Abstract:
An apparatus is for plasma dicing a semiconductor substrate of the type forming part of a workpiece, the workpiece further including a carrier sheet on a frame member, where the carrier sheet carries the semiconductor substrate. The apparatus includes a chamber, a plasma production device configured to produce a plasma within the chamber suitable for dicing the semiconductor substrate, a workpiece support located in the chamber for supporting the workpiece through contact with the carrier sheet, and a frame cover element configured to, in use, contact the frame member thereby clamping the carrier sheet against an auxiliary element disposed in the chamber.

Description:
BACKGROUND 
       [0001]    This invention relates to apparatus for plasma dicing a semiconductor substrate of the type forming part of a workpiece, the workpiece further comprising a carrier sheet on a frame member, wherein the carrier sheet carries the semiconductor substrate. The invention relates also to associated methods of plasma dicing. 
         [0002]    Semiconductor manufacture typically involves large numbers of silicon chips being processed in parallel on a single semiconductor wafer. Once the processing steps are completed, the wafer must be diced into discrete chips. Connections are then made to the chips which are subsequently packaged. Traditionally, the dicing step has been carried out using diamond saws which cut along scribe lines on the wafer. Recently, there has been much interest in providing new methods for dicing wafers. These offer the possibility of enhanced performance and lower cost. One such approach is to use lasers to carry out the dicing of the wafer. An alternative approach is to use plasma etching to dice the wafer. This has benefits in terms of minimising die edge damage, maximising the efficient use of the wafer surface by using very narrow scribe lines, and providing the option to use non-orthogonal scribe line layouts. Depending on the application requirement, the plasma etching can occur before or after thinning or grinding of the wafer. 
         [0003]    Silicon wafers are presented for dicing on a “frame and tape” carrier.  FIG. 7  is a semi-schematic representation of such an arrangement showing a silicon wafer  71  with discrete chips  73  separated by scribe lines  72 . The wafer  71  is adhered by an adhesive to a carrier tape  74  which is retained by an annular frame ring  76 . A plasma  9  is used to etch the exposed scribe lines in a vacuum system (not shown). The wafer  71 /carrier tape  74 /frame  76  forms a workpiece assembly which is placed on a platen  75 . The platen retains the workpiece, provides cooling, and optionally an RF bias to aid the etch process. Frequently, an electrostatic chuck (ESC) is used to improve heat coupling between the wafer and the platen. U.S. Pat. No. 8,691,702 discloses an ESC based platen assembly which can process taped wafers on a frame. A lift mechanism is provided which raises the workpiece to allow it to be moved to and from the platen. A frame cover spaced apart from the frame is provided which protects the lift mechanism and the frame. An extension of the frame cover or a separate component is provided to protect the tape in the vicinity of the frame. The ESC is provided with appropriate cooling channels to remove heat from the chuck. The removal of heat during the process in order to maintain suitable working temperatures is an important consideration. The carrier tape is at particular risk of overheating. Typically, the carrier tape is formed from a polymeric material such as a polyolefin (PO) polyvinyl chloride (PVC) or polyethylene terephthalate (PET) with a softening point of around 90° C. In practice, it is considered necessary to maintain the temperature of the carrier tape at 80° C. or less during processing to avoid damage to the tape or to the acrylic adhesive used on the tape. Thermal runaway is a constant risk as the thickness of the tape is typically less than 200 microns and therefore the tape has a low thermal capacity. 
         [0004]    It is desirable for economic reasons to dice wafers as quickly as possible while maintaining the process specification. This is typically achieved by running the plasma etch tool at a high RF power to increase the exposed material in the scribe lines of the semiconductor wafer. The use of high etch rate regimes increases the likelihood of overheating. Therefore, there is a strong desire to achieve high etch rates in plasma dicing to maximise throughput whilst avoiding damage to the workpiece, in particular avoiding damage to the carrier tape or the associated adhesive. It is particularly desirable to be able to maintain the temperature of the carrier tape at 80° C. or less. 
       SUMMARY 
       [0005]    The present invention, in at least some of its embodiments, addresses the above described problems and wants. 
         [0006]    According to the first aspect of the invention there is provided an apparatus for plasma dicing a semiconductor substrate of the type forming part of a workpiece, the workpiece further comprising a carrier sheet on a frame member, wherein the carrier sheet carries the semiconductor substrate, 
         [0007]    the apparatus comprising: 
         [0008]    a chamber; 
         [0009]    a plasma production device configured to produce a plasma within the chamber suitable for dicing the semiconductor substrate; 
         [0010]    a workpiece support located in the chamber for supporting the workpiece through contact with the carrier sheet; 
         [0011]    a frame cover element configured to, in use, contact the frame member thereby clamping the carrier sheet against an auxiliary element disposed in the chamber. 
         [0012]    The auxiliary element may be the workpiece support. 
         [0013]    The auxiliary element may be a shield ring which is disposed around the workpiece support. The shield ring may be in thermal contact with the chamber. Typically, the shield ring may be in thermal contact with one or more interior walls of the chamber. 
         [0014]    The apparatus may further comprise a heat shield disposed over the frame cover element to thermally shield the frame cover element from the plasma. 
         [0015]    The heat shield may be spaced apart from the frame cover element. The heat shield may be supported on the auxiliary element. 
         [0016]    The heat shield may be in contact with the frame cover element. Typically, in these embodiments, it is desirable to minimise the thermal contact between the heat shield and the frame cover element. The heat shield may comprise one or more protrusions which contact the frame cover element. In this way, thermal contact can be reduced. The protrusions may be of any suitable form, such as pins, one or more ridges, or pips. 
         [0017]    The heat shield may be formed from a ceramic material. The ceramic material may be alumina. 
         [0018]    In the other main embodiments, there is no heat shield present to thermally shield the frame cover element from the plasma. 
         [0019]    The apparatus may further comprise at least one clamp which applies a clamping force to the frame cover element to assist the clamping of the carrier sheet against the auxiliary element by the frame cover element. The at least one clamp may directly clamp the frame cover element, or if the heat shield is present, the at least one clamp may clamp the heat shield against the frame cover element to assist the clamping of the carrier sheet against the auxiliary element. The at least one clamp may be mounted on the workpiece support. 
         [0020]    The frame cover element may comprise a substantially flat lower surface which, in use, contacts the frame member. 
         [0021]    The frame cover element may comprise one or more protrusions which, in use, contacts the frame member. The protrusions may be of any suitable form, such as pins, one or more ridges, or pips. The auxiliary element may comprise one or more openings for receiving the protrusions of the frame cover element when a workpiece is not present on the workpiece support. 
         [0022]    The apparatus may be configured so that the frame cover element can be brought into thermal contact with the auxiliary element when a workpiece is not present on the workpiece support. 
         [0023]    The frame cover element may be formed from a metal or a ceramic material. An example of a suitable metal is aluminium. An example of a suitable ceramic material is alumina. 
         [0024]    The workpiece support may be an electrostatic chuck (ESC). The ESC may be a monopolar or a bipolar ESC. 
         [0025]    The apparatus may further comprise a lifting mechanism for lowering and raising the frame into and out of contact with the workpiece support and lowering and raising the frame cover element into out of contact with the frame member and, optionally, the workpiece support. 
         [0026]    The apparatus may be provided in combination with a workpiece comprising the semiconductor substrate and a carrier sheet on a frame member, wherein the carrier sheet supports the semiconductor substrate, the workpiece support supports the workpiece through contact with the carrier sheet, and the frame member is clamped by the frame cover element. 
         [0027]    The workpiece support may comprise a workpiece support surface which supports the workpiece through contact with the carrier sheet. The workpiece support surface has a periphery. The frame member may define a periphery of the workpiece. The periphery of the workpiece may be wholly located within the periphery of the workpiece support surface. 
         [0028]    The carrier sheet may comprise tape formed from a polymeric material, optionally with an adhesive. The polymeric material may be a PO, PVC or PET. 
         [0029]    According to a second aspect of the invention there is provided a method of plasma dicing a semiconductor substrate of the type forming part of a workpiece, the workpiece further comprising a carrier sheet on a frame member, wherein the carrier sheet carries the semiconductor substrate, the method comprising the steps of: 
         [0030]    providing an apparatus according to the first aspect of the invention; 
         [0031]    supporting the workpiece by placing the carrier sheet in contact with the workpiece support; 
         [0032]    clamping the carrier sheet against the auxiliary element by contacting the frame member with the frame cover element; and 
         [0033]    plasma dicing the semiconductor substrate. 
         [0034]    The workpiece support may comprise a workpiece support surface which supports the workpiece through contact with the carrier sheet. The workpiece support surface has a periphery. The frame member may define a periphery of the workpiece. The periphery of the workpiece may be wholly located within the periphery of the workpiece support surface. 
         [0035]    The workpiece support may be an electrostatic chuck and the portion of the electrostatic chuck which contacts the carrier film may be a substantially flat and featureless upper surface. 
         [0036]    The workpiece support may be an electrostatic chuck which acts as the auxiliary element, wherein the electrostatic chuck provides an additional, electrostatic clamping force acting on the frame member. 
         [0037]    After the step of plasma dicing the semiconductor substrate is completed, the workpiece may be removed from the chamber and the frame cover element may be brought into thermal contact with the workpiece support. 
         [0038]    Whilst the invention has been described above, it extends to any inventive combination of the features set out above, or in the following description, drawings or claims. For example, any feature described in relation to the first aspect of the invention is considered to be disclosed also in relation to the second aspect of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    Embodiments of apparatus and methods in accordance with the invention will now be described with reference to the accompanying drawings, in which: 
           [0040]      FIG. 1  is a cross sectional view of semiconductor substrate mounted in vacuum processing chamber with additional heat shield; 
           [0041]      FIG. 2  is a cross sectional view of vacuum processing chamber (from  FIG. 1 ) during a cooling stage, which occurs in between successive plasma treatments; 
           [0042]      FIG. 3  is a cross sectional view of a semiconductor substrate mounted in a vacuum processing chamber where clamping pins are used to clamp the annular frame to the dicing tape; 
           [0043]      FIG. 4  is a cross sectional view of the vacuum processing chamber (from  FIG. 3 ) during a cooling stage which occurs between successive plasma treatments, where clamping pins are lowered into recesses in the ESC; 
           [0044]      FIG. 5  is a cross sectional view of the heat shield optimised design in its lowered position; 
           [0045]      FIG. 6  is a cross sectional view of the heat shield optimised designs in its raised position; and 
           [0046]      FIG. 7  shows plasma dicing of a workpiece comprising a wafer supported on a tape and frame carrier. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0047]    A first embodiment of the invention is shown in  FIG. 1 . A semiconductor substrate  11  comprises scribe lines  12  and discrete semiconductor chips  13 . The semiconductor substrate  11  is typically composed of silicon, however, gallium arsenide and other III-V semiconductors may be used. The semiconductor substrate  11  is adhered to the dicing tape  14 , which is positioned on top of an electrostatic chuck  15 . An annular frame  16  is positioned on top of the dicing tape  14  such that the dicing tape  14  is fixed between the annular frame  16  and the electrostatic chuck  15 . The semiconductor substrate  11  is designed to accommodate a degree of non-concentricity, typically ±3 mm, in the positioning of the annular frame  16 . The frame assembly  17  comprises the semiconductor substrate  11 , the dicing tape  14 , and the annular frame  16 . The dicing tape  14  is typically composed of polyolefin, poly(vinyl chloride), or poly(ethylene terephthalate). The annular frame  16  is typically composed of stainless steel or plastic. The surface area of the frame assembly  17  and the electrostatic chuck  15  are selected so that the electrostatic chuck  15  extends beyond the diameter of the annular frame  16  and contains internal cooling channels  18  where a coolant gas is passed. A high voltage may be applied to the electrostatic chuck  15  to provide a first clamping force to the frame assembly  17 . The electrostatic clamping mechanism enables a good thermal contact to exist between the frame assembly  17  and the electrostatic chuck  15 . Furthermore, the electrostatic chuck  15  does not contain any surface features within the diameter of the annular frame  16  in order to maximise the thermal contact between the frame assembly  17  and the electrostatic chuck  15 . A good thermal contact between the frame assembly  17  and the electrostatic chuck  15  helps to keep the frame assembly  17  cool during plasma treatment and prevent thermal degradation of the dicing tape  14 . The annular frame  16  is shielded from direct exposure to the plasma  19  by use of a frame cover  110 . 
         [0048]    In one embodiment of the invention the frame cover  110  makes a good thermal contact with the annular frame  16 . The frame cover  110  may be provided additional protection from the plasma  19  by the use of a heat shield  111 . If a heat shield  111  is employed, the frame cover  110  is composed of a material with a high thermal conductivity, such as aluminium. However, if a heat shield  111  is not used, the frame cover  110  will be in direct contact with the plasma  19  and a material with a low thermal conductivity, such as alumina or other ceramic material, is preferred. The heat shield  111  is typically composed of a material with poor thermal conductivity, such as alumina or other ceramic material. The contact  112  between the heat shield  111  and the frame cover  110  is minimal or non-existent to minimise the thermal pathway between the heat shield  111  and the frame cover  110 . Consequently, the plasma  19  directly heats the heat shield  111  but that heat does not permeate towards the frame assembly  17 . The weight of the annular frame  16 , the frame cover  110 , and the heat shield  111  provides a second clamping mechanism of the dicing tape  14  to the electrostatic chuck  15 . The second clamping mechanism may also be achieved using an active clamping force. The active clamping force may be achieved through use of a platen mounted—weighted clamp or other clamping device. The active clamping force may be applied directly to the heat shield  111 , the frame cover  110 , the annular frame  16  or a combination thereof in order to achieve the second clamping mechanism of the dicing tape  14  to the electrostatic chuck  15 . The frame assembly  17  is removed from the vacuum processing chamber  113  after the plasma treatment is complete and a cooling stage is initiated as shown in  FIG. 2 . The frame cover  110  is cooled prior to the commencement of a successive plasma treatment. The cooling effect is achieved by lowering the frame cover  110  so that there is a large contact area between the frame cover  110  and the electrostatic chuck  15 . The frame cover  110  is composed of a material with a high thermal conductivity, such as aluminium, and forms a good thermal contact with the electrostatic chuck  15 . The good thermal contact between the frame cover  110  and the electrostatic chuck  15  allows the heat from the frame cover  110  to be dissipated easily and allow efficient cooling of the frame cover  110 . 
         [0049]    In a second embodiment of the invention, the frame assembly  17  is mechanically clamped from above to the electrostatic chuck  35  by the use of clamping pins  30  as shown in  FIG. 3 . The clamping pins  30  are protrusions from the underside of the frame cover  31  and form a poor thermal contact between the frame cover  31  and the annular frame  16 . The frame cover  31  may be provided with additional protection from the plasma  19  in the form of a heat shield  111 . If a heat shield  111  is employed, the frame cover  31  is composed of a material with a high thermal conductivity, such as aluminium. However, if a heat shield  111  is not used, the frame cover  31  will be in direct contact with the plasma  19  and a material with a low thermal conductivity, such as alumina or other ceramic material, is preferred. 
         [0050]    The frame assembly  17  is removed from the vacuum processing chamber  113  after the plasma treatment is complete and a cooling stage is initiated as shown in  FIG. 4 . The frame cover  31  is cooled prior to the commencement of a successive plasma treatment. The cooling effect is achieved by lowering the frame cover  31  so that it is in direct contact with the thermally regulated electrostatic chuck  35 . In this embodiment of the invention the clamping pins  30  insert into complementary recesses  32  located in the electrostatic chuck  35 . This ensures a high contact area and good thermal contact between the frame cover  31  and the electrostatic chuck  35  to allow efficient cooling of the frame cover  31  during the cooling stage. 
         [0051]    In a third embodiment of the invention, the annular frame  16  is positioned directly on top of a shield ring  50 . The shield ring  50  is an annular component surrounding the electrostatic chuck  55 . Thermal protection from the plasma  19  is provided by the use of a frame cover  51  and a heat shield  52  as shown in  FIG. 5 . The shield ring  50  is typically composed of a material with a high thermal conductivity, such as aluminium, and is kept in contact with the walls of the vacuum processing chamber  53 . The walls of the vacuum processing chamber  53  are maintained at a suitable temperature, typically about 55° C. The heat shield  52  provides thermal protection for the frame cover  51 . The heat shield  52  rests solely on the shield ring  50  and does not contact the frame cover  51  during the plasma treatment. The heat shield  52  is in contact with the shield ring  50  via a series of equidistantly dispersed protrusions such as pips  54 . The heat shield  52  may comprise an extension in the form of a tape cover  56  to give protection to the dicing tape  14 . The tape cover is typically composed of a material with a poor thermal conductivity, such as alumina or other ceramic material. The heat shield  52  and tape cover  56  are in direct line of sight to the plasma  19  and may reach temperatures in excess of 150° C. The heat shield  52  and tape cover  56  absorb the thermal radiation from the plasma  19  but heat is not readily transferred to the shield ring  50  due to the poor thermal contact of the protrusions  54  to the shield ring  50 . This helps to maintain the heat shield  52  and tape cover  56  at a high temperature. The heat shield  52  and tape cover  56  form a thermal shield which results in the frame cover  51  being cooler than in prior art designs. The relatively cool frame cover  51  then also forms a thermal shield around the frame  16 . This protects the frame  16  from radiative and convective heat transfer from the heat shield  52  and tape cover  56 . Advantageously, the high temperature of the heat shield  52  and tape cover  56  prevents the accumulation of particular deposits, which increases the useable lifetime in between cleaning procedures. The frame cover  51  makes contact with the annular frame  16  via a series of equidistantly dispersed protrusions such as pips  57  which act to mechanically clamp the annular frame  16  to the shield ring  50 . The mechanical clamping force may be as a result of the weight of the frame cover  51  or an additional clamping force may be employed. The contact area between the protrusions  57  and the annular frame  16  is minimal which results in a poor thermal contact. The combination of the frame cover  51  being relatively cool, and the poor thermal contact between the protrusions  57  and the frame  16  means that plastic frames can be used without substantial risk of damage. The use of metal frames is possible also. 
         [0052]    In order to remove the frame assembly  17  from the vacuum processing chamber  113  the annular frame  16  and the frame cover  51  are raised using lifting pins  60 .  FIG. 6  shows a cross-sectional view of the vacuum processing chamber with lifting pins  60  in the raised position. A poor thermal contact between the heat shield  52  and the frame cover  51  is maintained from the use of equidistantly dispersed protrusions such as pips  61 . The minimal contact area of the protrusions  61  minimises heat dissipation from the heat shield  52  to the frame cover  51 . Each set of protrusions  54 , and  57  has a corresponding recess for them to insert into, which allows the frame cover  51 , the heat shield  52  and the shield ring  50  to self-align when the lifting pins are utilised.