There are four primary ways to thin wafers, (1) mechanical grinding, (2) chemical mechanical planarization, (3) wet etching and (4) atmospheric downstream plasma dry chemical etching (ADP DCE). There are two groups that make up the four wafer thinning techniques: grinding and etching. To grind wafers, a wheel and water or chemical slurries combine to react with and thin the wafer, while etching is uses chemicals to thin the substrate.
Mechanical (conventional) grinding – This process has a high thinning rate, making it a very common technique. It uses a diamond and resin bonded grind wheel mounted on a high-speed spindle, similar to what is used in spin-on applications. 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 wafer is placed on a porous ceramic chuck and held in place by a vacuum. The wafer is situated with its back side facing up towards the grind wheel, while a grind tape is placed on the front side of the wafer to protect it from any damage during the thinning process. The two wheels rotate in opposite directions as deionized water sprays onto the wafer to make sure there is adequate lubrication between the grind wheel and substrate. This also controls the temperature and thinning rate to make sure that wafers are not cut too thin. In all, the process takes two steps:
- Coarse grinding that does most of the thinning at a rate of ~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, as well as 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, 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.
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