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Semiconductor Processes and Materials for Electronics

Wafers (Silicon and other Substrates)

The first step in the fabrication of an integrated circuit is to clean and prepare the silicon wafer substrate. Complete cleaning of semiconductor surfaces requires that particulates, organic films, and adsorbed metal ions be removed. Most cleaning procedures are based on immersion in liquid baths of semiconductor grade solvents or other chemical reagents. SEMI Standard Test Method G52-90 which utilizes ion chromatography is used to measure surface ionic contaminants on the wafers.

Semiconductor Ultrapure Water

Deionized (DI) water is used throughout the fabrication of an integrated circuit. In fact, an integrated circuit may be exposed to DI water hundreds of times during its manufacture. Because contaminants in DI water can distort normal dopant profiles, create inversion layers or cause shorts, or circuit malfunctions, it is extremely important that semiconductor pure water be maintained at its highest attainable quality. The SEMI recommendations for maximum acceptable levels of contaminants in Pure Water for Semiconductor Processing are shown in the SEMI Suggested Guidelines. A variety of technical papers published in Solid State Technology, Ultrapure Water, and other technical journals document case studies of how ion chromatography has been used to maintain the best attainable quality water in Semiconductor Fabrication facilities. Ion chromatography is the only technique recommended by SEMI for inorganic anions. On-line ion chromatography provides the capability to continuously monitor ions at low part per trillion levels.

Chemical Reagents

Chemical reagents, acids, bases and solvents are used to clean wafer surfaces and prepare the wafer for photoresist coating. Trace metals in these reagents can potentially cause yield and device reliability problems. Ion Chromatography allows the determination of parts per billion and even parts per trillion level concentrations of contaminants in chemical reagents. Microbore ion chromatography improves the detection limits for trace ions in concentrated reagents. Dionex solvent compatible ion exchange columns permit reliable determination of trace ions in semiconductor solvents, such as isopropanol, acetone and methanol.

Plating Bath Analysis

With the change from aluminum to copper interconnects, the analysis of the copper plating bath solution has become a critical step in chip production. With line widths moving to below 0.18 mm and aspect ratios >6, baths must be optimized for quality control, then monitored to assure individual additives and components remain at the correct concentrations. Contaminants in the baths must also be monitored to assure that they do not adversely affect bath performance or quality of the final product. Dionex instrumentation can monitor on-line or through grab samples to determine concentrations of additives, bath components, and bath contaminants to assure the quality of the plating deposition.

Photoresist Coating

After dehydrating the surface of the silicon wafer, a light sensitive coating is applied to the wafer for the printing of the circuit pattern. Trace anions and cations are routinely determined in photoresists by ion chromatography. The photoresists are first extracted with eluent, and then the extract is ready for analysis by IC. Ion chromatography has also been used to measure ionic contaminants in photoresist strippers and developers. Using solvent compatible columns, water-soluble strippers can be injected directly into the chromatographic system.

Photoresist Etch

The wafer, covered with the photoresist pattern, is etched with mixed acid etchants. The etch rate and depth is dependent upon the ratio of acids in mixed acid etchants. Wet chemistry methods for the determination of acetic, nitric, phosphoric, and hydrofluoric acids in the mixed etchants are both labor intensive and time consuming. Ion chromatography is rapid, accurate, and automated. SEMI has approved ion chromatography as an acceptable alternative to wet chemistry for assaying mixed acid etchants.

Ion Implantation

Ionized impurity atoms (boron, phosphorous, arsenic) are implanted into the pattern of the exposed silicon wafer. It has been shown that ion chromatography correlates closely with ICP for the determination of boron and phosphorous in chemically vapor-deposited borophosphosilicate glass (BPSG) films. Ion chromatography offers the advantage of being able to speciate the different oxidation state of phosphorous (+3 or +5) in the glass films. Different oxidation states of phosphorous have been shown to produce different glass film properties.

Die Attach

Silicon chips are attached to a package, which provides the contact leads to the chip. Die attach adhesives are used in this operation. Military specifications (MIL-STD-883C and MIL-A- 87172) require the use of ion chromatography to measure chloride, sulfate, and sodium in polymeric adhesives. Manufacturers of die attach adhesives use Dionex ion chromatography to comply with regulations.

Leadframes

Silicon chips are connected via a wire bond to leadframes. Leadframes connect the integrated circuit to the outside world. Contaminants are determined on the surfaces of leadframes according to SEMI Standard Test Method G52-90.

Wire Bond

Wire connections from the silicon chip are made to the leads of the package. Soldering flux residues are measured by extraction of the package and injection of the extract into an Ion Chromatograph. Ion Chromatography permits the identification of inorganic halide and organic acid residues. Even “Fluxless” soldering has been found to leave detrimental concentrations of ionic residues on devices.

Encapsulation

Silicon chips are encapsulated with plastic coating for mechanical and environmental protection. Contaminants entrained in the molding compounds can diffuse out of the molding compound into the integrated circuit and catalyze metallization corrosion. Contaminants are measured by ion chromatography according to SEMI Test Method G29-86.