Crack propagation methods

Abstract

To provide a wafer processing device and wafer processing method that can efficiently obtain a chip of stable quality.SOLUTION: A method for developing a crack inside a wafer is for performing a modified layer forming process with laser and for performing a grinding process from a backside to a target surface by combining rough grinding, fine grinding, and chemical mechanical polishing to remove a modified layer. In the rough grinding, the backside of the wafer is ground while a water supply is stopped, a wafer crack is developed due to thermal expansion, and the wafer is attached to expand tape, and the wafer is separated by a stretching process.SELECTED DRAWING: Figure 9

Description

Technical Field

[0001] The present invention relates to a technique for dicing a wafer having a laser modified region inside the wafer.

Background Art

[0002] In Patent Document 1, a semiconductor substrate placed with its back side facing upward is irradiated with a laser to form a modified region inside the substrate, an expandable tape is attached to the back side of the semiconductor substrate, and a knife edge is applied from above the expandable tape to cut the substrate based on the modified region, thereby cutting the semiconductor substrate into chips.

[0003] Also, in Patent Document 1, after irradiating a semiconductor substrate placed with its back side facing upward with a laser to form a modified region inside the substrate, the substrate is ground to make it thinner, an expandable tape is attached to the back side of the semiconductor substrate, and the substrate is cut based on the modified region by stretching the expandable tape.

[0004] In Patent Document 2, a semiconductor substrate placed with its back side facing upward is irradiated with a laser to form a modified region inside the substrate, thereby generating cracks in the thickness direction of the semiconductor substrate, and the back side of the substrate is ground and chemically etched to expose the cracks on the back side, thereby cutting the semiconductor substrate into chips. And in Patent Document 2, it is described that cracks occur in the thickness direction from the modified region naturally or by applying a relatively small force, for example, by generating thermal stress by applying an artificial force or a temperature difference to the substrate.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

[0006] In the invention described in Patent Document 1, the substrate is cracked by applying an external force locally using a knife edge. However, in order to apply this external force locally, bending stress and shear stress are applied to the substrate. However, it is difficult to distribute bending stress and shear stress uniformly across the entire surface of the substrate. For example, when bending stress or shear stress is applied to a substrate, the stress will concentrate at some weak point, making it impossible to efficiently and uniformly apply the minimum necessary stress to the desired area.

[0007] Therefore, if there is variation in the cracking of the substrate and the crack does not progress slowly, there is a problem that the substrate may break even within the chip. In addition, when cutting the substrate by applying stress locally and sequentially to the parts to be cut, for example when collecting many chips from a single substrate, there are many cutting lines, which leads to a problem of significantly reduced productivity.

[0008] Furthermore, when applying external force to break the substrate, if the substrate is not thinned, there is a problem in that a very large stress is required to break the wafer.

[0009] In particular, when the substrate thickness is sufficiently thick relative to the modification depth of the laser processing, applying external force may not result in a clean, perpendicular cut to the substrate due to the sudden effect of the external force. Therefore, it may be necessary to irradiate the substrate with several laser pulses in multiple stages in the thickness direction of the substrate.

[0010] Furthermore, in the inventions described in Patent Documents 1 and 2, the modified region formed inside the substrate by laser irradiation ultimately remains on the chip cross-section. As a result, dust may be generated from the modified region on the chip cross-section. In addition, if the chip cross-section is locally fractured, the fractured surface may trigger chip breakage. Consequently, there is a problem in that the flexural strength of the chip is reduced.

[0011] The invention described in Patent Document 2 states that cracks spontaneously occur in the thickness direction from the modified region, but on the other hand, there are cases where cracking does not occur spontaneously. In order to reliably obtain the stable effect of cracking, it is sometimes necessary to take arbitrary measures, and spontaneous cracking does not fall under the category of arbitrary measures.

[0012] Another possibility is to generate thermal stress by applying a temperature difference as a relatively small force, causing cracks to form in the thickness direction from the modified region. In this case, however, there is a significant problem in how to apply a uniform thermal gradient across the surface of the substrate. That is, even if a thermal gradient is artificially applied, heat conduction will disperse the heat within the substrate, partially mitigating the thermal gradient. Therefore, there is an extremely difficult problem in how to constantly form a stable thermal gradient (stable temperature difference) sufficient to cut a certain portion of the substrate.

[0013] Furthermore, in the invention described in Patent Document 2, after grinding the semiconductor substrate, the back surface is chemically etched. However, after grinding, grinding marks remain on the surface due to the fixed abrasive grains, and in addition, minute cracks are formed, leaving a processed and altered layer. When this surface is chemically etched, areas with large lattice distortions, such as minute cracks, are selectively etched. As a result, minute cracks are actually amplified and become larger cracks. Therefore, fracture may occur not only in the cutting initiation region but sometimes also from minute cracks formed by grinding and etching, making stable cutting difficult.

[0014] Furthermore, etching exacerbates surface irregularities on the substrate, resulting in a non-mirror-like surface. Consequently, irregularities remain on the divided chips, making it quite possible for them to break from areas with larger irregularities, i.e., microcracks, thus reducing the flexural strength of the chips.

[0015] Furthermore, when an etching solution acts on a laser-processed and modified area, large grain boundaries are formed because the modified area has melted, recrystallized, and solidified.

[0016] When an etching solution acts on these grain boundaries, the etching process proceeds in such a way that silicon grains peel off from the grain boundaries, further exacerbating the unevenness of the surface.

[0017] The polishing process is specifically described in the cited reference, p. 12, line 1, as consisting of grinding and chemical etching on the back surface. In the case of chemical etching, even if left untreated, the chemical etching solution penetrates into the cracks and dissolves the cracked areas.

[0018] In particular, if the crack extends to the surface of the substrate, the etching solution can penetrate between the chip and the film that adheres the chip, causing the chip to detach from the film.

[0019] Furthermore, etching can proceed along cracks even if they do not extend to the surface of the substrate. In particular, when etching is performed on a thinned wafer after grinding, the etching solution penetrates the cracks by capillary action, dissolving the crack tips and deepening the microcracks, while new etching solution enters them. In this case, if the etching solution acts near the device surface, it may dissolve the area near the device surface, and etching may progress into the interior of the element. Also, even if there are no problems at that time, etching solution remaining on the wafer wall may enter the interior of the device and cause defects later. Therefore, while processing distortion on the wafer surface is removed, it may exacerbate cracks and induce associated secondary problems.

[0020] In such cases, as a result, cracks may run ahead to the surface, allowing the etching solution to penetrate between some of the chips and the film, and there may be a problem that the chips peel off during processing. In particular, when the grinding is carried out and the substrate becomes thinner, cracks tend to progress with a little external force, and when the cracks reach the surface, chip peeling occurs. Therefore, when the substrate becomes thinner, it is necessary to perform a process so that the cracks do not progress further.

[0021] Patent Document 2 describes that cracks occur from the modified region by grinding the back surface of the substrate, but Patent Document 2 does not describe the method of fixing the substrate when grinding the substrate. As shown in FIG. 17, when the outer periphery of the substrate is supported by fitting the substrate into a retainer or the like, or as shown in FIG. 18, when only a part of the substrate is adsorbed, the substrate is not uniformly constrained over the entire surface. Therefore, even if the back surface of the substrate is ground in such a case, cracks do not occur from the modified region.

[0022] The present invention has been made in view of such circumstances, and an object thereof is to provide a wafer processing apparatus and a wafer processing method capable of efficiently obtaining chips of stable quality.

Means for Solving the Problems

[0023] A wafer processing apparatus for achieving the object of the present invention is a wafer processing apparatus for processing a wafer in which a modified region is formed by laser light inside. The apparatus includes a grinding unit that grinds the back surface of the wafer with a grinding wheel while adsorbing the surface of the wafer, and removes the modified region leaving minute cracks extending from the modified region, and a polishing unit that polishes the back surface of the wafer with a polishing cloth after grinding.

[0024] According to this apparatus, due to the grinding heat generated by grinding, it tends to thermally expand and spread in the radial direction together with the surface of the wafer being ground. However, on the other hand, the wafer tries to prevent the spread due to the expansion by the wafer vacuum chuck having a large heat capacity.

[0025] As a result, while the back surface of the wafer expands due to thermal expansion, the surface of the chucked wafer does not expand due to vacuum chucking and attempts to maintain its original state. As a result, the modified region formed inside the wafer acts to further expand the modified region according to the difference in expansion between the front and back surfaces of the wafer, and further cracks progress. Since the modified region includes portions that have been irradiated with laser light and once melted and recrystallized, the crystal grains are large and brittle. When such a modified region appears on the side surface of a future chip, dust is generated from the side surface of the chip, and large crystal grains may break off from the side surface of the chip. However, since the micro crack portion that has progressed from the modified region has a pure crystal plane, even if this plane appears on the side surface of a future chip, there is no dust generation from the side surface of the chip or breakage as large crystal grains.

[0026] In this way, while removing the wafer by grinding in the grinding process, micro voids are advanced in the thickness direction of the wafer to remove the modified region. Next, in order to make the processed damaged layer formed by grinding and the advanced micro voids prominent, chemical mechanical polishing is performed on the entire surface of the wafer (described in detail later), and the processed damaged layer is completely removed. As a result, only the micro voids remain on the surface, and the remaining region becomes a perfect mirror surface without remaining processing strain.

[0027] Thereafter, the unbroken wafer is placed on a mechanism for dicing the wafer, bending stress is applied to the wafer to dice the wafer, and then the expand film is pulled to separate the chips from each other. Thereby, it is possible to prevent the modified region formed by laser light from remaining on the cross section of the chip. Therefore, it is possible to prevent problems such as the chip breaking or dust generation from the chip cross section, and to efficiently obtain chips of stable quality.

[0028] In addition, since the back surface of the ground wafer is chemically mechanically polished and then the wafer is divided, the flexural strength of the chip can be increased. Here, chemical mechanical polishing is significantly different from the etching process in the polishing process shown in Reference 2.

[0029] First, the chemical solution used in this application is different from the etching solution in Reference Document 2. The etching solution in Reference Document 2 has the effect of naturally dissolving the substrate, i.e., etching, when the liquid acts on the substrate surface. As a result, even in areas where cracks have occurred, the etching solution penetrates and naturally etches the surrounding area, thus further enlarging the modified layer formed by the laser. Furthermore, as mentioned earlier, with conventional etching solutions, the etching solution penetrates too far into the cracks, causing the cracks to propagate and eventually penetrate into the area between the chip and the film. As a result, the etching solution can reach the chip surface, eroding the chip surface and potentially causing the individual chip devices to cease functioning.

[0030] In contrast, the abrasive (slurry) used in the chemical mechanical polishing of this invention does not have an etching effect under static conditions. That is, even if only the abrasive is supplied to the wafer, etching does not proceed at all, and it simply modifies the wafer surface. Therefore, even if the abrasive enters a crack in the wafer at the cutting line, it will not dissolve the surrounding silicon, and thus the crack will not propagate further.

[0031] Consequently, while cracks may propagate in the polishing process, i.e., etching process, described in Reference 2, in the polishing process of the present invention, the wafer is not etched at all even when the polishing agent is added and left to stand, so cracks hardly propagate. As a result, conventional problems such as the polishing agent penetrating and spontaneously detaching the chip from the film, or the polishing agent getting around to the chip surface and corroding the chip, causing the chip device to malfunction, do not occur.

[0032] The mechanism of chemical mechanical polishing is as follows: In polishing, an abrasive is first supplied to the wafer, which chemically modifies the wafer surface. Next, because the modified surface is softened, the abrasive particles contained in the slurry act on the wafer surface, allowing for efficient removal of the wafer surface even with very small stresses.

[0033] For example, in the process of removing silicon, a silica-based slurry is used. When a silica-based slurry is used, the following reaction occurs on the wafer surface.

[0034]

number

[0035] In other words, the wafer surface immediately after polishing typically contains Si atoms. These Si atoms are hydrated on the surface in water, and -OH groups are present on the Si atom surface. These OH groups attached to the Si substrate surface combine with the SiOH present in the silica sol in the liquid and the SiOH present on the surface of the silica particles.

[0036] However, etching does not proceed with this alone. Consequently, in this state, an abrasive pad containing a large amount of silica particles is moved relative to the substrate, and by acting on the substrate surface with a certain amount of momentum, the silica particles are mechanically removed while a chemical reaction proceeds under a certain temperature and pressure environment.

[0037] In this case, the silica sol or silica particles that have penetrated the crack do not have the effect of further propagating the crack. This is because the polishing pad does not enter the crack, so no mechanical action is performed, and as a result, no chemical removal action occurs.

[0038] As a result, the processed and altered layer on the substrate surface generated during grinding is removed by chemically and mechanically polishing only the substrate surface, due to the supply of a chemical slurry containing silica sol and silica particles, combined with the mechanical friction caused by the polishing pad contacting the substrate (Toshiro Doi, ed.: Detailed Explanation of Semiconductor CMP Technology (Kogyo Chosakai) (2001) pp. 40-42).

[0039] As a result, most of the processed and altered layer on the wafer surface is removed. Furthermore, the resulting wafer surface becomes mirror-like. The principle behind this mirror-like surface is as follows, when compared to the chemical etching described in Reference 2.

[0040] In the case of chemical etching, as mentioned earlier, lattice strains and crystal defects in crystals such as silicon are selectively etched. As a result, etch pits are formed, and large gouges occur along the grain boundaries, and these are, in principle, unavoidable.

[0041] In contrast, in the case of chemical mechanical polishing, as mentioned earlier, the chemical removal action does not occur unless a mechanical action is performed. The mechanical action, in this case, is rubbing the wafer surface with a polishing pad, and this mechanical action occurs with equal probability on all substrate surfaces, completely independent of the crystalline state of the substrate surface. Therefore, as the chemical action takes place while all surfaces are uniformly removed regardless of the crystalline state, the surface becomes uniform, free of crystalline defects, and mirror-like. Here, since the term "mirror-like surface" is ambiguous from a relative perspective, in this application, it is defined as a mirror-like surface obtained by chemical mechanical polishing.

[0042] By performing this type of chemical mechanical polishing, unlike in reference 2, it is possible to obtain an ideal mirror-like surface and prevent erosion caused by the penetration of etching solution between the chip and the film.

[0043] Furthermore, even after chemical mechanical polishing, the chip is not yet completely broken. Only minute voids that have progressed from the modified layer remain on the surface after some chemical mechanical polishing. Note that the modified layer had already been removed in the previous grinding process.

[0044] If the surface, which retains the micro-voids that have developed from this modified layer, is formed in a state similar to that of a surface subjected to grinding and etching as shown in Reference 2, it becomes impossible to clearly distinguish between the micro-voids that developed from the modified layer and the irregularities enhanced by the etching process after grinding. Therefore, when the chip breaks, it is not always the case that it breaks from the micro-voids, but in some cases, it may break from the irregularities that have been further enhanced by etching after grinding.

[0045] As in this invention, when chemical mechanical polishing is performed on the surface, the wafer surface after treatment will have almost no irregularities, and only minute voids that have propagated from the modified layer will remain. Therefore, when a fracture treatment is performed in this state, cracks will propagate further from the minute voids and cause fracture.

[0046] Furthermore, a wafer processing method for achieving the objectives of the present invention is a wafer processing method for processing a wafer in which a modified region has been formed inside with laser light, comprising: a grinding step in which the back surface of the wafer is ground with a grinding wheel while the surface of the wafer is adsorbed, and the modified region is removed while leaving minute cracks extending from the modified region; and a polishing step in which the back surface of the wafer is polished with an abrasive cloth after grinding. [Effects of the Invention]

[0047] According to the present invention, chips of stable quality can be obtained efficiently. [Brief explanation of the drawing]

[0048] [Figure 1] A diagram showing the overall configuration of the laser dicing apparatus 1. [Figure 2] A conceptual diagram showing the configuration of the driving means of the laser dicing apparatus 1. [Figure 3] A plan view showing the configuration of the drive mechanism of the laser dicing apparatus 1. [Figure 4] A diagram showing the overall configuration of grinding device 2. [Figure 5] A close-up view of part of the grinding device 2. [Figure 6] A close-up view of part of the grinding device 2. [Figure 7] A schematic diagram of the polishing stage of grinding device 2. [Figure 8] This diagram shows the details of the chuck of grinding device 2, with (a) being a plan view, (b) a cross-sectional view, and (c) a partial enlarged view. [Figure 9] A flowchart showing the processing flow for cutting semiconductor substrates. [Figure 10] A diagram illustrating the laser modification process. [Figure 11] A diagram illustrating the cutting line. [Figure 12] A diagram illustrating the grinding and decomposition process. [Figure 13] This diagram illustrates crack propagation during the grinding and removal process. (a) is a schematic diagram during grinding, (b) shows the condition of the back surface of wafer W, (c) shows the condition of the front surface of wafer W, and (d) shows a cross-sectional view of wafer W. [Figure 14] A diagram illustrating the surface state of the back surface of wafer W after the grinding and degrafting process. [Figure 15] A diagram illustrating the division and separation process. [Figure 16] A diagram illustrating the division and separation process. [Figure 17] A diagram showing the conventional fixing method used during substrate grinding. [Figure 18] A diagram showing the conventional fixing method used during substrate grinding. [Figure 19] A diagram illustrating the conditions for evaluating crack propagation. [Figure 20] A figure showing the evaluation results of crack propagation assessment. [Figure 21] A diagram showing a schematic of the splitting device 300. [Figure 22] An explanatory diagram showing another embodiment of the chuck bending mechanism. [Modes for carrying out the invention]

[0049] Preferred embodiments of the present invention will be described in detail below with reference to the attached drawings.

[0050] The present invention is carried out by a cutting apparatus comprising a laser dicing apparatus 1, a grinding apparatus 2, a transport apparatus (not shown) for transporting the wafer processed by the laser dicing apparatus 1 to the grinding apparatus 2, and a dividing apparatus for dividing the wafer ground by the grinding apparatus 2 into chips.

[0051] <About the device configuration> (1) Regarding the laser dicing device 1 Figure 1 shows an overview of the laser dicing apparatus 1. As shown in the figure, the laser dicing apparatus 1 of this embodiment mainly consists of a wafer moving unit 11, a laser head 40 consisting of a laser optics unit 20 and an observation optics unit 30, a control unit 50, and the like.

[0052] The wafer moving unit 11 consists of a suction stage 13 that holds the wafer W by adsorption, and an XYZθ table 12 provided on the main body base 16 of the laser dicing apparatus 1, which precisely moves the suction stage 13 in the XYZθ direction. The wafer W is precisely moved in the XYZθ direction shown in the figure by this wafer moving unit 11.

[0053] A wafer W has a BG tape B with adhesive material attached to one side of its surface, and is placed on the suction stage 13 with the back side facing upwards.

[0054] Alternatively, the wafer W may be placed on the suction stage 13 with a dicing sheet having an adhesive material attached to one side, and the wafer W integrated with the frame via this dicing sheet. In this case, the wafer is placed on the suction stage 13 with the surface facing upwards.

[0055] The laser optics unit 20 consists of a laser oscillator 21, a collimating lens 22, a half mirror 23, a condensing lens (focusing lens) 24, and a driving means 25 for moving the laser beam in minute parallel directions relative to the wafer W. The laser beam emitted from the laser oscillator 21 is focused into the wafer W through the optical system consisting of the collimating lens 22, the half mirror 23, and the condensing lens 24. The Z-direction position of the focusing point is adjusted by the Z-direction fine movement of the condensing lens 24 by the Z-fine movement means 27 described later.

[0056] The laser light conditions are as follows: the light source is a semiconductor laser-pumped Nd:YAG laser, the wavelength is 1064 nm, and the laser light spot cross-section is 3.14 × 10⁻⁶. -8 cm 2 The oscillation mode is a Q-switched pulse, the repetition frequency is 100 kHz, the pulse width is 30 ns, the output is 20 μJ / pulse, the laser light quality is TEM00, and the polarization characteristic is linear polarization. The conditions for the condenser lens 24 are a magnification of 50x, an NA of 0.55, and a transmittance of 60 percent relative to the laser light wavelength.

[0057] The observation optics unit 30 consists of an observation light source 31, a collimating lens 32, a half mirror 33, a condensing lens 34, a CCD camera 35 as an observation means, an image processing unit 38, a television monitor 36, and the like.

[0058] In the observation optics unit 30, illumination light emitted from the observation light source 31 passes through an optical system including a collimating lens 32, a half mirror 33, and a condensing lens 24 to irradiate the surface of the wafer W. The reflected light from the surface of the wafer W passes through the condensing lens 24, half mirrors 23 and 33, and condensing lens 34 before entering the CCD camera 35, which serves as an observation means, and an image of the wafer W's surface is captured.

[0059] This imaging data is input to the image processing unit 38 and used for aligning the wafer W, and is also displayed on the television monitor 36 via the control unit 50.

[0060] The control unit 50 consists of a CPU, memory, input / output circuit section, etc., and controls the operation of each part of the laser dicing apparatus 1.

[0061] The laser dicing apparatus 1 consists of a wafer cassette elevator (not shown), wafer transport means, control panel, and indicator lights, etc.

[0062] The wafer cassette elevator moves the cassette containing the wafers up and down to position it in the transport location. The transport means transports the wafers between the cassette and the suction stage 13.

[0063] The control panel is equipped with switches and display devices for operating various parts of the dicing device 10. Indicator lights show the operating status of the dicing device 10, such as processing in progress, processing completed, and emergency stop.

[0064] Figure 2 is a conceptual diagram illustrating the details of the driving means 25. The driving means 25 consists of a lens frame 26 that holds the condensing lens 24, a Z-axis fine-adjustment means 27 attached to the upper surface of the lens frame 26 that moves the lens frame 26 in the Z direction in the figure, a holding frame 28 that holds the Z-axis fine-adjustment means 27, and linear fine-adjustment means PZ1, PZ2, etc. that move the holding frame 28 in parallel with the wafer W.

[0065] The Z-axis fine adjustment mechanism 27 uses a piezoelectric element that expands and contracts when a voltage is applied. The expansion and contraction of this piezoelectric element causes the condensing lens 24 to be moved slightly in the Z direction, thereby precisely positioning the Z-axis position of the laser beam's focal point.

[0066] The retaining frame 28 is supported by two pairs of parallel springs made of four piano wires (not shown), and is freely movable in the XY direction, while its movement in the Z direction is restricted. However, the method of supporting the retaining frame 28 is not limited to this; for example, it may be supported by being sandwiched from above and below by multiple balls, which restricts movement in the Z direction while allowing movement in the XY direction.

[0067] The linear fine-adjustment means PZ1 and PZ2, like the Z-fine-adjustment means 27, use piezoelectric elements, with one end fixed to the case body of the laser head 40 and the other end in contact with the side surface of the holding frame 28.

[0068] Figure 3 is a plan view of the drive mechanism 25. As shown in Figure 3, two linear fine-adjustment mechanisms PZ1 and PZ2 are arranged in the X direction, with one end of each fixed to the case body of the laser head 40 and the other end in contact with the side surface of the holding frame 28. Therefore, by controlling the applied voltage, the condenser lens 24 can be reciprocated in the X direction, and the laser beam can be reciprocated or vibrated in the X direction.

[0069] Furthermore, a piezoelectric element may be used in either the linear fine-adjustment means PZ1 or PZ2, while the other is made of an elastic material such as a spring. Alternatively, three linear fine-adjustment means may be arranged around the circumference.

[0070] Laser light L is emitted from the laser oscillator 21 and irradiates the inside of the wafer W via an optical system including a collimating lens 22, a half mirror 23, and a condensing lens 24. The Z-direction position of the focal point of the irradiated laser light L is precisely set to a predetermined position inside the wafer by adjusting the Z-direction position of the wafer W using the XYZθ table 12 and controlling the position of the condensing lens 24 using the Z-fine adjustment means 27.

[0071] In this state, the XYZθ table 12 is fed in the X direction, which is the dicing direction, and the condensing lens 24 is moved back and forth by the linear micro-movement means PZ1 and PZ2 provided on the laser head 40, causing the laser beam L to vibrate parallel to the wafer W in the X direction or in any XY direction, and the focal point of the laser beam L vibrates minutely inside the wafer, forming a modified region P. As a result, a line of modified regions P due to multiphoton absorption is formed inside the wafer W along the cutting line of the wafer W.

[0072] Furthermore, if necessary, vibration in the Z direction may be added by the Z fine-adjustment means 27. Alternatively, the wafer W may be moved in the X direction while the laser beam L is slowly moved back and forth in the X direction, which is the processing direction, so that the laser beam L is repeatedly irradiated in a back-and-forth manner like a perforation.

[0073] When one modified region is formed along the cutting line, the XYZθ table 12 is indexed and advanced by one pitch in the Y direction, and a modified region is formed in the next line in the same manner.

[0074] Once modified regions are formed along all cutting lines parallel to the X direction, the XYZθ table 12 is rotated by 90°, and modified regions are formed along all lines perpendicular to the previous lines in the same manner.

[0075] (2) Grinding device 2 Figure 4 is a perspective view showing the overall configuration of the grinding apparatus 2. The main body 112 of the grinding apparatus 2 is equipped with an alignment stage 116, a rough grinding stage 118, a fine grinding stage 120, a polishing stage 122, abrasive cloth washing stage 123, abrasive cloth dressing stage 127, and a wafer washing stage 124.

[0076] The rough grinding stage 118, the fine grinding stage 120, and the polishing stage 122 are separated by a partition plate 125 (omitted in Figure 4), as shown in Figure 5, to prevent the processing fluid used in each of the stages 118, 120, and 122 from splashing onto adjacent stages.

[0077] As shown in Figure 5, the partition plate 125 is fixed to the index table 134 and is formed in a cross shape to partition the four chucks 132, 136, 138, and 140 installed on the index table 134.

[0078] The rough grinding stage 118 is a stage for rough grinding and, as shown in Figure 5, is enclosed by the side of the main body 112, the top plate 128, and the partition plate 125. The fine grinding stage 120 is a stage for fine grinding and, similar to the rough grinding stage 118, is enclosed by the side of the main body 112, the top plate 129, and the partition plate 125. Brushes (not shown) are provided on the top and side surfaces of the partition plate 125 to isolate the rough grinding stage 118 and the fine grinding stage 120 from the outside. In addition, through holes 128A and 129A are formed in the top plates 128 and 129 through which the heads of each stage are inserted.

[0079] The polishing stage 122 performs chemical mechanical polishing and is covered by a casing 126 having a top plate 126A, as shown in Figure 5, to isolate it from the other stages. The top plate 126A has through holes 126C through which the heads of each stage are inserted.

[0080] As shown in Figure 6, a brush 126B is attached to the side of the casing 126 through which the partition plate 125 passes. This brush 126B comes into contact with the upper surface 125A and side surface 125B of the partition plate 125 when the chuck 140 is in the machining position. As a result, when the chuck 140 is in the machining position, it is held in a nearly airtight state by the casing 126, partition plate 125, and brush 126B.

[0081] Since the polishing stage 122 performs chemical mechanical polishing, the polishing fluid contains a chemical abrasive. If grinding fluid is mixed with this polishing fluid, the concentration of the chemical abrasive decreases, resulting in a longer processing time. By keeping the polishing stage 122 in a nearly airtight state, it is possible to prevent the grinding fluid and processing debris used in the precision grinding stage 120 from entering the polishing stage 122, and also to prevent the polishing fluid used in the polishing stage 122 from splashing out of the polishing stage 122. Therefore, processing defects caused by the mixing of both processing fluids can be prevented.

[0082] Figure 7 is a structural diagram of the polishing stage 122. In the polishing stage 122, polishing is performed by the polishing cloth 516 and the slurry supplied from the polishing cloth 156, and the processed alteration layer that has formed on the back surface of the wafer W is removed by rough polishing and fine polishing. The processed alteration layer is a general term for streaks and processing distortions (crystal alteration) caused by grinding.

[0083] The abrasive cloth 156 of the polishing stage 122 is attached to a polishing head 161 connected to the output shaft 160 of the motor 158. Guide blocks 162, 162 constituting a linear motion guide are provided on the side of the motor 158, and the guide blocks 162, 162 are engaged with a guide rail 166 provided on the side of the support plate 164 so as to be able to move up and down. Therefore, the abrasive cloth 156 is attached to the support plate 164 together with the motor 158 so as to be able to move up and down.

[0084] The support plate 164 is provided at the tip of the horizontally positioned arm 168. The base end of the arm 168 is connected to the output shaft 174 of the motor 172 located inside the casing 170. Therefore, when the motor 172 is driven, the arm 168 can rotate around the output shaft 174. This allows the abrasive cloth 56 to be moved within the range of the polishing position (see solid line in Figure 3), the abrasive cloth cleaning position by the abrasive cloth cleaning stage 123 (see dashed line in Figure 3), and the dressing position by the abrasive cloth dressing stage 127. When the abrasive cloth 156 is moved to the abrasive cloth cleaning position, its surface is cleaned by the abrasive cloth cleaning stage 123 to remove any abrasive debris adhering to the surface. Examples of abrasive cloth 156 include foamed polyurethane and abrasive cloth, and the abrasive cloth cleaning stage 123 is provided with a removal member such as a brush to remove abrasive debris. This removal member is rotationally driven when the abrasive cloth 156 is cleaned, and the abrasive cloth 156 is similarly rotationally driven by the motor 158. The abrasive cloth dressing stage 127 uses the same material as the abrasive cloth 156, for example, foamed polyurethane.

[0085] Guide blocks 176, 176 constituting linear guides are provided on the side of the casing 170, and these guide blocks 176, 176 are engaged with a guide rail 180 provided on the side of the housing 178 for the screw feed device so as to be able to move up and down. A nut member 179 is also provided protruding from the side of the casing 170. The nut member 179 is screwed onto a threaded rod 181 of the screw feed device disposed inside the housing 178 through an opening (not shown) formed in the housing 178. The output shaft 184 of the motor 182 is connected to the upper end of the threaded rod 181. Therefore, when the motor 82 is driven and the threaded rod 181 is rotated, the casing 70 moves up and down due to the feeding action of the screw feed device and the linear action of the guide blocks 176 and the guide rail 180. As a result, the abrasive cloth 156 is moved a large amount in the vertical direction, and the distance between the abrasive head 161 and the wafer W is set to a predetermined distance.

[0086] The piston 188 of the air cylinder device 186 is connected to the upper surface of the motor 158 via a through hole 169 in the arm 168. A regulator 190 that controls the internal pressure P of the cylinder is also connected to the air cylinder device 186. Therefore, by controlling the internal pressure P with this regulator 190, the pressing force (contact force) of the polishing cloth 156 against the wafer W can be controlled.

[0087] In this embodiment, abrasive cloth 156 was used as the abrasive body, but the method is not limited to this. If it is possible to remove the processed and altered layer, for example, a grinding wheel or electrophoresis of abrasive grains may be used. When using a grinding wheel or electrophoresis of abrasive grains, quantitative polishing is preferable.

[0088] Returning to the explanation of Figure 4, the alignment stage 116 is a stage that aligns the wafer W, which has been transported from the laser dicing apparatus 1 by a transport device (not shown), to a predetermined position. The wafer W, which has been aligned on this alignment stage 116, is then held by a transport robot (not shown) and transported toward an empty chuck 132, where it is held by the suction surface of the chuck 132.

[0089] Chuck 132 is mounted on the index table 134, and chucks 136, 138, and 140, which have the same function, are mounted at 90-degree intervals on the circumference of the index table 134 centered on the rotation axis 135. A motor spindle (not shown) is connected to the rotation axis 135.

[0090] Chuck 136 is located at the rough grinding stage 118 in Figure 4, where the adsorbed wafer W is roughly ground. Chuck 138 is located at the fine grinding stage 120 in Figure 4, where the adsorbed wafer W is finish ground (fine grinding, spark-out) here. Chuck 140 is located at the polishing stage 122 in Figure 4, where the adsorbed wafer W is polished, removing the processed altered layer and variations in wafer W thickness caused by grinding.

[0091] Here, we will explain chucks 132, 136, 138, and 140. Since chucks 136, 138, and 140 have the same configuration as chuck 132, we will explain chuck 132, and omit the explanation for chucks 136, 138, and 140.

[0092] Figure 8 shows details of the chuck 132, where (a) is a plan view of the chuck 132, (b) is a cross-sectional view of section A-A' in (a), and (c) is an enlarged view of section B in (b).

[0093] The chuck 132 is constructed by fitting a mounting base 132a, made of a porous material (e.g., porous ceramics), into a chuck body 132b made of a dense material. Adsorption holes 132c are formed on the underside of the chuck body 132b into which the mounting base 132a is fitted, for vacuum adsorption. It is desirable that the chuck 132 be made of a material with low thermal conductivity.

[0094] As shown in Figure 8(c), the wafer W is placed on the mounting table 132a via the BG tape B. As shown in Figure 8(c), the mounting table 132a is formed such that when the wafer W is placed on the mounting table 132a, a portion of the outer circumference of the wafer W extends beyond the mounting table 132a, with a width x of approximately 1.5 mm. The wafer W used in this embodiment has a diameter of approximately 12 inches and a thickness t of approximately 775 μm.

[0095] As shown in Figures 2(a) and (b), the suction holes 132c are arranged to cover almost the entire surface of the mounting base 132a. A fluid coupling (not shown) is connected to the suction holes 132c, and a suction pump (not shown) connected to this fluid coupling draws in air. As a result, almost the entire surface of the wafer W is firmly vacuum-adhered to the surface of the mounting base 132a. This allows the wafer W and the mounting base 132a to be in close contact across their surface without any displacement.

[0096] As shown in Figure 7, chucks 132, 136, 138, and 140 are each connected to a spindle 194 and a motor 192 on their undersides, and are rotated by the driving force of these motors 192. The motors 192 are supported by the index table 134 via a support member 193. This eliminates the need to disconnect the spindle 194 from the chucks 132, 136, 138, and 140, or to connect the chucks 132, 136, 138, and 140 to the spindle 194 installed at the next movement position, each time the chucks 132, 136, 138, and 140 are moved by the motor 137.

[0097] The piston 119 of the cylinder device 117 is connected to the lower part of the motor 192. When this piston 119 is extended, it fits into and connects to recesses (not shown) formed at the lower part of the chucks 132, 136, 138, and 140. Then, the chucks 132, 136, 138, and 140 are moved upward from the index table 134 by the continuous extension of the piston 119 and are positioned for grinding by the cup-shaped grinding wheels 146 and 154.

[0098] The control unit 100 consists of a CPU, memory, input / output circuit section, etc., and controls the operation of each part of the grinding device 2.

[0099] The wafer W, held by suction in the chuck 132, has its thickness measured by a pair of measuring gauges (not shown) connected to the control unit 100. Each of these measuring gauges has a contact, one contact in contact with the upper surface (back surface) of the wafer W, and the other contact in contact with the upper surface of the chuck 132. These measuring gauges can detect the thickness of the wafer W as the difference between the in-process gauge readings, using the upper surface of the chuck 132 as a reference point. Note that the thickness measurement using the measuring gauges may be performed in-line. Furthermore, the method for measuring the thickness of the wafer W is not limited to this.

[0100] The control unit 100 rotates the index table 134 by 90 degrees in the direction of arrow R in Figure 4, so that the wafer W whose thickness has been measured is positioned on the rough grinding stage 118, and the back surface of the wafer W is roughly ground by the cup-shaped grinding wheel 146 on the rough grinding stage 118. As shown in Figure 4, this cup-shaped grinding wheel 146 is connected to an output shaft (not shown) of a motor 148 and is also attached to a grinding wheel feed device 152 via a support casing 150 for the motor 148. The grinding wheel feed device 152 moves the cup-shaped grinding wheel 146 up and down together with the motor 148, and this downward movement presses the cup-shaped grinding wheel 146 against the back surface of the wafer W. This performs rough grinding on the back surface of the wafer W. The control unit 100 sets the amount of downward movement of the cup-shaped grinding wheel 146 and controls the motor 148. The downward movement of the cup-shaped grinding wheel 46, i.e., the amount of grinding performed by the cup-shaped grinding wheel 146, is set based on the pre-registered reference position of the cup-shaped grinding wheel 146 and the thickness of the wafer W detected by the measuring gauge. The control unit 100 controls the rotation speed of the cup-shaped grinding wheel 146 by controlling the rotation speed of the motor 148.

[0101] After the back surface of the wafer W has been roughly ground on the rough grinding stage 118, the cup-shaped grinding wheel 146 moves away from the wafer W, and its thickness is measured by a measuring gauge (not shown) connected to the control unit 100. The control unit 100 rotates the index table 134 90 degrees in the direction of arrow R in Figure 4, so that the wafer W whose thickness has been measured is positioned on the fine grinding stage 120, where it is fine-ground and sparked out by the cup-shaped grinding wheel 154 on the fine grinding stage 120. The structure of this fine grinding stage 120 is the same as that of the rough grinding stage 118, so its explanation is omitted here. The amount of grinding by the cup-shaped grinding wheel 154 is set by the control unit 100, and the processing movement amount and rotation speed of the cup-shaped grinding wheel 154 are controlled by the control unit 100.

[0102] After the back surface of the wafer W has been precision-ground on the precision grinding stage 120, the cup-shaped grinding wheel 154 moves away from the wafer W, and its thickness is measured by a measuring gauge (not shown) connected to the control unit 100. When the index table 134 is rotated 90 degrees in the direction of arrow R in Figure 4 by the control unit 100, the wafer W whose thickness has been measured is positioned on the polishing stage 122, and chemical mechanical polishing is performed by the polishing cloth 156 on the polishing stage 122, and the back surface of the wafer W is mirror-finished. The vertical movement distance of the polishing cloth 156 is set by the control unit 100, and the position of the polishing cloth 156 is controlled by the motor 182 controlled by the control unit 100. In addition, the rotation speed of the motor 158, i.e., the rotation speed of the polishing cloth 156, is controlled by the control unit 100.

[0103] After polishing in the polishing stage 122, the control unit 100 rotates the arm 168, causing the polishing cloth 156 to retract from its position above the wafer W. The wafer W is then held by a robot's (not shown) hand (not shown) and transported to the wafer cleaning stage 124. The wafer cleaning stage 124 is a stage that has a rinse cleaning function and a spin drying function. Since the processed altered layer has been removed from the polished wafer W, it is not easily damaged and therefore does not get damaged during transport by the robot or during cleaning in the wafer cleaning stage 124.

[0104] The wafers W, after being cleaned and dried in the wafer cleaning stage 124, are held by a robot's hand (not shown) and stored in a designated shelf in a cassette (not shown).

[0105] (3) Regarding the splitting device Next, the splitting device (not shown) will be described. A conventional, ordinary splitting device can be used. For example, a splitting device with the following configuration, as disclosed in Table 2004-100240, can be used.

[0106] Specifically, the periphery of the dicing tape is fixed to a frame-shaped structure. A ring member abuts against the underside of the inner portion of the dicing tape's periphery. The upper outer edge of this ring member is smoothly chamfered. A UV light source (UV light irradiation means) is positioned below the dicing tape.

[0107] UV light is shone from a UV light source towards the dicing tape, while the frame is pushed downwards. The UV light can harden the adhesive on the dicing tape or alter its adhesive strength.

[0108] At the same time, a downward force is applied to the frame, pushing it downwards. This expands the dicing tape, widening the gaps between the chips. At this time, because the outer edge of the upper surface of the ring member is smoothly rounded, the dicing tape S expands smoothly.

[0109] Various known linear motion devices can be used as the mechanism for pushing down frame F. For example, a linear motion device consisting of a cylinder member (hydraulic, pneumatic, etc.), a motor, and a screw (a combination of a male screw as a shaft and a female screw as a bearing) can be used.

[0110] Irradiation conditions such as UV light irradiation intensity (power), wavelength range, and irradiation time can be selected as appropriate values ​​depending on the adhesive material of the dicing tape S, the wafer size, the size of the chip after dicing, etc.

[0111] A UV light source is not necessarily required; the chip can be properly diced by adjusting the adhesive strength of the dicing tape.

[0112] (4) Other embodiments of the splitting device Figure 21 is a schematic diagram of the splitting device 300. In Figure 21, (A) is a plan view of the splitting device 300, and (B) is a side view of the splitting device 300. The splitting device 300 mainly consists of a wafer chuck 304 for adsorbing and fixing the wafer 302, a bending means 306 for bending the wafer chuck 304, and a vacuum pump 308.

[0113] The wafer chuck 304 mainly consists of an independent suction chuck 310 having independent suction holes and a communicating groove chuck 312 having a helical groove communicating with the independent suction holes, with the independent suction chuck 310 and the communicating groove chuck 312 overlapping each other.

[0114] The thickness of the connecting groove chuck 312 and the independent suction chuck 310 is approximately 1 mm each, and when the two are stacked, the total thickness is approximately 2 mm. It is preferable that each be made from a resin material such as acrylic. However, it is not limited to resin materials; other materials such as ceramics may also be used, and any material that allows the wafer chuck to bend after vacuum-suction of the wear can be selected. Furthermore, the material of the wafer chuck 304 itself does not necessarily need to bend; it is sufficient if the wafer chuck 304 as a whole is configured and structured to bend.

[0115] The bending means 306 is a rod-shaped body with a rounded tip, and by pushing upward from below the center of the back surface of the wafer chuck 304, it can bend the wafer chuck 304 into a hemispherical shape.

[0116] The wafer 302 is placed on the wafer chuck 304 while still attached to an expanded film 314 whose outer circumference is fixed with a frame, for example, made of metal, and is then vacuum-suctioned. The expanded film 314 is preferably somewhat permeable, as this permeability allows the wafer 302 itself to be directly attracted to the wafer chuck 304 by the vacuum force.

[0117] The wafer chuck 304 is bent into a hemispherical shape by a bending mechanism while holding the wafer 302 in place. As a result, the wafer 302 is divided along the cutting line.

[0118] Next, other embodiments of the chuck bending mechanism will be described with reference to Figure 22. Figure 22 is an explanatory diagram showing other embodiments of the chuck bending mechanism. In this figure, everything except the chuck bending mechanism 320 is the same as in Figure 21, so the explanation will be omitted.

[0119] As shown in Figure 22, the chuck bending means 320 has a semi-circular shape. The splitting device 300 also includes means (not shown) for rotating the chuck bending means 320 relative to the wafer 302.

[0120] The division lines formed on the wafer 302 are often located along the X-axis and Y-axis, which are perpendicular to the differences within the wafer surface. Therefore, the wafer 302 is divided as follows: The chuck bending means 320 is positioned so that one side of the chuck bending means 320 is parallel to the X-axis. Next, the wafer 302 is positioned so that the division line is parallel to the X-axis and placed on the wafer chuck 304 and vacuum-suctioned.

[0121] The wafer 302 is divided in the X direction by pressing the chuck bending means 320 against the lower surface of the wafer chuck 304, thereby bending the wafer 302 together with the wafer chuck 304. Next, the chuck bending means 320 is separated from the wafer chuck 304, rotated 90 degrees so that one side is parallel to the Y axis, and pressed against the lower surface of the wafer chuck 304 again, thereby dividing the wafer 302 in the Y direction. As a result, the wafer 302 can be split along the division lines in both the X and Y directions.

[0122] Here, if the wafer 302 is not vacuum-adsorbed but is pressed against the chuck bending means 320 via an elastic film called a dicing film to bend the wafer 302, then if the wafer cracks first in a certain part, the wafer will bend sharply at the cracked part, and the curvature of the chuck bending means 320 will be absorbed there. As a result, the wafer may not bend in part and may conform to the chuck bending means 320 without being divided.

[0123] Therefore, when the wafer 302 is first vacuum-suctioned onto the wafer chuck 304, and the wafer 302 is straightened to a flat surface by the wafer chuck 304, and then the wafer chuck 304 is bent, each part of the wafer 302 conforms to the bending of the wafer chuck 304 while the wafer 302 remains suctioned. As a result, a uniform and constant bending stress is applied across the plane of the wafer 302, and the wafer 302 can be divided without any remaining fragments.

[0124] <Method for cutting semiconductor substrates> Next, we will explain the method for cutting semiconductor substrates. Figure 9 is a flowchart showing the processing flow of the semiconductor substrate cutting method.

[0125] (1) Laser modification process (Step S10) A wafer W with BG tape B attached to its surface is placed on the suction stage 13 of the laser dicing apparatus 1 with its back surface facing upwards. The following processes are performed by the laser dicing apparatus 1 and controlled by the control unit 50.

[0126] When laser light L is emitted from the laser oscillator 21, the laser light L is irradiated into the inside of the wafer W via an optical system including a collimating lens 22, a half mirror 23, and a condensing lens 24, and a modified region P is formed inside the wafer W.

[0127] In this embodiment, since the final thickness of the generated chip is approximately 50 μm, laser light is irradiated to a depth of approximately 60 μm to approximately 80 μm from the surface of the wafer W, as shown in Figure 10. This is because, in order to efficiently fracture the surface (device surface) of the wafer W, it is necessary to form a laser modification region at a relatively deep position close to the reference surface, which is the surface located on the back side of the wafer W surface by the thickness of the chip T.

[0128] The control unit 50 scans a pulsed processing laser beam L parallel to the surface of the wafer W to form a plurality of discontinuous modified regions P, P, ... inside the wafer W. Microscopic voids (hereinafter referred to as cracks) K are formed inside the modified regions P. Hereinafter, the region formed by the arrangement of the plurality of discontinuous modified regions P, P, ... is referred to as the modified layer.

[0129] Once the modified layer has formed along all of the cutting lines L shown in Figure 11, the process in step S10 is terminated.

[0130] (2) Grinding and removal process (Step S12) Once a modified region is formed along the cutting line L by the laser modification process (step S10), the wafer W is transported from the laser dicing apparatus 1 to the grinding apparatus 2 by a transport device (not shown). The following processes are performed in the grinding apparatus 2 and controlled by the control unit 100.

[0131] The transported wafer W is placed on the chuck 132 (examples include chucks 136, 148, and 140) with its back surface facing upwards, i.e., with the BG tape B attached to the surface of the wafer W facing downwards, and substantially the entire surface of the wafer W is vacuum-suctioned to the chuck 132.

[0132] The index table 134 is rotated around the rotation axis 135 to move the chuck 132 onto the rough grinding stage 118, and the wafer W is rough ground.

[0133] Rough grinding is performed by rotating the chuck 132 and the cup-shaped grinding wheel 146. In this embodiment, for example, a Tokyo Seimitsu Vitrified #325 is used as the cup-shaped grinding wheel 146, and the rotation speed of the cup-shaped grinding wheel 146 is approximately 3000 rpm.

[0134] After rough grinding, the index table 134 is rotated around the rotation axis 135 to move the chuck 132 into the fine grinding stage 120, and the chuck 132 is rotated while the cup-shaped grinding wheel 154 is rotated to fine grind the wafer W. In this embodiment, for example, Tokyo Seimitsu's resin #2000 is used as the cup-shaped grinding wheel 154, and the rotation speed of the cup-shaped grinding wheel 154 is approximately 2400 rpm.

[0135] In this embodiment, as shown in Figure 12, rough grinding and fine grinding are combined to grind to the target surface, that is, to a depth of approximately 50 μm from the surface of the wafer W. In this embodiment, approximately 700 μm is ground in rough grinding and approximately 30 to 40 μm is ground in fine grinding, but this is not strictly defined, and the grinding amount may be determined so that the time for rough grinding and fine grinding is approximately the same.

[0136] Therefore, as shown in Figure 12, the modified layer is removed in the grinding process, and no modified region P remains on the cross-section of the final product, the chip T, due to the laser light. As a result, the modified layer does not break apart from the cross-section of the chip, preventing the chip T from cracking or generating dust from the broken parts.

[0137] Furthermore, in this embodiment, the grinding and removal process includes a crack propagation step in which cracks in the modified layer propagate in the thickness direction of the wafer W. Figure 13 is a diagram illustrating the mechanism of crack propagation, where (a) is a schematic diagram during grinding, (b) is a view of the back surface of the wafer W, (c) is a view of the front surface of the wafer W, and (d) is a cross-sectional view of the wafer W during grinding.

[0138] During grinding, the grinding surface, i.e., the back surface of wafer W, as shown in Figure 13(a), expands due to the heat of grinding, as shown in Figure 13(b). In contrast, the surface opposite the grinding surface, i.e., the front surface of wafer W, is almost entirely held in place by vacuum suction using a vacuum chuck, as shown in Figure 13(c), and is physically constrained against lateral displacement to prevent displacement due to thermal expansion.

[0139] In other words, as shown in Figure 13(d), the back surface (grinding surface) of the wafer W tends to expand outward in the case of a disc shape due to thermal expansion (displacement due to thermal expansion), while the front surface (adhesion surface) of the wafer W is physically constrained to prevent each point on the wafer surface from shifting position as it tries to expand. As a result, strain is generated inside the wafer, and this internal strain causes cracks to propagate in the thickness direction of the wafer W. This internal strain acts uniformly between the part that expands due to thermal expansion and each point on the wafer surface that is physically constrained. Crack propagation due to internal strain is most efficient during the initial stage of grinding, i.e., rough grinding, when the amount of grinding is greatest and the frictional force is greatest, i.e., when frictional heat can be greatest.

[0140] The laser-modified region is formed at a relatively deep location, close to the thickness of the chip. Therefore, in the initial stages of grinding, the distance from the grinding surface to the laser-modified layer is relatively far, but the distance from the modified layer to the target surface is relatively close enough to induce crack propagation. For this reason, to promote crack propagation, it is desirable to induce thermal expansion of the wafer W by grinding heat in the initial stages of rough grinding.

[0141] Even if grinding is performed under conditions that cause thermal expansion of the wafer W, that is, conditions that generate a large amount of frictional heat (for example, by reducing the amount of grinding fluid), the shear stress from grinding is not immediately applied to the modified layer. In this embodiment, crack propagation is not caused by shear stress from grinding, but rather by thermal expansion due to grinding heat.

[0142] When crack propagation is induced by internal strain, cracks can be propagated uniformly at every point on the wafer W plane, regardless of the type of wafer W, without being affected by variations in rigidity within the wafer W plane. Therefore, it is possible to prevent stress from concentrating in areas with weak rigidity due to defects within the wafer W plane, as can occur when artificial stress is applied.

[0143] Furthermore, when an external force is applied artificially, stress concentrates in the weaker parts of the material, making it difficult to control the crack propagation to proceed uniformly and slowly, resulting in the wafer being completely fractured. In contrast, in the case of crack propagation due to internal strain in this embodiment, since the internal strain is due to the degree of thermal expansion, it is possible to propagate the crack subtly. That is, the crack can be propagated to the space between the target surface and the surface of the wafer W. Therefore, it becomes possible to efficiently divide the wafer in the division and separation process (step S18) which will be described later.

[0144] Furthermore, the internal strain due to thermal expansion of the wafer W is distinct from so-called thermal stress caused by temperature differences. Thermal stress is generated in proportion to the temperature gradient, but in this embodiment, the generated heat escapes to the chucks 132, 136, 138, and 140, so no thermal stress is generated.

[0145] (3) Chemical mechanical polishing process (Step S14) This process is performed by the grinding device 2 and controlled by the control unit 100.

[0146] After precision grinding, the index table 134 is rotated around the rotation axis 135 to move the chuck 132 onto the polishing stage 122. Chemical mechanical polishing is then performed using the polishing cloth 156 on the polishing stage 122. In the grinding removal process (step S12), the processed altered layer formed on the back surface of the wafer W is removed, and the back surface of the wafer W is polished to a mirror finish.

[0147] In this embodiment, a polyurethane-impregnated nonwoven fabric (for example, TS200L manufactured by Tokyo Seimitsu) is used as the abrasive cloth 156, colloidal silica is used as the slurry, and the rotation speed of the abrasive cloth 156 is approximately 300 rpm.

[0148] As a result of the grinding and removal process (step S12), numerous irregularities are formed on the back surface of the wafer W, as shown in Figure 14. When polishing is performed by chemical etching, the surface shape is preserved, which may cause cracks to form in the recesses, and the surface will not be mirror-finished. In contrast, in this embodiment, since chemical mechanical polishing is performed, the processing distortion caused by the processing is removed, the surface irregularities are removed, and the surface is mirror-finished.

[0149] In other words, to improve the quality of the final product, chip T, two processes are essential: a grinding and removal process using a grinding wheel, and a chemical mechanical polishing process using abrasive cloth with free abrasive particles containing a chemical solution.

[0150] (4) Expandable tape application process (Step S16) An expanded tape F is attached to the back surface of the wafer W that has undergone a chemical mechanical polishing process (step S14). The expanded tape is a type of elastic tape and is stretchable.

[0151] In this embodiment, since the back surface of the wafer W is made mirror-finished in the chemical mechanical polishing process (step S14), the adhesion between the expanded tape F and the wafer is significantly improved. Furthermore, the flexural strength of the final produced chip T can also be increased.

[0152] (5) Dividing and separating process (Step S18) In the expandable tape application process (step S16), the wafer W with the expandable tape F attached to its back surface is placed on the splitting apparatus with the front surface facing upwards, as shown in Figure 15. In the grinding and removal process (step S12), the crack has propagated toward the surface side from the target surface, so as shown in Figure 15, the propagated crack has formed on the side of the wafer W to which the expandable tape F is attached.

[0153] Subsequently, as shown in Figure 16, when the expandable tape F is expanded outward, the wafer W is fractured based on the propagated cracks. That is, the wafer W is fractured at the cutting line and divided into multiple chips T. Then, when the expandable tape F is further expanded, the individual chips T are separated.

[0154] In this embodiment, by propagating the crack downwards from the target surface during the grinding and removal process (step S12), the wafer W can be efficiently divided into chips T simply by pulling the expanded tape F during this splitting and separation process. Furthermore, since the wafer is not completely cut when the crack is propagated, the work efficiency is improved.

[0155] Furthermore, in this embodiment, since the back surface of the wafer W is mirror-finished in the chemical mechanical polishing process (step S14), the expanded tape F and the chip T do not shift and partially peel off during expansion.

[0156] <Evaluation of crack propagation by grinding> Next, the evaluation of crack propagation due to grinding in the above-mentioned grinding removal process (step S12) will be explained with reference to Figures 19 and 20. The grinding method, the splitting and separation method, and their conditions are basically the same as in steps S10 to S18 above. Figure 19 is a diagram showing the conditions for evaluating crack propagation, and Figure 20 is a diagram showing the evaluation results of the crack propagation evaluation.

[0157] In Figures 19(A), (B), and (C), the horizontal axis is common to all, with each position corresponding to the others, and represents the grinding time (s). In Figure 19(A), the vertical axis represents the cutting speed (grinding speed) (μm / s), in (B) the vertical axis represents the ON / OFF status of water supply to the grinding wheel during grinding, and in (C) the vertical axis represents the temperature (°C) on the back surface of the wafer W during grinding.

[0158] As shown in Figure 19(A), rough grinding was performed for a total of 710 μm while varying the grinding speed, followed by fine grinding for 13 μm (not shown), and then chemical mechanical polishing (not shown) for 2 μm. When working with wafers, as shown in Figure 19(B), a period of interruption was provided in the rough grinding process to stop the water supply to the grinding wheel or wafer. Water supply was stopped t1 seconds after the start of grinding and resumed t2 seconds after the start of grinding. Water was supplied at a flow rate of 10 L / min. Subsequently, steps S16 and S18 described above were performed to break the wafer, and the breakage state was observed and evaluated. A good breakage without chip cracking or dust generation was marked with ○, and a breakage with chip cracking or dust generation was marked with ×. The results are summarized in Figure 20.

[0159] As shown in Figure 19(C), the back surface temperature of the wafer W began to rise sharply t1 seconds after the start of grinding when the water supply was stopped, and began to fall immediately after t2 seconds after the start of grinding when the water supply was resumed.

[0160] As shown in Figure 20, when the back surface temperature of wafer W reached 70°C or higher due to grinding, the wafer fractured successfully. This is thought to be because the heat from grinding propagated the cracks formed by the laser.

[0161] Therefore, the grinding apparatus according to the present invention may include means for measuring the temperature of the wafer, means for turning the grinding wheel or water supply to the wafer ON and OFF during grinding, and means for controlling the water supply to the wafer to be turned OFF until the wafer temperature reaches a predetermined value after a predetermined time has elapsed from the start of grinding. This allows the cracks formed by the laser to propagate and the wafer to be fractured smoothly.

[0162] As explained above, according to this embodiment, since the cracks in the modified region formed by the laser light can be propagated by grinding, it is possible to prevent the modified region formed by the laser light from remaining on the cross-section of the chip T. Therefore, problems such as the chip T cracking or dust being generated from the cross-section of the chip T can be prevented. Consequently, chips of stable quality can be obtained efficiently.

[0163] (Note) As can be seen from the descriptions of the embodiments detailed above, this specification includes disclosures of a variety of technical ideas, including the inventions shown below.

[0164] (Note 1) A method for cutting a semiconductor substrate according to a first aspect of the present invention is characterized by comprising: a modified region formation step of forming a modified region inside the wafer by injecting laser light from the back surface of the wafer along a cutting line to form a modified region, thereby forming micro-vacancies within the modified region; an adsorption step of uniformly adsorbing substantially the entire surface of the wafer in which the modified region was formed in the modified region formation step onto a table independently of each region; a grinding step of grinding the wafer from the back surface after substantially the entire surface of the wafer has been adsorbed onto the table in the adsorption step to remove the modified region and to advance the micro-vacancies in the thickness direction of the wafer; a chemical and mechanical polishing step of the wafer in which the micro-vacancies have advanced in the thickness direction of the wafer in the grinding step; a cleavage step of cleavage along a cutting line based on the micro-vacancies remaining in the substrate; and a cleavage step of dividing the cleavage into a plurality of chips.

[0165] According to the semiconductor substrate cutting method of this first embodiment, a laser beam is incident on the back surface of the wafer along the cutting line to form a modified region inside the wafer, thereby creating minute voids within the modified region. The modified region is then removed by grinding the wafer from the back surface while the entire surface of the wafer in which the modified region has been formed is independently and uniformly adsorbed onto a table.

[0166] (Note 2) A method for cutting a semiconductor substrate according to a second aspect of the present invention includes: a modified region formation step of forming a modified region inside the wafer by injecting laser light from the back surface of the wafer along the cutting line to form a modified region, thereby forming minute voids within the modified region; a step of uniformly and independently adsorbing substantially the entire surface of the wafer on which the modified region has been formed in the modified region formation step onto a table; and, while the wafer is adsorbed, injecting the laser light to remove material up to the portion in front of the modified region formed inside the wafer, thereby removing material extending from the modified region. The method is characterized by comprising: a first grinding step of propagating microcracks in the depth direction of the substrate; a second grinding step of grinding off a modified region formed inside the wafer; a step of performing chemical mechanical polishing using a chemical slurry and polishing pad to modify the wafer surface, while leaving microcracks extending from the modified region, thereby removing the processed altered layer introduced in the first and second grinding steps and mirroring the surface; a step of cutting the wafer based on the microcracks that have propagated in the wafer thickness direction; and a step of separating the cut chips after cutting.

[0167] According to this second embodiment of the semiconductor substrate cutting method, a modified region is initially formed at a deep location inside the wafer, and by removing it while generating grinding heat in the first grinding process, it becomes possible to propagate the cracks formed in the modified region to an even deeper location within the wafer.

[0168] However, if the laser-modified region is ground down to the same extent as the unmodified region in the first grinding step, large grains may break off due to the larger grain boundaries in the modified region, and even more critical cracks may propagate along with these grains. Therefore, it is preferable to grind down to the portion just before the modified region where the crystallinity is constant in the first grinding step.

[0169] In the second grinding process, the laser-modified areas are primarily ground. At this stage, a grinding wheel with a higher grit size (i.e., a finer grit than in the first grinding process) is used. The grinding is performed more gently than in the first grinding process to avoid inducing critical cracks or defects originating from the modified areas. The second grinding process focuses solely on the modified areas, and the grinding rate is lower than in the first grinding process, resulting in a finer grind.

[0170] After the laser-modified areas are removed, chemical mechanical polishing is performed. In chemical mechanical polishing, a chemical solution is supplied to modify the wafer, while polishing pads made of polymers or nonwoven fabrics are pressed against the wafer to perform chemical and mechanical polishing.

[0171] If chemical mechanical polishing is performed on the modified region mentioned earlier, larger crystal grains may break off in that region. Since chemical mechanical polishing uses polishing pads made of nonwoven fabric or foamed polyurethane, if these broken crystal grains get into the surface of the pad, scratches will constantly be generated by the broken crystal grains during polishing. In such cases, the polished surface will be covered in scratches, without even achieving the goal of removing the processed altered layer and creating a mirror finish.

[0172] Therefore, when the material is introduced into the chemical mechanical polishing process, the modified layer introduced by the laser in the second grinding process must be completely removed.

[0173] (Note 3) The third aspect of the semiconductor substrate cutting method according to the second aspect of the present invention is characterized in that, in the modified region formation step, the modified region is formed to a depth of approximately 60 μm to approximately 80 μm from the surface of the wafer, and the first grinding step propagates the minute voids to a depth of approximately 50 μm from the surface of the wafer and the surface of the wafer.

[0174] As a result, a modified region is formed at a depth of approximately 60 μm to 80 μm from the wafer surface, and micro-vacancies are propagated to a depth of approximately 50 μm from the wafer surface and between the wafer surface and the wafer surface. Therefore, even if the workpiece is ground in such a way that no modified region formed by laser light remains on the cross-section of the chip, micro-vacancies can still be left in the workpiece.

[0175] (Note 4) The fourth aspect of the present invention relates to the method for cutting a semiconductor substrate, wherein the grinding step includes a step of interrupting the supply of water to the grinding wheel during grinding until the temperature of the wafer during grinding reaches 70°C or higher.

[0176] According to this fourth embodiment of the semiconductor substrate cutting method, the temperature of the wafer during grinding can be controlled. This allows minute voids to propagate in the thickness direction of the wafer.

[0177] (Note 5) The fifth aspect of the present invention relates to a method for cutting a semiconductor substrate, wherein the first grinding step includes a step of interrupting the supply of water to the grinding wheel during grinding until the temperature of the wafer during grinding reaches 70°C or higher, in the method for cutting a semiconductor substrate according to the second or third aspect described above.

[0178] According to this fifth embodiment of the semiconductor substrate cutting method, the temperature of the wafer during grinding can be controlled. This allows minute voids to propagate in the thickness direction of the wafer.

[0179] (Note 6) The method for cutting a semiconductor substrate according to the sixth aspect of the present invention is characterized in that, in the method for cutting a semiconductor substrate according to the first or fourth aspect described above, the dividing step includes the step of attaching an elastic tape to the surface of the wafer and the step of expanding the elastic tape.

[0180] According to this sixth embodiment of the semiconductor substrate cutting method, the wafer can be divided into multiple chips by attaching and expanding an elastic tape on the surface of the wafer.

[0181] (Note 7) A semiconductor substrate cutting apparatus according to a seventh aspect of the present invention comprises: a laser dicing means for injecting laser light from the back surface of a wafer along a cutting line to form a modified region inside the wafer; a grinding means for grinding the wafer from the back surface to remove the modified region; a transport means for transporting the wafer from the laser dicing means to the grinding means; and a dividing means for dividing the wafer along a cutting line, wherein the laser dicing means comprises a table on which the surface of the wafer is placed facing downward; an irradiation means for irradiating laser light toward the wafer to form the modified region; and a first control means for controlling the irradiation means so that the position from which the laser light is irradiated changes; and the grinding means comprises an suction table on which the surface of the wafer is placed facing downward and which adsorbs substantially the entire surface of the wafer; a grinding wheel for grinding the wafer; and a second control means for controlling the height and rotational speed of the grinding wheel.

[0182] This prevents modified regions formed by laser light from remaining on the cross-section of the chip. As a result, defects such as chip cracking or dust generation from the chip's cross-section are prevented, and chips of stable quality can be efficiently obtained.

[0183] (Note 8) The semiconductor substrate cutting apparatus according to the eighth aspect of the present invention is a semiconductor substrate cutting apparatus according to the seventh aspect described above, characterized in that the first control means controls the irradiation means to form the modified region to a depth of approximately 60 μm to approximately 80 μm from the surface of the wafer, and the second control means controls the height and rotation speed of the grinding wheel to propagate the minute voids in the modified region to a depth of approximately 50 μm from the surface of the wafer and the surface of the wafer.

[0184] This allows for the retention of minute voids in the workpiece even when the workpiece is ground in a way that removes any modified areas formed by laser light from the cross-section of the chip.

[0185] (Note 9) The semiconductor substrate cutting apparatus according to the ninth aspect of the present invention is characterized in that, in the semiconductor substrate cutting apparatus according to the seventh or eighth aspect described above, it further comprises means for measuring the temperature of the wafer during grinding, means for turning the grinding wheel or water supply to the wafer ON and OFF during grinding, and means for controlling the water supply to the wafer to be turned OFF until the wafer temperature reaches a predetermined value after a predetermined time has elapsed from the start of grinding.

[0186] This allows microscopic voids to propagate in the thickness direction of the wafer. [Explanation of Symbols]

[0187] 1…Laser dicing device, 2…Grinding device, W…Workpiece, B…BG tape, F…Expanded tape, 11…Wafer moving unit, 13…Adsorption stage, 20…Laser optics unit, 30…Observation optics unit, 40…Laser head, 50…Control unit, 118…Rough grinding stage, 120…Fine grinding stage, 122…Polishing stage, 132, 136, 138, 140…Chuck, 146, 154…Cup-shaped grinding wheel, 156…Abrasive cloth, 300…Dividing device

Claims

[Claim 1] A method for propagating a crack inside a wafer in the thickness direction of the wafer, The method includes a crack propagation step in which the crack is propagated by grinding the back surface of the wafer while the surface of the wafer is held, with the water supply stopped. Methods of crack propagation.

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