Strength inspection method for stacked chips

Abstract

To provide a laminated chip strength inspection method capable of easily comparing the bonding strength between laminated chips.SOLUTION: A method for inspecting the strength of a laminated chip in which two or more chips are laminated includes a modified layer forming step of forming a modified layer by positioning, inside the chip, a focused point of a laser beam having a wavelength transmissive to the chip and irradiating the chip with the laser beam, and a detachment inspection step of inspecting detachment of the laminated chip.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for inspecting the strength of a stacked chip in which two or more chips are stacked. [Background technology]

[0002] Wafers, on whose surface multiple devices such as ICs and LSIs are formed and partitioned by planned division lines, are divided into individual device chips using a dicing machine equipped with a cutting blade, a laser processing machine that irradiates a laser beam, or the like, and these are used in electrical equipment such as mobile phones and personal computers.

[0003] Furthermore, in order to improve the functions of device chips, a stacked wafer in which two or more wafers are stacked is divided into individual device chips by a dicing device or the like (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2005-116614 A Summary of the Invention [Problem to be solved by the invention]

[0005] Wafer stacking is performed by, for example, room temperature wafer bonding, underfill bonding, etc. Since stacked device chips formed by these bonding methods may peel off and break due to various external stresses such as heat, tension, shear, compression, and torsion, a stress test is performed using a stress tester to inspect the strength of the stacked device chips.

[0006] In order to evaluate the effects of the various external stresses described above, it is necessary to prepare a stress tester that can handle each type of external stress, which is uneconomical. In addition, when it is desired to perform strength testing on the bonding between stacked device chips in accordance with the bonding method and bonding conditions, a comparative evaluation is required based on quantitative values ​​detected by a tensile test, which is troublesome.

[0007] In addition, a test in which a stacked dummy wafer, which is formed by stacking two or more dummy wafers, is divided into individual stacked dummy chips using a dicing device or the like, and then the bonding method and bonding conditions used in stacking are selected, also has the same problems as the strength test of the stacked device chip described above.

[0008] The present invention has been made in consideration of the above-mentioned facts, and its main technical objective is to provide a strength inspection method for stacked chips, which makes it possible to easily compare the bonding strength between stacked chips, eliminates the need to prepare various types of stress testing machines to quantitatively detect the bonding strength between stacked chips, and thereby eliminates the problem of unbearable hassle. [Means for solving the problem]

[0009] In order to solve the above-mentioned main technical problem, according to the present invention, there is provided a strength inspection method for a stacked chip in which two or more chips are stacked, comprising a modified layer formation step of forming a modified layer by positioning a focal point of a laser beam having a wavelength that is transparent to the stacked chip inside the stacked chip and irradiating the laser beam to form a modified layer, and a peeling inspection step of inspecting the stacked chip for peeling.

[0010] In the modified layer forming step, the forming conditions of the modified layer can be appropriately set, and the power density of the laser beam can be appropriately set in the modified layer forming step.

[0011] In addition, the "stacked chip" in this invention includes at least a stacked device chip obtained by dividing a stacked wafer on which a plurality of devices are formed into individual device chips along the planned division lines, and a stacked dummy chip obtained by dividing a stacked dummy wafer into individual chips based on the dimensions of the intended device. Effect of the Invention

[0012] The strength inspection method for stacked chips of the present invention is a strength inspection method for stacked chips in which two or more chips are stacked, and includes a modified layer formation process in which a focused point of a laser beam having a wavelength that is transparent to the chip is positioned inside the chip and irradiated to form a modified layer, and a peeling inspection process in which peeling of the stacked chips is inspected.Therefore, it is possible to easily compare the strength of stacked chips having different bonding conditions, and there is no need to prepare various stress testing machines and compare the strength of stacked chips in quantitative values, which eliminates the problem of unbearable hassle. [Brief description of the drawings]

[0013] [Figure 1] FIG. 2 is a perspective view of a stacked chip to be inspected in the present embodiment. [Diagram 2] FIG. 2 is a perspective view of a laser processing apparatus suitable for carrying out the modified layer forming step of the present embodiment. [Diagram 3] FIG. 1A is a perspective view showing an embodiment of a modified layer forming step, FIG. 1B is a side view showing a modified layer formed on a laminated chip, and FIG. 1C is a side view showing peeling that has occurred on the laminated chip. [Figure 4] FIG. 11 is a side view of a stacked chip inspected after the first to third modified layer forming steps. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment of a method for inspecting strength of stacked chips according to the present invention will be described in detail with reference to the accompanying drawings.

[0015] FIG. 1 shows four stacked chips 10A to 10D to be subjected to the strength inspection of this embodiment. When obtaining the stacked chips 10A to 10D, for example, four types of stacked wafers are prepared by stacking two dummy wafers made of the same material (for example, silicon (Si)). The four types of stacked wafers are bonded by room temperature wafer bonding using a surface activation method, for example, and are prepared by bonding under four different bonding conditions (surface roughness when the bonding surface is polished, pressure applied when bonding, etc.) assumed when forming an actual stacked wafer. Once the four types of stacked wafers are prepared, they are divided into individual stacked chips by a dicing device or the like so as to have the expected device dimensions (for example, 20 mm x 20 mm), and the stacked chips 10A to 10D shown in the figure are formed. The stacked chips 10A to 10D thus formed are all stacked chips composed of a lower silicon chip 12a and an upper silicon chip 12b, and differ only in the bonding conditions of the bonding portion that forms the interface 14 between the lower chip 12a and the upper chip 12b.

[0016] 1, stacked chips 10A-10D are positioned at predetermined positions inside an opening Fa formed in an annular frame F, and are supported by the frame F via adhesive tape T attached with the lower chip 12a side facing downward. The stacked chips 10A-10D supported by the frame F are transported to a laser processing device 2 shown in FIG. 2, and the laser processing device 2 performs the stacked chip strength inspection method of the present embodiment.

[0017] 2 shows an overall perspective view of the laser processing device 2. The laser processing device 2 is provided with a laser irradiation means 4 arranged on a base 2a and irradiating a laser beam onto the stacked chips 10A to 10D as the inspection object, a holding means 20 holding the stacked chips 10A to 10D via a frame F and an adhesive tape T, an imaging means 6 imaging the frame F held by the holding means 20, a feed means 30 feeding the laser irradiation means 4 and the holding means 20 relatively for processing and moving the imaging means 6 and the holding means 20 relatively, and a frame 8 consisting of a vertical wall portion 81 erected on the side of the feed means 30 on the base 2a and a horizontal wall portion 82 extending horizontally from the upper end of the vertical wall portion 81.

[0018] An optical system (not shown) constituting the laser irradiation means 4 is housed inside the horizontal wall 82 of the frame 8. A condenser 42 constituting a part of the laser irradiation means 4 is disposed on the underside of the tip of the horizontal wall 82. The imaging means 6 is disposed at a position adjacent to the condenser 42 in the X-axis direction indicated by the arrow X in the figure. The plane defined by the X-axis direction and the Y-axis direction is substantially horizontal.

[0019] As shown in Fig. 2, the holding means 20 includes a rectangular X-axis direction movable plate 21 mounted on the base 2a so as to be movable in the X-axis direction, a rectangular Y-axis direction movable plate 22 mounted on the X-axis direction movable plate 21 so as to be movable in the Y-axis direction, a cylindrical support 23 fixed to the upper surface of the Y-axis direction movable plate 22, and a rectangular cover plate 24 fixed to the upper end of the support 23. A chuck table 25 is disposed on the cover plate 24 and extends upward through an elongated hole formed on the cover plate 24. The chuck table 25 is configured to be rotatable by a rotation drive means (not shown) housed in the support 23. A circular suction chuck 26 made of a porous material having air permeability and extending substantially horizontally is disposed on the upper surface of the chuck table 25. The suction chuck 26 is connected to a suction means (not shown) by a flow path passing through the support 23, and four clamps 27 are disposed at intervals around the suction chuck 26. The clamps 27 grip the frame F when the stacked chips 10A to 10D, which are held on the annular frame F via the adhesive tape T, are fixed to the chuck table 25.

[0020] The feed means 30 includes an X-axis feed means 31 and a Y-axis feed means 32. The X-axis feed means 31 converts the rotational motion of the motor 33 into linear motion via a ball screw 34, transmits the motion to the X-axis movable plate 21, and moves the X-axis movable plate 21 forward and backward in the X-axis direction along a pair of guide rails 2b, 2b arranged on the base 2a along the X-axis direction. The Y-axis feed means 32 converts the rotational motion of the motor 35 into linear motion via a ball screw 36, transmits the motion to the Y-axis movable plate 22, and moves the Y-axis movable plate 22 forward and backward in the Y-axis direction along a pair of guide rails 28, 28 arranged on the X-axis movable plate 21 along the Y-axis direction.

[0021] The laser processing apparatus 2 shown in FIG. 2 has a configuration generally as described above, and the method for inspecting strength of stacked chips according to this embodiment, which is carried out using this laser processing apparatus 2, will be specifically described.

[0022] The stacked chips A to D described with reference to FIG. 1 are placed on the suction chuck 26 of the chuck table 25 via an adhesive tape T, and are sucked and held by operating a suction means (not shown).

[0023] The stacked chips A to D held by the chuck table 25 are moved directly below the imaging means 6 by operating the above-mentioned feeding means 30, and imaged. The irradiation position of the laser beam on each of the stacked chips 10A to 10D is detected and stored in a control means (alignment) not shown.

[0024] After the above-mentioned alignment is performed, the feed means 30 is operated to move the chuck table 25 in the X-axis direction to position it directly under the condenser 42 of the laser irradiation means 4, as shown in Fig. 3(a). Next, the condenser 42 is moved vertically to position the focal point of the laser beam LB, which has a wavelength that is transparent to at least the upper chip 12b, by the laser irradiation means 4, at a predetermined depth in the upper chip 12b constituting the stacked chips 10A to 10D, as shown in Fig. 3(b). In the strength inspection method of this embodiment, the focal point is positioned in the upper chip 12b at a position closer to the interface 14 than the top surface.

[0025] As described above, once the focal point is positioned within the upper chip 12b, the X-axis feed means 31 and the Y-axis feed means 32 of the feed means 30 are operated while irradiating the laser beam LB having a wavelength that is transparent to at least the upper chip 12b, to form, for example, three modified layers 100 along the X-axis direction in each of the stacked chips 10A, 10B, 10C, and 10D as shown by the dashed lines in Fig. 3(a) (modified layer forming step). The processing conditions for the laser processing performed on the stacked chips 10A to 10D are set, for example, as follows.

[0026] Wavelength 1342nm Average power: 3W Repetition rate: 90kHz Pulse width 150nm Spot diameter φ10μm Machining feed speed 900mm / sec

[0027] Incidentally, when the modified layer forming process is performed on the stacked chips 10A-10D as described above, the modified layer 100 is formed on the interface 14 side in the upper chip 12b of each stacked chip 10A-10D, and the interface 14 side of the upper chip 12b expands in the direction in which the modified layer 100 is formed (the X-axis direction in this embodiment), so that the upper chip 12b generates a force that curves in a substantially U-shape as viewed from the Y-axis direction so that the interface 14 side is convex and the upper surface side is concave, and this force acts to peel the upper chip 12b from the lower chip 12a. Here, for example, if the force that tries to peel the upper chip 12b from the lower chip 12a at the end of the X-axis direction of the stacked chip becomes stronger than the bonding strength at the interface 14 of the stacked chip, peeling 14a will occur at the end of the X-axis direction as shown in FIG. 3(c).

[0028] As described above, after the modified layer forming step of forming three modified layers 100 on the stacked chips 10A to 10D is performed, as shown in FIG. 4(a), the interface 14 of each of the stacked chips 10A to 10D is inspected to check whether peeling 14a has occurred at the interface 14 as described based on FIG. 3(c) above (peeling inspection step). In this embodiment, the modified layer forming step and peeling inspection step cause peeling 14a to occur due to peeling of the end of the stacked chip 10C in the X-axis direction, but does not occur in the other stacked chips 10A, 10B, and 10D. As a result, the bonding strength at the interface 14 of the stacked chip 10C is weaker than that of the stacked chips 10A, 10B, and 10D, and the evaluation is determined to be × at the stage when the first modified layer forming step is performed.

[0029] As described above, after the first modified layer forming process and peeling inspection process are performed, if the strength inspection is to be continued for the stacked chips 10A, 10B, and 10D, in order to increase the stress that tries to peel the upper chip 12b from the lower chip 12a, i.e., the peel strength, a second modified layer forming process is performed to form a new modified layer 110 between the modified layers 100 in addition to the above three modified layers 100, as shown in FIG. 4(b). The processing conditions of the laser beam when forming the modified layer 110 are the same as those of the first modified layer forming process. This increases the density of the modified layer on the interface 14 side of the upper chip 12b of the stacked chips 10A, 10B, and 10D, and increases the peel strength that tries to peel the upper chip 12b from the lower chip 12a. After performing this second modified layer forming process, the peeling inspection process is performed again to inspect whether peeling 14a has occurred at the interface 14 of the stacked chips 10A, 10B, and 10D (peeling inspection process). In this embodiment, peeling 14a due to peeling as described based on Figure 3(c) occurred at the end in the X-axis direction of the stacked chip 10A shown in Figure 4(b), and it was determined that the bonding strength forming the interface 14 of the stacked chip 10A was weak compared to the stacked chips 10B and 10D, and the evaluation was given an X.

[0030] As described above, when the strength test is continued for the stacked chips 10B and 10D after the second modified layer forming step and peeling inspection step are performed, in order to increase the stress that tries to peel the upper chip 12b from the lower chip 12a, i.e., the peel strength, a modified layer forming step is performed to form a new modified layer 120 between the existing modified layers 100 and 110 in addition to the modified layers 100 and 110 as shown in FIG. 4(c). The processing conditions of the laser beam when forming the modified layer 120 are the same as those of the first and second modified layer forming steps. This further increases the density of the modified layer on the interface 14 side of the upper chip 12b of the stacked chips 10B and 10D, and further increases the peel strength that tries to peel the upper chip 12b from the lower chip 12a. After carrying out this third modified layer forming process, the above peeling inspection process is carried out to inspect whether peeling 14a has occurred at the interface 14 between the stacked chips 10B and 10D (peeling inspection process). In this embodiment, peeling 14a due to peeling as explained based on Fig. 3(c) has occurred at the end in the X-axis direction of the stacked chip 10D shown in Fig. 4(c), and it is determined that the bonding strength forming the interface 14 of the stacked chip 10D is weak relative to the stacked chip 10B, and the evaluation is x.

[0031] According to the above-described embodiment, in stacked chips 10A to 10D, it is easily determined that the bonding strength between lower chip 12a and upper chip 12b is 10B>10D>10A>10C, and when selecting a bonding method for two-layer wafers to be used in forming the stacked chip, the bonding conditions for the stacked wafers adopted in forming stacked chip 10B can be selected.

[0032] The laser processing conditions when carrying out the modified layer forming process of the present invention are not limited to the above embodiment. In the above embodiment, when the laser beam is irradiated to the laminated chips 10A to 10D, the peel strength for peeling the upper chip 12b from the lower chip 12a is adjusted by changing the interval of the modified layer formed on the upper chip 12b of the laminated chip, but the forming conditions of the modified layer when forming the modified layer and changing the peel strength can be appropriately set to other forms. For example, when forming the modified layer, the number of irradiations of the laser beam per predetermined area for forming the modified layer may be changed, the position in the depth direction for forming the modified layer may be changed to change the curvature direction of the upper chip 12b, or the modified layer may be formed so as to be concentrated in an arbitrary area (for example, the central area).

[0033] In order to change the peel strength, the power density of the laser beam irradiated in the modified layer forming step may be appropriately set.

[0034] According to this embodiment, it becomes possible to easily compare the strength of stacked chips having different bonding conditions, eliminating the need to prepare various types of stress testing machines and compare the strength of stacked chips in quantitative values, thereby eliminating the problem of unbearable hassle. [Explanation of symbols]

[0035] 2: Laser processing equipment 2a:Stationary base 2b: Guide rail 4: Laser irradiation means 42: Concentrator 6: Imaging means 8:Frame body 81: Vertical wall section 82:Horizontal wall 10A~10D: Stacked chips 12a: Lower chip 12b: Upper chip 14: Interface 14a: Peeling 20: Holding means 21:X-axis movable plate 22: Y-axis direction movable plate 23: Post 24: Cover plate 25: Chuck table 26: Suction chuck 27: Clamp 28: Guide rail 30:Transportation means 31: X-axis feed means 32: Y-axis feed means 33: Motor 34: Ball screw 35: Motor 36: Ball screw 100, 110, 120: Modified layer

Claims

1. A method for inspecting strength of a stacked chip in which two or more chips are stacked, comprising the steps of: a modified layer forming step of forming a modified layer by irradiating a laser beam having a wavelength that is transparent to the laminated chip with a focal point positioned inside the laminated chip; a peeling inspection step of inspecting peeling of the laminated chip; A method for inspecting the strength of a stacked chip comprising the steps of:

2. 2. The method for inspecting strength of laminated chips according to claim 1, wherein in the modified layer forming step, conditions for forming the modified layer are appropriately set.

3. 3. The method for inspecting strength of laminated chips according to claim 1, wherein the power density of the laser beam is appropriately set in the modified layer forming step.

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