A method for preparing frustum-shaped bump metal on a wafer

Abstract

This invention discloses a method for fabricating frustum-shaped bump metal on a wafer, belonging to the field of photolithography technology. The method includes: pre-processing a wafer to obtain a pre-processed wafer; performing photolithography and metal plating on the pre-processed wafer to obtain a wafer containing bump metal; and performing photolithography and metal etching on the wafer containing bump metal to obtain a wafer containing frustum-shaped bump metal. This invention can fabricate bump metal morphologies with consistent topography, uniform height, and small bottom, improving fault tolerance in subsequent flip-chip interconnect processes and solving the problem of short circuits between pixels caused by the pressure and compression of the bump metal during the bonding process.

Description

Technical Field

[0001] This invention relates to the field of photolithography technology, and more specifically to a method for preparing frustum-shaped bump metal on a wafer. Background Technology

[0002] Micro-LED, as a new type of display technology with high brightness, high contrast, high resolution, low power consumption, and self-emissive properties, can be widely used in AR / VR, micro-projection, 3D printing, automotive head-up displays and other related fields.

[0003] In existing technologies, the driving circuit and LED chip are generally fabricated separately, and the driving circuit and LED chip are combined through flip-chip interconnect technology to achieve one-to-one interconnection of pixel units. Although this technology saves the complex mass transfer process, it also introduces some problems that affect the effect of flip-chip interconnection. At present, the industry usually uses metal flip-chip interconnect technology to complete the one-to-one correspondence of pixel driving, thereby achieving high-precision, high-density pixel interconnection.

[0004] As Micro-LED microdisplay chips continue to be updated and iterated, the pixel pitch will become smaller and smaller, further limiting the height of the bump metal. At the same time, the warping of the driver chip and the LED chip surface will cause it to be impossible to guarantee the conduction of all pixels during the flip-chip interconnect process. Traditional bump metal fabrication processes generally require an additional high-temperature reflow process after the bump metal evaporation is completed. This makes it difficult to obtain a uniform and highly consistent bump morphology for high-density arrays with pixel pitch below 5µm. Therefore, the ability to fabricate highly uniform, consistent, and high-density bump metal is very important. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a frustum-shaped bump metal on a wafer, which solves the problem of short circuits between pixels caused by the bump metal being squeezed during the bonding process.

[0006] To solve the above-mentioned technical problems, the present invention provides a method for preparing a frustum-shaped bump metal on a wafer, comprising the following steps:

[0007] The wafer is preprocessed to obtain a preprocessed wafer;

[0008] The pre-treated wafer is subjected to photolithography and metal plating to obtain a metal wafer with bumps.

[0009] Photolithography and metal etching are performed on the bump metal wafer to obtain a wafer with inverted frustum-shaped bump metal.

[0010] Preferably, the pre-treated wafer is subjected to photolithography and metal deposition to obtain a bump-bumped metal wafer, specifically including the following steps:

[0011] After pretreatment, photoresist is coated onto the wafer, and the first photolithography is performed to obtain the wafer after the first photolithography.

[0012] After the first photolithography, a bottom metal layer is formed on the wafer. The photoresist is then removed to obtain a wafer containing the bottom metal layer.

[0013] After coating a photoresist onto a wafer containing an underlying metal layer, a second photolithography is performed to obtain a wafer after the second photolithography.

[0014] After the second photolithography, a bump metal layer covering the underlying metal layer is generated on the wafer. The photoresist is then removed to obtain a wafer with bump metal.

[0015] Preferably, the wafer containing the bumped metal is subjected to photolithography and metal etching to obtain a wafer containing the bumped metal in the shape of an inverted frustum, specifically including the following steps:

[0016] After coating a metal wafer with bumps with photoresist, a third photolithography is performed to obtain a wafer after the third photolithography.

[0017] After the third photolithography, the wafer is etched with metal to remove part of the bump metal layer, forming bumps with an inverted frustum structure. Then the photoresist is removed to obtain a wafer with bump metal in the shape of an inverted frustum.

[0018] Preferably, the wafer is pre-processed using one of the following two methods:

[0019] Method 1: Place the wafer on a 100℃ hot plate and bake for 300 seconds;

[0020] Method 2: Coat the wafer surface with HMDS and bake it on a hot plate at 120°C for 60 seconds.

[0021] Preferably, a bottom metal layer is formed on the wafer after the first photolithography, specifically including the following steps:

[0022] After the first photolithography, the wafer is first processed by a plasma cleaning device for 300 seconds, with an oxygen flow rate of 60-100 sccm and an argon flow rate of 30-50 sccm; after completion, it is immediately transferred to an evaporation machine to evaporate the bottom metal layer.

[0023] Preferably, the bottom metal layer includes an adhesion layer and a wetting layer; the adhesion layer is made of Ti / Cr and has a thickness of 30-50 nm; the wetting layer is made of Au and has a thickness of 100-300 nm.

[0024] Preferably, a bump metal layer is formed on the wafer after the second photolithography, specifically including the following steps:

[0025] After the second photolithography, the wafer is first processed by a plasma cleaning machine for 300 seconds with an oxygen flow rate of 60-100 sccm and an argon flow rate of 30-50 sccm. After completion, it is immediately transferred to an evaporation machine to deposit bump metal to form a bump metal layer.

[0026] Preferably, the thickness of the bump metal layer is 3-7 micrometers.

[0027] Preferably, the wafer needs to be cleaned before use and after removing the photoresist. The cleaning process specifically includes the following steps:

[0028] Clean the wafer with acetone and isopropanol for 5 minutes each, then rinse it with deionized water and dry it with nitrogen.

[0029] Preferably, the pattern of the photomask in the first photolithography is the target pattern;

[0030] The pattern of the photomask in the second photolithography is an overlay pattern; the orthographic projection of the overlay pattern can cover the orthographic projection of the target pattern in the first photolithography.

[0031] The pattern of the photomask in the third photolithography is the reverse target pattern; the orthographic projection of the reverse target pattern is the inverse of the target pattern in the first photolithography.

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] This invention enables the preparation of bump metal morphologies with consistent shape, uniform height, and small bottom, improving fault tolerance in subsequent flip-chip interconnect processes. Simultaneously, the frustum-shaped morphology of the bump metal significantly reduces the probability of pixel displacement during flip-chip interconnect processes, and solves the problem of short circuits between pixels caused by the pressure applied to the bump metal during bonding. Attached Figure Description

[0034] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0035] Figure 1 This is a schematic flowchart of a method for preparing a wafer-shaped frustum-shaped bump metal according to the present invention.

[0036] Figure 2 This is a schematic diagram of a frustum-shaped bump metal structure on a wafer. Detailed Implementation

[0037] Numerous specific details are set forth in the following description to provide a full understanding of the invention. However, the invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0038] The terminology used in one or more embodiments of this specification is for the purpose of describing particular embodiments only and is not intended to be limiting of the one or more embodiments of this specification. The singular forms “a,” “described,” and “the” as used in one or more embodiments of this specification and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used in one or more embodiments of this specification refers to and includes any or all possible combinations of one or more associated listed items.

[0039] It should be understood that although the terms first, second, etc., may be used to describe various information in one or more embodiments of this specification, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, first may also be referred to as second without departing from the scope of one or more embodiments of this specification, and similarly, second may also be referred to as first. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to a determination."

[0040] The following is in conjunction with the appendix Figure 1-2 The present invention will be described in further detail as follows:

[0041] like Figure 1 As shown, the present invention provides a method for preparing a frustum-shaped bump metal on a wafer, comprising the following steps:

[0042] The wafer is preprocessed to obtain a preprocessed wafer;

[0043] The pre-treated wafer is subjected to photolithography and metal plating to obtain a metal wafer with bumps.

[0044] Photolithography and metal etching are performed on the bump metal wafer to obtain a wafer with inverted frustum-shaped bump metal.

[0045] Preferably, the pre-treated wafer is subjected to photolithography and metal deposition to obtain a bump-bumped metal wafer, specifically including the following steps:

[0046] After pretreatment, photoresist is coated onto the wafer, and the first photolithography is performed to obtain the wafer after the first photolithography.

[0047] After the first photolithography, a bottom metal layer is formed on the wafer. The photoresist is then removed to obtain a wafer containing the bottom metal layer.

[0048] After coating a photoresist onto a wafer containing an underlying metal layer, a second photolithography is performed to obtain a wafer after the second photolithography.

[0049] After the second photolithography, a bump metal layer covering the underlying metal layer is generated on the wafer. The photoresist is then removed to obtain a wafer with bump metal.

[0050] Preferably, the wafer containing the bumped metal is subjected to photolithography and metal etching to obtain a wafer containing the bumped metal in the shape of an inverted frustum, specifically including the following steps:

[0051] After coating a metal wafer with bumps with photoresist, a third photolithography is performed to obtain a wafer after the third photolithography.

[0052] After the third photolithography, the wafer is etched with metal to remove part of the bump metal layer, forming bumps with an inverted frustum structure. Then the photoresist is removed to obtain a wafer with bump metal in the shape of an inverted frustum.

[0053] Preferably, the wafer is pre-processed using one of the following two methods:

[0054] Method 1: Place the wafer on a 100℃ hot plate and bake for 300 seconds;

[0055] Method 2: Coat the wafer surface with HMDS and bake it on a hot plate at 120°C for 60 seconds.

[0056] Preferably, a bottom metal layer is formed on the wafer after the first photolithography, specifically including the following steps:

[0057] After the first photolithography, the wafer is first processed by a plasma cleaning device for 300 seconds, with an oxygen flow rate of 60-100 sccm and an argon flow rate of 30-50 sccm; after completion, it is immediately transferred to an evaporation machine to evaporate the bottom metal layer.

[0058] Preferably, the bottom metal layer includes an adhesion layer and a wetting layer; the adhesion layer is made of Ti / Cr and has a thickness of 30-50 nm; the wetting layer is made of Au and has a thickness of 100-300 nm.

[0059] Preferably, a bump metal layer is formed on the wafer after the second photolithography, specifically including the following steps:

[0060] After the second photolithography, the wafer is first processed by a plasma cleaning machine for 300 seconds with an oxygen flow rate of 60-100 sccm and an argon flow rate of 30-50 sccm. After completion, it is immediately transferred to an evaporation machine to deposit bump metal to form a bump metal layer.

[0061] Preferably, the thickness of the bump metal layer is 3-7 micrometers.

[0062] Preferably, the wafer needs to be cleaned before use and after removing the photoresist. The cleaning process specifically includes the following steps:

[0063] Clean the wafer with acetone and isopropanol for 5 minutes each, then rinse it with deionized water and dry it with nitrogen.

[0064] Preferably, the pattern of the photomask in the first photolithography is the target pattern; the orthographic projection of the target pattern overlaps with the convex metal bumps to be generated in the shape of an inverted frustum.

[0065] The pattern of the photomask in the second photolithography is an overlay pattern; the orthographic projection of the overlay pattern can cover the orthographic projection of the target pattern in the first photolithography.

[0066] The pattern of the photomask in the third photolithography is the reverse target pattern; the orthographic projection of the reverse target pattern is the inverse of the target pattern in the first photolithography.

[0067] To better illustrate the technical effects of the present invention, the present invention provides the following specific embodiments to illustrate the above technical process:

[0068] Example 1: A method for preparing a frustum-shaped bump metal on a wafer, comprising the following steps:

[0069] Step 1: Clean the wafer and pre-treat it to increase surface adhesion;

[0070] Cleaning the wafer: Clean the wafer with acetone and isopropanol for 5 minutes each, then rinse with deionized water and dry with nitrogen.

[0071] Pre-treatment of wafers: Place the wafer on a hot plate at 100°C and bake for 300 seconds, or coat the wafer surface with HMDS (hexamethyldisilazane) and then place it on a hot plate at 120°C and bake for 60 seconds to increase the adhesion of the wafer surface.

[0072] Step 2: First photolithography, using the photolithography process to expose the parts of the metal to be vapor-deposited;

[0073] Using the pre-treated wafer obtained in step 1, a layer of photoresist is coated onto the wafer using a spin coater. The pattern on the photomask is then transferred onto the wafer through processes such as baking, exposure, and development.

[0074] The pattern on the first photolithography print is the target pattern; the orthographic projection of the target pattern overlaps with the convex metal bumps that need to be generated in the shape of an inverted frustum.

[0075] Step 3: Underlying metal vapor deposition;

[0076] The wafer obtained in step 2 is first processed by a plasma cleaning device for 300 seconds (oxygen flow rate 60-100 sccm, argon flow rate 30-50 sccm), and then immediately transferred to a vapor deposition machine to vapor deposit the bottom layer metal (first metal).

[0077] The underlying metal consists of an adhesion layer and a wetting layer; Ti / Cr is recommended for the adhesion layer, with a thickness of 30-50 nm; Au is recommended for the wetting layer, with a thickness of 100-300 nm.

[0078] The evaporation rate of Ti / Cr was controlled at 5 Å / s, and the evaporation rate of Au was controlled at 5 Å / s.

[0079] Step 4: Remove excess metal: Peel off the photoresist with the metal layer on the wafer, remove unwanted metal, and clean the wafer;

[0080] The wafer obtained in step 3 was soaked in N-methylpyrrolidone organic solvent for 3 hours for stripping. After stripping, it was ultrasonicated with acetone and isopropanol for 5 minutes each, cleaned with deionized water, and dried with nitrogen.

[0081] Step 5: Second photolithography to protect the alignment marks;

[0082] The wafer obtained in step 4 is coated with a layer of photoresist using a coating machine. After baking, exposure, and development, the pattern on the photomask is transferred to the wafer. The main purpose of this photolithography step is to protect the alignment marks on the wafer for alignment during the subsequent three photolithography steps.

[0083] The pattern on the photomask for the second photolithography is an overlay pattern; the orthographic projection of the overlay pattern can cover the orthographic projection of the target pattern in the first photolithography; the overlay pattern can be a circular pattern.

[0084] Step 6: Evaporate the metal bumps;

[0085] The wafer obtained in step 5 is first processed by a plasma cleaning machine for 300 seconds (oxygen flow rate 60-100 sccm, argon flow rate 30-50 sccm), and then immediately transferred to a thermal evaporation machine to deposit a 3-7 micrometer thick bump metal (second metal layer) on the wafer surface.

[0086] For the bump metal, a thermal evaporation device must be used, and the evaporation rate should be controlled at 5 angstroms / s to ensure that a dense metal film is obtained after evaporation. It is recommended to use Pb / Sn alloy, In metal or In / Sn alloy for the bump metal.

[0087] Step 7: Expose the alignment marks and clean the wafer;

[0088] The wafer obtained in step 6 was immersed in N-methylpyrrolidone organic solvent for 3 hours to remove metal. After that, it was sonicated with acetone and isopropanol for 5 minutes each, cleaned with deionized water, and dried with nitrogen.

[0089] Step 8: Third photolithography, protecting the alignment marks:

[0090] In step 7, a layer of photoresist is coated onto the wafer using a coating machine. The pattern on the photomask is then transferred to the wafer through processes such as baking, exposure, and development. This step requires the use of the alignment marks mentioned in step 5 to confirm the alignment accuracy, ensuring that the position with the alignment coordinates on the bottom is protected by photoresist on the top after the photolithography process.

[0091] The pattern on the photolithography plate of the third photolithography is the reverse target pattern; the orthogonal projection of the reverse target pattern is the inverse of the target pattern of the first photolithography.

[0092] Step 9: Chemical etching;

[0093] Chemical etching is performed using a metal-specific etching solution. The metal-specific etching solution reacts with the bumps that are not covered with photoresist, completely removing these parts of the bumps. In addition, the metal-specific etching solution also reacts slightly with the bumps covered with photoresist, reducing their diameter. The lower the height of the bump, the greater the diameter reduction, thus forming an inverted frustum-shaped bump.

[0094] Taking In metal and In / Sn alloy as examples, hydrogen peroxide and dilute hydrochloric acid are generally used as etching solutions; if some substrates do not have silicon compounds that need protection, hydrogen peroxide and hydrofluoric acid will also be used; it is generally recommended to control the etching rate at 30-50 nm / s for easy process control.

[0095] Step 10: Remove photoresist and clean the wafer;

[0096] The photoresist on the wafer surface was removed using N-methylpyrrolidone organic solvent, then soaked in acetone and isopropanol for 5 minutes each, rinsed with deionized water, and dried with nitrogen.

[0097] Step 11: Obtain a wafer containing frustum-shaped bumps. The thickness of the bumps with the frustum structure is the sum of the thicknesses of the bottom metal and the bump metal, such as... Figure 2 As shown.

[0098] In the several embodiments provided by this invention, it should be understood that the disclosed apparatus and methods can be implemented in other ways. For example, the apparatus embodiments described above are merely illustrative. For instance, the division of modules, units, or units is merely a logical functional division, and in actual implementation, there may be other division methods. For example, multiple units, modules, or components may be combined or integrated into another device, or some features may be ignored or not executed.

[0099] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions within the technical scope disclosed in the present invention should be covered within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a wafer inverted-trapezoidal bump metal, characterized by, The method comprises the following steps: Pre-treating the wafer to obtain a pre-treated wafer; Coating photoresist on the pre-treated wafer and performing first photoetching to obtain a first photoetched wafer; Forming a bottom metal layer on the first photoetched wafer and removing the photoresist to obtain a wafer containing a bottom metal layer; Coating photoresist on the wafer containing the bottom metal layer and performing second photoetching to obtain a second photoetched wafer; Forming a bump metal layer on the second photoetched wafer to cover the bottom metal layer and removing the photoresist to obtain a wafer containing a bump metal layer; Coating photoresist on the wafer containing the bump metal layer and performing third photoetching to obtain a third photoetched wafer; Performing metal etching on the third photoetched wafer to remove part of the bump metal layer, forming a wafer containing a bump metal layer with a truncated cone structure, and removing the photoresist to obtain a wafer containing a bump metal layer with a truncated cone structure; The pattern of the photoetching plate for the first photoetching is a target pattern; the orthographic projection of the target pattern overlaps the bump metal layer with the truncated cone structure to be formed; The pattern of the photoetching plate for the second photoetching is a covering pattern; The orthographic projection of the covering pattern covers the orthographic projection of the target pattern of the first photoetching; The pattern of the photoetching plate for the third photoetching is an inverse target pattern; the orthographic projection of the inverse target pattern is the inverse of the target pattern of the first photoetching.

2. The method for preparing the wafer frustum-shaped bump metal according to claim 1, characterized in that, The wafer is pre-treated by one of the following two methods: Method one: placing the wafer on a 100℃ hot plate and baking for 300s; Method two: coating HMDS on the wafer surface and placing it on a 120℃ hot plate and baking for 60s.

3. The method for preparing the wafer frustum-shaped bump metal according to claim 1, characterized in that, Forming a bottom metal layer on the first photoetched wafer, specifically comprising the following steps: The first photoetched wafer is first treated by a plasma cleaning device for 300s with an oxygen flow rate of 60-100sccm and an argon flow rate of 30-50sccm; and then transferred to an evaporation machine to evaporate a bottom metal layer.

4. The method for preparing a wafer bump metal layer with a truncated cone structure according to claim 3, wherein: The bottom metal layer comprises an adhesion layer and an infiltration layer; the adhesion layer is made of Ti / Cr with a thickness of 30-50nm, and the infiltration layer is made of Au with a thickness of 100-300nm.

5. The method for preparing the wafer frustum-shaped bump metal according to claim 1, characterized in that, Forming a bump metal layer on the second photoetched wafer, specifically comprising the following steps: The second photoetched wafer is first treated by a plasma cleaning device for 300s with an oxygen flow rate of 60-100sccm and an argon flow rate of 30-50sccm; and then transferred to an evaporation machine to evaporate a bump metal layer.

6. The method for preparing a wafer bump metal layer with a truncated cone structure according to claim 5, wherein: The thickness of the bump metal layer is 3-7 microns.

7. The method for preparing the wafer frustum-shaped bump metal according to claim 1, characterized in that, The wafer needs to be cleaned before use and after cleaning the photoresist, and the cleaning specifically comprises the following steps: Cleaning the wafer by ultrasonic cleaning with acetone and isopropyl alcohol for 5 minutes each, rinsing the wafer with deionized water, and drying the wafer with nitrogen.

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