Backside grinding is a crucial step in reducing the thickness of a semiconductor wafer. This step links the front-end and back-end processes to create a semiconductor chip with a reduced thickness. Thinner wafers allow higher chip stacking, but at a cost of lower performance. If you are considering backside grinding for your next semiconductor project, consider these tips. You will be glad you took the time to read this article.
Backside grinding
Backside grinding of a wafer reduces its thickness from approximately 800-700um to 80-70um. Wafers are then stacked in four to six layers, or up to sixteen or 32 layers for multichip packages. During this process, the wafer’s thickness must be below a specific level, or the next process becomes much more difficult, requiring more expensive and sophisticated machinery. This is where MTI Instruments comes in.
The method is particularly useful for protecting electronic devices and components positioned on the active surface of a wafer. The bumps on the active surface of a wafer can be damaged if they are not surrounded by a protective layer, such as a hot-melt adhesive. Backside grinding protects these devices by reducing the risk of them being damaged. When used for electronic devices and passive components, backside grinding is a good choice.
The backside grinding of a wafer can be divided into three processes: taping, backside grinding, and chip separation. Tape lamination, the first step of back grinding, involves attaching an adhesive tape to the front of the wafer. As the silicon compound spreads throughout the wafer during the back grinding process, it may cause it to break. This is especially important with larger wafers, as they are more fragile and more prone to breaking under the force. During this process, the wafer is also protected from ultraviolet light, which can damage the wafer.
Wet etching
Wet etching is a common process for semiconductors, but there are several differences between it and other techniques. A wet etch needs a mask that is not dissolved by the etching solution and adheres to the target layers. The mask material needs to be thin enough that it does not contaminate the etched material and must be sufficiently adhering to the wafer to prevent etching from taking place in the target layers.
Wet etching has many advantages, including high selectivity for most materials and the ability to process several wafers at a time. The disadvantages of wet etching include its limited resolution, the high cost of etchants, and problems with the resist losing its adhesion to the substrate. Additionally, the etching process can become incomplete due to the presence of bubbles or other irregularities in the wafer.
To do wet etching, researchers must prepare the etching solution. The LNF stocks buffered HF and commercial etchants. The LNF provides detailed instructions for conducting an etch using these two chemicals. There are references below for specific etch rates and selectivity. Researchers needing to etch materials other than silicon and glass should contact the LNF staff for guidance. The LNF has several tools for wet etching, including the CMOS-clean bench and Acid Bench 12.
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Poligrind
The process of double side grinding is more accurate than single side grinding and offers a cost advantage over CMP. With this technique, ground silicon wafers have identical thickness, which makes the transfer to polishing much easier. The double side grinding technology allows full tracking of wafers throughout the manufacturing process. Nitto designs and manufactures products for the production of electronics and electric materials. They developed Poligrind for wafer grinding.
The process involves placing the wafer on a porous ceramic rotating vacuum plate that faces a grind wheel. During this process, the wafer is washed with deionized water continuously. The grinding process is divided into two steps: a coarse grind removes the bulk of the material, while the fine grind removes the final 30um or less of material to produce a near-mirror finish. The conventional grind uses 1200 or 2000 grit sand to provide the desired surface finish, while poligrind has a near-mirror finish.
DISCO Corporation is one of the leading OEMs in the wafer grinding market. Their Precision Machines business unit produces grinding tools that are used in semiconductor manufacturing. The company has two types of Poligrind wafer grinders: the GF01 and IF series. They are both designed to provide high precision, consistent processing, and minimal lint. The UltraPoligrind wheel is also a chemical-free, metal-free grinding tool that is capable of producing thin backside wafers.
Poligrind vs CMP
When comparing Poligrind and CMP wafer grinding, it is necessary to consider the type of material being ground. The two methods use different thinning processes and both have their advantages and disadvantages. Both techniques produce the same final thickness but differ slightly in the type of material used. To determine which process is right for your project, read our comparison of Poligrind vs CMP wafer grinding.
In the first method, the wafer is ground with a high-grit sand, leaving visible grind marks. This process is also known as back grinding. It is the most expensive and provides the best surface finish, but the backside of the wafer has less planarization. In contrast, CMP uses a chemical slurry to flatten the wafer. Moreover, this process is dirtier than conventional grinding, which is why it is less cost-effective. It is also difficult to remove the backing film from the front surface of the wafer, and it leaves residue.
The cost of Poligrind vs CMP waffle grinding is relatively comparable, but the latter is better suited for exotic materials, such as niobium and silicon. The cost of Poligrind vs CMP wafer grinding depends on how thick the wafer is and how complex the final thickness is, so it is important to consider this factor when comparing the two methods.
TAIKO process
Optim Wafer Services offers a TAIKO process for wafer grinding. It is a method that leaves a silicon ring around the edge of the wafer, allowing for backside processing. The process doesn’t require any major modifications to the equipment used to grind wafers. Optim can perform this process on your own product or provide blank test wafers. Here are some advantages of this method.
TAIKO grinding is an excellent alternative to conventional backgrinding processes. This method preserves the outer edge of the wafer, but thinned out the inner portion. Because of this, the ring does not break during the backside grinding process, which decreases the risk of edge chipping and wafer breakage. Furthermore, this method prevents the outgassing during high-temperature processes. It also maintains the shape of the wafer without requiring a hard substrate.
The main challenge of this method is getting the ball to stick to ultra-thin wafers. This can be achieved by thinning the inner silicon wafer area and leaving the outer ring as a stiffening frame. The development of a TAIKO process for wafer balling started with a pilot line and was applied to a range of residual wafer thicknesses, wafer ball diameters, and balling pitch. The results proved that TAIKO grinds silicon wafers to a uniform thickness.
Diamond wheel
The use of a diamond wheel for wafer grinding enables the production of a high-quality surface. The grinding performance of diamond wheels depends on the wheel specifications and grinding conditions. The finishing surface roughness and abrasive protrusion height determine the grinding force and surface quality. The study aims to understand the dynamic behavior of a diamond abrasive during the process of wafer grinding services. It simulates the grinding process to examine the effectiveness of a certain diamond wheel.
There are four types of abrasives used for grinding wheels. The conventional abrasives used for grinding include silicon carbide, diamond and cubic boron nitride. Diamond wheels are the only ones made of diamond abrasives. Silicon carbide is not suitable for iron-based metals. Diamond wheels are used for the processing of silicon-based devices. A diamond wheel for wafer grinding requires special care in handling because of its high-speed and high-resistance.
A resin-bond diamond grinding wheel with high-concentration pores was first developed by Matsumoto et al. In addition, they used a pore-forming agent to enhance the self-sharpening capability of the diamond wheel. The pore-forming agent had a significant influence on the bond strength of the grinding wheel, whereas a gas-foaming pore-forming agent resulted in an improved grinding performance.
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