
         [0032]    [0032]FIG. 2 is a generalized block diagram of the cleaning apparatus of the present invention. Gas source  210  supplies a gas, such as nitrogen. An ionizing source  220  is used to ionize the gas provided by gas source  210 . As noted above, the gas can be bombarded with alpha particles or electrostatically ionized by the ionizing source  220 . The ionized gas  140  is then provided to the delivery device  130 , which directs the ionized gas  140  onto the reticle  110 . After the ionized gas  140  dislodges contaminants from the reticle  110 , the ionized gas  140  and the contaminants  150  are vacuumed by vacuum pump  230 , via contaminant collector  115 .  
         [0033]    [0033]FIG. 3 is a schematic diagram of an alpha particle ionizing source  220 , according to the present invention. Non-ionized gas  305  enters a chamber having a radiation source  310 , which produces alpha particles  315 . The alpha particles  315  bombard the gas (as illustrated generally at flow  320 ) to produce the ionized gas  140 , which then exits and is directed onto the reticle  110 . As discussed above, in one embodiment, the ionizing source  310  is a radioactive isotope such as Polonium or Americium. In an alternative embodiment, the radiation source  310  can be replaced by an electrostatic device used to ionize the gas.  
         [0034]    [0034]FIG. 4 is a cross-sectional view of an example embodiment of the present invention. The reticle  110  is shown in cross section and is moved across the exposure field  105  in a step and scan fashion. The light corresponding to the exposure field  105  travels from right to left in this figure and passes through the transmissive reticle, as illustrated by the arrows. (Of course, the present invention can also be adapted for lithography systems implementing a reflective reticle.) In this example embodiment, a single delivery tube  430  directs the ionized gas stream  140  onto the reticle  1   10 . Plural delivery tubes, including tubes on both sides of the exposure field, can be used. Particulate contaminants  150  are dislodged from the mask surface by the ionized gas stream  140  and removed via vacuum by contaminant collector tubes  445 . A single collector tube can be used. The delivery tube and collector tubes can have cylindrical, elliptical, rectangular, or the like, cross section.  
         [0035]    [0035]FIG. 5 is a perspective view of an example embodiment of the present invention. The reticle  110  is shown with its patterned side facing upward. The reticle  110  is moved underneath the exposure field  105  in step and scan directions  580 , as a portion of the mask is imaged onto the wafer (not shown). Gas source  210  supplies non-ionized gas to delivery tube  540 . Ionizing source  220  is not shown in this figure. However, in yet another embodiment, a radiation source can be located within delivery tube  540 , or located in the supply tubing between the gas source  210  and the delivery tube  540 . The alpha particle source can be Americium, Polonium, or any other radioactive isotope known to one skilled in the art to produce alpha particles, and can be adhesively affixed. The Delivery tube  540  contains a pattern of holes, slots, slits, or jets  550  and, in said yet another embodiment, an internal alpha particle source. The size, shape, number and location of the holes, slots, slits, or jets  550  used to deliver the ionized gas  140  will be determined by implementation requirements.  
         [0036]    For example, in one embodiment, holes, slots, slits, or jets  550  can be located directly underneath the delivery tube  540 . Example embodiment  500  can also contain a plurality of delivery tube  540 . Delivery tube  540  directs the ionized gas  140  onto the reticle  110 . The ionized gas  140  dislodges particulate contamination.  
         [0037]    The vacuum tubes  570  contain a pattern of holes, slots, slits, or jets  585 . The size, shape, number and location of the holes, slots, slits, or jets  585  used to deliver the ionized gas  140  will be determined by implementation requirements. For example, in one embodiment, holes, slots, slits, or jets  585  can be located directly underneath the vacuum tubes  570 . Example embodiment  500  can also contain only one of vacuum tubes  570 . Vacuum tubes  570  vacuum the particulate contaminants  150  from the patterned side of the reticle  110  and transport the particulate contaminants  150  to an exhaust pump  565 , where they are eventually filtered, or otherwise removed/discarded. In a further embodiment, the position of the delivery tube  540  and the vacuum tubes  570  can be reversed.  
         [0038]    In yet a further embodiment, automated cleaning can be performed by computer control to vary the amount of cleaning. For example, the gas flow can be adjusted based on system variables, such as the level of contamination (perhaps determined by sampling the gas prior to the exhaust pump), temperature, pressure, or the like variables.  
         [0039]    Conclusion  
         [0040]    Thus, according to the present invention particulate contaminants are continuously being removed from the reticle  110  in-situ during lithography. Alternatively, the above described cleaning can be done before, during and/or after imaging is being performed.  
         [0041]    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(s) that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.