Wafer grinding is a process for reducing the thickness of silicon wafers to make them thinner. Typically, the thickness of a wafer is 800-700um. The process thins a wafer to about a tenth of that thickness. There are multiple layers of silicon, but the total height of all the layers must be less than 1.4mm.
Process of thinning silicon wafers
The process of thinning silicon wafers varies according to the material that is to be thinned. The conventional methods involve mechanical grinding and chemical-mechanical polishing. Wet etching is also an option. This involves the application of abrasive slurry and a polishing pad to thin the silicon wafers. This process is more expensive and less clean than the conventional methods.
Backside thinning of fully processed IC wafers has become an increasingly popular technique in recent years. This technique is used to create thin ICs that can be processed at high speed. As an example, Figure 1 illustrates the thickness trend of finished ICs used in advanced package technologies. Various sources estimate the potential market for this technology to reach over 40% of the global wafer market by 2010.
Another method for thinned silicon wafers is dicing. The process begins by preparing dicing grooves on the front surface of the silicon wafer. The dicing grooves correspond to the projected chip thickness. Then, the thinned silicon wafer grinding is bonded to the carrier substrate. The process of dicing is a relatively simple method that can achieve a high level of uniformity.
The final process of semiconductor manufacturing involves lapping to remove saw marks and remove any microscopic cracks and surface damage. This step typically takes a week to a month.
Surface roughness is one of the most important parameters in direct wafer bonding. The work of adhesion between silicon wafers and the silicon substrate is proportional to the roughness of the silicon surface. A rougher silicon surface has a greater tendency to bond to metals.
Techniques used to thin wafers
There are many techniques for thinning wafers, with different levels of precision and throughput. Some methods require chemicals, while others require plasma or mechanical grinding. The most common technique is mechanical grinding, which utilizes a diamond-bonded grinding wheel and high-speed spindle to remove material from the wafer’s surface. The rate of material removal depends on the grind recipe and can be coarse, intermediate, or fine.
Mechanical grinding is the most common technique for thin wafers, with thinning rates of up to 5 um/s. This method is particularly effective when a large part of a wafer needs to be thinned, such as the entire wafer. However, this technique requires additional polishing to remove the damaged layer.
Stabilizing strips also play an important role. They prevent wafers from vibrating while being sliced, reducing total thickness variations. By eliminating vibrational stresses, a wafer dicing is able to be sliced at a much higher rate and with a substantially uniform thickness.
Wet etching is another technique for thinned wafers. This technique involves passing a thin stream of an etching agent over a rotating wafer. The process is useful for removing surface damage and can result in perfect flatness. The process is also helpful for selective thinning.
Impact of spindle grinding wheel on grind quality
The impact of the spindle grinding wheel on wafer grinding quality can be attributed to several factors. Increasing the speed of the spindle increases the speed at which the grinding wheel cuts the workpiece, increasing the number of effective abrasives per unit of time. However, increasing the speed also increases the generation rate of grinding debris.
The vibration of the spindle is responsible for creating waviness on the surface of the ground wafer. The amplitude of this waviness decreased with decreasing wheel speed.
The grinding wheel of a spindle is a critical part of a wafer machining process. A properly shaped grinding wheel is necessary to produce a planar surface. However, the vibration caused by the spindle wheel can seriously compromise the flatness of the fabricated wafer. The authors propose a mathematical model that accounts for the vibration-induced error in surface formation. This surface formation model also takes into account the overlapping effect of the grinding wheel.
Grinding force is also a critical factor that influences the TTV of a wafer. The force is directly related to the grit size of the wheel and the vertical pressure applied during grinding. The finer the grit, the smaller the scratches. Scratches reduce the strength of silicon, so a smooth surface is essential to minimize the amount of silicon exposed to the scratches.
A properly balanced grinding wheel improves both accuracy and speed. While traditional methods of balancing a spindle wheel are costly and require special knowledge, an automatic balancing system can permanently monitor spindle vibration and automatically correct any imbalance.
During the grinding process, excessive vibration reduces the spindle’s life and shortens its life. In addition, excessive vibration slows the process, resulting in lower throughput and reduced productivity. A second function of the spindle grinding wheel is to measure the axial force imparted to the workpiece.
Impact of lapping process on the flatness of the wafer
The lapping process can affect the flatness of a wafer by modifying the substrate’s surface roughness. This effect is dependent on the groove density. The denser the grooves are, the higher the MRR. In our experiment, we used groove densities less than 0.3. We found that MRR increased rapidly at this density, but increased gradually as we increased the groove density.
The lapping process is critical to the flatness of the silicon wafer. It includes statistical tools, such as multi-vari charts, that can help improve processes.
A proper process mapping can help you identify the factors that impact the flatness of a wafer. For instance, if the silicon wafer is not rotating in the lapping carrier at a high enough speed, uneven lapping can occur.
A proper lapping film thickness and film strength are important to achieve uniform surface roughness. If the film thickness is low, the component material will wipe closer to the lapping plate and will result in higher surface roughness measurements. Higher surface roughness means the wafer won’t be as flat.
Lapping silicon wafers requires a high-hardness lapping surface plate. This plate also provides equivalent flatness and reduced scratch occurrence to the conventional lapping surface plate. However, it is important to consider that the thickness of the high-hardness plate will increase as the silicon wafer diameter increases.
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