Laser cutting method

At least one exemplary embodiment is directed to a laser cutting method where a laser beam is condensed at internal points inside a substrate forming processing regions, and where the laser is swept along a cutting line, where the cutting line is associated with a recess on the substrate and where the recess can be formed contemporaneously with the formation of the processing regions.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a cutting method for cutting a workpiece with a laser beam, more particularly, though not exclusively, a laser beam condensed at an internal point of the workpiece.

2. Description of the Related Art

According to a conventional technique for cutting a workpiece (i.e., a member to be cut) with a laser beam, the workpiece such as a semiconductor substrate is cut by rotating, at a high speed, a circular blade having a width of several tens μm to several hundreds μm so that the abrasive material on the blade surface can grind the workpiece (semiconductor substrate). This method is generally referred to as a blade dicing method.

According to this method, to reduce heat generation or abrasion during the cutting operation, one can spray cooling water onto the cut surface. However, according to this method, finely powdered workpiece material or fine particles of the abrasive material generated during the cutting operation may be mixed with the cooling water and may spread in a wide region containing the cut surface.

To solve this problem, conventional systems perform the cutting operation in a dry environment without using cooling water. To this end, to cut a substrate, a processing method using a laser beam having a wavelength highly absorbable by the substrate can be employed to condense the laser beam on the substrate surface. However, according to this method, not only the portion directly irradiated with the laser beam but also its peripheral region on the substrate surface will melt. The electronic devices provided on the semiconductor substrate will be damaged.

On the other hand, there is a conventional processing method using a laser beam highly absorbable by a substrate to condense the laser beam inside the substrate so that the substrate can be cut by utilizing internal beam-condensing of the laser beam. For example, Japanese Patent Application Laid-open No. 2002-192370 and Japanese Patent Application Laid-open No. 2002-205180 discuss a processing method that uses a laser beam having a specific wavelength that shows high permeability relative to a substrate, i.e., a material to be cut, to condense the laser beam at an internal point of the substrate, thereby setting a start point of the cutting operation in a predetermined internal region of the substrate.

This processing method can realize an excellent cutting operation accompanied with less generation of heat or no hardening of dust particles on the surface, because no melt region is formed on the substrate surface.

Furthermore, Japanese Patent Application Laid-open No. 2002-205180 discusses a method for providing a plurality of property modified regions in the incident direction of a laser beam by changing the depth of the beam-condensing point.

However, according to the above-described conventional method, the start point of a cutting operation is limited to an internal region of the substrate where the laser beam is condensed. Hence, it can be difficult to appropriately control the direction or position of a crack growing from the start point of the cutting operation to the substrate surface.

Especially, in a workpiece made of a silicon wafer or a material having a similar crystal structure, the crack tends to grow along the crystal orientation. Therefore, according to the above-described laser processing method, if there is a small offset between a predetermined cutting line and a crystal orientation reaching the substrate surface that may be present due to manufacturing errors in the formation of the silicon substrate or elements, the crack may deviate from the predetermined cutting line in the process of growing toward the substrate surface and, as a result, may damage the logic circuits provided on a semiconductor element region.

SUMMARY OF THE INVENTION

At least one exemplary embodiment is directed to a laser cutting method for forming a crack connecting a surface of a workpiece (i.e., a member to be cut) to an internal processing region formed inside the workpiece while reducing the deviation of the crack from a predetermined cutting line on the workpiece surface.

At least one exemplary embodiment is directed to a laser cutting method configured to cut a workpiece by condensing a laser beam inside the workpiece from a surface of the workpiece to form an internal processing region therein. The laser cutting method includes a step of forming the internal processing region by condensing the laser beam at a predetermined depth from the workpiece surface so that the internal processing region extends in a depth direction of the workpiece, and a step of, along with the step of forming the internal processing region, forming a recessed portion at a position irradiated with the laser beam on the workpiece surface.

Further features of the present invention will become more apparent from the following detailed description of exemplary embodiments with reference to the attached drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following description of exemplary embodiment(s) is/are merely illustrative in nature and is in no way intended to limit the invention, its application, or uses.

Processes, techniques, apparatus, and materials as known by one of ordinary skill in the art may not be discussed in detail but are intended to be part of the enabling description where appropriate. For example certain lasers and optical lens systems may not be discussed in detail. However these systems and the methods to fabricate these system as known by one of ordinary skill in the relevant art is intended to be part of the enabling disclosure herein where appropriate.

Note that similar reference numerals and letters refer to similar items in the following figures, and thus once an item is defined in one figure, it may not be discussed for following figures.

Note that the non-limiting illustrative examples that follow discuss the use of a silicon substrate, however exemplary embodiments are not limited to silicon substrates, the substrate can be made of various materials (e.g., SiO2, other substrate materials as known by one of ordinary skill in the relevant art and equivalents).

Hereinafter, exemplary embodiments will be explained using, as a member to be cut, a silicon substrate10on which a plurality of semiconductor elements, such as logic elements10a, are formed. At least one exemplary embodiment proposes a cutting method for splitting the logic elements10aof the silicon substrate10into separate element chips.

In general, the substrate has a front surface and a back surface. A cutting operation of at least one exemplary embodiment can be applied to a silicon substrate10having a plurality of semiconductor circuits formed on its front surface and is carried out by irradiating the front surface of the silicon substrate with a laser beam.

Hereinafter, a surface on which the semiconductor circuits are formed is referred to as the front surface of the substrate. However, if simply referred to as a substrate surface, it includes both of the front surface and the back surface of the substrate. Especially, in a case where a workpiece is not differentiated in its front and back surface structures, the cutting method of at least one exemplary embodiment can be applied to the entire surface (e.g., front and back surface) of the substrate.

As shown inFIGS. 1A-1Cand2, a beam-condensing point of the laser beam is positioned inside the silicon substrate10, at a predetermined depth from the substrate surface11, to form an internal processing region being spaced from the substrate surface11on which logic circuits or other electronic devices are formed. The internal processing region is, for example, a region causing a change in crystal structure of the substrate material, or a softened or melted region, or cracks. According to at least one exemplary embodiment, cracks produced in the silicon substrate10facilitate a later-described internal processing operation.

According to at least one exemplary embodiment, a plurality of internal processing regions12(12ato12f,FIG. 2) are formed in a substrate (i.e., a member to be cut). Furthermore, shifting the laser beam relative to the substrate or vice versa can be performed to scan the beam-condensing point along a predetermined cutting line C. With this arrangement, a belt-like crack group is formed along the predetermined cutting line C (refer toFIG. 2).

A surface-processing trace11a, having a recessed shape, can preexist prior to the formation of the crack group or formed contemporaneously at a laser beam irradiation position on the substrate surface11along the predetermined cutting lines C(C1) and CC2) (FIG. 1A).

Then, after the surface-processing trace11ahas been formed and the internal processing operation for forming the cracks (crack group) inside the substrate has been performed, an external force can be applied to the substrate surface to trigger the cutting of element chips.

In this case, the force applied to the substrate surface concentrates on the surface-processing trace11a. Thus, the surface recess (e.g., the surface-processing trace11a) and the internal processing region (e.g., the internal processing regions12, especially an internal crack12fformed immediately below the surface-processing trace11a) are easily connected with each other. Thus, an actual cutting line appearing on the substrate surface11will not appreciably deviate from the predetermined cutting line C.

In other words, the crack formed on the substrate surface11substantially agrees in position with the surface-processing trace11a(e.g., with the groove bottom).

Such a crack is usually formed along a straight line. However, due to an unexpected chipping caused when the substrate has a crystal defect, the crack may appear along a zigzag line. Even in such a case, the region of the crack appearing on the substrate surface11is limited within the width of the recessed surface-processing trace11a.

This zigzag crack can include short cracks extending along different crystal planes peculiar to the silicon substrate. The region of the zigzag crack can be within the width of the surface-processing trace11aand is accordingly within the scribe width. Therefore, the actual cutting line does not deviate appreciably (e.g., where appreciably would be where the deviation cuts into a logic circuit) from the predetermined cutting line C.

FIGS. 1A and 1Billustrate a silicon substrate10, which has a related thickness (e.g., a thickness of 625 μm). As illustrated inFIG. 1C, a film2(e.g., an oxide film) having a related thickness (e.g., about 1 μm) is formed on the (100) crystal surface of a silicon wafer1. For example, a nozzle layer3, (e.g., an epoxy resin structural body that includes an ink or liquid discharge mechanism), a logic element (e.g., for driving the discharge mechanism), and its wiring can be disposed on the silicon wafer1. These members cooperatively constitute each logic element portion10a.

In the example of a nozzle layer3, a liquid supply port (i.e., ink supply port)4, serving as an opening portion, can be formed by applying anisotropic etching to the silicon wafer1, so that the liquid supply port4is positioned beneath the nozzle layer3incorporating the liquid discharge mechanism as described above. The nozzle layer3can be disposed between two predetermined cutting lines C so that the silicon wafer1can be cut into a plurality of element chips in the final stage of the manufacturing process. Each predetermined cutting line C can be formed along a crystal orientation of the silicon wafer1. In this example a clearance S between neighboring nozzle layers3is set to be a particular value (e.g., at least 400 μm).

FIG. 2illustrates a cross section of the silicon substrate10including the surface-processing trace1aformed as a recessed portion on the front surface11of the silicon substrate10together with the internal cracks12ato12fdisposed, as a plurality of internal processing regions, in the depth direction of the silicon substrate10along the predetermined cutting line C. A dicing tape T is bonded to the back surface of the silicon substrate10.

FIG. 3illustrates a flowchart explaining a cutting process for cutting the silicon substrate10into a plurality of logic element portions10aeach serving as an individual element chip. The cutting process shown in this flowchart includes a total of 6 processes including a tape mounting process S1as a first step, a wafer correcting process S2as a second step, a surface-processing trace and internal crack forming process S3as a third step, a cutting process S4as a fourth step, a repair process S5as a fifth step, and a pickup process S6as a sixth step, which are performed in this order.

Tape Mounting Process S1

As shown inFIG. 4, the silicon substrate10is first subjected to the tape mounting process for the purpose of preventing the elements from being separated before executing the cutting process. The tape mounting process includes a step of bonding a dicing tape T on the back surface of the silicon substrate10. The dicing tape T can be an adhesive member on which a dicing frame M is attached.

The nozzle layer3, (e.g., a resin layer formed on the surface of the silicon substrate10as described above), can cause heat shrinkage when it is cured. Accordingly, the entire body of the silicon substrate10is deformed as shown inFIG. 5A. If a later-described laser irradiation is applied to the deformed silicon substrate10, it is difficult to accurately perform the processing operation because the incident angle of a laser beam on the substrate surface11is different depending on the position of the substrate surface.

Therefore, one can improve the accuracy of cutting by reducing this deformation beforehand. Hence, as shown inFIG. 5B, a suction stage D can be placed behind the dicing tape T to pull the silicon substrate10by vacuum so that the silicon substrate10is flattened from the deformed condition.

Surface-Processing Trace and Internal Crack Forming Process S3

FIG. 6Aillustrates a processing apparatus50that can form the internal crack12shown inFIG. 2. The processing apparatus50includes a light source optical system (e.g.,51,51a, and51b), a beam-condensing optical system52, and an automatic stage mechanism53. The light source optical system includes a light source51, a beam expanding system51a, and a mirror51b. Furthermore, the beam-condensing optical system52includes a microscope objective lens52a, a mirror52b, and an automatic focusing mechanism52c.

Furthermore, the automatic stage mechanism53includes an X-stage53a, a Y-stage53b, and a fine adjustment stage53c. Furthermore, the processing apparatus50includes an alignment optical system (not shown) that can perform alignment of the silicon substrate10(i.e., workpiece W) by utilizing an orientation flat10b(refer toFIG. 1A) formed on the silicon substrate10.

Furthermore, the light source51can be a pulse laser (e.g., YAG laser) having a fundamental wavelength (e.g., of 1,064 nm for the YAG laser), with a pulse width (e.g., ranging from about 15 ns to about 1000 ns) and associated frequencies (e.g., ranging from 10 kHz to 100 kHz).

Note that although in the non-limiting examples that follow a YAG laser is discussed, exemplary embodiments are not limited to YAG lasers and any appropriate laser can be used. Selection of the laser beam should be determined with reference to a spectral transmission factor of the substrate. Therefore, one can use light in a predetermined wavelength region that can form a strong electric field in a beam-condensing point and is permeable into a substrate. For example, the fundamental wave of the pulse YAG laser beam used in the example of exemplary embodiments may not penetrate the entire body of the silicon substrate, depending upon the substrate thickness, and may not completely absorbed on the surface of the silicon substrate.

A laser beam L emitted from the light source51passes through the beam expanding system51aand enters the beam-condensing optical system52. Then, the microscope objective lens52aof the beam-condensing optical system52condenses the laser beam L at the workpiece W. Then, as shown inFIG. 6B, the laser beam L enters the workpiece W, via the substrate surface11on which the logic element portions10aare formed, the silicon substrate10being the workpiece W mounted on the automatic stage53.

The optical conditions in this case should be set in such a manner that the surface-processing trace11acan be present anywhere on the substrate surface11. In at least one exemplary embodiment the power can be increased in compensation of the energy loss of the processing laser beam L caused by the surface-processing trace11a, or one can select a light flux that can enter the substrate10without interfering with the surface-processing trace11a. In this manner, the light flux entering the silicon substrate10via the substrate surface11causes a refraction in the silicon substrate10and is condensed at a beam-condensing point A spaced by a predetermined depth (a) from the substrate surface11so as to form the internal crack12, which can include the beam-condensing point A.

In at least one exemplary embodiment the processing conditions are set considering the beam-condensing position, film arrangement of film2(e.g., oxide film), or the wavelength of a laser beam used, so that the crack edge of an uppermost internal crack12fas shown inFIG. 2is spaced by a chosen distance (e.g., of 10 μm or more) from the substrate surface11. This setting can be used to reduce the occurrence of the internal crack12fconnecting with the substrate surface11or with the surface-processing trace11a(when the surface-processing trace11ais being formed or after the surface-processing trace11ais already formed) during the processing operation. Furthermore, the substrate surface11may be damaged if laser irradiation conditions are improper.

The depth (a) of the beam-condensing point A can be controlled by shifting either the workpiece W (e.g., the silicon substrate10) or the beam-condensing optical system52in the optical axis direction, so that the beam-condensing position can be changed.

When ‘n’ represents the refractivity of the substrate10relative to a particular wavelength (e.g., 1064 nm) and ‘d’ represents a mechanical shift amount (e.g., the shift amount of either the silicon substrate10or the laser beam condensing optical system52when it is shifted in the optical axis direction), an optical shift amount of the beam-condensing point A can be expressed by a product of ‘n’ and ‘d’ (i.e., nd). The refractivity ‘n’ of the silicon substrate10is approximately 3.5 relative to the wavelength 1.1 μm to 1.5 μm. The comparison with an experimentally measured refractivity has revealed that ‘n’ is close to 3.5. For example, when the mechanical shift amount is 100 μm, the beam-condensing point of the laser beam is formed in a position at the depth of 350 μm from the substrate surface.

Furthermore, having the refractivity of approximately 3.5 is equivalent to having a large reflectance. In general, as the reflection in a vertical incidence is expressed by ((n−1)/(n+1))2, the silicon substrate10has the reflectance of approximately 30%. Theoretically, the rest of the energy can reach the beam-condensing point inside the silicon substrate. However, if the light absorption by the silicon substrate10itself is taken into consideration, the final energy available at the beam-condensing point A will be smaller than the theoretical value. According to an experimental measurement using a silicon substrate having a thickness of 625 μm, the transmission factor was approximately 20%.

When the laser beam L is condensed at the beam-condensing point A, a local change in crystal condition of the silicon can occur and, as a result, an internal crack12can be formed. According to experimental results, the crack length (b) was approximately 2 μm to 100 μm, which can depend upon beam-condensing intensity.

In this manner, the internal crack12is formed from an internal point inside the silicon substrate10. The internal processing operation can be performed, just beneath the predetermined cutting line C, by relatively shifting the beam-condensing point A along the predetermined cutting line C. As shown inFIG. 1A, the predetermined cutting lines C formed on the silicon substrate10include two kinds of cutting lines C(C1) and C(C2) that are substantially perpendicular to each other. The cutting line C(C1) is substantially parallel to the orientation flat10bformed as a reference on the silicon substrate10. The cutting line C(C2) is substantially perpendicular to the orientation flat10b.

The workpiece W (e.g., the silicon substrate10) can be mounted on the automatic stage53that is shiftable in the X and Y directions. Therefore, the positional adjustment of the workpiece W in the horizontal plane is feasible. On the other hand, to realize the positional adjustment of the workpiece W in the optical axis direction (i.e., the Z direction or the depth direction of the silicon substrate), it is possible to provide a Z-stage53c, on the automatic stage53or on the beam-condensing optical system52, that is capable of relatively shifting the automatic stage53in the Z direction. Thus, the clearance between the beam-condensing optical system52and the workpiece W is changeable.

The shifting speed in the X and Y directions can be determined considering the frequency or crack configuration. For example, the shifting speed can be in the range from 10 mm/sec to 100 mm/sec when the frequency is in the range from 10 kHz to 100 kHz. If the shifting speed exceeds 100 mm/sec, the internal processing regions will be formed at relatively long intervals in the shifting direction. Thus, the clearance between neighboring cracks formed along the same predetermined cutting line will become excessively wider in the depth direction. This can adversely affect the later-performed cutting operation.

Furthermore, the beam-condensing optical system52can include an observation camera52f, which can be equipped with a filter suitable for the laser output, which is disposed in a conjugated relationship with a workpiece irradiation point. The illumination for the observation uses a relay lens so that an illuminating light source is formed at the position of an entrance pupil of the microscope objective lens52aused for the beam-condensing, for example in at least one exemplary embodiment the arrangement can form a Koehler illuminating system.

In addition to the above-described observation optical system, an AF (i.e., Auto Focus) optical system (not shown) can be provided to measure the clearance between the system and the workpiece W. The AF optical system can obtain the contrast of an image when it is taken by the observation camera52f, and can also measures the focusing point or the inclination based on the obtained contrast. To measure this contrast in an actual operation, one can repeat a fine feeding operation to measure the distance from the workpiece W, so that an optimum position can be determined. The usage of such an AF action can also be used to determine the parallelization of the workpiece W orientation (e.g., the silicon substrate10).

In starting the internal processing operation, the following points should be considered.

(1) As shown inFIG. 7, the laser processing operation starts from the end point of the silicon substrate10being the workpiece W. In this case, the region near the end point is generally difficult to process compared with the central region. Hence, in accordance with at least one exemplary embodiment, the processing conditions can be changed so as to increase the laser energy when the region near the end point is processed.

(2) Furthermore, in the example where the substrate is cut into a plurality of rectangular chips as shown inFIG. 8, the predetermined cutting line C(C1) extending along the longer side of the chip is regarded as a first cutting direction and the internal crack12is formed along this line C1. Then, another internal crack12is formed along the predetermined cutting line C(C2) extending along the shorter side of the chip, which is regarded as a second cutting direction.

As described above, in the non-limiting example discussed herein, the crack length formed at one beam-condensing point is in the range from 2 μm to 100 μm. Meanwhile, the silicon substrate in this particular example has a thickness of 625 μm. Hence, to complete the cutting of this silicon substrate10, one can repeatedly perform the internal processing operation. Furthermore, regarding the order of the internal processing operation performed at one laser beam incident point, the internal processing operation can start with the far side (e.g., the inner side close to the back surface of the substrate10) and ends with the near side close to the front surface of the substrate10. This arrangement facilitates preventing the already formed internal processing region from interfering with the laser beam.

Furthermore, damage of the element chips disposed on the substrate surface can be reduced. The internal processing operation for forming internal cracks, in accordance with at least one exemplary embodiment, is constructed to facilitate the reduction of the occurrence of an internal crack, formed adjacent to the substrate surface, from reaching the substrate surface11. Furthermore, the processing conditions can be carefully determined to reduce the occurrence of an internal crack already existing in the vicinity of the beam-condensing point from developing due to laser irradiation heat and reaching the substrate surface.

However, in at least one exemplary embodiment the crack12acan reach the surface of the back surface of the substrate10. As shown inFIG. 2, a plurality of internal cracks12ato12fdisposed in the depth direction can be formed. Alternatively, the internal cracks12ato12fcan merge with each other. Furthermore, the uppermost internal crack12fclosest to the substrate surface11can be positioned at a chosen depth (e.g., the depth of 10 μm to 100 μm) from the substrate surface11of the silicon substrate10(refer to the distance (c) shown inFIG. 6B). In addition, in at least one exemplary embodiment the internal crack12fis not connected with the surface-processing trace11a.

A method for forming the surface-processing trace11aon the substrate surface11to accurately perform the operation for cutting the silicon substrate10into a plurality of logic element portions will now be described below. The surface-processing trace11afacilitates propagation of a crack along the predetermined cutting line C.

According to at least one exemplary embodiment, the surface-processing trace11acan be formed on the substrate surface11contemporaneously with the internal crack12fclosest to the substrate surface11at about the time the internal crack12fis formed in the substrate or prior to the internal crack12fformation. To this end, the depth of the laser beam-condensing point inside the substrate10is determined to be an appropriate value that is aligned with the surface-processing trace11a. Thus, the timing of forming the surface-processing trace11ais not limited to the timing of forming the internal crack12fclosest to the substrate surface.

For example, depending on the laser beam irradiation conditions, the surface-processing trace11amay be formed not only when the internal crack12fis formed but also when the internal crack12eis formed or before any crack is formed. In other words, the surface-processing trace11acan be formed when the laser beam irradiation for forming a plurality of internal cracks, e.g., the formation of the internal crack closest to the front substrate surface and the formation of another internal crack positioned closer to the back surface of the substrate, is performed.

Regarding the forming order of the internal cracks12ato12f, as shown inFIG. 9A, it is possible to form many layers (e.g., a total of 5 layers) of internal cracks (e.g.,12ato12e) before forming the internal crack12f. Alternatively, it is possible to successively form the remaining internal cracks12eto12ain this order after simultaneously forming the internal crack12fand the surface-processing trace11a. Thus various exemplary embodiments can have various orders of crack layer formation.

The length of each crack in the depth direction is variable (e.g., approximately 60 μm to approximately 70 μm). To form these internal cracks, the depth of the beam-condensing point from the substrate surface11can be successively changed at chosen intervals (e.g., of 95 μm). The clearance between neighboring internal cracks can be kept by appropriately determining the depth of the beam-condensing point. In the example, the internal crack12aformed at the deepest position (i.e., the position closest to the back surface of the substrate) is spaced by approximately 50 μm, at its end, from the back surface of the substrate, although other spacing can be chosen for other exemplary embodiments and examples.

The driving conditions of the laser processing apparatus for contemporaneously forming the internal crack12fand the surface-processing trace11aaccording to at least one exemplary embodiment were as follows (processing example A)Frequency: 100 kHzPulse width: 70 nsecShifting speed of Laser beam irradiation position: 100 mm/secOutput: 10 μJ

Under such conditions, the internal processing region (e.g., the internal crack) was formed by setting the beam-condensing point A positioned closest to the substrate surface11at the depth of 25 μm (e.g., DA1and DA2) from the substrate surface11. As a result, the internal processing region (i.e., internal crack region)12fhaving the length of approximately 10 μm to approximately 20 μm in the depth direction was obtained. The center of this crack was positioned at the depth of approximately 45 μm to approximately 50 μm (e.g., DA3) from the substrate surface11. In other words, the formed internal crack had its center being positioned at the point deeper than the depth of the beam-condensing point A (FIG. 9B).

Contemporaneously, the surface-processing trace11acan be formed on the substrate surface11along the predetermined cutting line C. The formed surface-processing trace11acan have a groove having a width (e.g., of approximately 5 μm to approximately 10 μm) and a depth (e.g., of approximately 3 μm to approximately 5 μm) (refer toFIG. 9B). In this manner, appropriately setting the laser beam irradiation conditions for forming the internal processing region makes it possible to contemporaneously form the surface-processing trace11atogether with the internal processing region. Thus, the surface-processing trace11acan be obtained by utilizing the laser beam irradiation that is performed to form the internal processing region. In at least one exemplary embodiment, the surface-processing trace11acan be formed before the cracks.

In at least one exemplary embodiment, the beam-condensing point depth of the laser beam, in forming the internal processing region12f, is determined depending on the selection of the laser processing apparatus, a laser oscillator, and a workpiece, so that the surface-processing trace11aand the internal processing region12fcan be simultaneously formed.

Results in accordance with a method in accordance with at least one exemplary embodiment are provided below. The results of such processing operations performed by changing the depth of the beam-condensing point to form the internal crack12fclosest to the substrate surface are given by way of example and are:Depth: 0 μm . . . only the surface-processing trace was formed.Depth: 25 μm . . . the surface-processing trace was formed together with the internal crack (processing example A, refer toFIG. 9B).Depth: 45 μm . . . the surface-processing trace was formed together with the internal crack (processing example B).Depth: 80 μm . . . only the internal crack was formed.

According to the above-described processing example B, the internal crack region was formed with its center being positioned at the depth of approximately 60 μm and the length of approximately 20 μm to approximately 30 μm.

From these results, the following information can be obtained. First, one should select a single laser beam wavelength that cannot penetrate the entire body of the silicon substrate for a given thickness and is not completely absorbed on the surface. Second, one should select appropriate laser processing conditions, such as a beam-condensing point depth, with reference to the reflection or absorption of a laser beam that may be changed according to the film characteristics of a substrate surface via which the laser beam enters the substrate body.

When such irradiation conditions are satisfied, the surface-processing trace and the internal crack of the silicon substrate can be contemporaneously formed. Thus in such an exemplary embodiment, it becomes possible to omit the process of scribing a groove with a scriber along a predetermined cutting line C on a substrate surface.

In the operation for contemporaneously processing the surface-processing trace and the internal crack, a threshold value of the energy for processing a silicon surface is smaller than a threshold value of the energy for forming the internal crack in the silicon substrate. This is one reason why such processing operations are feasible. The energy relationship described above has been confirmed by experimentation with respect to the example discussed above. Thus, experimentally it was confirmed, for the example discussed, that the output for forming the internal crack in the silicon substrate10was several times the energy for processing a silicon surface11. For example, according to the above-described processing example A, the laser beam energy on the substrate surface has exceeded a processing threshold value in the process of condensing the laser beam at the depth of 25 μm and the laser beam energy was partly used for a surface processing operation for forming the surface-processing trace.

FIG. 10illustrates an electron microscope photograph showing one example of the surface-processing trace11aformed together with the internal cracks. Changing the laser beam oscillating conditions makes it possible to contemporaneously change the conditions of the internal processing operation as well as the conditions of the surface processing operation. For example, the following results were obtained when the pulse width is set to 17 μm based on the processing system of the processing example A or B, without changing other conditions.Depth: 0 μm . . . only the surface-processing trace was formed.Depth: 25 μm . . . only the surface-processing trace was formed.Depth: 45 μm . . . the surface-processing trace was formed together with the internal crack.Depth: 80 μm . . . only the internal crack was formed.

The surface-processing trace11a, obtained together with the internal crack12in the above-described experiment, was connected to the internal crack12in a later-described cutting process. Thus, it becomes possible to realize a substrate cutting operation that reduces undesirable cracks that may deviate from the predetermined cutting line C. The undesirable cracks in this case include a crack that may damage an element chip disposed near the predetermined cutting line C.

Furthermore, in the process of contemporaneously forming the surface-processing trace11aand the internal processing region the beam-condensing point depth for forming the internal processing region closest to the substrate surface should be determined carefully, so as to reduce the occurrence of the surface-processing trace and the internal crack region from being connected with each other via the crack, (i.e., the internal crack region) of the substrate.

Cutting Process S4

According to at least one exemplary embodiment, in the silicon substrate10in which the surface-processing trace11aand a plurality of internal cracks12ato12fare formed along the predetermined cutting line C, the surface-processing trace11ais not connected with the internal crack12fpositioned beneath the substrate surface11. Therefore, at the time immediately after the laser processing operation is accomplished, the logic element portions10aof the silicon substrate10are not cut yet. Hence, cutting the silicon substrate10into a plurality of element chips is carried out next in the following manner.

For example, as shown inFIG. 11A, after the formation of the surface-processing trace11aand the internal crack12(e.g.,12a,12b,12c) is accomplished, the silicon substrate10is placed upside down on an elastic rubber sheet60of the cutting apparatus while the silicon substrate10is mounted on the dicing tape T. The elastic rubber sheet60is, for example, a silicon rubber or a fluorocarbon rubber. Next, the operation for cutting the silicon substrate10into element chips is carried out by applying a pressing force, from the back surface, to the silicon substrate10via the dicing tape T with a stainless roller61or other pressure device as know by one of ordinary skill in the relevant art and equivalents.

More specifically, as shown inFIG. 11B, the silicon substrate10is mounted on the rubber sheet60together with a protecting tape R that is bonded to the front surface of the substrate surface11to protect the substrate surface11, in such a manner that one predetermined cutting line C, for example the above-described first cutting direction, becomes parallel to a shaft of the stainless roller61. Next, a pressing force P is applied to the silicon substrate10with the roller61being rotated. The rubber sheet60positioned beneath the roller61is then deformed downward. The silicon substrate10receives an expansion force E acting along its front surface, e.g., the surface closer to the rubber sheet60. This expansion force E concentrates on a weakest portion of the substrate surface11, e.g., the surface-processing trace11aformed along the predetermined cutting line C1.

Accordingly, the groove of the surface-processing trace11ais forcibly widened in its width direction by the expansion force E applied by the roller61. Thus, the growth of a crack starts from the surface-processing trace11aon the front surface and propagates toward the back surface by successively connecting the internal cracks12f,12e,12d,12c,12b, and12aformed by the laser irradiation applied in the substrate. When the crack reaches the back surface, the silicon substrate10is cut into two pieces along the predetermined cutting line C1. In general, the growth of a crack occurs along a crystal orientation of the silicon substrate10. However, the above cutting operation is triggered by the surface-processing trace11a. Thus, the formed crack does not deviate appreciably from the predetermined cutting line C1on the substrate surface11.

According to the processing example A, there was appreciably no positional difference between the surface-processing trace11aand the internal crack12fin the direction parallel to the substrate surface. Therefore, in the cutting operation, the surface-processing trace11abecame a start point or one end of a crack that can be surely connected to the closest internal crack12fpositioned beneath the surface-processing trace11a. Furthermore, the connection between the internal crack12fand the internal crack12eand the connection between the internal crack12eand the internal crack12dsuccessively occurred.

As a result, the crack growth was approximately linearly propagated via the internal cracks sequentially disposed in the depth direction toward the back surface. Finally, a linear crack was formed from the internal crack12ato the back surface. As a result, the cut surface of the substrate was formed to extend along the predetermined cutting line substantially in parallel with the incident optical axis of the laser beam relative to the substrate. Furthermore, when the incident angle of the laser beam was substantially perpendicular to the substrate surface, the obtained cut surface was substantially perpendicular to the substrate surface.

Furthermore, on the substrate surface, the crack did not deviate appreciably from the surface-processing trace11a.

After the cutting operation is accomplished in the first cutting direction, the silicon substrate10is rotated by 90 degrees. Then, like the cutting operation performed along the first cutting direction, a pressing force is applied to the silicon substrate10with the roller61being rotated, thereby producing a crack which grows in the second cutting direction from the surface-processing trace11aand reaching the back surface of the silicon substrate10.

Through the above processes, the silicon substrate10is split into a plurality of element chips.

Repair Process S5

The crack formed in the above-described cutting process can connect the surface-processing trace11awith the internal crack12and can also grow inward to reach the back surface, thereby surely separating the silicon substrate10into individual element chips. However, if there is a portion that is not completely separated, one can perform the cutting operation again. As a method for performing such an additional cutting operation, it is possible to use a mechanism including, for example, a suction collet65and a pickup pin66as shown inFIGS. 12A and 12B. According to this mechanism, a pressing force is selectively applied to individual logic element portions10athat are not split yet, thereby completely accomplishing the cutting operation.

Pickup Process S6

The logic element portion10athat has been split as an independent element chip through the above-described cutting process S4and the repair process S5is then conveyed by the suction collet65and the pickup pin66as shown inFIGS. 12A and 12Band is separately stored. In this case, an expander can be used to widen the clearance between the element portions10abefore picking up them, so that the storage of each element portion10acan be surely carried out. Furthermore, sucking and removing the fine dusts generated during the pickup operation can be performed to minimize contamination that can result in the malfunction of circuits formed on each element portion10a.

According to the above-described exemplary embodiment, by appropriately setting the laser beam condensing point positioned in a workpiece, a recessed portion can be simultaneously formed on the surface of the workpiece in the process of forming an internal processing region inside the workpiece. Thus, at least one exemplary embodiment requires no independent process for forming such a recessed portion on the workpiece.

Furthermore, the contemporaneous formation of a recess portion and an internal processing region, can result in there not being a substantial horizontal positional difference between the two.

Furthermore, by connecting the internal processing region and the recessed portion with a crack growing between them, a reliable cutting operation can be performed which is configured to reduce a cut surface from deviating from a predetermined cutting line.

This application claims priority from Japanese Patent Application No. 2004-342246 filed Nov. 26, 2004, which is hereby incorporated by reference herein in its entirety.