Abstract:
An apparatus and method for cleaning the surface of a substrate using laser-induced plasma shockwaves and ultraviolet radiation is described. After defects such as organic, inorganic and metallic particles are detected during an inspection step, the substrate is mounted on a motorized stage inside a cleaning chamber. A laser beam is focused into a laser-cleaning nozzle within the chamber. The laser energy generates a laser-induced plasma shockwave inside the nozzle. The shockwave is amplified and exits the nozzle generating the necessary force to overcome the adhesion bond of the defects with the substrate. Coordinating defect locations from the preliminary inspection step the substrate is actively positioned only where defects are present for selective removal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a non-provisional application describing the same invention as an active provisional application, Ser. No. 61/278,628, filed on Oct. 8, 2009, and being filed within one year, hereby claims date priority therefrom. Said provisional application is incorporated herein by reference in its entirety. 
     
    
     TECHNICAL FIELD AND BACKGROUND 
       [0002]    The present description relates to surface cleaning, and more particularly, to a method and apparatus for non-contact laser induced shockwave treatment of contaminates on a substrate, such as, a semiconductor wafer or photomask for example. The terms “shock” and “wave” are used in combination herein as a single term “shockwave” to mean a traveling shock wave, that is a wave of energy that has a significant energy impulse. There are currently numerous methods used to clean substrate surfaces in the semiconductor industry including both chemical and mechanical cleaning techniques. For example, wet cleaning, megasonic and ultrasonic cleaning, brush cleaning, supercritical fluid cleaning and wet laser cleaning are all used to clean particles from the surface of a substrate. However, for sub-micron particulate these cleaning processes are ineffective as each has serious drawbacks requiring the use of cleaning tools and chemical agents that may introduce new contaminates or which may damage critical dimensions of a semiconductor or mask device. Furthermore, each of the above cleaning processes is directed to cleaning the entire surface of the substrate thereby increasing the probability of redeposition and damaging the substrate surface. 
         [0003]    In conventional cleaning of substrates, a wet cleaning method commonly referred to by the term “RCA cleaning” uses large-scale multi-tank immersion cleaning units. This procedure has been used for many years. In this technique, up to 50 substrates are immersed sequentially in aqueous solutions of: ammonium hydroxide plus hydrogen peroxide, hydrochloric acid plus hydrogen peroxide, and dilute heated hydrofluoric acid so as to remove particles, metallic contamination, and organic contamination. After each chemical processing step, the substrates are rinsed in pure water. Since this process uses a large amount of environmentally undesirable and expensive chemicals, and is not especially effective for smaller substrate features, alternative cleaning approaches are needed. 
         [0004]    Megasonic or ultrasonic cleaning removes organic films and particles from a photomask surface by the application of hydrostatic forces created in combination with the action of a chemical solution. However, both megasonic and ultrasonic cleaning techniques operate on the principle of chemical immersion which, undesirably, treats the entire substrate surface. 
         [0005]    Wet laser cleaning is also used to clean substrate surfaces. This cleaning technique entails cleaning the surface with a liquid, such as water or water and alcohol, wherein the solution is super-heated using a laser pulse as the heat source. In so doing, the solution rapidly expands propelling particle from a substrate surface. In this approach, the liquid solution can penetrate metal lines on a patterned substrate which can cause lifting of the metal lines off the substrate causing damage to the pattern and generating additional particulate. 
         [0006]    Relative larger lasers (600 mJ or greater) have been used to generate a laser induced plasma in air. These larger lasers generate radiation heat from the core of the laser induced plasma in excess of 15,000 K. In the case of laser induced plasma in air, the distance at which the radiation temperature drops below 1000 K is 1.5 to 5 mm from the center of the plasma core for I=1.3×10 13  and 2.3×10 14  W/cm2, respectively. The radiation heating from the laser induced plasma core can induce a considerable temperature rise on the substrate surface damaging thin films and sensitive structures. 
         [0007]    Other cleaning techniques include those that employ momentum transfer as a means to impinge and dislodge defects or contaminants from a surface. For example cryogenic aerosol cleaning uses pressurized frozen particles to remove surface contamination. Momentum transfer cleaning techniques are problematic for future generations of semiconductor technology as they increase the risk of physical damage to a substrate surface. Cryogenic cleaning can also electro-statically damage a surface of a substrate due to the presence of ions in the cleaning fluid. 
         [0008]    As manufacturers continue to decrease feature size, the need for, and cost of removal of substrate contamination grows. A more effective and efficient cleaning method and apparatus for removing contaminants from semiconductor and optics industry work products is needed. 
       SUMMARY 
       [0009]    The present apparatus and method provides a novel and greatly improved means for removing sub-micron particulate contamination from critical surfaces. The method employs a laser beam focused in a gaseous environment which results in a dielectric breakdown and ionization of the gas generating a rapidly expanding plasma at the focal point of the laser beam. Initially a release of electrons occurs due to the collision of photons with gas molecules. This creates a local high pressure plasma forming a shockwave which moves outward at supersonic velocity. With a Nd:YAG pulsed laser, these actions occur approximately in the first 100-150 ns of the arrival of the laser pulse at the focal point. The shockwave separates from the plasma within the first few microseconds of the process. 
         [0010]    The shockwave plays a critical role in breaking the bonds which hold particles to a substrate. A force moment is exerted on the particles due to collisions of those gas molecules which are adjacent to the particles, with the particles, the collisions delivering energy from the shockwave to the particles. The interaction of the shockwave energy with the substrate is a momentum transfer process which results in agitation of the particles and detachment from the substrate when the forces of agitation exceed the particle&#39;s adhesion forces. It has been found that particulate detachment is enhanced when the shockwave arrives at the substrate at an angle of between 30 and 45 degrees relative to the substrate surface. 
         [0011]    The presence of capillary forces and particle deformation significantly increases the adhesion force between particle and substrate. In order to increase the efficiency of particle detachment due to the laser induced shockwave cleaning, ultraviolet energy is used to advantage to desorb the substrate surface thereby reducing the capillary forces and related particle adhesion. 
         [0012]    Bearing in mind the problems and deficiencies of the prior art particulate removal processes, it is therefore an object of the presently described apparatus and method to provide improvements in contaminant removal from surfaces such as the substrate surfaces used in the manufacture of electronic components. A further objective is to use ultraviolet energy in removing organic contamination on substrate surfaces. Another objective is to provide a method and apparatus for using focused laser energy to create a shockwave for removing particles through a momentum transfer process. A further objective is to improve particulate removal by directing the shockwave at an acute angle to the substrate surface. It is another objective to provide a method and apparatus for removing contaminants from a substrate while preventing redeposition by sweeping detached particles to one side using a gas stream. Yet another objective is to provide a method and apparatus for removing targeted particulate from a substrate surface without the need to clean an entire substrate surface. 
         [0013]    The details of one or more embodiments of these concepts are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of these concepts will be apparent from the description and drawings, and from the claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0014]      FIG. 1  is a schematic diagram of an example, in accordance with the present description, of a laser induced shockwave surface cleaning apparatus and is shown as an elevational view; 
           [0015]      FIG. 2  is an example elevational view of a shockwave housing thereof in cross section showing a relationship between a laser beam, the housing and a substrate surface which may be placed on an angle relative to the housing; 
           [0016]      FIG. 3  is a graphical illustration in elevation view thereof of an example of the manner in which particles that are released from the substrate surface tend to initially move in a revolving torus shaped gas stream; 
           [0017]      FIG. 4  is a graphical illustration in elevation view thereof of an example of the manner in which particles that are released from the substrate surface are swept to one side by an injected gas stream; and 
           [0018]      FIG. 5  is a logic diagram defining an example process of the presently described method. 
       
    
    
       [0019]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0020]    Photolithographic surfaces such as the surfaces of photomasks, semiconductor wafers, and optical elements associated with, for instance, printing microelectronic features are susceptible to defect and residue formation during processing. Such defects may include haze, crystal growth, ionic residues, and oxides among others. The term “substrate  7 ” as applied herein shall mean a wafer, a photomask, an optical element and any other article that may, by its nature and use, require the removal of micro-particles  5  from its surface  6 , as shown in  FIGS. 3 and 4 . Substrates  7  may be of materials such as: quartz, metal, rubber, plastic, ceramic, and other organic or inorganic substances. The term “substrate surface  6 ” is applied herein to mean a surface of the substrate  7  that is generally planar, at least to micrometer dimensions, and able to be oriented facing generally upwardly as is shown in  FIG. 2 . The term “particle  5 ” is applied herein to mean any substance whatever that is foreign to the substrate  7  or foreign to the successful use of substrate  7 , and may include discrete solid bits of metals and non-metals, organic materials, dusts and debris, liquid droplets and residues therefrom and other similar foreign materials which are well known in the semiconductor fabrication and optics arts as well as in related fields. The presently described method is applicable to particles  5  and agglomerations of particles  5  in the range of a few nanometers to a few hundreds of nanometers. The particles  5  that are found on substrate surfaces  6  may be secured typically by mechanical, electrical and chemical bonds. Some of these defects may be classified as organic contamination and are also referred to herein as particles  5  as well. The presently described methods address such particles  5  and their adherent forces. 
         [0021]      FIG. 1  shows the presently described apparatus  10  which is particularly adapted for removing particles  5  from a surface  6  of a substrate  7 . The apparatus  10  may have a chamber  11  enclosed by chamber walls  12  which may support a window  14 , a blower nozzle  16 , a gas inlet  18 , a transfer door  20 , and an exhaust manifold  22 . A shockwave housing  24 , a substrate chuck  26  and a motorized stage  28  may be located within chamber  11 . Chamber  11  may be at or near atmospheric pressure with N 2  or air for instance. an atmosphere typically of a selected gas or gas mixture. Such chambers  11  are well known in the art. Chamber window  14 , blower nozzle  16 , gas inlet  18 , transfer door  20 , exhaust manifold  22 , substrate chuck  26  and motorized stage  28  are components that are well known in the field of this disclosure and could be selected by those of skill in the art by routine steps. Chamber  11  may be accessed through transfer door  20 . The gas inlet  18  is in gas communication with the shockwave housing  24  through conduit  19  which may be a stainless steel tube or a flexible tube. Process gas  4  may be used to supply one or more gas constituents directly to housing  24  in accordance with the presently described method. It is noted that fixture  35  secures housing  24  in place and allows housing  24  to be set at an angle relative to substrate  7 . This can be accomplished by routine mechanical engineering approaches well known to those of skill in the art. Substrate  7  is always held with surface  6  horizontal and facing upwardly. An inert gas  4  such as Ar, Kr, N 2 , He, and Ne or a reactive gas such as H2, O2, O3, NF3, C2F6, F2 and CL2 may be used alone or in combination in the present process and the combination of Ar and He has been found to be particularly effective. 
         [0022]    The transfer door  20  may be positioned and enabled for exchanging the substrate  7  with the substrate chuck  26  which can grip the substrate  7  by its edges or by suction, for example, as is well known in the art. Substrate transfers through such doors  20  is well known in the semiconductor and optics arts and are designed for maintaining the substrate  7  in a clean state during transfers, and also during manipulations in conjunction with the cleaning processes in general. The window  12  is made of a material that is transparent to the laser energy beam  32  used in the described method, and must be aligned with an entry channel  25  in housing  24  which may be in the range of 1 mm in diameter. It is pointed out, too, that channel  25  is formed clear through housing  24  so that beam  32  dos not impact housing  24 , but only the process gas  4  within. Housing  24  may be made of a structural material such as stainless steel or quartz glass, capable of withstanding the explosive forces of shockwave  29 A as will be described. 
         [0023]    The blower nozzle  16  may be aligned with the substrate chuck  26  for directing a gas stream  70  as shown in  FIG. 1  by arrows, from the blower nozzle  16  in parallel with the substrate surface  6  when the substrate  7  is secured and held in a preferred orientation by the substrate chuck  26 . This gas flow  70  relative to surface  6  is best shown in  FIG. 4 . A megasonic transducer  16 A may be engaged with blower nozzle  16  so that gas flow  70  is cavitated at between 800 and 4000 kHz with the energy released by transducer  16 A. This cavitation helps to reduce a gaseous laminar boundary layer adjacent to the surface of substrate  7  and this enhances the ability of the sonic shockwave  29 A to impact and remove particles  5 . This also helps to prevent redeposition of particles  5 , by delivering added gaseous floatation energy to particles  5  after they have been removed from surface  6  and are floating above it as shown in  FIG. 3 . 
         [0024]    The motorized stage  28 , preferably an X-Y-Z-e operating table may be able to position selected areas of surface  6  relative to a shockwave  29 A as shown in  FIG. 2 . During the cleaning process, the motorized stage  28  is moved continuously to expose a local area of the substrate  7  to a continuing series of shockwaves  29 A. The shockwave housing  24  may be located in a direct path between the blower nozzle  16  and the exhaust manifold  22  so that when particles  5  are liberated from selected areas of the substrate surface  6  they can be “blown,” by impingement of the gas stream  70 , toward exhaust manifold  22  for exiting from chamber  11 . 
         [0025]    A laser system  30  may be positioned outside chamber  11  adjacent to window  14 . Laser system  30  may include a laser beam generator and appropriate optics for expanding and focusing the beam  32 . As shown in  FIG. 2 , beam  32  is directed through channel  25  and focused at area  29  within the shockwave housing  24 . The impact of laser beam  32  on process gas  4  creates a rapid heating, ionization and expansion of the gas  4 . As shown in  FIG. 2 , a high velocity shockwave  29 A escapes housing  24  via outlet  23 . The interior surface of housing  24  may include a reflecting surface  24 A so that shockwave  29 A carries most of the energy delivered to the gas  4  by laser beam  32 . 
         [0026]    A chamber pressure control instrument  40  and a gas flow control instrument, such as a mass flow controller  44  and a gas inlet valve  46  operate under control of a system controller  50  to maintain a desired chamber gas pressure and gas throughput within chamber  11 . Such control is very well known in the art, and this description should be taken as only one possible example of the many approaches to gas pressure and gas flow control that are known. The system controller  50 , which may be a computer, may have an inlet port  52  for receiving substrate inspection data and a first outlet port  54  for delivering instructions to the motorized stage  28  and to pressure control instrument  40 , and a second outlet port  56  for delivering instructions to the laser system  30 . Signals between these components are made using common data signal cables as is well known in the art. System controller  50  is enabled for instructing motorized stage  28  to move substrate chuck  26  and substrate  7  to position selected areas of substrate surface  6  immediately below shockwave outlet  23  and then for instructing laser system  30  to release laser beam  32  for producing the shockwave  29 A. To enhance the laser induced shockwave formation and delivery to the surface  6 , a heavy gaseous atomic species such as Ar or Kr is used in this process and just prior to the delivery of the laser beam  32  into housing  24 , a steady stream of the process gas  4  is delivered to housing  24  from inlet  18  so that the pressure within housing  24  may be elevated at the time the incoming laser energy enters housing  24 . 
         [0027]    As shown in  FIG. 1 , an ultraviolet light source  60  may be positioned for directing ultraviolet energy  62  to the substrate surface  6 . In conjunction with the energy  62  one or more of the above defined process gases  4  may be used to desorb surface  6 . This process induces dissociation of certain gaseous species to produce one or more reactive gaseous components which reduces capillary forces between particles  5  and surface  6 . The ultraviolet radiation  62  may be selected in a wavelength range of from about 140 to 400 nanometers at an intensity of about 1 mW/cm2 and preferably higher then 10 mW/cm2. Examples of ultraviolet sources  60  that are effective in this process include high-pressure mercury lamps (wavelength of about 250-480 nm), low-pressure mercury lamps (wavelength of about 180-480 nm), UV light emitting laser diodes (wavelength of about 300-400 nm), metal halide lamps (wavelength of about 200-400 nm), Xe2 excimer lamps (wavelength of about 172 nm), Ar excimer lamps (wavelength of about 146 nm), KrCl excimer lamps (wavelength of about 222 nm), Xel excimer lamps (wavelength of about 254 nm), XeCl excimer lamps (wavelength of about 308 nm), ArF excimer lasers (wavelength of about 193 nm), and KrF excimer lasers (wavelength of about 248 nm). 
         [0028]    In an exemplary embodiment of the present method, the laser shockwave technique is employed for the removal of inorganic and metallic contamination, which we shall also refer to as particles  5 . In order to generate a laser induced plasma shockwave  29 A, a laser beam  32  may be generated by a Q-switched Nd:Yag laser with a fundamental wavelength of about 1064 nm. The laser beam  32  emerges from the laser system  30  where it has been expanded and focused by optics within the system  30 . The expanded and focused laser beam  32  passes through an optically transparent gas tight window  14 , which is mounted, on chamber wall  12 . The laser beam  32  may be directed parallel to the substrate surface  6  as shown in  FIG. 1 , and through housing  24  thereby generating the laser-induced plasma and shockwave  29 A. The shockwave  29 A propagates rapidly and with explosive force out of housing  24  to impinge on substrate  7  and thereby removes inorganic and metallic surface contaminants, i.e., particles  5 , which are adhered to the substrate surface  6 . The power density of the laser beam  32  at its focus point  29  is preferably about 1012 W/cm2. The plasma shockwave  29 A is the result of an intense electric field induced in the process gas  4  by the energy delivered by laser bean  32 . The gas  4  is rapidly heated, ionized and expanded, thereby producing the plasma shockwave  29 A which then propagates downwardly due to the confined space within the housing  24  and the reflective surface  27  within housing  24 . 
         [0029]    Using a 450 mJ laser, the delivered laser beam  32  has been found to be able to travel a distance of between 25 and 450 mm with appropriate effectiveness in the present process. The motorized stage  26  may be used to position the substrate  7  below the laser induced plasma shockwave exit  23  at a distance of between about 1 mm and 20 mm. In one application, the motorized stage  26  adjusts the z-axis height for a distance typically of about 5 mm from the exit point  23  of the housing  24  Due to the distance of the laser induced plasma  29  from the surface  6  shockwave pressure may be insufficient to remove particles  5  below about 50 nm in diameter. To increase the shockwave pressure sufficiently to overcome this problem, one or more of the above defined gases  4  may be used to generate a shockwave, Kr and particularly Ar gas shows higher pressures generated than other gases so that it is the preferred process gas in the presently described process. The interior surface of housing  24  may include a reflecting surface  24 A so that shockwave  29 A at outlet  23  carries most of the energy delivered to the process gas  4 . To further enhance the cleaning efficiency the shockwave housing  24  may be rotated to a shallow angle, between 20-60 degrees and preferably 45 degrees relative to the horizontal. The configuration shown in  FIG. 2  allows the use of a relatively lower power laser while generating a shockwave pressure sufficient to remove sub-50 nm sized particles. 
         [0030]    In another aspect of the present method it is desired to prevent redeposition of particles  5  that have been already removed from the substrate surface  6 . Referring to  FIG. 3  the impinging shockwave  29 A loosens and detaches particles  5  from the substrate surface  6 . Once the particles  5  are detached from the substrate surface  6 , the ionized gas stream behind the shockwave  29 A traps the suspended particles  5  in a toroidal shaped revolving gas envelope at velocities of several m/s. Therefore, the complete removal of particles  5  takes place over a timescale of milliseconds. The toroidal shaped gas envelope that is formed and related particle movement geometry adjacent to the substrate surface  6  produces a high probability of particle redeposition onto surface  6  as shown in  FIG. 3 . 
         [0031]    Referring to  FIG. 4  the chamber inlet gas nozzle  16  produces a unidirectional gas stream  70  within chamber  11  as indicated by the horizontal arrows in  FIGS. 1 and 4 . This gas stream  70  may be injected so as to move parallel to the substrate surface  6 , and, as a continuous flow over the substrate surface  6  which may have a velocity of 20 m/s for example. The gas stream  70  impinges on the ejected particles  5  sweeping them laterally away from the substrate surface  6 . In this approach, the ejected particles  5  are prevented from re-contacting the substrate surface  6 , but rather are forced to move laterally to be subsequently captured by exhaust pump  21 . When gas stream  70  is injected with transfer door open, it has been found that the efficiency of the particle sweeping process is enhanced. This result can be obtained by venting the chamber in other ways as well in order to reduce resistance to entry of stream  70 . 
         [0032]    The specific locations of particles  5  on the substrate surface  6  may be identified by well known inspection procedures and the information data concerning these locations may be transferred to the system controller  50  via input port  52  as shown in  FIG. 1 . The system controller  50  may then move the substrate  7  to position these particle locations sequentially under the shockwave housing  24  for exposure to the shockwave  29 A. To more efficiently accomplish the later, the substrate surface  6  may be associated with a virtual grid overlay as is well known in the art, and the particle locations then may be defined according to intersections on the grid overlay. For example, if during inspection a particle  5  is found on the substrate surface  6  at grid location X-26, Y-64; then, during the cleaning process, the system controller  50  directs the motorizing stage  28  to position the substrate so that location X-26, Y-64 is adjacent to the shockwave outlet  23  for exposure to the shockwave  29 A. The gas species and gas pressure above the substrate surface  6  and within the shockwave housing  24  may be controlled also by controller  50  to maximize the shockwave effect on the removal of the particles  5 . 
         [0033]    A number of embodiments have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of this disclosure. Accordingly, other embodiments are within the scope of the following claims.