Mechanical Grinding – This process is the most conventional thinning technique used today and is advantageous for its accuracy and high thinning rate. Mechanical grinding utilizes a diamond and resin bonded grind wheel mounted on a high-speed spindle. The grind recipe is responsible for the speed of the spindle as well as the rate of material removed.
To prepare for mechanical grinding, a layer of grinding tape is applied to the device (front) side of the wafer to protect it from any damage during the thinning process. The wafer is then placed on top of a porous ceramic chuck which holds the wafer in place by using a vacuum. The grinding wheel and the chuck rotate in opposite directions as deionized water is sprayed onto the wafer to provide cooling and wash away material particles generated during the grind. In all, the process takes two steps:
- Coarse Grinding – This step produces the majority of the material removal with removal rates of around ~5μm/sec.
- Fine grinding with a 1200 to 2000 grit sand & poligrind fine grind. This typically removes ~30µm or less of material at ≤1μm/sec and provides the final finish on the wafers.
- A 1200 grit sand leaves a rough finish with visible grind marks, while 2000 grit sand is less rough, but some grind marks are still apparent. Poligrind is a polishing tool that provides the most wafer strength and removes most subsurface damage.
Chemical Mechanical Planarization (CMP)
Chemical mechanical planarization (CMP) – This process flattens wafers and removes irregular topography on the surface. CMP is performed using a small particle abrasive chemical slurry and a polishing pad. This process provides more planarization than mechanical grinding, although it tends to be less clean.
Chemical mechanical planarization takes place in three steps:
- Mount the wafers to a backside film, like a wax mount, in order to hold them in place.
- Apply a chemical slurry from above and distribute it evenly with a polishing pad.
- Spin the polishing pad for about 60-90 seconds for each polish, depending on final thickness specifications.
- The thinning rate of CMP is slower than mechanical grinding, removing only a few microns per second. This results in near perfect flatness and a very controlled TTV.
Wet etching uses liquid chemicals, or etchants, to remove material from a wafer. This is useful in situations where only portions of the wafer need to thinning. By placing a hard mask on the wafer prior to etching, thinning will only occur on parts of the substrate without it. There are two ways to perform wet etching: isotropic (uniformly in all directions) and anisotropic (uniformly in vertical direction).
This process takes place in three steps:
- Liquid etchants diffuse onto wafer surface. The liquid etchant changes depending on the desired thickness and whether isotropic or anisotropic etching is necessary.
- In isotropic etching, the most common etchants are a combination of hydrofluoric acid, nitric acid, and acetic acid (HNA); the most common anisotropic etchants are potassium hydroxide (KOH), ethylenediamine pyrocatechol (EDP), and tetramethylammonium hydroxide (TMAH).
- A thin stream of etching agents sprays across the surface of a rotating wafer and the liquid etchant reacts with the substrate to thin it. The reaction rate can change depending on the etching agents used in the reaction, although most reactions remove ~10µm/min.
- Chemical byproducts diffuse from the surface of the wafer.
Atmospheric Downstream Plasma (ADP) Dry Chemical Etching (DCE)
ADP DCE is the newest wafer thinning technique and is a similar process to wet etching. Instead of using liquid, dry chemical etching uses plasma or etchant gases to remove material. This process uses a mixture of argon (Ar) and tetrafluoromethane (CF4) to thin the substrates. To perform the thinning process, either a high kinetic energy particle beam is shot at the target wafer or chemicals react with the wafer surface, or a combination of both. Dry etching removes ~20µm/min and there is no mechanical stress or chemicals necessary, so this method is able to produce very thin wafers with high yield.
Back grinding is a process that removes silicon from the back surface of a wafer. We provide grinding on our own substrates or on customer supplied wafers. We process bare and device patterned wafers with high yield and offer wafer thinning to customer specifications.
SVM Wafer Back Grinding Capabilities:
- Diameters: 25mm – 300mm
- Final wafer thickness for 50μm to 200μm: ≥ 50μm
- Final wafer thickness for 300mm wafers only: ≥ 80μm
- Back surface finish: ground, lapped, or polished
- Typical yield: ≥ 95%
SVM provides lapping for all wafer diameters 50mm to 300mm.
SVM provides wafer lapping services when there is a need to remove bulk amounts of material from a substrate. Bulk silicon removal is often a requirement in wafer reclaim and wafer thinning projects, as well as in major end uses like cell phones and modems.
What is wafer lapping?
Wafer lapping is a global planarization process that improves wafer flatness by removing surface damage, often from backside grinding. It is most common on silicon wafers, although certain applications require gallium arsenide (GaAs) and indium phosphide (InP) wafers to undergo this process as well. Lapping takes place between two counter-rotating cast iron plates and either an abrasive film or slurry. To adjust the penetration of the film/slurry, the wafers either spin faster or experience a heavier load to fit the target specification.
There are two ways to perform this process: free and fixed abrasive lapping.
Free abrasive lapping:
In free abrasive lapping, a slurry removes surface damage. The slurry consists of an abrasive powder floating in lapping oil. The abrasive powder is made with small particles (typically 5-20μm) of silicon carbide (SiC), aluminum oxide (Al2O3) or diamond, depending on the substrate material, diameter, and target thickness. Before depositing, the slurry spins in order to suspend the particles. When the slurry is ready, cast iron plates rotate slowly (< 80rpm) to distribute the film evenly across the wafer surface. After lapping, some wafers go through a second polishing to remove any remaining particles.
Fixed abrasive lapping:
Fixed abrasive lapping is the same process as free abrasive lapping. The only difference is that a thin SiC or other abrasive film deposits the particles on the substrate instead of using a slurry. The film consists of the same particles as free abrasive lapping on a thin polyester substrate. The film acts like sand-paper between the cast iron plates and the substrate, which rotates the same as free abrasive lapping. Ultimately, fixed abrasive lapping is much thicker than free abrasive lapping, which can produce superior flatness qualities and rounded edges.