Laser debonding for ultrathin and stacked fan out packages
BY ELISABETH BRANDL, THOMAS UHRMANN and MARTIN EIBELHUBER, EV Group, St. Florian, Austria
Fan-out packaging is an established technology for many mobile applications. Whereas early semiconductor packages have been single-chip packages, the continuing trend of expanding the wiring surface to support increased functionality has led to more complex packages, stacked packages, systems inpackageaswellashigh-performancepackages. With this development, fan-out technology is bridging a gap between cost-competitive packaging and high performance. For all aforementioned packages, temporary bonding will be needed, either to enable the thinning of wafers to address the need for smaller form factors, to achieve cost savings on mold materials or to serve as a processing platform for redistribution-layer (RDL) first processes.
Temporary bonding requires both a bonding and debonding process. Determining the right debonding technology can be difficult and confusing as every application from fan-out wafer-level packaging (FoWLP) to power devices has its own requirements in terms of process temperature, mechanical stress and thermal budget, to name just a few considerations. In this article, we will focus on laser debonding, where high- temperature compatible materials are available. We will point out for which applications the laser debond characteristics fit well.
To limit the thermal input associated with debonding, UV lasers are utilized for debonding where several materials from different temporary bonding material suppliers are available. To confine the maintenance effort to a minimum, a diode-pumped solid-state (DPSS) laser is the right choice in combination with beam-shaping optics for high process control and minimum heat input.
Challenges of temporary bonding for FoWLP
FoWLP has gained significant industry interest in part due to carrier, the requirements of the temporary bonding material in terms of chemical and thermal compat- ibility are high. Certain kinds of polyimides comply with this harsh environment and are also suitable for laser debonding.
By just comparing these two processes, the require- ments differ significantly even though both are FoWLP processes. By looking at the wide variety of semiconductor processes for various applications, it becomes clear that no single debonding process solution is compatible with all semiconductor processes, but rather several solutions are necessary. This is the reason why a variety of debonding processes (temporary bonding is characterized by the debonding technology) have been developed and are still in use today.
Comparison of the mainstream debonding technologies
The most common debonding methods are thermal slide-off debonding, mechanical debonding and UV laser debonding. These three methods are all in high- volume manufacturing and differ strongly in their process compatibility.
Thermal slide-off is a method that employs a thermo-plastic material as an adhesive interlayer between the device and carrier wafer. The debonding method uses the reversible thermal behavior of the thermoplastic material, meaning that at elevated temperatures the material experiences a drop in viscosity, which enables debonding to be accomplished by simply sliding the wafers off of each other. The character- istics of thermal slide-off debonding is bonding and debonding at elevated temperatures, which depending on the thermoplastic material being used can range between 130 and 350°C. Temperature stability depends in large part on mechanical stress, which can be observed due to the thermoplastic’s low viscosity at high temperatures .
Mechanical debonding is a method that is highly dependent on the surface properties of the wafers involved as well as the adhesion and cohesion of the temporary bonding material. For most material systems, a mechanical release layer is applied to achieve a controlled debonding mechanism. Key characteristics of mechanical debonding include processing at room temperature and a strong dependence on mechanical stress. Since mechanical debonding needs a low adhesion between the temporary bonding material and the wafer for a successful debond process, it can be tricky to use it for FoWLP applications. This is because the high wafer stress associated with FoWLP processing can lead to spontaneous debonding, even during the thinning process, which in turn can result in a drastic drop in yield .
Laser debonding is a technology that has been implemented with several different variations. The debond mechanism depends on the type of laser as well as the temporary bonding adhesive or the specific release layer used for the process. Infrared lasers work on the principle of the photo thermal process, where light is absorbed and transferred into heat, which leads to high temperatures within the bond interface. UV laser debonding typically uses the photo chemical process, where light is absorbed and the energy is used for breaking chemical bonds. Breaking the chemical bonds of a polymer results in the production of fragments of the original polymer. These fragments comprise gases, which increase the pressure within the interface to support the debonding process. For FoWLP applications, this method is a good fit due to the high adhesion of the temporary bonding adhesive to the wafers before the debonding process.
Optimized solution for FoWLP applications
UV lasers are advantageous for FoWLP processing due to their limited thermal input through the debonding process. The carrier wafer must be transparent to the UV laser’s wavelength to ensure efficient use of the laser energy and also ensure a higher lifetime of the carrier wafer. Two main types of UV lasers are available (solid-state laser and excimer laser), with each having several different wavelength options. Choosing a laser with a wavelength larger than 300nm is optimal for several reasons. First, commercially available laser debond materials effectively absorb and therefore debond at wavelengths higher than 300nm. Second, it allows a standard glass wafer to be used as the carrier since glass enables high transmission in this wavelength regime.
Solid-state lasers have the advantage of lower maintenance costs because they do not need halogen gas, which must be replaced on a regular basis. For solid-state lasers, the consumables are very low, and depending on the amount of power used by the laser there are examples of lasers used for laser debonding on a 24/7 basis that have required no laser consumables in the first five years of operation. Additionally, a smaller footprint can also be achieved due to a compact optical setup. Solid-state lasers typically have Gaussian beam profiles, pictured in FIGURE 3.
UV laser debonding is a threshold process, meaning that debonding occurs above a certain value of radiant exposure. In Figure 3, the area with the blue criss-cross lines indicates the radiant exposure, which is used for the debonding process. The energy that is below or above that value (areas in red in the picture) cannot be used for debonding and is typically trans- ferred into heat, which can lead to carbonization and particle creation. Because of the lack of sufficient energy at the edge of the Gaussian laser beam profile, a certain overlap of the pulses is necessary, which is an additional variable that must be optimized in order to achieve successful debonding without carbonization. Additionally, the excess energy in the beam center can cause carbonization. A Gaussian beam profile is not suitable to limit thermal effects during debonding.
Gaussian beam profiles can be transferred into quasi top hat beam profiles by using a proprietary optical setup for beam shaping. By employing this optical setup, a highly reproducible beam for debonding (whereby the beam shape does not change over time) is achieved with constrained thermal input similar to what is seen in the “top hat” beam profile in FIGURE 4. This gives tighter process control, which in combination with the high pulse repetition rate of this laser type and the ability to scan across the surface of a fixed wafer leads to a well-controlled, high-throughput debonding process. The scanning process is pictured in FIGURE 5 where -- in contrast to an excimer laser -- the wafer is fixed on a static stage and the laser spot is controlled by a galvo scanner over the wafer. leads to a well-controlled, high-throughput debonding process.
As shown in FIGURE 6, a test wafer is used to determine the optimum radiant exposure for debonding. Even with a top hat beam profile, it is important to use a radiant exposure value close to the debonding threshold to minimize heat effects . Small overlaps are necessary nonetheless because the adhesion between the temporary bonding material and the wafers is very high.
Temporary bonding for future FoWLP
Ultrathin and stacked fan-out packages, also called Package on package (PoP), are already on several industry roadmaps due to their ability to enable higher device densities. However, the need for reconstituted wafers to become even thinner for PoP versus current FoWLP will give rise to more challenges for temporary bonding. For example, the bow of the temporary bonded wafer stack consisting of a molded wafer and a carrier wafer must be minimized to ensure uniform thinning. The maximum total thickness variation (TTV) will also become tighter depending on the final thickness. As for every 3D application, questions regarding interconnects, such as choosing via first or via last, also arises for PoP, where several processes are also available and where no standard process exists that is employed by all fan-out packaging houses.
UV laser debonding is a suitable method for both chip- first and chip-last/RDL-first FoWLP processes because it offers debonding at room temperature, and because chemically stable materials are available. The UV laser debonding solutions presented in this article combine the advantages of the solid-state laser with low mainte- nance, low consumables costs and high pulse frequencies combined with high spatial control due to the special beam-shaping optics.
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