RADIATIVE WAFER CUTTING USING SELECTIVE FOCUSING DEPTHS

Abstract

Semiconductor wafer cutting is optimised by directing a plurality of laser beams at the wafer, with the laser beams being focused so that at least some of their respective focal points are located at different depths throughout the wafer.

Claims

1. A laser cutting apparatus for cutting a semiconductor wafer along a cut line of the wafer, comprising: a planar wafer support surface having a plane operative to support a semiconductor wafer thereon in use, a laser supply operative to produce a plurality of output laser beams, a beam focuser located in an optical path of each output laser beam operative to focus each said laser beam at a respective focal point, the wafer support surface being movable relative to the beam focuser in a direction parallel to the plane of the wafer support surface, and an actuator operative to relatively move the wafer support surface and beam focuser in a direction parallel to the plane of the wafer support surface so that in use the focal point of each output laser beam follows the cut line of the wafer during said relative movement, wherein the focal point of at least one output laser beam is located at a different distance from the plane of the wafer support surface than the focal point of at least one other output laser beam.

2. The laser cutting apparatus of claim 1, wherein the laser supply comprises a laser source operative to emit a source laser beam along an optical path and a beam divider located along the optical path of the source laser beam to split the source laser beam into the plurality of output laser beams.

3. A laser cutting apparatus according to claim 2, wherein the beam divider comprises a diffractive optical element.

4. The laser cutting apparatus of claim 3, wherein the diffractive optical element is operative to produce at least two output laser beams having different divergences.

5. The laser supply apparatus of claim 3, wherein the diffractive optical element is operative to produce at least two output laser beams having different propagation directions.

6. The laser cutting apparatus of claim 1, wherein the laser supply comprises a plurality of laser sources, each operative to produce a respective output laser beam.

7. The laser cutting apparatus of claim 6, comprising a plurality of beam focusers, each located along the optical path of a respective laser output beam.

8. The laser cutting apparatus of claim 1, wherein the focal point of each output laser beam is located within the semiconductor wafer in use, such that different output laser beams have respective focal points at different depths within the semiconductor wafer.

9. The laser cutting apparatus of claim 1, wherein the plurality of output laser beams form an array, with the focal point of each output laser beam in the array being spaced in the direction parallel to the plane of the wafer support surface.

10. The laser cutting apparatus of claim 9, wherein the arrangement of output laser beam focal points within the array form a linear profile, such that the spacing of adjacent focal points in the direction parallel to the plane of the wafer support surface is directly proportional to the spacing of those adjacent focal points in the direction orthogonal to the plane of the wafer support surface.

11. The laser cutting apparatus of claim 10, wherein the adjacent output laser beam focal points within the array are spaced by a Rayleigh length of the output laser beams.

12. The laser cutting apparatus of claim 9, wherein the arrangement of output laser beam focal points within the array form a non-linear profile, such that the spacing of adjacent focal points in the direction parallel to the plane of the wafer support surface is not directly proportional to the spacing of those adjacent focal points in the direction orthogonal to the plane of the wafer support surface.

13. A method of cutting a planar semiconductor wafer along a cut line of the wafer, comprising the steps of: a) supporting the semiconductor wafer within a laser cutting apparatus, b) directing a plurality of laser beams at the semiconductor wafer in a propagation direction substantially orthogonal to the plane of the semiconductor wafer, c) focusing the plurality of laser beams so that respective focal points of said plurality of laser beams are located within the semiconductor wafer, such that the focal point of at least one laser beam is located at a different depth of the semiconductor wafer than the focal point of at least one other output laser beam, and d) relatively moving the semiconductor wafer and the plurality of laser beams in a direction parallel to the plane of the semiconductor wafer such that the focal point of each laser beam follows the cut line of the wafer, so that the semiconductor wafer is cut along the cut line.

14. The method of claim 13, wherein step c) comprises focusing the plurality of laser beams such that the spacing of adjacent focal points in the direction parallel to the plane of the wafer support surface is directly proportional to the spacing of those adjacent focal points in the direction orthogonal to the plane of the wafer support surface, such that the arrangement of laser beam focal points forms a linear profile.

15. The method of claim 14, wherein the adjacent output laser beam focal points are spaced by a Rayleigh length of the laser beams.

16. The method of claim 13, wherein step c) comprises focusing the plurality of laser beams such that the spacing of adjacent focal points in the direction parallel to the plane of the wafer support surface is not directly proportional to the spacing of those adjacent focal points in the direction orthogonal to the plane of the wafer support surface, such that the arrangement of laser beam focal points forms a non-linear profile.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0037] The invention will now be described with reference to the accompanying drawings (not to scale), in which:

[0038] FIG. 1 schematically shows a sectional view of part of a known laser cutting apparatus;

[0039] FIG. 2 schematically shows the cutting apparatus of FIG. 1 during a cutting process;

[0040] FIG. 3 schematically shows a sectional view of alternative known laser cutting apparatus used for cutting foil-back wafer;

[0041] FIG. 4 schematically shows the cutting apparatus of FIG. 3 during a cutting process;

[0042] FIG. 5 schematically shows a top view of a wafer, illustrating a known cutting methodology;

[0043] FIG. 6 schematically shows an enlarged view of the wafer of FIG. 5;

[0044] FIG. 7 schematically shows a sectional view of a cut line of a wafer, cut with a known cutting apparatus;

[0045] FIG. 8 schematically shows the cut line of FIG. 7, along a section orthogonal to the cut line;

[0046] FIG. 9 schematically shows a sectional view of a cut line of a wafer, cut with a cutting apparatus in accordance with an embodiment of the present invention;

[0047] FIG. 10 schematically shows the cut line of FIG. 9, along a section orthogonal to the cut line;

[0048] FIGS. 11A-D schematically show sectional views along a cut line of a wafer, illustrating exemplary profiles of focal points in accordance with the present invention;

[0049] FIG. 12 schematically shows a laser cutting apparatus in accordance with an embodiment of the present invention;

[0050] FIGS. 13A, B schematically show DOEs suitable for use with the present invention;

[0051] FIG. 14 schematically shows part of a laser cutting apparatus in accordance with another embodiment of the present invention; and

[0052] FIG. 15 schematically shows part of a laser cutting apparatus in accordance with a further embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

[0053] FIGS. 9 and 10 show views similar to those of FIGS. 7 and 8 respectively, but with an improved laser beam focal point profile in accordance with an embodiment of the present invention. Here, an array of four output laser beams 22A-D illuminates a planar semiconductor wafer 21 to cause ablation of the same along a cut line. Although not shown for clarity, the wafer 21 is supported on a planar wafer support surface (24, see FIG. 12). The semiconductor wafer 21 may be provided with a carrying foil (not shown) similar to that shown in FIG. 1, or alternatively the wafer support surface 24 may include a jig (not shown) on which the wafer 21 is supported, similar to the arrangement shown in FIG. 3. The respective focal points of the laser beams 22A-D are spaced not only in the Y direction along the cut line, but also in the Z direction so that they are located at different distances relative to the wafer support surface. Moreover, the focal points all lie within the body of the uncut wafer. In this way, the focal points are positioned to be closer to the remaining material to be ablated by that respective laser beam.

[0054] FIGS. 11A-D schematically show sectional views along a cut line of a planar semiconductor wafer 21, illustrating exemplary profiles of focal points in accordance with the present invention. In these figures, the semiconductor wafer 21 is shown being cut by a linear array of eight output laser beams 22A-H in a cutting direction D. Laser beam 22A is therefore the leading laser beam of the array. Each output laser beam 22A-H is focused to a respective focal point 23A-H. The focal point of each output laser beam in the array is spaced from another output laser beam in a direction parallel to the plane of the wafer support surface.

[0055] The laser beams 22A-H undergo focusing such that the focal point of at least one output laser beam is located at a different distance from the plane of the wafer support surface 24 than the focal point of at least one other output laser beam. In preferred embodiments, such as those shown in FIGS. 11A-D, each of the laser beams 22A-H has a focal point 23A-H located within the semiconductor wafer 21 in use, such that different output laser beams have respective focal points at different depths within the semiconductor wafer.

[0056] The pitch between adjacent beams of the array in the Y direction may for example be in the range from about 5 m to about 400 m. The height differences in the Z-direction between adjacent beams may for example be in the range from about 5 m to about 100 m, which may be dependent on the thickness of the wafer 21.

[0057] In FIG. 11A, the focal points 23A-H are arranged in a linear profile, shown as a dotted line, such that the spacing of adjacent focal points in the direction parallel to the plane of the wafer support surface 24 is directly proportional to the spacing of those adjacent focal points in the direction orthogonal to the plane of the wafer support surface 24. Leading laser beam 22A is focused at a point near to the upper major surface of the semiconductor wafer 21, with each successive laser beam in the array being focused at successively lower points, until trailing laser beam 22H is focused at a point near to the lower major surface of the semiconductor wafer 21. With such a profile, it is clear that each laser beam will act to ablate semiconductor material at successively lower depths along the cut line. The focal point 23H of trailing beam 22H is selected to be sufficiently close to the lower surface of the semiconductor wafer 21 to ensure complete ablation of the semiconductor material throughout the depth of the cut line, thus providing singulation. Advantageously, the adjacent output laser beam focal points within the array are spaced by a Rayleigh length of the output laser beams.

[0058] In FIGS. 11B-D, the focal points 23A-H are arranged in non-linear profiles, such that the spacing of adjacent focal points in the direction parallel to the plane of the wafer support surface 24 is not directly proportional to the spacing of those adjacent focal points in the direction orthogonal to the plane of the wafer support surface.

[0059] In FIG. 11B, the spacings in the Z-direction between focal points of adjacent laser beams increases in a non-directly proportional arrangement from the leading beam to the trailing beam.

[0060] In FIG. 11C, a stepped profile is used, in which adjacent pairs of laser beams have their focal points at the same distance from the wafer support table, each pair having their focal points focused at successively lower points, until trailing laser beams 22H and its immediate precursor 22G are focused at a point near to the lower major surface of the semiconductor wafer 21.

[0061] In FIG. 11D, a more irregular profile is shown, the leading four laser beams 22A-22D having their focal points arranged in a linear profile, with the next three laser beams 22E-22G having their focal points at the same distance from the wafer support surface 24, and finally the trailing laser beam 22H being focused at a point near to the lower major surface of the semiconductor wafer 21.

[0062] FIG. 12 schematically shows a laser cutting apparatus in accordance with the present invention. A planar semiconductor wafer 21 is supported on a wafer support surface 24 of a wafer table 1, which forms part of a moveable stage assembly, controlled by a motion controller 12, with wafer 21 being held thereon by peripheral clamping or the like. The stage assembly comprises, for example, two separate linear motors (not shown) for independently driving the wafer table 1 along the X and Y axes. The wafer table 1 is caused to float smoothly over a reference surface (such as a polished stone surface) in the X-Y plane, for example with the aid of an air bearing or magnetic bearing (not shown). The exact position of the wafer table 1 is monitored and controlled with the aid of positioning instruments such as interferometers or linear encoders, for example (not shown). Focus control and/or level sensing systems (not shown) are employed to ensure that the surface of the wafer 21 is maintained at a desired level with respect to the laser projection system.

[0063] A pulsed laser source 14 is provided to emit a pulsed laser beam that propagates along an optical path. The laser source 14 is connected to a controller 13, such as a processor or computer, that can be used among other things to control laser parameters such as the pulse duration, pulse repetition rate and power or fluence of the beam.

[0064] A diffractive optical element (DOE) 26 is located along the optical path to split the laser beam into multiple output laser beams having different beam divergence angles, as will be described in more detail below.

[0065] A beam splitter/combiner 16, such as a partially-silvered or dichroic mirror, directs the output laser beams toward the wafer 21, while also permitting reflected light to be passed to a vision system (see below).

[0066] A beam focuser 27, such as a lens or concave mirror or the like, collects the output laser beams and focuses them for projection onto the wafer 21. At the point of impingement of the beams upon the wafer 21, light spots are formed according to the individual beam properties. Aberration and/or distortion correction may also be performed at this stage, as is known in the art. The beam focuser 27 is operative to focus the output laser beams at the required distance from the wafer support surface 24.

[0067] A vision system 18, optically connected to a digital camera 19, receives reflected light from the beam splitter/combiner 16. This is used to perform alignment and tracking operations of the beams relative to the surface of the wafer 21 as is known in the art. The use of a beam splitter/combiner 16 permits the camera 19 to be used in an on-axis arrangement, whereby it can observe the wafer 21 along an axis substantially co-linear with the beam. A portion of light emanating from the surface 4, due to reflection, will pass through the beam splitter/combiner 16 and be directed to the camera 19.

[0068] The controller 13 is used for controlling and processing images captured by the camera 19, and adapts the operation of the laser source 14 depending on the received image information.

[0069] FIGS. 13A-B schematically show two alternative DOE designs 26A, 26B suitable for use with the laser cutting apparatus described above. FIG. 13A shows a DOE 26A with which spots with different focal point depths can be realized by splitting and altering an incoming collimated source laser beam 28 into multiple output beams 22A-C of different divergences. The split output beams 22A-C have the same beam angle, and so propagate along the same longitudinal axis. If the output beams are to be spatially separated in the Y direction to form an array, then they may be passed through an additional DOE (not shown) as required. The output laser beams 22A-C pass through beam focuser 27, in this case a lens, which focuses the beams. Due to the differing divergences, the resulting focal points are spaced in the Z direction.

[0070] FIG. 13B shows an alternative DOE 26B which produces output beams with both different divergences and different beam angles, such that they propagate in different directions. When the output beams are focused by beam focuser 27, in this case a lens, an array of output laser beams 22A-C are produced, with their respective focal points being spaced in the Y direction. Due to the differing divergences, the resulting focal points are also spaced in the Z direction.

[0071] It is possible to design and fabricate DOEs to produce beam divergences and angular control with accuracies in the order of micro-radians. For focal lengths of 1 mm to 200 mm typically used in laser material processes, the design and fabrication errors would contribute to a geometric errors in the X, Y and Z axes of less than 4 m.

[0072] The design of the DOE requires the simulation of an inverted light propagation from the plane in which the pattern is to be created back to the plane of the DOE. The 3D spot distribution is firstly arranged based on the application need. A far-field electromagnetic wave profile is then calculated for an inverted light propagation to achieve the required phase change of an incoming coherent Gaussian laser beam. In designing the focused laser spots at different focus levels, the far-field information can be defined at certain nominal focus positions, whereby individual spots have differing divergences to reflect their corresponding focus positions.

[0073] The above-described embodiments make use of a DOE to act on optical rays propagating through free-space. However, various alternatives are possible within the scope of the present invention.

[0074] An alternative embodiment of the present invention is schematically shown in FIG. 14. For clarity, only components of the illumination system are shown, and it should be understood that the wafer support table and inspection system are as previously described.

[0075] Here, rather than splitting a single source laser beam to produce a plurality of output laser beams, a total of four laser sources 29A-D are provided, their outputs being controlled by controller 13. These are arranged to emit similar and generally parallel laser beams 30A-D. These are directed by beam splitter/combiner 16 towards respective beam focusers comprising individual lenses 31A-D. The lenses 31A-D have differing focal lengths, such that each output laser beam has a focal point at a different distance from the wafer support surface (not shown) in use.

[0076] A further alternative embodiment of the present invention is schematically shown in FIG. 15. For clarity, only components of the illumination system are shown, and it should be understood that the wafer support table, inspection system and laser controller are as previously described.

[0077] Here, similar to the previous embodiment, a total of four laser sources 29A-D are provided, their outputs being controlled by a controller (not shown). These are arranged to emit similar laser beams 30A-D. The laser beams 30A-D are guided by means of respective optical fibers 32A-D towards respective beam focusers comprising individual lenses 31A-D. The lenses 31A-D here have identical focal lengths, but the lenses are spaced in the Z direction, such that each output laser beam has a focal point at a different distance from the wafer support surface (not shown) in use. This embodiment therefore enables the Y-axis position of the laser spots to be precisely arranged without the use of diffractive elements, so that the ease of adjustment may be improved.

[0078] The above-described embodiments are exemplary only, and other possibilities and alternatives within the scope of the invention will be apparent to those skilled in the art.

[0079] For example, the beam focuser used in the embodiments shown in FIGS. 14 and 15 are freely interchangeable.

[0080] While the above-described embodiments show laser spot arrays with a similar Y-direction spacing, this spacing could be selected and/or varied as appropriate.

[0081] Optical fibers could be used to guide laser beams throughout any part of their optical paths, rather than free-space transmission.

[0082] The beam focuser (and optionally the laser supply) may be moved while the wafer support surface remains stationary, in order to effect cutting.

[0083] The apparatus and method of the present invention may be used both for singulation and scribing processes.

REFERENCE NUMERALS USED

[0084] 1wafer table [0085] 2semiconductor wafer [0086] 3foil [0087] 4first major surface [0088] 5second major surface [0089] 6, 6A, 6Bcut lines [0090] 7laser beam [0091] 8jig [0092] 9semiconductor devices [0093] 10A, 10Bdicing streets [0094] 11A-Dlaser spots [0095] 12motion controller [0096] 13controller [0097] 14laser source [0098] 15beam divider [0099] 16beam splitter/combiner [0100] 17beam focuser [0101] 18vision system [0102] 19camera [0103] 20A-Dlaser beams [0104] 21semiconductor wafer [0105] 22A-Houtput laser beams [0106] 23A-Hfocal points [0107] 24wafer support surface [0108] 26diffractive optical element [0109] 27beam focuser [0110] 28source laser beam [0111] 29A-Dlaser sources [0112] 30A-Dsource laser beams [0113] 31A-Dlenses [0114] 32A-Doptical fibers [0115] 33A-Dlenses [0116] Dcutting direction [0117] Twafer thickness