Processing of Semiconductor Chips

The best method for understanding the steps in manufacturing semiconductor chips is to see the video produced by SEMI called "Sillicon Run 1". Video ordering information: (phone) 1-408-943-6901 (fax) 1-408-943-7919 (online) http://dom.semi.org/webcatalog.nsf/itemList?OpenForm&group=1300. This takes you into a FAB unit to view from start to finish the methods used in making the chips. This overview video is a good starting point for understanding why high-purity electronic chemicals are important to this industry. Listed below is more information on processing steps for making semiconductor chips and where EastaPure electronic chemicals can be used.

Wafer Preparations and Pre-Coating

After silicone wafers are prepared, they must be cleaned of particles, particulate and films. This is accomplished by dipping the wafers into strong inorganic acids and peroxide solutions. The wafer is rinsed and spun dry in an atmosphere of filtered nitrogen. The cleaned and dried wafer must have an adhesion promoter applied to allow good uniform application of photoresist ink in the next process step.

Hexamethyldisilizane (HMDS) is typically selected as the adhesion promoter that is applied by spin coating, puddle/spin coating, or vapor priming onto a silicone wafer. The primed wafer after drying is ready for coating with the photoresist ink.

Photoresist inks are necessary to create pattern pathways for integrated circuits. A process called photolithography is used to expose a light source through photo mask and project this pattern onto the silicone wafer coated with a light-sensitive polymer and resin. The ingredients for the photoresist inks follows:

Diazonaphthoquinone (photoactive compound)
Novolac resin
Stabilizer
Polymerization inhibitors
Viscosity control agent
Dyes
Plasticizers
Solvents*

*Eastman has products such as EastaPure MAK, PM Acetate, n-Butyl Acetate, and EEP now being used in photoresist formulations.

The photoresist is applied to a spinning wafer with 2-4 ml used per layer of design. A typical chip will have 12 or more layers developed on top of each other with 24-48 ml of photoresist ink used per wafer. Since the solvent makes up 60%-70% of the ink, 14.4-33.6 ml of high purity solvent can be used per wafer.

New technology is driving the semiconductor industry to smaller designs (submicron -0.25 and less) for pathways, which is reducing the required amount of photoresist ink required/wafer: however, this ink is much higher price due to tighter (lower) levels of trace elements needed for processing the chips. So volumes will decrease while prices of solvents and other products increase.

¹ Information from SRI International "Electronic Chemicals" 1992

Application of photoresist ink to spinning wafer

Additional information, photographs, etc. available at: http://w3.kunsan.ac.kr/~bwolf/semi/_photoresist.html

Edge-Bead Removal

After the photoresist ink has been applied by the spin-coating process, the outer edges of the wafer have a slight buildup of photoresist ink. This extra coating must be removed by using high-purity solvent mixtures (the mixtures usually include solvents used in the photoresist ink) to level the edges to the same thickness as the other surface area on the wafers. Proper focus of light by photolithography cannot occur without level coatings on the wafer.

This requires spinning the wafer while high purity solvent mixtures are applied, usually 10-20 mL, to remove the beads. This process is done before the polymer is cross-linked (baked) or the design is applied by photolithography. This edge-bead removal is done for each layer, so, for a typical wafer, 120-240 ml of solvents would be used. We have the following high purity solvents for this process: EastaPure MAK, MPK, n-Butyl Acetate, Ethyl Acetate, PM Acetate and EEP (the EEP solvent is being used now and will make the EastaPure family in 2000).

Edge Bead Remover Equipment

Additional information, photograhs, etc. available at: http://www1.ushio.co.jp/catalog/300e/index.html

After the resist has been exposed to light, it is developed with tetramethylammonium hydroxide (TMAH). This is usually done with 50-100 mL/layer of TMAH. This mild base dissolves the acidic photosensitive polymeric-exposed regions (exposed to light through the photo mask) to make the patterns or pathways for the integrated circuits. This developer is used for positive photoresist and will probably continue to be used with new technology. We do not have chemicals for this process.

After the designs have been patterned by photolithography, and the polymer/resin cross-linked by baking, the wafer can be doped with p (boron) or n (phosphorous) channels by ion implantation. Upper level designs can be filled with dielectric films (insulators) or desired metals in the pathway. The photoresist must be removed by stripping before the new layers are made. Using strong chemicals (acids, oxidizers and bases) does this along with solvents to break up the cross-linked polymer/resin. Dissolve and remove by rinsing in high purity water.

The stripping process is usually a two-step process:

1st Step: Using plasma or piranha strip with acid/oxidizer (sulfuric acid/hydrogen peroxide). This is used to break the crossed linked polymer, which can be removed, in the next step. Some new technology is looking at ozone (O₃) and DI water (high purity) to replace the acid/oxidizer process.

2nd Step: N-methyl pyrrolidinone (NMP) has been used as a solvent with mixtures of organic amines (hydroxylamine, ethanolamine, 2-ethoxy 2-ethanol amine) or other strong base materials. These products are heated to 65-100°C for 30 minutes to break down the cross-linked polymers so they can be dissolved in the NMP and rinsed with DI water followed by drying (isopropyl alcohol can be used). To reduce the cost of stripping formulation, diethylene glycol ethers have been used such as DE and DM solvents. Eastman has both of these products along with DP and DB solvents. Ashland could use some of these products, since they make a series called ACT, Mallinckrodt Baker with PRS series, and EKC Technology with EKC series.

Some photoresist stripping is done using the plasma system (see plasma etching for details) or plasma/liquid chemical process.

Laboratory Stripping

Additional information, photograhs, etc. available at: http://www.engr.washington.edu/~cam/wtcmfl/photoresiststripbench.html

This process is used on the upper levels on the chip to give a smooth surface since the lower levels can produce uneven surfaces. The photolithography process of making patterns needs a mirror-like surface to develop the submicron width lines used in the computer chips used today. This precision polishing process only removes ten to hundreds of angstroms of material from the surface. This is done by mechanical action of polyurethane pads in aqueous (very high-purity water) slurries with suspended particles of aluminum oxide, zinc oxide, or manganese oxide. The future slurries might be made of organic polymers since the industry is moving from aluminum to copper that is more sensitive to the CMP process. Eastman does not have products for use in this area.

This is often called dry etching since liquid chemicals are not used, but rather hot plasma gas is used to remove the silicon and other materials. The gas contains oxygen or one of the halogenated hydrocarbon atoms such as fluorine, chlorine, bromine, or iodine. In the plasma reaction, the etching process is controlled by two factors: the type of plasma discharged (inert or reactive) and the amount of bombardment. The bombardment occurs in the plasma chamber where the wafer is placed and the plasma gas which is a mixture of electrically charged and neutral particles that reacts collectively to electromagnetic forces. Eastman does not have chemicals for this process. Because of good performance of this type of etching when compared to wet chemicals, there is a move to use this in the newer technologies (DUV chips with submicron lines). Wet etching is good for microchip dimensions of 3 µm and greater, but below this dry or plasma is suggested.

High-purity inorganic acids are used in the etching process on silicon chips. Acids such as nitric, hydrofluoric, hydrochloric, phosphoric, and sulfuric are commonly used for etching. To control etching rates, a mixed acid etchant (MAE) is used that includes the inorganic acids plus acetic acid. This can be used on the silicone wafer for initial polishing to provide a mirror-type surface. It can also be used for controlled etching of aluminum. Eastman provides EastaPure acetic acid, which is used in the MAE formulations.

Etching and Processing Equipment

Additional information, photographs, etc. available at: http://www.mff.ust.hk/photo.htm

Plasma or wet chemicals can do sidewall polymer removal. Both processes are used in the manufacture of semiconductor chips. Some of the same chemicals used in the photoresist stripping process could be used for sidewall polymer removal. Due to the cross-linking and baking process, these sidewalls are difficult to remove and strong chemicals must be used. Successful removal is needed to clear pathways for building the integrated circuits. See chemicals for stripping to see the very high purity solvents needed for this process.

A new system of using process gas with Excimer laser is being tested for the removal of sidewall polymers.