Patent ID: 12191204

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A method of processing a wafer and a laser processing apparatus suitable for performing the method according to a preferred embodiment of the present invention will be described in detail hereinbelow with reference to the accompanying drawings.

FIG.1illustrates in perspective a laser processing apparatus2according to the present embodiment. As illustrated inFIG.1, a workpiece to be processed by the laser processing apparatus2according to the present embodiment includes a wafer10shaped as a circular plate and held on an annular frame F by an adhesive tape T (see alsoFIG.2A) interposed therebetween.

The laser processing apparatus2includes at least a chuck table25for holding the wafer10thereon, a laser beam applying unit6for applying a laser beam to the wafer10held on the chuck table25to process the wafer10with the laser beam, a feed mechanism30for processing-feeding the chuck table25and the laser beam applying unit6relatively to each other, and a control unit100(seeFIG.3) for controlling operable components of the laser processing apparatus2that include the laser beam applying unit6and the feed mechanism30.

The chuck table25is included in a holding unit20. The holding unit20includes a rectangular X-axis movable plate21movably mounted on a base3for movement in X-axis directions along a pair of guide rails3aon the base3, a rectangular Y-axis movable plate22movably mounted on the X-axis movable plate21for movement in Y-axis directions along a pair of guide rails21aon the X-axis movable plate21, a hollow cylindrical support post23fixed on an upper surface of the Y-axis movable plate22, and a rectangular cover plate26fixed to an upper end of the support post23. The chuck table25includes a circular plate extending upwardly through an oblong hole defined in the cover plate26and is rotatable about its vertical central axis by rotary actuator means, not depicted. The chuck table25has a holding surface25amade of an air-permeable porous material and lying in a plane defined by the X-axis directions and the Y-axis directions. The holding surface25ais held in fluid communication with suction means, not depicted, through a fluid channel, not depicted, defined in and extending through the support post23. The X-axis directions represent directions indicated by the arrow X inFIG.1, and the Y-axis directions represent directions indicated by the arrow Y inFIG.1and extending perpendicularly to the X-axis directions. The plane defined by the X-axis directions and the Y-axis directions lies essentially horizontally. Z-axis directions represent vertical directions indicated by the arrow Z inFIG.1and extending perpendicularly to the X-axis directions and the Y-axis directions.

The feed mechanism30includes an X-axis moving mechanism31for moving the chuck table25of the holding unit20and a laser beam emitted from the laser beam applying unit6relatively to each other in the X-axis directions, and a Y-axis moving mechanism32for moving the chuck table25and the laser beam emitted from the laser beam applying unit6relatively to each other in the Y-axis directions. The X-axis moving mechanism31has a ball screw34extending in the X-axis directions over the base3and an electric motor33coupled to an end of the ball screw34. The ball screw34is operatively threaded through a nut, not depicted, mounted on a lower surface of the X-axis movable plate21. When the electric motor33is energized, it rotates the ball screw34about its central axis, and the nut converts the rotary motion of the ball screw34into linear motion that is transmitted to the X-axis movable plate21, moving the X-axis movable plate21in one or the other of the X-axis directions along the pair of guide rails3aon the base3. The Y-axis moving mechanism32has a ball screw36extending in the Y-axis directions over the X-axis movable plate21and an electric motor35coupled to an end of the ball screw36. The ball screw36is operatively threaded through a nut, not depicted, mounted on a lower surface of the Y-axis movable plate22. When the electric motor35is energized, it rotates the ball screw36about its central axis, and the nut converts the rotary motion of the ball screw36into linear motion that is transmitted to the Y-axis movable plate22, moving the Y-axis movable plate22in one or the other of the Y-axis directions along the guide rails21aon the X-axis movable plate21.

The laser processing apparatus2further includes a frame37erected from the base3at a position behind the holding unit20. The frame37includes a vertical wall37aextending upwardly, i.e., in one of the Z-axis directions, from an upper surface of the base3and a horizontal arm37bextending horizontally, i.e., in one of the Y-axis directions, from an upper end of the vertical wall37a. The horizontal arm37bincorporates therein the laser beam applying unit6and an image capturing unit7to be used in an alignment step. The laser beam applying unit6includes a beam condenser61disposed on a lower surface of the distal end of the horizontal arm37b. The image capturing unit7is disposed at a position spaced from the beam condenser61in the X-axis directions. The laser beam applying unit6includes means for applying a pulsed laser beam having a wavelength transmittable through the wafer10, and is set in operation to laser processing conditions for forming modified layers in the wafer10along projected dicing lines14thereon. The laser beam applying unit6, the image capturing unit7, the feed mechanism30, and the like which have been described above are electrically connected to the control unit100described below, and controlled by instruction signals sent from the control unit100to perform a laser processing operation on the wafer10.

FIGS.2A and2Billustrate in more specific detail the wafer10to be processed by the laser processing apparatus2according to the present embodiment. As understood inFIG.2A, the wafer10to be processed by the laser processing apparatus2according to the present embodiment has a plurality of devices12formed in respective areas on a face side10athereof that are demarcated by a plurality of projected dicing lines14A extending in a first direction indicated by the arrow X and a plurality of projected dicing lines14B extending in a second direction indicated by the arrow Y perpendicularly to the projected dicing lines14A extending in the first direction. The wafer10is held on the annular frame F by the adhesive tape T interposed therebetween, with the face side10aexposed upwardly and a reverse side10bthereof facing downwardly and affixed centrally to the adhesive tape T. The wafer10includes a silicon substrate, for example. According to the present embodiment, the method of processing a wafer will be described below as being carried out in a manner to process the wafer10with the laser beam that is applied to the face side10aof the wafer10as illustrated inFIG.2A. However, the present invention is not limited to such processing details. The method of processing a wafer may alternatively be carried out in a manner to process the wafer10with the laser beam that is applied to the reverse side10bof the wafer10as illustrated inFIG.2Bwhere the wafer10is held on the annular frame F by the adhesive tape T interposed therebetween, with the reverse side10bexposed upwardly and the face side10afacing downwardly and affixed centrally to the adhesive tape T.

An optical system of the laser beam applying unit6incorporated in the laser processing apparatus2according to the present embodiment will be described below with reference toFIG.3. The laser beam applying unit6illustrated inFIG.3includes at least a laser oscillator62for oscillating pulsed laser and emitting a pulsed laser beam LB0, a beam condenser61including an fθ lens66for converging the pulsed laser beam LB0emitted from the laser oscillator62and applying the converted pulsed laser beam to the wafer10held on the chuck table25, and undulating means64disposed between the laser oscillator62and the beam condenser61for undulating the pulsed laser beam on the face side10aof the wafer10. According to the present embodiment, the laser beam applying unit6also includes an attenuator63disposed between the laser oscillator62and the undulating means64for regulating the output power of the pulsed laser beam LB0emitted from the laser oscillator62and a reflecting mirror65for reflecting the pulsed laser beam LB0from the undulating means64toward the beam condenser61.

The undulating means64will be described in greater detail below. The undulating means64can include a spatial light modulator (SLM), for example, for electrically modulating the pulsed laser beam LB0emitted from the laser oscillator62to the undulating means64to control the wave front configuration of the pulsed laser beam LB0at a high speed as desired. Specifically, the undulating means64can be used to shift the pulsed laser beam LB0sideways so that the pulsed laser beam reflected by the reflecting mirror65can be shifted in the directions indicated by the arrow R1inFIG.3to thereby shift the position where the pulsed laser beam LB0is applied to the wafer10on the chuck table25to different positions LB1, LB2, and LB3in the Y-axis directions of the wafer10, for example. The undulating means64is not limited to a spatial light modulator and can include an acousto-optic device (AOD), a diffractive optical element (DOE), a galvanoscanner, a resonant scanner, or the like. The functional and operational details of the undulating means64will be described in detail later.

The image capturing unit7includes illuminating means for illuminating the wafer10held on the chuck table25, an ordinary image capturing device (CCD) for capturing images with a visible beam, infrared ray applying means for applying infrared rays to a workpiece, an optical system for catching infrared rays applied from the infrared ray applying means, and an image capturing device (infrared CCD) for outputting an electric signal representing infrared rays caught by the optical system, all not depicted.

The laser processing apparatus2according to the present embodiment has substantially structural details described above. The method of processing a wafer carried out by the laser processing apparatus2according to the present embodiment will be described below.

In preparation for the method of processing a wafer according to the present embodiment, the wafer10is delivered to the laser processing apparatus2and placed and held under suction on the chuck table25of the holding unit20, as illustrated inFIG.1. As described above with reference toFIG.2A, the wafer10has the devices12formed in the respective areas on the face side10athereof that are demarcated by the projected dicing lines14A extending in the first direction and the projected dicing lines14B extending in the second direction perpendicular to the first direction. Each of the projected dicing lines14A extending in the first direction and the projected dicing lines14B extending in the second direction has a width of 50 μm. Then, the feed mechanism30is actuated to position the wafer10held on the chuck table25directly below the image capturing unit7.

When the wafer10has been positioned directly below the image capturing unit7, the rotary actuator means for rotating the chuck table25and the feed mechanism30are appropriately actuated, and the image capturing unit7is energized to capture images of the face side10aof the wafer10positioned directly below the image capturing unit7. Then, the XY coordinates of the positions of the projected dicing lines14A extending in the first direction and the projected dicing lines14B extending in the second direction on the face side10aof the wafer10are detected from the captured images. At the same time, as illustrated inFIG.3, the XY coordinates of the centers A1through A3, Bl through B3, C1through C3of points where the projected dicing lines14A and the projected dicing lines14B intersect with each other are also detected. The detected XY coordinates are stored in a coordinate storage section, not depicted, of the control unit100. InFIG.3, only a portion of the face side10aof the wafer10is illustrated for illustrative purposes. Actually, however, the XY coordinates of the positions of all the projected dicing lines14A and the projected dicing lines14B which are formed on the face side10aof the wafer10and the XY coordinates of the centers of all the points where the projected dicing lines14A and the projected dicing lines14B intersect with each other are detected and stored in the coordinate storage section (alignment step). In a case where the wafer10with the reverse side10bfacing upwardly as illustrated inFIG.2Bis to be processed as a workpiece, the infrared ray applying means and the infrared CCD of the image capturing unit7are energized to capture infrared images of the projected dicing lines14A and the projected dicing lines14B on the face side10afacing downwardly from above the reverse side10bof the wafer10, and the XY coordinates of the positions of the projected dicing lines14A and the projected dicing lines14B and the XY coordinates of the points where they intersect with each other are detected from the captured infrared images.

Then, the feed mechanism30and the rotary actuator means described above are actuated to move the chuck table25to align the projected dicing lines14A in the first direction with the X-axis directions. Thereafter, the method of processing a wafer is carried out in the following description.

On the basis of the information of the XY coordinates of the positions of the projected dicing lines14A in the first direction as detected in the alignment step described above, the chuck table25is moved to position the beam condenser61of the laser beam applying unit6directly above a processing start position on a predetermined one of the projected dicing lines14A in the first direction on the wafer10. At this time, the undulating means64illustrated inFIG.3has been set to transmit the pulsed laser beam LB0emitted from the laser oscillator62straight through the undulating means64to the reflecting mirror65, which reflects the pulsed laser beam LB0downwardly as a laser beam LB1, indicated by the solid line inFIG.3, traveling centrally through the fθ lens66of the beam condenser61. As illustrated inFIG.4A, the pulsed laser beam LB1that has traveled through the fθ lens66is applied to the wafer10while its focused spot is being positioned inside the wafer10below the center of the projected dicing line14A in the first direction. As the wafer10as well as the chuck table25is processing-fed in the direction indicated by the arrow X1, which is one of the X-axis directions, the pulsed laser beam LB1is applied to the wafer10along the center of the projected dicing line14A in the first direction, thereby processing the wafer10along the projected dicing line14A to form a first modified layer110in the wafer10along the projected dicing line14A.

After the first modified layer110has been formed in the wafer10along the projected dicing line14A, the wafer10is indexing-fed in one of the Y-axis directions by a distance corresponding to the interval in the Y-axis directions between adjacent ones of the projected dicing lines14A, positioning an unprocessed adjacent one of the projected dicing lines14A directly below the beam condenser61. Then, in the same manner as the processing cycle described above, the pulsed laser beam LB1is applied to the wafer10while its focused spot is being positioned inside the wafer10below the center of the unprocessed adjacent projected dicing line14A. As the wafer10is processing-fed in the direction indicated by the arrow X1inFIG.4A, the pulsed laser beam LB1is applied to the wafer10along the center of the unprocessed adjacent projected dicing line14A in the first direction, thereby processing the wafer10along the projected dicing line14A to form another first modified layer110in the wafer10along the projected dicing line14A. The above sequence is repeated to form first modified layers110in the wafer10along all the projected dicing lines14A extending in the first direction (first modified layer forming step), as illustrated inFIG.4B. The first modified layers110according to the present embodiment are formed in the wafer10as extending along and through the centers of the projected dicing lines14A extending in the first direction, as viewed in plan. As illustrated inFIG.4B, the first modified layers110also extend through the centers A1through A3, Bl through B3, C1through C3of the points where the projected dicing lines14A and the projected dicing lines14B intersect with each other. The XY coordinates of the positions of all the first modified layers110formed in the wafer10are stored in the coordinate storage section, not depicted, of the control unit100.

When the first modified layers110have been formed in the wafer10along all the projected dicing lines14A in the first direction in the first modified layer forming step described above, the rotary actuator means is actuated to turn the chuck table25and hence the wafer10through 90 degrees in the direction indicated by the arrow R2inFIG.4Bto bring the projected dicing lines14A in the first direction and the first modified layers110into alignment with the Y-axis directions and also to bring the projected dicing lines14B in the second direction perpendicular to the projected dicing lines14A in the first direction into alignment with the X-axis directions. At this time, since the XY coordinates of the positions of the first modified layers110formed in the wafer10vary as the chuck table25is turned 90 degrees, the XY coordinates of the positions of the first modified layers110stored in the coordinate storage section of the control unit100are transformed into actual XY coordinates on the laser processing apparatus2by the control unit100. The transformed XY coordinates of the positions of the first modified layers110are stored in the coordinate storage section of the control unit100.

Then, the feed mechanism30described above is actuated to position a processing start position on a predetermined one of the projected dicing lines14B in the second direction on the wafer10directly below the beam condenser61of the laser beam applying unit6. When the pulsed laser beam emitted from the laser oscillator62is applied to the wafer10along the projected dicing line14B in the second direction, the control unit100sends an instruction signal to energize the undulating means64to shift the pulsed laser beam LB0to one side before reaching the reflecting mirror65, which reflects the pulsed laser beam LB0downwardly as a pulsed laser beam LB2, indicated by the dot-and-dash line inFIG.3, traveling off-center through the fθ lens66of the beam condenser61. In other words, the position of the focused spot of the pulsed laser beam LB2in the wafer10is shifted from the position of the focused spot of the pulsed laser beam LB1in the wafer10and stays in the projected dicing line14B in the second direction. As illustrated inFIG.5A, the pulsed laser beam LB2that has traveled through the fθ lens66is applied to the wafer10while its focused spot is being positioned inside the wafer10on the projected dicing line14B in the second direction. As the wafer10is processing-fed in the direction indicated by the arrow X2inFIG.5A, the pulsed laser beam LB2is applied to the wafer10along the projected dicing line14B in the second direction, thereby processing the wafer10along the projected dicing line14B to form a second modified layer120in the wafer10along the projected dicing line14B in the second direction. The formation of the second modified layer120in the wafer10along the projected dicing line14B will be described in greater detail below with reference toFIG.5Bwhich is an enlarged fragmentary plan view of a section S illustrated inFIG.5Awhere the second modified layer120is formed.

As illustrated inFIG.5B, the position where the second modified layer120is to be formed up to the first modified layer110is not along the center of the projected dicing line14B in the second direction, but along a line shifted toward one of two opposite devices12, i.e., the upper device120illustrated inFIG.5Baccording to the present embodiment, by a distance of 10 μm, for example, from the center of the projected dicing line14B. Then, as the wafer10is processing-fed in the direction indicated by the arrow X2, the pulsed laser beam LB2is applied to the wafer10to form the second modified layer120along the projected dicing line14B in the wafer10in the direction indicated by the arrow R3. When the focused spot of the pulsed laser beam LB2has reached the X coordinate xa of the first modified layer110, the undulating means64is energized to shift the pulsed laser beam LB0to the other side before reaching the reflecting mirror65, which reflects the pulsed laser beam LB0downwardly as a laser beam LB3, indicated by the two-dot-and-dash line inFIG.3, traveling off-center through the fθ lens66of the beam condenser61. The position of the focused spot of the pulsed laser beam LB3in the wafer10is shifted from the position of the focused spot of the pulsed laser beam LB2in the wafer10in the direction indicated by the arrow R4inFIG.5Bby a distance of 20 μm, for example, and stays in the projected dicing line14B in the second direction. Specifically, the focused spot of the pulsed laser beam LB3is shifted across a central point C1to a position spaced 10 μm from the central point C1. The pulsed laser beam LB3that has traveled through the fθ lens66is applied to the wafer10while its focused spot is being positioned inside the wafer10on the projected dicing line14B in the second direction. As the wafer10is processing-fed in the direction indicated by the arrow X2inFIG.5B, the pulsed laser beam LB2is applied to the wafer10along the projected dicing line14B in the second direction, thereby processing the wafer10along the projected dicing line14B to form a second modified layer120in the wafer10along the projected dicing line14B in the second direction. In this manner, the pulsed laser beam applied to the wafer10is undulated in a staggered pattern to prevent the second modified layer120from being formed straight in the wafer10along the projected dicing line14B, but to form the second modified layer120in the direction indicated by the arrow R3and then in the direction indicated by the arrow R5.

The undulating means64described above is adjusted in its operation each time the focused spot of the pulsed laser beam reaches the first modified layer110to form the second modified layer120undulated in the staggered pattern within the width of the projected dicing line14B in the second direction, as illustrated inFIG.6. After the second modified layer120has been formed in the wafer10along the projected dicing line14B in the second direction, as described above, the wafer10is indexing-fed in one of the Y-axis directions by a distance corresponding to the interval in the Y-axis directions between adjacent ones of the projected dicing lines14A, positioning an unprocessed adjacent one of the projected dicing lines14B directly below the beam condenser61. Then, in the same manner as the processing cycle described above, the pulsed laser beam is applied to the wafer10while its focused spot is being positioned inside the wafer10below the unprocessed adjacent projected dicing line14B. As the wafer10is processing-fed, the pulsed laser beam is applied to the wafer10along the unprocessed adjacent projected dicing line14B in the second direction, thereby processing the wafer10along the projected dicing line14B to form another staggered second modified layer120in the wafer10along the projected dicing line14B. The above sequence is repeated to form staggered second modified layers120in the wafer10along all the projected dicing lines14B extending in the second direction (second modified layer forming step), as illustrated inFIG.6.

Laser processing conditions in the first modified layer forming step and the second modified layer forming step which have been described above are established as follows, for example:Wavelength: 1342 nmAverage output power: 1.0 WRepetitive frequency: 90 kHzProcessing-feed speed: 700 mm/second

After the first modified layers110have been formed in the wafer10along the projected dicing lines14A in the first direction and the second modified layers120have been formed in the staggered pattern in the wafer10along the projected dicing lines14B in the second direction, as described above, the wafer10is delivered to expanding means, not depicted. The expanding means applies external forces to the adhesive tape T to which the wafer10is affixed, expanding the adhesive tape T horizontally to divide the wafer10into individual device chips along the first modified layers110and second modified layers120.

According to the embodiment described above, the second modified layers120are undulated in the staggered pattern at the points where the projected dicing lines14A in the first direction and the projected dicing lines14B in the second direction intersect with each other. Therefore, when external forces are exerted on the wafer10to divide the wafer into individual device chips, corners of the device chips are prevented from rubbing against each other and being chipped at the points where the first modified layers110and the second modified layers120intersect with each other, so that the device chips are free from the problem of a reduction in their quality.

The present invention is not limited to the embodiment described above. For example, according to the above embodiment, the first modified layers110are formed in the wafer10as extending along and through the centers of the projected dicing lines14A extending in the first direction. However, the first modified layers110may not necessarily be formed as extending through the centers of the projected dicing lines14A, but may be shifted sideways from the centers of the projected dicing lines14A. According to the above embodiment, furthermore, after the first modified layers110and the second modified layers120have been formed in the wafer10, the adhesive tape T to which the wafer10is affixed is horizontally expanded to divide the wafer10into individual device chips. However, the present invention is not limited to such details. Instead, a roller or the like may be pressed against the face side10aof the wafer10to apply external forces to the wafer10to thereby divide the wafer10into individual device chips.

The present invention is not limited to the details of the above described preferred embodiment. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.