Following on from the previous article which summarized the assembly process for conventional packages, this article will be the first of two episodes which focuses on the other main form of semiconductor packaging—wafer-level packaging (WLP). In particular, it will cover the five fundamental processes involved in WLP including photolithography, sputtering, electroplating, photoresist (PR) stripping, and metal etching.
Packaging With a Fully Intact Wafer
WLP refers to the process that is performed before the wafer is diced. It generally includes fan-in wafer-level chip scale packaging (WLCSP) and fan-out WLCSP in which the entire process is performed while the wafer is still fully intact. Nevertheless, redistribution layer (RDL) packaging, flip chip packaging, and through-silicon via1(TSV) packaging are also generally categorized as WLP even if only a part of their processes are performed before the wafer is diced. Depending on which of these types of packages is used, there are variations in the type of metal and pattern formed by electroplating2. However, they all follow a similar sequence during packaging as described below.
1 Through-silicon via (TSV): A type of vertical interconnect access (via) that completely passes through a silicon die or wafer to enable the stacking of silicon dice.
2 Electroplating: A reaction where oxidation occurs at the positive plate to produce electrons which are transmitted to a wafer with a solution that has metal ions that are negative plates to become metal.
After wafer testing is performed, a dielectric layer is created on the wafers as needed. The dielectric layer then exposes the chip pad again, following the first exposure during testing, with photolithography.
Afterwards, a metal layer is applied on the surface of the wafer through sputtering3. This metal layer enhances the adhesion of the electroplated metal layer that will be formed and acts as a diffusion barrier to prevent the development of chemicals within metals. It also functions as a pathway for electrons during the electroplating process, and applies photoresist to create an electroplating layer while a pattern is created through photolithography. A thick metal layer is then formed by electroplating. When electroplating is completed, the next step is to proceed with the PR stripping process while the remaining thin metal layers are removed by etching. As a result, the electroplated metal layers are formed on top of the wafers in the desired patterns. This pattern serves as the wiring for fan-in WLCSP, the pad redistribution in RDL packaging, and the bumps in flip chip packaging. The following sections will take a closer look at each of these processes.
3 Sputtering: A process in which plasma ions physically collide with a target and removes the target’s material so it can be deposited onto the wafer.
▲ Figure 1. The steps involved in various wafer-level packaging processes
Photolithography: Sketching Patterns on a Masked Wafer
Photolithography, a combination of “-litho” (stone) and “graphy (drawing),” refers to a printing technique. In other words, photolithography is a patterning process in which a photosensitive polymer called a photoresist is applied to the wafer and selectively exposed to light through a mask that has a desired pattern on it. The areas that are exposed to light are developed, and the required pattern or shape is created. The sequence of this process is shown in Figure 2.
▲ Figure 2. The steps of photolithography
In WLP, photolithography is primarily used to form patterns on dielectric layers, to create an electroplated layer by photoresist patterning, and to create metal wiring by etching diffusion layers.
▲ Figure 3. A comparison of photography and photolithography
To understand photolithography more clearly, it will be helpful to compare it with photography. As shown in Figure 3, photography uses sunlight as its light source to capture a photo of its subject, which could be an object, landscape, or a person. On the other hand, photolithography requires a specific light source to transfer patterns on a mask to an exposure tool. Lastly, the role of the film in a camera is equivalent to the photoresist that is applied to a wafer during photolithography. Consequently, there are three methods to apply a photoresist on the wafer as shown in Figure 4. They consist of spin coating, film lamination, and spray coating. After applying the photoresist, soft baking is performed to remove solvents to ensure that the viscous photoresist remains on the wafer and maintains its thickness.
As shown in Figure 5, spin coating places viscous photoresist onto the center of a spinning wafer so the photoresist is spread towards the edges due to centrifugal force. This makes the photoresist form a uniform thickness on the wafer. If the viscosity of the photoresist is high while the spin speed is low, the photoresist will be applied thickly. Conversely, if the viscosity is low and the spin speed is high, it is applied thinly. In the case of wafer-level packages, especially flip chip packages, they require a photoresist layer with a thickness ranging from 30 to 100 micrometers (μm) to form solder bumps. However, it is not easy to achieve the desired thickness in a single spin coating. In some instances, it is necessary to repeat the application of photoresist and soft baking more than once. Accordingly, when a thick photoresist is required, it is effective to use lamination as it makes the film the desired photoresist thickness from the start. It is also more cost-effective because there is no waste from the wafer during processing. However, if there are rough surfaces on the wafer’s structure, it can be difficult to adhere the film to the wafer which can lead to defects. For wafers that have very rough surfaces, a uniform thickness of photoresist can be achieved through spray coating.
▲ Figure 4. The three methods to apply photoresist
▲ Figure 5. An overview of spin coating
After the photoresist is coated and soft baked, the next step is to expose it to light. By shining light through a pattern formed on the mask, the photoresist on the wafer receives the image of the pattern. When using a positive photoresist that weakens when exposed to light, the mask needs to have holes in areas that are going to be removed. However, when using a negative photoresist that hardens when exposed to light, the mask must have holes in the areas that need to remain. For WLP, a mask aligner4 or a stepper5 is typically used as the process equipment for photolithography.
4 Mask aligner: One of the exposure tools that aligns the pattern on the mask and the wafer so that light can pass through them simultaneously.
5 Stepper: A machine where the stage moves in steps and photolithography is performed by a shutter that opens and closes to allow light to pass through.
Development is the process of dissolving the parts of the photoresist that have been weakened through photolithography with a developer solution. As shown in Figure 6, there are three types of development: puddle development that pours the developer onto the center of the wafer so it spins at a low speed, tank development that immerses multiple wafers in the developer at the same time, and spray development that sprays the developer onto the wafer. Figure 7 shows an overview of a chamber for puddle development. After the puddle development is finished, the photoresist takes on the desired pattern through photolithography.
▲ Figure 6. Three different methods of development
▲ Figure 7. An overview of a chamber for puddle development
Sputtering: Forming Thin Films on the Wafer
The process of sputtering is a type of physical vapor deposition6 (PVD) that forms a thin film of metal on a wafer. If the metal film formed on the wafer is below the bumps as seen in flip chip packages, it is called an under bump metallurgy (UBM). Typically, it is in the form of two or three layers of metal film, including an adhesion layer, a current carrying layer that provides electrons during electroplating, and a diffusion barrier with solder wettability7 that suppresses the formation of compounds between the plating layer and the metal. If the layers are comprised of titanium, copper, and nickel, the titanium acts as the adhesion layer, the copper acts as the current carrying layer, and the nickel acts as the diffusion barrier. Accordingly, the UBM has a significant impact on the quality and reliability of flip chip packages. As for metal layers like an RDL and a WLCSP that are used to form metal wiring, they usually consist of an adhesion layer and a current carrying layer that improves adhesion.
As Figure 8 shows the sputtering process, it starts with argon gas transforming into plasma8 and colliding with a target that has the same composition as the metal on which positive argon ions will be deposited. The impact of the collision removes the metal particles from the target so they are deposited on the wafer. The metal particles deposited by sputtering have a consistent directionality. Even though a flat plate is deposited with a uniform thickness, plates in the shape of a trench or vertical interconnect access (via) can have different results. Such irregular shapes can make the deposition thickness of the wall’s surface that is parallel to the metal deposition become thinner than the plate’s floor that is perpendicular to the metal deposition.
6 Physical vapor deposition (PVD): A process used to produce a metal vapor that can be deposited on electrically conductive materials as a thin and adhesive pure metal or alloy coating.
7 Wettability: The phenomenon where a liquid spreads on the surface of a solid due to the interaction between the liquid and the solid surface.
8 Plasma: A state of matter that is electrically neutral due to the coexistence of freely moving protons and electrons. When heat is continuously applied to a gaseous substance to raise its temperature, a collection of particles consisting of ions and free electrons is created. It is also called the “fourth state of matter” in addition to solid, liquid, and gas.
▲ Figure 8. The fundamentals of sputtering
Electroplating: Forming Metal Layers to Bond
Electroplating is the process of depositing metal ions of an electrolyte solution as metal on a wafer. This is possible through a reduction reaction using externally supplied electrons. In WLP, electroplating is used to form thick metal layers such as metal wiring for electrical connections or bumps for junctures. Just as Figure 9 illustrates, a metal undergoes oxidation at the anode to become an ion and releases electrons to the external circuit. The metal ions oxidized at the anode or present in the solution receive electrons and undergo a reduction reaction to become metal. In the electroplating process for WLP, the cathode plate becomes the wafer. The anode plate is made of the metal to be plated, but it also uses an insoluble electrode9 such as platinum. If the anode plate is made of the metal to be plated, metal ions are dissolved from the anode plate and continuously distributed to maintain a consistent ion concentration in the solution. However, if an insoluble electrode is used, metal ions expended while being plated on the wafer must be periodically replenished in the solution to maintain the ion concentration. Figure 10 below shows the electrochemical reactions that occur at the cathode and anode, respectively.
9 Insoluble electrode: An electrode used primarily in electrolysis and plating. It is neither chemically nor electrochemically soluble. Materials such as platinum are used for its creation.
▲ Figure 9. The process of electroplating
▲ Figure 10. Electrochemical reactions at the cathode and anode expressed as formulae
The equipment that electroplates a wafer is typically placed so the side of the wafer to be plated faces down while the anode is positioned below the solution. Electroplating happens when the solution flows toward the wafer and forcefully collides with the surface. At this point, patterns formed from photoresist can come into contact with the solution on the parts of the wafer to be plated. Electrons are distributed through the electroplating equipment at the edge of the wafer and eventually meet the metal ions in the solution at the patterned parts. They then combine with metal ions inside the solution where the patterns are formed to go through a reduction reaction and grow to form metal wiring or bumps.
PR Stripping and Metal Etching: Removing the Photoresist
Once the processes that use the photoresist pattern are complete, the photoresist must be removed via PR stripping. PR stripping is a wet process that uses a chemical solution called a stripper, and implements development methods such as puddle, tank, or spray. After a process like electroplating forms metal wiring or bumps, the metal film formed by sputtering must also be removed. This is necessary as the entire wafer will be electrically connected and result in a short circuit if the metal film is not taken off. The metal film is removed by wet etching with an acid-based etchant that can dissolve the metal. While the technique is similar to PR stripping, puddle development has been used more widely as metal patterns on the wafer have become finer.
A More Efficient and Reliable Packaging Process
WLP strives for efficiency, miniaturization, and reliability through the above mentioned stages that begin with sketching patterns through photolithography and culminate in removing the applied photoresist through PR stripping. The next episode will look into the different types of WLP that use technologies such as fan-in and fan-out WLCSP, RDL, flip chip, and TSV.
