Abstract:
In a post chemical-mechanical polishing (CMP) procedure for cleaning a workpiece, a cleaning solution is delivered to the core of a brush where the solution is absorbed by the brush and then applied by the brush onto the workpiece. The cleaning solution is uniformly applied to the workpiece. The volumes of solutions used in the scrubbing process is reduced. A thin oxide layer is etched. A hydrophilic surface state is maintained. The workpiece is then rinsed and dried in a centrifugal processing between upper and lower rotors. A high level clean is achieved while consumption of rinsing and drying fluids is reduced.

Description:
BACKGROUND OF THE INVENTION  
       FIELD OF THE INVENTION  
         [0001]    The field of the invention is systems and methods for processing a workpiece, such as a workpiece, a semiconductor wafer, and other flat media requiring low levels of contamination during the manufacturing process. The invention further relates to chemical-mechanical scrubbing, rinsing and drying a workpiece.  
           [0002]    In manufacturing semiconductor devices, flat panel displays, optical masks, and similar articles or devices, the surface of the article must be cleaned of contaminants. If not removed, contaminants may affect device performance characteristics and may cause device failure to occur at faster rates than usual.  
           [0003]    Cleaning silicon wafers in particular presents particular challenges. Silicon wafers often have a very thin oxide surface layer, such as a native or passivation oxide or a chemically grown oxide. Surface metal particles, which are contaminants which must be removed. These particles may be on top of the oxide, in the oxide or at the oxide/silicon interface. The oxide surface layer is typically less than 20 Angstroms (A) thick. Accordingly, a highly controlled etch must be used to remove contaminants from this layer. If the oxide is entirely removed, the surface becomes hydrophobic, and may become difficult to clean. Therefore, ideally, as much oxide as possible is etched away to remove contamination on the surface of the oxide layer, but without removing all of the oxide layer, to avoid having surface become hydrophobic.  
           [0004]    One well known cleaning technique uses a scrubber that scrubs a wafer or workpiece on one or both sides. The cleaning solution used in the scrubber depends on the contaminants to be removed, the type of wafer to be scrubbed, and/or the particular application. Where a high level clean is needed, a chemical solution may be used for scrubbing. Where a lower level clean, i.e., higher contamination levels, are acceptable, so that less contamination need be removed, water may be used for scrubbing.  
           [0005]    Two-sided scrubbers that use soft sponge like brushes to simultaneously clean both sides of the wafer or workpiece are widely and effectively used for cleaning silicon wafers via post chemical-mechanical planarization (CMP). Ammonium hydroxide solution is typically added to de-ionized water (DI water) during scrubbing to improve the cleaning performance especially in CMP cleaning. The ammonium hydroxide solution helps to remove slurry particles (and the metallic contamination associated with them) from the wafer surface. It also prevents brush loading by inducing a negative zeta potential on the particle, brush and the wafer surface.  
           [0006]    If contamination is under the workpiece surface, within the oxide layer, an etching solution may be needed to remove the contaminants, along with a thin oxide layer from the surface. Regardless of the solution used, the workpiece must be rinsed and dried, after the CMP or other cleaning steps, to remove the cleaning solution from the workpiece. Various rinser/dryer apparatus and methods have been used for this purpose. For example, well known spin rinser/dryers have been used in cleaning apparatus, including CMP apparatus, to rinse and dry workpieces. These spin rinser/dryers may operate on one workpiece at a time, or on a batch of workpieces. Typically, such spin rinser dryers have a rotor for holding the workpieces in a near vertical orientation. The rotor spins within a chamber. Nozzles within the chamber generally spray a rinsing fluid, such as DI water onto the spinning workpieces, to rinse away cleaning solutions, or other particles or droplets on the workpiece surfaces. The workpiece is then dried by spinning at high speed, to centrifugally remove the rinsing fluid. Drying gases may also be used.  
           [0007]    While this type of post CMP rinsing and drying has met with varying degrees of success, there remains a need for improved rinsing and drying methods and apparatus, for use after post CMP processes.  
         SUMMARY OF THE INVENTION  
         [0008]    In a method and system for cleaning workpieces, such as semiconductor wafer, a cleaning solution is provided to a core of a brush in a workpiece scrubber, to provide chemical mechanical scrubbing with in-situ etching of the workpiece with the brush. The thickness of an oxide etched is controlled as required by different applications. A thin native oxide on silicon may be etched, to remove surface contaminants without removing the entire film. This maintains a hydrophilic surface necessary for maintaining low levels of surface particles, especially in brush scrubbing systems, where the brush is in contact with the wafer surface during cleaning.  
           [0009]    Following this cleaning procedure, the workpiece or wafer is moved into a rinser/dryer. The rinser/dryer has upper and lower rotors or chamber members which are brought together or engaged to form a rinsing/drying chamber closely conforming to the shape of the workpiece. As the rinsing/drying chamber volume is small, the workpiece can be rinsed and dried in a highly controlled way. In addition, consumption of rinsing and drying liquids and/or gases is reduced.  
           [0010]    The invention resides as well in subcombinations of the systems and methods described. It is accordingly an object of the invention to provide improved rinsing and drying of workpieces, such as semiconductor wafers. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    In the drawings, where the same number indicates the same element in each of the views:  
         [0012]    [0012]FIG. 1 is a schematically illustrated side view of an apparatus according to the invention.  
         [0013]    [0013]FIG. 2 is a side view of the rinser/dryer shown in FIG. 1.  
         [0014]    [0014]FIG. 3 is a schematic view of liquid and gas supply lines to the rinser/dryer shown in FIGS. 1 and 2.  
         [0015]    [0015]FIG. 4 is cut away perspective view of the rinser/dryer shown in FIGS. 1 and 2.  
         [0016]    [0016]FIG. 5 is a section view taken along line  5 - 5  of FIG. 4.  
         [0017]    [0017]FIG. 6 is an enlarged view of components shown in FIG. 5.  
         [0018]    [0018]FIG. 7 is a bottom perspective view of the lower rotor shown in FIGS. 5 and 6.  
         [0019]    [0019]FIG. 8 is an enlarged view of components shown in FIG. 5 adjacent to the perimeter or edge of the workpiece.  
         [0020]    [0020]FIG. 9 is an enlarged view of alternative embodiment components shown in FIG. 5 adjacent to the perimeter or edge of the workpiece.  
         [0021]    [0021]FIG. 10 is a top perspective view of the lower rotor shown in FIGS. 5 and 6.  
         [0022]    [0022]FIG. 11 is an enlarged side view of the lever or arm shown in FIG. 10, with the arm in the up position.  
         [0023]    [0023]FIG. 12 is an enlarged side view of the lever or arm shown in FIGS. 10 and 1, with the arm in the down position. 
     
    
     DETAILED DESCRIPTION  
       [0024]    The system and methods described are useful for cleaning workpieces, such as silicon wafers where a controlled thin oxide etch is used to maintain a hydrophilic surface. A controlled removal of thin layers of oxide may be performed, regardless of whether the workpiece has been polished by CMP. A cleaning solution, such as dilute is preferably applied through the PVA brushes to the surface of the workpiece during the scrubbing cycle.  
         [0025]    To clean a workpiece having an ultrathin chemical oxide layer (typically less than 20 angstroms), very dilute HF (on the order of 0.005% HF) is used to perform an in-situ oxide etch with a controlled removal of less than approximately 15 angstroms. This removes contaminants, including particles and plated residues, on the surface of the oxide layer or within the oxide layer, without making the surface hydrophobic. A thin layer of oxide remains on the surface of the workpiece so that the surface remains hydrophilic.  
         [0026]    To clean thicker oxide layers, e.g., greater than approximately 30 angstroms, a controlled thin oxide etch may be used. For these applications, removal of metallic particle contamination (which may be incorporated into the oxide from the CMP polishing process) is important. The metallic particle contamination may diffuse into the miroelectronic devices on the workpiece and cause them to fail. Very dilute concentrations of HF (such as 0.005% HF) may be sufficient to remove metallic contamination, depending upon the depth of penetration of the contamination into the oxide layer. If the metallic contamination is more than 20 angstroms below the surface, a higher concentration of HF may be needed. The amount of oxide removed is determined by the concentration of HF delivered to the brush, the dispense flow rate, and time.  
         [0027]    For removing less than 15 angstroms of a native oxide layer, a 0.005% concentration of HF has a slow etch rate, with etch times of 20-60 seconds acceptable. The etch time is more critical when using higher concentrations. The concentration of HF can be adjusted to provide an oxide layer removal rate which is consistent with a desired workpiece throughput or production rate.  
         [0028]    For back-end CMP processes, removal of up to 100 angstroms of oxide may be required to adequately remove the metallic contamination incorporated therein by the polishing process. To remove this amount of oxide in less than approximately 40 or 50 seconds, the concentration of HF is increased to 0.5-1.0%.  
         [0029]    The CMP process is preferably performed in a scrubber that scrubs both sides of a workpiece simultaneously. The combination of mechanical double sided scrubbing with in-situ thin oxide etching allows multiple process steps to be accomplished within a single machine. This reduces handling requirements and the risks of contamination associated with handling and transport of workpieces between process machines.  
         [0030]    The cleaning processes includes the step of delivering a hydrofluoric acid (HF) solution to a core of a brush, such as a PVA brush, in a semiconductor workpiece scrubber. After delivering the hydrofluoric acid (HF) solution to the brush core, the HF solution is applied to the workpiece through the brush, followed by chemical mechanical scrubbing of the workpiece with the brush. The solution may be applied concurrently with the brush scrubbing of the workpiece.  
         [0031]    The concentration of the HF solution is in the range of approximately 0.005%-1.0% HF, depending upon application. The solution preferably includes a mixture of approximately 0.005 percent HF in water. The HF solution is applied for a predetermined amount of time, for example, 20-40, or 25-35 or about 35 seconds.  
         [0032]    The term wafer or workpiece here includes a bare or pure semiconductor workpiece, with or without doping, a semiconductor workpiece with epitaxial layers, a semiconductor workpiece incorporating one or more device layers at any stage of processing, other types of workpieces incorporating one or more semiconductor layers such as workpieces having semiconductor on insulator devices, or workpieces for processing other devices such as flat panel displays, multichip modules, optical masks, memory media, etc.  
         [0033]    [0033]FIG. 1 is a section view of a cleaning system  100  including several stations or sections within a housing or enclosure  101 . Each station performs one or more steps in the cleaning process In use, contaminated workpieces  50  are delivered into the cleaning system  100  typically after CMP or other processes resulting in contamination. The contaminated workpieces  50  within a cassette, box or tray  102  are moved into a loading station  104  of the system  100  via a door or window. Workpieces  50  are removed from the cassette  102  and placed, one at a time, into the first or outside brush station  106 , by a first transfer robot  108 . In the outside brush station  106 , a workpiece  50  is processed through a first scrub. The workpiece is treated with chemical solution, such as ammonium hydroxide, applied to the workpiece through brushes  110  during the first scrub.  
         [0034]    The scrubbed workpiece  50  is then transferred from the outside brush station  106  to an inside brush station  112  via the first transfer robot. In the inside brush station  112 , the workpiece  50  is processed through a second scrub. In the second scrub, the workpiece is preferably treated with a second solution, such as a very dilute HF solution. As in the first scrub step, the HF solution is applied to the workpiece through brushes  114 . HF may optionally be used in both scrub stations. Other solutions, such as water, citric acid, ammonium hydroxide, and ammonium citrate (or mixtures of them) may be used in either of the brush stations.  
         [0035]    After the second scrub the workpiece is transferred from the inside brush station  112  into a rinser/dryer  200 , via the first transfer robot  108 . The rinser/dryer  200  rinses, spins, and dries the now clean workpiece. Once the rinse, spin, and dry steps have been completed, the workpiece is then removed from the rinser/dryer  200  and placed into a clean cassette  120  at an output or unload station  122 , via a second transfer robot  118 . The transfer robots  108  and  118  each preferably have a robotic arm including an end effector or hand  116  which lifts the workpiece by its edges. The clean cassette  120  is then transferred to storage or to another cleaning or processing system, either by hand, or via a facility robot external to the system  100 .  
         [0036]    The brushes  110  and/or  114  may be PVA sponge brushes. During etching, the chemical solution is distributed to the rotating brushes so that the brushes are evenly soaked with the solution. The brushes are preferably saturated with the solution by absorbing the solution through the slots or holes in the brush core. The chemical solution is applied to the workpiece via the rotating bushes until the desired amount of the oxide layer is removed. Once the desired level is reached, the etch is stopped. To stop the etch, the chemical solution to the brushes is turned off. A water supply line is turned on to rinse the workpiece and stop the etching.  
         [0037]    With reference generally to FIGS.  2 - 6 , the rinser/dryer  200  has an upper chamber member or rotor  202  that includes an upper chamber wall  212 . A lower chamber member or rotor  204  similarly includes a lower chamber wall  214 . These walls  212 ,  214  open or separate to permit a workpiece  50  to be loaded into the rinser/dryer  200  by the second transfer robot  118 . The walls  212 ,  214 , move towards and engage each other to define a capsule assembly  216  containing the workpiece  50  in a processing position, between the walls  212 ,  214 .  
         [0038]    The capsule assembly  216  spins about a rotation axis A. A motor  222  in a head  220  rotates the upper rotor  202  around the axis A, along with the workpiece  50  and the lower rotor  204 . Specifically, as shown in FIGS. 4 and 5, the motor  222  drives a sleeve  223 , which is supported radially in the head  220 , by rolling-bearings  238 . The head  220  is pivotably supported on an armature  262  which is raised and lowered by an elevator  264 . The head  220  can be raised to separate the walls  212 ,  214 , of the rotors  202 ,  204  and can be lowered for bringing the walls  212 ,  214  towards each other.  
         [0039]    The upper chamber wall  212  has an inlet  237  for rinsing and drying fluids, which may be liquid, vaporous, or gaseous. The lower chamber wall  214  has an inlet  215  for such fluids, which for a given application may be similar fluids or different fluids. An upper nozzle  229  extends axially through the sleeve  223  So as not to interfere with the rotation of the sleeve  223 . The upper nozzle  229  directs streams of rinsing/drying fluids downwardly through the inlet  237  passing through the upper chamber wall  212 .  
         [0040]    The upper chamber wall  212  includes an array of similar outlets  245 , which are spaced similarly at uniform angular spacings around the vertical axis A. Preferably 36 outlets  245  are used. The outlets  245  are spaced outwardly from the vertical axis A by just slightly less than the workpiece radius. The outlets  245  are also spaced inwardly from the outer perimeter of the workpiece  50  supported in the capsule assembly  216  by a much smaller radial distance, such as a distance of approximately 1-5 mm.  
         [0041]    When the upper and lower rotors  202 ,  204  are brought together as shown in FIG. 6, an upper processing chamber or space  244  formed or is defined by the upper chamber wall  212  and by a first or top generally planar surface of the workpiece  50 . Similarly, a lower processing chamber or space  240  is defined or formed by the lower chamber wall  214  and a second generally planar surface of the workpiece  50  opposite the first side. The upper and lower processing chambers  244 ,  240 , are in fluid communication or connected with each other via an annular region  231  beyond the outer perimeter or cage of the workpiece and are sealed by an annular, compressible seal such as an O-ring  213 , surrounding the lower portion of the annular region  231 . The seal  213  prevents fluid leakage between the rotors  202  and  204 , forcing fluid to flow toward the outlets  245 .  
         [0042]    As illustrated in FIGS.  5 - 6 , the lower chamber wall  214  may have an annular sump  242  around the inlet  215 . The sump  242  is used to collect liquids and/or residual fluids supplied through the inlet  215 . If a liquid, for example, strikes and drops from a workpiece  50 , it is conducted toward the outlet  245  by centrifugal force as the capsule assembly  216  is rotated.  
         [0043]    The lower nozzle  226 , which is provided beneath the inlet  215  of the lower chamber wall  214 , includes two or more ports  227 , as shown in FIG. 6, for directing two or more streams of fluid upwardly through the inlet  215 . The ports  227  are oriented to cause the directed streams to converge approximately where the directed streams reach the lower surface of the workpiece  50 . The rinser/dryer  200  also includes a purging nozzle  230  as shown in FIG. 5, at a side of the lower nozzle  226 , for directing a stream of purging gas, such as nitrogen, across the lower nozzle  226 .  
         [0044]    Referring to FIG. 6, the rinser/dryer  200  may have the lower nozzle  226  and the purging nozzle  228  in a base  239  having a coaxial, annular plenum  232 . The plenum  232  has (e.g. four) drains  233  each of which is equipped with a valve, such as a pneumatically actuated poppet valve  234  for opening and closing the drain  233 . The drains  233  preferably provide separate paths for conducting liquid of different types to appropriate systems for storage, disposal, or recirculation.  
         [0045]    An annular shield or skit  236  may be provided and extending around and downwardly from the upper chamber wall  212 , above the plenum  232 . The skirt  236  rotates with the upper chamber wall  214  and upper rotor  202 . Each outlet  245  is oriented to direct fluids exiting the capsule assembly  216  against the inner surface of the annular skirt  236 . The inner surface is flared outwardly and downwardly to cause fluids to flow outwardly and downwardly toward the plenum  232  by centrifugal force. Thus, fluids tend to be swept through the plenum  232 , toward the drains  233 .  
         [0046]    As shown in FIG. 6, the upper rotor  202  has a ribbed surface  224  facing and closely spaced from a smooth lower surface  225  of the head  220 , in an annular region connecting with the plenum  232 . When the upper rotor  202  rotates, the ribbed surface  224  tends to cause air in the annular region to swirl, so as to help to sweep fluids through the plenum  232 , toward the drains  233 .  
         [0047]    Referring still to FIG. 6, the upper chamber wall  212  has spacers  281  that project downwardly to prevent lifting of the workpiece  50  from the processing position and from touching the upper chamber wall  212 . Similarly, posts  295  project upwardly from the lower wall  214  upwardly beyond the outer perimeter of the workpiece to prevent it from shifting off center from the vertical axis A.  
         [0048]    Referring to FIGS.  10 - 12 , the lower rotor  204  preferably includes a lifting mechanism  258  for lifting a workpiece  50  supported in the processing position to an elevated position. The lifting mechanism  258  lifts the workpiece  50  to the elevated position when the head  220  is raised above the base  239 , to open the capsule assembly  216 . The upper and lower chamber walls  212 ,  214  move away from each other as the capsule  216  opens. Lifting a workpiece  50  to the elevated position allows the second transfer robot to engage and the workpiece  50 .  
         [0049]    The lifting mechanism  258  includes lifting levers  272 . Each lifting lever  272  is pivotably mounted to the lower chamber wall  214  via a pivot pin  286  extending from the lifting lever  272  into a socket  282  in the lower chamber wall  214 . The levers  272  are pivotable between an operative (up) position and an inoperative (down) position. Each pivoting lever  272  is engaged by the upper chamber wall  212  when the upper and lower chamber walls  212 ,  214 , are brought together, whereby the pivoting lever  272  is pivoted into the inoperative or down position. Each lifting lever  272  is biased to pivot into the operative or up position, when not engaged by the upper chamber wall  212 .  
         [0050]    Thus, each lifting lever  272  is adapted to pivot from the up position into the down position as the upper and lower chamber walls  212 ,  214 , are closed, and to pivot from the down position into the up position as the upper and lower chamber walls  212 ,  214 , are opened. An arm  270  on each lifting lever  272  extends beneath the workpiece  50  supported in the processing position and lifts the workpiece  50  to the elevated position, when the lifting lever  272  is pivoted from the down position into the up position.  
         [0051]    The lifting levers  272  may be biased by an elastic member  278  (e.g. an O-ring) surrounding the lower chamber wall  214  and engaging the lifting levers  272 , via a hook depending from each lifting lever  272 .  
         [0052]    The elastic member  278  is maintained under comparatively higher tension when the upper and lower chamber walls  212 ,  214 , are closed, and under comparatively lower tension when the upper and lower chamber walls  212 ,  214 , are opened or spaced further apart.  
         [0053]    Referring momentarily to FIGS. 5, 6 and  7 , the upper and lower chamber walls  212 ,  214 , may also be releasably clamped to each other when in the closed state by a latching mechanism  250 . As shown in FIGS. 6 and 7, the latching mechanism includes a latching ring  252  that is carried on the rotor  204  and is adapted to engage a complementary shaped recess  254  the upper chamber wall  212 . The latching ring  252  is made from a resilient spring material (e.g. polyvinylidine fluoride) with an array of inwardly stepped sections  260 . This section  260  allows the latching ring  252  to deform from an undeformed condition in which the latching ring  252  has a first diameter, into a deformed condition in which the latching ring  252  has a comparatively smaller diameter. This deformation occurs when the stepped portions  260  are subject to radial inward directed forces. Upon removal of the forces, the latching ring  252  returns to the undeformed.  
         [0054]    The latching mechanism  250  further includes an array of latching cams  262 , each associated with a stepped section  260 . The latching cams  254  apply radial forces to the stepped portions  260 .  
         [0055]    As shown in FIGS. 5 and 6, the latching mechanism  250  further includes an actuating ring  256 , which is adapted to actuating the latching cams  262  as the actuating ring  256  is raised and lowered within a predetermined limited range of movement. The actuating ring  256  is raised up to actuate the latching cams  262 , and lowered to deactuate the latching cams. Pneumatic lifters  258  are adapted to raise and lower the actuating ring  256 . When the actuating ring  256  is raised, the upper and lower chamber walls  212 ,  214 , are released from each other so that the head  220  can be raised from the base  230  for opening the upper and lower chamber walls  212 ,  214 , or lowered onto the base for closing the upper and lower chamber walls  212 ,  214 .  
         [0056]    As shown in FIG. 6, pins  296  on the actuating ring  256  project upwardly and into apertures  253  in an aligning ring  257 , when the actuating ring  256  is raised. The aligning ring  257  is joined to, and rotates with, the lower rotor  204 . The pins  296  are withdrawn from the apertures  253  and clear the aligning ring  257  when the actuating ring  256  is lowered. When projecting into the respective apertures  253 , the pins  296  align the workpiece  55 , to facilitate unloading the workpiece via the second transfer robot  50 .  
         [0057]    In use, a wafer or workpiece  50  is transferred from the inside brush station  112  to the rinser/dryer  200  via the first transfer robot  108 . The rinser/dryer  200  is in the open position as shown in FIGS. 1 and 2. The upper and lower rotors  202  and  204  are spaced apart from each other. The lifting mechanism  258  is in the up position. The robot  108  moves the wafer  50  into the rinser/dryer  200  and sets it down on the lifting levers  272 . The elevator  264  moves the head  220  down. The upper rotor  202  engages the lower rotor  204 . The downward movement of the upper rotor  202  pushes the arms  270  down so that the wafer  50  comes to rest on the pins  295 .  
         [0058]    The circumferential edges of the wafer  50  are centered by the arm posts  274  which are located on a circle concentric with, and slightly larger than, the circle or which the pins  295  are located on. The spacers  281  on the upper rotor  202  come to rest on the top surface of the wafer  50 . This securely clamps the wafer  50  in place within the now closed capsule assembly  216 . The lifters  258  are lowered or released. The ring  256  moves down. The cams pivot inwardly (toward the axis A-A). The stepped sections  260  of the ring  252  flex outwardly into the recesses  254 . This locks the upper and lower rotors together. The flange  218  on the upper rotor  202  moves over the seal  213 . This seal stops fluid outflow from the chambers  240  and  244 , except via the outlet(s)  245 .  
         [0059]    The motor  222  is turned on to spin the capsule assembly  216  (including the upper and lower rotors  202  and  204 , and the wafer  50  held between them). A rinsing liquid, such as DI water, is introduced onto the upper and lower wafer surfaces via the inlets  237  and  215 . The liquid spreads and flows radially outwardly over the wafer surfaces via centrifugal force and drains out the capsule assembly  216  via the outlets  2345 . The rinsing liquid covers or flows over all areas of the workpiece, rinsing away process chemicals remaining on the workpiece from prior processes. The capsule  216  is then typically accelerated to a higher spin speed to remove remaining droplets of rinsing liquid, via centrifugal force. A drying gas may then be applied a gas supply, such as nitrogen supply  290  in FIG. 3. The drying gas, such as air or nitrogen, may be heated via a gas heater  292 . When the workpiece is dry, the elevator  264  lifts the head  220  to open the capsule  216 . As this occurs, the lifting mechanism  258  lifts the workpiece. The second transfer robot  118  removes the clean and dry workpiece  50  from the rinser/dryer  200 , and moves it into a clean cassette  120 .  
         [0060]    Thus, novel systems and methods have been described. Various changes, substitutions, and use of equivalent components and steps can of course be made without departing from the spirit and scope of the invention. The invention, therefore, should not be limited, except by the following claims and their equivalents.