What are the biggest technical challenges you encounter most in achieving high yield hybrid bonding?

At the end of the day, yield and cost must be addressed for die-to-wafer hybrid bonding to become cost-effective. The issue with die-to-wafer is as follows. First, you have to dice up the wafer, and in doing so, you remove some material. This removal generates particulates, and the current process is not perfect in eliminating 100% of these particles. Some particles generated during the dicing process get redeposited onto the die. Hybrid bonding is extremely sensitive to cleanliness, and any particulate generation results in defects or voids. If a "bubble" forms between the two pieces of silicon, it creates a void, leading to failure. Cleanliness is crucial; you need a super clean fab and a dicing process that generates minimal particles. Even if particles are generated, there must be a cleaning step to remove them as much as possible. We haven't yet reached the point where we can eliminate all the particulates formed. The second issue is warpage. Silicon contains various materials with different coefficients of thermal expansion, causing significant warping. Ideally, the wafer and die for hybrid bonding should be perfectly flat. Warpage leads to poor contact and void formation. Controlling die warpage is problematic, but there is ongoing innovation to address it. Warpage is a natural consequence of using different materials in silicon. During bonding, you must control it. At the wafer level, it is easier to manage due to the larger surface area. At the wafer's edges, you might have some open zones of voids, but not necessarily defects within the die or wafer interior. Die-to-wafer bonding is more complex due to limited real estate. The third issue is alignment. Let's consider an example of wafer-to-wafer bonding. Imagine you bring two wafers together, and there are 100 die on that wafer. You spend one minute aligning and bonding everything. Essentially, you have bonded 100 die in one minute. Now, if you switch to die-to-wafer bonding, you have diced up the wafer and pick up each die. If you spend one minute on each die for alignment and bonding, you multiply that by 100, resulting in 100 minutes. You cannot spend one minute on each die; you must work much faster. For die-to-wafer hybrid bonding to be effective, you need to bond approximately 7,800 die per wafer and achieve a run rate of 1,000 to 2,000 die attachments per hour. Consequently, you encounter many misalignment errors, and controlling warpage becomes challenging on a tool that must move quickly. These are engineering problems that need solutions.