Abstract:
A system for removing debris from a surface of a photolithographic mask is provided. The system includes an atomic force microscope with a tip supported by a cantilever. The tip includes a surface and a nanometer-scaled coating disposed thereon. The coating has a surface energy lower than the surface energy of the photolithographic mask.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to nanomachining processes. More particularly, the present invention relates to debris removal during and/or pursuant to nanomachining processes. 
     BACKGROUND OF THE INVENTION 
     Nanomachining, by definition, involves mechanically removing nanometer-scaled volumes of material from, for example, a photolithography mask, a semiconductor substrate/wafer, or some other monolith. For the purposes of this discussion, “substrate” will refer to any object upon which nanomachining may be performed. 
     Typically, nanomachining is performed by applying forces to a surface of a substrate with a tip (e.g., a diamond cutting bit) that is positioned on a cantilever arm of an atomic force microscope (AFM). More specifically, the tip is typically first inserted into the surface of the substrate. Then, the tip is dragged through the substrate in a plane that is parallel to the surface (i.e., the xy-plane). This results in displacement and/or removal of material from the substrate as the tip is dragged along. 
     As a result of this nanomachining, debris is generated on the substrate. More specifically, small particles may form during the nanomachining process as material is removed. These particles, in some instances, remain on the substrate once the nanomachining process is over. Such particles are often found, for example, in trenches and/or cavities present on the substrate. 
     In order to remove such debris, particularly in high-aspect photolithography mask structures and electronic circuitry, wet cleaning techniques are often used. More specifically, the use of chemicals in a liquid state and/or agitation of the overall mask or circuitry is typically employed. However, both chemical methods and agitation methods such as, for example, megasonic agitation, can destroy both high-aspect ratio structures and mask optical proximity correction features (i.e., features that are generally so small that these features do not image, but rather form diffraction patterns that are used beneficially by mask designers to form patterns). 
     In order to better understand why high-aspect shapes and structures are particularly susceptible to being destroyed by chemicals and agitation, one has to recall that such shapes and structures, by definition, include large amounts of surface area and are therefore very thermodynamically unstable. As such, these shapes and structures are highly susceptible to delamination and/or other forms of destruction when chemical and/or mechanical energy is applied. 
     In view of the above, other currently available methods for removing debris from a substrate make use of cryogenic cleaning systems and techniques. When employing such systems and techniques, the substrate containing the high aspect shapes and/or structures is effectively “sandblasted” using carbon dioxide particles instead of sand. 
     Unfortunately, even cryogenic cleaning systems and processes are also known to destroy high aspect features. In addition, cryogenic cleaning processes affect a relatively large area of a substrate (e.g., areas that may be approximately 10 millimeters across or more). Naturally, this means that areas of the substrate that may not need to have debris removed therefrom are nonetheless exposed to the cleaning process and to the potential structure-destroying energies associated therewith. 
     SUMMARY 
     At least in view of the above, what is needed are novel debris-removal methods and devices that are capable of cleaning substrates with high aspect structures, mask optical proximity correction features, etc., without destroying such structures and/or features. 
     According to one aspect of the present disclosure, a method for removing debris from a trench formed on a photolithographic mask is provided. The method includes positioning a tip within the trench, moving the tip within the trench to physically adhere debris to the tip, moving the tip with the adhered debris away from the trench, and removing the adhered debris from the tip. The tip includes a surface and a nanometer-scaled coating disposed thereon, and the coating has a surface energy lower than the surface energy of the photolithographic mask. 
     According to another aspect of the present disclosure, a system for removing debris from a surface of a photolithographic mask is provided. The system includes a pallet of soft material attached to a stage that supports the photolithographic mask, and an atomic force microscope including a tip, supported by a cantilever, to indent the soft material. The tip includes a surface and a nanometer-scaled coating disposed thereon, and the coating has a surface energy lower than the surface energy of the photolithographic mask. 
     According to another aspect of the present disclosure, a system for removing debris from a surface of a photolithographic mask is provided. The system includes an atomic force microscope including a tip supported by a cantilever. The tip includes a surface and a nanometer-scaled coating disposed thereon, and the coating has a surface energy lower than the surface energy of the photolithographic mask. 
     There has thus been outlined, rather broadly, certain embodiments of the invention in order that the detailed description thereof herein may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional embodiments of the invention that will be described below and which will form the subject matter of the claims appended hereto. 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-sectional view of a portion of a debris removal device according to an embodiment of the present invention. 
         FIG. 2  illustrates a cross-sectional view of another portion of the debris removal device illustrated in  FIG. 1 . 
         FIG. 3  illustrates a cross-sectional view of the portion of the debris removal device illustrated in  FIG. 1 , wherein particles are being imbedded in the patch. 
         FIG. 4  illustrates a cross-sectional view of the portion of the debris removal device illustrated in  FIG. 3 , wherein the tip is no longer in contact with the patch. 
         FIG. 5  illustrates a cross-sectional view of a tip according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will now be described with reference to the drawing figures, in which like reference numerals refer to like parts throughout.  FIG. 1  illustrates a cross-sectional view of a portion of a debris removal device  10  according to an embodiment of the present invention. The device  10  includes a nanometer-scaled tip  12  positioned adjacent to a patch  14  of low surface energy material. 
     Formed on the tip  12  is a coating  16 . Before forming the coating  16 , tip  12  may be pre-coated or otherwise surface treated to modify the surface energy of the tip  12  (e.g., to modify the capillary, wetting, and/or surface tension effects). When properly selected, the coating  16  allows the tip  12  to remain sharper for a longer period of time than an uncoated tip. For example, a PTFE-coated diamond tip can have a longer operating life than an uncoated diamond tip. 
     According to certain embodiments of the present invention, the coating  16  includes the same low surface energy material found in the patch  14 . Also, according to certain embodiments of the present invention, the tip  12  may be in direct contact with the patch  14  and the coating  16  may be formed (or replenished) on the surface of tip  12  by rubbing tip  12  against the patch  14 . Typically, rubbing tip  12  against the patch and/or scratching the pad  14  also causes enhanced surface diffusion of the low surface energy material over the surface of tip  12 . 
     According to certain embodiments of the present invention, the coating  16  and the patch  14  are both made from, or at least may include, polytetrafluoroethylene (PTFE) or some other similar material. According to other embodiments of the present invention, a metallic material, oxide, metal oxide, or some other high surface energy material may be disposed between the surface of tip  12  and the low-surface energy material coating  16 . Alternatively, the surface of tip  12  may be roughened or doped. The high surface energy material or tip treatment typically acts to bind the low-surface energy material coating  16  to tip  12  more strongly. 
     A high-surface energy pre-treatment is used without a low-surface energy coating  16  according to certain embodiments of the present invention. In such embodiments, the particles  20  discussed below may be embedded in some other soft targets (e.g., Au, Al) using similar methods to those discussed herein, or the tip may be a consumable. Also, other physical and/or environmental parameters may be modified (e.g., temperature, pressure, chemistry, humidity) to enhance tip treatment and/or particle pick-up/drop-off. 
     According to certain embodiments of the present invention, all of the components illustrated in  FIGS. 1 and 2  are included in an AFM. In some such configurations, the patch  14  is substantially flat and is attached to a stage that supports the substrate  18 . Also, according to certain embodiments of the present invention, the patch  14  is removable from the stage and may easily be replaced. For example, the patch  14  may be affixed to the AFM with an easily releasable clamp (not illustrated). 
       FIG. 2  illustrates a cross-sectional view of another portion of the debris removal device  10  illustrated in  FIG. 1 . Illustrated in  FIG. 2  is a substrate  18  that is typically positioned adjacent to the patch  14  illustrated in  FIG. 1 . Also illustrated in  FIG. 2  is a plurality of particles  20  that are present in a trench  22  that is formed on the surface of the substrate  18 . The particles  20  are typically attached to the surface via Van der Waals short-range forces. In  FIG. 2 , the tip  12  is positioned adjacent to the substrate  18 . In order to reach the bottom of the trench  22 , the tip  12  in the embodiment of the present invention illustrated in  FIGS. 1 and 2  is a high aspect ratio tip. Although a trench  22  is illustrated in  FIG. 2 , the particles  20  may be included in other structures. 
       FIG. 3  illustrates a cross-sectional view of the portion of the debris removal device  10  illustrated in  FIG. 1 , wherein particles  20  are being imbedded in the patch  14 . Then,  FIG. 4  illustrates a cross-sectional view of the portion of the debris removal device  10  illustrated in  FIG. 3 , wherein the tip  12  is no longer in contact with the patch  14 . 
     According to certain embodiments of the present invention, the device  10  illustrated in  FIGS. 1-4  is utilized to implement a method of debris removal. It should be noted that certain embodiments of the present invention may be used in conjunction with other particle cleaning processes, either prior or pursuant to the method discussed herein. It should also be noted that, although only one tip  12  is discussed herebelow, a plurality of tips may be used to implement certain embodiments of the present invention. For example, a plurality of tips could perform embodiments of the methods discussed herein in parallel and at the same time. 
     The debris method mentioned above includes positioning the tip  12  adjacent to one or more of the particles  20  (i.e., the pieces of debris) illustrated as being on the substrate  18  in  FIG. 2 . Then, the method includes physically adhering (as opposed to electrostatically adhering) the particles  20  to the tip  12  as also illustrated in  FIG. 2  as well as some possible repetitive motion of the tip when in contact with the particle(s) and surrounding surfaces. Pursuant to the physical adherence of the particles  20  to the tip  12 , the method includes removing the particles  20  from the substrate  18  by moving the tip  12  away from the substrate  18 , and moving the particles  20  to the patch  14 , as illustrated in  FIG. 3 . 
     According to certain embodiments of the present invention, the method also includes forming the coating  16  on a portion of the tip  12 . According to some of these embodiments, the coating  16  includes a coating material that has a lower surface energy than the substrate  18 . 
     In addition to the above, some embodiments of the method also include moving the tip  12  relative to the substrate  18  such that the tip  12  is adjacent to other pieces of debris (not illustrated). Then, these adhered other pieces of debris are removed from the substrate  18  by moving the tip  12  away from the substrate  18  in a manner analogous to what is shown in  FIG. 3 . 
     Once debris (e.g., the particles  20  discussed above, have been removed from the substrate  18 , some methods according to the present invention include the step of depositing the piece of debris in a piece of material positioned away from the substrate (e.g., the above-discussed patch  14 ). 
     Because the tip  12  may be used repeatedly to remove large amounts of debris, according to certain embodiments of the present invention, the method includes replenishing the coating  16  by plunging the tip  12  in the patch  14 . In these embodiments, low surface energy material from the patch would coat any holes or gaps that may have developed in the coating  16  over time. This replenishing may involve moving the tip  12  laterally within the patch  14  pursuant to plunging the tip  12  in the patch, rubbing the surface, or changing some other physical parameter (e.g., temperature). 
     It should be noted that certain methods according to the present invention include exposing a small area around a defect or particle to a low surface energy material before a repair in order to reduce the likelihood that the removed material will lump together and strongly adhere again to the substrate after the repair is completed. For example, a defect/particle and a approximately 1-2 micron area around the defect is pre-coated with PTFE according to certain embodiments of the present invention. In such instances, a tip  12  coated or constructed from a low surface energy material (e.g., a PTFE tip) can be used to apply a very generous amount of the low surface energy material to a repair area even when other repair tools (laser, e-beam) are being utilized. 
     According to certain embodiments of the present invention, the method includes using the patch  14  to push the particles away from the apex of the tip  12  and toward an AFM cantilever (not illustrated) that is supporting the tip  12 . Such pushing up of the particles  20  would free up space near the apex of the tip  12  physically adhere more particles  20 . 
     According to certain embodiments of the present invention, the tip  12  is used to remove nanomachining debris from high aspect ratio structures such as, for example, trench  22 , by alternately, “dipping” (or indenting) the tip  12  in a pallet of soft (i.e., “doughy”) material, typically found in the patch  14 . This soft material generally has greater adherence to the tip  12  and debris material (e.g., in the particles  20 ) than to itself. The soft material may also be selected to have polar properties to electrostatically attract the nanomachining debris particles  20  to the tip  12 . For example, a mobile surfactant may be used. 
     In addition to the above, according to certain embodiments of the present invention, the tip  12  may include one or more dielectric surfaces (i.e., electrically insulated surfaces). These surfaces may be rubbed on a similarly dielectric surface in certain environmental conditions (e.g., low humidity) to facilitate particle pick-up due to electrostatic surface charging. Also, according to certain embodiments of the present invention, the coating  16  attracts particles by some other short-range mechanism (e.g., hydrogen bonding, chemical reaction, enhanced surface diffusion). 
     Any tip that is strong and stiff enough to penetrate (i.e., indent) the soft pallet material may be used. Hence, very high aspect tip geometries are within the scope of the present invention. Once the tip is stiff enough to penetrate the soft (possibly adhesive) material, high aspect ratio tips that are strong and flexible are generally selected over tips that are weaker and/or less flexible. This allows for the boundaries of the cleaning vector box to be larger than the repair. Hence, according to certain embodiments of the present invention, the tip can be rubbed into the sides and corners of the repair trench  22 . A rough macro-scale analogy of this operation is a stiff bristle being moved inside a deep inner diameter. It should also be noted that, according to certain embodiments of the present invention, the tip  12  may be selected to be more “bristle-like” by including a plurality of rigid nanofibrils (e.g., carbon nanotubes, metal whiskers, etc.). The tip  12  may alternatively be selected to be more “mop-like” by including a plurality of flexible/limp nanofibrils (e.g., polymers, etc.). 
     According to certain embodiments of the present invention, the detection of whether or not one or more particles have been picked up involves performing a noncontact AFM scan of the region of interest (ROI) to detect particles. The tip  12  is then retracted from the substrate without rescanning until after treatment at the target. However, overall mass of debris material picked up by the tip may also be monitored by relative shifts in the tip&#39;s resonant frequency. In addition, other dynamics are used for the same function. 
     Instead of indenting in a soft material to remove particles  20  as discussed above and as illustrated in  FIG. 4 , the tip  12  may also be vectored into the patch  14  to remove the particles  20 . As such, if the tip inadvertently picks up a particle  20 , the particle  20  can be removed by doing another repair. Particularly when a different material is used for depositing the particles  20  by vectoring, then a soft metal such as a gold foil may be used. 
     In addition to the above, an ultra-violet (UV)-light-curable material, or similarly some other material susceptible to a chemically nonreversible reaction, may be used to coat the tip  12  and to form the coating  16 . Before the UV cure, the material picks up particles  20  from the substrate  18 . Once the tip  12  is removed from the substrate  18 , the tip  12  is exposed to a UV source where the material&#39;s properties would be changed to make it less adherent to the tip  12  and more adherent to the material in the patch  14 , where the. This, or some other, nonreversible process further enhances, or enables, the selectivity of particle pick up and removal. 
     Certain embodiments of the present invention provide a variety of advantatges. For example, certain embodiments of the present invention allow for active removal of debris from high aspect trench structures using very high aspect AFM tip geometries. Also, certain embodiments of the present invention may be implemented relatively easily by attaching a low surface energy or soft material pallet to an AFM, along with using a very high aspect tip and making relatively minor adjustments to the software repair sequences currently used by AFM operators. In addition, according to certain embodiments of the present invention, a novel nanomachining tool may be implemented that could be used (like nano-tweezers) to selectively remove particles from the surface of a mask which could not be cleaned by any other method. This may be combined with a more traditional repair where the debris would first be dislodged from the surface with an uncoated tip, then picked up with a coated tip. 
     Generally, it should be noted that, although a low surface energy material is used in the local clean methods discussed above, other possible variations are also within the scope of the present invention. Typically, these variations create a surface energy gradient (i.e., a Gibbs free energy gradient) that attracts the particle  20  to the tip  12  and that is subsequently reversed by some other treatment to release the particles  20  from the tip  12 . 
     For example, one variation that may be used includes using a high surface energy tip coating. Another variation includes pretreating the particles with a low surface energy material to debond the particles and then contacting the particles with a high surface energy tip coating (sometimes on a different tip). Still another variation includes making use of a chemical energy gradient that corresponds to a chemical reaction occurring between a tip surface coating and the particle surface to bond the two. This is either performed until a tip is exhausted or reversed with some other treatment. 
     According to still other embodiments of the present invention, adhesives or sticky coatings are used in combination with one or more of the above-listed factors. Also, the surface roughness or small scale (e.g., nanometer-scale) texture can be engineered to maximize particle clean process efficiency. 
     In addition to the above, mechanical bonding may be used, typically when the tip  12  includes fibrils that, analogously to a mop, are capable of mechanically entangling the particles  20 . The mechanical entanglement, according to certain embodiments of the present invention, is driven by and/or enhanced by surface energy or chemical changes with contact or environment. 
     According to still other embodiments of the present invention, the tip  12  may be coated with molecular tweezers (i.e., molecular clips). These tweezers are noncyclic compounds with open cavities capable of binding guests (e.g., the above-discussed particles  20 ). The open cavity of the tweezers typically binds guests using non-covalent bonding including hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π-π interactions, and/or electrostatic effects. These tweezers are sometimes analogous to macrocyclic molecular receptors except that the two arms that bind the guest molecules are typically only connected at one end. 
     In addition to the above, the particles  20  may be removed by the tip using diffusion bonding or Casimir effects. Also, as in the embodiments of the present illustrated in  FIG. 5 , bristles or fibrils  30  can be attached to the end of the tip  12 . Whether strategically or randomly placed, these bristles or fibrils  30  can enhance local clean in several ways. For example, an associated increase in surface area may be used for surface (short range) bonding to the particles. 
     According to some of embodiments of the present invention, fibrils  30  are engineered to be molecules that selectively (e.g., by either surface or environment) coil around and entangle a particle  20 , thus maximizing surface contact. Also, dislodging of the particles  20  occurs according to certain embodiments of the present invention, typically when stiff bristles  30  are attached to the tip  12 . However, fibrils  30  may also entangle a particle  20  and dislodge the particle  20  mechanically by pulling on the particle  20 . In contrast, relatively rigid bristles  30  typically allow the tip  12  to extend into hard-to-reach crevices. Then, by impact deformation stress of the bristles  30 , by surface-modification of the tip  12  to repel particles  20 , or by some combination, the particle  20  is dislodged. In addition, certain embodiments of the present invention mechanically bond the particles  20  to the tip  12 . When fibrils are on the tip  12 , entanglement of one or more of either the whole or frayed fibrils may occur. When bristles are on the tip  12 , the particle  20  may be wedged between (elastically) stressed bristles. 
     According to still other embodiments of the present invention, methods of debris removal include changing the environment to facilitate local clean. For example, gas or liquid media may be introduced or the chemistry and/or physical properties (e.g., pressure, temperature, humidity) may be changed. 
     In addition to the components discussed above, certain embodiments of the present invention include an image recognition system that identifies debris to be removed. As such, an automatic debris-removal device is also within the scope of the present invention. 
     According to certain embodiments of the present invention, a relatively soft cleaning tip is used to avoid unwanted damage to inside contours, walls, and/or bottom of a complex shape. When appropriate, a stronger force is used to bring the relatively soft tip into much stronger contact with the surface while also increasing the scan speed. 
     It should also be noted that a tip exposed to and/or coated with a low surface energy material can be used for other purposes besides removing debris (cleaning) of nanometer level structures. For example, such tips can also be used, according to certain embodiments of the present invention, to periodically lubricate micron level or smaller devices (like MEMS/NEMS) to contain chemical reactions. 
     The many features and advantages of the invention are apparent from the detailed specification, and thus, it is intended by the appended claims to cover all such features and advantages of the invention which fall within the true spirit and scope of the invention. Further, since numerous modifications and variations will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation illustrated and described, and accordingly, all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.