By Dr. Dietrich Tönnies, Suss MicroTec, Munich, Germany
Today, photolithography is employed in the majority of all bumping and redistribution technologies at the wafer level(1). Moreover, the use of photolithography will increase with the development of more complex IC architectures and with the transition to 300 mm wafers. While most wafer-level packaging (WLP) technologies(2) are more similar to front-end rather than backend technology, the cost of WLP equipment must be comparable to backend equipment before users will adopt it. At the same time, this equipment must address the specific technical requirements of WLP which, for example, derive from the fact that resist and metallization layers have to be processed with thicknesses that far exceed those in the front end. The move to 300 mm wafers is an important catalyst for wafer bumping and for the growth of wafer-level CSPs (WLCSPs). Furthermore, any technology that qualifies on 300 mm tools will also run on smaller wafer sizes. Last year Suss MicroTec (formerly Karl Suss) introduced its modular LithoPack series (Figure 1), offering a complete litho-graphy solution for wafer bumping and WLCSPs on 300 mm wafers. Fundamental Observations All photolithography processes are required to coat the wafer with a photosensitive layer, expose and develop it. For each of these process steps, the process designer can choose between two options: spin coating (liquid resist) or dry film lamination to apply the photosensitive layer; mask alignment or stepper for the exposure, and puddle- or spray development for the development step. Each of these techniques offers negative and positive attributes. While dry film resists have been successfully used in combination with solder pastes, liquid resists are more common with plating technology. Material suppliers, however, are currently very active in this area, and no final assessment of liquid resists versus dry-film resists is currently possible. For the development step, spray development is a must for thick resist applications (solder bumping) to guarantee the complete removal of the resolved resist out of the bump molds. Since spray development is also suitable for thinner resist layers, spray will probably become a standard development technique. Wafer Steppers During the current downturn, stepper suppliers have been looking for alternative markets and have shown interest in wafer-level packaging technology. Steppers, in fact, are employed by a number of companies in East Asia for gold bumping. Gold bumping illustrates why some companies are using steppers: This process is driven by display driver devices where the pitch is expected to fall to as low as 20 microns with a gap of 5 microns or less between adjacent gold bumps. These dimensions mark a resolution range where both mask aligners and steppers are economically acceptable, depending on the actual boundary conditions of the production environment. Five-micron features are about the smallest geometries that can be resolved by a mask aligner operating in the proximity mode. Tool Selection If the production volume of these ultrafine-pitch devices is high, and a yield loss cannot be tolerated, the stepper may be the preferred solution. On the other hand, if the exposure tool is mainly used for less-critical processes (or if a minor yield loss is acceptable because the die is cheap), the mask aligner may be the tool of choice. This is a decision that each gold bump operation has to make. While the gold bump industry is in transition from 150 mm to 200 mm wafers, the dominant technologies for 300 mm will be solder bumping and WLCSPs. These processes do not have the same resolution requirements needed for advanced gold bumping. In fact, only a small minority of the wafers that are solder bumped or packaged at the wafer level today are processed with steppers. Almost every lithography step is accomplished on mask aligners. But will the advantages the mask aligner offers for 200 mm wafers and smaller still exist for 300 mm technology?
Stepper suppliers argue that their tools are faster than mask aligners and that mask costs are lower2. Additionally, mask aligners have been accused of causing yield problems. While the latter is unlikely for solder bumping and WLCSPs, given that the mask aligner is the dominant exposure tool in this technology, we should take a closer look at the yield argument. In principle, when moving from 200 mm to 300 mm wafers, steppers will require twice as many shots to expose a full 300 mm wafer, because of doubled wafer area. To compensate for these added step-and-repeat times, intensity per shot must be at least doubled to obtain the same throughput achieved on a 200 mm wafer. This doubled intensity, however, does not seem to be available from the stepper suppliers. Figure 2 compares throughput between a 300 mm mask aligner and a 300 mm stepper. The stepper throughput is based on a 1750mW/cm2 exposure intensity per shot and a stage indexing time of 400 ms. Calculations are shown for 70, 100 and 130 shots per wafer.
The mask aligner throughput is based on a 90mW/cm2 full-field intensity, which corresponds to a 5 kW lamp. Depending on the exposure dose and the number of shots, the mask aligner has a two to five times higher throughput than a 300 mm stepper(3, 4). The LithoPack Concept In front-end technology, coating tracks that are linked to exposure tools can be found frequently; however, there are pros and cons to this clustering concept. Advantages include a higher level of auto-mation, reduced operator interaction, and the resulting improved process control. On the other hand, if either the track or the exposure system is down, the other tool has to wait. In addition, the throughput of coating track and exposure tool may not be well balanced, leaving one of the two machines unused for some amount of time. The choice between a fully clustered lithography system and stand-alone tools should be made on a case-by-case basis, depending on the process technology, production volume and the user's overall manufacturing philosophy. The LithoPack concept, referred to earlier, allows both the use of a fully clustered solution or a stand-alone coat, expose and develop system. Figure 3 shows one possible LithoPack configuration for 300 mm wafers.
The robot of a standard Brooks I/O with up to four load ports (e.g., two 300 mm FOUPs and two 200 mm open cassettes) feeds wafers into the coat/bake/ develop section of the cluster system. In this example, the tool consists of two spin-coating modules, one hot/cool plate stack and a develop module. |
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