Grinding method for workpieces

Abstract

To suppress defects that easily occur in a latter half period for polishing, in grinding a hard wafer or the like in multiple stages in a TAIKO process.SOLUTION: A grinding method for a work-piece comprises: a first grinding step of forming a ring-like reinforcement part, a first thin plate part positioned inside the ring-like reinforcement part in a radial direction and a second thin plate part positioned inside the first thin plate part in the radial direction, on a work-piece whose single crystal substrate has a silicon single crystal substrate, a compound semiconductor single crystal substrate, a single crystal sapphire substrate or a double oxide single crystal substrate, whose resistivity is 0.1 Ω.cm or less, using a first grinding wheel in which a plurality of first grinding stones are annularly arranged along a circumferential direction of a first base in a circular arc shape; and a second grinding step of grinding the first thin plate part and the second thin plate part, using a second grinding wheel in which a plurality of second grinding stones respectively including abrasive grain having average particle sizes smaller than average particle sizes of abrasive grain of the first grinding stones are arranged along a circumferential direction of a second base in a circular arc shape.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for grinding a workpiece, which forms a thin plate portion and a ring-shaped reinforcing portion surrounding the outer peripheral portion of the thin plate portion on the workpiece by grinding a single crystal substrate of the workpiece.

Background Art

[0002] With the spread of SiP (System in Package) in which a plurality of device chips thinned respectively are laminated to form one package, there is a demand for a grinding technology that can thinly process a wafer with high yield. As one of the grinding technologies for thinning a wafer, a grinding technology called TAIKO (registered trademark) (hereinafter, abbreviated as TAIKO process for convenience) is known.

[0003] In the TAIKO process, when grinding the back side of a wafer having a device region where a plurality of devices are formed on the front side, by grinding a predetermined region on the back side corresponding to the device region on the front side, the outer peripheral portion on the back side is left as an annular convex portion (see, for example, Patent Document 1).

[0004] Thereby, since the strength of the wafer can be increased compared to the case where the entire back side is uniformly thinned, there is an advantage that warping of the thinned wafer and cracking of the wafer during conveyance can be suppressed.

[0005] In the TAIKO process, first, rough grinding is performed on a predetermined region on the back side with a rough grinding wheel having a rough grinding abrasive, and then finish grinding is performed on the predetermined region after rough grinding with a finish grinding wheel having a finish grinding abrasive.

[0006] However, when performing multi-stage grinding in this manner (i.e., rough grinding and finish grinding in sequence), the condition of the grinding wheel tends to deteriorate during the latter half of the grinding process (i.e., finish grinding), especially when grinding hard wafers such as (1) highly-doped silicon wafers with relatively high impurity concentrations, and (2) compound semiconductor wafers (e.g., gallium nitride (GaN) and silicon carbide (SiC)).

[0007] When the condition of the grinding wheel deteriorates, problems such as increased grinding resistance and a sudden rise in the current required to drive the spindle can occur. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2007-19461 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] This invention has been made in view of the aforementioned problems, and aims to suppress defects that tend to occur in the latter half of grinding when grinding hard wafers and the like in a multi-stage process in the TAIKO process. [Means for solving the problem]

[0010] According to one aspect of the present invention, a method for grinding a workpiece, comprising grinding a single crystal substrate in the workpiece, wherein the single crystal substrate is a silicon single crystal substrate, a compound semiconductor single crystal substrate, or a complex oxide single crystal substrate with a resistivity of 0.1 Ω·cm or less, and the method comprises a holding step of holding the workpiece with a chuck table and a first grinding wheel in which a plurality of first grinding wheels are arranged in a ring along the circumferential direction of an annular first base, to form the ring-shaped reinforcing portion that is located on the outermost radial side of the workpiece. A method for grinding a workpiece is provided, comprising: a first grinding step of forming a portion on the workpiece, a first thin plate portion located inside the ring-shaped reinforcing portion in the radial direction and thinner than the ring-shaped reinforcing portion, and a second thin plate portion located inside the first thin plate portion in the radial direction and thinner than the first thin plate portion; and a second grinding step of grinding the first thin plate portion and the second thin plate portion after the first grinding step using a second grinding wheel in which a plurality of second grinding wheels, each containing abrasive grains having an average particle size smaller than the average particle size of the abrasive grains of the first grinding wheel, are arranged in a ring along the circumferential direction of an annular second base.

[0011] Preferably, the first grinding step includes an upper processing step for forming the ring-shaped reinforcing portion and the first thin plate portion, and a lower processing step for forming the second thin plate portion after the upper processing step.

[0012] Preferably, the first grinding step includes an internal processing step for forming the second thin plate portion, and an external processing step, after the internal processing step, for forming the first thin plate portion and the ring-shaped reinforcing portion.

[0013] Preferably, in the first grinding step, the single crystal substrate of the workpiece is ground in such a way that the rotational trajectories of the plurality of first grinding wheels do not pass along the extension of the rotation axis of the chuck table, thereby forming a cylindrical convex portion that is located inside the second thin plate portion in the radial direction and has the same thickness as the first thin plate portion, and in the second grinding step, the first thin plate portion, the second thin plate portion and the cylindrical convex portion are ground. [Effects of the Invention]

[0014] In the second grinding step of a workpiece grinding method according to one aspect of the present invention, first, the second grinding wheel is dressed for the first time on the upper surface side of the first thin plate portion. Then, the first thin plate portion can be ground with the second grinding wheel that has been dressed for the first time.

[0015] As grinding progresses, the second grinding wheel receives a second dressing on the upper surface of the second thin plate section. Then, the second thin plate section can be ground using the second grinding wheel, which has received its second dressing.

[0016] In other words, in the second grinding step, multiple dressings can be applied at time intervals through gradual grinding. In this way, the condition of the second grinding wheel can be restored at different timings in the second grinding step, which helps to suppress problems that tend to occur in the later stages of grinding. [Brief explanation of the drawing]

[0017] [Figure 1] This is a flowchart of the grinding method for a workpiece in the first embodiment. [Figure 2] This is a perspective view of the workpiece. [Figure 3] This is a diagram showing the holding step. [Figure 4] This is a diagram showing the first grinding step. [Figure 5] This is a cross-sectional view of a portion of the workpiece after the upper machining step. [Figure 6] Figure 6(A) is a partial cross-sectional side view showing the relative movement of the chuck table and the rough grinding unit along the horizontal direction, and Figure 6(B) is a partial cross-sectional side view showing the lower machining step. [Figure 7] This is a cross-sectional view of a portion of the workpiece after the lower machining step. [Figure 8] Figure 8(A) is a partial cross-sectional side view of the workpiece at the start of the second grinding step, and Figure 8(B) is a partial cross-sectional side view of the workpiece during the second grinding step. [Figure 9] It is a cross-sectional view of a part of the workpiece after the second grinding step. [Figure 10] It is a flowchart of a method for grinding a workpiece in the second embodiment. [Figure 11] FIG. 11(A) is a cross-sectional view of a part of the workpiece after the inner processing step, and FIG. 11(B) is a cross-sectional view of a part of the workpiece after the outer processing step. [Figure 12] It is a cross-sectional view of a part of the workpiece after the upper processing step. [Figure 13] FIG. 13(A) is a top view of the lower processing step in the third embodiment, and FIG. 13(B) is a partial cross-sectional side view of the lower processing step in the third embodiment. [Figure 14] It is a cross-sectional view of a part of the workpiece after the lower processing step. [Figure 15] FIG. 15(A) is a cross-sectional view of a part of the workpiece after forming a cylindrical convex portion in the inner processing step in the fourth embodiment, and FIG. 15(B) is a cross-sectional view of a part of the workpiece after the outer processing step in the fourth embodiment.

Embodiments for Carrying Out the Invention

[0018] Embodiments according to an aspect of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a flowchart of a method for grinding a workpiece 11 (see FIG. 2 etc.) in the first embodiment. As shown in FIG. 2, the workpiece 11 includes a disk-shaped single crystal substrate 13.

[0019] The single crystal substrate 13 of the present embodiment is a silicon single crystal substrate, a compound semiconductor single crystal substrate, or a complex oxide single crystal substrate having a resistivity of 0.1 Ω·cm or less. A silicon single crystal substrate having a resistivity of 0.001 Ω·cm or more and 0.1 Ω·cm or less is usually called a high-doped silicon wafer.

[0020] High-doped silicon wafers are harder than ordinary silicon single-crystal substrates (for example, those with a resistivity greater than 0.1 Ω·cm and 40.0 Ω·cm or less), so problems are more likely to occur during grinding (especially finish grinding) compared to grinding ordinary silicon single-crystal substrates.

[0021] Similarly, compound semiconductor single-crystal substrates and single-crystal sapphire substrates, which are made of compound semiconductor materials such as gallium nitride and silicon carbide, are harder than ordinary silicon single-crystal substrates, so problems are more likely to occur during grinding (especially finish grinding) compared to grinding ordinary silicon single-crystal substrates.

[0022] In contrast, wafers made of lithium tantalate (LT), lithium niobate (LN), etc. (i.e., complex oxide single crystal substrates) are not as hard as ordinary silicon single crystal substrates, but they are more prone to clogging during grinding, making them more susceptible to problems during grinding (especially finish grinding) compared to grinding ordinary silicon single crystal substrates.

[0023] On the surface 13a side of the single crystal substrate 13, multiple division lines (streets) 15 (not shown) are arranged in a grid pattern, and devices 17 such as ICs (Integrated Circuits) are formed in each small region partitioned by the multiple division lines 15. There are no restrictions on the type, quantity, shape, structure, size, arrangement, etc., of the devices 17 in the workpiece 11.

[0024] In this embodiment, a portion of the workpiece 11 is thinned by grinding a predetermined area on the back surface 13b that corresponds to the area on the surface 13a side where multiple devices 17 are formed (i.e., the device area). The predetermined area on the back surface 13b is concentric with the outer edge of the workpiece 11 and is a circular area smaller than the outer diameter of the workpiece 11.

[0025] Before grinding the single crystal substrate 13, a resin protective member 19 (see Figure 2) with approximately the same diameter as the single crystal substrate 13 is attached to the surface 13a side. Figure 2 is a perspective view of the workpiece 11, etc. The protective member 19 is, for example, a tape having a base layer and an adhesive layer, and the adhesive layer of the tape is attached to the surface 13a side of the single crystal substrate 13.

[0026] However, the protective member 19 may have only a base layer and no adhesive layer. In this case, the protective member 19 is attached to the surface 13a by heat compression. By attaching the protective member 19 to the surface 13a, the impact on the device 17 during grinding can be mitigated.

[0027] After attaching the protective member 19, the workpiece 11 is held in place by suction on the protective member 19 side using the chuck table 4 (see Figure 3) of the grinding device 2 (holding step S10). Figure 3 shows the holding step S10. Note that in Figure 3 and subsequent figures, the multiple planned division lines 15 and devices 17 are omitted.

[0028] The chuck table 4 includes, for example, a disc-shaped frame 6 made of non-porous ceramics. A circular recess is formed on the upper surface of the frame 6. A disc-shaped porous plate 8 made of porous ceramics is fixed to this recess.

[0029] The upper surface of the porous plate 8 has a conical shape in which the central part protrudes more than the outer periphery. Note that in Figure 3, the shape of the porous plate 8 is exaggerated for illustrative purposes. The amount of protrusion of the central part relative to the outer periphery on the upper surface of the porous plate 8 is, for example, between 10 μm and 30 μm.

[0030] The upper surface of the porous plate 8 and the upper surface of the frame 6 are substantially flush and function as a holding surface 4a for suction and holding the workpiece 11 via the protective member 19. The porous plate 8 is connected to a suction source (not shown), such as a vacuum pump, via a flow path or valve (not shown) provided inside the frame 6.

[0031] Therefore, when the workpiece 11 is placed on the upper surface of the porous plate 8 via the protective member 19, and then negative pressure from the suction source is transmitted to the porous plate 8, the workpiece 11 is held by suction on the holding surface 4a in accordance with the shape of the holding surface 4a, with the back surface 13b of the single crystal substrate 13 exposed upwards.

[0032] A rotational drive source (not shown), such as a motor, for rotating the chuck table 4 is provided at the lower part of the frame 6. The chuck table 4 is rotatable around the rotation axis 4b by the power of the rotational drive source.

[0033] The rotation axis 4b is tilted by a small angle with respect to the Z-axis direction (vertical direction, up and down direction) such that a portion of the holding surface 4a is approximately horizontal. The chuck table 4 is supported by a horizontal movement mechanism (not shown), which allows it to move along a horizontal direction perpendicular to the Z-axis direction.

[0034] After the holding step S10, a predetermined area on the back surface 13b side corresponding to the device area is ground (first grinding step S20). Figure 4 shows the first grinding step S20. As shown in Figure 4, a rough grinding unit (first grinding unit) 10 is positioned above the chuck table 4.

[0035] The rough grinding unit 10 includes, for example, a cylindrical spindle housing (not shown). A Z-axis movement mechanism (not shown) is connected to the spindle housing, and the rough grinding unit 10 is movable along the Z-axis direction by the Z-axis movement mechanism. A portion of a cylindrical spindle 12 is rotatably housed in the space inside the spindle housing.

[0036] The longitudinal direction of the spindle housing and the spindle 12 is aligned with the Z-axis direction. A rotational drive source (not shown), such as a motor, is provided on a portion of the upper side of the spindle 12. The lower end of the spindle 12 protrudes downward from the lower end of the spindle housing.

[0037] A disc-shaped mount 14, having a smaller diameter than the workpiece 11 (for example, having a diameter approximately half that of the workpiece 11), is fixed to the lower end of the spindle 12. An annular rough grinding wheel (first grinding wheel) 16 is fixed to the lower surface of the mount 14.

[0038] The rough grinding wheel 16 includes an annular wheel base (first base) 18 made of a metal such as an aluminum alloy. On the lower surface of the wheel base 18a, a plurality of rough grinding wheels (first grinding wheels) 18b are arranged in an annular shape at approximately equal intervals along the circumferential direction of the wheel base 18a.

[0039] The coarse grinding wheel 18b includes abrasive grains formed from cBN (cubic boron nitride), diamond, etc., and a bonding material such as a vitrified bond or resin bond for fixing the abrasive grains. The abrasive grains of the coarse grinding wheel 18b have a relatively large average particle size. For example, abrasive grains with a grit size from #240 to #1200 are used.

[0040] The grit size is specified in JIS R6001-2:2017 (Grit size of abrasives for grinding wheels - Part 2: Fine powder), a JIS standard established by the Japanese Industrial Standards Committee. For grit sizes not specified in JIS R6001-2:2017, the notation commonly used in the grinding wheel manufacturing and sales industry should be followed or equivalent.

[0041] Near the rough grinding wheel 16, a grinding water supply nozzle (not shown) is provided, which can supply a grinding liquid (typically pure water) to the rough grinding wheel 18b, etc. During grinding, this liquid is used to remove heat and grinding debris generated in the processing area.

[0042] When grinding the single crystal substrate 13 with the rough grinding wheel 16, first, the chuck table 4 is moved directly below the rough grinding unit 10. Specifically, the chuck table 4 is moved so that the rough grinding wheel 16 (i.e., all the rough grinding wheels 18b) is positioned directly above the circular area where the device 17 is formed.

[0043] Then, as shown in Figure 4, the chuck table 4 and the rough grinding wheel 16 are rotated in predetermined directions, and the rough grinding unit 10 is lowered at a predetermined speed (grinding feed rate) while grinding fluid is supplied from the nozzle (upper machining step S22).

[0044] In the upper machining step S22, the rotational speed of the chuck table 4 is set to 100 rpm or more and 600 rpm or less (typically 300 rpm), the rotational speed of the rough grinding wheel 16 is set to 1000 rpm or more and 7000 rpm or less (typically 4500 rpm), and the grinding feed rate is set to 0.8 μm / s or more and 10 μm / s or less (typically 6.0 μm / s).

[0045] Figure 5 is a schematic cross-sectional view showing a portion of the workpiece 11 after the upper machining step S22. In the upper machining step S22, the back surface 13b side corresponding to the device area is ground using the rough grinding wheel 16 to form a disc-shaped first thin plate portion 13c and a ring-shaped reinforcing portion 13d surrounding the outer periphery of the first thin plate portion 13c.

[0046] The first thin plate portion 13c and the ring-shaped reinforcing portion 13d constitute the first annular stepped portion 21a. The ring-shaped reinforcing portion 13d is the region that was not ground in the upper machining step S22 and is located on the outermost side in the radial direction 11a of the workpiece 11.

[0047] The first thin plate portion 13c is the area ground in the upper processing step S22, located inside the ring-shaped reinforcing portion 13d in the radial direction 11a, and is thinner than the ring-shaped reinforcing portion 13d. A rough surface region 13e is formed on the back surface 13b side of the first thin plate portion 13c due to grinding with the rough grinding wheel 18b.

[0048] The rough surface region 13e has larger irregularities than those formed on the ground surface of the workpiece 11 in the second grinding step S30, which will be described later. In one example, the irregularities are evaluated by the arithmetic mean roughness Ra, the maximum height Rz, etc.

[0049] Following the upper machining step S22 in which the first thin plate portion 13c and the ring-shaped reinforcing portion 13d are formed, the inside of the first thin plate portion 13c in the radial direction 11a is then ground using the rough grinding wheel 16 to form a second thin plate portion 13f (see Figure 7) which is thinner than the first thin plate portion 13c (lower machining step S24).

[0050] In the lower machining step S24, first, as shown in Figure 6(A), the chuck table 4 is moved horizontally in order to grind the inside of the first thin plate portion 13c in the radial direction 11a with the rough grinding wheel 16. This separates the outer surface of the rough grinding wheel 18b from the inner surface of the ring-shaped reinforcing portion 13d.

[0051] Figure 6(A) is a partial cross-sectional side view showing the relative movement of the chuck table 4 and the rough grinding unit 10 along the horizontal direction. When moving the chuck table 4 and the rough grinding wheel 16 relative to each other, the chuck table 4 and the rough grinding wheel 16 may be kept rotating or stopped.

[0052] There are no major restrictions on the speed or distance of movement, but for example, the speed of movement should be between 1.0 mm / s and 2.0 mm / s, and the distance of movement should be between 3.0 mm and 6.0 mm.

[0053] After moving horizontally, as shown in Figure 6(B), the rough grinding unit 10 is lowered relative to the chuck table 4 to perform further rough grinding on the first thin plate section 13c. Note that in Figures 6(A) and 6(B), the rough surface area 13e is omitted for clarity. In subsequent figures, the rough surface area 13e may also be omitted for convenience.

[0054] Figure 6(B) is a partial cross-sectional side view showing the lower machining step S24, and Figure 7 is a schematic cross-sectional view showing a part of the workpiece 11 after the lower machining step S24. The rotational speed of the chuck table 4, the rotational speed of the rough grinding wheel 16, and the grinding feed rate in the lower machining step S24 are the same as the values ​​used in the upper machining step S22 described above.

[0055] As described above, in the lower processing step S24, the inside of the first thin plate portion 13c is ground in the radial direction 11a to form the second thin plate portion 13f. At this time, a rough surface region 13e is also formed on the back surface 13b side of the second thin plate portion 13f, and a second annular stepped portion 21b is also formed, which is composed of the first thin plate portion 13c and the second thin plate portion 13f.

[0056] In addition, during the lower machining step S24, the speed at which the rough grinding unit 10 descends may be changed in accordance with the progress of grinding. Specifically, the grinding feed rate may be reduced as grinding progresses.

[0057] For example, the grinding feed rate is gradually reduced as the grinding progresses, starting at 6.0 μm / s, then 3.0 μm / s, and then 1.0 μm / s. By gradually reducing the grinding feed rate, the thickness of the second thin plate section 13f can be adjusted with high precision.

[0058] After the first grinding step S20, the rough grinding unit 10 is raised and moved away from the workpiece 11. Then, a finish grinding unit (second grinding unit) 20, which is different from the rough grinding unit 10, is placed above the workpiece 11 (see Figure 8(A)), and the first thin plate portion 13c and the second thin plate portion 13f are ground with the finish grinding unit 20 (second grinding step S30).

[0059] In this embodiment, the finish grinding unit 20 is mounted in the grinding device 2 together with the rough grinding unit 10. However, the finish grinding unit 20 may be mounted in a grinding device (not shown) different from the grinding device 2 that includes the rough grinding unit 10.

[0060] The finishing grinding unit 20 includes, for example, a cylindrical spindle housing (not shown). A Z-axis movement mechanism (not shown) is connected to the spindle housing, and the finishing grinding unit 20 is movable along the Z-axis direction by the Z-axis movement mechanism.

[0061] A portion of the cylindrical spindle 22 is housed in the space inside the spindle housing. The longitudinal direction of the spindle housing and the spindle 22 is aligned with the Z-axis direction. A rotational drive source (not shown), such as a motor, is provided on a portion of the upper part of the spindle 22.

[0062] The lower end of the spindle 22 protrudes downward from the lower end of the spindle housing. A disc-shaped mount 24, which has approximately the same diameter as the mount 14 described above, is fixed to the lower end of the spindle 22.

[0063] A finish grinding wheel (second grinding wheel) 26, which is approximately the same diameter as the rough grinding wheel 16 and has an annular shape, is fixed to the lower side of the mount 24. The finish grinding wheel 26 includes an annular wheel base (second base) 28 made of a metal such as an aluminum alloy.

[0064] On the lower surface of the wheel base 28a, multiple finishing grinding wheels (second grinding wheels) 28b are arranged in a ring shape at approximately equal intervals along the circumferential direction of the wheel base 28a. Each finishing grinding wheel 28b contains abrasive grains made of cBN (cubic boron nitride), diamond, etc., and a bonding material such as a vitrified bond or resin bond for fixing the abrasive grains.

[0065] The average particle size of the abrasive grains contained in the finishing grinding wheel 28b is smaller than the average particle size of the abrasive grains contained in the rough grinding wheel 18b. The average particle size is determined, for example, based on the frequency distribution of a group of particles expressed using a predetermined particle diameter (i.e., length) when the size of a single particle is expressed by this particle diameter.

[0066] There are several known methods for expressing particle size, including geometric diameter and equivalent diameter. Geometric diameters include the Ferret diameter, the maximum diameter in a given direction (i.e., the Krummbein diameter), the Martin diameter, and the sieve diameter, while equivalent diameters include the equivalent diameter of a projected area circle (i.e., the Heywood diameter), the equivalent diameter of an equisurface area sphere, the equivalent diameter of an equivolute sphere, the Stokes diameter, and the light scattering diameter.

[0067] When a frequency distribution is created for a group of particles, with particle diameter (μm) on the horizontal axis and frequency on the vertical axis, the average particle size is obtained, for example, by the average of the particle diameters in the weight-based or volume-based distribution. However, instead of the average particle size, the particle size of the abrasive grains may be determined based on the median diameter, which is the cumulative frequency of the particle group's frequency distribution that accounts for 50% of the total, or the mode diameter, which is the particle diameter with the highest frequency.

[0068] Even when based on median diameter or mode diameter, the particle size of the abrasive grains contained in the finishing grinding wheel 28b is smaller than the particle size of the abrasive grains contained in the rough grinding wheel 18b. In this embodiment, for example, abrasive grains with a grit size from #2000 to #10000 are used in the finishing grinding wheel 28b.

[0069] When grinding the first thin plate portion 13c and the second thin plate portion 13f with the finishing grinding wheel 26, first move the chuck table 4 directly below the finishing grinding unit 20. Specifically, adjust the position of the chuck table 4 so that all the finishing grinding wheels 28b are located further inward than the inner circumferential surface of the ring-shaped reinforcing portion 13d in the radial direction 11a (see Figure 8(A)).

[0070] Then, the chuck table 4 and the finish grinding wheel 26 are rotated in predetermined directions, and the finish grinding wheel 26 is lowered at a predetermined speed while grinding fluid is supplied from the nozzle. Figure 8(A) is a partial cross-sectional side view of the workpiece 11, etc. at the start of the second grinding step S30, and Figure 8(B) is a partial cross-sectional side view of the workpiece 11, etc. during the second grinding step S30.

[0071] In the second grinding step S30, the rotational speed of the chuck table 4 is set to 100 rpm or more and 600 rpm or less (typically 300 rpm), and the rotational speed of the finish grinding wheel 26 is set to 1000 rpm or more and 7000 rpm or less (typically 4000 rpm).

[0072] In the second grinding step S30, the first dressing is applied to the finish grinding wheel 28b in the rough surface region 13e on the first thin plate portion 13c. Therefore, as the grinding feed is advanced, the first thin plate portion 13c can be ground with the finish grinding wheel 28b that has been dressed for the first time.

[0073] As the grinding feed rate increases further, the finish grinding wheel 28b receives a second dressing in the rough surface region 13e on the second thin plate section 13f. Therefore, as the grinding feed rate increases, the first thin plate section 13c and the second thin plate section 13f can be ground with the finish grinding wheel 28b that has received a second dressing.

[0074] In the second grinding step S30, multiple dressings can be performed at time intervals through gradual grinding. In this way, the condition of the finish grinding wheel 28b can be restored at different timings in the second grinding step S30, thus suppressing defects that tend to occur during finish grinding.

[0075] In particular, when performing finish grinding on highly doped silicon wafers, compound semiconductor single crystal substrates, single crystal sapphire substrates, etc., the condition can be restored by performing multiple dressings at time intervals during the finish grinding process, thereby enabling proper finish grinding.

[0076] Furthermore, during the finish grinding of complex oxide single crystal substrates formed from LT, LN, etc., clogging is prone to occur. When clogging occurs, more heat is generated in the processing area compared to when clogging does not occur, and heat-sensitive materials such as LT and LN become more prone to cracking. However, by performing multiple dressings with time intervals between them during the finish grinding process, the condition can be restored, reducing heat generation and allowing for proper finish grinding.

[0077] Figure 9 is a schematic cross-sectional view showing a portion of the workpiece 11 after the second grinding step S30. In Figure 9, portions of the first thin plate section 13c and the second thin plate section 13f removed in the second grinding step S30 are shown by dashed lines.

[0078] As shown in Figure 9, a disc-shaped third thin plate portion 13g is formed inside the first thin plate portion 13c that remains unground in the radial direction 11a, and is larger in diameter and thinner than the second thin plate portion 13f. In addition, the second annular step portion 21b is removed, and instead, a third annular step portion 21c is formed, which is composed of the first thin plate portion 13c and the third thin plate portion 13g.

[0079] In the second grinding step S30, as described above, only the first thin plate portion 13c is ground first, and then the first thin plate portion 13c and the second thin plate portion 13f are ground. Since the area of ​​the surface to be ground when only the first thin plate portion 13c is ground is relatively small, the grinding feed rate of the finish grinding unit 20 may be increased compared to when grinding the first thin plate portion 13c and the second thin plate portion 13f.

[0080] For example, in the first half of finish grinding, where only the first thin plate section 13c is ground, the grinding feed rate is set to 0.8 μm / s or more and 5.0 μm / s or less (typically 1.6 μm / s), and in the second half of finish grinding, where both the first thin plate section 13c and the second thin plate section 13f are ground, the grinding feed rate is set to 0.1 μm / s or more and less than 0.8 μm / s (typically, the speed is reduced in two stages, first to 0.6 μm / s and then to 0.3 μm / s).

[0081] This allows for a reduction in grinding time and increased efficiency compared to fixing the grinding feed rate throughout the entire duration of the second grinding step S30 to the relatively slow grinding feed rate used in the latter half of finish grinding.

[0082] Furthermore, compared to fixing the grinding feed rate for the entire duration of the second grinding step S30 to the relatively high grinding feed rate used in the first half of finish grinding, the amount of scratches and distortions formed on the third thin plate section 13g can be significantly reduced.

[0083] (Second Embodiment) Next, a second embodiment will be described. Figure 10 is a flowchart of the grinding method for the workpiece 11 in the second embodiment. In the second embodiment, in the first grinding step S20 after the holding step S10, the second thin plate portion 13f described above is first formed as shown in Figure 11(A) (internal processing step S26).

[0084] Figure 11(A) is a schematic cross-sectional view showing a portion of the workpiece 11 after the internal machining step S26. In the internal machining step S26, the roughened surface region 13e described above is formed on the back surface 13b side of the second thin plate portion 13f.

[0085] Then, after the internal machining step S26, the first thin plate portion 13c and the ring-shaped reinforcing portion 13d are formed as shown in Figure 11(B) (external machining step S28). Figure 11(B) is a schematic cross-sectional view showing a part of the workpiece 11 after the external machining step S28.

[0086] After the external machining step S28, the second grinding step S30 is performed using the finish grinding wheel 26. In the second embodiment as well, the condition of the finish grinding wheel 28b can be restored multiple times with time intervals in between, so that defects that tend to occur during finish grinding can be suppressed.

[0087] (Third Embodiment) Next, a third embodiment will be described. The third embodiment is basically the same as the first embodiment, but differs from the first embodiment in that a cylindrical protrusion 13h (see Figure 13(A), etc.) is formed in the center of the back surface 13b in the lower processing step S24.

[0088] To explain step by step, in the upper processing step S22 of the third embodiment, the first thin plate portion 13c is formed, similar to the first embodiment. Figure 12 is a schematic cross-sectional view showing a part of the workpiece 11 after the upper processing step S22.

[0089] In the lower machining step S24 following the upper machining step S22, the rotation center of the spindle 12 is moved towards the center of the radial direction 11a of the workpiece 11, compared to the lower machining step S24 of the first embodiment (see Figure 6(B)) (see Figures 13(A) and 13(B)).

[0090] Figure 13(A) is a top view of the lower processing step S24 in the third embodiment. In Figure 13(A), radial saw marks formed on the rough surface region 13e of the first thin plate portion 13c, the second thin plate portion 13f, and the cylindrical convex portion 13h are shown.

[0091] Figure 13(B) is a partial cross-sectional side view of the lower machining step S24 in the third embodiment. In the lower machining step S24, the back surface 13b of the single crystal substrate 13 is ground while the rotational trajectories of the multiple rough grinding wheels 18b do not pass along the extension of the rotation axis 4b of the chuck table 4.

[0092] This forms a cylindrical protrusion 13h that is located inside the second thin plate portion 13f in the radial direction 11a and has the same thickness as the first thin plate portion 13c. Figure 14 is a schematic cross-sectional view showing a part of the workpiece 11 after the lower processing step S24.

[0093] In the subsequent second grinding step S30, the finish grinding unit 20 is used to first perform finish grinding on the first thin plate portion 13c and the cylindrical protrusion portion 13h, and then, by advancing the grinding feed, finish grinding is performed on the second thin plate portion 13f.

[0094] This removes the cylindrical protrusion 13h and forms the third thin plate portion 13g (see dashed line in Figure 14). In the third embodiment as well, the condition of the finishing grinding wheel 28b can be restored multiple times with time intervals in between, thus suppressing problems that tend to occur during finishing grinding.

[0095] In the third embodiment, the cylindrical protrusion 13h is formed in the first grinding step S20, which increases the volume of the workpiece 11 to be removed by grinding in the second grinding step S30 compared to the first and second embodiments.

[0096] However, since the area of ​​the rough surface region 13e is increased compared to the first embodiment, there is an advantage in that a higher dressing effect can be obtained when grinding the first thin plate portion 13c and the cylindrical convex portion 13h with the finishing grinding wheel 28b.

[0097] (Fourth Embodiment) Next, a fourth embodiment will be described. The fourth embodiment is basically the same as the second embodiment, but differs from the third embodiment in that a cylindrical protrusion 13i (see Figure 15(A), etc.) is formed in the central part of the back surface 13b side during the internal processing step S26.

[0098] To explain step by step, in the internal machining step S26 of the fourth embodiment, the second thin plate portion 13f is formed, similar to the second embodiment. However, in the internal machining step S26 of the fourth embodiment, the rotation center of the spindle 12 is moved to the center of the radial direction 11a of the workpiece 11, compared to the upper machining step S22 of the second embodiment.

[0099] By grinding the back surface 13b of the single crystal substrate 13 while the rotational trajectories of the multiple coarse grinding wheels 18b do not pass along the extension of the rotation axis 4b of the chuck table 4, a cylindrical protrusion 13i is formed which is located inside the second thin plate portion 13f in the radial direction 11a and has the same thickness as the first thin plate portion 13c.

[0100] Figure 15(A) is a schematic cross-sectional view showing a portion of the workpiece 11 after the cylindrical protrusion 13i has been formed in the internal machining step S26. In addition, in the internal machining step S26, a rough surface region 13e is also formed on the back surface 13b side of the second thin plate portion 13f.

[0101] Figure 15(B) is a schematic cross-sectional view showing a portion of the workpiece 11 after the external machining step S28 in the fourth embodiment. In the external machining step S28, the height of the cylindrical protrusion 13i is reduced to the same height as the cylindrical protrusion 13h shown in Figure 14. In addition, a rough surface region 13e is formed on the back surface 13b side of the first thin plate portion 13c and the cylindrical protrusion 13h.

[0102] In the subsequent second grinding step S30, the finish grinding unit 20 is used to first perform finish grinding on the first thin plate portion 13c and the cylindrical protrusion portion 13h, and then, by advancing the grinding feed, finish grinding is performed on the second thin plate portion 13f. This forms the third thin plate portion 13g (see the dashed line in Figure 15(B)).

[0103] In the fourth embodiment as well, the condition of the finishing grinding wheel 28b can be restored multiple times with time intervals between each restoration, thus suppressing problems that tend to occur during finishing grinding. Furthermore, in the fourth embodiment as well as in the third embodiment, the area of ​​the roughened surface region 13e is increased, which has the advantage of providing a higher dressing effect.

[0104] Furthermore, the structures, methods, etc., according to the above embodiments can be modified as appropriate without departing from the scope of the object of the present invention. In the above embodiments, the chuck table 4 was moved horizontally, but the rough grinding unit 10 and the finish grinding unit 20 may also be moved horizontally relative to the chuck table 4.

[0105] By the way, in the first grinding step S20 of the first and second embodiments described above, two stepped portions, the first annular stepped portion 21a and the second annular stepped portion 21b, are formed, but three or more stepped portions may be formed.

[0106] While forming three or more steps may slightly increase the processing time required for the first grinding step S20, it has the advantage of allowing more opportunities to restore the condition of the finishing grinding wheel 28b in the subsequent second grinding step S30.

[0107] Furthermore, the holding surface 4a of the chuck table 4 is not limited to a conical shape. When the chuck table 4 is viewed from a radial cross-section, the holding surface 4a may have a double-concave shape with a recess between the radial center and both ends.

[0108] Even when the holding surface 4a has a double-concave shape, when grinding the workpiece 11 held by the holding surface 4a sequentially with the rough grinding unit 10 and the finish grinding unit 20, grinding is performed with the rotating shaft 4b tilted appropriately. [Explanation of symbols]

[0109] 2: Grinding device, 4: Chuck table, 4a: Holding surface, 4b: Rotating shaft 6: Frame, 8: Porous board 10: Rough grinding unit, 12: Spindle, 14: Mount 11: Workpiece, 11a: Radial direction 13: Single crystal substrate, 13a: Front surface, 13b: Back surface 13c: First thin plate section, 13d: Ring-shaped reinforcement section, 13e: Rough surface area 13f: Second thin plate section, 13g: Third thin plate section, 13h, 13i: Cylindrical convex section 15: Planned division line, 17: Device, 19: Protective component 16: Rough grinding wheel (first grinding wheel) 18a: Wheel base (first base), 18b: Coarse grinding wheel (first grinding wheel) 20: Finishing grinding unit, 22: Spindle, 24: Mount 21a: First annular step section, 21b: Second annular step section, 21c: Third annular step section 26: Finishing grinding wheel (second grinding wheel) 28a: Wheel base (second base), 28b: Finishing grinding wheel (second grinding wheel) S10: Holding step, S20: First grinding step S22: Upper machining step, S24: Lower machining step S26: Internal machining step, S28: External machining step S30: Second grinding step

Claims

1. A method for grinding a workpiece, comprising grinding a single crystal substrate in the workpiece to form a thin plate portion and a ring-shaped reinforcing portion surrounding the outer periphery of the thin plate portion, The single crystal substrate is a silicon single crystal substrate, a compound semiconductor single crystal substrate, a single crystal sapphire substrate, or a complex oxide single crystal substrate with a resistivity of 0.1 Ω·cm or less, and the process includes a holding step of holding the workpiece with a chuck table, A first grinding step is performed to form on a workpiece a ring-shaped reinforcing portion located on the outermost side in the radial direction of the workpiece, a first thin plate portion located inside the ring-shaped reinforcing portion in the radial direction and thinner than the ring-shaped reinforcing portion, and a second thin plate portion located inside the first thin plate portion in the radial direction and thinner than the first thin plate portion. A second grinding step is performed, after the first grinding step, by using a second grinding wheel in which a plurality of second grinding wheels, each containing abrasive grains having an average particle size smaller than the average particle size of the abrasive grains of the first grinding wheel, are arranged in a ring along the circumferential direction of a second annular base, to grind the first thin plate portion and the second thin plate portion. A method for grinding a workpiece, characterized by comprising the following features.

2. The first grinding step is, An upper processing step for forming the ring-shaped reinforcing portion and the first thin plate portion, After the upper processing step, a lower processing step is performed to form the second thin plate portion, A method for grinding a workpiece according to claim 1, characterized by having the following features.

3. The first grinding step is, The internal processing step for forming the second thin plate portion, After the internal processing step, an external processing step is performed to form the first thin plate portion and the ring-shaped reinforcing portion, A method for grinding a workpiece according to claim 1, characterized by having the following features.

4. In the first grinding step, By grinding the single crystal substrate of the workpiece while the rotational trajectories of the plurality of first grinding wheels do not pass along the extension of the rotation axis of the chuck table, a cylindrical convex portion is formed that is located radially inside the second thin plate portion and has the same thickness as the first thin plate portion. In the second grinding step, A method for grinding a workpiece according to claim 2 or 3, characterized in that the first thin plate portion, the second thin plate portion, and the cylindrical convex portion are ground.

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