Substrate processing method

Abstract

A substrate has a first surface with at least one division line formed thereon and a second surface opposite the first surface. The substrate is processed by applying a pulsed laser beam from the side of the first surface. The substrate is transparent to the pulsed laser beam. The pulsed laser beam is applied at least in a plurality of positions along the at least one division line, a focal point of the pulsed laser beam located at a distance from the first surface in the direction from the first surface towards the second surface, so as to form a plurality of modified regions inside the substrate. Each modified region is entirely within the bulk of the substrate, without any openings open to the first surface or the second surface. Substrate material is removed along the at least one division line where the modified regions are present.

Claims

1. A method of processing a substrate, having a first surface with at least one division line formed thereon and a second surface opposite the first surface the method comprising: applying a pulsed laser beam to the substrate from the side of the first surface, wherein the substrate is made of a material which is transparent to the pulsed laser beam and the pulsed laser beam is applied to the substrate at least in a plurality of positions along the at least one division line, in a condition where a focal point of the pulsed laser beam is located at a distance from the first surface in the direction from the first surface towards the second surface, so as to form a plurality of modified regions inside the substrate along the at least one division line, the plurality of modified regions being formed inside the substrate so that adjacent modified regions do not overlap each other in the extension direction of the at least one division line, each modified region being arranged entirely within the bulk of the substrate, without forming any openings open to the first surface or the second surface; and removing substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.

2. The method according to claim 1, wherein the substrate is a single crystal substrate or a glass substrate or a compound substrate.

3. The method according to claim 1, wherein the modified regions comprise amorphous regions or regions in which cracks are formed, or the modified regions are amorphous regions or regions in which cracks are formed.

4. The method according to claim 1, wherein the substrate material is removed by cutting the substrate along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.

5. The method according to claim 1, wherein the substrate material is mechanically removed along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present, by mechanically cutting the substrate along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.

6. The method according to claim 1, further comprising grinding the second surface of the substrate to adjust the substrate thickness.

7. The method according to claim 6, wherein grinding the second surface of the substrate is performed after removing the substrate material along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.

8. The method according to claim 7, wherein the substrate material is removed along only a part of the thickness, in the direction from the first surface towards the second surface, of the substrate, and grinding the second surface of the substrate is performed along a remaining part of the thickness of the substrate, in which no substrate material has been removed, so as to divide the substrate along the at least one division line.

9. The method according to claim 1, wherein, in each of the plurality of positions along the at least one division line where the pulsed laser beam is applied, plural modified regions are formed, each modified region being arranged entirely within the bulk of the substrate, and the plural modified regions being arranged next to one another along the direction from the first surface towards the second surface.

10. The method according to claim 1, wherein the substrate material is removed along the entire extension, in the direction from the first surface towards the second surface, of the modified regions.

11. The method according to claim 1, wherein the at least one division line has a width in a direction perpendicular to the extension direction of the at least one division line, and the method further comprises applying the pulsed laser beam also in a plurality of positions along the width direction of the at least one division line, so as to form within the width of the division line a plurality of rows of modified regions, each row extending along the extension direction of the at least one division line, wherein the rows are arranged adjacent to each other in the width direction of the at least one division line.

12. The method according to claim 11, wherein the substrate material is removed along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present by mechanically cutting the substrate, using a cutting means, and a width, in the direction perpendicular to the extension direction of the at least one division line, of an area of the substrate, in which the rows of modified regions have been formed, is in a range of 90% to 110% of a width, in the direction perpendicular to the extension direction of the at least one division line, of the cutting means.

13. The method according to claim 12, wherein a row or rows of modified regions arranged closer to the center of the at least one division line, in the width direction of the at least one division line, is or are formed with a pulsed laser beam having a higher power than a pulsed laser beam used for forming a row or rows of modified regions arranged further away from the center of the at least one division line, in the width direction of the at least one division line.

14. The method according to claim 1, wherein the substrate material is mechanically removed along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present , only by mechanically cutting the substrate along the at least one division line where the modified regions arranged entirely within the bulk of the substrate are present.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Hereinafter, non-limiting examples of the invention are explained with reference to the drawings, in which:

(2) FIG. 1 shows an optical device wafer as a substrate to be processed by the method of the invention, wherein FIG. 1(a) is a perspective view of the wafer, and FIG. 1(b) is an enlarged cross-sectional view of the encircled region A in FIG. 1(a);

(3) FIG. 2 is a perspective view showing a condition in which the optical device wafer of FIG. 1(a) is attached to an adhesive tape supported by an annular frame;

(4) FIG. 3 is a perspective view of a part of a laser processing apparatus for applying a pulsed laser beam to the optical device wafer of FIG. 1(a);

(5) FIG. 4 is a side view for illustrating a step of forming a plurality of modified regions inside the optical device wafer of FIG. 1(a) according to an embodiment of the method of the present invention;

(6) FIGS. 5 (a) to 5(d) are schematic cross-sectional views illustrating the formation of a modified region inside the optical device wafer of FIG. 1(a);

(7) FIG. 6 shows a process of dividing the optical device wafer according to an embodiment of the processing method of the present invention, wherein FIGS. 6(a) and 6(b) are cross-sectional views illustrating a step of removing substrate material along a division line, and FIG. 6(c) is a cross-sectional view illustrating a grinding step;

(8) FIG. 7 shows steps of removing substrate material along a division line for two different embodiments of the present invention, wherein FIGS. 7(a) and 7(b) are cross-sectional views illustrating the substrate material removing step for one embodiment, and FIGS. 7(c) and 7(d) are cross-sectional views illustrating the substrate material removing step for another embodiment;

(9) FIGS. 8(a) and 8(b) illustrate steps of removing substrate material along a division line for further different embodiments of the present invention; and

(10) FIGS. 9(a) and 9(b) show cross-sectional views of examples of arrangements of rows of modified regions for different embodiments of the method of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(11) Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. The preferred embodiments relate to methods of processing an optical device wafer as a substrate.

(12) The optical device wafer may have a thickness before grinding in the m range, preferably in the range of 200 m to 1500 m.

(13) FIG. 1(a) is a perspective view of an optical device wafer 2 as a substrate to be processed by the processing method of the present invention. The optical device wafer 2 is a single crystal substrate.

(14) In other embodiments, the substrate to be processed by the processing method of the present invention may be a glass substrate or a compound substrate, such as a compound semiconductor substrate, e.g., a GaAs substrate.

(15) The optical device wafer 2 shown in FIG. 1(a) is substantially composed of a sapphire substrate with a thickness of, for example, 300 m. A plurality of optical devices 21, such as light emitting diodes (LEDs) and laser diodes, are formed on a front side 2a, i.e., a first surface, of the sapphire substrate. The optical devices 21 are provided on the front side 2a of the sapphire substrate in a grid or matrix arrangement. The optical devices 21 are separated by a plurality of crossing division lines 22 formed on the front side 2a of the sapphire substrate, i.e., on the front side of the optical device wafer 2.

(16) In the following, a preferred embodiment of the method of the present invention for processing the optical device wafer 2 as the substrate will be described with reference to FIGS. 2 to 6(c).

(17) First, a wafer supporting step is performed in such a manner that the optical device wafer 2 is attached to an adhesive tape, such as a dicing tape, supported by an annular frame. Specifically, as is shown in FIG. 2, an adhesive tape 30, e.g., a dicing tape, is supported at a peripheral portion thereof by an annular frame 3, so as to close an inner opening of the annular frame 3 by the adhesive tape 30. A back side 2b, i.e., a second surface, of the optical device wafer 2 is attached to the adhesive tape 30. Accordingly, the front side 2a of the optical device wafer 2 attached to the adhesive tape 30 is oriented upwards, as is shown in FIG. 2.

(18) FIG. 3 shows a part of a laser processing apparatus 4 for performing laser processing along the division lines 22 on the optical device wafer 2 after carrying out the wafer supporting step described above. As is shown in FIG. 3, the laser processing apparatus 4 includes a chuck table 41 for holding a workpiece, in particular, the optical device wafer 2, a laser beam applying means 42 for applying a laser beam to the workpiece held on the chuck table 41, and an imaging means 43 for imaging the workpiece held on the chuck table 41. The chuck table 41 has an upper surface as a holding surface for holding the workpiece thereon under suction. The chuck table 41 is movable by a feeding means (not shown) in a feeding direction which is indicated in FIG. 3 by an arrow X. Further, the chuck table 41 is movable by an indexing means (not shown) in an indexing direction which is indicated in FIG. 3 by an arrow Y.

(19) The laser beam applying means 42 includes a cylindrical casing 421 extending in a substantially horizontal direction. The casing 421 contains a pulsed laser beam oscillating means (not shown) including a pulsed laser oscillator and a repetition frequency setting means. Further, the laser beam applying means 42 includes a focusing means 422 mounted on a front end of the casing 421. The focusing means 422 comprises a focusing lens 422a for focusing a pulsed laser beam oscillated by the pulsed laser beam oscillating means.

(20) The numerical aperture (NA) of the focusing lens 422a of the focusing means 422 may be set so that the value obtained by dividing the numerical aperture of the focusing lens 422a by the refractive index (n) of the single crystal substrate is within the range of 0.2 to 0.85.

(21) The laser beam applying means 42 further includes a focal position adjusting means (not shown) for adjusting the focal position of the pulsed laser beam to be focused by the focusing lens 422a of the focusing means 422.

(22) The imaging means 43 is mounted on a front end portion of the casing 421 of the laser beam applying means 42. The imaging means 43 includes an ordinary imaging device (not shown), such as a CCD, for imaging the workpiece by using visible light, an infrared light applying means (not shown) for applying infrared light to the workpiece, an optical system (not shown) for capturing the infrared light applied to the workpiece by the infrared light applying means, and an infrared imaging device (not shown), such as an infrared CCD, for outputting an electrical signal corresponding to the infrared light captured by the optical system. An image signal output from the imaging means 43 is transmitted to a control means (not shown).

(23) When performing laser processing along the division lines 22 of the optical device wafer 2 by using the laser processing apparatus 4, a positioning step is performed in such a manner that the focusing lens 422a of the focusing means 422 and the single crystal substrate, i.e., the optical device wafer 2, are positioned relative to each other in the direction along the optical axis of the focusing lens 422a so that the focal point of the pulsed laser beam is located at a desired position in the direction along the thickness of the optical device wafer 2, i.e., at a desired distance from the front side 2a, i.e., the first surface, in the direction from the front side 2a towards the back side 2b, i.e., the second surface.

(24) When performing the processing method according to the current embodiment of the present invention, the optical device wafer 2 attached to the adhesive tape 30 is first placed on the chuck table 41 of the laser processing apparatus 4 shown in FIG. 3 in the condition where the adhesive tape 30 is in contact with the upper surface of the chuck table 41 (see FIG. 3). Subsequently, a suction means (not shown) is operated to hold the optical device wafer 2 through the adhesive tape 30 on the chuck table 41 under suction (wafer holding step). Accordingly, the front side 2a of the optical device wafer 2 held on the chuck table 41 is oriented upward. Although, for the purpose of better presentability, the annular frame 3 supporting the adhesive tape 30 is not shown in FIG. 3, the annular frame 3 is held by a frame holding means, such as clamps or the like, provided on the chuck table 41. Subsequently, the chuck table 41 holding the optical device wafer 2 under suction is moved to a position directly below the imaging means 43 by operating the feeding means.

(25) In the condition where the chuck table 41 is positioned directly below the imaging means 43, an alignment operation is performed by the imaging means 43 and the control means (not shown) in order to detect a subject area of the optical device wafer 2 to be laser processed. Specifically, the imaging means 43 and the control means perform image processing, such as pattern matching, in order to align the division lines 22 extending in a first direction on the optical device wafer 2 with the focusing means 422 of the laser beam applying means 42. In this way, alignment of a laser beam applying position is performed (alignment step). This alignment step is performed in a similar manner also for all the other division lines 22 extending in a second direction perpendicular to the first direction on the optical device wafer 2.

(26) After performing the alignment step detailed above for all of the division lines 22 on the front side 2a of the optical device wafer 2, the chuck table 41 is moved to a laser beam applying area where the focusing means 422 of the laser beam applying means 42 is located, as is shown in FIG. 4. One end (the left end in FIG. 4) of a predetermined division line 22 extending in the first direction is positioned directly below the focusing means 422. Further, the focal position adjusting means (not shown) is operated so as to move the focusing means 422 in the direction along the optical axis of the focusing lens 422a so that the focal point P of a pulsed laser beam LB to be focused by the focusing lens 422 is located at a desired distance from the front side 2a of the optical device wafer 2 in the direction from the front side 2a towards the back side 2b thereof, i.e., in the thickness direction of the optical device wafer 2 (positioning step).

(27) In this preferred embodiment, the focal point P of the pulsed laser beam LB is located inside the optical device wafer 2 at a position near the front side 2a, i.e., the upper surface, of the optical device wafer 2 to which the pulsed laser beam LB is applied. For example, the focal point P may be located at a distance from the front side 2a in the range of 5 m to 10 m.

(28) After performing the positioning step described above, a modified region forming step is performed in such a manner that the laser beam applying means 42 is operated to apply the pulsed laser LB from the focusing means 422 to the optical device wafer 2, thereby forming a modified region inside the optical device wafer 2, the modified region being arranged entirely within the bulk of the wafer 2, without forming any openings open to the first surface 2a or the second surface 2b of the wafer 2 (see also FIG. 5(d)).

(29) Specifically, the pulsed laser beam LB, which has a wavelength that allows transmission of the laser beam LB through the sapphire substrate constituting the optical device wafer 2, is applied to the optical device wafer 2 by the focusing means 422, and the chuck table 41 is moved at a predetermined feed speed in the direction indicated by an arrow X1 in FIG. 4 (modified region forming step). When the other end (right end in FIG. 4) of the predetermined division line 22 reaches the position directly below the focusing means 422, the application of the pulsed laser beam LB is stopped and the movement of the chuck table 41 is also stopped.

(30) By performing the modified region forming step detailed above along the predetermined division line 22, a plurality of modified regions 23 (see FIG. 5(d)) is formed in the optical device wafer 2 along the division line 22, wherein each modified region 23 is arranged entirely within the bulk of the wafer 2. Each modified region 23 is composed of a space 231, e.g., a cavity, inside the wafer material and an amorphous region 232 surrounding the space 231, as is schematically shown in FIG. 5(d) and will be further detailed below.

(31) The modified regions 23 may be formed along the division line 22 at predetermined, equidistant intervals in the extension direction of the division line 22. For example, the distance between adjacent modified regions 23 in the extension direction of the division line 22 may be in the range of 8 m to 30 m, e.g., approximately 16 m (=(work feed speed: 800 mm/second)/(repetition frequency: 50 kHz)).

(32) In this embodiment, the amorphous regions 232 of adjacent modified regions 23 are formed so as not to overlap each other (in this regard, see FIGS. 9(a) and (b)). Specifically, the distance between adjacent modified regions 23 is chosen so as to be slightly larger than the outer diameter of the amorphous regions 232. The amorphous regions of adjacent or neighbouring modified regions 23 are thus disconnected from each other.

(33) In other embodiments, the substrate may be, for example, a glass substrate and the modified regions may comprise or be regions in which cracks are formed in the glass substrate. The cracks formed in the glass substrate may be microcracks.

(34) It is sufficient to apply the pulsed laser beam LB once for the formation of each modified region 23, so that the productivity can be greatly enhanced. Furthermore, no debris is scattered in the modified region forming step, so that a degradation of the quality of the resulting devices can be reliably prevented.

(35) The formation of a modified region 23 inside the optical device wafer 2 is illustrated in FIGS. 5(a) to 5(d). The substrate of the wafer 2 is made of a material which is transparent to the pulsed laser beam LB, namely sapphire. Thus, the modified region 23 is formed in the wafer 2 by the application of the pulsed laser beam LB having a wavelength that allows transmission of the laser beam LB through the wafer 2. For example, the pulsed laser beam LB may have a wavelength in the infrared range, e.g., 1064 nm.

(36) The pulsed laser beam LB is applied to the wafer 2 from the side of the first surface 2a in a condition where the focal point P of the pulsed laser beam LB is located at a distance from the first surface 2a in the direction from the first surface 2a towards the second surface 2b (see FIG. 5(a)). Due to the application of the pulsed laser beam LB, the wafer material is locally heated up in the region inside the wafer 2 where the focal point P is arranged. The heated up region of the wafer 2 at the initial stage of the laser beam application is schematically indicated by a circle in FIG. 5 (b).

(37) As the application of the pulsed laser beam LB continues, the heated up region grows or expands in the direction towards the first surface 2a, as is indicated by an arrow in FIG. 5(c). When the laser beam application is stopped, the heated up wafer material cools down, resulting in the formation of the modified region 23 which is composed of the space 231 inside the wafer 2 and the amorphous region 232 fully surrounding the space 231 (see FIG. 5(d)). As is shown in FIG. 5(d), the modified region 23 is arranged entirely within the bulk of the wafer 2, without any openings open to the first surface 2a or the second surface 2b being formed.

(38) The division lines 22 have a width w in a direction substantially perpendicular to the extension direction thereof, as is schematically shown in FIG. 1(b). The modified region forming step detailed above is performed along the predetermined division line 22 two or more times, while slightly shifting the optical device wafer 2 relative to the laser beam applying means 42 in the indexing direction (indicated by the arrow Y in FIG. 3), so as to apply the pulsed laser beam LB also in a plurality of positions along the width direction of the division line 22. In this way, a plurality of modified regions 23 is formed also along the width direction of the division line 22. The modified regions 23 may be arranged with different distances between adjacent modified regions 23 in the extension direction and/or the width direction of the division line 22, as is schematically shown in FIG. 1(b).

(39) A plurality of rows of modified regions 23 may be formed within the width w of the division line 22, each row extending along the extension direction of the division line 22, wherein the rows are arranged adjacent to each other in the width direction of the division line 22. The rows of modified regions 23 may be equidistantly arranged in the width direction of the division line 22, as is schematically shown in FIGS. 6 and 7. Alternatively, the distances between adjacent rows of modified regions 23 in the width direction of the division line 22 may vary. For example, the distances between adjacent rows of modified regions 23 may be larger at or closer to the centre of the division line 22 than at positions further away from the centre, i.e., at the sides or edges in the width direction of the division line 22, as is shown in FIG. 8(a).

(40) Further, in each of the plurality of positions along the division line 22 where the pulsed laser beam LB is applied, plural modified regions 23 may be formed, each modified region 23 being arranged entirely within the bulk of the substrate, and the plural modified regions 23 may be arranged next to one another along the direction from the front side 2a towards the back side 2b, i.e., along the thickness direction of the wafer 2. In this way, plural layers of modified regions 23 can be formed, wherein the plural layers are stacked along the thickness direction of the wafer 2.

(41) In the embodiments shown in FIGS. 6 to 8, plural rows of modified regions 23 are arranged next to each other in the width direction of the division line 22, and plural layers of these plural rows are stacked along the thickness direction of the wafer 2.

(42) In other embodiments, a single row of modified regions 23 may be formed within the width w of the division line 22.

(43) After performing the modified region forming step a plurality of times along the predetermined division line 22 as detailed above, the chuck table 41 is moved in the indexing direction (indicated by the arrow Y in FIG. 3) by the pitch of the division lines 22 extending in the first direction on the optical device wafer 2 (indexing step). Subsequently, the modified region forming step is performed a plurality of times in the same manner as described above along the next division line 22 extending in the first direction. In this way, the modified region forming step is performed a plurality of times along all of the division lines 22 extending in the first direction. Thereafter, the chuck table 41 is rotated by 90, in order to perform the modified region forming step a plurality of times in the same manner as detailed above along all of the other division lines 22 extending in the second direction perpendicular to the first direction.

(44) The modified region forming step may be performed using a pulsed laser beam with a wavelength in the range of 300 nm to 3000 nm, a pulse width of 0.5 ps to 500 ps, an average power of 0.2 W to 10.0 W and a repetition frequency of 10 kHz to 100 kHz. The work feed speed with which the optical device wafer 2 is moved relative to the laser beam applying means 42 in the modified region forming step may be in the range of 500 mm/second to 1000 mm/second.

(45) If a semiconductor substrate is used as the substrate, e.g., the single crystal substrate, to be processed by the method of the present invention, a modified region 23 can be formed in a particularly efficient and reliable manner if the wavelength of the pulsed laser beam LB is set to a value which is two or more times the wavelength (reduced wavelength) corresponding to the band gap of the semiconductor substrate.

(46) After performing the modified region forming step in the manner detailed above, a step of dividing the optical device wafer 2 is carried out, as will be described in detail below with reference to FIGS. 6(a) to (c).

(47) FIG. 6(a) shows a cross-sectional view of a part of the optical device wafer 2 including a division line 22. As has been indicated above, a plurality of rows of modified regions 23, namely six rows of modified regions 23, are formed within the width of the division line 22, each row extending along the extension direction of the division line 22. The rows of modified regions 23 are equidistantly arranged adjacent to each other in the width direction of the division line 22. As is further shown in FIG. 6(a), three layers of six rows of modified regions 23 are stacked along the thickness direction of the wafer 2.

(48) While only one division line 22 is shown in FIG. 6(a), the remaining division lines 22 are provided with three layers of six rows of modified regions 23 in the same manner as shown in this figure.

(49) In the process of dividing the optical device wafer 2, substrate material is removed along the division line 22 first, using a cutting means 6, such as a rotating blade or a saw, as is schematically shown in FIGS. 6(a) and (b). As is shown in these figures, a width, in the direction substantially perpendicular to the extension direction of the division line 22, of an area of the optical device wafer 2 in which the rows of modified regions 23 have been formed is substantially the same as a width, in the direction substantially perpendicular to the extension direction of the division line 22, of the cutting means 6.

(50) The cutting means 6 is moved towards the front side 2a of the optical device wafer 2 and made to cut into the area of the wafer 2 in which the rows of modified regions 23 have been formed, as is indicated by the arrows in FIGS. 6(a) and (b). As is shown in FIG. 6(c), in the cutting step, the substrate material is removed along only a part of the thickness, in the direction from the front side 2a towards the back side 2b, of the optical device wafer 2. For example, the substrate material may be removed along approximately 50% of the thickness of the optical device wafer 2 in the cutting step.

(51) The cutting step is performed in the manner detailed above for all of the division lines 22 formed on the front side 2a of the optical device wafer 2. Subsequently, the back side 2b of the optical device wafer 2 is ground using a grinding apparatus (not shown), as is illustrated in FIG. 6(c). The grinding apparatus may include a chuck table (not shown) for holding a workpiece and a grinding means (not shown) for grinding the workpiece held on the chuck table. The chuck table may have an upper surface as a holding surface for holding the workpiece thereon under suction. The grinding means may include a spindle housing (not shown), a rotating spindle (not shown) rotatably supported to the spindle housing and adapted to be rotated by a driving mechanism (not shown), a mounter (not shown) fixed to the lower end of the rotating spindle and a grinding tool 8 (see FIG. 6(c)) mounted on the lower surface of the mounter. The grinding tool 8 may comprise a circular base 81 and abrasive elements 82 mounted on the lower surface of the circular base 81.

(52) Grinding of the back side 2b of the optical device wafer 2 is performed by holding the wafer 2 on the chuck table (not shown) of the grinding apparatus so that the front side 2a of the wafer 2 is in contact with the upper surface of the chuck table. Hence, the back side 2b of the wafer 2 is oriented upwards. Subsequently, the chuck table with the optical device wafer 2 held thereon is rotated around an axis perpendicular to the plane of the optical device wafer 2 and the grinding tool 8 is rotated around an axis perpendicular to the plane of the circular base 81. While rotating the chuck table and the grinding tool 8 in this manner, the abrasive elements 82 of the grinding tool 8 are brought into contact with the back side 2b of the wafer 2, thus grinding the back side 2b. Grinding is performed along a remaining part of the thickness of the optical device wafer 2, in which no substrate material has been removed in the cutting step, so as to divide the wafer 2 along the division lines 22.

(53) Dividing the optical device wafer 2 in this manner allows for individual chips or dies (not shown) with a high die strength and high quality side surfaces to be obtained in a particularly accurate, reliable and efficient manner.

(54) In particular, since plural layers of modified regions 23 are stacked along the thickness direction of the wafer 2, the back side 2b of the wafer 2 has been reduced in strength. Hence, the grinding process can be performed particularly efficiently, in particular, with an especially high grinding speed.

(55) In the following, further preferred embodiments of the present invention will be described with reference to FIGS. 7 and 8.

(56) These embodiments differ from the embodiments detailed above with reference to FIGS. 1 to 6 in the arrangement of the modified regions 23 and/or in the details of the step of removing substrate material along the division lines.

(57) The substrate material removing step, i.e., the cutting step, illustrated in FIGS. 7(a) and (b) differs from the substrate material removing step illustrated in FIGS. 6(a) and (b) mainly in that the width of the area of the optical device wafer 2 in which the rows of modified regions 23 have been formed is smaller than the width of the cutting means 6, as is shown in FIG. 7(a). Further, as is shown in FIG. 7(b), the substrate material is removed along the entire thickness of the optical device wafer 2, i.e., the wafer 2 is cut by the cutting means 6 along its entire thickness.

(58) Performing the substrate material removing step in this manner offers the benefit that the modified regions 23 can be prevented from remaining on the side surfaces 2c of the resulting chips or dies in a particularly reliable manner (see FIG. 7(b)). Hence, high quality chips or dies can be obtained in a simple way.

(59) The substrate material removing step, i.e., the cutting step, illustrated in FIGS. 7(c) and (d) differs from the substrate material removing step illustrated in FIGS. 6(a) and (b) mainly in that the width of the area of the optical device wafer 2 in which the rows of modified regions 23 have been formed is larger than the width of the cutting means 6, as is shown in FIG. 7(c). Further, the substrate material is removed along the entire thickness of the optical device wafer 2, i.e., the wafer 2 is cut along its entire thickness by the cutting means 6, as is shown in FIG. 7(d).

(60) In this way, the substrate material removing step can be particularly efficiently carried out, since it can be reliably ensured that substantially the entire width of the cutting area in which the cutting means 6 comes into contact with the optical device wafer 2 has been reduced in strength by the formation of the modified regions 23.

(61) Modified regions 23 remaining on the side surfaces 2c of the resulting chips or dies (see FIG. 7(d)) can be removed in an additional grinding or polishing step, if desired.

(62) As is schematically shown in FIGS. 6(a) and (b), 7(a), (c) and (d) and 8(a) and (b), the modified regions 23 are arranged entirely within the bulk of the wafer 2, without forming any openings open to the first surface 2a or the second surface 2b. The modified regions 23 are thus formed so as to extend along only a part of the thickness of the optical device wafer 2. Hence, the formation of the modified regions 23 can be performed in a particularly efficient manner.

(63) As is illustrated in FIG. 6(b), the substrate material may be removed along only a part of the thickness of the optical device wafer 2, e.g., the optical device wafer 2 may be cut along only a part of its thickness. In this case, the optical device wafer 2 may be divided, for example, by grinding the back side 2b thereof in the manner described in detail above.

(64) Alternatively, as is schematically shown in FIGS. 7(b) and (d), the substrate material may be removed along the entire thickness of the optical device wafer 2, e.g., the wafer 2 may be cut along its entire thickness.

(65) A distance between adjacent rows of modified regions 23 in the width direction of the division line 22 may be larger for rows of modified regions 23 arranged closer to the centre of the division line 22 than for rows of modified regions 23 arranged further away from the centre of the division line 22, as is schematically shown in FIG. 8(a). In this case, fewer rows of modified regions 23 have to be formed, thus enhancing the efficiency of the modified region forming step. Further, the smaller distance between adjacent rows of modified regions 23 away from the centre of the division line 22 ensures that damage to the side surfaces of the resulting chips or dies in the cutting process, such as chipping or cracking, can be reduced.

(66) Also in the embodiment shown in FIG. 8(a), three layers of six rows of modified regions 23 are stacked along the thickness direction of the wafer 2.

(67) The step of removing substrate material along the division line 22 may be performed with different material removal widths. For example, in a first material removing step, substrate material may be removed with a first width and, in a second material removing step, substrate material may be removed with a second width. The second removal width may be smaller than the first removal width.

(68) In particular, as is shown in FIG. 8(b), in the substrate material removing step, the optical device wafer 2 may be first cut with a first cutting means 6 along a part of its thickness. The width of the first cutting means 6 may be substantially the same as the width of the area of the optical device wafer 2 in which the rows of modified regions 23 have been formed. Subsequently, a remaining part of the area in which the rows of modified regions 23 have been formed may be cut using a second cutting means 6 (shown by a dotted line in FIG. 8(b)) with a width that is smaller than that of the first cutting means 6.

(69) A row or rows of modified regions 23 arranged closer to the centre of the division line 22, in the width direction of the division line 22 (see, for example, FIGS. 6(a) and (b), FIGS. 7(a) and (c) and FIGS. 8(a) and (b)), may be formed with a pulsed laser beam LB having a higher power than a pulsed laser beam LB used for forming a row or rows of modified regions 23 arranged further away from the centre of the division line 22, in the width direction of the division line 22.

(70) FIGS. 9(a) and 9(b) show examples of arrangements of rows of modified regions 23 for different embodiments of the method of the present invention. As is shown in these drawings, the modified regions 23 do not overlap with each other.

(71) FIG. 9(a) shows an example in which seven rows of modified regions 23 are arranged next to each other along the width direction of the division line 22. The division line 22 has a width w1 of approximately 100 m. A width w2, in the direction substantially perpendicular to the extension direction of the division line 22, of the area of the substrate 2, in which the rows of modified regions 23 have been formed, is approximately 48 m.

(72) A distance w3 between centres of adjacent modified regions 23 in the extension direction of the division line 22 is in the range of 8 m to 10 m. A distance w4 between adjacent rows of modified regions 23 in the width direction of the division line 22, i.e., between centres of the modified regions 23 of the adjacent rows, is in the range of 8 m to 10 m. The modified regions 23 have diameters d in the range of 2 m to 3 m.

(73) A distance w5 between outer edges of adjacent modified regions 23 in the width direction of the division line 22 is 1 m or more. A distance w6 between outer edges of adjacent modified regions 23 in the extension direction of the division line 22 is 1 m or more.

(74) Substrate material may be removed along the division line 22 where the modified regions 23 are formed, e.g., by using a cutting means (not shown), such as a blade or a saw. Particularly preferably, the cutting means may have a width, in the direction substantially perpendicular to the extension direction of the division line 22, which is slightly larger than the width w2 of the area of the substrate 2, in which the rows of modified regions 23 have been formed. For example, the cutting means may have a width of approximately 50 m.

(75) The arrangement of rows of modified regions 23 shown in FIG. 9(b) differs from that shown in FIG. 9(a) only in the number of rows of modified regions 23, the width w1 of the division line 22 and the width w2, in the direction substantially perpendicular to the extension direction of the division line 22, of the area of the substrate 2, in which the rows of modified regions 23 have been formed.

(76) Specifically, FIG. 9(b) shows an example in which three rows of modified regions 23 are arranged next to each other along the width direction of the division line 22. The division line 22 shown in FIG. 9(b) has a width w1 of approximately 50 m. The width w2 of the area of the substrate 2, in which the rows of modified regions 23 have been formed, is approximately 22 m.

(77) Substrate material may be removed along the division line 22 shown in FIG. 9(b) where the modified regions 23 are formed, e.g., by using a cutting means (not shown), such as a blade or a saw. Particularly preferably, the cutting means may have a width, in the direction substantially perpendicular to the extension direction of the division line 22, which is slightly larger than the width w2 of the area of the substrate 2, in which the rows of modified regions 23 have been formed. For example, the cutting means may have a width of approximately 25 m.

(78) In other embodiments, a single row of modified regions 23 may be formed within the width of the division line 22.

(79) In the embodiments shown in FIGS. 9(a) and (b), a single layer of the modified regions 23 can be formed in the wafer 2. Alternatively, plural layers of the modified regions 23 may be stacked along the thickness direction of the wafer 2, for example, in the manner shown in FIGS. 6 to 8.