(19)
(11)EP 2 956 959 B1

(12)EUROPEAN PATENT SPECIFICATION

(45)Mention of the grant of the patent:
22.04.2020 Bulletin 2020/17

(21)Application number: 14709000.5

(22)Date of filing:  12.02.2014
(51)International Patent Classification (IPC): 
H01L 21/78(2006.01)
H01L 21/683(2006.01)
H01L 21/67(2006.01)
H01L 21/3065(2006.01)
H01L 21/687(2006.01)
H01J 37/32(2006.01)
(86)International application number:
PCT/US2014/016074
(87)International publication number:
WO 2014/127027 (21.08.2014 Gazette  2014/34)

(54)

METHOD FOR PLASMA DICING A SEMI-CONDUCTOR WAFER

VERFAHREN FÜR PLASMA-SCHNEIDEN EINES HALBLEITERWAFERS

PROCÉDÉ POUR LE DÉCOUPAGE EN DÉS AU PLASMA D'UNE PLAQUETTE SEMI-CONDUCTRICE


(84)Designated Contracting States:
AL AT BE BG CH CY CZ DE DK EE ES FI FR GB GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR

(30)Priority: 14.02.2013 US 201313767459

(43)Date of publication of application:
23.12.2015 Bulletin 2015/52

(73)Proprietor: Plasma-Therm, LLC
St. Petersburg, FL 33716 (US)

(72)Inventors:
  • GAULDIN, Rich
    St. Petersburg, FL 33703 (US)
  • JOHNSON, Chris
    St. Petersburg, FL 33709 (US)
  • JOHNSON, David
    Cleveland, GA 30528 (US)
  • MARTINEZ, Linnell
    Lakeland, FL 33813 (US)
  • PAYS-VOLARD, David
    St. Petersburg, FL 33703 (US)
  • WESTERMAN, Russell
    Land O'Lakes, FL 34639 (US)
  • GRIVNA, Gordon, M.
    Mesa, AZ 85210 (US)

(74)Representative: Schmid, Wolfgang 
Lorenz & Kollegen Patentanwälte Partnerschaftsgesellschaft mbB Alte Ulmer Strasse 2
89522 Heidenheim
89522 Heidenheim (DE)


(56)References cited: : 
WO-A2-2012/125560
US-A- 5 880 924
US-A1- 2010 048 001
US-A- 4 230 515
US-A1- 2002 019 066
US-A1- 2010 216 313
  
      
    Note: Within nine months from the publication of the mention of the grant of the European patent, any person may give notice to the European Patent Office of opposition to the European patent granted. Notice of opposition shall be filed in a written reasoned statement. It shall not be deemed to have been filed until the opposition fee has been paid. (Art. 99(1) European Patent Convention).


    Description

    Field of the Invention



    [0001] The invention relates to a method for plasma dicing a substrate.

    Background



    [0002] Semi-conductor devices are fabricated on substrates which are in the form of thin wafers. Silicon is commonly used as the substrate material, but other materials, such as III-V compounds (for example GaAs and InP) are also used. In some instances (for example, the manufacture of LED's) the substrate is a sapphire or silicon carbide wafer on which a thin layer of a semi-conducting material is deposited. The size of such substrates ranges from 2 inches and 3 inches up to 200 mm, 300 mm, and 450 mm diameter and many standards exist (e.g., SEMI) to describe such substrate sizes.

    [0003] Plasma etching equipment is used extensively in the processing of these substrates to produce semi-conductor devices. Such equipment typically includes a vacuum chamber fitted with a high density plasma source such as Inductively Coupled Plasma (ICP) which is used to ensure high etch rates, necessary for cost-effective manufacturing. In order to remove the heat generated during the processing, the substrate is typically clamped to a cooled support. A cooling gas (typically Helium) is maintained between the substrate and the support to provide a thermal conductance path for heat removal. A mechanical clamping mechanism, in which a downward force is applied to the top side of the substrate, may be used, though this may cause contamination due to the contact between the clamp and the substrate. More frequently an Electrostatic chuck (ESC) is used to provide the clamping force.

    [0004] Numerous gas chemistries appropriate to the material to be etched have been developed. These frequently employ a halogen (Fluorine, Chlorine, Bromine, or Iodine) or halogen-containing gas together with additional gases added to improve the quality of the etch (for example, etch anisotropy, mask selectivity and etch uniformity). Fluorine containing gases, such as SF6, F2 or NF3 are used to etch silicon at a high rate. In particular, a process (Bosch or time division multiplexed "TDM") which alternates a high rate silicon etch step with a passivation step to control the etch sidewall, is commonly used to etch deep features into silicon. Chlorine and Bromine containing gases are commonly used to etch III-V materials.

    [0005] Plasma etching is not limited to semiconducting substrates and devices. The technique may be applied to any substrate type where a suitable gas chemistry to etch the substrate is available. Other substrate types may include carbon containing substrates (including polymeric substrates), ceramic substrates (e.g., AlTiC and sapphire), metal substrates, and glass substrates.

    [0006] To ensure consistent results, low breakage and ease of operation, robotic wafer handling is typically used in the manufacturing process. Handlers are designed to support the wafers with minimal contact, to minimize possible contamination and reduce the generation of particulates. Edge contact alone, or underside contact close to the wafer edge at only a few locations (typically within 3-6 mm of the wafer edge) is generally employed. Handling schemes, which include wafer cassettes, robotic arms and within process chamber fixtures including the wafer support and ESC, are designed to handle the standard wafer sizes as noted previously.

    [0007] After fabrication on the substrate, the individual devices (die or chips) are separated from each other prior to packaging or being employed in other electronic circuitry. For many years, mechanical means have been used to separate the die from each other. Such mechanical means have included breaking the wafer along scribe lines aligned with the substrate crystal axis or by using a high speed diamond saw to saw into or through the substrate in a region (streets) between the die. More recently, lasers have been used to facilitate the scribing process.

    [0008] Such mechanical wafer dicing techniques have limitations which affect the cost effectiveness of this approach. Chipping and breakage along the die edges can reduce the number of good die produced, and becomes more problematic as wafer thicknesses decrease. The area consumed by the saw bade (kerf) may be greater than 100 microns which is valuable area not useable for die production. For wafers containing small die (e.g., individual semiconductor devices with a die size of 500 microns x 500 microns) this can represent a loss of greater than 20%. Further, for wafers with many small die and hence numerous streets, the dicing time is increased, and productivity decreased, since each street is cut individually. Mechanical means are also limited to separation along straight lines and the production of square or oblong shaped chips. This may not represent the underlying device topology (e.g., a high power diode is round) and so the rectilinear die format results in significant loss of useable substrate area. Laser dicing also has limitations by leaving residual material on the die surface or inducing stress into the die.

    [0009] It is important to note that both sawing and laser dicing techniques are essentially serial operations. Consequently, as device sizes decrease, the time to dice the wafer increases in proportion to the total dicing street length on the wafer.

    [0010] Recently plasma etching techniques have been proposed as a means of separating die and overcoming some of these limitations. After device fabrication, the substrate is masked with a suitable mask material, leaving open areas between the die. The masked substrate is then processed using a reactive-gas plasma which etches the substrate material exposed between the die. The plasma etching of the substrate may proceed partially or completely through the substrate. In the case of a partial plasma etch, the die are separated by a subsequent cleaving step, leaving the individual die separated. The technique offers a number of benefits over mechanical dicing:
    1. 1) Breakage and chipping is reduced;
    2. 2) The kerf dimensions can be reduced to well below 20 microns;
    3. 3) Processing time does not increase significantly as the number of die increases;
    4. 4) Processing time is reduced for thinner wafers; and
    5. 5) Die topology is not limited to a rectilinear format.


    [0011] After device fabrication, but prior to die separation, the substrate may be thinned by mechanical grinding or similar process down to a thickness of a few hundred microns, or even less than a hundred microns.

    [0012] Prior to the dicing process, the substrate is typically mounted on a dicing fixture. This fixture is typically comprised of a rigid frame that supports an adhesive membrane. The substrate to be diced is adhered to the membrane. This fixture holds the separated die for subsequent downstream operations. Most tools used for wafer dicing (saws or laser based tools) are designed to handle substrates in this configuration and a number of standard fixtures have been established; however, such fixtures are very different from the substrates which they support. Though such fixtures are optimized for use in current wafer dicing equipment, they cannot be processed in equipment which has been designed to process standard substrates. Thus, current automated plasma etching equipment is not suitable for processing substrates fixtured for dicing and it is difficult to realize the benefits that plasma etch techniques should have for die separation.

    [0013] Some groups have contemplated using plasma to singulate die from wafer substrates. U.S. Patent 6,642,127 describes a plasma dicing technique in which the substrate wafer is first attached to a carrier wafer via an adhesive material, before plasma processing in equipment designed for processing silicon wafers. This technique proposes adapting the form factor of the substrate to be diced to be compatible with standard wafer processing equipment. While this technique allows standard plasma equipment to dice the wafer, the proposed technique will not be compatible with standard equipment downstream of the dicing operation. Additional steps would be required to either adapt the downstream equipment or revert the substrate form factor for standard downstream equipment.

    [0014] U.S. Patent Application 2010/0048001 contemplates the use of a wafer adhered to a thin membrane and supported within a frame. However, in the 2010/0048001 application, the masking process is achieved by adhering a mask material to the backside of the wafer and using a laser to define the etch streets prior to plasma processing. In contrast to standard dicing techniques which singulate the substrate from the front side, this technique introduces additional complex and expensive steps which may negate some of the advantages of plasma dicing. It also requires the additional demand of aligning the backside mask with the front side device pattern.

    [0015] US 2010/216313 A1, US 2010/048001 A1, WO 2012/125560 A2 disclose methods of plasma dicing a substrate.

    [0016] Therefore, what is needed is a plasma etching apparatus which can be used for dicing a semiconductor substrate into individual die and which is compatible with the established wafer dicing technique of handling a substrate mounted on tape and supported in a frame, and which is also compatible with standard front side masking techniques.

    [0017] Nothing in the prior art provides the benefits attendant with the present invention.

    [0018] Therefore, it is an object of the present invention to provide an improvement which overcomes the inadequacies of the prior art devices and which is a significant contribution to the advancement to the dicing of semiconductor substrates using a plasma etching apparatus.

    [0019] The present invention is as defined in the appended claims.

    Summary of the invention



    [0020] After device fabrication and wafer thinning, the front side (circuit side) of the substrate is masked using conventional masking techniques which protects the circuit components and leaves unprotected areas between the die. The substrate is mounted on a thin tape which is supported within a rigid frame. The substrate/tape/frame assembly is transferred into a vacuum processing chamber and exposed to reactive gas plasma where the unprotected areas between the die are etched away. During this process, the frame and tape are protected from amage by the reactive gas plasma. The processing leaves the die completely separated. After etching, the substrate/tape/frame assembly is additionally exposed to plasma which removes potentially damaging residues from the substrate surface. After transfer of the substrate/tape/frame assembly out of the process chamber, the die are removed from the tape using well known techniques and are then further processed (e.g., packaged) as necessary.

    [0021] The present invention provides a method for plasma dicing a substrate. The substrate can have a semiconducting layer such as Silicon and/or the substrate can have a III-V layer such as GaAs. The substrate can have a protective layer such as a photoresist layer that is patterned on a circuit side of the substrate. A process chamber having a wall with a plasma source adjacent to the wall of the process chamber is provided. The plasma source can be a high density plasma source. A vacuum pump in fluid communication with the process chamber and a gas inlet in fluid communication with the process chamber can be provided. A work piece support within the process chamber is provided. A work piece is formed by placing the substrate on a carrier support. The work piece has a support film, a frame and the substrate, the support film having some tension. The work piece can be formed by adhering the substrate to a support film and then mounting the substrate with the support film to a frame. The support film can have a polymer layer and/or a conductive layer. The support film can be standard dicing tape. The frame can have a conductive layer and/or a metal layer. The work piece is then loaded onto the work piece support for plasma processing. An RF power source can be coupled to the work piece support to create a plasma around the work piece. The support film is placed under additional tension by applying a force to the frame. The tensional force can be a mechanical force, a magnetic force and/or an electrical force. The support film can be elastically deformed by the tensional force. The support film cannot be plastically deformed by the tensional force. A heat transfer fluid can be introduced between the support film and the work piece. The heat transfer fluid can be a gas such as helium. The fluid pressure can be greater than one Torr and can be less than thirty Torr. An electrostatic or mechanical chuck can be incorporated into the work piece support whereby the chuck can clamp the support film to the chuck, The clamping of the work piece is performed after the additional tension is applied to the support film. The tensional force that is applied to the support film can be changed after the support film is clamped. The tensional force that is applied to the support film can be removed after the support film is clamped. The pressure within the process chamber can be reduced through the vacuum pump and a process gas can be introduced into the process chamber through the gas inlet. A plasma is generated through the plasma source whereby the work piece is etched through the generated plasma. A vacuum compatible transfer module can be provided that communicates with the process chamber. The work piece can be loaded onto a transfer arm in the vacuum compatible transfer module whereby the process chamber is maintained under vacuum during a transfer of the work piece from the vacuum compatible transfer module to the process chamber.

    BRIEF DESCRIPTION OF THE DRAWINGS



    [0022] 

    Figure 1 is a top down view of a semiconductor substrate illustrating individual devices separated by streets;

    Figure 2 is a cross-sectional view of a semiconductor substrate illustrating individual devices separated by streets;

    Figure 3 is a cross-sectional view of a semiconductor substrate mounted to tape and a frame;

    Figure 4 is a cross-sectional view of a semiconductor substrate mounted to tape and a frame being etched by a plasma process;

    Figure 5 is a cross-sectional view of separated semiconductor devices mounted to tape and a frame;

    Figure 6 is a cross-sectional view of a vacuum processing chamber;

    Figure 7 is a cross-sectional of a wafer/frame in process position;

    Figure 8 is an enlarged cross-sectional view of a frame and a cover ring in a vacuum processing chamber;

    Figure 9 is a cross-sectional view of a section of the inside the chamber with the cover ring mounted to a chamber wall;

    Figure 10 is a cross-sectional view of a section of the inside the chamber with the cover ring mounted to an internal heat sink;

    Figure 11 is a top down view of a semiconductor substrate mounted to tape and a frame supported by a transfer arm;

    Figure 12 is a cross-sectional view of a semiconductor substrate mounted to tape and a frame supported by a transfer arm;

    Figure 13 is a cross-sectional view of a wafer/frame in a transfer position;

    Figure 14 is a top view of a screen;

    Figure 15 is a cross-sectional view of an electrostatic chuck;

    Figure 16 is a schematic view of a chamber in a transfer position;

    Figure 17 is a cross sectional view of the work piece and work piece support;

    Figure 18 is a cross sectional view of the work piece and work piece support;

    Figure 19 is a cross sectional view of the work piece and work piece support;

    Figure 20 is a cross sectional view of the work piece and work piece support; and

    Figure 21 is a top down view of multiple semiconductor substrates mounted to tape and a frame.



    [0023] Similar reference characters refer to similar parts throughout the several views of the drawings.

    Detailed description of the invention



    [0024] A typical semiconductor substrate after device fabrication is illustrated in Figure 1. The substrate (1) has on its surface a number of areas containing device structures (2) separated by street areas (3) in which there are no structures which allows for separation of the device structures into individual die. Although silicon is commonly used as a substrate material, other materials chosen for their particular characteristics are frequently employed. Such substrate materials include gallium arsenide and other III-V materials or non-semi-conductor substrates on which has been deposited a semi-conducting layer.

    [0025] In the present invention, as is shown in a cross sectional view in Figure 2, the device structures (2) are then covered with a protective material (4) while the street areas (3) remain unprotected. This protective material (4) can be a photoresist, applied and patterned by well-known techniques. Some devices, as a final process step are coated with a protective dielectric layer such as silicon dioxide or PSG which is applied across the whole substrate. This can be selectively removed from the street areas (3) by patterning with photoresist and etching the dielectric material, as is well known in the industry. This leaves the device structures (2) protected by the dielectric material and the substrate (1) substantially unprotected in the street areas (3). Note that in some cases test features to check the wafer quality may be located in the street areas (3). Depending on the specific wafer fabrication process flow, these test features may or may not be protected during the wafer dicing process. Although the device pattern illustrated shows oblong die, this is not necessary, and the individual device structures (2) may be any other shape, such as hexagons, as best suits the optimum utilization of the substrate (1). It is important to note that while the previous example considers dielectric materials as the protective film, that the invention may be practiced with a wide range of protective films including semi-conductive and conductive protective films. Furthermore, the protective layer can consist of multiple materials. It is also important to note that some portion of the protective film may be an integral part of the final device structure, (e.g., a passivation dielectric, metal bonding pad, etc.)

    [0026] The substrate (1) may be thinned, typically by a grinding process, which reduces the substrate thickness to a few hundred microns to as thin as approximately 30 microns or less. As is shown in Figure 3, the thinned substrate (1) is then adhered to a tape (5) which in turn is mounted in a rigid frame (6) to form a work piece (1A). The tape (5) is typically made from a carbon-containing polymer material, and may additionally have a thin conductive layer applied to its surface. The tape (5) provides support for the thinned substrate (1) which is otherwise too fragile to handle without breakage. It should be noted that the sequence of patterning, thinning and then mounting is not critical and the steps may be adjusted to best fit the particular devices and substrate and the processing equipment used. It is important to note that while the previous example considers a work piece (1A) that is comprised of mounting a substrate (1) on an adhesive tape (5) which in turn is attached to a frame (6), that the invention is not limited by the configuration of the wafer and carrier. The wafer carrier can be comprised a variety of materials. The carrier supports the substrate during the plasma dicing process. Furthermore, the wafer need not be attached to the carrier using an adhesive - any method that holds the wafer to the carrier and allows a means thermal communication of the substrate to the cathode is sufficient, (e.g. an electrostatically clamped carrier, a carrier with a mechanical clamping mechanism, etc.)

    [0027] While the example above describes mounting a single substrate (1) on adhesive tape (5) that is supported by a frame (6) to form a work piece (1A), the invention can also be beneficially applied to a work piece (1A) that is comprised of more than one substrate (1) mounted on adhesive tape (5) which is supported by a frame (6) as is shown in Figure 21. The substrates (1) can be different sizes, shapes, thicknesses and/or materials. It is preferable that if the substrates are different materials that they etch in similar etch chemistries (e.g. Ge and Si both etch in fluorine-based chemistries). The substrates (1) may have different areas of exposed materials and/or different patterns. Some of the substrates (1) may be pieces of larger substrates. It is preferred that the substrates (1) be located inside the inner diameter of the support frame (6). In one embodiment, the outer diameter of the support frame (6) is smaller than the outer diameter of the work piece support. After mounting the substrate (1) with the tape (5) in the dicing frame (6), the work piece (1A) is transferred into a vacuum processing chamber. Ideally, the transfer module is also under vacuum which allows the process chamber to remain at vacuum during transfer, reducing processing time and preventing exposure of the process chamber to atmosphere and possible contamination. As shown in Figure 6, the vacuum processing chamber (10) is equipped with a gas inlet (11), a high density plasma source (12) to generate a high density plasma, such as an Inductively Coupled Plasma (ICP), a work piece support (13) to support the work piece (1A), an RF power source (14) to couple RF power to the work piece (1A) through the work piece support (13) and a vacuum pump (15) for pumping gas from the processing chamber (10). During processing, the unprotected areas of substrate (1) are etched away using a reactive plasma etch process (7) as shown in Figure 4. This leaves the devices (2) separated into individual die (8) as shown in Figure 5. In another embodiment of the invention, the unprotected areas of the substrate (1) are partially etched away using a reactive plasma etch process (7). In this case, a downstream operation, such as a mechanical breaking operation, can be used to complete the die separation. These downstream methods are well known in the art.

    [0028] While the previous example describes the invention using a vacuum chamber in conjunction with a high density plasma, it is also possible to etch the unprotected areas of the substrate using a wide range of plasma processes. For example, one skilled in the art can imagine variations of the invention using a low density plasma source in a vacuum chamber or even the use of plasmas at or near atmospheric pressures.

    [0029] When the substrate/tape/frame assembly (1A) is in the position for plasma processing, it is important that the frame (6) is protected from exposure to the plasma (7). Exposure to the plasma (7) will cause heating of the frame (6) which in turn will cause local heating of the mounting tape (5). At temperatures above approximately 100°C, the physical properties of the tape (5) and its adhesive capability may deteriorate and it will no longer adhere to the frame (6). Additionally, exposure of the frame (6) to the reactive plasma gas may cause degradation of the frame (6). Since the frame (6) is typically re-used after wafer dicing, this may limit the useful lifetime of a frame (6). Exposure of the frame (6) to the plasma (7) may also adversely affect the etch process: for example the frame material may react with the process gas, effectively reducing its concentration in the plasma which will reduce the etch rate of the substrate material, thus increasing process time. To protect the frame (6), a protective cover ring (20), as shown in Figures 6, 7 and 8, is positioned above the frame (6). The cover ring (20) does not touch the frame (6) since contact with the frame (6) (which would occur during transfer into the process chamber (10)) can generate undesirable particles.

    [0030] In Figure 8, dimension (A) represents the distance between the cover ring (20) and the frame (6). This dimension can range from less than approximately 0.5 mm to greater than approximately 5 mm with an optimal value of 1.5 mm. If the distance (A) is too large, plasma (7) will contact the frame (6) and the benefits of the cover ring (20) will be lost.

    [0031] It is important that the cover ring (20) is temperature controlled, otherwise its temperature will increase due to exposure to the plasma (7) and in turn heat the tape (5) and the frame (6) via radiational heating, causing degradation as noted above. For the case where the cover ring (20) is cooled, cooling of the cover ring (20) is accomplished by having it in direct contact with a cooled body, such as the process chamber wall (10W) shown in Figure 9 or a heat sink (30) located within the process chamber (10) shown in Figure 10. To ensure that heat is adequately removed from the cover ring (20) to the heat sink (30), the cover ring (20) should be made of a material that has good thermal conductivity. Such materials include many metals, for example Aluminum, but other thermally conductive materials, such as Aluminum Nitride and other ceramics can be used. The choice of the cover ring material is chosen to be compatible with the plasma process gases used. While Aluminum is satisfactory for Fluorine based processes, an alternate material, such as Aluminum Nitride, or the addition of a protective coating, such as Aluminum Oxide may be necessary when Chlorine based processes are used. Operation temperature of the cover ring (20) during plasma processing is typically less than 80°C which minimizes heat radiation to the tape (5) and the frame (6) and ensures that the tape (5) maintains its mechanical integrity. Alternatively, the cover ring (20) may be temperature controlled by bringing the cover ring (20) into contact with a temperature controlled fluid. This fluid can be a liquid or gas. In the case where the cover ring (20) temperature is controlled by a fluid, the cover ring (20) may contain a number of fluid channels to facilitate heat transfer. These fluid channels can be internal to the cover ring (20), externally attached, or some combination of the two.

    [0032] In one instance, the cover ring (20) can extend from the substrate diameter to the inner chamber diameter continuously. To avoid a loss in pumping conductance, which can adversely affect pressure control within the process chamber (10), a plurality of holes (21) can be added to the cover ring (20) which allows sufficient conductance of the process gas while still providing a path for heat removal from the cover ring (20). In Figures 9 and 10, a plurality of holes (21) arranged in a specific geometry is shown, but the density, size, pattern and symmetry of the holes (21) can vary depending on the process chamber (10) dimensions and the pumping conductance required.

    [0033] The substrate/tape/frame assembly (1A) is transferred both into and out of the process chamber (10) by a transfer arm (40) that supports the frame (6) and substrate (1) so that they are maintained coplanar as shown in Figures 11 and 12. The transfer arm (40) may support both the tape (5) and the frame (6) or the frame (6) alone, but it is important that the assembly (1A) not be supported beneath the substrate (1) area alone because of the fragile nature of thinned substrates (1). The transfer arm (40) has an alignment fixture (41) attached to it that aligns the frame (6) in a repeatable position before being transferred into the process chamber (10). The frame (6) can also be aligned by other techniques well-known in semiconductor processing (e.g., optical alignment). The alignment can also be performed on the substrate (1) by such well-known techniques. It is important that the substrate/tape/frame assembly (1A) be aligned before placement within the process chamber (10) to avoid mis-processing as explained below.

    [0034] In Figure 8, the dimension (D) represents the distance between the outer diameter of the substrate (1) and the inner diameter of the frame (6). This may be 20 mm to 30 mm (e.g., Disco Corporation dicing frame is 250 mm for 200 mm substrates, so that the dimension (D) is nominally 25 mm). During mounting of the wafer (1) on the tape (5) within the frame (6), the deviation of wafer (1) placement may be as much as 2 mm so that dimension (E), which is the distance between the substrate (1) outer diameter and the inner diameter of the cover ring (20) can also vary from assembly to assembly by up to 2 mm. If at some point (E) is less than zero the cover ring (20) will overlay the edge of the substrate (1). This point will be shadowed and prevented from etching, which can prevent die separation and cause problems in subsequent processing steps. Alignment of the substrate/tape/frame assembly (1A) prior to transfer is required to prevent such problems. Further, to additionally ensure that dimension (E) is not less than zero, the cover ring inner diameter should be greater than the diameter of the substrate (1) with a preferred diameter 5 mm greater than the substrate (e.g., 205 mm cover ring inner diameter for 200 mm substrate). Dimension (F) in Figure 8 represents the distance from the inner diameter of the cover ring (20) to the inner diameter of the frame (6). Alignment of the frame (6) prior to transfer into the process chamber (10) ensures that (F) remains constant for the entire circumference around the substrate (1) and that any portion of tape (5) that is not contacted by the Electrostatic chuck (ESC) (16) is shadowed from the plasma (7).

    [0035] When the substrate/tape/frame assembly (1A) is transferred into the process chamber (10), it is placed onto the lifting mechanism (17) and removed from the transfer arm (40). The reverse process occurs during transfer of the substrate/tape/frame assembly (1A) out of the process chamber (10). The lifting mechanism (17) touches the frame (6) area and provides no point contact to the substrate (1). Point contact to the substrate (1) can cause damage to the substrate (1), particularly after die separation and unloading of the substrate/tape/frame assembly (1A), since the flexibility of the tape (5) would cause the die to contact each other and damage to occur. Figure 13 shows the lifting mechanism (17) contacting the frame (6) from the underside: however the frame (6) can also be removed from the transfer arm (40) by contact with the top surface or outer diameter using a clamping device. To process the substrate (1), the frame (6), the work piece support (13), and the cover ring (20) move relative to each other. This can be accomplished by moving either the cover ring (20), the work piece support (13), or the lifting mechanism (17) or any combination of the three.

    [0036] While the tape (5) in the work piece (1A) is typically under some tension - there are often imperfections (ripples, etc.) in the tape that can make it difficult to clamp the work piece (1A) to the substrate support (13A) sufficiently for effective helium backside cooling. In order to facilitate clamping of the work piece (1A) to the work piece support (13) it is beneficial to construct the work piece support assembly (13A) such that the flexible tape (5) is placed under additional tension while the clamping force is applied to the work piece (1A). According to the invention, the additional tension is applied to the tape (5) by applying a force to the frame before the clamping force is applied. Once the tape (5) has been clamped, the additional tensioning force may be changed or removed.

    [0037] One way in which this additional tensioning may be accomplished to configure the work piece support assembly (13A) such that the surface defined by the frame / tape interface (50 as shown in Figure 17) is located at or below the surface defined by the substrate / tape interface (55 as shown in Figure 17). It is preferred that the some portion of the surface 50 is at least approximately 0.1mm below some portion of the surface 55. Some portion of the surface 50 can be at least approximately 1mm below the surface 55.

    [0038] In another embodiment, all of the surface 50 is below the surface 55. In this embodiment, it is preferred that the surface 50 is at least approximately 0.1mm below the surface 55. The surface 50 can be at least approximately 1mm below the surface 55.

    [0039] In the case where the tape (5) is adhered to both the bottom surface of the substrate (1) and the bottom surface of the frame (6) this may be accomplished by ensuring that the top surface of the electrostatic chuck (16) is located at or preferably above the plane defined by the lower surface of the bottom of the frame (6) as shown in Figure 17. In this configuration, it is preferred that the top surface of the ESC is at least 0.1mm above the bottom surface of the bottom of the frame (16). The work piece (1A) may remain in this configuration during plasma processing or the additional tension may be changed at some point in the process. This configuration is particularly beneficial when the clamping force is applied by an electrostatic chuck. The additional tensioning may be applied through a number of hardware configurations. Note that while Figure 17 shows the tape (5) being attached to the bottom of the support frame (6) that the method may still be beneficially applied to configurations where the tape (5) is applied to the top surface of the frame (6).

    [0040] The force required to apply the additional tension to the tape (5) is applied to the frame (6). The force may be applied to the top of the frame, the bottom of the frame or both. Some portion of the force required to apply the additional tension to the tape may be derived from the weight of the frame (6).

    [0041] In one configuration, the tape frame (6) is supported by the lift mechanism (17) during clamping. The top surface of the process kit (18) will be at or below the plane of the top surface of the electrostatic chuck (16). The process kit may be in contact with the tape (5) and/or the frame (6). In the cases where the process kit is not in contact with the work piece, it is preferred that the gap between the work piece (1A) and the process kit (18) is less than approximately 5 mm in order to prevent plasma formation in the space between the work piece (1A) and the process kit (18).

    [0042] In an alternate configuration, the tape frame is not supported by the lift mechanism (17) in order to tension the tape. In this configuration the frame (6) may be supported by the process kit (18), and/or a frame support member (17A) as shown in Figure 18.

    [0043] In yet another alternate configuration, the process kit may be incorporated into and/or replaced by extending the electrostatic chuck as shown in Figure 19. The tape frame (6) may be supported by the electrostatic chuck where the ESC surface supporting the substrate (1) is higher than the surface supporting the tape frame (6) placing the tape (5) under additional tension. In a preferred embodiment, a portion of the surface supporting the substrate (1) is at least 0.1mm higher than the surface supporting the tape frame (6).

    [0044] In yet another configuration, the inner diameter of the tape frame (6) is larger than the outer diameter of the work piece assembly (13A). In this configuration the frame may be held by the lift mechanism (17) and/or an external tape frame support (17A). Alternatively, the frame may be unsupported such that the weight of the frame contributes to the tensioning force.

    [0045] While the examples above describe tensioning the tape in conjunction with an electrostatic clamp, the invention may also be beneficially applied to other clamping configurations, including mechanical clamping. In another embodiment the invention may also be beneficially applied to a work piece support assembly that does not utilize a clamping mechanism.

    [0046] Figure 20 shows yet another configuration. In this configuration the flexible tape (5) is stretched across the top surface of the work piece support (13A) in order to form a seal between the tape (5) and the work piece support (13A). A heat transfer fluid, typically helium gas is introduced between the tape (5) and the work piece support (13A). The seal between the tape (5) and the work piece support (13A) needs to be sufficient to support a heat transfer fluid pressure of greater than approximately 1 Torr but less than approximately 30 Torr between the tape (5) and the work piece support (13A). It is preferable that the gas pressure behind the tape does not cause a separation between the tape (5) and the work piece support (13A) greater than approximately 100 microns as this would adversely affect the heat transfer between the substrate and the work piece support. It is desired that the areas of tape under the substrate (1) and tape areas of the tape that are exposed to the plasma be inside the seal created between the wafer support assembly (13A) and the tape (5). The force applied to the tape frame will put at least a portion of the tape (5) under tension - possibly deforming the tape (5). It is important to limit the applied force to the tape (5) such that the tape deformation does not preclude downstream packaging operations. Ideally, tensioning the tape (5) will result in only elastic deformation - though some amount of plastic deformation may be permissible provided it does not negatively impact downstream operations. The force required to create the seal between the tape (5) and the work piece support (13A) may be applied to the tape frame (6). The force can be magnetic, mechanical, electrostatic, or some combination of the three. The force may be applied to the top of the frame, the bottom of the frame or both. Alternatively, the force can be applied directly to the tape, preferably in the areas not overlapping the substrate (1) or the frame (6). In yet another embodiment, an electrostatic force may be applied to the tape underneath the area covered by the substrate (1) in order to minimize the gap between the tape (5) and the work piece support (13A).During plasma processing, heat is transferred to all of the surfaces the plasma (7) touches including the substrate (1), tape (5), and frame (6). The cover ring (20) will minimize the heat transfer to areas of the tape (5) and the frame (6), but the substrate (1) must remain exposed to the plasma (7) for processing. As shown in Figure 6, a conductive screen (25) (e.g., made from aluminum or aluminum coated with an appropriate plasma resistant coating) can be placed between the substrate (1) and the plasma (7). This will reduce ion bombardment on the substrate (1) and thus reduce heating of the substrate (1). Figure 14 shows the screen (25) is provided with a plurality of holes (26) which still allows neutral species from the plasma (7) to reach the substrate (1) such that the etch rate is only slightly reduced. Holes (27) allow for mounting of the screen (25) to the processing chamber (10).

    [0047] Additional cooling of the substrate (1) is provided by the use of an Electrostatic chuck (ESC) (16). Such ESCs (16) are commonly used in semiconductor processing to apply downward force to the substrate (1) while a pressurized gas such as Helium is maintained between the substrate (1) and the electrode. This ensures that heat transfer can occur between the substrate (1) and the electrode, which is cooled. Typically, ESCs (16) are the same diameter or smaller than the substrate (1) to prevent unwanted exposure of the ESC (16) surface to potentially corrosive plasma gases that can decrease the lifetime of the ESC (16). With a substrate/tape/frame assembly (1A), the area outside the diameter of the substrate (1) is tape (5). Using a typical ESC (16), because the cover ring (20) is larger than the diameter of the substrate (1), there would be an area of tape (5) exposed to the plasma process that is not being clamped and cooled by the ESC (16) or being shielded from the plasma (7) by the cover ring (20). Such an area of tape (5) would reach a high temperature and possibly fail. Thus, Figure 8 shows the use of an ESC (16) that is made purposely larger than the substrate diameter so that any tape (5) which is exposed to the plasma in region (E) is also clamped and cooled. This diameter can be extended outwards to the outer diameter of the frame (6), but is preferred to be 2 mm less than the inner diameter of the frame (6).

    [0048] In the case where the work piece (1A) contains more than one substrate (1), it is preferred that the ESC (16) extends beyond the edge of at least one substrate (1) - preferably extending beyond the edges of all substrates (1). In order to confine the cooling gas (typically helium) behind the substrates the tape (5) must form a sealing surface between the work piece support (1A) and the tape (5). This sealing surface is often called a seal band. The seal band is typically slightly higher than some portion of the area of the ESC that it circumscribes. In one embodiment the sealing surface is continuous and forms a shape that circumscribes all the substrates (1). In an another embodiment, the sealing surface may be discontinuous and circumscribes at least one region. It is preferred that a portion of the sealing band overlays a portion of ESC clamping electrode(s). In a preferred embodiment, all of the sealing band overlays a clamping electrode. The substrates (1) may overlay the sealing band(s) or alternatively, the sealing band(s) may lie outside the substrate(s) (1)

    [0049] Figure 8 shows a filler ring (18) that extends from the outer diameter of the ESC (16) to the lifting mechanism (17). This filler ring (18) is used to prevent the back surface of any exposed tape (5) from being contacted by the plasma (7). Although a separate filler ring (18) is shown, an extension of the ESC (16) would also prevent plasma (7) exposure to the backside of the tape (5). The filler ring (18) is typically made of a dielectric material, such as a ceramic (e.g., Aluminum Oxide) or a plastic material, (e.g., polytetrafluoroethylene (PTFE, Teflon)) selected for both its low thermal conductivity and its low electrical conductivity. Typical ESCs (16) used in semiconductor processing have a pattern of shallow features fabricated on their surface to facilitate Helium distribution or to minimize contact with the backside of a substrate (1) to reduce particle formation. Such an ESC (16) can be used for plasma dicing when a substrate (1) is separated into multiple die, providing the feature dimensions on the ESC surface are smaller than the die size. When the die size approaches and becomes smaller than the ESC feature size, the tape will now conform to the features and flex, possibly causing the die to touch each other which can cause damage. The use of a substantially coplanar ESC surface eliminates this problem. Note that though the preceding example describes an ESC that cools the substrate, for some materials (e.g. approximately 180°C for indium containing substrates) that require a higher temperature to facilitate the plasma etch process, a higher temperature controlled ESC (16) temperature may be desirable.

    [0050] A typical ESC (16) (coulombic design of Figure 15) consists of one or more electrodes (33) to which a high voltage (19) is applied, separated from the work piece support (13) by a thick insulating layer (32) and separated from the material to be clamped by a thin layer of dielectric material (34). The clamping force generated by electrostatic forces increases as the thickness of this dielectric layer (34) decreases and increases as the voltage applied increases. In the present instance, when the substrate (1) is mounted on an insulating tape (5), the thickness of the tape (5) adds to the total dielectric thickness interposed between the electrode (33) and the substrate (1). This total thickness should not be determined primarily by the tape thickness, since this is likely to vary, resulting in a variable clamping performance. Rather the ESC dielectric (34) should be relatively thick (of the order of a few 100 microns) to maintain a clamping performance independent of tape thickness. A high clamping force can be achieved by operating at a high clamping voltage (up to approximately 10 kV).

    [0051] During plasma processing, RF power (14) is coupled to the substrate (1) to control ion bombardment on the substrate (1) and control the etch characteristics. The frequency of this RF may vary from 100's of MHz down to a few hundred kHz. When etching a substrate material down to an insulating layer (in this instance the mounting tape), problems with the etch associated with charging of the insulating layer are well known. Such problems include localized severe undercutting at the substrate/insulator interface which is undesirable during die separation, since this affects the performance of the singulated die. As is well known in the art, such charging problems can be reduced by operating at low RF frequencies and additionally pulsing or modulating the RF power at low frequency. Since RF coupling at such low frequency is not efficient through a thick dielectric material (32), the RF coupling to the substrate (1) is preferably via the one or more ESC electrodes, for example via a coupling capacitor (35) rather than via the RF powered work piece support (13). To maintain uniform RF coupling to the substrate (1), the ESC electrode or electrodes should also be uniformly disposed behind the substrate (1). This is difficult to achieve if multiple electrodes are used, since the necessary gaps between the electrodes result in a local variation in the RF coupling which adversely affects the quality of the etch, particularly the undercutting at the substrate/tape interface. A preferred embodiment of the ESC design therefore incorporates a so called monopolar design, in which a single electrode is used to provide the clamping force. Additionally, there should be as few as possible penetrations through this electrode (for example as for pin lifts) since these penetrations will also disturb the RF coupling and degrade the etch performance.

    [0052] The substrate can be processed using techniques well known in the semiconductor industry. Silicon substrates are generally processed using a Fluorine based chemistry such as SF6. SF6/O2 chemistry is commonly used to etch Silicon because of its high rate and anisotropic profile. A disadvantage of this chemistry is its relatively low selectivity to masking material for example to photoresist which is 15-20:1. Alternatively a Timed Division Multiplex (TDM) process can be used which alternates between deposition and etching to produce highly anisotropic deep profiles. For example, an alternating process to etch Silicon uses a C4F8 step to deposit polymer on all exposed surfaces of the Silicon substrate (i.e., mask surface, etch sidewalls and etch floor) and then an SF6 step is used to selectively remove the polymer from the etch floor and then isotropically etch a small amount of silicon. The steps repeat until terminated. Such a TDM process can produce anisotropic features deep into Silicon with selectivities to the masking layer of greater than 200:1. This then makes a TDM process the desired approach for plasma separation of Silicon substrates. Note that the invention is not limited to the use of fluorine containing chemistries or a time division multiplex (TDM) process. For example, silicon substrates may also be etched with Cl, HBr or I containing chemistries as is known in the art.

    [0053] For III-V substrates such as GaAs, a Chlorine based chemistry is extensively used in the semiconductor industry. In the fabrication of RF-wireless devices, thinned GaAs substrates are mounted with the device side down onto a carrier, where they are then thinned and patterned with photoresist. The GaAs is etched away to expose electrical contacts to the front side circuitry. This well-known process can also be used to separate the devices by the front side processing described in the above mentioned invention. Other semiconductor substrates and appropriate plasma processes can also be used for the separation of die in the above mentioned invention.

    [0054] To further reduce the problems associated with charging at the substrate/tape interface, the process can be changed at the point at which the interface is exposed to a second process which has less tendency to undercut and is typically a lower etch rate process. The point in time at which the change takes place depends upon the substrate thickness, which is likely to vary. To compensate for this variability, the time at which the substrate/tape interface is reached is detected using an endpoint technique. Optical techniques which monitor the plasma emission are commonly used to detect endpoint and U.S. Patents 6,982,175 and 7,101,805 describe such an endpoint technique which is appropriate to a TDM process.

    [0055] After singulation of the semiconductor substrate there can be unwanted residues that exist on the devices. Aluminum is commonly used as an electrical contact for semiconductor devices and when exposed to Fluorine based plasmas a layer of AlF3 is formed on its surface. AlF3 is nonvolatile under normal plasma processing conditions and is not pumped away from the substrate and out of the system and remains on the surface after processing. AlF3 on Aluminum is a common cause of failure for devices because the bonding strength of wires to the electrical contacts is greatly reduced. Thus the removal of the AlF3 from the surface of the electrical contacts after plasma processing is important. Wet methods can be used; however, this becomes difficult because of the fragile nature of the separated die, and the possible damage to the tape causing die release. Therefore, the process can be changed to a third process while the substrate is still within the vacuum chamber, to a process designed to remove any AlF3 formed. U.S. Patent 7,150,796 describes a method for in-situ removal of AlF3 using an Hydrogen based plasma. Likewise, an in-situ treatment can be used to remove other halogen-containing residues when other halogen-containing gases are used to etch the substrate.


    Claims

    1. A method for plasma dicing a substrate, the method comprising:

    providing a process chamber having a wall;

    providing a plasma source adjacent to the wall of the process chamber;

    providing a work piece support within the process chamber;

    placing a work piece onto the work piece support, said work piece having a support film, a frame and the substrate, the support film of the work piece having some tension;

    clamping the work piece to the work piece support;

    placing the support film under additional tension by applying a force to the frame, said additional tension to the support film being applied before the clamping step;

    generating a plasma using the plasma source; and

    etching the work piece using the generated plasma.


     
    2. The method according to Claim 1 further comprising introducing a heat transfer fluid between the support film and the work piece support.
     
    3. The method according to Claim 2 wherein the heat transfer fluid is a gas.
     
    4. The method according to Claim 2 wherein the heat transfer fluid further comprising a fluid pressure greater than one Torr.
     
    5. The method according to Claim 1 wherein the support film is dicing tape.
     
    6. The method according to Claim 1 further comprising changing the additional tension applied to the support film after the clamping step.
     
    7. The method according to Claim 1 further comprising removing the additional tension applied to the support film after the clamping step.
     
    8. The method according to Claim 1 wherein the additional tension applied to the support film is mechanically actuated.
     
    9. The method according to Claim 1 wherein the additional tension applied to the support film is magnetically actuated.
     
    10. The method according to Claim 1 wherein the additional tension applied to the support film is electrostatically actuated.
     
    11. The method according to Claim 1 wherein the support film is elastically deformed by the application of the additional tension applied to the support film.
     
    12. The method according to Claim 1 wherein the support film is not plastically deformed by the application of the additional tension applied to the support film.
     
    13. The method according to Claim 1 wherein the clamping step uses an electrostatic chuck.
     
    14. The method according to Claim 1 wherein the clamping step uses a mechanical chuck.
     


    Ansprüche

    1. Verfahren zum Plasmaschneiden eines Substrats, wobei das Verfahren Folgendes aufweist:

    zur Verfügung Stellen einer Prozesskammer, die eine Wand aufweist;

    zur Verfügung Stellen einer Plasmaquelle benachbart zu der Wand der Prozesskammer;

    zur Verfügung Stellen einer Werkstückauflage innerhalb der Prozesskammer;

    Anordnen eines Werkstücks auf der Werkstückauflage, wobei das Werkstück eine Trägerfolie, einen Rahmen und das Substrat aufweist, wobei die Trägerfolie des Werkstücks eine gewisse Spannung hat;

    Klemmen des Werkstücks auf die Werkstückauflage;

    Bringen der Trägerfolie unter zusätzliche Spannung durch Aufbringen einer Kraft auf den Rahmen, wobei die zusätzliche Spannung auf die Trägerfolie vor dem Schritt des Klemmens aufgebracht wird;

    Erzeugen eines Plasmas unter Verwendung der Plasmaquelle; und

    Ätzen des Werkstücks unter Verwendung des erzeugten Plasmas.


     
    2. Verfahren nach Anspruch 1, welches des Weiteren das Einführen eines Wärmeübertragungsfluids zwischen die Trägerfolie und die Werkstückauflage beinhaltet.
     
    3. Verfahren nach Anspruch 2, wobei das Wärmeübertragungsfluid ein Gas ist.
     
    4. Verfahren nach Anspruch 2, wobei das Wärmeübertragungsfluid des Weiteren einen Fluiddruck größer als 1 Torr aufweist.
     
    5. Verfahren nach Anspruch 1, wobei die Trägerfolie eine Sägefolie ist.
     
    6. Verfahren nach Anspruch 1, welches des Weiteren das Verändern der zusätzlichen Spannung, die nach dem Schritt des Klemmens auf die Trägerfolie aufgebracht wird, beinhaltet.
     
    7. Verfahren nach Anspruch 1, welches des Weiteren das Wegnehmen der zusätzlichen Spannung, die nach dem Schritt des Klemmens auf die Trägerfolie aufgebracht wird, beinhaltet.
     
    8. Verfahren nach Anspruch 1, wobei die zusätzliche Spannung, die auf die Trägerfolie aufgebracht wird, mechanisch bewirkt wird.
     
    9. Verfahren nach Anspruch 1, wobei die zusätzliche Spannung, die auf die Trägerfolie aufgebracht wird, magnetisch bewirkt wird.
     
    10. Verfahren nach Anspruch 1, wobei die zusätzliche Spannung, die auf die Trägerfolie aufgebracht wird, elektrostatisch bewirkt wird
     
    11. Verfahren nach Anspruch 1, wobei die Trägerfolie durch das Aufbringen der zusätzlichen Spannung, die auf die Trägerfolie aufgebracht wird, elastisch deformiert wird.
     
    12. Verfahren nach Anspruch 1, wobei die Trägerfolie durch das Aufbringen der zusätzlichen Spannung, die auf die Trägerfolie aufgebracht wird, nicht plastisch deformiert wird.
     
    13. Verfahren nach Anspruch 1, wobei in dem Schritt des Klemmens ein elektrostatisches Spannmittel verwendet wird.
     
    14. Verfahren nach Anspruch 1, wobei in dem Schritt des Klemmens ein mechanisches Spannmittel verwendet wird.
     


    Revendications

    1. Procédé pour le découpage en dés au plasma d'un substrat, le procédé comprenant :

    la fourniture d'une chambre de traitement ayant une paroi ;

    la fourniture d'une source de plasma adjacente à la paroi de la chambre de traitement ;

    la fourniture d'un support de pièce à travailler dans la chambre de traitement ;

    le placement d'une pièce à travailler sur le support de pièce à travailler, ladite pièce à travailler ayant un film de support, un cadre et le substrat, le film de support de la pièce à travailler ayant une certaine tension ;

    le serrage de la pièce à travailler sur le support de pièce à travailler ;

    le placement du film de support sous une tension supplémentaire en appliquant une force sur le cadre, ladite tension supplémentaire sur le film de support étant appliquée avant l'étape de serrage ;

    la génération d'un plasma en utilisant la source de plasma ; et

    la gravure de la pièce à travailler en utilisant le plasma généré.


     
    2. Procédé selon la revendication 1, comprenant en outre l'introduction d'un fluide de transfert de chaleur entre le film de support et le support de pièce à travailler.
     
    3. Procédé selon la revendication 2, dans lequel le fluide de transfert de chaleur est un gaz.
     
    4. Procédé selon la revendication 2, dans lequel le fluide de transfert de chaleur comprend en outre une pression de fluide supérieure à un Torr.
     
    5. Procédé selon la revendication 1, dans lequel le film de support est une bande de découpage en dés.
     
    6. Procédé selon la revendication 1, comprenant en outre la modification de la tension supplémentaire appliquée au film de support après l'étape de serrage.
     
    7. Procédé selon la revendication 1, comprenant en outre la suppression de la tension supplémentaire appliquée au film de support après l'étape de serrage.
     
    8. Procédé selon la revendication 1, dans lequel la tension supplémentaire appliquée au film de support est actionnée mécaniquement.
     
    9. Procédé selon la revendication 1, dans lequel la tension supplémentaire appliquée au film de support est actionnée magnétiquement.
     
    10. Procédé selon la revendication 1, dans lequel la tension supplémentaire appliquée au film de support est actionnée électrostatiquement.
     
    11. Procédé selon la revendication 1, dans lequel le film de support est déformé élastiquement par l'application de la tension supplémentaire appliquée au film de support.
     
    12. Procédé selon la revendication 1, dans lequel le film de support n'est pas déformé plastiquement par l'application de la tension supplémentaire appliquée au film de support.
     
    13. Procédé selon la revendication 1, dans lequel l'étape de serrage utilise un mandrin électrostatique.
     
    14. Procédé selon la revendication 1, dans lequel l'étape de serrage utilise un mandrin mécanique.
     




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    Cited references

    REFERENCES CITED IN THE DESCRIPTION



    This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.

    Patent documents cited in the description