Method of making a peeled substrate using laser irradiation

The inventor of the present disclosure used the peeled substrate manufacturing apparatus 10 described in the above embodiments, held a silicon wafer as the substrate mounting member 12 on the stage surface 11 f of the XY stage 11 , and mounted and held a single crystal magnesium oxide wafer 20 u (hereinafter, simply referred to as a wafer 20 u ) as the single crystal substrate 20 on this silicon wafer.

Then, by the substrate manufacturing method described in the above embodiment, in order to sequentially form the processing marks K in an inner portion of each of irradiation experiment regions of the wafer 20 u , the laser beam B was applied to each irradiation experiment region of the wafer 20 u from the irradiated surface thereof, and at the same time, the condenser 14 and the wafer 20 u were two-dimensionally (in a plane form) moved relatively to each other.

In this example, the laser beam was applied in a line form (linearly), whereby one processing mark was formed, another processing mark was formed in parallel to this processing mark at a position apart therefrom at a designated amount of offset interval, and still another processing mark was further formed in a similar way at the designated amount of offset interval. Moreover, in this example, the irradiation test of the laser beam B was carried out for each of cases where the wavelength of the laser beam B was 1064 nm, 532 nm and 1024 nm. Irradiation conditions are shown in FIG. 3 .

As a result of observing the irradiated surface by an electron microscope after the irradiation, at 1064 nm, the laser beam B did not enter the wafer 20 u much, and aberration occurred on the surface of the wafer. At 532 nm, the laser beam B entered the wafer 20 u , and the processing marks were formed in the inner portion of the wafer; however, the processing marks were not very good since the irradiation energy was too strong. At 1024 nm, the laser beam B entered the wafer 20 u , the processing marks were formed in the inner portion of the wafer, and the processing marks were relatively good.

In a similar way to Example 1, the inventor of the present disclosure used the peeled substrate manufacturing apparatus 10 described in the above embodiments, held a silicon wafer on the stage surface 11 f of the XY stage 11 , and mounted and held the wafer 20 u (single crystal magnesium oxide wafer: crystal orientation 100, diameter 50.8 mm, thickness 300 μm) as the single crystal substrate 20 on this silicon wafer.

Then, the laser beam B was applied to each irradiation experiment region of the wafer 20 u , and at the same time, the condenser 14 and the wafer 20 u were moved relatively to each other in a plane form (two-dimensionally), whereby the processing marks K (for example, refer to FIG. 11 ) were sequentially formed in the inner portion of each irradiation experiment region. While three processing marks were formed in Example 1, 100 processing marks were formed in this example. Irradiation conditions are shown in FIG. 4 .

In this example, the wavelength of the laser beam B to be applied was set to 1024 nm based on the result of Example 1. Moreover, a defocus amount (DF) was set to 0.05 mm, and the processing marks K were formed in a line form (linearly) on a substantially intermediate position in a thickness direction of the wafer 20 u . At this time, the laser output was changed as a parameter, and the laser beam B was individually applied at 0.1 W, 0.3 W, 0.5 W and 1.0 W. Plan captured image views of the respective test pieces already irradiated are shown in FIG. 5 to FIG. 8 . Note that, in each of FIG. 6 to FIG. 8 , generation of a stripe pattern SP (rainbow pattern) on the front surface of the test piece suggests that the surface of the test piece is deformed since a crack propagates in the inner portion of the test piece.

Thereafter, in order to investigate a generation state of the crack due to the processing marks K, the test piece was cleaved for each irradiation experiment region so that the processing mark K was exposed, and a side cross section thereof was observed. Side cross section captured image views of the respective test pieces are shown in FIG. 9 to FIG. 12 . Note that generation of the stripe pattern SP (rainbow pattern) on the front surface of each test piece suggests that the surface of the test piece is deformed since the crack propagates in the inner portion of the test piece.

At 0.1 W of the laser output, the processing marks K were formed in the inner portion of the test piece; however, the propagation of the crack did not occur. At 0.3 W of the laser output, the processing marks K were formed in the inner portion of the test piece, and the propagation of the crack also occurred; however, peeling in such a propagated portion of the crack was not observed. At 0.5 W of the laser output, the processing marks K were formed in the inner portion of the test piece, the propagation of the crack also occurred, and the peeling in the propagated portion of the crack was observed. At 1.0 W of the laser output, the processing marks K were formed in the inner portion of the test piece, the propagation of the crack also occurred, the peeling in the propagated portion of the crack was observed, and a damage on the peeled surface was also observed since the irradiation energy was too strong.

The inventor of the present disclosure carried out this example by setting the laser output to 0.5 W based on the results of Example 2.

In a similar way to Example 2, the inventor of the present disclosure used the peeled substrate manufacturing apparatus 10 described in the above embodiments, held a silicon wafer on the stage surface 11 f of the XY stage 11 , and mounted and held the wafer 20 u (single crystal magnesium oxide wafer) as the single crystal substrate 20 on this silicon wafer.

Then, the condenser 14 and the wafer 20 u were moved in a planar shape (two-dimensionally) relatively to each other, whereby the laser beam B was applied in a planar shape to the wafer 20 u , and processing mark lines were formed. Irradiation conditions are shown in FIG. 13 .

In this example, when the laser beam B was applied, the line pitch rp was changed as a parameter, and the laser beam B was individually applied to irradiated regions 20 a to 20 c (refer to FIG. 14 ) so that the line pitch rp became 10 μm, 20 μm and 50 μm, respectively.

Then, each of the irradiated regions was cleaved to be used as such a test piece. In this cleavage, each of the irradiated regions of the wafer 20 u was cleaved using a glass cutter so that an elongated member (corresponding to a member shown as a dot-hatched region A in FIG. 15 ) with a short width of W 1 , which was elongated in a direction along an orientation flat, was formed. Moreover, both end portions in a longitudinal direction of each of the elongated members were cut off so that a center portion with a width of W 2 , which composed a center portion in the longitudinal direction in the elongated member, was left. In this way, final test pieces were obtained.

At the line pitch rp of 50 natural peeling did not occur from the irradiated surface even if the irradiated regions were cleaved into the elongated members. Note that, in this description, the natural peeling from the irradiated surface of the substrate means that a peeled substrate is obtained by two-dimensional peeling on the irradiated surface of the substrate even if force is not applied thereto.

Then, the elongated member was further cleaved to obtain a final test piece TP (refer to FIG. 16 ). Even in this final test piece TP, the natural peeling did not occur from the irradiated surface.

At the line pitch rp of 20 μm, when a final test piece was obtained by cleavage, then as shown in FIG. 17 , it was seen that the peeled substrate 20 p was naturally peeled in a half region of the irradiated surface 20 r (that is, a half region of the region A), and that the natural peeling occurred in the half region of the irradiated surface 20 r . Then, in a region as other half, it was confirmed that the peeled substrate 20 p was perfectly peeled from an elongated member body 20 m (refer to FIG. 2 ) though the natural peeling did not occur therein. Note that FIG. 17 shows the peeled substrate 20 p naturally peeled in the half region of the irradiated surface 20 r and a remaining test piece TPm covered with a protective film.

At the line pitch rp of 10 μm, when final test pieces were obtained by cleavage, then as shown in FIG. 18 , over the entire region of the irradiated surface 20 r (entire region of the region A), the peeled substrate was naturally cleaved, and was peeled off from the elongated member body 20 m (refer to FIG. 2 ). Hence, it was confirmed that such peeled substrates 20 p were perfectly peeled off from the elongated member body 20 m . Note that FIG. 18 shows such peeled substrates 20 p naturally peeled as a result of natural cleavage from the entire region of the irradiated surface 20 r and a remaining test piece TPm covered with a protective film.

As shown in FIG. 19 , the inventor of the present disclosure cut a single crystal magnesium oxide substrate having a square shape of 10 mm×10 mm when viewed from above (hereinafter, referred to as a test piece J 1 ) out of the single crystal magnesium oxide wafer 20 u having a diameter of 2 inches and a thickness of 300 μm. Note that, as shown in FIG. 19 , an experiment was carried out while making crystal orientations (100, 010 and the like) to correspond between the single crystal magnesium oxide wafer 20 u and the test piece J 1 . Note that a direction where the cleavage is easy to occur is the crystal orientation 100 (plane orientation 100).

(1) Irradiation Conditions

In this example, the peeled substrate manufacturing apparatus 10 described in the above embodiments was used, and as shown in FIG. 20A , at designated depth positions of the test piece J 1 , processing marks 22 c were formed at a designated dot pitch dp and a designated line pitch rp, whereby planar modified layers 22 were formed in an inner portion of this test piece J 1 . FIG. 20B shows a schematic cross-sectional view of the test piece J 1 in which the processing marks 22 c are formed, and FIG. 20C shows laser beam application conditions.

(2) Peeled Surface

After being irradiated with the laser beam, the irradiated surface (upper side) of the test piece J 1 was sandwiched by aluminum-made bases 24 u and 24 b via an adhesive. Both of the bases 24 u and 24 b are made of aluminum. As the adhesive, an epoxy adhesive was used, the base 24 u was adhered to the irradiated surface (upper side) of the test piece J 1 , and the base 24 b was adhered to a bottom surface side (lower side) of the test piece J 1 .

Then, these bases 24 u and 24 b are pulled in the vertical direction, whereby peeling force thereof from the modified layer 22 was measured, and a tensile rupture stress was calculated, which was necessary to separate, from the modified layer 22 , an upper test piece J 1 u having the irradiated surface (upper side) of the test piece J 1 and a lower test piece J 1 b having a bottom surface (lower side) of the test piece J 1 . As a result, the upper test pieces J 1 u and the lower test pieces J 1 b were able to be separated by a tensile tress of 0.3 MPa. Hence, the upper test pieces J 1 u and the lower test pieces J 1 b were able to be separated from the modified layer 22 by the tensile rupture stress that was much smaller than 12 MPa as a tensile rupture stress of the single crystal silicon substrate.

Then, the inventor of the present disclosure was able to observe, by the naked eye, that a stripe pattern was generated in each of a peeled surface J 1 us of the upper test piece J 1 u and a peeled surface J 1 bs of the lower test piece J 1 b (refer to FIG. 21 ).

Then, the peeled surface J 1 bs of the lower test piece J 1 b was captured by a SEM (scanning electron microscope) while sequentially shifting capturing positions from a point P 1 shown in FIG. 22 to a point P 2 shown in FIG. 22 . Capturing results are shown in FIG. 23 . Note that, in the captured image views attached to this description, crystal orientations are also shown as appropriate.

On the peeled surface J 1 bs , such a cyclic pattern J 1 bp was formed, in which a clouded portion J 1 bw , a transparent portion (smooth portion) J 1 bt and an interference portion J 1 bi appeared sequentially. Here, the clouded portion J 1 bw extended in a direction of the crystal orientation 011, and a direction of continuation in the cyclic pattern J 1 bp was a direction of the crystal orientation 01-1. Moreover, a large step difference BB (approximately 16 μm) was formed on the clouded portion J 1 bw , and a smooth surface F was formed on the transparent portion (smooth portion) J 1 bt.

Moreover, the inventor of the present disclosure increased a magnification of the SEM, and captured a clouded portion J 1 uw of the peeled surface J 1 us of the upper test piece J 1 u and a clouded portion J 1 bw of the peeled surface J 1 bs of the lower test piece J 1 b . Capturing results are shown in FIG. 24 . Crystal orientations in FIG. 24 are the same as the crystal orientations shown in FIG. 23 .

Moreover, the inventor of the present disclosure captured a transparent portion (smooth portion) J 1 ut of the peeled surface J 1 us of the upper test piece J 1 u and a transparent portion (smooth portion) J 1 bt of the peeled surface J 1 bs of the lower test piece J 1 b . Capturing results are shown in FIG. 25 .

Moreover, the inventor of the present disclosure captured an interference portion J 1 ui of the peeled surface J 1 us of the upper test piece J 1 u and an interference portion J 1 bi of the peeled surface J 1 bs of the lower test piece J 1 b . Capturing results are shown in FIG. 26 . Crystal orientations in FIG. 26 are the same as the crystal orientations shown in FIG. 25 .

In FIG. 24 to FIG. 26 , the inventor of the present disclosure captured each of the peeled surface J 1 us of the upper test piece J 1 u and the peeled surface J 1 bs of the lower test piece J 1 b at both magnifications of 1000 times and 10000 times.

With regard to the peeled surface J 1 bs of the lower test piece J 1 b , in the clouded portion J 1 bw , as shown in FIG. 24 , large hole portions BH were formed irregularly, and portions around the hole portions BH were formed into a rough irregular shape. In the transparent portion J 1 bt , as shown in FIG. 25 , a flat surface was formed, and no hole portions were formed. In the interference portion J 1 bi , substantially uniform hole portions SH were arrayed regularly. A dimension of the hole portions SH was much smaller than that of the hole portions BH. Moreover, traces of melting were observed in regions other than the hole portions SH.

Moreover, as shown in FIG. 27 , for the peeled surface J 1 bs of the lower test piece J 1 b , the inventor of the present disclosure measured a height change in a line region LS, which is linear when viewed from above and ranges from the clouded portion J 1 bw via the transparent portion J 1 bt to the interference portion J 1 bi , by a surface roughness meter. Capturing results are shown in FIG. 28 .

As shown in FIG. 28 , a gradient of the height change was lower in the interference portion J 1 bi than in the clouded portion J 1 bw . Moreover, in the transparent portion J 1 bt , a dent portion D was formed in at some midpoint thereof; however, no dent portion was formed in the interference portion J 1 bi.

(3) Conclusion

After being irradiated with the laser beam under the irradiation conditions of this example, the upper test piece J 1 u and the lower test piece J 1 b were separated, whereby the thin magnesium oxide single crystal substrate with less lattice defects were able to be obtained with ease.

Moreover, at the time of performing this separation, the upper test piece J 1 u and the lower test piece J 1 b were separated from the modified layer 22 by the very small tensile rupture stress as mentioned above. Hence, it is thought that the planar peeling is occurring in the modified layer 22 .

Moreover, it was seen that, in this separation, the peeled substrate was peeled while peeled tip end positions thereof are being repeated like waving in the vertical direction (a thickness direction of the test piece, that is, a thickness direction of the modified layer 22 ) in the modified layer 22 formed in the horizontal direction (substrate surface direction).

Moreover, in the peeled surface J 1 bs , the cyclic pattern J 1 bp was formed, in which the transparent portion J 1 bt , the interference portion J 1 bi and the clouded portion J 1 bw sequentially appeared and continued with one another. The direction of this continuation was directed to the [01-1] direction.

Then, from measurement results of the height change of this cyclic pattern, it is surmised that the clouded portion J 1 bw was generated near an upper end of the modified layer 22 (that is, an irradiated side with the laser beam), that the transparent portion J 1 bt was generated near a lower end of the modified layer 22 (that is, a remote side from the irradiated side with the laser beam), and that the interference portion J 1 bi was generated at an intermediate portion of the modified layer 22 (that is, an intermediate position in the thickness direction of the modified layer between the clouded portion J 1 bw and the transparent portion J 1 bt ).

Then, based on observation results of the peeled surface J 1 bs by the SEM and measurement results of surface roughness of the peeled surface J 1 bs , it was determined that, at the time of irradiating the test piece J 1 with the laser beam B, it was preferable to irradiate the test piece J 1 so that the interference portion J 1 bi was generated on the peeled surface J 1 bs since it was easy to reduce such irregularities of the peeled surface at the time of peeling the peeled surface.

The inventor of the present disclosure carried out an experiment in which the line pitch rp=4 mm in Example 4 (refer to FIGS. 20A to 20C ) was changed to a line pitch=7 mm, and performed peeling from the modified layer (separation of an upper test piece and a lower test piece) in a similar way to Example 4. As a result, a major part of the peeled surface of the lower test piece was such a transparent portion, and in addition, it was more difficult to peel the peeled surface than in the case of the line pitch rp=4 mm.

Therefore, it was surmised that narrowing the line pitch rp made it easy to generate the cyclic pattern J 1 bp , and that extreme widening of the line pitch rp made it easy to generate the transparent portion.

Accordingly, the inventor of the present disclosure examined a cause to generate the clouded portion J 1 bw . Then, the inventor of the present disclosure used a test piece J 2 cut out of the single crystal magnesium oxide wafer 20 u in the same manner as in the test piece J 1 used in Example 4, and as shown in FIG. 29 , further applied the laser beam to a region in which the peeled portion J 2 s was formed (that is, the same irradiated position was irradiated for the second time), and observed a surface of the region by the SEM. As a result, a white portion W as an image substantially coinciding with the clouded portion J 1 bw measured by Example 4 was observed. Hence, it is possible that the clouded portions J 1 uw and J 1 bw may be clouded since peeled regions are further irradiated with the laser beam.

Furthermore, the inventor of the present disclosure cut the test piece J 2 irradiated twice as described above, thereby exposing a cross section of the white portion W, and observing this cross section by the SEM. As a result, as shown in FIG. 30 , an image of the white portion W was such an image that a peeled portion J 2 v was further stacked on the peeled portion J 2 s developed earlier.

Moreover, as shown in FIGS. 31A and 31B , the inventor of the present disclosure carried out an experiment of irradiating the laser beam while shifting irradiated positions so that the irradiated positions do not overlap each other between first application of the laser beam and second application thereof. Specifically, a position of the test piece J 3 was set so that irradiation lines R 2 at the second application of the laser beam are located at intermediate positions between the adjacent irradiation lines R 1 and R 1 at the time of the first application of the laser beam. Note that, in a similar way to the test pieces J 1 and J 2 , the test piece J 3 was a piece cut out of the single crystal magnesium oxide wafer 20 u . Irradiation conditions are shown in FIG. 32 .

Here, in this example, a line pitch rp 1 at the first application (an interval between the adjacent irradiation lines at the first application) was 8 μm, and a line pitch rp 2 at the second application (an interval between the adjacent irradiation lines at the second application) was also 8 μm. Then, the irradiation lines R 2 at the second application were located at intermediate positions between the adjacent irradiation lines R 1 at the first application. That is, an interval rpm between the irradiation lines R 1 at the first application and the irradiation lines R 2 at the second application (that is, a line pitch between the first application and the second application) was 4 μm.

An irradiated surface of the test piece J 3 already subjected to the second laser beam application was captured from above using the SEM or the like. Capturing results are shown in FIGS. 33A and 33B . In FIG. 33A , linear white portions were observed in a part of the irradiated surfaces; however, no cyclic patter was generated.

Thereafter, a peeled surface J 3 s peeled from a processing layer in a similar way to Example 4 was observed by the SEM or the like. Capturing results are shown in FIG. 34 and FIGS. 35A and 35B . As seen from FIG. 35B , processing marks K 1 formed by the first application and processing marks K 2 formed by the second application were obviously different in shape from each other. Particularly at the second application, there were formed: annular marks K 2 c similar to outer peripheries of the hole portions SH (refer to FIG. 26 ) of the interference portion J 1 bi formed in Example 4; and square outer edge marks K 2 r formed so as to surround the respective annular marks K 2 c . Moreover, though irregular shapes were present on the peeled surface J 3 s , a maximum height of such irregularities was approximately H=2.62 μm (which was measured at a line U in FIG. 36 ).