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back-end.md [2026/05/05 19:06] – [Processes] arthurback-end.md [2026/05/06 17:29] (current) – [Processes] yusufabdillah
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-# Back-End+# Back-End-of-Line (BEOL) Sequence
  
 --- ---
- 
 ## 1. Wafer thinning ## 1. Wafer thinning
  
 +The wafer thinning process is essential for reducing the overall thickness of the silicon substrate to meet the requirements of modern, ultra-thin portable electronics.
 ### Processes ### Processes
  
 #### 1.1. First optical inspection #### 1.1. First optical inspection
  
-- Electricity+Before thinning, wafers undergo rigorous optical inspection to identify surface defects, particles, or structural anomalies like bubble inclusions that could cause failure. This step ensures that only high-quality wafers proceed to mechanical processing, preventing yield loss from pre-existing hidden voids. Automated scanners use laser light scattering to detect microscopic particles as small as 90 nm. These inspections characterize parameters like bow, warp, and total thickness variation (TTV). The data collected serves as a baseline for measuring material removal accuracy in subsequent steps.
  
-*Expliquer le process First optical inspection : 5-10 lignes*+- Electricity
  
 #### 1.2. Taping #### 1.2. Taping
 +
 +To protect the delicate integrated circuits on the active front side during mechanical thinning, a protective tape is applied. This tape acts as a buffer against mechanical stress and prevents contamination from the slurry or debris generated during grinding. Recent trends have seen an increase in the use of UV-sensitive tapes, which provide high adhesion during processing but can be easily removed later. The tape must be applied uniformly to avoid inducing asymmetric stress that could lead to wafer bowing. For MEMS devices with exposed membranes, specialized tapes with recessed areas may be used to avoid physical contact with fragile structures.
  
 - Electricity - Electricity
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 #### 1.3. Grinding #### 1.3. Grinding
 +
 +Mechanical back-grinding is the primary method for bulk material removal, often utilizing diamond tools to achieve high precision. The process typically involves two stages: rough grinding for rapid removal and fine grinding to achieve the final target thickness. High-precision grinders can routinely achieve a thickness tolerance better than 0.5 µm and minimal subsurface damage. After grinding, chemical-mechanical polishing (CMP) is often employed to remove the final layers of mechanical damage and achieve a mirror-like finish. This thinning is critical for technologies like Through-Silicon Vias (TSVs) and 3D chip stacking.
  
 - Electricity - Electricity
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 - Air - Air
 - Exhaust - Exhaust
 +
 +----
  
 ### Manufacturers ### Manufacturers
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 ## 2. Wafer bumping ## 2. Wafer bumping
  
-^^ Wafer bumping creates metal/solder bumps on pads (electroplated) and wafer-level packaging. Using polyimide to create dielectric/passivation structure and the metals stack to form the bump foundation and final solderable surface. To simplify, there are three main steps in this process: surface preparation and polyimide dielectric; pattern opening and plating areas with lithography; and solder deposition and reflow. +Wafer bumping refers to a packaging step that falls into the back-end-of-line (BEOL) category that creates small metal interconnects, known as bumps, on the surface of a processed semiconductor wafer. These bumps act as the main point of connection between each IC die and the final package substrate. Some of the metals that can be used in creating bumps include gold, tin, silver, and copper. To simplify the manufacturing process, there are three main steps including surface preparation and polyimide dielectric; pattern opening and plating areas with lithography; and solder deposition and reflow.
 ### Processes ### Processes
  
 #### 2.1. Plasma cleaning #### 2.1. Plasma cleaning
  
-^^ Plasma cleaning purpose is to remove organic residues, adsorbed moisture, and weak surface contamination, as well as to improve metal-polyimide adhesion. In wafer bumping, this step is critical before polyimide coating or metallization because adhesion defects at the wafer surface can cause delamination, voids, or lithography failures later in the stack. +Surface treatment through plasma cleaning is crucial in order to get rid of tiny particles, contaminants, and native oxides that could potentially affect either the adhesion quality or electrical properties of the product. In this process, the wafer goes into a vacuum chamber, where radio frequency (RF) power creates the plasma of reactive gas ions by ionizing gases. By impacting the wafer with mechanical force and reacting chemically, ions remove unwanted organic residue or thin film on the surface. This dry cleaning method is often preferred over wet cleaning because it minimizes damage to delicate structures and protects the silicon wafer from excessive chemical consumption [([[https://link.springer.com/referencework/10.1007/978-981-99-2836-1|Handbook of Integrated Circuit Industry]], Wang, Yangyuan, et al., eds. Springer Nature, 2023)]. Remote plasma systems may also be used to minimize the damage of ion bombardment and increase the reaction between chemical radicals. Effectively, this step prepares a pristine interface for the subsequent polyimide layer.
- +
-Oxygen or nitrogen plasma is used for organic removal, which changes how polyimide and copper surface interact. Optimizing process parameters can increase the interface shear strength of  PI-on-PI and PI-on-Cu structures, but must avoid damaging aluminum pads, alter Cu surface chemistry, or fragile passivation. ^^+
  
 Input Input
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 - Exhaust gases - Exhaust gases
 - Heat - Heat
 +
 +----
  
 #### 2.2. Polyimide Spin coating #### 2.2. Polyimide Spin coating
  
-^^ Polyimide is a common dielectric material used in wafer bumping because it provides electrical insulation, with mechanical stress relief, and protection between the redistribution layer and the bump structure. +Polyimide spin coating refers to the method by which a homogeneous insulating or passivating layer of polyimide can be applied on the wafer. In this step, a small quantity of the liquid form of polyimide resin is dispensed at the center of a spinning silicon wafer. Under centrifugal action, the resin gets distributed towards the edges, where the speed of rotation, viscosity, and temperature dictate the thickness of the deposited film. At this point, there is usually formation of an edge bead on the wafer edges as a result of solvent evaporation, which then removed using a solvent spray (EBR). Polyimides are preferred in bumping because of their flexibility, heat resistance, and stress-buffer capabilities [([[https://www.sciencedirect.com/book/edited-volume/9780128177860/handbook-of-silicon-based-mems-materials-and-technologies|Handbook of silicon based MEMS materials and technologies]], Tilli, Markku, et al., eds. Elsevier, 2020)].
- +
-Spin coating works by dispensing liquid polyimide precursor onto the wafer surface (RDL) followed by centrifugal force spreads the film. This dielectric layer also helps to relieve stress between silicon and the underbump metallization (UBM). +
- +
-A good polyimide coating needs to be uniform across the wafer, because thickness variation can cause problems via depth, stress, warpage, and bump height.^^+
  
 Input Input
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 - Wastewater - Wastewater
  
-#### 2.3. Polyimide Photolithography+----
  
-^^ When a polyimide is photoactive, it can be patterned directly with photolithography to form vias and openings through the dielectric material. If the polyimide is not photoactive, then a separate photoresist mask will be used to pattern the polyimide after it is coated and pre-baked.+#### 2.3. Polyimide Photolithography
  
-The goal is to expose parts of the seed layer or pad underneath for bump formation while leaving the rest of the wafer insulated and providing mechanical support. Proper alignment of the openings is critical because they must match the pad or redistribution features with good overlay, especially for fine-pitch wafers.^^+The photolithography technique involves defining locations on the polyimide film where openings are going to be etched. The wafer, which by now has been coated with a photosensitive polyimide film, is accurately positioned beneath a photomask bearing the layout using specialized equipment referred to as a stepper. An ultraviolet (UV) beam of light falls on the mask causing chemical modifications in the exposed parts of the polyimide film. Since polyimide often functions as a negative photoresist, the exposed regions become cross-linked and insoluble, while the unexposed areas remain dissolvable. Important factors for this stage include resolution, sensitivity, and alignment; otherwise, one might have lithography "hotspots," which will destroy any chance of establishing connections [([[https://link.springer.com/referencework/10.1007/978-981-99-2836-1|Handbook of Integrated Circuit Industry]], Wang, Yangyuan, et al., eds. Springer Nature, 2023)].
  
 Input Input
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 - Rinse wastewater - Rinse wastewater
 - Spent mask - Spent mask
 +
 +----
  
 #### 2.4. Polyimide Development #### 2.4. Polyimide Development
  
-^^ Development removes exposed or unexposed, depending on chemistry that is being used. The development process makes the openings that are needed for the metal to make contact on. The geometry of these openings can have a significant impact because it affects how the metal gets into the openings and the final shape of the bumps.^^+Development removes exposed or unexposed, depending on chemistry that is being used. The development process makes the openings that are needed for the metal to make contact on. The geometry of these openings can have a significant impact because it affects how the metal gets into the openings and the final shape of the bumps.
  
 Input Input
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 - Wastewater - Wastewater
 - Chemical waste - Chemical waste
 +
 +----
  
 #### 2.5. Polyimide Curing #### 2.5. Polyimide Curing
  
-^^ Curing is a process that changes the spin-coated precursor into a stable film by removing the solvents and finishing the imidization or crosslinking process. Curing conditions that are utilized have a direct impact on a number of important properties such as film stress, dielectric constant, mechanical hardness, moisture uptake and wafer bow. If the cure is incomplete or not uniform, residual solvent may be left in the film, leading to blistering or delamination of the film during later use. The cure is performed as multiple stages to minimize the occurrence of cracking: first there is a soft bake or intermediate drying stage followed by higher temperature curing under controlled ramp rates (up and down).^^+Curing is a process that changes the spin-coated precursor into a stable film by removing the solvents and finishing the imidization or crosslinking process. Curing conditions that are utilized have a direct impact on a number of important properties such as film stress, dielectric constant, mechanical hardness, moisture uptake and wafer bow. If the cure is incomplete or not uniform, residual solvent may be left in the film, leading to blistering or delamination of the film during later use. The cure is performed as multiple stages to minimize the occurrence of cracking: first there is a soft bake or intermediate drying stage followed by higher temperature curing under controlled ramp rates (up and down).
  
 Input Input
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 - Cured polyimide dielectric - Cured polyimide dielectric
 - Outgassed volatiles - Outgassed volatiles
 +
 +----
  
 #### 2.6. Seed layer deposition (PVD) #### 2.6. Seed layer deposition (PVD)
  
-^^ The seed layer is deposited by physical vapor deposition to create a thin conductive film over the wafer that allows for electroplating and serves as the first part of the UBM stack. In bumping, this layer is frequently Cu-based (often with barrier like Ti/Ni) or may consist of a multilayer stack such as Ti/Cu or equivalent materials. The barrier component adheres to the substrate wafer or dielectric while the conductive element ensures uniform current distribution during plating operations. If there are any discontinuities in the seed layer, then one might find missing bumps after plating due to non-uniformity; hence thickness and coverage over topography become critical parameters to control.^^+The seed layer is deposited by physical vapor deposition to create a thin conductive film over the wafer that allows for electroplating and serves as the first part of the UBM stack. In bumping, this layer is frequently Cu-based (often with barrier like Ti/Ni) or may consist of a multilayer stack such as Ti/Cu or equivalent materials. The barrier component adheres to the substrate wafer or dielectric while the conductive element ensures uniform current distribution during plating operations. If there are any discontinuities in the seed layer, then one might find missing bumps after plating due to non-uniformity; hence thickness and coverage over topography become critical parameters to control.
  
 Input Input
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 - Thin conductive seed film - Thin conductive seed film
 - Heat - Heat
 +
 +----
  
 #### 2.7. Spin coating #### 2.7. Spin coating
  
-^^ This second spin coating step is associated with coating the photoresist rather than applying the polyimide layer. A thick resist coating is necessary since plating masks require substantial thickness to create well-defined electroplating cavities characterized by a high aspect ratio and clean sidewalls. In wafer bumping, resist thickness, edge bead removal, and uniformity are critical as it determines the eventual bump diameter and whether plating will result in copper mushrooms or copper being undercuts.^^+This second spin coating step is associated with coating the photoresist rather than applying the polyimide layer. A thick resist coating is necessary since plating masks require substantial thickness to create well-defined electroplating cavities characterized by a high aspect ratio and clean sidewalls. In wafer bumping, resist thickness, edge bead removal, and uniformity are critical as it determines the eventual bump diameter and whether plating will result in copper mushrooms or copper being undercuts.
  
 Input Input
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 - Uniform resist film - Uniform resist film
 - Solvent vapor - Solvent vapor
 +
 +----
  
 #### 2.8. Photolithography #### 2.8. Photolithography
  
-^^ The photolithography process uses a mask to expose resist that defines the areas where copper or solder will be plated. The exposed areas on the photoresist film correlate with the location of the bumps or copper pillars, and it needs to be controlled because the plated feature size is directly related to the lithographic aperture.^^+The photolithography process uses a mask to expose resist that defines the areas where copper or solder will be plated. The exposed areas on the photoresist film correlate with the location of the bumps or copper pillars, and it needs to be controlled because the plated feature size is directly related to the lithographic aperture.
  
 Input Input
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 - Exposed resist pattern - Exposed resist pattern
 +
 +----
  
 #### 2.9. Development & soft baking #### 2.9. Development & soft baking
  
-^^ Once photoresist has been exposed to the photoresist analyzing light (UV), development opens vertical-walled cavities that define bump diameter and height. While soft bake is used to drive out the solvent and stabilize the resist before plating.^^+Once photoresist has been exposed to the photoresist analyzing light (UV), development opens vertical-walled cavities that define bump diameter and height. While soft bake is used to drive out the solvent and stabilize the resist before plating.
  
 Input Input
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 - Plating mold with openings - Plating mold with openings
 - Developer waste - Developer waste
 +
 +----
  
 #### 2.10. Plasma cleaning #### 2.10. Plasma cleaning
  
-^^ The second plasma cleaning is used to remove microscopic residue after the resist development step and to activate exposed surfaces. It also helps lower the amount of trapped contamination causing issues with solder wetting and seed etch uniformity in advanced packaging.^^+The second plasma cleaning is used to remove microscopic residue after the resist development step and to activate exposed surfaces. It also helps lower the amount of trapped contamination causing issues with solder wetting and seed etch uniformity in advanced packaging.
  
 Input Input
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 - Activated/cleaned plating surface - Activated/cleaned plating surface
 - Exhaust gases - Exhaust gases
 +
 +----
  
 #### 2.11. Copper plating #### 2.11. Copper plating
  
-^^ Copper plating is the step for making copper pillars or copper-supported bump structures, using the photoresist mold as a template.+Copper plating is the step for making copper pillars or copper-supported bump structures, using the photoresist mold as a template.
  
 Input Input
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 - Wastewater - Wastewater
 - Heat - Heat
 +
 +----
  
 #### 2.12. Resin stripping #### 2.12. Resin stripping
  
-^^ After plating, the thick resist is stripped, leaving free-standing copper pillars or copper pads and the exposed seed layer between them.+After plating, the thick resist is stripped, leaving free-standing copper pillars or copper pads and the exposed seed layer between them.
  
 Input Input
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 - Chemical waste - Chemical waste
 - VOCs - VOCs
 +
 +----
  
 #### 2.13. Wet etching #### 2.13. Wet etching
  
-^^ The exposed seed/UBM between bumps is etched away to electrically isolate bumps and define final pad dimensions.+The exposed seed/UBM between bumps is etched away to electrically isolate bumps and define final pad dimensions.
  
 Input Input
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 - Etched seed/metal waste in solution - Etched seed/metal waste in solution
 - Spent etchant - Spent etchant
 +
 +----
  
 #### 2.14. Bump reflow #### 2.14. Bump reflow
  
-^^ When solder is deposited on Cu pillars or pads, once the solder has gone through a reflow cycle in a controlled inert atmosphere, the solder will have melted into spherical forms or solder caps, as well as created stable intermettalics.+When solder is deposited on Cu pillars or pads, once the solder has gone through a reflow cycle in a controlled inert atmosphere, the solder will have melted into spherical forms or solder caps, as well as created stable intermetalics.
  
 Input Input
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 ## 3. Dicing ## 3. Dicing
- 
 ### Processes ### Processes
  
 #### 3.1. Wafer mounting #### 3.1. Wafer mounting
  
-^^ Before cutting, the wafer is mounted onto a dicing tape stretched over a metal or plastic frame. The tape keeps the wafer flat and stops the wafer pieces from moving while cutting. After cutting, the tape still holds the wafer pieces in place, so that each piece can be picked up easily.+^^ Before cutting, the wafer is mounted onto a dicing tape stretched over a metal or plastic frame. The tape keeps the wafer flat and stops the wafer pieces from moving while cutting. After cutting, the tape still holds the wafer pieces in place, so that each piece can be picked up easily.^^
  
 Input Input
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 #### 3.2. Laser grooving #### 3.2. Laser grooving
  
-^^ Laser grooving removes material in the scribe street, such as dielectrics, metals, and low-k stacks, to make a path for the blade or plasma that will be used later to separate the material. This helps to protect the BEOL and seal ring from cracking and coming apart. +^^ Laser grooving removes material in the scribe street, such as dielectrics, metals, and low-k stacks, to make a path for the blade or plasma that will be used later to separate the material. This helps to protect the BEOL and seal ring from cracking and coming apart. When choosing a process, it must balance the shape of the groove (for example U-shape) against narrow street size. Also, laser wavelength, beam size, multi-pass method, power; speed, frequency, pitting, and residues. Using ultrashort-pulse (USP) UV grooving can make the edges of the material better reduce the amount of burr, and supports the die be stronger when plasma dicing is combined.^^
- +
-When choosing a process, it must balance the shape of the groove (for example U-shape) against narrow street size. Also, laser wavelength, beam size, multi-pass method, power; speed, frequency, pitting, and residues. +
- +
-Using ultrashort-pulse (USP) UV grooving can make the edges of the material better reduce the amount of burr, and supports the die be stronger when plasma dicing is combined.^^+
  
 - Electricity - Electricity
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 - Lintec [RAD-2020F/12] (https://www.linteceurope.com/shop/equipment/uv-irradiation-system/rad-2020f-12/) : UV Irradiation - Lintec [RAD-2020F/12] (https://www.linteceurope.com/shop/equipment/uv-irradiation-system/rad-2020f-12/) : UV Irradiation
 - KLA [Orbotech & Lumina series] (https://www.kla.com/products/pcb-ic-substrate-manufacturing/inspection-metrology) : Automated Optical Inspection - KLA [Orbotech & Lumina series] (https://www.kla.com/products/pcb-ic-substrate-manufacturing/inspection-metrology) : Automated Optical Inspection
- 
 ### Sources ### Sources
  
 - IMEC - IMEC
 -  - 
 +
  
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