Briefly Describe The Four Main Methods Of Wafer Thinning

The four thinning techniques for wafer thinning consist of two groups: grinding and etching.
(1) Mechanical grinding
(2) Chemical mechanical planarization
(3) Wet etching
(4) Plasma Dry Chemical Etching (ADP DCE)
Grinding uses a combination of grinding wheels and water or chemical slurries to react with and thin the wafer, while etching uses chemicals to thin the substrate.
Grinding:
◆ Mechanical grinding
Mechanical (Conventional) Grinding – This process has a high rate of thinning, making it a very common technique. It uses a diamond and resin-bonded grinding wheel mounted on a high-speed spindle, similar to those used in spin-coating applications. The grinding recipe determines the speed of the spindle as well as the material removal rate.
To prepare for mechanical grinding, the wafer is placed on a porous ceramic chuck and held in place by vacuum. The backside of the wafer is placed towards the grinding wheel, while the abrasive belt is placed on the front side of the wafer to prevent any damage to the wafer during thinning. When deionized water is sprayed onto the wafer, the two gears rotate in opposite directions to ensure sufficient lubrication between the grinding wheel and the substrate. This also controls the temperature and thinning rate to ensure that the wafer is not ground too thin.
The process is a two-step process:
1. Coarse grinding does most of the refining at a rate of ~5μm/sec.
2. Fine grinding with 1200 to 2000 grit and poligrind. Typically removes ~30µm of material at a rate of ≤1µm/sec and provides a final finish on the wafer.
The 1200-grit has a rough finish with noticeable wear marks, while the 2000-grit is less rough but still has some wear marks. Poligrind is a polishing tool that provides maximum wafer strength and removes most subsurface damage.
◆ Chemical Mechanical Planarization (CMP)
Chemical Mechanical Planarization (CMP) – This process flattens the wafer and removes surface irregularities. CMP is performed using small particle abrasive chemical slurries and polishing pads. Provides more planarization than mechanical grinding.
CMP is divided into three steps:
1. Mount the wafer on a backside membrane, such as a wax holder, to hold it in place.
2. Apply the chemical slurry from above and distribute it evenly with a polishing pad.
3. Rotate the polishing pad for approximately 60-90 seconds per buff, depending on final thickness specifications.
CMP grinds at a slower rate than mechanical grinding, removing only a few microns. This results in near perfect flatness and controllable TTV.
Etching:
◆ Wet etching
Etching uses liquid chemicals or etchants to remove material from the wafer, which is useful when only portions of the wafer need to be thinned. By placing a hard mask on the wafer prior to etching, thinning occurs only on the part of the substrate where there is no substrate. There are two methods of performing wet etching: isotropic (uniform in all directions) and anisotropic (uniform in the vertical direction).
Liquid etchants vary depending on the desired thickness and whether isotropic or anisotropic etching is desired. In isotropic etching, the most common etchant is a combination of hydrofluoric acid, nitric acid, and acetic acid (HNA). The most common anisotropic etchants are potassium hydroxide (KOH), ethylenediaminecatechol (EDP) and tetramethylammonium hydroxide (TMAH). Most reactions proceed at a rate of ~10 μm/min, and the reaction rate may vary depending on the etchant used in the reaction.
◆ Plasma (ADP) Dry Etching (DCE)
ADP DCE is the latest wafer thinning technology, similar to wet etching. Instead of using liquids, dry chemical etching uses plasma or etchant gases to remove material. To perform the thinning process, a beam of highly kinetic particles can be fired at the target wafer, chemicals react with the wafer surface, or both are combined. The removal rate of dry etching is about 20μm/min, and there is no mechanical stress or chemicals, so this method can produce very thin wafers with high quality.