Abstract:
A method of processing wafers or batches of wafers sequentially in a vacuum chamber ( 10 ). The method involves removing a wafer ( 25 ) from a location on a support ( 11 ); chemically cleaning particles from the support to form volatile components; and placing the subsequent wafer on the cleaned support. The method is performed without a vacuum break.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to, and the benefit of, U.S. Provisional Patent Application Ser. No. 60/654,509, filed Feb. 22, 2006, titled “Methods and Apparatus for Processing Wafers,” the entire disclosure of which is hereby incorporated by reference in its entirety.  
       FIELD OF THE INVENTION  
       [0002]     This invention relates to a method and apparatus for processing wafers or batches of wafers sequentially in a vacuum chamber.  
       BACKGROUND OF THE INVENTION  
       [0003]     Particularly in the formation of MEMS it is becoming common for wafers to be processed on both sides including features to be etched through the full thickness of a wafer and to be significantly thinned, with the result that flakes or fragments of the wafer are more likely to become deposited on the upper surface of a wafer support. This causes problems when one tries to place the next wafer at the location, particularly, as is very common, if the wafers are clamped.  
         [0004]     Problems include: wafer mishandling possibly leading to mechanical breakage and/or poor thermal contact between the water and the support leading to process problems such as resist damage. Currently it tends to be that such incidents cause a subsequent wafer to be mishandled or are discovered on the inspection of the next wafer processed and then the processing chamber has to be opened up, necessitating a time consuming vacuum break, to allow cleaning to take place.  
         [0005]     It is also known from U.S. Pat. No. 6,256,186 to run a short (e.g. 5 sec) etch between wafers to remove polymer film material, which has become deposited on the platen during processing. This material will always be outside the wafer location and can only cause problems if a wafer is misplaced such that it is not entirely at the wafer location on the platen.  
       SUMMARY OF THE INVENTION  
       [0006]     According to one aspect the invention comprises a method of processing wafers or batches of wafers sequentially in a vacuum chamber including: 
        removing a wafer from a location on the support;     chemically cleaning particles from the support to form volatile components; and     placing the subsequent wafer on the cleaned support;     the method being performed without a vacuum break.        
 
         [0011]     The particles may be cleaned from the location.  
         [0012]     According to a further aspect invention comprises a method of removing particles from a wafer location on a wafer support including performing a chemical cleaning process on the location in the absence of a wafer from the location.  
         [0013]     In either aspect the method may further include detecting the presence of particles. For example the detection step may utilise visual recognition apparatus, in which case conveniently the surface of the support at the location may be of a colour contrast visually from the colour of the particles. The cleaning step may be performed in response to the detection of at least one particle and/or the cleaning step may be ended in response to the detection of the absence of particles from the location.  
         [0014]     In any of these cases the particles may have a dimension in the range of 100 μm to 5 mm and in particular 0.5 mm to 2 mm  
         [0015]     The method may include the step of assuming the particles are of the same material as the wafer and selecting a cleaning chemistry accordingly.  
         [0016]     In any case the platen may be biased during the cleaning step to induce ion bombardment and additionally or alternatively the chemical cleaning step may be plasma assisted. In that case, where there is ion bombardment, the same power source may be used for inducing bias and the plasma.  
         [0017]     For silicon wafers, XeF 2 , ClF 3  or SF 6  gas may be utilised for the chemical cleaning step with applied plasma if required, which may be regarded as an etch process.  
         [0018]     For aluminium or III/V semiconductors, Cl 2  and/or BCL 3  would be suitable. For organics, an oxidizing plasma e.g. O 2 /CF 4  mix would be suitable. In general a gas or gas mix—with applied plasma power as appropriate should be chosen by reference to the particulate material to be removed. Where particles are of more than one predominate material type then gas mixes or sequential processes may be selected as appropriate.  
         [0019]     In the case of at least SF 6  the chamber pressure may be at least about 30 mTorr for the cleaning step. Preferably the chamber pressure is in the range of about 30 mTorr to 80 mTorr and applied plasma power is required. For chemically active gases such as XeF 2  and ClF 3  no plasma power is required and a pressure of 5-50 Torr is appropriate.  
         [0020]     The cleaning step may take between about 1 minute and about 15 minutes and preferably is between about 5 minutes and about 7.5 minutes. Where there is no detection system, the cleaning step may take place between every wafer or batch of wafers or it may take place after a predetermined number of wafers being processed. However, as the dropping of particles from the wafer is a generally unpredictable event it is preferred that there is either a detection system or the operator makes an inspection between each wafer.  
         [0021]     The method may include clamping each wafer to the support for processing in which case a support may include an electrostatic chuck.  
         [0022]     It will be appreciated that by using chemical means to remove the particles, they can essentially be converted into volatile components that can be pumped out of the chamber without the need for a vacuum break.  
         [0023]     From a further aspect there is provided apparatus for cleaning particles from a substrate support including a vacuum chamber containing a support defining a substrate location; 
        a detection system for detecting particles on the substrate support at the substrate location; and     chemical cleaning apparatus for cleaning the support without a vacuum break in response to the detection of particles.        
 
         [0026]     Although the invention has been defined above it is to be understood that it includes any inventive combination of the features set out above or in the following description.  
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0027]     The invention may be performed in various ways as specific embodiments will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0028]      FIG. 1  is a schematic view of an “RIE” type apparatus for performing one embodiment of the invention; and  
         [0029]      FIG. 2  is a graph correlating the initial surface area of test particles and their etch rate. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     Referring to  FIG. 1 , a vacuum chamber  10  contains a wafer support  11 , having a wafer location A and being driven by an RF supply  12 , and a gas inlet  13  communicating with a shower head  14  for process gas. The chamber  10  also has an outlet  15 , controlled by valve  16 , to enable a pump  17  to evacuate it and to take away volatile gases. The gas inlet supply is controlled by a valve  18  and the apparatus includes a controller  19  which controls valve  16  and  18  and pump  17  in response to a visual recognition system  20 , which can observe the wafer location A through a window  21  by means of a sensor  22  and a suitable computing facility at  23 . A wafer handling device is schematically indicated at  24  for introducing and removing wafers  25  from the location A through a gate  26 .  
         [0031]     As will be indicated in more detail below the controller  19  also controls the wafer handling apparatus  24  and is responsive to the visual recognition system  20 .  
         [0032]     In one embodiment, this system comprises a window in the wall or ceiling of the chamber with an optical imaging apparatus that monitors the state of the support surface. This optical imaging apparatus may comprise a camera, a source of illumination and a control system with pattern recognition capability. The source of illumination may comprise an external light source, or it may comprise a plasma, initiated by applying power from a source to a suitable gas in the process chamber. In this latter embodiment, the action of illumination may be combined with an action of cleaning of the chamber or an action of declamping of a wafer.  
         [0033]     In a second embodiment, the presence of fragments on the support may be inferred by detecting the absence of these fragments from the processed wafer, for example by comparing an image of the processed wafer to a reference image.  
         [0034]     The subsystem that detects the presence of fragments on the support may also be used to determine the endpoint of the fragment removal process, or a separate endpoint system may be used to determine the completion of the fragment removal operation, or the processs may be operated for a predetermined length of time, according to the size or number of fragments to be removed.  
         [0035]     In one embodiment, the subsystem that detects or infers the presence of fragments on the support may make a record of this fact for future analysis.  
                                                           TABLE 1                           Dimensions and disappearance times of silicon fragments       removed using a SF 6  plasma etch.                    Initial                   L × W × H   mass   Initial surface   Disappearance       Sample #   (mm)   (mg)   area (mm 2 )   time (s)                    1   6.0 × 4.0 × 0.18   10.2   51.6   330       2   15.0 × 1.5 × 0.18    9.6   51.0   400       3   2.5 × 1.1 × 0.18   1.2   6.8   310       4   2.5 × 0.5 × 0.19   0.6   3.6   435       5   2.0 × 0.8 × 0.19   0.7   4.2   220        6*   13.5 × 5.2 × 0.65    107.1   164.9   1340       7   9.0 × 5.5 × 0.65   74.6   117.8   1085       8   6.0 × 3.9 × 0.65   35.7   59.7   990       9   3.5 × 3.0 × 0.66   16.1   29.6   757       10    7.0 × 1.6 × 0.65   16.9   33.5   762       11    5.0 × 1.0 × 0.65   7.6   17.8   610       12    1.6 × 1.6 × 0.62   3.7   9.1   522                 *Sample coated with oxide on one face             
 
       EXAMPLE  
       [0036]     The Applicants have demonstrated the effectiveness of this method in removing silicon fragments.  
         [0037]     Fragments with maximum dimensions ranging from 2 mm to 1 5 mm and masses from 0.5 mg to 100 mg were placed on the support, using an oxide-coated wafer as a carrier. This carrier is resistant to the etching action, and was used for convenience in the study. It does not form part of the invention.  
         [0038]     The fragments were then exposed to a plasma formed from SF 6  gas, with additional power applied to the support in accordance with a first embodiment of the invention. The time taken for these fragments to disappear are recorded in Table 1. The plot in  FIG. 2  shows that these disappearance times are correlated with the exposed surface area of the fragment, and can be interpreted with a constant etch rate of approximately 15 μm/min acting at all surfaces.  
         [0039]     The presence of an oxide coating on one surface of one of the fragments did not significantly inhibit the removal operation.  
         [0040]     The chamber used in this study was a specialised deep silicon etch tool which had a helicon type plasma source above the wafer, but it will be appreciated that the invention can be applied in any chamber capable of supporting an etching environment. For example Reactive Ion Etch (RIE), Inductively Coupled Plasma (ICP), diode, triode, microwave, remote and in fact any plasma source/chamber configuration suitable for etching wafers or layers upon a wafer. Similarly, although this study was restricted to removal of silicon fragments using a plasma formed from SF 6  gas, it will be appreciated that appropriate changes to the etching environment will allow fragments or particles of many other materials to be removed by the methods encompassed in this invention.  
         [0041]     Where SF 6  is used as a process gas the following conditions have been used: 
        500 sccm SF 6 —(a lower flow rate may be acceptable)     60 to 80 mTorr chamber pressure—(a pressure of &gt;30 mTorr is probably acceptable).     3 kW RF source—(lower powers may be acceptable).     75 W bias (200 mm) and 30 W for a 150 mm diameter wafer were used.        
 
         [0046]     The foregoing descriptions of specific embodiments of the present invention have been presented for the purpose of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications, embodiments, and variations are possible in lights of the above teaching. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.