Photoresist Processing


Lithography is the process of defining the regions or patterns on the wafer where material is to be deposited or removed, or where dopants are to be introduced. One important aspect of lithography is photoresist processing, which is the process of covering areas that either need to be subsequently removed or retained with a light sensitive film known as photoresist. The process of material removal following a photolithographic process is known as etching.


Photoresist layers have two basic functions: 1) precise pattern formation; and 2) protection of the substrate from chemical attack during the etch process. Typical resists consist of three components: 1) the resin, which serves as the binder of the film; 2) the inhibitor or sensitizer, which is the photoactive ingredient; and 3) the solvent, which keeps the resist in liquid state until it is processed. The pattern formed by the resist layer actually results only after the unwanted portions of the erstwhile uniformly distributed  resist have been removed, as explained in the following discussion on photoresist processing.


Photoresist processing, or simply resist processing, basically consists of six steps: 1) dehydration and priming; 2) resist coating; 3) soft baking; 4) exposure; 5) development; and 6) post-development inspection.


Prior to the application of resist to a wafer, the wafer must be free of moisture and contaminants, both of which cause a multitude of resist processing problems.  Dehydration baking is performed to eliminate any moisture adsorbed by substrate surfaces, since hydrated substrates result in adhesion failures. The bake is usually performed between 400 deg C to 800 deg C.  Convection ovens may be used for baking up to 400 deg C, while furnace tubes are used for 800 deg C baking. After dehydration baking, the wafer is coated with a pre-resist priming layer designed to enhance the adhesion properties of the wafer even further. One of the most common primers used for this purpose is hexamethyldisilazane (HMDS). Resist coating must follow as soon as possible after priming (within an hour after priming).


Resist coating, or the process itself of producing a uniform, adherent, and defect-free resist film of correct thickness over the wafer, is usually performed by spin-coating.  Spin-coating consists of dispensing the resist solution over the wafer surface and rapidly spinning the wafer until it becomes dry. Most spin-coating processes are conducted at final spin speeds of 3000-7000 rpm for a duration of 20-30 seconds.


Resist coating is followed by a soft bake, which is done to: 1) drive away the solvent from the spun-on resist; 2) improve the adhesion of the resist to the wafer;  and 3) anneal the shear stresses introduced during the spin-coating. Soft baking may be performed using one of several types of ovens (e.g., convection, IR, hot plate). Soft-bake ovens must provide well-controlled and uniformly distributed temperatures and a bake environment that possesses a high degree of cleanliness.  The recommended temperature range for soft baking is between 90-100 deg C, while the exposure time needs to be established based on the heating method used and the resulting properties of the soft-baked resist.


After a wafer has been coated with photoresist and subjected to soft baking, it has to undergo exposure to some form of radiation that will produce the pattern image on the resist.  The pattern is formed on the wafer using a mask, which defines which areas of the resist surface will be exposed to radiation and those that will be covered.  The chemical properties of the resist regions struck by radiation change in a manner that depends on the type of resist used.  Irradiated regions of positive photoresists will become more soluble in the developer, so positive resists form a positive image of the mask on the wafer.  Negative resists form a negative image of the mask on the wafer because the exposed regions become less soluble in the developer.


Development, which is the process step that follows resist exposure, is done to leave behind the correct resist pattern on the wafer which will serve as the physical mask that covers areas on the wafer that need to be protected from chemical attack during subsequent etching, implantation, lift-off, and the like.  The development process involves chemical reactions wherein unprotected parts of the resist get dissolved in the developer. A good development process has a short duration (less than a minute), results in minimum pattern distortion or swelling, keeps the original film thickness of protected areas intact, and recreates the intended pattern faithfully. 


Development is carried out either by immersion developing, spray developing, or puddle developing. Regardless of method used, it should always be followed by thorough rinsing and drying to ensure that the development action will not continue after the developer has been removed from the wafer surface. 


Post-development inspection, as the name implies, is an inspection conducted after development to ensure that the resist processing steps conducted earlier have produced the desired results.  This is typically done using an optical microscope, although SEM and laser-based systems are also used in some post-development inspection tasks. Items that this inspection step checks for include the following: 1) use of the correct mask; 2) resist film quality; 3) adequate image definition; 4) dimensions of critical features; 5) defects and their densities; and 6) pattern registration.    


Primary Reference:  S. Wolf and R. N. Tauber, "Silicon Processing for the VLSI Era Vol. 1", Lattice Press

Buy it now at:  Silicon Processing for the VLSI Era, Vol. 1: Process Technology


See Also:  Lithography/EtchWet Etching




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