Wafer protection film planarization method

Abstract

The invention provides a wafer protection film planarization method. The method comprises the following steps: providing a wafer attached with a protective film, wherein the surface of the wafer fluctuates on the surface of the protective film due to the height difference between a pattern area and a cutting channel area; and performing step-by-step cutting on the protective film for at least two times so as to remove the protective film with the preset total cutting amount and realize planarization. Single large cutting is changed into stepping operation of multiple small cutting, for example, the total cutting amount of 20 micrometers is divided into two times of 10 micrometers, and the rotating speed of the second cutting is preferably reduced, so that the extrusion force of the cutting tool on the protective film in the horizontal direction can be remarkably reduced, the film tension is effectively released, and the cutting efficiency of the protective film is improved. Therefore, the problem of film wrinkling caused by the height difference between the pattern and the cutting channel is solved, and the yield of subsequent ultra-thin wafer processing is improved.

Description

Technical Field

[0001] This invention relates to the field of semiconductor technology, and in particular to a method for planarizing a wafer protective film. Background Technology

[0002] With the continuous development of semiconductor manufacturing technology, especially with the increasing demand for thinner and more integrated chip packaging, the ability to reduce wafer thickness is constantly improving. Currently, wafer thickness is continuously decreasing, and in the future, thinning operations will be carried out to 50 micrometers (µm) or even lower.

[0003] In the fabrication of ultra-thin wafers, the impact of over-grinding on the back side becomes increasingly significant for large-volume chips, sometimes even leading to chip breakage. To address this issue, the industry typically introduces a film planarization process before the grinding process. This process usually uses a cutting tool (such as a sapphire blade) to perform micron-level thinning (cutting) of the protective film substrate attached to the front side of the wafer, eliminating surface unevenness of the protective film and ensuring uniform force during grinding of the back side of the wafer, thereby achieving a back side free of over-grinding after processing.

[0004] However, after the protective film is applied to a large chip, the surface of the protective film will also exhibit a state of undulation with the morphology of the underlying layer due to the significant height difference between the patterned area (i.e., the chip functional area) and the dicing area on the wafer surface.

[0005] like Figure 1 The diagram shown illustrates the cutting process in the prior art for membrane planarization. Figure 1 The gray structure at the bottom is a support platform, which supports a wafer with a flat lower surface and a raised pattern on the upper surface. A protective film covers the wafer. As can be seen, due to the raised pattern on the upper surface of the wafer, the top surface of the protective film also has corresponding undulations, that is, it bulges above the pattern area and collapses in the dicing area, which makes the surface of the protective film uneven.

[0006] In existing technologies (before improvement), a single, deep cut is typically used to eliminate the height difference caused by the protective film being lifted. For example... Figure 1 As shown in the diagram, the cutting tool directly performs a single cut of 20µm during operation. When the cutting tool acts directly on the uneven and elastic protective film surface with such a large cutting amount, it generates a huge horizontal compressive force (shear force) at the processing interface. Although this single large cutting method can remove material, the uneven stress release caused by the height difference of the protective film itself can easily lead to film wrinkling problems after the film planarization process.

[0007] The presence of film wrinkles can severely affect subsequent wafer thinning, transfer, and other related operations, thereby reducing product yield and production efficiency. Therefore, how to effectively improve film wrinkle abnormalities after the film planarization process and avoid stress concentration problems caused by single large cutting has become a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0008] This application provides a wafer protective film planarization method, which aims to solve the technical problem in the prior art where, in the film planarization process of large-volume chip processing, the height difference between the wafer surface pattern and the dicing path causes the protective film to be squeezed and wrinkled during a single cutting process, thus affecting subsequent process operations.

[0009] This application provides a wafer protective film planarization method, the method comprising the following steps:

[0010] Step 1: Provide a wafer with a protective film attached. The surface of the wafer has a patterned area and a dicing area. The protective film covers the patterned area and the dicing area and forms undulations on the surface due to the height difference between the patterned area and the dicing area.

[0011] Step 2: Perform at least two separate cuts on the protective film to remove a predetermined total amount of the protective film and flatten the surface of the protective film.

[0012] Preferably, in step two, the step-by-step cutting includes a first cutting and a second cutting.

[0013] Preferably, in step two, the cutting amount of the first cut is less than the predetermined total cutting amount.

[0014] Preferably, in step two, the predetermined total cutting amount is evenly distributed between the first cut and the second cut.

[0015] Preferably, in step two, the predetermined total cutting amount is 20 micrometers or more, the cutting amount of the first cut is 10 micrometers, and the cutting amount of the second cut is 10 micrometers.

[0016] Preferably, in step two, the first cutting is used to remove some undulations on the surface of the protective film and reduce the tension of the protective film.

[0017] Preferably, in step two, the cutting speed of the second cut is lower than the cutting speed of the first cut.

[0018] Preferably, in step two, the cutting speed of the first cut is between 1400 rpm and 1600 rpm, and the cutting speed of the second cut is between 1100 rpm and 1300 rpm.

[0019] Preferably, in step two, the cutting tool used to perform the cutting is made of sapphire.

[0020] Preferably, in step one, the wafer is 6 inches to 12 inches in size.

[0021] Preferably, the method is performed before the back side of the wafer is thinned and polished.

[0022] As described above, the wafer protective film planarization method of the present invention has the following beneficial effects:

[0023] The technical solution provided in this application, by changing the cutting method from the existing single cutting to step cutting (step cutting), such as changing a single large cutting to multiple small cuttings, can significantly reduce the extrusion force of the cutting tool (such as a sapphire cutter) on the protective film in the horizontal direction, thereby effectively improving the film wrinkling problem that occurs after the film planarization process; furthermore, by reducing the speed of the secondary cutting, the cutting process can be further stabilized, ensuring that the uniformity of the protective film thickness meets the benchmark requirements, eliminating wrinkling abnormalities, and improving the yield and efficiency of subsequent wafer processing. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a single cutting operation in the existing membrane planarization process;

[0025] Figure 2 The diagram shows a schematic flow chart of the wafer protective film planarization method in an embodiment of the present invention.

[0026] Figure 3 This is a schematic diagram illustrating the step-by-step cutting principle of the membrane planarization process in an embodiment of the present invention;

[0027] Figure 4 This diagram shows a comparison of the protective film thickness data after the film planarization process between the embodiments of the present invention and the prior art. Detailed Implementation

[0028] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0029] This application provides a method for planarizing a wafer protective film. This method can be performed in a wafer grinding apparatus. The apparatus typically includes a stage, a spindle, and a cutting unit mounted at the end of the spindle. The stage is used to fix and planarize the wafer using methods such as vacuum suction. The spindle drives the cutting unit to rotate at high speed, while the stage moves the wafer horizontally relative to the cutting unit, or the spindle moves relative to the stage, thereby removing material from the wafer surface through physical cutting. The cutting unit typically includes a tool holder and a cutting head fixed at its end; the cutting head is the component that directly contacts the workpiece and performs material removal.

[0030] In some embodiments, the grinding equipment used includes two bearing stages and two spindles. A dual-station configuration can significantly increase throughput, meaning that while one station performs roughing or first-step cutting, the other station can perform finishing or transfer operations, or the two stations can alternate performing the same processes, thereby processing more wafers per unit time.

[0031] The method includes the following steps, as illustrated in the flowchart below. Figure 2 As shown:

[0032] Step 1: Provide a wafer with a protective film attached. The wafer surface has a patterned area and a dicing area. The protective film covers the patterned area and the dicing area, and the height difference between the patterned area and the dicing area creates undulations on the surface.

[0033] In some embodiments, the wafer size is 6 inches to 12 inches. This method has good versatility, applicable not only to standard-sized wafers but also to the current mainstream large-size wafer processing, meeting the production needs of different process generations. The wafer is typically fixed on the stage with its back side down and the front side with the protective film applied up, ensuring stability under the cutting force of high-speed rotation.

[0034] In some embodiments, the wafer can be a semiconductor substrate such as a silicon wafer, germanium wafer, gallium nitride wafer, silicon carbide wafer, or silicon-on-insulator (SOI) wafer. The patterned area typically contains the functional circuitry structures of this large-volume chip, such as logic circuits, memory arrays, or power devices; these structures often have considerable thickness and complex morphologies. The dicing area is relatively low-lying, causing the protective film covering it to undulate with the underlying terrain rather than remain horizontal.

[0035] In some embodiments, the protective film may be polyolefin (PO), polyethylene terephthalate (PET), polyvinyl chloride (PVC), or other pressure-sensitive tapes suitable for semiconductor processes. The protective film typically possesses a degree of elasticity and ductility; if its surface undulations are not addressed, direct back-side grinding can lead to uneven stress. This method is performed prior to the thinning and grinding process on the back side of the wafer.

[0036] Step 2: Perform at least two separate cuts on the protective film to remove the predetermined total amount of protective film and achieve surface planarization. During the cutting process, the high-speed rotating spindle drives the cutter head to contact the protective film on the wafer surface. By breaking down the original single cutting process into multiple steps, the stress generated during the cutting process can be effectively dispersed, avoiding severe extrusion deformation between the tool and the soft protective film due to excessive instantaneous cutting volume. This significantly reduces residual stress after cutting and solves the film wrinkling problem.

[0037] In some embodiments, the cutting tool material used to perform the cutting includes sapphire. The cutting tip can be a hard cutting block embedded at the end of the tool shank. In specific applications requiring planarization of softer polymer protective films, sapphire tools, due to their specific crystal structure and cutting characteristics, are often proven to achieve smoother and less damaging surface finishes. Of course, as an alternative, other superhard materials such as single-crystal diamond, polycrystalline diamond, and cubic boron nitride can also be used as the cutting tool material, matched to the specific hardness of the protective film.

[0038] In some embodiments, step two, the step-by-step cutting includes a first cut and a second cut. This two-step method is the most direct and cost-effective optimization for existing single-cutting processes, and can significantly improve yield without significantly increasing process time. At the equipment operation level, this means that the spindle's feed motion in the vertical direction (Z-axis) is subdivided into two stages, rather than pressing down to the final depth all at once.

[0039] like Figure 3 The diagram illustrates the principle of step-by-step cutting in this embodiment. The left side of the diagram indicates the "film-forming area," which is the region where the upper protective film (tape) bulges due to the raised wafer pattern below. Unlike the prior art approach of a single large-volume cut, this embodiment employs a layered removal strategy. It can be seen that the tool performs two independent cutting trajectories above the protective film.

[0040] It should be noted that although the illustrations show an implementation of two cutting steps, step-by-step cutting is not limited to two steps. In some alternative embodiments, the step-by-step cutting in step two may also include three, four, or more cutting steps. Especially when the predetermined total cutting amount is large, or when the protective film material is soft and extremely sensitive to shear stress, a multi-step, step-like micro-cutting strategy may be more advantageous. For example, the total cutting amount can be divided into three cuts, with each cut decreasing or being equally distributed; or a polishing-style finishing with a very small cutting amount can be added after two large cutting amounts (rough grinding). As long as the core idea of ​​decomposing a single large cut into multiple small cuts to reduce instantaneous extrusion pressure is followed, it falls within the scope of protection of this application.

[0041] In some embodiments, in step two, the cutting amount of the first cut is less than the predetermined total cutting amount.

[0042] In some embodiments, in step two, the predetermined total cutting amount is evenly distributed between the first cut and the second cut.

[0043] In some embodiments, in step two, the predetermined total cutting amount is 20 micrometers or more, the cutting amount of the first cut is 10 micrometers, and the cutting amount of the second cut is 10 micrometers. For example... Figure 3 As indicated in the label, the specific process parameters are set to a single cut of 10 μm, with a total of two cuts. In practical applications, the total cutting amount can be adjusted according to the initial thickness of the protective film and the required final thickness, for example, it can be between 20 μm and 50 μm. Compared to the huge horizontal shear force caused by a single 20 μm cut in the prior art, this embodiment removes only 10 μm each time, significantly reducing the cutting resistance and the instantaneous disturbance to the film layer.

[0044] In some embodiments, in step two, the first cutting is used to remove some undulations on the surface of the protective film and reduce the tension of the protective film. For example... Figure 3 As shown, the first cut (i.e., the upper 10µm) mainly targets the "film-forming area" with the greatest surface undulations of the protective film. Although this roughing step may not completely eliminate all height differences, it can remove the largest protrusions on the outermost layer, while reducing the pressure of the tool on the film in the horizontal direction. This releases some of the tensile tension inside the protective film caused by adhesion and deformation, creating a more stable stress environment for subsequent fine machining.

[0045] In some embodiments, in step two, the cutting speed of the second cut is lower than that of the first cut. After the initial thinning is completed, reducing the speed of the second cut can further reduce friction and micro-vibration between the tool and the film surface, preventing the introduction of new wrinkles or ripples due to high-speed disturbances when the film tension has been released.

[0046] In some embodiments, in step two, the cutting speed of the first cut is 1500 rpm, and the cutting speed of the second cut is 1200 rpm. It should be noted that the above values ​​are typical parameters in preferred embodiments. To accommodate protective film materials of different hardness and different types of grinding machines, the above speed parameters can fluctuate within a certain range. For example, the cutting speed of the first cut can be set between 1400 rpm and 1600 rpm, and the cutting speed of the second cut can be set between 1100 rpm and 1300 rpm. As long as the cutting speed decreases in a stepwise manner with each cut, or at least the second cutting speed is lower than the first, the effect of improving surface quality and preventing wrinkles can be achieved. By modifying the software, such as editing the stepping operation menu, the machine can support the above-mentioned variable speed stepping operation mode. The spindle speed is precisely controlled, switching from high-speed cutting in the first stage to low-speed cutting in the second stage. Through this "reduction + speed reduction" method, not only is the deformation caused by single material removal physically reduced, but the cutting stability is also optimized through the adjustment of dynamic parameters. Figure 4 The "9-point thickness Raw Data" comparison chart shows that the horizontal axis represents 9 different measurement points on the wafer, and the vertical axis represents the residual protective film thickness (unit: μm). Comparing the "film thickness of this application embodiment" curve with the "film thickness of the prior art" curve, it can be seen that after performing two-step cutting, the final film thickness distribution of this application embodiment (fluctuating around 20 μm) is comparable to that of the prior art, and the thickness Match BSL (Base Line) is within the allowable error range of the process. This proves that the present method effectively solves the problem of abnormal film wrinkles without sacrificing the thickness uniformity of the planarization process, and can replace the existing processing solutions.

[0047] It should be noted that the illustrations provided in this embodiment are only schematic representations of the basic concept of the present invention. Therefore, the drawings only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0048] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

Claims

1. A method for planarizing a wafer protective film, characterized in that, At least including: Step 1: Provide a wafer with a protective film attached. The surface of the wafer has a patterned area and a dicing area. The protective film covers the patterned area and the dicing area and forms undulations on the surface due to the height difference between the patterned area and the dicing area. Step 2: Perform at least two separate cuts on the protective film to remove a predetermined total amount of the protective film and flatten the surface of the protective film.

2. The wafer protective film planarization method according to claim 1, characterized in that: In step two, the step-by-step cutting includes a first cutting and a second cutting.

3. The wafer protective film planarization method according to claim 2, characterized in that: In step two, the cutting amount of the first cut is less than the predetermined total cutting amount.

4. The wafer protective film planarization method according to claim 2, characterized in that: In step two, the predetermined total cutting amount is evenly distributed between the first cut and the second cut.

5. The wafer protective film planarization method according to claim 4, characterized in that: In step two, the predetermined total cutting amount is 20 micrometers or more, the cutting amount of the first cut is 10 micrometers, and the cutting amount of the second cut is 10 micrometers.

6. The wafer protective film planarization method according to claim 2, characterized in that: In step two, the first cutting is used to remove some of the undulations on the surface of the protective film and reduce the tension of the protective film.

7. The wafer protective film planarization method according to claim 1, characterized in that: In step two, the cutting speed of the second cut is lower than the cutting speed of the first cut.

8. The wafer protective film planarization method according to claim 7, characterized in that: In step two, the cutting speed of the first cut is between 1400 rpm and 1600 rpm, and the cutting speed of the second cut is between 1100 rpm and 1300 rpm.

9. The wafer protective film planarization method according to claim 1, characterized in that: In step two, the cutting tool used to perform the cutting is made of sapphire.

10. The wafer protective film planarization method according to claim 1, characterized in that: In step one, the wafer is 6 inches to 12 inches in size.

11. The wafer protective film planarization method according to claim 1, characterized in that: The method is performed before the thinning and polishing process on the back side of the wafer.

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