Metal bump flattening method and flip chip method

Abstract

The application discloses a metal bump flattening method and a flip chip method. The metal bump flattening method comprises the following steps: forming a metal bump on a substrate, wherein the substrate is a single chip or a wafer without being cut; and sequentially performing mechanical cutting and thinning on the metal bump, so as to remove tail filaments and necks on the top of the metal bump, retain a ball disc part of the metal bump, and control the consistency of the ball disc part within 2 microns. The flattening method solves the problem of insufficient consistency of large-scale metal bumps on small-size single chips, and the problem of open circuit or virtual connection caused by incomplete bonding of excessively low metal bumps in the ultrasonic thermal pressure flip chip process. The flattening operation can obviously improve the density of flip chip bumps, so that the welding strength of the prepared flip chip is high, and the electrical connectivity is good.

Description

Technical Field

[0001] This invention belongs to the field of flip chip technology, and specifically relates to a method for flattening metal bumps, as well as a flip chip method. Background Technology

[0002] Flip chip bonding, a microelectronic packaging technology, involves bonding a chip with bumped electrodes on its active surface (the surface of the active area) directly to the substrate's wiring layer. Compared to traditional wire bonding, the bumped electrodes in the flip chip bonding area can be arrayed across the entire surface through rewiring. Furthermore, flip chip bonding offers advantages over traditional wire bonding primarily in its higher density and increased I / O density per unit area. Since flip chip bump fabrication is primarily done on a wafer / chip basis, it offers higher production efficiency and reduces the cost of mass packaging compared to wire bonding, which is based on individual leads.

[0003] However, as the number of I / Os per unit area on the front side of the chip continues to increase, the impact on the ultrasonic thermocompression flip-chip bonding method for metal bumps is particularly severe, especially for large-scale metal bumps on small-sized single chips. Specifically, due to the increase in the number of bumps, firstly, the flip-chip contact area increases, increasing the difficulty of ultrasonic bonding; secondly, the increased number of bumps affects the consistency of the bumps, requiring a greater pre-compression force during ultrasonic thermocompression, and bumps with excessively low height are prone to open circuits. Summary of the Invention

[0004] In view of this, the present invention needs to provide a method for flattening metal bumps, which solves the problem of insufficient consistency of large-scale metal bumps on small-sized single chips, and the problem of open circuits or loose connections caused by incomplete bonding of excessively low bumps during ultrasonic thermoforming flip-chip bonding.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention provides a method for flattening metal bumps, comprising the following steps:

[0007] Metal bumps are formed on a substrate, which is a single chip or an undivided wafer;

[0008] The metal bumps are thinned by mechanical cutting. While maintaining consistency, the tail wire and neck of the metal bumps are removed, leaving the bump ball portion, and the consistency of the ball portion is controlled within 2μm.

[0009] In a further embodiment, the substrate is a single chip, and the metal bump flattening method includes the following steps:

[0010] Metal bumps are formed on a single chip;

[0011] The single chips with metal bumps are attached to a UV film to form an array, and the distance between adjacent single chips is ≥ 2 times the thickness of the single chip;

[0012] The metal bumps on the chip after mounting are thinned by mechanical cutting. While maintaining consistency, the tail wire and neck of the metal bump are removed, and the bump ball portion is retained. The consistency of the ball portion is controlled within 2μm.

[0013] In a further embodiment, the single chip is a silicon wafer with a single-sided dimension ≥5mm and a single-wafer area ≥25mm². 2 .

[0014] In a further embodiment, the number of metal bumps is ≥2000, and the spacing between the bumps is ≥150μm.

[0015] In a further embodiment, the metal bump is a gold bump, which is formed by bonding balls onto the substrate pads using gold wire bonding.

[0016] In a further embodiment, the gold wire bonding process involves gold wires with a diameter of 25 μm and / or 18 μm and a purity of ≥99.99%.

[0017] In a further embodiment, the metal bumps obtained by attaching balls with a wire diameter of 25 μm have a diameter of 70-120 μm and a corresponding height range of 35-65 μm; the metal bumps obtained by attaching balls with a wire diameter of 18 μm have a diameter of 45-60 μm and a corresponding height range of 15-20 μm.

[0018] The present invention further provides a flip-chip method, comprising the following steps:

[0019] The metal bumps on the substrate are leveled using the metal bump leveling method described above.

[0020] Pick up the flattened substrate;

[0021] After the substrate is picked up, it is connected to the substrate by ultrasonic thermo-press flip-chip welding.

[0022] In a further embodiment, the substrate is made of aluminum oxide or aluminum nitride, the height of the flip-chip bonding area on the substrate does not exceed 3μm, and the surface is wired using thin film or thick film processes; the flip-chip bonding pads on the substrate are made of gold with a thickness ≥1μm.

[0023] In a further embodiment, the ultrasonic thermo-press flip-chip bonding process specifically involves: placing the side of the substrate with metal bumps facing the substrate side, and then, under the influence of temperature and ultrasound, interconnecting the interface between the bonding pads and the metal bumps on the substrate after they have been leveled.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] By controlling the substrate consistency within an acceptable range, it ensures that the ultrasound can reach the interface between each bump and the substrate during the subsequent ultrasonic thermocompression flip-chip bonding process, guaranteeing the flip-chip bonding strength. Furthermore, it controls the highest point of the bumps on the substrate surface within a certain range, ensuring electrical continuity. This solves the problem of insufficient consistency of large-scale metal bumps on small-sized substrates, and the open circuit or poor connection issues caused by incomplete bonding of excessively low bumps during ultrasonic thermocompression flip-chip bonding. Attached Figure Description

[0026] Figure 1 This is a diagram of metal bump ball placement in a preferred embodiment of the present invention;

[0027] Figure 2 This is a schematic diagram of single-chip mounting in a preferred embodiment of the present invention;

[0028] Figure 3 for Figure 1 Morphology of the metal bumps formed in the process;

[0029] Figure 4 for Figure 2 Schematic diagram of leveling after single-chip mounting;

[0030] Figure 5 A topographical image of the flattened metal protrusions;

[0031] Figure 6 This is a schematic diagram of ultrasonic thermocompression flip welding. Detailed Implementation

[0032] The embodiments of the present invention are described in detail below. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0033] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.

[0034] The first aspect of this invention provides a method for flattening metal bumps. After preparing metal bumps on a substrate, the bumps are thinned by mechanical cutting to achieve flattening. This solves the problems of insufficient consistency of large-scale metal bumps on small-sized substrates used in actual production, and the open circuit or poor connection caused by incomplete bonding of bumps with too low a bonding depth during ultrasonic thermocompression flip bonding. Improving the consistency of bumps can increase the number of bumps in ultrasonic thermocompression flip bonding to a certain extent.

[0035] According to an embodiment of the present invention, the method for flattening metal bumps mainly includes the following steps:

[0036] S100. Form metal bumps on a substrate, wherein the substrate is a single chip or an undivided wafer.

[0037] The metal bumps are not particularly limited and can be composed of conventional bumps in the art. In some specific embodiments of the present invention, the metal bumps are gold bumps, which are formed by ball bonding on the substrate pads using conventional gold wire bonding technology. It is understood that different gold wires can form different bumps. According to embodiments of the present invention, in the gold wire bonding process, the diameter of the gold wire is 25 μm and / or 18 μm, and the purity is not less than 99.99%. The formed metal bumps are divided into two categories: metal bumps obtained by ball bonding with a 25 μm wire diameter have a diameter of 70-120 μm and a corresponding height range of 35-65 μm; metal bumps obtained by ball bonding with an 18 μm wire diameter have a diameter of 45-60 μm and a corresponding height range of 15-20 μm. According to embodiments of the present invention, the number of formed metal bumps is ≥2000, and the spacing between the bumps is ≥150 μm.

[0038] S200. Thin out the formed metal bumps, remove the top tail and neck of the metal bumps while maintaining consistency, retain the bump ball portion, and keep the consistency of the ball portion within 2μm.

[0039] In this invention, the metal bumps are thinned by mechanical cutting, specifically by grinding with a grinding wheel or cutting with a cutting tool. Mechanical cutting removes metal (such as gold or copper) from the substrate surface, thus preventing the grinding disc from becoming covered in metal during polishing, losing its self-sharpening properties, and hindering continuous processing. It also avoids burrs around the bumps after metal leveling, and even prevents cracks from appearing at the bump edges.

[0040] Furthermore, when the substrate is a single chip, the metal bump flattening method includes the following steps:

[0041] S100: Forming metal bumps on a single chip;

[0042] S200: Single chips with metal bumps are mounted onto a UV film to form an array. The distance between adjacent chips is ≥2 times the thickness of the single chip. Forming the single chips into an array facilitates subsequent mounting and pick-up, and the array structure facilitates subsequent inspection and marking. The mounting method is an automatic mounting method, and the mounting pressure remains consistent throughout the mounting process. Furthermore, the mounting process prevents the single chips from moving or tilting during subsequent processing. In addition, by controlling the spacing between adjacent chips, surface scratches caused by debris remaining between the chips during leveling and chip chipping due to edge contact during transport are avoided.

[0043] S300: Thin the metal bumps on the chip after mounting. Remove the tail wire and neck of the metal bump while maintaining consistency, retain the bump ball portion, and keep the consistency of the ball portion within 2μm.

[0044] According to an embodiment of the present invention, the single chip is a silicon wafer with a single-sided dimension ≥ 5 mm and a single-wafer area ≥ 25 mm². 2 .

[0045] A second aspect of this invention discloses a flip-chip method, comprising the following steps:

[0046] The metal bumps on the substrate are leveled using the metal bump leveling method described in the first aspect of this invention.

[0047] Pick up the flattened substrate;

[0048] After the substrate is picked up, it is connected to the substrate by ultrasonic thermo-press flip-chip welding.

[0049] According to an embodiment of the present invention, the substrate is aluminum oxide or aluminum nitride, the height of the flip-chip bonding area of ​​the substrate does not exceed 3μm, and the surface is wired using thin film or thick film processes; the material of the flip-chip bonding pads on the substrate is the same as the material of the metal bumps, preferably gold, and the thickness of the flip-chip bonding pad material is ≥1μm.

[0050] In a further embodiment, the ultrasonic thermo-press flip-chip bonding process specifically involves: placing the side of the substrate with metal bumps facing the substrate side, and then, under the influence of temperature and ultrasound, interconnecting the interface between the bonding pads and the metal bumps on the substrate after they have been leveled.

[0051] The present invention will be described below through specific embodiments. It should be noted that the specific embodiments below are for illustrative purposes only and do not limit the scope of the present invention in any way. In addition, unless otherwise specified, methods without specific conditions or steps are conventional methods, and the reagents and materials used can be obtained commercially.

[0052] Example 1

[0053] In this embodiment, the single chip used is a silicon wafer with a single-sided dimension of 5mm and a single-wafer area of ​​25mm². 2 .

[0054] Forming gold bumps

[0055] Chip balling is performed using a gold wire ball bonding machine. First, a ball is formed at the end of the gold wire using electrical discharge machining (EDM). Then, under the combined effects of heating, pressurization, and ultrasound, the gold ball is bonded to the chip's electrodes. Next, the cutting tool is lifted while the wire clamp remains open, and a section of the wire is fed out. Finally, the wire clamp closes, and the cutting tool moves left and right, cutting the gold wire through stretching and necking, completing the entire bump fabrication. (See [reference needed]). Figure 1 .

[0056] Chip Surface Mount

[0057] Using automated placement, the chip, with its ball-mounted structure facing up, is placed onto the UV film. The placement position is designed so that the distance between adjacent chips is ≤2 times the chip thickness. This avoids surface scratches caused by debris remaining between chips during leveling and chip chipping due to edge contact during transport. (See [reference needed]). Figure 2 .

[0058] Gold bump flattening

[0059] The gold bumps are thinned using a thinning device (grinding wheel or cutting tool). While maintaining consistency, the tail and neck of the bump are removed, retaining the ball-and-plate portion. During the removal of the tail and neck, the consistency of the remaining ball-and-plate portion is controlled to within 2μm. Figure 3 For details on leveling, please refer to [link / reference]. Figure 4 .

[0060] Chip pickup

[0061] Remove the processed chip for chip picking.

[0062] Ultrasonic thermocompression flip welding

[0063] After the samples are picked up, they undergo ultrasonic thermocompression flip welding. For details, please refer to [link / reference needed]. Figure 5 and Figure 6 .

[0064] Example 2

[0065] In this embodiment, an undivided 8-inch wafer is used as the substrate. Gold bumps are leveled on the entire wafer. The specific steps are as follows:

[0066] Forming gold bumps

[0067] Gold wire ball bonding machines are used to attach gold balls to undivided wafers. First, a ball is formed at the end of the gold wire using electrical discharge machining. Then, under the combined action of heating, pressure and ultrasound, the gold ball is bonded to the electrode of the chip. Next, the cleaver is lifted while the wire clamp remains open, and a section of the tail wire is fed out. Finally, the wire clamp is closed and the cleaver moves left and right, cutting the gold wire through stretching and necking, thus completing the fabrication of the entire bump.

[0068] Gold bump flattening

[0069] The entire wafer is fixed on the stage of the thinning equipment, and the gold bumps are thinned using the thinning equipment. While maintaining consistency, the tail wire and neck of the bump are removed, and the bump ball disk part is retained. When removing the tail wire and neck, the consistency of the remaining ball disk surface part is controlled within 2μm to achieve the flattening of the gold bumps.

[0070] Segmentation and Segmentation

[0071] The processed wafers are removed and then diced and picked up.

[0072] Ultrasonic thermocompression flip welding

[0073] The samples after picking up the wafers are subjected to ultrasonic thermo-press flip welding.

[0074] The flip chips obtained in Examples 1 and 2 were subjected to pull-out tests (referring to method 2031 in GJB548B-2005 Microelectronic Device Test Methods and Procedures) and shear tests (referring to method 2019.2 in GJB548B-2005 Microelectronic Device Test Methods and Procedures). The tests showed a significant increase in the bonding strength of the flip chips, with both the pull-out force and shear force significantly improved. This is mainly due to the improved bump consistency; furthermore, the improved bump consistency significantly increases the bump density and electrical connectivity of the flip-chip bonded chip.

[0075] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0076] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for leveling metal bumps, characterized in that, Includes the following steps: Forming metal bumps on a substrate, the substrate is a single chip, the single chip has a single side dimension ≥5mm, and a single chip area ≥25mm 2 The number of the metal bumps is ≥2000, and the pitch between the metal bumps is ≥150μm; the metal bumps are gold bumps, the gold bumps are formed by ball planting on the substrate pads through gold wire bonding, and the purity of the gold wire is ≥99.99% The single chips with metal bumps are attached to a UV film to form an array, and the distance between adjacent single chips is ≥ 2 times the thickness of the single chip; The metal bumps on the single chip after mounting are thinned by grinding with a grinding wheel. While maintaining consistency, the tail wire and neck of the metal bump are removed, and the bump ball portion is retained. The consistency of the ball portion is controlled within 2μm.

2. The method for leveling metal bumps as described in claim 1, characterized in that, The single chip is a silicon wafer.

3. The method for leveling metal bumps as described in claim 1 or 2, characterized in that, In the gold wire bonding, the diameter of the gold wire is 25 μm and / or 18 μm.

4. The method for leveling metal bumps as described in claim 3, characterized in that, Among the metal bumps, the diameter of the metal bumps obtained by attaching balls with a wire diameter of 25μm is 70-120μm, and the corresponding height range is 35-65μm; the diameter of the metal bumps obtained by attaching balls with a wire diameter of 18μm is 45-60μm, and the corresponding height range is 15-20μm.

5. A flip-chip method, characterized in that, Includes the following steps: The metal bumps on the substrate are leveled using the metal bump leveling method according to any one of claims 1-4; Pick up the flattened substrate; After the substrate is picked up, it is connected to the substrate by ultrasonic thermo-press flip-chip welding.

6. The flip-chip method as described in claim 5, characterized in that, The substrate is made of aluminum oxide or aluminum nitride, and the height of the flip-chip bonding area on the substrate does not exceed 3μm. The surface is wired using thin film or thick film technology. The flip-chip bonding pads on the substrate are made of the same material as the metal bumps, and their thickness is ≥1μm.

7. The flip-chip method as described in claim 5, characterized in that, The ultrasonic thermo-press flip-chip bonding process is as follows: the side of the substrate with metal bumps faces the substrate side, and under the action of temperature and ultrasound, the interface of the bonding pads and the interface of the metal bumps on the substrate are flattened and interconnected.

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