Wafer Cleaning Procedures; Photoresist or Resist Stripping; Removal of Films and Particulates

Wafer Cleaning

There are a number of wafer cleaning techniques or steps employed to ensure that a semiconductor wafer is always free of contaminants and foreign materials as it undergoes the wafer fabrication process. Different contaminants have different properties, and therefore have different requirements for removal from the wafer. Below are some examples of commonly-used methods for wafer cleaning.

Photoresist Stripping

Photoresist stripping, or simply 'resist stripping', is the removal of unwanted photoresist layers from the wafer. Its objective is to eliminate the photoresist material from the wafer as quickly as possible, without allowing any surface materials under the resist to get attacked by the chemicals used. Resist stripping can be classified into: 1) organic stripping; 2) inorganic stripping; and 3) dry stripping.

Organic stripping employs organic strippers, which are chemicals that break down the structure of the resist layer. The most widely-used commercially available organic strippers used to be the phenol-based ones, but their short pot life and difficulties with phenol disposal made low-phenol or phenol-free organic strippers the more popular choice nowadays.

Wet inorganic strippers, which are also known as oxidizing-type strippers, are used for inorganic stripping, usually to remove photoresist from non-metallized wafers, as well as post-baked and other hard-to-remove resists. Inorganic strippers are solutions of sulfuric acid and an oxidant (such as ammonium persulfate), heated to about 125 deg C.

Dry stripping pertains to the removal of photoresist by dry etching using plasma etching equipment. Its advantages over wet etching with organic or inorganic strippers include better safety, absence of metal ion contamination, decreased pollution issues, and less tendency to attach underlying substrate layers.

Chemical Removal of Film Contaminants

Chemically bonded films of contaminant material may be removed from a wafer surface by chemical cleaning. Chemical cleaning comes in various forms, depending on the nature of the film contaminants that need to be removed and from which surface. In general, however, chemical cleaning is performed with a series of acid and rinse baths. As an example, removal of film contaminants from a wafer with nothing but thermally grown oxide may consist of the following steps: preliminary cleaning, removal of residual organic contaminants and some metals, stripping of the hydrous oxide film created by the previous step, desorption of atomic and ionic contaminants, and drying. Storage of cleaned wafers must be avoided but, if necessary, must be done using closed glass containers inside a nitrogen dry box.

Sputter Etching of Native Oxide Films

A thin oxide layer grows over silicon or aluminum when these are exposed to air, forming SiO2 and Al2O3, respectively. These 'native' oxide layers need to be removed in places where they shouldn't be, because they exhibit adverse effects such as higher contact resistance or hampered interfacial reactions. In-situ sputter or plasma etching are the techniques commonly utilized to remove these native oxides from the wafer. "In-situ" means performing the sputter or plasma etching in the same vacuum environment where the overlying layer will be deposited.

Elimination of Particulates

The contamination of wafers with insoluble particulates is also a common problem. There are two frequently utilized techniques for removing particulates from a wafer: 1) ultrasonic scrubbing; and 2) a combination of mechanical scrubbing and high-pressure spraying.

Ultrasonic scrubbing consists of immersing the wafer in a liquid medium which is supplied by ultrasonic energy. The sonic agitation causes microscopic bubbles to form and collapse, creating shock waves that loosen and displace particles. Ultrasonic scrubbing requires a filtration system that removes the particles from the bath as they get detached. One drawback of ultrasonic scrubbing is that they can cause mechanical damage to substrate layers.

Mechanical scrubbing employs a brush that rotates and hydroplanes over a solvent applied on the wafer surface. This means that the brush does not actually contact the wafer, but the solvent moved by the rotating brush dislodges particles from the wafer surface. As this happens, high-pressure spraying of a D/I water jet over the wafer surface is done to help in clearing the wafer surface of particulate contamination.

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