Abstract:
Apparatus and a method for removing particles from the surface of a substrate include determining respective position coordinates of the particles on the surface. A beam of electromagnetic energy is directed via an optical cleaning arm at the coordinates of each of the particles in turn, such that absorption of the electromagnetic energy at the surface causes the particles to be dislodged from the surface substantially without damage to the surface itself.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/172,299, filed Dec. 16, 1999, and U.S. Provisional Patent Application No. 60/195,867, filed 7 Apr. 2000, which are incorporated herein by reference. This application further is a Continuation In Part of PCT Patent Application PCT/IL99/00701, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to processing of semiconductor devices, and specifically to methods and apparatus for removal of foreign particles and contaminants from solid-state surfaces, such as semiconductor wafers and lithography masks.  
       BACKGROUND OF THE INVENTION  
       [0003]     Removal of particles and contaminants from solid state surfaces is a matter of great concern in integrated circuit manufacture. This concern includes, but is not limited to, semiconductor wafers, printed circuit boards, component packaging, and the like. As the trend to miniaturize electronic devices and components continues, and critical dimensions of circuit features become ever smaller, the presence of even a minute foreign particle on a substrate wafer during processing can cause a fatal defect in the circuit. Similar concerns affect other elements used in the manufacturing process, such as masks and reticules.  
         [0004]     Various methods are known in the art for stripping and cleaning foreign matter from the surfaces of wafers and masks, while avoiding damage to the surface itself. For example, U.S. Pat. No. 4,980,536, whose disclosure is incorporated herein by reference, describes a method and apparatus for removal of particles from solid-state surfaces by laser bombardment. U.S. Pat. Nos. 5,099,557 and 5,024,968, whose disclosures are also incorporated herein by reference, describe methods and apparatus for removing surface contaminants from a substrate by high-energy irradiation. The substrate is irradiated by a laser with sufficient energy to release the particles, while an inert gas flows across the wafer surface to carry away the released particles.  
         [0005]     U.S. Pat. No. 4,987,286, whose disclosure is likewise incorporated herein by reference, describes a method and apparatus for removing minute particles (as small as submicron) from a surface to which they are adhered. An energy transfer medium, typically a fluid, is interposed between each particle to be removed and the surface. The medium is irradiated with laser energy and absorbs sufficient energy to cause explosive evaporation, thereby dislodging the particles.  
         [0006]     One particularly bothersome type of contamination that is found on semiconductor wafers and lithography masks is residues of photoresist left over from a preceding photolithography step. U.S. Pat. No. 5,114,834, whose disclosure is incorporated herein by reference, describes a process and system for stripping this photoresist using a high-intensity pulsed laser. The laser beam is swept over the entire wafer surface so as to ablate the photoresist. The laser process may also be effected in a reactive atmosphere, using gases such as oxygen, ozone, oxygen compounds, nitrogen trifluoride (NF 3 ), etc., to aid in the decomposition and removal of the photoresist.  
         [0007]     Various methods are known in the art for localizing defects on patterned wafers. A summary of these methods is presented in an article entitled “Defect Detection on Patterned Wafers,” in  Semiconductor International  (May 1997), pp. 64-70, which is incorporated herein by reference. There are many patents that describe methods and apparatus for defect localization, for example, U.S. Pat. Nos. 5,264,912 and 4,628,531, whose disclosures are incorporated herein by reference. Foreign particles are one type of defects that can be detected using these methods.  
         [0008]     U.S. Pat. No. 5,023,424, whose disclosure is incorporated herein by reference, describes a method and apparatus using laser-induced shock waves to dislodge particles from a wafer surface. A particle detector is used to locate the positions of particles on the wafer surface. A laser beam is then focused at a point above the wafer surface near the position of each of the particles, in order to produce gas-borne shock waves with peak pressure gradients sufficient to dislodge and remove the particles. It is noted that the particles are dislodged by the shock wave, rather than vaporized due to absorption of the laser radiation. U.S. Pat. No. 5,023,424 further notes that immersion of the surface in a liquid (as in the above-mentioned U.S. Pat. No. 4,987,286, for example) is unsuitable for use in removing small numbers of microscopic particles.  
         [0009]     Various methods are known in the art of surface contamination control using integrated cleaning. A summary of these methods is presented in an article entitled “Surface Contamination Control Using Integrated Cleaning” in  Semiconductor International  (June 1998), pp. 173-174, which is incorporated herein by reference.  
       SUMMARY OF THE INVENTION  
       [0010]     It is an object of some aspects of the present invention to provide methods and apparatus for efficient removal of contaminants from solid-state surfaces, and particularly for removal of microscopic particles from semiconductor wafers and other elements used in semiconductor device production. The wafers may be bare, or they may have layers formed on their surface, whether patterned or unpatterned.  
         [0011]     It should be noted that a substrate is henceforth broadly defined as any solid-state surface such as a wafer, which requires at least one contaminant or particle to be removed from its surface. It should be noted further that the word particle is used broadly to define any contaminant or other element, which requires removal from a substrate surface.  
         [0012]     It is a further object of some aspects of the present invention to provide improved methods and apparatus for targeted removal of contaminant particles from a surface based on prior localization of the particles.  
         [0013]     In preferred embodiments of the present invention, a cleaning module is employed to remove particles from a substrate surface. The cleaning module comprises a moving chuck, on which the substrate is mounted, and a moving optical cleaning arm, positioned over the chuck. The chuck holds the substrate, most preferably by suction, and comprises a motorized system which rotates the chuck about a theta (θ) axis or, alternatively, on x-y axes. The moving arm comprises optics, through which electromagnetic radiation, preferably a laser beam, is conveyed and directed onto the substrate to clean the substrate surface. The arm preferably rotates about a phi ( 101  ) axis passing through its base, parallel to but displaced from the θ axis of the chuck. Alternatively, the arm may move on x-y axes. Alternatively, the optical arm may be stationary, and only the chuck moves the substrate so as to place a particle directly under the arm. Similarly, the chuck may be stationary, and only the optical arm moves so as to position itself above a particle on the substrate surface.  
         [0014]     The arm motion is preferably coordinated with movement of the moving chuck so that the laser beam can be directed locally at any point on the wafer surface. The cleaning module is connected to an electromagnetic energy source via a radiation guide, which is coupled to convey the energy to the optics of the moving arm. The cleaning module and laser module are herein termed a “particle removal unit”.  
         [0015]     In some preferred embodiments of the present invention, the arm further comprises channels for vapor or gas-phase transport to the substrate, and suction systems for transfer of gases and residuals from the substrate surface. In one such embodiment, vapor, preferably water vapor, is conveyed to the substrate via the channels in the cleaning arm. In another such embodiment, vapor such as alcohol, or an alcohol:water mixture, is conveyed via the channels in the cleaning arm. A vapor film is thus deposited onto the substrate, which condenses into a thin liquid film. Subsequently, when the electromagnetic energy impinges on the substrate, the liquid film evaporates explosively, as described, for example, in the above-mentioned U.S. Pat. No. 4,987,286. The particle residuals and gas-phase matter are then preferably removed via the cleaning arm. The water vapor thus serves two purposes: to dislodge the particle from the substrate surface by explosive evaporation of the liquid, and to cool the substrate surface, so as to minimize damage.  
         [0016]     In some preferred embodiments of the present invention, the particle removal unit is connected to a particle localization unit. The particle localization unit preferably provides the particle removal unit with the coordinates of one or more particles. The contaminated area of the substrate is positioned under the cleaning arm by moving both the substrate and the cleaning arm according the coordinates of the particle. Laser energy is conveyed from the electromagnetic energy source, via the energy guide and the cleaning arm, and then targets the particle according to the information received from the particle localization unit. The energy is fired so as to remove the particle from the substrate surface. The particle removal unit lifts the particle, preferably by suction, and conveys it away from the substrate.  
         [0017]     In some preferred embodiments of the present invention, the electromagnetic energy source comprises a multi-wavelength laser source. Preferably, the source combines ultraviolet laser radiation and infrared radiation, most preferably from an Optical Parametric Oscillator (OPO).  
         [0018]     In some other preferred embodiments of the present invention, a laser source such as an Er:YAG laser (at 2.94 micron wavelength, for example) may be directed directly from the electromagnetic energy source via the optical arm to the substrate.  
         [0019]     The different wavelengths are used individually or in combination, in order to match the energies required to remove a specific type of contaminant from a defined solid-state surface. The infrared radiation is preferably used in conjunction with the vapor film described above.  
         [0020]     In some preferred embodiments of this invention, the particle removal unit is integrated into a metrology tool, cluster tool, or other process tool for microelectronics fabrication on a semiconductor wafer. Preferably, the cleaning module is connected to other processing units by a clean wafer transfer system. This integration of the cleaning module in the process system is made possible by the novel, compact design of the moving chuck and arm, making the cleaning module far more compact and non-intrusive than laser-based cleaning units known in the art. The proximity of the particle removal unit to a particle localization unit and/or to other process tools enables fast and effective removal of particles without adding a separate cleaning process step. This integrated laser cleaning reduces the amount of inter-step substrate handling, and thus reduces process time and costs and increases process yield.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0021]     The present invention will be more fully understood from the following detailed description of the preferred embodiments thereof, taken together with the drawings in which:  
         [0022]      FIG. 1  is a schematic pictorial illustration of a particle removal unit, constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0023]      FIG. 2  is a schematic top view of a cleaning module used in the unit of  FIG. 1 , constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0024]      FIG. 3  is a schematic, sectional view of the cleaning module of  FIG. 2 , illustrating removal of a contaminant particle from the substrate;  
         [0025]      FIG. 4  is a schematic, sectional view of the cleaning module of  FIG. 2 , showing further details of its construction;  
         [0026]      FIG. 5 . is a schematic, sectional view of a cleaning module for removal of particles from a substrate during a manufacturing process, in accordance with a preferred embodiment of the present invention;  
         [0027]      FIG. 6  is a graph showing a water absorption spectrum as a function of wavelength, useful in understanding a preferred embodiment of the present invention;  
         [0028]      FIG. 7  is a simplified block diagram illustrating a laser source coupled to an optical parametric oscillator, constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0029]      FIG. 8  is a schematic view of a particle removal unit and a particle localization unit integrated into a semiconductor wafer processing cluster tool, constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0030]      FIG. 9  is a flow chart showing a method of substrate cleaning, in accordance with a preferred embodiment of the present invention; and  
         [0031]      FIG. 10  is a flow chart showing a method of substrate cleaning, in accordance with another preferred embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0032]      FIG. 1  is a schematic pictorial illustration of a particle removal unit  10 , constructed and operative in accordance with a preferred embodiment of the present invention. Unit  10  comprises an in situ particle removal module  20 , also referred to herein as a cleaning module. Module  20  comprises a substrate-holding chuck  25  on which a substrate  30  is mounted, and a cleaning arm  40 . A single wavelength laser or a multi-wavelength laser module  60  generates a laser beam, which is conveyed to arm  40  via a radiation guide  50 .  
         [0033]     Substrate  30  is preferably a semiconductor wafer, although the methods and apparatus described hereinbelow are similarly applicable to substrates of other kinds. Module  20  is preferably integrated in situ in a metrology or process tool or with other semiconductor processing equipment, as described hereinbelow. Laser module  60  is preferably remote from the process tool.  
         [0034]      FIG. 2  is a schematic top view of module  20 , in accordance with a preferred embodiment of this invention. Substrate  30  is placed on chuck  25 , which rotates about a theta (θ) axis  80  at the center of the substrate. Arm  40  moves about a phi (φ) axis  90 , which is parallel to but displaced from the θ axis. Arm  40  comprises optics  72  for conveying a laser beam  75  received via radiation guide  50  to the coordinates of a particle on substrate  30 . The laser beam may be used in this manner to clean selected points on the substrate, which have been identified as the location of undesired particles, or to scan over and clean the entire substrate.  
         [0035]     Preferably, arm  40  also comprises an inlet channel  70  for conveying gas or vapor to substrate  30 . Additionally or alternatively, the arm comprises a suction channel  95  and a suction port  85  for removing particle debris, contaminants, liquid and gases from the area of the substrate. Suction port  85  comprises a nozzle, preferably constructed with an aperture of 0.5-3 cm diameter, most preferably 0.5 cm diameter. The nozzle is preferably positioned at a tilt of 25 to 60 degrees and a distance of up to 4 cm from the substrate surface, most preferably approximately 2 cm from the surface.  
         [0036]      FIG. 3  is a schematic, sectional view showing details of module  20  and illustrating removal of a contaminant particle  110  from substrate  30 . The laser beam is directed by optics in arm  40  onto the area in which particle  110  is located. A steam pulse is directed onto the area of the particle from inlet channel  70  before laser beam  75  is fired, with contaminated gas, liquid and solid products being removed simultaneously via suction port  85 . Preferably, dry gas is conveyed via inlet channel  70  subsequent to the steam flow. The dry gas preferably impinges on substrate&#39;s  30  surface, and then preferably flows a suction nozzle (not shown) via tubing to the suction gas outlet  135 . in order to dry the tubing.  
         [0037]     In other preferred embodiments of this invention, suction is preferably started prior to activating the electromagnetic energy source, preferably laser. The time delay before activating the energy source is preferably 0 to 5 seconds, and most preferably, 0.5 seconds. This enables gas flow lines into the suction nozzle to form. Thereafter, when substrate  30  surface is irradiated, and one or more particles  110  are released, they exhibit a drift diffusion. The particle drift diffusion is controlled by the suction and dry gas flow rate.  
         [0038]      FIG. 4  is a schematic, sectional view of cleaning module  10 , showing further details of its construction, in accordance with a preferred embodiment of the present invention. Substrate  30  is preferably held on chuck  25  by a suction mechanism  125 . Preferably, a coolant channel  120  conveys a coolant  122 , such as water, to chuck  25  in order to cool substrate  30 , and thus to prevent thermal damage. Suction channel  95  is connected to a suction gas outlet  135 . Rotation of chuck  25  is controlled by a motor  140 . Although module  10  is shown here as an independent unit, in an alternative embodiment of the preferred embodiment, arm  40  is incorporated in an existing process chamber or metrology tool and makes use of a rotating chuck or X-Y stage that is already present in the system.  
         [0039]      FIG. 5  is a schematic, sectional view of cleaning module  20 , in accordance with another preferred embodiment of the present invention. In this embodiment, wafer  30  is mounted on an x-y stage or platform  111 . Cleaning arm  40  may rotate about the φ axis, or it may be fixed, since the x-y stage allows the laser beam to reach all areas of the wafer surface without the necessity of scanning the laser beam, as well. Alternatively, chuck  25  is enabled to reach every position under arm  40  by moving along radius r, and rotating about theta axis  80 . The configuration of  FIG. 5  is useful in the context of particle detection tools, which commonly include an x-y stage already.  
         [0040]      FIG. 6  is a graph showing a water absorption spectrum as a function of the wavelength of the incident radiation, useful in understanding aspects of the present invention. In order to achieve high absorption of the laser beam in a water film deposited on wafer  30 , wavelengths of 10.6 μm and 2.95 μm are preferred, as they are points of strong absorption. The 2.95 μm absorption is more than one order of magnitude stronger than absorption at 10.6 μm. Preferably, laser module  60  is designed to generate a tuned, pulsed laser beam at wavelengths that are tailored according to the particular particle removal application, including both vapor-assisted and dry methods. Different process stages and contaminant types typically require different methods and different wavelengths for optimal cleaning. Thus, module  60  is preferably able to generate both ultraviolet and infrared (IR) radiation, which is most preferably tunable to the water absorption peak at 2.95 μm.  
         [0041]      FIG. 7  is a simplified block diagram illustrating elements of multi-wavelength laser module  60 , constructed and operative in accordance with a preferred embodiment of the present invention. A Nd:YAG laser source  170  emits a laser beam at 1.06 μm, which is directed into an optical parametric oscillator (OPO)  180 . The OPO down-converts the laser frequency so as to emit a beam in the mid-IR, at one of the wavelengths at which water has an absorption peak, as shown in  FIG. 6 . Alternatively, a pulsed C 0   2  laser (10.6 μm wavelength) can be used instead of the OPO. Beam shaping optics  190  direct the IR beam into radiation guide  50 , which then carries the beam to arm  40 . Preferably, module  60  also includes an ultraviolet (UV) laser, such as a Lambda Physik (Gottingen, Germany) LPX315 IMC excimer laser. Alternatively, a Nd:YAG laser in its fourth harmonic may be employed. The UV laser is highly efficient for cleaning bare silicon, while OPO  180  can generate radiation in the strong absorption region of water (2.95 μm) such that “explosive evaporation” conditions are reached and efficient particle cleaning achieved when UV cleaning is ineffective or unsatisfactory for other reasons. Alternatively, an Er:YAG laser may be employed.  
         [0042]     In another preferred embodiment of this invention, the OPO and the UV laser operate simultaneously to deliver both IR and UV radiation. The OPO and laser are controlled in order to deliver radiation in amounts that will be sufficient for cleaning but below the damage threshold of the device. Proper control of the IR and UV sources enables particle removal with a lower total amount of energy imparted to substrate  30  than when only a single laser wavelength is used, as in systems known in the art. Lower energy deposition in the substrate reduces the possibility of thermal or radiation damage during cleaning.  
         [0043]      FIG. 8  schematically illustrates integration of particle removal unit  10  into a cluster tool  210  for semiconductor wafer processing, in accordance with a preferred embodiment of the present invention. Preferably, cluster tool  210  also comprises a particle localization unit  230 , which is used to provide coordinates of particles that must be removed from wafer  30  by unit  10 . A typical example of particle localization unit  230  is the KLA-Tencor “Surfscan” system.  
         [0044]     Wafers are transferred to cleaning module  20  from other process elements in the cluster tool, in order to remove contaminants from the wafers before or after other processing steps. A mechanical wafer transfer unit  222  transfers wafer  30  via a clean wafer transfer system  232  to and from the other process elements. These typically include a process etch unit  224 , a deposition unit  226 , a lithography unit  228 , and the like. After each process or cleaning step, mechanical wafer transfer unit  222  may transfer substrate  30  to the next process unit, or to particle localization unit to locate any further particles, and then to the cleaning module  20  to be cleaned again. When particle removal unit  10  receives information concerning the location of particles from particle localization unit  230 , it can then perform very localized cleaning, and does not need to clean the whole wafer surface.  
         [0045]     At the end of all the unit processes in the cluster tool, mechanical wafer transfer unit  222  transfers substrate  30  via clean wafer transfer system  232  to the cluster tool exit.  
         [0046]     Thus, the laser-cleaning system comprising particle localization unit  230  and particle removal unit  10  can be used to clean a substrate in situ. This cleaning may take place at the front end of a process line [FEOL], at the back end of the line [BEOL], simultaneously with, during, or after a process, simultaneously with a measuring process, or prior, during, or after a measuring process. Process examples include, but are not limited to, pre-deposition, post-deposition, before and after lithography, development and etch processes, and before, during and after measurement processes. Two typical options are exemplified in  FIGS. 9 and 10 .  
         [0047]      FIG. 9  is a flow chart showing a typical sequence of substrate cleaning employing particle removal unit  10  in situ prior to a process in accordance with a preferred embodiment of the present invention. particle localization unit  230  checks substrate  30  surface for particle  110 . Particle  110  may be external contaminant such as dust, microbe, photoresist residues from prior processing, and the like. When particle localization unit  230  finds one or more particles  110  on the surface of substrate  30 , it transfers substrate  30  to particle removal unit  10 . Particle localization unit  230  relays coordinates of particle  110  to particle removal unit  10 . Preferably, cleaning arm  40  rotates about phi (φ) axis  90  to the area of particle  110  on substrate  30 . Substrate  30 , mounted on substrate chuck  25 , may also move about theta (θ) axis  80  according to the coordinates of particle  110  received from particle localization unit  230 .  
         [0048]     Cleaning arm  40  then conveys steam  70  to the surface of substrate  30 . The water vapor condenses on impact with substrate  30 , and a liquid film is formed. The liquid film may cover parts or all of the surface of substrate  30 . Laser beam  75 , is conveyed from multi-wavelength laser module  60  via radiation guide  50  and through cleaning arm  40  onto the liquid film. The liquid film explosively evaporates, dislodging particle  110  from the surface of substrate  30 . Particle  110  and/or particle remnants are preferably carried by airflow, or sucked into the channel in cleaning arm  40  and are ejected at suction gas outlet  135  of cleaning module  20 .  
         [0049]     The above process is repeated until all particles have been removed from the substrate surface.  
         [0050]     Particle localization unit  230  preferably has electromechanical systems for substrate transfer. Substrate transfer may alternatively be manual, or be part of mechanical wafer transfer unit  222  of cluster tool  210 . The wafer may be transferred to a holding stage or to another process unit, such as a process etch unit  224 , a deposition unit  226 , a lithography unit  228 , or the like.  
         [0051]      FIG. 10  is a flow chart illustrating a typical sequence of substrate cleaning employing particle removal unit  10  in situ simultaneously with a metrology process, in accordance with a preferred embodiment of the present invention. For example in a slow measuring process, such as microscopic measurement of dimensions of elements on substrate  30 , or electrical measurements on a substrate, it is preferable to utilize the time to remove the particles simultaneously. The sequence of  FIG. 10  is thus substantially similar to that of  FIG. 9 , except that in  FIG. 10  the particle location and removal processes are interleaved, rather than serial. While unit  10  is operating, a metrology tool, such as a remote microscope, takes measurements of various elements on the surface of substrate  30 . It is preferable that the movement of the microscope is coordinated with that of arm  40 . The metrology tool continues to take measurements until all particles have been removed, and no more measurements are required.  
         [0052]     It will be appreciated that the preferred embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.